Myocardial strain measured in survivors of acute ST-elevation myocardial infarction: implementation and prognostic significance of novel magnetic resonance imaging methods

Background: Cardiac Magnetic Resonance (CMR) has utility in the risk stratification of patients post ST elevation myocardial infarction (STEMI). Myocardial strain is theoretically more linked to left ventricular pump function than left ventricular ejection fraction (LVEF). There are a number of CMR strain techniques including bespoke methods such as displacement encoding with stimulated echoes (DENSE) and cine derived methods such as feature-tracking. Whilst cine-derived strain is more appealing for imaging in real-world practice, there are concerns on accuracy, especially on a myocardial segmental level. Deformation-tracking is a new technique based on tissue-tracking from cine imaging which has been developed in our group and is theoretically more accurate at identifying myocardial displacement and shortening than other commercial cine-strain techniques. Hypothesis: Compared with standard methods for imaging heart function, novel strain methods have superior diagnostic and prognostic performance. Objectives: (1) I aimed to compare circumferential strain derived from DENSE, deformation-tracking and feature-tracking in a group of 81 healthy volunteers, and in a group of STEMI patients. I investigated the relationship between strain age and sex in the healthy volunteers. (2) I also investigated the comparative performance of the three strain techniques and LV surrogate outcomes (LVEF, LV end diastolic volume indexed to body surface area, infarct size) as well as composite health outcomes (major adverse cardiac events) at 4 years in the STEMI patients. (3) I investigated the incremental predictive utility of segmental circumferential strain over infarct size to predict segmental functional improvement by wall-motion scoring at 6 months in patients with STEMI, and the influence of infarct characteristics (microvascular obstruction, intra-myocardial haemorrhage) on segmental circumferential strain at 6 months. (4) I investigated the utility of feature-tracking derived global longitudinal strain in this STEMI group. (5) Finally, I performed a de-novo study implementing a new DENSE technique in a group of STEMI patients and compared deformation-tracking and feature-tracking against this new technique. Methods: 1. Healthy Volunteers Study: 81 participants underwent multi-parametric CMR at 1.5T. 2. STEMI population 1: 324 patients underwent a similar multi-parametric CMR at 3 days and 295 at 6 months post STEMI. Composite health outcomes that are pathophysiologically linked to STEMI were collected by an independent team. 3. STEMI population 2: 50 patients underwent a multi-parametric CMR at 1 day and 6 months post STEMI. This protocol included the new 2D-Spiral DENSE sequence. The imaging analyses were performed using standardised methods. Health outcomes were analysed and adjudicated by an independent team blinded to the rest of the study. Statistical analyses were carried out under the supervision of a biostatistician. Results: The main findings of this thesis are: 1. Deformation-tracking performed well when compared with a reference method (DENSE) in a large group of healthy volunteers. The advantage of utilising a cine-strain derived method is that this would obviate the need for bespoke strain sequences being acquired, limiting the total duration of an CMR scan, and making strain more accessible in the clinical setting. 2. Global circumferential strain with DENSE provides incremental prognostic value over infarct size and pathologies revealed by contrast-enhanced CMR for LV surrogate outcomes. Strain imaging with DENSE has emerging potential as a new reference test for prognostication in patients after an acute STEMI. 3. Global circumferential strain with DENSE provides incremental prognostic value over infarct size and pathologies revealed by contrast-enhanced CMR for MACE. Conclusions: The data presented in this thesis indicate that CMR strain imaging may be clinically useful in the assessment of patients following an acute STEMI. This indicates that strain should be more widely used in clinical studies as both global and segmental strain provide incremental utility over more commonly used markers of prognosis for global and regional LV function, as well as major adverse cardiac events. 2D-Spiral DENSE is a new technique, which I have demonstrated, to be feasible to acquire in STEMI patients and has the potential to investigate LV pump function in more detail than conventional methods.