A calibration phantom for absolute quantitation of radionuclide uptake by SPECT

Absolute quantitation of radionuclide uptake in small volumes or organs is of interest for nuclear medicine procedures including the estimation of radiation dose in radioimmunotherapy. The use of a calibration phantom for absolute quantitation of a radionuclide in small volumes in a transverse section image of a SPECT phantom with a rotating gamma camera has been investigated. The calibration phantom is a set of 4 tubes filled with a suitable range of specific activity of the radionuclide in solution. This is scanned simultaneously with the SPECT phantom or patient. The tubes can either be placed at appropriate points on the patient boundary or enclosed in a tissue equivalent crescent-shaped phantom which is either attached to the underside of the SPECT table or placed on the patient to preserve the ellipse for attenuation compensation. A region of interest (ROI) for each tube and volume in the transverse section image was defined with an appropriate threshold of about 10% and the variation of counts in the ROI versus specific activity was found to be linear over the range 1 to 100..mu..Ciml/sup -1/. However, the counts from the calibration phantom were found to depend on the total activity in the transverse section slice,more » the size and effective attenuation coefficient of the transverse section, the reconstruction filter used and the accuracy of boundary detection for attenuation compensation. Preliminary results indicate that this SPECT calibration phantom can be used for absolute quantitation of radionuclide uptake in a selected site in the body, provided attenuation compensation is satisfactory. Also, the method will simplify determination of cumulated activity for radiation dose calculations.« less