1. CHAPTER SIXTEEN
PHYSICS OF DIAGNOSTIC
X-RAYS
Lecturer. Sura A. Obaid
2016-2017

2. Physics of Diagnostic X-Rays
• X-rays discovered in 1895 by Wilhelm
Konrad Roentgen, are electromagnetic waves,
• short wavelength (0.001 to 10 nm),
• travel at the speed of light,
• higher energy (200 eV - 1 MeV),
• penetrate solid objects,
• and reacts with photographic film.

3. •In quantum physics named photons
•The higher energy of x-ray photons has higher
penetration power through materials.

4. The field of radiology has three major branches:-
● Diagnostic radiology.
● Radiation therapy.
●Nuclear medicine.

5. Production of x-rays:
X-ray are produced when fast moving electrons are suddenly
stopped by impact on a metal target. The kinetic energy of the
electrons is converted into X-rays.
*No more than 1% energy deposited as x-ray
•(99%) high energy electron beam striking atom of material target
produced heat .
•
The production of x-ray is depend on the following;
A- Accelerated electrons (accelerated by103 to 106 V).
B- A target which can stop these electrons suddenly.

6. • Electrons move between the cathode and the
anode because there is a potential difference in
charge between the electrodes.

7. There are two different atomic processes that can produce
x-ray photons:
Bremstrahlung x-ray radiation:
• electromagnetic radiation produced by the acceleration or
especially the deceleration of a charged particle after
passing through the electric and magnetic fields of a
nucleus) , sometimes is called the white radiation.
• When the fast electrons (80%) get close enough to
the nucleus of a target atom to be diverted from its
path and it’s a photon that has some of its energy.

9. Characteristic x-ray radiation,
• sometimes fast electrons strikes a K-shell electron in
a target atom and knocks it out of its orbit and free of
the atom. The vacancy in the K-shell is filled almost
immediately when an electron from an outer shell of
the atom falls into it.
• An x-ray photon emitted when an electron falls from
the L level to the K level is called a
Kαcharacteristic x-ray , and that emitted when an
electron falls from M shell to the K shell is call a
Kβcharacteristic x-ray

11. • If the entire electron energy is converted to that of the x-
ray photon, the energy of the x-ray photon is related to
the excitation potential V by the relation:
E = h fmax
= h c / λmin = e V
Where e is the electron charge =1.6 x 10-19 C.
Thus, x-ray wavelength λ ;
λmin = h c / e V
Inserting the values of the constants h, c, and e, we have;
λmin (nm) = 1.243 / V (kV)
(λmin) which is named here, the cutoff wavelength.

12. The intensity of X-rays
is defined as the energies of x-rays passing through the unit
area with its normal line along the propagating direction of
x-rays per unit time.
• To increase the intensity of x-rays, we need to
increase:
• (1) The tube current (more electrons).
• (2) The tube voltage (increasing the energy of each x-
ray photon).

14. X-ray tube principle
To satisfy these conditions, the following was done:
1) Electron emitter: is a filament which emits electrons by heat.
2) The target: is to stop the fast electrons suddenly.
3) High voltage: is between the filament and target (cathode and anode) to
accelerate the electrons to high speed.
4) The target and the filament are kept in vacuum to allow the accelerated
electrons to move freely not to be obstructed by air atoms.

15. Having done all the above points we can produce x-ray by:
Heating the filament will emit electrons (thermo-ionic emission).
These electrons is accelerated to a very fast speed by keeping the
potential between the target and the filament (anode and cathode)
very high of order of hundred thousand of volts (in kV).
Then; these electrons will strike the target.
►►The choice of target should be of high melting point and high
efficiency to produce x-ray for this reason tungsten chosen because:
• 1) High melting point (3410 C).
• 2) It has high atomic number( Z).

16. In the diagnostic range of x-ray, the tube
voltage is in general
between 20 – 100 KV.
The power put into the surface of the target can be
quite large
P = I V
Where I, is current in amperes V, is in volts.
The power at the target of an x-ray tube with a
current of 1A operating at 100Kv is :-
P = I V = 1* 105= 100Kw
over 99 % of this power appears as heat

17. To avoid over heating without increase the blurring of the x-
ray image by increase surface area of striking of target by
three techniques:
1. Focal spot size (Line Focus Principle): is
illustrated in figure below, because of the
angle of the target, typically 10 to 20 ͦ ,
therefore, Large area of target are struck but
small projected focal spot.

18. Small focal spot
large focal spot
1-produce less blurring and
better visibility of detail,
produce sharper mages
2- have a greater heat
3- Small focal spots are used
for mammography which has
few exposures, but very sharp
images
1-produces more blurring of
the x-ray image.
2- large focal spots have a
greater heat-dissipating
capacity.
3- are used in fluoroscopy
which has continuous
exposure, but lesser
resolution
Mostx-raytubeshavetwofocalspotsizes(smallandlarge),which
canbeselectedbytheoperatoraccordingtotheimaging
procedure.

20. 2- Target material:
 tungsten
3-The rotating target:
Rotating anode x-ray tube, the normal rotational rate of
the anode is 3600 (rpm) and the heat is spread over a large
area as the anode rotates. This high speed is to make the
electrons strike the target on area of one evolution at least.

21. X-ray absorption
When x-ray penetrates through matter part
of the x-ray beam will be absorbed and
scattered. The mod of absorption is given in
the following equation:
I = Io e-µ x
I is the emerging beam intensity, Io is the
original beam intensity.
x is the thickness of the absorber and µ is the
attenuation coefficient,.
µ = µs + µa

22. The amount of attenuation that occurs depends on the
intensity of the original x-ray beam and the physical
properties of the object through which the x-ray beam
passes.

23. X-Half value layer (HVL or x1/2)
The HVL or x1/2of an x-ray beam is the thickness of
a given material that will reduce the beam intensity
by one half.
x goes to x1/2
when I = Io / 2
I = Io e-µ x
must be equal to; Io / 2 = Io exp.(-µ . x1/2)
1/2 = exp.[-µ . x1/2]
by taking ln function for both sides yield,
ln (1/2) = ln [exp.(-µ . x1/2)]
-ln 2 = - µ . x1/2
0.693 = µ .x1/2 x1/2 = 0.693 / µ

24. Forms of x-ray Interactions:
*Photoelectric Effect:
X-rays in the diagnostic range can undergo ionizing interactions
with inner shell electron of the target atom. It is not scattered but
totally absorbed.
A photoelectric interaction can not occur unless the incident x-ray has
energy equal to or greater than the electron binding energy.

25. *Compton Effect:
• Moderate energy x-ray photon throughout the diagnostic x-ray
range can interact with outer shell electron. In this interaction the
photon strikes the free electron, give part of its energy to the
electrons and the photon will scatter with less energy (fmin , λmax )
in a different direction.

26. *Pair Production:
This is the third type of interaction occurs when a high energy
photon interacts with strong electric field of nucleus it will
disappear and form a pair of particles(electrons e- , positron e+) it
has a threshold value of 1.02 MeV equivalent to mass energy of
both electron and positron (0.51+0.51) =1.02 MeV (which is a
characteristic energy for pair production). This type of interaction
is proportional with Z. As Z increases, the atomic electric field of
the nucleus increase and the pair interaction increase.

27. How are these interactions related to
diagnostic radiology?
pair production is of no use in diagnostic
radiology because of the high energies
needed
the photoelectric effect is more useful than
the Compton Effect because it permits us to
see bones and other heavy materials such as
bullets in the body.

28. X-ray beam filtration
• Filters are added into the beam and the choice of filter is to
absorb the low energy of x-ray more than higher energy.
*Image improvement:
To increase the image sharpness by:
1-Reduce the focal spot size by angulations of the target between
10o-20o, called line focus principal.
2-Using the small focal spot filament this will reduce the focal spot
size by reducing the size of electron beam striking the target.
P = (D/L) x I
The effect of the focal spot size
in forming penumbra (P) is
shown in figure below:

29. *Contrast improvement:
The scattered radiation can be very destructive to the x-ray image. It
is mainly reduce the contrast.
• To eliminate the scattered radiation:
►Reduce the beam size: The beam size should not be large because
it will increase the scattered radiation and the patient dose ( area of
the beam)
►The use of compression: can reduce the tissue volume(thickness
of the Patient’s body)
Figure: shows the thickness of the
Patient’s body and area of the beam.

30. The use of the grids:
A device composed of lead strips held by plastic
strips. The strips are aligned so that the primary beam
of x-ray from the source will go through the plastic
strips and strike the film while most of the scattered
radiation will strike the lead strips and be absorbed.

31. Because the grid lines might show the
radiograph and disturbs the radiograph image
• *A thinner strips were made it can reach 4 grids
per mm.
• **The moving grids, this type the grids move fast
during the exposure and the grids lines blurred.
• ***Another type of the grid is the focused grid in
which the grid lines are angulated towards the
beam direction to prevent the grid cutoff which
occur in the linear grid.

32. As the grid lines will prevent part of the primary
beam, not scattered, the tube exposure should be
increased when grid is used.

33. • X-ray contrast media
1) Purpose:
• a) Makes internal organs denser.
• b) Blocks passage of x-rays to film.
• c) Provides a clearer image of organs and tissues.
2) Types:
• 1) Gases. b) Heavy metal salts. c) Iodine compounds
An oily mist containing iodine is sometimes sprayed into
lungs to make the airways visible.
Radiologist gives barium compound orally to see parts of
the upper gastrointestinal tract (upper G/T).

34. • Basic Chest x-ray Interpretation
(Explanation)
Different tissues in our body absorb x-rays at
different extents:
• ## Bone- high absorption (white).
• ## Tissue- somewhere in the middle absorption
(grey).
• ## Air- low absorption (black).

35. The film taken under full inspiration, so, 10 posterior
ribs should be visible. We say the posterior here,
because when x-ray beams pass through the anterior
chest on to the film under the patient, the ribs closer
to the film (posterior) are most apparent. A really
good film will show anterior ribs too, there should be
6 to qualify as a good inspiratory film.

36. •Fluoroscopy
• 1-The conventional fluoroscopy, is composed
from screen coated with fluorescent material, it gives
yellow when struck by x-ray.
• The screen is covered by lead glass, which
absorbs all the transmitted radiation. Because the
light emitted from the fluoroscopic screen is
weak, the radiologist has to view the image with
his night vision, which it is 1000 times more
sensitive than for day vision.

37. • Some radiologist in some cases increase the x-ray
exposure such as (kV or MA) which makes the
image brighter, but in this way the x-ray hazard is
increased for both patient and the operators.
• This is against the protection rules so until 1960 it
was stopped
2- The image intensifier:
As the conventional fluoroscopy gives very weak
light a device called the image intensifier has been
constructed, it Consist of fluorescent screen on
which the x-ray pattern is directed as the
conventional fluoroscopy.

38. • The corresponding pattern light emitted from this
screen is incident upon a second
screen(photocathode) which emits electron (by
photoelectric effect) and the photocathode is in
intimate contact with the fluorescent screen.
• The increased brightness also makes it possible to
take movies of the fluoroscopic images. The movie
camera is sometimes replaced by TV camera. This
can enable us to (1) video tape recording for latter
study or (2) transfer the image to TV monitors
during fluoroscopy

39. •Tomography x-ray of the Body
In taking ordinary radiography objects in the
path of x-ray beam are superimposed the
shadow of organ of interest may be masked
or disturbed by other organs. Tomography is
a technique that to show one plane clearly
and blur out all shadows over and under this
plane of interest (Plane of Cut) also called
tomography section.

40. Tomography x-ray
Linear tomography gives longitudinal section other
types called (Transverse axial tomography) can give a
transverse plane of cut by changing the direction of the
x-ray beam and the film position or in planning of
cancer therapy.

41. • Computerized Axial Tomography (CAT)
or(CT-Scanners)
• The conventional tomography was dramatically
improved in 1972, this type of tomography does not
use an x-ray film, and it consists of an x-ray tube
work sat relatively high potential (~140KV).
• The opposite side of the tube a scintillation detector
is placed which can register the attenuation for x-ray
beam then stored in the computer memory.

42. • The total time for scan takes about 4 min. This
longtime is not suitable for scans that need
breathe holding. It is difficult to hold the
breath more than 30sec.
• New generation of modern scanners now is
being developed with a fan shape x-ray beam
and multi detectors, these new generations
scanners can finish the scan in less than 30 sec,
latest generations scanner stakes 1-2 sec.

43. •Xeroradiography
• The principle of this radiography that is
consists of selenium coated plate depth in a
light tight cassette, the plate is positive
charged and because the selenium is an
insulator charges will stay without
movement, when an x-ray pattern fall on it
electrons will be released and neutralize
the positive charges and a charge pattern
will be on the selenium sheet according to
the x-ray pattern.

44. • Advantages and disadvantages of
xeroradiography
• Although xeroradiography has low contrast but it can
give sharp radiograph due to the edge enhancement
effect. This effect is due to the charge distortion on the
edge between two differently exposed parts and
consequently more powder will be accumulated on the
edge giving sharp edge or edge enhancement. This is
in addition to the high latitude for xeroradiography
which can give acceptably densities even for thick
parts.
• One main disadvantage is that xeroradiography less
sensitive than ordinary radiography so it needs higher
exposures sometimes ten times greater than ordinary
radiography.

45. • Applications of x-rays to medicine
• 1) Treatment
• The x-rays used in clinic treatment is mainly for
cancer. It is found that x-rays can induce a series of
biological effects in human body (ionizing, Compton
effect, producing electron-positron pairs).
• They can damage especially the biological tissue
cells which are active in fission. Cancer cells are
such cells. Therefore, the x-rays can kill them or at
least can reduce their fission speed.

46. 2) Diagnosis
Fluoroscopy and photography: As different parts
and organs in human body have different
absorbing abilities of x-rays, the homogeneous
intensity of x-rays will be not homogeneous after
penetrating human body. If the non-homogeneous
x-rays are projected onto fluorescent screen, the
image of the organs can be formed on the screen.
This is called x-ray fluoroscopy.
If the transmitted x-rays irradiate on a negative
film, the picture can be seen after development.
The technique is called x-ray photography.

47. • 3) X-CTBasic principle of
CT(Computerized Tomography):
• As different tissue has different absorbing
coefficient and different thickness, the coefficient
is taken as a parameter. The principle is to set up
the distribution of the coefficient in each part of
every layer of the material in question (state) and
using computer to reconstruct the image of the
material.
• 4) CT Scanner CT scanner (Computerized
Tomography Scanner):an x-ray machine that
can produce stereographic images (former name:
CAT scanner or Computer-Aided Tomography
scanner.