GB2513583A

GB2513583A - MRI gradient system with improved performance

Info

Publication number: GB2513583A
Application number: GB1307740.9A
Authority: GB
Inventors: Michael Ben Sellers
Original assignee: DBDT Ltd
Current assignee: DBDT Ltd
Priority date: 2013-04-30
Filing date: 2013-04-30
Publication date: 2014-11-05
Also published as: GB201307740D0

Abstract

A gradient system for use in an MRI machine includes a gradient coil assembly and a gradient amplifier. The gradient coil assembly comprises three independent and orthogonal gradient coils wherein two transverse coils are oriented such that they produce linear magnetic field gradients along axes (a-axis and b-axis) that are at 45 degrees to the sagittal and coronal directions. The gradient amplifier is configured to provide simultaneous near-identical currents to the transverse coils such that the composite resultant field gradients may be produced that align with axes that are perpendicular to the sagittal and coronal planes i.e. the conventional x-axis and y-axis. Such a gradient system produces transverse gradients that are comparable to conventional coil assemblies but approximately 1.4 times stronger without demanding larger peak currents from the gradient amplifier.

Description

Mill Gradient System with Improved Performance

Field of Invention

The invention relates to a gradient system for MRI with improved performance by utilizing multiple gradient coils simultaneously.

Background of Invention

Known Magnetic Resonance Imaging MRI) machines utilize gradient coils to generate linear magnetic field gradients along predetermined axes. Further, gradient coil assemblies generally comprise 3 independent coils, each producing a linear magnetic field gradient along a predetermined axis. Further, each of these predetermined axes is orthogonal. The inventor herein has recognized that at least two of said independent coils may be utilized simultaneously to improve the performance of the gradient system by producing a stronger gradient without increasing the cunent demanded from each axis of the gradient amplifier and without increasing the time taken to change from zero culTent to maximum current, the rise time.

During gradient system operation independent gradient amplifiers are used to apply a voltage to each independent coil in order to drive electric current through the windings. The maximum gradient strength is determined by the coil's geometry and the maximum electric current which the amplifier is capable of supplying. The rise time is determined by the impedance of the coil and the applied voltage. It is desirous in clinical MRI to maximize the gradient strength and minimize the rise time within the physiological limits that patients can endure. This places significant demands on the gradient amplifier to produce large currents at high voltages.

Thus, the inventor herein has recognized that there is a need to improve gradient coil performance without increasing the demands on the gradient amplifier.

Summary of Invention

The foregoing problems are greatly reduced by a gradient system designed in accordance with the exemplary embodiments disclosed herein.

For the purposes of this embodiment, the z-axis is defined as being perpendicular to the imaging transverse plane and approximately along the axis of symmetry of the MRT machine. The x-axis is perpendicular to the imaging sagittal plane and approximately horizontal. The y-axis is perpendicular to the coronal plane and approximately vertical.

A gradient coil assembly in accordance with exemplary embodiments includes a gradient tube extending along the z-axis. The tube includes three independent and orthogonal gradient coils which may be actively shielded to reduce the amount of stray field that interacts with other components of the MRI machine. The Z gradient coil comprises axisymmetric windings distnbuted along the length of the tube and produces a linear magnetic field gradient given by Ci = where G is constant over the desired Field of View (FoV) along the z-axis. The transverse gradient coils A and B compnse saddle coils distnbuted around the tube oriented such that they produce linear magnetic field gradients that are at 45 degrees to the x-axis and the y-axis. Thus, transverse coil A produces a linear magnetic field gradient given by GA = Z where GA is constant over the desired FoV along the a-axis which is disposed at 45 degrees to the positive x-axis and positive y-axis. Transverse coil B produces a linear magnetic field gradient given by GB = where GB is constant over the desired FoV along the b-axis which is disposed at 45 degrees to the negative x-axis and positive y-axis.

A method for operating a gradient system in accordance with exemplary embodiments is provided. The method inc'udes simultaneously energizing the transverse coils A and B to produce composite field gradients. By the principle of vector summation, energizing the transverse gradient coils A and B simultaneously with almost identical magnitudes of electric culTent yields composite linear magnetic field gradients that can be aligned with the x-axis and y-axis. Further, by energizing coils A and B with predetermined current directions, it is possible to produce positive and negative magnetic field gradients along the x-axis and y-axis. Further, the composite field gradient magnitude will be larger than its component parts by a factor of SQRT(2). Thus, a gradient coil assembly designed and operated according to this invention will produce magnetic field gradients along the x-axis and y-axis that are 1.414 times larger than conventional gradient coils without the need for higher peak currents from the gradient amplifier. Further, these brger fie'd gradients can be achieved without any associated increase in rise time because the impedance of each coil has not changed.

An alternative embodiment of this invention involves planar gradient coils for open MRI. Gradient systems for open MRI generally comprise 2 gradient coil assemblies juxtaposed across a patient gap. Although the axis convention for open MRI is different to cylindrical MRI. it will be readily understood by those skilled in the art that the transverse gradient coils for open MRI can be oriented at 45 degrees to the x-axis and y-axis and simultaneously energized to produce a field gradient that is 1.414 times larger than that for conventional coils.

Other systems and/or methods according to the embodiments will be or become apparent to one with skill in the art upon review of the following drawings and detailed description. It is intended that at all such additional systems, methods, and/or computer program products be within the scope of the present invention, and be protected by the accompanying claims.

Bnef Description Drawings

Figure 1 is a section view of an MRI machine.

Figure 2 is a section view of a gradient coil assembly in the MRI machine.

Figure 3 is an end view of a gradient coil assembly in the MRI machine.

Figure 4 is a vector diagram of a composite magnetic field gradient

Description of an Embodiment

Referring to the drawings, identical reference numerals represent identical components in the various views.

Referring to Figure 1, an MRI machine is provided for generating images of a person 18. The MRI machine may comprise a superconducting magnet 12, a gradient coil assembly 22, an RF body cofi 62, a gradient amplifier unit 14 and a system controller 16.

Superconducting magnet 12 is provided to generate a constant magnetic field that is homogeneous over the FoV 52 of the MRI machine for the purpose of imaging person 18.

Gradient coil assembly 22 has 3 independent and orthogonal coils A, B and Z. Coil A, coil B and coil Z generate magnetic fie'd gradients respectively along predetermined a-axis, b-axis and z-axis in response to signals Ia, Tb and Iz from the individual amplifier channels contained in gradient amplifier unit 14.

Referring to Figure 2, gradient coil assembly 22 is actively shielded, thus includes an inner gradient coil assembly 23, an outer gradient cofi assembly 25, and resin 54 disposed throughout the gradient coil assembly 22 to hold it together.

Inner gradient coil assembly 23 includes an inner gradient tube 24, an inner Z coil 27, an inner A coil 28, an inner B coil 29 and resin 54 to hold it together.

Inner gradient tube 24 is provided to be disposed within an outer gradient tube 26 and is disposed about the z-axis 58. Gradient tube 24 may be constructed from a fiber composite material such as glass reinforced plastic (GRP).

Inner Z coil 27 is provided to generate a magnetic field gradient along a predetermined z-axis58. Coil 27 may include a plurality of copper conductors disposed in a plurality of grooves (not shown) formed in tube 24.

Outer gradient coil assembly 25 includes an outer gradient tube 26, an outer Z coil 40, an outer A coil 42, an outer B coil 44 and resin 54 to hold it together.

Outer gradient tube 26 is disposed around an inner gradient tube 24 and is disposed about z-axis 58. Gradient tube 26 may be constructed from a fiber composite material such as glass reinforced plastic (GRP).

Outer Z coil 40 is provided to generate a magnetic field gradient along the z-axis that is disposed to provide electromagnetic shielding of the inner Z coil 27 such that any magnetic flux straying outside of the gradient coil assembly 22 is minimized.

Coil 40 may include a plurality of copper conductors disposed in a plurality of grooves (not shown) formed in tube 26.

Referring to Figure 3, inner A coil 28 is provided to generate a magnetic field gradient along a predetermined a-axis 56-Coil 28 may comprise a plurality of saddle coils that subtend approximately 180 degrees with gaps 67 between them. Inner A coil 28 may be disposed a predetermined radial distance away from inner Z coil 27.

Inner B coil 29 is provided to generate a magnetic field gradient along a predetermined b-axis 57. Coil 29 may comprise a plurality of saddle coils that subtend approximately 180 degrees with gaps 68 between them. Coil 29 may be disposed a predetermined radia' distance away from coil 28.

Outer A coil 42 is provided to generate a magnetic field gradient along the a-axis 56 that is disposed to provide electromagnetic shielding of the inner A coil 28 such that any magnetic flux straying outside of the complete gradient coil assembly is minimized. Coil 42 may comprise a plurality of saddle coils that subtend approximately 180 degrees. Coil 42 may be disposed a predetermined radial distance away from coil 40.

Outer B coil 44 is provided to generate a magnetic field gradient along the b-axis that is disposed to provide electromagnetic shielding of the inner B coil 29 such that any magnetic flux straying outside of the complete gradient coil assembly is minimized. Coil 44 may comprise a plurality of saddle coils that subtend approximately 180 degrees. Coil 44 may be disposed a predetermined radial distance away from coil 42.

Gradient amplifier 14 may be operated by the system controller 16 such that a predetermined electric current 1A flows through A coil assembly, comprising inner A coil 28 and outer A coil 42, while a predetermined electric current 1R simultaneously flows through B coil assembly, comprising inner B coil 29 and outer B coil 44. In practice, the magnetic field gradient produced by the current 1A in A coil is given by GA = IA 1\where A is the efficiency of the A coil. Similarly, the field gradient produce by B in B coil is given by GB = B B where 4 is the efficiency of the B coil. Note that because of their different radii, is unlikely to be equal to Referring to Figure 4, if A coil and B coil are simultaneously energized to produce identical positive gradients along their respective a-axis and b-axis, that is GA (72) = GB (73), the composite field gradient along the y-axis is given by Gy (74) here: C.,. =CJA.cos4S+GB -cos45 Thus (i =2G\.cos45=V-GA=1.4l4.GA Further, the respective signs of GA and GB, and therefore the signs of A and B, may be chosen to produce positive or negative linear magnedc field gradients along the X and Y axes as desired.

The gradient coil assembly and method related thereto provides a substantial advantage over known assemblies and methods. In particular, the inventive gradient coil assembly ufilizes a combination of transverse coils to produce a composite gradient field in the X and Y directions thereby increasing the achievable gradient strength by a factor of 1.414 without placing increased demands on peak current from the gradient amplifier 14.

While the invention is described with reference to an exemplary embodiment, it will be understood by those skilled in the art that various changes may be made and equivalence may be substituted for elements thereof without departing from the scope of the invenfion. In addition, many modifications may be made to the teachings of the invention to adapt to a particular situation without departing from the scope thereof.

Therefore, it is intended that the invention not be limited to the embodiment disdosed for carrying out this invention, but that the invention includes all embodiments falling within the scope of the intended claims. Moreover, the use of the term's first, second, etc. does not denote any order of importance, but rather the term's first, second, etc. are used to distinguish one element from another.