Imaging
Radiation Minimisation and Outcome Maximisation in Cardiac Computed Tomography
James P Earls1 and Jonathon A Leipsic2
1. Director, Cardiovascular Computed Tomography and Magnetic Resonance Imaging, Fairfax Radiology Consultants; 2. Chairman, Department of Radiology, University of British Columbia, Vancouver
Abstract
Recent reports have raised general awareness that cardiac computed tomography (CT) has the potential for relatively high effective radiation doses. While the actual amount of risk this poses to the patient is controversial, the increasing concern has led to a great deal of research on new CT techniques capable of imaging the heart at substantially lower radiation doses than was available only a few years ago. Methods of dose reduction include optimised selection of user-defined parameters, such as tube current and voltage, as well as use of new technologies, such as prospective triggering and iterative reconstruction. These techniques have each been shown to lead to substantial reduction in radiation dose without loss of diagnostic accuracy. This article will review the most frequently used and widely available methods for radiation dose reduction in cardiac CT and give practical advice on their use and limitations.
Keywords
Cardiac, multidetector-row computed tomography (MDCT), dual-source computed tomography (DSCT), coronary artery disease, radiation dose, CT techniques
jpearls@yahoo.com
Radiation Dose Concerns in Cardiac Computed Tomography
Computed tomography (CT) utilisation for general medical imaging and for dedicated cardiac indications has come under a great deal of scrutiny in the past few years.1–3
Concern has arisen due to both the
increased utilisation of CT, with more than 70 million scans performed in the US in 2007, and the increasing radiation dose. A recent report on ionising radiation exposure from medical imaging to the American population estimated that the collective radiation dose received increased by 700% between 1980 and 2006.4
CT has experienced an
annual growth rate of >10% and accounted for approximately 50% of the total collective dose in 2006.
With the recent advances in CT gantry technology, including slip rings, multisegmented detector arrays and sub-second gantry rotation, there have been significant improvements in image acquisition time, enabling coronary CT angiography (CCTA).5
In 2006,
cardiac CT was estimated to have resulted in 1.5% of the collective CT dose, but with significant further improvements in scanner technology and greater adoption of cardiac CT in clinical practice this number will almost certainly increase.
Cardiac CT provides excellent cross-sectional anatomical detail of the coronary arteries, the heart and the surrounding structures.
as well as to provide a moderate descriptor of However, this exceptional anatomical detail and spatial
It
is unique in its ability to non-invasively image coronary atherosclerosis,6,7 stenosis.8
resolution has come at the price of a high historical radiation dose. The international Prospective Multicenter Study On RadiaTion Dose
© T O UCH BRIEFINGS 2010
Estimates Of Cardiac CT AngIOgraphy I (PROTECTION I) trial surveyed 50 coronary CT angiography sites worldwide and reported median doses of 12mSv per exam.9
The majority of scans were performed
with retrospective gating; use of prospective gating was limited to 6% of studies as the data were collected prior to widespread clinical release of this technique.
Radiation risk is a controversial topic in CT. The controversy stems from the fact that a direct link between radiation from medical imaging and development of solid-organ cancers has never been established in the <20mSv effective dose range encountered in cardiac CT. Despite the lack of direct evidence, most physicians and imaging societies advocate the adoption of the linear no-threshold hypothesis, which states that there is no threshold below which radiation cannot cause malignancy and that the risk of malignancy increases in a linear fashion with higher doses of radiation. This hypothesis suggests that it is reasonable to linearly extrapolate the risk of malignancy induction from higher doses to the risk from lower dose exposures. It is this default position that requires that physicians take a conservative approach and adhere to the ‘as low as reasonably allowed’ (ALARA) principle, all the while maintaining diagnostic efficacy.
Imaging physicians must balance the benefits of the information obtained from CCTA with the potential risk of the radiation used to generate the images. If there is no proven benefit to performing an exam, regardless of the radiation dose the risk is too high and it should not be performed. Lowering the radiation dose of an appropriately ordered CT to a point where the scan loses diagnostic capability also does not ultimately benefit the patient. Likewise,
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