MRI differentiates AMI from myocarditis

Nov 20, 2005

Myocarditis or acute myocardial infarction? That is the question for some clinicians when emergency coronary angiography is normal. In fact it can be hard to distinguish the two very different heart problems. But MRI can provide a definitive answer, according to researchers in Paris, who employed early- and late-perfusion MRI sequences in their noninvasive protocol, and found significant differences between the two conditions.

"Acute myocarditis may mimic acute myocardial infarction (AMI) when the patient has various combinations of chest pain, hemodynamic instability, ischemia-like ... ECG changes, biochemical marker (troponin I and T and/or creatinine kinase) changes, and segmental wall abnormalities at presentation," wrote Dr. Jean-Pierre Laisssy, Ph.D.; Dr. Fabien Hyafil; Dr. Laurent Feldman, Ph.D.; and colleagues from Hôpital Bichat in Paris.

The patient profiles differ -- patients with myocarditis tend to be younger and have fewer coronary risk factors than AMI patients -- but definitive diagnoses are hard to come by.

The gold standard, endomyocardial biopsy, can usually establish a diagnosis. But the invasive procedure has several drawbacks, the team wrote in the October 2005 issue of Radiology (Vol. 237:1, pp. 75-82).

For example, if the biopsy sample comes from a noninvolved area, the test can render a false-negative diagnosis. In addition, the invasive procedures can produce severe side effects, and the risk-to-benefit question can be difficult to resolve considering that myocarditis often resolves spontaneously. Finally, there are no specific histological criteria to predict the acute phase of chronic myocarditis with left ventricular dysfunction, and the biopsy is contraindicated in areas suspected of ischemia, the authors wrote.

In an effort to develop a definitive noninvasive test, the authors examined 55 patients with a clinical presentation suggestive but not typical of AMI, all of whom had undergone emergency coronary angiography that did not show evidence of acute coronary syndrome. At the final diagnosis, 31 patients had AMI and 24 had myocarditis.

MRI was performed on a 1.5-tesla scanner (TwinSpeed, GE Healthcare, Chalfont St. Giles, U.K.) equipped with high-performance gradients. Scout images were acquired, followed by first-pass perfusion (T1-weighted multishot gradient-echo echo-planar IR sequence with interleaved notched saturation; 6.6/1.3/240 repetition time msec, echo time msec, inversion time msec; flip angle of 25°), cine balanced steady-state free precession images (4.2/1.5 repetition time msec/echo time msec; flip angle of 60°, 7-8-mm section thickness). Five to eight 10-mm-thick short-axis sections were acquired every two heartbeats, according to the authors.

Following administration of gadolinium chelate (0.05 mmol of gadoterate meglumine per kilogram of body weight) before and again after the first-pass acquisition, delayed enhancement images were obtained with the patient in diastole using the same T1-weighted multishot gradient-echo echo-planar IR sequence. Additional T-1 weighted black-blood acquisitions were performed as needed.

Three experienced readers who were blinded to the clinical results worked independently and in consensus. They examined the MR images in continuous cine loops, segment by segment, to evaluate segmental left-ventricular first-pass perfusion contractile function abnormalities, noting the number and distribution of involved segments, their transmural extents, and the shapes of the highly enhancing areas.

The x² test was used to assess the locations of the abnormalities, and the Mann-Whitney U test was used to assess the numbers of involved segments, the authors wrote. Coronary angiography was the reference standard.

According to the results, the MR imaging patterns showed clear differences between the two cardiac disease groups (p < 0.05).

"All the patients with AMI had a segmental early subendocardial defect, with corresponding segmental subendocardial or transmural delayed high enhancement in a predominantly anteroseptal or inferior vascular distribution in 28 patients," the team wrote. In addition, all of the AMI patients had stenosis of at least the infracted coronary artery. "All but one of the patients with myocarditis had no early defect and focal or diffuse nonsegmental nonsubendocardial delayed enhancement predominantly in an inferolateral location."

Univariate analyses showed that involvement of myocardial segments 5, 7, and 11 was the most significant parameter for differentiating between the two diseases (p < 0.05), whereas estimating equation analyses found this correlation to be nonsignificant (p = 0.06).

Involvement of the left circumflex artery area was the most discriminant parameter (p < 0.001) when vascular areas were considered. A generalized estimating equation analysis showed a significantly different repartition of the involved segments within the three vascular areas (p = 0.002).

In delayed enhancement images, involved myocardial regions showed a highly enhancing band in all AMI patients, while 83% of myocarditis patients had highly enhancing nodules.

"Finally, 31 (100%), 12 (39%), and 31 (100%) patients with AMI had, respectively, first-pass segmental hypoperfusion, delayed subendocardial high enhancement in a segmental vascular distribution, or a highly enhancing band pattern at presentation," the authors wrote. "Conversely, 23 (96%), 19 (79%), and 20 (83%) patients with myocarditis had, respectively, normal first-pass perfusion imaging findings, nonsubendocardial delayed high enhancement, or highly enhancing nodules at presentation."

Delayed enhancement imaging has been found to be an important method of evaluating cardiac viability in myocardial ischemia, the authors stated. The results of the present study show that several MR patterns, both on first-pass perfusion and delayed-enhancement images, can help distinguish acute myocarditis from AMI.

"Myocarditis is characterized by nodular delayed enhancement in a diffuse, predominantly inferolateral subepicardial location in nonvascular territories; AMI is associated with early subendocardial perfusion defects and subendocardial or transmural delayed enhancement of a smaller number of segments, all in a vascular distribution," they reported.

The patients with myocarditis were younger and had fewer cardiovascular risk factors.

As for limitations, the study had a disproportionately high prevalence of myocarditis. In addition, because there was no reference standard for diagnosing myocarditis, its final diagnosis was based on the discordance between increased troponin levels and negative results on extensive coronary workup.

Owing to the results, MRI should gain an important role in the differential diagnosis of myocarditis or AMI, particularly when angiography results are normal, the authors wrote.

By Eric Barnes
AuntMinnie.com staff writer
November 21, 2005

Study quantifies prevalence of noncardiac findings on cardiac MR, January 25, 2005

Myocardial perfusion measured by MRI useful in detecting heart disease, August 4, 2003