Despite some concerns about an inverse relationship between dose reduction and image quality, it is possible to apply an 80 percent CCTA dose reduction protocol and maintain image quality, according to a study published in the August issue of Academic Radiology.
Although several studies have evaluated low-dose coronary CT angiography (CCTA), none have yet examined low-dose and full-dose protocols in the same group of patients, explained Nina V. Gagarina, MD, PhD, of the department of radiology at First Moscow State Medical University in Moscow, and colleagues.
The researchers sought to fill the gap and designed a study to quantitatively compare image quality for low-dose (tube voltage 100 kV) and full-dose (tube voltage 120 kV) protocols on a 320-slice CT scanner.
The study enrolled 70 patients with intermediate probability of coronary artery disease between June and September 2009. All eligible patients had sinus rhythm and body mass index (BMI) less than 30 kg/m 2. Mean age was 57.3 years, 73 percent of patients were male and mean body mass index was 25.8 kg/m 2. Measured heart rates were approximately 49 to 64 beats per minute.
Gagarina and colleagues found that the low-dose protocol reduced the standard CCTA radiation dose by an average of 80 percent. That is, mean patient dose was 4.9 mSv using the standard protocol and 0.98 mSv for the low-dose protocol.
“Nevertheless, in the visual evaluation of hemodynamically significant stenosis, there were no differences between standard and low-dose protocols,” wrote Gagarina.
Three radiologists, blinded to the dose protocol, evaluated the studies using a four-point scale, producing a mean score of 3.32 for the standard protocol and 3.29 for the low-dose protocol. Image quality and BMI weakly correlated for both protocols, stated the researchers.
In addition, there was a strong inverse correlation between image quality and heart rate variability for the low-dose protocol at r=-0.58, but not for the standard-dose protocol. Heart rate variability was 2.79 +/- 1.46 beats per minute for the standard protocol and 2.53 +/-1.54 beats per minute for the low-dose protocol.
“The quantitative assessments … demonstrated that of the 12 anatomic regions analyzed contrast-to noise-ratio (CNR) loss only occurred in the ascending aorta, left ventricle and in the right coronary artery (RCA) middle. Considering that the ascending aorta and left ventricle still have very high CNR values at the low-dose protocol, we conclude that the result in only one of 12 anatomic regions (RCA middle) would indicate some loss of image quality,” wrote Gagarina.
The researchers emphasized, “Prospective ECG-gated CCTA with tube voltage 100 kV can be applied without loss of image quality on patients with BMI of less than 30, but is more sensitive to heart rate variability than the full-dose protocol.”