The new 320-detector row CT scanner from Toshiba Medical Systems has been shown to deliver consistently excellent image quality and iodinated contrast opacification, as well as a low radiation dose to image the coronary arteries, according to researchers from Brigham and Women’s Hospital.
While the current investigation used a high iodine load as well as prospective ECG-gating phase window and craniocaudal coverage to achieve optimal results, future work will focus on lowering contrast and radiation dose while maintaining image quality, according to lead author Frank Rybicki, MD, PhD, director of the applied imaging science laboratory at BWH.
The study was published online March 27 in the International Journal of Cardiovascular Imaging.
The development of wide area detector CT enabled greater coverage per gantry rotation. Expansion from a prototype 256-detector row to a 320-detector row system (AquilionOne, Dynamic Volume CT) has enabled whole heart coverage, in theory reducing patient irradiation by eliminating helical oversampling, according to the study.
Investigators noted two theoretical advantages of the 320-detector row system in comparison with 64- and 32x2-detector systems with respect to image quality:
- It eliminates “stair-step” artifacts inherent in 64-slice technology that images sub-volumes of the entire cardiac volume over multiple gantry rotations.
- The subsecond acquisition of the entire cardiac volume allows the contrast bolus to be imaged at a single time point. This enables, for the first time, accurate measurements of the physiologic coronary opacification with iodinated contrast.
For the study, Rybicki and colleagues retrospectively evaluated images from 40 consecutive patients referred for the evaluation of chest pain, dyspnea, and/or pre-surgical evaluation to exclude CAD.
Researchers compared the radiation dose for prospective versus retrospective ECG-gating, two body mass index categories, and single versus two heartbeat acquisition. They correlated CT findings with coronary catheterization in four patients.
All patients were imaged at 120 kV and one of two mA settings: 400 (n=28) or 580 (n=12). The staff considered both body mass index and the geometry of the patients’ thorax in deciding whether to use the higher mA setting.
Nearly 90 percent of arterial segments had excellent image quality. The most common reason for image degradation was cardiac motion. However, motion rendered only a single segment in a single patient unevaluable, according to the study.
Thirty-six segments were degraded by image noise with significantly more segments imaged at 580 mA. Five additional segments had beam hardening artifact from heavily calcified lesions.
The iodinated contrast opacification (80 ml iopamidol 370 mg I/ml followed by 40 ml saline) was almost universally considered excellent.
Radiation doses were greater for retrospective ECG-gating, larger patients, and those imaged with two heartbeats. For the most common protocol (120 kV, 400 mA, prospective ECG-gating, 60–100% phase window, 16 cm craniocaudal coverage, single heartbeat), the mean dose was 6.8 mSv.
Future efforts to reduce dose will include decreasing the phase window from 40% to 10%, or lower. Moreover, the investigators noted that most patients can be imaged using only a 14 cm (280 detectors) craniocaudal field of view, further reducing dose. With all other factors remaining the same, these two modified protocols would reduce dose to 4 mSv.
Efforts to lower the radiation dose in coronary CTA have led physicians away from retrospective