All X-ray units do not always produce the same quality of X rays for a given voltage. This may be due to incorrect calibration, age of apparatus, waveform and other causes. Unless the beam quality is known, dose measurements and tests on radiographic recording systems may be invalid. It has been shown (Ardran and Crooks, 1963) that a difference of 10 kV can affect the patient integral dose by 20-40 per cent. Diagnostic X-ray beam quality is usually expressed by means of the kV or kVp and the filtration or, alternatively, the half-value thickness (HVT) in millimetres of aluminium. However, HVT alone is not a good indicator of beam quality for this purpose: a simple experiment will show that it is possible, with different kVp's and filtrations, to produce X-ray beams with the same HVT but which give radio graphs of quite different quality. This point has been stressed by Holm and Moseley (1964)
Beam Quality Defining Factor (HVL):
Half Value Layer (HVL) is used to define the X-ray beam quality. The HVL of an x-ray beam is the amount or thickness of absorbing material or filtration that must be placed in the beam to reduce the transmission of the beam by one half. The material predominantly used to determine the HVL of diagnostic xray equipment is aluminum, and hence HVL is expressed in terms of mm of aluminum. Medical diagnostic x-ray machines typically have HVLs ranging from 2.3 to 5 mm Al. Total filtration and HVL are sometimes erroneously used interchangeably. They are not
equivalent terms. The introduction of a known thickness of aluminum will not increase the HVL by a corresponding amount of millimeters of aluminum. For example, in the following table at
80 kVp, 1.5 mm Al total filtration results in an HVL of 1.8. An increase of 1 mm Al to a total of 2.5 mm Al changes the HVL to 2.4 mm, an increase of only 0.6 mm Al. While the total filtration doesn’t typically change, the HVL measured will vary depending on the kVp and the type of high voltage generator (i.e., single phase, three phase, high-frequency, etc.) used in the machine.
1. What is the range of HVL value in medical diagnostic?
a) 2.0mm to 5.5mm Al
b) 2.3mm to 5.0mm Al
c) 1.5mm to 5.0mm Al
d) 2.5mm to 5.0mm Al
b) 2.3mm to 5.0mm Al
5. The Physics of Radiation Therapy, 4th Edition by F.M.Khan.
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