3D imaging of radioactivity in man measured with a whole body counter

In the Medical Physics Department of the University of Insubria, Varese, Italy, a whole body counter is in use, for clinical and radioprotection measurements. It consists of a scanning bed, four opposite (anterior-posterior and lateral-lateral) NaI(T1) detectors and a shielding based on the shadow-shield principle. By moving the bed on which the patient lies in the supine position, the longitudinal profiles of the counts measured by each probe along the patient axis are obtained. Making the assumption that radioactivity is distributed in N voxel sources located in N selected positions in the patient, this distribution is calculated by solving an over-determined system of linear equations. The solution can be calculated using different methods. An iterative method and a regularization technique are presented. The algorithm proposed allows the evaluation of the distribution of the radioactivity in 3D in voxels with dimensions ranging from 15 to 20 mm, depending on the size of the patient. The 3D distribution of the radioactivity and the knowledge of the time of the intake allow the assessment of the effective dose.