Date of Award

Fall 1993

Document Type

Thesis - Restricted

Degree Name

Master of Science (MS)

Department

Biomedical Engineering

First Advisor

Heinen, James A.

Abstract

This thesis pertains to medical diagnostic Magnetic Resonance Imaging (MRI). As the time required for pulse sequences in MRI examinations decreases, the non-ideal aspects associated with flat-top readout gradients (those attributed to a band-limited rectangular pulse) become of concern. Furthermore, the cost, in terms of design time, hardware, and reliability of such a flat-top system is excessive, since, with respect to the gradient-generating subsystem, an infinite bandwidth is the ultimate goal. Also, new pulse sequences have been devised that decrease scan time, utilizing sinusoidal readout gradients [18]. With that in mind, there is considerable advantage associated with an MRI system that allows for imaging- that is, receiving echoes-while the readout gradient amplitude varies. The goal of this thesis is to investigate the above-mentioned problem in detail and to propose a modification which, when performed on a current MR digital receiver, will transform the MR system from one which requires flat-top readout gradients to one which allows the readout gradient to assume any pre-defined function. With such an MR system, the readout gradient may be a sinusoid, in which case the bandwidth of the gradient-generating subsystem is minimized. The proposed modification is to replace a standard digital finite impulse response (FIR) filter with a new type of digital FIR filter that uses a warped time axis in the development of its coefficients and allows for these coefficients to change with every desired output point. Furthermore, the filter is specified such that the output sampling period (the time between output points) may be time varying and not, necessarily, a multiple of the input sampling frequency. We call this filter a time-warping digital FIR (TWF) filter.

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