Abstract
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Objectives: Accurate correction for attenuation by the thoracic structures is necessary for high quality Rb-82 myocardial perfusion studies. PET scanners that use Ge-68 for transmission scans thus need to acquire transmission images with adequate counts while keeping the scan time to a minimum. This study attempts to utilize count statistics as a guide to determine the optimal transmission scan time for Rb-82 studies.
Methods: 20 mCi and 6 mCi of Rb-82 were put into the ventricular wall and chest cavity of a Data Spectrum anthropomorphic phantom. A 1.5-cm defect was placed in the mid anterior wall and a 2.0 cm defect in the basal inferior wall to simulate infarcts. An emission scan was acquired on the GE Advance NXi scanner for 5 minutes in 2D mode 3 minutes post activity injection. New sources of Ge-68 were used for transmission scans which were acquired for 0.5, 1, 2, 4, 6, 8, 10 minutes resulting in 18, 36, 73, 146, 219, 292, 366 million counts, respectively. 7 sets of images were reconstructed using Filtered Backprojection and corrected for attenuation by each of the transmission scans. The reconstructed images were evaluated by the Emory Tool Box, contrast ratios of the defects, and for artifacts in the images.
Results: While both defects were identified by the Emory Tool Box in all image sets, severe streak and uniformity artifacts were noted in the images corrected by the 18M and 36M count transmission scans, and improved as the counts increased. Scans with more than 73M counts did not show remarkable differences visually. Depending on the cardiac orientation, contrast ratio of the anterior defect increased from 6% to 18%; inferior defect contrast increased from 13% to 16% when counts increased from 18M to 73M. Contrast increased by less than 5% when counts increased from 73 M to 366M.
Conclusions: For patients approximating the size of this standard phantom, 73M counts in the transmission scan were adequate for attenuation correction of the Rb-82 myocardial perfusion images. There was no substantial gain in the image quality or lesion detectibility by lengthening the transmission scan. Our data suggested that high quality Rb-82 images can be obtained on actual myocardial perfusion studies by collecting approximately 73M counts for the transmission scan. A given patient’s transmission scan time can be approximated by multiplying the phantom scan time to obtain 73M counts by the ratio of the patient’s body mass index to that of the phantom and the correction factor for decay of the Ge-68 activity since the phantom measurement.
- Society of Nuclear Medicine, Inc.