Ongoing technical developments in computed tomography (CT) have established coronary CT angiography as a robust noninvasive imaging test of the heart and coronary arteries, with ever-improving image quality and diagnostic accuracy.1–3 However, concerns have been raised regarding radiation exposure from cardiac CT, which has traditionally been comparatively high relative to other CT applications, although the dose levels compare well with other, longer-established cardiac imaging tests. Considering the increasing number of cardiac CT investigations around the globe, however, and the expected sustained growth, the contribution of cardiac CT to radiation exposure of the general population is not negligible. Although there is no evidence that the level of radiation that is ordinarily applied at cardiac CT bestows any kind of risk, the current uncertainties regarding the biological effects of radiation, our commitment to the ALARA (As Low As Reasonably Achievable) principles, and increasing public radiation awareness behoove us to keep radiation dose in medical imaging to a minimum.
When entering into the discussion of radiation dose for cardiac CT, it is important to be cognizant of the protean nature of effective radiation dose values, which are rather indiscriminately reported in the literature. Depending on their ideological provenience, investigators use a variety of chest CT conversion factors [eg, 0.014 or 0.017 mSv/(mGy×cm)] for converting absolute dose length product (DLP) measures at cardiac CT into millisievert. This practice may not be appropriate for the specific scenario of cardiac CT (which is different from chest CT) and limits the comparison of different studies and scan acquisition protocols. The most reliable and independent parameters for determining radiation exposure at cardiac CT are the CT dose index and DLP. In many countries, diagnostic reference level values are established on the basis of the CT dose index and DLP for CT investigations of different areas of the body, which should not be exceeded by a CT scan. However, diagnostic reference level values are currently not available for cardiac CT.
Retrospective electrocardiogram (ECG) gating has been the traditional method of ECG synchronization with mechanical CT scanners, which comprises simultaneous acquisition of the ECG signal combined with constant x-ray emission during slow-pitch spiral/helical scan acquisitions.4–8 Because of this constant, slow-pitch application of radiation, which enables the acquisition of oversampled data sets that allow interrogation of the entire cardiac anatomy across the RR interval, cardiac CT has traditionally been considered radiation dose intensive, especially in light of the small volume covered (ie, only approximately 12 to 13 cm in the z axis for adult hearts). The relatively high radiation dose is the result of the particular requirements of this imaging test, in which, more than with other applications, diagnostic evaluation of the small and rapidly moving coronary arteries depends on the combination of high temporal resolution, spatial resolution, and signal-to-noise ratio.7 Image noise is mainly influenced by the number of x-ray photons reaching the detector, which, in turn, is a factor of object attenuation and tube voltage and is proportional to the slice width, tube current, and amount of time required to acquire the projection data needed for image reconstruction.9 For cardiac CT, the best possible temporal resolution is required to minimize the artifacts resulting from cardiac motion. This goal is achieved by using only projections gathered in a defined, short time window for image reconstruction, which, in turn, requires relatively high x-ray tube settings to ensure sufficient photon flux during this narrow acquisition window. High temporal resolution is ordinarily achieved by fast gantry rotation times8; however, in most of the available CT scanners, this necessitates the use of a slower pitch to avoid discontinuities in the anatomic coverage of the heart. A slow pitch, in turn, yields a higher radiation dose. Thus, the high temporal, spatial, and contrast resolution required for cardiac CT is obtained at the expense of an increase in radiation exposure. Increased awareness of these relationships has prompted the development of a variety of strategies aimed at improving the efficiency of ECG-synchronized acquisition techniques. With appropriate application of these techniques, a more efficient utilization of x-ray photons does not necessarily sacrifice the image quality of a cardiac CT study.10
This article intends to summarize the most important tools currently available to reduce the radiation dose from cardiac CT. Some of these have only recently become available with the ever-increasing sophistication of cardiac CT systems. However, many radiation protection strategies are surprisingly basic and are easy to implement, albeit often underutilized.
PATIENT SELECTION AND APPROPRIATE INDICATION
Certainly, the primary and most effective strategy to avoid unnecessary radiation exposure to the population is to exclude patients with inappropriate indications or those for whom cardiac CT will not likely affect management. Appropriate indications for a variety of clinical scenarios can be found in numerous national and international guidelines.1,3
Briefly, asymptomatic individuals with cardiovascular risk factors indicating an intermediate risk for cardiovascular events are potential candidates for undergoing low-dose coronary calcium screening scans for finer risk stratification and for determining the level of aggressiveness of risk factor modification. In individuals with a high risk for cardiovascular events, such as patients with diabetes, the role of coronary calcium screening for customizing the therapeutic regimen is still debated.
Contrast-enhanced coronary CT angiography is ordinarily considered appropriate in certain symptomatic patients, such as in the setting of a prior equivocal or nondiagnostic physiological test, in whom cardiac CT scanning is used for confirming or excluding coronary artery disease. According to current guidelines, cardiac CT is appropriate in patients with low or intermediate pretest probability of coronary artery disease who present with atypical angina, acute chest pain, chest pain syndrome, or suspected coronary anomalies. Conversely, patients with obvious clinical signs and symptoms of typical angina pectoris or acute myocardial infarction or those with known coronary artery disease and recurrent symptoms should not undergo cardiac CT, but rather should proceed immediately to invasive workup and therapy, if warranted.
Coronary CT angiography may be particularly helpful in women with ambiguous cardiac symptoms, in whom this test can safely rule out coronary artery disease as the source of their symptoms. In this population, the differential diagnosis of symptomatic chest pain is broader than in their male counterparts, so that noninvasive testing with CT holds definite advantages. However, especially in this population, the expected diagnostic benefit of the test must outweigh the theoretical risk of radiation exposure because of the inclusion of the radiosensitive breast tissue within the scan range.11
Most importantly, because of radiation concerns and principal considerations, coronary CT angiography is considered inappropriate as a screening tool in asymptomatic individuals with low and intermediate cardiac risk (class III, level of evidence C).12,13 Whether coronary CT angiography has incremental value for risk stratification, risk modification, and therapeutic monitoring in asymptomatic individuals at high risk is a topic of ongoing research.
SCAN COVERAGE OPTIMIZATION
The scan coverage is planned using the topogram or “scout” view. As the total radiation dose delivered is directly proportional to the scan coverage, precise collimation is important to minimize the DLP. With the improved capabilities of CT systems, there is a general tendency to increase the area of coverage, due to faster acquisition and lesser limitation of tube power. If the z axis of the scan is too long, for example, including the upper part of the abdomen or the neck, there will be unnecessary radiation delivered to the abdominal organs or the thyroid gland. However, if the scan length is too short, excluding a portion of the coronary artery tree, the examination would not be complete and repeat scanning would be required, resulting in increased radiation and contrast media volume to the patient. For dedicated coronary artery imaging (ie, excluding coronary artery bypass and “triple rule out” acute chest pain studies), scanning should begin at the level of the bifurcation of the trachea in most cases (Fig. 1), but the exact starting position is dependent on individual anatomy. For example, in thin patients with a small, vertically oriented heart, scanning may start lower than the carina without the risk of missing the proximal coronary artery tree. It is also important to be able to stop the scan during the acquisition once the entire cardiac anatomy is covered, so as to avoid unnecessary radiation to the upper abdomen. However, not all systems currently in use allow manual interruption of the CT acquisition. Adjustment of scan length, and thus optimization of the radiation dose, requires careful attention of the CT technologist. Scan length optimization solely based on the topogram may be difficult, as the position of the heart may vary with diaphragmatic motion, and some patients may have difficulty in keeping the same respiratory state for successive acquisitions. If a noncontrast calcium scoring scan is acquired preceding coronary CT angiography, as is the practice at many institutions, the landmarks obtained at calcium scoring can be advantageously used for more exact planning of the scan range of the contrast-enhanced study (Fig. 1).
Finally, image acquisition with multidetector-row CT systems is ordinarily associated with z axis overscanning to enable interpolation of data for image reconstruction. Thus, there is typically an overexposure beyond the boundaries of the volume to be imaged. With more recent CT systems, however, this latter issue is increasingly addressed with the implementation of sophisticated collimators, which aim at keeping overexposure outside the target anatomy to a minimum.
RETROSPECTIVE VERSUS PROSPECTIVE ECG SYNCHRONIZATION
The choice of the ECG synchronization method at cardiac CT should be informed by various factors, such as patients' heart rate or rhythm, and the clinical question. Until recently, the most frequently used scan technique for ECG synchronization at cardiac CT has been retrospective ECG gating (Fig. 2). The ECG signal is continuously recorded while a low-pitch spiral/helical CT scan with continuous emission of x-rays is acquired. Because the spiral/helical pitch used for this technique is very low compared with a standard CT examination (usually around 0.3), the radiation exposure may be up to 3 times higher than that of a standard CT examination of the same anatomic region. This scan technique is still commonly used in patients with an irregular heart rhythm or in whom the functional assessment of the myocardium or the valves is desired. However, the increased radiation burden with the use of this technique has to be individually justified by the additional diagnostic benefit of functional assessment, particularly if other radiation-free tests such as echocardiography or magnetic resonance imaging are available.
More recently, prospective ECG triggering (Fig. 3), also referred to as the “step-and-shoot mode,”14 which had been the standard acquisition technique of electron beam CT, has been reintroduced for the purpose of ECG synchronization at cardiac CT with mechanical CT systems. This scan mode eliminates redundant radiation that occurs from low-pitch cardiac spiral/helical scanning, as radiation is exclusively applied during a defined, predetermined phase of the cardiac cycle (typically during diastole with slower heart rates and during systole with faster heart rates), while the x-ray tube is completely shut off during the remainder of the cardiac cycle. With this technique, the effective radiation dose equivalent can be an order of magnitude lower than with traditional retrospective ECG gating. However, prospective ECG triggering may suffer from image degradation when more heartbeats are necessary for the acquisition of the entire scan volume. Accordingly, prospective ECG triggering will only provide adequate image quality if the detector of the CT system is wide enough to scan the entire heart within a limited number of cardiac cycles, as is the case with the newer 64-slice and higher CT systems. More importantly, the use of prospective ECG triggering has traditionally been restricted to patients with regular sinus rhythm, whereas this acquisition technique has been considered unsuitable for patients with arrhythmia, because the occurrence of the desired cardiac phase for image acquisition becomes increasingly unpredictable with varying length RR intervals. Accordingly, patients with arrhythmia have traditionally not benefited from this radiation protection strategy and had to be imaged with conventional retrospective ECG gating, which leaves the operator with more flexibility to choose the portion of the cardiac cycle with the least cardiac motion for image reconstruction.
There have been various technical attempts at improving the robustness of prospective ECG triggering vis-à-vis faster and more irregular heart rates. These include prolonging the acquisition interval during the RR cycle (ECG padding) to provide more flexibility in choosing the most suitable phase of image reconstruction (Fig. 4). Although ECG padding provides greater flexibility and improves the robustness to account for variations in heart rhythm, it also increases radiation exposure by widening the temporal window during which the x-ray tube is turned on. A more sophisticated method to account for heart rate variability is adaptive online monitoring of the ECG. This technique detects the occurrence of extra systoles and automatically rejects arrhythmic beats to ensure that image acquisition occurs only during the desired cardiac phase (Fig. 5). Finally, recent developments have successfully addressed the inability of obtaining functional information with prospective ECG triggering. Hybrid acquisition techniques (Fig. 6) use a short, prospectively ECG-triggered pulse of nominal tube output radiation to supply morphologic data flanked by phases of low-radiation image acquisition covering end-systole and end-diastole, which enables functional evaluation. However, again, as this involves exposure through a longer period of the cardiac cycle, this approach is associated with an increase in radiation compared with traditional prospective ECG triggering, albeit with dose levels that are still substantially lower than with conventional, retrospectively ECG-triggered techniques and ECG-dependent tube current modulation (see below).
HIGH-PITCH VERSUS BROAD DETECTOR COVERAGE
Dual-source CT (DSCT) was introduced in 2005 and provides particularly high temporal resolution for coronary CT angiography. The system uses 2 x-ray tubes and 2 detectors arranged at an angle of 90 degrees. Therefore, only one-quarter rotation of the gantry is necessary to acquire the x-ray data for one cross-sectional image. This effectively doubles the temporal resolution when compared with single-source CT at the same rotation speed. With the introduction of second-generation DSCT, the so-called “high-pitch single heartbeat acquisition” became feasible. In early reports, the ability to perform ECG-triggered spiral data acquisition using very high-pitch values (3.0 or more) has been described for DSCT.15,16 The high pitch allows image acquisition of the entire volumetric data set of the heart within a single cardiac cycle (Fig. 7). As the pitch is inversely related to radiation exposure, this scan mode is associated with approximately one tenth the exposure of a retrospectively gated spiral scan and half to one third the dose of a prospectively ECG-triggered scan. In combination with low tube voltage techniques (see below), effective radiation doses of 1 mSv or less can be routinely obtained (Fig. 8). When applied to the entire chest for the purpose of “triple-rule-out” acute chest pain studies (Fig. 9), the effective radiation dose equivalent still remains at below 2 mSv, depending on the x-ray tube setting. However, as the heart must remain motion-free for the entire scan time (approximately 270 ms), this mode has been predominantly applied in patients with heart rates below 65 beats per minute.16 More recent reports suggest the feasibility of using this technique also in patients with faster heart rates, with data acquisition during systole instead of the usual diastolic scan window. Similar to second-generation DSCT, broad detector 320-row CT scanners allow image acquisition of the entire heart within a single heartbeat. With this technology, the entire z axis of the heart is covered by a broad (ie, 16 cm) detector, with a recently published study demonstrating that low radiation dose single-heartbeat acquisition in patients with a heart rate <65 beats per minute is indeed feasible with 320-row CT.17
ECG-DEPENDENT TUBE CURRENT MODULATION
ECG-dependent tube current modulation is currently the most widely used technical tool to reduce radiation dose without sacrificing image quality at retrospectively ECG-gated acquisitions. Radiation may be reduced by up to 50% by the application of a simple principle: radiation dose is decreased during the phases of the cardiac cycle that are not anticipated to be useful for morphologic evaluation, and the full dose is delivered only during the cardiac phase that is expected to yield diagnostic results for morphologic evaluation18–20 (Fig. 10). Some CT manufacturers allow adjustment of the temporal window receiving full dose, which can be beneficial for optimizing the performance of ECG-dependent tube current modulation. For example, if the cardiac rhythm is regular, sinusoidal, and slow, there is a high probability that the most suitable phase for reconstruction will be found in diastole.10 In such cases, it is advisable to apply the full dose during end-diastole, representing a temporal window of only 20% of the full RR interval. The remaining 80% of the cardiac cycle receives reduced tube current. For faster heart rates, the most suitable phase for reconstruction becomes more difficult to predict and may vary from 30% to 80%. In this scenario, it is advisable to set the full dose period from 30% to 80%, which increases the likelihood of including the optimal reconstruction phase in the full-dose window, albeit with reduced radiation savings. Future developments should aim at providing operators with the ability to adjust the temporal amplitude of the modulation more flexibly across the RR cycle. For example, with faster heart rates, it may be desirable to have available both mid-diastolic and end-systolic images at full dose.
It is also important to remember that the effectiveness of ECG-dependent tube current modulation varies with heart rate: the slower the heart rate, the more efficient the modulation, because the full dose phase is proportionally shorter within the RR interval. Consequently, the use of rate-controlling agents for lowering the heart rate will lengthen the diastolic phase and thus enhance the effectiveness of ECG-dependent tube current modulation with lower incident radiation doses.
ECG-dependent dose modulation is activated prospectively, according to an automated analysis of the preceding heart beats. One drawback associated with the use of ECG-dependent dose modulation is that, similar to prospective ECG triggering, the exact occurrence of systolic and/or diastolic phases cannot be reliably predicted in patients with arrhythmia or premature ventricular contractions. In these cases, there is a risk of incurring image reconstructions during a low radiation dose phase, with consequent increases in image noise and loss of image quality within the z-extent of the data acquired during the arrhythmic beat. Thus, in cases of cardiac arrhythmia, the use of ECG-dependent tube current modulation should be used with caution. Similar automated online ECG-monitoring techniques for prospective ECG triggering, however, this limitation is increasingly addressed by the recent availability of sophisticated algorithms that automatically detect premature contraction of the heart and suspend ECG-dependent tube current modulation midscan in a real-time manner. Despite these current limitations, ECG-dependent tube current modulation has become one of the most successful and widely used techniques for radiation protection in clinical routine and is reported to be used in 82% of patients undergoing cardiac CT21 without impairment of image quality.
ANATOMICAL TUBE CURRENT MODULATION
Radiation dose can be substantially reduced by anatomy-adapted tube current modulation.22–24 The pixel noise in a CT image is largely attributable to projections in which the greatest attenuation occurs; therefore, the intensity of the radiation can be reduced in projections with less attenuation. This technique modulates the emitted radiation according to the patient's morphology in the x-y plane, along the z axis, or both and allows substantial reduction in radiation exposure. Modulation along the z axis uses attenuation values obtained from the topogram/scout view. However, this dose-sparing technique is ordinarily not fully compatible with the ECG-dependent tube current modulation described above, which takes priority in ECG-gated protocols. Accordingly, anatomical tube current modulation currently plays a limited role in cardiac CT compared with the more widespread application of this technique in general body CT imaging.
Another approach that has been described for general thoracic CT applications to save radiation, in particular for dose-sensitive organs such as the breast tissue, is the use of bismuth shielding. Although substantial reduction of direct radiation exposure can be achieved with this shielding, the artifacts that arise from such shielding would not be acceptable for any cardiac CT study; therefore, this approach currently has no role in cardiac CT. However, as an electronic alternative to physical shielding, some newer scanners allow reduction of the x-ray beam at the time point when the x-ray tube is within a certain angle above the patient. Effective reduction in radiation exposure is comparable with bismuth shielding without compromising image quality. Currently, such techniques are only available for chest CT but not yet for ECG-gated cardiac scans, although their implementation is expected in the very near future.
A simple, useful, and perhaps underestimated way to reduce radiation exposure involves lowering the tube voltage, the square of which is proportional to the radiation dose.25 Reduced tube voltage causes an increase in image noise26,27; thus, the use of low tube voltage protocols is recommended only in normal weight or underweight patients to maintain a diagnostic contrast-to-noise ratio.25,28 In addition, artifacts from metallic hardware (eg, pacemakers, defibrillators) and coronary artery stents are aggravated at lower tube voltages compared with standard tube settings, so that a low kilovoltage setting should be used with caution in such scenarios. When using ECG-controlled tube current modulation retrospectively, ECG-gated coronary CT angiography protocols with 64-slice CT systems are associated with effective doses of 10 to 12 mSv21 and 7 to 9 mSv with DSCT.29 By reducing the tube voltage to 100 kV, the dose from retrospectively ECG-gated coronary CT angiography can be lowered to approximately 5 to 7 mSv.25 Finally, a combination of low voltage techniques with prospective ECG triggering further lowers the effective dose to 1 to 3 mSv.30 It should be noted that the latter effective dose values are lower than those from standard single-phase chest and abdomen CT examinations and also lower than the levels of background radiation from natural sources that each individual is exposed to each year (ie, ca. 3.5 mSv).
EMERGING RADIATION PROTECTION STRATEGIES
Iterative reconstruction techniques were developed almost 2 decades ago and their advantages over traditional filtered back-projection reconstruction have long been recognized. However, only recently has the increasing performance of mainframe computers enabled their entry into clinical routine. Iterative reconstruction methods may substantially reduce image noise and thereby may also allow substantial reduction in radiation dose. It is foreseeable that the ongoing refinement of this and other, more novel, raw data reconstruction methods will increasingly replace current approaches and further reduce radiation exposure while maintaining (or improving) diagnostic image quality.
In the future, ever-expanding automation will increasingly rationalize the choice of appropriate image acquisition protocols and replace the subjective selection of scan parameters by the operator. For example, automatic scan range determination, automated tube voltage and scan mode selection, and radiation modulation techniques will be increasingly implemented with the goal of providing the optimal trade-off between diagnostic image quality and radiation dose.
Finally, we are currently witnessing the entry of cardiac CT into new territories, such as dual-energy imaging of the heart or perfusion imaging of the myocardium. These new techniques will require particular attention to radiation matters for their optimal application and acceptance in routine clinical scenarios.
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