A DPA Technique for Simultaneously Measuring Bone, Soft Tissue, and Fat Content

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Abstract
Although conventional dual photon absorptiometry (DPA) techniques can be used to estimate fat and soft tissue content, such estimates are not possible where bone is present. We propose a method that can make these estimates in the presence of bone in the extremities of the body. The combination of this method with the conventional method should yield fat and soft tissue composition from most points within the body. The proposed technique simultaneously measures thicknesses of bone, soft tissue, and fat. These thicknesses are determined from a combination of gamma-ray transmission data at two energies and a measurement of total tissue thickness. To test the technique, a feasibility study was performed with known thicknesses of aluminum (simulating bone), lucite (simulating tissue), and polyethylene (simulating fat). A variety of thicknesses of each material were employed (Al: 0-1.3cm, lucite: 0-5cm, polyethylene: 0-5cm). The accuracies (standard errors of the estimates) of the calculated versus true thicknesses of aluminum, lucite, and polyethylene were 0.6%, 2.6%, and 2.5%, respectively. The estimates of “bone” thickness were insensitive to the presence of varying thicknesses of “fat.” (In contrast, application of the conventional DPA method to the same gamma-ray transmission data yielded underestimates in “bone” thickness due to “fat” by as much as 11%.) For a 60 minute (whole body) scan time, the reproducibility of the From the Department of Radiology, University of Washington, Seattle, Washington. Reprint requests: Mitchell M. Goodsitt, Ph.D., Department of Radiology, SB-05, University of Washington, Seattle, WA 98195. Received January 16, 1989, and accepted for publication, after revision, May 25, 1989. measurements of the thicknesses of aluminum, lucite, and polyethylene were 0.4%, 1.0%, and 1.3%, respectively. All of these values are in a clinically useful range. Precision could be further improved by replacing the DPA system with a high output dual photon x-ray absorptiometry system.
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