Calculation and Measurement of Calibration Factors for Bone-Surface Seeking Low Energy Gamma Emitters and Determination of 241Am Activity in a Real Case of Internal Contamination

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Abstract
A voxel phantom has been developed to simulate in vivo measurement systems for calibration purposes. The calibration method presented here employs a mathematical phantom, produced in the form of volume elements (voxels), obtained through magnetic resonance images of the human body. The voxel phantom has a format of 871 'slices' each of 277 x 148 picture elements. The calibration method uses the Monte Carlo technique to simulate the tissue contamination, to transport the photons through the tissues and to simulate the detection of the radiation. The program was applied to obtain calibration factors for the in vivo measurement of 241Am deposited on the cortical bone surface of a real contamination case, and to the simulation of in vivo measurements of 241Am deposited on the cortical bone surface of three head phantoms, as measured with a germanium detector. The calculated and real activities in all four cases were found to be in good agreement. The results indicate that mathematical phantoms could eventually substitute for physical phantoms in the calibration of in vivo measurements for low energy radionuclides.

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