The determination of body composition (e.g., fat mass, lean mass, etc.) of a living human subject has been recognized as having medical utility for diverse predictive, diagnostic, and monitoring purposes. Body composition can also be of interest where x-rays irradiate non-human subjects or inanimate objects. For some purposes, whole body composition is measured or estimated while for other purposes the composition of parts of the body are of interest, such as limbs, hips, etc.
Systems using dual-energy x-ray absorptiometry (DXA) have long been used to measure or estimate parameters such as bone mineral density (BMD), e.g., systems commercially available from the assignee hereof under trade designations such as the QDR 4500 and QDR 2000 product lines. Other types of systems have been used for BMD measurement to a lesser extent, such as quantitative computer-aided tomography (QCT) and single photon absorptiometry (SPA) using isotopes as radiation sources. DXA systems also have been used to measure or estimate body composition, both for the whole body and for regions thereof. See, e.g.: Kelly T L, Berger N and Richardson T L, Appl. Radiat. Isot., Vol. 49, No. 5/6, pp. 511-513, 1988; Fuerst T and Genant H K (1996) Evaluation of body composition and total bone mass with the Hologic QDR 4500, Osteoporisis International 6, s202; Prince R, Price R. Gutteridge D, Retallack R, Dick I, Lemmon J, Hall S, LeDain S 1995 Comparison of bone mineral density measurement between the Hologic QDR2000 and QDR4500A, J Bone Miner Res 10 (Suppl 1): s272; Kelly T (1996) Whole Body Enhancements: Free software upgrades available for QDR-4500A and QDR-4500W users with and without body composition option QDR Insights, New Developments in Bone Mineral Measurements, Vol. 7, p. 15. Some DXA systems use a single, pencil beam shaped beam of radiation that scans the body, typically in a rectilinear fashion, and take dual-energy measurement at each of the many pixel positions arranged in a rectangular pixel matrix. Others, such as the QDR-4500A systems use a wider, fan-shaped distribution of x-rays, and can scan the entire body, typically in three scans along the length of the body, combined to simulate the effect of scanning with a single fan-shaped distribution that is sufficiently wide to encompass the entire body width, as described in commonly assigned U.S. Pat. No. 5,748,705. The patent and publications cited above are hereby incorporated by reference in this patent specification as though fully set forth herein.
When pencil-beam systems are used for body composition measurements, the attenuation measurement for all the pixels are obtained by measuring the intensity of x-rays that travel along essentially parallel paths. However, when a system with a fan-shaped x-ray distribution is used, there are geometric and other factors that can complicate body fat computations and introduce inaccuracies. In an effort to account for such factors, Hologic released a body composition option for its 4500A system. The option has been used commercially in this country since its introduction in 1996, but it is believed that a need still remains to improve body composition analysis in systems using fan-shaped distributions of x-rays.