medical equipment lec 4

1. Medical Equipment III
X-ray

2. Detailed Block Diagram
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3. X-ray Quality vs. Quantity
 Quality = penetrating power (energy)
 Quantity = # of X-rays in beam
 ↑kVp = ↑ speed of e- = ↑ quality
 ↑ mA = more e- hit target = ↑ quantity
 ↑s = longer exposure time = ↑ quantity

4. Problem:
 Only x-rays of sufficient energy (quality) can
transmit through body to create an image.
 Low-energy x rays are attenuated to a greater
extent than those of high energy (photoelectric
effect).
 Thus, low-energy x-rays don’t contribute
substantially to image, but add to patient radiation
dose.
 Therefore, we need to reduce low-energy (low-
quality) x-rays, and at the same time have the
right quantity of x-rays hitting the body part.

5. Collimators
 The coverage of the X-ray beam is determined by the bevel
angle of the X-ray tube anode.
 As the beam reaches the patient, the beam can be much
wider than the field-of-view (FOV) being imaged.
 This leads to:
 Increased patient dose
 Increased Compton scattered X-rays
 A collimator (also called a beam-restrictor) is placed under
the port of the X-ray tube to restrict the dimensions of the
beam to the imaging FOV.
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Medical Equipment IV Spring 2018 Shereen M. El-Metwally and Inas A.
Yassine

6. Collimators

7. Filtration
 Is X-ray beam attenuation as it traverses several
attenuating materials before reaching patient.
 Three kinds of filtration:
 Inherent – due to glass envelope of tube, oil
insulation and the exit window in the tube housing
 Added – aluminum shielding that blocks low energy
x-rays.
 Special – used to image body parts that have
varying thickness or density.
 Filtration “hardens” the x-ray beam.
 i.e., x-ray beam has a higher average energy and is
able to penetrate more dense (i.e., harder) substances
such as bone.

8. “Special Filtration”
Equalization filters are sometimes used to compensate for the large
differences in x-ray transmission

9. 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

10. Grids
 Part of the “bucky” that holds the film cassette (detector)
 Purpose: reduces scattered radiation that hits film to
improve contrast.
 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).
 Tradeoff between contrast and signal intensity.
 “Bucky factor” is the factor by which the patient dose
increases while using an anti-scatter grid in order to
produce a certain signal intensity in the presence and

12. Since the X-ray beam is in fact slightly diverging, the anti-scatter
grid can also be manufactured at the same diverging angle