The disclosed technology relates generally to MRI safety evaluation, and more specifically to acousto-optical sensors for measuring localized radio frequency (RF) electric and magnetic fields and associated temperatures indicative of specific absorption rates (SAR).
Magnetic resonance imaging (MRI) utilizes a combination radio frequency (RF) waves and magnetic fields to non-invasively create detailed images of tissues, organs, etc., of a subject's body. During an MRI examination, three types of fields are typically employed to produce three dimensional images: (1) a strong static magnetic field (typically up to 3 T) that generates a net proton magnetization vector in the human body; (2) a gradient magnetic field, typically in the frequency range of 100 to 1,000 Hz, which is utilized to localize aligned protons inside the body, thus allowing spatial reconstruction of tissue sections into images; and (3) a radio frequency (RF) electromagnetic wave, typically in the frequency range of 10 to 400 MHz, which is utilized to energize the magnetization vector allowing conversion of tissue properties into images. Different levels of contrast are based on the different magnetic properties and physical structure of the biological tissues.
While it is well established that the non-ionizing electromagnetic energy produced during an MRI scan can impose fewer health risks compared with the ionizing radiation associated with x-rays, local RF-induced currents and/or specific absorption rates (SAR) of tissue, fluid, implants, etc., under MRI can lead to hot spots and burns, as discussed in “Biological Effects and Safety in Magnetic Resonance Imaging: A Review,” (Hartwig, et al., Int J Environ Res Public Health, 2009), which is incorporated herein by reference in its entirety.
Certain types of monitoring equipment and/or implantable devices can include elongated conductors that can heat up in the presence of the MRI fields (for example, due to RF-induced eddy currents) and cause burns upon contact with the patient. U.S. Patent Publication No. US2017/0143234, which is incorporated herein by reference in its entirety, discloses a device for active device visualization under MRI and provides certain solutions to address the risk of RF-induced heating by using acousto-optical sensors and optical fiber.
Another major recognized mechanical risk associated with MRI is the presence of metallic (ferromagnetic) devices that can be subject to the attractive and rotational forces caused by the static field. Thus, the presence of certain devices and implants, such as catheters, heart valve prostheses, coronary artery stents, aortic stent grafts, pacemakers, implantable defibrillators, etc., can create thermal and mechanical safety hazards in the MRI environment.
To assess the potential dangerous biological effects associated with MRI on biological tissues, there is a need for small MRI dosimeters that can measure localized field strengths, associated thermal responses, and/or evaluate effects caused by the presence of an implant during MRI scans. However, currently available dosimeters are bulky, and typically include cabling with conductors that can heat up and/or have associated output signals corrupted by the RF field induced over the conductive lines. These dosimeters, are, therefore, not suitable for in-situ measurements of RF fields over the patient's body. Certain exemplary implementations are disclosed herein to address the above-referenced needs and risks.
In addition to SAR measurements during MRI scans, there is also a need for mapping RF magnetic fields, so-called B1 fields, both during MRI scans as well as during the testing of implants for MRI safety classification and qualification. For example, the test methods for ISO (the International Organization for Standardization) standards such as ISO/TS 10974:2018 “Assessment of the safety of magnetic resonance imaging for patients with an active implantable medical device,” requires that B1 fields be measured. Accurate and local measurements of RF Electric (E) field and B1 field measurements are critical for MRI safety determination.
Miniature RF E field and B1 field sensors placed on flexible catheters or surgical tubes are also desired in characterizing and mapping the RF induced heating effects during the manufacture and testing of medical implants such as cardiac pacemakers and neurostimulation devices implanted in the central nervous system (brain and spinal cord). The RF induced currents on these implant wires depend on their exact shape and location in the body (or phantoms during testing). Mapping the variation of RF induced currents over these wires with a high spatial resolution during testing these devices under MRI fields through E field and B1 field measurements help determine the safety of particular implant and preferred shape. Currently available bulky dosimeters are not suitable for this purpose.
During MRI scans or MRI device safety testing, simultaneous measurement of RF field components at multiple points on the patient body or along the implant wiring is desired as these field components can show significant spatial variation.
Briefly described, certain exemplary implementations of the disclosed technology include acousto-optical sensors for measuring localized RF fields and associated temperatures indicative of specific absorption rates and/or presence of implantable devices. Some or all of the above needs may be addressed by certain implementations of the disclosed technology.
According to an exemplary implementation of the disclosed technology, a method is provided for measuring a local E-field during an MRI scan. The method includes: providing an acousto-optical sensor, the acousto-optical sensor including: an optical fiber including a distal end; an acousto-optical sensor region disposed towards the distal end of the optical fiber, the acousto-optical sensor region including an electro-mechanical conversion assembly comprising: an antenna configured to receive E-field radio-frequency (RF) energy and to produce a corresponding electrical signal; and an ultrasonic transducer in mechanical communication with the acousto-optical sensor region, wherein the ultrasonic transducer is in electrical communication with the antenna, and wherein the ultrasonic transducer is configured to elastically modulate the acousto-optical sensor region by acoustic waves generated responsive to an electrical signal corresponding to RF energy received at the antenna. The method includes positioning the acousto-optical sensor at a first location of a body; receiving, with the antenna, MRI RF energy localized at the first location; interrogating, with a light source, and via the optical fiber, the acousto-optical sensor region; detecting, with a photodetector, interrogation light reflected from the acousto-optical sensor region; and outputting an E-field signal corresponding to the detected interrogation light reflected from the acousto-optical sensor region, wherein the E-field signal corresponds to an E-field of the received MRI RF energy at the first location.
According to another exemplary implementation of the disclosed technology, a method is provided for measuring a local B-field during an MRI scan. The method includes providing an acousto-optical sensor, the acousto-optical sensor including: an optical fiber including a distal end; an acousto-optical sensor region disposed towards the distal end of the optical fiber, the acousto-optical sensor region including an electro-mechanical conversion assembly comprising: a loop antenna configured to receive B-field radio-frequency (RF) energy and to produce a corresponding electrical signal; and an ultrasonic transducer in mechanical communication with the acousto-optical sensor region, wherein the ultrasonic transducer is in electrical communication with the loop antenna, and wherein the ultrasonic transducer is configured to elastically modulate the acousto-optical sensor region by acoustic waves generated responsive to an electrical signal corresponding to RF energy received at the loop antenna. The method includes positioning the acousto-optical sensor at a first location of a body; receiving, with the loop antenna, MRI RF energy localized at the first location; interrogating, with a light source, and via the optical fiber, the acousto-optical sensor region; detecting, with a photodetector, interrogation light reflected from the acousto-optical sensor region; and outputting a B-field signal corresponding to the detected interrogation light reflected from the acousto-optical sensor region, wherein the B-field signal corresponds to a B-field of the received localized MRI RF energy at the first location.
According to another exemplary implementation of the disclosed technology, a method is provided for attaching one or more acousto-optical sensors on a surface of a body. The method includes: providing an acousto-optical sensor, the acousto-optical sensor including a first optical fiber including a distal end; a first acousto-optical sensor region disposed at a first position towards the distal end of the first optical fiber, the first acousto-optical sensor region including an electro-mechanical conversion assembly comprising: a first antenna configured to receive radio-frequency (RF) energy and to produce a corresponding first electrical signal; and a first ultrasonic transducer in mechanical communication with the first acousto-optical sensor region, wherein the first ultrasonic transducer is in electrical communication with the first antenna, and wherein the first ultrasonic transducer is configured to elastically modulate the first acousto-optical sensor region by acoustic waves generated responsive to an electrical signal corresponding to RF energy received at the first antenna. The method includes positioning the acousto-optical sensor on a surface of a body, and securing the acousto-optical sensor to the surface of the body
According to another exemplary implementation of the disclosed technology, a method is provided for measuring a local temperature and one or more of a local E-field or a local B-field during an MRI scan. The method includes providing a combined thermo-optical and acousto-optical sensor, the combined thermo-optical and acousto-optical sensor including: an optical fiber including a distal end; a fiber Bragg grating (FBG) disposed towards the distal end of the optical fiber; an electro-mechanical conversion assembly comprising: an antenna configured to receive radio-frequency (RF) energy and to produce a corresponding electrical signal; and an ultrasonic transducer in mechanical communication with the FBG, wherein the ultrasonic transducer is in electrical communication with the antenna, and wherein the ultrasonic transducer is configured to elastically modulate the FBG by acoustic waves generated responsive to an electrical signal corresponding to RF energy received at the antenna. The method includes positioning the combined thermo-optical and acousto-optical sensor at a first location of a body; receiving, with the antenna, MRI RF energy at the first location; interrogating, with a light source, and via the optical fiber, the FBG; detecting, with a photodetector, an interrogation signal based on light reflected from the FBG; processing the interrogation signal; and outputting, based on processing the interrogation signal, one or more of: a field signal corresponding to the received MRI RF energy at the first location; and a temperature signal corresponding to a wavelength shift in the light reflected from the FBG.
According to another exemplary implementation of the disclosed technology, a method is provided for measuring local temperature and one or more of a local E-field or local B-field during an MRI scan. The method includes providing a combined thermo-optical and acousto-optical sensor, the combined thermo-optical and acousto-optical sensor including: an optical fiber including a distal end; a fiber Bragg grating (FBG) disposed towards the distal end of the optical fiber; a GaAs-based temperature detector disposed at the distal end of the optical fiber; and an electro-mechanical conversion assembly comprising: an antenna configured to receive radio-frequency (RF) energy and to produce a corresponding electrical signal; and an ultrasonic transducer in mechanical communication with the FBG, wherein the ultrasonic transducer is in electrical communication with the antenna, and wherein the ultrasonic transducer is configured to elastically modulate the FBG by acoustic waves generated responsive to an electrical signal corresponding to RF energy received at the antenna. The method includes positioning the combined thermo-optical and acousto-optical sensor at a first location of a body; receiving, with the antenna, MRI RF energy at the first location; interrogating, with a light source, and via the optical fiber, the FBG and the temperature detector; detecting, with a photodetector, an interrogation signal based on light reflected from the FBG and the temperature detector; processing the interrogation signal; and outputting, based on processing the interrogation signal, one or more of: a field signal corresponding to the received MRI RF energy at the first location; and a temperature signal corresponding to a spectrum shift in the light reflected from the temperature detector.
According to another exemplary implementation of the disclosed technology, a method is provided for mapping effects of MRI over an implant. The method includes: providing an acousto-optical sensor, the acousto-optical sensor including: an optical fiber including a distal end; an acousto-optical sensor region disposed towards the distal end of the optical fiber, the acousto-optical sensor region including an electro-mechanical conversion assembly comprising: an antenna configured to receive radio-frequency (RF) energy and to produce a corresponding electrical signal; and an ultrasonic transducer in mechanical communication with the acousto-optical sensor region, wherein the ultrasonic transducer is in electrical communication with the antenna, and wherein the ultrasonic transducer is configured to elastically modulate the acousto-optical sensor region by acoustic waves generated responsive to an electrical signal corresponding to RF energy received at the antenna. The method includes mounting at least a distal end of the acousto-optical sensor in a housing configured to move the acousto-optical sensor around at least a portion of the implant; sequentially positioning the mounted acousto-optical sensor at a plurality of locations within a body in a region of the implant; sequentially receiving, with the antenna, MRI RF energy localized at the corresponding plurality of locations; sequentially interrogating, with a light source, and via the optical fiber, the acousto-optical sensor region; sequentially detecting, with a photodetector, corresponding interrogation light reflected from the acousto-optical sensor region; and sequentially outputting field signals corresponding to the sequentially detected interrogation light reflected from the acousto-optical sensor region, wherein the field signals correspond to the received MRI RF energy at the plurality of locations.
According to another exemplary implementation of the disclosed technology, a method is provided for simultaneously mapping effects of MRI over an implant. The method includes: providing an acousto-optical sensor, the acousto-optical sensor including: an optical fiber including a distal end; a plurality of acousto-optical sensor regions disposed towards the distal end of the optical fiber, each of the plurality of acousto-optical sensor regions including an electro-mechanical conversion assembly comprising: a fiber Bragg grating (FBG); an antenna configured to receive radio-frequency (RF) energy and to produce a corresponding electrical signal; and an ultrasonic transducer in mechanical communication with the FBG wherein the ultrasonic transducer is in electrical communication with the antenna, and wherein the ultrasonic transducer is configured to elastically modulate the FBG by acoustic waves generated responsive to an electrical signal corresponding to RF energy received at the antenna. The method further includes mounting at least a distal end of the acousto-optical sensor in a housing configured to position and move the acousto-optical sensor around at least a portion of the implant; positioning the mounted acousto-optical sensor within a body in a region of the implant such that each of the plurality of acousto-optical sensor regions are disposed at a plurality of corresponding locations; receiving, with each antenna of the plurality of acousto-optical sensor regions, MRI RF energy localized at the plurality of corresponding locations; interrogating, with a light source, and via the optical fiber, the acousto-optical sensor; detecting, with a photodetector, corresponding interrogation light reflected from the acousto-optical sensor; and outputting field signals corresponding to the detected interrogation light reflected from the acousto-optical sensor, wherein the field signals correspond to the received MRI RF energy at the plurality of corresponding locations.
Other implementations, features, and aspects of the disclosed technology are described in detail herein and are considered a part of the claimed disclosed technology. Other implementations, features, and aspects can be understood with reference to the following detailed description, accompanying drawings, and claims.
Reference will now be made to the accompanying figures and flow diagrams, which are not necessarily drawn to scale, and wherein:
The disclosed technology relates to sensor devices, systems, and methods that may be utilized for evaluating safety in certain magnetic resonance imaging (MRI) applications. Certain exemplary implementations of the disclosed technology may include acousto-optical sensors for measuring localized RF fields and/or associated temperature increases indicative of specific absorption rates (SAR). Certain aspects of the disclosed technology are related to U.S. Patent Publication No. US2017/0143234, the contents of which is incorporated by reference in its entirety as if set forth in full.
U.S. Patent Publication No. US2017/0143234 describes an acousto-optical (AO) catheter probe incorporating an active receiver that can modulate interrogation light at a frequency of the localized MRI gradient field. The reflected and modulated light can be utilized to determine the location of the probe. The probe can include a receiver coil in communication with a piezoelectric transducer that is coupled to an acousto-optical sensor region of an optical fiber. For example, the acousto-optical sensor region can include a fiber Bragg grating (FBG). The optical fiber serves as a transmission line that enables the elimination of typically elongated lead wire conductors that can heat up (and damage surrounding tissue) in the presence of the MRI equipment's electromagnetic RF field. The piezoelectric transducer is directly in contact with the optical fiber over the FBG region and generates acousto-optical modulation signals directly on the fiber. Using a thin film piezoelectric layer directly deposited on the fiber partially or fully over the circumference, the elastic waves generated by the piezoelectric layer may be cylindrically focused on the core of the optical fiber where it is most effective. This technique presents efficient acousto-optical modulation at a target frequency for locating the receiver coil position.
The disclosed technology includes certain advancements and improvements, that when combined with the technology disclosed in U.S. Patent Publication No. US2017/0143234 may be utilized to produce an improved device that can address certain challenges, limitations, and issues associated with prior devices.
RF-induced heating can impact patients during an MRI as the locally induced current distribution over the patient's body can cause temperature rise leading to hot spots and burns. This is because the specific absorption rate (SAR) values reported by the MRI manufacturers are obtained over phantoms, and the estimated SAR values may not reflect the real situation with the variety of the patients scanned.
Certain features and devices disclosed herein can include an MRI dosimeter configured to measure local radio frequency (RF)-induced currents and/or associated increases in temperature (essentially SAR) over an area on the patient's body with a low profile, non-invasive and MRI compatible sensor. Certain exemplary implementations of the disclosed technology can include methods and techniques that enable measurements of local electric and/or magnetic fields and/or the associated temperature effects thereof as MRI fields interact with bodily implants, associated lead conductors, phantoms, etc. Certain exemplary implementations of the disclosed technology may be utilized for implantable device evaluation (for example, within phantoms) with increased resolution and/or reduced RF interference.
Certain elements of the disclosed technology may further utilize electrical-to-mechanical energy conversion via a piezoelectric transducer for receiver signal extraction. In accordance with certain exemplary implementations, acousto-optical modulation on the optical fiber (i.e., mechanical-to-optical signal conversion) may be utilized for signal detection and transmission. In certain exemplary implementations, a fiber Bragg grating (FBG) may be utilized in conjunction with the piezoelectric transducer. The resulting combination may yield improved devices that can include MRI safe active receivers and/or location markers without conducting transmission lines and without compromising mechanical performance.
The receiver antenna 108 may be connected to an ultrasonic transducer 112, which may be in intimate contact with the FBG 114 region. In certain exemplary implementations, the ultrasonic transducer 112 may be a piezoelectric transducer that is driven by the receiver antenna 108. Certain exemplary implementations of the device 102 can include a rigid connection support 110 for holding the receiver antenna 108 and/or the associated short interconnections 109 to the transducer 112. In certain implementations, the optical fiber 106 may be held in place with one or more O-rings 107 or spacers.
Certain implementations may include additional antenna connected in parallel with the piezoelectric transducer 112. In certain exemplary implementations, one or more antennae may be utilized to provide additional sensors along the length of the device 102.
In certain exemplary implementations, light 205 emitted from the laser source 206 (such as a swept laser) may traverse the optical fiber 106 and a portion of the incident light may be reflected (as a function of wavelength) by the FBG 114 such that reflected light 207 is modulated corresponding to the RF signal 202 frequency, the wavelength of the laser source 208, and as a function of the reflectivity curve (see
According to another exemplary implementation of the disclosed technology (not shown), the piezoelectric transducer 112 may be mechanically coupled directly to the optical fiber 106. In this embodiment, the corresponding acoustic waves 204 produced in the optical fiber 106 and generated by the piezoelectric transducer 112 may modulate the elastic properties of the fiber at the RF frequencies (via the AO effect) corresponding to the RF signal 202, which in turn can be detected by laser-based interferometric sensing. Because the optical fiber 106 is not conductive, the sensor is immune to RF interference and heating along its length.
In certain exemplary implementations, the distal tip electrodes 302 may be connected directly to the piezoelectric transducer 112. In other exemplary implementations (as shown in
In accordance with certain exemplary implementations of the disclosed technology, in addition to sensing a local E-field, the acousto-optical MRI dosimeter device 102 can also have an integrated temperature sensor for RF safety monitoring. In certain exemplary implementations, this temperature sensor can be the same FBG 114 as discussed above. When the same FBG 114 is used for both RF field (E-field and/or B-field) and temperature sensing, the slowly varying wavelength shift due to temperature change can be used as an indicator of temperature. Alternatively, the low slope region of the FBG reflectance spectrum (such as shown in
In yet another exemplary embodiment, the temperature sensing FBG can be disposed on a separate optical fiber and packaged next to the RF sensing FBG. In this case, the temperature sensing FBG can be interrogated using a separate system, but the readings from both the temperature sensing FBG and the RF sensing FBG can be synchronized and displayed together to observe the correlation between the sensors.
Yet in another alternative embodiment, a temperature sensor may be integrated into the acousto-optical based MRI dosimeter device, for example, by attaching the temperature sensor at the end of the optical fiber. In certain exemplary implementations, a reflection spectrum corresponding the localized temperature may be monitored to provide absolute temperature information, for example, as discussed in “A New Fiber Optical Thermometer and its Application for Process Control in Strong Electric, Magnetic, and Electromagnetic Fields,” Roland et al, Sensor Letters, Vol. 1, pp. 93-98, 2003, which is incorporated by reference herein as if presented in full. Such gallium arsenide (GaAs) crystal-based optical temperature sensors are well known, but they have not previously been integrated with acousto-optical RF sensors.
In accordance with certain exemplary implementations of the disclosed technology, a broadband light source or a swept laser source may be coupled into the fiber and the reflectance spectrum may be measured. Since GaAs behaves as a temperature sensitive cut-off filter in which the crystal absorbs some light and transmits other light depending on temperature, the characteristic edge, or transition wavelength, between the reflected and transmitted spectrum is directly related to the band gap energy, and hence the absolute temperature. In certain exemplary implementations, photo energy from the light source may excite electrons from the valence to the conduction band of the semiconductor crystal. The required amount of energy is equal to the so-called band-gap energy, Egap. The well-known underlying principle of operation is based on the temperature dependence of the band gap of the direct (zone center) GaAs intrinsic gap; Egap=1.423 eV, corresponding to 872 nm at 300° K; dEgap/dT=−0.452 meV/° K at 300° K. According to an exemplary implementation of the disclosed technology, a GaAs sensor crystal may be placed in a medium of unknown temperature and the reflectance spectrum may be measured. In certain exemplary implementations, the wavelength position of the characteristic edge may be analyzed to determine the temperature.
RF-induced heating over interventional devices during an MRI scan is subject to guidelines and regulations by the US Food and Drug Administration by the International Electrotechnical Commission. The guidelines are summarized in Table 1.
Most of the frequently used active implantable medical devices (AIMD) such as cardiac pacemakers, defibrillators and neurostimulators should be evaluated in terms of their availability for MRI studies. Electromagnetic field and implantable device interactions may induce current/heating. Implant lead malfunctions, unexpected pacing behavior, inappropriate shocks, ventricular defibrillation, and even deaths have been reported. The American Society of Testing Materials (ASTM) designates devices as MR safe, MR conditional, and MR unsafe through ASTM F2053-13 “Standard Practice for Marking Medical Devices and Other Items for Safety in the Magnetic Resonance Environment” Standard. “MR Safe” medical devices are composed of nonmetallic materials and systems. Therefore, it is very difficult to design an implantable pacemaker or ICD system that can be assigned as “MR Safe”. “MR Conditional” refers to devices which pose no known hazards to patients when MRI scans are performed with specific conditions and the approval of a device requires strict definition of these conditions. Thus, each medical device should be tested extensively to determine safe MRI scan conditions.
The presence of ferromagnetic device components in the strong static and gradient magnetic fields of the MR system can lead to unintentional movement or vibration of the implant devices. Accordingly, implantable medical devices should be manufactured and tested using MR Compatible materials. The long conductors of pacing leads may behave as antennas and interact with RF waves during MR scans. Through this interaction, the transfer of RF energy to pace leads may induce heat and the amount of temperature rise is dependent on factors including pulse sequence parameters (i.e. flip angle, TR/TE ratio), the whole body averaged, and local specific absorption rates (SAR) associated with a given sequence, lead factors such as lead design, length, placement configuration, and orientation.
Many factors such as lead length, lead configuration within the phantom, phantom shape and MR surface coil position affect induced heating over pace leads. Mechanical and electrical requirements of lead design have prompted multilayer and non-homogenous device design—such as alternating winding configurations of lead wires. It has been shown that increasing the lead insulation thickness along the lead wire can cause increased heating in that section. Therefore, it is very important to have a heating profile of overall lead wires instead of just the lead tip. Another study has shown that when the lead wire is near the phantom edge, the RF induced heating over the lead wire also increases.
The interaction of an AIMD and the RF field of an MR scanner is a complicated process and depends on AIMD design, AIMD location within the body, bird cage, and surface coils design, patient size, anatomy and tissue properties. Depending on the specific conditions, in vivo temperature rise variation may span several orders of magnitude. For example, a pacemaker lead implanted in an anatomical region that receives minimal RF exposure may have lower RF induced heat compare to a shorter neurostimulator lead implanted in a region receiving higher RF exposure. Therefore, FDA asks all AIMD manufacturers to test their devices according to an additional standard, the ISO/TS 10974 “Assessment of the safety of magnetic resonance imaging for patients with an active implantable medical device.” The tests for this standard include both E-field and B1-field measurement, which can be achieved using dipole, monopole, and/or microstrip antennas for E-field, and coil or loop-type antennas which can be wire wound or printed on printed circuit boards or different dielectric substrates for B-field. However, current commercially available probes tend to have a large diameter probe tip, a rigid probe shaft, and bulky circuitry, which can limit tests, such as measuring the RF induced current variation along a small profile lead wires. Also, because the long probe shaft may interact with RF waves, users should calibrate their probes for each MRI in vitro phantom tests. Certain exemplary implementations of the disclosed technology can provide a compact, flexible and RF immune sensor to map the E-field or current distribution over these AIMD leads, including the distal tip electrode and the overall lead shaft.
The RF induced heating issue can be particularly problematic when the patients have implanted medical devices having metallic components, which can locally amplify or change the induced E-fields. The current and temperature distribution over these implants should be carefully measured for each device in a variety of configurations to mimic clinical use conditions within phantoms. For example, a lead may be tested by moving an RF sensor along the length of the lead, or by passing the lead through a sensing coil and moving the sensing coil along the lead.
Certain exemplary implementations of the disclosed technology may be utilized to evaluate active implantable medical device leads for MRI safety evaluation. Furthermore, as indicated in
In clinical use, RF-induced heating of tissue surrounding an active implantable medical device can be caused by increased local SAR that arises from induced current along the conductive structures. Furthermore, eddy currents induced by the MRI gradient field can cause heating of the implantable device and/or associated lead wires. According to an exemplary implementation of the disclosed technology, acousto-optical based MRI dosimeter devices (such as the device 102 discussed herein) may be used to map/measure the SAR locally over the skin.
In accordance with certain exemplary implementations of the disclosed technology, the acousto-optical based MRI dosimeter disclosed herein may include an integrated temperature sensor to provide temperature information. In certain exemplary implementations, the disclosed technology may allow investigation of factors, such as the relationship between the SAR and the squared electric field. Other factors may be investigated utilizing the disclosed technology, such as B field coupling, standing wave formation, and gradient coil effects.
In accordance with certain exemplary implementations of the disclosed technology, the one or more of the features of the enhanced devices 700, 706, as discussed above with reference to
In certain exemplary implementations, the distance between the distal end 704 and the notch 702 or ring feature 710 can be optimized based on the acoustic fields generated at the Larmor frequency. Since these geometrical features are far away from the core region of the optical fiber 106, the propagating light 405, 407 and its interaction with the FBG 114 may not be adversely affected by the acoustic resonators 701, 708. There are many different ways of implementing such acoustic resonators 701, 708 such as reflectors formed by small but periodic perturbations of the structure of the optical fiber 106, where the periodicity is determined by the wavelength of the acoustic waves 404. In certain exemplary implementations, a quality factor of acoustic resonators 701, 708 may be adjusted so that the bandwidth of the device is still large enough to cover the typical MRI signal bandwidth of about 100 kHz.
In certain exemplary implementations, the antenna is selected to be one of a dipole antenna, a monopole antenna, or a microstrip antenna.
Certain implementations can include calibrating the acousto-optical sensor, which can include receiving, with the acousto-optical sensor, test RF energy having a known field strength; outputting a test signal corresponding to detected interrogation light reflected from the acousto-optical sensor region, wherein the test signal corresponds to the received test RF energy; determining a calibration coefficient, the calibration coefficient comprising a ratio of the test signal amplitude and the known field strength, and applying the calibration coefficient to the E-field signal to produce a calibrated output signal.
In certain exemplary implementations, the first location is on the surface of the body. In other implementations, the first location is within the body. In certain exemplary implementations, the body is a human patient or a phantom.
According to an exemplary implementation of the disclosed technology, the acousto-optical sensor region comprises a fiber Bragg grating (FBG).
Certain implementations can include calibrating the acousto-optical sensor, which can include receiving, with the acousto-optical sensor, test RF energy having a known field strength; outputting a test signal corresponding to detected interrogation light reflected from the acousto-optical sensor region, wherein the test signal corresponds to the received test RF energy; determining a calibration coefficient, the calibration coefficient comprising a ratio of the test signal amplitude and the known field strength, and applying the calibration coefficient to the B-field signal to produce a calibrated output signal.
In certain exemplary implementations, securing the acousto-optical sensor to the surface of the body can include covering at least a portion of the acousto-optical sensor and at least a portion of the surface of the body with biocompatible adhesive tape.
In certain exemplary implementations, the first antenna is a loop antenna configured to receive localized MRI B-field RF energy. The first antenna is chosen from one of a dipole antenna, a multipole antenna, or a microstrip antenna configured to receive localized MRI E-field RF energy.
Certain exemplary implementations may include providing a second antenna configured to receive radio-frequency (RF) energy for elastically modulating a second acousto-optical sensor region by acoustic waves generated responsive to an electrical signal corresponding to RF energy received at the second antenna. In certain exemplary implementations, the second antenna is a loop antenna configured to receive localized MRI B-field RF energy. In certain exemplary implementations, the first antenna is chosen from one of a dipole antenna, a multipole antenna, or a microstrip antenna configured to receive localized MRI E-field RF energy.
In certain exemplary implementations, providing the acousto-optical sensor can further include providing: a second optical fiber including a distal end; a second acousto-optical sensor region disposed towards the distal end of the second optical fiber, the second acousto-optical sensor region including an electro-mechanical conversion assembly comprising: a second antenna configured to receive radio-frequency (RF) energy and to produce a corresponding second electrical signal; and a second ultrasonic transducer in mechanical communication with the second acousto-optical sensor region, wherein the second ultrasonic transducer is in electrical communication with the second antenna, and wherein the second ultrasonic transducer is configured to elastically modulate the second acousto-optical sensor region by acoustic waves generated responsive to the electrical signal received at the second antenna. In certain exemplary implementations, the second antenna may be a loop antenna configured to receive localized MRI B-field RF energy. In certain exemplary implementations, the first antenna is chosen from one of a dipole antenna, a multipole antenna, or a microstrip antenna configured to receive localized MRI E-field RF energy.
In certain exemplary implementations, the acousto-optical sensor regions include a fiber Bragg grating (FBG).
In certain exemplary implementations, the antenna is a loop antenna configured to receive localized MRI B-field RF energy, and wherein the field signal corresponds to the localized MRI B-field RF energy.
In certain exemplary implementations, the antenna is chosen from one of a dipole antenna, a multipole antenna, or a microstrip antenna configured to receive localized MRI E-field RF energy, and wherein the field signal corresponds to the localized MRI E-field RF energy.
In certain exemplary implementations, the temperature signal corresponds to a slowly varying wavelength shift in the light reflected from the FBG, wherein the slowly varying wavelength shift is characterized by a component of the interrogation signal having a bandwidth less than 100 Hz, and wherein the slowly varying wavelength shift corresponds to one or more of a thermal expansion and a thermally-induced refractive index change of the FBG.
In certain exemplary implementations, the antenna is a loop antenna configured to receive localized MRI B-field RF energy, and wherein the field signal corresponds to the localized MRI B-field RF energy.
In certain exemplary implementations, the antenna is chosen from one of a dipole antenna, a multipole antenna, or a microstrip antenna configured to receive localized MRI E-field RF energy, and wherein the field signal corresponds to the localized MRI E-field RF energy.
In certain exemplary implementations, the antenna is a loop antenna configured to receive localized MRI B-field RF energy, and wherein the field signals correspond to localized MRI B-field RF energy measurements at the corresponding plurality of locations.
In certain exemplary implementations, the antenna is chosen from one of a dipole antenna, a multipole antenna, or a microstrip antenna configured to receive localized MRI E-field RF energy, and wherein the field signals correspond to the localized MRI E-field RF energy measurements at the corresponding plurality of locations.
In certain exemplary implementations, the acousto-optical sensor comprises a fiber Bragg grating (FBG).
Certain exemplary implementations can further include: processing the field signals; and outputting, based on processing the field signals, a plurality of temperature signals corresponding to the plurality of locations, wherein the temperature signals corresponds to a slowly varying wavelength shift in the light reflected from the FBG.
In certain exemplary implementations, the acousto-optical sensor may be disposed in housing is characterized by a tube shape.
In certain exemplary implementations, the antenna is a loop antenna configured to receive localized MRI B-field RF energy, and wherein the field signals correspond to localized MRI B-field RF energy measurements at the plurality of corresponding locations.
In certain exemplary implementations, the antenna is chosen from one of a dipole antenna, a multipole antenna, or a microstrip antenna configured to receive localized MRI E-field RF energy, and wherein the field signals correspond to the localized MRI E-field RF energy measurements at the plurality of corresponding locations.
In accordance with certain exemplary implementations of the disclosed technology, the method 1600 can further include processing the field signals; and outputting, based on processing the field signals, a plurality of temperature signals corresponding to the plurality of locations, wherein the temperature signals corresponds to a slowly varying wavelength shift in the light reflected from the FBG.
Certain exemplary implementations of the disclosed technology may include an optical fiber having a distal end; an acousto-optical sensor region disposed at the distal end of the optical fiber; an electro-mechanical conversion assembly in communication with the acousto-optical sensor region, the electro-mechanical conversion assembly including: one or more antennae disposed on the mounting tube structure, the one or more antennae configured to receive radio-frequency (RF) electromagnetic energy and produce a corresponding electrical signal; and an ultrasonic transducer in mechanical communication with the acousto-optical sensor region, wherein the ultrasonic transducer is in electrical communication with the one or more antennae, and wherein the ultrasonic transducer is configured to elastically modulate the acousto-optical sensor region by acoustic waves generated responsive to the electrical signal received from the one or more antennae.
In certain implementations, the device can include a resonator feature in communication with the optical fiber, wherein the resonator feature is configured to at least partially reflect the generated acoustic waves for enhanced modulation of the acousto-optical sensor region. In certain exemplary implementations, the resonator feature can include an acoustical discontinuity comprising a notch formed by removal of at least a portion of cladding from the optical fiber. In certain exemplary implementations, the resonator feature can include an acoustical discontinuity comprising deposition of a ring of material on the optical fiber. Yet, in certain implementations, the resonator uses radial vibration resonances of the optical fiber under the piezoelectric transducer over the FBG region.
In accordance with certain implementations of the disclosed technology, the one or more antennae can include at least a first antenna and a second antenna, wherein the second antenna may be oriented in an orthogonal direction with respect to the first antenna.
In certain exemplary implementations, the acousto-optical sensor region can include a first fiber Bragg grating (FBG) and a second FBG. In certain exemplary implementations, the electromagnetic conversion assembly can include a first piezoelectric transducer in mechanical communication with the first FBG and in electrical communication with the first antenna. The electromagnetic conversion assembly can include a second piezoelectric transducer in mechanical communication with the second FBG and in electrical communication with the second antenna.
In accordance with certain implementations of the disclosed technology, the one or more antennae can include one or more of: a patch antenna; a coil antenna; a loop antenna, a monopole antenna, a microstrip antenna, and a dipole antenna.
In accordance with certain exemplary implementations of the disclosed technology, the acousto-optical sensor region can include a fiber Bragg grating (FBG).
In accordance with certain exemplary implementations of the disclosed technology, the ultrasonic transducer can include a piezoelectric transducer.
In accordance with certain exemplary implementations of the disclosed technology, the optical fiber can include at least one proximal end configured for coupling with an external light source for interrogation of the acousto-optical sensor region.
In accordance with certain exemplary implementations of the disclosed technology, the optical fiber can include at least one proximal end configured for coupling with a photodetector to receive interrogation light reflected from the acousto-optical sensor region.
In accordance with certain exemplary implementations of the disclosed technology, the optical fiber and the electro-mechanical conversion assembly are configured to reduce MRI RF-induced heating of the device.
In certain exemplary implementations, acousto-optical sensor region and/or the optical fiber may include a resonator feature that can be configured to at least partially reflect the generated acoustic waves to enhance a modulation amplitude of the acousto-optical sensor region. In certain implementations, the resonator feature includes an acoustical discontinuity that can include one or more of: a notch formed by removal of at least a portion of cladding from the optical fiber, and deposition of a material on the optical fiber. In some embodiments, the piezoelectric thin film transducer on the fiber can serve as the resonator using the radial vibration modes of the composite optical fiber/thin film transducer structure.
In certain implementations, the acousto-optical sensor region can include a first fiber Bragg grating (FBG) and a second FBG. In certain exemplary embodiments, the electromagnetic conversion assembly can include a first piezoelectric transducer in mechanical communication with the first FBG and in electrical communication with one or more of the first antenna and the second antenna. In certain example implementations, the electromagnetic conversion assembly can include a second piezoelectric transducer in mechanical communication with the second FBG and in electrical communication with the second antenna.
Numerous specific details of the disclosed technology are set forth herein. However, it is to be understood that implementations of the disclosed technology may be practiced without these specific details. In other instances, well-known methods, structures, and techniques have not been shown in detail in order not to obscure an understanding of this description. References to “one implementation,” “an implementation,” “exemplary implementation,” “various implementations,” etc., indicate that the implementation(s) of the disclosed technology so described may include a particular feature, structure, or characteristic, but not every implementation necessarily includes the particular feature, structure, or characteristic. Further, repeated use of the phrase “in one implementation” does not necessarily refer to the same implementation, although it may. The use of “exemplary” herein carries the same meaning as “example,” and is not intended to mean “preferred” or “best.”
Throughout the specification and the claims, the following terms take at least the meanings explicitly associated herein, unless the context clearly dictates otherwise. The term “connected” means that one function, feature, structure, or characteristic is directly joined to or in communication with another function, feature, structure, or characteristic. The term “coupled” means that one function, feature, structure, or characteristic is directly or indirectly joined to or in communication with another function, feature, structure, or characteristic. The term “or” is intended to mean an inclusive “or.” Further, the terms “a,” “an,” and “the” are intended to mean one or more unless specified otherwise or clear from the context to be directed to a singular form.
As used herein, unless otherwise specified the use of the ordinal adjectives “first,” “second,” “third,” etc., to describe a common object, merely indicate that different instances of like objects are being referred to and are not intended to imply that the objects so described must be in a given sequence, either temporally, spatially, in ranking, or in any other manner.
It is also to be understood that the mention of one or more method steps does not preclude the presence of additional method steps or intervening method steps between those steps expressly identified. Similarly, it is also to be understood that the mention of one or more components in a composition does not preclude the presence of additional components than those expressly identified.
The materials described as making up the various elements of the invention are intended to be illustrative and not restrictive. Many suitable materials that would perform the same or a similar function as the materials described herein are intended to be embraced within the scope of the invention. Such other materials not described herein can include but are not limited to, for example, materials that are developed after the time of the development of the invention.
While certain implementations of the disclosed technology have been described in connection with what is presently considered to be the most practical and various implementations, it is to be understood that the disclosed technology is not to be limited to the disclosed implementations, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the scope of the appended claims. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.
This written description uses examples to disclose certain implementations of the disclosed technology, including the best mode, and to enable any person skilled in the art to practice certain implementations of the disclosed technology, including making and using any devices or systems and performing any incorporated methods. The patentable scope of certain implementations of the disclosed technology is defined in the claims and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal language of the claims.
This application claims priority to U.S. Provisional Patent Application No. 62/695,260, entitled “Acousto-optical Sensors for MRI Safety Evaluation,” filed 9 Jul. 2018, the contents of which are also incorporated by reference in their entirety as if set forth in full. This application is also related to U.S. patent application Ser. No. 15/303,002, entitled “Interventional MRI Compatible Medical Device, System, and Method,” filed 10 Apr. 2015, and published as U.S. Patent Publication No. US2017/0143234 on 25 May 2017, the contents of which are also incorporated by reference in their entirety as if set forth in full.
This invention was made with government support under Grant No. EB017365 awarded by the National Institutes of Health. The government has certain rights in the invention.
Filing Document | Filing Date | Country | Kind |
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PCT/US2019/040982 | 7/9/2019 | WO | 00 |
Number | Date | Country | |
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62695260 | Jul 2018 | US |