Correcting the Magnification Error of Fan Beam Densitometers

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Abstract
Dual-energy X-ray absorptiometry (DXA), using a narrow pencil-shaped X-ray beam coupled to a single detector, has been used extensively. More recently, DXA using a fan- shaped X-ray beam coupled to an array of detectors has been introduced. This new generation of scanners causes an inherent magnification of scanned structures as the distance from the X-ray source decreases. This magnification, which occurs in the medial-lateral direction but not in the craniocaudal direction, does not affect bone mineral density (BMD). There are, however, significant changes of bone mineral content (BMC), bone area, and parameters of hip geometry, with varying distance of the bone scanned from the X-ray source. Variability of soft tissue thickness in vivo, by altering the distance of the skeleton from the scanning table and X-ray source, may cause clinically significant errors of BMC, bone area, and proximal femur geometry when measured using fan-beam densitometers. We analyzed the geometry of Lunar and Hologic fan beam scanners to derive equations expressing the true width of scanned structures in terms of the apparent width and machine dimensions. We also showed mathematic ally that performing an additional scan, at a different distance from the X-ray source than the first scan, provides simultaneous equations that can be solved to derive the real width of a scanned bone. This hypothesis was tested on the Lunar Expert using aluminium phantoms scanned at different table heights. There was an excellent correlation, r = 0.99 (p < 0.001), between the predicted phantom width and the measured phantom width. In conclusion, this study shows that the magnification error of fan beam DXA can be corrected using a dual scanning technique. This has important implications in the clinical usefulness of BMC and geometrical measurements obtained from these scanners.