Electricity: measuring and testing – Particle precession resonance – Spectrometer components
Reexamination Certificate
2000-11-30
2003-07-29
Arana, Louis (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Spectrometer components
C324S303000
Reexamination Certificate
active
06600319
ABSTRACT:
FIELD OF THE INVENTION
This invention is generally in the field of Nuclear Magnetic Resonance (NMR) based techniques, and relates to a device and method for magnetic resonance imaging (MRI). Although not limited thereto, the invention is particularly useful for medical purposes, to acquire images of cavities in the human body, but may also be used in any industrial application.
BACKGROUND OF THE INVENTION
MRI is a known imaging technique, used especially in cases where soft tissues are to be differentiated. Alternative techniques, such as ultrasound or X-ray based techniques, which mostly utilize spatial variations in material density, have inherently limited capabilities in differentiating soft tissues.
NMR is a term used to describe the physical phenomenon in which nuclei, when placed in a static magnetic field, respond to a superimposed alternating (RF) magnetic field. It is known that when the RF magnetic field has a component directed perpendicular to the static magnetic field, and when this component oscillates at a frequency known as the resonance frequency of the nuclei, then the nuclei can be excited by the RF magnetic field. This excitation is manifested in the temporal behavior of nuclear magnetization following the excitation phase, which in turn can be detected by a reception coil and termed the NMR signal. A key element in the utilization of NMR for imaging purposes is that the resonance frequency, known as the Larmor frequency, has a linear dependence on the intensity of the static magnetic field in which the nuclei reside. By applying a static magnetic field of which the intensity is spatially dependent, it is possible to differentiate signals received from nuclei residing in different magnetic field intensities, and therefore in different spatial locations. The techniques which utilize NMR phenomena for obtaining spatial distribution images of nuclei and nuclear characteristics are termed MRI.
In conventional MRI techniques, spatial resolution is achieved by superimposing a stationary magnetic field gradient on a static homogeneous magnetic field. By using a series of excitations and signal receptions under various gradient orientations, a complete image of nuclear distribution can be obtained. Furthermore, it is a unique quality of MRI that the spatial distribution of chemical and physical characteristics of materials, such as biological tissue, can be enhanced and contrasted in many different manners by varying the excitation scheme, known as the MRI sequence, and by using an appropriate processing method.
The commercial application of MRI techniques suffers from the following two basic drawbacks: the expenses involved with purchasing and operating an MRI setup; and the relatively low signal sensitivity which requires long image acquisition time. Both of these drawbacks are linked to the requirement in standard MRI techniques to image relatively large volumes, such as the human body. This necessitates producing a highly homogeneous magnetic field over the entire imaged volume, thereby requiring extensive equipment. Additionally, the unavoidable distance between a signal receiving coil and most of the imaging volume significantly reduces imaging sensitivity.
There are a number of applications in which there is a need for imaging relatively small volumes, where some of the above-noted shortcomings may be overcome. One such application is geophysical well logging, where the “whole body” MRI approach is obviously impossible. Here, a hole is drilled in the earth's crust, and measuring equipment is inserted thereinto for local imaging of the surrounding medium at different depths.
Several methods and apparatuses have been developed, aimed at extracting NMR data from the bore hole walls, including U.S. Pat. Nos. 4,350,955; 4,629,986; 4,717,877; 4,717,878; 4,717,876; 5,212,447; 5,280,243; and “Remote ‘Inside Out’ NMR”, J. Magn. Res., 41, p. 400, 1980; “Novel NMR Apparatus for investigating an External Sample”, Kleinberg et al., J. Magn. Res., 97, p. 466, 1992.
The apparatuses disclosed in the above documents are based on several permanent magnet configurations designed to create relatively homogeneous static magnetic fields in a region external to the apparatus itself RF coils are typically used in such apparatuses to excite the nuclei in the homogeneous region and, in turn, receive the created NMR signal. To create an external region of a homogeneous magnetic field, the magnetic configurations have to be carefully designed, to reconcile the fact that small deviations in structure may have a disastrous effect on magnetic field homogeneity. It turns out that such a region of a homogeneous magnetic field can be created only within a narrow radial distance around a fixed position relative to the magnet configuration, and that the characteristic magnetic field intensities created in this region are generally low. As a result, such apparatuses, although permitting NMR measurements, have only limited use as imaging probes for imaging extensive regions of bore-hole walls.
With respect to medical MRI-based applications, the potential of using an intra-cavity receiver coil has been investigated, and is disclosed, for example, in the following publications: Kandarpa et al., J. Vasc. and Interventional Radiology, 4, pp. 419-427, 1993; and U.S. Pat. No. 5,699,801. Different designs for catheter-based receiver coils are proposed for insertion into body cavities, such as arteries during interventional procedures. These coils, when located close to the region of interest, improve reception sensitivity, thus allowing high-resolution imaging of these regions. Notwithstanding the fact that this approach enables the resolution to be substantially improved, it still suffers from two major drawbacks: (1) the need for bulk external setup in order to create the static homogeneous magnetic field and to transmit the RF excitation signal; and (2) the need to maintain the orientation of the coil axis within certain limits relative to the external magnetic field, in order to ensure satisfactory image quality. Because of these two limitations, the concept of an intra-cavity receiver coil is only half-way towards designing a fully autonomous intra-cavity imaging probe.
U.S. Pat. No. 5,572,132 discloses a concept of combining the static magnetic field source with the RF coil in a self contained intra-cavity medical imaging probe. Here, several permanent magnet configurations are proposed for creating a homogeneous magnetic field region external to the imaging probe itself, a manner somewhat analogous to the concept upon which the bore-hole apparatuses are based. Also disclosed in this patent are several RF and gradient coil configurations that may be integrated in the imaging probe in order to allow autonomous imaging capabilities. The suggested configurations, nevertheless, suffer from the same problems discussed above with respect to the bore-hole apparatuses, namely: a fixed and narrow homogeneous region to which imaging is limited, and low magnetic field values characteristic of homogeneous magnetic field configurations.
SUMMARY OF THE INVENTION
There is accordingly a need in the art to improve MRI based techniques, by providing a fully autonomous intra-cavity MRI probe and an imaging method.
The present invention is based on the realization that rather than attempting to overcome problems of non-homogeneity of the magnetic field, this non-homogeneity may be used to the advantage of high-resolution imaging. The imaging probe according to the invention comprises all components necessary to allow magnetic resonance measurements and imaging of local surroundings of the probe, obviating the need for external magnetic field sources. The imaging method is based on the non-homogeneous static magnetic field created by permanent magnets and on a high sensitivity RF coil block, all located in the imaging probe itself. This makes the imaging probe an autonomous high-resolution magnetic resonance imaging device, capable of imaging the medium surrounding the probe.
There is thus provided according t
Arana Louis
TopSpin Medical (Israel) Limited
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