THE EFFECT OF FAT ON THE COHERENT-TO-COMPTON SCATTERING RATIO IN THE CALCANEUS - A COMPUTATIONAL ANALYSIS

The coherent-to-Compton scattering ratio (CCSR) is a technique that has been proposed for measuring trabecular bone mineral density (TBMD). This paper investigates the effect of fat on the CCSR and its correlation to the error in TBMD measurements. It is a computational study to determine the relationship between the magnitude of fat error and the momentum-transfer variable x, which represents the incident photon energy and the scattering angle. Variation in fat content contributes significantly to the error in CCSR measurements. When employing a typical Am-241 source (E(gamma) = 59.45 keV), the resulting error decreases with increasing momentum-transfer variable or angle. For example, the error ranges from +14 mg/cc at an angle of 45 degrees (x = 18.3) to +3 mg/cc at an angle of 135 degrees (x = 44.3) for an osteoporotic trabecular region (100 mg/cc mineral) of a calcaneus that contains 6% less fat than a calibration standard. The error is about 0.3-1.2 mg/cc less for regions containing 2-3X more bone mineral and is reduced and opposite in sign for regions containing about 7% more fat than the calibration standards (e.g., -9 mg/cc at 45 degrees and -1.5 mg/cc at 135 degrees). Others have shown that the intrinsic sensitivity of the CCSR method for measuring TBMD at a given photon energy generally increases with increasing detector angle. Thus large angles are advantageous both for reduced sensitivity to fat variation and increased sensitivity to bone mineral variation. The primary disadvantage is reduced count rates that degrade precision unless long counting times are employed. For experimental studies, a compromise angle must be chosen to insure adequate counting statistics for reasonable precision and examination times while providing moderate mineral sensitivity and moderate fat error.