CMR of Myocardial Infarction

Coronary artery disease and ischemic heart disease are relatively common in patients with HIV infection [3, 4]. The prevalence of ischemic heart disease and its related mortality are increasing among HIV-positive patients [5,6].

Cine-CMR with steady-state free preces­sion allows for accurate assessment of LV global and regional LV and RV function. Myocardial infarction appears as hypokine­sia or akinesia during systole, which occurs in a myocardial wall supplied by an epicar­dial coronary artery (Fig. 1). Myocardial thinning may occur during systole. Besides rapid imaging of cardiac morphology, cine- CMR also permits the assessment of mitral valve regurgitation. Mechanical complica­tions occurring after acute MI may be depicted by CMR in the cine mode. They include pericardial effusion, LV aneurysm, intracavitary thrombus, and mitral regurgi­tation (Fig. 2). Acute infarcts may be recog­nized as hyperintense signal on breath­hold ECG-gated T2-weighted black-blood fast spin-echo CMR images during the first days after the onset of MI (Fig. 3) [13]. This technique, however, does not identify chronic infarcts and may overestimate infarct size by including area at risk. In addition, T2-weighted images often have a low signal-to-noise ratio. Dynamic ultrafast multislice CMR of myocardial perfusion allows for the detection of microvascular obstruction after MI (Fig. 4) [14], or may add significant diagnostic value over usual clinical and biological markers after non-

Fig. 1 HIV-infected 36-year old male with HAART who suffered a first recent anterior acute MI. Still images extracted from a cine- CMR sequence, performed at day 3, in the short axis (upper panel) and 4-chamber views (lower panel), at end-diastole (left) and end-systole (right), showing a segmental akinesia in the mid-anterior (upper panel, arrows) and apicolat­eral walls (lower panel, arrows), corresponding to the left anterior descending coronary territory

Fig.

Fig. 3 42-year-old male with HIV and HAART. Pre-contrast T2- weighted black-blood fast spin-echo CMR image in the short­axis view showing a hypersignal in the inferolateral wall (arrow) 2 days after the onset of acute MI

Fig. 4 Still frames in the short-axis view extracted from dynamic first-pass myocardial perfusion imag­ing, acquired 5,10 and 15 s after gadolinium injec­tion, showing the distribution of contrast agent from RV (5 s) to LV cavity (10 s) and ultimately within the myocardium (15 s). Microvascular obstruction is displayed as hyposignal in the anterolateral wall (arrows) in this 53-year-old male who suffered acute MI and underwent successful angioplasty of the left anterior descending 48 hours before

ST elevation acute coronary syndromes [15]. Direct high-resolution CMR imaging of MI with the so-called “delayed-enhancement technique” after Gadolinium contrast injection is well standardized and carries important clinical implications for the diagnosis of myocardial viability [16]. With this imaging sequence, Gadolinium-DTPA enhanced imaging provides great contrast and high resolution images in which the infarct appears as a hyperenhanced region

relative to noninfarcted tissue on inversion­recovery images acquired 10-15 min after contrast injection (Fig. 5). Indeed, the dif­ferences observed after MI in myocardial wash-in/wash-out kinetics of Gadolinium enable to differentiate three patterns [14]: normal nonischemic myocardium is char­acterized by rapid wash-in of the contrast agent on first-pass images (30 min) due in a large part to an increase of the distribution volume.

Fig. 5a-d Late inversion-recovery Gadolini- um-enhanced CMR images in the four- chamber view (a-b) showing late hyperen­hancement of the apex and apical septum (arrows) at day 3 in a HIV infected patient who presented acute anterior MI. The infarct is transmural at the apex and suben­docardial at mid-septum. c-d Late inver­sion-recovery Gadolinium-enhanced CMR images in the short axis view in a HIV infected patient who presented inferior MI, showing subendocardial late hyperenhance­ment in the posterior wall (arrows) at day 2

The third pattern corresponds to microvascular obstruction when perfusion is not adequate at the tissue level despite reopening of the culprit coronary artery. In this case, gadolinium wash-in is dramatical­ly delayed and the signal is very low (black) on first-pass myocardial perfusion images relative to noninschemic or necrotic but reperfused myocardium. It is well estab­lished from contrast-echocardiography and contrast-enhanced CMR that microvascular obstruction identified at the tissue level during the days following MI is a predictor of poor outcome and adverse LV remodel­ling [17, 18]. Delayed-enhanced CMR has important clinical implications for detec­tion of infarct size, which is the strongest determinant of prognosis in these patients, and for detection of myocardial viability [16]. Infarct imaging by CMR is very sensi­tive and can depict subtle non-transmural infarcts or even infarctlets [19, 20]. The method has been validated against PET for detection of viability and has been recog­nized as the standard of reference for viabil­ity detection by the ESC Consensus Panel report [21, 22].

Finally, common features of infarct imag­ing by CMR are characterized by the pres­ence of delayed enhancement of the infarct- ed tissue, which occurs in a specific coro­nary territory and predominates in the subendocardium. The delayed-enhancement within the infarcted myocardium corre­sponds typically to a single area of hypersig­nal that extends towards the subepicardium as a wavefront. These patterns are observed in recent, as soon as a few hours after the onset of MI, and in chronic MI. The trans­mural extent of MI serves as an index of tis­sue viability, with graded probability of sub­sequent recovery after revascularization for a given myocardial segment that is inverse­ly proportional to the initial transmural extent [16].