Review of new advances in cardiac imaging in 2012

    There were many important publications in cardiac imaging in 2012, with a focus on cardiovascular magnetic resonance imaging (CMR) and cardiac computed tomography (CT). This article focuses on the important research in cardiac tomography, with an emphasis on new evidence supporting clinical practice, such as stress perfusion imaging and other new methods that can advance clinical practice.

　　1. Perfusion Imaging

　　Arguably, the research findings that had the greatest impact on cardiac imaging in 2012 did not come from imaging papers, but from the FAMEⅡ trial. Although invasive pressure wire assessments have been used to diagnose myocardial ischemia and can improve the prognosis of patients undergoing revascularization, some non-invasive functional ischemia tests can also produce the same results. The FAMEⅡ trial survey found that there is increasing evidence that the presence of myocardial ischemia is the basis for determining whether revascularization is needed, rather than the imaging findings of angiography. This will inevitably increase the demand for imaging modalities that can reliably show myocardial ischemia among clinicians. Although the choice of functional ischemia tests is still limited by local medical equipment and expertise, the supporting evidence for each imaging modality that can detect functional ischemia is increasing.

　　2. Cardiovascular Magnetic Resonance

　　The two largest clinical studies to date compared cardiovascular magnetic resonance (CMR) and single-photon emission computed tomography (SPECT) with invasive coronary angiography (CA) as the reference standard and published their results in 2012. CE-MARC enrolled 752 patients with suspected angina at a single-UK center. CMR testing used a multiparameter protocol that included adenosine stress myocardial perfusion, delayed gadolinium enhancement, and whole-heart coronary angiography. SPECT was performed with cardiac gating and a standard 2-day stress/rest mode using 99mTc tetrofosmin. The other study, MR-IMPACT II, ​​was a multicenter study that included 533 patients at 33 centers in Europe and the United States. The centers used a CMR protocol similar to the above, but without whole-heart coronary angiography. SPECT was performed in different formats according to the clinical routine of each institution, including gated, nongated, and adenosine/physical stress protocols. Both clinical studies have shown that the sensitivity of cardiovascular magnetic resonance imaging is significantly higher than that of SPECT, but it does not show a clear advantage in specificity (in MR-IMPACTⅡ, the specificity of CMR was significantly lower than that of SPECT). After combining the results of these trials, it was found that CMR can be used as a safe alternative to SPECT, which can avoid the radiation exposure of SPECT. Jogiya et al. conducted a meta-analysis of PET, CMR, and SPECT, and the results of the analysis further supported the above findings.

Results of the CE-MARC and MR-IMPACT clinical trials

　　Jogiya et al published the results of three-dimensional (3D) perfusion imaging with MRI at 3.0 T, which suggests that the application of stress CMR has a promising future. This study compared 3D CMR sequences with fractional flow reserve assessed by pressure wire in 53 patients after angiography. The sensitivity, specificity, and diagnostic accuracy of CMR were 91%, 90%, and 91%, respectively, and for coronary arteries, 79%, 92%, and 88%, respectively. As new technologies and other new areas of advances are applied to clinical practice, the accuracy of diagnosis can be further improved.

　　3. Computed tomography

　　The current multi-slice spiral CT device has high spatial and temporal resolution and has formed a powerful clinical tool-CTCA, which can evaluate the coronary arteries non-invasively. Recently, some people have advocated that CT can be used as an alternative means to evaluate myocardial perfusion status: Dr. Lima released the preliminary results of the CORE-320 study at the 2012 ESC meeting: a total of 381 patients participated in the trial. The researchers combined CT coronary angiography (CTA) with another CT perfusion (CTP) sequence and compared it with the standard model of invasive CA and SPECT MPI. CTP significantly increased the accuracy of CTA in diagnosing blood flow-restricted coronary artery disease (AUC: 0.87 vs. 0.81, P < 0.001), and the radiation dose was lower than that of invasive angiography and MPI.

　　An alternative method for assessing functional coronary artery stenosis is the CT-based fractional flow reserve (FFRCT) measurement. This new method uses computational fluid dynamics to calculate the pressure gradient across coronary artery lesions during standard CT coronary angiography. In the multicenter DeFACTO study, the authors compared the results of FFRCT with the gold standard invasive FFR, and the results showed that the combination of FFRCT and CTA can improve the accuracy and discrimination of diagnosing functional stenosis compared with CTA alone (AUC: 0.81 vs. 0.68, P, 0.001). Although the results of this trial did not meet the pre-specified primary endpoint, it also confirmed that this CT method can replace invasive examinations for direct assessment of myocardial perfusion.

　　The evidence base for the use of cardiac CT in the emergency department has also been further developed, driven by the release of results from two large clinical trials, ACRIN-PA and ROMICAT II. These studies enrolled a total of 2370 low-risk patients, some of whom were likely to be admitted to emergency departments in the United States for acute coronary syndrome. They also compared traditional care with a new clinical protocol that included early CT. The results showed a significant reduction in hospitalizations and a significant increase in emergency department discharges in the group that underwent CT, without any adverse clinical outcomes. Although these results are promising, it remains to be seen whether they will be replicated effectively in Europe, where treatment pathways are different.

Results of ACRIN-PA and ROMICATII clinical trials

　　4. Cardiac Nuclear Medicine

　　The major advance in this area is the introduction of a new generation of gamma cameras that use cadmium zinc telluride (CZT) detector technology that improves sensitivity and spatial resolution compared to conventional systems. These new detectors improve image quality, imaging speed, and radiation dose reduction. Several studies have been conducted to investigate these advantages. Two studies have demonstrated important advances in low-dose stress imaging: Duval et al studied 71 patients weighing less than 200 pounds, while Gimelli et al studied 148 obese patients. Both groups were able to maintain good sensitivity and specificity while reducing radiation dose by more than 50% compared to standard examinations.

Comparison of low-dose CZT-SPECT results between obese and non-obese patients

　　In nuclear cardiology, a new area of ​​research may be the combination with CT to provide both functional and anatomical information to further guide clinical care. Fiechter et al investigated the efficacy and impact of combining a CZT gamma camera with a 64-slice CT scanner on clinical practice. Patients were classified as having matched malperfusion if they had malperfusion consistent with coronary artery stenosis, with 62 patients having matched malperfusion and 8% not having matched malperfusion. Revascularization was required in 91% (21 patients) of the group with perfusion defects matched to coronary stenosis and in the group without matched malperfusion, respectively. Although the authors emphasize the potential for selection bias, the results of this study are an important step toward large randomized studies of hybrid imaging.

　　Conclusions: 2012 saw the publication of several important studies that have enhanced the role of functional imaging in ischemic heart disease. The CE-MARC and MR-IMPACT II studies suggest that stress perfusion CMR is playing an increasing role in the clinic. Similarly, the ACRIN-PA and ROMICAT II studies suggest that coronary CT is playing an increasing role in the emergency department. In addition, there have been significant advances in the study of new clinical modalities such as hybrid cardiac imaging and perfusion CT.