Magnetic field sensing with a miniature electro-optic sensor for interventional magnetic resonance imaging

Abstract

A magnetic field sensor comprising an optical microresonator, an optical fiber, and an antenna. The optical microresonator has a first side and a second side. The optical fiber is coupled to and extends from the first side. The antenna has a first end, a second end, and length therebetween. The first end of the antenna is coupled to the first side of the optical microresonator and the second antenna is coupled to the second side of the optical microresonator. The length of the antenna is positioned proximate to the optical microresonator.

Description

MAGNETIC FIELD SENSING WITH A MINIATURE ELECTRO-OPTIC SENSOR FOR INTERVENTIONAL MAGNETIC RESONANCE IMAGINGCROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of priority to U.S. Provisional Patent Application No. 63 / 644,724, filed May 9, 2024; and U.S. Provisional Patent Application No. 63 / 551,167, filed February 8, 2024; the entire contents of each of these applications are hereby incorporated by reference.BACKGROUNDField of the Disclosed Subject Matter

[0002] The disclosed subject matter relates to a magnetic field sensor for interventional magnetic resonance imaging.Description of Related Art

[0001] Technologies that enable precise localization and real-time tracking of interventional devices, such as catheters, under magnetic resonance imaging (MRI) are necessary to guide minimally invasive interventions for high-risk patients. At present, MRI- based catheter tracking relies on passive and active devices. However, current and emerging devices have demonstrated shortcomings due to their poor contrast generation, radiofrequency (RF)-induced heating inside the body or large size.SUMMARY

[0002] The purpose and advantages of the disclosed subject matter will be set forth in and apparent from the description that follows, as well as will be learned by practice of the disclosed subject matter. Additional advantages of the disclosed subject matter will be realized and attained by the methods and systems particularly pointed out in the written description and claims hereof, as well as from the appended drawings.

[0003] To achieve these and other advantages and in accordance with the purpose of the disclosed subject matter, as embodied and broadly described, the disclosed subject matter includes a magnetic field sensor, comprising an optical microresonator, an optical fiber, and an antenna. The optical microresonator has a first side and a second side. The optical fiber is coupled to and extends from the first side. The antenna has a first end, a second end, and alength therebetween. The first end is coupled to the first side of the optical microresonator and the second end is coupled to the second side, with the length of the antenna positioned proximate to the optical microresonator.

[0004] The magnetic field sensor may further comprise an external module operably coupled to the optical microresonator. The external module may comprise a laser, a circulator, and a receiver. The laser interrogates the optical microresonator with an incident light signal. The circulator separates an optical signal including a reflected light signal from the optical microresonator and the incident light signal emitted by the laser. The receiver processes the reflected light signal. The receiver may comprise a photodetector (e.g., a transimpedance photodetector) and a bandpass filter. The photodetector converts the reflected light signal into an electrical signal. The bandpass filter filters the electrical signal. The receiver may comprise a phase shifter. The phase shifter alters the phase of the filtered electrical signal. The external module may comprise a data acquisition system for data analysis of the electrical signal. The optical fiber may comprise a fiber housed in a V-groove. The optical fiber may comprise a single mode optical fiber or a multi-mode optical fiber. The optical microresonator may comprise a first optical mirror, a second optical mirror, and a piezoelectric layer disposed between the first optical mirror and the second optical mirror. The optical microresonator may comprise a first and a second electrode, with the first and second optical mirror positioned between the first and second electrode. The first electrode may comprise a metal (e.g., aluminum, titanium, gold, and / or platinum). The second electrode may comprise a transparent conductive oxide (e.g., zinc oxide or indium tin oxide). The first end of the antenna may be electrically coupled to the first electrode and the second end of the antenna may be electrically coupled to the second electrode. The first electrode may be adjacent to the first optical mirror. The second electrode may be spaced from the second optical mirror. The magnetic field sensor may comprise a substrate and an antireflection coating layer. The antireflection layer may be disposed on the substrate. The substrate and the antireflection coating layer may be positioned between the second electrode and the second optical mirror. The substrate may comprise silicon. The piezoelectric layer may comprise aluminum nitride, scandium-doped aluminum nitride (ScAln), barium titanate (BTO), lead zirconate titanate (PZT), or lithium niobate (LiNbCf). The first optical mirror may comprise a Distributed Bragg Reflector mirror. The second optical mirror may comprise a Distributed Bragg Reflector mirror. The first optical mirror may comprise alternating layers of silicon and silicon dioxide or alternating layers of silicon and nitride. The antenna may bepositioned in a coaxial configuration around the optical fiber. The antenna may comprise a coil antenna or dipole antenna.

[0005] The disclosed subject matter includes a method of manufacturing an optical microresonator. A substrate having a first side and a second side is provided. An antireflection coating layer is deposited on the first side of the substrate. A first optical mirror is deposited on the antireflection coating layer. A piezoelectric layer is deposited on the first optical mirror. A second optical mirror is deposited on the piezoelectric layer. A first conductive layer is deposited on the second optical mirror. A second conductive layer is deposited on the second side of the substrate.

[0003] It is to be understood that both the foregoing general description and the following detailed description are exemplary and are intended to provide further explanation of the disclosed subject matter claimed.

[0004] The accompanying drawings, which are incorporated in and constitute part of this specification, are included to illustrate and provide a further understanding of the method and system of the disclosed subject matter. Together with the description, the drawings serve to explain the principles of the disclosed subject matter.BRIEF DESCRIPTION OF THE DRAWINGS

[0005] A detailed description of various aspects, features, and implementations of the subject matter described herein is provided with reference to the accompanying drawings, which are briefly described below. The drawings are illustrative and are not necessarily drawn to scale, with some components and features being exaggerated for clarity. The drawings illustrate various aspects and features of the present subject matter and may illustrate one or more implementation(s) or example(s) of the present subject matter in whole or in part.

[0006] FIG. 1 is a schematic view of an implementation of a magnetic field sensor, in accordance with the present disclosure.

[0007] FIG. 2A is a schematic view of an implementation of an antenna and an optical microresonator (OMR), in accordance with the present disclosure.

[0008] FIG. 2B is a graph illustrating electro-optic transduction, in accordance with the present disclosure.

[0009] FIG. 3 is a schematic illustrating a process for fabricating an OMR, in accordance with the present disclosure.

[0010] FIG. 4A is a photograph of an implementation of a magnetic field sensor, in accordance with the present disclosure.

[0011] FIG. 4B is a photograph of a microfabricated die with different size OMRs, in accordance with the present disclosure.

[0012] FIG. 5A is a graph of the measured reflectance spectra, in accordance with the present disclosure.

[0013] FIG. 5B is a graph of the measured frequency response of the 320pm OMR, showing 1700 Q and 108MHz BW, in accordance with the present disclosure.

[0014] FIG. 6 is a block diagram of a test setup and an implementation of an external module, in accordance with the present disclosure.

[0015] FIG. 7 is a graph of the measured sensor output versus magnetic field at 64MHz, in accordance with the present disclosure.

[0016] FIG. 8 is a graph of the measured input-referred spectral density with a 300nT, 64MHz magnetic field applied to the sensor, in accordance with the present disclosure.

[0017] FIG. 9A is a graph of the measured sensor output in response to a 300nT magnetic field input with varying phase at 64MHz, in accordance with the present disclosure.

[0018] FIG. 9B is a graph of the measured sensor output in response to a 300nT magnetic field input with varying frequency, in accordance with the present disclosure.DETAILED DESCRIPTION

[0019] The various concepts introduced above and discussed in greater detail below may be implemented in any of numerous ways, as the described concepts are not limited to any particular manner of implementation. Examples of specific implementations and applications are provided primarily for illustrative purposes.

[0020] References herein to positions of elements (e.g., “top”, “bottom”) are merely used to describe the orientation of various elements in the FIGURES. It should be noted that the orientation of various elements may different according to other exemplary implementations, and that such variations are intended to be encompassed by the present disclosure.

[0021] Prostate cancer (“PCa”) is the second most common cancer in men worldwide. In 2020, -1,400,000 patients with PCa were diagnosed, and more than 375,000 deaths werereported globally due to PCa. Prostate biopsies can provide a definitive diagnosis of malignancies. At present, transrectal ultrasound (“TRUS”) imaging is the predominant technology used in clinical practice to guide prostate biopsies in at-risk populations. However, the TRUS-guided biopsy has demonstrated shortcomings due to its low sensitivity for clinically significant PCa, resulting in high false-negative results of up to 52%; this is because US imaging suffers from low contrast. In addition, TRUS-guided prostate biopsy procedures have associated risks of complications such as bleeding and infection, as it is performed by inserting a biopsy needle into the prostate about 10-12 times through the wall of the rectum to collect tissue samples from suspicious regions.

[0022] Compared to ultrasound imaging, magnetic resonance imaging (MRI) provides high tissue contrast and, as a result, produces more detailed high-contrast images of the prostate, enabling more accurate identification of potentially malignant lesions that require sampling with a biopsy. Compared to the conventional TRUS-guided biopsy, MRI-guided prostate biopsies (fusion and in-bore techniques) have been recently demonstrated to increase clinically significant PCa detection rates by 32% and reduce the number of biopsy cores required for the optimal detection of PCa.

[0023] To maximize diagnostic yield in biopsy procedures, the probe of the RF magnetic field sensor can be designed in the form of a biopsy needle with a diameter smaller than 1mm; this can enable active tracking of the needle position and orientation relative to the target lesions in real-time with high accuracy under an MRI system during biopsy procedures. Compared to existing image-guided prostate biopsy approaches, this advancement would allow safer and more accurate sampling of clinically significant prostate lesions, potentially reducing the number of biopsy cores required for diagnosis and false negative results in biopsy.

[0024] Accurate real-time tracking of catheters with clinical imaging modalities is essential to guide and transform targeted interventions for patients with major diseases such as cardiovascular disease. X-ray fluoroscopy is currently the predominant imaging modality that allows in vivo tracking of tungsten or BaSCU-doped catheters in real time. However, X-ray fluoroscopy suffers from low soft tissue contrast and hence requires expert practitioners to determine the catheter position and orientation relative to anatomical structures. In addition, X-ray ionizing radiation raises health and safety concerns for patients and medical professionals during medical procedures.

[0025] Unlike X-ray fluoroscopy, magnetic resonance imaging (MRI) has shown the ability to provide high soft tissue contrast without ionizing radiation. MRI-based catheter tracking in the body has been demonstrated by using passive and active devices. Existing passive devices often formed by magnetic materials suffer from orientation-dependent artifacts and poor contrast, limiting their deployment in clinical practice. On the other hand, most existing active devices rely on RF receivers based on either coil or loop antennas to detect MRI- induced RF magnetic fields from the tissue and transmit the detected signals to the MR scanner outside the body to enable localization. Although this approach provides relatively accurate localization and tracking of the catheters, these active devices have an inherent limitation of RF-induced heating inside the body. This arises because these devices convey the detected radiofrequency (RF) signals on long electrical cables to the MRI system. Several methods, including parallel RF transmission and saline coolant flow through the catheter, have been demonstrated to reduce RF heating but with minimal success.

[0026] The RF-induced heating issue led to the development of active tracking devices based on the transduction of an electrical RF signal in the optical domain. These devices rely on either an optoelectronic circuit or a Fiber Bragg grating (FBG) modulator to convert the RF signal to an optical signal and carry the resulting signal on a compact optical fiber instead of electrical cables, eliminating the RF heating safety risk. However, these devices are currently limited by localized tissue heating due to circuit power dissipation or large size, frequently causing post-operative complications.

[0027] The disclosed subject matter relates to a magnetic field sensor for tracking the position of the catheter in real-time during interventional MRI, while avoiding the drawbacks of the active devices mentioned above. The magnetic field sensor may include a miniature millimeter (mm)-sized probe that includes a coil loop antenna and a microfabricated optical microresonator (OMR). The magnetic field sensor can perform active catheter tracking for image-guided interventional procedures in 1.5T MRI. The magnetic field sensor can enable operation with virtually zero probe heat-up inside the body and hence eliminate the heating safety risk. Implementations of the present disclosure open the door to the realization of sensors with sub-mm-scale probes that can enable safer, minimally invasive MRI-guided catheter-based interventions and thus improve patient outcomes.

[0028] Referring now to Fig. 1, a schematic view of an implementation of a magnetic field sensor for use in interventional MRI. As shown in Fig. 1, the magnetic field sensor includes a probe 102 with an antenna 106 (e.g., loop antenna, coil antenna, dipole antenna), an opticalmicroresonator (OMR) 108 coupled (e.g., mechanically coupled, electrically coupled) to the antenna, and an optical fiber 110 coupled (e.g., mechanically coupled, optically coupled) to the OMR 108. The size of the probe can be minimized through the compact integration of the antenna, the OMR, and the fiber, with the antenna primarily determining the probe size.

[0029] The probe 102 can be positioned on the body of a patient, for example, while the patient is in an MRI machine, and may be held within a catheter, such as at the catheter tip. An external module 104 can be positioned a distance from the patient’s body. The external module 104 can be operably coupled to the OMR 108.

[0030] The optical fiber 110 can receive incident light (e.g., emitted by a laser) and transmit reflected light. The optical fiber 110 can include a single mode fiber or a multi-mode fiber. The optical fiber 110 can include a V-groove fiber (including the fiber and a substrate 112). A V-groove fiber refers to an optical fiber that is positioned within (e.g., housed within) a V- shaped groove formed on the surface of a substrate 112. The substrate 112 may be formed of a metal alloy and / or a polymer. The substrate 112 is configured to hold the optical fiber and align the fiber by guiding the optical fiber 110 along the groove’s edges. The substrate 112 can be rectangular. The optical fiber 110 can extend beyond an end (e.g., longitudinal end) of the substrate 112.

[0031] The optical fiber 110 can be coupled to and extend from the first side 114 of the OMR 108. An adhesive (e.g., epoxy, optical adhesive) can be used to couple the optical fiber 110 and the OMR 108. The optical fiber 110 can be oriented at an angle relative to the first side 114 of the OMR. In some implementations, the optical fiber 110 is perpendicular to the first side. In some implementations, the optical fiber 110 is oriented at an angle between 85 to 95 degrees. The antenna 106 is positioned around the optical fiber 110. The antenna 106 can encircle the optical fiber 110 such that there is a gap (e.g., annular gap) between the antenna 106 and the exterior surface of the optical fiber 110. The antenna 106 can be positioned in a coaxial configuration around the optical fiber 110. In some implementations, the optical fiber 110 extends beyond the antenna 106. In some implementations, the antenna 106 is a 14-tum coil antenna. In some implementations, the antenna 106 is a coil antenna with a diameter between 1.0 mm and 3 mm. In some implementations, the antenna 106 is a coil antenna with a length between 5 mm and 7 mm. The size of the antenna can be chosen to maintain a reasonable trade-off between its magnetic field sensitivity and the impact on the probe size.

[0032] Still referring Fig. 1, with additional reference to Fig. 2A, the OMR 108 has a first side 114, a second side 116, and a thickness between the two sides. The OMR 108 caninclude a resonant cavity based on piezoelectric actuation, operating as an optical intensity modulator. The OMR 108 can include a multi-layered structure. The OMR 108 can include a pair of electrodes 202a, 202b, a pair of optical mirrors 204a, 204b disposed between the electrodes 202a, 202b, and a piezoelectric layer 206 disposed between the optical mirrors 204a, 204b. The first electrode 202a can be adjacent to and contact the first optical mirror 204a. The OMR 108 can include bond pads disposed on at least one of the electrodes, providing an interface for electrical connections. The OMR 108 can be different shapes such as rectangular, cylindrical, or an irregular shape. The cross-section of the OMR 108 can be about 1X1 mm2.

[0033] In some implementations, the second electrode 202b is adjacent to and contacts the second optical mirror 204b. In some implementations, the second electrode 202b is spaced a distance from the second optical mirror 204b. In some implementations, the OMR 108 includes an antireflection coating layer and / or a substrate disposed between the second electrode 202b and the second optical mirror 204b. The OMR 108 can be configured to operate at a 1310 nm wavelength. In some aspects, operation at wavelengths > 1pm minimizes the absorption of the materials (e.g., ITO, a-Si, and SiO2) used in the microfabrication of the OMR. At the operating wavelength, the substrate of the OMR may be transparent, which can cause a portion of the light incident upon the probe to be reflected off the bottom electrode surface, degrading the sensor’s Q-factor and, therefore, modulation gain p. Therefore, the OMR 108 can include an antireflection coating layer, eliminating the effect of back reflection on OMR performance.

[0034] In some implementations, the piezoelectric layer 206 is formed of aluminum nitride (AIN). In some implementations, the piezoelectric layer 206 is formed of scandium-doped aluminum nitride (ScAlN). In some implementations, the piezoelectric layer 206 is formed of lead zirconate titanate (PZT). In some implementations, the piezoelectric layer 206 is formed of lithium niobate (LiNbCh). In some implementations, the piezoelectric layer 206 is formed of zinc oxide. The optical mirrors 204a, 204b can include Distributed Bragg Reflector mirrors. The optical mirrors 204a, 204b can include alternating layers of silicon layers and silicon dioxide layers. The optical mirrors 204a, 204b can include alternating layers of silicon layers and silicon nitride layers.

[0035] The antenna 106 has a first end 118 and a second end 120, with the first end 118 of the antenna 106 coupled to the first side 114 of the OMR 108 and the second end 120 of the antenna 106 coupled to the second side 116 of the OMR 108. The first end 118 of the antenna106 can be electrically coupled to the first electrode 202a of the OMR 108, while the second end 120 of the antenna 106 can be electrically coupled to the second electrode 202b of the OMR 108. An adhesive (e.g., conductive epoxy) can be used to couple the antenna 106 and the OMR 108.

[0036] In operation, the antenna 106 detects and converts an MRI-induced RF magnetic field from the tissue to an electrical signal, which in turn modulates the OMR 108. The antenna 106 receives the magnetic field component of the RF field emitted by the nearby hydrogen atoms in tissue during MRI and produces an RF voltage due to Faraday’s law of induction. The OMR can enhance the signal-to-noise ratio (SNR) of the detected radio frequency (RF) signals. This can enable the miniaturization of the antenna and, hence, the probe size.

[0037] The induced RF voltage on the antenna 106 modulates the OMR resonant wavelength (Ares). This modulation results in a shift in the reflectance (R) spectra and, hence, a change in light intensity (Pout) reflected off the OMR. A laser, optically coupled to the OMR, can be used to provide an optical signal that interacts with the OMR, enabling an optical readout of this modulation.

[0038] To maximize the modulation depth (DR) and hence the SNR of the RF signal, the OMR can be operated with an incident light wavelength (Ain) at the maximum slope of its R curve. This electro-optic transduction enables the transmission of the magnetic field- modulated optical signal on a single optic fiber. Fig. 2B is a qualitative graph of reflectance (R) as a function of wavelength (A). Fig. 2B shows an electro-optic transduction enabled by an OMR operated with an incident light wavelength inat the steepest slope of the R curve to maximize sensitivity. The electrical signal from the antenna causes a shift in the OMR R spectra, as shown in Fig. 2B by the dashed lines 210, 212, modulating the reflected light intensity.

[0039] The fiber 110 (e.g., single mode fiber) carries the optical signal, in which the probe position information is encoded, to the external module 104 outside of the body. The RF signal is encoded in the optical domain to an optoelectronic circuit in the external module 104 for data decoding and further MRI integration. In various aspects, this allows the sensor to operate with virtually zero probe power / heat-up inside the body and allows miniaturization of the probe diameter. The probe diameter can be less than 2mm, while achieving a competitive resolution of 20pT-\ / OHz. These features provide potential capabilities for use in interventional MRI procedures with minimal safety risk and tissue invasiveness.

[0040] The magnetic field sensor can operate with a laser wavelength Xinwhere the reflectance (R) slope is the steepest near the resonance wavelength XrCs. A voltage (Vin) applied across the electrodes 202a, 202b induces an electric field inside the piezoelectric layer 206 that causes it to change its thickness due to piezoelectric effect, resulting in a shift in the Aes and, hence, a modulation in the R. The modulation depth (AR) can be estimated by:

[0041] Here, P is the OMR modulation gain, Rmax is the resonant dip amplitude, Q and t are the device quality factor and the cavity layer thickness, d33 is the piezoelectric coefficient, and no and r?3 are the unperturbed refractive index and Pockels coefficient of the piezoelectric layer (e.g., aluminum nitride film). The OMR operated with a light intensity Pin exhibits a relative change in the reflected light intensity, AP0Ut = AR*Pin.

[0042] Referring now to Fig. 3, a schematic view illustrating a process for fabricating an OMR is shown. The steps of process 300 presented below are intended to be illustrative. The process 300 may be accomplished with one or more additional steps not described and / or without one or more of the steps described. Additionally, the order in which the steps of process 300 are illustrated in Fig. 3 and described below is not intended to be limiting.

[0043] The OMR 108 can be fabricated on a substrate 303 (e.g., bare p-Si wafer) using standard microfabrication techniques. The substrate 303 includes a first side, a second side, and a thickness between the two sides. The substrate may be a semiconductor. At step 302, an antireflection coating (ARC) layer 304 is deposited on a first side of a substrate 303 (e.g., silicon substrate) and the second optical mirror 204b is deposited on the ARC layer 304. The ARC layer 304 can include a single layer or multiple layers. For example, the ARC layer 304 can include two layers, with a first layer including titanium and the second layer including silicon (e.g., amorphous silicon). In some implementations, the thickness of the first layer of the ARC layer 304 is between 40 nm to 60 nm. In some implementations, the thickness of the second layer of the ARC layer 304 is between 150 nm and 250 nm. In some implementations, the second optical mirror 204b includes a Distributed Bragg Reflector mirror. The second optical mirror 204b can include multiple alternating layers of materials with different refractive indices. A first subset of the layers may have the same thickness, while a second subset of the layers may have a thickness different from the first subset. In someimplementations, the second optical mirror 204b includes nine quarter- wavelength layers. In some implementations, the second optical mirror 204b includes multiple alternating layers of silicon (e.g., amorphous silicon) and silicon dioxide. In some implementations, the layers of silicon (e.g., amorphous silicon) each have a thickness between 80 nm and 100 nm. In some implementations, the layers of silicon dioxide each have a thickness between 230 nm and 250 nm.

[0044] At step 305, a piezoelectric layer 206 is deposited on the second optical mirror 204b and the first optical mirror 204a is deposited on the piezoelectric layer 206. Depositing the piezoelectric layer 206 on the second optical mirror 204b can include sputtering piezoelectric material (e.g., aluminum nitride) to form a thin layer. In some implementations, the thickness of the piezoelectric layer 206 is between 300 nm and 320 nm. The first optical mirror 204a can include multiple alternating layers of materials with different refractive indices. A first subset of the layers may have the same thickness, while a second subset of the layers may have a thickness different from the first subset. In some implementations, the first optical mirror 204a includes nine quarter- wavelength layers. In some implementations, the first optical mirror 204a includes multiple alternating layers of silicon (e.g., amorphous silicon) and silicon dioxide.

[0045] At step 307, a first conductive layer 308 is deposited on the second side of the substrate, forming the second electrode 202b. The first conductive layer 308 can be made of one or more materials (e.g., a combination thereof), including, but not limited to, titanium, aluminum, indium tin oxide, copper, gold, nickel, and silver. Depositing the conductive layer 308 can include sputtering a conductive material to form a thin layer. The conductive layer 308 can include a single layer or multiple layers. For example, the first conductive layer 308 can include two layers, with a first layer including titanium and second layer including aluminum. In some implementations, the thickness of the first layer is between 5 nm and 15 nm. In some implementations, the thickness of the second layer is between 90 nm and 110 nm. The first conductive layer provides an electrical contact to the substrate.

[0046] At step 309, a portion of the first optical mirror 204a and the piezoelectric layer 206 is removed. Removing a portion of the second optical mirror 204a and piezoelectric layer 206 can include etching their respective layers. Material can be removed to define a raised region 310 and a region 311 surrounding the raised region.

[0047] At step 312, a second conductive layer 313 is deposited on the first optical mirror 204a, forming the first electrode 202a. The second conductive layer 313 can contact surfacesof the raised region 310, which include the upper surface of the first optical mirror 204a and side surfaces of the first optical mirror and the piezoelectric layer 206. The second conductive layer 313 can additionally contact the surrounding region 311, which includes the upper surface of the second optical mirror 204b. The second conductive layer 313 can be made of one or more transparent conductive materials (e.g., indium tin oxide, zinc oxide, fluorinedoped tin oxide, aluminum-doped zinc oxide, etc.). The conductive layer 313 can include a single layer or multiple layers. In some implementations, the thickness of the second conductive layer 312 is between 50 nm and 70 nm. Depositing the second conductive layer 313 can include sputtering and patterning (e.g., lithographic pattering) of the second conductive layer to form a transparent electrode.

[0048] At step 314, one or more bond pads 315 (e.g., two bond pads) are formed on the second conductive layer 313. Forming the bond pads 315 on the second conductive layer 313 can include evaporation and patterning of a conductive material (e.g., metal alloy(s)) on the second conductive layer. The bond pads 315 can be formed of one or more materials (e.g., a combination thereof), including, but not limited to, titanium, aluminum, indium tin oxide, copper, gold, nickel, and silver. In some implementations, the thickness of the bond pads 315 is between 250 nm and 350 nm. The bond pads provide an interface for electrical connections between the OMR and other elements (e.g., the antenna, the external module). In some implementations, the bond pads 315 are positioned on the surrounding region 311. In some implementations, the bond pads 315 are positioned on the raised region 310. In some implementations, the bond pads 315 are positioned on the first conductive layer 308.

[0049] Referring now to Fig. 4A, a photograph of an implementation of a probe of a magnetic field sensor is shown. The probe shown in Fig. 4A was fabricated with a die shown on Fig. 4B. The size of the die was 5* 5mm2and contained different-sized OMRs, which were used to compare the performance of the OMRs. The fiber (e.g., V-groove single-mode fiber) was aligned and epoxied onto a 320pm-diameter OMR on the die using UV-curable epoxy (NOA61). The antenna was attached to the OMR using conductive epoxy (MGC 8331) cured at room temperature. The size of a die with a single OMR can be reduced to 1 x 1 mm2, which can enable a smaller 1.8mm diameter probe, limited by the antenna size.

[0050] Referring now to Figs. 5A and 5B, graphs of the measured reflectance spectra and the measured optical frequency response of the 320 pm-diameter OMR are shown, respectively. The reflectance spectra and optical frequency were obtained using a custom- built reflectometer with a tunable laser (Santec TSL-570) and a power meter (SantecMPM210). The probe exhibited a high Q of 1700, yielding a P of 1.5x 10-3 V-l, which was calculated using the measured device parameters and Equation 2. The high p demonstrated high transduction sensitivity that greatly enhanced the RF signal from the antenna and its SNR. This resulting enhancement enables the miniaturization of the antenna and, hence, the probe. The 320pm-diameter OMR had a bandwidth (BW) of 108MHz, enabling operation inside a 1.5T MRI system with a Larmor frequency of about 64MHz. The OMR operating BW is set by its electrical BW, limited by RC time constant, where R is the resistance from the conductive layer (e.g., the second conductive layer), and C is the capacitance of the layers between the electrodes. This reveals that reducing the probe size increases its BW. Although a 320 pm-diameter OMR was used, a smaller OMR with a higher BW can be used to enable operation in MRI with magnetic fields > 1.5T.

[0051] Referring now to Fig. 6, the magnetic field sensor can include an external module 602 communicatively coupled to the OMR 108. The external module 602 can include a tunable laser 604 (e.g., Santec TSL-570) configured to interrogate the OMR 108 with the antenna 106 in the probe tip (e.g., the end of the optical fiber extending from the antenna). A circulator 606 (e.g., Thorlabs CIR1310) can be optically coupled to the laser 604 and the OMR 108 such that the incident light emitted by the laser 604 is directed to the OMR 108 and reflected light from the OMR 108 is directed to a receiver of the external module. In this way, the circulator 606 separates incident light from the reflected light. Fibers or waveguides can be used to optically couple the circulator 606, the laser 604, and the OMR 108.

[0052] The receiver can be operably coupled to the circulator 606 to read out and process these intensity-modulated optical signals from the probe. The receiver can include a photodetector 608 (e.g, photodiode), a bandpass filter 610, a phase shifter 612, an MRI scanner 614, and / or a data acquisition system (DAQ) 616. In some implementations, the photodetector 608 includes a InGaAs balanced photodetector with a 350MHz BW (e.g, Thorlabs PDB435C). The photodetector 608 can be optically coupled to the circulator 606 such that reflected light is routed to the photodetector 608. The photodetector 608 converts the optical signal into an electrical signal proportional to the intensity of the incoming reflected light. A transimpedance amplifier (TIA) 618 can be electrically coupled to the photodetector 608, converting the photocurrent into a voltage signal that mirrors the modulation of the reflected optical signal, thus representing the RF signal. This resulting RF signal can be filtered with the bandpass filter 610 (e.g., 55-67MHz) and then phase-shifted with the phase shifter 612. The phase shifter 612 can be used to remove a constant phaseoffset in the magnetic field sensor output, which is introduced by the OMR. This phase shift can compensate for the distortion caused by the OMR’s RC-limited bandwidth. In some implementations, the constant phase shift is between 15° and 25°. and 25°. In some implementations, the constant phase shift is about 18°. The bandpass filter 610 and phase shifter 612 can be implemented in hardware (e.g., analog filter circuit or as part of an integrated circuit) and / or software e.g., via a computer or digital signal processor). After filtering and / or phase-shifting, the RF signal can be sent to the DAQ 616 for data analysis and the MRI scanner 614 for tracking the position of the probe tip. The filtered RF signal can be sent to the MRI scanner through its coil plug.

[0053] Fig. 6 also illustrates a test setup which was used to characterize the magnetic field sensor. The setup employs Helmholtz coils that are driven by a signal generator to generate RF magnetic fields at about 64MHz (1.5T MRI Larmor frequency), simulating RF echoes from the tissue. Experiments were conducted with the sensor operated with an incident light power of ImW at a wavelength of 1310.5 nm (that is, the maximum slope of the R curve shown in Fig. 5A).

[0054] The SNR of the magnetic field sensor operated with an incident light power Pin is bounded by its magnetic field sensitivity and two main sources of noise - shot noise and TIA noise at the photodetector - which is given bywhere S and N represent the signal and noise power at the TIA input, and 1 is the responsivity of the photodetector. Equation 3 reveals that the sensor SNR is dependent on not only the antenna gain (Gant) and the OMR modulation gain (P) through AR (=p-Gant B, where B is the magnetic field) but also Pin and BW. As expected, a larger Pin results in a better SNR in the electro-optic sensor with a fixed 108MHz BW.

[0055] Fig. 7 shows the measured sensor response as a function of the input magnetic field in the range of 50-300nT. The sensor demonstrated 5.2V / mT sensitivity with a linearity of -0.99. Fig- 8 shows the sensor input-referred noise spectrum with a 64MHz, 300nT magnetic field input, demonstrating that the sensor achieves an SNR of 43 dB and a minimum detectable magnetic field of 20pT / Hz with ImW incident optical power and a 108MHz BW. The measured noise floor of the sensor is in excess of the shot noise limit (6.7pA / Hz) by7dB, dominated by the TIA noise. Note that the input-referred spectral noise was computed from the data collected by dividing the sensor output voltage by its sensitivity.

[0056] Phase and frequency encoding techniques were used in MRI to detect and track the in vivo position of the catheter tip in the body. These techniques relied on an RF field sensor that had a linear phase response and a flat frequency response over a certain BW (e.g., RF signal BW is 100kHz for a 1.2m long 1.5T MRI bore). The sensor was tested with a magnetic field input at various phases and frequencies. Fig. 9A and Fig. 9B show the measured output responses to a 300nT magnetic field input at different phases from 0-60° at 64MHz and at different frequencies between 63.88-64.12MHz, respectively. The sensor exhibited a linear input-output phase relationship with R2 > 0.99, a flat frequency response over 240kHz with a standard deviation of 1.4%, and a 20pT / Hz resolution, verifying the probe’s ability to be localized by an MRI scanner (a 1.5T MRI system).

[0057] The disclosed system may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The foregoing implementation are therefore to be considered in all respects illustrative, rather than limiting of the invention. Having thus described several illustrative implementations, it is to be appreciated that various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to form a part of this disclosure, and they are intended to be within the spirit and scope of this disclosure. While some examples presented herein involve specific combinations of functions or structural elements, it should be understood that those functions and elements may be combined in other ways according to the present disclosure to accomplish the same or different objectives. In particular, acts, elements, and features discussed in connection with one implementation are not intended to be excluded from similar or other roles in other implementations. Additionally, elements and components described herein may be further divided into additional components or joined together to form fewer components for performing the same functions. Accordingly, the foregoing description and attached drawings are by way of example only, and they are not intended to be limiting.

Claims

CLAIMS1. A magnetic field sensor, comprising: an optical microresonator having a first side and a second side; an optical fiber coupled to and extending from the first side; and an antenna having a first end, a second end, and a length therebetween, the first end coupled to the first side of the optical microresonator and the second end coupled to the second side, with the length of the antenna positioned proximate to the optical microresonator.

2. The magnetic field sensor of claim 1, further comprising an external module operably coupled to the optical microresonator.

3. The magnetic field sensor of claim 2, the external module comprising: a laser interrogating the optical microresonator with an incident light signal; a circulator separating an optical signal including a reflected light signal from the optical microresonator and the incident light signal; and a receiver processing the reflected light signal.

4. The magnetic field sensor of claim 3, the receiver comprising: a trans-impedance photodetector converting the reflected light signal into an electrical signal; and a bandpass filter filtering the electrical signal.

5. The magnetic field sensor of claim 4, the receiver further comprising: a phase shifter altering the phase of the filtered electrical signal.

6. The magnetic field sensor of claim 4, the external module further comprising: a data acquisition system for data analysis of the electrical signal.

7. The magnetic field sensor of claim 1, the optical fiber housed in a V-groove.

8. The magnetic field sensor of claim 1, the optical fiber comprising one of a single mode optical fiber and a multi-mode optical fiber.

9. The magnetic field sensor of claim 1, the optical microresonator comprising: a first and a second optical mirror; and a piezoelectric layer disposed between the first and the second optical mirror.

10. The magnetic field sensor of claim 9, the optical microresonator further comprising: a first and a second electrode, the first and second optical mirror positioned between the first and second electrode.

11. The magnetic field sensor of claim 10, the first electrode comprising one of aluminum, gold, platinum, and titanium.

12. The magnetic field sensor of claim 10, the second electrode comprising a transparent conductive oxide.

13. The magnetic field sensor of claim 12, the second electrode comprising one of indium tin oxide and zinc oxide.

14. The magnetic field sensor of claim 10, the first end of the antenna electrically coupled to the first electrode; and the second end of the antenna electrically coupled to the second electrode.

15. The magnetic field sensor of claim 10, the first electrode adjacent to the first optical mirror; and the second electrode spaced from the second optical mirror.

16. The magnetic field sensor of claim 15, further comprising: a substrate; and an antireflection coating layer disposed on the substrate; wherein the substrate and the antireflection coating layer are positioned between the second electrode and the second optical mirror.

17. The magnetic field sensor of claim 16, the substrate comprising silicon.

18. The magnetic field sensor of claim 9, the piezoelectric layer comprising one of aluminum nitride (AIN), scandium-doped aluminum nitride (ScAlN), barium titanate (BTO), lead zirconate titanate (PZT), and lithium niobate (LiNbCh).

19. The magnetic field sensor of claim 9, the first and the second optical mirror comprising Distributed Bragg Reflector mirrors.

20. The magnetic field sensor of claim 9, the first and the second optical mirrors each comprising one of alternating layers of silicon and silicon dioxide and alternating layers of silicon and silicon nitride.

21. The magnetic field sensor of claim 1, the length of the antenna positioned around the optical fiber.

22. The magnetic field sensor of claim 21, the antenna positioned in a coaxial configuration around the optical fiber.

23. The magnetic field sensor of claim 1, the antenna comprising one of a coil antenna and a dipole antenna.

24. A method of manufacturing an optical microresonator, comprising: providing a substrate having a first side and a second side;depositing an antireflection coating layer on the first side of the substrate; depositing a first optical mirror on the antireflection coating layer; depositing a piezoelectric layer on the first optical mirror; depositing a second optical mirror on the piezoelectric layer; depositing a first conductive layer on the second optical mirror; and depositing a second conductive layer on the second side of the substrate.

Images