Abstract:
In one example, an antenna is disclosed comprising a first, outer conductor, at least one inner conductor within the first outer conductor, and a second outer conductor surrounding the first outer conductor. The second outer conductor may define a plurality of holes therethrough. The at least one inner conductor may comprise a plurality of inner conductors. At least two of the plurality of inner conductors may be connected to each other in series, across a capacitor. At least two of the plurality of inner conductors may be connected to each other in parallel, across a capacitor. The at least one inner conductor may be sufficiently surrounded by the first outer conductor to shield the inner conductor from receiving a radiofrequency signal.

Description:
The present application is a division of application Ser. No. 10/996,575, which was filed on Nov. 24, 2004, is assigned to the assignee of the present invention and is incorporated by reference herein. Application Ser. No. 10/996,575 is a continuation of application Ser. No. 09/738,235, which was filed on Dec. 15, 2000, and issued on Jan. 25, 2005 bearing U.S. Pat. No. 6,847,210, which is also assigned to the assignee of the present invention and is incorporated by reference herein. Application Ser. No. 09/738,235 claims the benefit of Application No. 60/172,199, which was filed on Dec. 17, 1999 and is assigned to the assignee of the present invention. 

   FIELD OF THE INVENTION 
   This invention relates to radio frequency receiving and transmitting antennas, and, more particularly, to receiving and transmitting radio frequency antennas for use in magnetic resonance imaging. 
   BACKGROUND OF THE INVENTION 
   Magnetic resonance imaging (“MRI”) is a well known, highly useful technique for diagnosing abnormalities in biological tissue. MRI can detect abnormalities which are difficult or impossible to detect by other techniques, without the use of x-rays or invasive procedures. 
   MRI uses changes in the angular momentum or spin of the atomic nuclei of certain elements within body tissue in a static magnetic field after excitation by radio frequency energy, to derive images containing useful information concerning the condition of the tissue. During an MRI procedure, the patient is inserted into an imaging volume containing a static magnetic field. The vector of the angular momentum or spin of nuclei containing an odd number of protons or neutrons tends to align with the direction of the magnetic field. A transmitting antenna within the imaging volume emits a pulse or pulses of radio frequency energy having a particular bandwidth of frequency, referred to as the resonant or Larmor frequency, shifting the vectors of the nuclei out of alignment with the applied magnetic field. The spins of the nuclei then turn or “precess” around the direction of the applied primary magnetic field. As their spins precess, the nuclei emit small radio frequency signals, referred to as magnetic resonance (“MR”) signals, at the resonant or Larmor frequency, which are detected by a radio frequency receiving antenna tuned to that frequency. The receiving antenna is typically positioned within the imaging volume proximate the patient. Gradient magnetic fields are provided to spatially encode the MR signals emitted by the nuclei. After the cessation of the application of radio frequency waves, the precessing spins gradually drift out of phase with one another, back into alignment with the direction of the applied magnetic field. This causes the MR signals emitted by the nuclei to decay. The MR signals detected by the receiving antenna are amplified, digitized and processed by the MRI system. The same antenna may act as the transmitting and receiving antenna. Hydrogen, nitrogen-14, phosphorous-31, carbon-13 and sodium-23 are typical nuclei detected by MRI. Hydrogen is most commonly detected because it is the most abundant nuclei in the human body and emits the strongest MR signal. 
   The rate of decay of the MR signals varies for different types of tissue, including injured or diseased tissue, such as cancerous tissue. By known mathematical techniques involving correlation of the gradient magnetic fields and the particular frequency of the radio frequency waves applied at various times with the rate of decay of the MR signals emitted by the patient, it is possible to determine the concentrations and the condition of the environment of the nuclei of interest at various locations within the patient&#39;s body. This information is typically displayed as an image with varying intensities, which are a function of the concentration and environment of the nuclei of interest. Typical MRI systems are the Quad 7000 and Quad 12000 available from FONAR Corporation, Melville, N.Y., for example. 
   The quality of the magnetic resonance image is directly related to the characteristics of the receiving and transmitting antenna. Significant electrical characteristics of the antenna include its sensitivity, Q factor and the signal-to-noise ratio. 
   Sensitivity is the signal voltage generated in the receiving antenna by MR signals of a particular magnitude. The higher the sensitivity within the region to be imaged, the weaker the signals which can be detected. The sensitivity of the antenna is preferably substantially uniform with respect to MR signals emanating from all volume elements within the region of the subject which is to be imaged. 
   The Q or quality factor, which is closely related to the sensitivity of the antenna, is a measure of the ability of the antenna to amplify the received signal. The Q-value of the antenna can be lowered by a patient proximate or within an antenna, due to capacitive and to a lessor extent the inductive coupling between the patient and the antenna. Antennas must therefore have a high Q-value when they are unloaded and the Q-value must not become too diminished by the presence of the patient. On the other hand, the coil must couple well with the region of a patient&#39;s anatomy which is to be imaged. 
   Signal-to-noise (“S/N”) ratio is the ratio between those components in the electrical impulses appearing at the antenna terminals representing the detected MR signals and the components representing spurious electromagnetic signals in the environment and internally generated thermal noise from the patient. To optimize the S/N ratio, the antenna should have low sensitivity to signals from outside the region to be imaged and to thermal noise. To enhance both S/N ratio and sensitivity, the antenna is “tuned” or arranged to resonate electrically at the frequency of the MR signals to be received (the Larmor frequency), which is typically several megahertz or more. Neither the coil size nor geometry of the antenna can be allowed to create an inductance or self-capacitance which prevents tuning to the desired frequency. 
   The antenna must also meet certain physical requirements. The antenna should have a high filling factor, which maximizes the amount of tissue which fits within the volume detected by the windings of the coil. The antenna must also fit within the relatively small imaging volumes typically provided for receiving the subject within the magnet assembly, along with other components of the system and the subject. The antenna should not cause significant discomfort to the subject. Additionally, the antenna should be easy to position with respect to the subject, and be relatively insensitive to minor faults in positioning relative to the subject. 
   These numerous considerations often conflict with one another and therefore must be balanced during the design process. 
   The sensitivity and S/N ratio of MRI radio frequency receiving antennas have been improved by positioning a first coil, tuned to resonate at the Larmor frequency of the species of interest, proximate the part of the subject which is to be imaged, and positioning a similarly tuned second coil, typically a single loop, adjacent to the first coil. The second coil is connected to the preamplifier of the MRI system. The first and second coils are inductively coupled to each other. MR signals emitted by the patient induce voltages in the first winding, causing current to flow within the winding. The current generates a magnetic field which induces voltage in the second winding. The MR signals may induce voltages in the second coil, as well. The voltages induced in the second coil are processed by the MRI system. Use of such first and second coils amplify the MR signals, and the second coil filters spurious signals outside of the frequency band of the Larmor frequency. See, for example, U.S. Pat. No. 5,583,438 and U.S. Pat. No. 5,575,287, assigned to the assignee of the present invention. 
   Radio frequency antenna coils may be used in a variety of configurations. For example, the coil may be receiving coil, as discussed above. The receiving coil may be part of an array of receiving coils, such as in the primary and secondary coil arrangements, discussed above. The receiving coil may also act as the transmitting coil of the MRI system. A pair of receiving coils can also be arranged 900 with respect to each other to enable quadrature detection, which improve the signal-to-noise ratio. 
   SUMMARY OF THE INVENTION 
   In accordance with an embodiment of the invention, an antenna is disclosed comprising a first, outer conductor, at least one inner conductor within the first outer conductor, and a second outer conductor surrounding the first outer conductor. The second outer conductor may define a plurality of holes therethrough. The at least one inner conductor may comprise a plurality of inner conductors. At least two of the plurality of inner conductors may be connected to each other in series, across a capacitor. At least two of the plurality of inner conductors may be connected to each other in parallel, across a capacitor. The at least one inner conductor may be sufficiently surrounded by the first outer conductor to shield the inner conductor from receiving a radiofrequency signal. 
   In accordance with another embodiment if the invention, an antenna is disclosed comprising a first conductor and a plurality of second conductors substantially encased by the first conductor. The plurality of second conductors may be sufficiently encased by the first conductor to shield the second conductors from receiving a radiofrequency signal. A third conductor may be provided substantially encasing the first conductor. The third conductor may define a plurality of holes therethrough. At least two of the plurality of inner conductors may be connected to each other in series and at least two of the plurality of inner conductors may be connected to each other in parallel. 
   In accordance with another embodiment of the invention, an antenna is disclosed comprising a first coaxial cable unit lying in a first plane and a second coaxial cable unit lying in a second plane perpendicular to the first plane. The first coaxial cable unit may comprise a first, inner conductor and a second, outer conductor substantially surrounding the first inner conductor. The second coaxial cable unit may comprise a third, inner conductor and a fourth, outer conductor, substantially surrounding the third inner conductor. The first coaxial cable unit may define a region comprising a perpendicular projection of the first coaxial cable unit and the second coaxial cable unit may lie within the region. 
   In accordance with another embodiment of the invention, an antenna is disclosed comprising a first coaxial cable unit comprising a first, inner conductor and a second, outer conductor substantially surrounding the first inner conductor. The first and second conductors lie in a first plane. A second coaxial cable unit is provided comprising a third, inner conductor and a fourth, outer conductor, substantially surrounding the third inner conductor. The third and fourth conductors lie in a second plane different than the first plane. 
   The antennas may be used as receiving and/or transmitting antennas. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a plan view of a basic coaxial cable unit of an MRI antenna in accordance with one embodiment of the present invention; 
       FIG. 2  is a cross-sectional view of the basic coaxial cable unit  10 , along line  2 - 2  in  FIG. 1 ; 
       FIG. 3  shows the basic coaxial cable unit of  FIG. 1 , with electrical connections to form an antenna in accordance with one embodiment of the present invention; 
       FIG. 4  is a schematic diagram of a circuit corresponding to the antenna of  FIG. 3 ; 
       FIG. 5  shows the basic unit of  FIG. 1 , with a shielding adapter; 
       FIG. 6  shows another shielding technique; 
       FIG. 7  shows a variation of the basic unit of  FIG. 1 , comprising two sections of coaxial cable, and their electrical connections; 
       FIG. 8  is a schematic diagram of a circuit corresponding to the configuration of  FIG. 7 ; 
       FIG. 9  is a schematic representation of an antenna array  100  appropriate for imaging of a hand, wrist or toe, using three basic coaxial cable units, in accordance with the present invention; 
       FIG. 10  is a schematic representation of the side view of the antenna array of  FIG. 9 ; 
       FIG. 11  is a schematic representation of a circuit corresponding to the antenna array of  FIG. 9 ; 
       FIG. 12  is a perspective view of the antenna array of  FIG. 9 , encased in a base of dielectric material, in use with a patient; 
       FIG. 13  is a schematic diagram of an antenna array in accordance with another embodiment the present invention, which is particularly suited for imaging the head and neck; 
       FIG. 14  is a side view of the antenna array of  FIG. 13 , wherein four coaxial units are each in respective vertical, parallel planes, and the center of each unit lies along the same axis; 
       FIG. 15  is a schematic diagram of a circuit corresponding to the antenna array of  FIG. 11 ; 
       FIGS. 16-18  show coaxial cable units in various test configurations; 
       FIG. 19  shows a test configuration of a coaxial cable unit of the present invention; 
       FIG. 20  shows an antenna array comprising two concentric coaxial cable units lying in the same plane; 
       FIG. 21  is a schematic diagram of a circuit corresponding to the configuration of  FIG. 20 ; 
       FIG. 22  shows a quadrature antenna system comprising the hand, wrist and toe antenna array of  FIGS. 9-12  and a pair of rectangular coaxial cable units; 
       FIG. 23  is a top view of the quadrature antenna system of  FIG. 22 , showing the rectangular coaxial cable units; 
       FIG. 24  shows a quadrature antenna system of  FIG. 23 , including additional coaxial cable units coupled to the rectangular coaxial cable units; 
       FIG. 25  is a side view of the quadrature antenna system of  FIG. 24 , along line  25 - 25  in  FIG. 24 ; 
       FIG. 26  is a side view of the quadrature antenna system of  FIG. 24 , along line  26 - 26  of  FIG. 25 ; 
       FIG. 27  is a schematic diagram of the preamplifier section of an MRI system for use with the quadrature antenna system of  FIGS. 22-26 ; 
       FIG. 28  is a side view of the gap between the magnet poles of an MRI system; 
       FIG. 29  is a plan view of a coaxial cable unit in accordance with another embodiment of the invention, including four inner conductors; 
       FIG. 29   a  is a cross-sectional view of the coaxial cable unit of  FIG. 29 , along line  29 - 29 ; 
       FIG. 30  shows a preferred connection scheme for the inner conductors of the configuration of  FIG. 29 ; 
       FIG. 31  shows three coaxial cable units as in  FIG. 29 , separated by an insulator; 
       FIG. 32  shows the three coaxial cable units of  FIG. 31  and preferred connections between their inner and outer conductors; 
       FIG. 33  is a schematic diagram of a circuit corresponding to the antenna array of  FIG. 31 ; 
       FIG. 34  shows a triaxial cable unit in accordance with another embodiment of the present invention; 
       FIG. 34   a  is a cross-sectional view of the triaxial cable unit of  FIG. 34 , along line  34   a - 34   a;    
       FIG. 35  is a schematic representation of the triaxial cable unit of  FIG. 34 ; 
       FIG. 36  is a schematic representation of the thiaxial cable unit of  FIG. 34 , wherein the inner conductors are connected in parallel; 
       FIG. 37  shows a coaxial cable unit as in  FIG. 3 , wherein the ends of the inner conductor are connected across a capacitor to a radio frequency power source; 
       FIG. 38  is a schematic representation of the coaxial cable unit of  FIG. 37 ; and 
       FIG. 39  is a schematic representation of portions of an MRI system, showing in particular a connection between an antenna or antenna array  802  in accordance with the present invention with certain components of the MR1 system; 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   In examples of embodiments of the present invention, antennas for use in magnetic resource imaging comprise inductively coupled first and second windings tuned to the same frequency, typically the Larmor frequency of species of interest. In a receiving antenna, the second winding is connected to the receiver subsystem of an MRI system. It is believed, without limiting the scope of the invention, that in antennas of embodiments of the present invention, the first winding substantially shields the second winding from direct reception of magnetic resonance (“MR”) signals. However, since the windings are inductively coupled, MR signals detected by the first winding induce voltage signals in the second winding. Since both windings are tuned to the same frequency, highly filtered signals are provided for analysis by the MRI system. The first and second windings may form a coaxial cable. Multiple coaxial cables may be suitably coupled to form antenna arrays for use with different parts of the body. 
   In a transmitting antenna, the second winding is connected to the radio frequency power source in the radio frequency transmitting section of the MR1 system. Voltage signals provided to the second winding induce voltage signals and current flow in the first winding, causing the emission of highly filtered radio frequency signals. 
     FIG. 1  is a plan view of a basic coaxial cable unit  10  of an MRI antenna in accordance with one embodiment of the present invention. The unit  10  comprises an inner conductor  12  and an outer conductor  14  coaxially arranged, as shown in the cross-sectional view of  FIG. 2 . The inner conductor  12  and the outer conductor  14  are separated by a dielectric material  24 , such as Teflon®, for example, forming a coaxial cable unit  10 . The coaxial conductors  12 ,  14  are inductively coupled to each other. Preferably, the inner and outer conductors  12 ,  14  are tightly coupled to each other. More preferably, they are critically coupled to each other. 
   The inner conductor  12  has two ends  16 ,  18  and the outer conductor  14  has two ends  20 ,  22 . A plurality of basic units  10  may be connected or coupled to each other in different combinations to form an antenna array, as discussed below. 
   A body part to be imaged is received in the region  26  bounded by the coaxial cable unit  10 . In  FIG. 1 , the coaxial conductors are in the shape of a ring. Other shapes may be used, dependent on the body part being imaged. 
   In one example, the outputs  16 ,  18  of the inner conductor  12  are connected to each other through a capacitor C 1  and the outputs  20 ,  22  of the outer conductor  14  are connected to each other through a capacitor C 2  to form an antenna  11 , as shown in  FIG. 3 . The capacitors C 1 , C 2  have values such that the inner and outer conductors  12 ,  14  are tuned to the Larmor frequency of the species of interest, such as hydrogen. The capacitor C 1  may be connected to a variable capacitor (“varactor”) in the preamplifier of the receiver system of an MR1 system, or it may be the varactor in the preamplifier. Each conductor  12 ,  14  may be copper tubing. As shown in  FIG. 2 , the inner diameter “d 1 ” of the inner conductor  12  may be about 0.125 inches. The diameter “d 2 ” of the outer conductor  14  may be about 0.25 inches. The wall thickness of each conductor  12 ,  14  may be about 0.032 inches. 
   The coaxial cable unit  10  may be made from readily available soft copper refrigeration tubing of appropriate diameters. Such tubing may be obtained from Metal Product, Wynne, Ark., for example. The tubing corresponding to the inner conductor  12  is covered by a Teflon® tubing, such as TFT70C polytetrafluoroethylene, available from AIN Plastics, Inc., Mount Vernon, N.Y., for example. The Teflon® covered inner conductor  12  is inserted through the tubing corresponding to the outer conductor  14 , forming the coaxial cable unit  10 . The coaxial cable unit  10  may also be flexible. One-quarter inch, High Power, High Temperature Dielectric Coaxial Cable, Andrew HSTI-50 HELIAX, available from Andrew Corporation, Orland Park, Ill., may be used, for example. 
   The inner conductor  12  is shielded from direct reception of the MR signals by the outer conductor  12 . However, voltage signals induced in the outer conductor  14  induce voltage signals in the inner conductor  12 . The outer conductor  14  may, therefore, be modeled as a primary winding of a transformer while the inner conductor  12  may be modeled as the secondary winding of the transformer. 
     FIG. 4  is a schematic diagram of a circuit corresponding to the antenna of  FIG. 3 . The inner conductor  12  and the outer conductor  14  are represented as inductors L 1 , L 2 , respectively. The capacitors C 1  and C 2  are shown, as well. The inductor L 1  and the capacitor C 1  form a first circuit “A” and the inductor L 2  and the capacitor C 2  form a second circuit “B”. The inductors L 1 , L 2  are inductively coupled to each other and have a distributed capacitance Cd. The two circuits are tuned to the same, Larmor frequency, forming anti-resonant or parallel resonant, double-tuned circuits. The circuits A, B form a bandpass network which filters frequencies outside of the bandpass of the two circuits. The bandpass of the circuit B (corresponding to the inner conductor  12 ) is narrower than the bandpass of the circuit A (corresponding to the outer conductor  14 ), as is known in the art. Noise and other signals outside of the narrower bandwidth of the inner conductor  12  are therefore filtered. As mentioned above, in the embodiment of  FIG. 3 , the inner conductor  12  is connected to the receiver subsystem of an MRI device, providing highly filtered signals for analysis. 
   The outer conductor  14  also shields the inner conductor  12  from capacitive coupling with the body of the patient over most of its length. As shown in  FIG. 3 , however, the inner conductor  12  may not be effectively shielded by the outer conductor  14  in the gap  28  between the ends of the outer conductor  14 . To improve the shielding effect of the outer conductor  14  on the inner conductor  12 , the ends  20 ,  22  of the outer conductor  14  may be connected through a copper tube adaptor  30 , which closes the gap  28  between the ends of the inner and outer conductors  12 ,  14 , as shown in  FIG. 5 . The adaptor  30  acts as a capacitive connection between the ends  20 ,  22  of the outer conductor  14 , as well as shielding the outputs  16 ,  18  of the inner conductor  12 . The adaptor  30  also minimizes field distributions in the gap  28  between the ends of the conductors  12 ,  14 . 
   Two capacitors C 1 , C 2  are provided within the adaptor  30 , electrically connecting in series the first and second ends  16 ,  18  of the inner conductor  12  and the first and second ends  20 ,  22  of the outer conductor  14 , respectively. The exposed ends,  16 ,  18  of the inner conductor  14  and the capacitors C 1 , C 2  are shielded by the adaptor  30 . 
   The direct exposure of the inner conductor  12  to MR signals from within the area  28  may also be minimized by distancing the exposed ends  16 ,  18  from the source of the MR signals in the region  28  and positioning more of the outer conductor  14  between the inner conductor  12  and the source of MR signals within the coil, as shown in  FIG. 6 . 
   Dependent on the overall length of the coaxial ring and the magnetic field strength of the magnet used in the MR1 system, in order to tune the antenna  11  of  FIG. 3  to the Larmor frequency, the inductance of the inner and outer conductors  12 ,  14  of the coaxial cable may need to be decreased. The inductance may be decreased by decreasing the lengths of the inner and outer conductors  12 ,  14 , as is known in the art. For example, the inner and outer conductors  12 ,  14  may be split into two sections  34 ,  36 , as shown in  FIG. 7 , to facilitate their being tuned to the higher frequencies used in higher field strength magnets. The ends of the inner conductor  12  of a first section  34  are connected in series through respective capacitors C 1 , C 3  to the ends of the inner conductor  12 ″ of the second section  36 . The ends of the outer conductor  14 ″ of the first section  34  are similarly connected in series to the ends of the outer conductors  14 ″ of the second section  36  through respective capacitors C 2 , C 4 . The values of the capacitors C 1 , C 2 , C 3 , C 4  are adjusted to tune the conductors  12 ′,  12 ″,  14 ′,  14 ″ to the desired frequency. Either the capacitor C 1  or the capacitor C 3  may be connected to the variable capacitor in the preamplifier of the MRI system, or may be the variable capacitor in the preamplifier. 
     FIG. 8  is a schematic diagram of the circuit corresponding to the configuration of  FIG. 7 . In this case, the inner conductors  12 ′,  12 ″ correspond to the inductors L 1 , L 2  which are connected in series through capacitors C 1 , C 3 . The outer conductors  14 ′,  14 ″ correspond to the inductors L 3 , L 4 , respectively, which are connected in series through the capacitors C 2  and C 4 . As above, the outer conductor  14  (L 3 , L 4 ) and the inner conductor  12  (L 1 , L 2 ) are inductively coupled. 
     FIG. 9  is a schematic diagram of an antenna array  100  appropriate for imaging of a hand, wrist or toe, using three coaxial cable ring units  102 ,  104 ,  106  as in  FIG. 1 , in accordance with one embodiment of the present invention. The coaxial ring units  102 ,  104 ,  106  lie in three respective parallel, vertical planes P 1 , P 2 , P 3 , respectively, as shown in  FIG. 10 . To better accommodate the hand, wrist or toe, such that the outer conductors are close to the body party, each unit has a rectangular shape. The antenna array  100  may be used for imaging other body parts, as well. The shape of each unit may be varied as appropriate to accommodate and maintain suitable proximity to the body part. 
   Each unit  102 ,  104 ,  106  has an inner conductor  12 ,  12 ′,  12 ″, respectively, and an outer conductor  14 ,  14 ′,  14 ″. A first, top end  16  of the inner conductor  12  of the first unit  102  is electrically connected in series to a first, bottom end  16 ′ of the inner conductor  12 ′ of the second unit  104  through a capacitor C 1 . The second, top end  18 ′ of the inner conductor  12 ′ is electrically connected in series to the bottom end  16 ″ of the inner conductor  12 ″ of the third unit  106  through a capacitor C 2 . The second, bottom end  18  of the inner conductor  12  of the first unit  102  and the second, top end  18 ″ of the inner conductor  12 ″ of the third unit  106  provide an output  108  of the antenna array  100 . The output  108  may be connected across a capacitor (not shown) which may be connected to or may be the varactor of the preamplifier of the receiver system of the MRI device. The output  108  may be connected to the preamplifier through a BNC connector acting as a capacitive module. 
   The top end  20  of the outer conductor  14  of the first unit  102  is electrically connected in series to the bottom end  20 ′ of the outer conductor  14 ′ of the second unit  104 , through a capacitor C 3 . The top end  22 ′ of the outer conductor  14 ′ is electrically connected in series to the first, bottom end  20 ″ of the outer conductor  14 ″ of the third unit  106  through a capacitor C 4 . The bottom end  22  of the outer conductor  14  of the first unit  102  and the second, top end  22 ″ of the third unit  106  are electrically connected in series through the capacitor C 5 , which is part of a BNC connector acting as a capacitive module. 
   The antenna array  100  of  FIG. 9  may be represented as three-double tuned transformers connected in series, as shown in  FIG. 11 . The inductors L 1 , L 2 , L 3  correspond to the inner conductors  12 ,  12 ′,  12 ″ of the first, second and third units  102 ,  104 ,  106 , respectively, and the inductors L 4 , L 5 , L 6  correspond to the outer conductor  14 ,  14 ′ and  14 ″. In one implementation, for use in an MRI system including a magnet with a field strength of from about 6,000 Gauss to about 8,000 Gauss, capacitor C 1 =68 picofarads; capacitor C 2 =56 picofarads; capacitor C 3 =22 picofarads; and capacitor C 4 =22 picofarads. The inner and outer conductors  12 ,  14  were tuned to the Larmar frequency of hydrogen for the particular magnet field strength, which in the range of 6,000 to 8,000 Gauss is between about 25-35 megahertz. The length “D 1 ” ( FIG. 9 ) of each coaxial cable unit  102 ,  104 ,  106  was 7 inches. The distance “D 2 ” ( FIG. 10 ) between the center of adjacent coaxial cable units was 1.5 inches. The Q of the antenna array  100  was found to be about 197 while the S/N ratio was found to be 320 to 1. 
   While an antenna array comprising three coaxial cable units preferred, the antenna array  100  could comprise the two coaxial cables  102  and  106 , in which case, the first end  16  of the inner conductor  12 ″ of the first coaxial cable  102  would be connected to the first end  16  of the inner conductor  12  coaxial cable unit  106  and the first end  20  of the outer conductor  14  would be connected to the first end  20 ″ of the outer conductor  14 ″ coaxial cable unit  106 . The outputs of the antenna array would be the same. 
   The antenna array  100  is preferably encased in a base  100   a  of a rigid, dielectric material, such as the fire resistant polymers polyvinyl chloride, polytetrafluoroethylene and fluorinated ethylene propylene, as shown in  FIG. 12 . Polymers having low dielectric constants are particularly preferred. In  FIG. 12 , the first, second and third coaxial cable units  102 ,  104 ,  106  are shown in phantom. The coaxial cable units  102 ,  104 ,  106  are coaxially aligned along an axis A through the center of the base I  00   a , as discussed above with respect to  FIG. 11 . The base  100   a  is connected to a plate  100   b  which may be positioned on the patient bed within the gap of the MRI magnet. A patient&#39;s hand (“h”) is shown, partially inserted into a region within each of the coaxial cable units  102 ,  104 ,  106 , for magnetic resonance imaging of the fingers, for example. A strap  100   c  may be provided to restrain movement of the hand h during imaging. 
   Flexible coaxial cable units can also be supported by a flexible belt. An example of a flexible coaxial cable is described above. The flexible belt can comprise cross-linked polyethylene foam, available from Contour Fabricators, Inc., Grand Blanc, Mich. 
     FIG. 13  is a schematic diagram of an antenna array  200  in accordance with another embodiment the present invention, which is particularly suited for imaging the head and neck.  FIG. 14  is a side view of the array, wherein four coaxial ring units  202 ,  204 ,  206 ,  208  are each in respective vertical, parallel planes P 1 , P 2 , P 3 , P 4  and the center of each unit lies along the same axis “A”. 
   Returning to  FIG. 13 , because larger conductors are needed to surround the head, each coaxial ring unit preferably has two sections to lower the inductance of the conductor, as discussed above with respect to the embodiment of  FIG. 7 . Each section has an outer conductor with first and second ends and an inner conductor with first and second ends. 
   In the first unit  202 , the upper section  202   a  has an inner conductor  210  with first and second ends  210   a ,  210   b  and an outer conductor  214  with first and second ends  214   a ,  214   b . Similarly, the lower section  202   b  has an inner conductor  216  with first and second ends  216   a ,  216   b  and an outer conductor  218  with first and second ends  218   a ,  218   b . The first end  210   a  of the inner conductor  210  is electrically connected in series to the first end  216   a  of the inner conductor  216  through a capacitor C 1 . The first end  214   a  of the outer conductor  214  is electrically connected in series to the first end  218   a  of the outer conductor  218  through a capacitor C 2 . The second end  210   b  of the inner conductor  210  and the second end  216   b  of the inner conductor  216  provide an Output. The Output would be connected to a varactor in the preamplifier of the receiver system of the MRI device, optionally through a capacitor. The second end  214   b  of the outer conductor  214  is electrically connected in series to the second end  218   b  of the outer conductor  218  through a capacitor C 3  of a BNC connector capacitive module. 
   In the second unit  204 , the upper section  204   a  has an inner conductor  220  with first and second ends  220   a ,  220   b  and an outer conductor  222  with first and second ends  222   a ,  222   b . Similarly, the lower section  204   b  has an inner conductor  224  with first and second ends  224   a ,  224   b  and an outer conductor  226  with first and second ends  226   a ,  226   b . The first end  220   a  of the inner conductor  220  is electrically connected in series to the first end  224   a  of the inner conductor  224  through a capacitor C 4 . The first end  222   a  of the outer conductor  222  is electrically connected in series to the first end  226   a  of the outer conductor  226  through a capacitor C 5 . The second end  220   b  of the inner conductor  220  and the second end  224   b  of the inner conductor  224  are electrically connected in series through a capacitor C 14 . The second end  222   b  of the outer conductor  222  is electrically connected in series to the second end  226   b  of the outer conductor  226  through a capacitor C 6 . 
   In the third unit  206 , the upper section  206   a  has an inner conductor  228  with first and second ends  228   a ,  228   b  and an outer conductor  230  with first and second ends  230   a ,  230   b . Similarly, the lower section  206   b  has an inner conductor  232  with first and second ends  232   a ,  232   b  and an outer conductor  234  with first and second ends  234   a ,  234   b . In the fourth unit  208 , the upper section  208   a  has an inner conductor  236  with first and second ends  236   a ,  236   b  and an outer conductor  238  with first and second ends  238   a ,  238   b . The lower section has an inner conductor  240  with first and second ends  240   a ,  240   b  and an outer conductor  242  with first and second ends  242   a ,  242   b.    
   The first end  228   a  of the inner conductor  228  of the third unit  206  is electrically connected in series to the first end  232   a  of the inner conductor  232  through a capacitor C 7 . The first end  230   a  of the outer conductor  230  is electrically connected in series to the first end  234   a  of the outer conductor  234  through a capacitor C 8 . The second end  230   b  of the outer conductor  230  is electrically connected in series to the second end  232   b  of the inner conductor  232  through a capacitor C 9 . The second end  228   b  of the inner conductor  228  is electrically connected in series to the second end  242   b  of the outer conductor  242  of the lower section  208   b  of the fourth unit  208  through a capacitor C 10 . The second end  234   b  of the outer conductor  234  is electrically connected in series to the second end  236   b  of the inner conductor  236  of the upper section  208   a  of the fourth unit  208  through a capacitor C 11 . The second end  238   b  of the outer conductor  238  is electrically connected in series to the second end  240   b  of the inner conductor  240  through a capacitor C 12 . The first end  236   b  of the inner conductor  236  is electrically connected in series to the first end  240   a  of the inner conductor  240  through a capacitor C 13 . The first end  238   a  of the outer conductor  238  is electrically connected in series to the first end  242   a  of the outer conductor  242  through a capacitor C 14 . 
   In operation, a magnetic field is generated between the third unit  206  and the fourth unit  208 . The fourth unit  208 , which has a smaller diameter than the third unit  206 , reflects the magnetic field towards the third unit  206 . The first and second units  202 ,  204  act as receiver coils and resonators within the magnetic field created by the third and fourth units. 
     FIG. 15  is a corresponding circuit diagram of the coaxial cable units of  FIG. 11 . The inductors of  FIG. 13  correspond to the inner and outer conductors of  FIG. 11 , as follows: 
   
     
       
             
             
             
           
         
             
                 
                 
             
           
           
             
                 
               L 1   
               Inner Conductor 210 
             
             
                 
               L 2   
               Inner Conductor 216 
             
             
                 
               L 3   
               Outer Conductor 214 
             
             
                 
               L 4   
               Outer Conductor 218 
             
             
                 
               L 5   
               Inner Conductor 220 
             
             
                 
               L 6   
               Inner Conductor 224 
             
             
                 
               L 7   
               Outer Conductor 222 
             
             
                 
               L 8   
               Outer Conductor 226 
             
             
                 
               L 9   
               Inner Conductor 228 
             
             
                 
               L 10   
               Inner Conductor 232 
             
             
                 
               L 11   
               Outer Conductor 230 
             
             
                 
               L 12   
               Outer Conductor 234 
             
             
                 
               L 13   
               Inner Conductor 236 
             
             
                 
               L 14   
               Inner Conductor 240 
             
             
                 
               L 15   
               Outer Conductor 238 
             
             
                 
               L 16   
               Outer Conductor 242 
             
             
                 
                 
             
           
        
       
     
   
   The corresponding capacitors in  FIGS. 11 and 13  are commonly identified. 
   In one configuration of the head and neck antenna array  200  for the use in an MRI system with a magnet with a field strength of between about 6,000 Gauss to about 8,000 Gauss, the capacitors C 1 , C 2 , C 4 , C 9 , C 11  and C 14  had a value of 220 picofarads (“pf”). Capacitor C 3  had a value of 470 pf and Capacitor C 10  had a value of 820 pf. The inner and outer conductors are preferably tuned to a frequency between about 25 to about 35 MHz. The first, second and third coaxial cable units  202 ,  204 ,  206  had an outer height “h 1 ” ( FIG. 13 ,  FIG. 14 ) of about 11 inches. The fourth coaxial ring unit  208  had an outer diameter “D 2 ” of about 9 inches. The distance “D” between adjacent coaxial cable units was 2 inches. ( FIG. 14 ) Each coaxial cable unit was critically coupled to an adjacent coaxial cable unit. Such an antenna array  200  was found to have a Q of about 220 and a S/N ratio of about 360 to 1. 
   When positioned over the head of a patient, the third unit  204  is preferably positioned over the eyes of the patient. 
   The antenna array  200  may be encased in a base  200   a  of rigid dielectric material, as discussed above with respect to  FIG. 13 . 
   Testing of several of the configurations discussed above show significant improvements in Q and S/N ratio. The Q of the antenna may be measured by a network analyzer, such as the 3577A, available from the Hewlett Packard Company. The S/N ratio may be measured by performing a phantom scan by an MRI system, as is known in the art. In one test, the ends  20 ,  22  of an outer conductor  14  were electrically connected in series across a variable capacitor C 1 , as shown in  FIG. 16 . The range of the variable capacitor C 1  was from 0 to about 30 picofarads. The outer conductor was tuned to a frequency between 25 to 35 MHz. The Q and S/N ratio of the coaxial cable in  FIG. 16  were found to be 59 and 180, respectively. In  FIG. 17 , the ends  16 ,  18  of the inner conductor  12  were connected across the variable capacitor C 1 , and the ends  20 ,  22  of the outer conductors were connected across a capacitor C 2  having a value of 10 picofarads. The inner and outer conductors  12 ,  14  were tuned to the frequency of between about 25 to 35 MHz. The Q of the circuit was 72 and the S/N ratio was 220, approximately a 22% improvement in both characteristics. The use of multiple coaxial cable units has been found to further improve the Q and S/N ratio of the antenna. As mentioned above, the Q of the hand, wrist and toe antenna array  100  of  FIG. 10 , for example, was found to be about 197, about a 233% improvement. The S/N ratio was found to be about 320, about an 82% improvement. 
   When only the inner ends  16 ,  18  of the conductor  12  of a coaxial cable unit were connected across the capacitor C 1 , and a capacitor C 3  of 120 picofarads, and the inner conductor was tuned to a frequency between about 25 to 35 MHz, as in  FIG. 18 , the Q of the circuit  28  was 103. The S/N ratio was 160. In the configuration of  FIG. 19 , the ends  16 ,  18  of the inner conductor  12  and the ends  20 ,  22  of the outer conductor  14  were connected through capacitors C 1 , C 4 , C 5  and C 6 , respectively. The capacitors C 4 =72 pf, the capacitor C 5 =68 pf and the capacitor C 6 =47 pf. Both conductors were tuned to a frequency between about 25 to 35 MHz. The Q of the circuit was 128 and the S/N ratio was 200 to 1, approximately a 25% improvement. As mentioned above, the use of multiple coaxial cables further improves the Q and S/N of the antenna. The Q of the head antenna array  200 , with four coaxial cables, was 220, about a 113% improvement over the configuration of  FIG. 18 . The S/N ratio was 360 to 1, about a 125% improvement. 
   The basic coaxial cable unit  10  of  FIG. 1  may also be used in antennas in other configurations, in accordance with the present invention. For example, in  FIG. 20 , an antenna array  400  comprises two concentric coaxial cable units  402  and  404 , lying in the same plane. The inner coaxial cable unit  402  comprises an inner conductor  406  and an outer conductor  408 . The inner conductor  406  has a first end  414  and a second end  418 . The outer coaxial cable unit  404  comprises an inner conductor  410  and an outer conductor  412 . The inner conductor  410  has a first end  416  and a second end  420 . The outer conductor  412  has a first end  422  and a second end  424 . 
   The first end  414  and the second end  418  of the inner conductor  406  of the inner coaxial cable unit  402  are electrically connected in parallel to the first end  416  and the second end  420 , respectively, of the inner conductor  410  of the outer coaxial cable  404 . A capacitor C 1  is electrically connected in parallel to the inner conductors  406  and  410 . 
   The first end  422  of the outer conductor  412  is electrically connected to the second end  424  of the outer conductor  412 , through a capacitor C 2 . A portion  408   a  of the outer conductor  408  of the inner coaxial cable unit  402  is also directly electrically connected to an adjacent portion  412   a  of the outer conductor  412  and a portion  408   b  of the outer conductor  408  is directly electrically connected to a portion  412   b  of the outer conductor  412   b , through electrical contacts  411 ,  413 , respectively. Insulation (not shown) may be provided between the outer conductors  408  and  412  along the remainder of their lengths. 
   The inner conductors  406 ,  410  of the two coaxial cables  402 ,  404  provide the output of the antenna  400 . The capacitor C 1 , may be connected to a variable capacitor in the preamplifier of the MRI system, or be the variable capacitor of the preamplifier. 
   As above, the values of the capacitors C 1 , and C 2  are such that the circuit including the inner conductors and the circuit including the outer conductors, are tuned to the same Larmor frequency. 
     FIG. 21  is an schematic diagram of a circuit corresponding to the configuration of  FIG. 20 , wherein the inductor L 1 , represents the outer conductor  412  of the outer coaxial cable unit  404  and the inductor L 2  represents the outer conductor  408  of the inner coaxial cable unit  402 . The inductor L 3  represents the inner conductor of  410  of the outer coaxial cable unit  404  and the inductor L 4  represents the inner conductor  406  of the inner coaxial cable unit  402 . 
   The antennas and antennas arrays of the present invention may also be used in a quadrature arrangement. For example, in  FIG. 22 , a quadrature antenna system  450  is shown comprising the hand, whist and toe antenna array  100  of  FIGS. 9-12  and a pair of rectangular coaxial cable units  452   a ,  452   b .  FIG. 23  is a top view of the antenna arrays  100  and  452 , showing the rectangular coaxial cable unit  452 . In  FIG. 23 , the rectangular coaxial cable coil  452  lies in a flat plane A of the page. In  FIG. 22 , the flat plane A is perpendicular to the page and is perpendicular to the parallel planes P 1 , P 2 , P 3 , of the antenna array  100 . 
   In  FIG. 22 , the antenna array  100  and the rectangular or coaxial cable  452  are separated by a dielectric material  454 , such as plastic. The dielectric material  454  may be in the form of a plate. The rectangular coaxial cable  452  may also be supported by a plate  456  of plastic or other dielectric material. The plates  502 ,  503  may be 1 inch thick, for example. 
   Returning to  FIG. 23 , the rectangular coaxial cable unit  452  preferably includes two sections  452   a  and  452   b  to lower the inductance of the coaxial cable  452 . The two sections  452   a ,  452   b  each have an inner conductor  12 ′,  12 ″, and an outer conductor  14 ′,  14 ″, as in  FIG. 7 . Opposing ends  16   a ,  18   a  and  16   b ,  18   b  of the inner conductors  12 ′ and 12″ are connected to each other through capacitors C 1 , and C 4 , while the opposing ends  20   a ,  22   a  and  20   b ,  22   b  of the outer conductors  14 ′  14 ″, are connected to each other through capacitors C 2 , C 3 . The output of the rectangular coaxial cable unit  452  is across the capacitor C 1 . As above, the capacitor C 1 , may be connected to a varactor in the preamplifier of the MRI system or may be the varactor in the preamplifier. 
   A capacitor C 5  is connected across the output  108  of the antenna array  100  (see  FIG. 9 ). The details of the connections of the inner and outer conductors of each coaxial cable unit of the array  100 , and of the output across the capacitor C 5 , are not shown in  FIG. 23 . As above, the capacitor C 5  may also be connected to or be a varactor in the preamplifier of the MRI system. 
   The range of the rectangular coaxial cable units  452   a ,  452   b  may be extended by providing additional coaxial cable units  454 ,  456 , perpendicular to the coaxial cable units  452   a ,  452   b .  FIG. 24  shows a quadrature antenna system  450 ′ of  FIG. 23 , which includes additional coaxial cable units  454 ,  456 . The coaxial cable unit  454  includes an inner conductor  458  and an outer conductor  460 . The coaxial cable unit  456  includes an inner conductor  462  and an outer conductor  464 .  FIG. 25  is a side view of the quadrature antenna system  450 , along line  25  in  FIG. 26 .  FIG. 26  Is a side view of the quadrature antenna system  450 , along arrow  26  in  FIG. 26 . The height “h” of the coaxial cable units  454 ,  456  may be 4 inches, for example. The antenna array  100  is not shown in  FIGS. 25 and 26 . 
   As shown in  FIGS. 24 and 25 , a first end  460   a  of the outer conductor  460  of the coaxial cable unit  454  is connected to the second end  22   b  of the inner conductor  12 ″ of the coaxial cable unit  452   b  through a capacitor C 10 . The first end  458   a  of the inner conductor  458  is directly connected to the first end  18   b  of the outer conductor  14 ″. 
   The second end  460   b  of the outer conductor  460  is directly connected to the second end  20   b  of the coaxial cable unit  452   a . The second end  16   b  of the inner conductor  12 ′ of the coaxial cable unit  452   a  and the second end  458   b  of the inner conductor  458  of the coaxial cable unit  454  are connected across a capacitor C 11 . 
   As shown in  FIGS. 24 and 26 , a first end  462   a  of the inner conductor  462  of the coaxial cable unit  456  is connected to the first end  18   a  of the inner conductor  12 ″ of the coaxial cable unit  452   b  through a capacitor C 12 . The second output of the quadrature antenna system is provided across the capacitor C 12 . The first end  464  of the outer conductor  464   a  is connected to the second end  22   a  of the outer conductor  14 ″ through a capacitor C 14 . 
   The second end  462   b  of the inner conductor  462  is directly electrically connected to the first end  16   a  of the inner conductor  12 ′ of the coaxial cable unit  452   a . The second end  464   b  of the outer conductor  464  is directly connected to the first end  20   a  of the outer conductor  14 ′. 
   The connections within the antenna array  100  are the same as in  FIG. 23  and  FIG. 9 . 
     FIG. 27  is a schematic diagram of the preamplifier section  470  of an MRI system for use with the quadrature antenna systems  100  and  452 . Two preamplifiers;  472 ,  474  are shown, each providing an input  476 ,  478  to a phase shifter signal combiner  480 , which provides an output  482  to the signal processing portion of an MRI system. The output from the antenna array  100  may be provided to the preamplifier  472  and the output from the rectangular coaxial cable antenna  452  may be provided to the preamplifier  472  for example. The phase shifter may be a Mini-Circuit 15542 ZSCQ-2-50, available from Mini-Circuit, Brooklyn, N.Y., for example. 
     FIG. 28  is a side view of the gap  490  between the magnet poles  492 ,  494  of an MRI system, such as the Quad 7000 available from FONAR, Corporation, Melville, N.Y. The quadrature antenna system  450  rests on a patient bed  496  portion of which is above the pole  494 . Preferably, the quadrature antenna system  450  is supported at an angle with respect to the patient bed  496 . An angle of about 15° may be provided, for example. The quadrature antenna system  450  may be supported at an angle by a wedge  498 , for example. 
   The head and neck antenna array  200  may also be used in a quadrature arrangement by adding a coaxial cable unit perpendicular to the planes of the coaxial cable units of the antenna array  200 , above or below the antenna array. 
   It is known that the voltage induced in a secondary winding of a transformer may be increased by increasing the number of turns in the secondary winding. The voltage induced in the inner conductor of the coaxial cable unit  10  of  FIG. 1  may therefore be increased by connecting a plurality of inner conductors in series. The signal-to-noise (S/N) ratio is thereby improved. However, it is also known that connecting inductors in series increases the resistance of the circuit, decreasing the Q of the circuit. In addition, the inductance is also increased, preventing tuning of the circuit to high frequencies for use with magnets of high field strengths, as discussed above. Therefore, it is preferred to also provide inner conductors connected in parallel, to lower the resistance and inductance of the circuit including the inner conductors. Parallel connection also increases current flow, which also improves the signal-to-noise (S/N) ratio. 
     FIG. 29  is a plan view of a coaxial cable unit  500  in accordance with another embodiment of the invention, including multiple inner conductors. In the configuration of  FIG. 29 , four inner conductors  504 ,  506 ,  508 ,  510  are shown. More or fewer inner conductors may be provided.  FIG. 29   a  is a cross-sectional view of the coaxial unit  500  along line  29   a - 29   a  in  FIG. 29 . Preferably, the inner conductors  504 ,  506 ,  508 ,  510  are closely and tightly bundled by slightly twisting them together. The inner conductors may be solid and are covered by a dielectric material, such as Teflon tubing (not shown). 
   The ends  502   a ,  502   b  of the outer conductor are connected across a capacitor C 1 , as shown in  FIG. 29 . 
   A preferred connection scheme for the inner conductors is shown in  FIG. 30 , where the inductors L 2 , L 3 , L 4  and L 5  correspond to the inner conductors,  504 ,  506 ,  508 ,  510 , respectively. The inductors L 2  and L 3  are preferably connected in series through capacitor C 2 . The inductor L 3  and the inductor L 4  are also preferably connected in series, through a capacitor C 3 . The inductors L 4  and L 5  are preferably connected in parallel. The parallel connector inductors L 4 , L 5  are electrically connected to the inductor L 3  though a capacitor C 3 . The resistance and inductance are not, therefore, increased as much as if all of the inductors were connected in series. 
   The circuit including the outer conductor and the circuit including the inner conductor are tuned to the same Larmor frequency, as discussed, above. As above, the capacitor C 4  may be connected to a varactor in the preamplifier of the MRI system or may be the varactor. 
   An antenna array may also be provided with multiple coaxial cable units  500 .  FIG. 31  shows three coaxial cable units  520 ,  522  and  524 , separated by an insulator  526  such as Teflon®. Each coaxial cable  520 ,  522 ,  524 , may be copper tubing with a diameter of about 0.25 inches. The Teflon® may be 1 mm thick, for example.  FIG. 33  shows the three coaxial cable units  520 ,  522 ,  524  and the connections between their inner and outer conductors. Those connections will be explained with respect to the electrical schematic diagram of  FIG. 33 , wherein the top box corresponds to the coaxial cable unit  520 , the middle box corresponds to the coaxial cable unit  522  and the lower box corresponds to the coaxial cable unit  524 . 
   The outer conductors of each coaxial cable unit  520 ,  522 ,  524  are represented as inductors L 1 , L 2  and L 3 , respectively, in  FIG. 33 . The inductors L 1 , L 2  and L 3  are preferably connected in parallel across a capacitor C 1 . The parallel connection of the outer conductors provides better shielding of the inner conductors. Alternatively, the outer conductors may be connected in series. 
   In the coaxial cable unit  520 , two of the inner conductors are preferably connected in series and two are preferably connected in parallel. In  FIG. 33 , the inductors L 4 , L 5 , L 6  and L 7  represent the four inner conductors of the first coaxial cable unit  520 . The two inductors L 4  and L 5  are connected in series across a capacitor C 2 . The two inductors L 6  and L 7  are connected in parallel. The parallel connected inductors L 6 , L 7  are connected to the inductor L 5  through a capacitor C 3 . 
   In the second coaxial cable unit  522 , two pairs of the inner conductors are connected in parallel. In  FIG. 33 , the inner conductors are represented by the inductors L 8 , L 9 , L 10  and L 11 , respectively. The inductors L 8  and L 9  are connected in parallel and the inductors L 10  and L 11  are connected in parallel. One pair (L 8 , L 9 ) is connected to the other pair (L 10 , L 11 ) across a capacitor C 4 . The pair of parallel connected inductors (L 6 , L 7 ) in the first coaxial cable unit is also connected to one of the pairs of parallel connected inductors (L 8 , L 9 ) in the second coaxial cable unit  522  through a capacitor C 5 . 
   In the third coaxial cable unit  524 , three of the inner conductors are preferably connected in parallel. In  FIG. 33 , the inner conductors are represented as the inductors L 12 , L 13 , L 14 , and L 15 , respectively. The inductor L 12  is connected to the three parallel connected inductors L 13 , L 14 , L 15  through a capacitor C 6 . While the capacitor C 6  is shown within the box  524  representing the third coaxial cable unit  524 , it is most readily provided between the second and third coaxial cable units  522 ,  524 , as shown in  FIG. 32 . One of the pairs of parallel connected inductors (L 10 , L 11 ) in the second coaxial cable unit  522  is also connected in parallel to a first one of the inductors L 12  in the third coaxial cable unit  524 . 
   One end the first inductor L 4  of the first coaxial cable unit  520  and one end of the three parallel connected inductors L 13 , L 14 , L 15  in the third coaxial cable unit  524  are connected across a capacitor C 7  to provide an output of the antenna array. As discussed above, the capacitor C 7  may be connected to a varactor in the preamplifier of the MRI system or may be the varactor in the preamplifier. 
   As above, the circuit of the connected inner conductors is tuned to the same frequency as the circuit including the outer conductors. Such a configuration was found to have a signal-to-noise (S/N) ratio of about 700 and a Q of about 200. 
   The outer conductor  504  of the coaxial cable unit  502  may have an inner diameter “D 1 ” of 0.25 inches, as shown in  FIG. 29   a . The outer diameter “D 2 ” ( FIG. 29 ) of the coaxial cable unit ring may be 6¾ inches, for example. Such a coaxial cable unit is appropriate for imaging a knee, for example. The inner conductors  504 ,  506 ,  508  and  510  may each have an outer diameter of 0.74 millimeters. As stated above, the inner conductors  504 ,  506 ,  508 ,  510  are preferably tightly bundled. 
   For coaxial cable units with diameters D 1  ( FIG. 29 ) of greater than about 8½ inches, or for use with high magnetic field strengths and higher Larmor frequencies, it is preferred to connect all of the inner conductors in parallel, to facilitate tuning. A parallel connection scheme is discussed below with respect to  FIG. 36 . 
   In another embodiment, an additional outer conductor may be added to the coaxial cable unit  500  of  FIG. 29 , forming a triaxial cable unit  600 , as shown in  FIG. 34 . Two outer conductors  602 ,  604  are provided with multiple inner conductors  606 ,  608 ,  610 ,  612 . The outermost, first outer conductor  602 , which surrounds the second outer conductor  604 , has openings  614  distributed around its periphery.  FIG. 34   a  is a cross-sectional view of the triaxial cable unit  600  along line  34   a - 34   a  of  FIG. 34 , showing the various components. 
   Ends of the first outer conductor  602  are connected through a capacitor C 2 . Ends of the second outer conductor  604  are also connected to each other through a capacitor C 1 . The first and second outer conductors  602 ,  604  are both tuned to the Larmor frequency of the species of interest. Depending on the diameter D of the ring  600 , and/or the magnetic field strength, the inner conductors  606 ,  608 ,  610 ,  612  are connected to each other as described above with respect to  FIG. 30 , or in parallel, as discussed below. 
     FIG. 35  is a schematic representation of a circuit corresponding to the triaxial cable unit of  FIG. 34 , wherein the first outer conductor  602  is represented by the inductor L 1 , the second outer conductor  604  is represented as the inductor L 2  and the inner conductors  606 ,  608 ,  610 ,  612  are represented as inductors L 3 , L 4 , L 5 , L 6 , respectively. As discussed above, ends of the outer conductors  602 ,  604  (L 1 , L 2 ) are connected across the capacitors C 1 , C 2 , respectively. Two of the inner conductors  606 ,  608  (L 3 , L 4 ) are connected in series and the other two inner conductors  610 ,  612  (L 5 , L 6 ) are connected in parallel, as shown in  FIG. 35 . 
     FIG. 36  is a schematic representation of the triaxial cable unit  600  of  FIG. 34 , wherein the inner conductors  606 ,  608 ,  610 ,  612  (L 3 , L 4 , L 5 , L 6  are connected in parallel. 
   In one configuration, the triaxial cable unit ring has an inner diameter “D” of 8 inches. The inner diameter “d 1 ” ( FIG. 34   a ) of the first conductor is ⅜ inch. The inner diameter “d 2 ” of the second conductor is ¼ inch. The holes  614  in the first conductor  602  have diameters of about 2 mm. and are separated by a length “L” of about 1 inch. 
   Because of the holes  614  through the first, outermost conduct  602 , it is believed, without limiting the scope of the invention, that magnetic resonance signals are detected by both the first and second outer conductors  602  and  604 , and that in addition, since the first and second outer conductors  602 ,  604  are inductively coupled, voltage signals which are a function of the magnetic resonance signals detected by the first outer conductor  602 , are induced in the second outer conductor  604 . The second outer conductor is also inductively coupled to the inner conductors  606 ,  608 ,  610 ,  612  and induces voltage signals in the inner conductors which are provided to the MRI system for processing. While the embodiment of  FIGS. 34-36  has multiple inner conductors  504 ,  506 ,  508 ,  510 , a second outer conductor  604  may be used in a triaxial cable with only one inner conductor. 
   As mentioned above, the coaxial cable units of the present invention may also be used as transmitting antennas.  FIG. 37  shows a coaxial cable unit  700  as in  FIG. 3 , wherein the ends  16 ,  18  of the inner conductor  12  are connected across a capacitor C 1 , to a radio frequency power source. The ends  20 ,  22  of the outer conductor  14  are connected across a capacitor C 2 . The inductance of the inner and outer conductors  12 ,  14  and the values of the capacitors are adjusted so that the respective circuits are tuned to substantially the same frequency. 
     FIG. 38  is a schematic representation of the coaxial cable unit of  FIG. 37 , where the inner conductor  12  is represented by the inductor L 1 , and the outer conductor  14  is represented by L 2 . In this case, a time varying voltage in the inner conductor  12  (L 1 ) induces a time-varying voltage in the outer conductor  14  (L 1 ), causing emission of radio frequency pulses from the outer conductor  14  (L 1 ). The first circuit A comprising the inductor L 1 , and the capacitor C 1 , filters the driving signal from the RF power source to within a first bandwidth. The second circuit B comprising the inductor L 2  and the capacitor C 2  filters the signal received from the first circuit A to within a narrower bandwidth. The signal transmitted by the circuit B therefore includes less noise than conventional transmitting antennas. 
   Any of the coaxial cable configurations discussed above can act as both a receiving and transmitting antenna. 
     FIG. 39  is a schematic representation of portions of an MRI system  800 , showing in particular a connection between an antenna or antenna array  802  in accordance with the present invention with certain components of the MRI system. A signal processing system  804 , a computer  806 , an NMR controller  808 , a gradient coil subsystem  810 , a receiver subsystem  812  including a preamplifier  116 , and an image display system  114  of the MRI system  800  are also shown. The computer  806  controls the overall operation of the MRI system  800 . The NMR controller  808  stores the timing of the scanning sequences and controls implementation of the scanning sequence. The signal processing system  804  typically includes a variable amplifier, a frequency down converter, an analog-to-digital converting array and a digital data processor (not shown), as is known in the art. The gradient coil subsystem  810  includes the gradient coils and a gradient waveform generator, which outputs particular waveforms for a desired scanning sequence to the gradient coils under the control of the NMR controller  808 , also as is known in the art. The particular ends of the inner conductor of the coaxial cable unit providing the output of the antenna array  802  are connected to the receiver system  812  of the MRI system  800  through the port  818 . 
   As discussed above, the presence of a patient provides a load on the antenna array  802  which lowers the antenna&#39;s Q. The presence of the patient also shifts the resonant frequency of the antenna array  802 , which may require returning to the desired Larmor frequency. A varactor, or vanable capacitor  820  is therefore provided between the capacitor C 1  of the antenna  802  and the preamplifier  816  parallel to the capacitor C 1 , to enable returning of the antenna array  802  when the antenna array is positioned with respect to the patient, as is known in the art. Alternatively, C 1 , may be the varactor  520 . A back diode  822  is preferably provided parallel to the varactor  820  to prevent the passage of excessive voltage to the preamplifier  816 , also as is known in the art. Voltage greater than about 0.7 volts is typically blocked by the back diode  822 . The varactor  820  is controlled by the computer  806 . The port  818  may be connected to the varactor  820 , back diode  552  and preamplifier  810  through a short, low capacitance cable, or other appropriate means. 
     FIG. 39  also shows an optional connection between the capacitor C 6  and the antenna  802 , and the RF transmitting subsystem  824  of the MRI system, through the optional port  826 . The RF transmitting subsystem includes an RF power source (not shown) for driving the transmitting antenna, as is known in the art. The RF transmitting subsystem  824  can also be connected to the capacitor C 1 , through a switch controlled by the computer  806 . The computer  806  would switch the connection between the RF subsystem  824  and the antenna  802 , and the preamplifier  820  and the antenna  802 , at appropriate times. The RF transmitting subsystem  824  is controlled by the NMR controller  808  and the computer  806  of the MRI system  800 . 
   A more complete description of the structure and operation of the MRI system may be found in U.S. Pat. No. 6,025,717, assigned to the assignee of the present invention and incorporated by reference, herein. 
   The above embodiments are examples of antennas and magnetic resonance imaging systems in accordance with the present invention. It will be recognized by those skilled in the art that changes may be introduced to those embodiments without going beyond the scope of the invention, which is defined by the claims, below.