Our main findings are summarized as follows. First, AAR by T2 weighted MRI correlates well with angiographic measures of myocardial jeopardy such as the APPROACH lesion score. Second, measurement of AAR and IS in the early post-infarct period enabled estimation of myocardium salvaged in all patients, regardless of presentation type or past history of MI. Third, in a multivariable analysis, IS was predicted by MRI-derived AAR and also by the sum of coronary and collateral artery flow grades at initial angiography.
Determination of initial AAR and myocardial salvage has several applications for clinical and research purposes; however, measurement of these variables has limited feasibility (3
). While AAR measurement is possible following technetium perfusion tracer injection during coronary occlusion (21
), this approach has been difficult to implement in both clinical practice and large multicenter studies. MRI may now enable AAR delineation and estimation of salvage. First, recent technical advances have overcome problems due to signal drop-off (4
) and cardiorespiratory motion (22
). Second, AAR measurements by T2 weighted MRI has been validated in reperfused (7
) and non-reperfused (23
) MI. Third, since MVO and intra-myocardial hemorrhage may cause AAR to be underestimated by MRI (24
), our approach to image analyses was designed to minimize this problem. Fourth, our study was prospectively performed in a broad range of patients with acute MI. Previous studies have had several exclusion criteria (17
), such as symptoms > 24 h from PCI, history of prior MI, signs of clinical instability (9
), or persistent TIMI flow at angiography. In order to enhance the applicability of our findings to clinical practice, we included all MI patients who would consent to MRI regardless of presentation type or success of reperfusion. Our results indicate that T2 weighted MRI enables AAR estimation even in patients with a second MI.
In line with our first hypothesis, we found that AAR estimated by T2 weighted MRI was a predictor of the APPROACH Lesion Score, which is an anatomical and prognostically validated measure of the extent of myocardial jeopardy. Additionally, AAR was a multivariable predictor of infarct size.
Correlations and limits of agreement between T2-weighted MRI and the angiographic estimates of AAR leave questions regarding which answer is correct. The Bland Altman analysis suggests either T2-weighted MRI overestimates AAR or that the APPROACH Lesion Score AAR underestimates AAR or some combination of these two factors. However, the relationships between IS derived by MRI with AAR derived by MRI, the APPROACH Lesion Score, the DUKE Jeopardy Score and the ST elevation jeopardy score () provide an independent metric to help resolve these possibilities. As infarct size is physiologically a subset of the AAR, the expected relationship between AAR and infarct size involves all points at or below the line of identity. Of all metrics analyzed, T2-weighted MRI had the fewest underestimations of AAR (1 underestimate), APPROACH was almost as good on this metric (5 underestimates) while the Duke Jeopardy Score and the ST Elevation Score underestimated AAR in 10 and 19 cases, respectively. While these data cannot unequivocally resolve which method provides the best measurement of AAR, all analyses indicate that T2-weighted MRI provides a good measure of AAR.
Consistent with previous studies in which salvage has been measured by nuclear perfusion imaging (25
) and also with what might be expected from a biological perspective, coronary flow grades at baseline and post-procedure were predictors of salvage derived by MRI. Furthermore, given the pathophysiologic importance of no-reflow (26
), which is also a determinant of IS (27
), we also found that AAR and IS were larger in patients with MVO compared to in those with no evidence of microvascular injury. These results were also observed in analyses restricted to STEMI patients.
We confirmed the recent observations by Ortiz-Pérez et al
) who demonstrated that IS estimated by MRI closely matched angiographic estimates of jeopardy in a group of patients undergoing primary angioplasty for a first MI. Our results extend this analysis since AAR derived by T2 weighted MRI represents all of the ischemic territory, including viable and infarcted myocardium, permitting estimation of myocardial salvage. Our results extend those of Carlsson et al
) and O'Regan et al
) who used black blood T2 weighted MRI to estimate myocardial salvage in MI patients. This method may be less applicable to clinical practice since it has lower diagnostic accuracy than bright blood T2-weighted SSFP (4
Confirming our second hypothesis, T2 weighted MRI enabled delineation of the acute infarct territory in patients with prior MI. This is consistent with the observations of Abdel-Aty et al
) who found that T2 weighted black blood MRI combined with delayed enhancement permitted discrimination of acute versus chronic MI with a specificity of 96% in 57 infarct zones when evaluated by 2 blinded observers. Prior MI was also a predictor of myocardial salvage. One explanation for this result may be due to a pre-conditioning effect from chronic myocardial ischemia, or enhanced coronary flow grades related to collateral artery supply (29
). Several other observations merit comment. We found that coronary flow grade at initial angiography, represented by the sum of the TIMI and Rentrop collateral flow grades, was a negative multivariable predictor of IS, which is consistent with previous observations (17
). Our study extends these earlier findings since coronary flow grades pre- and post-procedure were also predictors of myocardial salvage derived by MRI. Our results add to the role of MRI in post-infarct imaging, where the utility of MRI to discriminate acute from chronic MI (9
), and depict adverse characteristics, such as LV remodeling (30
), has already been established. To our knowledge, T2-weighted SSFP methods are being developed by several MRI vendors.
We also found that there was a fairly wide scatter for AAR estimates using the ECG or DUKE angiographic risk score compared to by the APPROACH lesion score or contrast-enhanced MRI (). While all of these variables correlated with AAR estimated by T2-weighted MRI, the magnitude of the differences in AAR estimates between the variables (agreement) varied and appeared best for contrast-enhanced MRI and least good for the ECG.