Efficiently shielded MRI gradient coil with discretely or...

Electricity: measuring and testing – Particle precession resonance – Spectrometer components

Reexamination Certificate

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C324S309000, C324S319000

Reexamination Certificate

active

06479999

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to the magnetic resonance arts. It finds particular application in conjunction with medical magnetic resonance imaging and will be described with particular reference thereto. It is to be appreciated, however, that the invention also finds application in conjunction with other types of magnetic resonance imaging systems, magnetic resonance spectroscopy systems, and the like.
In magnetic resonance imaging, linear magnetic field gradients are used for spatial encoding. Gradient coils are used to produce the linear magnetic field gradients. Gradient coils are generally designed to provide an imaging field-of-view (FoV) that is fixed in size. For example, in whole-body applications the gradient coil will typically be designed to produce sufficiently linear or uniform magnetic field gradients over a 50 cm diameter spherical volume (DSV). For a dedicated cardiac scanner, however, the DSV may be 35 cm. For a dedicated head system the linear gradients would typically be designed to produce sufficiently linear magnetic field gradients over a 25 cm DSV. Of course, some models of scanner are designed with slightly larger or smaller DSVs. As the useful DSV is made smaller, the stored energy of the gradient coil is reduced, which allows for higher performance, namely, higher peak gradient strengths and faster gradient coil switching. Outside the substantially linear region of the gradient field (i.e., the “useful” DSV), and to a lesser extent within, the magnetic field gradients produce image distortion. Software-based distortion correction schemes have been developed to correct for non-uniformities within the useful DSV, as well as to expand somewhat the useful imaging FoV beyond the linear region.
In each dedicated case noted above, the gradient coil is generally a unique electromechanical structure and gradient coils with a defined DSV are known and utilized throughout the industry. For example, the most common is a self-shielded symmetric gradient coil design for whole-body imaging applications. Dedicated head and cardiac/head coil designs have emerged to enhance performance (peak strength and switching rate) over a reduced imaging DSV. Generally, body access is desirable for patient comfort reasons, although dedicated head gradient designs continue to be discussed for advanced neuro/brain research applications.
Gradient coils are heavy electromechanical devices, unlike most RF surface coils, which can be easily removed and replaced with different RF coils between imaging procedures (except for body RF transmit coils, which are typically fixed in the imaging system). A gradient coil, due to its high power nature and the high forces created when it is energized, is firmly fixed within an MRI system. As such, a dedicated gradient coil tends to make the MRI system a dedicated imaging system, limiting its scope of clinical application. Thus, accommodating both large and small FoV applications has generally required either separate dedicated machines, which is expensive, or the use of dedicated insertable coils for the smaller volumes, which are heavy and difficult to insert or replace.
More recently, dual or twin gradient designs have been described in the literature that attempt to combine both large volume and high-performance small volume imaging capabilities into a single gradient coil electromechanical package. Katznelson et al., in U.S. Pat. No. 5,736,858, describe a means for providing two gradient coils, which can be configured to allow for two different useful DSVs. Each gradient axis, x, y, and z, has two gradient coil sets. One gradient coil set is designed to produce a linear magnetic field gradient over a first DSV, and a second gradient coil set is designed to produce a second linear magnetic field gradient, such that when the second gradient coil is driven in series with the first gradient coil, there results a second DSV that is larger than the first DSV. In this scheme, the DSV can take two discrete values but is not continuously variable. The first gradient coil has lower stored energy and can be switched faster than the second gradient coil alone or when the two gradient coils are connected in series. In another embodiment, the first coil produces a gradient for use in small FoV applications and the second coil produces a gradient for use in conventional, large FoV applications and a single amplifier means and a switching means allows for one or the other coil to be used separately. In the preferred embodiment, the first coil is used for fast-switching, small FoV imaging and both coils together are used for larger FoV imaging and/or to produce very high gradient strengths, which may find use in diffusion imaging applications. A key point is that each coil is designed so as to produce, alone or in combination, a linear gradient magnetic field over one of two possible imaging DSVs. In the preferred embodiment the two coils are used together (in series) to produce a relatively large DSV. In the alternate embodiment, each coil can be used individually to create reasonably non-distorted magnetic resonance images over two differently sized DSVs. Each coil is self-shielded or actively-shielded in design to minimize eddy current effects. A drawback of this approach is that two full-power gradient coils are layered within one electromechanical assembly. This consumes a great deal of radial space, particularly when the two coils occupy different radial positions within the electromechanical structure. Since the delivered power increases with R
5
, even a small increase in gradient coil diameter has significant power ramifications. Also, cooling of the two coils becomes an issue, as does the ability to fit in other components such as passive and electrical shim coils.
It has also been proposed by Kimmlingen et al. (“Gradient system with continuously variable field characteristics,” ISMRM 2000 (April, 2000, Denver meeting)) to take a standard whole body coil with a large field of view and identify a subset of the primary coil windings that would produce a linear gradient in a smaller FoV, but with comparable (about 20% less) peak gradient strength and substantially lower inductance (about 45% less), allowing for faster gradient switching. A generally corresponding subset of the shield was also selected analogously. A switching means or a dual amplifier design, to feed both coil sections separately, would be provided such that either the subset or all of the windings could be utilized, and the amount of current to subset or other windings could be adjustable, depending on the size of the FoV. The primary advantage is that the primary and shield coils occupy the same radial position with the normal six layers, making cooling and construction easier and more cost-effective. A disadvantage of this approach is that when some of the coil windings were taken away to provide for the smaller FoV, some gradient strength was lost. Another disadvantage is that shielding is compromised since only the combined coils were optimally shielded, leading to increased eddy current effects.
Petropoulos, in U.S. Pat. No. 6,049,207, describes a dual gradient coil assembly with two primary coils and one common shield coil. Each primary coil produces a linear magnetic field gradient over differently sized DSVs when operated with the common shield coil. The residual eddy current effects are not equal for the two coils; one inevitably is better than the other. However, this is minimized by constraining each continuous current primary coil and common shield coil combination to have an integer number of turns before discretization. The approach of having one common shield does save some radial space for manufacture. However, two high power primary coils are still required.
The present invention contemplates a new and improved gradient coil system which provides a selectively or continuously variable imaging field of view, and which overcomes the above-referenced problems and others.
SUMMARY OF THE INVENTION
In a first aspect of the present invention, a m

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