Performance of a triple-detector, multiple-pinhole SPECT system with iodine and indium isotopes

SPECT imaging with isotopes other than ^99mTc is useful for developing new molecular probes for in-vivo imaging. We have recently modified a standard 3-head gamma camera system for μSPECT with 6 pinhole tungsten collimators using a 3D-OSEM reconstruction algorithm. We have previously reported on the ^99mTc performance of this μSPECT imager. We have recently measured the performance of the μSPECT system for imaging with ¹²⁵I, ¹²³I, and ¹¹¹In. The smallest of the interchangeable apertures (0.8-mm) was used for all tests. Point sources consisted of droplets or ion exchange beads with ¹²³I and ¹¹¹In. The imaging of ¹²⁵I was simulated using the 30-keV X-rays from ¹²³I. The volume source was a 30 cc syringe containing 25 ml. Point and volume projection data were acquired separately. The projection data sets of point and volumes were scaled and added before reconstruction. The pinhole-collimated ¹²³I energy spectrum demonstrated 18% contribution from high-energy photons detected in the 159-keV, 20% wide window, whereas a low-energy parallel-hole collimator allowed 60% contamination. The point-source sensitivity at 159 keV was comparable to that measured previously for ^99mTc (18 cpm/kBq), and was 3.6 times higher than that measured in the X-ray region (30-keV, 50% window). Point sensitivity for ¹¹¹In was 39 cpm/kBq. Volume sensitivities were 1.4 cpm/kBq at 30 keV, 4.3 cpm/kBq at 159 keV and 14 cpm/kBq for ¹¹¹In. With 1 iteration, the average FWHM resolution at 30 keV was 1.05 mm, at a background noise level of 40%. Average resolution for ¹²³I and ¹¹¹In were both measured to be 1.2 mm at a 10% noise level with 1 iteration and decreased to 0.9 mm at 25% noise level and 3 iterations. The μSPECT system performs comparably for ^99mTc, ¹²³I and ¹¹¹In and acceptably for ¹²⁵I. New iodinated probes and mice containing ¹¹¹In are now being imaged with the system.