A New Anthropometric Calibration Phantom for In Vivo Measurement of Bone Seeking Radionuclides

A new anthropometric phantom having the shape of the adult human knee and containing a removable femur, patella, tibia, and fibula, has been developed for use in calibrating in vivo measurements of bone-seeking radionuclides. The shell of the phantom is assembled in three interlocking sections so that the skeletal components can easily be exchanged. All the materials used in the phantom have the same density, attenuation coefficient, and effective Z as that of human muscle and trabecular bone. A precisely known quantity of one or more radioactive materials is added to the trabecular bone substitute when molding the skeletal components for the phantom. An array of germanium or Phoswich detectors can easily be positioned on the top or sides of the knee phantom for calibration measurements. Estimates of the total skeletal content can be determined by measuring activity in the knee and adjusting the result for the fraction of skeleton monitored. The bones contained in the knee phantom represent approximately 10.7% of the total skeletal mass or approximately 12.4% of the total skeletal surface area. The counting efficiency for Am in the knee (per unit detector surface area) is approximately 6 x 10 cpm/Bq/mm using an array of germanium or Phoswich detectors. A single knee phantom exhibits approximately the same counting efficiency as the conventional skull phantom, suggesting that the knee is a desirable alternative to the head, especially if contamination is present on the hair or face or if the person is uncomfortable with detectors surrounding their head. Intercomparison measurements using skull and knee phantoms demonstrate that measurement efficiencies are equivocal. Measurement sensitivity can be substantially improved by placing detectors over both knees rather than around the head since a larger fraction of the total body activity would be monitored.