Cardiac magnetic resonance imaging: the new reference method for infarct characterization and prognostication after myocardial infarction?

Nuclear cardiology laboratories provide diagnostic services as part of standard care in many hospitals worldwide. In 2008, a world-wide survey of nuclear cardiology procedures was reported by the Working Group on Nuclear Cardiology for the International Atomic Energy Agency. High nuclear cardiology utilization (.1000 procedures/100 000 population) was reported in the USA, Canada, and Israel, and moderate-high utilization (250–999 procedures/100 000) was reported in most Western European countries, Australia, and Japan. Lower utilization rates were reported in other countries in all continents globally (Figure 1). Historically, in many hospitals, radionuclide testing has been the reference diagnostic method for the assessment of infarct size, myocardial viability, and ischaemia after acute myocardial infarction (MI). 3 In contrast to North American guideline recommendations (see Supplementary data online, Table S1), recent European guidelines for the management of patients with ST elevation myocardial infarction (STEMI) accord radionuclide testing a more restricted indication and alternative diagnostic modalities have greater prominence. The evidence-base for imaging infarct size with Tc99mSestamibi-SPECT and late gadolinium enhancement cardiac magnetic resonance (CMR) are well established. However, the comparative diagnostic accuracy and prognostic significance of infarct size measured by SPECT and CMR are uncertain. Our commentary is stimulated by the publication by Hadamitzky et al. who retrospectively analysed 281 acute STEMI patients treated by primary angioplasty who had undergone both Tc99mSestamibi-SPECT and CMR on a 1.5 T scanner at a median 5.0 (IQR 4.1–5.9) days post-MI. In multivariable analyses, the strongest predictor for the primary outcome was microvascular obstruction (P , 0.0001), followed by infarct size by CMR (P 1⁄4 0.0043) and infarct size by SPECT (P 1⁄4 0.012) (all P-values corrected for clinical risk). However, in a multivariate model including clinical and periprocedural parameters, microvascular obstruction was the only significant predictor in addition to clinical risk as measured by the GRACE score. In addition to the authors three main conclusions, we would add a fourth. Given the prognostic importance of microvascular obstruction revealed by CMR, over-and-above infarct size, CMR has superior diagnostic utility over SPECT for prognostication post-MI. Infarct size varied according to the method of analysis. Compared with SPECT, infarct sizewith CMR was greater when a ×4 SD threshold was used to discriminate infarct tissue from non-infarct tissue, whereas the amount of infarct size was smaller vs. SPECT when ×5 SD, ×6 SD, or ‘full-width half maximum’ methods were used. Clearly, technical factors underlie these differences. On the one hand, smaller MIs may be missed with SPECT due to limited spatial resolution. On the other hand, with CMR, viable oedematous tissue may be included in the infarct area when a less restrictive approach to CMR image analysis (e.g. ×2 SD) is used. In fact, the size of infarction as revealed by late gadolinium enhancement within the first week post-MI may over-estimate actual infarct size compared with when infarct size is re-assessed 6 months later. Therefore, the timing of the scan post-MI and the approach to infarct size measurement should be standardized. Hadamitzky et al. also observed that infarct size by CMR using the 6 SD threshold had a stronger relationship with clinical outcomes comparedwith infarct sizebyotherCMR methods orSPECT. The superior correlation between late enhancement at higher thresholds and health outcomes may be explained by the fact that the restrictive approach provides a more accurate assessment of actual (final) infarct size. This study’s results support the adoption of ×5 to ×6 SD difference in infarct zone signal intensity vs. mean signal intensity of the remote zone. Hadamitzky’s study has several important and obvious limitations, but these limitations do not outweigh the originality of the data and importance of the paper. Instead, the reader should understand the limitations of the paper and interpret its results accordingly. The limitations are the retrospective nature of the analysis, the selection of