how does a radiograph form?

2.  A picture produced on a sensitive surface by a form of radiation other
than visible light specifically : an X-ray or gamma ray is called as
radiographic image or simply radiograph
 It aids in diagnosis of a pathological condition

3. There are five possible mechanisms of interaction of a photon with
matter:
coherent scattering
photoelectric effect
Compton scattering
pair production
photodisintegration

4. • All significant scattered radiation encountered in diagnostic radiology
results from Compton scattering
• An incoming x-ray photon interacts with a peripheral shell electron of
a tissue atom
• The peripheral shell electron is ejected, and the original photon is
scattered at a different angle and at a lower energy
• The scattered electron is called either a Compton electron or a recoil
electron

5. • The probability of a Compton reaction is independent of atomic
number and depends mainly on the physical density of the tissue
• The independence of Compton scattering on atomic number is
disadvantageous with reference to producing a radiograph because all
tissues tend to absorb x-rays similarly
• With Compton absorption being independent of atomic number,
differential absorption by different tissue types is diminished, and image
contrast is reduced

7. • The energy of the primary x-ray beam is deposited within the atoms
comprising the tissue, some x-ray photons are completely absorbed
• Complete absorption of the incoming x-ray photon occurs when it has
enough energy to remove (eject) an inner-shell electron
• The ejected electron is called a photoelectron and quickly loses energy by
interacting with nearby tissues
• The ability to remove (eject) electrons, known as ionization, is one of the
characteristics of x-rays in the diagnostic range, this x-ray interaction with
matter is known as the photoelectric effect

8. • With the photoelectric effect, the ionized atom has a vacancy, or electron
hole, in its inner shell
• An electron from an outer shell drops down to fill the vacancy
• Because of the difference in binding energies between the two electron
shells, a secondary x-ray photon is emitted
• This secondary x-ray photon typically has very low energy and is unlikely
to exit the patient

10. • The process of image formation is a result of differential absorption of the
x-ray beam as it interacts with the anatomic tissue
• Differential absorption is a process whereby some of the x-ray beam is
absorbed in the tissue and some passes through (transmits) the anatomic
part
• The term differential is used because varying anatomic parts do not
absorb the primary beam to the same degree
• Anatomic parts composed of bone absorb more x-ray photons than parts
filled with air
• Differential absorption of the primary x-ray beam creates an image that

12. • To make a radiograph the patient is placed between the x-ray tube and
the film cassette or imaging plate
• X-rays produced in a diagnostic x-ray tube have a broad spectrum of
energies
• Very low-energy x-rays serve no useful purpose because they are all
absorbed by the patient and make no useful contribution to the image
• Therefore, filters are placed in the x-ray tube housing

13. • To remove these very low-energy x-rays from the x-ray beam before they
ever strike the patient
• The geometric shape of the x-ray beam striking the patient is configured
to the size needed by the beam-shaping collimator
• Three x-rays pass through the collimator opening
• The left x-ray penetrates the patient and will be
recorded on the x-ray film
• The middle x-ray hits a structure within the
patient and is absorbed
• The x-ray on the right hits the patient and is
scattered

15. o Filters
o Beam collimator
o Grid
o FFD

16. Filters:
• Placed between patient and x-ray tube
• Primary purpose is to remove less energetic x-rays from the primary
beam which have no chance to reach the film
• Aluminum and copper and commonly combination of both which is good
and inexpensive is used
 Atomic No. of Aluminum 13 and Cu is 29
Beam Collimators:
• Collimation refers to the regulation of x-ray beam by beam restricting
devices
to restrict it to the site of the part of the patient under examination e.g.

17. Grid:
• Grid is a flat plate made of lead and aluminum and is placed between the
part
to be examined and cassette so as to absorb scatter radiation falling on the
film
Focal spot film distance:
• The advantage of keeping FFD long is optimization of detail
• The advantage of keeping FFD short is decreased radiographic technique
(mAs) requirements

18. • Radiographic opacity is a measure of the capacity of a tissue or structure
to block x-rays
• Where x-rays readily reach the film, the film appears black after
processing
• Radiographic opacity therefore depends on subject density; the greater
the subject density, the less radiation reaches the film

19.  The measure of the degree of blackness on a processed film and is
directly related to the number of x-rays reaching the film
1. The degree of film blackness is affected by the number of x-rays striking
the film, which is related directly to the x-ray machine output (mAs)
2. Film blackness is also affected by the energy of the x-ray beam, which is
controlled by the kVp setting

20. 3) As x-ray energy increases, a greater proportion of the x-rays will penetrate
the patient, increasing film blackness
4) The distance from the x-ray tube to the film also affects film blackness.
This distance is referred to as the focal spot film distance (FFD)
5) As the FFD increases, film blackness decreases because the intensity of x-
rays in the x-ray beam (x-rays/unit area) decreases
 The inverse square law, which is described by the equation

21. Where I is intensity in terms of number of x-rays/unit area, and d is
distance
In the equation, I1 is the intensity at d1, and I2 is the intensity at d2
Therefore, as FFD increases, intensity and film blackness decrease and as
FFD decreases, intensity and film blackness increase
These changes in intensity are a function of the square of the distance, not
simply the distance

22.  A direct relationship exists between the milli amperage needed to
maintain the same x-ray intensity and distance, so the equation Is As
follows:

23. 1) Motion:
• Motion, leading to image unsharpness , is the biggest enemy of detail in
veterinary radiology
• Short exposure times are used to minimize the effect of motion
• When exposure time becomes very short, the mA must be large;
otherwise, the mAs will be too low to produce the necessary film blackness
• Intensifying Screen used to overcome this issue

24. 2) Focal Spot Size:
• Use of the small focal spot results in improved detail
• The disadvantage of the small focal spot is that lower mAs values must
be used to prevent the filament from overheating
• With a large focal spot, edges of anatomic structures are projected more
unsharply comparatively

25. 3) Intensifying Screens:
• Converting the x-ray energy into visible light and then using the visible
light to expose the film is a convenient way to increase the efficiency of
radiographic production
• Lower mAs values are needed to create a radiograph when intensifying
screens are used compared with using the x-rays to expose the film directly

26. 4) Grids:
• A grid is a flat, rectangular plate with alternating lead and aluminum strips
• Some x-rays interacting with the patient will be scattered and absorbed in
the grid, preventing them from reaching the film
• the mAs must be increased when grids are used to compensate for the
portion of the primary x-ray beam absorbed by the grid