Introduction to Medical Imaging (MED 2019)

Class 1
General introduction to medical imaging &
related safety issues

presented by Dr. Brian Kot

School of Medical and Health Sciences

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Tung Wah College
 Introduction
• Medical imaging of the human body requires some form of energy
• In the medical imaging techniques used in radiology, the energy
used to produce the image must be capable of penetrating tissues
• Visible light, which has limited ability to penetrate tissues at depth,
is used mostly outside of the radiology department for medical
imaging

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• Visible light images are used in dermatology (skin photography),
gastroenterology and obstetrics (endoscopy), and pathology (light
microscopy)

• All disciplines in medicine use direct visual observation, which

also utilizes visible light

• In diagnostic radiology, the electromagnetic spectrum outside the

visible light region is used for medical imaging, including:

– X-rays in mammography and computed tomography (CT);

– Radiofrequency (RF) in magnetic resonance imaging (MRI);
– Gamma rays in nuclear medicine
– Mechanical energy, in the form of high-frequency sound
waves, is used in ultrasound imaging 6
• With the exception of nuclear medicine, all medical imaging
requires that the energy used to penetrate the body’s tissues also
interacts with those tissues

• If energy were to pass through the body and not experience some
type of interaction (e.g., absorption or scattering)
•  the detected energy X contain any useful information regarding
the internal anatomy
•  X construct an image of the anatomy using that information

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• In nuclear medicine imaging, radioactive substances are
injected or ingested, and it is the physiological interactions
of the agent that give rise to the information in the images

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 The modalities
• Different types of medical images can be made by varying the types
of energies and the acquisition technology used
• The different modes of making images are referred to as modalities
• Each modality has its own applications in medicine
 Radiography
 Fluoroscopy
 Mammography
 Computed Tomography (CT)
 Magnetic Resonance Imaging (MRI)
 Ultrasound Imaging
 Nuclear Imaging
 Combined Imaging Modalities
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 What a Nobel path you tread
• Roentgen (1901, physics): discovery of x-radiation
• Rabi (1944, physics): nuclear magnetic resonance (NMR)
methodology
• Bloch and Purcell (1952, physics): NMR precision
measurements
• Cormack and Hounsfield (1979, medicine): computed
assisted tomography (CT)
• Ernst (1991, chemistry): high-resolution NMR spectroscopy
• Laterbur and Mansfield (2003, medicine): discoveries
concerning magnetic resonance imaging (MRI)

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 Radiography
• Radiography was the first medical imaging technology, made
possible when the physicist Wilhelm Roentgen discovered x-rays
on 8th Nov 1895
• Radiography is performed with an x-ray source on one side of the
patient and a (typically fat) x-ray detector on the other side
• A short-duration (typically less than ½ second) pulse of x-rays is
emitted by the x-ray tube, a large fraction of the x-rays interact in
the patient, and some of the x-rays pass through the patient and
reach the detector, where a radiographic image is formed

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• The homogeneous distribution of x-rays that enters the patient is
modified by the degree to which the x-rays are removed from the
beam (i.e., attenuated) by scattering and absorption within the tissues
• The attenuation properties of tissues such as bone, soft tissue, and air
inside the patient are very different, resulting in a heterogeneous
distribution of x-rays that emerges from the patient
• The radiographic image is a picture of this x-ray distribution

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• The detector used in radiography can be photographic film
(e.g., screen-film radiography) or an electronic detector
system (i.e., digital radiography)

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• Transmission imaging refers to imaging in which the
energy source is outside the body on one side, and the
energy passes through the body and is detected on the
other side of the body
• Radiography is a transmission imaging modality

• Projection imaging refers to the case when each point on

the image corresponds to information along a straight-line
trajectory through the patient
• Radiography is also a projection imaging modality

• Radiographic images are useful for a very wide range of

medical indications, including the diagnosis of broken
bones, lung cancer, cardiovascular disorders, etc.
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 Fluoroscopy
• Fluoroscopy refers to the continuous acquisition of a
sequence of x-ray images over time, essentially a real-time
x-ray movie of the patient
• It is a transmission projection imaging modality, and is, in
essence, just real-time radiography
• Fluoroscopic systems use x-ray detector systems capable
of producing images in rapid temporal sequence

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• Fluoroscopy is used for positioning catheters in arteries,
visualizing contrast agents in the GI tract, and for other
medical applications such as invasive therapeutic
procedures where real-time image feedback is necessary
• It is also used to make x-ray movies of anatomic motion,
such as of the heart or the esophagus

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 Mammography
• Mammography is radiography of the breast, and is thus a transmission
projection type of imaging
• To accentuate contrast in the breast, mammography makes use of much
lower x-ray energies than general purpose radiography, and
consequently the x-ray and detector systems are designed specifically
for breast imaging

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• Mammography is used to screen asymptomatic women for
breast cancer (screening mammography) and is also used to
aid in the diagnosis of women with breast symptoms such as
the presence of a lump (diagnostic mammography)

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• Digital mammography has eclipsed the use of screen-film
mammography in the United States, and the use of computer-aided
detection is widespread in digital mammography
• Some digital mammography systems are now capable of tomosynthesis,
whereby the x-ray tube (and in some cases the detector) moves in an arc
from approximately 7 to 40 degrees around the breast
• This limited angle tomographic method leads to the reconstruction of
tomosynthesis images, which are parallel to the plane of the detector,
and can reduce the superimposition of anatomy above and below the in-
focus plane.

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 Computed tomography
• Computed tomography (CT) became clinically available in the early
1970s, and is the first medical imaging modality made possible by
the computer
• CT images are produced by passing x-rays through the body at a
large number of angles, by rotating the x-ray tube around the body

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• A detector array, opposite the x-ray source, collects the transmission
projection data
• The numerous data points collected in this manner are synthesized
by a computer into tomographic images of the patient
• The term tomography refers to a picture (graph) of a slice (tomo)
• CT is a transmission technique that results in images of individual
slabs of tissue in the patient
• The advantage of CT over radiography is its ability to display three-
dimensional (3D) slices of the anatomy of interest, eliminating the
superposition of anatomical structures and thereby presenting an
unobstructed view of detailed anatomy to the physician

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• There are a number of different acquisition modes available on
modern CT scanners, including dual-energy imaging, organ perfusion
imaging, and prospectively gated cardiac CT
• While CT is usually used for anatomic imaging, the use of iodinated
contrast injected intravenously allows the functional assessment of
various organs as well

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 Magnetic resonance imaging
• Magnetic resonance imaging (MRI) scanners use magnetic fields
that are about 10,000 to 60,000 times stronger than the earth’s
magnetic field
• Most MRI utilizes the nuclear magnetic resonance properties of the
proton, i.e., the nucleus of the hydrogen atom, which is very
abundant in biological tissues
• The proton has a magnetic moment and, when placed in a 1.5 T
magnetic field, the proton precesses (wobbles) about its axis and
preferentially absorbs radio wave energy at the resonance frequency
of about 64 million cycles per second (megahertz—MHz)

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• Patient is placed in the magnetic field, and a pulse of radio waves is
generated by antennas (“coils”) positioned around the patient
• The protons in the patient absorb the radio waves, and subsequently
reemit this radio wave energy after a period of time that depends upon
the spatially dependent magnetic proper-ties of the tissue
• The radio waves emitted by the protons in the patient are detected by
the antennas that surround the patient
• By slightly changing the strength of the magnetic field as a function
of position in the patient using magnetic field gradients, the proton
resonance frequency varies as a function of position, since frequency
is proportional to magnetic field strength
• The MRI system uses the frequency and phase of the returning radio
waves to determine the position of each signal from the patient

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• MRI produces a set of tomographic images depicting slices through
the patient, in which each point in an image depends on the
micromagnetic properties of the tissue corresponding to that point
• Different types of tissue such as fat, white and gray matter in the
brain, cerebral spinal fluid, and cancer all have different local
magnetic properties, images made using MRI demonstrate high
sensitivity to anatomical variations and therefore are high in contrast
• MRI has demonstrated exceptional utility in neurological imaging
(head and spine) and for musculoskeletal applications such as imaging
the knee after athletic injury

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 Ultrasound imaging
• Mechanical energy in the form of high-frequency (“ultra”) sound can be
used to generate images of the anatomy of a patient
• A short-duration pulse of sound is generated by an ultrasound transducer
that is in direct physical contact with the tissues being imaged
• The sound waves travel into the tissue, and are reflected by internal
structures in the body, creating echoes
• The reflected sound waves then reach the transducer, which records the
returning sound
• This mode of operation of an ultrasound device is called pulse echo imaging

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• The sound beam is swept over a slice of the patient line by line using a
linear array multielement transducer to produce a rectangular scanned
area, or through incremental angles with a phased array multielement
transducer to produce a sector scanned area
• The echo amplitudes from each line of ultrasound are recorded and used
to compute a brightness mode image with grayscale-encoded acoustic
signals representing a tomographic slice of the tissues of interest

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• Ultrasound is reflected strongly by interfaces, such as the surfaces and
internal structures of abdominal organs
• Ultrasound is thought to be less harmful than ionizing radiation to a
growing fetus  preferred in obstetrical patients
• An interface between tissue and air is highly echoic  very little sound
can penetrate from tissue into an air-filled cavity
• Less utility in the thorax where the air in the lungs presents a barrier that
the sound beam cannot penetrate
• An interface between tissue and bone is also highly echoic  brain
imaging impractical in most cases

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 Doppler Imaging Mechanism

Transducer

Pulse

Echo

Blood flow direction

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 Spectral Doppler Trace

Doppler signal:
artifactual vs phasic waveform
(true flow)

Spectral trace allows

differentiation of arterial from
venous flow and allows
measurement of parameters
such as the resistive index
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 Nuclear medicine imaging
• A chemical or other substance containing a radioactive isotope is
given to the patient orally, by injection or by inhalation
• Material distributed itself according to the physiological status of
the patient
• A radiation detector is used to make projection images from the x-
and/or gamma rays emitted during radioactive decay of the agent
• Nuclear medicine produces emission images (as opposed to
transmission images), because the radioisotopes emit their energy
from inside the patient

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• Functional imaging
• provide information regarding the physiological conditions in the patient
• For example, thallium tends to concentrate in normal heart muscle, but
in areas that are infarcted or are ischemic, thallium does not concentrate
as well
• These areas appear as “cold spots” on a nuclear medicine image, and are
indicative of the functional status of the heart

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Single Photon Emission Computed Tomography
• Tomographic counter-part of nuclear medicine planar imaging, just like
CT is the tomographic counterpart of radiography
• A nuclear camera records x- or gamma-ray emissions from the patient
from a series of different angles around the patient
• These projection data are used to reconstruct a series of tomographic
emission images
• SPECT images provide diagnostic functional information similar to
nuclear planar examinations; however, their tomographic nature allows
physicians to better understand the precise distribution of the
radioactive agent, and to make a better assessment of the function of
specific organs or tissues within the body

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 Positron Emission Tomography
• Positrons are positively charged electrons, and are emitted by some
radioactive isotopes such as fuorine-18 and oxygen-15
• These radioisotopes are incorporated into metabolically relevant compounds,
such as 18F-fuorodeoxyglucose (18FDG), which localize in the body after
administration
• The decay of the isotope produces a positron, which rapidly undergoes a very
unique interaction: the positron (e+) combines with an electron (e-) from the
surrounding tissue, and the mass of both the e- and the e+ is converted by
annihilation into pure energy

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• many of the elements that emit positrons (carbon, oxygen, fuorine) are quite
physiologically relevant (fluorine is a good substitute for a hydroxyl group),
and can be incorporated into a large number of biochemicals
• The most important of these is 18FDG, which is concentrated in tissues of
high glucose metabolism such as primary tumors and their metastases
• PET scans of cancer patients have the ability in many cases to assess the
extent of disease, which may be underestimated by CT alone, and to serve
as a base-line against which the effectiveness of chemotherapy can be
evaluated

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SPECT – CT scan

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PET – CT scan

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 The limiting spatial resolutions of various medical imaging modalities
The resolution levels achieved in typical clinical usage of the modality are listed
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 Contrast
• Any substance that renders an organ or structure more visible
than is possible without its addition
• Allows visualization of structures that can not be seen well or at
all under normal circumstances

• Contrast media is needed because:

– Soft tissue has a low absorption/interaction ratio

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Type of contrast Positive contrast medium

• Negative • Non-soluble
– Air – Absorbed - Oily &/or viscous
– Oxygen – Not absorbed - Inert (Barium)
– Carbon Dioxide
• Water-soluble
– Nitrous Oxide – Non-injectible - Oral
– Injectible – Intravenous
• Positive
– Barium • Ionic
– Iodine • Non-Ionic
• Low Osmolality

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 Routing of contrast media
• Direct
– Barium studies
– Myelography
– Angiography
– Arthrography
• Indirect
– Intravenous Pyelogram (IVP)
– Oral Cholecystogram (OCG)

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 Precautions
• Screening - Medical History
• Pre-Testing???
• Special Considerations
– General anesthesia
– Pregnancy
– Nursing mothers
– Small children

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 Technologist’s responsibilities
• Medical history
• Familiar with contrast media
• Know location of emergency supplies
• Know emergency procedures

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 Radiation safety

As Low As Reasonably Achievable (ALARA)

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Class schedule and topics

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Any Questions?