RF COIL FOR IMAGING SYSTEM

Abstract: An RF coil suitable for use in imaging systems is provided which coil has a dielectric filled cavity formed by a surrounding conducting enclosure, the conducting enclosure preferably being patterned to form continuous electrical paths around the cavity, each of which paths may be tuned to a selected resonant frequency. The patterning breaks up any currents induced in the coil and shortens path lengths to permit higher frequency, and thus higher field strength operation. The invention also includes improved mechanisms for tuning the resonant frequency of the paths, for selectively detuning the paths, for applying signal to the coil, for shortening the length of the coil and for controlling the field profile of the coil and the delivery of field to the object to the image.

Description: RF COIL FOR IMAGING SYSTEM Field of the Invention This invention relates to imaging systems employing radio frequency (RF) coils for RF field generation, and more particularly to RF coils for use in such systems which coils facilitate higher frequency, higher efficiency, higher energy operation, permit use of larger coils, facilitate flexibility in coil design to accommodate a variety of applications and provide enhanced signal- to-noise performance so as to achieve among other things improved MRI. fMRI and MR spectroscopic imaging, all the above being achieved without significant increase in cost. The invention applies similarly to EPR or ESR. Background of the Invention Nuclear magnetic resonance (NMR) or magnetic resonance imaging (MRI). functional MRI (fMRI). electron spin resonance (ESR) or electron paramagnetic resonance (EPR) and other imaging techniques using RF field generating coils are finding increasing utility in applications involving imaging of various parts of the human body, of other organisms, whether living or dead, and of other materials or objects requiring imaging or spectroscopy. For purposes of this application. RF shall be considered to include frequencies from approximately 1 MHz to 100GHZ, the upper ranges of which are considered to be microwaves. While existing such systems are adequate for many applications, there is often a need for higher signal-to-noise and improved spectral resolution in such imaging so as to permit higher spatial resolution, higher image contrast, and faster imaging speed. In fMRI applications for example, where multiple images may be taken over time and a difference image generated to permit visualization of small changes in blood oxygen use over time in the body being imaged, differences between successive images may be very small, requiring high signal-to-noise to permit detection. A major limitation to higher resolution, and/or faster imaging is an insufficient signal to noise ratio. If the image signal intensity is below the noise level, an image can not be made. It is therefore important in high resolution systems to design an RF coil to maximize signal and to minimize noise. The RF coil of such a system should also be designed to minimize eddy currents propagating therein which are induced by time transient currents in gradient coils or by other causes. The signal-to-noise ratio (SNR) and spectral resolution are increased by increasing the magnetic field strength of the system, generally expressed in tesla (T). The SNR benefits of image speed, spatial resolution, and contrast are also increased with the magnetic field strength. However, the frequency of which the nuclei of atoms in the body resonates varies as a function of the applied magnetic field, with each atomic species having a unique magnetic field dependent resonant frequency referred to as the Larmor frequency. For the human body which is composed primarily of hydrogen atoms in water, fat and muscle tissue, these hydrogen nuclear (proton) frequencies are approximately 64 MHZ for a field strength of 1.5 T. 170 MHZ (4 T), 175 MHZ (4.1 T), 300 MHZ (7 T). 340 MHZ (8 T) and 400 MHZ (9.4 T). Other species of atomic nuclei in a body would resonate at other frequencies for a given field strength. However, while conventional birdcage coils in existing MRI and related systems might resonate at a frequency of 170 MHZ (4 T) for example, the conventional birdcage coil with lumped elements (reactance) will operate very inefficiently, radiating much of its energy like an antenna, rather than conserving its energy like a "coil". At higher frequencies still, such lumped element coils of human head or body dimensions will not reach the Larmor resonant frequency required, limiting the magnetic field strength at which such MRI or EPR systems can operate. Further, since frequency is a function of the electrical path lengths (measured in wavelengths) in the RF coil. higher frequency, and thus higher field strength operation, has been previously achievable only with very small coils which are not always useful for imaging a human being or other larger objects. A need therefore exists for an RF coil design which provides short electrical path lengths and shields against radiative losses, while still permitting an RF coil to be constructed with physical dimensions sufficient to image a human body and/or other larger objects with high frequency RF energy, thus permitting high field strength operation. It is also desirable to be able to tune each path of an RF coil to a precise resonant frequency, to be able to provide two or more resonant frequencies for different paths on the coil, and to be able to easily adjust/retune the resonant frequency of a path or paths. Still another potential problem in operating these imaging systems, especially at high fields, is in driving the RF coil in a manner so as to achieve a homogeneous RF field, even when a body is positioned in the field, or to achieve some other desired field profile. Many factors influence field profile or contours including the manner in which the coils are driven, the geometric and frequency dependent electrical properties of the anatomy or object, and the frequency dependent properties of the coil circuits. Techniques for controlling these and other factors to achieve a homogenous or other desired field profile are therefore desirable. Also, while in many systems the same coil is used for both the transmitting of RF energy and the receiving thereof, the coils being switched between transmit and receive circuitrv. there are manv applications where the homogeneous excitation of NMR signal is ed with a large volume coil and a small local receive coil having very short path lengths is used for ach-ieving high SNR operation, such local receive coil being placed as close to the region of the body being imaged as possible. However, having both the large transmit coil and the local RF receive coil tuned to the same frequency results in the coils being destructiveh coupled enz's Law for example), this defeating enhanced operation from the local receive coil It is therefore desirable to be able to quickly detune the large RF transmit coil duπng a receive operation b> a local RF receive coil and vice versa. Improved s of achieving this objective. particularh in an RF coil providing the characteristics previously indicated, are therefore desirable Finally, some of the advantages of having a local receive coil, and in particular the ability to place the RF coil closely adjacent to a region where imaging is desired, could be achieved if the RF coil were designed so as to localize both the transmission and reception of RF energy. While coils adapted for performing this function in certain specialized situations have existed in the past, a more general purpose design for RF coils to facilitate their use in producing localized RF fields and the localized reception of RF (NMR) signal, is desirable in order to achieve the enhanced SNR benefits of higher image signal, resolution, speed and contrast. While some of the advantages indicated above are achieved by distπbuted impedance RF coils disclosed in US Patents 5.557,247 and 5.744.957. which patents e the same inventor as this invention, the systems taught in these patents, and in particular the RF coils thereof, do not provide optimum performance in all situations, and improvements are possible on vaπous aspects of these RF coils, including edd\ current suppression, design of the coil for optimum positioning in a greater number of cases, improved control of field profile, improved tuning options and improved detuning in situations where the use of two or more coils is desired Summary of the Invention In accordance with the above, this invention provides an RF coil for use in an imaging system, which coil has a cavity formed as a conductive enclosure in which resonant field can be excited, the enclosure being formed at least in part of an electrical conductor patterned to form RF conducm e paths around the cavity At least one tuning mechanism may be provided which determines a resonant frequency, or frequencies for such paths The tuning mechanism may be fixed, resulting in a preselected resonant frequenc\ tor the path, or \ aπable to provide a tunable resonant frequency or frequencies The tuning mechanism ma\ reactn e ad]ust the electrical length of each path to tune the path The path reactance may also be adjusted to achieve a selected field profile for the coil The tuning mechanism ma\ tune all the paths to the same resonance frequenc\ or may selectiv ely tune the paths to resonate at two or more different frequencies In particular, alternate ones of the paths mav be tuned to resonate at a first frequency and the remaining ones of the paths tuned to resonate at a second frequencv The coil may also include a dielectric which at least substantialK fills the cavity the thickness of the conductor for at least selected portions of the enclosure may be substantially greater than one skin depth at the resonant frequenc} . the dielectric filling the cavity having a dielectric constant different from that of air This results in signals of different frequencies propagating on the outer and inner surfaces of the conductor Each of the N paths on the coil mav have at least one non-conductι\ e gap formed therein, and the tuning mechanism may include a reactance anαVor an impedance across at least selected ones of said gaps The reactance impedance for at least selected ones of the gaps may be variable to control the resonant frequency for the corresponding path The reactance for at least some embodiments includes a capacitor the capacitance of which may be varied and/or an inductor the inductance of which may be varied The variable impedance and/or reactance may be controlled to tune, retune anαVor detune the path in which it is located Where the enclosure is formed of an outer wall, inner wall, and side walls, end conductiv e lands may be formed on each of the walls, with corresponding lands on each wall being connected to form the paths and the gaps being formed in the conductor for each of the paths for at least one of the walls For some embodiments, the gaps are formed in the outer wall conductor tor each path The resonant frequencv of the paths may be determined by distributed capacitance and distributed inductance for the path, the distributed capacitance being determined by the area of the electπcal conductor for each path, a dielectric fill material for the cavity and/or the dimensions of the dielectric fill material The electrical conductor forming each path may be a thin foil, the distributed inductance for the path being a function of the path length At least one reactance component may be provided in at least selected ones of the paths, the reactance component being either distributed or lumped \. distributed or discrete reactance may be selected to achieve a desired resonant frequency for the paths The paths ha e a cumulative reactance which includes at least in part the distributed capacitance/inductance, the cumulative reactance for the paths being tuned to result in D different resonant frequencies tor the coil, every Dth path s mmetπcally spaced around the coil being tuned to the same trequencv The coil may include a circuit which applies RF signal to and/or receives RF signal from M selectively spaced ones of said paths, where M is an integer and 1<M<N. The RF signals may be phase shifted corresponding to a phase shift for the corresponding paths to provide circular or other polarization for the coil. Each RF signal is preferably reactively coupled to the corresponding path, the coupling reactance for each path being variable for some embodiments to independently match/tune the path. In particular, the coupling reactances may be impedance matched to different loading conditions for the coil. For some embodiments, the -RF coil may be used to transmit and/or receive RF signals, but not both simultaneously, and includes a detuning mechanism for the paths, the detuning mechanism being operative when the RF coil is not in the one of the transmit/receive modes for which it is being used. The detuning mechanism may include a mechanism for altering the path length and/or impedance for each path to be detuned, and in particular may include for some embodiments a PIN diode circuit for each path which facilitates rapid switching to a changed impedance state sufficient to effect the path detuning. Alternatively, the RF drive signals may be phase-shifted corresponding to the phase shift for the paths to which they are applied to provide circular polarization for the coil, the detuning mechanism including circuitry which reverses the phase of the RF drive signals. For an enclosure which is formed of an outer wall, inner wall, and side walls, within conductive lands being formed for each wall, corresponding lands on each wall being connected to form the paths, at least the outer wall may have two conductive layers separated by a dielectric. the two conductive layers each being slotted to form a pattern of lands, with slots on each layer being overlaid by lands of the adjacent layer. The degree of overlap for the lands of said layers is at least one factor controlling coil resonant frequency. At least one of the side walls may also have an aperture through substantially the center thereof through which a body to be analyzed may be passed to an area inside the inner wall, the conductive layer on the inner wall being patterned to provide a selected magnetic flow pattern in said aperture. One of the side walls may also be closed, the closed side wall being slotted to form a land pattern covering at least most of the wall. The imaging system may also have at least one gradient coil which induces low frequency eddy currents in the RF coil, the slotting on at least the outer wall and side walls resulting in the breaking up of and substantial attenuation of such eddy currents w ithout substantial attenuation of RF currents and fields. The electrical conductor for at least the outer wall and side walls may be a conductive layer which is thin enough to attenuate low frequency eddy currents while still conducting RF currents For such embodiment, the conductor layer has a thickness substantially equal to one skin depth at the resonance frequencv to which the coil is tuned, which thickness is substantially equal to approximatelv 5 microns for an illustrativ e embodiment For some embodiments, each of the paths has at least one urcumterential/azimut-hal slot formed therein to break the path into smaller paths Λ fixed v aπable αnαVor switched reactive coupling and/or an impedance coupling may be connected across each of the circumferential slots Where a reactiv e coupling is utilized, such coupling is a capacitive coupling for illustrative embodiments An RF drive signal input is provided to at least one of the paths, the path inductively coupling an RF driv e signal and a path to adjacent paths The dielectnc mateπal filling the cavitv may prov ide a selected path capacitance, and thus a selected resonant frequency A mechanism mav be provided for controlling the dielectnc fill of the cavity and thus the resonant frequencv of the coil The electrical conductor may be patterned to form N conductive lands for the enclosure. each of a selected width, and the number N of conductiv e paths and the idth of conductive lands for each path may be selected to achiev e a desired resonant frequency and a desired field contour The enclosure is preferably formed to break induced eddy currents and/or to shape the RF magnetic field patterns For some embodiments, a lid is mounted to at least one end of the coil The lid may be conductive, non-conductive or segmented to be partiallv conductive \ pluralitv of sample spaces may also be formed in the dielectnc at a selected portion of the enclosure such portion being one of the side walls for an illustrative embodiment The sample spaces extend at least part way into the dielectric from the side wall Alternatively , the open center chamber or aperture of the coil may contain a dielectnc which preferably fills such chamber and a plurality of sample spaces may penetrate such dielectric At least a portion of at least selected ones of the paths may be formed as conductive tubes or coaxial tube conductors For embodiments having a close end wall, the closed end wall functions as an RF mirror, the end wall having a radial slotting pattern covering at least most ol the wall for an embodiment where the electrical conductors for each wall are slotted For some embodiments, field applied to at least one of the electrically conducting paths causes an alternating magnetic field in the cav ity and at least one aperture is formed in at least the electncal conductor through which magnetic field mav be applied to an adjacent body For some such embodiments, the coil is shaped to conform to the body being imaged, the surrounding walls including connected top and bottom walls, with the at least one aperture being formed in only the bottom wall. The coil may be flexible to conform to a surface of a body being imaged and the at least one apertures may be arranged to be adjacent the areas to be imaged of the body being imaged. Where the areas to be imaged are at least one projection on a body, such projection may extend into the cavity through an adjacent aperture. For such embodiments, the dielectric may be conformable to an outer surface of a projection extending into the cavity so as to minimize discontinuity between the projection and the dielectric. Where the coil is formed is a closed loop, an aperture may be formed in only one of an inner or outer wall of the coil. Apertures may be arranged to be adjacent areas to be imaged of a body being imaged. Various features of the invention, such as the detuning mechanism, may be employed independent of other features of the invention. Another potentially independent feature is the dielectric material filling the cavity being utilized to control the resonant frequency for one or more of the paths. In particular, the dielectric material may be different in different areas of the cavity so as to selectively shape the coil field. A mechanism may also be provided for controlling the dielectric fill of the cavity and thus the resonant frequency of the coil, for example the amount of fluid in the cavity being controlled where a fluid dielectric is employed. Acoustic damping material may also be provided as a fill for at least a portion of the cavity. A dielectric material may also be selectively positioned between the coil and a body to be imaged to control and/or shape the field applied to the body from the coil. The dielectric material preferably substantially fills the space between the coil and at least a selected area of the body, the dielectric constant of the dielectric substantially matching that of the body in such area. Where a selected area is the area to be imaged, the dielectric concentrates and directs the field to such area. In its broadest sense, the invention includes a conductive enclosure which is patterned to suppress low frequency currents and EMI noise. In accordance with still another aspect of the invention, the RF coil includes a cavity formed by at least an inner and an outer conductor, a dielectric material tilling the cavity and at least one sample space formed in the coil. The sample space may for example be formed in the dielectric material, projecting therein from a wall of the enclosure. For some embodiments, the coil is a transmission line stub, the inner and outer conductor of the coil being the inner and outer conductors of the transmission line stub, respectively. A conductive cap may short one end of the transmission line stub. The sample space is preferably located at a distal end of the stub, the sample space extending trom such distal end into the dielectnc mateπal or a hollo ed-out portion of the center conductor The stub is tuned and matched so that maximum current, and therefore maximum RF field, occur at such distal end The foregoing and other objects features and advantages of the inv ention will be apparent in the following more particular description of prefened embodiments ot the inv ention as illustrated in the accompany ing drawings In the Drawings Figs 1A and I B are perspectiv e views of RF coils in accordance with illustrative embodiments of the invention Fig 2A is a sectional v iew taken generallv along the line 2-2 ol I lg 1 A. Fig 2B is the same sectional v iew as f ig 2 \ but tor an alternativ e illustrative embodiment Figs 3A and 3B are illustrations of slotted conductor configurations for an outside wall and an inside wall, respectively , of an illustrative coil having double-sided conductor clad dielectric substrate on both walls Figs 3C and 3D are illustrations of slotted conductor configurations tor an alternative embodiment Figs 4A and 4B are illustrations ot slotted conductor patterns for the back wall and front wall, respectively , of an illustrative coil embodiment having double-sided conductors on each of these walls and a closed back wall The tront and back end walls ot Figs 4Λ and 4B are preferablv utilized in conjunction with the inside and outside walls of Tigs 3 \. 3B or of Figs 3C, 3D to form a conductor pattern lor an RF coil embodiment Fig 5 A is a semi-schematic diagram of a coil for an illustrativ e embodiment illustrating a novel drive mechanism for such coil and a nov el detuning mechanism for the coil Fig 5B is an enlarged schematic v iew ot a detuning circuit for an embodiment Fig 6 is a top perspective view of a receiver coil illustrating detuning mechanism Fig 7 A. and 7B are a top perspectiv e and bottom perspectiv e view respectiv ely. of an RF coil for another embodiment of the invention Fig 8 is a planar rear v iew of a coil in accordance w ith the teachings ot this invention with a patient therein and a field * focusing dielectric pillow Figs. 9A. 9C and 9E are perspective views of three additional embodiments of the invention and Figs. 9B. 9D and 9F are sectional views of Figs. 9A. 9C and 9E. respectively. Figs. 10A-10C are perspective views of three coaxial stub embodiments of the invention. Fig. 10D is a cross-sectional view of a generalized coaxial stub embodiment. Detailed Description Figs. 1A and 2 A show an illustrative embodiment of the invention which overcomes many of the problems discussed earlier. In particular, the RF coil 10 shown in these figures has a conducting cavity formed as a conductive enclosure 12 in which resonant field can be excited, the enclosure being formed by a surrounding, conducting wall 16. which wall may be supported by a non-conducting wall 14. Conducting wall 16 may be a whole wall which is at least selectively patterned as described later, or may be formed of conducting tubes, coaxial tubes as in the 5.557.247 patent or other appropriate spaced components. Cavity/enclosure 12 is filled with air or another dielectric material and supporting wall 14 may also be formed of a dielectric material having a dielectric constant which substantially matches that of the material in cavity 12. Cavity 12 may also be defined by a solid piece of dielectric material of the appropriate shape. The enclosure or cavity 12 may have a circular (Fig. 1 B) or elipsoidal (Fig. 1 A) transaxial cross-section for many applications, and an elongated axial cross-section, shown for example in Fig. 2A or Fig. 2B. so that the cavity has an outer conducting wall 16O. an inner conducting wall 161. a front conducting wall 16F. and a rear conducting wall 16R. ( 1 a.bδa.bDisclosure) As indicated above, each of the conducting walls 16 may be supported by a corresponding nonconducting wall 14. or only selected ones of the conducting walls may be so supported. In accordance with the teachings of this invention, each of the conductor walls 16O. 161. 16F, 16R is slotted or otherwise patterned to form a plurality of lands 18 separated by nonconducting slots 20. While in the figures, the slots 20 are shown as being substantially parallel to each other and to the axis 21 (Fig. 2A) of the coil in each of the inner and outer walls, and continuous with substantially radial slots in the front and rear walls, this is not a limitation on the invention, and other patterns are possible. The exact pattern of slots and lands formed for a given coil may vary with application and with the desired field profile for the coil. While there is no limit to the width of the slots and lands, narrow slots and wide lands provide better Faraday shielding. Wide slots and narrower lands allow RF magnetic flux to pass and allow for visual, auditory, physical.and other access to/from the coil's interior through it's inner and or outer walls. Thus, for one embodiment, conducting cavity walls 160, 16F. and 16R have narrow slots and wide lands, and conducting wall 161 has narrow lands and wide slots. For this embodiment, conducting wall 161 may for example be formed of coaxial tube conductors. One of the objectives of the coils shown in Figs. 1 A . I B. 2A. and 2B is to suppress eddy currents in the coil, and in particular in the outer wall thereof, caused by the proximity of the RF coil to gradient coils for the imaging system, and to suppress other low frequency noise in the conductor, such eddy currents/low frequency noise causing image blurring and therefore adversely affecting the signal-to-noise ratio and resolution achievable from a system employing the coil. One way in which such eddy cunents have been suppressed in the past is to have the thickness of the conductor for at least outer wall 160 of the coil, and preferably for all walls of the coil, thin enough so as to attenuate the low frequency eddy currents induced by the gradient coil, while still conducting RF cuπents. This is possible since the skin depth required to conduct signal decreases with increasing frequency, so that if the thickness of the conductor 16 is substantially equal to one skin depth at the resonant frequency to which the coil is tuned, this frequency being a relatively high RF frequency, then the conductor will not pass, and will suppress or attenuate the low frequency gradient field induced signals or other low frequency noise signals. However, while this mechanism preserves the coil's RF efficiency while attenuating switched gradient induced eddy currents, it alone is not sufficient to fully suppress gradient and/or other low frequency noise for some applications such as fMRI. This objective is facilitated by the slotting or dividing of at least outer wall conducting 160 and preferably by the slotting of this wall and at least end conducting walls 16F and 16R. The slotting of the front and rear conducting walls is desirable to prevent switched gradient induced eddy current flow through or around the ends of the coil. The nanower this slotting (i.e. the greater number there are of nonconducting slots 20, and therefore the narrower the width of each land 18). the more effective this eddy current suppression becomes. The combination of the conductor thickness being substantially equal to one skin depth at the resonant frequency and the slotting of conductor 16. preferably for at least outer wall 160 and end walls 16F and 16R. provides a substantial elimination of all eddy current induced/low frequency noise in RF coil 10. and thus far clearer images and/or faster imaging, then can otherwise be obtained. Further, in order to achieve increased field strength to 4T. 7T. 9.4T or even higher, it is necessary to be able to operate RF coil 10 with increasingly high frequencies. For example, as previously mdicated. for a field strength of 4T. coil 10. when used in an MRI embodiment on the human body , must have a resonant frequencv of about 170 MHZ. and this frequency goes to 400 MHZ for a 9 4T field strength However, for a coil to resonate at these higher frequencies, the reactance of the coil (l e its inductance and capacitance) must be relativ elv low Such low reactances are either not achiev able, or are achiev able onlv tor coils so small as to have limited practical application, when lumped inductors and capacitors are used in conventional lumped element circuit designs for the coil Therefore, distributed capacitance and inductance has been used in distributed element circuit designs to facilitate desired lower reactances However, while a coil 10 such as that shown in Fig 1 with distributed reactance can otter higher frequency performance than coils operating with lumped reactance components ev en such head and body sized coils have difficulty operating w ith maximum efficiencv at Larmor frequencies corresponding to field strengths above 4 or 5T The coil 10 reduces this problem bv breaking the conducting wall to form a large number of continuous paths, the lands 1 8 of the walls being connected or formed into N such conductiv e paths Breaking the conducting wall to form N conductive paths also improves the homogeneity of the field, the higher the v alue of N. the more homogeneous the field N may for example be 16 or 24 Fig 1 further shows a circumferential or azimuthal slot 22 being formed in for example the center of outer conducting wall 16O, which slot is covered by a collar 24 of a conductiv e mateπal (the collar 24 being shown in dotted lines in Fig 1 so that the structure thereunder may be more easily v lewed) The slot 22 further breaks up the paths, thus shortening the individual paths and reducing the inductance thereof The greater number of breaks in each of N paths formed around coil 10 the lower the inductance, and thus the higher the resonant frequencv for such coil Collar 24 mav be moved to ary the capacitance formed by the collar, slot 24 and lands 18 of conductor 16 to vary the capacitance of each path, and thus fine tune the resonant frequencv thereof Tuning could similarly be performed by vaπable or fixed capacitors bridging the gap or gaps in each path Capacitance tor the paths is also determined by the thickness of dielectric filled cav ity 12. by the dielectric mateπal in this cavity, by the area and thickness of conductiv e wall 16. etc Distributed inductance is determined pπmanlv bv the umntemipted conductor length and by the width of each path Thus, assuming that all paths are to operate at the same resonant frequency , the slotting of conductor 16 is selected so that all of the lands 20 are ot equal width, however, in applications where different paths are to hav e different resonant frequencies, for example e ery other path having a first resonant frequencv and the remaining paths a second resonant frequency , every other path could be of a first width to prov ide the first resonant frequency and the interv ening paths ot a second width to provide the second resonant frequency Vanous parameters of the paths may also be selected or adjusted, including capacitance, inductance, phase, and conductor thickness of at least selected walls, to control relative current earn ing or otherwise control field contours or profiles ithin the coil While only a single circumterential/azimuthal slot 22 on the outer wall 160 is shown in Figs 1 A and 2A. this is not a limitation on the invention, and higher frequency operation can be achieved by having a plurality of circumferential slots 22 for each path For example, gaps might be provided at selected one or more of points A. B, C in Fig I B Further, there are some advantages in having slot 22 on outer wall 160 of the coil, including the operation of the timing πng or capacitors 24 being easier from this location, and that the E-field applied to a patient/body in the recess 26 formed bv inner wall 161 is significantlv reduced, thus reducing detuning caused by the presence of the patient body in the coil interior and reducing E-lield induced noise and heating in the patient or load This is a problem particularly at higher frequencies since more EM energy is lost (radiated) at higher frequencies for a given coil Howev er, slots 22 may be at a plurality of locations on outer wall 160 and/or inner wall 161. on sidewalls 16F and or 16R and/or at the intersections of these various walls (See points B. C. Fig IB) In some applications these azimuthal slots may be bridged by fixed, v aπable. or sw itched discrete inductance and/or capacitance components (see A in Fig IB), or by distnbuted components similar to collar 24. but fixed rather than moveable Fig 2B illustrates a number ot variations w hich are possible in practicing the teachings of this invention First, the cavity 12 for Fig 2A is assumed to be tilled w ith a dielectric, the thickness and other dimensions, including volume, of the cav ity and the dielectric constant of the dielectric being two of the factors which determine a resonant frequencv toi each path Fig 2B illustrates the cavity 12" as being filled with a fluid, for example a liquid or a gas. having a known dielectric constant However, a tube 30 connected to a suitable pump may increase or decrease the quantity of fluid dielectnc in cavity 12" and/or the fluid pressuie in the cavity and/or may alter the spacing between at least semi-elastic cavity walls Anv ot these changes alter the dielectric constant or v olume of the dielectric in cav ity 12", and thus can be used to control the resonant frequency or to tune the resonant frequencv of the coil Second, the embodiment of Fig 2B has more clearly defined walls which may for example be separately formed as discussed in conjunction with Figs 3 \--4B These separate conductiv e layers may envelop a solid dielectric cavity core or a fluid tilled, tor example air filled, cavity core Corresponding lands on adiacent walls mav then be electrically connected, for example directlv . capacitiv ely . or lnductivel . to form the N continuous electrical paths around the coil While for reasons indicated above, conducting wall 16 would generally be far too thin to provide structural integπty for the coil without a supporting dielectric substrate 14. this is not a limitation on the invention and electrically , all that is required to define cavity 12 is a surrounding conducting wall 16 While one objective of the invention is to provide distπbuted capacitances and inductances to achieve higher frequency and thus higher field strength operation, in some applications it may be desired to operate a coil of this invention at a lower icsonant frequency to, for example, permit operation at a lower field strength for a giv en application, while still achieving the other advantages of this invention Adding discrete reactance, for example the added lumped (fixed, variable, or switched) capacitance (or inductance) elements 32 shown in Fig. 2B. may be utilized both to achieve a desired reduced frequencv operation and to tune the paths to a desired lower resonant frequency A capacitor 32 could lor example be provided for each of the N paths or. where operation at more than one frequency is prov ided. the capacitor 32 might only be in the paths to be operated at the lower of the frequencies. 01 the capactance might be one value tor one frequency, and another value for another frequencv 1 or example, if every other path is to hav e a resonant frequency at a first higher frequencv and the remaining alternate paths to be resonant at a second lower frequencv . capacitors 32 might appeal onlv tor the alternate paths to be operated at the second lower frequency . or capacitors ot alternating v alues may appear on alternating elements to tune two frequencies for the coil Further w hile the embodiment of Fig. 2B has only a single azimuthal slot 34 for each path and a single lumped capacitance or inductance element 32. additional azimuthal slots could also be prov ided tor this embodiment to for example shorten path lengths, and these slots or gaps could be bridged bv appropnate lumped reactance elements, lumped capacitance elements being shown for bridging gaps 34. or could be bπdged by distributed reactance elements such as tor the gaps of 1 ig 2 \. 3B Except for the differences discussed abov e, the embodiments of Fig 2 A and 2B are othei ise substantially the same and operate in substantially the same manner to achiev e the benefits ot this invention Fig 3 A illustrates a conducting wall 16 which is typically used as an outer wall 160, but sometimes used as an inner wall 161 for a coil 10 Fig 3B illustrates a transmission line conductor 161 which is most often used at the inside wall for the volume coils 10 and 10' These conductors differ from the conductors 16 discussed earlier in that each is made up of an inner layer and an outer laver separated bv a dielectnc to form transmission line elements For the wall 16 of Fig 3 A. the slots for the outer lay er are shown in solid lines while the slots for the bottom or inner layer are shown in dotted lines The figure therefore show s these layers as being staggered so that the slot of one layer is overlaid by a land of the other layer This provides superior RF efficiency and Faraday shielding for the confinement ot fields generated by the RF coil Similarly, for Fig 3B which shows a conductor preferably used lor the inside wall of the coil to form conducting wall 161. the solid outer conductor is slotted both longitudinally and circumferentially, while the dotted bottom conductor is slotted onlv longitudinally, with lands on each side of the conductor overlying slots on the opposite side The double-sided conductors, with lands on the conductor for one side overlying slots for the conductor on the other side, also applies for the two end walls as illustrated in Figs 4A and 4B. Fig 4 A being for example conducting rear wall 16R. and Fig 4B showing conducting front wall 16F of a coil suitable for example for head imaging While not shown, slots may also be provided in various of the radial lands for the end walls to shorten path lengths The degree of overlap foi lands on opposite sides of each double-sided conductor controls capacitance, and thus resonant frequency, for the conductive paths The degree of ov erlap on the outside and end walls ot the cavity improve the RF conduction, efficiency and shielding of the cav ity In addition to oi in lieu of. overlapping conductive lands on double sided cavity walls, capacitors can be used to budge the slots or gaps between the lands to provide additional RF conduction or alternate RT paths across these slots Figs 3C and 3D show a conducting outside wall 16 O' and a conducting inside wall 16 I' for an alternative embodiment of the invention W all 16 O' is slotted into multiple overlapping layers, as for wall 16 O (Fig 3 A) but w ith additional azimuthal slotting Bridging capacitance can be added across the horizontal azimuthal gaps at tor example points A. B. and/or C, or anywhere else such gaps appear, and can function to tune the coil/path to a desired frequency Placing lumped or distributed capacitance at these outside wall positions has the added benefit of moving capacitiv e sections of the coil away from the patient ' s head or body Wall 16 I' is shown as single-sided printed line segments This inside wall embodiment places the more inductive inside wall surface ot the coil nearer to the coil load ( for example the patient), while the more capacitiv e outside and/or end walls of the coil are away from the load The end walls of Figs 4A. 4B mav be used with the outer and inner walls 16 O' and 16 I' and the walls 16 O, 16 I may be used together or with other inner/outer wall embodiments depending on application Fig 4A also illustrates another optional feature of the inv ention in that the conducting rear wall 16R can be an open wall like Figure 4B. or a closed wall, this closed wall acting as an RF mirror which permits the length of the cavity to be shorter The shortened coil facilitates decreased electrical path lengths for all circuits, thus facilitating higher efficiency, higher frequency operation and thus higher field strength operation In this case, α phvsically shortened coil is also an electπcally shortened coil (in wavelengths) and therefoie a more efficient coil for a given higher frequency and larger dimension such as a human head sized coil operation at 128 MHz or higher Particularly where the coil is being utilized to image the head or biain of a patient, the coil being shortened in this way provides ergonomic benefits in that it permits at least the patients mouth and sometimes nose/eyes, to be outside of the coil, reducing the claustrophobic feeling sometimes experienced by patients in such imaging machines, and also facilitating easier breathing by the patient through the mouth or nose if exposed This design also permits an optical minor to be mounted within visual range of the patient to permit visual stimuli to be provided to the patient, something which is required for various brain imaging applications, without requiπng a transparent section in the coil Since it is necessaiv to have the coil's current pass through a conductiv e film or screen covering these transparent l egions, the patient is not afforded an unobstructed view ot visual stimuli for example in some fMRI studies The shortened coil is thus highly advantageous However, if the coil design is such as to extend over the patient's eyes, patient v isibility may be enhanced by providing thin conductoi tor the conducting wall 16 in areas over the patient ' s eyes, the conducting wall 16 in this ai ea toi example being formed by thin, substantially parallel tubes or coaxial conductors Mternativ eh a transparent section such as a view port over the face of a human can be prov ided in v anous ways, including 1) through widened slots (thin conductors) for the inside and outside w alls ov er the face, 2) through widened slots or gaps between elements in the inside wall and a continuous or slotted conductive screen window in the outside wall, or 3) through transparent conductive elements, continuous or slotted, for inside and outside walls of the cavity As seen in Fig 4A. the rear wall has a unique slotting pattern w ith the slotting extending over substantially the entire end wall The rear wall with this unique slotting pattern contributes to the resonant frequency of each path. The coil with the closed end wall is also more efficient in that it limits radiation loss which might otherwise occur through the end wall, the closed end wall also enhancing coil symmetry and thus facilitating tuning of the paths. This follows from the fact that all of the coil elements or paths are symmetrically referenced to the same ground plane. Finally, since the patient ' s mouth can extend beyond the coil, a bite bar can be provided to reduce patient head movement, something which facilitates signal averaging and diminishes motion artifacts. Without the ability to maintain the patient ' s head absolutely still, applications involving multiple images, such as for example fMRI. can provide erroneous results. Fig. 5A illustrates a number of features of the invention, including preferred ways for applying RF drive signals and for outputting received RF signals from the coil, and techniques for switched detuning of the coil in applications where the coil is being used for either transmit or receive but not both simultaneously . In particular, instead of applying drive signal to only one of the paths 18 and relying on inductive coupling to couple the RF signal to the remaining paths. Fig. 5A shows a technique for applying RF drive signal simultaneously to multiple paths. In particular, an RF drive (transmit) signal on line 40 is passed through transmit/receive switches 42 to a 1 0° splitter 44 which divides the signal into two signals 180° out of phase with each other. The outputs from splitter 44 are split again into quadrature hybrids 46A and 46B to drive four line elements or paths 18 separated by 90° azimuth angles. Since the electrical phase difference for the signals applied to each of the paths corresponds to the angular separation between the paths, the transmit mode is circularly polarized. A second 1 80° splitter 48 combines the quadrature combined receive channels passing through the quadrature hybrids 46 to a common receive channel 50 which is passed through switches 42 to receive output line 52. Transmit and receive lines 40. 52 are decoupled from the power amplifier and the preamplifier respectively by transmit receive switches 42. In a body coil application for example, these switches will have the attributes of low loss, high speed and very high power ratings, while requiring low switch bias voltages. All of the components of the drive circuit should be nonmagnetic and can be mounted close to the back of coil 10. While for the illustrative embodiment shown in Fig. 5A. circularly polarized drive signals have been applied to four evenly spaced paths on coil 10 for improved homogeneity, all that is required for circular polarization, is that the paths to which signal is applied be in relative quadrature phase. The number of paths coupled electrically to the power amplifier or power amplifiers and the receiv er or receivers may be any number between l and N where N is the number of elements or paths 18 in Figure 5 A Thus, assuming 16 paths tor an illustrative embodiment, signal could be applied to 2. 4. 8. or 16 paths for balanced quadrature operation, or could be applied to another number of coils Transmit signal amplitude, phase angle, and dnve paths can be selected for maximum homogeneit . or for targeting a desired legion of interest in the body or other test object Similarly , the receiv e paths and phase angles can be chosen for overall homogeneity . or for highest sensitivity reception from a specific region of interest. In addition, while signals are shown applied to a sidewall of the paths, signal can be applied at vaπous points on the paths, it being cunently preferred that they be applied to inner conducting wall 161 at a point near its junctions with a sidewall Further, the signal on each of the lines is shown as being capacitiv elv coupled to the conesponding path through a variable capacitor 54 While capacitiv e coupling is shown, any reactive coupling (capacitive or inductiv e) can be used Operation at two or more frequencies can be achieved for the coils in Figures 1. 5a. by changing the electrical path lengths for alternating paths for each frequency desired For example, for a double tuned coil, all odd numbered paths would be adjusted or tuned to one electπcal length or frequency All even numbered paths would be tuned to a second frequency This tuning can be achieved by adding or subtracting, inductance or capacitance in the respectiv e paths Operation at two different frequencies may also be achieved by having at least one wall of the resonant cavitv ot greater than one skin depth, and by having a dielectnc constant within the cav ity different than that ot air outside ot the cavity Thus results in a signal wavelength on the inner surface of the conductiv e cav ltv all facing the inner dielectric to be different ( in frequency) to the signal wav elength on the cav ity wall facing the outer air dielectric This results in the cavity being resonant at two different liequencies In the discussion so tar. it has been assumed that coil 1 0 is being used both as a transmit coil and as a receive coil However, in some applications, particularly where homogeneous excitation of. and high sensitivity detection from, localized regions ot interest (ROI) is required, separate coils may be utilized for transmit and for receiv e The coil 10 would more typically be used as a transmit coil, with for example a phased arrav or other appropriate l ecei e coil such as is shown in Fig 6 being placed adjacent the area being imaged on the body The ieceiv e coil may have varying numbers of coil loops of v arying shapes depending on application In some applications a coil 10 might only be used as a recei e coil One problem when separate transmit and receiv e coils are used together is that destructive reactive coupling mav occur between the two coils which can interfere w ith the imaging and eliminate the sensitivitv benefits achiev able from hav ing a separate receiv e coil It is therefore necessary to RF field decouple the transmit and receiv e coils from each other This field decoupling can be accomplished bv oπenting the spatial position of one coil relative to the other, by manipulating the electrical phase relations of one coil relativ e to the other, by changing the field amplitude of one coil relative to the other, by changing the resonant frequency of one coil relative to the other anαVor bv temporal separation ot the field of one coil relative to the other by any combination of the above techniques While mechanical means, including relative spatial manipulations of the two coils or mechanical switching or reorienting ot the phase, amplitude and/or frequencv of the coil, cunent v oltage and RF fields might be utilized to effect the field decoupling of the two coils, tor pretened embodiments the decoupling is accomplished electncally or electronicallv The actuation or control of such decoupling mav be by PIN diodes, solid state switches such as transistors, and semiconductor relay s tube switches, electromechanical relays, varistors. etc In addition to the " active electronic components indicated above, "passive ' components may also be used, including small signal diodes, hmiter diodes, rectifier diodes, etc . these components often being used together with quarter-wave circuits Further, by the general methods above, coil coupling can alternativ elv be maximized for some applications Tor example, it may be desirable for coil 10 to be stionglv coupled to a remote or implanted surface coil where transmission-line coupling mav be impractical Because of its speed power handling, compactness and non-magnetic packaging, the PIN diode is a good choice for manv decoupling circuit implementations, including ones inv olving a coil 10 Such PIN diode circuits can be used to change the electrical length ot a coil or its individual paths, and to thus change the resonant frequency of one coil relativ e to the other coil, decouplmg in this case being effected by frequency shifting \ PIN diode circuit can also be used to open circuit or short circuit a coil or indiv ldual paths thereof to etfectiv elv switch the coil on or off. therebv decoupling it from the other coil Similarly . PIN diodes mav be used to shift the phase of coil cunents to minimize the coupling between two coils Fig 5 A shows one wav in which a PIN diode 56 mav be utilized to detune the paths 18 of coil 10. a separate PIN diode sw itched circuit 56 being pro ided tor each path 18 for this embodiment Each PIN diode shorts two points on the conespondmg path hen conducting, for example a point on an inner wall to a point on a side wall or outer w ail therebv alteπng the effective length, phase or impedance of the path and thus its resonant frequencv Fig 5B shows that this detuning technique can be used w ith a con of the ty pe shown in U S Patent No 5.557.247. each transmission line element 57 hav ing an inner conductor 59 and an outer conductor 61 separated bv a dielectric 63 Each PIN diode 56 for this embodiment is connected through a solder post 60 to short outer conductor 61 through choke coil 62 to a conductor 64 on rear wall 68 of the coil The diodes are cunent forward biased to short the path, thus dramaticallv alteπng the coil's resonant frequency , and thus decoupling the coil from another coil, a transmit or receive coil of the same operational frequencv The resonant frequency of each path may be quickly restored by voltage back biasing the diode to disconnect the conductor 61 from the cavitv wall This PIN diode switching approach is effectivelv changing the impedance across an equivalent gap or azimuthal slot located in the paths at position C" in Figure IB This or a similar approach using a PIN diode circuit to change the impedance ( higher or lower) in some or all of the paths can be affected at other gap positions in the paths such as A or B While in Figs 5A and 5B PIN diodes 56 are used to short paths 18 for detuning, PIN diodes could also be used to effect detuning by placing the diodes in the path, for example two walls being connected through a PIN diode for each path, the path being open circuited for detuning The PIN diodes could be used to quicklv switch reactance into or out of each path to change its resonant frequency, or the PIN diodes could be utilized to effect detuning in other ways Further, for the embodiment shown in Fig 5 A. detuning mav be effected bv reversing the phase for the RF drive signals applied to the v arious paths 18 so that electrical phase is out of phase rather than in phase with the azimuthal separation of the paths In the discussion so far. coil 10 has been assumed to hav e a closed tubular configuration with an RF field mode M=l or greater, so that field is applied to a bod\ position w ithin the coil However, this is not a limitation on the inv ention and the coil could be designed to operate in an M=0 mode for example, as taught in U S Patent No 5 744 957 In particular bv tor example, not slotting the inner wall 161 of the coil, or bv having a two lav er ov erlapping inner conductor as shown in Fig 3A. the RF field can be confined in the cav πv 1 to circulate therein, field not exiting the cavity except where an opening is prov ided in one of the walls ot the cavity through which the field mav exit or through which a bodv to be imaged mav be inserted into the cavity to be exposed to the field circulating therein Refemng to Figs 7 \ and 7B the coil 70 may be flat as shown, may hav e a slight curv ature mav be flat w ith a circular elliptical or other appropπate shape rather than a rectangular shape as shown or mav hav e some other appropπate shape so as to fit on the body being imaged with minimal spacing, therebv achieving optimal coupling between the coil and the bodv to be imaged, eliminating losses resulting trom dielectnc constant mismatches and spaces between the coil and the bodv Coil 10 mav tor example be flattened to achiev e a flat shape The resonant cav itv of this inv ention mav thus have a wide vaπetv of sizes and shapes, modes of operation, conductor patterning apertures etc Any cavity coil geometry is allowable provided that an RF field can be generated therein which can be made useful for MRI or EPR imaging applications One or more openings 72 may be provided in a wall of coil 70 which wall is to be adjacent to or in contact with the bodv being imaged, each opening 72 being adjacent a portion of such body on which imaging is desired Holes are preferablv at B l magnetic field nodes of the cav itv wall Where the portion of the body on which imaging is desired is a projection on the body, for example a woman s breasts opening 72 may be positioned as shown in Fig 7B to permit such projections to enter cavitv 12 through the openings so as to be in the field path in the cavity The dielectric material in the cavity may be shaped or deformable to fit projections extending into the cavitv minimizing dielectnc (impedance) mismatch Openings 72 are strategicallv located and dimensioned to both encompass the body portions to be imaged and to be properlv phased Openings 72 might also be used on an inside or outside wall of a coil 10 designed to operate in M = 0 mode or in a side wall In one application, apertures in for example a side wall of a coil are each dimensioned to hold an experimental mouse or the mouse s head onlv to permit a pluralitv ot mice to be batch/simultaneouslv imaged In particular, referring to Figs 1 A and I B an embodiment of the invention is shown which is suitable for batch nuclear magnetic resonance ( NMR) study of multiple laboratory samples, which samples may for example be held in test tubes, or of lab animals such as mice The coil 100 is of the ty pe prev iouslv descπbeα and has a cavity which is filled with a dielectnc matenal. which matenal can be a gas such as air a fluid or a solid The dielectric material is preferablv matched to the sample to minimize the electrical impedance boundary between the sample and the cavitv for improv ed performance \ solid dielectric can serve as the support wall for sample spaces 102 As for prev ious embodiments 160. 161. 16F. and 16R are the outer wall, inner wall, front wall and rear wall respectiv elv ot the cavity For this embodiment, these walls mav be slotted to break up eddv currents and shield the sample, as for the pπor embodiments, or mav be continuous The space or recess 26 inside wall 161 is not used for this embodiment While end wall 16F may be a conducting w all, for example having slotted conductors between the sample spaces 102 as shown in Fig 9B either in addition to or instead of wall 16F being conducti e, a conductive lid 104 may be prov ided which is mounted to wall 16F in a manner to provide a good electrical connection, for example being press-fitted The lid aids the coil ' s performance by completelv enclosing the coil cav ity 12 and thus more efficiently sealing energy in the coil Where tront wall 16F is conductiv e. the lid could be non- conductive The lid could also be segmented, but this would require greater care in mounting the lid to assure the lid segments align with the coil segments The embodiment of Figs 9C and 9D is substantially the same as the embodiments previously described, for example in conjunction with Figs 1 -4B. and in particular has an inner wall configuration which is substantiallv the same as that ot Fig 3C I " or this embodiment, the useful space where a sample would be placed is the center space 26 requiring inner wall 161 to be constructed of conductiv e lands, tubes or transmission line elements 1 8 separated by slots 20 wide enough to allow magnetic flux to efficiently fill space 26 Outei wall 160 and end walls 16F and 16R could be any of the configurations previously described, depending on application Figs. 9C and 9D further illustrate the use of a lid 104 with these embodiments of the invention, which lid may be conductive or non-conductive and serves the same functions as those performed by the lid 104 of Fig 9 A Fig 9D also illustrates an optional solid back wall 106 which also may be continuously conductiv e or slotted for the reasons prev louslv discussed Figs 9E and 9T illustrate still another embodiment ot the invention For this embodiment, center space 26 is filled with a dielectric 108 hav ing sample spaces 1 10 formed therein Sample spaces 1 10 may extend completely through dielectric 1 08 or mav extend only partly through the dielectric as for spaces 102 While for the embodiment shown, dielectric 108 is solid, the gas/air or liquid dielectric is utilized, sample spaces 1 10 can be formed of non- conducting walls or the sample can be immersed in the gas or liquid dielectric For some embodiments, sample spaces 1 10 may be tubes through which sample passes in a continuous NMR morutonng process of a gas. liquid or solid sample flow or conv e ance The cavity walls 16 for the embodiment of Figs 9E. 9F would be substantiallv the same as those for the embodiment of Figs 9C. 9D and, as for this pπor embodiment, will v a v w ith application Also. as for the prior embodiments shown in Fig 9. the lid 104 is optional performing the same functions if utilized as tor the prior embodiments Fig. 10 illustrates several transmission line stub embodiments of the invention which may for example be used for NMR microprobe applications, permitting small samples to be efficiently measured. Each of the transmission line stub embodiments 120. 120A- 120C. may be capped with a conductive cap 104 to short-circuit its center conductor 122 with its outer conductor 124. Alternatively, the cap may be spaced or constructed in a manner such that the center conductor and outer conductor of the transmission line are left open-circuited or the cap may be eliminated completely. The stub is preferably tuned and matched such that the maximum current, and therefore maximum RF magnetic field, is located at the sample end of the stub. For the current to peak at the end, the length of the stub should be approximately a one half wavelength or be electrically adjusted to be. a full wavelength increment of the resonant frequency for an open stub and one quarter wavelength or three quarter wavelength for a shorted stub. The stub is typically connected to a transmission line to or from the NMR system by a coaxial connector 126 which is best seen in cross-sectional view 10D. A variety of coaxial connectors known in the art might be utilized. The sample spaces may be located within the dielectric space 128 between the conductors as shown in Figs. 10A and 10B. or may be located in a hollow, slotted center conductor as shown in Fig. IOC. More particularly, outer wall 124 is the outer conductor or shield of a typical coaxial transmission line. The inner wall is a center conductor 122 of a typical coaxial line. The dielectric may be, as for prior embodiments, gas. liquid or solid and impedance matched to the sample. In a typical coaxial line, the dielectric is solid and the sample space would be cut or drilled into the dielectric material. The sample space 130A for the embodiment of Fig. 10A is the space between the inner and outer conductors at the end of the coil. The sample space 130B for the embodiment of Fig. 10B are multiple sample spaces formed in the dielectric around the center conductor. For the embodiment of Fig. I OC. the center conductor 122 is a slotted or element conductor, the sample space 130C being within the hollow end of the center conductor. While in the discussion above it has been assumed that the number of lands on each wall of the coil is the same so that N continuous RF electrical paths are formed around the coil, this is not a limitation on the invention. In particular, the number of lands formed on each wall of the coil may not be the same. Thus, the outer wall may have a first number of lands N, and the inner wall mav have a second number of lands N-,. The side walls mav also have N, lands for reasons previously indicated \ may . for example, be selected at least in part to effectiv ely breakup up low frequency eddy and other curcents induced in the coil while \ is generally selected to achieve a desired magnetic field pattern Ev en where N, and V are not equal adjacent paths on the walls are still connected to form a pluralitv ot continuous electric paths around the coil, these paths providing various ones of the advantages previously indicated As has been indicated earlier, one advantage of a coil 10 in accordance with the teachings of this invention is that it can provide a uniform, homogeneous field inside the coil for imaging purposes While such a homogeneous field is advantageous in many applications, there are applications where some other field pattern is desirable Achiev mg such a patterned field through use of spacing and polanzation of the paths to which signals are applied and the phasing of such signals has been discussed earlier The field may also be patterned bv the choice positioning, and control of the dielectric in cavity 12 to obtain a desired field pattern Still another way of controlling field pattern is illustrated in Fig 8 where a dielectric "pillow " 80 is shown inside of coil 10 which dielectnc is not part of cavity 12 and is selected to prov ide a good dielectnc match with body /patient 82 and/or with the dielectric in cav ity 12 The effect ot dielectric pillow 80 is to concentrate or otherwise manipulate the RF magnetic flux in a region ot interest in the patient's head where, for this embodiment, imaging is desired Dielectric inserts could be otherwise positioned between coil 10 and the portion of a body on which imaging is to be performed, or even within cavity 12. to concentrate or manipulate field in such areas, thereby enhancing measurement sensitivity in these regions of imaging anαVor to minimize field coupling to areas which are not to be imaged The shape of the insert, as well as its dielectric constant, is a factor in achieving the desired control and shaping of the Rf field, the shape of the insert also being useful to restrain body motion, which motion, as previously indicated, can ad ersely affect imaging. Another potential problem with MRI and other imaging systems utilizing RF coils is that rapidly switched currents in the field gradients can generate intense acoustical noise Such noise is often annoying to a patient or even painful One av in which such noise can be reduced is by utilizing an acoustic damping material in cavity 12 as at least part of the dielectric therein, such acoustic dampening material either forming the entire dielectric, or being used in conjunction with other dielectric material in order to achieve a desired dielectric constant or pattern of dielectnc constants in the cavity so as to prov ide a desired resonant frequency . field pattern and/or other features of the invention Thus, while the v ention has been particularly shown and described above with reference to illustrative and prefened embodiments, the foregoing and other changes of form and detail may be made therein by one skilled in the art while still remaining within the spirit and scope of the invention, which is to be defined only by the appended claims What is claimed is

Claims: Claims 1 An RF coil for use in an imaging system including a cavity formed as a conductive enclosure in which resonant fields can be excited, said conductiv e enclosure being formed at least in part of an electncal conductor patterned to form RF conductiv e paths around the cavity: 2. An RF coil as claimed in claim 1 including at least one tuning mechanism which determines a resonant frequency for said paths 3 An RF coil as claimed in claim 2. wherein said tuning mechanism reactively adjusts the electrical length of each said path to tune the path to a said resonant frequency 4. An RF coil as claimed in claim 3 wherein reactances ot said paths are adjusted to achieve a selected field profile for the coil 5. An RF coil as claimed in claim 3 wherein said at least one tuning mechanism tunes all said paths to resonate at the same frequency 6. An RF coil as claimed in claim 3 wherein said at least one tuning mechanism selectively tunes said paths to resonate at at least two different frequencies 7. An RF coil as claimed in claim 6 wherein alternate ones of said paths are tuned to resonate at a first frequenc . and wherein remaining ones ot said paths are tuned to i esonate at a second frequency. 8. An RF coil as claimed in claim 2 including a dielectric at least substantially filling said cavity. 9. An RF coil as claimed in claim 8 wherein the thickness of the conductor for at least selected portions of said enclosure is substantially greater than one skin depth at said resonant frequency, and wherein the dielectric filling said cavity has a dielectric constant different from that of air. whereby signal of different frequencies propagate on outer and inner surfaces of said conductor. WO OO'72033 26 PCT/USOO/14046 10 An RF coil as claimed in claim 2 wherein each ot said N paths has at least one nonconductive gap formed therein, and wherein said tuning mechanism includes at least one of a reactance and an impedance across at least selected said at least one gap 1 1 An RF coil as claimed in claim 10 wherein said reactance impedance for said at least selected gap is variable to control the resonant frequencv for the conespondmg path 12 An RF coil as claimed in claim 1 1 wherein said reactance for at least some of said paths includes a capacitor, the capacitance of which may be varied 13 An RF coil as claimed in claim 1 1 wherein said reactance for at least some of said paths includes an inductor, the inductance of which mav be varied 14 An RF coil as claimed in claim 1 1 wherein the variable impedance reactance is controlled to at least one of tune, retune and detune the path in which it is located 15 An RF coil as claimed in claim 10 wherein said enclosure is formed of an outer wall, inner wall and side walls. N conductiv e lands being formed for each said wall, with conespondmg lands on each wall being connected to form said paths, and wherein said gap is formed in the conductor for each of said paths for at least one of said walls 16 An RF coil as claimed in claim 15 wherein said gaps are formed in the outer wall conductor for each path 17 An RF coil as claimed in claim 1 wherein each said paths has a resonant frequency determined bv a distributed capacitance and a distributed inductance for the path 18 An RF coil as claimed in claim 17 wherein said distributed capacitance is determined by at least one of the area of the electrical conductor for each path a dielectric till material in said cavity, and dimensions of said dielectric fill material 19. An RF coil as claimed in claim 17 wherein the electπcal conductor forming each path is a thin foil, the distπbuted inductance of the path being a function of the path length 20. An RF coil as claimed in claim 17 including at least one reactance component in at least selected ones of said paths, said reactance component being one of distributed and lumped. 21. An RF coil as claimed in claim 20 wherein said at least one discrete reactance is selected to achieve a desired resonant frequency reduction for the paths 22. An RF coil as claimed in claim 17 wherein each said paths has a cumulative reactance, said cumulative reactance including at least in part said distributed capacitance'inductance, the cumulative reactances for the paths being tuned to result in D different resonant frequencies for the coil, every Dth path symmetrically spaced around the coil being tuned to the same frequency 23. An RF coil as claimed in claim 1 including a circuit which applies RF signal to and/or receives RF signals from M selectively spaced ones of said paths, where M is an integer and l≤M≤N. 24. An RF coil as claimed in claim 23 wherein the RF signals are phase shifted conesponding to a phase shift for the conespondmg paths to provide circular polarization for the coil 25. An RF coil as claimed in claim 23 wherein each Rr signal is reactivel coupled to the conespondmg path 26. An RF coil as claimed in claim 25 wherein coupling reactance for each path can be varied to independently match/tune the path 27. An RF coil as claimed in claim 26 wherein the coupling reactances are impedance matched to different loading conditions for the coil 28. An RF coil as claimed in claim 23 wherein said RF coil is used to at least one of transmit and receive RF signals, but not both simultaneously , and including a detuning mechanism for said paths, said detuning mechanism being operative when said RF coil is not in the one of transmit/receive mode for which it is being used. 29. An RF coil as claimed in claim 28 wherein said detuning mechanism includes a mechanism for alteπng at least one of the path length and impedance for each path to be detuned. 30. An RF coil as claimed in claim 29 wherein said detuning mechanism includes a PIN diode circuit for each path which facilitates rapid switching to a changed impedance state sufficient to effect the path.detumng 31. An RF coil as claimed in claim 23 wherein the RF drive signals are phase shifted conespondmg to the phase shift for the paths to which they are applied to provide circular polarization for the coil, and wherein said detuning mechanism includes circuitry which reverses the phase of the RF drive signals 32. An RF coil as claimed in claim 1 wherein said enclosure is formed of an outer wall, inner wall and side walls, N conductive lands being formed for each said wall, with conesponding lands on each wall being connected to form said paths, wherein at least said outer wall has two conductive layers separated bv a dielectric, and wherein said two conductive layers are each slotted to form a pattern of lands, slots on each layer being overlaid by lands of the adjacent layer. 33. An RF coil as claimed in claim 32 wherein the degree of overlap for the lands of said layers controls coil resonant frequency 34. An RF coil as claimed in claim 1 wherein said enclosure is formed of an outei wall, inner wall and side walls. N conductiv e lands being formed for each said wall, with corresponding lands on each wall being connected to form said paths, and wherein at least one of said side walls has an aperture through substantially the center thereof through which a body to be analyzed may be passed to an area inside said inner wall, die conductive layer on said inner wall being patterned to provide a selected magnetic flux pattern in said aperture 35 .An RF coil as claimed in claim 1 wherein said enclosure is formed of an outer wall, inner wall and side walls. N conductiv e lands being formed tor each said wall, with corresponding lands on each wall being connected to form said paths, and w herein one of said side walls is closed, said closed side wall being slotted to form a land pattern cov ering at least most of said wall. 36. An RF coil as claimed in claim 1 wherein said enclosure is formed of an outer wall, inner wall and side walls. N conductiv e lands being formed for each said wall, with corresponding lands on each wall being connected to form said paths, wherein said imaging system has at least one gradient coil which induces low frequencv eddv cunents in said RF coil, and wherein the slotting on at least said outer wall and side walls results in the breaking up ot and substantial attenuation of said eddy cunents without substantial attenuation ot RF cunents and fields. 37. A RF coil as claimed in claim 36 wherein the electrical conductor for at least said outer wall and said side walls is a conductive layer which is thin enough to attenuate said low frequency eddy cunents while still conducting said RF cunents 38. An RF coil as claimed in claim 37 wherein said lav er has a thickness substantially equal to one skm depth at the resonant frequency to which said coil is tuned 39. An RF coil as claimed in claim 37 wherein said lav er has a thickness substantially equal to approximately 5 microns 40. An RF coil as claimed in claim 1 w herein each ot said paths has at least one circumferential/ zimuthal slot formed therein to break said path into smaller subpaths 41 An RF coil as claimed in claim 40 including one ot a fixed, v ariable, and switched reactive coupling and an impedance coupling across each ot said circumterential slots 42. .An RF coil as claimed in claim 41 wherein said reactiv e coupling is a capacitiv e coupling. 43. An RF coil as claimed in claim 1 including an RF drive signal input to at last one of said paths, said paths inductively coupling an RF drive signal on a path to adjacent paths. 44. An RF coil as claimed in claim 1 including a dielectric material filling the cavity, providing a selected path capacitance and thus a selected resonant frequency. 45. An RF coil as claimed in claim 44 including a mechanism for controlling the dielectric fill of said cavity and thus at least one of the resonant frequency of the coil and impedance matching the coil to a body being imaged. 46. .An RF coil as claimed in claim 1 wherein said electrical conductor is patterned to form N conductive lands for the enclosure, each of a selected width, and wherein the number N of conductive paths and the width of conductive lands for each path are selected to achieve at least one of a desired resonant frequency and a desired field contour. 47. An RF coil as claimed in claim 1 wherein said enclosure at least one of breaks induced eddy currents and shapes RF magnetic field patterns. 48. An RF coil as claimed in claim 1 including a lid mounted to at least one end of the coil. 49. An RF coil as claimed in claim 48 wherein said lid is at least partially conductive. 50. An RF coil as claimed in claim 1 including a dielectric material filling said cavity, and a plurality of sample spaces formed in said dielectric at a selected portion of said enclosure. 51. An RF coil as claimed in claim 50 wherein said enclosure is formed of an inner wall, an outer wall and side walls, and wherein said sample spaces are formed in one of said side walls and extend at least part way into the dielectric from said side wall. 52. An RF coil as claimed in claim 1 wherein said coil has an open center chamber and including a dielectric in said center chamber, and a plurality of sample spaces penetrating the dielectric. 53. An RF coil as claimed in claim 1 wherein at least a portion ot at least selected ones of said paths is formed of one of conductive tubes and coaxial tube conductors 54 An RF coil for use in an imaging system including a cavity formed as a conductive enclosure defined by an outer wall, an inner wall, a first end wall and a second end wall, said inner wall defining a sample-receiving chamber, said first end wall being open adjacent said chamber to provide access thereto and said second end wall being closed to seal the chamber at its second end, the closed second end wall functioning as an RF signal minor; at least one electrical conductor for each of said walls, electrical conductors on adjacent walls being electπcally coupled to form at least one continuous electrical path around the cavity; and a port through which RF signal is applied to at least one said electrical paths. 55. An RF coil as claimed in claim 54 wherein the electrical conductors for each said wall are slotted to form a pattern of conductive lands on each wall, the pattern on said second end wall being a radial slotting pattern covering at least most of said wall 56 An RF coil for use in an imaging system including a dielectric filled cavity formed as wall sunounding the dielectric, which w all is formed at least in part of an electrical conductor, said electncal conductor forming at least one continuous electπcally conducting path around said cavity, field applied to said at least one electrically conducting path causing a selected alternating magnetic field in said cavity , and at least one aperture formed in at least said electrical conductor through which said magnetic field may be applied to an adjacent body . 57. An RF coil as claimed in claim 56 wherein said coil is shaped to a body surface being imaged, said sunounding wall including connected top and bottom walls, said at least one aperture being formed in only said bottom wall 58. An RF coil as claimed in claim 57 wherein said coil is flexible to conform to a surface of a body being imaged. 59. An RF coil as claimed in claim 57 wherein said at least one aperture are arranged to be adjacent areas to be imaged of a body being imaged. 60. An RF coil as claimed in claim 59 wherein said areas to be imaged are at least one projection on said body, a projection to be imaged extending into said cavity through an adjacent aperture. 61. An RF coil as claimed in claim 60 wherein said dielectric is conformable to an outer surface of a projection extending into said cavity so as to minimize discontinuity between the projection and the dielectric. 62. An RF coil as claimed in claim 56 wherein said coil is in the form of a closed loop, said surrounding wall including connected inner and outer walls, said at least one aperture being formed in only one of said inner wall and said outer wall. 63. An RF coil as claimed in claim 56 wherein said at least one aperture are arranged to be adjacent areas to be imaged of a body being imaged. 64. An RF coil as claimed in claim 56 including a lid mounted to at least one end of the coil. 65. An RF coil as claimed in claim 64 wherein said lid is at lest partially conductive. 66. An RF coil as claimed in claim 56 including a plurality of sample spaces formed in said dielectric at a selected portion of said enclosure. 67. An RF coil for use in an imaging system including: N continuous RF electrical paths formed for said coil, each of which path has a resonant frequency; and a circuit which applies RF dπve signal to and/or receiv es Rr signals from M selectively spaced ones of said paths, where M is an integer and l≤M≤N 68. An RF coil as claimed in claim 67 wherein the -RF signals are phase shifted conesponding to a phase shift for the conesponding paths to provide circular polarization for the coil. 69. An RF coil as claimed in claim 67 wherein said tuning mechanism reactively adjusts to achieve a selected field profile 70. An RF coil as claimed in claim 67 wherein each RF drive signal is at least one of reactively coupled and reactively decoupled to the conesponding paths 71. An RF coil as claimed in claim 70 wherein coupling leactance for each path can be varied to at least one of independently tune the path and decouple the path 72. An RF coil as claimed in claim 71 wherein D different frequencies are applied to the coil, each frequency being applied to every D' h path evenly spaced along the coil, each of the paths being tuned to the frequency applied thereto. 73. An RF coil as claimed in claim 67 wherein said RF coil is used to at least one of transmit and receive RF signals but not both simultaneously, and including a detuning mechanism for said paths, said detuning mechanism being operativ e when said RF coil is not in the one of the transmit receive mode for which it is being used. 74. An RF coil as claimed in claim 73 wherein said detuning mechanism includes a mechanism for altering at least one of the path length and path impedance tor each path to be detuned. 75. An RF coil as claimed in claim 74 wherein said detuning mechanism includes a PIN diode circuit which facilitates rapid switching to a changed impedance state sufficient to effect the path detuning. 76 An RF coil as claimed in claim 67 wherein the Rr driv e signals are phase shifted conesponding to the phase shift for the paths to which they are applied to provide circular polanzation for the coil, and wherein said detuning mechanism includes circuitry which reverses the phase of the RF drive signals 77 An RF coil as claimed in claim 67 wherein said RF dπv e signals are of selected amplitude to provide a desired RF field profile 78 An RF coil for use in an imaging s stem which coil is used to at least one of transmit and receive RF signals, but not both simultaneously, including N continuous electrical paths formed for said coil, each of which path has a resonant frequency, and a detuning mechanism for said paths, said detuning mechanism being operative when said RF coil is not in the one of the transmit/receive mode for which it is being used 79. An RF coil as claimed in claim 78 wherein said detuning mechanism includes a mechamsm for altering at least one of the RF electrical path length and impedance for each path to be detuned 80 An RF coil as claimed in claim 79 wherein said detuning mechanism includes a PIN diode circuit for each path which facilitates rapid switching to a changed impedance state sufficient to effect the path detuning 81. An RF coil as claimed in claim 78 wherein RF drive signals are applied to M selectively spaced ones of said paths, where M is an integer and l≤M≤N and wherein the RF dπve signals are phase shifted conesponding to the phase shift for the paths to which thev are applied to provide circular polarization for the coil, said detuning mechanism including ciicuitry which reverses the phase of the RF drive signals 82. An RF coil as claimed in claim 78 wherein said RF coil is used for transmit only, including a separate RF receive coil, and a detuning mechanism for said receiv e coil which is operative when said receive coil is not in receive mode. 83. An RF coil as claimed in claim 82 wherein said detuning mechanism for said receive coil includes a mechanism for altering at least one of path length and impedances for paths of said receive coil. 84. An RF coil for use in an imaging system including: a cavity formed as a conductive enclosure defined by a sunounding conductive wall, said conductive wall being patterned to form at least one continuous electrical path around said cavity; and a dielectric material filling said cavity . said dielectric material having a dielectric constant which provides a selected capacitance and wavelength, and thus a selected resonant frequency, for said at least one path. 85. An RF coil as claimed in claim 84 wherein the dielectric material is different in different areas of the cavity so as to selectively shape the coil field. 86. An RF coil as claimed in claim 84 including a mechanism for controlling the dielectric fill of said cavity and thus the resonant frequency of the coil. 87. An RF coil as claimed in claim 86 wherein said dielectric material is a fluid, resonant frequency being controlled by controlling the amount of said fluid in the cavity. 88. An RF coil as claimed in claim 84 including an acoustic damping material filling at least a portion of said cavity. 89. An imaging system including: an RF coil having at least one continuous electrical path: a mechanism which determines a resonant frequency for said coil: and dielectric mateπal selectively positioned between said coil and a body to be imaged and shaped to at least one of control and shape the field applied to the body trom the coil. 90 A system as claimed in claim 89 wherein the dielectric material substantially fills the space between the coil and at least a selected area of the bod . and wherein the dielectnc constant of the dielectric substantially matches that of the body in said area 91. A system as claimed in claim 90 wherein said selected area is the area to be imaged, said dielectric concentrating and directing field to said area. 92. An RF coil for use in an imaging system including a resonant cavity of a selected size and shape which is formed by a surrounding conductive enclosure, the conductive enclosure being patterned to suppress low frequency eddy currents and EMI noise. 93. An RF coil as claimed in claim 92 including at least one mechamsm for detuning the coil. 94 An RF coil for use in an imaging system including a cavity formed by at least an inner and an outer conductor. a dielectric material filling said cavity: and at least one sample space formed in said coil 95. An RF coil as claimed in claim 94 wherein said at least one sample space is formed m said dielectnc material, projecting therein from a wall of said enclosure 96. An RF coil as claimed in claim 92 wherein said coil is a transmission line stub, said inner and outer conductor being inner and outer conductors, respectiv ely . ot the transmission line stub. 97. An RF coil as claimed in claim 96 including a conductiv e cap shorting an end of said transmission line stub 98. An RF coil as claimed in claim 96 wherein said at least one sample space is located at a distal end of said stub, the sample space extending from said distal end into one of said dielectric material and a hollowed out portion of said center conductor. 99. An RF coil as claimed in claim 98 wherein said stub is tuned and matched so that maximum current, and thus maximum RF field occur at said distal end of the stub.

Inventors: VAUGHAN JOHN T, VAUGHAN, JOHN, T.
Applicants: GEN HOSPITAL CORP, THE GENERAL HOSPITAL CORPORATION
Application year: 2000
Main classification: G01R33
IPC codes: A61B5, G01R33, H01F5