Electricity: magnetically operated switches – magnets – and electr – Magnets and electromagnets – Magnet structure or material
Reexamination Certificate
2000-11-20
2001-09-25
Barrera, Ramon M. (Department: 2832)
Electricity: magnetically operated switches, magnets, and electr
Magnets and electromagnets
Magnet structure or material
C324S318000, C324S320000
Reexamination Certificate
active
06294972
ABSTRACT:
BACKGROUND OF THE INVENTION
The field of the invention is nuclear magnetic resonance imaging methods and systems. More particularly, the invention relates to a local coil for acquiring NMR images of a selected part of a subject such as the human brain.
Any nucleus which possesses a magnetic moment attempts to align itself with the direction of the magnetic field in which it is located. In doing so, however, the nucleus precesses around this direction at a characteristic angular frequency (Larmor frequency) which is dependent on the strength of the magnetic field and on the properties of the specific nuclear species (the magnetogyric constant &ggr; of the nucleus). Nuclei which exhibit this phenomena are referred to herein as “spins”.
When substance such as human tissue is subjected to a uniform magnetic field (polarizing field B
0
), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. A net magnetic moment M
z
is produced in the direction of the polarizing field, but the randomly oriented magnetic components in the perpendicular, or transverse, plane (x-y plane) cancel one another. If, however, the substance, or tissue, is subjected to a magnetic field (excitation field B
1
) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, M
z
, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment M
t
, which is rotating, or spinning, in the x-y plane at the Larmor frequency. The practical value of this phenomenon resides in the signal which is emitted by the excited spins after the excitation signal B
1
is terminated. There are a wide variety of measurement sequences in which this nuclear magnetic resonance (“NMR”) phenomena is exploited.
The Larmor frequency of the NMR signals is determined by the magnitude of the polarizing magnetic field B
0
and one of the design objectives of an MRI system is that it produce a homogeneous polarizing magnetic field throughout a volume of a specified size (e.g., a 50 cm sphere). To this end it is well known in the art to use shims which alter the magnetic field in order to make it homogenous. Such shimming may be accomplished actively by using shim coils which conduct the appropriate current are described in U.S. Pat. Nos. 4,949,043; 4,862,087 and 5,490,509. In the alternative, shimming may be accomplished passively by judiciously placing carbon steel shims at locations within the polarizing magnetic field as described in U.S. Pat. Nos. 5,677,854; 5,532,597 and 5,349,297. Such shimming is performed as part of the system calibration procedure and it is performed without a subject in the bore of the magnet.
It is well known in the art that the SNR of images produced by MRI systems can be increased by using small RF coils which are designed to couple solely with the tissues in the particular region of interest. Such “local” or “surface” coils have been designed for various parts of the human anatomy, such as knees, shoulders, neck, breasts, hands and head. Of particular relevance to the present invention are local head coils which employ the so-called “bird cage” RF coil described by C. E. Hayes et al,
J. Magn. Reson.
63, 622-628 (1985) and U.S. Pat. No. 5,372,137.
The concept of acquiring NMR image data in a short time period has been known since 1977 when the echo-planar pulse sequence was produced by Peter Mansfield (J. Phys. C.10:L55-L58, 1977). In contrast to standard pulse sequences, the echo-planar pulse sequence produces a set of NMR signals for each RF excitation pulse. These NMR signals can be separately phase encoded so that an entire scan of 32, 64 or 128 views can be acquired in a single pulse sequence of 20 to 100 mili-seconds in duration. The advantages of echo-planar imaging (“EPI”) are well-known, and there has been a long felt need for apparatus and methods which will enable EPI to be practiced in a clinical setting. Other echo-planar pulse sequences are disclosed in U.S. Pat. Nos. 4,678,996; 4,733,188; 4,176,369; 4,355,282; 4,588,948 and 4,752,735.
Functional magnetic resonance imaging (fMRI) is a method to map human brain function. See for example, U.S. Pat. No. 5,603,322. This method is dependent on the fact that highly localized changes in blood flow, blood volume and blood oxygenation occur that are a consequence of brain neuronal activity. The method of blood oxygen level dependent (BOLD) imaging has been found to be particularly effective in fMRI. Gradient-recalled (GR) rather than spin echo (SE) recalled pulse sequences are required for optimum sensitivity to BOLD contrast. As a result, the majority of fMRI studies to date have been performed using GR EPI.
The images acquired with EPI pulse sequences are very sensitive to local B
0
field inhomogeneities that arise from susceptibility differences between bone, tissue and air when a subject is placed in the otherwise homogeneous B
0
field. At high fields such as 3 Tesla where functional MRI is practiced, a 1 ppm deviation in B
0
magnetic field strength translates to a misplacement of 5 pixels in the phase encoding direction. It has become apparent, therefore, that further shimming must be performed after the subject to be imaged is placed in the MRI system.
One solution to this problem is to provide active shim coils that are located close to the subject as described in U.S. Pat. No. 6,023,167. The shim coils are integral with the local RF coil and a procedure is conducted after the subject is in place to determine the amount of current to apply to each shim coil to offset B
0
field inhomogeneities. This solution requires many shim coils, each with a separate power supply. This is not only expensive, but the wiring needed to couple the shim coils to the remotely located power supplies is very cumbersome.
SUMMARY OF THE INVENTION
The present invention relates to a local coil assembly for acquiring NMR signals from a region of interest in a subject, and particularly to the production of passive shims for such a coil assembly. The local coil assembly is used to acquire measurement signals with the subject in place, and these signals are used to reconstruct a map which indicates changes in the polarizing magnetic field (&Dgr;B
0
) at locations throughout the region of interest, a ferroshim insert matrix is calculated from the &Dgr;B
0
map, a passive shim assembly is constructed with ferroshims placed at locations indicated by the ferroshim insert matrix; and the passive shim assembly is inserted into the local coil assembly to improve the homogeneity of the static polarizing field.
A general object of the invention is to compensate the polarizing magnetic field for inhomogeneities in the region of interest caused by the subject being imaged. A passive shim assembly is produced by acquiring measurements with the subject in place and producing a passive shim from information derived from these measurements. The passive shim customizes the local coil for the subject's particular anatomy.
Another object of the invention is to produce a set of local coils which are shimmed to offset variations in static magnetic field strength caused by common anatomic characteristics of subjects. Local coils with passive shim assemblies designed to compensate for different anatomical characteristics (e.g., short neck in head imaging) are produced. These local coils may be used as is with subjects having the corresponding anatomic characteristic, or they can be further customized to the subject by acquiring measurements and calculating further shim information. The further shim information may be used to alter the passive shim assembly, or it may be used to determine the currents in active shim coils.
The foregoing and other objects and advantages of the invention will appear from the following description. In the description, reference is made to the accompanying drawings which form a part hereof, and in which there is shown by way of illustration a preferred embodiment of the invention.
Hyde James S.
Jesmanowicz Andrzej
Punchard William F. B.
Starewicz Piotr M.
Barrera Ramon M.
Quarles & Brady LLP
The MCW Research Foundation Inc.
LandOfFree
Method for shimming a static magnetic field in a local MRI coil does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method for shimming a static magnetic field in a local MRI coil, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method for shimming a static magnetic field in a local MRI coil will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2458962