WO2007106601A2

WO2007106601A2 - Mri coil with whisper gallery mode photonic sensor

Info

Publication number: WO2007106601A2
Application number: PCT/US2007/006795
Authority: WO
Inventors: Randy G. Duensing; Jovan Jevtic
Original assignee: Invivo Corporation
Priority date: 2006-03-15
Filing date: 2007-03-15
Publication date: 2007-09-20
Also published as: WO2007106601A3

Abstract

A whispering gallery mode optical resonator (26) is constructed from an electro-optic material allowing direct electrical modulation of a light signal. Modulation frequencies of greater than the bandwidth of the modes (42) of the whispering gallery mode optical resonators are made possible by means of a resonance splitting technique that provides a second passband mode offset from the primary passband modes of the whispering gallery mode optical resonators to accommodate a modulation sideband (44).

Description

MRI Coil With Whisper Gallery Mode Photonic Sensor

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR

DEVELOPMENT

loooi] --

CROSS REFERENCE TO RELATED APPLICATION

[0002] This application claims the benefit of U.S. provisional application 60/782,419 filed March 15, 2006 hereby incorporated by reference.

BACKGROUND OF THE INVENTION

[0003] The present invention relates to magnetic resonance imaging (MRI) and in particular to an electo-optical interface for connecting NMR receiving coils used in MRI imaging to an MRI machine.

[0004] Magnetic resonance imaging can provide sophisticated images of the human body by detecting faint nuclear magnetic resonance ("NMR") signals, primarily from concentrations of hydrogen protons in the tissues of the body. In MRI, a patient is located in a strong, polarizing, magnetic field and hydrogen protons of the patient's tissues are excited into precession with a radio frequency ("RF") pulse. A series of applied gradient magnetic fields are switched on and off to spatially encode the precessing protons by phase and frequency. A sensitive antenna is then used to detect the NMR signals which are reconstructed into images. [0005] MRI machines normally provide an integral antenna as part of the magnet assembly that may be used both for the RF excitation pulse and for detecting the NMR signal. Preferably, however, the NMR signals will be detected using one or more "local coils", being one or more small antennas that may be positioned near the patient to provide for improved signal to noise ratio in the detection of the NMR signals.

[0006] Typically a shielded cable is attached to the local coil to receive a signal from preamplifiers built into the local coil that amplify the signal before transmitting it to the MRI machine. The shielded cable may connect to a termination box on the MRI machine (a "dog house") often at the end of the patient table, where signals from the shielded conductor are routed to the MRI processing electronics. The termination box may also provide a source of electrical power, transmitted through the shielded cable to the local coil, to power the preamplifiers. In addition, the shielded cable may conduct other electrical signals to the local coil including active decoupling signals communicating with decoupling circuits in the local coil to detune the local coil during the RF excitation pulse to prevent excessive current conduction in the local coil during that time period. The termination box may also provide a separate electrical connector for a second shielded cable passing to the local coil and conducting an RF excitation pulse to the local coil when the local coil operates both in a receive and transmit mode.

[0007] The MRI machine presents a difficult environment in which to transmit the faint NMR signal from the local coils to the MRI acquisition circuitry. Shielding of the cable may not be fully effective in preventing electrical interference with the NMR signal by the switched fields used during the imaging process. In addition to interference, these switched fields can promote high currents on the shield that may cause heating and possible risk to the patient. [0008] Shielded cables, furthermore, are relatively bulky and inflexible, in part, as a result of the necessary electrical shielding and electrical and thermal insulation, the latter needed to guard the patient against shield heating. This problem of managing shielded cables is, of course, exacerbated for multi-channel coils which employ separate shielded cables for each channel. The rigidity of these shielded cables can cause storage problems when multiple coils must be stored on-site, for example, in the limited space of the MRI room.

[0009] One promising solution to the problems of shield currents and electrical interference is that of transmitting the NMR signals optically, for example, over optical fibers. However, this approach faces a number of practical problems including the high cost of optical modulation circuitry suitable to provide high signal-to-noise transmission of the NMR signal, a cost that is multiplied by the number of channels of the local coil. Conventional light modulators, for example, laser diodes or Mach-Zender modulators, cannot receive the unamplified NMR signal directly from the coil and thus require preamplifiers and associated matching and protection circuitry. Laser diode modulators require a source of electrical power at the coil, typically conducted along the same cable as the optical fiber, obviating some of the benefits of a fiber optic system. SUMMARY OF THE INVENTION

[0010] The present invention employs a "whispering gallery mode" optical resonator that may be electrically modulated by the NMR signals to convert NMR signals to optical signals for transmission over an optical fiber. Extracting the modulated signal from the whispering gallery mode optical resonator is made possible by a resonance splitting technique which, in a preferred embodiment, couples the optical resonator to a second resonator. A sideband of the modulation is aligned with the secondary resonance for extraction. The optical resonator may receive optical power to be modulated from a remote light source, eliminating the need for electrical power conductors. A resonant step-up transformer may allow the direct modulation of the optical resonator with the received NMR signal without the need for preamplifiers that introduce noise and require electrical power.

[0011] Specifically then, the present invention provides an MRI coil incorporating an electro- optic sensor accommodating radio modulation detection speeds. The electro-optic sensor includes a photomodulator formed of an optical material that provides a change in an effective light path through the optical material in response to a radio frequency electrical signal applied to the optical material. The optical material is shaped to provide a whispering gallery mode resonator entrapping light introduced into the optical material at frequencies of whispering gallery resonances. The electro-optic sensor further includes a mechanism for resonant peak splitting of at least one whispering gallery resonance to allow passage of sidebands of light photons as modulated by a radio frequency modulating electric signal applied to the optical material.

[0012] It is thus one aspect of at least one embodiment of the present invention to make use of a whispering gallery mode optical resonator to detect modulation that would normally be outside of the resonant pass band of the optical resonator. Typical modulation side bands of an NMR signal will be 64 to 256 MHz from the center frequency, while the resonant passband of a high Q whispering gallery mode optical resonator may be as little as 30 MHz. [0013] The mechanism for resonant peak splitting may be a supplemental optical resonator coupled to the optical material.

[0014] It is thus another aspect of at least one embodiments of the invention to provide a means for peak splitting that may flexibly accommodate modulation sidebands by control of the coupling coefficient between the resonators. [0015] The supplemental optical resonator may be a second whispering gallery mode resonator.

[0016] It is thus another feature of some embodiments of the invention to provide symmetrical coupled oscillators for ease of fabrication and design.

[0017] The optical material may provide a change of index of refraction with applied electrical voltage. The optical material may be LiNbCβ.

[0018] It is another feature of some embodiments of the invention to employ well-known electro-optical materials to provide for direct modulation of a whisper gallery device.

[0019] The invention may further include a light source, a reference optical device, a beam splitter dividing light from the light source between the optical material and the reference optical device, and an analyzer receiving light from the optical material and the reference optical device to extract the radio frequency modulating electrical signal.

[0020] It is thus an feature of some embodiments of the invention to provide for a simple phase sensitive demodulator suitable for detecting phase shifts caused by the whispering gallery mode optical resonator. It is another feature of at least one embodiment of the invention to provide a demodulator using balanced light paths that reduce the effect of other changes in parameters such as temperature, light source phase, or the like.

[0021] The mechanism for resonant peak splitting may separate the whispering gallery resonance into two peaks spaced by a frequency of 64 to 256 MHz.

[0022] It is thus another feature of some embodiments of the invention to provide a photo modulator using whispering gallery mode operation particularly suited for radio frequency detection.

[0023] These particular features and advantages may apply to only some embodiments falling within the claims and thus do not define the scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Fig. 1 is a fragmentary perspective view of an MRI machine showing a local coil providing multiple channels of received NMR signals such as may incorporate the present invention;

[0025] Fig. 2 is a schematic representation of one channel of the local coil of Fig. 1 showing a loop antenna and a photo modulator receiving NMR signals from the loop antenna as boosted by a resonant step up transformer; [0026] Fig. 3 is a elevational cross-section of the photo modulator of Fig. 3 showing two coupled whispering gallery mode optical resonators, one communicating with input and output optical fibers;

[0027J Fig. 4 is a plan view of the photo modulator of Fig. 3 showing a path of light coupled into the whispering gallery mode optical resonator through a first optical fiber and extracted from the whispering gallery mode optical resonator through a second optical fiber and the coupling of the two whispering gallery mode resonators;

[0028] Fig. 5 is a plot of a spectrum of the resonant passbands of a single whispering gallery mode resonator of Figs. 3 and 4 aligned with a plot of the spectrum of a modulated signal produced by electrical modulation of one of the whispering gallery mode resonators with an

NMR signal showing the position of modulation side bands outside the resonant passband of the whispering gallery mode resonator;

[0029] Fig. 6 is a figure similar to that of Fig. 5 showing peak splitting of the resonant passbands of one whispering gallery mode optical resonator by coupling to a second whispering gallery mode optical resonator such as aligns one of the split resonant peaks with a modulation side band; and

[0030] Fig. 7 is a schematic representation of an optical system employed to detect phase modulation by the photomodulator of Figs. 2-4.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT [0031] Referring now to Fig. 1, an MRI machine 10 may include a polarizing magnet assembly 12, typically providing a superconducting toroidal magnet, a whole body radio frequency transmit and receive coil, and magnetic gradient coils (not shown) as are known in the art. These radio frequency and magnetic gradient coils communicate with control electronics 14 which control the same transmitted radio frequency signals and gradient coils and receive NMR signals from the radio frequency receiving coil to reconstruct magnetic resonance images of a patient contained in a bore of the polarizing magnet assembly.

[0032] The radio frequency receiving coils of the polarizing magnet assembly 12 may be supplemented or replaced by one or more local coils 15 also providing for the receipt of NMR signals and communicating with the control electronics 14. The local coils 15 may provide for multiple channels of reception each generally associated with an independent receiving antenna (not shown in Fig. 1). In the preferred embodiment, the present invention provides one or more photo modulators 24 associated with each channel of the local coil 15 so that it may communicate independently received NMR signals over optical fibers of an optical cable 18 to the control electronics 14.

[0033] Referring now to Fig. 2, as mentioned, the local coil 15 may provide for multiple channels of reception each associated with an antenna loop 16 tuned to receive NMR signals typically in a range of 64 to 128 MHz for current strength of polarizing magnet assemblies 12. The antenna loops 16 may connect to a resonant step up autotransformer 19 formed by a parallel connection of an inductor 20 and capacitor 22.

[0034] The inductor 20 and capacitor 22 are tuned to block current flow in the antenna loops 16 at the resonant frequency of the protons in the polarizing magnet assembly 12. This blocking of current decreases inductive coupling between multiple coils. The antenna loop 16 may attach to the inductor at inner inductor taps defining the step-up voltage of the autotransformer 19. Alternatively, a standard two winding transformer may be used. The capacitor 22 will normally be on the order of 10 pF. Notably, the tuning of the inductor 20 and the capacitor 22 is largely independent of the frequency at which the local coil 15 will operate and decoupling diodes or the like are not required.

[0035] Referring now to Figs. 2, 3 and 4, the dielectric of the capacitor 22 may be an optical whispering gallery mode (WGM) optical resonator 26 receiving the NMR signal from the antenna loop 16 and modulating an optical signal received by the WGM optical resonator 26 over the optical cable 18, with the modulated optical signal then transmitted back over the optical cable 18.

[0036] As is understood in the art, a WGM optical resonator provides a semispherical volume of transparent material having an index of refraction greater or less than the surrounding material to cause total internal reflection of entrapped light. At integer multiples of wavelengths of the entrapped light, equaling the path length around the equator of the semispherical volume, the light becomes trapped within the volume for time scales on the order of microseconds producing an effective quality factor Q of over 10⁹.

[0037] In the present invention, light may be introduced from a first optical fiber 30 into the WGM resonator 26 by means of a coupling element 32 which may, for example, be one or more prisms (shown schematically as a single prism) or a bevel cut end of the optical fiber 30 or other mechanisms known in the art. A second prism of the coupling element 32 or other coupling mechanism may extract some of the light trapped in the WGM resonator 26 to provide a return optical signal through optical fiber 34.

[0038] Referring now to Fig. 5, in isolation, a single WGM resonator 26 will provide a mode spectrum 40 having a number of resonant passbands 42 associated with integer multiples of a fundamental mode fo. These resonant passbands 42 represent frequencies of light that can be entrapped in a whispering gallery mode by the WGM resonator 26. Each of these resonant passbands 42 has a bandwidth of approximately 30 MHz or less for typically obtainable Q values.

[0039] As shown in Fig. 3, the WGM resonator 26 may be sandwiched between an upper electrode 36 and a lower electrode 38 of the capacitor 22 so that electrical voltage may be applied as received to an optical material of the WGM resonator 26 from the antenna loop 16. In the preferred embodiment, the optical material of the WGM resonator 26 may be a material such as the LiNbO3 which has a property of changing its index of refraction as a function of applied voltage. The voltage applied to the electrodes 36 and 38 thus causes a change in the index of refraction of the optical material of the WGM resonator 26.

[0040] As will be understood to those of ordinary skill in the art, a change in index of refraction of the WGM resonator 26 will cause a slight shift in the resonant passbands 42, resulting in a change in a wavelength of the light that can be entrapped by the WGM resonator 26.

[0041] This modulation of the resonant passbands 42 modulates the entrapped light to generate modulation side bands 44 on either side of a center frequency 46 (f_x) of the entrapped light. The modulation side bands 44 will be approximately equal to the center frequency 46 of the light plus and minus the modulation frequency (f_st_>) which may range from 64 to 128 MHz for current MRI machines 10. These side bands 44 are outside of the resonant passbands 42 and thus effectively suppressed by the WGM resonator 26.

[0042] Referring again to Figs. 3 and 4, in a preferred embodiment of the invention, the suppression of the modulation side bands 44 is overcome by evanescent coupling 50 between a supplemental optical resonator and the WGM resonator 26 by proximity characterized by a coupling coefficient k.

[0043] Referring now to Fig. 6, this coupling of the supplemental optical resonator 48 and WGM resonator 26 causes a splitting of the resonant passbands 42 into passband 42a and passband 42b separated as a function of the coupling coefficient k. The coupling coefficient k is adjusted so that resonant passband 42a is centered on the center frequency 46 of the light and resonant passband 42b is centered at one side of band 44 to provide for single sideband modulated output along the optical fiber 34 of the optical cable 18. .

[0044] Referring now to Fig. 7, the demodulation of the signal from the photo modulator 24 may be accomplished by a number of techniques, the most simple of which provides for a light source 52 that is split by a beam splitter 54 or the like to provide one light beam passing down optical fiber 30 to the photo modulator 24, and a second beam passing down a second light fiber 56 to a reference optical element 58 providing a constant and known phase shift. Light modulated by the photo modulator 24, and passing through optical fiber 34, is combined with light through the reference optical element 58, and passing through optical fiber 60, by beam combiner 62. The light of the combined beams is received by a photodetector 64 which may distinguish phase modulation by a change in amplitude caused by interference between the beams from optical fiber 34 and optical fiber 60. Other methods of phase detection may also be used. [0045] The present invention provides a number of advantages over prior art devices including the elimination of conventional loop interface circuits that require many components, including preamplifiers that must be adjusted for each antenna loop 16 separately and which impose intrinsic limitations on the amount of isolation that can be achieved between antenna loop 16. The present invention may be made to operate over a wide range of frequencies without significant adjustment. Further, the present invention does not require a separate source of electrical power and may be used with multiple channel coils without concern about electrical interference.

[0046] The present invention is not limited to use with MRI local coils but has particular application to any direct conversion of the electrical signal having a modulation frequency of greater than approximately 30 MHz and less than approximately 10 GHz necessary for the side bands 44 to reach the next resonant passbands 42 on a device of practical size. [0047] It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein, but include modified forms of those embodiments including portions of the embodiments and combinations of elements of different embodiments as come within the scope of the following claims.

Claims

CLAIMSWhat we claim is:

1. An electro-optic sensor accommodating radio frequency modulation speeds and comprising: a photomodulator (24) formed of an optical material (26) that provides a change in effective light path through the optical material in response to a radio frequency electrical signal applied to the optical material, wherein the optical material is shaped to provide a whispering gallery mode resonator entrapping light introduced into the optical material at frequencies of whispering gallery resonances, further including means (48) for resonant peak splitting of at least one whispering gallery resonance (42) to allow passage of a sideband (44) of light photons entrapped by the optical material as modulated by a radio frequency modulating electric signal applied to the optical material.

2. The electro-optic sensor of claim 1 wherein the means for resonant peak splitting is a supplemental optical resonator (48) coupled to the optical material.

3. The electro-optic sensor of claim 2 wherein the supplemental optical resonator is a second whispering gallery mode resonator.

4. The electro-optic sensor of claim 1 wherein the optical material provides a change of index of refraction with applied electrical voltage.

5. The electro-optic sensor of claim 1 where in the optical material is LiNbO3.

6. The electro-optic sensor of claim 1 further including: a light source (52); a reference optical device (58); a beam splitter (54) dividing light from the light source to the optical material and the reference optical device; an analyzer (62, 64) receiving light from the optical material and the reference optical device to extract the radio frequency modulating electrical signal.

7. The electro-optic sensor of claim 6 wherein the reference optical device is a second whisper gallery mode resonator.

8. The electro-optic sensor of claim 1 wherein the means for resonant peak splitting separates at least one whispering gallery resonance into two peaks spaced by a frequency of 64 to 256 MHz.

9. The electro-optic sensor of claim 1 further including: a support structure (15) positionable adjacent to a patient undergoing an MRI scan; at least one resonant electrical loop antenna (16) attached to the support structure for receiving NMR electrical signals from the patient during an MRI scan; a coupling circuit (19) coupling the NMR electrical signal to the optical material as a radio frequency modulating signal.

10. An NMR reception coil for a magnetic resonance imaging machine comprising: a support structure (15) positionable adjacent to a patient undergoing an MRI scan; at least one resonant electrical loop antenna (16) attached to the support structure for receiving NMR electrical signals from the patient during an MRI scan; a photo modulator (24) receiving the NMR electrical signals to modulate optical properties of a whispering gallery mode resonator to provide an optical signal encoding the NMR electrical signal for transmission to processing circuitry via an optical cable.

11. A method of optically encoding a radio frequency electrical signal comprising the steps of:

(a) introducing light to a photomodulator (24) formed of an optical material that provides a change in effective light path through the optical material in response to a radio frequency electrical signal applied to the optical material, wherein the optical material is shaped to provide a' whispering gallery mode resonator (26) entrapping light introduced into the optical material at frequencies of whispering gallery resonances;

(b) splitting the whispering gallery resonances (42) to allow passage of sidebands (44) of light photons entrapped by the optical material as modulated by a radio frequency modulating electric signal applied to the optical material;

(c) applying a radio frequency modulating electrical signal to the optical material; and

(d) detecting light from the optical material to extract the electrical signal.

12. The method of claim 11 wherein the step of splitting the resonances couples an optical resonator (48) to the optical material.

13. The method of claim 12 wherein the optical resonator is a second whispering gallery mode resonator.

14. The method of claim 11 further including the steps of: directing a portion of the light to reference optical device (58); combining the light from the reference optical signal and the optical material before step (d).

15. The method of claim 11 wherein the step of splitting the resonances separates at least one resonance into two peaks spaced by a frequency of 64 to 256 MHz.