Myocardial lipids—techniques and applications of proton magnetic resonance spectroscopy of the human heart

Ectopic storage of lipids within the myocardium provides a readily available pool of fatty acids, and is crucial for the heart to maintain myocardial energy homeostasis during physiological challenges. Disturbed systemic and myocardial lipid metabolism and excess ectopic myocardial lipid accumulation or “myocardial steatosis” have been implicated in the pathogenesis of cardiomyopathy and heart failure in various diseases. Localized proton magnetic resonance spectroscopy (1H-MRS) of the human heart allows for noninvasive quantitative assessments of the intracellular neutral lipid droplets that store (excess) fatty acids as triglycerides. This chapter presents techniques and applications of 1H-MRS of the heart. Methods of localized signal acquisition are introduced, particularly focusing on challenges and solutions that arise from the heart's beating motion as well as respiration. Aspects of myocardial metabolite content quantification are addressed so that results from in vivo studies can be put in perspective. The insights into human myocardial lipid metabolism in health and disease, obtained with 1H-MRS, are discussed. As such, this chapter presents how 1H-MRS has become a valuable tool to investigate ectopic myocardial lipid storage in many different conditions and pathologies

Atlases of cardiac fiber differential geometry

Studies of intra-species cardiac fiber variability tend to focus on first-order measures such as local fiber orientation. Recent work has shown that myofibers bundle locally into a particular type of minimal surface, the generalized helicoid model (GHM), which is described by three biologically meaningful curvature parameters. In order to allow intra-species comparisons, a typical strategy is to divide the parameters of the generalized helicoid by heart diameter. This normalization does not compensate for variability in myocardial shape between subjects and makes interpretation of intra-species results difficult. This paper proposes to use an atlas of rat and dog myocardium, obtained using diffeomorphic groupwise Log-demons, to register all hearts in a common reference shape to perform the normalization. In this common space the GHM is estimated for all hearts and compared using an improved fitting method. Our results demonstrate improved consistency between GHM curvatures within a species and support a direct relation between myocardial shape and fiber curvature in the heart

In vivo proton T-1 relaxation times of mouse myocardial metabolites at 9.4 T

PurposeProton magnetic resonance spectroscopy (H-1-MRS) for quantitative in vivo assessment of mouse myocardial metabolism requires accurate acquisition timing to minimize motion artifacts and corrections for T-1-dependent partial saturation effects. In this study, mouse myocardial water and metabolite T-1 relaxation time constants were quantified. MethodsCardiac-triggered and respiratory-gated PRESS-localized H-1-MRS was employed at 9.4 T to acquire signal from a 4-mu L voxel in the septum of healthy mice (n=10) while maintaining a steady state of magnetization using dummy scans during respiratory gates. Signal stability was assessed via standard deviations (SD) of zero-order phases and amplitudes of water spectra. Saturation-recovery experiments were performed to determine T-1 values. ResultsPhase SD did not vary for different repetition times (TR), and was 13.1 degrees 4.5 degrees. Maximal amplitude SD was 14.2%+/- 5.1% at TR=500 ms. Myocardial T-1 values (mean +/- SD) were quantified for water (1.71 +/- 0.25 s), taurine (2.18 +/- 0.62 s), trimethylamine from choline-containing compounds and carnitine (1.67 +/- 0.25 s), creatine-methyl (1.34 +/- 0.19 s), triglyceride-methylene (0.60 +/- 0.15 s), and triglyceride-methyl (0.90 +/- 0.17 s) protons. ConclusionThis work provides in vivo quantifications of proton T-1 values for mouse myocardial water and metabolites at 9.4 T. Magn Reson Med 73:2069-2074, 2015. (c) 2014 Wiley Periodicals, In

Dynamic MR imaging of cerebral perfusion during bicycling exercise

Habitual physical activity is beneficial for cerebrovascular health and cognitive function. Physical exercise therefore constitutes a clinically relevant cerebrovascular stimulus. This study demonstrates the feasibility of quantitative cerebral blood flow (CBF) measurements during supine bicycling exercise with pseudo-continuous arterial spin labeling (pCASL) magnetic resonance imaging (MRI) at 3 Tesla. Twelve healthy volunteers performed a steady-state exercise-recovery protocol on an MR-compatible bicycle ergometer, while dynamic pCASL data were acquired at rest, during moderate (60% of the age-predicted supine maximal heart rate (HRmax)) and vigorous (80% of supine HRmax) exercise, and subsequent recovery. These CBF measurements were compared with 2D phase-contrast MRI measurements of blood flow through the carotid arteries. Procedures were repeated on a separate day for an assessment of measurement repeatability. Whole-brain (WB) CBF was 41.2 ± 6.9 mL/100 g/min at rest (heart rate 63 [57-71] beats/min), remained similar at moderate exercise (102 [97-107] beats/min), decreased by 10% to 37.1 ± 5.7 mL/100 g/min (p = 0.001) during vigorous exercise (139 [136-142] beats/min) and decreased further to 34.2 ± 6.0 mL/100 g/min (p < 0.001) during recovery. Hippocampus CBF decreased by 12% (p = 0.001) during moderate exercise, decreased further during vigorous exercise (-21%; p < 0.001) and was even lower during recovery (-31%; p < 0.001). In contrast, motor cortex CBF increased by 12% (p = 0.027) during moderate exercise, returned to resting-state values during vigorous exercise, and decreased by 17% (p = 0.006) during recovery. The inter-session repeatability coefficients for WB CBF were approximately 20% for all stages of the exercise-recovery protocol. Phase-contrast blood flow measurements through the common carotid arteries overestimated the WB CBF because of flow directed to the face and scalp. This bias increased with exercise. We have demonstrated the feasibility of dynamic pCASL-MRI of the human brain for a quantitative evaluation of cerebral perfusion during bicycling exercise. Our spatially resolved measurements revealed a differential response of CBF in the motor cortex as well as the hippocampus compared with the brain as a whole. Caution is warranted when using flow through the common carotid arteries as a surrogate measure for cerebral perfusion

Dynamic magnetic resonance measurements of calf muscle oxygenation and energy metabolism in peripheral artery disease

Background: Clinical assessments of peripheral artery disease (PAD) severity are insensitive to pathophysiological changes in muscle tissue oxygenation and energy metabolism distal to the affected artery. Purpose: To quantify the blood oxygenation level-dependent (BOLD) response and phosphocreatine (PCr) recovery kinetics on a clinical MR system during a single exercise-recovery session in PAD patients. Study Type: Case–control study. Subjects: Fifteen Fontaine stage II patients, and 18 healthy control subjects. Field Strength/Sequence: Interleaved dynamic multiecho gradient-echo 1H T 2* mapping and adiabatic pulse-acquire 31P-MR spectroscopy at 3T. Assessment: Blood pressure in the arms and ankles were measured to determine the ankle-brachial index (ABI). Subjects performed a plantar flexion exercise-recovery protocol. The gastrocnemius and soleus muscle BOLD responses were characterized using the T 2* maps. High-energy phosphate metabolite concentrations were quantified by fitting the series of 31P-MR spectra. The PCr recovery time constant (τ PCr) was derived as a measure of in vivo mitochondrial oxidative capacity. Statistical Tests: Comparisons between groups were performed using two-sided Mann–Whitney U-tests. Relations between variables were assessed by Pearson's r correlation coefficients. Results: The amplitude of the functional hyperemic BOLD response in the gastrocnemius muscle was higher in PAD patients compared with healthy subjects (–3.8 ± 1.4% vs. –1.4 ± 0.3%; P < 0.001), and correlated with the ABI (r = 0.79; P < 0.001). PCr recovery was slower in PAD patients (τ PCr = 52.0 ± 13.5 vs. 30.3 ± 9.7 sec; P < 0.0001), and correlated with the ABI (r = –0.64; P < 0.001). Moreover, τ PCr correlated with the hyperemic BOLD response in the gastrocnemius muscle (r = –0.66; P < 0.01). Data Conclusion: MR readouts of calf muscle tissue oxygenation and high-energy phosphate metabolism were acquired essentially simultaneously during a single exercise-recovery session. A pronounced hypoxia-triggered vasodilation in PAD is associated with a reduced mitochondrial oxidative capacity. Level of Evidence: 2. Technical Efficacy: Stage 1 J. Magn. Reson. Imaging 2020;51:98–107

Small animal cardiovascular MR imaging and spectroscopy

The use of MR imaging and spectroscopy for studying cardiovascular disease processes in small animals has increased tremendously over the past decade. This is the result of the remarkable advances in MR technologies and the increased availability of genetically modified mice. MR techniques provide a window on the entire timeline of cardiovascular disease development, ranging from subtle early changes in myocardial metabolism that often mark disease onset to severe myocardial dysfunction associated with end-stage heart failure. MR imaging and spectroscopy techniques play an important role in basic cardiovascular research and in cardiovascular disease diagnosis and therapy follow-up. This is due to the broad range of functional, structural and metabolic parameters that can be quantified by MR under in vivo conditions non-invasively. This review describes the spectrum of MR techniques that are employed in small animal cardiovascular disease research and how the technological challenges resulting from the small dimensions of heart and blood vessels as well as high heart and respiratory rates, particularly in mice, are tackled. (C) 2015 Elsevier B.V. All rights reserve

A 72-channel receive array coil allows whole-heart cine MRI in two breath holds

Background: A new 72-channel receive array coil and sensitivity encoding, compressed (C-SENSE) and noncompressed (SENSE), were investigated to decrease the number of breath-holds (BHs) for cardiac magnetic resonance (CMR). Methods: Three-T CMRs were performed using the 72-channel coil with SENSE-2/4/6 and C-SENSE-2/4/6 accelerated short-axis cine two-dimensional balanced steady-state free precession sequences. A 16-channel coil with SENSE-2 served as reference. Ten healthy subjects were included. BH-time was kept under 15 s. Data were compared in terms of image quality, biventricular function, number of BHs, and scan times. Results: BHs decreased from 7 with C-SENSE-2 (scan time 70 s, 2 slices/BH) to 3 with C-SENSE-4 (scan time 42 s, 4–5 slices/BH) and 2 with C-SENSE-6 (scan time 28 s, 7 slices/BH). Compared to reference, image sharpness was similar for SENSE-2/4/6, slightly inferior for C-SENSE-2/4/6. Blood-to-myocardium contrast was unaffected. C-SENSE-4/6 was given lower qualitative median scores, but images were considered diagnostically adequate to excellent, with C-SENSE-6 suboptimal. Biventricular end-diastolic (EDV), end-systolic (ESV) and stroke volumes, ejection fractions (EF), cardiac outputs, and left ventricle (LV)-mass were similar for SENSE-2/4/6 with no systematic bias and clinically appropriate limits of agreements. C-SENSE slightly underestimated LV-EDV (-6.38 ± 6.0 mL, p < 0.047), LV-ESV (-7.94 ± 6.0 mL, p < 0.030) and overestimated LV-EF (3.16 ± 3.10%; p < 0.047) with C-SENSE-4. Bland-Altman analyses revealed minor systematic biases in these variables with C-SENSE-2/4/6 and for LV-mass with C-SENSE-6. Conclusions: Using the 72-channel coil, short-axis CMR for quantifying biventricular function was feasible in two BHs where SENSE slightly outperformed C-SENSE