Use of a gamma camera to estimate bone mineral content based on the dual-photon technique

Bone mineral content evaluations using a point source of 153Gd of strength 100 mCi or less and a Siemens large field of view gamma camera equipped with a converging collimator is reported. Both 44 and 100 ke V images are digitised in 128 x 128 matrices and processed in a Siemens MicroDelta System attached to a VAX 11-750 computer. Shorter imaging time, low source strength and the usage of existing nuclear medicine equipment are the major highlights of the technique. The effective focal distance (EFD) of the collimator used is experimentally determined with a 57Co source and found to be 75 cm. Focal distances are also calculated from measured values of magnification factors at different points on the collimator face to assess the degree of misalignment of converging holes. Due to the high count rate of the system, corrections for dead time losses and image shrinkages are applied. Two additional low-activity point sources of 153Gd, which serve as live time meters of the system, are used for this purpose. In addition, due to the non-homogeneity of the converging collimator, uniformity corrections for each field of view are found to be essential. In external standards made of various amounts of hydroxyapatite, a correlation coefficient greater than 0.990 with a standard error of the estimate (SEE) 0.0225 is obtained between measured and actual values. In a limited clinical trial, high-resolution images of human spines are acquired and quantitative estimates of bone mineral content are evaluated.