WO2018192233A1 - Procédé de prédétermination de la température de tissus entourant des implants actifs sous résonance magnétique et système d'imagerie par résonance magnétique - Google Patents

Procédé de prédétermination de la température de tissus entourant des implants actifs sous résonance magnétique et système d'imagerie par résonance magnétique Download PDF

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WO2018192233A1
WO2018192233A1 PCT/CN2017/114778 CN2017114778W WO2018192233A1 WO 2018192233 A1 WO2018192233 A1 WO 2018192233A1 CN 2017114778 W CN2017114778 W CN 2017114778W WO 2018192233 A1 WO2018192233 A1 WO 2018192233A1
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sequence
temperature
test sequence
scanned
temperature rise
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PCT/CN2017/114778
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Chinese (zh)
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姜长青
张锋
丁建琦
董延涛
李路明
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清华大学
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    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
    • A61B5/00—Measuring for diagnostic purposes; Identification of persons
    • A61B5/05—Detecting, measuring or recording for diagnosis by means of electric currents or magnetic fields; Measuring using microwaves or radio waves 
    • A61B5/055—Detecting, measuring or recording for diagnosis by means of electric currents or magnetic fields; Measuring using microwaves or radio waves  involving electronic [EMR] or nuclear [NMR] magnetic resonance, e.g. magnetic resonance imaging
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R33/00—Arrangements or instruments for measuring magnetic variables
    • G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance

Definitions

  • the present application relates to the technical field of medical devices, and in particular to a method for pre-evaluating tissue temperature around an active implant under MR based on magnetic resonance (MR) temperature measurement technology and a magnetic resonance imaging system using the same .
  • MR magnetic resonance
  • Magnetic Resonance Imaging Compared with other imaging technologies (such as X-ray, CT, etc.), Magnetic Resonance Imaging (MRI) has obvious advantages: magnetic resonance imaging is clearer, has high resolution to soft tissue, and The human body has no ionizing radiation damage. Therefore, magnetic resonance imaging technology is widely used in the clinical diagnosis of modern medicine. It is estimated that at least 60 million cases are examined annually using MRI technology.
  • a high-intensity uniform static magnetic field B 0 A high-intensity uniform static magnetic field B 0 , a gradient field G, and a radio frequency (RF) magnetic field for exciting nuclear magnetic resonance signals.
  • RF radio frequency
  • the gradient field G z is applied in the direction of the static magnetic field so that the spatial positions of the different layers have different magnetic field strengths; at the same time, the RF field RF with a certain frequency of a certain frequency is applied, and the frequency and bandwidth of the RF signal are
  • the Larmor frequency in the layered space corresponds, so that only the hydrogen nuclei in the tissue in a particular layer in the layering direction can be excited to generate a signal. After the signal is excited, it begins to decay.
  • the excited nuclear magnetic signal can be locally peaked, called echo; usually, the signal is collected before and after the echo occurs.
  • the phase encoding and frequency encoding gradient fields are used to spatially encode the signals.
  • the phase-encoding gradient magnetic field is superimposed along the direction of the static magnetic field (the magnetic field gradient is usually along the y-axis), and is turned off after a certain period of time.
  • the signals at different positions in the phase encoding direction have different phases.
  • a gradient magnetic field is applied similarly in the frequency encoding direction (the frequency encoding gradient direction is usually along the x-axis) such that signals in different positions have different frequencies in the frequency encoding direction.
  • the phase and frequency of the signal contain spatial position information of the signal, and the intensity of the signal reflects the anatomical structure or physiological state of the human tissue at the position.
  • signal acquisition is started: the magnetic resonance signals are read in N equidistant time steps, and the resulting data is stored in one line of k space. Then repeat the above process, only need to select different gradient field G y intensity in the phase encoding stage, and the read data exists as a corresponding position of another line of k space until the k space is filled.
  • a total of one digital matrix with N x N data points is obtained, from which an image can be constructed in the image space by two-dimensional Fourier transform.
  • an implantable medical device such as a cardiac pacemaker, a defibrillator, a vagus nerve stimulator, a spinal cord stimulator, a deep brain stimulator, etc.
  • IMD implantable medical device
  • the three magnetic fields used may pose a significant safety risk to the patient.
  • One of the most important hidden dangers is the inductive heating of implantable medical devices in the RF field, especially for those with elongated conductive structures, such as deep brain electrical stimulator extension leads and electrode leads, cardiac pacemaker electrodes. line.
  • Patients with these implantable medical devices in their body may experience severe temperature rise at the tip of the elongated conductive structure in contact with the tissue during MRI scans. Such temperature rise may cause serious injury to the patient.
  • IMD implants require MRI during the life of the device, and the safety hazards associated with RF magnetic field induction have led to rejection of these patients.
  • the reason for the induced heating of the elongated conductive structure under the RF magnetic field is the coupling between the elongated conductive structure and the RF magnetic field.
  • the coupling between the elongated conductive structure and the radio frequency magnetic field generates an induced current in the elongated conductive structure, and the induced current is mainly transported into the tissue through the portion of the conductive structure tip that is in contact with the human tissue to form a concentrated distribution of the induced electric field.
  • Human tissue has a higher electrical resistivity and produces more Joule heat.
  • the tissue heating caused by the RF induced electric field can be characterized by the bioheat transfer formula.
  • the heat transfer formula is:
  • T is the tissue temperature
  • Q is the energy of RF induction deposition
  • S is the heat generated by metabolism
  • ⁇ is the density
  • C is the specific heat capacity
  • k is the thermal conductivity
  • ⁇ is the blood perfusion rate
  • subscript b is the nature of the blood, such as T b is the local blood temperature.
  • the RF fever of implantable medical devices is related to a variety of factors, including the size and distribution of the magnetic field, the physical characteristics of the human tissue, the location and routing of the medical device in the human body, the location of the patient within the magnetic resonance scanner, and Position and so on. It is not yet possible to accurately predict the RF rise of medical devices before each scan.
  • the present application proposes a method for predicting the radio frequency rise of a medical device.
  • the method calibrates the patient's safety in performing magnetic resonance scanning in the current state by applying a test sequence and measuring the temperature prior to the formal scan. By correlating with the radio frequency energy of the magnetic resonance scan, the safety of the subsequent magnetic resonance imaging scan can be judged.
  • Temperature measurements can be made by temperature sensors integrated into implantable medical devices such as thermocouples, RTDs, and the like. It can also be achieved by temperature sensitive magnetic resonance imaging methods. A variety of MR parameters exhibit temperature sensitivity, and these temperature-sensitive parameters can be used to obtain tissue temperature changes. For example, the proton resonance frequency changes with temperature, using a gradient The phase map obtained by the echo (GRE) sequence also changes, and the phase change and temperature change satisfy the following relationship:
  • ⁇ is the phase difference between the two phase diagrams before and after
  • ⁇ T is the temperature difference between the two image acquisition times before and after
  • ⁇ is the temperature-dependent chemical transfer coefficient of water molecules
  • B 0 is the static magnetic field strength
  • ⁇ is the gyromagnetic ratio
  • TE It is the echo time.
  • the present application proposes a method for real-time calibration and prediction of radio frequency rise safety of an implantable medical device and a magnetic resonance imaging system implementing the same.
  • the method first applies a test sequence through a magnetic resonance imaging system to detect temperature changes around the implanted medical device before and after the test sequence, and correlates with the RF magnetic field energy applied by the test sequence, thereby combining the time and space of the temperature rise.
  • the law of change can predict the safety of the RF rise caused by the implanted medical device based on the RF energy to be applied by the subsequent scan.
  • a magnetic resonance magnetic field is excited in the scanning cavity by the magnetic resonance imaging system, and the magnetic field distribution matches the distribution of the radio frequency magnetic field generated by the subsequent scanning.
  • the test sequence uses the same RF transmit coil as the subsequent scan sequence to ensure uniformity of the excitation magnetic field.
  • the spatial RF magnetic field is excited by applying an RF excitation signal to the transmitting coil.
  • the excitation signal is typically a modulated pulse having a center frequency of the Larmor frequency ⁇ 0 and has a certain bandwidth ⁇ , typically ⁇ is much smaller than ⁇ 0 .
  • the bandwidth is adjusted according to the needs of magnetic resonance imaging, such as layer thickness to be excited, field of view (FOV) size, and the like.
  • the RF signal used should have a frequency similar to the subsequent scan. In general, more than 80% of its energy should be distributed within the frequency band of ⁇ 0 ⁇ 20% ⁇ 0 . If the frequency deviation is too large, the resulting magnetic field distribution may be greatly different, resulting in inaccurate prediction.
  • Detecting temperature changes around the implanted medical device can be accomplished by temperature sensitive magnetic resonance imaging.
  • temperature-sensitive magnetic resonance imaging parameters there are many temperature measurement methods.
  • the type of the temperature measurement sequence is generally a gradient echo sequence (GRE sequence) or a plane echo sequence (EPI sequence).
  • the temperature measurement sequence can also be based on the Proton density, that is, according to the Boltzmann distribution, the proton density is inversely proportional to the absolute temperature, so the proton density weighted MRI image can be used to calculate the temperature of the measured object.
  • the temperature measurement sequence can also be based on the T1 relaxation time of the water molecule, ie the spin-lattice relaxation in the biological tissue is caused by the dipole interaction between the biological macromolecule and the water molecule, which depends on the temperature, When the temperature variation range is small, the T1 relaxation time is almost linear with the temperature T, so the temperature can be measured by detecting T1.
  • the temperature measurement sequence can also be based on the diffusion coefficient (Diffusion Coefficient), that is, in the strong magnetic field environment of MRI, the diffusion of water molecules in the tissue causes the signal phase dispersion in the direction of the diffusion gradient, which leads to the attenuation of the nuclear magnetic signal, the degree of attenuation and the diffusion coefficient. It is proportional and affected by temperature, so MRI imaging can be used to obtain the diffusion coefficient under different temperature conditions, and then the temperature change can be obtained. This application does not limit temperature measurement The method of quantity.
  • the RF magnetic field strength B 1 excited by the RF transmitting coil is proportional to the excitation voltage (or current) of the coil.
  • the induced electric field E is proportional to B 1
  • the energy absorbed by the human body is proportional to E 2 .
  • the absorption rate (SAR) is expressed. SAR stands for RF power absorbed per unit mass in W/kg. Therefore, these parameters are capable of characterizing the energy of the RF magnetic field.
  • the RF rise around the implanted medical device is closely related to these parameters.
  • the Q in the formula (1) can be represented by SAR.
  • the parameters related to the RF magnetic field energy under different scanning parameters can be obtained, such as The RF magnetic field size B 1 , the root mean square of the RF magnetic field B 1+rms or SAR.
  • the RF magnetic field size B 1 the root mean square of the RF magnetic field B 1+rms or SAR.
  • the corresponding parameter size related to the RF energy can be obtained.
  • the size of the same RF energy related parameter of the subsequent sequence to be scanned can be obtained.
  • the temperature rise result of the test sequence can be measured, and the relationship between the subsequent scan sequence and the RF energy related parameter in the test sequence can be compared. Whether the subsequent scan sequence is safe.
  • the parameter setting of the subsequent scanning sequence is determined by the actual demand, for the patient implanted with the medical device, in order to ensure the safety, the parameter needs to be lowered, which may result in poor image quality and cannot meet the requirements of actual clinical diagnosis.
  • the test sequence is only to calibrate the patient's RF temperature rise safety of the medical device in the current state, without having to consider the image quality problem, so it can be flexibly adjusted. In actual use, the test sequence can use a shorter time, smaller RF energy to ensure patient safety. It is possible to gradually increase the RF energy by applying the test sequence multiple times, thereby achieving the temperature measurement sensitivity requirement on the basis of ensuring the patient's safety, and obtaining an accurate assessment of the temperature rise around the implanted medical device in the current state of the patient.
  • the relationship between the temperature rise around the implantable medical device and the parameters related to the RF energy, and the variation of the temperature rise with time and space can be obtained through experimental experience, or by fitting empirical formulas through experimental results, or by model. The analysis was obtained.
  • Equation (1) is a commonly used bioheat transfer model in which energy deposition Q is caused by an induced electric field around a implantable medical device, that is, biological tissue absorbs radio frequency energy, which can be represented by SAR.
  • This model characterizes the spatial and temporal variations in temperature. Approximate, ignoring the effects of metabolism and tissue inhomogeneity, considering the blood temperature as a benchmark, we can get the formula (3):
  • the equation (3) has a homogeneous characteristic, that is, the temperature rise ⁇ T of a certain spatial point and a certain time point is proportional to the SAR.
  • the SAR is further proportional to the square of the parameters such as the induced electric field E, the induced current density J, the magnetic field B 1 , and B 1+rms .
  • the present application provides a method for pre-evaluating tissue temperature around an active implant under MR, the method being applicable to a magnetic resonance imaging system for generating a temperature measurement sequence, a test sequence, and a sequence to be scanned
  • the method includes: Step S20, before performing the test sequence, using the temperature measurement sequence to perform M temperature measurement, M ⁇ 1; Step S30, performing a test sequence, using the temperature measurement sequence for N temperature measurement, N ⁇ 0; Step S40, after performing the test sequence, performing P temperature measurement using the temperature measurement sequence, P ⁇ 1; and step S50, calculating the safety index according to the RF energy correlation parameter of the test sequence and the RF energy correlation parameter of the sequence to be scanned, and performing the threshold with the threshold value Compare, if it is safe, scan it, otherwise, refuse to scan.
  • the present application also provides a magnetic resonance imaging system, including: an MR scanning device for generating a temperature measurement sequence, a test sequence, and a sequence to be scanned; an MR control unit for controlling the MR control unit The MR scanning device performs scanning using the temperature measuring sequence, the test sequence and the sequence to be scanned; and a data processing unit for processing the scanning result of the temperature measuring sequence and the test sequence, and pre-evaluating the MR under the above method. Tissue temperature around the active implant.
  • FIG. 1 is a schematic structural view of a deep brain electric stimulator used in an embodiment of the present application.
  • FIG. 2 is a schematic block diagram of a magnetic resonance imaging system according to an embodiment of the present application.
  • FIG. 3 is a schematic diagram of a method for testing temperature by a magnetic resonance imaging method of a magnetic resonance imaging system according to an embodiment of the present application.
  • FIG. 4 is a flow chart of a method for determining an artifact region of the active implant in accordance with an embodiment of the present application.
  • FIG. 5 is a schematic structural diagram of a field drift correcting device used in an embodiment of the present application.
  • FIG. 6 is a schematic diagram of selecting a plurality of points in a central region of a corresponding image of a field drift correction container when the temperature variation caused by field drift is corrected according to an embodiment of the present application.
  • the present application provides a method of pre-evaluating tissue temperature around an active implant under MR and giving a safety assessment and a magnetic resonance imaging system employing the same.
  • the active implant can be a cardiac pacemaker, a defibrillator, a deep brain electrical stimulator, a spinal cord stimulator, a vagus nerve stimulator, a gastrointestinal stimulator or other similar implantable medical device.
  • the present application is only described by taking a deep brain electrical stimulator as an example, and the present application is further described with reference to the accompanying drawings.
  • the deep brain electrical stimulator 10 includes an external programmer 11 and a pulse generator 12 implanted in the body, and an extension lead 14 and a stimulation electrode 16.
  • the external programmer 11 controls the pulse generator 12 for generating a pattern of current pulses that are transmitted through the extension lead 14 to the electrode contacts 18 of the stimulation electrode 16 through which stimulation of a particular core can be achieved.
  • the purpose of treating the disease when an MR scan is performed on a patient implanted with the deep brain electrical stimulator 10, the elongated extension lead 14 and the stimulating electrode 16 absorb electromagnetic energy as an antenna, and generate heat at the electrode contact 18, which is safe. Hidden dangers. To ensure the safety of these patients when scanning MR, monitoring and safety assessments of the temperature around the electrode contacts 18 of these patients can be performed using the methods and systems provided herein.
  • the magnetic resonance imaging system 20 provided by the present application includes an MR scanning device 22, an MR control unit 24, and a data processing unit 26.
  • the MR scanning device 22 mainly comprises a coil for generating a static magnetic field, a coil for generating a gradient field, a coil for generating a radio frequency field, a radio frequency transmitting and receiving coil for different parts, an MR scanning bed and supporting automatic electrical equipment.
  • the MR control unit 24 includes MR device control software and image reconstruction processing software. The MR device control software can set the scan parameters and set the scan sequence.
  • the data processing unit 26 is equipped with data processing software, and the MR control unit 24 transmits the acquired temperature measurement image to the data processing unit 26 in real time.
  • the data processing software includes an electrode identification module 261, a temperature calculation module 262, a tissue thermal diffusion simulation module 263, an interface temperature reverse seeking module 264, and a tissue damage evaluation module 265.
  • the data processing unit 26 calculates the temperature distribution of the region of interest according to the temperature measurement image before and after the test sequence, and gives the calculated safety index, and determines the safety of the subsequent scan according to the threshold value set by the program, and feeds back to the MR control unit 24 in time. . If the security indicator exceeds the threshold, the MR scanning device 22 refuses to perform a subsequent scan of the patient, otherwise, a scan is performed.
  • M temperature measuring unit is included before the test sequence, M ⁇ 1, and N temperature measurement may be included in the test sequence, N ⁇ 0, and P temperature measurement is included after the test sequence, P ⁇ 1.
  • Perform multiple measurements before the test sequence ie M>1, and use the average of the M temperature measurements as a reference.
  • the RF energy correlation parameters of the test sequence are obtained, for example, the SAR value is SAR1, and the SAR value of the sequence to be scanned is SAR2.
  • the temperature rise at the same position at the same time is 1 times the SAR2/SAR of the test sequence.
  • the safety of subsequent magnetic resonance scans is judged by analyzing safety indicators.
  • the safety indicator can be a temperature rise value, or a thermal cumulative dose value, or other parameters associated with tissue damage.
  • the thermal cumulative dose is typically characterized by a cumulative equivalent number of minutes Celsius (CEM43, Cumulative Equivalent Minutes@43 °C).
  • the following provides a method for estimating the temperature rise of different sequences and different positions of the sequence to be scanned.
  • the electromagnetic field and heat transfer numerical model are established.
  • the electric field distribution in different heating modes can be obtained by using the current density J at the conductive part-tissue interface of the active implant as a parameter, and the heat transfer diffusion law can be obtained.
  • the current density J 0 for example, 1000 A/m 2
  • the temperature change matrix st_P is as follows (4):
  • st_P(i,j) represents the temperature change value of the position in the standard diffusion model corresponding to the i-th temperature measurement moment of r j .
  • ⁇ T 1 P
  • ⁇ T 0 st_P
  • ⁇ can be obtained in the sense of least squares, that is, the equation (6)
  • the temperature change curve of the highest temperature rise point is the tissue interface temperature change curve.
  • a method of pre-evaluating the temperature of tissue surrounding a metal implant during MR scanning using a magnetic resonance imaging system 20 provided by the present application is described below.
  • the method includes the following steps:
  • Step S10 performing a positioning scan using a positioning sequence to determine an implant position and a temperature selection layer including the region of interest;
  • Step S20 before performing the test sequence, performing M temperature measurement, M ⁇ 1;
  • Step S30 implementing a test sequence, calculating a radio frequency energy correlation parameter Seq_1 (R1, t1), performing N temperature measurements in the process, N ⁇ 0;
  • Step S40 after performing the test sequence, performing P temperature measurement, P ⁇ 1;
  • Step S50 calculating a radio frequency energy correlation parameter Seq_x (Rx, tx) of the sequence to be scanned, calculating a safety indicator, and comparing with the threshold, if it is safe, performing scanning, otherwise, rejecting scanning.
  • the positioning sequence is generally a sequence for determining the position of the patient each time the nuclear magnetic scan, such as a Survey sequence.
  • the radio frequency energy is low and the scanning time is relatively short, and the patient-safe sequence can be further scanned to improve the imaging resolution and accurately locate the implant and the region of interest.
  • the SAR value of the sequence should not exceed 0.4 W/kg, and the length of one scan should not exceed 10 min.
  • a suitable gradient echo sequence can be selected, for example, using the T1W_3D_TFE sequence.
  • the temperature measurement in the steps S20, S30, and S40 may be a nuclear magnetic temperature measurement method in which a temperature sensitive parameter is used as a measurement object.
  • M temperature measurement sequence scanning is performed on the temperature measurement layer determined in step S10 to obtain a magnetic resonance signal matrix S 1 ⁇ S M , Leaf transformation to obtain a complex map, and then obtain a reference phase map
  • the average value of M measurement data can be used as the reference phase.
  • a phase map can be obtained for each temperature measurement of S30 and S40. Then, the temperature rise distribution ⁇ T map is obtained according to the equation (8).
  • ⁇ is the chemical transfer coefficient of water molecules related to biological tissue temperature, about 0.01ppm/°C for human tissue
  • B 0 is the static magnetic field strength of magnetic resonance equipment
  • ⁇ is the gyromagnetic ratio
  • hydrogen protons in human water molecules are 42.58MHz/T
  • TE is the echo time.
  • the scan sequence may use a gradient echo GRE sequence or a plane echo EPI sequence, and the reference scan parameter settings are as shown in Table 1.
  • Voxel (mm) 0.5 to 3) ⁇ (0.5 to 3) (0.5 to 3) (0.5 to 3) ⁇ (0.5 to 3)
  • phase difference can be calculated as shown in the following equation (9)
  • the RF energy-related parameters R1 and Rx in steps 30 and 50 may be local or whole body average SAR values, or the amplitude of the radio frequency magnetic field B 1 , or the root mean square value of the radio frequency magnetic field B 1+rms , or the driving voltage of the radio frequency transmitting coil. Or current, or RF transmit power, etc.
  • the RF temperature rise around the implant has a corresponding relationship with the RF energy correlation parameter, and the temperature rise of the subsequent sequence to be scanned can be calculated by the temperature measurement result of the test sequence and the magnitude relationship of R1 and Rx. And safety indicators.
  • R1 and Rx are only related to the sequence design parameters of the test sequence and the sequence to be scanned, so its calculation can be performed at any time after determining the test sequence and the sequence to be scanned.
  • R1 and Rx can also be obtained in real time by detecting the drive parameters of the RF transmit coil, such as voltage, current or transmit power.
  • R1 and Rx can also be obtained by setting an electric field or a magnetic field sensor in real time.
  • the safety indicator in step 50 is the maximum temperature rise, or the thermal cumulative dose value, or other parameters associated with tissue damage in the region of interest or an area surrounding the implant.
  • the safety indicator is the maximum temperature rise of the area where the temperature rise of the implant surface is most severe, or the value of the thermal cumulative dose, or other parameters associated with tissue damage.
  • the thermal cumulative dose is typically characterized by a cumulative equivalent number of minutes Celsius (CEM43, Cumulative Equivalent Minutes@43 °C).
  • the safety of the scan is judged by the safety index K associated with tissue thermal damage, and K is related to the magnitude and duration of the scanned RF energy, ie:
  • R is the RF energy related parameter
  • t is the scan duration
  • the correlation f is related to various factors, including RF frequency, RF transmit coil structure, implant geometry, structure, material physical properties, and implant location. , pose and distribution path, the position and posture of the patient relative to the coil, the tissue characteristics of the implant site, the blood flow, and the initial state of the scan.
  • the specific form of f is difficult to give accurately in advance, but f is determined after the scanning conditions are fixed, that is, when the patient is ready to scan in the magnetic resonance machine.
  • the test sequence Seq_n(Rn, tn) can be applied and temperature measurement is performed to obtain the security index Kn, thereby determining the security of the sequence Seq_x (Rx, tx) to be scanned. Sex.
  • the test sequence since the RF energy correlation parameter Seq_i (R1, t1) of the test sequence is predetermined, and for security, the test sequence needs to select a sequence with a small RF energy or a short time for security. Ensure patient safety is uncertain. Therefore, if the RF temperature rise around the implant is small, such as less than the accuracy of the temperature rise measurement, the effective temperature rise may not be detected by one test because the effective RF temperature rise around the implant cannot be measured.
  • another embodiment of the present application further provides a method for evaluating a radio frequency rise of an implant by performing a test sequence multiple times.
  • the method includes the following steps:
  • Step S10 performing a positioning scan using a positioning sequence to determine an implant position and a temperature selection layer including the region of interest;
  • Step S20 before performing the test sequence, performing M temperature measurement, M ⁇ 1;
  • Step S30A implementing a test sequence Seq_i (Ri, ti), its RF energy related parameter Ri, duration ti, and performing N temperature measurements, N ⁇ 0;
  • Step S40 after performing the test sequence, performing P temperature measurement, P ⁇ 1;
  • Step S50A determining whether an effective temperature rise is detected, and if so, determining whether the sequence to be scanned is safe according to the detected temperature rise, and determining whether to scan according to the judgment result; if not, determining the temperature to be measured as the temperature rise, determining the to-be-scanned Whether the sequence is safe, if yes, perform scanning, otherwise, proceed to step S60;
  • Step S60 resetting the test sequence Seq_i+1 (Ri+1, ti+1), the radio frequency energy related parameter Ri+1, the duration is ti+1, and repeating steps S20 to S50A until the security of the sequence to be scanned can be determined. until.
  • the temperature selection layer should be as close as possible to the surface of the implant with severe heat generation, such as the surface of the brain deep electrical stimulation electrode. Due to the different physical properties of the implant and the biological tissue, especially the magnetization coefficient of the metal part is different, the magnetization of the static magnetic field in the magnetic resonance environment causes the surrounding magnetic field to be distorted, thereby causing distortion of the image signal around the implant, which is represented as an image. Artifacts. Often the signals in this part of the area are difficult to extract useful information. Therefore, the selection of the evaluation area typically determines the artifact area of the active implant.
  • the induced implant interacts with the radio frequency magnetic field of the magnetic resonance to produce an induced electric field that is strongest at the tip end surface of the elongated conductor structure, resulting in the highest temperature rise and decreasing with heat conduction to the surroundings.
  • temperature rise is more likely to occur. Therefore, assessing the safety requires determining the assessment area around the implant artifacts as close as possible to the highest temperature rise of the implant surface and extracting temperature information from the temperature measurement data.
  • the evaluation area selects an area that is a certain distance outward from the edge of the artifact. Preferably, this distance is from 1 to 6 mm.
  • the artifact edge 42 can be detected by a threshold method.
  • the internal signal strength of the artifact is I0
  • the signal strength of the surrounding area is I1
  • a certain value I2 between I0 and I1 is set as a threshold. Above this threshold, it is considered as an artifact. Outside the assessment area.
  • (I2-I0) / (I1-I0) is from 0.3 to 0.5.
  • the edge detection 42 can also be determined using an edge detection algorithm. The determination process is as shown in FIG. 4.
  • the artifact edge 42 can be determined using the canny algorithm, the sober algorithm, and the Roberts algorithm. An area other than the artifact edge 42 is selected as the evaluation area. Further preferably, the artifact edge 42 belongs to a transition region of the metal artifact region 40 to the tissue signal 44, and the classification algorithm determines the type of the pixel edge 42 of the artifact edge, and if it belongs to the tissue signal 44, it is included in the evaluation region. Preferably, the artifact edge 42 can be classified by using a Bayesian classification algorithm to determine the category of the pixel point covered by the artifact edge 42, the tissue signal 44 or the artifact area 40, so that the artifact area is taken from the image. 40 is determined.
  • step S50A the method of determining whether the effective temperature rise is detected is determining the temperature distribution around the artifact area of the active implant according to the scan result of the temperature measurement sequence, and determining whether the temperature change edge_T of the edge of the artifact area is greater than A threshold T 0 , if yes, is considered to be effective to detect temperature rise.
  • the following steps are included:
  • Step S501A determining an artifact area of the active implant
  • Step S502A determining a temperature distribution around the artifact area
  • Step S503A determining whether the temperature change edge_T of the edge of the artifact area is greater than a threshold value T 0 .
  • test sequence Seq_i+1 (Ri+1, ti+1) is reset.
  • R or lengthening t a higher temperature rise is obtained, so that the temperature rise is effectively detected under the premise of satisfying safety, and thus the safe line of the sequence to be scanned can be accurately determined.
  • the patient first installs a field bleaching correction device in a suitable area around the scanning site, such as around the head, before performing the MR scanning.
  • the field drift correction device is used to provide a reference reference for the magnetic resonance signal around the scanning site, and to remove the influence of the magnetic field drift when analyzing the temperature rise.
  • the field drift correction device 30 includes a set of containers 34.
  • the set of containers 34 are prepared from a non-magnetic material.
  • the non-magnetic material may be nylon, polypropylene, plexiglass or the like.
  • the set of containers 34 contains a homogeneous medium such as physiological saline, agar gel, Hydroxy Ethyl Cellulose gel or the like.
  • the homogeneous medium is also provided with a substance that adjusts the relaxation time of the medium, such as CuSO 4 or other transition metal salt, to facilitate magnetic resonance imaging.
  • the medium within the container 34 should be maintained at the same temperature as the environment in which the MR device is located.
  • the container 34 is a plastic test tube composed of four non-magnetic materials, each of which is filled with agar.
  • four flexible tubes 36 can be used to evenly hoop the four tubes around the head 32 so that the orientation of the four tubes is substantially parallel to the orientation of the stimulation electrode 16 and that the electrode contacts 18 are located.
  • the temperature measurement layer contains four in-tube materials.
  • the tube can be secured by a rigid shelf that can be retracted.
  • the static magnetic field generated by the MRI scanner may drift, causing a phase change, which may cause the temperature distribution obtained in the above steps to be inaccurate. Therefore, preferably, it is necessary to correct the temperature change caused by the field drift.
  • the static magnetic field drift has a distribution in space, and this distribution can be approximated by a polynomial approximation.
  • the general correction needs to select at least 3 positions for 1st-order plane correction. In particular, for the case where the measurement area is small relative to the static magnetic field and the 1st order item has little influence, the 0th order correction can be directly performed by subtracting the mean c, and at least 1 point is selected. As shown in FIG.
  • each field drift correction container 34 selects a number of points corresponding to the central area 46 of the image. Another method of correcting field drift does not rely on field-floating correction containers. At least one reference area is selected from the tissue signal MRI image (the MRI image includes an amplitude map, a phase map, and a temperature profile), and the reference region should contain at least one pixel. The present application does not limit the shape, size, and selection method of the reference area. It is easy to understand that the reference area used in the field drift correction method herein may also include an image area corresponding to the floating correction container.
  • the tissue in the selected reference area should not be heated or cooled during the scanning process, and the signal in the reference area should be relatively uniform (the tissue signal includes the amplitude signal, the phase signal and the temperature signal), and the reference area is representative.
  • the tissue signal includes the amplitude signal, the phase signal and the temperature signal
  • the reference area is representative.
  • ⁇ 3 points are selected, and the position information and temperature change information of each point are stored in the matrix A(i, j, ⁇ T), and the pseudo-temperature variation distribution map caused by the field drift is obtained by linear interpolation.
  • the calculation process can be solved by solving the problem:
  • the first column of [i j 1] n ⁇ 3 is A(:,1)
  • the second column is A(:,2)
  • the third column is all 1.
  • ⁇ T correction (i, j) ⁇ T map (i, j) - z(i, j) (13).
  • n is the number of points selected in all reference regions, ie the number of elements in the quantity B(i).
  • the method for performing safety evaluation according to the temperature change curve T'(t) at the time of MR scanning includes: performing heat accumulation amount and/or maximum temperature rise of the active implant surface with a safety threshold value. Comparison. Specifically, the thermal accumulation amount CEM 43 of the active implant surface is compared with a preset threshold value threshold_CEM 43 while comparing the highest temperature rise ⁇ T max of the active implant surface with a preset maximum temperature rise threshold threshold_ ⁇ T Max , any one of which exceeds a threshold, the data processing unit 26 issues a hazard warning to the MR control unit 24 in time, and the MR scanning device 22 automatically refuses to scan the patient. If neither of them exceeds a threshold, the MR scanning device 22 can perform an MR scan on the patient.
  • thermal damage depends not only on the temperature but also on the temperature duration, the so-called heat accumulation.
  • CEM 43 The more commonly used thermal cumulant model is CEM 43 , which is calculated as

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Abstract

L'invention concerne un procédé de prédétermination de la température de tissus entourant des implants actifs sous résonance magnétique (RM), applicable à un système d'imagerie par résonance magnétique (IRM) (20), qui est utilisé pour générer une séquence de test de température, une séquence de test et une séquence à balayer, le procédé comprenant : étape S20 : avant la mise en œuvre de la séquence de test, utilisation de la séquence de test de température pour effectuer un test de température M fois, où M ≥ 1 ; étape S30 : mise en œuvre de la séquence de test, et utilisation, au cours de ce processus, de la séquence de test de température pour effectuer un test de température N fois, où N ≥ 0 ; étape S40 : après la mise en œuvre de la séquence de test, utilisation de la séquence de test de température pour effectuer un test de température P fois, où P ≥ 1 ; et étape S50 : calcul d'un coefficient de sécurité sur la base de paramètres de corrélation d'énergie radiofréquence pour la séquence de test ainsi que de paramètres de corrélation d'énergie radiofréquence pour la séquence à balayer ; et comparaison du coefficient de sécurité à un seuil ; si pas de danger, mise en œuvre du balayage ; sinon, renoncer au balayage.
PCT/CN2017/114778 2017-04-18 2017-12-06 Procédé de prédétermination de la température de tissus entourant des implants actifs sous résonance magnétique et système d'imagerie par résonance magnétique WO2018192233A1 (fr)

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