Determination of gamma camera's calibration factors for quantitation of diagnostic radionuclides in simultaneous scattering and attenuation correction.

OBJECTIVE: The characterization of cancerous tissue and bone metastasis can be distinguished by the accurate assessment of accumulated uptake and activity from different radioisotopes. The various parameters and phenomena such as calibration factor, Compton scattering, attenuation and penetration are influenced intrinsic this quantity, and the qualification of images as well.

METHODS: The camera calibration factor (CF) translates reconstructed count map into absolute activity map, is determined by both planar and tomographic scans using different phantom geometries. In this study, the CF for radionuclides of Tc-99m and Sm-153 in soft tissue and bone was simulated by Monte Carlo method, and experimental results were obtained in equivalent tissue and bone phantoms. It may be employed to correct simultaneously the scattering and attenuation rays interacted to the camera, leading to corrected counts. Also, the target depth (d) may be estimated by combination of scattering and photoelectric functions, which we have published before.

RESULTS: The calibrated equations for soft tissue phantom for the radionuclides were obtained by RTc = -10d+ 300 and RSm = -8d + 100, and the relative errors between the simulated and experimental results were 4.5% and 3.1%, respectively. The equations for bone phantom were RTc = -30d + 300 and RSm = -10d + 100, and the relative errors were 5.4% and 5.6%. The R and d are in terms of cpm/mCi and cm. Besides, the collimators' impact was evaluated on the camera response, and the relevant equations were obtained by Monte Carlo method. The calibrated equations as a function of various radiation angles on the central of camera's cells without using collimator indicated that both sources have the same quadratic coefficient by -2E-08 and same vertical width from the origin by 8E-05.

CONCLUSION: The presented procedure may help to determine the absorbed dose in target and likewise to optimize treatment planning.