Abstract:
An insertable wand is used in a multi-nuclear NMR probe enabling the probe to detect and obtain data from various combinations of nuclei. The particular combination of frequencies is determined by various electrical components, and parts within the wand that are designed to cooperate with a tube, to form an adjustable ¼ wave assembly. The adjustable ¼ wave assembly component in the wand comprises a metal rod with an adjustable conductive collar and spring contacts such that when inserted into the tube, the rod and the tube to form an adjustable ¼ wave circuit or a ¼ wave shorted stub. The tube may form part of the wand or the probe. When the wand is plugged into the probe, the combination of the NMR coil within the probe and the adjustable ¼ wave shorted stub provides means for the NMR circuit to resonate at two separate frequencies.

Description:
FIELD OF THE INVENTION 
     This invention is in the field of nuclear magnetic resonance (NMR) apparatus and relates to an adjustable plug-in wand to facilitate either single-frequency operation or multi-nuclei operation of the probe wherein the particular choice of nuclei is determined by the plug-in wand. 
     BACKGROUND OF THE INVENTION 
     NMR spectrometers typically include a probe containing the sample to be analyzed, a superconducting magnet for generating a static magnetic field B 0 , and console unit containing the electronic equipment needed to operate the spectrometer system. The probe contains one or more radio-frequency (RF) coils surrounding the sample for generating time-varying magnetic fields B 1  perpendicular to the static magnetic field B 0 . For multi-nuclear probes several different RF fields may be applied simultaneously or consecutively to stimulate the resonance of two or more nuclei which may be in the sample. 
     The multiple tuning of the probe is achieved with the aid of one or more additional RF coils and capacitors that are removed physically from the RF coils containing the NMR sample. Typically a spectrometer system is designed to detect protons and deuterium, which is used for set-up and spectrometer frequency control. These two frequencies may be provided by one RF coil by double tuning. Another coil disposed at right angles to this first coil may be double tuned to detect phosphorus-31 and carbon-13. It is often desirable to use the same probe to detect a different second set of nuclei, for example sodium-23 and chlorene-35. Existing probes are generally constructed to operate on a pre-selected one or two frequencies (in addition to protons and deuterium), therefore two or more probes are needed for applications requiring up to four additional frequencies. 
     U.S. Pat. No. 5,982,179 “NMR Circuit Switch”, assigned to the Assignee of the present invention, describes an NMR probe with a stepped cavity for locating switch components therein. For single frequency operation a capacitor switch is activated upon insertion of a capacitor stick. For double frequency operation a ¼ wave stick incorporating a metallic threaded screw that closes a threaded connection switch thereby connecting capacitors internal to the probe and the ¼ wave center connector of the switch to operate cooperatively to permit double frequency operation. To permit operation at different pairs of frequencies an extension stick is used in place of the ¼ wave stick and an external section of wave tube is mounted on the extension stick wherein part of the wave tube is in the probe and part on the stick. For each pair of additional frequencies a different stick and extension tube is required. 
     SUMMARY OF THE INVENTION 
     There is therefore a need for a single probe design that facilitates single tune and double tune operation of any pre-selected set of nuclei, that is capable of operating at one or two pre-selected sets of frequencies. 
     It is a feature of the present invention to provide a probe with a set of wands, each of which provides the tuning for a single or for double frequency operation. The wands for double frequency operation have one or more capacitors and a central conducting rod with an adjustable conductive collar having spring finger contacts that in cooperation with a conducting tube form an adjustable ¼ wave stub and produce double tune operation. The tube may either be fixed to the wand or to the probe. Wands for different set of double frequencies contain capacitors with different values and with the conductive collar set to different positions. 
     In one embodiment of this invention, the wands for different single frequencies or different sets of double frequencies may be identical in their construction making them easier to construct. The values selected for capacitors within the wand, the way of their connection, and the position of the adjustable collar determine the different frequencies. Single frequency operation is obtained by different electrical connections within the wand. In a preferred embodiment clips hold the capacitors, permitting them to be changed. An electrical jumper may be used in place of a capacitor, or the clip may be left vacant thereby, changing the circuit configuration. 
     The insertion of the wand into the probe requires no turning or rotating. The wand is inserted directly in the probe: the end of the wand plugs into a keyed electrical socket within the probe. The electrical plug and socket permits changing the probe configuration and operating frequencies, without requiring the wand to be rotated. All the wands can be of the same size and length and still have means for providing for different resonant frequencies of the ¼ wave shorted stub. 
     An additional feature of the present invention is the low manufacturing cost of the probe and wand, as the mechanical parts of all probes and wands are identical. The electrical socket within the probe and mating electrical plug on the wand provides a low cost method of electrically coupling the wand to the probe. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A schematically shows a ¼ wave assembly component contained in a wand. 
     FIG. 1B is a detailed view of a conductive collar part of the wand. 
     FIG. 1C schematically shows a conductive tube surrounding the ¼ wand assembly. 
     FIG. 2A schematically shows clips used to make electrical contact and retain the capacitors and/or conducting jumpers within the wand. 
     FIG. 2B is a side view of a pair of clips connecting to, and mechanically supporting, a capacitor. 
     FIG. 3 schematically shows a conductive tube attached to an electrical socket which is located within the probe that receives the wand. 
     FIG. 4 schematically shows an electrical circuit contained within the wand. 
     FIG. 5 schematically shows the electrical circuits contained in the probe. 
     FIG. 6A is a schematic of a first circuit configuration within the wand for single frequency operation. 
     FIG. 6B is a circuit of the equivalent circuit for a probe and wand, with the wand configuration of FIG.  6 A. 
     FIG. 7A is a schematic of a second circuit configuration within the wand for single frequency operation at a lower frequency. 
     FIG. 7B is a circuit of the equivalent circuit for a probe and wand, with the wand configuration of FIG.  7 A. 
     FIG. 8A is a schematic of a first circuit configuration within the wand for double frequency operation. 
     FIG. 8B is a circuit of the equivalent circuit for a probe and wand, with the wand configuration of FIG.  8 A. 
     FIG. 9A is a schematic of a second circuit configuration within the wand for double frequency operation. 
     FIG. 9B is a circuit of the equivalent circuit for a probe and wand, with the wand configuration of FIG.  9 A. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1A illustrates the adjustable wand  10  used to select the frequency range of operation of a probe and thereby select the particular nuclei the probe will detect. It determines whether the RF probe coil tunes to a single frequency or is tuned to two frequencies. Double frequency operation is achieved using a ¼ wave assembly. The ¼ wave assembly comprises conducting rod  11 , shorting stub  12  and conducting tube  37  of FIG.  1 C. Shorting stub  12  comprises collar  13 , spring contact  15 , and set screw  17 . Conducting rod  11  and collar  13  are made of metals with high electrical conductivity such as copper or aluminum. Spring contact  15  may consist of a helical wound coil spring made of a non-magnetic spring metal such as phosphor bronze or non-magnetic stainless steel. 
     FIG. 1B is a plan view of collar  13  and spring contact  15 . When the wand is inserted into conducting tube  37 , the spring loops are compressed making good electrical contact between collar  13  and conducting tube  37 . Alternatively a series of spring contact fingers (not shown) could be used to provide electrical contact between collar  13  and conducting tube  37  of FIG. 1C. A handle  27  is located at the lower end of conducting rod  11  to facilitate the insertion of the wand into the probe. 
     The upper end of conducting rod  11  supports platform  19  and electrical plug  20 . Electrical plug  20  comprise pins  21 ,  22 ,  23  (labeled  1 ,  2 , and  3  respectively) and insulating plug body  25 . The geometrical arrangement of the pins is such that there is only one orientation in which pins will match the corresponding socket on the probe. Platform  19  is made of a dielectric material and is fixed to the end of conducting rod  11 . Platform  19  supports up to three capacitors  31 ,  32 ,  33 . For some wand configurations, electrical jumpers replace one or more capacitors or one or more capacitors are left out with no connection as is explained below. Capacitor  31  is connected between pin  21  and conducting rod  11 ; capacitor  32  between pins  21  and  22 ; capacitor  33  between pin  23  and conducting rod  1 . In one embodiment for single frequency operation a dielectric rod replaces conducting rod  11 , and in this case one side of capacitors  31  and  33  are still connected together. 
     FIG. 1C is a perspective view of a preferred embodiment wherein conducting tube  37  is fixed to wand  10 . In this embodiment the position of conductive collar  13  is first adjusted to the required position and then conductive tube  37  is slipped over wand  10  and secured by machine screws  36  threaded into insulating plug body  25  of electrical plug  20 . 
     FIG. 2A is a preferred embodiment incorporating an electrical printed circuit  64  printed on platform  19 . Small spring clips  60  hold capacitor  31 ,  32 , and  33  in place and provide electrical contact to them. FIG. 2B is a side view showing capacitor  32  supported by two spring clips  60 . Spring clips  60  are soldered or otherwise fixed to printed circuit  64 . Electrical contact is provided by extension of plug pins  21 ,  22 , and  23  down through holes  61 ,  62 , and  63  respectively, where a solder connection is made to the printed circuit  64 . In some configurations one or more clips are left empty, and some clips require an electrical jumper (not shown) to be inserted in place of one or more capacitors. The jumper is a metal object with the same dimensions as a capacitor so it fits within the same space and can be held by clips  60 , to provide a low resistance connection between the two clips. Specific circuit configurations of the wand are shown in FIGS. 6A through 9B. The spring clip mountings permit the wand configuration to be easily changed. Alternatively surface amount capacitors may be used and soldered directly to the printed circuit eliminating the need for spring clips  60 . Similarly electrical jumpers may also be soldered directly to the printed circuit, or the connection may be left open. 
     FIG. 3 is preferred embodiment wherein conducting tube  37  is fixed to electrical socket  38 , which is mounted in the probe. Electrical socket  38  comprises connector receptacles  41 ,  42  and  43  (labeled  1 ,  2 , and  3 ) keyed to receive respective pins  21 ,  22  and  23  of electrical plug  20 . Connector receptacles  41 ,  42 , and  43  are held in place by insulating material  39  of electrical socket  38 . Conducting tube  37  is fixed to electrical socket  38  by machine screws  36  that screw into tapped holes in insulating material  39 . Conducting tube  37  is a metal tube made of copper or some other metal of high electrical conductivity. Electrical connection is made between conducting tube  37  and connector receptacle  43  (See FIG.  5 ). When adjustable wand  10  is inserted into conducting tube  37  as shown in FIG. 4, spring  15  establishes a low conductivity electrical connection tube  37  and collar  13  and in cooperation with conducting rod  11  form a ¼ wave shorted stub. 
     FIG. 4 is a schematic diagram of the electrical circuit within the wand  10  showing capacitors  31 ,  32 ,  33  and their connections to pins  21 ,  22 ,  23  (labeled  1 ,  2 ,  3  on the plug) and to rod  11 . In the preferred embodiment spring clip connectors  60  are used to hold the capacitors and to permit their ease of replacement, and for the insertion of an electrical jumper in the place of a capacitor, and for the establishment of an electrical open circuit. 
     FIG. 5 a schematic diagram of the probe incorporating conducting tube  37  with electrical socket  3 S that receives the wand plug and the remaining electrical circuit of the probe. The NMR sample is contained in within NMR probe coil  45  that is located in the magnet in the region containing the most homogeneous magnetic field B 0 . The outer shell of conducting tube  37  and connector receptacle  43  are connected to probe ground  50 . (If the conducting tube  37  is mounted on the wand its electrical ground is established tough pin  23  which plugs into connector receptacle  43  and thereby to probe ground  50 ). One terminal of circuit variable capacitor Cs  47  and one terminal of wave variable capacitor Ct  46  are connected to probe ground  50 . Input and out signals from and to the console (not shown), are made via a coaxial cable connected to probe cable connector  49 . Shield connection  51  of probe cable connector  49  goes to probe ground  50 , and the electrically active center wire  52  connects to one side of match variable capacitor Cm  48 . The other terminal of match variable capacitor Cm  48  connects to the ungrounded terminal of circuit tune capacitor Cs  47 , to connector receptacle  42  and to the probe coil  45 . The other side of probe coil  45  is connected to the ungrounded terminal of wave variable capacitor  46 , and to connector receptacle  41 . 
     The connection arrangement contained in the wand determines whether single frequency of double frequency operation is selected and the values of capacitors contained in the wand determines which nuclei will be detected by the probe. The following figures illustrate how single and double frequency operation is determined and the capacitors that determine the operating frequencies. FIGS. 6A-B and  7 A-B show the connection arrangement for single frequency operation and FIGS. 8A-B and  9 A-B for double frequency operation. 
     Single frequency operation of FIG. 6A is achieved by placing electrical jumpers  131  and  133  in place of capacitor  31  and  33  of FIG.  4 . Connections to the probe are made through pins  21 ,  22 ,  23  (labeled  1 ,  2 ,  3  respectively). The circuit is left open in place capacitor  32  of FIG.  4 . This combination of connections within the wand when inserted into the probe yields a first circuit configuration for single frequency operation. 
     FIG. 6B is the electrical circuit resulting from the connections selected in FIG.  6 A. In this selection the ¼ wave shorted stub is not in the circuit, and is in fact shorted out by jumper  133  of FIG.  6 A. If desired a non-conducting rod  111  could replace conducting rod  11  as it does not enter the circuit. Collar  13  with spring contact  15  may also be eliminated. Conducting tube  37  may also be eliminated in the embodiment where tube  37  is normally attached to plug insulator  25  of wand  10 . NMR probe coil  45  is tuned by capacitor Cs  47  and matched by capacitor Cm  48 . Connection to the console (not shown) are make through coaxial cable connector  49 . Shield connection of connector  49  is attached to probe ground  50 . 
     FIG. 7A is an alternative wand configuration for single frequency operation. It is identical to the wand of FIG. 6A with the change that capacitor  32  is fixed therein. Capacitor  32  is introduced in place of the empty clip of FIG.  6 A. This has the property of lowering the resonant frequency of the Probe. For example, at a field strength of 9.4 T (400 MHz proton field), the circuit of FIG. 6B might be tuned for phosphorus 31 at 162 MHz. Carbon 13 at 100.6 MHz could be observed with the same probe using the wand of FIG. 7A by proper choice of capacitor  32 . In this configuration ¼ wave shorted stub is not in the circuit being shorted out by electrical jumper  133 . Electrical jumper  131  in series with jumper  133  shorts out capacitor Ct  46  of FIG.  5 . Plug pins  21 ,  22 ,  23  (labeled  1 ,  2 , and  3  respectively) furnish electrical connection means to the probe. The equivalent circuit FIG. 7B shows capacitor  32  is in parallel with circuit variable capacitor Cs  47 . The other components of FIG. 7B are identical with those of FIG. 6B, with NMR probe coil  45 , matching capacitor Cm  48 , connector  49  and ground  50 . 
     FIG. 8A is a wand configuration for a double tuned circuit. Here the jumper  133  of FIG. 7A is removed thereby unshorting the ¼ wave structure Capacitors  31  and  33  optimize the coupling of the ¼ wave structure to the probe circuit pins  21 ,  22 ,  23  (labeled  1 ,  2 ,  3  respectively) provide coupling between the wand and the probe circuit. FIG. 8B is the equivalent circuit for the combination of wand configuration of FIG.  8 A and the probe circuit (FIG.  4 ). In this configuration length of the ¼ wave shorted stub is adjusted by moving shorting stub  12  (FIG.  1 A). This is done by loosening set screw  17  and moving collar up or down on rod  11  to the desired position and then tightening set screw  17  (FIG. 1A,  4 ). Further adjustment is achieved by wave variable capacitor Ct  46 . Conducting rod  11 , shorting stub  12  and conducting tube  37  comprize a ¼ wave structure. Capacitors  31  and  33  optimize the coupling of the ¼ wave structure to the probe circuit. Probe circuit further comprizes NMR probe coil  45 , wave variable capacitor Ct  46 , circuit variable capacitor Cs  47  and match variable capacitor Cm  48 . Connection to the console (not shown) is made through coaxial cable connector  49 . Probe ground connections  50  are made connector  49 , conducting tube  37 , and capacitors  33 , Ct  46 , and Cs  47 . 
     FIG. 9A is an alternative wand configuration for a double timed circuit. Capacitor  32  is introduced in place of the empty clip of FIG.  8 A. The other components of FIG.  9 A and their labeling is unchanged from FIG.  8 A. The additional capacitor  32  has the property of lowering the resonant 
     frequencies of the probe. The equivalent circuit FIG. 9B shows capacitor  32  is in parallel NMR probe coil  45 . The other components of FIG.  9 B and their labelings are unchanged from FIG.  8 B. 
     Although the invention has been described herein in its preferred form, those skilled in the art will recognize that many variations and magnifications may be made thereto without departing from the spirit and scope of the invention as defined in the claims.