Radiation Protection of Patients Undergoing Cardiac Computed Tomographic Angiography

Abstract

INTEREST IN THE USE OF COMPUTED TOMOGRAPHY (CT) for cardiac evaluation has increased rapidly since the introduction of 64-slice scanners. Reflecting this, the installation base of CT scanners in US cardiology practices has tripled in the past 2 years. Reports of the high diagnostic performance of coronary CT angiography (CTA), and especially its high negative predictive value in populations with low-to-intermediate prevalence of coronary disease, have been tempered by a concern about its high radiation dose to patients and the attendant risk of cancer. Despite a number of single-center studies that have reported a wide range of effective doses for coronary CTA, the existing literature does not adequately answer the questions of what radiation doses patients actually receive in clinical practice, and what factors are associated with higher radiation dose. Such information should help practitioners develop protocols that are in accordance with the goal of maintaining radiation exposure to patients as low as reasonably achievable (the ALARA principle). The Prospective Multicenter Study On Radiation Dose Estimates Of Cardiac CT Angiography In Daily Practice I (PROTECTION I), an observational study of worldwide cardiac CTA practice in 2007 reported by Hausleiter and colleagues in this issue of JAMA, represents an effort to fill this gap. The primary outcome measure used to quantify radiation dose in PROTECTION I is the dose-length product (DLP), a CT-specific term unfamiliar to many physicians. The DLP is a reflection of the total amount of radiation deposited over the entire set of images comprising a patient’s CT series, reported in mGy cm. Better known is the effective dose, a measure applicable beyond the confines of CT and reported in millisieverts (mSv). Effective dose weights the concentrations of energy deposited in each organ from a radiation exposure using factors reflecting the type of radiation and the relative detriment to each organ of potential radiation-associated mutagenic changes. Although effective dose can be compared between different types of exposures, the factors used in its determination to weight each organ are approximate population averages, and therefore it is imprecise to report the effective dose of an individual patient’s study. Thus, in characterizing the amount of radiation from a single CTA study, DLP is a more objective physical metric than effective dose, and the reason PROTECTION I is replete with DLP data. However, effective dose is appropriate to refer to in a population of patients and is especially useful for comparing between different types of exposures in populations with similar age and sex distributions. As in numerous previous studies, effective dose of CTA is estimated in PROTECTION I by multiplying a median DLP by a conversion factor suggested by the European Commission. Hausleiter et al present a number of interesting and surprising findings about radiation dose from cardiac CTA. The estimated overall median effective dose for CTA, excluding calcium scoring when performed as part of the same study, was 12 mSv, somewhat less than the value reported in several earlier studies using 64-slice scanners. A few factors may account for this. Although the conversion factor used in PROTECTION I to estimate effective dose from DLP was derived using single-slice scanners and chest rather than cardiac scan sequences, this factor is the most current available. The value is approximately 20% lower than that used in previous studies, and thus would be expected to result in effective doses that are correspondingly lower. Additionally, as pointed out by Hausleiter et al, invitation of study sites on the basis of previous publications may have introduced a bias favoring more expert centers, which would be expected to be more proficient in managing radiation dose. Indeed, dosereduction techniques were used in at least 80% of patients undergoing 64-slice CTA for coronary assessment. Consistent with the suggestion of lower dose in centers with greater expertise, Raff et al recently reported a reduction in median effective dose of CTA from 25 mSv in the initial month to 13 mSv in the ninth month of a statewide quality improvement program in Michigan, with no change in image quality. This program used a collaborative