Abstract
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Objectives: The acquisition method and placement within the protocol of the transmission map during cardiac PET/CT can affect the clinical outcome of myocardial perfusion studies. CT imaging proves beneficial for assessment of coronary calcification but fails to account for respiratory averaging when correcting PET emission studies. Mismatch of the heart between the transmission/emission sequences due to diaphragm or patient motion commonly leads to artifacts. Three different CT protocols: gated for coronary artery scoring, fast, and slow for temporal blurring were compared to determine which provided the most accurate attenuation correction. Differences in mismatch between rest and stress were also examined to determine if a second transmission scan was necessary.
Methods: 28 patients with paired gated (326ms diastolic, 120kVp, 280mA, 2.6sec) and slow (0.562:1 pitch, 120kVp, Auto-mA, 16sec) CT acquisitions and 14 patients with paired fast (0.938:1 pitch, 120kVp, Auto-mA, 4.8sec) and slow CT acquisitions and 35 patients with Ge-68 transmission scans (5min) (no CT) were studied. All transmission scans were performed under free-breathing prior to Rb-82 myocardial perfusion studies using adenosine pharmacological stress. The magnitude of the misalignment was assessed using Gould’s method (J. Nucl. Med. 24 (6), 2004) to determine the volume of cardiac tissue in the emission scan that overlaid lung tissue in the CT scan.
Results: The cardiac volume that overlaid lung was smallest for the slow CT acquisition in all cases: normal CT (p<0.01 rest, p<0.001 stress); gated CT (p<0.03 rest, p<0.05 stress). No significant difference existed between the slow CT and Ge-68 transmissions. The fractions of overlaying cardiac volume greater than 0.1 mL were 56%, 42%, and 21% for the fast, gated and slow transmission scans, respectively. The difference in the volume of overlaid cardiac tissue between rest/stress emissions was not significantly different for any of the CT protocols. However, there was a trend toward the overlap volume being greater for the rest studies.
Conclusions: The low temporal resolution slow CT protocol was the best method for minimizing cardiac misalignment between transmission and emission studies. The trend toward less misalignment in the stress study suggests downward movement of heart away from the lung and suggests that a second transmission scan is not needed. If desired, a second gated CT can be acquired for quantification of calcium burden. Although reduced, the large fraction of studies with overlap warrants an external or manufacturer provided solution to reorient misaligned studies.
- Society of Nuclear Medicine, Inc.