Multiple-echo proton spectroscopic imaging using time domain parametric spectral analysis

Multiple-Echo Proton Spectroscopic Imaging Using Time Domain Parametric Spectral Analysis Andreas P. Kiefer, Varanavasi Govindaraju, Gerald B. Matson, Michael W. Weiner, Andrew A. Maudsley A multiple-echo MR spectroscopic imaging (MRSI) method is presented that enables improved metabolite imaging in the presence of local field inhomogeneitiesand measurement of transverse relaxation parameters. Short echo spacing is used to maximize signal energy from inhomogeneouslyline-broadened resonances, and time domain parametric spectral analysis of the entire echo train is used to obtain sufficient spectral resolution from the shortened sampling periods. Optimal sequence parameters for 'H MRSl are determined by computer simulation, and performance is compared with conventional single-echo acquisition using phantom studies at a field strength of 4.7 T.A preliminaryexample for use at 1.5 T is also presented using phantom and human brain MRSl studies. This technique is shown to offer improved performance relative to single-echo MRSl for imaging of metabolites with shortened T2* values due to the presence of local field inhomogeneities. Additional advantages are the intrinsic measurement of metabolite T2 values and determination of metabolite integrals without T2 weighting, thereby facilitating quantitative metabolite imaging.

Key words: proton spectroscopic imaging; brain; parametric spectral analysis.

INTRODUCTION

A common concern for in vivo spectroscopic measurements is local magnetic-field inhomogeneity that results in shortened metabolite T,* values, lineshape distortions, and spectral overlap. Spatially localized field inhomogeneities may be particularly severe at regions of rapidly changing susceptibility, such as at the interfaces between air, tissue, and bone (1-4), that generally cannot be corrected using conventional shimming methods. Although it may be possible to perform localized MRS measurements in these regions by optimizing the Bo field over a single small selected volume, this approach cannot be used for MR spectroscopic imaging (MRSI) studies where spectra are acquired over a wide field of view and MRM 39528438 (1998) From the Departments of Radiology (A.P.K., V.G., A.A.M.), Pharmaceutical Chemistry (G.B.M.), and Medicine (M.W.W.), University of California San Francisco, MR Unit (114M), DVA Medical Center, San Francisco, California. Address correspondence to: Andrew A. Maudsley, Ph.D., MR Unit, 114M, DVA Medical Center, 4150 Clement St., San Francisco, CA 94121. Received June 2, 1997; revised August 27, 1997; accepted September 15, 1997. This work was supported by PHS grants CA48815, AG12119 (A.A.M.), AG10897 (M.W.W.), and the Department of Veterans Affairs Medical Research Service (G.B.M.). 0740-3194/98 $3.00 Copyright 0 1998 by Williams & Wilkins All rights of reproduction in any form reserved.

a wide range of T,* values may be present over the imaged region. Spectra obtained in the presence of field inhomogeneity have increased spectral overlap and decreased signalto-noise ratio (SNR) in the frequency domain, also seen as a reduction in the total signal energy in the time domain. It is well known that when T,"