Hologic DISCOVERY A, Technical Manual

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Discovery QDR Series Technical Manual
1-3
Before passing through the patient, the beam is filtered through a rotating drum in which
alternating segments having radio-opacities equivalent to tissue, bone and air are located.
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 detectors, 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 Discovery 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 L and H are the logarithms of the sample attenuation at high (140kVp) and low
(100kVp) energies, respectively. The constant k depends on the tissue attenuation
characteristics of the beam. In the Discovery, k is continuously measured using the
“tissue” segment in the filter wheel.
The program works in the following manner:
1. Load preliminary scan and obtain regions of interest from operator.
2. Estimate k as an average value of:
k = [L
tissue
- L
air
] / [H
tissue
- H
air
]
where L
tissue
indicates a low-energy measurement with tissue-equivalent material
interposed by the filter drum, and L
air
, H
tissue
and H
air
are similarly defined.
Note: The subscript "air" designates the filter drum segment that is empty (i.e., contains
neither bone- nor tissue-equivalent material).
3. Using this value of k, calculate Q for each point scanned using the formula given
above (Q = L - kH). This array of Q values constitutes a "Q scan". Displays the Q
scan.
4. Compile a histogram of the Q values. Because a large portion of the scan con-
tains soft tissue only, this histogram will have a large peak. Choose a threshold
value just above this peak, and apply that value to discriminate, point by point in
the Q scan, between "bone" points (whose Q is above the threshold) and "non-
bone" points (whose Q is below the threshold).