Patent Application
20110299565
The invention relates to medical devices used in diagnostic and therapeutic procedures and in particular to a system and method for monitoring temperature of a medical device in a magnetic resonance imaging environment.
MRI has achieved prominence as a diagnostic imaging modality, and increasingly as an interventional imaging modality. The primary benefits of MRI over other imaging modalities, such as X-ray, include superior soft tissue imaging and avoiding patient exposure to ionizing radiation produced by X-rays. MRI's superior soft tissue imaging capabilities have offered great clinical benefit with respect to diagnostic imaging. Similarly, interventional procedures, which have traditionally used X-ray imaging for guidance, stand to benefit greatly from MRI's soft tissue imaging capabilities. In addition, the significant patient exposure to ionizing radiation associated with traditional X-ray guided interventional procedures is eliminated with MRI guidance.
MRI uses three fields to image patient anatomy: a large static magnetic field, a time-varying magnetic gradient field, and a radiofrequency (RF) electromagnetic field. The static magnetic field and time-varying magnetic gradient field work in concert to establish both proton alignment with the static magnetic field and also spatially dependent proton spin frequencies (resonant frequencies) within the patient. The RF field, applied at the resonance frequencies, disturbs the initial alignment, such that when the protons relax back to their initial alignment, the RF emitted from the relaxation event may be detected and processed to create an image.
Each of the three fields associated with MRI present safety risks to patients when a medical device is in close proximity to or in contact either externally or internally with patient tissue. One important safety risk is the heating that can result from an interaction between the RF field of the MRI scanner and the medical device (RF-induced heating), especially medical devices which have elongated conductive structures with tissue contacting electrodes, such as electrode wires in pacemaker and implantable cardioverter defibrillator (ICD) leads, guidewires, and catheters. Thus, as more patients are fitted with implantable medical devices, and as use of MRI diagnostic imaging continues to be prevalent and grow, the need for safe devices in the MRI environment increases.
A variety of MRI techniques are being developed as an alternative to X-ray imaging for guiding interventional procedures. For example, as a medical device is advanced through the patient's body during an interventional procedure, its progress may be tracked so that the device can be delivered properly to a target site. Once delivered to the target site, the device and patient tissue can be monitored to improve therapy delivery. Thus, tracking the position of medical devices is useful in interventional procedures. Exemplary interventional procedures include, for example, cardiac electrophysiology procedures including diagnostic procedures for diagnosing arrhythmias and ablation procedures such as atrial fibrillation ablation, ventricular tachycardia ablation, atrial flutter ablation, Wolfe Parkinson White Syndrome ablation, AV node ablation, SVT ablations and the like. Tracking the position of medical devices using MRI is also useful in oncological procedures such as breast, liver and prostate tumor ablations; and urological procedures such as uterine fibroid and enlarged prostate ablations.
In many of the foregoing cases, elongated or large surface area metallic structures may be present in interventional devices that are used during a procedure to deliver therapy or provide a diagnosis, implanted devices that are placed within the body to provide therapy or deliver a diagnosis, or the tools used to deploy or deliver the interventional or implanted device to the patient. Examples of interventional devices having metallic structures may include plaque excision devices, embolic traps, electrophysiology catheters, biopsy needles/tools, and stem cell delivery catheters. Examples of implanted devices having metallic structures may include cochlear implants, pacemakers, implantable cardioverter defibrillators, Insulin pumps, nerve stimulators, lead wires, prosthetic heart valves, hemostatic clips, and non-ferromagnetic stapedial implants. Finally, examples of deployment or delivery tools having metallic structures may include catheters, sheaths, introducers, guidewires, transseptal devices, and trochars.
As appreciated by those skilled in the art, these metallic structures may undergo heating during an MRI scanning process. This heating may be caused by numerous factors, including but not limited to eddy currents from MRI gradient switching, RF induced heating due to electromagnetic interactions between the metallic structure and the MRI transmit coil, and large current densities at metal/tissue interfaces (where heating may occur in both the metallic structure as well as the connected tissue). In all of these cases, it may be important to monitor the device temperature at a single or multiple points such that a safe level of device heating may be maintained.
In some of the foregoing cases, the interventional procedure may also include delivery of ablative therapy in the form of either heat, such as by radiofrequency delivery, laser delivery, microwave delivery, or highly focused ultrasound delivery, or freezing, such as by delivery of a cryogenic fluid. When the interventional procedure includes the delivery of ablative energy, it may be especially important to monitor the temperature of the therapy delivery point such that the therapy can be appropriately titrated. Thus, temperature monitoring is an important step for interventional procedures performed under MRI guidance.
Numerous methods and devices for measuring temperature are known and used in the medical device field. One exemplary device for measuring temperature is a thermocouple. Generally speaking, a thermocouple may be any conductor that generates a voltage when subjected to a thermal gradient. Thermocouples typically use two dissimilar metals to create a circuit in which the two legs generate different voltages that may be measured to determine a temperature value. Thermopile devices operate in a similar manner and are constructed by connecting a plurality of thermocouples in series or parallel. Another exemplary device for measuring temperature is a resistance thermometer or resistance temperature detector (RTD). This type of device operates by exploiting the predictable change in electrical resistance of materials with changing temperature, and is typically made of platinum. Yet another exemplary device for measuring temperature is a thermistor. Thermistors utilize a type of resistor that exhibits a varying resistance according to its temperature. Both positive and negative coefficient devices exist (PTC and NTC). As opposed to RTDs which are formed from pure metals, thermistors are generally formed from a ceramic or polymer.
One exemplary method of measuring temperature is known as radiation thermometry. Every object emits radiant energy, and the intensity of this radiation per unit area is a function of its temperature. In radiation thermometry, infrared thermometers are used to measure intensity of radiation. Radiation thermometry is also commonly referred to as optical pyrometry, radiometric temperature measurement, infrared thermometry, optical fiber thermometry, two color radiation thermometry, and infrared thermometry. Another exemplary method of measuring temperature is based upon the semiconductor absorption theory, and may be referred to as the method of “spectral analysis.” Spectral analysis uses gallium arsenide (GaAs) tipped fibers, and operates on the absorption/transmission properties of gallium arsenide crystal semiconductors. As the crystal temperature increases, its transmission spectrum shifts to a higher wavelength. The relationship between temperature and the wavelength at which the absorption shift takes place is predictable. The temperature value may be obtained by analyzing the absorption spectrum. Yet another method of measuring temperature is known as fluoroptic thermometry. When thermo-sensitive phosphor is stimulated with red light it emits light over a broad spectrum in the near infrared region. The time required for the fluorescence to decay is dependent upon the sensor's temperature. The measured decay time may be converted to temperature using a calibrated conversion table.
The foregoing known devices and methods for measuring temperature have numerous disadvantages and limitations. Thermocouples are inaccurate, susceptible to MRI-induced heating due to their metallic nature, and require conductive leads that can create a non-MRI safe condition. Resistance thermometers or RTDs require conductive leads that can create a non-MRI safe condition and are mechanically fragile. Thermistors also require conductive leads that can create a non-MRI safe condition and are mechanically fragile. With regard to radiation thermometry, radiation amplitude at body temperatures is small and requires large area detectors. Further, it is difficult to provide sufficient lensing at the tip of the catheter. Spectral analysis is expensive, potentially toxic in the body due to the use of gallium arsenide, and the fibers are difficult to manufacture. Fluoroptic thermometry is also an expensive and inaccurate process that requires calibration before each use. Further, it is difficult to localize the temperature measurement point, and process testing cannot be exposed to ambient light.
Current technologies for measuring temperature in an MRI environment are inadequate. Therefore, what is needed is a real-time temperature measurement system that is MRI safe, accurate, biocompatible, and cost effective.
The present invention solves the foregoing needs by providing a novel MRI compatible temperature measurement system and method for a medical device. In one exemplary embodiment, a temperature monitoring system is provided that includes an optical transmit/receive unit, an elongate optical fiber having a proximal end, a distal end, and an inner core extending between the proximal end and the distal end, and one or more fiber Bragg grating elements formed in the inner core of the optical fiber. The optical fiber is operably coupled to the transmit/receive unit at the proximal end. At least a portion of the optical fiber is also operably coupled to a medical device and is structured to measure temperature at one or more temperature sensing locations on the medical device.
In accordance with another aspect of the present invention, a method of estimating temperature is provided that generally includes the steps of selecting a plurality of known calibration temperature values, determining a bulk wavelength for each of the calibration temperature values, formulating a calibration data set that includes the plurality of known temperature values and the corresponding plurality of bulk wavelengths, and using the calibration data set to determine an estimated current temperature value based upon a current bulk wavelength, wherein the current temperature value is estimated based upon one or more data points in the calibration data set.
The fiber 14 of the temperature monitoring system 10 may be structured such that it is completely removable from the catheter 22 and may be reused in a different catheter or another type of medical device. Alternatively, as illustrated in
The FBG element 18 positioned or embedded within the main body 24 of the catheter 22 allows a user such as a surgeon to monitor temperature during a medical procedure. As will be appreciated by those skilled in the art, one or more FBGs may be used to monitor temperature during therapy delivery. Alternatively or additionally, one or more FBGs may be used to monitor medical device heating during scanning such that safe levels of heating may be maintained.
Generally speaking, an FBG is one type of distributed Bragg reflector that is constructed in a segment of optical fiber and is structured to reflect predetermined wavelengths of light and to transmit all others therethrough. This selective reflection is accomplished by adding a periodic variation to the refractive index of the optical fiber core, thereby creating a wavelength specific dielectric mirror. Thus, FBGs act as “filters” to block or reflect certain wavelengths.
FBGs are typically formed in an optical fiber by either “writing” or “inscribing” the periodic (or aperiodic) variation of refractive index into the core of the optical fiber using an ultraviolet source. The methods used to create the variations include “interference” and “masking.” The interference method, which may be useful for uniform gratings, utilizes an ultraviolet laser that is split into two separate beams that interfere with one another to create a periodic intensity distribution along the interference pattern. The magnitude of the refractive index is dependent upon the intensity of the laser light used. The masking method, which is well-suited for the manufacture of chirped FBGs, utilizes a photomask placed between an ultraviolet light source directed at the fiber and creates a grating structure based upon the intensity of the light that impinges upon the fiber. In another common method, an ultraviolet laser beam may be operated to “write” the grating into the fiber point-by-point.
As appreciated by those skilled in the art, FBGs operate on a principle known as “Fresnel reflection,” wherein light traveling between media of different refractive indices may be both reflected and refracted at the interface. The grating of the FBG element includes a varying sinusoidal refractive index over the length of the element. The wavelength reflected by the grating, which is known as the Bragg wavelength, may be approximated as follows:
Bragg wavelength=2ηΛ,
where η represents the average refractive index in the grating of the fiber and Λ represents the grating period.
The refractive index and the grating period are determined by the structure of the FBG element. Generally speaking, there are six known and common structures for FBGs, including chirped, superstructure, Gaussian apodized, discrete phase shift, uniform positive-only index change, and raised-cosine apodized.
Because the Bragg wavelength is sensitive to temperature, FBGs may be used as sensing elements in optical fiber sensors. In a FBG element, the measurand causes a shift in the Bragg wavelength. The relative shift in the Bragg wavelength due to an applied strain (ε) and a change in temperature (ΔT), may be approximated as follows:
Relative shift in Bragg wavelength=CSε+CTΔT,
wherein CS is the coefficient of strain and CT is the coefficient of temperature.
Based upon the foregoing relationship, FBGs may be used to directly sense the temperature and determine changes in temperature. Various other methods of estimating temperature with FBGs are also possible. A more detailed, exemplary method for estimating temperature using FBGs will be described in further detail to follow.
The fiber 14 of the temperature monitoring system 10 may be either a single-mode or multi-mode fiber optic cable. As appreciated by those skilled in the art, single-mode fiber optical cables are structured for carrying only a single ray or mode of light, which may contain a variety of different wavelengths. Single-mode cables have a small light carrying core, and are well-suited for long distance transmissions. Conversely, multi-mode fiber optic cables have a relatively larger light carrying core, and are well-suited for short distance transmissions.
Although only one FBG element 18 is illustrated in
The optical transmit/receive circuitry 12 is illustrated as being external to the catheter 14 of
In operation, the scan generator 34 may tune the light source 30 by sweeping it across a predetermined range so that the wavelength of light being transmitted down the fiber 14 is known at all times. When the wavelength emitted by the light source 30 matches the specified Bragg wavelength of the FBG element 18, light is reflected back along the fiber 14 towards the detector 38. The scan generator 34 is operable to transmit a timing signal to the processor 36. This timing signal allows the processor to create a “spectrum” based upon the “intensity” versus “time” information it has received. The processor may be operable to identify various characteristics of the spectrum such as peak positions, which may then be used to estimate temperature.
As further illustrated in
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Although the various embodiments of medical devices were described above as including a single fiber element, temperature monitoring systems utilizing multiple fiber elements each having one or more FBG elements therein are also possible. Thus, a single medical device such as an ablation catheter may be structured with two or more fibers positioned or embedded therein. This type of design may be used for measuring the temperature of one or more therapy delivery points or one or more locations for safety monitoring during therapy delivery or delivery of a medical device using MRI guidance. Further, although the fiber and FBG elements of the temperature monitoring systems have been generally described as embedded or removably positioned within the medical devices, they may alternatively be fixedly or removably coupled to an outer surface of the device without departing from the intended scope of the present invention.
As will be appreciated by those skilled in the art, the optical fiber may be positioned or embedded within a device, positioned on an outer surface of a device, or any combination thereof without departing from the intended scope of the present invention. For example, in one exemplary embodiment the fiber may be partially exposed to the exterior of the device. In another exemplary embodiment the device may include a fiber with at least one portion completely positioned/embedded within the device and at least one additional portion positioned on the exterior of the device. Thus, numerous alternative designs are contemplated and within the intended scope of the present invention.
As a Bragg diffraction grating system does not posses the technical shortcomings of other temperature measuring techniques inside an MRI system, another alternative embodiment of the present invention may include external in vitro or in vivo temperature measurement of a medical device. In this embodiment, a fiber optic cable having one or more FBG elements is placed external to the medical device. As will be appreciated by those skilled in the art, this embodiment may be useful in determining the safety of a medical device in MRI with regard to joule heating at tissue/electrode interfaces, dielectric heating along the length of a metallic structure, gradient induced heating and the like.
Although the temperature monitoring system of the present invention has been described with reference to a discrete number of medical devices, those skilled in the art will appreciate that the temperature monitoring system may be incorporated into any medical device that is used in an MRI environment. Thus, the embodiments set forth herein have been described merely for purposes of example and not limitation.
Now that several exemplary embodiments of the temperature monitoring system have been described with reference to various medical devices, one exemplary method of operating the temperature monitoring systems to determine temperature measurements will be described in detail. The exemplary method of the present invention may generally be separated into three processes, including determining bulk wavelength 200, calibrating temperature 300, and measuring temperature 400. Each of these processes will now be described with reference to
Starting at the minimum wavelength, the optical transmit/receive unit transmits narrowband (or broadband) light into the proximal end of a fiber containing one or more FBG elements at step 206. The light reflected off of the one or more FBG elements is received and measured by a photo detector in the optical transmit/receive unit at step 208, and the magnitude and transmit wavelength, λtx, are recorded into memory at step 210.
A processor then determines whether the transmit wavelength, λtx, is greater than or equal to the maximum wavelength, λmax, at step 212. If the transmit wavelength, λtx, is determined to be less than the maximum wavelength, λmax, the transmit wavelength, λtx, is incremented by the wavelength step, λstep, at step 214 and the process 200 enters a loop 216 where steps 206-212 are repeated for transmit wavelengths from λmin to λmax at incremental steps of λstep. Once the processor determines that the transmit wavelength, λtx, is greater than or equal to the maximum wavelength, λmax, at step 212, this portion of the process is complete and a data set now exists consisting of transmit wavelengths and associated received light magnitudes. An exemplary data set is represented by the graph in
Although one exemplary method of forming the data set represented by the graph in
For purposes of discussion and not limitation, the bulk wavelength may be defined as a single wavelength value that represents the center wavelength of the received light. To find the bulk wavelength, the wavelength at which the magnitude is maximum is first identified at step 218. This step is depicted graphically in
As will be appreciated by those skilled in the art, bulk wavelength may be calculated using numerous alternative methods without departing from the intended scope of the present invention. For example, bulk wavelength may be determined using peak detection (i.e. finding the absolute peak magnitude value), filtered peak detection (i.e. filtering the wavelength magnitudes followed by finding the absolute peak magnitude value), filtered center of mass (i.e. filtering the wavelength magnitudes followed by finding the center of mass of the magnitudes), or the like. Thus, the bulk wavelength process 200 is one of many processes that may be used, and was discussed herein for purposes of example and not limitation.
Turning next to
Turning next to
Although several exemplary steps were described with reference to the bulk wave determination, temperature calibration, and temperature measurement processes, those skilled in the art will appreciate that the order and number of steps may be modified without departing from the intended scope of the present invention. Thus, the exemplary steps were provided merely for purposes of example and not limitation.
As will further be appreciated by those skilled in the art, the processes previously described may be embodied as a system, method or computer program product. Accordingly, the present invention may take the form of an entirely hardware embodiment, an entirely software embodiment (including firmware, resident software, micro-code, etc.) or an embodiment combining software and hardware aspects that may all generally be referred to as a “circuit,” “module” or “system.” Furthermore, the present invention may take the form of a computer program product embodied in any tangible medium of expression having computer usable program code embodied in the medium.
The processes comprising the method of the present invention have been described with reference to flow diagrams illustrating exemplary steps. It will be understood that each block of the flowchart diagrams, and combinations of blocks in the flowchart diagrams, can be implemented by computer program instructions. These computer program instructions may be provided to a processor of a general purpose computer, special purpose computer, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, create means for implementing the functions/acts specified in the flowchart diagram block or blocks.
These computer program instructions may also be stored in a computer-readable medium that can direct a computer or other programmable data processing apparatus to function in a particular manner, such that the instructions stored in the computer-readable medium produce an article of manufacture including instruction means which implement the function/act specified in the flowchart block or blocks.
The computer program instructions may also be loaded onto a computer or other programmable data processing apparatus to cause a series of operational steps to be performed on the computer or other programmable apparatus to produce a computer implemented process such that the instructions which execute on the computer or other programmable apparatus provide processes for implementing the functions/acts specified in the flowchart diagram block or blocks.
Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in and detail without departing from the spirit and scope of the invention.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/US10/26225 | 3/4/2010 | WO | 00 | 8/22/2011 |
| Number | Date | Country | |
|---|---|---|---|
| 61157471 | Mar 2009 | US |
