Author + information
- Published online February 3, 2020.
- Troy M. LaBounty, MD∗ ()
- ↵∗Address for correspondence:
Dr. Troy LaBounty, University of Michigan Medical Center, 1500 East Medical Center Drive, SPC 5853, Room 2365, Ann Arbor, Michigan 48109-5853.
“I didn’t come this far to only come this far”
—Tom Brady (1)
With the development of 64-detector computed tomography (CT), imaging the heart and coronary arteries became feasible with a single breath hold. However, initial techniques used multiple overlapping acquisitions of the heart to provide imaging throughout the cardiac cycle and did not routinely apply radiation dose-reduction techniques, exposing patients to significant radiation doses. A 2007 study by Einstein et al. (2) estimated that a cardiac CT study could expose the female breast to an organ dose of 77 mSv, and that lifetime attributable cancer risk could reach 1 in 143 in the case of a 20-year-old female.
Studies such as this one raised awareness of the potential risks from the ionizing radiation associated with CT and prompted implementation of radiation dose-reduction strategies. The PROTECTION I (Prospective Multicenter Study On Radiation Dose Estimates Of Cardiac CT Angiography In Daily Practice) study examined the radiation doses of coronary CT angiography (CCTA) performed in 2007 at 50 international sites and reported a median dose of 12 mSv. They observed a large variation in median site radiation dose (up to a 6-fold difference), and demonstrated the effectiveness of several dose-reduction strategies including reduced scan length, tube current modulation, prospective electrocardiogram gating, and reduced tube voltage (3). A multicenter study by Raff et al. (4) established the effectiveness of implementing a dose-reduction strategy with site feedback, observing a reduction in radiation dose from 21 mSv to 10 mSv without adverse changes in image quality.
As we fast forward a decade, new methods of radiation dose reduction, such as iterative reconstruction (5), have been developed, and contemporary scanner platforms now typically provide tools to help reduce radiation dose while preserving image quality (6). The PROTECTION VI study prospectively evaluated 4,502 patients undergoing CCTA in 61 hospitals from 32 countries during 2017 and evaluated scan data as well as image quality. This study observed a 78% reduction in radiation dose compared with the 2007 findings, with no change in the rate of non-diagnostic studies; the median estimated radiation dose was only 2.7 mSv using the conversion factor of 0.014 mSv/mGy/cm, which has typically been used until recently (7). However, there was a 37-fold difference in median site radiation dose (ranging from 0.8 to 29.3 mSv), demonstrating incomplete use of dose-reduction strategies (8).
Recent data suggest that we have been systematically underestimating the radiation doses associated with CCTA on modern CT scanners, and support the use of a higher conversion factor of 0.026 mSv/mGy/cm to convert the reported dose-length product to estimate the effective radiation dose of CCTA (9). This results in estimated radiation doses nearly double previous estimates; when we apply this conversion factor to data from the PROTECTION VI study, the median dose is instead 5.1 mSv, and median site doses range from 1.5 to 54.3 mSv. The large variation and the high end of this range are concerning and suggest that much more needs to be done to make sure sites are using appropriate dose-reduction strategies.
In this issue of iJACC, Stocker et al. (10) report the findings from a substudy of PROTECTION VI that examines the current use and effectiveness of reduced tube voltage on CCTA. This study examined 4,006 subjects undergoing CCTA at 61 international sites, and compared image quality, radiation dose, and contrast volume in patients imaged using very low (≤80 kVp), low (90 to 100 kVp), conventional (110 to 120 kVp), and high (≥130 kVp) tube voltage. Low or very low tube voltage was used in 56% of studies, with site use ranging from 0% to 100%. The use of very low or low tube voltages were associated with radiation dose reductions of 68% and 50%, respectively, and contrast volume reductions of 25% and 13%, respectively; there was no difference in diagnostic image quality and lower tube current was associated with improved signal to noise ratio. Using a conversion factor of 0.026 mSv/mGy/cm, estimated median doses were 2.5, 4.1, 8.1, and 16.3 mSv for very low, low, conventional, and high tube voltages, respectively.
Given the exponential relationship of kVp and radiation dose (10), reduced tube voltage is one of the most important means of lowering the radiation dose of CCTA. Based on body mass index, 58% of patients scanned using a conventional protocol and 44% of patients scanned using a low kVp protocol may have been appropriate for lower kVp imaging. Although obese patients may need increased tube voltage for adequate image quality, 33% of overweight individuals and 20% of subjects with a normal body mass index were imaged using conventional or high tube voltage. Of the 61 sites, 6 exclusively used conventional or high kVp, and 20 additional sites used low kVp for less than one-half of studies.
The importance of this study is that it highlights the wide disparity between sites in the use of reduced tube voltage and the resulting radiation exposure to patients. Critically, this data represents sites that participated in this study and knew in advance that their scanner protocols and radiation doses would be reported. It is possible and even likely that “real world” radiation doses for CCTA are higher. The algorithm proposed in this paper to use body mass index and patient characteristics to select an appropriate tube voltage is helpful, although achieving widespread use of such algorithms remains a challenge.
This study demonstrates the effectiveness of reduced tube voltage and its importance in lowering the radiation dose associated with CCTA. However, the main takeaway point is that even 10 years after the PROTECTION I study, a large number of sites are not adequately using reduced tube voltage and are exposing patients to higher radiation doses than in many cases may be needed. It is also possible that the excess in radiation dose may be greater among “real world” sites not participating in a prospective study of CCTA radiation dose. Although large improvements have been demonstrated over the past decade, it is clear that many sites have been left behind and have not implemented adequate radiation dose-reduction strategies for CCTA. These findings should prompt action by scanner manufacturers, cardiovascular societies, researchers, and regulators to uniformly improve the safety of patients imaged using CCTA. This study highlights the need for continued action so that all sites performing CCTA use strategies to minimize the radiation dose of CCTA and optimize patient safety.
↵∗ Editorials published in JACC: Cardiovascular Imaging reflect the views of the author and do not necessarily represent the views of iJACC or the American College of Cardiology.
Dr. LaBounty has reported that he has no relationships relevant to the contents of this paper to disclose.
- 2020 American College of Cardiology Foundation
- ↵Tom Brady’s Inspirational Playoff Quote: ‘I Didn’t Come This Far To Only Come This Far.’ CBS Boston [online]. January 6, 2016. Available at: https://boston.cbslocal.com/2016/01/06/tom-bradys-inspirational-playoff-quote-i-didnt-come-this-far-to-only-come-this-far/. Accessed April 4, 2019.
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