Patent Publication Number: US-8989841-B2

Title: Interface devices for use with intracavity probes for high field strength magnetic resonance systems

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This present application is a divisional of U.S. application Ser. No. 11/719,253 filed 14 May 2007 (now U.S. Pat. No. 7,885,704), which is a national stage entry of PCT International Application No. PCT/US2005/041912 filed 15 Nov. 2005, which is a continuation-in-part of PCT International Application PCT/US03/07774 filed 13 Mar. 2003 and which claims the benefit of U.S. Provisional Application 60/628,166 filed on 15 Nov. 2004, all of which incorporated by reference herein and assigned to the assignee of the invention disclosed below. 
    
    
     FIELD OF THE INVENTION 
     The present invention generally relates to systems and methods of obtaining images and spectra of intracavity structures using magnetic resonance (MR) systems. More particularly, the invention pertains to an intracavity probe capable of being inserted into various bodily openings, such as the rectum, vagina, mouth, etc., to obtain high resolution images of and spectroscopic results for regions of interest therein. Even more particularly, the invention relates to interface devices designed to interface such an intracavity probe with 2.0 Tesla to 5.0 Tesla MR systems to obtain such high resolution images and spectroscopic results for such regions of interest. 
     BRIEF DESCRIPTION OF RELATED ART 
     The following background information is provided to assist the reader to understand the invention disclosed below and the environment in which it will typically be used. The terms used herein are not intended to be limited to any particular narrow interpretation unless clearly stated otherwise, either expressly or impliedly, in this document. 
     Magnetic resonance imaging (MRI) is a noninvasive method of producing high quality images of the interior of the human body. It allows medical personnel to see inside the human body without surgery or the use of ionizing radiation such as X-rays. The images are of such high resolution that disease and other forms of pathology can often be visually distinguished from healthy human tissue. Magnetic resonance techniques and systems have also been developed for performing spectroscopic analyses by which the chemical content of tissue or other material can be ascertained. 
     MRI uses a powerful magnet, radio waves and computer technology to create detailed images of the soft tissues, muscles, nerves and bones in the human body. It does so by taking advantage of a basic property of the hydrogen atom, an atom that is found in abundance in all cells within the human body. In the absence of a magnetic field, the nuclei of hydrogen atoms spin like a top, or precess, randomly in every direction. When subject to a strong magnetic field, however, the spin-axes of the hydrogen nuclei align themselves in the direction of that field. This is because the nucleus of the hydrogen atom has what is referred to as a large magnetic moment, which is basically a strong inherent tendency to line up with the direction of the magnetic field. Collectively, the hydrogen nuclei of the area to be imaged create an average vector of magnetization that points parallel to the magnetic field. 
     A typical MRI system, or scanner, includes a main magnet, three gradient coils, a radio frequency (RF) antenna (often referred to as the whole body coil), and a computer station from which an operator can control the overall MRI system. The chief component of the MRI system, however, is the main magnet. It is typically superconducting in nature and cylindrical in shape. Within its cylindrical bore (into which patients are placed during an MRI procedure), the main magnet generates a strong magnetic field, often referred to as the B 0  field, which is both uniform and static (non-varying). This B 0  magnetic field is oriented along the longitudinal axis of the bore, referred to as the z direction, which compels the magnetization vectors of the hydrogen nuclei in the body to align themselves in that direction. In this alignment, the nuclei are prepared to receive RF energy of the appropriate frequency from the whole body coil. This frequency is known as the Larmor frequency and is governed by the equation ω=ΥB 0 , where ω is the Larmor frequency (at which the hydrogen atoms precess), Υ is the gyromagnetic constant, and B 0  is the strength of the magnetic field. 
     The RF antenna, or whole body coil, is generally used both to transmit pulses of RF energy and to receive the resulting magnetic resonance (MR) signals induced thereby in the hydrogen nuclei. Specifically, during its transmit cycle, the body coil broadcasts RF energy into the cylindrical bore. This RF energy creates a radio frequency magnetic field, also known as the RF B 1  field, whose magnetic field lines are directed in a line perpendicular to the magnetization vector of the hydrogen nuclei. The RF pulse (or B1 field) causes the spin-axes of the hydrogen nuclei to tilt with respect to the main (B 0 ) magnetic field, thus causing the net magnetization vector to deviate from the z direction by a certain angle. The RF pulse, however, will affect only those hydrogen nuclei that are precessing about their axes at the frequency of the RF pulse. In other words, only the nuclei that “resonate” at that frequency will be affected, and such resonance is achieved in conjunction with the operation of the three gradient coils. 
     The gradient coils are electromagnetic coils. Each gradient coil is used to generate a linearly varying yet static magnetic field along one of the three spatial directions (x, y, z) within the cylindrical bore known as the gradient B 1  field. Positioned inside the main magnet, the gradient coils are able to alter the main magnetic field on a very local level when they are turned on and off very rapidly in a specific manner. Thus, in conjunction with the main magnet, the gradient coils can be operated according to various imaging techniques so that the hydrogen nuclei—at any given point or in any given strip, slice or unit of volume—will be able to achieve resonance when an RF pulse of the appropriate frequency is applied. In response to the RF pulse, the precessing hydrogen atoms in the selected region absorb the RF energy being transmitted from the body coil, thus forcing the magnetization vectors thereof to tilt away from the direction of the main (B 0 ) magnetic field. When the body coil is turned off, the hydrogen nuclei begin to release the RF energy in the form of the MR signal, as explained further below. 
     One well known technique that can be used to obtain images is referred to as the spin echo imaging technique. Operating according to this technique, the MRI system first activates one gradient coil to set up a magnetic field gradient along the z-axis. This is called the “slice select gradient,” and it is set up when the RF pulse is applied and is shut off when the RF pulse is turned off. It allows resonance to occur only within those hydrogen nuclei located within a slice of the area being imaged. No resonance will occur in any tissue located on either side of the plane of interest Immediately after the RF pulse ceases, all of the nuclei in the activated slice are “in phase,” i.e., their magnetization vectors all point in the same direction. Left to their own devices, the net magnetization vectors of all the hydrogen nuclei in the slice would relax, thus realigning with the z direction. Instead, however, the second gradient coil is briefly activated to create a magnetic field gradient along the y-axis. This is called the “phase encoding gradient.” It causes the magnetization vectors of the nuclei within the slice to point, as one moves between the weakest and strongest ends of the gradient, in increasingly different directions. Next, after the RF pulse, slice select gradient and phase encoding gradient have been turned off, the third gradient coil is briefly activated to create a gradient along the x-axis. This is called the “frequency encoding gradient” or “read out gradient,” as it is only applied when the MR signal is ultimately measured. It causes the relaxing magnetization vectors to be differentially re-excited, so that the nuclei near the low end of the gradient begin to precess at a faster rate, and those at the high end pick up even more speed. When these nuclei relax again, the fastest ones (those which were at the high end of the gradient) will emit the highest frequency of radio waves. 
     Collectively, the gradient coils allow the MR signal to be spatially encoded, so that each portion of the area being imaged is uniquely defined by the frequency and phase of its resonance signal. In particular, as the hydrogen nuclei relax, each becomes a miniature radio transmitter, giving out a characteristic pulse that changes over time, depending on the local microenvironment in which it resides. For example, hydrogen nuclei in fats have a different microenvironment than do those in water, and thus transmit different pulses. Due to these differences, in conjunction with the different water-to-fat ratios of different tissues, different tissues transmit radio signals of different frequencies. During its receive cycle, the body coil detects these miniature radio transmissions, which are often collectively referred to as the MR signal. From the body coil, these unique resonance signals are conveyed to the receivers of the MR system where they are converted into mathematical data corresponding thereto. The entire procedure must be repeated multiple times to form an image with a good signal-to-noise ratio (SNR). Using multidimensional Fourier transformations, an MR system can convert the mathematical data into a two- or even a three-dimensional image. 
     When more detailed images of a specific part of the body are needed, a local coil is often used in addition to, or instead of, the whole body coil. A local coil can take the form of a volume coil or a surface coil. A volume coil is used to surround or enclose the volume to be imaged (e.g., a head, an arm, a wrist, a leg, a knee or other region of interest). A surface coil, however, is merely fitted or otherwise placed against a particular surface of the patient so that the underlying region of interest can be imaged (e.g., the abdominal, thoracic and/or pelvic regions). In addition, a local coil can be designed to operate either as a receive-only coil or a transmit/receive (T/R) coil. A receive-only coil is only capable of detecting the MR signals produced by the human body (in response to the B 1  magnetic field generated by the MR system during a scanning procedure). A T/R coil, however, is capable of both receiving the MR signals as well as transmitting the RF pulses that produce the RF B 1  magnetic field, which is the prerequisite for inducing resonance in the tissues of the region of interest. 
     It is well known in the field of MRI to use a single local coil, whether surface or volume, to detect the MR signals. According to the single coil approach, a relatively large local coil is used to cover or enclose the entire region of interest. Early receiving coils were just linear coils, meaning that they could detect only one of the two (i.e., vertical M X′ , and horizontal M Y′ ) quadrature components of the MR signals produced by the region of interest. Subsequent receiving coils, however, employed quadrature mode detection, meaning that they could intercept both the vertical and horizontal components. Compared to linear receiving coils, quadrature receiving coils enabled MRI systems to provide images for which the SNR was much improved, typically by as much as 41%. Even with the improvement brought with quadrature mode detection, the single coil approach still provided images whose quality invited improvement. The disadvantage inherent to the single coil approach is attributable to just one coil structure being used to acquire the MR signals over the entire region of interest. 
     Phased array coils were developed to overcome the shortcomings with the single coil approach. Instead of one large local coil, the phased array approach uses a plurality of smaller local coils, with each such coil covering or enclosing only a portion of the region of interest. In a system having two such coils, for example, each of the coils would cover or enclose approximately half of the region of interest, with the two coils typically being partially overlapped for purposes of magnetic isolation. The two coils would acquire the MR signals from their respective portions simultaneously, and they would not interact adversely due to the overlap. Because each coil covers only half of the region of interest, each such coil is able to receive the MR signals at a higher SNR ratio for that portion of the region of the interest within its coverage area. The smaller local coils of the phased array thus collectively provide the MRI system with the signal data necessary to generate an image of the entire region of interest that is higher in resolution than what can be obtained from a single large local coil. 
     One example of a phased array coil is the Gore® torso array produced by W. L. Gore and Associates, Inc. The torso array contains four surface coils, two of which disposed in an anterior paddle and the other two disposed in a posterior paddle. The two paddles are designed to be placed against the anterior and posterior surfaces, respectively, of the patient about the abdominal, thoracic and pelvic regions. The torso array is designed for use with an MR system whose data acquisition system has multiple receivers. The four leads of the torso array, one each from the two anterior surface coils and the two posterior surface coils, can be connected to separate receivers, with each receiver amplifying and digitizing the signal it receives. The MR system then combines the digitized data from the separate receivers to form an image whose overall SNR is better than what could be obtained from a single local coil, or even two larger anterior and posterior local coils, covering the entire region of interest alone. 
     It is also well known to obtain images of internal bodily structures through the use of intracavity probes. An example of a prior art intracavity probe can be found in U.S. Pat. Nos. 5,476,095 and 5,355,087, both of which are assigned to the assignee of the present invention and incorporated herein by reference. The prior art probe disclosed in those patents is designed to be inserted into bodily openings such as the rectum, vagina, and mouth. Those patents also disclose interface devices that are designed to interface the prior art intracavity probe with MR imaging and spectroscopy systems. A method of using the intracavity probe is disclosed in U.S. Pat. No. 5,348,010, which is also assigned to the assignee of the present invention and incorporated herein by reference. 
     The prior art probe, operated in conjunction with its associated interface unit, allows an MR system to generate images of, and spectroscopic results for, various internal bodily structures such as the prostate gland, colon or cervix. Examples of such prior art probes include the BPX-15 prostate/endorectal coil (E-coil), the PCC-15 colorectal coil, and the BCR-15 cervix coil, all of which are part of the MRInnervu® line of disposable coils produced by Medrad, Inc. of Indianola, Pa. Examples of such interface units include the ATD-II and the ATD-Torso units, also produced by Medrad, Inc. 
     The ATD-II unit is used to interface the prior art probe with one receiver of an MR system to provide images or spectra of the region of interest, namely, the prostate gland, colon or cervix. The ATD-Torso unit is used to interface not only the prior art probe but also the Gore® torso array with multiple receivers of the MR system. When connected to such a probe and the torso array, the ATD-Torso unit allows the MR system to provide images or spectra not only of the prostate gland, colon or cervix but also of the surrounding anatomy, i.e., the abdominal, thoracic and pelvic regions. 
     Despite their widespread acceptance and good reputation in the marketplace, these prior art intracavity probes and interfaces units nevertheless have a few shortcomings First, the prior art probe and its associated interface units (i.e., ATD-II and ATD Torso units) are designed to operate only with 1.0 or 1.5 Tesla MR systems. Consequently, they are not suitable for use with MR systems designed to operate at higher field strengths, such as the 2.0 to 5.0 Tesla and particularly 3.0 Tesla MR systems that are capable of producing even higher quality images and spectrographic results. Second, as a result of that design constraint, the prior art intracavity probe was designed with a coil loop that exhibits a 750 to 1000 ohm output impedance. Consequently, the interface units for the prior art probe had to include a π network or similar circuitry to match the high output impedance of the coil loop to the low input impedance (e.g., 50 ohms) required by various MR systems. Third, the design of the prior art probe allowed the tuning of its coil loop to deviate from the operating frequency of the MR system, the extent to which depending on the particular conditions (e.g., patients) in which the probe was used. Therefore, the prior art interface units for the prior art probe typically had to include tuning circuitry so as to assure that the intracavity probe could be tuned to the operating frequency of the MR system under all operating conditions. 
     OBJECTIVES OF THE INVENTION 
     It is, therefore, an objective of the invention to provide an intracavity probe, capable of being used with magnetic resonance (MR) systems designed to operate at 2.0 to 5.0 Tesla field strengths at least and particularly at 3.0 Tesla field strengths. 
     Another objective of the invention is to provide an intracavity probe having a coil loop with a broader frequency response than prior art intracavity probes, with little or even no sacrifice of signal-to-noise ratio, thereby obviating the need to tune the coil loop on a per patient or per coil basis as is required of such prior art probes. 
     Yet another objective is to provide an interface device that interfaces such an intracavity probe with such an MR system to obtain high resolution images of and spectroscopic results for the region of interest, without the need to tune the probe. 
     Still another objective is to provide an interface device that is designed to interface not only such an intracavity probe but also a phased array coil system, such as the Gore® torso array, with such an MR system. 
     A further objective of the invention is to provide a method of obtaining images and/or spectra of a region of interest within a cavity of a patient using such an intracavity probe, an interface device and an MR system. 
     One other objective of the invention is to provide a method of making such an intracavity probe for use with such an MR system with which to obtain images and/or spectra of a region of interest within a cavity of a patient. 
     Yet another objective is to provide an intracavity probe that is disposable in that it does not contain the relatively expensive decoupling components, which are instead incorporated into a reusable interface device with which the probe shall interface. 
     Still another objective is to provide an intracavity probe as an endorectal probe designed to be inserted into the rectum to obtain images and/or spectra of the male prostate gland. 
     A further objective is to provide an intracavity probe capable of being inserted into any one or more of various bodily openings, such as the rectum, vagina, mouth, etc., to obtain high resolution images of and spectroscopic results for the region of interest. 
     In addition to the objectives and advantages listed above, various other objectives and advantages of the invention will become more readily apparent to persons skilled in the relevant art from a reading of the detailed description section of this document. The other objectives and advantages will become particularly apparent when the detailed description is considered along with the drawings and claims presented below. 
     SUMMARY OF THE INVENTION 
     The foregoing objectives and advantages are attained by the various embodiments and related aspects of the invention summarized below. 
     In one aspect of a presently preferred embodiment, the invention provides an intracavity probe for use with a magnetic resonance (MR) system for obtaining images or spectra of a region of interest within a cavity of a patient. The probe includes a coil loop and an output cable. Designed to receive MR signals from the region of interest, the coil loop has a plurality of capacitors including first and second drive capacitors and a tuning capacitor. The first and second drive capacitors are serially connected within the coil loop and at a junction node thereof form a virtual ground for electrically balancing and impedance matching the coil loop. The two drive capacitors are of approximately equal value. The tuning capacitor is serially connected within the coil loop diametrically opposite the junction node of the drive capacitors. The tuning capacitor has a value selected to resonate the coil loop at an operating frequency of the MR system. The output cable connects the coil loop to an interface device for the intracavity probe. The output cable at one end connects across one of the drive capacitors and at its other end has a plug for connection to the interface device. The output cable has an electrical length of n(λ/2)+S L  wherein n is an integer, λ is a wavelength of the operating frequency of the MR system, and S L  is a supplemental length whose reactance is equal in magnitude to that of one of the drive capacitors. 
     In a broader application, the invention provides a magnetic resonance (MR) system comprising an MR scanner, an intracavity probe and an interface device. Designed to be inserted within a cavity of a patient, the intracavity probe includes a shaft, an inflatable balloon and a coil loop. The balloon is connected to a distal end of the shaft, and the coil loop secured within the balloon approximate an underside of its anterior surface. The anterior surface of the balloon is conformable to an interior contour of the cavity, and a posterior surface of the balloon is used for positioning the balloon within the cavity. When the balloon is inflated, the posterior surface presses against a wall of the cavity that is generally opposite a region of interest within the cavity. This forces the anterior surface of the balloon against the interior contour of the cavity thereby bringing the coil loop approximate the region of interest for optimal reception of MR signals therefrom. The coil loop has a plurality of capacitors including first and second drive capacitors and a tuning capacitor. The first and second drive capacitors are serially connected within the coil loop and at a junction node thereof form a virtual ground for electrically balancing and impedance matching the coil loop. The two drive capacitors are of approximately equal value. The tuning capacitor is serially connected within the coil loop diametrically opposite the junction node of the drive capacitors. The tuning capacitor has a value selected to resonate the coil loop at an operating frequency of the MR system. The MR scanner is used to generate image(s) or spectra of the region of interest using the MR signals received by the coil loop from the region of interest. The interface device has a probe interface circuit for electrically interconnecting the intracavity probe and the MR system. The probe interface circuit features a PIN diode capable of being biased by the MR system by which the coil loop can be (i) coupled to a probe input port of the MR system during a receive cycle thereof and (ii) decoupled from the probe input port during a transmit cycle thereof. 
     In another aspect of the presently preferred embodiment, the invention provides an interface device for interfacing an intracavity probe with a (probe) input port of a magnetic resonance (MR) system which is not equipped with its own preamplifier. The probe has a output cable for connecting its coil loop to the interface device. The interface device includes a PIN diode and a preamplifier. The MR system can bias the PIN diode so that the coil loop is (i) coupled to the probe input port during a receive cycle of the MR system and (ii) decoupled from the probe input port during a transmit cycle of the MR system. The preamplifier provides gain and impedance matching between an anode of the PIN diode and the probe input port of the MR system so that with enhancement of signal-to-noise ratio the MR signals received by the coil loop are passed to the probe input port of the MR system. 
     In yet another aspect of the presently preferred embodiment, the invention provides an interface device for interfacing both an intracavity probe and a coil system with a magnetic resonance (MR) system. The intracavity probe features an output cable for connecting a coil loop of the probe to the interface device. The interface device allows the probe via its output cable to be interfaced with a (probe) input port of the MR system which is equipped with its own preamplifier. The interface device includes a PIN diode and an array interface circuit. The MR system can bias the PIN diode so that the coil loop is (i) coupled to the probe input port during a receive cycle of the MR system and (ii) decoupled from the probe input port during a transmit cycle of the MR system. The array interface circuit is used to electrically interconnect the coil system and the MR system. The array interface circuit includes first and second series resonant networks, a pair of 1/4  wavelength networks and a 1/4  wavelength combiner. The first series resonant network is for conveying MR signals from a first coil of the coil system to the first coil input port of the MR system. The second series resonant network is for conveying MR signals from a second coil of the coil system to the second coil input port of the MR system. One of the 1/4  wavelength networks is for receiving MR signals from a third coil of the coil system, and the other 1/4  wavelength network is for receiving MR signals from a fourth coil of the coil system. The 1/4  wavelength combiner is used to combine the MR signals received from the pair of 1/4  wavelength networks and convey the combined MR signals to the third coil input port. 
     The invention also provides a preferred method of obtaining images or spectra of a region of interest within a cavity of a patient using a magnetic resonance (MR) system. The method includes the steps of providing an intracavity probe and providing an output cable. The intracavity probe shall have (i) a flexible shaft, (ii) an inflatable balloon that connects to an end of the flexible shaft, and (iii) a coil loop secured within the balloon approximate an underside of an anterior surface thereof and capable of receiving MR signals from the region of interest. The anterior surface of the balloon is conformable to a contour of the cavity, and a posterior surface of the balloon features at least a pair of undulating folds. The coil loop has a plurality of capacitors including first and second drive capacitors and a tuning capacitor. The first and second drive capacitors are serially connected within the coil loop and at a junction node thereof form a virtual ground for electrically balancing and impedance matching the coil loop. The two drive capacitors are of approximately equal value. The tuning capacitor is serially connected within the coil loop diametrically opposite the junction node of the drive capacitors. The tuning capacitor has a value selected to resonate the coil loop at an operating frequency of the MR system. The method also includes the step of providing an output cable for connecting the coil loop to an external circuit with which said intracavity probe is connected to the MR system. Another step involves inserting the intracavity probe into a position within the cavity of the patient so that the anterior surface of the balloon is in proximity to the region of interest. The next step involves inflating the balloon and thereby force the undulating folds to unfold against a wall of the cavity generally opposite the region of interest. This forces the anterior surface of the balloon against the contour of the cavity and securely positions the coil loop approximate the region of interest for optimal reception of the MR signals therefrom. Subsequent steps involve inducing the region of interest to emit the MR signals, and using the coil loop to sense the MR signals induced within the region of interest. The method also includes the step of generating image(s) and spectra of the region of interest using the MR signals received therefrom. 
     The invention further provides a preferred method of making an intracavity probe for use with a magnetic resonance (MR) system with which to obtain images or spectra of a region of interest from within a cavity of a patient. The method includes the steps of choosing a size of a coil loop of the probe to permit the probe to be suitable for insertion into the cavity, and temporarily inserting a variable capacitor in serial connection within the coil loop. The method also involves the steps of subjecting the coil loop to an operating frequency of the MR system, and then adjusting the variable capacitor to a resonance value at which the coil loop resonates. At this operating frequency, the capacitive reactance of the coil loop will equal in magnitude the inductive reactance of the coil loop. Related steps involve measuring a quality factor of the coil loop when the coil loop is loaded, and then determining a series resistance of the coil loop using the quality factor so measured and an inductive reactance of the coil loop when loaded. The matching value for a matching capacitor is then calculated, so as to match an output impedance of the probe with an impedance required by an external circuit with which the intracavity probe shall interface. The method also includes the step of inserting two drive capacitors of the matching value into the coil loop in series with each other to form a junction node whereat the drive capacitors connect. The junction node is connectable to a shield conductor of an output cable, and an opposite node of one of the drive capacitors is connectable to a center conductor of the output cable. A tuning capacitor is then selected so that a total capacitance of the coil loop is equal to the resonance value. The variable capacitor is then replaced with the tuning capacitor. The tuning capacitor is serially connected within the coil loop diametrically opposite the junction node of two drive capacitors. The junction node thus forms a virtual ground for electrically balancing the coil loop. 
     It should be understood that the present invention is not limited to the presently preferred embodiment(s) and related aspects discussed above. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention and its presently preferred and alternative embodiments will be better understood by reference to the detailed disclosure below and to the accompanying drawings, wherein: 
         FIG. 1  is a schematic diagram of a coil loop and an output cable of an intracavity probe according to one aspect of the presently preferred embodiment of the invention; 
         FIG. 2  is a perspective view showing the intracavity probe of  FIG. 1  in its fully assembled and fully equipped state; 
         FIG. 3  is a cross-sectional view of the intracavity probe taken through line  3 - 3  of  FIG. 2  showing a distal end of the probe and inflatable balloon(s) attached thereto; 
         FIG. 4  is partial cross-sectional view of the intracavity probe taken through line  4 - 4  of  FIG. 2  showing its shaft in cross-section and the two lumens defined therein and an anti-migration disc snapped onto the shaft; 
         FIG. 5  is a cross-sectional view of the distal end of the intracavity probe taken through line  5 - 5  of  FIG. 3  showing its outer and inner balloons, its coil loop situated between the balloons, and its shaft with the two lumens defined therein; 
         FIG. 6  is a cross-sectional view of the distal end of the intracavity probe taken through line  6 - 6  of  FIG. 3  showing its coil loop situated atop an anterior surface of the inner balloon; 
         FIG. 7  is a cross-sectional view of the shaft of the intracavity probe of  FIG. 2  illustrating the two lumens defined therein and a flexible tip at its distal end; 
         FIG. 8  is a schematic diagram of an interface device according to another aspect of the presently preferred embodiment of the invention wherein, in its single-receiver version, the interface device has a probe interface circuit for interfacing the intracavity probe of  FIGS. 1-7  with a (probe) input port of a magnetic resonance (MR) system which is not equipped with its own preamplifier; 
         FIG. 9  is a schematic diagram of an interface device according to yet another aspect of the presently preferred embodiment of the invention wherein, in its multiple-receiver version, the interface device has (i) a probe interface circuit for interfacing the intracavity probe of  FIGS. 1-7  with a (probe) input port of an MR system equipped with its own preamplifier and (ii) an array interface circuit for interfacing a phased array coil system, such as the Gore® torso array, with the (coil) input ports of the MR system; 
         FIG. 10  is a perspective view of the interface device in its single-receiver version of  FIG. 8 , which is designed to interface the intracavity probe to the MR system via a (probe) input port thereof that is not equipped with a preamplifier; 
         FIG. 11  is a perspective view of the interface device in its multiple-receiver version of  FIG. 9 , which is designed to interface the intracavity probe and a phased array coil system, such as the Gore® torso array, with the Phased Array Port of the MR system; 
         FIG. 12  is a schematic diagram of a coil loop and an output cable of an intracavity probe, and a decoupling diode of an interface device corresponding thereto, according to a first alternative embodiment of the invention; 
         FIG. 13  is a schematic diagram of a coil loop and an output cable of an intracavity probe, and decoupling diodes of an interface device corresponding thereto, according to a second alternative embodiment of the invention; and 
         FIG. 14  is a schematic diagram of a coil loop and an output cable of an intracavity probe, and a decoupling diode of an interface device corresponding thereto, according to a third alternative embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In all of its embodiments and related aspects, the present invention disclosed below is ideally used with magnetic resonance (MR) systems designed to operate at 3.0 Tesla field strengths, though it is also applicable to those operable from approximately 2.0 to 5.0 T. For purposes of illustration below, the invention will be described in the context of the 3.0 T systems produced by General Electric Medical Systems (GEMS). 
       FIGS. 1-7  illustrate one aspect of a presently preferred embodiment of the invention, namely, an intracavity probe, generally designated  1 . The probe is intended for use with an MR system to obtain images or spectra of a region of interest within a cavity of a patient. It is described herein in a specific implementation, i.e., as an endorectal probe designed to be inserted into the rectum to obtain images and/or spectra of the male prostate gland. Although presented herein as an endorectal probe, it should be understood that the invention is equally capable of being adapted to obtain images of and/or spectra from other regions of interest such as those accessible through the mouth, the vagina or other orifices penetrable by an intracavity probe. The principles presented herein may also be applied to MR imaging or spectroscopic techniques appropriate for the arteries, veins, and other structures of the body. Whatever the application, the receiving coil within intracavity probe will need to be housed in, or otherwise incorporated into, a package appropriately designed to conform to the target anatomy. 
     In its most novel aspects as best shown in  FIG. 1 , intracavity probe  1  includes a coil loop  2  and an output cable  3 . Ideally made of a conductive material that is flexible, coil loop  2  is preferably a single turn coil capable of picking up radio frequency (RF) signals. Designed to receive magnetic resonance RF signals from the region of interest, coil loop  2  has a plurality of capacitors including first drive capacitor  21 , second drive capacitor  23 , and tuning capacitor  24 . The first and second drive capacitors are serially connected within coil loop  2 . As is explained below, the junction node  22  at which drive capacitors  21  and  23  connect forms a virtual ground for electrically balancing and impedance matching the coil loop  2 . Tuning capacitor  24  is also serially connected within coil loop  2  but diametrically opposite the junction node  22  of capacitors  21  and  23 . Tuning capacitor  24  is selected to resonate coil loop  2  at an operating frequency of the MR system, which for a 3.0 Telsa scanner would be approximately 128 MHz. 
     Output cable  3  is designed to connect coil loop  2  to an interface device for the intracavity probe  1 . Such an interface device, such as either of the ones disclosed below, at its other end in turn connects to a probe input port of the MR system  10 , as shown in  FIGS. 8 and 9 . Encased within an insulating sheath, output cable  3  has a shield conductor  31  and a center conductor  32  insulatively disposed therein. The shield conductor  31  connects to the junction node  22 , and the center conductor  32  connects to a node of one of the drive capacitors  21  and  23  opposite junction node  22 , as shown in  FIG. 1 . In addition, for reasons detailed below, output cable  3  preferably has an electrical length of n(λ/2)+S L , where n is an integer, λ is the wavelength of the operating frequency of MR system  10 , and S L  is a supplemental length. 
       FIG. 2  shows the intracavity probe 1 of the present invention in fully assembled form, and  FIGS. 3-7  illustrate various partial cross-sectional views thereof. Intracavity probe  1  includes a flexible shaft  40  and inner and outer balloons  50  and  60 . The shaft  40  has a distal end whose tip  41  is preferably substantially more flexible than the remainder of the shaft, and indeed may be bonded thereto as indicated at reference numeral  15 . The use of such a flexible tip  41  will reduce not only the discomfort felt by the patient but also the likelihood of perforating nearby tissue during use of the probe. 
     Inner balloon  50  connects to the distal end of shaft  40  and encloses tip  41  thereof, as best shown in  FIG. 3 . Inner balloon  50  is generally cylindrical in shape except for a substantially planar section on its anterior surface  51 . It can be anchored to shaft  40  by a clamp  16  and by an interference fit with the distal end of shaft  40 . Coil loop  2  itself is preferably encased within 5K volt insulation over which shrink wrap or similar tubing is used, thus providing a double layer of insulation. A non-stretchable material  55 , which is preferably composed of an adhesive-backed cloth, can then be used to attach the coil loop  2  to the anterior surface  51  of inner balloon  50 , thus securing the coil loop  2  between the inner and outer balloons  50  and  60 . 
     Outer balloon  60  also connects to the distal end of shaft  40 , enclosing both coil loop  2  and inner balloon  50 . It can be anchored to shaft  40  by a clamp  17  and by an interference fit with the distal end. Outer balloon  60  has anterior and posterior surfaces  61  and  62 . The anterior surface  61  is preferably saddle-shaped to conformably fit against a correspondingly-shaped interior surface/contour of the cavity, which in the case of the prostate probe will be the rectal prostatic bulge inferior to the ampulla of the rectum. The posterior surface  62  features at least one pair of undulating folds  63  projecting therefrom. As described below, these folds  63  enable the outer balloon  60  to properly position the coil loop  2  in operative proximity to the rectal prostatic bulge of the patient when the inner balloon  50  is inflated, which optimizes the coupling between coil loop  2  and the target anatomy. In addition, as shown in  FIG. 5 , lateral indentations  64  are preferably provided within outer balloon  60  intermediate the anterior and posterior surfaces  61  and  62 . These indentations  64  essentially form a shelf on which the sides of coil loop  2  rest during assembly of the probe  1 . They essentially serve as a means of positioning the coil loop between those surfaces when balloons  50  and  60  are in the uninflated state. The balloons  50  and  60  are each preferably made of a medical-grade latex or other appropriate elastomeric material. Such material should, of course, be non-paramagnetic and exhibit low dielectric losses. 
     Flexible shaft  40  defines two lumens  42  and  44 , as best shown in  FIGS. 3 ,  4 ,  5  and  7 . Within its cylindrical wall near its distal end, shaft  40  also defines a hole  43  in communication with lumen  42 , as shown in  FIG. 7 . Lumen  42  and hole  43  together serve as a passageway for the air or other gas pumped into and expelled out of inner balloon  50  when inflated and deflated, respectively. Further away from its distal end, shaft  40  defines another hole  45  in its cylindrical wall. Lumen  44  and hole  45  act as the conduit through which output cable  3  is routed from coil loop  2 . Output cable  3 , as shown in  FIG. 2 , has a plug  35  at its proximal end to connect the intracavity probe  1  with the appropriate interface device. 
     Intracavity probe  1  further includes an anti-migration disc  46 , an introducer  48 , and a handle  49 . Fixed to the proximal end of shaft  40 , the handle  49  enables the probe  1  to be easily manipulated as its distal end, inclusive of outer balloon  60  secured thereon, is inserted into the rectum and appropriately aligned within the cavity as described below. The introducer  48 , also referred to as a dilator element, is designed to be easily slid over the entire length of shaft  40 . Preferably funnel-shaped, the introducer  48  can be used to manually dilate the anal sphincter to allow outer balloon  60  to be easily positioned within the cavity. Without introducer  48 , the anal sphincter would contract around shaft  40  and interfere with the ability to rotationally and longitudinally position the intracavity probe  1  within the cavity. The anti-migration disc  46 , composed of a semi-rigid plastic or other suitable polymer, is preferably semi-spherical in shape. As shown in  FIGS. 2 and 4 , the disc  46  defines a slot  47 . This slot allows the disc  46  to be snapped onto shaft  40 . When affixed to shaft  40  adjacent the anal sphincter after the probe has been inserted into the rectum, the anti-migration disc  46  prevents the probe  1  from migrating superiorly due to the normal peristaltic activity of the colon. 
     Intracavity probe  1  also includes a means for controlling inflation of inner balloon  50 . The inflation control means preferably takes the form of a compressible inflator cuff  70 , a tube  71 , and a stop cock  72 . A syringe of suitable size could be used in lieu of cuff  70 . Tube  71  connects the inflator cuff  70 , or syringe, to the lumen  42  at the proximal end of shaft  40 . The stop cock  72  is connected in series with tube  71  and serves to control whether air is pumped to or released from inner balloon  50 . The probe  1  also preferably features a scale  14  printed on an outer surface of shaft  40 . Scale  14  provides an indication of not only the distance that shaft  40  has been inserted into the-cavity but also the rotational orientation of the distal end for proper alignment of the saddle-shaped anterior surface  61  of outer balloon  60  with the prostate. 
     In operation, the distal end of intracavity probe  1  is inserted into the cavity via the rectum while inner balloon  50 , and outer balloon  60  surrounding it, are in the uninflated state. Once the distal end is inserted, the introducer  48  can be used to keep the anal sphincter dilated and thereby enable shaft  40 , and its balloon-enclosed distal end, to be easily manipulated within the cavity. With the distal end inserted and the introducer  48  in place, the scale  14  on shaft  40  can then serve as a guide to enable the clinician or other medical personnel to more accurately position the probe both rotationally and longitudinally within the cavity adjacent the region of interest. Once the intracavity probe  1  is correctly positioned, the introducer  48  can be pulled inferiorly along the shaft, thereby allowing the sphincter to contract around shaft  40 . This contraction assists in holding the intracavity probe  1  in place. The anti-migration disc  46  can then be snapped onto the shaft  40  adjacent the sphincter to assure that the intracavity probe  1  stays in position during the MR scanning procedure. 
     Before inflating the balloons, the stop cock  72  must be switched to the open state. By pumping inflator cuff  70 , the inner balloon  50  will inflate via tube  71 , stop cock  72 , and lumen  42  and hole  43  in shaft  40 . As the inner balloon inflates, the non-stretchable material  55  that secures coil loop  2  to the anterior surface  51  of inner balloon  50  also focuses inflation of the inner balloon posteriorly so as to inflate into the undulating folds  63  of outer balloon  60 . As the undulating folds  63  inflate, they soon force the posterior surface  62  (i.e., folds  63 ) of outer balloon  60  to abut against a wall of the cavity opposite the region of interest. As inner balloon  50  continues to inflate, the force of inflation is then directed towards the underside of the anterior surface  61  of outer balloon  60 . The anterior surface  51  of inner balloon  50 , with coil loop  2  attached thereto, thus forces the saddle-shaped anterior surface  61  of outer balloon  60  against the correspondingly-shaped interior contour of the cavity, namely, the prostatic region of the rectum. Once the balloons at the distal end are fully inflated, the coil loop  2  will be situated approximate the prostate gland for optimal reception of the MR signals therefrom during the MR scanning procedure. The stop cock  72  can then be switched to the closed position, thereby allowing the clinician to disconnect the inflator cuff  70  without deflating the balloons  50  and  60 . The intracavity probe  1  can then be connected to the appropriate interface device via the plug  35  of output cable  3 . 
     When the scanning procedure is completed, the clinician need only switch the stop cock  72  to the open position to deflate inner balloon  50  and outer balloon  60  therewith. Whether or not the anti-migration disc  46  is removed from shaft  40 , the balloon-enclosed distal end can then be removed from the rectum merely by gently pulling on the handle  49  of the intracavity probe  1 . 
     Alternatively, the intracavity probe  1  may employ a single balloon in lieu of the double balloon version described above. It may be composed of a single-ply medical-grade latex material or other suitable elastomeric material. In this arrangement, the balloon still connects to the distal end of flexible shaft  40 , and the balloon will preferably have anterior and posterior surfaces identical to those described for the double balloon version. The coil loop  2 , however, will ideally be bonded or otherwise secured to the underside of the anterior surface  61  of the balloon. The coil loop  2  could also be encapsulated within the anterior surface  61  during the process of manufacturing the balloon. For example, coil loop  2  could be placed on a surface of the balloon and then the balloon could be redipped to place another ply of material over the outer surface of the balloon, thus covering coil loop  2  and creating the anterior surface  61  described above. However manufactured, when the inflatable balloon is inserted into the cavity and inflated, the undulating folds  63  will press against the wall of the cavity opposite the region of interest. Upon full inflation of the balloon, its anterior surface  61  will then be forced against the correspondingly-shaped interior contour of the cavity thereby bringing the coil loop  2  into operative proximity with the region of interest (i.e., the prostate gland) wherefrom it can best receive the MR signals. 
     The invention further provides a preferred method of designing the intracavity probe  1 . Variations on this method, which will become apparent to skilled artisans upon reading this document, are also contemplated by the present invention. The first step of the preferred method involves choosing the size of a loop of wire that will form the basis for coil loop  2 . For an intracavity probe designed for imaging the prostate, the wire loop should be sized so that the distal end of the probe, inclusive of the two balloons between which coil loop  2  will be situated, can be inserted into the rectum with minimal discomfort to the patient. The next steps involve temporarily inserting a variable capacitor within the wire loop, and then subjecting the loop to the operating frequency of the MR system  10 . For 3.0 Tesla scanners for which the present invention is particularly well suited, the operating frequency would be approximately 128 MHz. For the GEMS 3.0 T Signa® scanner, the operating frequency is actually closer to 127.74 MHz. For the Siemens 3.0 T scanner, the operating frequency is 123.2 MHz. 
     While the wire loop is being subjected to RF energy at the designated operating frequency, the variable capacitor should be adjusted to a value, hereinafter referred to as C RV , at which the wire loop will resonate. Once resonance is achieved, the capacitive and inductive reactances of the wire loop will, of course, be equal in magnitude at the operating frequency. For the purposes of the following calculations, 10 picofarads (pF) is an ideal value for C RV  to establish resonance within the wire loop according to the presently preferred method of designing the intracavity probe  1 . 
     Once C RV  has been established, the quality factor of the loop can be measured while the loop is operating under loaded conditions. There are several known techniques for measuring the quality factor. One such technique involves making an S 21  response measurement using two test probes and a network analyzer, with the two test probes being connected to ports  1  and  2 , respectively, of the network analyzer. With the loops of the two test probes positioned at right angles to each other, the wire loop of the present invention is placed between them. This arrangement allows RF energy supplied to the loop of the first test probe to be induced within the wire loop, which in turn induces an RF signal in the loop of the second test probe. The two test probes then convey their respective RF signals to the network analyzer, which displays the resulting frequency response curve graphically in terms of amplitude versus frequency. Using the displayed signal, the quality factor can be ascertained by locating the center frequency of the frequency response curve and dividing it by the 3 dB bandwidth (i.e., the band between the 3 dB (half power) points at the high-pass and low-pass ends of the curve). For a 3.0 Tesla scanner, the quality factor of the loop will lie between 10 and 20. More typically, the quality factor of the loop under loaded conditions will be:
 
 Q   Loaded =15 (measured)
 
     The next step of the method involves determining the series resistance, R S , of the loop. The series resistance represents the equivalent resistive losses exhibited by the loop due to its presence within the cavity of the patient. R S  is thus not a physical component, only the effect the patient has on the loop. It reduces the quality of coil loop  2  by partially dissipating the energy within it. It can be calculated from the equation:
 
 R   S = X   L   /Q  
 
where Q is the quality factor measured above and X L  is the inductive reactance of the wire loop when loaded. As noted above, the capacitive and inductive reactances of the loop are equal in magnitude at resonance:
 
 X   L   =X   P  
 
 X   L =2 πfL   COIL  and  X   P =1/(2 πfC   RV )
 
where f is the operating frequency of the MR system  10 . Consequently, the inductive reactance of the loop, X L , can be calculated from:
 
 X   L =1/(2 πfC   RV )=1/(2π×128×106×10×10 −12 )=124.34Ω.
 
     Consequently, the series resistance of the loop will be:
 
 R   S   =X   L   /Q   Loaded =124.34Ω/15=8.29Ω.
 
     The method also requires the step of matching the output impedance of intracavity probe  1  with the impedance required by the external circuit with which the intracavity probe shall interface. The external circuit can take the form of one of the interface devices disclosed herein, and will typically require an impedance of 50 Ω. Consequently, this step of the method includes devising an impedance matching network to match the impedance required by the external circuit, R P , to the series resistance of the loop, R S . In this impedance matching network, the quality of the series and parallel legs of the matching network, as represented by Q P =R P /X P  and Q S =X S /R S , are equal. Consequently, R S  and R P  are related by the equation:
 
 R   P =( Q   2 +1)R S  
 
where R P  can also be referred to as the equivalent parallel resistance. Given that the quality of the series and parallel legs of the matching network are equal, the quality of the matching network can then be calculated from:
 
 Q=Q   S,P =( R   P   /R   S −1) 1/2 =(50Ω/8.29Ω−1) 1/2 =2.24.
 
     The parallel reactance, X P , associated with R P  in the impedance matching network can then be calculated from:
 
 X   P   =R   P   /Q =50Ω/2.24=22.32Ω.
 
     The value of the matching capacitor can then determined from the parallel reactance:
 
 C   P =1/(2 πfX   P )=1/(2π×128×10 6 ×22.32)=55.7 pF.
 
     Another step involves inserting two capacitors of the matching value into the wire loop in series with each other. These are the two drive capacitors, C D1  and C D2 , respectively designated  21  and  23 , shown in  FIG. 1 . Using the above calculations, the drive capacitors  21  and  23  together have an effective value of 27.85 pF. Junction node  22  is formed at the site at which drive capacitors  21  and  23  connect. The shield conductor  31  of output cable  3  connects to junction node  22 , and the center conductor  32  connects to the node on the other side of either drive capacitor  21  or drive capacitor  23 . Therefore, according to the above calculations, the value of drive capacitor  21 , C D1 , is thus what enables the coil loop  2  to appear as a 50 ohm source to the interface device or other external circuit. This allows a 50 ohm coaxial cable to be used as the output cable  3 . 
     A further step involves selecting a tuning capacitor, C TUN , so that the total capacitance of the wire loop is equal to the resonance value, C RV . The total capacitance of the wire loop, C RV , can be determined from:
 
1 /C   RV =1 /C   TUN +1 /C   D1 +1 /C   D2 .=1 /C   TUN +2 /C   D  
 
where C D =C D1 =C D2 . The value of the tuning capacitor, C TUN , can then be calculated as follows:
 
     
       
         
           
             
               
                 
                   
                     C 
                     TUN 
                   
                   = 
                   
                     
                       ( 
                       
                         
                           C 
                           RV 
                         
                         * 
                         
                           C 
                           D 
                         
                       
                       ) 
                     
                     / 
                     
                       ( 
                       
                         
                           C 
                           D 
                         
                         - 
                         
                           2 
                           ⁢ 
                           
                             C 
                             RV 
                           
                         
                       
                       ) 
                     
                   
                 
               
             
             
               
                 
                   = 
                   
                     
                       ( 
                       
                         10 
                         × 
                         
                           10 
                           
                             - 
                             12 
                           
                         
                         ⁢ 
                         F 
                         × 
                         55.7 
                         × 
                         
                           10 
                           
                             - 
                             12 
                           
                         
                         ⁢ 
                         F 
                       
                       ) 
                     
                     / 
                     
                       ( 
                       
                         
                           55.7 
                           × 
                           
                             10 
                             
                               - 
                               12 
                             
                           
                           ⁢ 
                           F 
                         
                         - 
                         
                           2 
                           × 
                           10 
                           × 
                           
                             10 
                             
                               - 
                               12 
                             
                           
                           ⁢ 
                           F 
                         
                       
                       ) 
                     
                   
                 
               
             
             
               
                 
                   = 
                   
                     15.6 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     pF 
                   
                 
               
             
           
         
       
     
     The variable capacitor is then removed from the wire loop and replaced with the tuning capacitor, C TUN . Designated as C T  in  FIG. 1 , the tuning capacitor  24  is serially connected within the wire loop diametrically opposite the junction node  22 . Junction node  22  thus forms a virtual ground for electrically balancing the coil loop, as the electric field there is effectively zero and the voltage drop across each drive capacitor is equal but opposite in sign. This configuration results in symmetry of the electric fields relative to the patient during the receive cycle of MR system  10 . It renders coil loop  2  particularly sensitive to the magnetic field, but not the electric field, components of the MR signals emitted by the region of interest. It thus enables coil loop  2  to receive the MR signals with a greater signal-to-noise ratio than prior art probes. It also does so with greater safety, as the voltages induced in the coil loop will be equal and half of what they would otherwise be if the coil loop were totally unbalanced. 
     Due to the high operating frequency (e.g., 128 MHz for a 3.0 T MR system) and very low operating Q (i.e., between 10-20) of coil loop  2 , there is no need to tune coil loop  2  on a per patient or per-coil basis, unlike the probe disclosed in U.S. Pat. Nos. 5,476,095 and 5,355,087. On the basis of the above calculations including the quality factor of the loaded coil loop, the bandwidth of coil loop  2  will nominally be +/−4.25 MHz. Consequently, assuming the coil loop will be built with +/−2% components, the tuning shift from probe to probe should be a maximum of approximately +/−1.85 MHz, which is substantially less than the 3 dB bandwidth of the coil loop even without the effects of the low input impedance preamplifier explained below. The tuning is essentially fixed without material compromise due to the low Q of coil loop  2  under loaded conditions. 
     Output cable  3  preferably has an electrical length of n(λ/2)+S L , where n is an integer, λ is the wavelength of the operating frequency of MR system  10 , and S L  is a supplemental length. As best shown in  FIG. 1 , the entire length of output cable  3  extends from coil loop  2  to its plug  35 . Plug  35  represents the point at which the output cable connects to the PIN diode  33 , also referred to as the decoupling diode, of the interface device or other external circuit. The n(λ/2) part yields a section whose length is one-half the operating wavelength, which will effectively appear as zero electrical length. The value of n will typically need only be equal to 1, as coil loop  2  will in practice always be reasonably close to the circuit to which it will connect. S L  represents an additional section of output cable  3  whose inductive reactance is ideally equal in magnitude to the capacitive reactance of first capacitor  21  across which the terminals of cable  3  connect. The net effect is that the entire length of output cable  3  exhibits an inductive reactance equal to the capacitive reactance of first capacitor  21 . 
     Supplement length S L  thus inherently acts as an inductor, hereinafter referred to as L D , which affects the operation of intracavity probe  1 . During the transmit cycle of MR system  10 , the MR system will decouple coil loop  2  of intracavity probe  1  from the MR system by forward biasing PIN diode  33  with a 200 mA current (see, e.g.,  FIG. 8 ). This will effectively short circuit PIN diode  33 , and leave the inherent inductor, L D , of output cable  3  and first drive capacitor  21 , C D1 , as a parallel resonant circuit. The high impedance of this parallel resonant circuit approximates an open circuit, which effectively opens coil loop  2  and thus decouples the intracavity probe  1  from the probe input port of the host MR system  10 . Conversely, during the receive cycle, the MR system will couple the intracavity probe  1  to the MR system by reverse biasing decoupling diode  33  with −5V DC. This will effectively cause output cable  3  to act as a 50 ohm transmission line rather than an inductor, L D . This will allow coil loop  2  to detect the MR signals generated within the region of interest by the resonance-inducing RF pulse transmitted by the body coil of MR system  10  (or other external coil). The MR signals will then be passed to the interface device via the conductors of cable  3 . 
     Drive capacitors C D1  and C D2  will typically have values in the range of approximately 62 pF to 82 pF. Similarly, tuning capacitor  24 , C TUN , will preferably be in the range of approximately 12 to 15 pF. Better decoupling (higher open-circuit impedance) during the transmit cycle can be obtained using a value for C D1  in the lower end of the preferred range. Such a lower value for drive capacitor  21  would then also increase the source impedance that coil loop  2  presents to the interface device during the receive cycle. Furthermore, the exact length of S L  will depend on the particular coil loop used within intracavity probe  1 . For a coil loop that would be only lightly loaded during use, for example, drive capacitors of, say, 120 pF may be used, in which case S L  would be shorter. Conversely, for a coil loop that would be more heavily loaded, drive capacitors of 40 pF might be used, in which case S L  would be longer. 
     The intracavity probe  1  described above is particularly well suited for use as an endorectal coil probe with the 3.0 T MR systems produced by GEMS, although it should be understood that the probe can be used for other applications as well. 
       FIGS. 8 and 9  depict two other aspects of the preferred embodiment of the invention, both of which designed to interface intracavity probe  1  with the GEMS MR system. In its first aspect, the interface device interfaces the intracavity probe with one receiver of the MR system, and is thus referred to as the single-receiver version. In its second aspect, the interface device interfaces both intracavity probe  1  and an external coil to the MR system using multiple receivers, and is referred to as the multiple-receiver version. As is well known, the typical GEMS Signa® system features four receivers and eight input ports. Receiver  0  can connect to Ports  1  or  5 , receiver  1  to Ports  2  or  6 , receiver 2 to Ports  3  or  7 , and receiver  3  to Ports  4  or  8 . In the standard configuration, the GEMS MR system has a preamplifier in each input port, except for Ports  1  and  8 . 
       FIGS. 8 and 10  illustrate the interface device, generally designated  100 , according to its single-receiver version. By its connector  102 , interface device  100  is designed to interconnect intracavity probe  1  via output cable  3  to Port  1  of the host MR system  10 , which is not equipped with its own preamplifier. Consequently, interface device  100  includes PIN diode  33  and a preamplifier  101 . PIN diode  33  is connected across the input socket  103  of interface device  100  into which plug  35  of output cable  3  plugs. This design choice allows PIN diode  33  to be physically remote from the intracavity probe  1 , thus allowing it to be reused as part of the interface device after the probe  1  is disposed. The preamplifier includes a GASFET  110  and a series resonant input circuit  130 . The series resonant circuit  130  includes an input capacitor C P  and an input inductor L P  at the junction of which the gate of GASFET  110  is also connected. The GASFET has its source connected to biasing resistor R B  and its drain linked to coupling capacitor C C  and an RF choke RFC 2 . According well known circuit design principles, resistor R B  should be selected so that the current flowing through GASFET  110  will provide a good gain and a low noise figure. RFC 2  allows DC power to be fed to the drain of GASFET  110  without shorting out the MR RF signals output by preamplifier  101  during the receive cycle of MR system  10 . A cable trap  115  is preferably employed on the other side of capacitor C C  to block undesirable cable currents. 
     When interface device  100  is connected to the MR system via probe cable  150  and connector  102 , the drain is linked to Port  1  of MR system  10  via coupling capacitor C C  and cable trap  115 . The drain is also linked to a DC power source in MR system  10  via the RF choke RFC 2 . Bypass capacitor C B2  connects between this RF choke and ground, and therefore carries any non-DC components to ground. Interface device  100  also includes a bypass capacitor C B1  and RF choke RFC 1 . Bypass capacitor C B1  connects between ground and a biasing line  121  with which MR system  10  is able to bias PIN diode  33 . C B1  thus serves to carry any non-DC components away from the biasing line and decoupling diode  33 . RFC 1  connects between the anode of PIN diode  33  and bypass capacitor C B1 , and thus presents a high impedance to RF frequencies without appreciably limiting the flow of the biasing currents. Interface device  100  also preferably includes a preamp protection diode D PP  and a bypass capacitor C B3 . Diode D PP  protects preamplifier  101  during the transmit cycle of the MR system. Bypass capacitor C B3  connects between the anode of the preamp protection diode D PP  and ground. RFC 3  prevents any RF currents from preamplifier  101  from flowing to MR system  10 , while allowing the flow of the biasing currents on biasing line  121 . 
     During the transmit cycle, MR system  10  will forward bias diodes D D  and D PP  via biasing line  121 . Situated across the connector  103  of device  100  into which plug  35  of output cable  3  plugs, PIN diode D D  will thus decouple intracavity probe  1  as explained above. Meanwhile, preamp protection diode D PP  will effectively short circuit the gate of GASFET  110 , which prevents the transmitted RF pulse signal from damaging preamplifier  101 . During the receive cycle, MR system  10  will reverse bias those diodes, effectively turning them off. The series resonant circuit  130  will provide optimum impedance to GASFET  110  when coil loop  2  is operating under loaded conditions. Coupled to the gate of GASFET  110 , the series resonant circuit  130  will provide preamplifier  101  with a relatively low input impedance, which serves to broaden the frequency response of coil loop  2 . This broader frequency response offsets the fixed tuning scheme, which makes the tuning of coil loop  2  far less critical when compared to the probe disclosed in U.S. Pat. Nos. 5,476,095 and 5,355,087. More specifically, with coil loop  2  acting as a 50 ohm input, series resonant circuit  130  will provide a high impedance (˜1000 to 2000 ohms) to GASFET  110  while appearing as a very low impedance (˜1 to 5 ohms) to coil loop  2 . This will effectively cause coil loop  2  to decouple somewhat, which broadens its frequency response without sacrificing the signal-to-noise ratio. Along with its series resonant input circuit  130 , preamplifier  101  will thus provide gain and impedance matching between the anode of decoupling diode  33  and Port  1  so that the MR signals detected by coil loop  2  are passed to Port  1  of the MR system with enhanced signal-to-noise ratio. 
     Interface device  100  also preferably features circuitry  160  to prevent the MR system  10  from performing a scanning procedure when the intracavity probe  1  is not connected to the interface device. Such circuitry  160  could create a driver fault within the MR system  10  to prevent it from undertaking a scan when the probe is disconnected. An audible alarm or display  161  as part of circuitry  160  through which to notify medical personnel of such a fault is also preferable. 
       FIGS. 9 and 11  illustrate the interface device, generally designated  200 , according to its multiple-receiver version. By its connector  202 , interface device  200  is designed to interface not only intracavity probe  1  but also a phased array coil system  80  with the Phased Array Port of the GEMS 3.0 T Signa® MR system. The Phased Array Port is typically composed of four ports (e.g., Ports  2 ,  4 ,  5 , and  7 ), all of which are accessible via a single connector. The prior art Gore® torso array is one such phased array coil system  80  that itself can be plugged via its single connector  81  into the Phased Array Port. If the Gore® torso array were to be used as coil system  80 , coil elements A 1  and A 2  of  FIG. 9  would be the two surface coils in the anterior paddle  82 , and coil elements P 1  and P 2  the two surface coils in the posterior paddle  83 . Those two paddles each have two coil elements whose leads are routed by means of two cables  84 , 85  to single connector  81 . It is by connector  81  that the Gore® torso array  80  normally plugs into the Phased Array Port of the host MR system, with each of its four coil elements being interconnected with one of the four system ports. Interface device  200 , however, when used with intracavity probe  1  and the Gore® torso array, will interface five coil elements (i.e., coil loop  2  and coil elements A 1 , A 2 , P 1  and P 2 ) to the four-receiver Phased Array Port of MR system  10 . Interface device  200  combines the four-coil torso array with the receive-only endorectal coil  1  to enable high resolution imaging of the prostate along with phased array imaging of the pelvic region. 
     Interface device  200  includes a probe interface circuit  210  and an array interface circuit  240 . Probe interface circuit  210  includes PIN diode  33  and a cable trap  211 . PIN diode  33  is connected across the input socket  203  of device  200  into which plug  35  of output cable  3  plugs. Probe cable  213 , also referred to herein as circuit length  213 , is used to link the decoupling diode  33 —and therethrough coil loop  2  of intracavity probe  1 —with a first port (i.e., Port  7 ) of MR system  10 . Cable trap  211  prevents undesired current from flowing on the shield conductor of the probe cable. As shown in  FIG. 9 , the circuit length  213  preferably has an electrical length of n(λ/2), where n is an integer and λ is the wavelength of the operating frequency of the MR system. This makes circuit length  213  effectively appear to have zero electrical length. 
     The array interface circuit  240  is used to electrically interconnect phased array coil system  80  and MR system  10 . It includes first and second series resonant networks  242  and  252 , two 1/4  wavelength networks  261  and  262 , and a 1/4  wavelength combiner  271 . Assuming coil system  80  takes the form of the Gore® torso array, series resonant network  242  will convey the MR signals from anterior coil element A 1  to a second port (i.e., Port  4 ) of MR system  10 . Similarly, the other series resonant network  252  will pass the MR signals from anterior coil element A 2  to a third port (i.e., Port  2 ). As illustrated in  FIG. 9 , one 1/4  wavelength network  261  is situated to receive MR signals from posterior coil element P 1 , and the other 1/4  wavelength network  262  is configured to receive MR signals from posterior coil element P 2 . Preferably of the Wilkinson type, the 1/4  wavelength combiner  271  is connected to the outputs of both 1/4  wavelength networks  261  and  262 . It combines the MR signals received from those two networks and conveys the resulting MR signals to a fourth port (i.e., Port  5 ) of MR system  10 . 
     The first series resonant network  242  includes capacitor C R1  and RF choke RFC 5 . Similarly, the second series resonant network  252  includes capacitor C R1  and RF choke RFC 6 . The values of C R1  and C R2  are selected so that each capacitor tunes out the inductance inherent in its respective circuit path. First and second networks  242  and  252  are thus series resonant at the operating frequency of MR system  10  (i.e., they act as if having a length of n(λ/2) where n=0). This enables coil system  80  and MR system  10  to operate electrically as if there were no length to the networks  242  and  252 . In addition, RF choke RFC 5  is disposed in parallel with capacitor C R1 , as choke RFC 6  is with capacitor C R2 . This is because, along the circuit paths of series resonant networks  242  and  252 , MR system  10  will convey biasing signals to the decoupling diodes in coil system  80  for anterior coil elements A 1  and A 2 . Chokes RFC 5  and RFC 6  allow those biasing signals to pass from Ports  4  and  2  to those decoupling diodes. 
     Furthermore, as shown in  FIG. 9 , the length of the circuit path from the input for coil element P 1  (through network  261  and combiner  271 ) to Port  5  is ideally one-half the operating wavelength (i.e., nλ/2). The same length applies for the circuit path extending from the input for coil element P 2  to Port  5 . Consequently, these circuit paths will effectively appear as zero electrical length, which permits the beneficial effects of the low impedance preamplifier in Ports  5  to be reflected back to their respective inputs. In addition, MR system  10  conveys biasing signals to the decoupling diodes for posterior coil elements P 1  and P 2 . An RF choke and related circuitry within combiner  271  and network  261  allow biasing signals to pass from Port  5  to the decoupling diode for coil element P 2 . An RF choke RFC 7  and related circuitry allow biasing signals to pass from Port  8  to the decoupling diode for coil element P 1 . The biasing signals for coil element P 1  are sourced from Port  8  so that it is independent of that for coil element P 2 . 
     During the transmit cycle, MR system  10  will forward bias decoupling diode D D  with the decoupling voltage, which is preferably superimposed on the signal line of cable  213 . Situated across the connector  203  of device  200  into which plug  35  of output cable  3  plugs, PIN diode D D  will thus decouple intracavity probe  1  as explained above. MR system  10  will also simultaneously forward bias the decoupling diodes of the four coil elements A 1 , A 2 , P 1 , and P 2  in coil system  80 . This will cause those decoupling diodes to short circuit, thereby yielding parallel resonant circuits of high impedance, which will effectively open circuit the four coil elements of coil system  80 . In this manner, the host MR system  10  will thus decouple both the intracavity probe  1  and the torso array  80  from the Phased Array Port of the MR system. Conversely, during the receive cycle, MR system  10  will reverse bias PIN diode D D  of probe  1  and the decoupling diodes of coil system  80 , effectively turning them off. This will couple intracavity probe  1  and torso array  80  to the Phased Array Port. This will allow coil loop  2  and coil elements A 1 , A 2 , P 1  and P 2  to detect the MR signals emitted from their respective regions of interest (e.g., prostate and surrounding abdominal, thoracic and pelvic regions) in response to the resonance-inducing RF pulse(s). The MR signals will then be routed through interface device  200  in the aforementioned manner and passed via connector  202  to the Phased Array Port of the host MR system  10 . 
     Interface device  200  also preferably features circuitry  280  to prevent the MR system from performing a scanning procedure when the intracavity probe  1  is not connected to the interface device. Such circuitry  280  could include a probe sense line connected to the socket  203  into which the plug  35  of intracavity probe  1  plugs. When the probe  1  is connected to interface device  200  (i.e., plug  35  inserted into socket  203 ), the probe sense line would be grounded. Circuitry  280  would then detect the ground and pass an appropriate signal to Port  1  to enable the MR system to begin the scanning procedure. Should the intracavity probe not be connected to the interface device, circuitry  280  would detect the resulting open circuit and respond by altering the state of Port  1  to prevent the MR system from undertaking the scan. An audible alarm or display  281  as part of circuitry  280  through which to notify the clinician of such a fault is also preferable. Various other methods of determining whether the probe is connected to the interface device are, of course, also contemplated by the present invention. 
       FIG. 12  illustrates the intracavity probe, and the relevant part of the interface device corresponding thereto, according to a first alternative embodiment of the invention. Specifically,  FIG. 12  shows coil loop  2  a connected through output cable  3   a  to the decoupling diode D D  of the interface device. Output cable  3   a  is unbalanced, with its shield conductor  31   a  connected to junction node  22   a  and its center conductor  32   a  connected to the node on the other side of drive capacitor C D1 . Unlike the previously disclosed preferred embodiment, however, output cable  3   a  has an electrical length of only n(λ/2). This is because the supplemental length S L  has been incorporated within the interface device. This can be accomplished as shown in  FIG. 12 , for example, by assuring that the electrical length from the input socket to the decoupling diode D D  is equal to S L . When output cable  3   a  of the probe is plugged into the interface device, the total electrical length from the coil loop  2   a  to PIN diode D D  is then equal to n(λ/2)+S u  Although this embodiment puts S L  in the interface device rather than in output cable  3   a , it still allows the intracavity probe and its corresponding interface device to operate in the same manner as the presently preferred embodiment of the invention during both the transmit and receive cycles of the MR system. 
       FIG. 13  illustrates the intracavity probe, and the relevant part of the interface device corresponding thereto, according to a second alternative embodiment of the invention. Specifically,  FIG. 13  shows coil loop  2   b  linked to the decoupling diodes D D1  and D D2  of the interface device through a balanced output cable  3   b . At one end of output cable  3   b , the first and second center conductors  32   b  and  34   b  are connected to the nodes on opposite sides of drive capacitors C D1  and C D2 , respectively. When plugged into the input socket of the corresponding interface device, output cable  3   b  at its proximal end has its first and second center conductors  32   b  and  34   b  electrically linked to the anodes of diodes D D1  and D D2 , respectively, with its shield conductor  31   b  grounded with the cathodes of the two decoupling diodes. Unlike the previously disclosed preferred embodiment, output cable  3   b  has an electrical length of only n(λ/2), as S L  has again been incorporated within the interface device. Such use of a balanced output cable  3   b  allows better decoupling (e.g., 2×1500 Ω across each drive capacitor) than the unbalanced output cable  3   a  used in the first alternative embodiment. 
       FIG. 14  illustrates the intracavity probe, and the relevant part of the interface device corresponding thereto, according to a third alternative embodiment of the invention. The coil loop  2   c  of the probe is linked to the decoupling diode D D  of the interface device through a balanced output cable  3   c . Unlike the previous embodiments, coil loop  2   c  is constructed with only one drive capacitor C D , with the tuning capacitor C T  positioned within the wire loop diametrically opposite it. The values of drive capacitor C D  and tuning capacitor C T  can be calculated generally according to the foregoing method so as to enable the coil loop  2   c  not only to appear as a 50 ohm source to the interface device but also to resonate at the operating frequency of the MR system. At one end of output cable  3   c , the first and second center conductors  32   c  and  34   c  are connected across drive capacitor C D . When plugged into the input socket of the interface device, output cable  3   c  at its proximal end has its first and second conductors  32   c  and  34   c  electrically linked to the anode and cathode, respectively, of decoupling diode D D  and its shield conductor  3   c  grounded with the interface device. Unlike the previously disclosed preferred embodiment, output cable  3   c  has an electrical length of only n(λ/2), as S L  has again been incorporated within the interface device. 
     As should be apparent to persons of ordinary skill in the field of magnetic resonance imaging and spectroscopy, the intracavity probe in any of the above embodiments may be constructed with two or more coil loops arranged in a phased array configuration. In addition, two or more coil loops in a single intracavity probe can be oriented cooperatively to provide quadrature coverage of the region of interest. The output cable of such intracavity probes will have to be modified accordingly to properly link the coil loops to the interface device. 
     The presently preferred and alternative embodiments for carrying out the invention have been set forth in detail according to the Patent Act. Persons of ordinary skill in the art to which this invention pertains may nevertheless recognize alternative ways of practicing the invention without departing from the spirit of the following claims. Consequently, all changes and variations which fall within the literal meaning, and range of equivalency, of the claims are to be embraced within their scope. Persons of such skill will also recognize that the scope of the invention is indicated by the following claims rather than by any particular example or embodiment discussed in the foregoing description. 
     Accordingly, to promote the progress of science and the useful arts, we secure for ourselves by Letters Patent exclusive rights to all subject matter embraced by the following claims for the time prescribed by the Patent Act.