Surgery – Diagnostic testing – Detecting nuclear – electromagnetic – or ultrasonic radiation
Reexamination Certificate
2000-11-06
2004-03-09
Lateef, Marvin M. (Department: 3737)
Surgery
Diagnostic testing
Detecting nuclear, electromagnetic, or ultrasonic radiation
C600S407000, C600S409000, C600S410000, C600S421000, C324S318000, C324S322000, C128S899000
Reexamination Certificate
active
06704594
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 a 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 perpendicular in direction 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 MRI-based systems suffer from the relatively low signal sensitivity that requires long image acquisition time. Moreover, these systems are expensive and complicated in operation. These drawbacks become more essential when an MRI system is used for imaging relatively large volumes, such as the human body. This necessitates producing a highly homogeneous magnetic field over the entire imaged volume, leading to extensive equipment size. 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 of 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 el 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 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 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 regions of the 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 in proximity of 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 the bulky 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 insure satisfactory image quality. Because of these two limitations, the concept of 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 the low magnetic field values characteristic of homogeneous magnetic field configurations.
The main reason why the above-mentioned apparatuses, as well as most all NMR or MRI setups, prefer operation with a substantially homogeneous static magnetic field, is that such a setup allows operating in a narrow frequency bandwidth (typically a few Hz). Working in a narrow frequency band allows easier electronic circuit design for tuning and matching the receiver and transmitter channels, but, most importantly, it results in low noise or, rather, high signal to noise ratio (SNR) per NMR signal (spin-echo, for example). Since the field is substantially homogeneous, the duration of the NMR signal is typically a few milliseconds. The down side of this conventional scheme of working in homogeneous fields is that field homogeneity is hard to achieve over extensive volumes, especially for “inside-out” apparatuses.
SUMMARY OF THE INVENTION
The present invention takes an advantage of a technique described in a co-pending patent application assigned to the assignee of the present application. This technique is based on the operati
Alexandrowicz Gil
Blank Aharon
Golan Erez
Browdy and Neimark , P.L.L.C.
Lateef Marvin M.
Pass Barry
TopSpin Medical (Israel) Limited
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