X-rays are produced in two ways when electrons hit a tungsten target: characteristic x-rays which only occur above 70kVp, and bremsstrahlung x-rays which are produced by electrons slowing down. The quality and quantity of x-rays can be adjusted using exposure factors like kVp, mA, and time. X-rays that pass through the body may be absorbed, scattered, or transmitted to the detector. Additional devices like collimators, filtration, and grids are used to improve image quality by reducing scattered radiation and adjusting the x-ray beam.

1. Medical Equipment III
X-ray
Inas A. Yassine
Systems and Biomedical Engineering Department,
Faculty of Engineering - Cairo University
iyassine@eng.cu.edu.eg

2. Detailed Block Diagram
Machine Learning Spring 2014 Inas A.Yassine 2

3. Characteristic Radiation

4. Bremsstrahlung Interaction

5. Coverage

6. Hitting the target
§ e-’s hitting the target creates x-rays two
different ways:
§ Characteristic x-rays – are due to the material
the e-’s hit (tungsten). Only occurs above 70
kVp
§ Bremsstrahlung (braking) x-rays – due to
slowing down of e- beam.
§ < 70 kVp – 100% of X-rays are of this type
§ > 70 kVp – 85% of X-rays are of this type

7. Anode Heel Effect

8. Energy Spectrum of X-ray

9. Exposure Factors:
§ kVp – kilovoltage peak
§ mA – miliamps (current)
§ s – seconds (duration of exposure)
mAs – product of mA and s
Exposure factors are set by
radiographer

10. X-ray Quality vs. Quantity
§ Quality = penetrating power / energy
§ Quantity = # of X-rays in beam
§ ↑kVp = ↑ speed of e- = ↑ quality
§ ↑ kVp = efficiency of x-ray production = ↑
quality
§ ↑ mA = more e- hit target = ↑ quantity
§ ↑s = longer exposure time = ↑ quantity

11. X-ray Imaging
§ The tube – X-rays are produced
§ The body – X-rays interact with the
body
§ The image – X-rays interact with film,
Detectors
§ Film processing, Signal analysis

12. Interactions in the Body:
§ Three things can happen
to x-rays as they hit the
body:
§ Absorption (Photoelectric
effect) – x-ray is absorbed
by tissues – does not
contribute to image.
§ Scatter (Compton effect)
– contributes to “fog”
§ Transmission – penetrates
through body to hit
radiographic film.

13. Photoelectric Effect

14. Compton Scattering

15. X- Ray Attenuation

16. Problem:
§ Only x-rays of sufficient energy (quality) can
transmit through body to create an image.
§ Low energy x-rays don’t contribute to the image,
but add to patient radiation dose.
§ Also, different thicknesses, and composition of
body parts will determine amount of x-ray
penetration.
§ Therefore we need to reduce low energy (low
quality) x-rays, but at the same time have the right
quantity of x-rays hitting the body part.

17. Collimators
§ Is located under the port of the X-ray tube.
§ Has a light in it for radiographer to see
where x-rays would hit the patient
§ Purpose- restricts beam
§ ↓patient dose
§ ↓scatter (↑contrast)
§ Collimation should be visible on a minimum
of three sides of the film

18. Collimators

19. Filtration
§ How we fix the problem is with filtration
§ Three kinds of filtration:
§ Inherent – due to tube housing, insulation, etc.
§ Added – aluminum shielding that blocks low
energy x-rays.
§ Special – used to image body parts that have
varying thickness or density.
§ Filtration is measured in terms of “half-value
layer”

20. “Special Filtration”

21. X-ray Imaging
§ The tube – X-rays are produced
§ The body – X-rays interact with the body
§ The image – X-rays interact with film,
Detectors
§ Film processing, Signal analysis

22. Grids
§ Part of the “bucky” that hold the film
cassette
§ Reduces scatter radiation that hits film
§ Grid is made of lead strips
§ Grid ratio – height/width of interspace
§ Hitting prep button causes grid to vibrate
to blur out grid lines (doesn’t show up on
film)