Radiation From Cardiac CT Can Be Slashed Without Losing Image Quality

Reed Miller

December 10, 2009

December 10, 2009 (Chicago, Illinois) — New research shows that a novel retrospective gating technique can almost halve radiation dose from coronary CT angiography (CCTA) in certain patients, without compromising the diagnostic value of the images [1].

Dr Gudrun Feuchtner (Innsbruck Medical University, Austria) and colleagues compared a 100-kV-tube-voltage CCTA protocol with the standard 120 kV in 103 patients undergoing retrospective ECG-gated helical 64-slice CCTA. Feuchtner presented results of the study December 1 at the Radiological Society of North America (RSNA) 2009 Scientific Assembly, and the paper was published online August 9, 2009 in the European Journal of Radiology.

The resulting images from both groups were of similar quality. Image noise was slightly higher in the 100-kV group, but within acceptable ranges for diagnostic image quality, and the contrast-to-noise ratio was slightly better with the 100-kV protocol. Importantly, the radiation dose was 47% lower in the 100-kV group than in the 120-kV group, an average of 7.1 mSv compared with an average of 13.4 mSv.

Feuchtner explained to heartwire the advantage of retrospective ECG-gating is that it provides full coverage of the entire cardiac cycle, whereas prospective ECG-gating takes images only at certain points in the cardiac cycle, so it is generally appropriate only for diastolic image acquisition in patients with stable heart rates under 70 bpm.

A retrospective gating protocol is usually necessary in patients with high heart rates in order to reconstruct an image of the coronaries during systole or for CT functional evaluation, such as scans of hearts with wall-motion abnormalities, left-ventricular dysfunction, or valvular dysfunction. However, the usual disadvantage of retrospective gating is that it requires a relatively high dose of 9 to 14 mSv compared with 2 to 4 for prospective gating, she explained. She noted that a very new CT system based on dual-step adaptive pulsing, developed by Siemens, allows for coverage of data during the entire cardiac cycle in a prospective gating protocol, but the scientific data are lacking on whether that technique will be effective in patients with high heart rates.

"The main message is that the 100-kV is a very useful protocol, with the advantage that, when you use it in the retrospective mode, you can have the full functional data and the systole," Feuchtner told heartwire . "The 100-kV protocol is a very useful method and saves almost 50% of the radiation dose."

In the study, 12 coronary segments (1.4%) were uninterpretable, including three distal segments that were uninterpretable due to increased image noise in a patient with a body-mass index (BMI) of 27.5 kg/m2. Other segments were uninterpretable due to artifacts such as motion and/or calcium in patients with calcium scores >100 Agatston units. Therefore, the conclusions of the study can be applied only to patients with a BMI under 25 kg/m2 and a low calcium load, Feuchtner said.

"It's very important that all cardiologists and everyone who uses the scan be aware of this . . . particularly for younger patients, age 40 or less, for whom the radiation dose is more of an issue of concern than in the older patients. We know that the younger the patients are, the higher the risk of cancer. In these patients, we should take care to use the 100-kV protocol."

Feuchtner pointed out that this study complements research led by Dr Paul Stolzmann (University Hospital Zurich, Switzerland) and supported by the Swiss National Science Foundation. Stolzmann and colleagues evaluated a 100-kV protocol with a prospective "step-and-shoot" CCTA method. Results of their study, published in 2008 in Radiology [2], showed no significant differences in image quality between the 100-kV and 120-kV protocols, but the mean estimated effective dose was 1.2 mSv for scans with the 100-kV protocol and 2.6 mSv with the 120-kV protocol.

"So for nonobese patients, 100 kV is enough, using not only retrospective ECG-gated protocols but prospective as well," Stolzmann told heartwire . "We don't know where the next threshold is going to be, but . . . there are no signs of a need to reduce the kilovoltage any further."

Feuchtner reported having no potential conflicts of interest. Stolzmann reported no conflicts of interest.

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