Lowering radiation dose for integrated assessment of coronary morphology and physiology: First experience with step-and-shoot CT angiography in a rubidium 82 PET-CT protocol
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Background. Reduction of radiation exposure from computed tomography coronary angiography (CTA) will be a key factor for more liberal use in cardiac hybrid positron emission tomography (PET)-computed tomography (CT). We report our initial experience with a new algorithm for low-dose CTA based on a prospectively gated step-and-shoot technique. This limits acquisition to the diastolic phase and minimizes exposure time versus the previous standard of retrospectively gated helical acquisitions.
Methods and Results. In 15 consecutive patients referred for integrated functional and morphologic workup by rubidium 82 perfusion PET-CTA, step-and-shoot CTA (SnapShot Pulse; GE Medical Systems) (120 kV, 600–800 mA) was acquired on a 64-slice GE Discovery Rx VCT PET-CT scanner and compared with a group of patients with conventional helical CTA (120 kV, with modulation of the milliampere level) who were matched with regard to clinical variables. Effective dose was estimated from dose-length product. The American Heart Association 15-segment coronary tree model was used to determine study interpretability. Potential for fusion with Rb-82 perfusion PET was tested by use of commercial software. In addition, direct dose measurements were conducted by use of an anthropomorphic phantom for more accurate dosimetry. The dose-length product-derived effective patient dose for step-and-shoot and helical CTA was 5.5±0.1 mSv versus 20.5±3.5 mSv (P<.0001). The mean number of evaluable segments per patient for the best phase of helical CTA was 12.5±2.8 (83.3%±18.7%) versus 13.3±2.2 (88.7%±14.7%) (P=not significant vs helical) for step-and-shoot CTA. Review of multiple phases increased the number for helical CTA to 13.7±1.7 (91.3%±11.3%;P=not significant vs step-and-shoot CTA, for which this was not an option). Semiautomated fusion with corresponding PET was feasible for all studies. Phantom data confirm effective doses of 5.4 mSv for step-and-shoot CTA and 19.6 mSv for helical acquisition.
Conclusions. Low-dose prospectively gated CTA reduces radiation exposure by nearly 70% versus the previous standard of helical acquisition, without significant loss in interpretability and integrative potential with Rb-82 perfusion PET. This represents a step toward a broader, routine integration of CTA and perfusion PET for assessment of coronary morphology and physiology by cardiac PET-CT.
Key WordsPositron emission tomography-computed tomography computed tomography coronary angiography radiation dose rubidium 82
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