In 2010, a consensus document on appropriate use criteria for CCTA was created jointly by the American College of Cardiology Foundation, the Appropriate Use Criteria Task Force, the Society of Cardiovascular Computed Tomography, the American College of Radiology, the American Heart Association, the American Society of Echocardiography, the American Society of Nuclear Cardiology, the North American Society for Cardiovascular Imaging, the Society for Cardiovascular Angiography and Interventions, and the Society for Cardiovascular Magnetic Resonance.32 In this document, the appropriateness of CCTA was assessed for a number of clinical indications, with each indication classified as appropriate, inappropriate, or uncertain. CCTA was classified as appropriate in the emergent setting of acute symptoms with suspicion of ACS in a low-risk to intermediate-risk patient [defined by a <10% (low) or 10% to 20% (intermediate) 10-y absolute coronary heart disease risk].33
Four new category I Current Procedural Terminology codes were introduced in 2010 to report cardiac CT and CCTA examinations. Of the 4, 2 can be used in the setting of acute chest pain. Calcium CT (75571) and CCTA (75574) together add to the noninvasive diagnostic algorithm of acute chest pain workup.
In 2012, the ACR published appropriateness criteria in the clinical scenario of a patient with acute nonspecific chest pain at low probability of CAD and determined that CCTA is usually appropriate (rating: 7) in that specific clinical setting.34 In comparison, MPI and ICA were rated “may be appropriate” (rating: 6) and “usually not appropriate” (rating: 1), respectively. However, CCTA has a lower rating than both MPI and ICA when the suspicion for ACS is high.
A consensus must be reached among emergency physicians, cardiologists, and radiologists regarding the evaluation of acute chest pain patients to mitigate overuse of CCTA in the ED. Developing a clear algorithm with well-defined clinical categories (negative, low risk, medium risk, high risk) that corresponds to imaging findings would help optimize patient management. For instance, we consider any patient with coronary stenosis >70% (>50% in the left main coronary artery) to be at high risk and requiring hospital admission (Fig. 3). Having a CT scanner capable of high-quality cardiac scans in close proximity to the ED allows for close patient monitoring and throughput. These patients often receive nitroglycerin and/or β-blocker, requiring the education and vigilance of the staff in potential drug complications (eg, hypotension, interaction of nitroglycerin with phosphodiesterase inhibitor). Diagnostic algorithms should also take into consideration a patient’s pretest probability of CAD. Otherwise healthy 25-year-old patients will have an extremely low pretest probability, and thus CCTA should be avoided in these patients, as it is extremely unlikely to provide any new useful information. Likewise, elderly patients with multiple risk factors for CAD, such as long-standing diabetes, should also be avoided. These patients are very likely to have extensive coronary artery calcifications, and it is doubtful that CCTA further clarifies the cause of their acute symptoms.
Patients whose CCTA studies demonstrate normal coronary arteries or nonsignificant stenosis (generally <50%) can be safely discharged from the ED. Cardiology consultation in an outpatient setting should be considered for patients with mild coronary stenosis. When moderate stenosis (50% to 70%) is found, the patient is admitted for observation and often undergoes further evaluation with MPI or diagnostic catheterization.
Although careful attention is paid to coronary assessment, a number of noncoronary pathologies can present in acutely symptomatic patients. Other causes of chest pain include pneumonia, PE, rib fracture, pneumothorax, aortic dissection, and pericardial disease. Therefore, reconstructions with full field of view to include the lung fields are often obtained to assess for causes of chest pain other than ACS.
A variety of MDCT systems are tailored for coronary imaging. Wide-detector scanners include the 256-slice MDCT (Brilliance iCT; Philips Medical Systems) and 320-slice MDCT (Aquilion One; Toshiba Medical Systems), which cover 8 and 16 cm, respectively, thereby reducing misregistration or “slab” artifact. The 320-slice MDCT has a better spatial resolution at 0.5 mm and covers the whole heart, whereas the 256-slice scanner uses double Z-sampling and offers improved temporal resolution (0.27 s rotation time). Several dual-source 2×128-slice scanners (SOMATOM Definition Flash and Force, Siemens Medical System, GE Revolution, and other new scanners utilizing spectral CT currently in development) offer dual-energy acquisition in addition to improved temporal resolution (generally <0.28 s rotation time). The selection of MDCT therefore depends on institutional and imager preference. It should be noted that, although many advanced scanners and techniques are available to optimize CCTA, diagnostic studies generally can be obtained with using conventional scanners (64 slices and higher) located in close proximity to the ED.
Two scanning protocols are available in the ED setting: dedicated CCTA and a whole chest scan called “triple rule-out.” The triple rule-out protocol provides a global view of the major vasculature, enabling assessment for CAD, PE, and aortic dissection. This is achieved by using a single acquisition with a biphasic contrast injection (resulting in higher contrast volume) including full concentration intravenous contrast administered in the first phase, followed by a second phase of dilute contrast (50:50 with saline).35 With this protocol, the pulmonary vasculature, aorta, and coronary arteries are each adequately opacified during a single scan acquisition. The longitudinal extent of the scan is widened to that of a conventional chest CT, which results in a longer scan compared with dedicated CCTA. Studies have shown no significant difference in the image quality of the coronary arteries between the 2 protocols.35,36 One study showed that noncoronary reasons for chest pain were diagnosed in 11% of ED patients at low to moderate risk for ACS using a triple rule-out study.37 Use of a triple rule-out scan may be useful when the clinical signs and symptoms of PE and ACS overlap, presenting the clinician with similar pretest probabilities for both. Madder and colleagues found a poor diagnostic yield of triple rule-out for PE and aortic dissection; however, Schertler and colleagues suggested that utilizing an evidence-based method for developing a pretest probability for PE that includes Wells criteria and D-dimer improved the diagnostic yield of the triple rule-out. In cases in which both PE and ACS are considered equally likely, and with appropriate patient selection, the triple rule-out study may be superior to a CCTA given the improvement in the detection of PE.38,39
The increasing use of CCTA has created considerable concern over the potential for radiation-induced malignancies due to medical imaging. To that end, radiation dose should be carefully monitored. Historically, CCTA studies have an average dose of 12 mSv.37,40 As the radiation dose is proportional to the square of the tube voltage, we routinely obtain diagnostic scans at a tube potential setting of 100 kV in patients with a body mass index of <40 kg/m2. When the body mass index is >40 kg/m2, an increased tube voltage of 120 kV is needed to preserve image quality. More recent advances have led to drastic reductions in radiation dose. Under ideal conditions and appropriate technique, submilliSievert scans are possible.41 The correct choice of scanning strategy is also important for limiting dosage. Approximately 80% dose reduction can be achieved by using prospective triggering rather than retrospective gating.28,42 In our experience, diagnostic triple rule-out scans can be achieved at an effective dose of 1.4 mSv by utilizing a high-pitch protocol.43 Nonetheless, use of a dedicated CCTA instead of a “triple rule-out” scan can also reduce the dose by a significant amount.44
Filtered back projection (FBP) was the obligatory algorithm for image reconstruction for many years, because processing algorithms were bottlenecked by inadequate computational power. More recently, computationally intense algorithms have been devised called iterative reconstruction. Iterative reconstruction utilizes a more advanced geometric model than filtered back projection and can provide similar image quality with up to 44% reduction in radiation dose (Fig. 4).45,46
In addition to luminal assessment, CCTA is capable of characterizing plaques (ie, fatty, fibrous, or calcified) and identifying high-risk lesions. Motoyama et al47 found that plaques that demonstrated positive remodeling and low-attenuation (<30 HU) on CCTA were associated with a higher risk of subsequent development of ACS compared with those that did not. Madder et al48 showed good sensitivity (53% to 81%) and specificity (82% to 95%) using CCTA to identify evidence of plaque disruption (eg, plaque ulceration and intraplaque dye penetration) and also reported that these disrupted lesions were larger, more likely to be positively remodeled, and contained more low-attenuation plaque (<50 HU). In practice, we view noncalcified plaque and positive remodeling as a concerning finding that we report routinely.
FFR is a technique used during ICA to gauge the hemodynamic significance of specific coronary plaques by measuring pressure differences across a stenosis. When coronary angiographic findings are equivocal for significant stenoses, FFR is considered the gold standard for determining the need for treatment. The mean pressure in the coronary artery distal to the stenosis is divided by the aortic pressure during maximal vasodilation. FFR values of 0.8 to 1.0 are highly accurate for the absence of ischemia, whereas FFR<0.75 is associated with positive ischemic results that can benefit from revascularization (values between 0.75 and 0.8 represent indeterminate FFR results).49,50 Its analogue in CCTA (denoted FFRCT) attempts to provide a noninvasive calculation of FFRCT by applying computational fluid dynamics. The addition of FFRCT criteria (FFRCT≤80%) to CT criteria (stenosis ≥50%) was shown to improve detection of hemodynamically significant lesions as compared with CT criteria alone in the multicenter Determination of Fractional flow reserve by Anatomic Computed Tomographic Angiography (DeFACTO) trial.51 Further studies are necessary to determine whether routine application of FFRCT improves patient outcome as applied to the ED setting.
The potential for assessing myocardial perfusion on CT has generated much interest. Resting CT MPI cannot reliably exclude ACS, as reported in 1 study in which CTP detected only 3 of 9 patients with ACS.52 Adenosine stress CTP has showed more promise. Rochitte et al53 demonstrated that the combination of CTA and adenosine stress CTP (using a 320-slice scanner capable of volumetric scans) permitted accurate diagnosis of patients with hemodynamically significant CAD, using ICA and MPI as reference standards. It remains unclear whether CCTA/CTP has an incremental value over CCTA in the setting of low-risk to intermediate-risk patients with acute chest pain in the ED setting.
Substantial evidence in the form of multiple large randomized controlled trials has validated the utility of CCTA to exclude ACS in low-risk to intermediate-risk ED patients. Imaging specialists are advised to work with ED physicians and cardiologists before implementation of a CT-based program in the ED. The balance between patient safety and diagnostic utility of CT must be evaluated in each clinical scenario to optimize patient care.
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