Myocardial Perfusion Imaging with Ultra-high Resolution SPECT/CT System in Rats using Multiple Pinhole Collimator: Comparison with High-resolution Autoradiography

Objectives: Ultra-high resolution SPECT/CT system using multiple pinhole collimators for small animal can offer the resolutions of less than a millimeter. To determine the accuracy of ultra-high resolution SPECT/CT system, we compared the size of myocardial infarction (MI) area by high-resolution autoradiography and in-vivo SPECT/CT using rat model with coronary artery occlusion and reperfusion.

Methods: After thoracotomy, the left coronary artery was occluded for 20 min (n=4) or 30 min (n=5). After reperfusion, the rats were allowed to recover for 2 month. Then the rats were injected with ^99mTc-MIBI (185MBq) to perform in-vivo SPECT/CT and high-resolution autoradiography. The MI area on SPECT/CT images (U-SPECT system, MIlabs, the Netherlands) was analyzed by Amide's a Medical Image Data Examiner (AMIDE) software; region of interests (ROI) were set on whole heart to evaluate the maximum counts in the heart. The thresholds were set at 25% 40%, 50%, 60%, and 75% to the maximum counts, and MI ROIs was delineated manually using three-dimensional series of ROIs. The MI area on autoradiography was analyzed by Image J software. The maximum count in the heart was assessed, and threshold was set at 50% of the maximum count. The MI ROI was set in each image which was cut in every 1mm. Regression analysis was performed to confirm the MI size relations between SPECT/CT and autoradiography.

Results: MI demonstrated as perfusion defect was confirmed in every rat on both SPECT and autoradiography images. MI sizes in autoradiography were correrated very well to that in SPECT/CT images with thresholds at 25%, 40%, 50%, and 60% to the maximum counts; r²=0.76, 0.83, 0.88, and 0.52, P<0.05, respectively). However, small MI was not recognized as perfusion defect on images when the threshold was set to 25% to the maximum counts. In some cases, it was difficult to determine the MI due to inhomogeneous myocardial uptake when the threshold was set at 75% to the maximum counts (n.s.).

Conclusion: These data indicate that ultra-high resolution SPECT/CT can evaluate the severity of MI in rats, which were well correlated with the results of autoradiography. Research Support: This work has been supported by Grants-in-Aid for scientific research (No. 26461847) from the Ministry of Education, Culture, Sports, Science, and Technology, Japan.

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