Gradient coil apparatus and method of micro-imaging

Details Gradient coil apparatus and method of micro-imaging Gradient coil apparatus and method of micro-imaging

2001-06-06

2003-05-13

Lefkowitz, Edward (Department: 2862)

Electricity: measuring and testing

Particle precession resonance

Spectrometer components

C324S309000

Reexamination Certificate

active

06563315

ABSTRACT:

BACKGROUND OF INVENTION
The present invention relates generally to magnetic resonance imaging (MRI), and more particularly, to an apparatus and method to perform micro-imaging by switching from a whole-body set of gradient coils to a micro-imaging gradient coil.
When a 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. If 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, or “longitudinal magnetization”, M
Z
, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment M
t
. A signal is emitted by the excited spins after the excitation signal B
1
is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (G
x
G
y
and G
z
) are employed using gradient coils. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
The use of gradient coils to generate a gradient field about the bore of a magnet for imaging is known in the art of nuclear magnetic resonance imaging. Generally, a patient is positioned on an examination table and inserted into a superconducting magnet having a cylindrical bore therethrough. The superconducting magnet provides the uniform magnetic field B
0
through the bore. The gradient coils extend around the bore and are energized to impose time varying magnetic fields on the uniform magnetic field.
While the use of gradient coils around the bore of the superconducting magnetic is practical during a whole-body imaging scan, it is not very efficient during micro-imaging of localized regions within a smaller field-of-view (FOV), such as the imaging of a finger. One of the problems with using whole-body gradient coils during micro-imaging is that the gradient fields extend through the bore of MRI device. The extension of the gradient fields over a large volume causes non-linearities in the gradient fields within the FOV. Another problem with whole-body gradient coils is dB/dt effects during imaging of human patients. Such dB/dt levels are regulated for patients under examination. That is, the speed at which the gradient coils are switched (i.e. ramped up and down) must be carefully monitored and controlled. These regulations limit the applied strength of the time-varying magnetic fields generated by the superconducting magnet and gradient coils during imaging.
It would therefore be desirable to have a device that provides substantially linear gradient magnetic fields during micro-imaging in a localized region or micro-FOV, that also decreases the amount of magnetic field gradient passing through a patient during a micro-imaging scan of the patient as compared to a whole-body imaging scan.
SUMMARY OF INVENTION
The present invention provides a local RF/gradient coil assembly and method of micro-imaging solving the aforementioned problems.
The invention includes the use of a magnet to produce a uniform background magnetic field for MR imaging of an object or anatomy in a localized region or micro-FOV. Upon selection of a micro-imaging scan, a computer activates a local RF/gradient coil assembly positioned within a bore of the magnet. The local RF/gradient coil assembly is configured such that, upon energization, substantially linear magnetic field gradients are created along three axes in a localized region or micro-FOV adjacent to the local RF/gradient coil.
In accordance with one aspect of the present invention, a device for micro-imaging includes a local RF/gradient coil assembly comprising a first gradient coil arranged to conduct current in opposite directions, a second gradient coil arranged to conduct current in one direction, and a third gradient coil arranged to conduct current in two planes. The first, second, and third coils are arranged to be electrically separated, and produce magnetic field gradients in different directions, such as along the axes of an x-y-z coordinate system. The first, second, and third coils are also positioned about one another to form a single local RF/gradient coil assembly.
In accordance with another aspect of the present invention, an MRI apparatus to acquire images includes a magnetic resonance imaging (MRI) system having a gradient coil positioned within a bore of a magnet to impress a polarizing magnetic field, and an RF transceiver system. The MRI system further includes an RF switch controlled by a pulse module to transmit RF signals to an RF coil mechanically connected to the gradient coil. The design of the gradient coil includes a first coil aligned along a first, or z, axis, and a second coil also aligned along the z-axis. The second coil is substantially positioned within the first coil. The gradient coil additionally includes a third coil having a plurality of bi-planar coils, wherein both the first and second coils are partially enclosed by the third coil. The first, second, and third coils are configured to provide a gradient magnetic field in different directions.
In accordance with yet another aspect of the present invention, a method of acquiring MR data from a localized region is disclosed comprising the steps of applying a uniform magnetic field to anatomy or an imaging object and locating the local gradient coil adjacent to a surface of a localized FOV of the imaging object. The localized FOV or micro-FOV is positioned in a region external to the local gradient coil. The method also includes the step of generating a substantially linear gradient over the localized FOV on three axes, such as x, y, and z of a three dimensional orthogonal coordinate system.
The invention further includes a micro-imaging gradient coil comprising a means for creating dB
z
/dz, dB
z
/dy, and dB
z
/dx gradients on a localized region or FOV. Each of the gradients are created in different directions, such as along the axes of a three dimension orthogonal coordinate system, and are superimposed over a background magnetic field. The invention further comprises a means for locating the micro-imaging gradient coil adjacent to a surface of the localized FOV of the imaging object, such that the localized FOV is positioned in a region external to the micro-imaging gradient coil.
Various other features, objects and advantages of the present invention will be made apparent from the following detailed description and the drawings.


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