WO2018192233A1 - Method for pre-assessing temperature of tissues surrounding active implants under magnetic resonance and magnetic resonance imaging system - Google Patents

Abstract

A method for pre-assessing the temperature of tissues surrounding active implants under magnetic resonance (MR), applicable to a magnetic resonance imaging (MRI) system (20) which is used to generate a temperature-testing sequence, a test sequence and a sequence to be scanned, the method comprising: step S20: before executing the test sequence, use the temperature-testing sequence to perform temperature testing M times, wherein M≥1; step S30: executing the test sequence, and during the process of which, use the temperature-testing sequence to perform temperature testing N times, wherein N≥0; step S40: after executing the test sequence, use the temperature-testing sequence to perform temperature testing P times, wherein P≥1; and step S50: calculate a safety index on the basis of radio frequency energy correlation parameters of the test sequence as well as radio frequency energy correlation parameters of the sequence to be scanned; and compare the safety index to a threshold; if safe, perform scanning; otherwise, reject scanning.

Description

Method for pre-evaluating tissue temperature around active implant under MR and magnetic resonance imaging system

Related application

This application claims priority to Chinese Patent Application No. 201710252095.X, entitled "A Method for Pre-Assessing Tissue Temperature Around Active Implants Under MR and Magnetic Resonance Imaging System", filed on April 18, 2017 The entire disclosure is hereby incorporated by reference.

Technical field

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 .

Background technique

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.

There are three magnetic fields that work when MRI is working. 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. The specific imaging process is briefly described as follows: First, under the action of the static magnetic field B ₀ , the hydrogen nuclei in the human body precession along the direction of the static magnetic field. According to the Larmor theorem, the precession frequency of the hydrogen nuclei is ω=γB, where ω is The dynamic frequency, γ is the gyromagnetic ratio, and B is the magnetic field strength; that is, the precession frequency is proportional to the magnetic field strength. In order to excite the signal in a specific layer, 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. By the combination of the RF magnetic field and the gradient magnetic field, the excited nuclear magnetic signal can be locally peaked, called echo; usually, the signal is collected before and after the echo occurs. Within the layer being excited, in order to distinguish signals at different locations, the phase encoding and frequency encoding gradient fields are used to spatially encode the signals. Before the signal is read out, 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. At this time, the signals at different positions in the phase encoding direction have different phases. Following the frequency encoding, 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. After the above spatial encoding process, 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. At the same time as frequency encoding, 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. Thus, 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.

If the patient is equipped with an implantable medical device (IMD), such as a cardiac pacemaker, a defibrillator, a vagus nerve stimulator, a spinal cord stimulator, a deep brain stimulator, etc., it is required for MRI work. 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. However, most patients with 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:

Where T is the tissue temperature, Q is the energy of RF induction deposition, S is the heat generated by metabolism, ρ is the density, C is the specific heat capacity, k is the thermal conductivity, ω is the blood perfusion rate, and subscript b is the nature of the blood, such as T _b is the local blood temperature. The electric field induced by the RF magnetic field causes the tissue to heat up and change in accordance with the laws of biological heat transfer.

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.

However, when the position and posture of the patient in the magnetic resonance scanner are determined, the degree of radio frequency heating of the implanted medical device is related to the radio frequency energy applied by the magnetic resonance scanner at this time. Therefore, 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:

Where Δφ is the phase difference between the two phase diagrams before and after, ΔT is the temperature difference between the two image acquisition times before and after, α is the temperature-dependent chemical transfer coefficient of water molecules, B ₀ is the static magnetic field strength, γ is the gyromagnetic ratio, TE It is the echo time.

Summary of the invention

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.

In the test sequence, 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. In general, 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 ω ₀ and has a certain bandwidth Δω, typically Δω is much smaller than ω 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. In the test sequence, 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 ω ₀ ± 20% ω 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. There are many temperature-sensitive magnetic resonance imaging parameters, and there are many temperature measurement methods. For example, in the temperature measurement method using the proton resonance frequency as the temperature sensitive parameter, 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 , and 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. By detecting the excitation signal size or transmission power of the RF transmitting coil (proportional to the square of the excitation signal), or by detecting the feedback sensor, combined with the previously calibrated model, 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. When the test sequence is performed, the corresponding parameter size related to the RF energy can be obtained. According to the same method, the size of the same RF energy related parameter of the subsequent sequence to be scanned can be obtained. Since the temperature rise around the implantable medical device is positively correlated with the RF energy related parameter, 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.

Since 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 ΔT 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 . By using this feature, combined with the space and time distribution law of heat transfer, the temperature rise of other time and other areas can be calculated according to the temperature rise of a certain time and a certain area.

In particular, 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.

DRAWINGS

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.

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.

Main component symbol description

Deep brain stimulator 10

External programmer 11

Pulse generator 12

Extension wire 14

Stimulation electrode 16

Electrode contact 18

Magnetic resonance imaging system 20

MR scanning equipment 22

MR control unit 24

Data processing unit 26

Electrode identification module 261

Temperature calculation module 262

Tissue Thermal Diffusion Simulation Module 263

Interface temperature reverse module 264

Tissue damage assessment module 265

Field drift correction device 30

Head 32

Container 34

String 36

Artifact area 40

Artifact edge 42

Organizational signal 44

Central area 46

detailed description

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. Wherein 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.

Referring to FIG. 1, 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. However, 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.

Referring to FIG. 2, 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.

As shown in FIG. 3, 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. By comparing the temperature measurement results in the test sequence and the test sequence with the temperature measurement results before the temperature measurement sequence, the temperature rise during and after the test sequence can be obtained. 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. According to the formula (3), 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. Specifically, 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. In this embodiment, the current density J ₀ , for example, 1000 A/m ² , is used as a standard thermal diffusion model, and the position is calculated as r=(r ₁ , r ₂ , . . . r _m ), corresponding to k temperature measurement moments. The temperature change matrix st_P is as follows (4):

It can be understood that 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 . According to the formula (3), it can be seen that ΔT=a·J ² , and further, the formula (5) can be obtained:

Here, ΔT ₁ =P, ΔT ₀ = st_P, λ can be obtained in the sense of least squares, that is, the equation (6)

The minimum value, so that the above formula is equal to zero, the extreme point can be obtained.

The value of the formula (7),

will

Bringing into the thermal diffusion simulation model st_P(i,j), the thermal diffusion model corresponding to the experiment is obtained, and the temperature variation curve of the highest temperature rise point can be extracted from the model. 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. In one embodiment, 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.

In the step S10, 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. Preferably, 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 reference scan parameters are set as follows: 3D acquisition mode, repetition time TR=6~12ms, echo time TE=2~6ms, flip angle FA=6~12° , layer thickness ST = 1 ~ 3mm, layer spacing SS = 0 ~ 3mm, voxel (0.4 ~ 2) mm × (0.4 ~ 2) mm, field of view FOV = 180 ~ 350mm, scanning time 2 ~ 8min. It will be appreciated that this step may be omitted if the implant location and the temperature selective layer containing the region of interest are known in advance.

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. Taking the temperature measurement method based on the proton resonance frequency drift as an example, in step S20, 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

For the case of M>1, the average value of M measurement data can be used as the reference phase.

For each temperature measurement of S30 and S40, a phase map can be obtained.

Then, the temperature rise distribution ΔT _{map is} obtained according to the equation (8).

Where α 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, γ is the gyromagnetic ratio, and 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.

Table 1 main parameters of the sequence example

	GRE sequence	EPI sequence
TR(ms)	100~300	100~300
TE(ms)	10~50	10~50
Acceleration factor	-	2 to 20
Flip angle FA (°)	10~40	10~40
Field of view FOV (mm)	(150～300)×(150～300)	(150～300)×(150～300)
Acquisition matrix	(64 to 512) × (64 to 512)	(64 to 512) × (64 to 512)
Voxel (mm)	(0.5 to 3) × (0.5 to 3)	(0.5 to 3) × (0.5 to 3)
Layer thickness (mm)	1 to 5	1 to 5

Since the value of the phase is usually [-π, π] in the actual MRI image, the edge phase will jump, resulting in so-called phase winding. Therefore, the above method of directly subtracting the phase difference may cause a large error. To avoid phase winding, the phase difference can be calculated as shown in the following equation (9)

among them

It is a complex exponential form of the scanning phase signal twice, and Im() and Re() respectively represent the imaginary part and the real part of the complex number. Expand the above formula to obtain the following formula (10)

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. According to the formula (3) and the foregoing analysis, 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. In general, the relative magnitude relationship between R1 and Rx is 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. Preferably, 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:

K=f(R,t)(11)

Where R is the RF energy related parameter, t is the scan duration, and 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. Since the security of the sequence Seq_x (Rx, tx) to be scanned is unknown, 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. By properly designing the application of test sequences, patient safety can be greatly ensured.

It can be understood that, in the foregoing embodiment, 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.

To this end, 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. In one embodiment, 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.

In the above steps, 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. In general, 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. . For example, in the deep brain electrical stimulation electrode contacts, 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.

Preferably, 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, and 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. Preferably, (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. Preferably, 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.

In 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. Specifically, 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 .

If the effective temperature rise is not detected, or the temperature rise confidence is low, the test sequence Seq_i+1 (Ri+1, ti+1) is reset. By increasing 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. There may be multiple setting methods, such as fixed t, ie ti+1=ti, let Ri+1=a*Ri, a is a proportional coefficient, preferably between 1.1 and 2; another setting method fixes R, ie Ri+ 1=Ri, let ti+1=b*ti, b be a proportional coefficient, preferably between 1.1 and 2; another setting method simultaneously adjusts R, t, so that Ri+1=a*Ri, ti+1=b *ti, the proportional coefficient a is preferably 0.8 to 3, b is preferably 0.5 to 2, and when R is increased, that is, a>1, t can be appropriately shortened, that is, b<1, to ensure safety, and similarly, when the time t is extended , that is, b>1, then R can be appropriately lowered, that is, a<1.

In the step S10, preferably, 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. As shown in Figure 5. 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. Generally, 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. In this embodiment, the container 34 is a plastic test tube composed of four non-magnetic materials, each of which is filled with agar. At the time of installation, 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. Alternatively, 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. 6, in the temperature profile, 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. Preferably, 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. . For For the first-order correction, ≥3 points are selected, and the position information and temperature change information of each point are stored in the matrix A(i, j, ΔT), 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:

Among them, the first column of [i j 1] _n×3 is A(:,1), the second column is A(:,2), and the third column is all 1. Solving the fitting plane z(i,j)=a·i+b·j+c in the sense of least squares, and subtracting z from the original temperature variation profile, the corrected actual temperature distribution ΔT _{correction is obtained} . That is, (13),

ΔT _correction (i, j) = ΔT _map (i, j) - z(i, j) (13).

For the 0th-order correction, select several points in all reference areas, store the temperature change information of each point in the vector B(i), calculate the average value of the temperature change information of the selected points, and find the cause of the field drift. The pseudo temperature change z is calculated as follows (14):

Where n is the number of points selected in all reference regions, ie the number of elements in the quantity B(i). By subtracting z from the original temperature profile, the corrected temperature distribution is obtained (15):

ΔT _correction (i,j)=ΔT _map (i,j)-z(15)

Further, in the step S50A, 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 ΔT _{max of the} active implant surface with a preset maximum temperature rise threshold threshold_ΔT _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.

It can be understood that the thermal damage depends not only on the temperature but also on the temperature duration, the so-called heat accumulation. The more commonly used thermal cumulant model is CEM ₄₃ , which is calculated as

Wherein, when T(t)>43°C, R=0.5; when T(t)<43°C, R=0.25.

The above-mentioned embodiments are merely illustrative of several embodiments of the present application, and the description thereof is more specific and detailed, but is not to be construed as limiting the scope of the claims. It should be noted that for those of ordinary skill in the art, A number of variations and modifications may be made without departing from the spirit of the present application, and these are within the scope of the present application. Therefore, the scope of the invention should be determined by the appended claims.

Claims (10)

1. 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 comprising:

   Step S20, before performing the test sequence, using the temperature measurement sequence to perform M temperature measurement, M≥1;

   Step S30, the test sequence is implemented, and the temperature measurement sequence is used for N temperature measurement, N≥0;

   Step S40, after performing the test sequence, using the temperature measurement sequence to perform P temperature measurement, P≥1;

   Step S50: Calculate the safety indicator according to the RF energy correlation parameter of the test sequence and the RF energy correlation parameter of the sequence to be scanned, and compare with the threshold. If it is safe, scan it, otherwise, the scan is rejected.
2. The method for pre-evaluating the tissue temperature around the active implant in the MR according to claim 1, wherein the step of performing temperature measurement before the M temperature measurement further comprises the step S10: performing a positioning scan by using the positioning sequence. The implant location and the temperature selective layer containing the region of interest are determined.
3. The method of pre-evaluating tissue temperature around an active implant under MR according to claim 2, wherein the safety indicator is a temperature rise value or a heat cumulative dose value.
4. The method for pre-evaluating tissue temperature around an active implant under MR according to claim 3, wherein the radio frequency energy correlation parameter of the test sequence is Seq_n (Rn, tn); the radio frequency of the sequence to be scanned The energy correlation parameter is Seq_x (Rx, tx); the test sequence matches the radio frequency magnetic field distribution of the sequence to be scanned.
5. The method for pre-evaluating tissue temperature around an active implant under MR according to claim 4, wherein the method for calculating a safety index according to a radio frequency energy correlation parameter of a test sequence and a radio frequency energy correlation parameter of a sequence to be scanned To: Calculate the temperature rise caused by the sequence to be scanned based on the temperature rise caused by the test sequence.
6. The method for pre-evaluating tissue temperature around an active implant under MR according to claim 5, wherein the method for calculating the temperature rise caused by the sequence to be scanned according to the temperature rise caused by the test sequence is: determining whether to detect The temperature rise caused by the test sequence, if yes, calculate the temperature rise caused by the sequence to be scanned based on the detected temperature rise and determine whether the sequence to be scanned is safe; if not, determine whether the temperature rise caused by the temperature measurement accuracy is the test sequence It can be judged whether the sequence to be scanned is safe, and if so, scan is performed; otherwise, the RF energy-related parameters of the test sequence are reset, and steps S20, S30 and S40 are repeated until the temperature rise caused by the test sequence is detected.
7. The method for pre-evaluating tissue temperature around an active implant under MR according to claim 6, wherein the method of determining an implant position and a temperature-selective layer comprising a region of interest comprises: utilizing edge detection The algorithm determines the edge of the artifact and uses the outer edge as the region of interest.
8. The method for pre-evaluating tissue temperature around an active implant under MR according to claim 7, wherein the method for determining whether the temperature rise caused by the test sequence is detected is determined according to a scan result of the temperature measurement sequence. The temperature distribution around the artifact area of the active implant, and determining whether the temperature change edge_T of the edge of the artifact area is greater than a threshold value T ₀ , and if so, the temperature rise is considered to be effectively detected.
9. A magnetic resonance imaging system characterized in that it comprises:

   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 scanning device to scan using the temperature measurement sequence, the test sequence, and the sequence to be scanned;

   a data processing unit for processing scan results of the temperature measurement sequence and the test sequence, and pre-evaluating the tissue surrounding the active implant under MR using the method according to any one of claims 1 to 8. temperature.
10. The magnetic resonance imaging system according to claim 9, wherein said data processing unit compares the heat accumulation amount CEM _{43 of the} active implant surface with a threshold threshold_CEM ₄₃ set in advance while comparing the active implants The highest temperature rise ΔT _max of the surface of the object and the preset highest temperature rise threshold threshold_ΔT _max , any one of which exceeds the threshold value, and the data processing unit issues a danger warning to the MR control unit in time, and the MR scanning device automatically The patient is refused to scan; if neither of them exceeds the threshold, the MR scanning device scans the patient.

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