Relationship between radioactivity concentration ratio and cross-talk correction effect for simultaneous 99mTc and 18F acquisition using small-animal SPECT-PET/CT system

1824 Objectives: The 18F energy spectrum for a small-animal SPECT-PET/CT system with a clustered multi-pinhole collimator is composed 511 keV annihilation radiation photo-peaks, and additional 170 keV backscatter photo-peak. If 99mTc and 18F scans are simultaneously performed, the 99mTc image has the influence of cross-talk occurred by overlapping the 170 keV backscatter photo-peak of 18F and 141 keV photo-peak of 99mTc. Although the influence of cross-talk has to accurately correct, the effect of correction has not been fully evaluated. The objective of this study was to reveal the influence and the correction effect of cross-talk and different radioactivity concentration ratio. Methods: Small-animal SPECT-PET/CT scanner used a triple-detectors gamma camera (VECTor+/CT, MILabs B.V., Netherlands) with high-energy ultrahigh-resolution rat and mouse type clustered multi-pinhole collimator (HE-UHR-RM). A fillable cylindric chamber 30 mm (body part) of NEMA-NU4 phantom filled in 99mTc or both of 99mTc and 18F solution. Furthermore, two small-region chambers were filled with non-radioactive water and 99mTc attenuated half radioactive concentration of body part. The radioactive concentration of 99mTc and 18F were approximately 6.0 and 78.0 MBq/mL, and 18F/99mTc ratio set 0.4-13 using radioactive decay. All data were acquired using list-mode of 30 min/frame, and total acquisition time was 14 hours. Main energy windows of 99mTc and 18F were 141 keV ± 10% and 511 keV ± 10%, in addition, the sub energy window of 7% window width set on the upper and lower sides of main energy window both 99mTc and 18F using triple energy window (TEW) technique. Transverse image was reconstructed using pixel-based ordered subset expectation maximization (POSEM) algorithm, and the number of subset and iteration was 32 and 8. The reconstructed transverse image was added all slices including two small-region chambers to perform the image analysis, thereby used integral value in circular-shape region of interest (ROI) drawing on the body part and two small-region chambers of added transverse image. The scatter content ratio by cross-talk calculated from 99mTc integral values of simultaneous 99mTc and 18F scan and independent 99mTc scan. Furthermore, the effect of cross-talk correction defined by 99mTc integral values of simultaneous 99mTc and 18F scan with and without TEW correction. We compared the 99mTc image of simultaneous 99mTc and 18F scan reference to the 99mTc image of the independent 99mTc scan. Results: The scatter content ratio for the body part and small-region chamber with 99mTc solution was gradually increased 10-50% as higher 18F/99mTc ratio, whereas the scatter content ratio small-region chamber with non-radioactive water was exponentially increased as higher 18F/99mTc ratio. However, all scatter content ratio conversely decreased at 18F/99mTc ratio of more than 8. Although the integral value of body part and small-region chamber with 99mTc solution became similar value at the independent 99mTc scan, the small-region chamber with non-radioactive water could not correct scatter by cross-talk at the 18F/99mTc ratio of more than 2. Conclusion: We revealed the influence and correction effect of cross-talk and different radioactivity concentration ratio. The 99mTc image simultaneously acquired 18F/99mTc ratio of less than 2 could exactly correct scatter by cross-talk from 18F.