CA1236880A - Nuclear magnetic resonance radio frequency antenna - Google Patents

Abstract

Nuclear Magnetic Resonance Radio Frequency Antenna Abstract A nuclear magnetic resonance radio frequency coil.
The disclosed coil provides high frequency resonance signals for perturbing a magnetic field within the coil The coil is impedance matched and tuned with adjustable capacitors. A balanced configuration is achieved with a co-axial cable chosen to phase shift an energization signal coupled to the coil. The preferred coil is a thin metallic foil having a shorting conductor, four wing conductors, and uniquely shaped parallel cross conductors connecting the shorting and wing conductors.
When mounted to a rf transmissive plastic substrate and energized the coil produces a homogenous field within a region of interest the size of a patient head. A semi-circular balanced feedbar arrangement is used to minimize undesired field contributions.

Description

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9-874 Description Nuclear Magnetic Resonance Radio Frequency Antenna Technical Field The present invention relates to nuclear m2gne.ic resonance imaging and more particularly to an improve resonator for applying radio frequency pulses and receive in low level RF signals over a region of interest.
Background Art . . _ In medical applications, nuclear magnetic Rosen No can indicate variations in the distribution of atomic substances in slices or volumes of interest with-in a patient. Such variations can be displayed in a wry similar to the distributions provided by a kiwi.-iced tomography system. In a nuclear magnetic resonance examination, magnetic and of fields rather than x-radi2;io.-scan the body. Resonances caused by these fields are detected as induced signals in one or more detector coil systems. The outputs from these coils are then stored and analyzed so that NOR distributions can be displayed.
Techniques for producing these images are well Nina in the art and disclosed in various printed put cations end U.S. patents. Several proposals for ape-tusk to utilize these procedures are embodied, for exar,?le,in U.S. Patent Nos. 4,454,474 to Young, 4,384,255 to Yours et at, and 4,379,262 to Young.
The techniques disclosed in the above mentioner prior art patents involve selection of a planer slice of interest in the body and application of a strong magnetic field gradient in a direction perpendicul2 I
the slice. This field is perturbed in a perpendicular direction in the plane of the slice. The direction ox the perturbation is continuously varied by a procedure documented in the literature.

, 1~3613~0 The effect of this perturbation is to introduce a dispersion in nuclear resonance frequencies which rev -.
to their original unperturbed state in ways ch2r2cteris- c of the structure within the slice of interest. eye..
of this procedure for different directions can size many signals for each slice of interest which are then used to construct cross-sectional images descriptive o' the internal structure of the patient slice.
The radio frequency perturbation excites the nuclei by realigning the macroscopic magnetization or magnetic moment within the cross-section of interest. This ratio frequency energization is performed at the Armor frozen cry. This frequency is related to a constant descriptive of the nuclei making up the region of interest end the magnetic field gradient imposed during perturbation.
Experience in NOR imaging indicates that the scan-nine times can be reduced and special resolution of OR
images can be increased by increasing this field to higher levels. Since the Armor frequency of a given nuclei is directly proportional to the field strength, this increase in field strength must be accompanied by higher frequencies for I energization. In the prior art this energization us accomplished with suitably designed energization coils which generate pert~Jrbztion fields and in some instances are also used for detecting signals caused by resonances set up within the region of interest.
Transmission and reception of radio frequency skis-nets for NOR imaging requires a resonant radi2tins sac lure, often called an of coil, meeting sever cry Thea resonant point of the structure must be high eno~c to alloy proper tuning at the frequency OX interest on the structure must have sufficiently high "Q" to Provide good signal to noise performance in the receive mode.

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Generally, in small volume nuclear magnetic resow-ante systems-an unbalanced feed system and coil configure anion is used. A simple reactive element is used as an impedance matching component and a second reactive eye-mint is used in parallel with the coil structure Titan the coil to an appropriate frequency.
For large volume nuclear magnetic resonance applique-lions, however, such as a head imaging system a balanced coil system is preferred. This is preferable since under sample loading the coil system will be less in-flounced than an asymmetrical system.
As the frequency of operation is raised, however, the effectiveness of a symmetrical matching system is limited by a variety of factors. The reactive components Buick unmanageably small and are also subjected to extremely high peak voltages. The stray capacitance of the RF coil network eventually makes it impossible to match the network to a useful impudence. In addition to problems in achieving proper energization frequencies, use of larger RF coils creates problems in achieving a uniform magnetic field over the region to be perturbed.
various prior art proposals to provide new and improved OF energization and detection coils are discus-sod in the literature. A publication entitled "Slotted Tube Resonator: A new NOR probe head at high observing frequencies" by Schneider and Dullenkopf discusses a resonator for use at high frequencies. This work was the first of a number of similar prior art publications discussing NO resonator structures. Much of this work, however, has been conducted with extremely small dime-signal structures which do not encounter the difficulties encountered when imaging a cross-section of a head.
The task of converting a resonator coil for use in small structure anuses into a device suitable for NOR medical imaging is not a straight forward extension of this prior work.

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, ,, ~Z368~0 Disclosure of Invention The invention is particularly suited or No iris of a human head. A resonator having a length and Doria of approximately 30 centimeters is constructed Jo provide a homogeneous magnetic field in the region of the head that does not unreasonably degrade with sample loading and is easily tuned over a wide frequency range.
An antenna or coil arrangement constructed in accord-ante with the invention both transmits and receives high frequency energy in the range of 30 to 95 muggier.
The disclosed resonator includes a cylindrical base of a diameter suitable for enclosing the human head and a metallic foil coupled to the base and forming the antenna structure with a resonant frequency of about 30 to I
megahertz. The self resonant frequency of the structure is well in excess of 100 megahertz. This resonant struck lure includes a pair of diametrically opposed ark electrical conductors with each conductor subtending an arc of between 75 and 85 degrees. Short circuiting conductors interconnect these conductors at one end and wing strips extend circumferential from each of the conc~c.ors at the other end of the resonator. Enrages-lion signals are applied to the resonator through conduct live 'eel skis which interconnect the wing strips.
The arrangement between feed strips, wing strips and conductor strips causes uniform magnetic field ~lthi~
the region of interest, i.e. the head. In a preferrer embodiment, the conductor strips are circumferentic i;
spaced parallel conductor strips with each strip inquiries-in in cross-sectional area from its center towards each end. This configuration of the conductor Sue us enhance_ tune uniformity o' the magnetic 'to`, w-affecting the tunability of the structure.
An interface circuit is preferably coupled between the disclosed resonator and a standard 50 ohm input .

123~8~30 cable. The resonator also acts as a pick up coil so that resonances within a patient slice of-lnterest incus electrical signals which are detected, amplified, end utilized in constructing an NOR image- To achieve Roy-impedance matching and resonance the disclosed resonators coupled to the 50 ohm input cable through three adjust.-able capacitors and a half wavelength Boolean coaxial cable.
The resonator is a quarter wavelength antenna which can ye easily tuned and matched in a transmit mode of operation and effectively coupled to a preamplifier for generating output signals for use in NO imaging.
From the above it should be appreciated that one object of the invention is a antenna structure suitable for of signal generation and reception at high frequencies with a geometry large enough for head imaging. Other objects, advantages and features of the invention will Buick better understood when a detailed description of a preferred embodiment of the invention is described in conjunction with the accompanying drawings.
Brief Description of the Drawings inure 1 is a perspective view of an NOR imaging station.
Figure 2 is a perspective view of a resonator Poe for providing of signals in the vicinity of a patient's head.
Figure 3 is a top plan view of a foil con inured to form the Figure 2 resonator.
Figure 4 is a schematic circuit diagram OX the resonator showing use of three adjustable capacitor for tuning and impedance matching.
Figure 5 is a schematic showing an entire I
coil system for both transmitting and receiving resonance signals from within a region of interest.

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~23~8~30 Figure 6 shows a filter for reducing transmitter noise. I/
Figures 7-9 are graphical representations showing magnetic field uniformity in the region encircled by the antenna.
Best Modifier Carrying out the Invention Turning now to the drawings and in particular issuer 1, an imaging station for an NOR scanner 10 is disclosed.
The scanner 10 includes a large encircling magnet 12 for generating magnetic fields of between 1.5 and 2 Tussle within a patient aperture 14. Shown positioned in proximity to the magnet 12 is a patient couch 16 having a headrest 18. The patient is positioned on the couch in a prone position and then moved into the patient aperture 14 for NOR scanning.
During a head scan a probe coil or resonator 20 is moved on rollers 22 so that the patient's head is post-toned within the coil 20. In accordance with techniques well known in the nuclear magnetic resonance imaging art, the magnet 14 is energized to produce a strong magnetic field having a gradient to selectively choose

2 slice or region of patient interest. With the probe coil 20 encircling the patient's head, the coil is ever-gibed with a high frequency (between 30 and 95 megahertz) signal which sets up-a time varying magnetic field in the region of interest. Various techniques are known within the art for pulsing the probe coil in ways to produce meaningful resonance information which can be utilized in NOR imaging. The particular configuration of the disclosed coil 20 allows high frequency enters zap lion necessary to cause resonance of the spin soys e, a the high r,agne,ic fields generated by the masse' 12.
At such high frequencies, the disclosed prove 20 produces uniform magnetic fields which do not exhibit undue Q
degradation with sample loading.

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123~80 Turning now to Figures 2-4, details of the construe-lion of the probe coil are discussed. A cylindrical base 30 formed from an acrylic material forms a surface to which a metallic foil can be affixed. The base 30 has physical dimensions such that a patients head can be inserted within the base and the probe coil 20 ever-gibed in conjunction with generation of the high strength magnetic field. Two copper foil resonator sections 32 having a thickness of .0635 millimeter are affixed to an outer surface of the base 30 in a configuration shown in Figure 2. The foils are self adhesive with a backing layer which is removed prior to application to the base 30. One of the foil sections 32 is shown in plan view prior to mounting to the substrate 30 in Figure 3. The physical dimensions of this foil are shown in that figure.
The thickness of the foil is chosen to be approxi-mutely seven spin depths at the resonant frequency.
Use of this thickness causes the resonator 20 to be essentially transparent to the high strength field grad-tents generated by the magnet 12. This minimizes thegener~tion of eddy currents within the foil by this high strength magnetic field gradient which would be undesirable since the induced eddy currents would produce their awn magnetic field in addition to the desired homogeneous of field.
Etch foil segment 32 includes a shorting strip 34 and a wins strip 36. These two strips 34, 36 are inter-connected by conductor strips 38 which are parallel to each other and nonuniform in width along their length.
Preferably these conductor strips 38 are narrow in ye middle and widen as they approach the shorting sir id 34 and wing strip 36. As seen most clearly in Fissure 2, when affixed to the outside surface of the substrate 30, the two foil segments 32 contact each other at the ends of the shorting strips 34 and define a 1 cm gay between the ends of the wing strips 36.

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lZ36880 An end portion aye of each wing 36 disconnected by a feed bar 40 having a midpoint 42 connected to an inter-face circuit 110 (Figure 5) by copper strips (not shown).
The feed bars both energize the probe and transmit resow 5 nuance signals generated from within the patient region of interest. The feed bars 40 form a semicircle 2 cm wide each of the same thickness as the foil and are mounted to an acrylic substrate.
The resonator 20 is energized with a high frequency output from a transmitter 112. A preferred transmitter is available from Amplifier Research under Model No. 2000 Lo and produces an alternating current voltage a few hundred volts in magnitude. In order to interface the resonator 20 to a standard 50 ohm unbalanced transmission line 114 a half wavelength Boolean 115 (Figures 4 and I
is utilized. The Boolean 115 is constructed from 50 ohm coaxial cable, with a velocity factor of 0.80 or 0.66.
The total length of the Boolean is 1.875 meter using a velocity factor of 0.80 at 64 megahertz. Since the signal traveling in this cable is delayed by one half wavelength the phase of the voltage at one end of the cable is 180 shifted from the other end. Thus, the voltages at each end of the cable are equal in amplitude and opposite in phase. The current at an input node 116 divides equally between the load and the Boolean phzs-in line. Thus, the resonator matching network sees a voltage double that of the input voltage, and a query, equal to half the applied current. This causes the impedance at the output of the matching network to be four times the input line impedance.
A m2tchins network 120 having three adjustable capacitors 122, 12~, 126 is used to tune the resonator 20 and impedance match the high impedance resonator 20 with the 200 ohm balanced input. Model Number COCA 125 vacuum capacitors from ITT Jennings of San Jose, CalifOrnL2 .. .

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are preferred. Representative values of these capacitors are 60 pick farads for the parallel capacitor 122 and 12 picofarads for the two series capacitors 124, 126.
These values are representative and are tuned to optimize performance of the resonator 20.
To utilize the resonator 20 as both a transmitter and receiver, a multiplex circuit 130 (Figure 5) couples the resonator and balancing network to both the transmitter 112 and a preamplifier 132. The multiplex network includes a plurality of diodes 134 and two quarter wave-length cables 136, 138.
In the transmit mode the large magnitude signals from the transmitter 112 forward bias the diodes 134.
The charter wavelength cable 136 consumes no net power since the cable inverts the terminating impedance and no signal from the transmitter reaches the preamplifier 132.
In a receive mode the goal is to couple induced signals in the resonator 20 to the preamplifier 132.
these signals see a half wavelength cable since the two qua.terw~ve cables 136, 138 act as a single half wave cable.
A noise filter circuit 140 figure 6) couples the transmitter 112 to the multiplexer circuit 130 and in-eludes a plurality of diodes 142 and two quarter wave-length cables 144, 146 which function in a way similar to the diodes 13~ and cables 136, 138 of the multiplex circuit 130. In the transmit mode the diodes are for-ward biased, shorting the cable 146 to a quarter wave-length cable 144. No net power is consumed by the cable 144. In 2 receive mode the two cables 14q, 146 act as a single half wavelength cable. Any noise from the transmitter is blocked since the half wavelength cable presents a virtual short.

~2;~6~30 The preamplifier 132 is coupled to other apparatus known in the nuclear magnetic resonating art for convert-in signals from the resonator into signals suitable for imaging. The resonator 20 has an unloaded "On of about 300 and a loaded "Q" of approximately 50. A very good match to the 50 ohm transmission line is achieved with reflected power levels under two percent.
Field uniformity is presented in Figures 7-9 where a plot of variations and magnetic field strength with position in the X, Y, and Z directions as defined in Figure 1 are disclosed. The origin of this co-ordinate axis is a point centered within the resonator 20 halfway between the shorting and wing conductors. The data presented in Figures 7-9 was generated with a probe coil energization of 64.5 megahertz, an unloaded "Q" of 260 and a loaded "Q" of 55. The X and Y uniformity in field is excellent and by properly positioning the resow-atop 20 along the Z axis uniformity within a region of interest as defined by the field gradient of the magnet 12 con be achieved.
The disclosed design fulfills all the requirements for hush quote head imaging at field strengths of 1.5 Tussle. The operating parameters of the resonator 20, however, should not be viewed as limiting the invention and field strengths of 2.0 Tussle and resonance frequent ales of 85 megahertz are possible. It is the intent that the invention cover 211 modifications and/or alter-anions following within the spirit or scope of the eye-dyed claims.

Claims (2)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. In a nuclear magnetic scanner, field modifying apparatus comprising:
an electrically unbalanced generator for generating a high frequency electrical signal;
an rf signal coil having two balanced inputs coupled to said generator for converting said high frequency electrical signal into a magnetic field over a volume of interest; and an interface circuit interposed between said generator and said coil to impedance match and tune said coil, said interface including a .lambda./2 co-axial cable to alter the output impedance and balance to ground the output of said generator coupled across an adjustable capacitive coupling circuit and said two inputs, said cable in combination with said capacitive circuit providing a balanced energization to said two coil inputs,

2. The field modifying apparatus of Claim 1, wherein the coil acts as both transmitter and receiver and additionally comprising a multiplexer to send and receive rf signals to and from said coil.