Determining body composition using fan beam dual-energy...

Surgery – Diagnostic testing – Detecting nuclear – electromagnetic – or ultrasonic radiation

Reexamination Certificate

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C378S054000

Reexamination Certificate

active

06233473

ABSTRACT:

FIELD
This patent specification pertains to the field of using x-rays to determine internal characteristics of patients or objects, and more specifically to using dual-energy x-ray absorptiometry to determine whole body and regional composition. Still more specifically, this specification is in the field of using a fan-shaped distribution of x-rays for such purposes.
BACKGROUND
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.
SUMMARY
This patent specification describes a new approach to body composition analysis in DXA systems using a fan-shaped distribution of x-rays that accounts not only for the factors previously considered in the 1996 option for the QDR 4500A systems, but also for mass magnification effects that the system geometry entails. The new approach makes use of the realization that accuracy can be improved significantly by taking into account the apparent changes in measured mass with changes of the location of the mass along the raypaths from the x-ray source to the x-ray detector, and by finding an effective way to make corrections for such changes in apparent mass.
When a fan-shaped distribution of x-rays is used, e.g., with a DXA system that includes a patient table on which a supine patient reclines, a magnification effect takes place that causes a mass element nearer the table surface to be weighted more heavily (to appear to have more mass) than an identical mass element further away from the table surface. Thus, one unit of mass can be measured correctly as one unit if on the table surface but as less than a unit of mass if at some height above the table surface. For a supine patient on the table surface, a mass element at the patient's back can appear to have more mass than an identical mass element at the patient's abdomen. While in known earlier work the measured mass was calibrated to be nominally correct for the average-size subject, the mass for thin subjects could be overestimated and the mass for obese subjects could be underestimated.
The detailed disclosure set forth herein describes the recognition of the cause for this inaccuracy and a solution that makes the body composition more accurate.


REFERENCES:
patent: 6090849 (2000-07-01), Teeter et al.
patent: 6123451 (2000-09-01), Schaefer et al.

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