A magnetic resonance imaging method and apparatus

Abstract: A coil assembly (40) includes a first, birdcage type head coil assembly (42) dimensioned to receive a patient's head and a second, neck coil assembly (44) including an anterior coil portion (44a) and a posterior coil portion (44b) dimensioned to receive the patient's neck region. The head and neck coils are partially overlapped. A first cable extends (98a) from the anterior coil portion past the birdcage head coil assembly and a second coaxial cable (98b) extends from the posterior portion past the birdcage head coil assembly. A first decoupling circuit (104a) is disposed in the first coaxial cable beyond a guard ring (106) and a second decoupling circuit (104b) is disposed in the second coaxial cable adjacent the region of overlap between the head and neck coil assemblies.

Description: The present invention relates to a magnetic resonance imaging method and apparatus. It finds particular application in conjunction with split-top insertable radio frequency coils for magnetic resonance imaging of the head and neck and will be described with particular reference thereto. It is to be appreciated, however, that the present invention will also find application in other multiple coil techniques, spectroscopy, phased array coils, imaging for other than medical diagnostic purposes, and the like. Conventionally, magnetic resonance imaging systems generate a strong, uniform static magnetic field B 0 in a free space or bore of a magnet. This main magnetic field polarizes the nuclear spin system of an object in the bore to be imaged. The polarized object then possess a macroscopic magnetic moment vector pointing in the direction of the main magnetic field. In a superconducting main magnet assembly, annular magnets generate the static magnetic field B 0 , along a longitudinal or z-axis of the cylindrical bore. To generate a magnetic resonance signal, the polarized spin system is excited by applying a radio frequency field B 1 , perpendicular to the z-axis. Typically, a radio frequency coil for generating the radio frequency field is mounted inside the bore surrounding the sample or patient. In a transmission mode, the radio frequency coil is pulsed to tip the magnetization of the polarized sample away from the z-axis. As the magnetization precesses around the z-axis back toward alignment, the precessing magnetic moment generates a magnetic resonance signal which is received by the radio frequency coil in a reception mode. For imaging, a magnetic field gradient coil is pulsed for spatially encoding the magnetization of the sample. Typically, the gradient magnetic field pulses include gradient pulses pointing in the z-direction but changing in magnitude linearly in the x, y, and z-directions, generally denoted G x , G y , and G z , respectively. The gradient magnetic fields are typically produced by a gradient coil which is located inside the bore of the magnet and outside of the radio frequency coil. Conventionally, when imaging the torso, a whole body radio frequency coil is used in both transmit and receive modes. By distinction, when imaging the head, neck, shoulders, or an extremity, the whole body coil is often used in the transmission mode to generate the uniform B 1 excitation field and a local coil is used in the receive mode. Placing the local coil close to the imaged region improves the signal-to-noise ratio and the resolution. (US-A-5166618, in which a local neck coil is passively decoupled from the RF excitation field by means of diodes). In some procedures, local coils are used for both transmission and reception. One drawback to local coils it that they tended to be relatively small. The whole body coils are typically used for elongated regions, such as the spine. One technique for adapting surface coils for imaging an elongated region is illustrated in U.S. Patent No. 4,825,162 of Roemer, in which a series of surface coils are lapped to construct a phased array. Other radio frequency coil designs include a multi-modal coil known as the "birdcage" coil. See, for example, U.S. Patent No. 4,692,705 of Hayes. Typically, a birdcage coil has a pair of end rings which are bridged by a plurality of straight segments or legs. In a primary mode, currents in the rings and legs are sinusoidally distributed which results in improved homogeneity along the axis of the coil. Homogeneity along the axis perpendicular to the coil axis can be improved to a certain extent by increasing the number of legs in the coil. Typically, a symmetric birdcage coil has eight-fold symmetry. Such a symmetric birdcage coil with N legs (where N is an even integer) exhibits N/2 mode pairs. The first (N/2)-1 mode pairs are degenerate, while the last mode pair is non-degenerate. The primary mode of such an eight-fold symmetric birdcage coil has two linear modes which are orthogonal to each other. The signals from these two orthogonal or quadrature modes, when combined, provide an increased signal-to-noise on the order of 40%. The simplest driven current pattern or standing waves defined by superpositions of degenerate eigenfunctions. For a low-pass birdcage of N meshes driven at is lowest non-zero eigenfrequency, the current in the n-th mesh is given by sin(2πn/N+). The phase angle  determines the polarization plane of the resulting B 1 radio frequency field and can be varied continuously by suitable application of driving voltages. The alignment and isolation of the two linear modes of a birdcage coil are susceptible to sample geometry. That is, the sample dominates the mode alignment and isolation between the two linear modes. Birdcage coils, like other magnetic field coils, undergo mutual inductive coupling when positioned adjacent each other. As the coils approach each other, the mutual inductive coupling tends to increase until a "critical overlap" is reached. At the critical overlap, the mutual inductance drops to a minimum. As the coils are moved towards a complete coincidence from the critical overlap, the mutual inductive coupling again increases. See, "Optimized Birdcage Resonators For Simultaneous MRI of the Head and Neck", Leussler, SMRM, 12th Annual Meeting, Book of Abstracts, page 1349 (1993). In one multiple coil birdcage design, two birdcage coils have been overlapped to a point of minimum mutual inductance. A symmetric coil is used to image the head and an asymmetric coil was used to image the neck. Capacitive elements are added to provide the necessary phase shifting through the leg sections of unequal length. The coils are mounted in a rigid frame for optimum symmetry and a fixed geometric position. The coils are isolated from each other by the critical overlapping as well as by the addition of neutralization capacitors. See for example, U.S. Patent No. 4,769,605 issued September 6, 1988 to Timothy R. Fox. Building an asymmetric coil design is fairly complicated and time-consuming. The phase shift from one section to another in the birdcage coil needs to be maintained for optimum coil performance. The critical overlapping between the two birdcage coils reduces the mutual coupling between the coils to a certain extent. Introducing different coil samples into the two birdcage coils alters the alignment of their linear modes and the mode isolation in either of the coils will change. This change, in turn, affects the symmetry and therefore the mutual coupling between the coils. The greater the mutual coupling between the coils, the larger the noise correlation between the coils and therefore the lower the combined signal-to-noise ratio. Electrical optimization of such a coil design is very complex. The tuning, matching, and isolation process is iterative and thus time-consuming. More specifically, the two linear modes of each birdcage coil need to be tuned, matched, and aligned to their respective coupling points on the coil and isolated from one another and from the two linear modes of the neck coils. Such a complex and iterative tuning, matching, and isolation process is not readily amenable to mass production. "Novel Two Channel Volume Array Design For Angiography of the Head and Neck", Reykowski, et al., SMR 2nd Annual Meeting, Book of Abstracts, pp. 216 (1994), discloses a birdcage coil in combination with two volumetric Helmholtz coils arranged such that the B 1 fields of the two Helmholtz coils are diagonal and perpendicular to one another. The two quadrature combined outputs, one from the birdcage coil and one from the Helmholtz coils are interfaced to two channels of the system. By orienting the two Helmholtz coils such that their B 1 fields are orthogonal, coupling is reduced and the noise correlation therebetween held to a minimum. However, when these two volume coils are overlapped with a quadrature head coil, they experience the same difficulties discussed above in conjunction with the multiple birdcage coils. That is, when different sampled geometries are introduced, the isolation between the individual volume Helmholtz coils and the head coil change, causing a change in isolation, resulting in an increased noise correlation between all coils and a lower combined signal-to-noise ratio. Manufacturability of the coil is again complex and time-consuming. The problem of coil interaction generally exists whenever two or more volume coils of different geometries are used. Electrical optimization of these coil designs is often complicated and iterative, hence time-consuming. Different sample geometries introduced into the designs alter coil-to-coil isolation, resulting in different noise correlations between the coils from one patient to another. "Head and Neck Vascular Array Coil For MRI", Srinivasan, et al., SMR 2nd Annual Meeting, Book of Abstracts, pp. 1107 (1994) disclose a combination birdcage coil and quadrature volume coil pair. In the described coil design, the coils maintained different current distributions with preferred mode orientations independent of one another. The coil consists of a birdcage coil and a quadrature volume coil pair. The quadrature volume coil pair consists of at least two surface coils of the distributed type, that maintain preferred mode orientations with respect to one another at all times. The birdcage coil maintains an eight-fold symmetry; whereas, the surface coil maintains a two-fold symmetry. After a nominal overlap is achieved between the coils of this design, only one iteration of tuning is required to retune all coils to the magnetic resonance frequency. In accordance with one aspect of the present invention, a magnetic resonance apparatus includes a magnet for generating a temporally constant, uniform magnetic field through an examination region, at least one radio frequency coil which performs at least one of (1) transmitting radio frequency signals into the examination region to induce and manipulate resonance of dipoles disposed therein and (2) receiving radio frequency signals from resonating dipoles in the examination region, and a processor for processing the received magnetic resonance signals, the radio frequency coil including: a first coil assembly; a second coil assembly partially overlapping the first coil assembly in a common overlap region, characterised by a first electronic circuit mounted adjacent to and connected to the second coil assembly; a first coaxial cable extending from the first electronic circuit past and contiguous to the first coil assembly, to a region on a side of the first coil assembly opposite from the second coil assembly; and a first coil-to-coil decoupling circuit connected to the first coaxial cable for inhibiting the first and second coil assemblies from coupling to each other along the first coaxial cable. The second coil assembly may include a first coil connected to a first coaxial cable and a second coil, the radio frequency coil further including: a second electronic circuit disposed adjacent to and contiguous with the second coil; a second coaxial cable extending from the second coil past and contiguous to the first coil assembly to the opposite side thereof; and a second coil-to-coil decoupling circuit connected to the second coaxial cable for inhibiting the second coil and the first coil assembly from coupling to each other along the second coaxial cable. The second coil-to-coil decoupling circuit may be disposed in a common plane with the overlap region. The first coil assembly may include a birdcage style coil and a guard ring disposed adjacent the birdcage style coil on a side opposite from the first volume coil assembly, the first coil-to-coil decoupling circuit being disposed on an opposite side of the guard ring from the birdcage coil. The coil-to-coil decoupling circuits may each include a case having a spool-like cable guide therein about which the coaxial cable is looped to form an inductance, the case further having a pair of coaxial cable receiving shoulders for receiving the coaxial cable and locking it against longitudinal movement. At least one tuning capacitor may be connected with the looped coaxial cable adjacent the cable guide. An electrically conductive radio frequency shield may shield the case. In accordance with another aspect of the present invention, a method of magnetic resonance imaging is provided in which a temporally constant uniform magnetic field is generated through a head and neck examination region, magnetic field gradients are applied across the examination region, radio frequency signals are transmitted into the examination region to induce and manipulate magnetic resonance of dipoles therein, radio frequency signals are received from the resonating dipoles with a radio frequency coil assembly that has a first coil assembly around the head region and a second coil assembly around the neck region, the second coil assembly partially overlapping the first coil assembly in a common overlap region, and the received radio frequency signals are processed into an image representation, the method further comprising receiving magnetic resonance signals from resonating dipoles in at least one of the head and neck regions with corresponding first and second coil assemblies to generate a volumetric image representation of at least one of the head and neck regions, including a first electronic circuit mounted adjacent to and connected to the second coil assembly, and a first coaxial cable extending from the first electronic circuit past and contiguous to the first coil assembly, to a region on a side of the first coil assembly opposite from the second coil assembly, and inhibiting the first and second coil assemblies from coupling to each other using a first coil-to-coil decoupling circuit connected to the first coaxial cable. The second coil assembly may be operated alone to image only the neck region or the first neck coil assembly may be operated alone to image only the head region. The first and second coil assemblies may be tuned individually while separated, the first and second coil assemblies being overlapped and fixed in their overlapped state. After overlapping, the coil assemblies may be retuned in a single iteration. Thereafter, a patient is received in the coil assemblies for imaging. One advantage of the present invention is that it provides easy and efficient patient handling procedures. Another advantage of the present invention resides in its ease of tuning, matching, and mode isolation adjustment. The present invention is also advantageous due to the possibility of several operating modes. The invention will now be described further by way of example with reference to the accompanying drawings in which: FIGURE 1 is a diagrammatic illustration of a magnetic resonance imaging system with an insertable head and neck coil in accordance with the present invention; FIGURE 2 is a perspective view of the head and neck coil of FIGURE 1; FIGURE 3 is an electrical diagram and side view of the head coil of FIGURE 1 and associated preamplifier and decoupling circuitry; FIGURE 4 is a perspective view of the head coil of FIGURE 1 with the top panel removed to illustrate the location of preamplifier, interface, and decoupling circuitry; FIGURE 4A is a detailed view of the anterior neck coil region of FIGURE 4; FIGURE 4B is a detailed view of the coupling circuitry region of FIGURE 4; FIGURE 5 is a diagrammatic view illustrating the effective electrical circuitry taken cross-sectionally through the guard ring of FIGURE 3; FIGURE 6 is a perspective view of the head coil of FIGURE 1 viewed from below with the lower casing removed to expose interconnect and decoupling circuitry location; FIGURE 6A is a detailed view of the posterior neck coil and decoupling circuit region of FIGURE 6; FIGURE 7 is a flow chart illustrative of a preferred manufacturing procedure; FIGURE 8A is a perspective view of an exemplary one of the decoupling circuitry of the preceding FIGURES; FIGURE 8B is a perspective view of the decoupling circuit of FIGURE 8A with the cover removed to show internal detail; FIGURE 8C is an equivalent circuit of the decoupling circuit of FIGURES 8A and 8B; FIGURE 9 is a diagrammatic illustration of an alternate embodiment of the present invention; and, FIGURE 10 is a diagrammatic illustration of yet another alternate embodiment of the present invention. With reference to FIGURE 1, a plurality of primary magnetic coils 10 generate a uniform, temporally constant magnetic field B 0 along a longitudinal or z-axis of a central bore 12. In a preferred superconducting embodiment, the primary magnet coils are supported by a former 14 and received in a toroidal helium vessel or can 16 . The vessel is filled with helium to maintain the primary magnet coils at superconducting temperatures. The can is surrounded by a series of cold shields 18 which are supported in a vacuum dewar 20 . Of course, annular resistive magnets, C-magnets, and the like are also contemplated. A whole body gradient coil assembly 30 includes x, y, and z-coils mounted along the bore 12 for generating gradient magnetic fields, G x , G y , and G . z Preferably, the gradient coil assembly is a self-shielded gradient coil that includes primary x, y, and z-coil assemblies 32 potted in a dielectric former and secondary x, y, and z-coil assemblies 34 that are supported on a bore defining cylinder of the vacuum dewar 20 . A whole body radio frequency coil 36 is mounted inside the gradient coil assembly 30 . A whole body radio frequency shield 38 , e.g., copper mesh, is mounted between the whole body RF coil 36 and the gradient coil assembly 30 . An insertable radio frequency coil 40 is removably mounted in the bore in an examination region defined around an isocenter of the magnet 10 . In the embodiment of FIGURE 1, the insertable radio frequency coil is a head and neck coil for imaging one or both of patient's head and neck. An operator interface and control station 50 includes a human-readable display, such as a video monitor 52 , and an operator input means including a keyboard 54 , a mouse 56 , a trackball, light pen, or the like. A computer control and reconstruction module 58 includes hardware and software for enabling the operator to select among a plurality of preprogrammed magnetic resonance sequences that are stored in a sequence control memory. A sequence controller 60 controls gradient amplifiers 62 connected with the gradient coil assembly 30 for causing the generation of the G x , G y , and G z gradient magnetic fields at appropriate times during the selected gradient sequence and a digital transmitter 64 which causes a selected one of the whole body and insertable radio frequency coils to generate B 1 radio frequency field pulses at times appropriate to the selected sequence. Resonance signals received by the coil 40 are demodulated by a digital receiver 66 and stored in a data memory 68 . The data from the data memory are reconstructed by a reconstruction or array processor 70 into a volumetric image representation that is stored in an image memory 72 . The image is reconstructed from the birdcage and quadrature coil signals when a combined head and neck image is wanted. Alternately, signals from the head and neck coils can be reconstructed separately to make separate images. It is to be appreciated, that for a reconstruction processor and image memory that has a fixed size, e.g., 1024x1024x1024, that the resolution of the resultant image will be higher when the imaged volume is smaller. A video processor 74 under operator control converts selected portions of the volumetric image representation into slice images, projection images, perspective views, or the like as is conventional in the art for display on the video monitor 52 . With continuing reference to FIGURE 1 and further reference to FIGURES 2 and 3, the preferred insertable radio frequency coil 40 includes a birdcage coil 42 and a quadrature coil pair 44 including an upper or anterior coil 44a and a lower or posterior coil 44b. The patient's head is received within the birdcage coil with the anterior coil 44a wrapping around the patient's upper shoulder onto the patient's chest and the posterior coil 44b wrapping around the lower side of the patient's shoulders and along the patient's back. Each of the coils has outputs for two linear modes, preferably orthogonal modes. The birdcage coil and the coils in the quadrature coil pair have capacitors or inductive elements added at appropriate locations such that each operates in a low pass, high pass, band pass, or band stop configuration. In the preferred split-top embodiment, an upper half of the birdcage coil and the anterior coil are housed in a first or removable housing portion 80 . To reduce claustrophobic effects on the patient, the upper housing portion has windows 82 between adjacent legs of the birdcage coil. The upper housing portion is removably received on a lower housing portion 84 which rests on the patient support. The upper and lower housing portions are interconnected by mechanical pins or connectors (not shown). Electrical connectors 86 , such as pin, contacts, capacitive couplings, or the like, which may be the same as the mechanical pin or connector assemblies, electrically interconnect end rings 88a , 88b of the birdcage coil 42. Preferably, a mechanical latch 90 holds the first and second portions of the insertable coil assembly together. Adapter tabs or other interconnectors (not shown) are associated with the insertable coil and the patient support to assure accurate alignment of the head coil assembly with the magnetic resonance system. An electrical plug or socket 92 is disposed adjacent a rear end of the insertable coil for interconnection with a matching socket or plug arrangement in the patient support. This enables built-in cable handling assemblies to be provided below the patient support to facilitate operation and use. Although not shown, it is understood that pads are also provided within the coil to immobilize the patient during scanning and to help with patient comfort. The upper and lower coil portions include formers on which copper foil coils are supported, e.g., fiber reinforced plastic. The coil formers are fastened to inner portions of the housing for rigidity and for maintaining the coil's shape and structural integrity. Initially, the head coil formers are fixed to the housing. During manufacture and initial calibration, the neck coil formers and the carried neck coil 44 are shifted until a point of minimum mutual inductance with the head coil 42 is reached. At this point, the neck coil formers are fixed to the head coil formers, fixing the relative positions of the neck and head coils. The inner housings are contoured to fit the anatomy under investigation. The outer housings cover the internal parts and electronic assemblies, as well as provide mechanical strength to the coil construction. Between the inner and outer housings, skeletons are provided to add rigidity to the coil structure. With particular reference to FIGURES 3, 4, 4A and 4B, the head portion of the coil is of a birdcage design. The anterior neck coil 44a is etched on a flexible PC board 94a and mounted on its respective coil former. The coil former and PC board are fastened into the upper housing 80 after the appropriate overlap has been set. The PC board also carries an electronic assembly 96a including matching and decoupling electronics, a preamplifier protection circuit, a preamplifier, and a coaxial cable support assembly. The anterior neck coil is tuned and matched to the magnetic resonance frequency prior to overlapping with the birdcage coil 42. A 50 Ohm coaxial cable 98a passes through an S-shaped or extension region 100a , rides on a support bridge 102a that originates on the anterior neck side and extends along a central plane of the birdcage head coil, and connects to a decoupling circuit 104a. The decoupling circuit is located beyond or rearward of a guard ring 106 of the birdcage coil. The shield of the coaxial cable past the anterior decoupling circuit 104a is connected to the guard ring. As illustrated in FIGURE 5, the anterior coaxial cable navigates over the guard ring to the bottom of the coil assembly to an interconnect or output board 108 and the plug or socket 92 . With particular reference to FIGURES 6 and 6A, the posterior neck coil 44b is etched on a circuit board 94b which is fastened to its respective former. The former is adjustably positioned, then fixed to become an integral part of the lower housing portion 84 . An electronic circuit 96b is mounted on the posterior coil circuit board 94b . The circuit again includes matching and decoupling electronics, a preamplifier protection circuit, and a preamplifier. A coaxial cable 98b extends from the posterior circuitry 96b through an S-shaped expansion region 100b along a central plane of the birdcage head coil on a bridge 102b to a posterior decoupling circuit 104b which is disposed in the plane of the overlap between the birdcage head coil 42 and the posterior neck coil 44b . The coaxial cable 98b further extends from the decoupling circuit to the interconnect board 108 . The shield of the posterior coaxial cable is again connected with the guard ring 106 thus shorting them together. With reference to FIGURE 7, the neck and birdcage coils are each tuned 110 , 112 and the orientation of their B 1 fields is adjusted 114 , 116. The tuned coils are overlapped 118 a selected amount, e.g., to a point of minimum mutual inductance. The S-shaped extension region 100a , 100b of the cables facilitate ready positioning of circuit boards 94a , 94b relative to the birdcage coil. Once overlapped to the selected point, the coils are fixed together 120 . The birdcage coil and the neck coils are retuned 122, 124 in a single iteration to the magnetic resonance frequency. In most cases, the birdcage coil needs most of the retuning and the neck coil little or none. The extent of the retuning depends mainly on the proximity of the coaxial cables 98a and 98b to the birdcage coil 42. In most cases, the neck coil frequencies remain the same. Because only one iteration is needed to retune all coils to the same magnetic resonance frequency after achieving the selected overlap, the number of steps needed for optimization during manufacturing is reduced. With reference to FIGURES 8A, 8B, and 8C, the anterior and posterior decoupling circuits 104a and 104b each include a housing having a top cover 130a and a bottom cover 130b within which a circular spool 132 is supported. The coaxial cable extends through a first immobilizer or guide 134a, around the spool, and exits by a second immobilizer or guide 134b. Fixed and variable capacitors are soldered across the turn(s) of the coaxial cable to tune the decoupling circuits close to the magnetic resonance frequency. When the top and bottom covers are closed, the immobilizers grip the coaxial cable sufficiently tightly that it is locked against sliding into or out of the housing. The immobilizers not only reduce any torque from being transmitted to the electronics within the decoupling circuit, but also protect against changes in inductance due to changes in the tightness of the winding around the spool. After the decoupling circuit housing is closed, a sheet of copper foil 136 is wrapped around the housing and the immobilizers. Once the decoupling circuit is positioned in place, the foil is pierced through a small hole 138 to gain access to a trimming capacitor and the decoupling circuit is tuned to the magnetic resonance frequency. After fine tuning, the access opening to the trimming capacitor is foil covered as well. The foil covering functions as a radio frequency shield to isolate the decoupling tank circuit (FIGURE 8C) for efficient decoupling circuit operation. Tuning the decoupling circuits to the magnetic resonance frequency serves two major functions. The primary function is to present a high impedance (Z) for currents flowing in the shields of the coaxial cables during RF transmit. This prevents the formation of closed loops inside the magnet bore. The second function of each decoupling circuit is slightly different. The posterior neck coil has its second mode tuned to the magnetic resonance frequency. The posterior decoupling circuit isolates the currents flowing in the region where the shield of the coaxial cable is exposed to the birdcage coil from the currents flowing in the shield exposed to the posterior neck coil. For the coaxial cable 98a above the anterior neck coil 44a , the central plane is a plane of symmetry. That is, the central plane is a virtual ground. The anterior neck coil has its first, primary mode tuned to the magnetic resonance frequency. The anterior coaxial cable is transparent to the anterior neck coil. However, when the coaxial cable is guided across the birdcage coil 42, the cable is no longer at a virtual ground plane and currents are induced in its shield. These circulating currents are substantially attenuated by the anterior decoupling circuit 104a . Again, the anterior decoupling circuit is before the guard ring 106 of the birdcage coil. This stops induced RF currents on the shield of the anterior coaxial cable from being communicated to the guard ring through the shorting connection. The guard ring is also broken by shorting capacitors (not shown) to reduce gradient induced eddy currents. The decoupling circuits are shielded to minimize their interaction with the body coil 36 during RF transmit, with the individual coils in the insertable coil 40, and to reduce any irradiation from the shields of the straight segments of the coaxial cable. Additional decoupling circuits may also be employed to provide further barriers to the transmission of stray radio frequency signals. If the decoupling circuits are eliminated completely, the coaxial cables would carry currents in their shields. Further, the interaction between the coils in the array would not be minimized. Rather, the two coils would talk to each other through the shield, causing a disadvantageous transfer of noise between the coils. The presence of the decoupling circuits as illustrated maintains the signal-to-noise ratio of the coils in its different operating modes within a few percent. The signal-to-noise ratio and uniformity of the illustrated insertable coil is similar to that of a standard quadrature head coil without neck coils. The signal-to-noise ratio of the posterior neck coil is similar to that of a C-spine element in a cervical-thoracic-lumbar array coil. However, the coverage of the neck coils is greater in the present design. The coil has three modes -- (1) head and neck, (2) head only, and (3) neck only. See TABLE 1 below. In the head and neck mode, the birdcage head coil and the neck coils are operated together to image from the aortic arch to the top of the head. In the head and neck mode, the signal-to-noise ratio is still high as is the B 1 homogeneity. This head and neck imaging mode enables blood flow to be measured and monitored as it flows from the aortic arch beyond the circle of Willis in the brain in a single image. This high signal-to-noise ratio and uniform coverage is important in the imaging of skull-base tumors that are difficult to image with either a head coil alone or a neck coil alone. The neck coils 44 are decoupled during the head only mode and the head coil 42 is decoupled during the neck only mode. Operating Modes CH1 CH2 CH3 CH4 Head Only ON ON OFF OFF Neck Only OFF OFF ON ON Head/Neck ON ON ON ON The coils 42, 44 are actively decoupled during body transmit. The coil is interfaced to the magnetic resonance system via an interface. Individual channel device drivers in the system transmit/receive interface circuit are programmed to provide different sets of voltages in the three operating modes for the insertable radio frequency coil 40 , viz, the head only, neck only, and head and neck mode. In the head mode, the neck coils are actively decoupled and only the head coil is resonant at the magnetic resonance frequency. Similarly, int he neck mode, the head coil is actively decoupled and only the neck coils are resonant at the magnetic resonance frequency. In the head and neck mode, all coils in the insertable radio frequency coil 40 are resonant at the larmor frequency and receive magnetic resonance signals. It is to be appreciated that various linear or quadrature surface coils may be overlapped with the birdcage coil. Such surface coils are preferably shaped in conformity with the surface of the subject adjacent the region of interest. The electrical circuits 96a, 96b of the preferred embodiment include a preamplifier and output for the anterior neck coil and a preamplifier and output for the posterior neck coil. A birdcage coil output circuit 96c includes two preamplifiers connected to the birdcage coil to provide 90°, quadrature outputs. These four preamplified signals in the illustrated embodiment are conveyed to the radio frequency receiver 66 which demodulates the four resonance signals. Alternately, the quadrature resonance signals can be shifted by 90° and combined at the insertable coil rather than after demodulation. As yet another alternative, the signals can be digitized at the surface coil and digital signals sent to the receiver. In the above-described preferred embodiment, the birdcage coil has eight-fold symmetry and the neck coils two-fold symmetry. However, other symmetries are also contemplated. In the embodiment of FIGURE 9, a birdcage coil 140 is again utilized to provide uniform coverage over the brain. A pair of loop-type anterior neck coils 142a , 142b are positioned over the patient. A distributed type posterior neck coil 144 is positioned below the patient's neck. This coil combination is operable in a head-only mode, a neck-only mode, a posterior neck-only mode, an anterior neck and arch-only mode, a left or right interior neck-only mode, a neck and head combined mode, or a combination of the above. With reference to FIGURE 10, a birdcage coil 150 with a pair of quadrature outputs provides uniform head coverage. Two loop type coils 152a , 152b provide anterior neck and arch coverage. Helmholtz and loop type coils 154a , 154b provide uniform posterior neck coverage. The birdcage, the anterior neck coils, and the posterior neck coils may each be interfaced with one or two channels of the receiver to operate singly or in various combinations as discussed above. Other alternate embodiments include birdcage coils that have other than eight-fold symmetry and neck coils with other than two-fold symmetry. The birdcage coil may be circularly cylindrical, elliptically cylindrical, or have other geometries. The neck coil is contoured in such a way as to provide a high signal-to-noise ratio and uniform coverage over its imaging field of view. The coils in the volume quadrature pair can also be of the loop type, Helmholtz type, figure-8 type, distributed type, or combinations thereof. The signals from the individual coils can be combined prior to or after quadrature combination or prior to or after preamplification. The signals may also be combined digitally post-data acquisition. As another alternative, the individual coils may be tuned to one or more selected magnetic resonance frequencies. In yet another alternate embodiment, the birdcage volume coil is combined with several quadrature pairs in a cascade manner to cover an elongated anatomy under investigation. Decoupling circuits of other designs for inhibiting the flow of radio frequency currents and different numbers of decoupling circuits may also be utilized. The insertable coil need not be in a split mechanical package. For example, the coil may be slid over the patient's head and neck or other portions of the patient's anatomy as may be appropriate to the coil design and application.

Claims: A magnetic resonance apparatus which includes a magnet (10) for generating a temporally constant, uniform magnetic field through an examination region, at least one radio frequency coil (40) which performs at least one of (1) transmitting radio frequency signals into the examination region to induce and manipulate resonance of dipoles disposed therein and (2) receiving radio frequency signals from resonating dipoles in the examination region, and a processor for processing the received magnetic resonance signals, the radio frequency coil including: a first coil assembly (42); a second coil assembly (44a, 44b) partially overlapping the first coil assembly (42) in a common overlap region, characterised by a first electronic circuit (96a) mounted adjacent to and connected to the second coil assembly; a first coaxial cable (98a) extending from the first electronic circuit past and contiguous to the first coil assembly (42), to a region on a side of the first coil assembly opposite from the second coil assembly; and a first coil-to-coil decoupling circuit (104a) connected to the first coaxial cable (98a) for inhibiting the first and second coil assemblies from coupling to each other along the first coaxial cable. A magnetic resonance apparatus as claimed in claim 1, wherein the second coil assembly includes a first coil (44a) connected to a first coaxial cable (98a) and a second coil (44b), the radio frequency coil further including: a second electronic circuit (96b) disposed adjacent to and contiguous with the second coil (44b); a second coaxial cable (98b) extending from the second coil (44b) past and contiguous to the first coil assembly (42) to the opposite side thereof; and a second coil-to-coil decoupling circuit (104b) connected to the second coaxial cable (98b) for inhibiting the second coil (44b) and the first coil assembly (42) from coupling to each other along the second coaxial cable. A magnetic resonance apparatus as claimed in claim 2, wherein the second coil-to-coil decoupling circuit (104b) is disposed in a common plane with the overlap region. A magnetic resonance apparatus as claimed in claim 2 or 3, wherein the first coil assembly (42) includes a birdcage style coil (42) and the apparatus further includes a guard ring (106) disposed adjacent the birdcage style coil (42) adjacent the opposite side from the second coil (44b), sheaths of the first and second coaxial cables being electrically connected to the guard ring (106) and the first coil-to-coil decoupling circuit (104a) being disposed on a side of the guard ring (106) opposite from the birdcage style coil. A magnetic resonance apparatus as claimed in any one of claims 2 to 4, further including: a lower housing portion (84) which supports the second coil (44b), a lower part of the first coil assembly (42), the second electronic circuit (96b), and the second decoupling circuit (104b); an upper housing portion (80) which supports the first coil (44a), an upper part of the first coil assembly (42), the first electronic circuit (96a), and the first decoupling circuit (104a), the upper and lower housing portions being selectively mechanically couplable to provide a unitary coil construction and decouplable to facilitate patient access; and, electrical connections (86) for interconnecting the upper and lower parts of the first coil assembly (42). A magnetic resonance apparatus as claimed in any one of claims 2 to 5, the first and second decoupling circuits each including: a case (130a, 130b) having a spool-like construction (132) therein, the case having first and second coaxial cable receiving guides (134a, 134b) for receiving the coaxial cable and locking the coaxial cable against longitudinal movement, the coaxial cable being wrapped around the spool (132) to form an inductance and being interconnected with at least one adjustable tuning capacitor; an electrically conductive layer (136) for coating and shielding the case. A method of magnetic resonance imaging in which a temporally constant uniform magnetic field is generated through a head and neck examination region, magnetic field gradients are applied across the examination region, radio frequency signals are transmitted into the examination region to induce and manipulate magnetic resonance of dipoles therein, radio frequency signals are received from the resonating dipoles with a radio frequency coil assembly that has a first coil assembly (42) around the head region and a second coil assembly (44a, 44b) around the neck region, the second coil assembly partially overlapping the first coil assembly in a common overlap region, and the received radio frequency signals are processed into an image representation, the method further comprising receiving magnetic resonance signals from resonating dipoles in at least one of the head and neck regions with corresponding first and second coil assemblies to generate a volumetric image representation of at least one of the head and neck regions, including a first electronic circuit (96a) mounted adjacent to and connected to the second coil assembly, and a first coaxial cable (98a) extending from the first electronic circuit past and contiguous to the first coil assembly (42), to a region on a side of the first coil assembly opposite from the second coil assembly, and inhibiting the first and second coil assemblies from coupling to each other using a first coil-to-coil decoupling circuit (104a) connected to the first coaxial cable (98a). A magnetic resonance imaging method as claimed in claim 7, further comprising: decoupling the first coil assembly (42) and operating the second coil assembly alone to image only the neck region; decoupling the second coil assembly (44a, 44b) and operating the first coil assembly alone to image only the head region; operating the first and second assemblies (42, 44a, 44b) simultaneously to image the neck and head region simultaneously. A magnetic resonance imaging method as claimed in claim 7 or 8, further comprising: tuning the first coil assembly (42) and the second coil assembly (44a, 44b) while the first and second coils are separated; overlapping the first and second coil assemblies (42, 44a, 44b) and fixing the first and second coil assemblies together in an overlapped state; retuning the first and second coil assemblies once; and, positioning a patient's head and neck in the first and second coil assemblies in preparation for the magnetic resonance exciting step. A magnetic resonance imaging method as claimed in any one of claims 7 to 9, wherein at least one coaxial cable (98a, 98b) extends from the second coil assembly (44a, 44b) along the first coil assembly (42) and has a radio frequency decoupling circuit (104a, 104b) of adjustable frequency therein, the method further comprising adjusting the frequency of the decoupling circuit (104a, 104b) to match a received magnetic resonance frequency to prevent the first and second coil assemblies from coupling.

Inventors: ELEK ROBERT A, HENDERSON ROBET G, SRINIVASAN RAVI, ELEK, ROBERT A., HENDERSON, ROBET G., SRINIVASAN, RAVI
Applicants: MARCONI MEDICAL SYS INC, MARCONI MEDICAL SYSTEMS, INC.
Application year: 1996
Main classification: G01R33
IPC codes: G01R33, A61B5