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

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.