Simultaneous SPECT and CT with an opposed dual head gamma camera system and conventional parallel hole collimators

All examinations aiming at quantitative assessment of activity concentrations suffer from non-uniform attenuation due to inhomogeneous tissue density in various body regions. In such applications, a combined transmission tomography with SPECT has demonstrated great clinical advantages since it allows a pixel by pixel correction for attenuation based on measured data. We have developed a method for transmission studies that is applicable to dual head detector systems with conventional parallel-hole collimators. A scanning line-source (Am-241) is mounted close to one camera head with its beam directed towards the other head. Two energy discrimination windows are used for each camera head. The camera-head near to the transmission source registrates SPECT-data in one window and the spill-over of scattered emission photons into the Am-window. The opposed camera registrates SPECT-data in one window and the transmission data from the Am-source in another window. Post acquisition corrections are made for differences in camera uniformity and for loss of SPECT-data due to shadowing of the field-of-view during the transverse movement of the transmission device (about 5%). The height of the collimated slit source is only 30 mm to allow a setting of the camera head with the transmission device close to the patient.