In this issue of Academic Radiology , Hoffmann et al ( ) demonstrated a potential for computed tomographic angiography (CTA) to triage patients without catheter coronary angiography. In this study, 94 patients were included in the analysis to evaluate CT angiography as compared to catheter coronary angiography. This study excluded 24% of patients who did not have complete datasets due to artifacts in at least one major segment (AHA 15 segment model). However, the use of 16 slices in this study may partially explain the image quality issues seen in this study. Several studies with 64 slice CT have demonstrated improved image quality and significantly higher numbers of evaluable segments. A meta-analysis of 16 and 64 slice CT demonstrates improved overall image quality with 64 slice CT. ( ) Nonetheless, even with current 64 slice systems, 5-7% of segments are non-evaluable, so some patients would still be excluded from such an analysis as performed by Hoffman et al. Primary reasons for exclusion on cardiac CT remains dense coronary calcification, motion artifacts and image acquisition artifacts.
In the overall assessment of whether CTA could triage patients to revascularization or no revascularization as well as invasive catheterization (CATH), there was excellent agreement. In this study, there was 93% agreement between catheterization (CATH) and multi-slice computed tomography (MSCT) (87 of 94). Both tests also agreed 100% in the exclusion of coronary artery disease. Agreement between CATH and MSCT was 90% for the decision between PCI and CABG (36 of 40). MSCT correctly diagnosed 4 stenoses in the left main and did not miss any left main stenosis. While MSCT and CATH had good overall agreement, even a 7% discrepancy would leave most cardiologists uncomfortable with this decision tree. Larger trials with 64 slice CT should be performed to see if the improved image quality leads to even higher agreement.
A major significant difference between the cardiac surgeon and the inverventional cardiologist involved the type of invasive therapy recommended. There was only 52% agreement with regard to the type of revascularization, while guideline-based therapeutic decisions on the basis of MSCT and CATH agreed in 90% of cases ( ).
The decisions were quite divergent, with the cardiologist choosing percutaneous coronary intervention (PCI) over coronary artery bypass grafting (CABG) (78% of the time) as compared with the cardiac surgeon (recommended CABG 62% of the time). If the patient undergoes coronary angiography and the interventional cardiologist feels this person is appropriate for PCI, often this is done at the same time. Whether the primary choice of PCI or CABG represents best practice patterns or physician (or patient) preference is difficult to assess, and largely divergent at different centers. Many sites do present difficult cases at joint catheterization conferences, where both surgeons and interventional cardiologists can discuss patient management. Thus, the idea that a non-invasive angiogram can reveal the coronary anatomy with sufficient accuracy is appealing. In these cases, the patient can be presented to both specialists, and the best decision concerning revascularization can be determined. Since there was 100% agreement in the triage of patients not to revascularize, the study will most likely have good clinical correlation and outcomes compared directly with invasive coronary angiography. This study, in concordance with prior studies, demonstrates a very high negative predictive power for the presence of obstructive coronary artery disease (CAD) (100% in this study). Thus, the strength of CT angiography remains in its ability to rule out disease (negative tests), so that further evaluation (including stress testing, functional tests and angiograms) can be safely avoided. This remains a primary use of CTA in recently published Appropriateness Criteria and American Heart Association Guidelines ( ). Other studies have demonstrated this high negative predictive power, giving clinicians confidence that a negative CTA (no obstructive disease seen), will result in a normal or near normal CATH. Two outcome studies have demonstrated 100% short term event-free survival after a normal or near normal CTA, so safety concerns are somewhat alleviated that high risk anatomy is being missed ( ).
However, one of the cardinal limitations of CT angiography, even with 64 slice CT, is the spatial resolution. CTA uses a 512×512 matrix for evaluation, while new CATH labs have better spatial resolution (1024×1024 resolution) ( ). Furthermore, slice thickness (currently limited to 0.5 mm) will not allow for accurate assessment of 1 mm vessels (which may be good targets of revascularization with bypass surgery). Newer scanners (dual detector systems or 256 detector scanners) do not improve the spatial resolution or slice thickness of CT angiography, so they will probably not address this current limitation of CTA.
The authors concluded “MSCT could be established as a diagnostic tool to replace catheter angiography in specific patient groups and to thus reduce the incidence of catheter-related complications. The results presented here suggest that MSCT would allow reliable triage of patients with suspected coronary artery disease.” This may prove to be the case when image quality is good or excellent, but this was a minority of cases in this study. Furthermore, in cases triaged to PCI, they would be undergoing coronary angiography during the PCI procedure regardless. Thus, the persons who would safely be able to avoid coronary angiography would be those with non-obstructive disease (true negatives) and those triaged to CABG. Further multicenter trials will need to demonstrate that the algorithm of patient triage based solely on CT angiography, is both safe and effective to confirm these preliminary findings.
References
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