OPTIMIZATION OF PULSE ENVELOPES FOR MRI APPLICATIONS

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The application of magnetic resonance as a clinical diagnostic tool has become common place in recent years. The large majority of these applications are in diagnostic imaging techniques. The great advantage of magnetic resonance imaging is that the contrast can be made to depend on a wide variety of parameters such as flow, relaxation, proton density, or diffusion. New imaging techniques continue to develop at an astonishing rate. High resolution magnetic resonance spectroscopy is also finding applications in the clinical environment. High resolution spectra of tissue extracts, bulk tissue samples and bodily fluids have been used as screening tests for disease as well as drug abuse. The arena which shows the greatest unrealized potential is the combination of spectroscopic and imaging techniques. In vivo spectroscopy could provide concentration information for a variety metabolites in a non destructive, non invasive manner. This potential has gone largely un realized due to a variety of technical difficulties. All of these techniques rely heavily on the implementation of a variety of composite and shaped RF pulses. The response characteristic of the nuclear spin system to these pulses ultimately determines the quality of the data attained in these studies. Recent studies suggest that considerable improvements in the quality of clinical magnetic resonance data can be attained by proper pulse envelope design. Optimized pulse envelopes could reduce signal loss due to incomplete refocusing on spin echis and poor phase dispersion as well as shorten echo times, improves spatial location, and frequency selectivity. Here we propose implementing numerical optimization in pulse envelop design. The optimization techniques will be studied in detail in order to provide a most efficient algorithm. Several relatively simple target pulse profiles will be investigated during the algorithm development. These early investigations will involve the design of composite and magnitude shaped pulses. Upon full optimization of the pulse design method, more complex pulses will be investigated. The fully optimized codes will be used to design a variety of pulses in which both the magnitude and the phase of the pulse are shaped. The pulse envelopes will be implemented in the particular applications for which they were designed. A comparison of the quality of the data attained with the optimized and standard pulse envelopes will be performed.