DISTRIBUTED PHASE RF COIL

Abstract: A distributed phase RF coil arrangement for exciting a sample under NMR analysis with an RF field and for detecting nuclear magnetic resonance of said sample. The distributed phase RF coil arrangement comprises a coil former which defines an NMR sample region therewithin. A portion of the coil former is provided with shielding of the NMR sample region from RF irradiation. The remaining portion of the coil former includes a Faraday shield for electrostatically decoupling the sample. An electrically con­ ductive loop which, for example, has the geometry of a toroid is wound about the former and has alternating shielded and unshielded segments for magnetically coupling normal components of the RF field with the sample. Due to the orientation of the shielded and unshielded segments and their respective lengths, phase delays inherent at high frequencies, along the loop between adjacent pairs of unshielded segments of said loop, or at points thereon, are made equal to the angular displacements of said segments, or between said points, thereby generating a desired magne­ tic field having a relatively substantial normal component throughout the NMR sample region.

Description: DISTRIBUTED PHASE RF COIL . v Technical Field This invention relates to nuclear magnetic resonance (NMR), and in particular, to RF coil designs particularly well suited for NMR imaging. The RF coil provides the dual function of a transmitting or exciting coil for applying an RF-magnetic field to a sample under NMR analysis and of a receiving coil for detecting the radio frequency magnetization of the sample. Background Art In recent years, advances in the art of superconducting magnets have permitted NMR spectrometers to be built which operate at frequencies as high as 600 megahertz. At such frequencies, the performance of the RF coil in the spectrometer becomes a major limiting factor in the overall system perfozmance. The RF coil, which surrounds the sample under analysis, has the function of magnetically coupling the spectrometer to the sample. An example of a prior art RF coil circuit for a nuclear magnetic resonance probe having a tuning range of 6 to 32 megahertz is disclosed in U.S. Patent No. 4,095,168. At high frequencies, the distributed capacitance of such coils becomes significant so that even a single turn coil becomes "self-resonant". When this limit is approached, the homogeneity of the RF field decreases, due to the phase shift in different parts of the coil. Morever, as the number of turns of the coil is reduced, the homogeneity as well as efficiency of the coil, both in generating fields and in picking up the signal, is diminished. A further problem occuring in high frequency superconducting magnets is that the static magnetic field is co-axial with the axis of the cylindrical bore of the magnet. Since the RF field must be perpendicular, i.e., normal, to this axis, the conventional coil design is to place one loop on each side of the cylindrical sample. This inherently inefficient configuration, and variations of it, is the current state of the art in NMR spectrometers. In the separate but related field of NMR imaging, the sample sizes, frequencies, and magnet geometry are such that exactly the same problems occur. Again, the same general coil designs are the state of the art. In NMR imaging, the homogeneity of the irradiating field and the uniformity of the sensitivity to the signal are even more important than in conventional NMR spectroscopy. Typically, homogeneity can be improved by increasing the diameter of the RF coil and moving it away from the sample. This approach, however, is inherently inefficient and results in decreased magnetic coupling of the coil with the sample. / Typically, the NMR signal improves as the strength of the static magnetic field and the operating frequency are increased. In NMR imaging, the phenomenon which limits the useful operating frequency is the RF absorption effects in the sample. With medical applications, the frequency and coil size at which this limit is approached is such that conventional RF coils suffer from all of the problems discussed above. At any field strength, the efficiency and uniformity of the coupling between the sample and spectrometer have a significant effect on the quality of the resultant image. Summary of the Present Invention We have discovered an RF coil arrangement which generally overcomes the problems noted above; principally, by recognizing the phase shift or propagation delay inherent in an RF coil conductor and incorporating this phenomenon into the design in such manner that the delay at each increment of the loop is predetermined and contributes to a high level of magnetic coupling with the sample. Thus, the propagation delay, inherent in the coil conductor, is incorporated into the design of the coil rather than being treated as a limiting phenomenon. The delays are calculated and used to put the correct current in the correct place. In other words, the electrical phase of the current at each segment of wire which contributes to the magnetic field is in the correct position, due to the geometry of the coil and the distribution of the electrical delays, to produce the desired magnetic field. The result can be considered part RF coil and part resonant transmission line. In a preferred embodiment, a distributed phase RF coil arrangement is provided for at least one of (a) exciting a sample under NMR analysis. with an oscillating magnetic field such as an-RF field and (b) for detecting nuclear magnetic resonance of the sampler and adapted for a relatively high degree of magnetic coupling with the sample. The RF coil arrangement comprises at least one electrically conductive loop defining an NMR sample region therewith in, the loop having alternating shielded and unshielded segments. The shielded segments define inactive portions of the loop and connect pairs of unshielded segments. The unshielded segments define the active portions of the loop and magnetically couple to the sample. Due to the length of each inactive portion, the length of each active portion and the inductive coupling among the active portions, the propagation delay between points on said active portions of the loop results in a phase shift which at the resonant, or operating, frequency of the coil arrangement is generally equal to the angular displacement between said points of said active portions, thereby generating a desired magnetic field having a relatively substantial normal component relative to the static magnetic field throughout the NMR sample region. Similarly, the loop is sensitive to a magnetic field having a normal component. In a preferred embodiment, the RF coil arrangement is generally cylindrical and further comprises an insulating means for supporting the electrically conductive loop, electrically conductive means for shielding a portion of the NMR sample region from RF irradiation, said shielded portion of the NMR sample region defining therewithin a first inactive portion and thereby delimiting the non-shielded active portion of the sample region, and means electrically connected to said shielding means for electrostatically decoupling a sample within the active portion of the NMR sample region from the loop. This novel design has the advantages of providing both a uniform irradiating field when used as a transmitting coil and a uniform sensitivity when used as a receiving coil. In imaging applications, this results in a superior image and increased efficiency by permitting the coil to be placed closer to the sample. This novel design has many more turns than the traditional coil for the same frequency and size. In addition to increasing the uniformity as mentioned above, this feature increases the efficiency of the coil. When used as an irradiating coil, this means that the same amount of current produces a higher magnetic field. When used as a receiving coil, this means that the same amount of sample magnetization produces a larger emf. Brief Description of the Drawings Fig. 1 is a plan view, partly in section, of a coil arrangement according to the present invention. Fig. 2 is.a section taken along line 2-2 of Fig. 1. Fig. 3 is a development view showing the conductive loop of the coil arrangement of Fig. 1 as it would look if removed from the coil former and laid out flat. Fig. 4 is a block diagram of a portion of an NMR system including an RF coil arrangement. Fig. 5 is an example in schematic form of a coupling network. Fig. 6 is illustrative of a pair of linear transmission lines. Fig. 7 is a graph of the waveforms of the transmission lines shown in Fig. 6. Fig. 8 is a diagrammatic representation in perspective of the coil arrangement shown in Fig. 1. Fig. 9 is an end view of Fig. 8. Fig. 10 shows an alternate winding of the conductive loop. Fig. 11 is an end view of Fig. 10. Fig. 12 is a diagrammatic representation of a section taken along line 12-12 of Fig. 10. Detailed Description of the Invention Referring now to Fig. 1, there is shown a distributed phase RF coil arrangement generally designated as 10 incorporating features of the present invention. The RF coil arrangement 10 includes a coil support or coil former 12 which, in one model of the invention, is a 3-1/4 inch diameter, 9 inch long, insulating tube made of glass and open at both ends. At either end of the glass tube 12 there is provided a 3 inch wide strip of thin brass foil 14 which is soldered end to end along line 16 to form a tight fit around the glass tube 12. The sample that is to undergo NMR analysis is inserted through one end of the glass tube 12, generally in the central region thereof. The two brass foil strips 14 provide an RF shield at each end of the glass tube 12 thereby defining an active portion 18 in the central region of the glass tube 12. Between the two strips of brass foil 14, a plurality of wires 20 are wound around the glass tube 12. The plurality of wires 20 is preferably copper or other conductive material and soldered by cross wire 22 to form closed loops. The closed loops of wires 20 are spaced at about 1/8 inch intervals over the 3 inch portion of the glass tube within the two brass foil strips 14. Each end of the cross wire 22 is soldered to one of the brass foil strips 14 and also has a discontinuity 24 therebetween. The plurality of wires 20 together with cross wire 22 forms a Faraday shield" for electrostatically decoupling a sample within the active portion 18 of the NMR sample region from electric fields of the coil arrangement 10 and for shielding the coil from electrostatic atmospheric noise. Moreover, the Faraday shield permits the magnetic field to pass-through to the sample under NMR analysis. This electric decoupling reduces the RF energy applied to the sample since the energy applied is related to the product of the electric and magnetic fields. The shield also decreases the effect of the sample on the tune of the RF coil arrangement 10. This is crucial since, for example, a slightly conductive sample can increase the distributed capacitance of a coil arrangement and thereby shift its resonant frequency. The Faraday shield together with brass foil strips 14 form a shield between the sample under NMR analysis and the RF coil arrangement 10. This shield does not shield the NMR sample from the desired component of the magnetic field which is the portion normal to the axis of the glass tube 12. The coil arrangement 10 further includes a closed conductive loop 26 preferably disposed about said glass tube having the general configuration of a toroid. For additional clarity, conductive loop 26 is shown in Fig. 3 as it would look if removed from glass tube 12 and laid out flat. The conductive loop 26 includes shielded coaxial cable segments 28 and unshielded wire segments 30. The unshielded segments 30 comprise the active wires while the shielded coaxial cables 28 form the return or inactive wire portions. Both ends of the shield of each shielded segment 28 are soldered to brass foil strips 1 at contact points 32. The discontinuity 24 in the cross wire 22 ensures that there is no shorted conducting path parallel to the active wires and between them and the sample. Any such shorted conducting path would tend to reduce the magnetic coupling between the active wires and the sample. In connection with this model of the RF coil arrangement 10, which is approximately 9 inches long, the shielded segments of the wire are lifted up and away from the glass tube 12 to a distance approximately 1-1/4 inches therefrom. The shielded segments 28 are 3/16 inch outer diameter rigid coaxial cable. Each segment of the shielded wire is approximately 6 inches long. The active wire segments 30 are roughly 3 inches long and extend across the closed loop wires 20. The resonant frequency of this example is about 60 megahertz Current propagating along active wires 30 produces a magnetic field in the sample. Similarly, the resonant magnetic field of the sample produces a current in the active wire segments 30. The active wire segments 30 are arranged in generally axial relation with the glass tube 12 to produce lines of flux orthogonal to said axis. Preferably, the active wires 30 are in parallel relation to each other and are slightly angled relative to the axis of tube 12. The combination of the angle of the active wire segments, together with the distance of each of the return wire segments 28, produce the desired effect of overcoming the problem of the phase delay inherent in the distributed capacitance of the conductive loop 26 In other words, the phase delay between adjacent pairs of active segments 30 of the conductive loop 26 is generally equal to the angular displacement between said pair of active segments thereby generating a magnetic field having a relatively substantial normal component throughout the active portion 18 of said NMR sample region. As further shown in Fig. 3, the RF coil arrangement 10 is connected to a spectrometer (not shown) by conductor 34 which is a coaxial cable. The shielded segment 36 of conductor 34 is soldered to the brass foil 14. A portion of the shielding is removed to create active segment 37 of conductor 34 and to permit the active segment 37 to inductively couple to the RF coil arrangement 10. The end of active segment 37 of conductor 34 is# soldered to the brass foil 14 at point 38. The active segment 37 is positioned to provide the desired coupling. Preferably, this is accomplished by placing active segment 37 adjacent to one of the active segments 30 of conductive loop 26. The orientation of active segment 37 of conductor 34 affects the coupling achieved so that, together with the conductor 34 and variable capacitor 80 (shown in Fig. 5) the impedance at 81 can be set to 50 ohms topermit impedance matching. Other modes of coupling to the resonant RF coil arrangement can be made. One example is to couple at two points 90 degrees apart. The nuclear spins of the NMR #sample are-sensitive only to a rotating magnetic field, or to a circularly polarized magnetic field, By connecting to the RF coil arrangement's conductive loop at two places and driving the two points with a 90 degree phase difference, the field generated is circularly polarized and is, as a result, more effective. The resultant field is twice as strong. Similarly, during the detecting mode, since the NMR magnetization of the sample is circularly polarized, the detection efficiency is thereby improved. For added shielding, such as for example, from outside noise, an additional thin brass cylindrical shield 42 is wrapped around the outside of return shielded wire segments 28 and is soldered to brass foil strips 14. The conductive loop 26 described in connection with the preferred embodiment discussed hereinabove, consists of eight active segments 30 and eight return segments 28. Other arrangements are contemplated, however, wherein the number of either active or return segments is some other number. In addition, there may be more than one conductive loop 26. The coupling between the conductive loop 26 and an NMR analysis system as shown in part in Fig. 4 is provided by a conductor 34 which has shielded segment 36 and active segment 37 connected to the RF coil arrangement at connection 38. Adding another conductive loop 26A (a portion of which is shown in Fig. 3) to the RF coil arrangement 10 further increases the uniformity of the RF field and increases the efficiency of the magnetic coupling to the sample by providing more "turns" of the conductive loop. The second conductive loop 26A, essentially identical to conductive loop 26, is assembled about glass tube 12, but offset therefrom by one-half turn spacing. In such a dual conductive loop embodiment, one of the conductive loops can be considered a driven conductive loop while the other one undriven. Coupling between the unshielded or active wires 30 of the driven loop induces current in the undriven loop such that both loops 26 and- 26A contribute to the RF field that is coupled to the sample. Conversely, the NMR signal is coupled to both conductive loops 26 and 26A and transmitted to an NMR-analysis system. The RF coil arrangement 10 has a resonant frequency determined by its dimensions and in particular by the total length of the shielded and unshielded wire segments of conductive loop 26. In considering the operation of the conductive loop refer to Fig. 6 which consists of two transmission lines C and C'. When driven at point A with a frequency such that the length of the line is ?'/2, the signal in the upper line C' will meet the signal from the lower line C at point B with exactly the same amplitude. Point B will appear as an open circuit for signals on each line. A reflection of the current in each. line will occur and will propagate back to point A. The total path traveled by the signal is > , i.e., a 360 degree phase shift. The impedance at point A then will also appear nearly as an open circuit and a standing wave will occur on each line C and C' such that relative voltage maxima occur at points A and B and current maximum at C and C'. The voltage and current wave forms are graphically presented at Fig. 7. Now, if this transmission line is wound upon a coil former so that the turns are uniformly distributed and sections of the line are unshielded so as to no longer bound the elctromagnetic field between the inner and outer conductors, the configuration of Fig. 8 results. Points A and B are on opposite sides of the coil, as are points C and C' as shown in Fig. 8. Thus, it can be seen that as a first approximation, the total length of all segments must approximate#ly equal > at the frequency of interest. The number of turns and the length of the shielded wires can be varied within that limit. Because the field from one unshielded wire is coupled to the adjacent unshielded wires an increase in effective inductance occurs and the frequency of resonance is reduced. The amount of induction is related to the number of turns and the relative lengths of the unshielded wire. When the transmission line is configured as shown in Fig. 8, the current in each successive segment is shifted by an amount equal to its angular rotation. (See Fig. 9.) Alternatively, if the transmission line is formed as shown in Fig. 10 then the current in segment XX as shown in Fig. 12 is shifted from segment YY by 1800 + 80. Similarly, its position on the former is shifted by 1800 + 80 as shown in Fig. 11 and the same conditions for a uniform RF field are established as in Fig. 8. It is possible to couple RF power into the coil by means of coupling loop near an unshielded wire at point C or C'. Coupling is as in a transformer and the high impedance of the coil is lowered by the square root of the turns ratio to values suitable for low impedance transmission lines. Matching can be done either at the coil or at some other point such as the signal separation point 40. Other configurations of the RF coil arrangement are contemplated when the NMR sample to be imaged would not conveniently fit into a cylindrical shape such as provided within glass tube 12. Examples of such shapes are a sphere for NMR analysis of round shaped objects. Another example would be a cup shaped configuration which would be advantageous when imaging, for example, a breast. Referring now to an overview of an NMR system as shown in Fig. 4, the RP coil arrangement 10 is shown as it interrelates with other components of the system. The system includes a magnet 50 which is powered by a magnet power supply 52. There is further included an RF pulse gate 54 for gating radio #requency electrical signals. The radio frequency signals are amplified by a radio frequency power amplifier 58. A coupling network 40 couples the radio frequency-excitation signals with the RF coil arrangement 10. Similarly, detection is accomplished by the combination of the RF coil arrangement 10 in its detecting mode, the coupling network 40 and a low noise amplifier 60. The coupling network 40 during detection serves to separate the detected signals from the excitation signals, and the low noise amplifier 60 for amplifying -the detected NMR signals. Fig. 5 shows an example of a coupling network 40. Variable capacitor 80 permits adjustment of the RF coil impedance to a value of 50 ohms for proper operation with commercially available RF power amplifier 58 and low noise amplifier 60.

Claims: Claims: 1. A distributed phase RF coil arrangement for at least one of exciting a sample under NMR analysis with an oscillating magnetic field such as an RF field and for detecting nuclear magnetic resonance of the sample, and adapted for a relatively high degree of magnetic coupling with the sample, said arrangement comprising: : at least one electrically conductive loop defining an NMR sample region therewithin, said loop having alternating shielded and unshielded segments of predetermined lengths, said shielded segments defining inactive portions of said conductive loop and connecting pairs of unshielded segments, said unshielded segments defining active portions of the conductive loop for magnetically coupling with the sample, said unshielded segments adapted for current propagation along each of said active portions such that the propagation delay between points on said active portions is generally proportional to the angular displacement between said points of said active portions. 2. A distributed phase RF coil arrangement according to claim 1 further comprising: a) insulating means for supporting said electrically conductive loop; b) electrically conductive means for shielding a portion of the NMR sample region from RF irradiation, said shielded portion of the NMR sample region defin-ing therewith in a first inactive portion and thereby delimiting the non-shielded active portion of the sample region; and c) means electrically connected to said shielding means for electrostatically decoupling a sample within the active portion of the NMR sample region. 3. A distributed phase RP coil arrangement according to claim 2 wherein said decoupling means decouples the NMR sample from electric fields of said conductive loop and said conductive loop from electrostatic atmospheric noise. 4. A distributed phase RF coil arrangement according to any of claims 1 through 3 wherein said shielded segments connect adjacent pairs of unshielded segments. 5. A distributed phase RF coil arrangement according to any of claims 1 through 3 wherein said shielded segments connect diametrically opposed pairs of unshielded segments. 6. A distributed phase RF coil arrangement for at least one of exciting a sample under NMR analysis with an oscillating magnetic field such as an RF field and for detecting nuclear magnetic resonance of said sample, and adapted for a relatively high level of magnetic coupling with said sample, which comprises: : a) a coil support defining an NMR sample region therewithin; b) electrically conductive means for shielding a portion of the-NMR sample region from RF irradiation, said shielded portion of the NMR sample region defining therewith in a- first inactive portion and thereby delimiting the non-shielded active portion of the sample region; c) means electrically connected to said shielding means for decoupling a sample within the active portion of the NMR sample region from electric fields of the coil generated by an RP magnetic field and for shielding said sample from axial components of said RF magnetic field and from electrostatic atmospheric noise; and d) an electrically conductive loop peripherally disposed about said coil support having alternating shielded and unshielded segments for magnetically coupling normal components of said RF field with said sample wherein: i) said shielded segments define inactive regions of said conductive loop and connect pairs of unshielded segments; and ii) said unshielded segments define active regions of said conductive loop, said unshielded segments being disposed about the active portion of the NMR sample region in generally axial relation with said coil former adapted for current propagation along each of said active region segments such that the phase delay between adjacent pairs of active segments of said coil is generally proportional to the angular displacement between said pair of active segments, thereby generating a magnetic field having a relatively substantial normal component throughout the active portion of said NMR sample region. 7. A distributed phase RF coil arrangement according to claim 6 wherein said coil support is a cylindrically shaped insulator. 8. A distrlbuted phase RF coil arrangement according to claim 6 or 7 wherein said decoupling means comprises a plurality of wires wound about said coil support about the active portion of the NMR sample region. 9. A distributed phase RF coil arrangement according to claim 6, 7 or 8 which further-comprises a second electrically conductive loop peripherally disposed about said coil support having alternating shielded and unshielded segments for magnetically coupling normal components of said RF field with said sample. 10. A distributed phase RF coil arrangement according to any of claims 6 to 9 wherein the normal component of said magnetic field is generally uniform through said NMR sample region.

Inventors: GAUSS, ROBERT CHARLES, HINSHAW, WALDO STEPHEN
Applicants: TECHNICARE CORPORATION
Application year: 1981
Main classification: G01N24
IPC codes: A61B10, G01R33, H01F5, G01N24, A61B5