Dosimetry is important in the evaluation of new radiopharmaceuticals and in radionuclide therapy treatment planning. The base is an accurate in vivo measurement of the spatial and temporal distribution of the administered radioactivity. This can be made by planar whole-body activity quantification using the conjugate-view method where a geometrical-mean image is corrected for attenuation by a measured transmission image. We present a new method to measure whole-body transmission images using the X-ray unit on a single photon emission computed tomography/computed tomography (SPECT/CT) scintillation system. Method: Two general problems are associated with this method: (1) the divergence of the X-ray beam and (2) the scaling of the resulting image. We have applied linear transformations and scale by a system-generated factor to obtain images corresponding to a radiological depth at 70 keV. We validated the method by measurements of the radiological depth for different types of phantoms, and in terms of activity quantification accuracy when applying the method for attenuation correction. Results: An accuracy of within ±4% was found for both the radiological depth measurements and for the activity-quantification measurements. Conclusions: The advantage with the proposed method is that it is fast (2 minutes for a 2-minute scan), it has good spatial resolution and signal-to-noise ratio, and it is easy to carry out for the patient.
