Quantitative evaluation of ischemic myocardial scar tissue by unenhanced T1 mapping using 3.0 Tesla MR scanner

PURPOSE

We aimed to use a noninvasive method for quantifying T1 values of chronic myocardial infarction scar by cardiac magnetic resonance imaging (MRI), and determine its diagnostic performance.

MATERIALS AND METHODS

We performed cardiac MRI on 29 consecutive patients with known coronary artery disease (CAD) on 3.0 Tesla MRI scanner. An unenhanced T1 mapping technique was used to calculate T1 relaxation time of myocardial scar tissue, and its diagnostic performance was evaluated. Chronic scar tissue was identified by delayed contrast-enhancement (DE) MRI and T2-weighted images. Sensitivity, specificity, and accuracy values were calculated for T1 mapping using DE images as the gold standard.

RESULTS

Four hundred and forty-two segments were analyzed in 26 patients. While myocardial chronic scar was demonstrated in 45 segments on DE images, T1 mapping MRI showed a chronic scar area in 54 segments. T1 relaxation time was higher in chronic scar tissue, compared with remote areas (1314±98 ms vs. 1099±90 ms, P < 0.001). Therefore, increased T1 values were shown in areas of myocardium colocalized with areas of DE and normal signal on T2-weighted images. There was a significant correlation between T1 mapping and DE images in evaluation of myocardial wall injury extent (P < 0.05). We calculated sensitivity, specificity, and accuracy as 95.5%, 97%, and 96%, respectively.

CONCLUSION

The results of the present study reveal that T1 mapping MRI combined with T2-weighted images might be a feasible imaging modality for detecting chronic myocardial infarction scar tissue.In ischemic cardiac diseases, differentiating viable from nonviable myocardial tissue is important for clinical decision-making (1). The viable myocardium with decreased blood-flow such as hibernating or stunned myocardium will recover function following coronary revascularization, whereas during the chronic stage of infarction, a dense fibrotic scar replaces the infracted myocardium and this scar tissue will not recover function (2). Sometimes more aggressive treatment is needed to improve blood flow such as angioplasty, stenting, and coronary artery bypass surgery. Therefore, evaluation of viability is critical for myocardium.Positron-emission-tomography, single-photon-emission computed tomography and dobutamine echocardiography are noninvasive techniques for assessing myocardial viability. Although these techniques have proven use in clinical practice, there are several limitations that may reduce their diagnostic accuracy (3). Infracted or noninfracted myocardium is determined arbitrarily within a viewing window (i.e., the range of gray/color values to be selected for viewing) by these techniques (4). Therefore, assessment of myocardial viability will vary qualitatively and subjectively due to the pitfalls of the techniques.Recently, delayed contrast-enhancement (DE) cardiac magnetic resonance imaging (MRI) has been used for an increasing number of clinical applications in cardiac diseases. However, there are conflicting reports concerning the utility of contrast-enhanced MRI after infarct healing. Firstly, this method makes a qualitative assessment of myocardial fibrosis, similar to previously mentioned methods. In addition, with conventional DE-MRI sequences, signal intensity is expressed on an arbitrarily scale that differs from one imaging to another. Therefore, this method is not suitable for direct signal quantification (5). Furthermore, concerns about the increased risk of gadolinium-induced nephrogenic systemic fibrosis have increased gradually in populations with impaired renal function (6, 7).T1 mapping is a novel cardiac MRI technique and it allows measurements of absolute T1 relaxation times for each pixel (8). This technique involves the sampling of signal recovery during multiple measurements following a preparation pulse. The resulting relaxation time is then determined for each pixel contained in a parametric image, referred to as T1 mapping (9). In contrast to other imaging techniques, T1 mapping achieves signal quantification (in milliseconds) on a standardized scale. It reflects tissue changes without need for signal intensity thresholds, post-processing, reference region-of-interest, or contrast material. The quantitative evaluation of myocardial T1 values (T1 mapping) has been used recently to identify patients with diffuse fibrosis or myocarditis, or acute and chronic myocardial infarction (MI) (4, 10). In addition, although previous studies show that MI quantification can be achieved using a magnetic field strength of 1.5 Tesla (T) (7), to our knowledge, these values have not yet been reported on chronic infarction for 3.0 T cardiac MRI.The purpose of this study was to use high-resolution unenhanced T1 mapping to investigate the feasibility of using 3.0 T cardiac MRI to detect scar areas in myocardium resulting from chronic MI.