# Patent

## Title
Quadrature and linear RF coil array for MRI of human spine and torso

## Abstract

A radio frequency (RF) coil array includes a plurality of RF coil sections arranged in a superior-inferior direction. Each RF coil section includes a first linear coil element, a loop-saddle coil quadrature pair and a second linear coil element configured in an overlapping arrangement in a left-right direction. The position of the first linear coil element and the second linear coil element on the left and right may be shifted in the superior-inferior direction with respect to the center loop-saddle coil quadrature pair.

## Background
CROSS REFERENCE TO RELATED APPLICATION
The present application claims the benefit of U.S. Provisional Application Ser. No. 61/291,039, filed Dec. 30, 2009, herein incorporated by reference in its entirety.


FIELD OF THE INVENTION
The present invention relates generally to a magnetic resonance imaging (MRI) system and in particular to a radio frequency (RF) coil array using loop-saddle quadrature pairs and linear side coils.
BACKGROUND OF THE INVENTION
Magnetic resonance imaging (MRI) is a medical imaging modality that can create pictures of the inside of a human body without using x-rays or other ionizing radiation. MRI uses a powerful magnet to create a strong, uniform, static magnetic field (i.e., the “main magnetic field”). When a human body, or part of a human body, is placed in the main magnetic field, the nuclear spins that are associated with the hydrogen nuclei in tissue water become polarized. This means that the magnetic moments that are associated with these spins become preferentially aligned along the direction of the main magnetic field, resulting in a small net tissue magnetization along that axis (the “z axis,” by convention). An MRI system also comprises components called gradient coils that produce smaller amplitude, spatially varying magnetic fields when a current is applied to them. Typically, gradient coils are designed to produce a magnetic field component that is aligned along the z axis, and that varies linearly in amplitude with position along one of the x, y or z axes. The effect of a gradient coil is to create a small ramp on the magnetic field strength, and concomitantly on the resonant frequency of the nuclear spins, along a single axis. Three gradient coils with orthogonal axes are used to “spatially encode” the MR signal by creating a signature resonance frequency at each location in the body. Radio frequency (RF) coils are used to create pulses of RF energy at or near the resonance frequency of the hydrogen nuclei. The RF coils are used to add energy to the nuclear spin system in a controlled fashion. As the nuclear spins then relax back to their rest energy state, they give up energy in the form of an RF signal. This signal is detected by the MRI system and is transformed into an image using a computer and known reconstruction algorithms.
As mentioned, RF coils are used in an MRI system to transmit RF excitation signals and to receive MR signals emitted by an imaging subject. Various types of RF coils may be utilized in an MRI system such as a whole-body coil and RF surface (or local) coils. Typically, the whole-body RF coil is used for transmitting RF excitation signals, although a whole-body RF coil may also be configured to receive MRI signals. One or more (e.g., an array) surface coils can be used as receive coils to detect MRI signals or, in certain applications, to transmit RF excitation signals. Surface coils may be placed in close proximity to a region of interest in a subject and, for reception, typically yield a higher signal-to-noise ratio (SNR) than a whole-body RF coil.
An array of surface RF coils can be used for “parallel imaging,” a technique developed to accelerate MR data acquisition. In parallel imaging, multiple receive RF coils acquire (or receive) data from a region or volume of interest. For example, to perform parallel imaging for the human spine and torso, a three-dimensional (3D) RF coil array is used. A 3D RF coil array typically consists of an anterior two-dimensional (2D) RF coil array and a posterior 2D RF coil array. Many existing 2D RF coil arrays for MRI are constructed using linear loop coils and a single layer structure. In order to improve SNR of an RF coil array, multiple layer coil configurations have been developed (for example, two-layer loop-saddle quadrature pairs) and used in, for example, one-dimensional (1D) RF coil arrays.
Human spine and torso imaging require different designs of RF coil geometry to achieve the best performance for each of the different anatomies. RF coils dedicated for spine imaging usually have multiple layers of smaller coil elements, for example, loop-saddle quadrature pairs, optimized for providing the best SNR for the spine at a shallow depth close to the coil elements. RF coils for torso imaging, on the other hand, use large size coil elements to accomplish better signal penetration for the deep tissue of the human body. In addition, the coverage in the right-left (R-L) direction of a dedicated spine coil is smaller than that of a dedicated torso coil. Therefore, the R-L coverage of a dedicated spine coil is too small to be adequate for whole body imaging. On the other hand, while a dedicated torso coil can provide large enough R-L coverage for whole body imaging, its performance for spine imaging is lower than that of a dedicated spine coil.
It would be desirable to provide a general purpose RF coil array that combines the best imaging capabilities of spine RF coils and torso RF coils into one single RF coil array system.
BRIEF DESCRIPTION OF THE INVENTION
In accordance with an embodiment, A radio frequency (RF) coil array includes a plurality of RF coil sections arranged in a superior-inferior direction. Each RF coil section includes a first linear coil element, a loop-saddle coil quadrature pair and a second linear coil element that are configured in an overlapping arrangement in a left-right direction.
In accordance with another embodiment, A radio frequency (RF) coil array includes a first linear coil element, a loop-saddle coil quadrature pair, and a second linear coil element. The first linear coil element, the loop-saddle coil quadrature pair and the second linear coil element are arranged in a left-right direction.



BRIEF DESCRIPTION OF THE DRAWINGS
The invention will become more fully understood from the following detailed description, taken in conjunction with the accompanying drawings, wherein like reference numerals refer to like parts, in which:
FIG. 1 is a schematic block diagram of an exemplary magnetic resonance imaging (MRI) system in accordance with an embodiment;
FIG. 2 shows an RF coil array in accordance with an embodiment;
FIG. 3 shows an RF coil tile or coil section including a loop-saddle coil quadrature pair and two linear coil elements in accordance with an embodiment;
FIG. 4 shows an RF coil array in accordance with an embodiment;
FIG. 5 shows an RF coil tile or coil section including a loop-saddle coil quadrature pair and two linear coil elements in accordance with an embodiment;
FIG. 6 shows an RF coil tile or coil section including a loop-saddle coil quadrature pair and two linear coil elements in accordance with an embodiment;
FIG. 7 shows an RF coil tile or coil section including a loop-saddle coil quadrature pair and two linear coil elements in accordance with an embodiment;
FIG. 8 shows a loop coil element with active decoupling in accordance with an embodiment;
FIG. 9 shows an asymmetric saddle coil element with active decoupling in accordance with an embodiment; and
FIG. 10 shows a saddle coil element with active decoupling in accordance with an embodiment.



DETAILED DESCRIPTION
FIG. 1 is a schematic block diagram of an exemplary magnetic resonance imaging (MRI) system in accordance with an embodiment. The operation of MRI system 10 is controlled from an operator console 12 that includes a keyboard or other input device 13, a control panel 14, and a display 16. The console 12 communicates through a link 18 with a computer system 20 and provides an interface for an operator to prescribe MRI scans, display resultant images, perform image processing on the images, and archive data and images. The computer system 20 includes a number of modules that communicate with each other through electrical and/or data connections, for example, such as are provided by using a backplane 20a. Data connections may be direct wired links or may be fiber optic connections or wireless communication links or the like. The modules of the computer system 20 include an image processor module 22, a CPU module 24 and a memory module 26 which may include a frame buffer for storing image data arrays. In an alternative embodiment, the image processor module 22 may be replaced by image processing functionality on the CPU module 24. The computer system 20 is linked to archival media devices, permanent or back-up memory storage or a network. Computer system 20 may also communicate with a separate system control computer 32 through a link 34. The input device 13 can include a mouse, joystick, keyboard, track ball, touch activated screen, light wand, voice control, or any similar or equivalent input device, and may be used for interactive geometry prescription.
The system control computer 32 includes a set of modules in communication with each other via electrical and/or data connections 32a. Data connections 32a may be direct wired links, or may be fiber optic connections or wireless communication links or the like. In alternative embodiments, the modules of computer system 20 and system control computer 32 may be implemented on the same computer system or a plurality of computer systems. The modules of system control computer 32 include a CPU module 36 and a pulse generator module 38 that connects to the operator console 12 through a communications link 40. The pulse generator module 38 may alternatively be integrated into the scanner equipment (e.g., resonance assembly 52). It is through link 40 that the system control computer 32 receives commands from the operator to indicate the scan sequence that is to be performed. The pulse generator module 38 operates the system components that play out (i.e., perform) the desired pulse sequence by sending instructions, commands and/or requests describing the timing, strength and shape of the RF pulses and pulse sequences to be produced and the timing and length of the data acquisition window. The pulse generator module 38 connects to a gradient amplifier system 42 and produces data called gradient waveforms that control the timing and shape of the gradient pulses that are to be used during the scan. The pulse generator module 38 may also receive patient data from a physiological acquisition controller 44 that receives signals from a number of different sensors connected to the patient, such as ECG signals from electrodes attached to the patient. The pulse generator module 38 connects to a scan room interface circuit 46 that receives signals from various sensors associated with the condition of the patient and the magnet system. It is also through the scan room interface circuit 46 that a patient positioning system 48 receives commands to move the patient table to the desired position for the scan.
The gradient waveforms produced by the pulse generator module 38 are applied to gradient amplifier system 42 which is comprised of Gx, Gy and Gz amplifiers. Each gradient amplifier excites a corresponding physical gradient coil in a gradient coil assembly generally designated 50 to produce the magnetic field gradient pulses used for spatially encoding acquired signals. The gradient coil assembly 50 forms part of a resonance assembly 52 that includes a polarizing superconducting magnet with superconducting main coils 54. Resonance assembly 52 may include a whole-body RF coil 56, surface or parallel imaging coils 76 or both. The coils 56, 76 of the RF coil assembly may be configured for both transmitting and receiving or for transmit-only or receive-only. A patient or imaging subject 70 may be positioned within a cylindrical patient imaging volume 72 of the resonance assembly 52. A transceiver module 58 in the system control computer 32 produces pulses that are amplified by an RF amplifier 60 and coupled to the RF coils 56, 76 by a transmit/receive switch 62. The resulting signals emitted by the excited nuclei in the patient may be sensed by the same RF coil 56 and coupled through the transmit/receive switch 62 to a preamplifier 64. Alternatively, the signals emitted by the excited nuclei may be sensed by separate receive coils such as parallel coils or surface coils 76. The amplified MR signals are demodulated, filtered and digitized in the receiver section of the transceiver 58. The transmit/receive switch 62 is controlled by a signal from the pulse generator module 38 to electrically connect the RF amplifier 60 to the RF coil 56 during the transmit mode and to connect the preamplifier 64 to the RF coil 56 during the receive mode. The transmit/receive switch 62 can also enable a separate RF coil (for example, a parallel or surface coil 76) to be used in either the transmit or receive mode.
The MR signals sensed by the RF coil 56 or parallel or surface coil 76 are digitized by the transceiver module 58 and transferred to a memory module 66 in the system control computer 32. Typically, frames of data corresponding to MR signals are stored temporarily in the memory module 66 until they are subsequently transformed to create images. An array processor 68 uses a known transformation method, most commonly a Fourier transform, to create images from the MR signals. These images are communicated through the link 34 to the computer system 20 where it is stored in memory. In response to commands received from the operator console 12, this image data may be archived in long-term storage or it may be further processed by the image processor 22 and conveyed to the operator console 12 and presented on display 16.
As mentioned, RF coils (for example, RF body coil 56 and a surface coil or coils 76) may be used to transmit RF excitation pulses and/or to receive MR signals. An array of RF surface coils 76 may be used, for example, for parallel imaging applications to receive data from a region or volume of interest. FIG. 2 shows an RF coil array in accordance with an embodiment. The 2D RF coil array 200 consists of multiple tiles or coil sections 202 arranged in the superior-inferior (S-I) direction. Each tile (or coil section) 202 consists of a center quadrature coil pair 204 and two linear coil elements 206, 208, one on each of the right and left sides of the center quadrature coil pair 204. The center quadrature coil pair 204 allows the RF coil array 200 to perform as a spine imaging coil and the two linear coil elements 206, 208 extend the right-left (R-L) coverage of the RF coil array for torso imaging. RF coil array 200 can be used for an anterior coil array and/or for a posterior coil array. The coil elements of the RF coil array 200 naturally decouple from one another. Accordingly, all the coil elements in a tile 202 can be used simultaneously or all the coil elements in the RF coil array 200 can be used simultaneously.
In the embodiment shown in FIG. 2, the quadrature coil pair 204 is a loop-saddle coil quadrature pair and each linear coil elements 206, 208 is an asymmetric saddle coil. A first asymmetric saddle coil 206 is on the left side of the loop-saddle coil quadrature pair 204 and a second asymmetric saddle coil 208 is on the right side of the loop-saddle coil quadrature pair 204. As described further below with respect to FIGS. 3-7, the two side linear coil elements 206, 208 can be various coil topologies, including, but not limited to, loop, saddle, asymmetric saddle, saddle train, higher order lobe coils, etc. In various embodiments, each of the two side linear coil elements can be the same coil topology or different coil topologies.
FIG. 3 shows an RF coil tile or coil section including a loop-saddle coil quadrature pair and two linear coil elements in accordance with an embodiment. Tile or coil section 302 consists of a center loop-saddle coil quadrature pair 304, a first asymmetric saddle coil 306 on the left side of the loop-saddle coil quadrature pair 304 and a second asymmetric saddle coil 308 on the right side of the loop-saddle coil quadrature pair 304. The loop-saddle coil quadrature pair 304 consists of a loop coil 310 and a saddle coil 312. Each asymmetric saddle coil 306, 308 has a small wing 314 and a large wing 316. The width (D1) of the large wing 316 is larger than the width (D2) of the small wing 314 (e.g., D1/D2>1). The shape of the two wings 314, 316 for each asymmetric saddle coil 310, 312 can be arbitrary.
In tile or coil section 302, the left side asymmetric saddle coil 306 and the right side asymmetric saddle coil 308 may be shifted in the superior-inferior (S-I) direction with respect to the center loop-saddle coil quadrature pair 304. Referring again to FIG. 2, the side linear coil elements 206, 208 in each tile 202 may be shifted in the S-I direction with respect to the loop-saddle coil quadrature pair 204 in the respective tile 202. The center of each of the left and right asymmetric saddle coils 206, 208 is lined up with an overlap region 220 between the center loop-saddle coil quadrature pairs 204 in adjacent tiles 202 in the S-I direction, as indicated by the horizontal dashed line 222 in FIG. 2. In this configuration, the left and right side asymmetric saddle coils 206, 208 compensate for the signal drop off at the overlap region 220 between two adjacent center loop-saddle coil quadrature pairs 204 so that the image uniformity in the S-I direction is improved. In addition, the mutual inductance between each side linear coil element and its nearest neighbor coil elements can be minimized simultaneously. This allows all the coil elements in the coil array to be used simultaneously to improve imaging efficiency and performance.
An exemplary number of tiles or coil sections 202 are shown in FIG. 2. Fewer or more tiles 202 can be used in the RF coil array 200. For example, tiles 202 can be repeated in the S-I direction to form a larger RF coil array system for covering a larger field-of-view (FOV). The RF coil array 200 can be used for both spine and torso imaging and provides improved image quality and uniformity and improved efficiency of the imaging. The RF coil array 200 can be used for three-dimensional (3D) parallel imaging for both spine imaging and torso imaging.
As mentioned above, the side linear coil elements in the RF coil array can be various coil topologies. FIG. 4 shows an RF coil array in accordance with an embodiment. The 2D RF coil array 400 consists of multiple tiles or coil sections 402 arranged in the superior-inferior (S-I) direction. Each tile (or coil section) 402 consists of a center loop-saddle coil quadrature pair 404, a first loop coil 406 on the left side of the loop-saddle coil quadrature pair 404 and a second loop coil 408 on the right side of the loop-saddle coil quadrature pair 404. FIG. 5 shows an RF coil tile or coil section including a loop-saddle coil quadrature pair and two linear coil elements in accordance with an embodiment. The center loop-saddle coil quadrature pair 504 consists of a loop coil 510 and a saddle coil 512. A first loop coil 506 is positioned on the left side of the loop-saddle coil quadrature pair 504 and a second loop coil 508 is positioned on the right side of the loop-saddle coil quadrature pair 504. In tile 502, the first loop coil 506 and the second loop coil 508 may be shifted in the superior-inferior (S-I) direction with respect to the center loop-saddle coil quadrature pair 504. Referring again to FIG. 4, the center of each of the first and second loop coils 406, 408 is lined up with an overlap region 420 between the center loop-saddle coil quadrature pair 404 in adjacent tiles 402 in the S-I direction, as indicated by the horizontal dashed line 422 in FIG. 4. In this configuration, the first and second loop coils 406, 408 compensate for the signal drop off at the overlap region 420 between two adjacent center loop-saddle coil quadrature pairs 404 so that the image uniformity in the S-I direction is improved. In addition, the mutual inductance between a loop coil and its nearest neighbor coil elements can be minimized simultaneously. This allows all the coil elements in the coil array to be used simultaneously to improve imaging efficiency and performance. An exemplary number of tiles 402 are shown in FIG. 4. Fewer or more tiles 402 may be used. For example, the tiles 402 can be repeated in the S-I direction to form a larger RF coil array system for covering a larger field-of-view (FOV). The RF coil array 400 can be used for both spine and torso imaging and provides better imaging quality and efficiency of the imaging. The RF coil array 400 can be used for three-dimensional (3D) parallel imaging for both spine imaging and torso imaging.
Additional examples of coil topologies that can be used for side linear coil elements are shown in FIGS. 6 and 7. FIG. 6 shows an RF coil tile or coil section including a loop-saddle coil quadrature pair and two linear coil elements in accordance with an embodiment. Tile 602 includes a center loop-saddle coil quadrature pair 604 having a loop coil 610 and a saddle coil 612. An asymmetric saddle coil 606 is positioned on the left side of the loop-saddle coil quadrature pair 604. An oval-shaped loop coil 608 is positioned on the right side of the loop-saddle coil quadrature pair 604. FIG. 7 shows an RF coil tile or coil section including a loop-saddle coil quadrature pair and two linear coil elements in accordance with an embodiment. Tile 702 includes a center loop-saddle coil quadrature pair 704 having a loop coil 710 and a saddle coil 712. A first loop coil 706 is positioned on the left side of the loop-saddle coil quadrature pair 704 and a second loop coil 708 is positioned on the right side of the loop-saddle quadrature pair 704. The first loop coil 706 and the second loop coil 708 are different sizes. In addition, in tile 702, the two side loop coils 706, 708 are not shifted in the S-I direction with respect to the center loop-saddle quadrature pair 704, rather, in accordance with alternative embodiments, the edges of the first loop coil 706 and the second loop coil 708 are lined up with the edges of the center loop-saddle coil quadrature pair in the S-I direction.
In other embodiments, each coil element of the RF coil array, for example, as shown in the embodiments of FIGS. 2 and 4 is equipped with multiple active decoupling (AD) networks or switches as shown in FIGS. 8-10. FIG. 8 shows a loop coil element with active decoupling, FIG. 9 shows an asymmetric saddle coil element with active decoupling and FIG. 10 shows a saddle coil element with active decoupling in accordance with various embodiments. Referring to FIGS. 8-10, the active decoupling networks or switches 850, 950, 1050 can be controlled, i.e., turned on/off, using a DC voltage/current. Accordingly, a programmable DC power supply 852, 952, 1052, e.g., a programmable multiple coil (MC) bias DC power supply of a MRI scanner system, is connected to the active decoupling networks 850, 950, 1050. When the active decoupling networks or switches 850, 950, 1050 of a coil element (for example, loop coil 800, asymmetric saddle coil 900, saddle coil 1000) are turned on, the coil element will be effectively desensitized and become electrically and magnetically transparent to a coil system that is used nearby. If all the active decoupling networks or switches of an RF coil array system are turned on, the entire RF coil array system will become electrically and magnetically transparent to other coil systems that are used in its vicinity. The other coil system can either be resonated at the same frequency of the RF coil array system or at a different resonating frequency, for example, at the Larmor frequency of Carbon-13.
This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to make and use the invention. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal language of the claims. The order and sequence of any process or method steps may be varied or re-sequenced according to alternative embodiments.
Many other changes and modifications may be made to the present invention without departing from the spirit thereof. The scope of these and other changes will become apparent from the appended claims.

## Claims

1. A radio frequency (RF) coil array comprising:
a plurality of RF coil sections arranged in a superior-inferior direction, each RF coil section comprising a first linear coil element, a loop-saddle coil quadrature pair and a second linear coil element configured in an overlapping arrangement in a left-right direction, wherein at least one of the first linear coil element or the second linear coil element is shifted in the superior-inferior direction with respect to the loop-saddle coil quadrature pair.

2. An RF coil array according to claim 1, wherein the plurality of RF coil sections are configured in an overlapping arrangement.

3. An RF coil array according to claim 2, wherein a position of a center of the first linear coil element is shifted in the superior-inferior direction to align with an overlap region between adjacent RF coil sections.

4. An RF coil array according to claim 2, wherein a position of a center of the second linear coil element is shifted in the superior-inferior direction to align with an overlap region between adjacent RF coil sections.

5. An RF coil array according to claim 1, wherein at least one of the first linear coil element and the second linear coil element is a loop coil.

6. An RF coil array according to claim 1, wherein at least one of the first linear coil element and the second linear coil element is an asymmetric saddle coil.

7. A radio frequency (RF) coil array comprising:
a first linear coil element;
a loop-saddle coil quadrature pair;
a second linear coil element, wherein the first linear coil element, the loop-saddle coil quadrature pair and the second linear coil element are arranged in a left-right direction, and at least one of the first linear coil element or the second linear coil element is shifted in a superior-inferior direction with respect to the loop-saddle coil quadrature pair.

8. An RF coil array according to claim 7, wherein a first portion of the first linear coil element overlaps a first portion of the loop-saddle coil quadrature pair.

9. An RF coil array according to claim 7, wherein a first portion of the second linear coil element overlaps a second portion of the loop-saddle coil quadrature pair.

10. An RF coil array according to claim 7, wherein a portion of the center of the first linear coil element is shifted in the superior-inferior direction with respect to a position of the loop-saddle coil quadrature pair.

11. An RF coil according to claim 10, wherein a portion of the center of the second linear coil element is shifted in the superior-inferior direction with respect to a position of the loop-saddle coil quadrature pair.

12. An RF coil array according to claim 7, wherein the first linear coil element, the loop-saddle coil quadrature pair and the second linear coil element are configured to form a coil section.

13. An RF coil array according to claim 12, further comprising a plurality of coil sections aligned in the superior-inferior direction.

14. An RF coil array according to claim 13, wherein the plurality of coil sections are configured to provide spine imaging.

15. An RF coil array according to claim 13, wherein the plurality of coil sections are configured to provide torso imaging.

16. An RF coil array according to claim 13, wherein the plurality of coil sections are configured as a posterior imaging array.

17. An RF coil array according to claim 13, wherein the plurality of coil sections are configured as an anterior imaging array.

18. An RF coil array according to claim 7, wherein at least one of the first linear coil element and the second linear coil element is a loop coil.

19. An RF coil array according to claim 7, at least one of the first linear coil element and the second linear coil element is an asymmetric saddle coil.