Abstract:
A multimode radio frequency coil (50 1 ) receives resonance signals from a region of interest while allowing arbitrary placement of the coil. A peripheral electrical conductor (62) is divided into four symmetric segments by capacitors (76), (78), (80), (82). A pair of crossing conductors (64, 66) are connected between 90° offset diagonally opposite portions of the peripheral loop (62). The crossing conductors include capacitors (68, 70) when not connected and include capacitors (68, 70, 86, 88) when connected at their midpoints. With this configuration, the coil supports orthogonal modes (72, 74) within the plane of the coil and, additionally, a third orthogonal mode (84) perpendicular to the plane of the coil. To image an extended region, a plurality of coils are overlapped to minimize mutual inductance relative to a first mode. An adjustable capacitor (90) across one of the coils adjusts mutual inductance relative to the second mode. A pair of half wavelength conductors (94, 96) are connected diagonally across the coils and are interconnected by an adjustable capacitor (98) for adjusting the third mode.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to the magnetic resonance arts. It finds particular application in improving signal-to-noise ratio (&#34;SNR&#34;) , reducing image acquisition time, and allowing arbitrary coil and patient positioning when overlapping multimode surface coils are used in conjunction with magnetic resonance imaging (&#34;MRI&#34;) and will be described with particular reference thereto. However, it is to be appreciated that the present application will also find application in conjunction with other magnetic resonance imaging and spectroscopy systems in which the B 0  primary magnetic field is orthogonal to the plane of the radio frequency coils. 
     Conventionally, magnetic resonance imaging procedures include disposing a patient in a substantially uniform, primary magnetic field B 0 . Magnetic resonance is excited in dipoles which preferentially align with the B 0  field by transmitting radio frequency excitation signals into the examination region and receiving radio frequency magnetic resonance signals emanating from the resonating dipoles. 
     Most commonly, the B 0  field is generated along the central bore of an annular magnet assembly, i.e., the B 0  field aligns with the central axis of the patient. Linear and quadrature radio frequency and gradient magnetic field coils surround the bore. Both types of coils are sensitive to placement relative to the main field and must be designed to be orientation specific. 
     In order to examine larger regions of patients disposed in the bore of a horizontal B 0  field imager, surface coils consisting of a plurality of loop coils have been used. See, for example U.S. Pat. No. 4,825,162 of Roemer and Edelstein. More specifically, a series of loop coils are partially overlapped in order to examine contiguous regions. As explained mathematically by Grover in &#34;Inductance Calculations&#34; (1946) and summarized in the Roemer and Edelstein patent, the mutual inductance between adjacent coils is minimized when the coils are positioned by a slight overlap. Although the use of overlapped loop coils with the induction minimized enables a larger area to be examined, each coil must be linear. That is, each coil must be sensitive to only one component and not sensitive to the orthogonal component such that no quadrature detection is provided. 
     To achieve an optimum SNR, specific magnetic coils are used for a unique magnetic field of orientation. More specifically, the coils have only two usable polarization modes. For example, if a magnetic field B 0  is oriented in a y-plane, magnetization exists in an x-z plane. No sensitivity to magnetization exists in the y-plane. To measure magnetization, one of the three following techniques is used. 
     In the first technique, a scan is performed at one spatial position, and then a second scan is performed with the coil at a second position. The second position overlaps, but substantially covers, the first position. This first technique typically involves physically moving the coil or using a type of switchable coil. 
     In the second technique, several linearly polarized coils are placed in an overlapping fashion with minimal mutual inductance. The outputs to the coils are connected to receivers and A/D converters. Thus, the resultant image has an SNR of a small linearly polarized coil with the field of view of a large coil similarly polarized. 
     The third technique builds on the second technique. It uses small quadrature coils placed in an overlapping fashion with minimum mutual inductance. The outputs of the coils are connected to receivers and A/D converters. As in the second technique, the resultant image has an SNR of a small linearly polarized coil with the field of view of a large coil similarly polarized. 
     In all three of the techniques mentioned above, the coil configurations are sensitive to the main field orientation. Linear coils may be positioned in two orthogonal planes relative to the main field without risking a decrease in sensitivity. Quadrature coils may only be positioned in one orthogonal plane relative to the main field without losing sensitivity. 
     These and other current techniques in the art lack the ability to place coils arbitrarily relative to the main field while preserving an optimal SNR. Each of the three configurations described above may only be optimized to work in a specific imaging plane relative to the main magnetic field. The linear versions require two orthogonal configurations to maintain optimal sensitivity, while the quadrature version may only require one such configuration to achieve the same benefit. 
     Devices achieving three orthogonal planes of sensitivity using a single element, or multiple overlapping elements, have previously not been available. Similarly, no coil, which can be arbitrarily placed relative to the main field, is known. 
     The present invention provides a new and improved apparatus and method which provides improved SNR over a given field of view without limiting the placement of the coil relative to the main field. 
     SUMMARY OF THE INVENTION 
     In accordance with one aspect of the present invention, a radio frequency coil is provided. The coil receives magnetic resonance signals from an extended region of interest while allowing arbitrary placement of the coil relative to a temporally constant main magnetic field. The radio frequency coil comprises a first capacitor electrically connected at a first location on a perimeter of the coil and a second capacitor electrically connected at a second location on the perimeter of the coil. A first segment of the coil is defined to be located between the first and second capacitors. A third capacitor is electrically connected at a third location on the perimeter of the coil. A second segment of the coil is defined to be located between the second and third capacitors. A fourth capacitor is electrically connected at a fourth location on the perimeter of the coil. A third segment of the coil is defined to be located between the third and fourth capacitors and a fourth segment of the coil is defined to be located between the fourth and first capacitors. A first electrical conductor, electrically connected between mid-points of the first and third segments of the coil, has a first radio frequency output defined therealong. A second electrical conductor, electrically connected between mid-points of the second and fourth segments of the coil, has a second radio frequency output defined therealong. A fifth capacitor is electrically connected along the first electrical conductor. A sixth capacitor is electrically connected along the second electrical conductor. Leads for receiving a first component of the resonance signal are connected across the fifth capacitor. Leads for receiving a second component of the resonance signal are connected across the sixth capacitor. Leads for receiving a third component of the magnetic field are connected across opposite peripheral segments of the coil. 
     In accordance with a more limited aspect of the invention, the first and second electrical conductors cross and are not electrically connected to each other. 
     In accordance with another aspect of the invention, the first and second electrical conductors cross and are electrically connected adjacent mid-points thereof. 
     In accordance with another aspect of the invention, the radio frequency coil includes a seventh capacitor, electrically connected along the first electrical conductor. 
     In accordance with another aspect of the invention, the radio frequency coil includes an eighth capacitor, electrically connected along the second electrical conductor. 
     One advantage of the present invention is its ability to receive signals simultaneously with three orthogonal components. Another advantage resides in improved signal-to-noise ratios (&#34;SNR&#34;) and reduced acquisition times, using an arbitrary coil placement and arbitrary patient placement relative to the magnetic field orientation. 
     Another advantage of the present invention is its ability to shape the coils arbitrarily to fit an arbitrary anatomical area of interest and preserve SNR&#39;s. 
     Still further advantages of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the preferred embodiments. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention may take form in various components and arrangements of components, and in various steps and arrangements of steps. The drawings are only for purposes of illustrating a preferred embodiment and are not to be construed as limiting the invention. 
     FIG. 1 is a diagrammatic illustration of a magnetic resonance system in accordance with the present invention; 
     FIG. 2 is an enlarged view of a coil in the coil assembly of FIG. 1; 
     FIGS. 2A, 2B and 2C illustrate three orthogonal modes of sensitivity of the coil of FIG. 2; 
     FIG. 3 is an enlarged view of a second embodiment of a coil in the coil assembly of FIG. 1; 
     FIG. 4 is an array of two coils; 
     FIG. 5 is a chart summarizing the isolation techniques between the various modes of operation for the array of coils shown in FIG. 4; 
     FIG. 6 is a graph illustrating a typical mutual inductance between the primary modes for the coils in the array of FIG. 4; 
     FIG. 7 illustrates a schematic representation of one method to interface a coil to an NMR system; 
     FIG. 8 illustrates capacitive coupling for the three modes; 
     FIG. 9 illustrates an embodiment of the invention in which three flat coils are arbitrarily contoured and overlapped for imaging an abdomen of a patient; 
     FIG. 10 illustrates another embodiment of the invention in which four curved and/or flat coils are overlapped and arbitrarily positioned on the patient for spine or leg imaging; 
     FIG. 11 illustrates another embodiment of the invention in which two cupped coils are used for imaging anatomies of a patient which fit into the coils; 
     FIG. 12 illustrates another embodiment of the invention in which two multimode coils are arbitrarily contoured for surrounding a sample. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to FIG. 1, an imaging region 10 is defined between pole pieces 12, 14. The pole pieces are interconnected by a ferrous flux path 16, such as a C or U-shaped iron element. Superconducting electrical windings (not shown) extend around the flux path 16 for inducing the magnetic flux in the ferrous flux path 16 and the B 0  field across the pole faces. Passive or active shims are disposed at the pole pieces or in the ferrous flux path adjacent the pole pieces to render the vertical B 0  field more linear across the imaging region 10. 
     For imaging, magnetic field gradient coils 20, 22 are disposed at the pole pieces 12, 14. In the preferred embodiment, the gradient coils are planar coil constructions which are connected by gradient amplifiers 24 to a gradient magnetic field controller 26. The gradient magnetic field controller, as is known in the art, causes current pulses which are applied to the gradient coils such that gradients in the uniform magnetic field are created along the longitudinal or z-axis, the vertical or y-axis, and the transverse or x-axis. 
     In order to excite magnetic resonance in dipoles of a subject disposed in the examination region 10, radio frequency coils 30, 32 are disposed between the gradient coils and the imaging region. A radio frequency transmitter 34, preferably a digital transmitter, causes the radio frequency coils to transmit radio frequency pulses requested by a radio frequency pulse controller 36 to be transmitted into the imaging region 10. A sequence controller 40, under operator control, retrieves an imaging sequence from a sequence memory 42. The sequence controller 40 provides the sequence information to the gradient controller 26 and the radio frequency pulse controller 36 such that radio frequency and gradient magnetic field pulses are generated in accordance with the selected sequence. 
     A radio frequency surface coil assembly 50 is disposed along a region of interest of the subject. Typically, the radio frequency coils 30, 32 are general purpose coils and are built-in. On the other hand, specialty surface coils are removable for greater flexibility. However, the surface coil 50 and the below-described alternate embodiments can be the only radio frequency coils in the system. In the embodiment of FIG. 1, the surface coil assembly 50 is an elongated spine coil that is disposed on a patient supporting surface immediately below the spinal column of a patient resting on the patient supporting surface. The surface coil assembly 50 with radio frequency receivers 52 demodulates the radio frequency resonance signals received by the built-in and/or removable radio frequency coils. The surface coil assembly 50 is an array of coils 50 1 , 50 2 , 50 3 , 50 4 , each connected with one or more receivers 52. Signals from the receivers are digitized with an array of analog-to-digital converters 54 and processed by a reconstruction processor 56 into volumetric image representations which are stored in a volumetric image memory 58. A video processor 59, under operator control, withdraws selected image data from the volume memory and formats it into appropriate format for display on a human-readable display 61, such as a video monitor, active-matrix monitor, liquid crystal display, or the like. 
     Although illustrated in conjunction with a C-magnet that generates a y-directed magnetic field, it is to be appreciated that the below described radio frequency coils are equally usable in the bore of annular magnets that generate a z-directed magnetic field. 
     With reference to FIGS. 2, 2A, 2B and 2C, each coil of the coil array 50 includes a peripheral loop 62. In the illustrated embodiment, the peripheral loop 62 is square and is defined by four linear conductors 62a, 62b, 62c, and 62d. A pair of electrical connectors 64, 66 interconnect opposite points on the loop. More specifically, the conductor 64 connects 180° opposite points of the peripheral loop 62; and, the conductor 66 connects a second pair of 180° opposite points on the peripheral loop 62. The second pair of 180° opposite points are offset by 90° from the first pair of oppositely disposed points. In the preferred square embodiment, the conductor 64 connects mid-points of conductor segments 62a and 62c; and the conductor 64 connects mid-points of the conductor segments 62b, 62d. 
     A capacitor or capacitive coupling 68 is connected in the first conductor 64 and a capacitor or capacitive coupling 70 is disposed in the electrical connector 66. These two capacitors enable the coil to support a first mode (FIG. 2A), which is sensitive to a magnetization component 72 in the z-direction and a second mode (FIG. 2B) which are primarily sensitive to magnetization components 74 in the x-direction of FIG. 1. Each quadrant of the outer loop 62 is broken by one of capacitors 76, 78, 80, and 82. These capacitors enable the coil to support a third mode (FIG. 2C) which is primarily sensitive to a magnetization 84 along the y-axis. In this manner, the coil is sensitive to the three mutually orthogonal modes. 
     When the coil 50 1  is oriented to lie in the x-z plane and a z-directed magnetic field is applied, modes 74, 84 have maximum sensitivity and mode 72 has minimum sensitivity. This will also be true if coil 50 1  is turned 90° to lie in the z-y plane, another 90° to lie in the x-z plane, and another 90° to lie in the z-y plane. When coil 50 1  is rotated clockwise within each plane in each of these orientations, the modes 72, 74 will alternate in sensitivity. However, the quadrature gain remains. When the coil is tipped such that it is not parallel to the z-axis, the mode 72 increases, reaching a maximum with the coil perpendicular to the z-axis. 
     When a magnetic field is applied in the y-direction with the coil 50 1  lying in the x-z plane, the modes 72, 74 have maximum sensitivity and mode 84 has minimum sensitivity. When the coil 50 1  is then rotated in the plane clockwise 90°, modes 72, 74 have maximum sensitivity and mode 84 has minimum sensitivity. Because of the three-modes of sensitivity, the coil 50 1  achieves quadrature gain at any arbitrary angle of orientation relative to the main field. 
     With reference to FIG. 3, in a second embodiment, the connectors 64, 66 are electrically connected to one another at their mid-points 85. Additional capacitors 86, 88 are added to the connectors 64, 66 to provide a capacitive coupling between the midpoint 85 and each interconnection with the peripheral conductor 62. Connecting the conductors 64, 66 at their mid-points improves stability. 
     With reference to FIG. 4, when two of the coils 50 1 , 50 2  are arranged in an array, they are partially overlapped. The degree of overlap in the z-direction (in the illustrated orientation) is adjusted to minimize the mutual inductance relative to mode 72. An adjustable tuning capacitor 90 is added to adjust mutual inductance relative to mode 74. A pair of half wavelength conductors 94, 96 are connected across diagonally opposite corners of each of coils 50 1  and 50 2 . The half wavelength conductors are connected by an adjustable tuning capacitor 98 for adjusting mutual inductance relative to mode 84. To facilitate interconnection of the half wavelength conductors 94, 96 while maintaining symmetry, each of capacitors or capacitive couplings 76, 78, 80, and 82 are divided into a balanced pair of capacitors or capacitive couplings 76a, 76b; 78a, 78b; 80a, 80b; and 82a, 82b. 
     With reference to FIG. 7, each of the three orthogonal modes are preferably sampled separately to provide three discrete signal components. More specifically, the mode 72 is sampled by a pair of leads connected across the capacitor 68 (or 86) and connected with a matching and tuning circuit 100. The output signals are amplified by an amplifier 102 and, preferably, digitized by an analogue digital converter 104 to provide a digital output of mode 72. Analogously, a pair of leads are connected across capacitor 70 (or 88) to sample mode 74. The sampled signal is adjusted by a tuning or matching circuit 106, amplified by a preamplifier 108, and digitized by an analog to digital converter 110. Analogously, a pair of conductors are connected with diagonally opposite points of the outer loop 62 to sample the mode 84. As illustrated in FIG. 4, capacitors 76 and 78 may be split into a pair of matched capacitors and the sampling leads connected therebetween. The leads are connected with a tuning and matching circuit 112 which includes a half wavelength section to compensate for the half wavelength difference encountered between the sampling points for the mode 84. The output signals are boosted by a preamplifier 114 and digitized by an analog digital converter 116. Preferably, each of the three digital output signals are conveyed to a different digital receiver 52 for demodulation. 
     FIG. 5 summarizes the isolation techniques between the various modes of operation. 
     Instead of using the isolation techniques described above (spatial position, variable capacitors, etc.), it is to be understood that other forms of isolation techniques may also be used. For example, extra loops may be added to the coil 50 1  to cancel the presence of certain modes of the coil 50 2 . These alternative isolation techniques exist and may be identified or practiced by those skilled in the art. FIG. 6 illustrates typical mutual inductance between the primary x, y and z modes 74, 84, and 72 of coils 50 1 , 50 2 . 
     FIG. 8 illustrates the coil 50 1  including inductive couplers associated with capacitors 68, 70, 76, 80. The inductive couplers transfer outputs from the capacitors 68, 70, 76, 80. 
     Other embodiments, which use phase shifters and combiners for combining signals prior to digitizing from each of the individual multimode coils within the array are also contemplated. In this embodiment, a single combined output signal is supplied to the receiver 52 for each coil. 
     FIG. 9 illustrates an embodiment of the invention in which three flat coils 50 1 , 50 2 , 50 3  are arbitrarily contoured and overlapped for imaging an abdomen of a patient. The contoured flat coils 50 1 , 50 2 , 50 3  can also image other cylindrical or elliptical body shapes, such as knees, torsos and heads, while allowing an arbitrary orientation of a patient relative to the main magnetic field. 
     FIG. 10 illustrates another embodiment of the invention in which four flat coils 50 1 , 50 2 , 50 3 , 50 4  are arranged in an array and curved to match the contour of a leg. Again, the coils can be curved and arbitrarily positioned. 
     FIG. 11 illustrates another embodiment in which coils 50 1 , 50 2  are cupped to match corresponding anatomies of the patient (e.g., breasts) . The cupped coils allow the patient to be positioned supine, prone, on his/her side, or other arbitrary positions during the imaging. 
     FIG. 12 illustrates another embodiment of the invention in which the multimode coils 50 1 , 50 2  are arbitrarily contoured. More specifically, the multimode coils 50 1 , 50 2  are contoured to fit volumes such as a pelvis, torso, or cardiac areas of a patient 118. 
     Embodiments which use multimode elements, constructed out of individual double D loops and butterflies, are also contemplated. 
     Modifying any of the above embodiments using double tuned coils, in place of any of the coils disclosed in this application, for multichannel spectroscopy imaging is also contemplated. 
     Using any of the above combinations as transmit/receive coils or using particular parts of the above disclosed coils as transmitters is also contemplated. 
     The invention has been described with reference to the preferred embodiment. Obviously, modifications and alterations will occur to others upon reading and understanding the preceding detailed description. It is intended that the invention be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.