Three-Dimensional NMR Microscopy of Rat Spleen and Liver zyxwv Xiaohong Zhou, Richard L. Magin, Jay C. Alameda, Jr., Harry A. Reynolds, Paul C. Lauterbur zyxwvu Three-dimensional microscopic NMR images of spleen and liver specimens from rats injected wlth dextran magnetite par- ticles and from controls were obtained at 4.7 T, using a spe- cially designed probe in conjunction with a 3D filtered back projection reconstruction algorithm. All of the images were reconstructed as 643 arrays with (25 ~ m ) ~ isotropic voxels. With the aid of the MR contrast agent, the red pulp and mar- ginal zone of the spleen and the portal triad of the liver could be distinguished from the surrounding tissue in Tz-weighted images. For mature rat spleen, natural contrast in Tz-weighted images was found to distinguish the same features. Histolog- ical examinations of the tissues with and without contrast agent were also performed using an optical microscope. Mi- croscoplc NMR images, despite their lower resolution, clearly revealed many features seen in the optical images. Key words: dextran magnetile; microscopy; liver; spleen; projection reconstruction. INTRODUCTION It has long been understood that a successful NMR mi- croscope would have substantial value in biological and m.edica1 investigations, since it could provide informa- tion not easily obtained by other techniques. Compared with conventional forms of microscopy, NMR micros- copy has two major advantages. First, NMR imaging pro- vi des an opportunity to obtain three-dimensional infor- mation without sectioning, and second, image contrast can be based on a variety of NMR parameters, e.g., proton density, spin-lattice relaxation time, spin-spin relaxation time, and diffusion coefficient. However, at its current stage of development, the resolution of NMR microscopy is low. Signal-to-noise ratio (SNR) and molecular diffu- sion are generally considered to be the major factors that limit the resolution (1-8). The inherently low SNR of NMR makes it difficult to obtain three-dimensional iso- tr'opic high resolution in NMR microscopy, while diffu- MRM 30:92-97 (1993) From the Biomedical Magnetic Resonance Laboratory (X.Z., P.C.L.), the Departments of Electrical and Computer Engineering (R.L.M., J.C.A.), Nu- clear Engineering (R.L.M., J.C.A.), and Veterinary Pathobiology (H.A.R.), University of Illinois at Urbana-Champaign, Urbana, Illinois. Received September 14, 1992; revised March 2, 1993; accepted March 12, 1993. Address correspondence to: Richard L. Magin, Ph.D., Department of Elec- trical and Computer Engineering, University of Illinois, 1406 W. Green Street, Urbana, zyxwvutsrqponm IL 61801. This work was supported by the University of Illinois, NIH NCRR Biomedical Research Technology Grant 1 P41PR05964, NSF Biol. Instr. and Resources Grant 9102419, and the Servants United Foundation. Present address of X.Z.: Department of Radiology, Box 3302, Duke Univer- sity Medical Center, Durham, NC 27710. Copyright Q 1993 by Williams zyxwvutsrqpo 8 Wilkins All rights of reproduction in any form reserved. 0740-3194193 zyxwvutsrqpo $3.00 sion can blur the boundaries of different compartments in the image. In addition, the application of NMR micros- copy to biological systems is complicated by motion ar- tifacts and lack of intrinsic imaging contrast. While mo- tion artifacts can be reduced by signal processing and gated acquisition of data, natural contrast in most tissues is not sufficient to take full advantage of the resolving power of NMR microscopy. Therefore, as is the case in light microscopy where specific tissue stains are com- mon, MR contrast agents will often be necessary. Most microscopic NMR imaging, with few exceptions (3, 8-10), has been accomplished with slice selection methods, resulting in highly anisotropic voxels and very poor resolution perpendicular to the slices. In many bio- medical applications it is essential to have high isotropic resolution so as to visualize complex structures while minimizing confusing partial volume effects. In these cases, three-dimensional volume imaging should be the method of choice. Three-dimensional volume imaging is particularly advantageous in projection reconstruction pulse sequences where gradient switching, which is re- quired in phase-encoding methods and two-dimensional projection reconstruction, is not necessary. This not only minimizes gradient-induced vibrations and eddy current problems, but also allows a very short zyxw TE time to be used in the pulse sequence to obtain an appropriate T,-weight- ing from samples with short T2 values. Using 3D projec- tion reconstruction, we have recently obtained images with isotropic spatial resolution on the order of (10 ~ m ) ~ (8, 11). This new technical development makes it possi- ble to visualize microscopic tissue organization, pro- vided that adjacent regions exhibit sufficiently different NMR properties to establish contrast. Splenic tissue has separate regions (red pulp, white pulp, and marginal zone) that exhibit distinct NMR prop- erties, making it suitable for microscopic studies. Splenic macrophages, located in the red pulp and marginal zone, efficiently take up particulate MR contrast agents. In ad- dition, the iron content of the red pulp increases with age and the ratio of white to red pulp increases with inflam- matory disease. The liver is organized in hexagonal lob- ules with a central vein surrounded by portal triads (each consisting of a small portal vein, a branch of the hepatic artery, and a bile ductule). Blood flows from the periph- ery of the lobule radially via the hepatic sinusoids to the center where it exits through the central vein. In the liver, varying populations of phagocytic Kupffer cells (mac- rophages that can accumulate particulate MR contrast agents) line the sinusoids throughout the organ and can provide the basis for contrast in microscopic NMR imaging. 92