UNIT - V:Principles of Medical Imaging

Radiography:
 An NDT method that utilizes x-rays or gamma radiation to detect discontinuities in materials and to
present their images on recording medium.
 X-rays are discovered in 1895 & Natural radiations are discovered in 1896 as uranium radioactivity
 All materials can be easily examined
 Radiography uses penetrating radiations, Components stop or absorb some radiations which is caused
by differences in material density and thickness.
 Absorption differences can be recorded on film or electronically.
 X-rays & Gamma rays are the main sources of radiations.
 Discontinuities & hidden flaws are easily detected.

 X-rays & Gamma rays are electromagnetic type of radiations of shorter wavelength than visible light.
 ƛ(visible)= 600 A , ƛ(x-rays)= 1 A , ƛ(gamma rays)= 0.0001 A
 X-rays & Gamma rays are different because of difference in source of origin.

I. X-ray imaging:
Equipments that replace certain critical physiological functionalities, or provide needed pain
therapy.

Properties of X-rays

 The X-rays in the medical diagnostic region have wavelength of the order of 10-l0 m. They
propagate with a speed of 3 x 1010 cm/ s and are unaffected by electric and magnetic fields.
 They have short wavelength and extremely high energy.
 X-rays are able to penetrate through materials which readily absorb and reflect visible light.
 X-rays are absorbed when passing through matter. The extent of absorption depends upon the
density of the matter.
 X-rays produce secondary radiation in all matter through which they pass. This secondary
radiation is composed of scattered radiation, characteristic radiation and electrons.
 X-rays produce ionization in gases and influence the electric properties of liquids and solids.
The ionizing property is made use of in the construction of radiation-measuring instruments.
 X-rays also produce fluorescence in certain materials to help them emit light. Fluoroscopic
screens and intensifying screens have been constructed on the basis of this property. X-rays
affect photographic film in the same way as ordinary visible light.
 Production of X-rays

 X-rays are produced whenever electrons collide at very high speed with matter and are thus
suddenly stopped. The energy possessed by the electrons appears from the site of the
collision as a parcel of energy in the form of highly penetrating electromagnetic waves (X-
rays) of many different wavelengths, which together form a continuous spectrum.
 X-rays are produced specially constructed glass tube, which basically comprises,
(i) a source for the production electrons,
(ii) a energy source to accelerate the electrons,
(iii) a free electron path,
(iv) a mean t focusing the electron beam and
(v) a device to stop the electrons.

Stationary mode tubes and rotating anode tubes are the two main types of X-ray tubes:
 Stationary Anode Tube

 Fig. shows the basic components of a stationary anode X-ray tube. The normal tube is a
vacuum diode in which electrons are generated by thermionic emission from the filament of
the tube.
 The electron stream is electrostatically focused on a target on the anode by means of a
suitably shaped cathode cup.
 The kinetic energy of the electrons impinging on the target is converted into X-rays. Most
electrons emitted by the hot element become current carriers across the tube.

 Some X-ray tubes function as a triode with a bias voltage applied between the filament and
the cathode cup.
 The cathode block, which contains the filament, is usually made from nickel or from a form
of stainless steel. The filament is a closely wound helix of tungsten wire, about 0.2 mm thick,
the helix diameter being about 1.0-1.5 mm.
 The target is normally comprised of a small tablet of tungsten about 15mm wide, 20mm long
and 3mm thick soldered into a block of copper. Tungsten is chosen since it com bines a high
atomic number (74)—making it comparatively efficient in the production of X-rays. It has a
high melting point (3400°C) enabling it to withstand the heavy thermal loads.
 Copper being an excellent thermal conductor, performs the vital fu nction of carrying the heat
rapidly away from the tungsten target. The heat flows through the anode to the outside of the
tube, where it is normally removed by convection. Generally, an oil environment is provided
for convection current cooling.
 In addition, the electrodes have open high voltages on them and must be shielded. The tube
will emit X-rays in all directions and protection needs to be provided except where the useful
beam emerges from the tube.
 In order to contain the cooling oil and meet the above-mentioned requirements, a metal
container is provided for completely surrounding the tube. Such a container is known as a
‘shield’.
Rotating Anode Tube
 The construction of a rotating-anode x-ray tube

 The filament is constructed from a spiral of tungsten wire (melting point 3410 °C), which is
set in a nickel block. This block supports the filament and is shaped to create an electric field
that focuses the electrons into a slit beam.
 The anode has a bevelled edge, which is at a steep angle to the direction of the electron beam.
The exit window accepts x-rays that are approximately at right angles to the electron beam so
that the x-ray source as viewed from the receptor appears to be approximately square even
though the electron beam impinging on the target is slit-shaped.

 The choice of the anode angle will depend upon the application, with the angle being varied
according to the requirements of field and focal spot sizes and tube output. For general-
purpose units, an angle of about 17° is appropriate.

 Most of the energy in the electron beam is deposited in the target in the form of heat. The use
of a slit source of electrons helps by spreading out the target area and this idea can be
extended by using a rotating anode, so that the electron beam impinges on the bevelled edge
of a rotating disc and the target area is spread out over the periphery of the disc.
 A rotation speed of about 3000 RPM and an anode diameter of 10 cm are used in general-
purpose units.

X-RAY MACHINE
 Block diagram of basic x-ray machine.

Parts

1. X-Ray tube 2. High Tension Supply 3. Collimator

4. Patient Table. 5. Grid. 6. Radiographic film

1. X ray Tube – For details refer stationary anode tube and rotating anode tube given above.

It is an important component of x-ray machine which is inaccessible as it is contained in a

protective housing. It is a vacuum tube.
There are two primary parts.
1) the cathode
2) the Anode.

2. Operating Console

It is an apparatus in X-Ray machine that allows to control the x-ray tube current and voltage.
The Console Controls are: -

1. Voltage compensator. 2. kV Meter. 3. mA Meter. 4. Exposure time.

1. Voltage Compensator
Because of variations in power distribution to the hospital and in power consumption by the
various sections of the hospital, the voltage to the x-ray unit may vary by 5%, which will
result in large variations in x-ray output.

High Tension Supply with Rectifier

Power supply system consists of Autotransformer

 The power supplied to x-ray machine is delivered to a special transformer called an

Autotransformer. It works on the principle of electromagnetic induction but is very different
from conventional transformer.
 It has only one winding and one core. The single winding has number of connections, or
electric taps. The purpose to use the Autotransformer is to overcome induction losses. Its
value ranges from 0 to 400V.
 Used for producing high voltage which is applied to the tube’s anode and cathode and
comprises a high voltage step-up transformer followed by a recifier.
 Self Rectification Circuit for High Voltage Generation

The high voltage is produced using a step-up transformer whose primary is connected to
auto-transformer. The secondary of the H.T. transformer can be directly connected to the
anode of the x-ray tube which will conduct only during the half cycles when the cathode is
negative with respect to anode or target.

 The current through the tube follow, the H.T. pathway and is measured by a mA meter.
 A kV selector switch enables to change voltage between exposures. The voltage is measured
with the help of a kV meter.
 The exposure switch controls the timer and thus the duration of the application of kV.

 To compensate for mains voltage variations, a voltage compensator is used in the circuit

 The filament is heated with 6 to 12 V of ac supply at a current of 3 to 5 amperes. The

filament temperature determines the tube current or mA, and, therefore, the filament
temperature control has an attached mA selector. The filament current is controlled by using,
in the primary side of the filament transformer, a variable choke or a rheostat. The rheostat
provides a stepwise control of mA and is most commonly used in modem machines.
 These machines have maximum tube currents of about 20 mA and a voltage of about 100 kV.
When self-rectification is used, it is necessary to use a parallel combination of a diode and
resistance, in series with the primary of the H.T. transformer for suppressing higher inverse
voltage likely to appear during the non-conducting half-cycle of the x-ray tube. This helps to
reduce the cost and complexity of the x-ray machines.
 A preferred method of providing high voltage dc to the anode of the x-ray tube is by using a
bridge rectifier using four valve tubes or solid state rectifiers. This results in a much more
efficient system than with half wave of self-rectification methods.
 The kV meter is connected across primary of the H.T. transformer. It actually measures volts
whereas it is calibrated in kV by using an appropriate multiplication factor of turns-ratio of
the transformer.
 In order to obtain the load voltage which varies with the tube current, a suitable kV meter
compensation is provided in the circuit. The kV meter compensator is ganged to the mA
selector mechanically. Therefore, the mA is selected first and the kV setting is made
afterwards during operation of the machine.
 Moving coil meters are used for making current I (mA) measurements, for shorter
exposures, a mAs meter is used which measures the product I of mA and time in seconds.
 The exposure time is generally controlled by using some form of timing arrangement coupled
with a contactor which supplies the H.T. to the anode of the x-ray tube only during that time.
 Collimator: The Collimator is attached to the x-ray tube below the glass window where the
useful beam is emitted. Lead shutters are used to restrict the beam. Its purpose is to minimize
field of view, to avoid un necessary exposure by using lead plates.
 Grid: By virtue of function and material, collimator and grid are same but they have different
location. It is made up of lead. It is located just after patient. It is used to destroy scattered
radiation from the body.
 Radiographic Film: Two types of x-ray photon are responsible for density, contrast and
image on a radiograph. Those that pass through the patient without interacting and those that
are scattered in the patient through compton interaction. Together these x-rays that exit from
the patient and intersect the film are called Remnant x-rays.

APPLICATIONS OF X-RAYS IN MEDICINE

1. X-rays are used in medicine for medical analysis. Dentists use them to find complications,
cavities and impacted teeth. Soft body tissues are transparent to the waves. Bones and teeth
block the rays and show up as white on the black background.

2. A mammogram:
(Also called a mammography exam) is a safe, low-dose x-ray of the breast. A high-quality
mammogram is the most effective tool for detecting breast cancer early. Early detection of
breast cancer may allow more treatment options. Using a low-dose x-ray, the mammogram
machine takes a snapshot of the inside of a woman’s breast. The machine holds and
compresses the breasts so that images at different angles can be taken. Doctors and nurses
examine these snapshots, looking for signs of abnormalities such as lumps, which could be
tumors.

3. Skeletal system:
A standard radiograph is usually the first course of action when a patient is suffering from a
suspected bone injury. The excellent natural contrast provided by bone produces clear images
with good resolution. Two views at right angles to each other are generally required and can
lead to the diagnosis of fractures, dislocations, spinal injuries and so on. Other abnormalities,
ranging from tumours and cysts in the spine to arthritis (figure 2.23), can also indicated.

4. The chest:

A standard chest X-ray is the commonest means of detecting lung cancer and other
abnormalities (figure 2.24). Difficulties sometimes arise due to liie inevitable obstruction of
the heart.
 5. Circulatory system:
An artificial contrast medium, typically an organic iodine compound, is injected into the
blood vessel to be examined. The structure and effective flow diameter of both arteries and
veins can be examined, allowing the diagnosis of blood vessel blockages and heart disease

6. Dental studies:
Most dental practices now have small X-ray units, to investigate problems with the
overcrowding or uneven growth of teeth, particularly in juveniles, or with the growth of
wisdom teeth. Surgery or orthodontal treatment may then be recommended.

7. Foreign bodies:
It is amazing what people, particularly children, will swallow! A standard radiograph can
help to identify the shape and position of such objects to assist with their removal.

COMPUTED TOMOGRAPY

Limitations of X-rays

1. The super-imposition of the three-dimensional information onto a single plane makes

diagnosis confusing and often difficult.
2. The photographic film usually used for making radiographs has a limited dynamic range
and, therefore, only objects that have large variations in X-ray absorption relative to their
surroundings will cause sufficient contrast differences on the film to be distinguished by the
eye. Thus, whilst details of bony structures can be clearly seen, it is difficult to discern the
shape and composition of soft tissue organs accurately.
3. In such situations, growths and abnormalities within tissue only show a very small contrast
difference on the film and consequently, it is extremely difficult to detect them, even after
using various injected contrast media.
4. The problem becomes even more serious while carrying out studies of the brain due to its
overall shielding of the soft tissue by the dense bone of the skull.

Basic Principle of CT

 In computed tomography (CT), the picture is made by viewing the patient via X-ray imaging
from numerous angles, by mathematically reconstructing the detailed structures and
displaying the reconstructed image on a video monitor.
 Computed tomography differs from conventional X-ray techniques in that the pictures
displayed are not photographs but are reconstructed from a large number of absorption
profiles taken at regular angular intervals around a slice, with each profile being made up
from a parallel set of absorption values through the object.
 In computed tomography, X-rays from a finely collimated source arc made to pass through a
slice of the object or patient from a variety of directions. For directions along which the path
 length through-tissue is longer, fewer X-rays are transmitted as compared to directions where
there is less tissue attenuating the X-ray beam. In addition to the length of the tissue
traversed, structures in the patient such as bone may attenuate X-rays more than a similar
volume of less dense soft tissue.
 In principle, computed tomography involves the determination of attenuation characteristics
for each small volume of tissue in the patient slice, which constitute the transmitted radiation
intensity recorded from various irradiation directions. It is these calculated tissue attenuation
characteristics that actually compose the CT image.

If a slice of heterogeneous tissue is irradiated given below, and we divide the slice into
volume elements or voxels with each voxel having its own attenuation coefficient, it is
obvious that the sum of the voxel attenuation coefficients for each X-ray beam direction can
be determined from the experimentally measured beam intensities for a given voxel width.
However, each individual voxel attenuation coefficient remains unknown.
Computed tomography uses the knowledge of the attenuation coefficient sums derived from
X-ray intensity measurements made at all the various irradiation directions to calculate the
attenuation coefficients of each individual voxel to form the CT image.

X-rays incident on patient from different directions. They are attenuated by different
amounts, as indicated by the different transmitted X-ray intensities

Block Diagram of the CT System

Figure below shows a block diagram of the system. The X-ray source and detectors are
mounted opposite each other in a rigid gantry with the patient lying in between, and by
moving one or both of these around and across the relevant sections, which is how the
measurements are made.
 The X-ray tube and the detector are rigidly coupled to each other. The system executes
translational and rotational movement and transradiates the patient from various angular
projections. With the aid of collimators, pencil thin beam of X-ray is produced.

 A detector converts the X-radiation into an electrical signal. Measuring electronics then
amplify the electrical signals and convert them into digital values. A computer then processes
these values and computes them into a matrix-line density distribution pattern which is
reproduced on a video monitor as a pattern of gray shade.

 In one system which employs 18 traverses in the 20s scanning cycle, 324,000 (18 x 30 x 600)
X-ray transmission readings arc taken and stored by the computer. These arc obtained by
integrating the outputs of the 30 detectors with approximately 600 position pulses.
 The position pulses are derived from a glass graticule that lies between a light emitting diode
and photo-diode assembly that moves with the detectors. The detectors arc usually sodium-
iodide crystals, which are thallium-doped to prevent an after-glow. The detectors absorb the
X-ray photons and emit the energy as visible light. This is converted to electrons by a photo-
multiplier tube and then amplified. Analog outputs from these tubes go through signal
conditioning circuitry that amplifies, clips and shapes the signals.
 A relatively simple analog-to-digital converter then prepares the signals for the computer.
Simultaneously, a separate reference detector continuously measures the intensity of the
primary X-ray beam .The set of readings thus produced enables the computer to compensate
for fluctuations of X-ray intensity. Also, the reference readings taken at the end of each
 traverse are used to continually calibrate the detection system and the necessary correction is
carried out.
 After the initial pre-processing, the final image is put onto the system disc. This allows for
direct viewing on the operator’s console. The picture is reconstructed in either a 320 x 320
matrix of 0.73 mm squares giving higher spatial resolution or in a 160x 160 matrix of 1.5 mm
 squares which results in higher precision, lower noise image and better discrimination
between tissues of similar density.
 Each picture clement that makes up the image matrix has a CT number, say between -1000
and + 1000, and therefore, takes up one computer word. A complete picture occupies
approximately 100 K words, and u pto eight such pictures can be stored on the system disc.
There is a precise linear relationship between the CT numbers and the actual X-ray
absorption values, and the scale is defined by air at -1000 and by water at 0.

Image Reconstruction
The formation of a CT image is a distinct three phase process.

1. The scanning phase produces data, but not an image.

2. The reconstruction phase processes the acquired data and forms a digital image.
3. Digital-to analog conversion phase: The visible and displayed analog image (shades of
gray) is produced by the digital-to analog conversion phase.

1. The scanning phase

During the scanning phase a fan-shaped x-ray

beam is scanned around the body. The amount
of x-radiation that penetrates the body along
each individual ray (pathway) through the body
is measured by the detectors that intercept the
x-ray beam after it passes through the body.
The projection of the fan-shaped x-ray
beam from one specific x-ray tube focal spot
position produces one view. Many
views projected from around the patient's body
are required in order to acquire the necessary
data to reconstruct an image.Each view produces one "profile" or line of data as shown here.
The complete scan produces a complete data set that contains sufficient information for
the reconstruction of an image. In principle, one scan produces data for one slice image.

2. Image Reconstruction Phase

Image reconstruction is the phase in which the scan data set is processed to produce an
image. The image is digital and consist of a matrix of pixels.

Filtered back projection is the reconstruction method used in CT.

"Filtered" refers to the use of the digital image processing algorithms that are used to improve
image quality or change certain image quality characteristics, such as detail and noise.

"Back projection" is the actual process used to produce or "reconstruct" the image.
Back projection Principle

We start with one scan view through a body section

(like a head) that contains two objects. As we know,
the data produced is not a complete image, but
a profile of the x-ray attenuation by the objects.

Let's now take this profile and attempt to draw an

image by "back projecting" the profile onto our image

surface.

We have now rotated the x-ray beam around the

body by 900 and obtained another view. If we
now back project this profile onto our image area we
see the beginnings of an image showing the two
object. Two views does not give us a high-quality
image. Several hundred views are used to produce
clinical CT images. A part of the reconstruction
process is the calculation of CT number values for
each image pixel.

Image Reconstruction Computer, used in CT scanners.

This method enables pictures to be reconstructed within a few seconds. Figure shows a block
diagram image reconstruction computer, used in CT scanners.

SCANNING SYSTEM
 Biomedical Engineering Module V S5-ECE, 2017

 The purpose of the scanning system is to acquire enough information to reconstruct a picture
for an accurate diagnosis. A sufficient number of independent readings must be taken to
allow picture reconstruction with the required spatial resolution and density discrimination
for diagnostic purposes. The readings are taken in the form of ‘profiles’.
 When a plane parallel X-ray beam is passing through a required section, a profile is defined
as the intensity of the emergent beam plotted along a line perpendicular to the X-ray beam.
This profile represents a plot of the total absorption along each of the parallel X-ray beams. It
thus follows that the higher the number of profiles obtained, the better is the resulting picture.

Types of Scanning Systems

1. First Generation—Parallel Beam Geometry: In the

basic scanning process, a collimated X-ray beam passes
through the body and its attenuation is detected by a
sensor that moves on a gantry along with the X-ray tube.
The tube and detector move in a straight line, sampling
the data 180 times. At the end of the travel, a 1° tilt is
made and a new linear scan begins. This assembly travels
180°around the patient’s position. This arrangement is
known as Traverse and Index' and was used in the
earliest commercial system.

This procedure results in 32,400 independent measurements of attenuation, which are

sufficient for the systems computer to produce an image. Obviously, this is a fairly slow
procedure and requires a typical scan time of 5 minutes. It is essential for the patient to keep
still during the entire scan period and for this reason, the early scanners were limited in their
use to only brain studies.
2. Second Generation—Fan Beam, Multiple Detectors: An
improved version of the traverse-index arrangement
consists in using a bank of detectors and a fan beam of X-
rays. This system effectively takes several profiles with
each traverse and thus permits greater index angles. For
example, by using a 10° fan beam, it is possible to take 10
profiles, at 1° intervals, with each traverse and then index
through 10° before taking the next set of profiles.
Therefore, a fu II set of 180 profiles can be obtained with
18 traverses. This method has permitted a reduction in the
scan time, and at the rate of approximately 1 s for each traverse, it has led to the systems
operating in the 18-20 s range.
3. Third Generation—Fan Beam, Rotating
Detectors: The main obstacle for a further
increase in speed with the conventional
computer tomographs arises from the
mechanically unfavorable multiple alterations
between the translational and rotational
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ECE, MBITS
 movement of the measuring system. But using a fan-shaped beam and an array of detectors,
larger steps can be taken and the scanning process speeded up linear scanning movement can
be avoided by using a sufficiently wide fan-shaped X-ray beam which encompasses the
whole object cross-section, and a multiple detector system mechanically tied to the tube
which permits a simultaneous measurement of the whole absorption profile in one projection
direction (Fig. 20.6(c)). Also, on account of the largeness of the measuring system consisting
of X-ray tube and detectors, the rotational movement must not be stepwise but continuous.

4. Ultrafast Electron Beam CT Scanner

Comparison of Electron Beam Tomography and conventional CT

Electron Beam Tomography Conventional CT
In this electron beam sweeps back and In this X ray tube and X ray detector are
forth through a magnetic field. The mounted across each other on a circular
impact of electron beam on a semi frame and rotate around the patient.
circular tungsten array underneath the
patient generates X-rays and the X ray
detectors are mounted on a semi circular
array above the patient.
Light weight Heavy moving parts weighing 250 kg
Takes only 50ms with electron beam Takes 1 sec or more to take all the
tomography. snapshots

Schematic of ultrafast electron beam CT scanner

 The detector array consists of two continuous ranges of 216° with 432 channels each.
Luminascent crystals coupled to silicon photo-diodes are used.
 The scanning electron beam emitted by an electron gun is accelerated by 130-140 kV,
electromagnetically focused and deflected over a target in a typical time of 50-100 ms.
 It was originally designed for cardiac examinations. The unit was equipped for this purpose
with four anode rings and two detector rings which enabled eight contiguous slices, an area of
approximately 8x8 mm. to be scanned without movement of the patient.
5. Spiral /Helical Scanning. This is a scanning technique in which the X-ray tube rotates
continuously around the patient while the patient is continuously translated through the fan
beam. The focal spot therefore, traces a helix around the patient. The projection data thus
obtained allow for the reconstruction of multiple contiguous images. This operation is often
referred to as helix, spiral, volume, or three-dimensional CT scanning. This technique has
been developed for acquiring images with faster scan times and to obtain fast multiple scans
for three-dimensional imaging to obtain and evaluate the Volume at different locations.

Figure illustrates the spiral scanning technique, which causes the focal spot to follow a spiral
path around the patient. Multiple images are acquired while the patient is moved through the
gantry in a smooth continuous motion rather than stopping for each image.

The projection data for multiple images covering a volume of the patient can be acquired in a
single breath hold at rates of approximately one slice per second.

The reconstruction algorithms are more complex because they need to account for the spiral
or helical path traversed by the X-ray source around the patient (Kalender,1993).

Detectors used in CT:

 For a good image quality, it is important to have a stable system response and in that,
detectors play a significant role. The detectors used in CT systems must have a high
overall efficiency in order to minimize the patient radiation dose, have large dynamic
range, he very stable with time and insensitive to temperature variations within the
gantry.
 Figure shows the three types of detectors commonly used in CT scanners. Fan-beam
rotational scanners mostly employ xenon gas ionization detectors. The schematic
diagram of the detector shows that X-rays enter the detector through a thin aluminium
window. The aluminium window is a part of a chamber that holds the xenon gas,
which fills the entire chamber. Only one gas volume is present so that all detector
elements arc under identical conditions of pressure and gas purity.
 Processing System
A typical data acquisition system is shown in Fig. It
consists of precision pre-amplifiers, current to
voltage convertor, analog integrators, multiplexers
and analog-to-digital convertors. Data transfer rates
of the order of 10 Mbytes/s are required in some
scanners. This can be accomplished with a direct
connection for systems having a fixed detector
array. The third generation slip ring systems make
use of optical transmitters on the rotating gantry to
send data to fixed optical receivers.

Applications of CT

1. Unlike other medical imaging techniques, such as conventional x-ray imaging (radiography),
CT enables direct imaging and differentiation of soft tissue structures, such as liver, lung
tissue, and fat.
2. CT is especially useful in searching for large space occupying lesions, tumors and
metastasis and can not only reveal their presence, but also the size, spatial location and
extent of atumor.
3. CT imaging of the head and brain can detect tumors, show blood clots and blood vessel
defects, show enlarged ventricles (caused by a build up of cerebrospinal fluid) and image
other abnormalities such as those of the nerves or muscles of the eye.

4. Due to the short scan times of 500 milliseconds to a few seconds, CT can be used for all
anatomic regions, including those susceptible to patient motion and breathing. For
example, in the thorax CT can be used for visualization of nodular structures, infiltrations
of fluid, fibrosis (for example from asbestos fibers), and effusions (filling of an air space
with fluid).

5. CT has been the basis for interventional work like CT guided biopsy and minimally
invasive therapy. CT images are also used as basis for planning radiotherapy cancer
treatment. CT is also often used to follow the course of cancer treatment to determine how
the tumor is responding to treatment.
6. CT imaging provides both good soft tissue resolution (contrast) as well as high
spatial resolution. This enables the use of CT in orthopedic medicine and imaging
of bony structures including prolapses (protrusion) of vertebral discs, imaging of
complex joints like the shoulder or hip as a functional unit and fractures,
especially those affecting the spine. The image postprocessing capabilities of CT
- like multiplanar reconstructions and 3-dimensional display (3D) - further
enhance the value of CT imaging for surgeons. For instance, 3-D CT is an
invaluable tool for surgical reconstruction following facial trauma.
I. MAGNETIC RESONANCE IMAGING:

Magnetic Resonance Imaging (MR1) or Nuclear Magnetic Resonance (NMR) Tomography

has emerged as a powerful imaging technique in the medical field because of its high
resolution capability and potential for chemical specific imaging.

Comparison of NMR system and XRAY and CT

1. Similar to the X-ray computerized tomography (CT), MRI uses magnetic fields and radio
frequency signals to obtain anatomical information about the human body as cross-sectional
images in any desired direction and can easily discriminate between healthy and diseased
tissue.
2. MRI images are essentially a map of the distribution density of hydrogen nuclei and
parameters reflecting their motion, in cellular water and lipids.
3. The total avoidance of ionizing radiation, its lack of known hazards and the penetration of
bone and air without attenuation make it a particularly attractive non-invasive imaging
technique.
4. CT provides details about the bone and tissue structure of an organ whereas NMR
highlights the liquid-like areas on those organs and can also be used to detect flowing liquids,
like blood.
5. A conventional X-ray scanner can produce an image only at right angles to the axis of the
body, whereas the NMR scanner can produce any desired cross-section, which offers a
distinct advantage to and is a big boon for the radiologist.

Basic Principle

MR1 systems provide highly detailed images of tissue in the body. The systems detect and
process the signals generated when hydrogen atoms, which are abundant in tissue, are placed
in a strong magnetic field and excited by a resonant magnetic excitation pulse.

All materials contains nucleus that have a combination of protons and neutrons. It possesses a
spin and the amount of spin give rise to a magnetic moment. The magnetic moment has a
magnitude and direction. In tissues Magnetic moments of nuclei making up the tissue are
randomly aligned and net magnetization=0.
 Random alignment of magnetic moments of the nuclei making up the
tissue, resulting in a zero net magnetization.

When a material is placed in a magnetic field B0, some of the randomly oriented nuclei
experience an external magnetic torque which tends to align the individual parallel or anti-
parallel magnetic moments to the direction of an applied magnetic field. This gives a
magnetic moment that accounts for the nuclear magnetic resonance signal on which the
imaging is based. This moment is in the direction of applied magnetic field Bo. With the
magnetic moments being randomly oriented with respect to one another, the components in
the X-Y plane cancel one another out while the Z components along the direction of the
applied magnetic field add up to produce this magnetic moment M0 shown in Figure given
below.

The application of external magnetic

field causes the nuclear magnetic
moments to align themselves,
producing a net moment in the direction
of the field B0.

NMR Resultant Signal Pick up by the Instrument

 When a nucleus with a magnetic moment is placed in an externally applied magnetic
field, the energy of the nucleus is split into lower (moment parallel with the field) and
higher (anti-parallel) energy levels. The energy difference is such that a proton with
specific frequency (energy) is necessary to excite a nucleus from the lower to die
higher state.
 The excitation energy E obtained by the application of external RF signal, and is
given by the Planck's equation
E = hωo Where h is Planck's constant. This energy is usually supplied by an RF
magnetic field. ωo = Frequency of appied RF.
 The excited proton tends to return or relax to its low-energy state with spontaneous
decay and re-emissions of energy at a later time Y in the form of radio wave photons.
This decay is exponential in nature and produces a “free induction decay” (FID)
 signal (Fig. below) that is the fundamental form of the nuclear signal obtainable from
an NMR system.
 To summarize, if in a static field, RF waves of
the right frequency are passed through the
sample of interest (or tissue), some of the
parallel protons will absorb energy and be
stimulated or excited to a higher energy in the
anti-parallel direction. Sometime later, the RF
frequency absorbed will be emitted as
electromagnetic energy of the same frequency
as the RF source. The amount of energy
required to flip protons from the parallel to the
anti-parallel orientation is directly related to the
magnetic field strength; stronger fields require more energy or higher frequency
radiation. This is picked up by the instrument and then processed.

BASIC NMR COMPONENTS

The basic components of an NMR

imaging system are shown in Fig. These
are:
1. Magnet: Provides a strong uniform,
steady, magnet field B0.
2. RF transmitter, which delivers radio-
frequency magnetic field to the sample.
3. Gradient system, which produces
time-varying magnetic fields of
controlled spatial non-uniformity;
4. Detection System, which yields the
output signal; and
5. Imager system, including the
computer, which reconstructs and
displays the images.

1. Imager System
 The imaging sequencing in the system is provided by a computer. Functions such as
gates and envelopes for the NMR pulses, blanking for the pre-amplifier and RF power
amplifier and voltage waveforms for the gradient magnetic fields are all under
software control.
  The computer also performs the various data processing tasks including the Fourier
transformation, image reconstruction, data filtering, image display and storage.
Therefore, the computer must have sufficient memory and speed to handle large
image arrays and data processing, in addition to interfacing facilities.

2. The Magnet:

 In magnetic resonance tomography, the base field must be extremely uniform in space
and constant in time as its purpose is to align the nuclear magnets parallel to each
other in the volume to be examined.
 Also, the signal-to-noise ratio increases approximately linearly with the magnetic
field strength of the basic field, therefore, it must be as large as possible.
 Four factors characterize the performance of the magnets used in MR systems; viz.,
field strength, temporal stability, homogeneity and bore size.
 The gross non-homogeneities result in image distortion while the bore diameter limits
the size of the dimension of the specimen that can be imaged.
 Such a magnetic field can be produced by means of four different ways, viz.,
permanent magnets, electromagnets, resistive magnets and super-conducting magnets.
 Permanent Magnet: In case of the permanent magnet, the patient is placed in the gap
between a pair of permanently magnetized pole faces. Permanent magnet materials
normally used in MRI scanners include high carbon iron alloys such as alnico or
neodymium iron.. Although permanent magnets have the advantages of producing a
relatively small fringing field and do not require power supplies, they tend to be very
heavy (up to 100 tons) and produce relatively low fields of the order of 0.3 T or less.
 Electromagnets: Make use of soft magnetic materials such as pole faces which
become magnetized only when electric current is passed through the coils wound
around them. Electromagnets obviously require external electrical power supply.
 Resistive magnets: make use of large current-carrying coils of aluminium strips or
copper tubes. In these magnets, the electrical power requirement increases
proportionately to the square of the field strength which becomes prohibitively high as
the field strength increases. Moreover, the total power in the coils is converted into
heat which must be dissipated by liquid cooling.
 Superconductive magnets. Most of the
modem NMR machines utilize
superconductive magnets. These magnets
utilize the property of certain materials,
which lose their electrical resistance fully
below a specific temperature. The
commonly used superconducting material
is Nb Ti (Niobium Titanium) alloy for
which the transition temperature lies at 9 K
(-264°C). In order to prevent
superconductivity from being destroyed by
 an external magnetic field or the current passing through the conductors, these
conductors must be cooled down to temperatures significantly below this point, at
least to half of the transition temperature. Therefore, superconductive magnet coils are
cooled with liquid helium which boils at a temperature of 4.2 K (-269°C).

RF Transmitter System

The system consists of an RF transmitter, RF power amplifier and RF transmitting coils.

1. RF Transmitter System
 In order to activate the nuclei so that they emit a useful signal, energy must be
transmitted into the sample. This is what the transmitter does.
 The RF transmitter consists of an RF crystal oscillator at the Larmor frequency. The
RF voltage is gated with the pulse envelopes from the computer interface to generate
RF pulses that excite the resonance.

2. RF Power Amplifier

 These pulses are amplified to levels varying from 100W to several kW and are fed to
the transmitter coil.

3. RF Transmitting Coils

 The coil generates RF field perpendicular to the direction of main magnetic field.
 Coils are tuned to the NMR frequency and are usually isolated from the remaining
system using RF shielding cage.

Detection System
 Block diagram is shown in above fig.
 The function of detection system is to detect the nuclear magnetization and generate
an output signal for processing by the computer.

 The receiver coil usually surrounds the sample and acts as an antenna to pick up the
fluctuating nuclear magnetization of the sample and converts it to a fluctuating output
voltage V(i).

 NMR signal is given by

Where M(t, x) is the total magnetization in a volume and

Bc(x) the sensitivity of the receiver coil at different points in space.
Bc(x) describes the ratio of the magnetic field produced by the receiver coil to the
current in the coil.
 The receiver coil design and placement is such that Bc(x) has the largest possible
transverse component. The longitudinal component of Bc(x) contributes little to the
output voltage and can he ignored.
 The RF signals constitute the variable measured in magnetic resonance tomography.
These are extremely weak signals having amplitude in the nV (nano-Volt) range thus
requiring specially designed RF antennas. The sensitivity of an MR scanner therefore
depends on the quality of its RF receiving antenna. For a given sample magnetization,
static magnetic field strengths and sample volume, the signal-to-noise-ratio (SN R)of
the RF signal at the receiver depends in the following manner upon the RF-receiving
antenna.

 This implies that the SNR of an MR scan can be improved by maximizing

magnetization to coil volume.
 Some of the commonly available coils are:
 Body Coils: Constructed on cylindrical coils forms with diameter ranging from 50 to
60 cm entirely surround the patient's body.
 Head Coils: Designed only for head imaging, with typical diameter of 28 cm.

 Surface coils:
Orbit/ear coil: flat, planar ring-shaped coil with 10 cm diameter;
Neck coil: flexible, rectangular shaped surface coil (10 cm x 20 cm) capable of
adaptation to the individual patient anatomy; and
Spine coil: cylindrical or ring-shaped coil with 15 cm diameter.

 Organ-enclosing coils:
Breast coil: cylindrical or ring-shaped coil with 15 cm diameter.
 Helmholtz-type coil: a pair of flat ring coils each having 15 cm diameter with
distance between the two coils variable from 12 to 22 cm.

Matching Network

 Following the receiver coil is a matching network which couples it to the pre-
amplifier in order to maximize energy transfer into the amplifier. This network
introduces a phase shifty to the phase of the signal.

Pre-amplifier: The pre-amplifier is a low-noise amplifier which amplifies the signal and
feeds it to a quadrature phase detector.

Quadrature phase detector

 The detector accepts the RF NMR signal which consists of a distribution of
frequencies centred around or near the transmitted frequency w and shifts the signal
down in frequency by w.
 The detector circuit accepts the inputs, the NMR signal V(t) and a reference signal,
and multiplies them, so that the output is the product of the two inputs. The frequency
of the reference signal is the same as that of the irradiating RF pulse. The output of
the phase-sensitive detector consists of the sum of two components, one a narrow
range of frequencies centred at 2w0, and the other, a narrow range centred at zero.
 The low pass filter following the phase-sensitive detector removes all components
except those centred at zero from the signal.
ADC
 It is necessary to convert the complex (two-channel) signal to two strings of
digital numbers by analog-to-digital converters. The A-D converter output is passed,
in serialdata form to the computer for processing.

Gradient System for Spatial Coding:

 Spatial distribution information can be obtained by using the fact that the resonance
frequency depends on the magnetic field strength. By varying the field in a known
manner through the specimen volume, it is possible to select the region of the
specimen from which the information is derived on the basis of the frequency of the
signal. The strength of the signal at each frequency can be interpreted as the density of
the hydrogen nuclei in the plane within the object where the magnetic field
corresponds to that frequency.
 . The imaging methods differ mainly in the nature of the gradient time dependence
(static, continuously time-depended or pulsed), and in the type of NMR pulse
sequence employed.
 Spatial information and therefore images obtained by super-imposing a linear
magnetic field gradient on the uniform magnetic field applied to the object to be
imaged. When this is done, the resonance frequencies of the processing nuclei will
depend primarily on the positions along the direction of the magnetic gradient.
  This produces a one-dimensional projection of the structure of the three- dimensional
object. By taking a series of these projections at different gradient orientations, a two
or even three-dimensional image can be produced.
 In NMR systems, for spatially resolving the signals emitted by the object, the initially
homogeneous magnetic field B0 is overlaid in all three spatial dimensions, X, Y, Z
with small linear magnetic fields-gradient fields G.
 These gradient fields are produced with die aid of current carrying coils and can be
switched on or off as desired, both during the application of the RF energy and also in
any phase of the measuring procedure.

A block diagram of gradient control system is shown in Fig. given below. The hardware can
be broken down into four sub-system.

Block diagram of gradient control system. Each X,Yand Z coil pair has its own control
circuit.

1. Serial Parallel Computer

 The first sub-system includes the interface between the computer and the gradient
control system. Its primary function is to allow the independent positioning of the
three planes (X, Y and Z).
2. The digital oscillator
 Consists of a 555 timer followed by shift registers A digital oscillator facilitates
varying itis output frequency over an extremely wide range through the use of a single
control
 The 8-bit input from the interface circuit is used directly to one attenuator while the
same 8-bits are inverted to control the second attenuator. The output of the attenuators
is then voltage-amplified by two op amps prior to the driven circuits.
 Current control used to adjust the static field gradients be available for setting the DC
levels upon which the alternating gradients are superimposed .
  An op amp serves the differential voltage drop across a dummy load and produces an
output which is then DC coupled to the drivers.
 The high current drivers use a conventional design with a single op amp providing the
input to a driver and a complimentary pair of power transistors to provide a sufficient
current to the gradient coil.
 In typical scanners, gradient coils have an electric resistance of about l Ohm and an
inductance of 1 mH. The gradient fields are required to be switched from 0 to 10 mT/
m in about 0.5 ms. The current switches from O to about 100 A in this interval. The
power dissipation during the switching interval is about 20 kW. This places very
strong demands on the power supply and it is often necessary to use water cooling to
prevent overheating of the gradient coils.
 With well-designed coils, errors resulting from non-linear gradients will perhaps not
be evident in a medical image since the image will remain clear and will not contain
rigidly shaped objects or those with sharp edges for close comparison. But these
gradient coils are usually designed to optimize linearity in the central region. Away
from the centre, gradient linearity becomes progressively worse. Without restoration,
the image will not give accurate information on the outer regions. Therefore, non-
linear field gradients result in a geometrical distortion of the image reconstructed from
projections.
Imager System:
The imager system includes the computer for image processing, display system and control
console. The timing and control of RF and gradient pulse sequences for relaxation time
measurements and imaging, in addition to FT image reconstruction and display necessitate
the use of a computer.
The computer is the source of both the voltage waveforms of all gradient pulses and the
envelopes of the RF pulses. A general purpose mini-computer of the type used for a ('AT
scanner is adequate for these purposes.

BIOLOGICAL EFFECTS OF NMR IMAGING

The three aspects of NMR imaging which could cause potential health hazard are:

(i) Heating due to the rf power.

A temperature increase produced in the head of NMR imaging would be about
0.3°C. This does not seem likely to pose a problem.
(ii) Static magnetic field:
No significant effects of the static field with die level used in NMR are known,
but the possible side effects of electromagnetic fields are decrease in cognitive
skills, mitotic delay in slime moulds, delayed wound healing and elevated serum
triglycerides.
(iii) Electric current induction due to rapid change in magnetic field:
It is believed that oscillating magnetic field gradients may induce electric currents
strong enough to cause ventricular fibrillation. However, no damage due to NMR
 from exposures has been reported. It is suggested that fields should not vary at arate faster
than 3 tesla/s.

ADVANTAGES OF NMR IMAGING SYSTEM

1. The NMR provides substantial contrast between soft tissues that are nearly identical.
2. NMR uses no ionizing radiation and has minimal hazards for operators of the machines and for
patients.
3. Unlike CT, NMR imaging requires no moving parts, gantries or sophisticated crystal detectors.
4. The system scans by superimposing electrically controlled magnetic fields consequently, scans
in any pre-determined orientation are possible.
5. With the new techniques being developed, NMR permits imaging of entire three- dimensional
volumes simultaneously instead of slice by slice, employed in other imaging systems.
6. In NMR both biochemical (spectroscopy) and spatial information (imaging) can be
obtained without destroying the sample.

PET (Positron Emission Tomography)

• Positron Emission Tomography (PET) is a nuclear imaging technique that produces a 3-D image of
functional processes in the body by detecting the radiation emitted by photons .
• The system detects pairs of gamma rays emitted indirectly by positron emitting radionuclide
(tracer), which was previously injected in body on a biologically active molecule,
• 3-D images of tracer concentration within the body are then constructed by computer analysis.

• Injection of Short lived Radioactive Isotope in body. most commonly used is FDG (fluoro-2-
deoxyglucose).
• Wait till tracer gets accumulated in tissues of interests.
• Subject is placed in the imaging scanner
• Tissue concentration is recorded with time.
• As isotope decays in body, it releases a positron in body. On interaction with an electron, it produces a
pair of photons.
PET scanner detect these photons and with the help of a computer creates pictures offering details on both
the structure and function of organs and tissues in your body.

Applications of PET
• PET is used in following areas.
• Neuroimaging
• Clinical oncology (medical imaging of tumors).
• Musculo-skeletal imaging
• Cardiology
• Pharmacology
• Neuropsychology

S P E C T(SINGLE PHOTON EMISSION COMPUTED TOMOGRAPHY

)
SPECT is short for single photon emission computed tomography. As its name suggests (single photon
emission) gamma rays are the sources of the information rather than X-ray emission in the conventional
CT scan.

Similar to X-ray, CT, MRI, etc SPECT allows us to visualize functional information about patient’s
specific organ or body system.
⚫ SPECT is a technology used in nuclear medicine where the patient is injected with a
radiopharmaceutical which will emit gamma rays.
⚫ We seek the position and concentration of radionuclide distribution by the rotation of a photon
detector array around the body which acquires data from multiple angles.

⚫ Each of the cameras collects a matrix of values which correspond to the number of gamma counts
detected in that direction at the one angle.
 ⚫ Images can be reprojected into a three dimensional one that can be viewed in a dynamic rotating
format on computer monitors, facilitating the demonstration of pertinent findings to the referring
physicians
ADVANTAGES
⚫ Localization of defects is more precise and more clearly seen by the inexperienced eye.
⚫ Extend and size of defects is better defined.
⚫ Images free of background.

SPECT APPLICATIONS

⚫ Heart imaging
⚫ Brain Imaging
⚫ SPECT imaging
⚫ Tumor detection
⚫ Bone Scans

ULTRASONIC IMAGING SYSTEMS

 The term ultrasound refers to acoustical waves above the range of human hearing
(frequencies higher than 20.000 Hz).
 Medical ultrasound systems operate at frequencies of up to 10 MHz or more.
 An ultrasonic wave is acoustical; i.e., it is a mechanical wave in a gaseous, liquid, or solid
medium. Such mechanical waves consists of alternating areas of higher and lower pressures,
called compression and rarefaction zones, respectively.
 Ultrasonic imaging is used in medicine, engineering, geology, and other scientific areas.
Radio signals are electromagnetic waves, while medical ultrasound signals are acoustical.

Comparison of Ultrasound and Radio Waves

Ultrasound Waves Electromagnetic Waves

1. The acoustical signal requires a medium 1. Electromagnetic signal can propagate in
in which to propagate. outer space, where no known medium
exists.
If an alternating current (ac) oscillation of, But if that same 2500 kHz ac signal were
say, 2500 kHz, were connected to an applied to an ultrasound transducer, then an
appropriate antenna, then an acoustical signal would be launched.
electromagnetic (radio) wave would be
launched.

Properties of Ultrasound and X-rays

1. Ultrasound rays are Non-invasive, While X-rays are invasive.

2. Ultrasound rays are Externally applied and non-traumatic, also apparently safe at the
acoustical intensities. While X-rays only respond to atomic weight differences and often
require the injection of a more dense contrast medium for visualization of non-bony tissues.
3. Diagnostic ultrasound is applied for obtaining images of almost the entire range of internal
organs in the abdomen
4. These include the kidney, liver, spleen, pancreas, bladder, major blood vessels and of
course, the foetus during pregnancy.
5. It has also been usefully employed to present pictures of the thyroid gland, the eyes, the
breasts and a variety of other superficial structures.
6. In a number of medically meaningful cases, ultrasonic diagnostics has made possible the
detection of cysts, tumours or cancer in these organs.
 7. The main limitation of ultrasound is that it is almost completely reflected at boundaries
with gas and is a serious restriction in investigation of and through gas-containing structures.
8. Ultrasonic waves are sound waves associated with frequencies above the audible range and
generally extend upward from 20 kHz.

Characteristics

1. Ultrasonic waves can be easily focused, i.e., they are directional and beams can be
obtained with very little spreading.
• They are inaudible and are suitable for applications where it is not advantageous
to employ audible frequencies.
• By using high frequency ultrasonic waves which are associated with shorter
wavelengths, it is possible to investigate the properties of very small structures.
• Information obtained by ultrasound, particularly in dynamic studies, cannot be
acquired by any other more convenient technique.

Use of ultrasound in Medical Field

 The use of ultrasound in the medical field can be divided into two major areas: the
therapeutic and the diagnostic.
 The major difference between the two applications is the ultrasonic power level at which the
equipment operates.
 In therapeutic applications, the systems operate at ultrasonic power levels of up to several
watts per square centimeter while the diagnostic equipment operates at power levels of well
below 100 in W/ cm'.
 The therapeutic equipment is designed to agitate the tissue to the level where thermal heating
occurs in the tissue, and experimentally has been found to be quite successful in its effects for
the treatment of muscular ailments such as lumbago.
 For diagnostic purposes, on the other hand, as long as a sufficient amount of signal has
returned for electronic processing, no additional energy is necessary. Therefore, considerably
lower ultrasonic power levels are employed for diagnostic applications.

PROPAGATION OF ULTRASONIC THROUGH TISSUES AND REFLECTIONS

Ultrasound waves are vibrations or disturbances consisting of alternating zones of

compression and rarefaction in a physical medium such as gas, liquid, or solid matter.
Wavelength, frequency, and velocity

Frequency is defined as cycles per unit of time.

All waves, including both acoustical and electromagnetic (or ocean waves, for that matter)
possess three related attributes: frequency (F). wavelength (X), and velocity (V).
 Frequency: is defined as the number of complete cycles per unit of time. Figure given above
shows one complete cycle.
In terms of alternating current, the cycle consists of one entire positive excursion followed by
one complete negative excursion of the voltage or current. In terms of sound and ultrasound
waves, the cycle consists of one complete zone of compression followed by one complete
zone of rarefaction.
The basic unit of cycles is the hertz (Hz), which equals one cycle per second (1 Hz = 1 cps);

Wavelength: is the distance traveled by one cycle propagating away from the source and is
expressed in meters (m), or subunits centimeters (cm) or millimeters (mm). The wavelength
is also the distance between successive identical features on successive cycles.
one wavelength is expressed in terms of the distance between two positive peaks, between
two negative peaks, or between negative- going crossings of the zero baseline.

Velocity: is the speed of propagation of the wave. In radio signals, the velocity is the speed
of light (c), or 300.000,000 m/s. In human tissue, ultrasound propagates at a much slower
rate. i.e.. around 1500 m/s.
For all forms of wave, the relationship between frequency, wavelength, and velocity is:
V=F (17-1)
Where
V is the velocity of the wave in meters per second (m/s)
Lambda is the frequency in hertz (Hz)
 is the wavelength in meters (m)
The period of the wave is die lime required to complete one cycie and can be measured in
terms of either time (T) or angle (one cycle = 2pi radians).
The type of the wave refers to the method of propagation.

The two forms are

1. Longitudinal Propagation
In the longitudinal form, the waves propagate in the same direction as the zones of
compression and rarefaction.

2. Transverse Propagation
In transverse propagation, the waves propagate in a direction orthogonal (at right angles) to
the direction of the zones of compression and rarefaction. Transverse propagation occurs
when the wave propagate along the surface of the medium, as on the surface of a container of
water or the surface of a bone.

In medical ultrasound both forms are seen. While the main mode is longitudinal propagation,
a mode conversion to transverse propagation can occur. Mode conversion is associated with a
significant loss of signal level.

Reflection of Ultrasound
 Ultrasound travels freely through fluid and soft tissues. However, ultrasound bounces back (is
reflected back) as echoes when it hits a more solid (dense) surface.
For example, the ultrasound will travel freely though blood in a heart chamber. But, when it
hits a solid valve, a lot of the ultrasound echoes back. Another example is that when
ultrasound travels though bile in a gallbladder it will echo back strongly if it hits a solid
gallstone. So, as ultrasound 'hits' different structures of different density in the body, it sends
back echoes of varying strength.

Ultrasonography (sonography) uses a probe containing multiple acoustic transducers to send

pulses of sound into a material. Whenever a sound wave encounters a material with a
different density (acoustical impedance), part of the sound wave is reflected back to the probe
and is detected as an echo. The time it takes for the echo to travel back to the probe is
measured and used to calculate the depth of the tissue interface causing the echo. The greater
the difference between acoustic impedances, the larger the echo is. If the pulse hits gases or
solids, the density difference is so great that most of the acoustic energy is reflected and it
becomes impossible to see deeper.

Figure given below illustrates the situation for reflection and refraction.

Reflection and refraction of waves

 At the boundary between two zones of different density,

some of the wave energy is reflected back into the
original medium, and some propagates into the second
medium but is refracted (i.e.. changes its direction of
travel).

 if the incident wave impinges on the surface or boundary at an angle of 90 degrees (i.e.. it is
coincident with the normal line), it will be reflected back on itself. But if the angle is other
than 90 degrees, then the reflected wave will travel away from the surface at the same angle.
 Refraction phenomena affect the portion of the incident wave that enters the second medium.
We may infer the behavior of ultrasound waves
 REAL-TIME ULTRASONIC IMAGING SYSTEMS

 One of the serious limiting factors in B-scanning is the length of time taken to complete a
scan. This results in blurring and distortion of the image due to organ movement, as well as
being tedious for the operator.
 Elimination of motion artifact is important in conventional B-scanning but is critical if
rapidly moving areas such as the chambers of the heart are to be made visible.
 Rapid scanning techniques have been developed to meet these needs. The approaches used
include fast physical movement of a single transducer.
 Alternatively, electronic methods using arrays of transducers which can be triggered in
sequence or in groups, may be utilized. In these systems, electronic manipulation allows the
beam to be swept rapidly through the area of interest.
 Finally, instruments in which an array of transducers is combined with mechanized motion,
have also been introduced. These instruments are called real-time scanners as there is
negligible time delay between the input of data and the output of processed data in such
systems.
 A very important property of ultrasonography is the short image reconstruction time of about
20 to 100 ms, permitting the real-time scanning and observation of processes in the organs of
the body. Consequently, sonography is well-suited to the fast, interactive screening of even
larger organ regions and the display of dynamic processes, such as cardiology.

The real-time systems, therefore, have the following characteristics:

Possibility of studying structure in motion—this is important for cardiac and foetal structures.

During observation, the scan plane can be easily selected since the echo image appears
instantaneously on the display.

Requirements of Real Time Ultrasonic Imaging Systems

The primary requirements of an ultrasonic imaging system are:

1. High resolution

High resolution or the ability of the system to resolve fine spatial dimensions is a key
performance requirement. A resolution of 1-3 mm in all three spatial dimensions is desirable
for a number of diagnostic studies like the early detection of tumours or other pathological
conditions.

2. Long range

The required depth range varies considerably for different anatomical studies. For example, a
range of 25-30 cm is desirable for abdominal and obstetrical studies. For cardiac studies, the
distance from the chest wall to the posterior heart wall is 15 cm or more. In superficial organs
like the breast, thyroid, carotid and femoral arteries and in infant studies, the range of depth is
3-10 cm.

3. Adequate field of view

The field of view should be large enough to display the entire region under examination and
to provide a useful perspective view of an organ of interest. When viewing small superficial
structures such as the thyroid, a field of approximately 5 cm * 5 cm can encompass the
desired region. For cardiac imaging, sector scans are preferable to rectilinear scans since a
large structure is to be viewed through a small window. An angle ol 60° is adequate to
simultaneously view most of the heart.
4. Sufficiently high frame rate and high detectivity

In real-time imaging systems, the frame rate (the rate at which the image is
retreated) should Ire rapid enough to resolve the important motions and to obtain
the image without undesirable smearing. Most of these requirements arc met with a
frame rate of about 30 frames/s.

This also satisfies the requirements for flicker-free display and is compatible with
standard television formats. For some special studies, greater frame rates are often
required for the data acquisition mode. These frames would then be stored and
played back at about 30 frames/s to provide a slow motion presentation.

5. Detectivity

It is the ability of an imaging system to effectively capture, process and display the
very wide dynamic range of signals which may, in turn, help to detect ah
image, lesion or other abnormal structure or process. Poor system detectivity
manifests itself in lack of fidelity or picture quality which is often apparent in the
visual displays of ultrasonic images.

BIOMEDICAL TELEMETRY SYSTEM

⚫ The term telemetry is derived from the two Greek terms: “tele” and “metron”, which
mean “remote” and “measure”.
⚫ In general, a physical variable or quantity under measurement, whether local or
remote, is called a measurand.
⚫ Telemetry is a technology that allows the remote measurement and reporting of
information of interest to the system designer or operator.
⚫ Literally, biotelemetry is the measurement of biological parameters over a distance.
Elements of Telemetry
 1. Transducer or Sensor:
• Converts the physical variable to be telemetered into an electrical quantity.
2. Signal Conditioner-1:
• Converts the electrical output of the transducer (or sensor) into an electrical
signal compatible with the transmitter.
3. Transmitter:
Its purpose is to transmit the information signal coming from the signal conditioner-1 using a
suitable carrier signal to the receiving end.
⚫ The transmitter may perform one or more of the following functions:
⚫ (i) Modulation: Modulation of a carrier signal by the information signal.
⚫ (ii) Amplification: As and if required for the purpose of transmission.
⚫ (iii) Signal Conversion: As and if required for the purpose of transmission.
⚫ (iv) Multiplexing: If more than one physical variables need to be telemetered
simultaneously from the same location, then either frequency-division multiplexing
(FDM) or time-division multiplexing (TDM) is used.
⚫ Receiver: Its purpose is to receive the signal(s) coming from the transmitter
(located at the sending end of the telemetry system) via the signal transmission
medium and recover the information from the same.
⚫ It may perform one or more of the following functions:
1. Amplification
2. Demodulation:
3. Reverse Signal Conversion
De-multiplexing
⚫ Signal Conditioner-2: Processes the receiver output as necessary to make it suitable
to drive the given end device.
⚫ End Device: The element is so called because it appears at the end of the system.
⚫ End device may be performing one of the following functions:
1. Analog Indication:
2. Digital Display
Digital Storage

Telemetry Classification Based on Transmission Medium

Divided into 2

1. Wired Telemetry 2. Wireless Telemetry

COMPONENTS OF WIRED BIOTELEMETRY SYSTEM

General Block Diagram

ANALOG SYSTEM [WIRED]

DIGITAL SYSTEM [WIRED]

Elements of Wireless Bio-Telemetry

ELEMENTS OF WIRELESS BIO-TELEMETRY

⚫ A typical biotelemetry system comprises:
1. Sensors appropriate for the particular signals to be monitored
2. Battery-powered, Patient worn transmitters
3. A Radio Antenna and Receiver
4. A display unit capable of concurrently presenting information from
multiple patients
1. Wireless Bio-telemetry Transmitter

2. Wireless Bio-telemetry Receiver

 Physiological signals are obtained from the subject by means of appropriate

transducers. The signal is then passed through a stage of amplification and processing
circuits that include generation of a subcarrier and a modulation stage for
transmission.
 The circuitry which generates the carrier and modulates it constitutes the transmitter.
Equipment capable of receiving the transmitted signal and demodulating it to recover
the information comprise the receiver. By tuning the receiver to the frequency of the
desired RF carrier, that signal can be selected while others are rejected.
 The range of the system depends upon a number of factors, including the power and
frequency of the transmitter, relative locations of the transmitting and receiving
antennas, and the sensitivity of the receiver.
 The receiver consists of a tuner to select the transmitting frequency, a demodulator to
separate the signal from the carrier wave, and a means of displaying or recording the
 signal. The signal can also be stored in the modulated state by the use of a tape
recorder, as shown in the block diagram.
 The biotelemetry system use two modulators. The physiological signals are used to
modulate a low-frequency carrier called sub-carrier in the audio frequency range. The
RF carrier is then modulated by the sub-carrier and transmitted. The double
modulation gives better interference free performance in transmission and enables the
reception of low frequency biological signals. The sub modulator can be FM or PWM
but the final modulator is practically always FM system.
 If several physiological signals are to be transmitted simultaneously, each signal is
placed on a sub-carrier of a different frequency using FM or AM. The sub-carriers are
added together to give a composite signal in which none of the parts overlap in
frequency. The composite signal modulates the RF carrier by the same or different
method. This process of transmitting many channels of data on a single RF carrier is
called frequency division multiplexing (FDM). This is more efficient and less
expensive than employing a separate transmitter for each channel.
 At the receiver, a multiplexed RF carrier is first demodulated to recover each of the
separate sub-carriers which are then demodulated to retrieve the original physiological
signals. Both FM/AM (sub-carrier is frequency modulated and RF carrier is amplitude
modulated) and FM/FM systems are used in biotelemetry, the later more extensively.
 The modulation system is selected based on the size, complexity, noise transmission
and other operational problems.

APPLICATION OF TELEMETRY IN MEDICINE

1. Telemetry of ECGs from Extended Coronary Care patients

 To make monitoring possible for the cardiac patients, some hospitals have extended
coronary-care units equipped with patient-monitoring systems that include telemetry.
In this arrangement, each patient has ECG electrodes taped securely to his chest.
 The electrodes are connected to a small transmitter unit that also contains the signal-
conditioning equipment. The transmitter unit is fastened to a special belt worn around
the patient’s waist.
 Batteries for powering the signal conditioning equipment and transmitter are also
included in the transmitter package. These batteries must be replaced periodically.
 The output of each receiver is connected to one of the ECG channels of the patient
monitor.

2. Telemetry for ECG Measurements During Exercise

 For certain cardiac abnormalities, such as ischemic coronary artery disease, diagnostic
procedures require measurement of the electrocardiogram while the patient is
exercising, usually on a treadmill or a set of steps. Although such measurements can
be made with direct-wire connections from the patient to nearby instrumentation, the
connecting cables are frequently in the way and may interfere with the performance of
 the patient. For this reason, telemetry is often used in conjunction with exercise ECG
measurements.
 The transmitter unit used for this purpose is similar to that described earlier for extended
coronary care and is normally worn on the belt. Care must be taken to ensure that the
electrodes and all wires are securely fastened to the patient, to prevent their swinging during the
movement of the patient.

3. Telemetry for Emergency Patient Monitoring

 In many areas ambulances and emergency rescue teams are equipped with equipment to allow
electrocardiograms and other physiological data to be transmitted to a nearby hospital for
interpretation.
 Two-way voice transmission is normally used in conjunction with the telemetry to facilitate
identification of the tele-metered information and to provide instructions for treatment.
 Through the use of such equipment, ECGs can be interpreted and treatment begun before the
patient arrives at the hospital of this type requires a much more powerful transmitter than the
two applications previously described. Often the data must be transmitted many miles and
sometimes from a moving vehicle.
 To be effective, the system must be capable of providing reliable reception and reproduction of
the transmitted signals regardless of conditions.

4. Telephone Links
One application involves the transmission of ECGs from heart patients and (particularly) pacemaker
recipients. In this case the patient has a transmitter unit that can be coupled to an ordinary telephone.
The transmitted signal is received by telephone in the doctor’s office or in the hospital. Tests can be
scheduled at regular intervals for diagnosing the status and potential problems indicated by the ECGs.
TELEMEDICINE AND TELE NURSING
Telemedicine is defined as the use of tele communications, the deliver health care expert
sharing of medical knowledge with persons are distance locations.
It is transfer of medical data for
Consultations Diagnosis
Support for Clinical Case
Containing Medical Educations