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In the October 2006 issue of Diagnostic Imaging, I wrote a brief article describing the benefits of using prospective gating when performing coronary CT angiography.
In the October 2006 issue of Diagnostic Imaging, I wrote a brief article describing the benefits of using prospective gating when performing coronary CT angiography.1 Since then, a number of software and technique improvements have occurred that make possible the use of prospectively gated CCTA in the everyday clinical setting. In our practice, prospective gating is used in 98% of our CCTA patients as the initial and only scan technique. It has become rare for us to default initially to retrospective gating or reinject a patient to do a retrospectively gated CCTA because of a failed prospectively gated acquisition.
Prior to prospectively gated CCTA, the most effective radiation dose reduction technique was ECG dose modulation, used when performing retrospectively gated CCTA. It reduces dose by decreasing the mA of the x-ray beam in the systolic and near-systolic portions of the cardiac cycle, when the data are unlikely to be used to postprocess the coronary artery images. This results in a reduction in radiation exposure of approximately 30%. What prevents this technique from reducing radiation dose further is that the x-ray beam remains on throughout the cardiac cycle, even during the reduced exposure phases, when no images will be reconstructed. This results in unnecessary radiation being delivered to the patient and is inherent in all spirally acquired CCTA exams.
Prospective gating avoids this extraneous radiation dose by completely turning off the x-ray beam during most of the cardiac cycle. The portion of the cardiac cycle that is to be radiated is selected before the scan, hence the term prospective. The radiated window (length of time the tube is turned on) may be left wide, making it possible to reconstruct vessels in a range of cardiac phases, though this comes at the expense of an increased radiation dose. Before scanning a patient, the user must select the phase of the cardiac cycle in which the reconstruction will be centered. I routinely use the 75% phase, which, in patients with heart rates < 65 bpm, ensures scanning takes place during diastole.
In the prospectively gated CCTA protocol, there is a default time added to the window for tube on-time (dynamic padding), based on the patient's heart rate. However, if the patient's heart rate is < 65 bpm, one can safely conclude that the postprocessing will be successful; i.e., diagnostic images of all 15 American Heart Association coronary artery segments will be obtained with the single 75% phase available for reconstruction. For this reason, I always override the default padding and manually enter a 10-msec padding surrounding the 75% phase. For example, in a patient with a heart rate of 60 bpm, the duration of the cardiac cycle is 1000 msec. By radiating only 10 msec before and 10 msec after the 75% phase, only 2% of the cardiac cycle beyond the window-centered at the 75% phase-is radiated.
Using prospectively gated CCTA with the default padding reduces radiation by 52% to 74% compared with retrospectively gated CCTA, with no loss of image quality. By manually decreasing the padding to 10 msec, the radiation exposure can be reduced by up to 90%. This dramatic reduction in radiation exposure more than compensates for the "inconvenience" of administering beta blockers to bring the heart rate down to 65 bpm or less. Prospectively gated CCTA may, in fact, be used in patients with heart rates between 65 bpm and 70 bpm but is somewhat less consistent in obtaining all 15 AHA coronary artery segments at postprocessing. Functional evaluation of the heart is not possible, since images are not collected at all phases of the cardiac cycle.
Before the topics of spatial and temporal resolution as they pertain to CCTA can be considered, images with a signal-to-noise ratio adequate to visualize the coronary arteries must be obtainable. The demands for an increased SNR are most apparent in obese and morbidly obese patients. In these patients, the use of 650 to 800 mA may be necessary to obtain diagnostic images. In scanners whose maximum tube current is less than this, it may be necessary to increase voltage to 140 kVp from the usual 120 kVp. Each incremental increase of 20 kVp results in a 38% increase in radiation exposure. Conversely, a reduction of 20 kVp decreases radiation exposure by 38%.
It is important to titrate your technical factors to the minimum needed for patients of different body mass indices. Body mass index is the only variable used to select the kVp and mA for both prospectively and retrospectively gated CCTA. From the development of prospectively gated CCTA has come the realization that image quality can be preserved in many patients by using 100 kVp instead of the 120 kVp I routinely used with retrospectively gated CCTA, resulting in ultralow radiation dose CCTA. In many patients, the dose from this scan is at or less than that needed for coronary artery calcium scoring.
In my opinion, retrospectively gated CCTA was unfairly labeled as having excessive radiation exposures. In fact, the radiation dose resulting from retrospectively gated CCTA is in the range of technetium-99m sestamibi SPECT nuclear stress tests and less than the radiation exposure resulting from thallium-201 SPECT or SPECT dual-isotope nuclear stress tests.2,3 Prospectively gated CCTA has made this point moot. Taken to an extreme, 100-kVp, 10-msec padded prospectively gated CCTA in a patient with a BMI < 24 results in a 90% dose reduction when compared with Tc-99m sestamibi SPECT (see table).
Prospectively gated CCTA should become the first-line test in any patient suspected of having coronary artery disease, especially given its superior sensitivity to a stenosis of > 50% (SPECT studies usually are not positive until there is at least a 70% stenosis).4,5 Also in CCTA's favor are its ability to detect eccentric plaque invisible to stress tests and many coronary catheterizations (the same eccentric plaques whose rupture accounts for up to 85% of all myocardial infarctions) and its relatively low cost-and now the dramatic dose reduction that is possible with the modality. The only downside I see is the risk that comes with injecting iodinated contrast and the lack of guidelines for determining which patients should be on statins when plaque is detected in patients with normal cholesterol profiles.6,7
With the introduction of 64-slice CT scanners using a 4-cm detector composed of 0.625-mm elements, expanded coverage in the z-axis became possible. Simultaneous arterial-phase opacification of the thoracic aorta, coronary, and pulmonary arteries also became possible. Called the triple rule-out because of its diagnostic capability to rule out aortic pathology, coronary artery disease, and pulmonary emboli in one scan, this exam has drawn major interest from physicians, especially those in an emergency department setting.
In the ER, establishing the etiology of a patient's symptoms is often difficult, resulting in erroneous discharge of 4% of patients with an acute myocardial infarction.8 These patients have a threefold increase in mortality over the next 72 hours compared with their cohorts who are correctly admitted to the hospital. The cost to the patient with retrospectively gated CCTA triple rule-out exams was radiation exposure in the 20 to 30 mSv range.
With the dramatic reduction in dose possible with prospectively gated CCTA, it is now a reality to perform triple rule-out exams with more acceptable levels of radiation exposure. This may usher in the era of CTA exams with even greater z-axis coverage. Quadruple rule-out exams (adding carotid stenosis), which extend coverage from the skull base through the thorax, may be useful in patients with syncope, transient ischemic attacks, and cerebrovascular accidents resulting from carotid stenoses (Figures 1 and 2).
Quad rule-out exams require 20 cc more contrast than triple rule-out exams and a 20-second-long breath-hold, but they result in a radiation dose of 5.8 to 10 mSv depending on the patient's BMI. In the future, it is conceivable that the circle of Willis could be added to the exam, which may diagnose an embolic cardiovascular accident but also the possible origin of the embolus. In essence, this would be a "half-body CCTA." At the moment it exists only in theory, but it may be brought to a clinical reality with further improvements in z-axis coverage.
Prospective gating is now achievable in the overwhelming majority of patients needing CCTA. It requires a heart rate of < 65 bpm, which is easily and safely obtained with the administration of beta blockers. Given the massive reduction in radiation exposure, I feel that prospectively gated CCTA will soon become the standard of care and will likely force the hand of all CT vendors to develop this capability. I question whether the additional expense of dual-source CT scanners can be justified in an era when the use of beta blockers can bring heart rates low enough that it's possible to reduce radiation exposures to levels never before seen in cardiac CT and CCTA. Given that the radiation dose is now far less than that used with SPECT studies, CCTA should quickly become the initial exam used in patients suspected of having CAD.
Dr. Dowe is medical director of coronary CTA at Atlantic Medical Imaging in Galloway, NJ.