MRI endocavitary RF coils

Abstract: An endocavity RF coil assembly for an MRI apparatus includes a reusable probe (30). The reusable probe has a hollow outer cover (60) having a closed distal end and (62) an open proximate end (64). The distal end (62) is formed to fit into a cavity of a subject being examined. An active RF coil element (72) is rigidly formed about an internal sleeve (70) which is located within the distal end (62) of the outer cover (60). A tuning and matching circuit is disposed within the outer cover (60) on the proximate end (64) side of the active RF coil element (72). The tuning and matching circuit is arranged on a printed circuit board (74) and attached to the active RF coil element (72). A closure (90) is connected to the proximate end (64) of the outer cover (60), and seals the proximate end (64) of the outer cover (60) closed.

Description: The present invention relates to magnetic resonance imaging. It finds particular application in conjunction with medical magnetic resonance imaging systems. In magnetic resonance imaging, a substantially uniform main magnetic field is generated within an examination region. The main magnetic field polarizes the nuclear spin system of a subject being imaged within the examination region. Magnetic resonance is excited in dipoles which align with the main magnetic field by transmitting radio frequency excitation signals into the examination region. Specifically, radio frequency pulses transmitted via a radio frequency coil assembly tip the dipoles out of alignment with the main magnetic field and cause a macroscopic magnetic moment vector to precess around an axis parallel to the main magnetic field. The precessing magnetic moment, in turn, generates a corresponding radio frequency magnetic signal as it relaxes and returns to its former state of alignment with the main magnetic field. The radio frequency magnetic resonance signal is received by the radio frequency coil assembly, and from the received signals, an image representation is reconstructed for display on a human viewable display. In certain medical MRI applications, it is advantageous to perform imaging scans over a limited field of view and depth of penetration of specific regions of the patient being examined. Such regions may include the anus, or the prostate. RF receive coils of the intracavitary or endocavitary type are generally used to image these regions as the proximity of the coils in such applications provides improved signal-to-noise ratio over a limited field of view and depth of penetration. Previously, intracavitary or endocavitary RF receive coils made use of an active RF coil element contained within an inflatable non-permeable balloon. An electrical cable interfaced the active RF coil to external electrical circuitry that was used to interface the coil with the magnetic resonance imaging system. The device would be inserted into a cavity associated with the region of interest, for example a patients rectum, and the balloon would then be inflated. Finally, the external electrical interface would be used to tune and match the coil to the MRI system. Typically, such endocavitary coils were disposable and would not be reused for multiple scans. Additionally, the active RF coil element was not rigidly formed, and as such, each individual probe had to be tuned and matched. Another previous form of an endocavitary probe was reusable for a limited number of times before disposal. This form employed additional latex sheathing which could be disposed of between patient uses. However, the device was reused for only a limited number of times. The previous endocavity RF coil probes employed external tuning and matching circuitry as part of the interface with the MRI system. The electronic tuning procedures were performed for each individual patient the coil was use on. As the active RF coil element was not fixed in location within the probe and could not be repeatedly fixed in position relative to the anatomy being imaged, image performance was not repeatable. In Proc. ISMRM, 1996, p. 1437 a solid coil structure for use in MRI is shown. The coil consist of an elongated saddle, 60 mm in length, with an outside diameter of 7 mm. The coil is fitted with a seamless cover having an outside diameter of 8 mm. The cover is removable for sterilization. Both coil former and cover are made from acetal Homopolymer. Fig. 1 shows an intra-urethal coil with an elongated cover for the saddle coil, a tuning and matching network inside another cover of larger diameter but shorter length and an extension for a clamp with a diameter between the diameters of the coil cover and the tuning network cover. Further a cable to the preamplifier is shown. In MRQ 11 (1995), p. 45 to 56 in chapter "COILS" cylindrical saddle geometry receiver coils are described, being 9 mm in diameter and 50 to 70 mm in length and wound on a former. The coils were placed inside a rigid protective tube with a slightly enlarged and rounded proximal end. An integral 10 mm square hybrid circuit was incorporated to allow tuning and matching as well as to provide decoupling. The coils were connected to a preamplifier by a nonmagnetic coaxial cable. The present invention contemplates an endocavity RF coil assembly for an MRI apparatus which overcomes the above referenced disadvantages. Ways of carrying out the invention will now be described in detail, by way of example, with reference to the accompanying drawings, in which: FIGURE 1 is a diagrammatic illustration of a MRI apparatus; FIGURE 2a is an illustration of an anal probe in accordance with the endocavity RF coil assembly of the present invention; FIGURE 2b is an illustration of an outer cover of the anal probe in accordance with the present invention; FIGURE 2c is an illustration of an internal sleeve of the anal probe in accordance with the present invention; FIGURE 3 is an illustration of a decontamination vessel in accordance with the endocavity RF coil assembly of the present invention; FIGURE 4 is an illustration of an adjustable clamp in accordance with the endocavity RF coil assembly of the present invention; and FIGURE 5 is an illustration of the electronic coupling between a probe and the clamp in accordance with the present invention. With reference to FIGURE 1, a main magnetic field control 10 controls superconducting or resistive magnets 12 such that a substantially uniform, temporally constant main magnetic field is created along a z axis through an examination region 14 . A magnetic resonance echo means applies a series of radio frequency (RF) and magnetic field gradient pulses to invert or excite magnetic spins, induce magnetic resonance, refocus magnetic resonance, manipulate magnetic resonance, spatially and otherwise encode the magnetic resonance, to saturate spin, and the like to generate magnetic resonance imaging and spectroscopy sequences. More specifically, gradient pulse amplifiers 20 apply current pulses to selected ones or pairs of whole-body gradient coils 22 to create magnetic field gradients along x, y and z-axes of the examination region 14 . A digital radio frequency transmitter 24 transmits radio frequency pulses or pulse packets to a whole-body RF coil 26 to transmit RF pulses into the examination region. A typical radio frequency pulse is composed of a packet of immediately contiguous pulse segments of short duration which taken together with each other and any applied gradients achieve a selected magnetic resonance manipulation. The RF pulses are used to saturate, excite resonance, invert magnetization, refocus resonance, or manipulate resonance in selected portions of the examination region. For whole-body applications, the resonance signals are commonly picked up by the whole-body RF coil 26 . For generating images of limited regions of the subject, local coils are commonly placed contiguous to the selected region. For example, a receive-only local radio frequency coil can be used to receive resonance signals introduced by body-coil RF transmissions, such as an endocavitary RF coil assembly probe 30 . The resultant radio frequency signals are picked up by the whole-body RF coil 26 or other specialized RF coils and demodulated by a receiver 32 preferably including a preamplifier (not illustrated). A sequence control circuit 40 controls the gradient pulse amplifiers 20 and the transmitter 24 to generate any of a plurality of multiple echo sequences such as echo planar imaging, echo volume imaging, gradient and spin echo imaging, fast spin echo imaging, and the like. For the selected sequence, the receiver 32 receives a plurality of data lines in rapid succession following each RF excitation pulse. An analog-to-digital converter 42 converts each data line to a digital format. The analog-to-digital converter is disposed between the radio frequency receiving coil and the receiver for digital receivers and is disposed downstream from the receiver for analog receivers. Ultimately, the radio frequency signals received are demodulated and reconstructed into an image representation by a reconstruction processor 50 which applies a two-dimensional Fourier transform or other appropriate reconstruction algorithm. The image may represent a planar slice through the patient, an array of parallel planar slices, a three-dimensional volume, or the like. The image is then stored in an image memory 52 where it may be accessed by a display, such as a video monitor 54 which provides a man-readable display of the resultant image. With reference to FIGURE 2a-c, an endocavity RF coil assembly is used to pick up radio frequency signals generated by relaxing magnetic dipoles. The endocavity RF coil assembly includes a reusable probe 30 which is inserted into a cavity of a subject being examined so as to be contiguous to a region of interest, such as the anus . The coil's proximity to the region of interest provides for a relatively high signal-to-noise ratio. In one preferred embodiment, the reusable probe 30 is constructed from a hollow outer cover 60 having a closed distal end 62 and an open proximate end 64 , the closed distal end 62 formed to fit into the cavity of the subject being examined. At the proximate end 64 of the outer cover 60 , on an outside surface thereof, a number of grooves 66 are cut. The grooves 66 extend completely around the outside surface of the outer cover 60 . Further, flat grooved quadrants 68 are also cut out of the proximate end 64 on the outside surface of the outer cover 60 . Preferably, the outer cover 60 is constructed of medical grade ABS plastic. The ABS material is appropriately flame retardant and is specifically intended for limited exposure to mucus membrane or blood barrier contact. An internal sleeve 70 , preferably also made out of medical grade ABS material, is formed to be seated within the hollow outer cover 60 . An active RF coil element 72 is positioned within machine grooves along the internal sleeve 70 at the end located within the distal end 62 of the outer cover 60 . The end of the internal sleeve which is located within the proximate end 64 of the outer cover 60 holds a tuning and matching circuit which is arranged on a printed circuit board 74 . The printed circuit board 74 is attached to the active RF coil element 72 . The electronic components mounted on the printed circuit board 74 that make up the tuning and matching circuit are preferably non-magnetic or minimally magnetic. The printed circuit board layout for electronic component placement and the distance of the printed circuit board 74 from the active coil element 72 are specifically designed to reduce or eliminate image anomalies caused by the magnetic content of the electronic components used. A specially constructed RF cable 80 connects the output of the tuning and matching circuit to the MRI system preamplifier. Preferably, the cable 80 has been constructed from a non-magnetic version of a standard RG174 cable. Over the outer cable insulation, a non-toxic PVC outer jacket has been moulded. The PVC jacket provides an appropriate non-toxic contact with a patient and also provides an electrical field insulating distance between the patient and the outer electrical shield of the cable thereby preventing the risk of RF burn. An over-moulded form (closure formation) 90 seals the open proximate end of the hollow outer cover. Preferably, the over-moulded form 90 is constructed of PVC plastic, and can be moulded or fashioned by any appropriate technique. The over-moulded form 90 is formed over the outer surface of the proximate end 64 and covers the grooves 66 and flat grooved quadrants 68 . The RF cable 80 extends from the tuning and matching circuit out of the open proximate end 64 through the over-moulded form 90 forming a fluid tight seal therewith. As a portion of the over-moulded form 90 is embedded in the circumferential grooves 66 formed on the outer surface of the hollow outer cover 60 , a tight mechanical seal is provided therebetween which is appropriately resistant to the ingress of fluid. The interface between the over-moulded form 90 and RF cable 80 is appropriately resistant to the ingress of fluid due to a chemical PVC-to-PVC seal provided therebetween. Further, being that the over-moulded form 90 is formed around the flat grooved quadrants 68 , the over-moulded form 90 will not rotate with respect to the outer cover 60 . Referring to FIGURE 3 the endocavity RF coil assembly includes a decontamination vessel 140 for disinfecting the outer cover of the probes. The decontamination vessel 140 is compatible with all embodiments of the probes. An upper lip 142 is arranged at the top of the vessel such that it cradles the over-moulded form 90 . The upper lip 142 can be constructed to cooperate with the over-moulded form 90 by having the same shape. The upper lip 142 holds the probe at a prescribed height so that the outer cover is fully immersed in a decontamination fluid which resides in a reservoir 144 of the decontamination vessel 140 . The disinfecting fluid may be glutaraldehyde, alcohol, hydrogen peroxide, or like disinfectant. The rectangular throat 146 allows the outer cover of the probes 30 to extend into the disinfecting fluid but will not allow the over-moulded form 90 to pass through. A cap 148 is also provided for sealing the decontamination vessel 140 when not in use. The decontamination vessel 140 also has an overflow spillway 150 which prevents the decontamination fluid from rising above the prescribed level. That is to say, if the level of the decontamination fluid is too high when the probe 30 is inserted, as the probe 30 displaces the fluid and the level rises, it spills through the overflow spillway 150 rather than rising above the predetermined level. This prevents the decontamination fluid from contacting the over-moulded form 90 or specially constructed RF cable 80 , to protect the PVC plastic parts from exposure to the decontamination fluid which might be damaging thereto and further protect the seals between the over-moulded form 90 and the outer cover and the seal between the over-moulded form 90 and the specially constructed RF cable 80 from possible ingress of fluid. With reference to FIGURE 4, an adjustable clamp 160 having jaws 162 adapted to selectively engage the over-moulded form 90 is used to hold the probe 30 in a fixed relationship within the cavity of the subject being examined. The adjustable clamp jaws 162 are selectively engaged by turning a backnut 164. The jaws are formed to the shape of the over-moulded form 90 to allow maximum freedom in the positioning of the probe 30 . With reference to FIGURE 5, in one preferred embodiment, the probe does not include a specially constructed RF cable. Rather, the over-moulded form 90 completely seals the open end of the hollow outer cover. An electrical component 170 , such as an inductive coil, within the over-moulded form 90 is connected to the tuning and matching circuit which is in turn connected to the active RF coil element 72 of the probe 30 . The electrical component 170 in the over-moulded form 90 is electronically coupled to electrical components 172 , such as inductive coils, housed within the jaws 162 of the adjustable clamp 160 . The electrical components 172 in the jaws 162 of the adjustable clamp 160 are then in turn connected to a preamplifier of the MRI apparatus via cable 172 . In this manner, the probe is connected to the preamplifier of the MRI apparatus without the use of extraneous RF cables. While the illustrated over-moulded form 90 has been depicted as being spherical in shape, the over-moulded form 90 may take on other appropriate shapes. For example, in one preferred embodiment, the over-moulded form 90 takes the shape of a polyhedron. Each side of the polyhedron is marked with appropriate indicia. In this manner, the over-moulded form 90 can be exploited for indexing the relative orientation of the probe 30 and the active RF coil element 72 rigidly secured therein with respect to the patient. In another alternate embodiment, the probe 30 may include materials in its construction which emit a limited nuclear magnetic resonance signal. Again, this allows the relative position and orientation of the probe 30 within the subject to be determined relative to the anatomy being imaged. One advantageous aspect of the present invention is the fact that the probe 30 is sealed to prevent the ingress of fluids. While the specifically constructed RF cable 80 has been described as connecting the tuning and matching circuit of the probe 30 to the preamplifier of the magnetic resonance imaging system, in one preferred embodiment, the preamplifier is integrated into the probe 30 and mounted on the same printed circuit board 74 as the tuning and matching circuit. One advantage of the described endocavitary coils is that the probes are reusable. Another advantage is that they can be manufactured to meet the electrical and mechanical required compliance standards such as UL 544 and IEC 601. Another advantage is that the active RF coil element location may be fixed with respect to the anatomy being imaged. Another advantage is that external circuitry for performing tuning and matching adjustments on a per-patient basis is eliminated. Another advantage is the reduction of motion artifacts by clamping the probe in a fixed position while performing the scan. Another advantage is the prevention of the ingress of fluids into the interior of the reusable probe. Another advantage is the protection of subjects from RF burn.

Claims: An endocavity RF coil assembly for an MRI apparatus, the assembly comprising: a reusable probe (30) designed for examination of the anus of a subject and having a hollow outer cover (60) with a closed distal end (62) and an open proximate end (64), the distal end (62) formed to fit into the anus of the subject being examined; an active RF coil element (72) rigidly formed about a former (70) which is located within the distal end (62) of the outer cover (64); a tuning and matching circuit disposed within the outer cover (60) on the proximate end side of the active RF coil element (72), the tuning and matching circuit being connected to the active RF coil element (72) and being arranged on a printed board circuit (74); and a closure (90) connected to the proximate end (64) of the outer cover (60), characterized in that a plurality of grooves are cut (66) into the radially outermost surface of the outer cover (60) at the proximate end (64) thereof, the grooves (66) completely encircling the outer cover (60), and in that the closure (90) is formed over the radially outermost surface of the proximate end of outer cover (60) such that the distal end of the closure (90) is embedded in the grooves (66) to seal the connection between the closure (90) and the outer cover (60), and such that the closure (90) either seals the open end of the hollow outer cover (60) completely closed or is penetrated, in a fluid-tight seal manner, by a non-magnetic RF cable (80) connected to the tuning and matching circuit. An endocavity RF coil assembly as claimed in claim 1, wherein the RF cable (80) has an outer cable insulation and an MR compatible and biocompatible outer jacket moulded over the outer cable insulation. An endocavity RF coil assembly as claimed in claim 1 or claim 2, further including: a decontamination vessel (140) for disinfecting the outer cover (60) of the probe (30), the decontamination vessel (140) having an upper lip (142) and a reservoir which holds a decontamination fluid at a prescribed level, the upper lip being arranged such that it cradles the closure (90) at a prescribed height so that the outer cover (60) is fully immersed in the decontamination fluid and the closure (90) is not in contact with the decontamination fluid. An endocavity RF coil assembly as claimed in claim 4, wherein the decontamination vessel (140) has an overflow spillway (150) such that the decontamination fluid is prevented from rising above the prescribed level. An endocavity RF coil assembly as claimed in any one of claims 1 to 4, further including: an adjustable clamp (160) having jaws (162) adapted to selectively engage the closure (90) such that the probe (30) is held in a fixed relationship within the subject being examined. An endocavity RF coil assembly as claimed in claim 5, wherein the closure (90) seals the open end of the hollow outer cover completely closed and includes at least one electrical component (170) connected to the tuning and matching circuit, such that when the closure (90) is engaged with the adjustable clamp (160) the electrical component (170) in the closure (90) is electronically coupled to at least one electrical component (172) in the jaws (160) of the adjustable clamp (160) which is arranged to be connected to a preamplifier of the MRI apparatus. An endocavity RF coil assembly as claimed in any one of claims 1 to 6, wherein the probe (30) includes material which emits a limited magnetic resonance signal which is used to track a position of the probe (30) relative to the subject being examined. An endocavity RF coil assembly as claimed in any one of claims 1 to 7, wherein the printed circuit board (74) is positioned and electronic component are located thereon such that image anomalies caused thereby are eliminated. An endocavity RF coil assembly as claimed in any one of claims 1 to 8, wherein the closure is a moulded formation. An endocavity RF coil assembly as claimed in claim 9, wherein the closure is bulbous. A method of using the endocavity RF coil assembly as claimed in any one of the previous claims, the method comprising: (a) positioning the reusable probe inside the anus of a subject being examined: (b) exciting magnetic resonance in dipoles of the subject; (c) receiving via the reusable probe magnetic resonance signals emitted from excited dipoles as they relax; and (d) generating human viewable images from the received magnetic resonance signals.

Inventors: BRAUM WILLIAM O, CARLON JOHN T, MOLYNEAUX DAVID A, REISKER THEODORE J, STERN BENJAMIN R, BRAUM, WILLIAM O., CARLON, JOHN T., MOLYNEAUX, DAVID A., REISKER, THEODORE, J., STERN, BENJAMIN R.
Applicants: KONINKL PHILIPS ELECTRONICS NV, KONINKLIJKE PHILIPS ELECTRONICS N.V.
Application year: 1998
Main classification: G01R33
IPC codes: A61B5, H01F6, G01R33, H01F7