Unusual pattern of enhancement in cryptococcal meningitis: in vivo findings with postmortem correlation.

Leptomeningeal enhancement on CT and MR imaging studies secondary to cryptococcal meningitis is an uncommon finding. In immunocompromised patients, this meningitis incites only a mild inflammatory reaction and abnormalities are often absent on imaging studies. We recently encountered two patients with cryptococcal meningitis in whom postcontrast MR imaging showed thick enhancing subarachnoid spaces. Both had cryptococcal meningitis at autopsy. In a different patient with cryptococcal meningitis, postmortem MR imaging and pathologic examinations showed that these areas of enhancement correspond to abundant mucoid material secreted by the yeasts.

Signal-to-noise ratio comparison between surface coils and implanted coils.

The signal-to-noise ration (SNR) from implanted coils is widely known to be superior to that from surface coils. This article addresses the quantitative aspects of this improvement by explicitly evaluating the magnetic vector potential in a conducting medium of finite extent for both implanted and surface coils. The predictions of the model are tested with actual image data from spin warp experiments on gelatin phantoms. The authors derived a simplified expression that yields the gain in SNR of an implanted coil relative to that of a surface coil and is valid in many practical situations.

Magnetic resonance microscopy of the rat carotid artery at 300 megahertz.

RATIONALE AND OBJECTIVES: Magnetic resonance microscopy (MRM) has evolved from a technical curiosity to a tool with which researchers can study important disease models. But MRM is not simply an extension of clinical magnetic resonance imaging. In this article, the unique adaptations of MRM required in the study of carotid artery disease are outlined. The techniques of MRM are integrated into a specific model of carotid artery disease in the rat to allow in vivo studies of vascular wall thickening after removal of the vascular endothelium. METHODS: Imaging was performed at 300 MHz in 250-gm Sprague-Dawley rats using surgically implanted radiofrequency coils to localize the region of interest and to provide an increase in the signal-to-noise ratio over that of volume or surface coils. A three-dimensional Fourier encoding sequence was modified with flow-dephasing gradients to minimize signal and artifacts from moving blood. RESULTS: In vivo images were acquired with spatial resolution of 25 x 25 x 400 microns and signal-to-noise ratio more than sufficient to define the morphology of the vascular wall. Significant changes in the intensity and distribution of signal were visible in the area surrounding the vessel after angioplasty. CONCLUSIONS: Signal-to-noise improvements from surgically implanted coils coupled to a three-dimensional radiofrequency-refocused sequence with flow-dephasing gradients were sufficient to define the wall of the carotid artery. The diffusion-weighted pulse sequence detects signal changes in the area surrounding the vessel after angioplasty. The MRM techniques described and the contrast observed allow us, for the first time to follow in vivo the early stage of developing atherosclerosis in the vessel wall and closely surrounding tissue.

Corrections for nonuniform sampling distortions in magnetic resonance imagery.

A technique for estimating and correcting nonuniform sampling distortions in magnetic resonance imagery is presented. The technique consists of first estimating the sampling grid distortion using a parametric model, and then estimating the uniformly sampled data by interpolation on the nonuniform grid. Parameterized models for two particular sampling grid distortion are presented. Maximum likelihood and maximum a posteriori estimators are developed for these distortions. Using computer simulated data, these estimators are shown to perform well.