Miniature self‐contained intravascular magnetic resonance (IVMI) probe for clinical applications

Abstract

A miniature (1.73 mm in diameter) NMR probe, which contains a magnet and a radiofrequency (RF) coil, is presented. This probe is integrated at the tip of a standard catheter and can be inserted into the human coronary arteries, creating local magnetic fields needed to obtain the NMR signal from the blood vessel walls, without the need for external magnet or RF coils. The basic theory governing the probe performance in terms of signal-to-noise-ratio and contrast parameters is presented, along with measured results from test samples. The NMR signal can be analyzed to obtain tissue contrast parameters such as T 1 , T 2 and the diffusion coefficient, which may be used to detect lipid-rich vulnerable plaques in the coronary arteries. 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 (1,2). 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 more 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 hand-held 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 for NMR measurement or NMR imaging without a sample-surrounding magnet is termed "inside-out," or ex situ (3-7). Using the inside-out setup it is usually impossible to create highly homogeneous fields outside the magnet. Several types of systems that operate in an inside-out geometry were designed and built, demonstrating measurement capabilities of relaxation parameters (8,9), diffusion coefficients (10), spectroscopic data (11), and a capability of 3D imaging (12). In the case of microscopic MRI, where very high field gradients are required to obtain high resolution, inside-out systems were found very useful and enabled the acquisition of images with a resolution of about 1 m, outside a superconducting magnet (5,13), or near a fine magnetic tip (14,15).The efforts described above were mainly directed toward materials science applications or related subjects, with appreciable achievements. However, until now there has been no demonstration of the full potential of this method to approach clinically significant problems, for which current MRI systems cannot provide an answer due to limitations in resolution, SNR, gradient magnitude, complexity, local measurement flexibility, and even cost. The unavailability of such a demonstrati...

Advances in Intravascular Imaging

“…In 2005, a miniature (1.73 mm in diameter) MRI probe that contained both magnet and coil (ie, no need either external magnet or coil) was reported with an estimated resolution of 100 m ex vivo ( Figure 2B, bottom). 29,30 Lipid fraction was calculated based on diffusion coefficient of the tissue with depth information (0 to 100 m or 100 to 250 m from the surface of the plaque). However, long acquisition time with occlusion balloon was a technical limitation for in vivo patient evaluation.…”

Ex situ endorectal MRI probe for prostate imaging

“…The information about the static field, B 0 , and the RF field, B 1 , of the transmit and receive coils and their electrical properties can be compiled with the aid of a 3D NMR simulation (15,23) to extract the theoretical SNR and the geometry of the volume sections around the probe from which the signal is obtained. Fig.…”

“…Since the probe has a relatively large static gradient, it can be used to obtain both diffusion-and/or T 2 -weighted images. For example, for a CPMG sequence with restively short 1 and 2 , the summed echo signal is mostly T 2 -weighted, while for large 1 and 2 , the summed signal is heavily diffusion-weighted (15,16).…”

“…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)(16)(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).…”

“…These performances, especially if considered in conjunction with a general-purpose clinical use, are not sufficient. Nevertheless, as was shown in the case of the miniature intravascular probe (15), if one tailors a specific probe design to a very specific clinical application, ex situ NMR/MRI can be a viable solution that complements the current conventional MRI capabilities.…”