Cardiac magnetic resonance imaging (MRI) is a proven technique for the evaluation of myocardial structure and function. An introduction to cardiac physiology is presented in Chapter 1, wherein a measure of LV rotational mechanics called LV twist is defined. LV twist is the apparent rotation of the LV apex relative to the LV base and provides insight into LV function beyond that traditionally reported in measures like ejection fraction. Importantly, cardiac MRI techniques are very well suited to evaluating cardiac structure and function and these methods are outlined in Chapter 2. In particular, MRI tagging can be used to non-invasively generate myocardial tissue landmarks that facilitate the qualitative and quantitative assessment of left ventricular (LV) myocardial deformation in both research and clinical settings. This thesis focuses on several developments. First, we developed a new tagging technique called complementary radial tagging (CRT) that generates a tagging pattern that better matches the annular shaped LV myocardium in the short-axis view. CRT also has better tag contrast during late diastolic phases and can be used to accurately measure the LV mechanics during all cardiac phases (Chapter 3).
In parallel with the development of the CRT technique we used conventional clinical tagging protocols to measure LV rotational mechanics quantitatively. Myocardial fibrosis is known to frequently occur in patients with Duchenne Muscular Dystrophy (DMD). The consequences of myocardial fibrosis on LV mechanics, however, are incompletely understood. In Chapter 4 we report on the LV rotational mechanics in patients with DMD with or without myocardial fibrosis. The results suggested that both DMD and the presence of myocardial fibrosis independently worsen LV rotational mechanics.
In Chapter 5 we report on the effects of conventional cardiac MRI exams, which require repeated breath holding and place a burden on some clinical patients, on measure of LV rotational mechanics. This breath hold paradigm presents two problems for patients with DMD who typically develop progressive respiratory impairment and the signs and symptoms of cardiac involvement at an early age. Currently, clinical protocols use both breath-hold and free breathing techniques, as needed, to acquire data. Chapter 5 compared the LV rotational mechanics between these techniques in healthy subjects and patients with DMD. It was found that free-breathing significantly decreases estimates of LV twist compared to breath hold measures. The results suggest that when using quantitative imaging biomarkers of LV rotational mechanics to monitoring disease progression or the response to therapy, especially in patients with DMD for whom decline in respiratory function is certain, it is important to use a free-breathing strategy for all studies to facilitate intra-subject longitudinal comparisons.
In Chapter 6, LV rotational mechanics were further evaluated in patients with mitral regurgitation. In patients with mitral regurgitation LV twist decreases, while CL-shear angle (an alternate measure of LV rotational mechanics) pseudo-normalized due to subtle changes in the heart’s geometry.
Lastly, in Chapter 7, a technique for acquiring two slices simultaneously – Controlled Aliasing In Parallel Imaging Results In Higher Acceleration (CAIPIRINHA) was implemented in a cardiac MRI tagging sequence. The application of CAIPIRINHA to a LV tagging sequence is shown to achieve similar estimates of peak LV twist in a single breath hold, which simplifies the exam and avoids measurement differences that may arise from data acquired in different breath holds.
In conclusion, this thesis reports on several technical developments and clinical applications related to estimating quantitatively the function of the left ventricle. Taken together these developments can be combined to provide fast and accurate estimates of LV rotational mechanics that provide insight to LV function beyond traditional measures.