Simultaneous acquisition of 99mTc- and 123I-labeled radiotracers using a preclinical SPECT scanner with CZT detectors

Simultaneous acquisition of Tc-99m and I-123 was evaluated using a preclinical SPECT scanner with cadmium zinc telluride (CZT)-based detectors. 10-ml cylindrical syringes contained about 37 MBq Tc-99m-tetrofosmin (Tc-99m-TF) or 37 MBq I-123-15-(p-iodophenyl)-3R,S-methyl pentadecanoic acid (I-123-BMIPP) were used to assess the relationship between these SPECT radioactive counts and radioactivity. Two 10-ml syringes contained 100 or 300 MBq Tc-99m-TF and 100 MBq I-123-BMIPP to assess the influence of Tc-99m upscatter and I-123 downscatter, respectively. A rat-sized cylindrical phantom also contained both 100 or 300 MBq Tc-99m-TF and 100 MBq I-123-BMIPP. The two 10-ml syringes and phantom were scanned using a pinhole collimator for rats. Myocardial infarction model rats were examined using 300 MBq Tc-99m-TF and 100 MBq I-123-BMIPP. Two 1-ml syringes contained 105 MBq Tc-99m-labeled hexamethylpropyleneamine oxime (Tc-99m-HMPAO) and 35 MBq I-123-labeled N-omega-fluoropropyl-2 beta-carbomethoxy-3 beta-(4-iodophenyl) nortropane (I-123-FP-CIT). The two 1-ml syringes were scanned using a pinhole collimator for mice. Normal mice were examined using 105 MBq Tc-99m-HMPAO and 35 MBq I-123-FP-CIT. The relationship between SPECT radioactive counts and radioactivity was excellent. Downscatter contamination of I-123-BMIPP exhibited fewer radioactive counts for 300 MBq Tc-99m-TF without scatter correction (SC) in 125-150 keV. There was no upscatter contamination of Tc-99m-TF in 150-175 keV. In the rat-sized phantom, the radioactive count ratio decreased to 4.0 % for 300 MBq Tc-99m-TF without SC in 125-150 keV. In the rats, myocardial images and radioactive counts of Tc-99m-TF with the dual tracer were identical to those of the Tc-99m-TF single injection. Downscatter contamination of I-123-FP-CIT was 4.2 % without SC in 125-150 keV. In the first injection of Tc-99m-HMPAO and second injection of I-123-FP-CIT, brain images and radioactive counts of Tc-99m-HMPAO with the dual tracer in normal mice also were the similar to those of the Tc-99m-HMPAO single injection. In the first injection of I-123-FP-CIT and second injection of Tc-99m-HMPAO, the brain images and radioactive counts with the dual tracer were not much different from those of the I-123-FP-CIT single injection. Dual-tracer imaging of Tc-99m- and I-123-labeled radiotracers is feasible in a preclinical SPECT scanner with CZT detector. When higher radioactivity of Tc-99m-labeled radiotracers relative to I-123-labeled radiotracers is applied, correction methods are not necessarily required for the quantification of Tc-99m- and I-123-labeled radiotracers when using a preclinical SPECT scanner with CZT detector.