X-Ray Shadowgraph Imaging Technique Lecture

MEDICAL IMAGING
X-RAY IMAGING TECHNIQUE
BY
DR RAMY ABDLATY

CONTENTS
• What is an X-ray?
• X-ray energy (E)
• X-ray penetration
• Generation of X-rays
• X-ray Machine
• Fluoroscopy
• Advantages and disadvantages

WHAT IS X-RAY?
• X-rays and visible
light rays are both
wavelike forms of
electromagnetic
energy carried by
particles called
photons (little
packets of
energy).
• The difference,
between X-rays
and visible light
rays, is the energy
level of the
individual
photons.

X-RAY ENERGY (E)
• X-rays are the second highest energetic electromagnetic
radiation after Gamma rays.
• E = h * γ, where (h) is blanks constant (6.626068 × 10-34 m2 kg / s) and,
(γ) is the wave frequency
• c =  * γ, where (c) is the speed of light (3 X 108 m/s) and, () is the
wavelength

X-RAY PENETRATION
• When a photon collides with an atom, it may absorb the photon's energy by boosting an
electron to a higher level. For this to happen, the energy level of the photon has to match
the energy difference between the two electron positions. If not, the photon can't shift
electrons between orbitals.
• The atoms of human body tissues absorb visible light photons very well. The energy level of
visible photons fits with various energy differences between electron positions.
• Radio waves have less energy to move electrons between orbits in larger atoms, so they pass
through most stuff.
• X-ray photons also pass through most things, but for the opposite reason: They have too
much energy.
• X-ray penetrates structures within human body and creates images on film or
screen. X-ray imaging is a painless and noninvasive way of medical imaging.

X-RAY PENETRATION
• X-rays can, however, knock an electron away from an atom altogether. Some of the energy
from the X-ray photon works to separate the electron from the atom, and the rest sends the
electron flying through space.
• A larger atom is more likely to absorb an X-ray photon in this way, because larger atoms have
greater energy differences between orbitals -- the energy level more closely matches the
energy of the photon.
• Smaller atoms, where the electron orbitals are separated by relatively low jumps in energy,
are less likely to absorb X-ray photons.
• Soft tissues, in our bodies, are composed of smaller atoms, and so it does not absorb X-ray
photons particularly well.
• Calcium atoms that make up our bones are much larger, so they are better in absorbing X-ray
photons.

GENERATION OF X-RAYS
• The generation, of the X-rays, is based on the
existence of extremely high voltage difference
between the anode and the cathode in the X-ray
machine.
• The result is an accelerated electron beam hitting
the anode. This situation result in two types of
interactions which generate the X-rays photons.
• In the first, the accelerated electron interacts
directly with an electron in an orbital shell of a
tungsten atom of the anode. The orbital electron
is displaced but the orbital gap is rapidly filled by
an electron from a more distant orbit. The
difference in the energies of the two electron
orbits is equal to an x-ray photon energy.

• The second and more frequent interaction is
termed Bremsstrahlung. This German word
means "braking" radiation and that
describes what happens to the incoming
electron.
• In this interaction, the accelerated electron
passes relatively close to the nucleus of a
tungsten atom in the anode. The path of the
accelerated electron is affected by the
nucleus with a resulting change in direction
and the kinetic energy of the electron is
dissipated.
• The difference in the kinetic energy before
and after interacting with the nucleus is
radiated as an X-rays photon.

• The first, a tungsten atom of the anode
has an inner orbit’s electron displaced
but the orbital gap is rapidly filled by an
electron from a more distant orbit. The
characteristic x-ray are distinguishing for
the anode material.
• The second and more frequent
interaction is termed Bremsstrahlung
that describes what happens to an
incoming electron relatively close to the
nucleus of a tungsten atom in the
anode. The X-ray photons produced in
this manner range in energy from near
zero up to the energy of the electrons.

X-RAY MACHINE
• The function of the X-ray machine is to provide a sufficient intensity of
electron flow in controlled manner to produce an X-ray beam of desired
quantity and quality.
• Every X-ray machine regardless of its design has three principal parts:
1. X-ray tube assembly.
2. High voltage generator.
3. Control Console.

X-RAY MACHINE
• In dental and portable x-ray machines, the
three components are housed compactly.
1. X-ray tube assembly.
2. High voltage generator.
3. Control Console.

X-RAY MACHINE
• Conventional x-ray machines have the
head of the X-ray tube located in one
room with the control console in an
adjacent room, usually equipped with a
viewing window.
• The high voltage generator is usually
housed in container, perhaps 3 feet on a
side, located in the corner of the
examining room. Some newer versions
locate these generators out of sight
above the room.

X-RAY MACHINE
1-TUBE
• The X-ray tube consists of two primary parts:
Cathode and Anode.
• There are two basic types of microfocus X-ray
tubes: solid-anode tubes and metal-jet-anode
tubes.
• The X-ray tube is always mounted inside a metal
housing designed to control two serious hazards:
1. Excessive radiation exposure
2. Electric shock
• These two requirements were found to be
achieved by the use of Lead as a casing for the X-
ray tube.

X-RAY MACHINE
1-TUBE
• When X-ray is produced, there exist X-ray photons in all directions. In order
to force the X-ray photons to propagate in only one direction (the patient
direction) a specially designed port, called the "window" is used.
•
• Properly designed protective housing reduces the level of radiation leakage
to less than 100 mR/hr at 1M, when the machine is operated at maximum
condition. The protective housing around X-Ray tubes contains oil that is
sealed in and serves as both: electrical insulator, and thermal cushion. Some
protective housings have a cooling fan to air-cool the tube or the oil in which
the tube is bathed.

X-RAY MACHINE
II-CATHODE
• The cathode is the negative side of the x-ray tube and has two primary
parts: Filament and Focusing cup.
1. Filament:
• The filament is a coil of wire. When current through the filament is sufficiently intense; the
outer-shell electrons of the filament atoms are boiled off and ejected from the filament.
This phenomenon is known as thermoionic emission.
• Filaments are usually made of tungsten which provides higher thermoinonic emission
than other metals. Its melting point is 3380 ْC and therefore it is not likely to burn out
like the filament of a light bulb.

X-RAY MACHINE
II-CATHODE
2. Focusing Cup:
• The filament is embedded in a round-shaped metal container called focusing cup. The
effect of using the focusing cup is to focus the electron beam on certain area of the
anode, as shown in figure.
(a) (b)
The effect of adding the focusing cup around the cathode
filament

X-RAY MACHINE
III-ANODE
• The anode is the positive side of the X-ray tube. There are two types of anodes:
1. Stationary
2. Rotating.
• Stationary anode X-ray tube is used in dental X-ray and portable machines since high
tube current and power are not required.
• Most general purpose X-ray tubes use the rotating anode type because they must be
capable of producing high intensity X-ray beams in a short time.
• When the electrons hit the anode, more than 95% of their kinetic energy is converted
into heat. This heat must be conducted away quickly before it can melt the anode.
Copper is the most common material used for manufacturing the anode.

X-RAY MACHINE
III-ANODE
• Target: is the area of the anode which is struck by the electrons from the
cathode. In stationary anodes the target consists of a tungsten alloy metal
embedded in the copper anode. In rotating anode, the entire disc is the target.
• Tungsten is the material of choice for the target for three main reasons:
1. Atomic number: high atomic number results in higher efficiency X-ray production and in
higher energy X-rays.
2. Thermal conductivity: tungsten has a thermal conductivity nearly equal to that of copper.
3. High melting point: Tungsten has a high melting point (3380 ْC compared to 1070 ْC of
copper).

X-RAY MACHINE
III-ANODE
• Line focus principle: The focal spot is the area of the target from which x-ray is emitted.
Radiology requires small focal spots because the smaller the focal spot the sharper the
radiographic image.
Effective focal spot size
Target angle
Electron beam size
Actual focal spot size

X-RAY MACHINE
III-ANODE
• Unfortunately, as the size of the focal spot decreases, the heating of the target is
concentrated to smaller areas.
• Therefore, the line focus principle appears to solve this problem. It is a design
introduced to the x-ray tube to allow a large area of heating while maintaining a small
focal spot by angling the target.
• It is based on making the effective area of the target much smaller than the actual area
of electron interaction, which is the area projected onto the patient and the film.
• The lower the target angle, the smaller the effective focal spot size.

X-RAY MACHINE
CONTROLS OF THE X-RAY TUBE
• There are three important controls which directly affect the quality of the
resultant X-rays film. These controls are:
1. Kilo-voltage potential (KVp)
2. Milliamperage (mA)
3. Milliampere seconds (mA.s)

X-RAY MACHINE
1. Kilo-voltage potential (KVp)
• The energy (the penetrating power) of the x-ray beam is controlled by the voltage
adjustment.
• This control is usually labeled in keV (thousand electron volts) and sometimes
referred to as KVp (kilo-voltage potential). Do not be confused, just remember it is
the difference in potential between the cathode and the anode can be controlled.
• The higher the voltage setting, the more energetic will be the beam of x-ray. A
more penetrating beam will result in a lower contrast radiograph than one made
with an x-ray beam having less penetrating power. It is probably obvious that the
more energetic the beam, the less effect different levels of tissue density will have
in attenuating that beam.

X-RAY MACHINE
2. Milliamperage (mA)
• This control determines how much current is allowed to flow through the filament
which is the cathode side of the tube.
• If more current (and therefore more heating) is allowed to pass through the
filament, more electrons will be available in the "space charge" for acceleration to
the target and this will result in a greater flux of photons when the high voltage
circuit is energized.
• The effect of the mA circuit is quite linear. If you want to double the number of X-
rays photons produced by the tube, you can do that by simply doubling the mA.
• Changing the number of photons produced will affect the blackness of the film but
will not affect the film contrast.

X-RAY MACHINE
3. Milliampere seconds (mA.s)
• The third control of the x-ray tube which is used for medical imaging is the
exposure timer.
• This is usually denoted as an "S" (exposure time in seconds) and is combined with
the mA control. The combined function is usually referred to as mAs or milliampere
seconds so, if you wanted to give an exposure using 10 milliampere seconds you
could use a 10 mA current with a 1.0 second exposure or a 20 mA current for a 0.5
second exposure or any combination of the two which would result in the number
10.
• Both of these factors and their combination affect the film in a linear way. That is, if
you want to double film blackness you could just double the mAs.

X-RAY MACHINE
RADIOGRAPHIC GRID
• One of the problems in getting a sharply
defined image in clinical radiology is the
presence of scattered or secondary
radiation.
• These photons are created in the body of
the patient or closely surrounding objects
by the interaction of that material and the
primary X-Ray photons coming from the x-
ray tube.
• Several possible interactions occur in the
diagnostic energy range.

X-RAY MACHINE
RADIOGRAPHIC GRID
• At relatively low energies, the photoelectric effect is probable. The photoelectric effect is
actually the desirable, photon/tissue interaction because there is complete absorption of
the photon with no production of a secondary photon.
• The common tissue interaction at the photon energies used for the majority of clinical
procedures is called the Compton effect or coherent scattering. In Compton effect, a
secondary photon, with lower energy and an altered direction than the primary photon,
is produced at the site of interaction. Secondary photons, if reached the film, would
actually produce erroneous information by recording gray tone variation (and therefore
indicating relative tissue densities) at some distance from the site at which the
photon/tissue interaction actually occurred. The net result of allowing a significant
number of secondary photons to reach the film is a reduction in image sharpness,
resulting in a loss of spatial resolution

X-RAY MACHINE
RADIOGRAPHIC GRID
• Several methods have been devised to reduce the problem of
scattered radiation:
1. The simplest and most direct is to simply limit the field of exposure. If a
small image area is adequate to make the clinical diagnosis, the image
area should be "coned down" to that small size. For instance, if you
want to image the gallbladder, you will get a much sharper picture if
you bring the shutters down to include an area only the size of the
gallbladder instead of including the entire upper abdomen on the
image. Just remember that the smaller the area of the x-ray beam the
fewer scattered photons you will produce.

X-RAY MACHINE
RADIOGRAPHIC GRID
2. In the typical clinical imaging situation, the most common method of reducing scatter is
to use a radiographic grid. The grid looks like a flat metallic plate the size of the x-ray film.
However, it is more complicated than that. It actually is composed of alternating
radiopaque (lead) and radiolucent (aluminum) strips. These are arranged to let x-ray
photons come between the strips. The edge of these strips is turned towards the source of
x-rays and in the most commonly used grid, the focused grid, the angle of the strips is
arranged to match the divergence of the x-ray beam. The arrangement of the radiographic
grid will give the highest probability for primary X-ray photons passing between the lead
grid strips reaching the film, while the off-focus or secondary photons are likely to interact
in the lead strips and never reach the film.

X-RAY MACHINE
RADIOGRAPHIC GRID
The use of this radiographic grid will greatly improve
image sharpness when a relatively thick body part is
being imaged.
There is always a trade off, since the grid stops some of
the photons which contribute to film blackening. if you
just add a radiographic grid without changing the tube
settings, the film will be greatly underexposed.
Using a grid is usually combined with increasing the
number of photons produced by the x-ray tube in
order to get the correct film exposure. Remember, the
position of the grid is between the patient and the
film.

X-RAY MACHINE
RADIOGRAPHIC GRID
3. The third method of reducing scatter or at least reducing the
probability that scattered photons reach the film is to use an air
gap. This is infrequently used in clinical radiography but can
still be used when magnification of the image is helpful.
Ordinarily, the film is positioned as close to the patient's body
as possible for the radiography of any body part. With an air
gap technique, the film is moved several inches away from the
patient's body. That separation, (because secondary photons
are likely to be lower energy and moving at a greater angle
than primary photons) will result in a decreased probability of
the secondary photon hitting the film. Remember the
secondary photon emissions are produced from almost a point
source and it diverges as it goes towards the patient.

X-RAY MACHINE
FLUOROSCOPY
• Fluoroscopic imaging is useful to radiograph a dynamic situation. For
example, it is used to evaluate the gastrointestinal tract (GI-tract) and also
to record the motion of ureters, diaphragms, in order to reach a
diagnostic decision.
• A fluoroscope is a radiographic machine which has an x-ray tube mounted
in a way that the beam can pass through the patient and be recorded on
a fluorescent screen.
• In modern fluoroscopes, the observer looks at a video image produced by
a camera focused on the screen. Fluoroscopes allow the operator to take
"snap shot" pictures of any observed abnormality on a spot film.
• This equipment is attached to an x-ray table which allows the operator to
tilt the patient in various directions and the x-ray tube is most commonly
positioned under the table with the spot film device and the fluorescent
screen including an image intensifier being above the patient if the
patient is lying supine on the table.

X-RAY MACHINE
FLUOROSCOPY-CONTRAST MEDIUM
• In an X-ray picture, soft tissue doesn't show up clearly. To
focus on organs, or to examine the blood vessels that
make up the circulatory system, doctors must introduce
contrast media (CM) into the body.
• CM are liquids that absorb X-rays more effectively than
the surrounding tissue. To bring organs in the digestive
and endocrine systems into focus, a patient will swallow a
CM mixture, typically a barium compound. For examining
blood vessels or other elements in the circulatory system,
CM will be injected into the patient's blood.
• CM are often used in conjunction with a fluoroscope.
Fluoroscopy can help doctors to trace the passage of CM
through the body, which can be recorded as still X-ray
images on film or a moving X-ray video.

X-RAY MACHINE
GENERATIONS OF THE MACHINE
1- The conventional X-ray Machine
is an analog machine which
depends on chemical process for
producing X-ray images.
• This generation has several limits:
1. It takes long time during X-ray
exposure
2. The developing process of X-
ray films is delicate and time
consuming
3. The produced X-ray films are
hard to recycle
4. It produces low image quality

X-RAY MACHINE
II- Computed Radiography (CR)
Machine is used since 1980’s.
1. It uses stimulated luminescence
screens to capture x-ray image
instead of films
2. The CR cassette goes into a reader
to convert the data into a digital
image using photostimulable
phosphor (PSP)
3. CR cassette and reader is
distinguished by: light weight, made
from plastic, and contains chip to
record patient data
4. It produces low image quality

X-RAY MACHINE
II- Computed Radiography (CR)
Machine has the following advantages.
1. The same plate can be used multiple
times
2. Plate exposure times are less than
film
3. X-ray images don’t require a dark
room and chemicals for film
production
4. It produces digital images that can
be store and processed
electronically
When X-ray is irradiated to the plate, electrons generated in the plate are
accumulated. A laser beam scans (excites) the image formed on the plate,
causing visible light to be emitted according to the amount of accumulated X-
rays. A PMT is then used to convert this weak visible light into electrical
signals, which are then digitally processed to reconstruct an image.

X-RAY MACHINE
II- Computed Radiography (CR) Machine has
the following limitations.
1. The imaging plates may be damaged
during transportation
2. Malfunctions of rollers in digitizers
3. Cracked or weakened lead coating on the
back of the cassette
4. Defective scanning results in an alteration
in image contrast

X-RAY MACHINE
III- Digital Radiography (DR) Machine
1. This technology uses a new machine
construction that is different from
the conventional X-ray one.
2. Simply, it uses a sensor to capture a
radiographic image, breaking it into
electronic signals and storing the
image on a computer.
3. Once the image is stored, it can be
displayed, transported, and
processed.
4. Exposure times for DR are 50% -
80% less than that required for
radiography using conventional X-
ray film.

X-RAY MACHINE
PROBLEMS OF X-RAY
• X-rays are ionizing radiations.
• Ionizing radiations knock electrons off the atom to create an ion, an electrically-charged
atom.
• An ion's electrical charge can lead to unnatural chemical reactions inside cells.
• For example, the charge can break DNA chains. A cell with a broken strand of DNA will
either die or the DNA will develop a mutation.
• If a lot of cells die, the body can develop various diseases.
• If the DNA mutates, a cell may become cancerous, and this cancer may spread.
• If the mutation is in a sperm or an egg cell, it may lead to birth defects. Because of all
these risks, doctors use X-rays sparingly today.

X-Ray Shadowgraph Imaging Technique Lecture

X-Ray Shadowgraph Imaging Technique Lecture

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