Edward G. Walsh - US grants

technology /technique development; male; female; nervous system; model design /development; human subject; nuclear magnetic resonance spectroscopy; cardiovascular system; biomedical resource; Mammalia;

High-field NMR imaging provides improved ability to detect small contrast changes associated with physiological function (e.g. perfusion). Techniques have been developed and implemented at 4.1T which have demonstrated improved contrast characteristics for ventricular function imaging using RF tagging. RF tagging deposits a grid of lines in the field of view at the start of the cardiac cycle which then distort as the myocardium moves across the cardiac cycle. This grid can be used in automated analysis of ventricular motion. This improvement resulted from the reduced magnetic relaxation rate at 4.1T vs clinical field strengths, the improved signal-to-noise ratio inherent at the higher field, and to radio frequency excitations developed at UAB which themselves improve tag line persistence. Animal studies have been carried out at heart rates from 75 to 130bpm demonstrating persistence of tag lines to end diastole. Myocardial perfusion has been imaged using an arterial spin-inversion technique which provides a quantitative model for perfusion estimation by comparing images in which arterial blood is magnetically inverted vs a control image. Signal differences of 1-3% have been visualized with high field imaging of animal models.

It is known that the presence of infarcted regions of myocardium can result in fatal arrhythmias with little or no warning. This project seeks to correlate the 3-dimensional configuration of infarcts with onset of sudden arrhythmia. This will be done initially using high-resolution (0.2-0.3mm spatial resolution) isotropic 3D NMR imaging at high field of fixed human hearts. High field imaging will permit acquisition of images with adequate signal-to-noise ratio while maintaining very high spatial resolution. Initial experiments have demonstrated the ability to clearly visualize infarcted regions within (what had been) normal tissue of fixed hearts (formalin) using T1 weighted NMR imaging. This study may lead to high resolution ventricular function imaging of paced infarct animal models using imaging techniques developed under Core Project III (Imaging of Physiological Function).

Ordinarily, the lack of higher k-space values causes a blurring artifact in the reconstructed images in which sharp discontinuities create spatial oscillations into nearby regions. In some cases - such as lipids in the scalp - these discontinuities con interfere with low-level signal measurements in nearby regions. If the general location of these bright signals is known, we can postprocess the image to "steer" the oscillations in a different direction. We have developed a simple approach to solving this problem. By specifying the location of the bright interference, the phase response of a filter is optimized to reduce the oscillations in the region of interest. At present, out technique has been verified in 1-D simulations. It holds promise for significantly reducing the oscillations caused by nearby bright objects. The computational complexity - once the filter has been designed - is on the same order as the original reconstruction from k-space samples. We are currently extending the method to 2-D and more general interference patterns. We investigated the use of a local variance model of the image to indicate a priori knowledge of discontinuities in the original. The model separately represented variations in the horizontal direction and the vertical direction. We assumed that an accurate model could be derived from a standard MR scout image of the object. The local variance model was incorporated into the reconstruction by penalizing roughness in a way inversely related to the local variance of the scout image. In this way, the reconstruction formula was made to be shift variant, allowing for the possibility of spectral extrapolation from the observed k-space samples. Our experiments confirmed that this is a very effective form of image model for the type of prior information that is available from an anatomic scout image. While higher resolution of fine details was somewhat limited, the reconstruction was dramatically sharper near boundaries in the image so that blurring of edges was minimal.[unreadable]

Generation of uniform radio frequency magnetic fields over a volume sufficient for whole body imaging at high field is not possible with conventional coil designs as seen in clinical scanners. This project will result in the development and implementation of an efficient, slotted cavity resonator design, which will provide for uniform imaging of the human thorax. Dr. Vaughan has successfully developed head coils of this design for use at field strengths as high as 9.4T. This experience, along with experience at UAB in successfully imaging most of the human heart using a surface coil for transmit and receive indicate high potential for successful implementation of a large volume coil for human imaging. This work is currently supported by two Small Business Technology Transfer grants with Enon Inc. (Dr. Vaughan as principal investigator).