JOURNAL OF MAGNETIC RESONANCE 129, 161–164 (1997) ARTICLE NO. MN971257 Diffusion Measurement in Phantoms and Tissues Using SLIM Localization Yihong Yang, 1 Su Xu, 1 M. Joan Dawson, and Paul C. Lauterbur Biomedical Magnetic Resonance Laboratory, University of Illinois at Urbana – Champaign, Urbana, Illinois 60801 Received February 17, 1997; revised August 12, 1997 A new approach to efficient localized diffusion measurements scopic image reconstruction. The accuracy of SLIM, includ- has been developed and evaluated on phantoms and isolated tis- ing SLIM studies of inhomogeneous compartments, has been sues. The combination of a diffusion-sensitive pulse sequence with verified using simulations and experiments on phantoms SLIM (spectral localization by imaging) makes efficient and accu- ( 15, 16 ), as well as in 31 P spectroscopy studies of human rate localized water and metabolite diffusion measurements possi- brain ( 17 ) and excised uterus ( 18 ). 1 H SLIM has been com- ble with a substantial improvement in spatial or time resolution bined with quantum coherence transfer methods for imaging compared to standard methods. Phantom experiments showed lactate in muscle and brain ( 19, 20 ). The pulse sequence that diffusion of substances present in relatively low concentration used in the present experiments for localized diffusion mea- within small compartments can be measured accurately by this surement of water or metabolites is shown in Fig. 1. For the method, suggesting potential applications for diffusion measure- ments of metabolites in vivo. Experiments on excised rat uterine metabolite diffusion study, this was combined with a water horns demonstrated the ability of this method to measure localized suppression scheme consisting of frequency-selective RF diffusion of water within irregularly shaped regions of biological pulses and crusher gradients. Phase-encoded spectroscopic samples. Accurate diffusion measurements were achieved in the signals were combined with a high-resolution water image localized regions with acquisition times less than would have been to achieve spatial localization. A series of localized spectra required by standard diffusion imaging methods.  1997 Academic with varied diffusion weightings was obtained by SLIM Press analysis, and the diffusion coefficients in each of the local- ized regions were calculated from the intensities of the spec- tra. The sequence can easily be modified for measurements Diffusion-weighted imaging ( 1–4 ), which has proved in a 2D slice by replacing the hard RF pulses with slice- useful in a number of neurological ( 5–8 ) and other ( 9 ) selective pulses along with slice-selective gradients, and applications, could benefit from improvements of spatial eliminating the phase-encoding gradients in the slice direc- and / or temporal resolution. Similarly, measurement of me- tion. tabolite diffusion in tissues has been conducted predomi- Both phantom and biological experiments were performed nantly by spectroscopic techniques ( 10–13 ) but remains on a Varian VXR-500S spectrometer equipped with a Doty challenging due to the typically low concentration of tissue microimaging probe. The phantom was composed of 100 metabolites. We show here that the combination of a diffu- mM creatine and 100 mM lactic acid (both dissolved in sion-sensitive pulse sequence with SLIM ( spectral localiza- 99.9% D 2 O) in two separate capillaries (1-mm inner diame- tion by imaging) localization makes efficient water and me- ter) inserted parallel to the long axis of a tube (5-mm inner tabolite diffusion imaging possible, with a substantial im- diameter) filled with 99.9% D 2 O (Fig. 2a). The total abso- provement in spatial or time resolution. This method was lute amount of each metabolite was 785 mmol. A proton validated by phantom experiments on metabolite diffusion and was applied to measurement of water diffusion in rat spectrum of the entire phantom obtained with a single-pulse uteri. sequence is shown in Fig. 2b, and peaks in the spectrum are SLIM is a non-Fourier-based localized spectroscopy identified for the two components. method ( 14, 15 ) which enhances effective resolution by em- The diffusion gradient strength varied from 0 to 16.8 G/ ploying a priori information, in this case the high-resolution cm with six levels. The gradient pulse duration ( d ) was 7 proton image, to constrain signals in the resulting spectro- ms and the gradient separation ( D ) was 14 ms. For each diffusion gradient strength, phase-encoding gradients ( 4 1 4 ) were applied along orthogonal directions in the transverse 1 Current address: National Institutes of Health, Laboratory of Diagnostic plane, with a step size of 0.114 G/cm and a duration of 4.1 Radiology Research, OIR, Building 10, Room B1N-256, Bethesda, MD 20892. ms in both directions. Echo time (TE) and repetition time 161 1090-7807/97 $25.00 Copyright  1997 by Academic Press All rights of reproduction in any form reserved.