X-ray

EM Spectrum
Electromagnetic wave spectrum
 Nature of X-ray
• X-rays are electromagnetic radiation located at the low wavelength
end of the electromagnetic spectrum.
• The X-rays in the medical diagnostic region have wavelength of the
order of 10^-10m.
• They propagate with a speed of 3 × 10^10 cm/s and are unaffected
by electric and magnetic fields.
• According to the quantum theory, electromagnetic radiation
consists of photons, which are conceived as ‘packets’ of energy.
Their interaction with matter involves an energy exchange and the
relation between the wavelength and the photon is given by,
 Properties of X-Ray
• Because of short wavelength and extremely high energy, X-rays are
able to penetrate through materials which readily absorb and reflect
visible light. This forms the basis for the use of X-rays for
radiography.
• 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.
 Units of X-ray
• The International Commission on Radiological Units and
Measurements has adopted Rontgen as a measure of the quantity
of x-radiation. This unit is based on the ability of radiation to
produce ionization and is abbreviated ‘R’.
• One R is the amount of x-radiation which will produce 2.08 ×10^9
Ion pairs per cubic centimeter of air at standard temperature (0°C)
and pressure.
• Other units derived from the Rontgen are the millirontgen (mR =
1/1000 R) and the micro rontgen.
• The biological effects of X-rays are due to energy imparted to
matter: Therefore, these effects are more closely correlated with
the absorbed dose than with exposure.
• The unit of absorbed dose is rad. One rad is the radiation dose
which will result in an energy absorption of 1.0 ×10^-2 J/kg in the
irradiated material. It is approximately equal to the dose absorbed
by soft tissue exposed to one Rontgen of X-rays.
• The Rontgen and the absorbed dose D are related as D = f R
where f is a proportionality constant and depends upon both the
composition of the irradiated material and quality of the radiation
beam.
• The value of f for air is 0.87 rad/R. For soft tissues, f = 1 rad/R
and hence the absorbed dose is numerically equal to the exposure.
• for bone, f is larger but significantly decreases with an increase in
kV. Therefore, if the contrast requirements permit, the patient’s
absorbed dose can be decreased by using suitably high kV.
• Rontgens express incident energy,
• Rads give an indication of how much of this incident energy is
absorbed and
• Rems are a measure of the relative biological damage caused.
X-ray
 Diagnostic Range

50 keV < E < 150 keV

 ≈ 0.5%

Rotate anode to prevent melting

What parameters do we have to play with?

1. Current
Units are in mA
2. Time
Units · sec

3. Energy ( keV)
Interactions between X-rays and Matter
Photons with energy less than 13.6 eV are nonionizing. These
photons cannot eject an electron from its atom, but are only able
to raise it to a higher energy shell, a process called excitation.
Ionizing photons can interact with matter in different ways.
 Photoelectric Effect
 The photoelectric effect is a phenomenon in which electrons
are ejected from the surface of a metal when light is incident
on it. These ejected electrons are called photoelectrons.
 It is important to note that the emission of photoelectrons and
the kinetic energy of the ejected photoelectrons is dependent
on the frequency of the light that is incident on the metal’s
surface.
 The process through which photoelectrons are ejected from the
surface of the metal due to the action of light is commonly
referred to as photoemission.
 The photoelectric effect occurs because the electrons at the
surface of the metal tend to absorb energy from the incident
light and use it to overcome the attractive forces that bind them
to the metallic nuclei.
Pair production
Photodisintegration
 Characteristic Radiation
• If the electron interacts with an inner-shell electron of
the target atom rather than with an outer-shell
electron, characteristic x-rays can be produced.
• Characteristic x-rays result when the interaction is
sufficiently violent to ionize the target atom through
total removal of an inner-shell electron.
• Characteristic x-rays are emitted when an outer-shell
electron fills an inner-shell void.
• The effective energy of characteristic x-rays increases
with increasing atomic number of the target element.
 White or Bremsstrahlung Radiation
• The production of heat and characteristic x-rays involves
interactions between the projectile electrons and the
electrons of x-ray tube target atoms.
• A third type of interaction in which the projectile
electron can lose its kinetic energy is an interaction with
the nuclear field of a target atom.
• In this type of interaction the kinetic energy of the
projectile electron is also converted into electromagnetic
energy.
• A projectile electron that completely avoids the orbital
electrons as it passes through a target atom may come
sufficiently close to the nucleus of the atom to come under the
influence of its electric field
• Because the electron is negatively charged and the nucleus is
positively charged, there is an electrostatic force of attraction
between them.
• The closer the projectile electron gets to the nucleus, the more
it is influenced by the electric field of the nucleus. This field is
very strong because the nucleus contains many protons and the
distance between the nucleus and projectile electron is very
small.
• As the projectile electron passes by the nucleus, it is slowed
down and changes its course, leaving with reduced kinetic
energy in a different direction. This loss of kinetic energy
reappears as an x-ray.
• Bremsstrahlung x-rays are produced when a projectile electron is
slowed by the nuclear field of a target atom nucleus.
• when an x-ray imaging system is operated at 70 kVp, electrons
from the cathode have kinetic energies from zero to 70 keV.
• An electron with kinetic energy of 70 keV can lose all, none, or
any intermediate level of that kinetic energy in a bremsstrahlung
interaction. Therefore the bremsstrahlung x-ray produced can
have any energy up to 70 keV.
• This is different from the production of characteristic x-rays,
which have very specific energies.
• In the diagnostic range, most x-rays are bremsstrahlung x-rays.
• Maximum x-ray energy is associated with the minimum x-ray
wavelength (λmin).
• The electrodes are generally sealed in a vacuum, which
allows independent control of the number and speed of
the accelerated electrons striking the tungsten anode.
• The presence of gas can result in variation in the
number of electrons and reduced speed. The cathode is
composed of two elements, the filament, made of
tungsten, and a metallic cup for focusing the electrons
emitted by the filament.
• The filament is a helical coil of tungsten wire of about
0.2 mm in diameter. The coil is approximately 0.2 cm
in diameter and 1 cm in length. When the current is fed
through the wire, it becomes heated. The heat will be
absorbed by the electrons in the wire.
• When the temperature reaches a certain level, the electrons absorb
enough energy to overcome the surface barrier and to escape from
the metal. These escaped electrons form a cloud around the
filament and are called space charge.
• They prevent the electrons within the wire from escaping the
filament, and the effect of the space charge on limiting the
emission of more electrons from the filament is called the space
charge effect.
• The electrons stay around the filament because the loss of
electrons causes the filament to become positive. These escaped
electrons can then be accelerated toward the anode by applying a
high voltage potential.
• Tungsten is desirable in X-ray application because it has a high
melting point (3370°C) and little tendency to vaporize, and it is
strong.
X-ray Tube
• The external structure of the x-ray tube consists of three parts: the
support structure, the protective housing, and the glass or metal
enclosure. The internal structures of the x-ray tube are the anode
and the cathode.
• Ceiling Support System
• Floor-to-Ceiling Support System
• C-Arm Support System
Filament: tungsten
Cathode block: nickel or SS
Anode block: Copper
Target area: Tungsten, 15mm wide, 20mm
long, 3mm thick
 The cathode is the negative side of the x-ray tube; it has two
primary parts, a filament and a focusing cup.
 The filament is approximately 2 mm in diameter and 1 or 2 cm
long, 0.2mm thick.
 An x-ray tube filament emits electrons when it is heated. When
the current through the filament is sufficiently high, the outer-
shell electrons of the filament atoms are “boiled off” and ejected
from the filament. This phenomenon is known as thermionic
emission.
• Filaments are usually made of thoriated tungsten.
• Tungsten provides for higher thermionic emission than
other metals. Its melting point is 3410°C; therefore, it
is not likely to burn out like the filament of a light
bulb.
• Also, tungsten does not vaporize easily. If it did, the
tube would become gassy quickly, and its internal parts
would be coated with tungsten. The addition of 1% to
2% thorium to the tungsten filament enhances the
efficiency of thermionic emission and prolongs tube
life.
• Tungsten vaporization with deposition on the inside of
the glass enclosure is the most common cause of tube
failure.
• The filament is embedded in a metal shroud called
the focusing cup.
• The focusing cup is negatively charged so that it
electrostatically confines the electron beam to a small
area of the anode.
• The effectiveness of the focusing cup is determined
by its size and shape, its charge, the filament size and
shape, and the position of the filament in the focusing
cup.
• The x-ray tube current is adjusted by controlling the
filament current.
• When emitted from the filament, electrons are in the
vicinity of the filament before they are accelerated to
the anode.
• Because these electrons carry negative charges, they
repel one another and tend to form a cloud around the
filament. This cloud of electrons, called a space charge,
makes it difficult for subsequent electrons to be emitted
by the filament because of electrostatic repulsion. This
phenomenon is called the space charge effect.
• A major obstacle in producing x-ray tubes with currents
that exceed 1000 mA is the design of adequate space
charge– compensating devices.
• Thermionic emission at low kVp and high mA can be
space charge limited.
• Most diagnostic x-ray tubes have two focal spots— one large and
the other small. The small focal spot is used when better spatial
resolution is required. The large focal spot is used when large
body parts are imaged and when other techniques that produce
high heat are required.
• Small focal spots range from 0.1 to 1 mm; large focal spots
range from 0.3 to 2 mm.
 ANODE
• The anode is the positive side of the x-ray tube.
• There are two types of anodes, stationary and rotating.
• Stationary anode x-ray tubes are used in dental x-ray imaging
systems, some portable imaging systems, and other special-purpose
units in which high tube current and power are not required.
• General-purpose x-ray tubes use the rotating anode because they must
be capable of producing high-intensity x-ray beams in a short time.
• The anode is the positive side of the x-ray tube; it conducts
electricity and radiates heat and x-rays from the target.
• The anode serves three functions in an x-ray tube.
• The anode is an electrical conductor. It receives electrons
emitted by the cathode and conducts them through the tube to the
connecting cables and back to the high voltage generator. The
anode also provides mechanical support for the target.
• The anode also must be a good thermal dissipater. When the
projectile electrons from the cathode interact with the anode,
more than 99% of their kinetic energy is converted into heat. This
heat must be dissipated quickly. Copper, molybdenum, and
graphite are the most common anode materials.
• Higher tube currents and shorter exposure times are possible with
the rotating anode.
• The target is the area of the anode struck by the electrons from
the cathode.
• In stationary anode tubes, the target consists of a tungsten alloy
embedded in the copper anode.
• In rotating anode tubes the entire rotating disc is the target .
• Alloying the tungsten (usually with rhenium) gives it added
mechanical strength to withstand the stresses of high-speed
rotation and the effects of repetitive thermal expansion and
contraction.
• High-capacity x-ray tubes have molybdenum or graphite layered
under the tungsten target.
• Both molybdenum and graphite have lower mass density than
tungsten, making the anode lighter and easier to rotate.
• Specialty x-ray tubes for mammography have molybdenum or
rhodium targets principally because of their low atomic number
and low K-characteristic x-ray energy.
• Tungsten is the target material of choice for general
radiography for three main reasons:
• 1. Atomic number—Tungsten’s high atomic number,
74, results in high-efficiency x-ray production and in
high-energy x-rays.
• 2. Thermal conductivity—Tungsten has a thermal
conductivity nearly equal to that of copper. It is
therefore an efficient metal for dissipating the heat
produced.
• 3. High melting point—Any material, if heated
sufficiently, will melt and become liquid. Tungsten has
a high melting point (3400°C compared with 1100°C
for copper) and therefore can stand up under high tube
current without pitting or bubbling.
• Heat capacity can be further improved by increasing the speed of
anode rotation. Most rotating anodes revolve at 3400 rpm
(revolutions per minute). The anodes of high-capacity x-ray tubes
rotate at 10,000 rpm.
• The stem of the anode is the shaft between the anode and the rotor.
It is narrow so as to reduce its thermal conductivity. The stem
usually is made of molybdenum because molybdenum is a poor
heat conductor.
• The rotating anode is powered by an electromagnetic induction
motor.
• An induction motor consists of two principal parts separated from
each other by the glass or metal enclosure.
• Outside glass, stator, consists of a series of electromagnets equally
spaced around the neck of the tube. Inside the glass or metal
enclosure is a shaft made of bars of copper and soft iron fabricated
into one mass. This part is called the rotor.
• The induction motor works through electromagnetic induction,
similar to a transformer. Current in each stator winding induces
a magnetic field that surrounds the rotor.
• The stator windings are energized sequentially so that the
induced magnetic field rotates on the axis of the stator. This
magnetic field interacts with the ferromagnetic rotor, causing it
to rotate synchronously with the activated stator windings.
• The focal spot is the area of the target from which x-rays are
emitted. Radiology requires small focal spots because the
smaller the focal spot, the better the spatial resolution of the
image. Unfortunately, as the size of the focal spot decreases, the
heating of the target is concentrated onto a smaller area. This is
the limiting factor to focal spot size.
• Before the rotating anode was developed, another design was
incorporated into x-ray tube targets to allow a large area for heating
while maintaining a small focal spot. This design is known as the
line-focus principle.
 Line Focus Principle
• Large focal spot is preferred because it allows the accumulation of
larger amounts of heat. However, a small focal spot is needed to
generate better images.
• The anode angle, which is the angle between the slanted target
surface and the plane perpendicular to the electron beam, typically
varies from 5° to 15°. The effective focal size, f , is related to the
length of actual focal size, F, on the anode by the following
equation: f=F sinƟ
• Where Ɵ is the anode angle. It is evident that a larger anode angle
provides a larger area for bombardment but it also produces a larger
apparent focal spot.
• The angle is limited by the so-called Heel effect. The intensity
leaving the X-ray tube is not uniform; it is smaller in the anode
direction because photons travelling in certain directions have to
travel a longer path in the anode than others.
• This heat can be dissipated in one of three ways: radiation, conduction, or
convection.
• Radiation is the transfer of heat by the emission of infrared radiation.
Heat lamps emit not only visible light but also infrared radiation.
• Conduction is the transfer of energy from one area of an object to
another. The handle of a heated iron skillet becomes hot because of
conduction.
• Convection is the transfer of heat by the movement of a heated substance
from one place to another. Many homes and offices are heated by the
convection of hot air.
 X-ray Tube Ratings
• Filament temperature controlled by the filament current (if), the
potential difference between the anode and cathode (tube voltage,
Vt),
• The number of electrons bombarding the anode target (tube current,
mA), and the target material.
• Target Material The higher the atomic number, the greater the
efficiency of Xray production. For example, platinum (atomic
number 78) produces more white radiation than tungsten (atomic
number 74) at the same tube current and potential.
• Tube Voltage The tube voltage Vt can be either dc or ac following
full-wave or half-wave rectification. For ac generators, it is usually
measured in terms of peak voltage applied or kilovolts peak (kVp).
The intensity is proportional to the square of kVp. The maximum
energy produced also depends on the voltage. Typically the tube
voltage ranges from a few kilovolts peak up to 150 kVp.
• Tube Current The number of X-ray photons produced depends
on the number of electrons striking the target and therefore
should depend on the tube current. It was found that the intensity
is linearly proportional to the tube current. Typically the tube
current ranges from a few milliamperes to a few hundred
milliamperes.
• Filament Current The tube current increases initially as the tube
voltage is increased at a fixed filament current. However, as the
potential difference is further increased, a point will be reached
after which an increase in potential difference has no effect on
tube current. This point is called the saturation voltage.
• In this region, the current is limited by the filament temperature
or the filament current. Typical values of filament current are a
few amperes which can be dc or ac.
 FACTORS AFFECING THE X-RAY EMISSION SPECTRUM

• Effect of mA and mAs:

• If one changes the current from 200 to 400 mA while all other
conditions remain constant, twice as many electrons will flow from
the cathode to the anode, and the mAs will be doubled. This
operating change will produce twice as many x-rays at every
energy.
• A change in mA or mAs results in a proportional change in the
amplitude of the x-ray emissionspectrum at all energies.
• Effect of kVp:
• When kVp is increased, the relative distribution of emitted x-ray
energy shifts to the right to a higher average x-ray energy. The
maximum energy of x-ray emission always remains numerically
equal to the kVp.
• A change in kVp affects both the amplitude and the position of the
x-ray emission spectrum.
In the diagnostic range a 15% increase in kVp is equivalent to
doubling the mAs.
• FOUR PRINCIPAL FACTORS INFLUENCING THE SHAPE
OF AN X-RAY EMISSION SPECTRUM
• 1. The projectile electrons accelerated from cathode to anode do not
all have peak kinetic energy. Depending on the types of rectification
and high-voltage generation, many of these electrons may have
very low energies when they strike the target. Such electrons can
produce only heat and low-energy x-rays.
• 2. The target of a diagnostic x-ray tube is relatively thick.
Consequently, many of the bremsstrahlung x-rays emitted result
from multiple interactions of the projectile electrons, and for each
successive interaction, a projectile electron has less energy.
• 3. Low-energy x-rays are more likely to be absorbed in the target.
• 4. External filtration is always added to the x-ray tube assembly.
This added filtration serves selectively to remove low-energy x-rays
from the beam.
• Effect of Added Filtration
• Adding filtration to the useful x-ray beam reduces x-ray beam
intensity while increasing the average energy.
• The result of added filtration is an increase in the average energy of
the x-ray beam with an accompanying reduction in x-ray quantity.
• Adding filtration is sometimes called hardening the x-ray beam
because of the relative increase in average energy.
• Effect of Target Material
• The atomic number of the target affects both the number (quantity)
and the effective energy (quality) of x-rays. As the atomic number
of the target material increases, the efficiency of the production of
bremsstrahlung radiation increases, and high-energy x-rays
increase In number to a greater extent than low-energy x-rays.
• Increasing target atomic number enhances the efficiency of x-ray
production and the energy of characteristic and bremsstrahlung x-
rays.
• Effect of Voltage Waveform
• There are five voltage waveforms: half-wave–rectified, full-
wave–rectified, three-phase/six-pulse, three-phase/ 12-pulse,
and high-frequency waveforms.
• Because of reduced ripple, operation with three-phase power
or high frequency is equivalent to an approximate 12%
increase in kVp, or almost a doubling of mAs over single-
phase power.
 X-ray quantity is the number of x-rays in the useful beam.
 When SID is increased, mAs must be increased by SID2 to
maintain constant exposure to the image receptor.
 Adding filtration to the useful x-ray beam reduces patient dose.
• X-RAY QUALITY
• Penetrability:
• As the energy of an x-ray beam is increased, the penetrability is
also increased. Penetrability refers to the ability of x-rays to
penetrate deeper in tissue. High-energy x-rays are able to penetrate
tissue more deeply than low-energy x-rays. The penetrability of an
x-ray beam is called the x-ray quality. X-rays with high
penetrability are termed high-quality x-rays. Those with low
penetrability are low-quality x-rays.
• Half-Value Layer
• Although x-rays are attenuated exponentially, high energy x-rays
are more penetrating than low-energy x-rays. Whereas 100-keV x-
rays are attenuated at the rate of approximately 3%/cm of soft
tissue, 10-keV x-rays are attenuated at approximately 15%/cm of
soft tissue. X-rays of any given energy are more penetrating in
material of low atomic number than in material of high atomic
number.
• In radiography the quality of x-rays is measured by the half-value
layer (HVL). Therefore the HVL is a characteristic of the useful
x-ray beam. The HVL of an x-ray beam is the thickness of
absorbing material necessary to reduce the x-ray intensity to half of
its original value. A diagnostic x-ray beam usually has an HVL in
the range of 3 to 5 mm Al or 3 to 6 cm of soft tissue.
• STEPS TO DETERMINE THE HALF-VALUE LAYER
• 1. Determine the x-ray beam intensity with no absorbing
material in the beam and then with different known
thicknesses of an absorber.
• 2. Plot the ordered pairs of data (thickness of absorber, x-
ray quantity).
• 3. Determine the x-ray quantity equal to half the original
quantity and locate this value on the y- or vertical axis of
the graph.
• 4. Draw a horizontal line parallel to the x-axis from point A
in step 3 until it intersects the curve (B).
• 5. From point B, drop a vertical line to the x-axis.
• 6. On the x-axis, read the thickness of the absorbe required
to reduce the x-ray intensity to half of its original value
point (C). This is the HVL.
X-Ray machine
 High voltage generation
• Voltages in the range of 30–200 kV are required for the production
of X-rays.
• A high ratio step up transformer is used, the ratio would be in the
range of 1:500 so that an input of 250 V would produce an output
of 125 kV.
• Self-rectified Circuit (One Pulse):
• These machines have maximum tube currents of about 20 mA and
a voltage of about 100 kV.
• Full Wave Rectification X-ray Circuit (Two Pulse):
• By using a full wave bridge rectifier circuit, the exposure
time for the same radiation output is reduced by half in
comparison to the one pulse system.
• Three Phase Power for X-ray Generation:
• The X-ray circuits based on a single phase supply provide a
pulsating voltage to the anode.
• Disadvantages:
• The intensity of radiation produced is lower because no radiation is
generated during a large portion of the exposure time.
• When the tube voltage is appreciably lower than the peak voltage,
the X-rays produced are of low energy and get mostly transformed
into heat at the anode.
• A considerable part of the radiation produced is absorbed by the
filter or the tube housing and patient resulting in a comparatively
poor quality image.
• Three phase supply can result in steady power to the X-ray tube
instead of pulsating power. The three phase equipment is more
efficient than the single phase equipment of the same-rated
capacity.
• Using different types of three phase
transformers and rectifier configurations,
6 pulses or 12 pulses of applied anode
voltage can be obtained.
• Six-rectifier Circuit (Six Pulse): The
primary supply is delta-connected
whereas the secondary is y-connected.
The secondary is connected to a six-
rectifier arrangement for full wave
rectification of the three phase
transformer output voltage. The
rectification process produces the six-
pulse voltage waveform with a voltage
ripple of 13% of the maximum value as
compared to 100% of the single phase
full wave rectified voltage system.
• Twelve Rectifiers Circuit (Twelve Pulse): A further refinement of
the three phase generator is to use a combination of both star and
delta windings on the secondary side.
X-RAY
 Filters
• The X-ray photons emitted by the anode of an X-ray generator
during an Xray examination are absorbed by the X-ray tube itself.
• The added absorbers usually are thin sheets of metal placed
between the X-ray source and the patient. Aluminum is an
excellent absorber for low-energy X-rays, while copper is most
useful for high-energy X-rays.
• copper is never used alone as an absorber.
• the aluminum layer is placed below the copper layer to absorb this
radiation.
• An aluminum layer 3 mm thick can attenuate more than 90% of the
X-ray energy at 20 keV.
 Beam Restrictors and Grids
• There are three types of X-ray beam restrictors: aperture diaphragms,
cones and cylinders, and collimators.
• The basic function of a beam restrictor is to regulate the size and
shape of the beam.
• A closely collimated beam can reduce patient exposure and generate
less scatter radiation.
• The aperture diaphragm, sheet of lead with a hole in the center whose
size and shape determines those of the X-ray beam. The width of the
penumbra Ρ is related to the source diameter by the following
equation,

where D is the width of the source, / and L are the distances

between the source
and the object, and between the object and the detector,
respectively.
• Cones and cylinders are sometimes also used as beam restrictors,
but they, along with the diaphragm, suffer from a major drawback:
Only a limited number of beam sizes can be obtained. The
collimator is the most popular beam restrictor for two reasons: The
X-ray field size is adjustable, and a light beam can be used to
indicate the exact size of the field.
• The X-ray beam size is adjusted by the movable aperture
diaphragm.
• The Xray field is illuminated by a light beam from a light bulb
located within the collimator the same distance from the center of
the mirror as the X-ray source.
• Scattered X-rays are noise that degrades image quality and
increases patient exposure and therefore should be minimized. The
most effective way of removing scatter radiation is the
radiographic grid.
 GRID TYPES

• Parallel Grid
Most grids have frequencies in the range of 25 to 45 lines per
centimeter. Grid frequency can be calculated if the widths of the grid
strip and of the inter space are known.
Grid frequency is computed by dividing the thickness of one line pair
(T + D), expressed in μm, into 1cm.
• The grid is composed of a series of lead foil strips separated by X-ray
transparent spacers which are either aluminum or organic material. The
grid blocks the scattered radiation while letting the primary radiation
pass.
• The grid ratio is defined as h/g, that is, the ratio between the height of
the lead strips and the width of the gap between them. This ratio usually
ranges from 4 to 16. For a grid with a height of 2 mm, lead strip width
of 0.05 mm, and strip gap width of 0.25 mm, the grid ratio is 8.
• It then becomes quite apparent that the higher the grid ratio, the better
the grid function in removing scatter radiation.
• increasing patient exposure.
• linear grid.
• When the grid strips are focused toward the X-ray source, the grid is
called a focused grid.
• If the lead strips are thin enough, their image on the film or the detector
may be negligible. However, if the image quality requirement
necessitates thick lead strips, the grid may be moved during exposure to
blur out the image of the grid lines.
 Intensifying Screens,

• the process of converting X-ray photons into visible photons is

called fluorescence, the intensifying screen is one form of
fluorescent screens.
• An intensifying screen is basically a layer of phosphor with a
thickness range of 0.05 mm to 0.3 mm that emits light photons
when struck by X-ray photons, with some supporting material.
• The substrate material of approximately a few tenths of a
millimeter thick should be relatively transparent to X-ray.
• the protective layer is light transparent. The light-reflective layer of
0.025 mm thick prevents the light photons from going in the
backward direction.
• the X-ray photons are high-energy photons and light photons are
low-energy photons, a small number of X-ray photons can generate
a large number of light photons.
• A number of phosphors have been used for intensifying screens.
The most popular phosphor for intensifying screens has been
calcium tungstate (CaW04) and terbium-activated rare-earth
oxysulfide. CaW04 , which has a Κ edge of 50.2 keV, emits light
in the blue region of the visible light spectrum with a wavelength
range of 350 to 580 nm and a peak wavelength of 430 nm, which
is not optimal for visualization by the eye but very desirable for X-
ray film exposure.
• The efficiency of CaW04 is relatively low, about 5%. The newer
gadolinium (Gd2C>2S), lanthanum (LaOBr), or yttrium
oxysulfide phosphor (Y202S:Tb) can achieve a better efficiency
of 15, 12, or 18%.
• The rare-earth phosphors are more absorbent in midrange energy
between 50 and 60 keV than CaW04 , and Gd202S has a Κ edge at
50.2 keV. CaW04 screens have been largely replaced by rareearth
screens.
• The rare-earth screens emit narrow bands of light in the green or
blue region of the light spectrum which necessitates the use of a
photographic film sensitive to light in these regions.
• The speed of an intensifying screen for a given phosphor is
determined primarily by its thickness, phosphor grain size, and the
ability of the light photons to escape from the screen.
• The thicker the phosphor, the higher the speed. This is because a
thicker layer will absorb more X-ray photons and, thus, emit more
light photons, which darken the film in a shorter time.
• However, the increase in thickness also results in a degradation in
image quality, such as sharpness and contrast, due to more scattering
or diffusion of the light photons.
• A phosphor crystal closer to the film will blacken a smaller area of
the film, whereas a crystal farther from the film will blacken a larger
area.
• A light-absorbing dye may sometimes be added to the phosphor to improve
the resolution by preferentially absorbing the light photons emitted laterally.
 Image Intensifier

• The image produced on the image-intensifying screen is typically

very weak and can be visualized only when the room is darkened.
To brighten the image, a device called the image intensifier can be
used.
• It is a vacuum tube with the following components: (1) input
phosphor and photocathode, (2) focusing plates, (3) an anode, and
(4) output phosphor.
• The X-ray photons that have propagated through the patient will
be absorbed by the fluorescent screen of 15 to 35 cm diameter
with almost spontaneous emission of light photons. The light
photons strike the photocathode kept at ground potential, causing
it to emit electrons in a number proportional to the brightness of
the screen.
• The photocathode is usually made of a photoemissive metal such
as antimony and cesium compounds. The electron beam will be
accelerated and focused onto the output fluorescent screen by the
• The anode has a positive potential about 25 kV. The
output fluorescent screen of 1.5 to 2.5 cm diameter, when
struck by these high-energy electrons, produces an image
much brighter than the image generated by a
conventional fluoroscope with similar image quality
because,
• (1) the electrons gain energy during acceleration to the
anode; and
• (2) the output screen is much smaller than the input
screen.
• In the modern image intensifier, cesium iodide, which is
more efficient and thinner than zinc cadmium sulfide, is
used as the input phosphor and zinc cadmium sulfide is
used as the output phosphor.
 X-ray Detector
• X-ray films:
• The X-ray film is a photographic film consisting of a
transparent plastic substrate made of acetate or polyester
usually coated on both sides with a light-sensitive emulsion.
• The most important components of the emulsion are silver
halide crystals with grain sizes from 0.1 to 1μm , mostly silver
bromide, and gelatin.
• The silver bromide crystal, upon receiving a light photon,
yields a free electron that can combine with a silver ion to form
a silver atom. Silver iodide 1-10% is used as a sensitizer. The
silver atom is black.
• exposure of the film to light causes it to darken.
• The blackening process of the film is directly related to the
energy of the incident light, which is the product of intensity of
the light and time of exposure.
• Optical Density The photographic density or optical density used
to measure the film blackness is defined by the following equation,

where D denotes the optical density, h and /t are incident and transmitted light
intensities,

 This measure actually reflects the light opacity of the medium or

the blackening of the film.
 Higher density means a darker file or less light transmission. In
X-ray, a film with a density of 2 appears black when viewed on a
standard light box, while a density between 0.25 to 0.3 appears to
be white or transparent.
• An unexposed X-ray film after being processed shows a density
usually less than 0.22, called the base plus fog density owing to
• (1) the base material on which the film emulsion is coated; and (2)
silver halides developed without exposure, called film fog.
• One of the reasons for expressing the density in terms of a
logarithmic scale is that the physiological response of the eye to
the light intensity is logarithmic.
• Characteristic Curve The relationship between density and film
exposure is called the characteristic curve, or the Η and D curve, of
the film.
• It can easily be seen that even at zero exposure, the density is not
zero because of the base material and film fog.
• In the mid-portion of the curve, which is the most important part of
the curve, the density is approximately linearly proportional to the
log exposure.