The clinical utility of cardiovascular magnetic resonance

Mordi, Ify Raphael (2015) The clinical utility of cardiovascular magnetic resonance. MD thesis, University of Glasgow.

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The use of cardiovascular magnetic resonance (CMR), particularly in the cardiovascular research setting, has grown exponentially in the past 20 years. While CMR is increasingly used in clinically, it has yet to be incorporated into routine clinical practice and guidelines in the majority of conditions. One of the main reasons for this is the relative paucity of evidence for its diagnostic and prognostic utility and cost-effectiveness compared to more established non-invasive imaging techniques such as echocardiography and nuclear imaging. For CMR to become part of routine clinical care, more evidence of its utility is required.

The aim of this thesis was to demonstrate the clinical utility of CMR by using it to examine 5 clinical questions that pose a relevant dilemma to physicians in a cohort of patients referred for clinically indicated CMR studies.

CMR has two major advantages over echocardiography, the most commonly used technique in our centre (and most worldwide). Firstly, it is the gold standard for assessment of left ventricular volumes and function, and secondly, it has the ability to characterise the myocardium using specific imaging sequences and intravenous gadolinium contrast (known as late gadolinium enhancement – LGE). The first study in this thesis explored the potential benefit of a CMR protocol using these benefits to predict prognosis in an unselected cohort of patients. In this study I found that the assessment of myocardial function using ejection fraction and deformation imaging (strain) and assessment of the presence of fibrosis using LGE had incremental prognostic significance in addition to clinical predictors of outcome in all patients, including in those with ejection fractions greater than 35% (commonly thought to predict higher risk patients).

During scanning of this cohort of patients, it became apparent that CMR imaging of myocardial scar in patients with a history of prior myocardial infarction (MI) had the ability to identify fat within the infarcted territory, known as lipomatous metaplasia, which had been recognised pathologically but is not identified by echocardiography. The tissue characterisation ability of CMR has for the first time allowed this to be identified non-invasively ante-mortem. Pathological studies had suggested that lipomatous metaplasia was associated with adverse remodeling, while recent animal studies had suggested that the presence of myocardial fat within infarcts was pro-arrhythmogenic. In this study I showed that the presence of lipomatous metaplasia was indeed independently associated with mortality and ventricular arrhythmias, suggesting that it perhaps provides an arrhythmogenic substrate.

The next study explored the use of LGE in addition to established clinical markers in patients undergoing implantable cardioverter-defibrillator (ICD) implantation for ischaemic or dilated cardiomyopathy. These patients met the current clinical criteria for ICD implantation and also had testing of NT-proBNP, a marker of cardiac strain that is associated with adverse prognosis. The patients all underwent pre-implantation CMR. I found that the presence of LGE and a high NT-proBNP was associated with a higher risk of death and ICD activation, perhaps hinting at a role for CMR in providing further risk stratification in this group of patients.

Following on from this, I looked at the diagnostic capabilities of CMR. Characterisation of patients with mildly impaired left ventricular systolic function is important as early identification of cardiomyopathy can potentially allow early institution of life-saving therapies. Mild left ventricular impairment can however also be associated with the normal myocardial adaptations to exercise, known as athlete’s heart. This poses a diagnostic dilemma, which may not be easily solved using current imaging techniques. I found that the use of a further CMR parameter to characterise tissue, T1 mapping, was able to discriminate between patients with early DCM and exercisers with normal physiological myocardial adaptation, perhaps providing a solution to this diagnostic challenge.

The final study explored the utility of dobutamine stress CMR (DSCMR) to diagnose significant coronary artery disease (CAD) in patients with left bundle branch block (LBBB) and clinically suspected CAD. CAD is the most common cause of LBBB, yet LBBB causes myocardial abnormalities that can make it difficult diagnose CAD non-invasively, leading many patients to be referred for invasive coronary angiography (ICA) to confirm the diagnosis. Despite this, a substantial proportion of the patients with LBBB will not have significant CAD, meaning that ICA would be unnecessary. This study compared DSCMR with dobutamine stress echocardiography, with ICA as the gold standard. I found that DSCMR was significantly more accurate in diagnosis than dobutamine stress echocardiography, perhaps providing a technique that could be used as a gatekeeper to ICA.

This thesis shows that CMR can provide important diagnostic and prognostic information in a variety of cardiac conditions and can potentially help guide clinical decision-making. Larger studies should be performed to confirm these findings, allowing for determination of cost-effectiveness and incorporation of CMR into routine clinical management.

Item Type: Thesis (MD)
Qualification Level: Doctoral
Keywords: Cardiovascular magnetic resonance, CMR, cardiovascular imaging
Subjects: R Medicine > R Medicine (General)
Colleges/Schools: College of Medical Veterinary and Life Sciences > School of Cardiovascular & Metabolic Health
Supervisor's Name: Tzemos, Dr. Nikolaos and Berry, Prof. Colin
Date of Award: 2015
Depositing User: Dr Ify Mordi
Unique ID: glathesis:2015-6200
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 20 Mar 2015 12:23
Last Modified: 20 Mar 2015 12:51

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