Thin extended-cavity RF coil for MRI

Systems and methods for reducing an amount of space occupied by a radio frequency coil assembly in a magnetic resonance imaging system are provided. In one embodiment, a radio frequency coil assembly for a magnetic resonance imaging system includes a radio frequency coil disposed cylindrically around a patient space and a radio frequency shield disposed cylindrically around the patient space and electrically coupled to the axial ends of the radio frequency coil. The radio frequency shield may be configured to extend behind the radio frequency coil, and the axial length of the radio frequency shield may be at least two times the axial length of the radio frequency coil.

BACKGROUND

The subject matter disclosed herein relates generally to magnetic resonance imaging systems and, more particularly, to a radio frequency coil assembly for magnetic resonance imaging systems.

Magnetic resonance imaging (MRI) systems have become ubiquitous in the field of medical diagnostics. An MRI system may generally be cylindrical in shape and a patient may be placed within an imaging volume of the cylinder to be scanned. Based on a primary magnetic field, a radio frequency (RF) pulse, and time-varying magnetic gradient fields that interact with specific nuclear components in the patient, the MRI system may produce internal images of the patient. A radiologist may employ such images for research purposes or for diagnosis of disease.

When the patient is placed in the imaging volume of the MRI system, the patient may experience claustrophobia and/or discomfort due to the closeness of the scanner. Although techniques have been developed to increase the amount of available imaging volume space, such techniques may reduce the performance of one or more components of the MRI system.

BRIEF DESCRIPTION

Embodiments of the presently disclosed subject matter may generally relate to radio frequency coils for use in magnetic resonance imaging systems. In one embodiment, a radio frequency coil assembly for a magnetic resonance imaging system includes a radio frequency coil disposed cylindrically around a patient space and a radio frequency shield disposed cylindrically around the patient space and electrically coupled to the axial ends of the radio frequency coil. The radio frequency shield may be configured to extend behind the radio frequency coil away from the patient space, and the axial length of the radio frequency shield may be at least two times the axial length of the radio frequency coil.

In another embodiment, a magnetic resonance imaging system includes a gradient coil assembly disposed cylindrically around a patient space and a radio frequency coil assembly disposed cylindrically around the patient space and entirely within the cylinder formed by the gradient coil assembly, and the radio frequency coil assembly has a transverse length of less than two centimeters.

In a further embodiment, a magnetic resonance imaging system includes a cylindrical radio frequency coil assembly having a transverse thickness of two centimeters or less.

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 coils26,28,30, and32of the scanner12may be controlled by external circuitry to generate desired pulsed fields, and to induce 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 an upper quadrant of the cylindrical scanner12with an embodiment of a thin extended-cavity RF body coil. As shown inFIG. 2, the RF coil assembly32, an inner gradient coil assembly58, an outer gradient coil assembly60, and the main magnet24may be disposed cylindrically around the imaging volume18. The inner gradient coil set58and the outer gradient coil set60may each include a z-gradient coil26, x-gradient coil28, and y-gradient coil30.

The RF coil assembly32may include an RF coil62and an RF shield64. The RF coil62may include a plurality of rungs made of conductive material (e.g., copper), each of which may be controlled individually by the RF transmit/receive interface42to transmit and/or receive RF signals. Each of the conductive rungs of the RF coil62may be electrically coupled to an RF shield64via one or more capacitive elements. To form an RF coil cavity around the RF coil62, the RF shield64may include a front shield pane66, a side shield pane68, and a rear shield pane70. The RF coil cavity formed by the RF shield64may be filled with a low dielectric material, such as a dielectric epoxy.

The RF coil62may have a length72, which may be, for example, approximately 30 cm. The RF shield64may have a length74, which may be greater than the RF coil length72. For example, the RF shield length74may be approximately 2-3, 3-4, or 4-5 times greater than the RF coil length72, and may be, for example, approximately 90 cm. A cavity height76may represent the height of the RF coil cavity that is formed by the RF coil62and the front66, side68, and rear70panes of the RF shield64. The cavity height76may correspond to the radial, or transverse, length of the side panel68, and may also represent the radial distance from the RF coil62and the front shield pane66(both of which may be at approximately the same radial distance from the center of the scanner12) to the rear shield pane70.

To provide additional space in the imaging volume18, the cavity height76may be made very small relative to the lengths72and74. The cavity height76may be, for example, less than 2 cm when the RF shield length74is greater than the RF coil length72. A scanner length78may represent the axial length of the scanner12, which may be, for example, approximately 140 cm, and the imaging volume18may have a radius to the inner gradient coil set58of approximately 30 cm. Thus, the thin extended-cavity RF coil assembly32illustrated inFIG. 2may occupy approximately 3.3% of the possible imaging volume when the cavity height76is approximately 1 cm, rather than occupy approximately 6.7% when the cavity height76is 2 cm.

Varying the dimensions of the RF shield64may reduce the amount of space the RF coil assembly32occupies in the imaging volume18while maintaining a desired efficiency. It should be understood that the total magnetic flux in the scanner12is approximately proportional to a cross section of the RF coil cavity that is formed by the RF coil62and the front66, side68, and rear70panes of the RF shield64, multiplied by a current of the RF coil62. Similarly, the magnetic field at the isocenter is proportional to the total magnetic flux. As such, the coil losses are proportional to a circumference of the RF coil assembly32, multiplied by current squared. Thus, efficiency may described by the following equation:

Due to the relationship described in Equation (1) above, the following equation may be expressed relating RF shield length74(L) and cavity height76(h) to efficiency:

The RF coil assembly32may achieve a given efficiency at an RF shield length74and cavity height76. As shown in Equation (2), the RF coil assembly32may maintain the efficiency when the cavity height76is halved and the RF shield length74is multiplied by four. Rather than maintain an RF shield length74of approximately 30 cm and a cavity height76of approximately 2-3.5 cm, the cavity height76may be reduced to approximately 1 cm, while the RF shield length72may be extended to approximately 90 cm, which may cause only a minor reduction in efficiency. Additionally, it should be appreciated that because the thin extended-cavity RF coil assembly32does not intersect the gradient coil sets58or60, neither RF uniformity nor imaging volume may be comparably adversely impacted. Moreover, maintaining the RF coil assembly32apart from the gradient coil sets58or60may simplify the construction and installation process.

FIG. 3provides a three-dimensional view of the RF coil assembly32in which one of the rear shield panes70has been removed to expose elements of the underlying RF coil assembly32. As shown inFIG. 3, elements of the RF coil assembly32include the RF coil62, which may include a plurality of conductive rungs80(e.g., 16 or 32 rungs), which may be connected to capacitive elements82and separated by gaps84, and the RF shield64, which may include the shield panes66,68, and70. An additional conductive shield extension86may be coupled to the outer axial edges of the RF coil assembly32.