320-detector row CT delivers image quality, low dose
Three-dimensional volume rendering acquired in a single heart beat with 320-detector CT (AquilionOne Dynamic Volume CT, Toshiba Medical Systems). The volume has no stair-step artifact typically noted in 64-slice systems since the entire heart is imaged within the 16 cm craniocaudal coverage of the 320-detector system. (All images: F. Rybicki)  
Curved multiplanar reformatted image from the 320-detector row CT acquisition of the left anterior descending coronary artery clearly demonstrating a proximal stenosis. This patient also has hypertrophy of the left ventricle. One of the advantages of cardiac CT is full depiction of the myocardium as well as the coronary arteries.
Curved multiplanar reformatted image from a 320-detector row CT acquisition of the left circumflex demonstrates a stenosis >70 percent, confirmed at coronary catheterization.
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 ECG-gating without dose modulation to retrospective ECG-gating with dose modulation and subsequently to prospective ECG-gating. The disadvantage of prospective CT gating, regardless of the CT technology, is the inability to assess cardiac function because image acquisition does not span the entire heartbeat.

In this study, researchers did not use dose modulation for those patients who were imaged with retrospective ECG-gating because they wanted to optimize visualization of myocardial and valve motion throughout the cardiac cycle.

One problem the investigators encountered was finding an accurate way to measure radiation dose. The standard measurement, CT dose index 100 (CTDI100), uses an ionization chamber and a dosimetry phantom that are smaller than the primary x-ray beam of the 320-detector row scanner. To overcome this problem, researchers used newly-developed longer chambers and phantoms, and they derived a scaling factor to estimate dose using traditional methods. 

“This is an important issue in future investigations as the dose in CT coronary angiography continues to be scrutinized,” Rybicki said.