As Cury explained to heartwire, all imaging modalities that have been used to triage possible ACS patients in the emergency setting—MRI as well as cardiac CT and nuclear perfusion imaging—have struggled to distinguish acute from prior MI. For their study, Cury et al were interested specifically in patients with chest pain, negative cardiac biomarkers, and no ECG changes indicative of acute ischemia.
The patients in this study were all patients who had been in the emergency department for at least 12 hours waiting for changes in cardiac enzymes and waiting for a stress test," he said. "So by doing MRI early on, we were able to detect myocardial injury before the rise of cardiac enzymes. There were some patients who had increased cardiac enzymes at a later stage, but we were able to detect both edema and necrosis before the increase in cardiac enzymes."
MR protocol includes information on edema, LV wall thinning
In their protocol, Cury et al used T2-weighted imaging to look for myocardial edema and assessment of left ventricular wall thickness to check for myocardial thinning, in addition to first pass perfusion, cine function, and delayed-enhancement MRI. "The beauty of this is the combination of delayed enhancement looking at myocardial necrosis and T2-weighted imaging looking at myocardial edema," Cury explained. "By combining this information, we can differentiate patients with acute vs old MI and which patients had unstable angina vs non-STEMI. Unstable angina is very difficult for different modalities to detect, and we were able to detect it in the great majority of cases," Cury said.
The entire cardiac MR protocol, on average, took just over 30 minutes per patient. A diagnosis of ACS was subsequently confirmed by chart review.
Out of 62 patients presenting with possible ACS symptoms, the new cardiac MR protocol increased the specificity, positive predictive value, and overall accuracy, as compared with the conventional cardiovascular magnetic resonance protocol (which did not include T2-weighted imaging or LV-wall assessment). Cury et al's protocol also significantly improved on information derived from clinical risk assessment alone or from clinical risk assessment combined with traditional cardiac risk factors.
Diagnostic accuracy of new protocol, vs standard
Diagnostic accuracy
Standard MR protocol* (%)
Standard MR protocol plus T2-weighted and LV wall thickness (%)
Sensitivity
85
85
Specificity
84
96
Positive predictive value
58
85
Negative predictive value
95
96
Accuracy
84
93
*Cine wall motion, perfusion, and delayed-enhancement MRI
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"At this point, just using initial clinical risk assessment based on ECG and cardiac enzymes, we are not doing a great job, and that's why many physicians are just admitting patients for catheterization, or, in some cases, 2% to 4% of patients are discharged when they actually have ACS," he explained. "So we think this is really impressive: MRI remained a significant predictor above and beyond traditional cardiac risk factors and clinical risk assessment. Specifically, if you have a positive MRI, you have 120 times higher likelihood of having ACS as compared with patients with normal MR, after adjusting for traditional cardiac risk factors and clinical risk assessment."
How MR might fit in
So how would cardiac MR fit into emergency-room triage? Cury told heartwire that if a patient had a positive stress test, he or she would go directly to the cath lab. If not, the patient could undergo MRI and if this turned up myocardial edema or necrosis or a regional wall-motion abnormality, they, too, could go directly to the cath lab. But patients in whom there is no evidence of myocardial injury could likely be sent home or undergo later stress testing, although Cury emphasized that this proposed strategy would require further, prospective testing.
Cury also clarified that MR might carve out its own niche distinct from cardiac CT, which has increasingly been shown to play an important role in emergency-room triage.
"CT will, I think, be the first-line test in the acute chest pain setting for patients with low likelihood of having ACS, whereas MRI, I believe, will be used more in patients with intermediate likelihood of ACS," he said. "In the future, I can see the emergency physicians stratifying the patient as low, intermediate, or high risk based on risk factors and type of chest pain, etc, and patients with low risk would probably go to CT, whereas in patients with intermediate risk or even high risk but no diagnostic ECG and negative enzymes, MRI would be a better test."
At present, Cury acknowledged, most hospital emergency departments would not have easy access to MR, but down the road, newer acute chest pain and stroke centers might consider including dedicated MR machines.
Cury disclosed having no conflicts of interest.
Source
1. Cury RC, Shash K, Nagurney JT, et al. Cardiac magnetic resonance with T2-weighted imaging improves detection of patients with acute coronary syndrome in the emergency department. Circulation 2008; DOI: 10.1161/CIRCULATIONAHA.107.740597. Available at: http://circ.ahajournals.org.
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