Abstract:
An NMR probe, lacking electrical balance in respect of the RF coil thereof, includes a conducting shield of axial extent greater than λ/4 from the ground plane closure of said shield of the probe (λ being the highest resonant frequency of interest), the components of the probe other than the coil being confined to the axial region less than λ/4 from the ground plane and spaced apart from the shield.

Description:
FIELD OF THE INVENTION 
     The invention is in the field of magnetic resonance, in general, and specifically pertains to improved balanced circuit performance for magnetic resonance probes. 
     BACKGROUND OF THE INVENTION 
     Coupling an RF source through a transmission line to a load traditionally involves a careful matching of the load impedance to the transmission line impedance, which is again matched to the RF source. Tuning the load to the RF source frequency is a separate degree of (RF) freedom obtained through an independent adjustment. Both of these operations maximize the dissipation of the available RF power in the load. The impedance and frequency response are not the sole features which demand attention, especially as the wavelength approaches the physical dimensions of the load. It is well known in the radio communications arts that when the load is an antenna, the resulting radiation field is the ultimate design goal and the field distribution and polarization distribution will be severely affected by the electrical symmetry properties of the radiator. It has long been known that the radiation field symmetry properties are disturbed for the case where an unbalanced source drives a balanced load, or a balanced source drives an unbalanced load. A balanced load is one wherein there exists a plane of electrical symmetry such that this locus may be characterized by a static electrical potential, e.g., a virtual ground. A transmission line of choice for contemporary installations is the coaxial cable, which presents an electrically unbalanced symmetry with respect to ground. The NMR coil, as thus driven, is unbalanced unless additional circuit elements are present to restore balance. One effect of imbalance in the load is that in addition to radiation from the load, radiation occurs from this unbalanced transmission line which now supports asymmetrical RF currents, and the geometrical properties of the net radiation field will be distorted. In the time reversed case where the NMR probe is receptive to resonance de-excitation, from a sample within the coil, the resulting signal will be similarly degraded. 
     RF communications technology has long dealt with the problem through interposition of a balanced/unbalanced conversion device, in popular parlance, a “balun”. At lower frequencies this took the form of a transformer circuit, center tapped for the balanced terminals and driven from the unbalanced terminals. At higher frequencies, e.g., hundreds of MHz, transformer coupling is not practical. The prior art has developed several approaches to balun devices and these are summarized by Stutzman and Thiele, “Antenna Theory and Design”, pp.183-187 (John Wiley and Sons, 1998). 
     A balanced NMR probe is desirable for decreasing the potential differences between portions of the coil (load) and ground and especially desirable as the wavelength associated with probe operation approach the physical dimensions of the load. A balanced probe yields a symmetric field distribution whereas the unbalanced circuit distributes the RF magnetic field asymmetrically and therefore, non-uniformly over the sample volume. Moreover, an unbalanced circuit imposes the full potential difference from maximum to ground across the sample, resulting in a larger RF electric field leading to undesirable sample heating, greater likelihood of arcing to nearby surfaces, and higher voltage tolerances upon capacitors in the circuit. Balanced NMR probes are well known, but these are commonly achieved with additional, usually lumped, circuit components. Murphy-Bosch and Koretsky, J. Mag. Res., v.54, 526-532 (1983); Probe structure often includes aspects that contribute to imbalance, e.g., internal unbalanced transmission line structures. The investigation of magnetic resonance in solid samples often utilizes transmission line components to realize high RF power required for these samples. Without additional circuit elements, a multi-resonant probe may exhibit excellent tuning and matching at its several ports while remaining essentially imbalanced. With differing RF field distributions, the concurrent resonant excitations will be spatially distributed differently, and thus the interaction of the resonant spin systems will be reduced. 
     SUMMARY OF THE INVENTION 
     Among the several objects and advantages of the invention, there is provided a balanced NMR probe achieved by surrounding (laterally and at the ground plane with an open opposite end) an unbalanced probe structure including coaxial transmission lines having outer conductor(s) with a conducting surface spaced from any outer conductor(s) and extending from the ground plane of the unbalanced probe structure by an amount λ/4 where λ is the wavelength associated with the resonant frequency of the probe. (The term λ/4 is used in an approximate sense for reasons which will be discussed below.) The tuning and matching components of the probe circuit are constrained to placement within the resonant λ/4 length of the above mentioned conducting material with the coil disposed just beyond the λ/4 extension. This surrounding conducting material, of at least S/4 extension, functions as a balun and imposes an RF symmetry on the circuit by providing (crudely speaking) virtual parallel λ/4 transmission lines to ground on either side of the coil. The surrounding conductor desirably extends further than λ/4 to provide RF shielding in the usual manner. 
     When the coil is multiply tuned (additionally resonant at much lower frequency), the low frequency resonance (λ much larger than dimensions of the probe), the lower frequency RF magnetic field intensity is distributed as a half sinusoid over the coil dimensions. The symmetrization of the high frequency resonant mode aligns the high frequency RF field as a half sinusoid superposed on the low frequency distribution. Thus, experiments based upon interaction of the respective resonating spin systems are optimized. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 depicts the system forming the context of the invention. 
     FIG. 2 a  is an NMR probe of prior art. 
     FIG. 2 b  shows the improved NMR probe of the invention. 
     FIG. 3 a  is an equivalent circuit for FIG. 2 a.    
     FIG. 3 b  is the equivalent circuit for the inventive embodiment of FIG. 2 b.    
     FIG. 4 a  compares the axial RF magnetic field distributions for a double resonant probe of prior art. 
     FIG. 4 b  is the same comparison as FIG. 4 a  for the present invention. 
     FIG. 4 c  is the same as FIG. 4 a  for a prior art probe with variable pitch solenoid. 
     FIG. 4 d  is the same comparison as FIG. 4 b  for the present invention with a variable pitch solenoid. 
     FIG. 5 is another embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In FIG. 1 there is shown in block form a typical NMR apparatus. A magnet  10  having bore  11  provides a main magnetic field. In order to control the magnetic field with precision in time and direction, there are provided magnetic field gradient coils (not shown). These are driven by gradient power supplies  16 ,  18  and  20 , respectively. Additionally, other shimming coils (not shown) and power supplies (not shown) may be required for compensating residual undesired spatial inhomogeneities in the basic magnetic field. An object for analysis (hereafter “sample”) is placed within the magnetic field in bore  11  and the sample is subject to irradiation by RF power, such that the RF magnetic field is aligned in a desired orthogonal relationship with the magnetic field in the interior of bore  11 . This is accomplished through a transmitter coil  12  in the interior of bore  11 . Resonant signals are induced in a receiver coil, proximate the sample within bore  11 . The transmitter and receiver coils may be the identical structure, or separate structures. 
     As shown in FIG. 1, RF power is provided from first transmitter  24   a , and is amplified by an amplifier  31  and then directed via transmit/receive (T/R) isolator  27  to the probe  12  that includes RF transmitter coil located within the bore  11 . The transmitter  24  may be modulated in amplitude or frequency or phase or combinations thereof, either upon generation or by a modulator  26 . Additional transmitter  24   b /modulator  26   b  components are often employed to independently excite different gyromagnetic resonators, e.g., protons and C 13 . These independent excitations are conveniently supported by a multiply resonant coil as described herein. Transmit and receive functions are not concurrently active. The identical coil within probe  12  may be employed for both functions if so desired. Thus, the T/R isolator  27  is provided to separate the receiver from the transmitter. In the case of separate transmitter and receiver coils, element  27  will perform a similar isolation function to control receiver operation. 
     The modulator  26  is responsive to controller  38  including pulse programmer  29  to provide RF pulses of desired amplitude, duration and phase relative to the RF carrier at preselected time intervals. The pulse programmer may have hardware and/or software attributes. The pulse programmer also controls the gradient power supplies  16 ,  18  and  20 , if such gradients are required. These gradient power supplies may impose gradient pulses or maintain selected static gradients in the respective gradient coils if so desired. 
     The transient nuclear resonance waveform is processed by receiver  28  and further resolved in phase quadrature through phase detector  30 . The phase resolved time domain signals from phase detector  30  are presented to Fourier transformer  32  for transformation to the frequency domain in accordance with specific requirements of the processing. Conversion of the analog resonance signal to digital form is commonly carried out on the phase resolved signals through analog to digital converter (ADC) structures which may be regarded as a component of phase detector  30  for convenience. 
     It is understood that Fourier transformer  32  may, in practice, act upon a stored (in storage unit of processor  34 ) representation of the phase resolved data. This reflects the common practice of averaging a number of time domain phase resolved waveforms to enhance the signal-to-noise ratio. The transformation function is then applied to the resultant averaged waveform. Display device  36  operates on the acquired data to present same for inspection. Controller  38 , most often comprising one or more digital processors, controls and correlates the time critical operations, such as the performance of pulse sequences. Controller  38  ordinarily incorporates an independent time base for maintaining synchrony with resonant spin systems. Overall operation of the entire apparatus within processor  34  includes input  37  from operating personnel, non-time critical calculation and output for further processing or display. 
     FIG. 2 a  is a representational illustration of a known NMR probe (here, a dual resonant circuit) that employs transmission line tuning. The corresponding equivalent circuit is shown at  3   a . While nominally a balanced circuit in form, the transmission line elements  52  and  54  are quantitatively distinct to serve their respective tuning/matching functions and RF balance is perturbed. (It is known to provide trimming networks of capacitors, for example, which restore balance.) Moreover, the intrinsic nature of unbalanced transmission lines introduces imbalance to the probe as a whole. The prior art probe of FIG. 2 a  employs a coaxial line  82  shorted at one end and length slightly more than λ/4 (resonant frequency=600 MHz) to drive a load, here, solenoidal coil  58  disposed proximate the open aperture of the λ/4 stubs  82  and  84 . In one example, the coil  58  is wound from wire approximately 0.2λ in length. A variable tap point  60  is provided for impedance matching to a 50Ω feed for RF (600 MHz) power applied at F 1 . In order to tune the transmission line  82 , a sliding dielectric tuning slug  68 ′ is provided. A sliding ground plane is an alternate way to accomplish this function. At the open end of transmission line  82 , the center conductor  64  connects to one end  66  of the solenoid  58 . In similar fashion, dielectric tuning slug  68  is provided to adjust the capacitance of transmission line  84 , the center conductor  70  of which connects to the other end  72  of solenoid  58 . This low impedance transmission line  84  provides sufficient shunt capacitance to resonate coil  58  at the desired lower frequency F 2 , matched to its low frequency source with coupling capacitor  81 . A shield  74  extends axially to limit radiation, in the region of coil  58  per conventional usage. Dual resonant probes of this form have been made and sold by Varian Chemagnetics and its predecessors under the product designation “T 3 ”. 
     In discussions related to multi frequency probes implemented with λ/4 transmission lines, the actual transmission line is usually not exactly λ/4. At resonance such a line presents an effective open circuit to the resonant traveling wave. Thus in FIG. 3 a  for example, the transmission line  84  providing capacitance to ground on the undriven side of coil  58  must not present an excessive impedance to the F 1  port. It is common in such circumstance to reduce (or increase) the length of the actual λ/4 line by 10%-20% to provide an adequate path to ground for the RF (high frequency) power applied to the F 1  port. Lumped capacitance may be added to the affected leg of the circuit to improve the efficiency of the circuit. 
     Turning now to FIG. 2 b , the invention is implemented in a preferred embodiment by introducing a spacing  76 , of length at least λ/4, between shield  74  and the transmission line structures  82  and  84  of the prior art probe whereby the RF field in the general space  76  is perturbed to remove the outer conductors of the transmission lines  82  and  84  from ground at the high frequency resonance F 1 . The shield  74  thus forms an outer conductor of an effective open transmission line of length λ/4 with a load (coil  58 ) disposed axially beyond the aperture (at λ/4). The conducting sleeve  74  may be extended axially beyond the λ/4 point to function as an RF shield for the coil  58 . 
     FIG. 3 a  is a simplified equivalent circuit for the prior art probe of FIG. 2 a . At the outset it is recognized that lumped component representations are mere cartoons of the complex traveling wave dynamics of these circuits: however, the main points of the discussion are advanced thereby. A tapped transmission line  82 - 82 ′ provides a high frequency input at the tap point  60  for excitation of the tuned circuit comprising coil  58  through low impedance Z 1 . (predominantly distributed capacitance) to ground. Concurrently low frequency RF is introduced through matching capacitor  81  through coil  58  to ground through Z 2 . The impedances Z 1  and Z 2  each include all couplings to ground  110 . These couplings are predominantly frequency dependant and may also include parasitic effects. The low impedance tuning capacitor  84  is such that that one may regard the proximate end of coil  58  to be at ground potential shorting Z 1 . The circuit is inherently asymmetric. Consider now the inventive arrangement as illustrated in FIG. 3 b  wherein the local ground  112  is isolated from ground  110  by the λ/4 line  174  which presents a very high impedance at frequency F 1 . The impedance Z 1  is no longer shorted to ground and coil  58  is essentially flanked by impedances Z 1  and Z 2  with the result that for substantially equal total impedances to the right and to the left of coil  58 , a virtual ground plane is established in the axial median plane of the coil  58  intermediate its ends. One may regard the functional effect of the λ/4 stub  174  as restoring balance to the previously unbalanced circuit of FIG. 3 a.    
     As the wavelength of the RF excitation supported by the probe approaches the dimensions of the load, the distortion of the RF field distribution in the near field region becomes significant, as parasitic currents, depending upon frequency dependent effects, become large. The effect of the λ/4 length shield (disposed as described with respect to the probe circuit components) serves two functions: it is the outer conductor of a λ/4 transmission line stub, and in its further extension, is an effective RF shield. 
     The prior art NMR probe provides a graphic demonstration of the effect upon the high frequency RF field distribution for 600 and 150 MHz as the resonant wavelength approaches the dimensions of the load. The curves there shown are the normalized RF magnetic field distributions on the axis of the known NMR probe of FIG.  2 A and the difference in RF magnetic field intensity at the two frequencies is shown as the field deviation. The curve labeled 600 MHz (H 1 ) is shifted substantially with respect to the 150 MHz (C 13 ) curve. It should be readily apparent that the deviation measures a degree of loss in efficiency where experiments depend upon the mutual excitation of both H 1  and C 13  such as cross polarization and the like. The displacement of these distributions is clearly disadvantageous for experiments where there is an interaction between hetronuclear spins (such as H 1  and C 13 ). Various polarization transfer experiments are easily seen to suffer from this lack of alignment of the RF field distributions for the two resonant nuclei. These field distributions are obtained by a field perturbation method well known in the art. (see Ginzton, Microwave Measurements, McGraw Hill Book Co., Inc. 1957). For an NMR measurement, the known probe of FIG. 2 a  achieves a signal-to-noise ratio (SNR) for hexamethylbenzene of 197 in a cross polarization magic angle spinning experiment of 4 scans and 3KHz spinning rate. 
     The present invention provides alignment of high frequency (600 MHz) with low frequency (150 MHz) RF current distributions as shown in FIG. 4 b . At 150 MHz, the (low frequency) RF current distribution is substantially unaffected by the presence of the λ/4 line formed by shield  74  and spacing  76  from the probe circuit components. That is, the parasitic effects in this apparatus are negligible compared to those obtaining at the high frequency. This low frequency is such that the associated wavelength is large compared with dimensions of the load and the parasitic effects are negligible. The corresponding NMR measurement demonstrates a value of SNR=320 for an improvement of 62% as a direct result of the improved field overlap. The general result of this invention applies to any pair of resonant frequencies having a sufficiently large ratio wherein the shorter wavelength resonance is asymmetrically distributed along the load while the longer wavelength resonance remains distributed with substantial symmetry. That is, the parasitic effects are relatively negligible for the low frequency resonance and quite substantial for the high frequency resonance. The invention is applied in this circumstance to re-align the high frequency RF field distribution along the axis to match the low frequency RF field distribution. The range of value for such ratio is not so much a quantitative specification as it is a specification of the context in which the multi-resonant circuit operates and the sensitivity of associated apparatus. 
     FIG. 4C is similar to FIG. 4A with the difference that the solenoidal coil  58  (FIG. 2A) is replaced by a variable pitch solenoidal coil. Such variable pitch coils exhibit a less dense axial winding density in the region intermediate the end regions of the coil as compared with the winding density in these end regions, in order to obtain improved axial RF field homogeneity. It is apparent that the axial RF field distribution at high frequency is quite distorted whereas the same coil in the inventive structure supports a substantially symmetrized, e.g., balanced RF field distribution as shown in FIG.  4 D. 
     Turning now to FIG. 5, an alternate embodiment is shown wherein the tap  60 ′ is provided to drive the outer conductor  83  of transmission line  82  (instead of providing the tap to the inner conductor  64  of FIG. 2 b ). 
     In another embodiment, it is recognized that at sufficiently high frequencies of operation, it is desirable to restore balance to each of the ports of a multi-resonant probe, when the lower frequency field distribution is also substantially distorted by parasitic effects. 
     The obvious case of physical interest in the field of NMR occurs for  1 H and  13 C for which the nuclear magnetic moments, and thus the resonant frequencies, are related by a factor of 4. Thus, in this embodiment, the shield  74  may exhibit an extension of ¾λ for the (as projected onto conductors  70  and  64 , for example) for the high (proton) frequency F 1  while concurrently presenting a length of λ/4 to the low ( 13 C) frequency, F 2 . In principle, this embodiment is effective for any pair of nuclear species wherein the nuclear magnetic moments are in the ratio of 2 −2N . 
     It is apparent that the ability to superimpose the RF current distributions (and hence, the corresponding RF magnetic fields, B 0 ,) for concurrently excited nuclei, maximally fulfills the Hartman-Hahn condition for interaction of two (or more) spin systems. 
     Independently, the symmetrization of the RF magnetic field about a median plane of the coil, reduces the magnitude of variation of B 1  in a central region of the sample. This reduction is owing to the symmetrization of the z dependence of the RF magnetic field on opposite sides of the median plane. 
     In the particular application for NMR probes, it is especially useful that the desirable result is obtained by a single, simple structural entity with minimal disturbance of the circuit. this single structural entity, e.g., a conductive sleeve, offers both implementation of a λ/4 open transmission line and, in further axial extension, an effective RF shield. 
     Although this invention has been described with reference to particular embodiments and examples, other modifications and variations will occur to those skilled in the art in view of the above teachings. For example, much of the discussion has employed the example of a dual resonant probe. The invention may be usefully employed to restore electrical balance to single, triple, or other multiple resonant NMR probes. It should be understood that, within the scope of the appended claims, this invention can be practiced otherwise than as specifically described.