Simultaneous SPECT and CT with shutter controlled radionuclide sources and parallel collimator geometry

A method for combined single photon emission computed tomography (SPECT) and transmission tomography has been developed for application to parallel collimator geometry. The technique is based on a specially designed, shutter operated device with radionuclide line sources for the transmission studies. The present device operates with four 1-GBq, 40-cm-long line sources of ⁹⁹Tc^m. Using the same radionuclide for SPECT, emission data are acquired with the shutter closed. At the same angle, transmission data are subsequently acquired with the shutter open. The total acquisition time is prolonged and the transmission data must be corrected before reconstruction. Using different emission and transmission photon energies, the shutter may be open during most of the SPECT acquisition time and closed only for collecting crosstalk correction data. Experiments are performed with a transmission source with lower photon energy, 0.8 GBq ¹⁵³Gd line sources (97 and 103 keV). Simulations of transmission data indicate that the activity of ¹⁵³Gd might be about 2.5 times that of ⁹⁹Tc^m and about six times higher for ²⁴¹Am for the same image noise. Simulations show that the total activity in uniform line sources may be reduced without increasing the noise by choosing line sources with an activity distribution that is averaged inversed to the photon transmission through the object