Task 1, MMed Physics

Dr Yacoob OMAR CARRIM                                           20 Mar. 11

Question 1

Explain the purpose and the application of each of the following components in the x-ray beam:

1.       The Filter

2.       The Grid

3.       The Intensifying Screen

The Filter: The main function of filters in diagnostic Radiology is:

1)      to reduce the patient’s radiation  dose.

2)      Heavy metal filtering may also be sued for improving image contrast.

a)      Remove higher energy photons from the x-ray beam, except for molybdenum, have atomic numbers greater than 60.

Christenson’s Physics of Diagnostic Radiology, Thomas S Curry, et al, Fourth Edition pages 87, 90.

The Grid:

1)      Grids are used to improve radiographic image contrast by absorbing secondary radiation or scatter radiation before it reaches the film.

2)      Improvement in image contrast comes with increasing patient exposure.

The Intensifying Screen:

1)      Intensifying screens are used because they decrease the x-ray dose to the patient.               

2)      Intensifying screens have increased sensitivity,

3)      Intensifying screens allow for better contrast,

4)      Shorter patient exposure times,

5)      Minimises patient motion

Christenson’s Physics of Diagnostic Radiology, Thomas S Curry, et al, Fourth Edition pages 118-135

Question 2: Explain 5 ways in which scatter can be reduced

Scattered Radiation is undesirable in diagnostic radiology because it reduces contrast.

The most important scatter is the scatter radiation that results from Compton scatter.

The ratio of scatter to primary radiation exiting a patient can be 5:1 or even greater.

1)      Decrease field size: Scatter increases with increasing field size. Decreasing field size limits scatter.

2)      Decrease part Thickness: Scatter increases with increasing patient thickness. Decreasing part thickness decreases scatter. Compression of the breast in mammography is an applied example.

3)      Decrease energy: Scatter increases with increasing kVp as increasing kVp increases the amount of Compton Scattering and decreases the amount of Photoelectric attenuation. Lowering the kVp will enhance the patient tissue differential contrast, by increasing the photoelectric effect and reduce scatter.  Lowering kVp is not a practical way to reduce scatter.

4)      Decrease patient mass irradiated: Collimation reduces the total patient mass irradiated and, therefore, reduces the scatter.

5)      Use antiscatter grids: Grids are used to improve radiographic image contrast by absorbing secondary radiation or scatter radiation before it reaches the film.

6)      Air gaps: Air gaps between the patient and the cassette reduce scatter.

a)      Major drawbacks, larger x-ray tube output required

b)      Magnification imaging, with additional focal spot blurring

Christenson’s Physics of Diagnostic Radiology, Thomas S Curry, et al, Fourth Edition, Chapter 5

Question 3: Briefly discuss the following: Advantages of an intensifying screen-film combination compared with just a film

The main advantage of an intensifying screen-film combination compared to film alone is

1)      increased sensitivity,

2)      better contrast,

3)      Lower patient x-ray dose is required,

4)      Shorter patient exposure times,

5)      Minimises patient motion

The screens have a much higher (510 times) X-ray absorption efficiency than a photographic film and will also produce a great number of light photons per X-ray photon absorbed, thus yielding a more efficient film exposure.

The X-ray exposure can often be reduced by a factor of 10 - 50, depending on the screen characteristics.

The sensitivity can be varied using a different screen thickness.

The thicker the screen, the higher the absorption efficiency. However, a thicker screen will also result in an impairment of spatial resolution. This comes from the fact that the light produced in the screen will have a longer distance to travel before hitting the film, and the light will therefore be more diffused than for a thinner screen.

More sensitive screens can also be produced using larger phosphor crystals in the screen material. This will lower the spatial resolution and will also increase the noise level.

Due to the higher atomic number of the screen material compared to the silver halide grains, the film-screen combination is always relatively more sensitive to higher energy X-rays than film alone. This extends the use of film-screen combinations to higher kVp’s compared to film alone.

Using just film alone:

1)      The sensitivity of film to direct exposure alone is low.

2)      Increase the required patient x-ray dose

3)      Increased patient exposure times with

4)      increased risk of movement artefacts from increased patient movement.

Christenson’s Physics of Diagnostic Radiology, Thomas S Curry, et al, Fourth Edition pages 118-135

Question 4: Briefly discuss the factors affecting film contrast?

1)      The kVp is important in producing proper subject contrast.

a)      Too much kVp would diminish the differential attenuation of subject tissue and may reduce the quality of the image. 

2)      The mAs determines the number of x-rays in each part of the attenuated beam to be sufficient to produce the correct overall density in the processed film.

i)        Too little mAs results in an underexposed film, and

ii)       Too much mAs results in an overexposed film

3)      Exposure Time

4)      Focus film distance

5)      The shape of the characteristic curve of the film

i)        The shape of the characteristic curve tells us how much change in film density will occur as film exposure changes. 

ii)      Film gamma: the maximum slope of the characteristic curve.

1.       )

iii)    Average gradient (Normal range density 0.25 to 2.0).

(1)    If the average gradient is greater than 1, the film will exaggerate the subject contrast.

(2)    If the average gradient is equal to 1, the film will not exaggerate the subject contrast.

(3)    If the average gradient is less than 1, the film will decrease the subject contrast.

6)      Film density

a)      The slope of the characteristic curve (i.e film contrast) changes with density.

7)      Screen or direct x-ray exposure

a)      No Intensifying Screen: 

i)        More mAs is required

ii)       Direct x-ray exposure may require between 15 and 50  times more exposure

iii)    The radiation dose to the patient must be increased

iv)    Contrast is lower

v)      More sensitive to secondary radiation or low energy x-ray beams, Recording of scatter radiation may impairing contrast.

b)      Intensifying Screen:

i)        Intensifying screens may require between 15 and 50  times less exposure

ii)       Patient dosage may be reduced

iii)    Contrast is higher

iv)     Less sensitive to secondary radiation or low energy beams, enhancing contrast

8)      Film processing (development)

Increasing the time or temperature of development :

i)        increases the average gradient of the film  (film contrast)

ii)      Increases the average film speed (increasing density for a given exposure), and

iii)    Fog will also be increased with increasing the time or temperature of development

Christenson’s Physics of Diagnostic Radiology, Thomas S Curry, et al, Fourth Edition , pages 153-156, 200