Ex situ endorectal MRI probe for prostate imaging

Abstract

A unique ex situ MRI probe, which examines samples external to its geometry, is presented. The probe is intended to be used for imaging the prostate gland via an endorectal pathway. It has a semicylindrical shape with a length of 6 cm and typical diameter of ϳ3 cm. The probe's imaging field of view spans almost along its entire length and up to a distance of 2 cm away from its surface, with an angular sector of ϳ90°. The detailed design of the probe is presented, followed by a set of representative results obtained by the current bench prototype of this system. NMR and its descendant MRI are usually pursued in a setup based on a highly homogenous static magnetic field, B 0 , with variance Ͻ1 ppm, creating nuclear spin precession at a corresponding narrow band of frequencies (1). The homogeneous static field setup has several advantages, such as the ability to obtain chemical shifts while ensuring good signal-to-noise ratio (SNR) due to the small bandwidth involved. However, this setup suffers from the need to employ large magnets that usually surround the examined sample/object. In specific cases, such as clinical MRI systems, the large magnet, and the corresponding large radiofrequency (RF) and gradient coils, are a major factor in the relative complexity and the high cost of such systems. If one is interested only in a specific small region within the body, it could be highly advantageous to obtain NMR information by using either a noninvasive handheld probe or an intracavity self-contained (magnet ϩ RF and gradient coils) NMR probe, thus avoiding the requirement for a large external magnet. Such an approach to NMR measurement or NMR imaging without a sample-surrounding magnet is termed "inside-out," or ex situ (2-6). In the "inside-out" setup it is very difficult to create highly homogeneous fields without making significant compromises regarding field magnitude. Several types of systems that operate in an "inside-out" geometry have been designed and built, demonstrating measurement capabilities of relaxation parameters (7,8), diffusion coefficients (9), spectroscopic data (10), and three-dimensional (3D) imaging (11). In the case of microscopic MRI, where very high field gradients are required to obtain a high resolution, "inside-out" systems were found to be very useful and enabled the acquisition of images with a resolution of 1 m or better, outside a superconducting magnet (4,12), or near a microscopic magnetic tip (13,14).The efforts described above were mainly directed toward materials science applications or related subjects, with appreciable achievements. In the field of medicine, recent work has demonstrated the use of a miniature intravascular ex situ NMR probe with one-dimensional imaging capability for the detection of vulnerable plaque in the coronary arteries (15-17). Larger nonimaging and/or one-dimensional imaging probes have been shown to be useful in obtaining interesting clinical results regarding the examination of tendon anisotropy, skin conditions, and breast implants (18). Despite this ...

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