In vivo assessment of the performance of strain-encoded MRI (DENSE) in healthy subjects and patients with myocardial infarction

Introduction: In patients with myocardial infarction (MI), regional left ventricular contractile function has important prognostic value. Displacement ENcoding with Stimulated Echoes (DENSE) is an MRI technique which has been developed to allow quantitative assessment of myocardial strain. To date, much of the research performed with DENSE has been methods development, and its application in a routine clinical setting has been incompletely investigated. The purpose of the research presented within this thesis was to investigate variations in strain within the healthy heart, and then to assess the in vivo performance of DENSE strain imaging in acute and chronic myocardial infarction (MI). Methods: 80 healthy subjects (M:F = 40:40, age 43 +/- 17 years) were recruited from the community. 50 male patients (age 56 +/- 10 years) were recruited from the clinical service and scanned within 7 days of myocardial infarction (“acute MI”), and invited to return for a follow-up scan after 6 months (“chronic MI”). MR imaging was performed on a 1.5T Siemens Avanto scanner, using an imaging protocol which included DENSE, cine, late gadolinium enhancement (LGE, patients only) and T2-weighted imaging acquired from left ventricular (LV) short-axis slices in both basal and mid-ventricular positions, which were divided into 6 segments for analysis. The percentage of LGE and T2 hyperenhancement within each segment were calculated, and the area at risk (acute MI) and myocardial salvage index (chronic MI) were determined. DENSE images were analysed to obtain values for strain parameters relating to circumferential strain (Ecc). Strain measurements obtained from healthy subjects were used to investigate the variations in Ecc with age, gender, slice position and myocardial segment. Strain measurements obtained from MI patients were used to investigate the relationships between Ecc and the extent of myocardial infarction, area at risk and/or salvage, and to determine whether DENSE strain measurements are informative in acute and chronic MI. Results: Comparison of DENSE strain measurements in healthy subjects revealed statistically significant differences between males and females, and between measurements obtained from basal and mid-ventricular short-axis slice positions. These differences must be taken into account to allow appropriate analysis of DENSE data in patients. DENSE was found to be informative in both acute and chronic MI. At both time points, strain measurements can be used to distinguish between myocardial segments with 0%, 50% infarction. There is the potential for the development of reference ranges which could be applied to strain measurements from future MI patients to allow assessment of the extent of infarction. In acute MI, four additional applications were identified: i) comparison with references ranges, established from strain measurements in healthy subjects, can be used to identify the presence of infarction with high specificity and moderate to high sensitivity, ii) peak Ecc can be used to distinguish between segments categorised as remote and adjacent, iii) strain measurements in the acute setting may provide prognostic information relating to the potential progression or recovery of contractile abnormalities in the chronic setting, iv) peak Ecc may allow a preliminary assessment of LV ejection fraction. Sensitivity for the detection of injured but non-infarcted segments was low. In chronic MI, two additional applications were identified: i) strain recovery can be detected in infarcted myocardial segments, and also in non-infarcted segments which are located adjacent to infarcted segments, which could improve identification of changes in contractile function compared to conventional qualitative analysis of cine imaging, ii) strain measurements can be used to distinguish between segments in which the extent of infarction has increased and those in which it has decreased. The relationships with myocardial salvage index were not found to be informative. Conclusions: DENSE images were successfully acquired and analysed from both healthy subjects and patients with myocardial infarction, which indicates that the technique is feasible in different clinical settings. DENSE strain measurements were found to be informative in both acute and chronic MI, and can provide insight into the presence and extent of infarction and the progression or recovery of contractile abnormalities.