Use of X-ray CT-defined regions of interest for the determination of SPECT recovery coefficients

For accurate activity per unit volume measurements in SPECT, recovery coefficients are usually applied based on the size and shape of objects being imaged to properly account for the resolution limitations of the gamma camera. Because of noise and limited spatial resolution, determination of object sizes and boundaries can be difficult using the SPECT images alone. The authors therefore have developed a technique which determines activity concentrations for SPECT using regions of interest (ROIs) obtained from coregistered X-ray CT images. In this study, experimental phantoms containing cylindrical and spherical objects were imaged on a combined X-ray CT/SPECT system and reconstructed data volumes were registered using the known geometry of the system. ROI's were defined on the registered CT images and used to help quantify activity concentration in localized regions and to measure object volumes. The authors have derived the recovery curves for these objects and scan technique. They have also tested a technique that demonstrates activity quantitation without the need for object and size dependent recovery coefficients in the case of low background.