X-Ray Scanning Theory; Figure 1-1. Qdr 4500 - Hologic QDR 4500 Technical Manual

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QDR 4500 Technical Manual
Operator's Console
Scanner Unit
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Figure 1-1. QDR 4500

X-RAY SCANNING THEORY

An X-ray source, consisting of a generator and tube in a common, shielded enclosure, is mounted
beneath the patient. It generates a narrow, tightly collimated, fan shaped beam of X-rays which
alternate, at power line frequency, between 100kVp and 140kVp. The source is at one end of a
C-arm. At the other end, above the patient, is a crystal/solid state detector. During a scan, the C-
arm and table move under computer control to guide the beam over the desired scan area.
Before passing through the patient, the beam is filtered through a rotating drum, in which
alternating segments have radio-opacities equivalent to tissue, bone and air. When finally
intercepted by the detector, the beam contains information about the X-ray absorbing
characteristics of both the patient and the calibration materials in the filter drum. An A/D
converter, fed by the detector, supplies a complex digital signal to the computer, which uses that
signal both to construct the screen display and as the basis for its computations of BMC and
BMD.
The QDR 4500 computer algorithm is based on the principle that bone attenuates the X-ray beam
differently at high and low energies. The bone mineral content of any sample point can be
computed from:
Q = L - kH
where H and L are the logarithms of the sample attenuation at high (140kVp) and low (100kVp)
energies, respectively, and the constant k depends on the tissue attenuation characteristics of the
beam. In the QDR 4500, k is continuously measured using the "tissue" segment in the filter
wheel.
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