Interleaved gradient coil for magnetic resonance imaging

Gradient coils for generating gradient magnetic fields in a magnetic resonance imaging system are provided. In one embodiment, a gradient coil for a magnetic resonance imaging system may include a plurality of turns formed generally in a figure-eight. The figure-eight may form a first section configured to overly a section of a first adjacent coil, a second section configured to underly another section of the first adjacent coil, a third section configured to overly a section of a second adjacent coil, and a fourth section configured to underly another section of the second adjacent coil.

BACKGROUND

The subject matter disclosed herein relates generally to magnetic resonance imaging systems and, more particularly, to gradient coils for use in magnetic resonance imaging systems.

Magnetic resonance imaging (MRI) systems enable imaging based on a primary magnetic field, a radio frequency (RF) pulse, and time-varying magnetic gradient fields that interact with specific nuclear components in an object, such as hydrogen nuclei in water molecules. The magnetic moments of such nuclear components may attempt to align with the primary magnetic field, but subsequently precess at a characteristic frequency known as the Larmor frequency. An RF pulse at or near the Larmor frequency of such nuclear components may cause the magnetic moments to be rotated. When the RF pulse has ended, the magnetic moments may attempt to realign with the primary magnetic field, emitting a detectable signal.

At least three discrete gradient coils (x, y, and z) may produce time-varying magnetic gradient fields (Gx, Gy, and Gz) calculated to enable detection of signals from a specified slice of the object. One problem that may arise is that the gradient fields produced by the respective gradient coils may vary. Such variations may make signal localization more difficult.

BRIEF DESCRIPTION

Embodiments of the presently disclosed subject matter may generally relate to gradient coils in magnetic resonance imaging systems. In one embodiment, a gradient coil for a magnetic resonance imaging system may include a plurality of turns formed generally in a figure-eight.

In another embodiment, a gradient coil for a magnetic resonance imaging system may include four self-similar coils, each coil having a plurality of turns formed generally in a figure-eight. The coils may be interleaved in a generally cylindrical configuration with a first section of each coil overlying a corresponding underlying section of a preceding coil, a second section underlying a corresponding overlying section of the preceding coil, a third section underlying a corresponding overlying section of a subsequent coil, and a fourth section overlying a corresponding underlying section of the subsequent coil.

In a further embodiment, a gradient coil for a magnetic resonance imaging system may include first and second X coils disposed at diametrically opposite locations in a generally cylindrical configuration and first and second Y coils disposed at diametrically opposite locations in the generally cylindrical configuration and displaced at 90 degrees with respect to the X coils. The coils may be interleaved with an overlying section of each X coil overlying a corresponding underlying section of each adjacent Y coil and an underlying section of the same X coil underlying a corresponding overlying section of the same adjacent Y coil.

DETAILED DESCRIPTION

Turning now to the drawings, and referring first toFIG. 1, a magnetic resonance imaging (MRI) system10is illustrated diagrammatically as including a scanner12, scanner control circuitry14, and system control circuitry16. While the MRI system10may include any suitable MRI scanner or detector, in the illustrated embodiment the system includes a full body scanner comprising an imaging volume18into which a table20may be positioned to place a patient22in a desired position for scanning. The scanner12may additionally or alternatively be configured to target certain anatomy, such as the head or neck.

The scanner12may include a series of associated coils for producing controlled magnetic fields, for generating radio frequency (RF) excitation pulses, and for detecting emissions from gyromagnetic material within the patient in response to such pulses. In the diagrammatical view ofFIG. 1, a main magnet24is provided for generating a primary magnetic field generally aligned with the imaging volume18. A series of gradient coils26,28and30are grouped in one or more gradient coil assemblies for generating controlled magnetic gradient fields during examination sequences as described more fully below. An RF coil32is provided for generating RF pulses for exciting the gyromagnetic material. Power may be supplied to the scanner12in any appropriate manner, as indicated generally at reference numeral34. In the embodiment illustrated inFIG. 1, the RF coil32may also serve as a receiving coil. Thus, the RF coil32may be coupled with driving and receiving circuitry in passive and active modes for receiving emissions from the gyromagnetic material and for applying RF excitation pulses, respectively. Alternatively, various configurations of receiving coils may be provided separate from RF coil32. Such coils may include structures specifically adapted for target anatomies, such as head coil assemblies, and so forth. Moreover, receiving coils may be provided in any suitable physical configuration, including phased array coils, and so forth.

In a present configuration, the gradient coils26,28, and30may be formed of conductive wires, bars or plates which are wound or cut to form a coil structure which generates a gradient field upon application of control pulses. The placement of the coils within the gradient coil assembly may be done in several different orders and with varying configurations, and the scanner12may further include complementary gradient coils (in the manner described below) to shield the gradient coils26,28, and30. Generally, a z-gradient coil26may be positioned at an outermost location, and is formed generally as a solenoid-like structure which has relatively little impact on the RF magnetic field. The gradient coils28and30may be x-axis and y-axis coils respectively.

The gradient coils26,28, and30of the scanner12may be controlled by external circuitry to generate desired fields and pulses, and to read signals from the gyromagnetic material in a controlled manner. When the material, typically bound in tissues of the patient, is subjected to the primary field, individual magnetic moments of the paramagnetic nuclei in the tissue partially align with the field. While a net magnetic moment is produced in the direction of the polarizing field, the randomly oriented components of the moment in a perpendicular plane generally cancel one another. During an examination sequence, the RF coil32may generate an RF pulse at or near the Larmor frequency of the material of interest, resulting in rotation of the net aligned moment to produce a net transverse magnetic moment. This transverse magnetic moment precesses around the main magnetic field direction, emitting RF signals that are detected by the scanner12and processed for reconstruction of the desired image.

The gradient coils26,28, and30may serve to generate precisely controlled magnetic fields, the strength of which vary over a predefined field of view, typically with positive and negative polarity. When each gradient coil26,28, or30is energized with known electric current, the resulting magnetic field gradient is superimposed over the primary field and produces a desirably linear variation in the axial component of the magnetic field strength across the field of view. The field may vary linearly in one direction, but may be homogenous in the other two. The three gradient coils26,28, and30have mutually orthogonal axes for the direction of their variation, enabling a linear field gradient to be imposed in an arbitrary direction with an appropriate combination of the three gradient coils26,28, and30.

The pulsed gradient fields may perform various functions integral to the imaging process. Some of these functions are slice selection, frequency encoding and phase encoding. These functions can be applied along the x-, y- and z-axes of the original coordinate system or along other axes determined by combinations of pulsed currents applied to the individual field coils.

The slice select gradient field may determine a slab of tissue or anatomy to be imaged in the patient, and may be applied simultaneously with a frequency selective RF pulse to excite a known volume of spins that may precess at the same frequency. The slice thickness may be determined by the bandwidth of the RF pulse and the gradient strength across the field of view.

The frequency encoding gradient, also known as the readout gradient, is usually applied in a direction perpendicular to the slice select gradient. In general, the frequency encoding gradient is applied before and during the formation of the MR echo signal resulting from the RF excitation. Spins of the gyromagnetic material under the influence of this gradient are frequency encoded according to their spatial position along the gradient field. By Fourier transformation, acquired signals may be analyzed to identify their location in the selected slice by virtue of the frequency encoding.

Finally, the phase encode gradient is generally applied before the readout gradient and after the slice select gradient. Localization of spins in the gyromagnetic material in the phase encode direction is accomplished by sequentially inducing variations in phase of the precessing protons of the material using slightly different gradient amplitudes that are sequentially applied during the data acquisition sequence. The phase encode gradient permits phase differences to be created among the spins of the material in accordance with their position in the phase encode direction.

A great number of variations may be devised for pulse sequences employing the exemplary gradient pulse functions described above, as well as other gradient pulse functions not explicitly described here. Moreover, adaptations in the pulse sequences may be made to appropriately orient the selected slice and the frequency and phase encoding to excite the desired material and to acquire resulting MR signals for processing.

The coils of the scanner12are controlled by the scanner control circuitry14to generate the desired magnetic field and radiofrequency pulses. In the diagrammatical view ofFIG. 1, the control circuitry14thus includes a control circuit36for commanding the pulse sequences employed during the examinations, and for processing received signals. The control circuit36may include any suitable programmable logic device, such as a CPU or digital signal processor of a general purpose or application-specific computer. Further, the control circuit36may include memory circuitry38, such as volatile and/or non-volatile memory devices for storing physical and logical axis configuration parameters, examination pulse sequence descriptions, acquired image data, programming routines, and so forth, used during the examination sequences implemented by the scanner12.

Interface between the control circuit36and the coils of the scanner12may be managed by amplification and control circuitry40and by transmission and receive interface circuitry42. The amplification and control circuitry40includes amplifiers for each gradient field coil26,28, and30to supply drive current in response to control signals from the control circuit36. The receive interface circuitry42includes additional amplification circuitry for driving the RF coil32. Moreover, where the RF coil32serves both to emit the RF excitation pulses and to receive MR signals, the receive interface circuitry42may include a switching device for toggling the RF coil between active or transmitting mode, and passive or receiving mode. A power supply, denoted generally by reference numeral34inFIG. 1, is provided for energizing the primary magnet24. Finally, the scanner control circuitry14includes interface components44for exchanging configuration and image data with the system control circuitry16.

The system control circuitry16may include a wide range of devices for facilitating interface between an operator or radiologist and the scanner12via the scanner control circuitry14. In the illustrated embodiment, for example, an operator workstation46is provided in the form of a computer workstation employing a general purpose or application-specific computer. The operator workstation46also typically includes memory circuitry for storing examination pulse sequence descriptions, examination protocols, user and patient data, image data, both raw and processed, and so forth. The operator workstation46may further include various interface and peripheral drivers for receiving and exchanging data with local and remote devices. In the illustrated embodiment, such devices include a monitor48, a conventional computer keyboard50, and an alternative input device such as a mouse52. A printer54is provided for generating hard copy output of documents and images reconstructed from the acquired data. In addition, the system10may include various local and remote image access and examination control devices, represented generally by reference numeral56inFIG. 1. Such devices may include picture archiving and communication systems, teleradiology systems, and the like.

FIG. 2illustrates a cross-sectional view of the scanner12, particularly employing interleaved gradient coils28and30in accordance with one embodiment. In the cross-sectional view ofFIG. 2, inner and outer gradient coil sets58and60are disposed radially around the inner volume18and between the main magnet24and the RF coil32. The inner gradient coil set58and the outer gradient coil set60each includes the z-gradient coil26, x-gradient coil28, and y-gradient coil30.

Because the scanner12employs interleaved gradient coils, a portion of the x-gradient coil28may be located above a portion of the y-gradient coil30on one side of each gradient coil set58or60, while a portion of the x-gradient coil28may be located below a portion of the y-gradient coil30on the other. As such, a mean radial distance from the center of the inner volume18for the x-gradient coil28and the y-gradient coil30for each gradient coil set58or60may be effectively the same. Thus, the x-gradient coil28and y-gradient coil30of the inner gradient coil set58may have an effective radial distance62from the center of the inner volume18. Similarly, the x-gradient coil28and y-gradient coil30of the outer gradient coil set60may have an effective radial distance64from the center of the inner volume18.

FIG. 3illustrates an interleaved figure-eight gradient coil66, which may be, for example, the x-gradient coil28or the y-gradient coil30. The interleaved figure-eight gradient coil66may generally be wound in the shape of a figure-eight having a first underlying portion68, a first overlying portion70, a second underlying portion72, and a second overlying portion74. Each overlying portion70and74may be at the same outer radial distance and each underlying portion68and72may be at the same inner radial distance from the center of the inner volume18. Steps76and78may connect overlying and underlying portions of each lobe of the interleaved figure-eight gradient coil66, which may represent bends in the coils in a transverse direction toward or away from the inner volume18, as shown. As will be described below, when a plurality of interleaved figure-eight gradient coils66are fitted together, the second overlying portion74of one coil66may cover the second underlying portion72of another coil66. Similarly, the first underlying portion68of the one coil66may be covered by the first overlying portion70by the other coil66. Such a configuration is described in greater detail below with reference toFIG. 5.

FIG. 4illustrates a three-dimensional view of an x-gradient coil28or a y-gradient coil30formed from two of the interleaved figure-eight gradient coils66disposed diametrically opposite to one another. As illustrated inFIG. 4, each interleaved figure-eight gradient coil66may include a plurality of figure-eight coil conductors80. The coil conductors80may be formed of any conductive material (e.g., copper), and may be solid or hollow. If the conductors80are hollow, a liquid coolant may be circulated through the conductors80, as described further below. The figure-eight coil conductors are wrapped as shown inFIG. 4, creating the various portions68-74.

FIG. 5is a schematic view of a configuration interleaving x- and y-gradient coils28and30have the same effective radial distance using four interleaved figure-eight gradient coils66. Of the four interleaved figure-eight gradient coils66shown inFIG. 5, the first and third may form the x-gradient coil28, while the second and fourth may form the y-gradient coil30. The coils66of the x-gradient coil28may interleave with the coils66of the y-gradient coil30in the manner depicted. Thus, the first underlying portion68of each coil66may fit beneath the first overlying portion70of an adjacent coil, while the second overlying portion74may fit over the second underlying portion72of the adjacent coil.

FIGS. 6-8illustrate interleaved gradient coils for use in the scanner12. Turning first toFIG. 6, an interleaved Golay gradient coil84having a concentric coil arrangement of two distinct lobes per coil half may be interleaved in a manner similar to the interleaved figure-eight gradient coil66ofFIG. 3. As such, the interleaved Golay gradient coil84may include a first overlying portion86, a first underlying portion88, a second overlying portion90, and a second underlying portion92. Each overlying portion86and90may be at the same outer radial distance and each underlying portion88and92may be at the same inner radial distance from the center of the inner volume18. Steps94and96may connect overlying and underlying portions of each side of the interleaved Golay gradient coil84, which may represent bends in the coils in a transverse direction toward or away from the inner volume18, as shown. It should be appreciated that a plurality of interleaved gradient coils84may be fitted together. For example, the first overlying portion86of one coil84may cover the first underlying portion88of an adjacent coil84, while the second underlying portion92of the one coil84may be covered by the second overlying portion90of the adjacent coil.

FIG. 7illustrates a three-dimensional view of an x-gradient coil28or a y-gradient coil30formed from two interleaved gradient coils84disposed diametrically opposite to one another. As illustrated inFIG. 7, each interleaved Golay gradient coil84may include a plurality of coil conductors98arranged in concentric coil windings. The coil conductors98may be formed of any conductive material (e.g., copper), and may be solid or hollow. If the conductors98are hollow, a liquid coolant may be circulated through the conductors98. The coil conductors98may be wrapped as shown inFIG. 7to form the Golay gradient coil84.

FIG. 8is a schematic view of a configuration for interleaving x- and y-gradient coils28and30that may be formed from interleaved gradient coils84. Of the four interleaved gradient coils84shown inFIG. 8, the first and third may form the x-gradient coil28, while the second and fourth may form the y-gradient coil30. The coils84of the x-gradient coil28may interleave with the coils84of the y-gradient coil30in the manner depicted. Thus, the first underlying portion88of each coil84may fit beneath an adjacent first overlying portion86, while the second overlying portion90may fit over the adjacent second underlying portion92.

FIGS. 9 and 10describe a hybrid gradient coil102having elements of the interleaved figure-eight gradient coil66and the interleaved Golay gradient coil84. Turning toFIG. 9, the interleaved hybrid gradient coil102may include a plurality of coil conductors104formed in the shape generally depicted therein. The coil conductors104may be formed of any conductive material (e.g., copper), and may be solid or hollow. If the conductors104are hollow, a liquid coolant may be circulated through the conductors104. The coil conductors104may be wrapped as shown inFIG. 9andFIG. 10, having some turns in the manner of the figure-eight gradient coil66and some turns in the manner of the Golay gradient coil84. Due to the shape of the conductors104near its equator, the hybrid gradient coil102may be more efficient than a conventional Golay coil.

The hybrid gradient coil102may be configured to be interleaved in a manner similar to the interleaved figure-eight gradient coil66and the interleaved Golay Golay gradient coil84. As such, the hybrid gradient coil102may include a first overlying portion106, a first underlying portion108, a second overlying portion110, and a second underlying portion112. Each overlying portion106and110may be at the same outer radial distance and each underlying portion108and112may be at the same inner radial distance from the center of the inner volume18. It should be appreciated that a plurality of interleaved hybrid gradient coils102may be fitted together. For example, the first overlying portion106of one coil84may cover the second underlying portion112of an adjacent coil102, while the first underlying portion108of the one coil102may be covered by the second overlying portion110of the adjacent coil. Such features may be more readily apparent as illustrated inFIG. 10, which represents a schematic view of an x-gradient coil28or a y-gradient coil30formed from two interleaved hybrid gradient coils102disposed diametrically opposite to one another.

FIG. 11illustrates a manner of employing the figure-eight gradient coil66to evenly distribute coolant throughout the figure-eight coil conductors80. The interleaved figure-eight gradient coil66may include an inlet118that may supply a coolant to various inflow turns120, which may include, for example, water with corrosion inhibitors. To evenly apply the coolant throughout the coil66, the inflow turns120may be alternatingly located between outflow turns122, which may return the coolant in a heated state to an outlet that may be located behind the inlet118, from which heat may be extracted before the coolant is recycled through the coil66. Both the inlet118and the outlet may terminate at a service end of the scanner12.

It should be noted that outer turns of the coil conductors80on one side of the interleaved figure-eight gradient coil66are connected to the inner turns of the other side. As such, each figure-eight coil conductor80may have approximately the same length. Accordingly, each cycle of coil conductors80from the inlet118to the outlet behind the inlet118may relatively evenly relieve heat dissipated by the figure-eight gradient coil66. Additionally, the configuration ofFIG. 11may further enable a parallel electrical connection.

FIG. 12illustrates another manner of employing the figure-eight gradient coil66to distribute coolant throughout the figure-eight coil conductors80. The interleaved figure-eight gradient coil66may include a terminal124, through which coolant may flow via various inflow turns120and outflow turns122. A complementary terminal behind the terminal124may similarly supply and receive coolant from the conductors80.

Outer turns of the coil conductors80on one side of the interleaved figure-eight gradient coil66may be connected to the inner turns of the other side. As such, despite additional bends of the conductors80, each figure-eight coil conductor80may have a very similar length. Accordingly, each cycle of coil conductors80from the terminal124to the complementary terminal may relatively evenly relieve heat dissipated by the figure-eight gradient coil66. In a manner not unlike the configuration ofFIG. 11, the configuration ofFIG. 12may further enable a parallel electrical connection. The terminal124and the complementary terminal may serve as an electrical input and output, providing a signal to the figure-eight gradient coil66.

FIG. 13depicts a configuration for parallel electrical connection of the figure-eight gradient coil66. As shown inFIG. 13, a manner of parallel connection may include an electrical input126and an electrical output128. WhileFIG. 13illustrates a connection to three of the six cycles of coil conductors80, the electrical input126and output128may electrically connect any number of conductors. For example, the electrical input126and output128may alternatively connect all six cycles at the upper and lower portions of the rightmost lobe.