While CCTA is associated with a relatively high patient radiation dose, it is also particularly amenable to dose reduction techniques. What's more, as demonstrated in a study led by Gilbert Raff of William Beaumont Hospital (Royal Oak, MI), this reduction can be achieved without significantly affecting diagnostic image quality (JAMA 301 2340).
Working as part of the Advanced Cardiovascular Imaging Consortium (a Michigan-based collaborative quality improvement programme), Raff and colleagues examined 4995 patients undergoing CCTA for suspected coronary artery disease.
The year-long study, conducted at 15 hospital imaging centres, was divided into three distinct periods. After the first two months (the control period), the researchers analysed the baseline characteristics of the patients, radiation doses and image quality metrics. Based on these data, they developed a best-practice model for radiation dose reduction during CCTA scan acquisition. These recommendations were disseminated to each participating site during the next eight months (the intervention period).
Dose reduction schemes included minimizing the length of the scan range, heart-rate reduction, electrocardiographic-gated tube current modulation, and reducing tube voltage in suitable patients. During the final two months of the study (the follow-up period), measurement data were collected without further interventions.
Taking effect
Results revealed that patients' estimated median radiation dose in the follow-up period was reduced by 53.3% compared with the control period. The dose-length product decreased from 1493 mGy×cm to 697 mGy×cm, and the effective dose reduced from 21 mSv to 10 mSv. All sites achieved an increase in the number of patients receiving the target dose of less than 15 mSv.
Importantly, there were no significant changes in median image quality rating. During the follow-up period, 769 of 835 patients (92%) had diagnostic-quality scans, compared with 554 of 620 patients (89%) in the control period.
The researchers note that the strongest factor associated with achieving the target dose was reducing tube voltage from 120 kVp to 100 kVp (which increased the number of cases meeting the target from 13% to 43%), followed by a site study volume of more than 30 scans per month. Among patient-related variables, reduction in heart rate demonstrated a strong positive association with decreased radiation dose.
"Baseline data obtained in this study support the concern that CCTA testing may result in relatively high radiation doses, but the rapid improvement in radiation dose in the first three intervention months (a decrease of 48%) demonstrates how use of existing technology and technical methods is associated with dramatically reduced dosages," the researchers write. "The importance of radiation-reduction techniques must be emphasized during physician and technologist training."
Emergency measures
Elsewhere, a study performed at the University of Washington School of Medicine (Seattle, WA) reveals a means of reducing radiation dose when evaluating patients with non-specific chest pain using whole-chest multi-detector CT (MDCT). The researchers demonstrated that combining MDCT with prospective electrocardiogram (ECG) triggering, as opposed to using retrospective ECG gating, can reduce patient dose by 71% (AJR 192 1662).
Prospective ECG triggering enables the CT scanner to monitor the ECG signal of several heart beats in real time and turns the radiation beam on only during alternate beats. Retrospective ECG gating also monitors the ECG signal of heartbeats, but turns the radiation beam on throughout several consecutive beats, thus delivering more radiation.
The study included 72 emergency department patients with non-specific chest pain referred for whole-chest 64-slice MDCT. A total of 41 patients were scanned using retrospective gating and 31 using prospective triggering. For retrospective gating, the mean effective radiation dose was 31.8±5.1 mSv. Prospective triggering reduced this dose to 9.2±2.2 mSv.
In addition, the image quality as scored by two independent reviewers was slightly better with prospective triggering. Two of 512 segments imaged with retrospective gating were non-evaluable (0.4%), while two of 394 segments imaged with prospective triggering were non-evaluable (0.5%). Prospectively triggered images were 2.2 times as likely as retrospectively gated images to receive a high image-quality score for each segment, after adjustment for segment differences.
"As our study suggests, patients benefit from MDCT scans using ECG triggering because it delivers a much lower radiation dose - opening the utility of CT to more patients. This lower dose is particularly useful when non-specific indications suggest scanning of the whole chest, such as in the 'triple rule-out'," said lead author William P. Shuman. "Emergency rooms face severe congestion and cost constraints, plus a need for accurate, swift diagnosis. Lower dose, high image quality ECG triggered CT will help significantly on all of those fronts."