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.

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.