Kristy Hendrich

Bioenergetic abnormalities associated with severe left ventricular hypertrophy.

Transmurally localized 31P-nuclear magnetic resonance spectroscopy (NMR) was used to study the effect of severe pressure overload left ventricular hypertrophy (LVH) on myocardial high energy phosphate content. Studies were performed on 8 normal dogs and 12 dogs with severe left ventricular hypertrophy produced by banding the ascending aorta at 8 wk of age. Spatially localized 31P-NMR spectroscopy provided measurements of the transmural distribution of myocardial ATP, phosphocreatine (CP), and inorganic phosphate (Pi); spectra were calibrated from measurements of ATP content in myocardial biopsies using HPLC. Blood flow was measured with microspheres. In hypertrophied hearts during basal conditions, ATP was decreased by 42%, CP by 58%, and the CP/ATP ratio by 32% in comparison with normal. Increasing myocardial blood flow with adenosine did not correct these abnormalities, indicating that they were not the result of persistent hypoperfusion. Atrial pacing at 200 and 240 beats per min caused no change in high energy phosphate content in normal hearts but resulted in further CP depletion with Pi accumulation in the inner left ventricular layers of the hypertrophied hearts. These changes were correlated with redistribution of blood flow away from the subendocardium in LVH hearts. These findings demonstrate that high energy phosphate levels and the CP/ATP ratio are significantly decreased in severe LVH. These abnormalities are proportional to the degree of hypertrophy but are not the result of persistent abnormalities of myocardial perfusion. In contrast, depletion of CP and accumulation of Pi during tachycardia in LVH are closely related to the pacing-induced perfusion abnormalities and likely reflect subendocardial ischemia.

Phase-modulated rotating-frame spectroscopic localization using an adiabatic plane-rotation pulse and a single surface coil

Spectroscopic spatial localization based on B1 gradients is generally performed as an amplitude-modulated experiment using a single surface coil, with a consequent loss in signal-to-noise (SN) ratio. The phase-modulated implementation of such experiments with a single surface coil requires the ability to apply both B1-dependent rotations and B1-independent 90° plane rotations using the same surface coil. It is demonstrated that this is now possible using adiabatic plane-rotation pulses. These pulses can execute a plane rotation of any angle despite the presence of large B1 inhomogeneities; therefore, they can be used to perform a 90° plane rotation throughout the sensitive volume of a surface coil. A new phase-modulated, spatial-localization experiment based on B0 and B1 gradients (RAPP-ISIS) is introduced. It is shown, using phantom and in vivo, intact animal studies, that the technique provides excellent spectroscopic localization and the expected gain in SN compared to its amplitude-modulated analogue.

Effects of dobutamine on myocardial blood flow, contractile function, and bioenergetic responses distal to coronary stenosis: Implications with regard to dobutamine stress testing

To determine the effects of dobutamine stimulation on myocardium distal to a coronary stenosis, transmural spatially localized phosphorus 31 nuclear magnetic resonance measurements of myocardial high-energy phosphate compounds (adenosine triphosphate and phosphocreatine), inorganic phosphate, and blood flow and systolic wall thickening were made in 8 open-chested dogs. Data were collected under (1) control conditions, (2) after the application of a moderate coronary stenosis, (3) during infusion of dobutamine with continuing stenosis, and (4) after the release of the stenosis with continuing dobutamine. Stenosis was associated with concordant reductions of subendocardial blood flow, wall thickening, and high-energy phosphate, and mild elevation of inorganic phosphate; subepicardial measurements were essentially unchanged. During dobutamine infusion, blood flow increased in all myocardial layers. Wall thickening returned to control values in the subendocardium and increased nonsignificantly in the subepicardium. Additional loss of high-energy phosphate occurred only in the subepicardium. The data suggest that improved contractile function associated with dobutamine infusion resulted from the inotropic effects of dobutamine and was made possible by the improved blood flow it produced. The data indicate that measurements of blood flow and contractile function do not reliably predict the transmural myocardial metabolic responses to inotropic perturbations in the hypoperfused heart. Taken together, the present findings yield insights with regard to the interpretation of diagnostic dobutamine stimulation testing with single photon emission tomography, radionuclide angiography, and echocardiography.

B1 voxel shifting of phase-modulated spectroscopic localization techniques☆

Localization techniques based on B1 gradients have generally been implemented as amplitude-modulated experiments. Recent introduction of adiabatic plane-rotation pulses has permitted the execution of B1-based localization in a phase-modulated form using a single surface coil, resulting in gains in signal-to-noise ratio with multivoxel capabilities. In this phase-modulated form, the voxel positions can also be shifted arbitrarily to desired regions in B1 with appropriate postacquisition data processing. The resultant “voxel-shifted” spectra are identical to spectra acquired with parameters optimized for the desired B1 region. This technique of B1 voxel shifting is presented and demonstrated on both phantoms and in vivo data sets.

Functional Imaging of the Brain by Nuclear Magnetic Resonance

Publisher Summary Evolution of magnetic resonance imaging (MRI) has resulted in visualization of the anatomical structures in the living human brain with high spatial resolution and contrast. The structural information with detail that is evident in such magnetic resonance images has been complemented with biochemical and metabolic information following the development of magnetic resonance spectroscopy methods that provide chemical shift and spatial encoding. The noninvasive nature of these spectroscopic methods has enabled the investigation of intracellular metabolism and bioenergetics in preparations ranging from intact cells in suspension to humans. Among the plethora of biomedical applications with magnetic resonance, an avidly pursued new dimension is the acquisition of physiological information, such as tissue perfusion and function. A recent development in this new dimension is the ability to obtain functional maps that depict regions of the human brain that are activated during the performance of a specific task and permit the investigation of the extraordinary capabilities unique to the human brain. The basic MRI methods under development are blood oxygenation level–dependent (BOLD) contrast imaging, imaging based on first-pass exogenous vascular contrast agents, and blood perfusion imaging using inversion-recovery methods. This chapter focuses on BOLD-based functional brain mapping.