Strain-encoded cardiac MRI as an adjunct for dobutamine stress testing: incremental value to conventional wall motion analysis

Abstract

Background: High-dose dobutamine stress MRI is safe and feasible for the diagnosis of coronary artery disease (CAD) in humans. However, the assessment of cine scans relies on the visual interpretation of regional wall motion, which is subjective. Recently, strain-encoded MRI (SENC) has been proposed for the direct color-coded visualization of myocardial strain. The purpose of our study was to compare the diagnostic value of SENC with that provided by conventional wall motion analysis for the detection of inducible ischemia during dobutamine stress MRI.

Methods and results: Stress-induced ischemia was assessed by wall motion analysis and by SENC in 101 patients with suspected or known CAD and in 17 healthy volunteers who underwent dobutamine stress MRI in a clinical 1.5-T scanner. Quantitative coronary angiography deemed as the standard reference for the presence or absence of significant CAD (> or =50% diameter stenosis). On a coronary vessel level, SENC detected inducible ischemia in 86 of 101 versus 71 of 101 diseased coronary vessels (P<0.01 versus cine) and showed normal strain response in 189 of 202 versus 194 of 202 vessels with <50% stenosis (P=NS versus cine). On a patient level, SENC detected inducible ischemia in 63 of 64 versus 55 of 64 patients with CAD (P<0.05 versus cine) and showed normal strain response in 32 of 37 versus 34 of 37 patients without CAD (P=NS versus cine). Quantification analysis demonstrated a significant correlation between strain rate reserve and coronary artery stenosis severity (r(2)=0.56, P<0.001), and a cutoff value of strain rate reserve of 1.64 was deemed as a highly accurate marker for the detection of > or =50% stenosis (area under the curve, 0.96; SE, 0.01; 95% CI, 0.94 to 0.98; P<0.001).

Conclusions: The direct color-coded visualization of strain on MR images is a useful adjunct for dobutamine stress MRI, which provides incremental value for the detection of CAD compared with conventional wall motion readings on cine images.

Figure 1. Illustration of the strain and strain rate response during inotropic stimulation

Quadratic interpolation was used to generate strain curves in a patient without significant CAD by coronary angiography (a–c) and in a subject with single-vessel CAD of the LAD (d–f). In the first subject myocardial strain rate increased by ∼2-fold (curved red arrow in c), and peak systolic strain remained constant (black arrow in c), while in the ‘ischemic territory’, myocardial strain decreased during peak stress (black arrow in f), and strain rate remained unchanged (dotted red circle in f).

Figure 2. Strain-Encoded MRI detects ischemic myocardium in the lateral wall, which is overlooked by wall motion assessment

Wall motion analysis detected stress induced ischemia in the left ventricular apex (f, red arrows) and in the mid-ventricular septum (h, red arrow). SENC confirmed the presence of inducible ischemia in these segments (g, red arrows) and additionally detected reduced strain response in the mid-ventricular lateral wall (i, yellow arrows). Coronary angiography yielded 2-vessel CAD, with high grade lesions (hatched circles) in both the left anterior descending (e), and in the left circumflex coronary artery (j).

Figure 3. Quantification of myocardial strain/strain rate

Normal and non-ischemic segments (supplied by vessels with <50% diameter stenosis) had significantly higher strain and strain rate response during inotropic stimulation compared to ischemic segments (a and c, p<0.001 for both). Furthermore, ischemic segments without new/worsening WMA during stress had significantly higher strain/strain rate response compared to those with new/worsening WMA (b and d, p<0.001).

Figure 4. Correlation between myocardial strain and stenosis severity

Scatter plots showing the relation of myocardial strain to stenosis severity demonstrated weak non-linear correlations for S_reserve (a), (r²=0.38), and for SR_reserve (d), (r²=0.56). SR_reserve was reduced already with moderate coronary stenosis (e), while a significant reduction in S_reserve required higher grade stenotic lesions (b). ROC analysis showed that SR_reserve, with a cut-off value of SR_reserve>1.64, is a highly accurate parameter for the detection of coronary artery stenosis≥50% (f).

Figure 5. SR_reserve for the detection of inducible ischemia in segments with resting WMA

SR_reserve correlated significantly with stenosis severity both in segments with and without baseline WMA, (a and c). Furthermore, in segments with resting WMA, the detection of anatomically significant CAD was similar to that in segments without resting WMA (b and d).