CN106667487B - A kind of method and magnetic resonance imaging system monitoring active implantation material surrounding tissue temperature - Google Patents

Abstract

The present invention relates to a method for monitoring the temperature of tissue surrounding an active implant based on magnetic resonance thermometry and using a magnetic resonance imaging system; the magnetic resonance imaging system is used to generate at least one A sequence 2 for research or other purposes and a sequence 3 for measuring temperature distribution; the method includes the following steps: Step S11, using the sequence 2 to scan, and interspersing the scanning of the temperature measurement sequence 3 in the sequence 2; and In step S12, a safety assessment is performed according to the scanning results of the temperature measurement sequence 3. The method can effectively monitor the radiofrequency temperature rise of patients with implanted medical devices during MRI scanning, and eliminate potential safety hazards.

Description

A method and magnetic resonance imaging system for monitoring the temperature of tissue surrounding an active implant

technical field

The present invention relates to the technical field related to medical devices, in particular, to a method for real-time monitoring of tissue temperature around active implants under MR based on magnetic resonance (Magnetic Resonance, MR) temperature measurement technology and a magnetic resonance imaging system using the method .

Background technique

Compared with other imaging techniques (such as X-ray, CT, etc.), Magnetic Resonance Imaging (MRI) has significant advantages: MRI is clearer, has a high resolution for soft tissues, and There is no ionizing radiation damage to the human body. Therefore, magnetic resonance imaging technology is widely used in the clinical diagnosis of modern medicine. Today, it is estimated that at least 60 million cases worldwide are examined using MRI technology every year.

When MRI works, three magnetic fields come into play. 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 precess along the direction of the static magnetic field. According to Larmor's theorem, the precession frequency of the hydrogen nuclei is ω=γB, where ω is the γ is the gyromagnetic ratio, and B is the magnetic field strength; that is, the frequency of precession is proportional to the magnetic field strength. In order to excite the signal in a specific layer, a gradient field _Gz is applied in the direction of the static magnetic field, so that the spatial positions of different layers have different magnetic field strengths; at the same time, a radio frequency field RF with a certain frequency and a certain bandwidth is applied, and the frequency and bandwidth of the RF signal are the same as The Larmor frequency in the layer selection space corresponds, so only the hydrogen nuclei in the tissue in a specific layer in the layer selection direction can be excited and generate signals. After the signal is excited, it begins to attenuate continuously. Through the combination of radio frequency magnetic field and gradient magnetic field, the excited nuclear magnetic signal can have a local peak, which is called an echo; usually the signal is collected around the time when the echo appears. Within the excited layer, the signals are spatially position encoded using phase-encoded and frequency-encoded gradient fields in order to distinguish signals at different positions. 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 it 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. Followed by frequency encoding, a gradient magnetic field is similarly applied in the direction of frequency encoding (the gradient direction of frequency encoding is usually along the x-axis), so that signals at different positions in the direction of frequency encoding have different frequencies. After the above spatial encoding process, the phase and frequency of the signal contain the spatial position information of the signal, and the strength of the signal reflects the anatomical structure or physiological state of the human tissue at this position. Simultaneously with frequency encoding, start signal acquisition: read magnetic resonance signals in N equidistant time steps, and store the obtained data in one line of k-space. Then repeat the above process, only need to select different gradient field G _y intensities in the phase encoding stage, and store the read data as another line of k-space in the corresponding position until the k-space is filled. In total, this results in a digital matrix with N×N data points, from which an image can be constructed in image space by means of a two-dimensional Fourier transformation.

If the patient has an implantable medical device (IMD), such as a pacemaker, defibrillator, vagus nerve stimulator, spinal cord stimulator, deep brain electrical stimulator, etc., MRI is required for work The three magnetic fields used could pose a significant safety risk to the patient. One of the most important hidden dangers is the induced heating of implantable medical devices in radio frequency fields, especially for those with long and thin conductive structures, such as extension leads and electrode leads of deep brain electrical stimulators, pacemaker electrodes Wire. During an MRI scan of a patient with these implanted medical devices, a severe temperature rise may occur at the point where the tip of the elongated conductive structure contacts the tissue, and such temperature rise may cause serious injury to the patient. However, most patients implanted with IMDs require MRI examinations during the life cycle of the device, and the safety hazards caused by radio frequency magnetic field induction lead to the rejection of these patients for examination.

The reason for the induction heating of the elongated conductive structure under the radio frequency magnetic field is the coupling between the elongated conductive structure and the radio frequency 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 to the tissue through the part where the tip of the conductive structure is in contact with human tissue, forming a concentrated distribution of the induced electric field. The higher resistivity of human tissue will generate more Joule heat.

The tissue heating caused by radio frequency induced electric field can be described by the biological heat transfer formula, and the heat transfer formula is:

Where T is the tissue temperature, Q is the energy deposited by radio frequency induction, S is the heat generated by metabolism, ρ is the density, C is the specific heat capacity, ω is the perfusion rate of the blood, and the subscript b represents the property of the blood, such as T _b is the local blood temperature. The electric field induced by the radiofrequency magnetic field causes tissue heating, which varies according to the law of bioheat transfer.

Since the most serious radio frequency temperature rise usually occurs at the tip of the slender conductor structure of implantable medical devices, and is also affected by factors such as material biocompatibility, sensor size, and electromagnetic interference under MRI, traditional temperature sensors such as thermocouples , thermal resistance, etc. are difficult to integrate. Even if it can be integrated, because it needs to be applied under MRI, there is still the problem of real-time interaction between measurement data and the outside world. Therefore, after these implantable medical devices are implanted into the patient's body, there is no effective monitoring method for the radio frequency induction temperature rise of the MRI scan.

And the data scanned by MRI itself may provide a real-time, non-invasive way to monitor temperature. A variety of MR parameters exhibit temperature sensitivity, and tissue temperature changes can be obtained using these temperature-sensitive parameters. For example, the proton resonance frequency will change with the change of temperature, and the phase diagram obtained by using the gradient echo (GRE) sequence will also change, and the phase change and temperature change satisfy the following relationship:

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

At present, MR thermometry has been successfully applied to radiofrequency ablation damage, focused ultrasound treatment of tumors and other aspects. In these applications, the heating source comes from an external therapeutic device. MR thermometry is only used as a monitoring method. For the radio frequency heating of implantable medical devices under nuclear magnetic resonance, the heating comes from the MRI scan itself, and the scanning sequences have their own purposes, and the parameters are different. They are very different from the scanning of MR thermometry, and cannot be obtained from other scanning sequences. to obtain temperature information.

In addition, metal conductors in implantable medical devices will also cause distortion of the surrounding magnetic field due to magnetization under the magnetic field of MRI, which will cause image artifacts and cause MRI signals near the conductor to be lost or severely distorted. The radio frequency induction heating is the most serious near the conductor, which is also a problem in the current application of MR thermometry.

Therefore, the present invention proposes a method and device for interspersing MR temperature measurement sequence with general-purpose scanning, so as to monitor the radiofrequency temperature rise of patients with implanted medical devices during MRI scanning. Furthermore, a method and device for inversely calculating the surface temperature of the MRI image using effective data other than instrument artifacts using heat transfer laws are proposed.

Contents of the invention

Based on this, the present invention proposes a method and a magnetic resonance imaging system for real-time monitoring of tissue temperature around a metal implant under MR and giving a safety assessment based on MR thermometry.

A method of monitoring the temperature of tissue surrounding an active implant based on magnetic resonance thermometry and employing a magnetic resonance imaging system; the magnetic resonance imaging system is used to generate at least one Sequence 2 for purposes and a sequence 3 for measuring temperature distribution; the method includes the following steps: step S11, scanning by using sequence 2, and interspersing the scanning of temperature measurement sequence 3 in sequence 2; and step S12, Carry out a safety assessment based on the scan results of temperature measurement sequence 3.

According to the above method for monitoring the temperature of the tissue around the active implant, wherein the method of using sequence 2 for scanning and interspersing the scanning of temperature measurement sequence 3 in sequence 2 includes: dividing sequence 2 into i parts, And each part contains n ₁ , n ₂ ,..., n _i TR units respectively, where TR is the interval time between two excitation pulses in sequence 2; sequence 3 is interspersed between the beginning and end of sequence 2 and each part scanning, and the time intervals between each part of sequence 2 and the previous and subsequent sequence 3 are Δt _1a , Δt _1b , Δt _2a , Δt _2b , . . . , Δt _ia , Δt _ib .

According to the above method for monitoring tissue temperature around an active implant, wherein, the time intervals Δt _1a , Δt _1b , Δt _2a , Δt _2b , . . . , Δt _ia , Δt _ib are all zero.

According to the above-mentioned method for monitoring the temperature of the surrounding tissue of the active implant, the step S11 further includes: before the sequence 2 scanning, first use the temperature measurement sequence 3 to perform a temperature measurement scan on the temperature measurement layer to obtain the initial temperature or temperature related information.

According to the above-mentioned method for monitoring tissue temperature around the active implant, the step S12 includes the following steps: step S121, determine the evaluation area; step S122, determine the temperature rise distribution of the evaluation area; step S123, calculate the safety index; and Step S124, comparing the security index with a security threshold.

According to the above method for monitoring tissue temperature around the active implant, the step S121 includes: using an edge detection algorithm to determine the edge of the artifact, and taking the outside of the edge as an evaluation area.

According to the above-mentioned method for monitoring tissue temperature around the active implant, wherein, in the step S122, determining the temperature rise distribution in the evaluation area includes: each time the sequence 3 is scanned, the data processing unit of the magnetic resonance imaging system receives a set of Data, starting from the second sequence 3 scan, the results of each sequence 3 scan are differentiated from the previous results to obtain the temperature rise distribution.

According to the above method for monitoring the temperature of the tissue around the active implant, the step S124 includes: comparing the calculated thermal accumulation CEM ₄₃ with a preset threshold threshold_CEM ₄₃ , or comparing the calculated maximum temperature rise ΔT _max with The pre-set maximum temperature rise threshold threshold_ΔT _max is compared, or both are compared at the same time; if any one exceeds the threshold, the data processing unit of the magnetic resonance imaging system sends a danger warning to the MR control unit in time, and automatically stops the scanning of the MR scanning device.

According to the above-mentioned method for monitoring tissue temperature around an active implant, wherein, the magnetic resonance imaging system further includes a sequence 1 for positioning or other scanning purposes; step S10 is further included before step S11, and sequence 1 is used to perform Positioning and scanning to determine the layer selection for temperature measurement and imaging.

A magnetic resonance imaging system comprising: an MR scanning device for generating at least one sequence 2 for clinical examination or scientific research or other purposes and a sequence 3 for measuring temperature distribution ; an MR control unit, the MR control unit is used to control the MR scanning device to scan using sequence 2 and sequence 3; and a data processing unit, the data processing unit is used to process the scanning results of the temperature measurement sequence 3, wherein, The magnetic resonance imaging system has the function of monitoring the tissue temperature around the active implant, and the method for the magnetic resonance imaging system to monitor the tissue temperature around the active implant is any one of the above methods.

Compared with the prior art, the method for monitoring the tissue temperature around the active implant by the magnetic resonance imaging system provided by the present invention can effectively monitor the radio frequency temperature rise of a patient with an implanted medical device during MRI scanning, eliminating potential safety hazards .

Description of drawings

FIG. 1 is a schematic structural diagram of a deep brain electrical stimulator used in an embodiment of the present invention.

Fig. 2 is a block diagram of a magnetic resonance imaging system provided by an embodiment of the present invention.

FIG. 3 is a schematic structural diagram of a field drift correction device used in an embodiment of the present invention.

FIG. 4 is a schematic diagram of a scanning mode of temperature measurement sequence 3 interspersed with sequence 2 in an embodiment of the present invention.

FIG. 5 is a schematic diagram of a scanning mode in which the temperature measurement sequence 3 is continuously interspersed in the sequence 2 according to an embodiment of the present invention.

FIG. 6 is a schematic diagram of the temperature measurement sequence scanning performed interspersed between layers and the temperature measurement sequence scanned interspersed between rows adopted in the embodiment of the present invention.

FIG. 7 is a flow chart of a method for performing temperature measurement sequence scanning interspersed between layers in an embodiment of the present invention.

FIG. 8 is a flow chart of a method for performing temperature measurement sequence scanning interspersed between lines in an embodiment of the present invention.

Fig. 9 is a flowchart of a method for determining the artifact region of the active implant according to an embodiment of the present invention.

10 is a schematic diagram of selecting several points in the central area of the image corresponding to the field drift correction container when correcting the temperature change caused by the field drift according to the embodiment of the present invention.

Description of main component symbols

Deep Brain Stimulator 10

External programmer 11

Pulse generator 12

Extension lead 14

Stimulating electrodes 16

Electrode contacts 18

Magnetic resonance imaging system 20

MR scanning equipment 22

MR control unit 24

Data processing unit 26

Field drift correction device 30

head 32

container 34

string 36

Artifact Area 40

Artifact edges 42

Organization Signals 44

central area 46

The following specific embodiments will further illustrate the present invention in conjunction with the above-mentioned drawings.

Detailed ways

The invention provides a method for real-time monitoring of tissue temperature around an active implant under MR based on MR temperature measurement and a safety assessment and a magnetic resonance imaging system using the method. The active implant can be a cardiac pacemaker, defibrillator, deep brain stimulator, spinal cord stimulator, vagus nerve stimulator, gastrointestinal stimulator or other similar implanted medical devices. The present invention is only described by taking the deep brain electrical stimulator as an example, and the present invention is further described in conjunction with the accompanying drawings.

Please refer to FIG. 1 , the deep brain electrical stimulator 10 includes: an external programmer 11 , a pulse generator 12 implanted in the body, an extension wire 14 and a stimulating electrode 16 . The external programmer 11 controls the pulse generator 12 to generate a certain pattern of current pulses, which are transmitted to the electrode contacts 18 of the stimulating electrodes 16 through the extension wires 14, and stimulate specific nuclei through the electrode contacts 18 to achieve purpose of treating disease. However, when the patient implanted with the deep brain electrical stimulator 10 is performing an MR scan, its slender extension wire 14 and the stimulating electrode 16 will absorb electromagnetic wave energy like an antenna, and generate heat at the electrode contact 18, which is safe. Hidden danger. In order to ensure the safety of these patients during MR scanning, the temperature around the electrode contacts 18 of these patients can be monitored and evaluated for safety by using the method and system provided by the present invention.

Referring to FIG. 2 , the magnetic resonance imaging system 20 provided by the present invention includes: an MR scanning device 22 , an MR control unit 24 , and a data processing unit 26 .

The MR scanning device 22 mainly includes 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 suitable for different parts, an MR scanning bed and supporting automatic electrical equipment.

The MR control unit 24 includes MR equipment control software and image reconstruction processing software. The MR equipment control software can set scanning parameters and set scanning sequences. In particular, the MR equipment control software integrates a magnetic resonance scanning scheme capable of real-time monitoring of temperature changes in specific anatomical regions of the subject. The protocol includes at least one sequence 2 for clinical examination or scientific research or other purposes, and one sequence 3 for measuring temperature distribution. These two sequences are interspersed during scanning.

Generally, the solution also includes a sequence 1 for positioning or other scanning purposes, the purpose of the sequence 1 scanning is to determine the region of interest, especially the region where the implant is located. Whenever the relative position of the subject in the MR scanning device 22 changes or the positioning center of the MR scanning device 22 changes or other situations that may cause the position of the region of interest to change, it is necessary to re-scan the sequence 1 to reposition area of interest. Generally, the sequence 1 should be the first scan sequence performed each time an MRI examination is performed. If there is no situation that may cause the position of the region of interest to change after the first sequence 1 scan, then the subsequent There is no need to repeat the sequence 1 scan during the scan. The present invention does not limit the parameters and types of sequence 1.

The purpose of the sequence 2 is to examine or diagnose the subject or to conduct scientific research, and the scanning result has clinical significance or scientific value. The parameters and types of the sequence 2 are generally set by medical workers or equipment operators, which are not limited in the present invention. It can be understood that due to the interaction between the radio frequency magnetic field of the MRI system and the implanted medical device, during the scanning process of sequence 2, a temperature rise may occur in a specific anatomical region of the subject. When the temperature rise exceeds a certain threshold or the temperature accumulates If the heat exceeds the safe limit, it may cause local tissue damage of the subject and threaten the life, health and safety of the subject. It is therefore necessary to monitor the temperature changes in specific anatomical regions of the subject in real time during the sequence 2 MRI scan. The scanning solution provided by the present invention is to intersperse the scanning of the temperature measurement sequence 3 in the sequence 2 .

The sequence 3 is a temperature-sensitive sequence, and when the temperature in a specific anatomical region of the subject changes, the scanning results of the sequence 3 will change accordingly. According to different temperature-sensitive physical parameters used, there are many types of sequence 3. For example, in a temperature measurement method using proton resonance frequency as a temperature sensitive parameter, the type of temperature measurement sequence is generally a gradient echo sequence (GRE sequence) or a planar echo sequence (EPI sequence). The temperature measurement sequence can also be based on the proton density (Proton density), that is, according to the Boltzmann distribution, the proton density is inversely proportional to the absolute temperature, so the temperature of the measured object can be calculated by using the MRI image weighted by the proton density. The thermometry sequence can also be based on the T1 relaxation time of water molecules, that is, the spin-lattice relaxation in biological tissues is caused by the dipole interaction between biological macromolecules and water molecules, which depends on temperature, When the temperature range is small, the T1 relaxation time is almost linearly related to 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 will cause the phase dispersion of the signal in the direction of the diffusion gradient, which will lead to the attenuation of the NMR signal. The degree of attenuation is proportional to the diffusion coefficient. Proportional and affected by temperature, MRI imaging can be used to obtain the diffusion coefficient under different temperature conditions, and then obtain the temperature change. The present invention does not limit the parameters and types of sequence 3 . Each time the temperature measurement sequence 3 is scanned, the temperature distribution of a specific area at a time point can be obtained. It can be understood that, for the sake of "real-time" temperature measurement, the time interval of the sequence 3 scanning should not be too long, and the duration of the sequence 3 itself should not be too long. Preferably, the time interval of scanning in sequence 3 should be controlled within 6 minutes, and the duration should be controlled within 2 minutes. Further preferably, the time interval of scanning in sequence 3 should be controlled within 3 minutes, and the duration should be controlled within 30 seconds. Sequence 3 shouldn't produce large energy deposits either. Preferably, the local SAR value of sequence 3 should be less than 0.4W/kg. Further preferably, the local SAR value of sequence 3 should be less than 0.1W/kg. In this way, no additional energy deposition is generated, and no additional safety risk is caused to the patient. In practical applications, several scans of sequence 3 should be able to faithfully reflect the time course of temperature change in a specific area. It can be understood that the temperature measurement sequence 3 is interspersed in the sequence 2, and the results of the sequence 3 are processed in real time, so that real-time monitoring of the temperature change of a specific anatomical region of the subject can be realized during the magnetic resonance scanning process.

Introduce various parameters that the present invention will relate to below:

t1: Duration of sequence 1 scan.

t2: the duration of the uninterrupted scanning of sequence 2, that is, the time from the start of scanning to the end of scanning assuming that the scanning of sequence 2 is uninterrupted.

t3: Duration of each sequence 3 scan.

Δt: The temperature measurement interval time of sequence 3, that is, the time from the start of one sequence 3 scan to the next sequence 3 scan. The selection of the temperature measurement interval Δt depends on the specific scanning situation. If the temperature rises slowly in the scanning situation involved, a longer Δt can be used to monitor the temperature change; but if the temperature rises rapidly or the time gradient of the temperature is large in the scanning situation involved, A shorter temperature measurement time interval Δt is required, so that on the one hand, the resolution of the temperature measurement time can be improved, and on the other hand, the temperature information can be fed back in time to ensure the safety of the subjects. Generally, when a patient carrying a deep brain electrical stimulator 10 scans in a 3T environment, Δt selects a value within the range of 10 seconds to 6 minutes, because the temperature rise at the electrode contact 18 is very fast at this time, in order to improve the accuracy of the measurement results. Accuracy is typically measured at short intervals, such as 10 seconds.

T _slice : The time required for sequence 2 to scan slice 1.

TR: repetition time in sequence 2, that is, the interval time between two excitation pulses in sequence 2.

The data processing unit 26 is equipped with temperature calculation software based on MR image information, and the MR control unit 24 transmits the collected and reconstructed temperature measurement images to the data processing unit 26 in real time. The data processing unit 26 calculates the temperature distribution of the region of interest according to the temperature measurement image, and provides a safety index for evaluating safety at this time. The safety index can be a certain temperature rise value, or a thermal cumulative dose value, commonly used 43 Cumulative Equivalent Minutes in Celsius (CEM43, Cumulative Equivalent Minutes@43°C), the safety index can be the maximum value of the region of interest at this time, the maximum value of the surface of the implant (such as an electrode) calculated at this time, the calculated The maximum value of the region of interest after a certain moment, the calculated maximum value of the surface of the implant (such as an electrode) after a certain moment, etc. Judging the safety of the magnetic resonance scan at this time according to the safety threshold set by the program, and feeding back to the MR control unit 24 in time. If the safety index exceeds the threshold, the MR control unit 24 stops the MR scanning of the MR scanning device 22, otherwise, continues the scanning.

The following introduces a method for real-time monitoring of tissue temperature around metal implants under MR and giving a safety assessment when using the magnetic resonance imaging system 20 provided by the present invention to perform head MR scans on patients with active implants. The method includes the following steps:

Step S10, using Sequence 1 to perform positioning scanning to determine the temperature measurement layer selection and imaging layer selection;

Step S11, using sequence 2 to scan, and interspersed with sequence 2 to scan the temperature measurement sequence 3; and

In step S12, a safety assessment is performed according to the scanning results of the temperature measurement sequence 3.

In the step S10, preferably, before the MR scan, the patient installs a field drift correction device on a suitable area around the scan site, such as around the head. The field drift correction device is used to provide a reference of the magnetic resonance signal around the scanning site, and remove the influence of the magnetic field drift when analyzing the temperature rise. As shown in Figure 3. The field drift correction device 30 includes: a set of containers 34 . The set of containers 34 is made of non-magnetic materials. The non-magnetic material can be nylon, polypropylene, plexiglass and the like. The set of containers 34 is filled with a uniform medium, such as physiological saline, agar gel, hydroxyethyl cellulose (Hydroxy Ethyl Cellulose) gel and the like. Generally, the homogeneous medium is also equipped with substances that adjust the relaxation time of the medium, such as CuSO ₄ or other transition metal salts, to facilitate magnetic resonance imaging. The medium in the container 34 should be kept at the same temperature as the environment in which the MR device is located. In this embodiment, the container 34 is four plastic test tubes made of non-magnetic material, and each test tube is filled with agar. When installing, four test tubes can be evenly hooped around the head 32 with two elastic soft strings 36, so that the orientation of the four test tubes is basically parallel to the orientation of the stimulating electrodes 16, and the position of the electrode contacts 18 is ensured. The temperature selection layer contains four test tube contents. Optionally, the fixing method of the test tube can also be fixed by using a flexible hard shelf.

Generally, a positioning scan sequence 1 is performed first during scanning to roughly observe the region of interest and the position of the implant to be monitored. Sequence 1 may be multiple sequences, which are used to further determine the area to be scanned subsequently. Based on the scan results of sequence 1, the region of interest for sequence 2 and the area to be monitored for sequence 3 are determined. Taking the scanning of the implanted deep brain electrical stimulator system as an example, the temperature rise of the electrode contact surface is usually the most serious and needs to be monitored. Therefore, the area where the electrode contact 18 is located is selected as the scanning area of sequence 3 to determine the temperature measurement layer. The imaging layer selection of sequence 2 is determined according to the actual diagnosis or research needs, and there is no limitation here. The scanning area and parameter settings of Sequence 2 and Sequence 3 are independent of each other and do not interfere with each other. It can be understood that, if the location of the implant is known in advance, this step S10 can be omitted.

Table 1 lists some main sequence parameters that may be used as examples, and the application is not limited to those listed in the table.

Table 1 Example of the main parameters of the sequence

In the step S11, before the scanning sequence 2, the temperature measurement sequence 3 is used to conduct a temperature measurement scan on the temperature measurement layer to obtain the initial temperature or temperature-related information. Generally, gradient echo sequence (GRE) or echo planar imaging (EPI) sequence can be used as the temperature measurement sequence to scan the temperature measurement layer, and the obtained phase map can be used as the initial reference phase map

The scanning mode of the temperature measurement sequence 3 interspersed with the scanning sequence 2 is shown in FIG. 4 . Divide the sequence 2 into multiple parts, each part is composed of several units, each unit contains a series of specific radio frequency pulses and time series changes of the gradient magnetic field within a TR time, and can collect a set of data to form a sequence 2 part of the image k-space. Then, the scan of sequence 3 is interspersed between the beginning and end of sequence 2 and between each part to form temperature rise monitoring. As shown in FIG. 4 , sequence 2 is composed of i parts, and each part contains n ₁ , n ₂ , . . . , n _i TR units. It can be understood that n ₁ TR + n ₂ TR + . . . + n _i TR = t2. The time intervals between each part of sequence 2 and the previous and subsequent sequence 3 are Δt _1a , Δt _1b , Δt _2a , Δt _2b , . . . , Δt _ia , Δt _ib . In particular, as shown in FIG. 5 , in another embodiment, there may be no time interval between each part of sequence 2 and sequence 3 , and the scanning is continuous. More specifically, the intervals between sequences 3 are equal, that is, Δt ₁ =Δt ₂ =...=Δt _i =Δt. The number of units contained in each part of sequence 2 is also equal, that is, n ₁ =n ₂ =...=n _i . Because the initial segment of sequence 2 may contain different shimming pulses, flipping pulses, etc. from the subsequent part, and the number of units it contains may not necessarily be an integer multiple of i, so n ₁ , n ₂ ,...,n _i can be different, Δt ₁ , Δt ₂ , . . . , Δt _i may also be different.

Each time sequence 3 is scanned, a set of data can be obtained, which is transmitted to the data processing unit 26 for processing, and temperature-related information of the scanning area of sequence 3 is obtained. Starting from the second sequence 3 scan, the results of each sequence 3 scan can be compared with the results of the first sequence 3 scan, and the temperature rise distribution of the sequence 3 scan area can be obtained through data processing. Furthermore, the safety index used to evaluate safety at this time is given. The safety index can be temperature rise value or thermal cumulative dose value, which is usually characterized by cumulative equivalent minutes at 43 degrees Celsius (CEM43, Cumulative Equivalent Minutes@43°C), The safety index can be the maximum value of the scanning area of sequence 3, the maximum value of the surface of the implant (such as an electrode) estimated at this time, the maximum value of the scanning area of sequence 3 after a certain moment of calculation, the maximum value of the implant (such as an electrode) after a certain moment of calculation, or the maximum value of the implant after a certain moment of calculation The maximum value on the surface of the entry (such as an electrode), etc. Judging the safety of the magnetic resonance scan at this time according to the safety threshold set by the program, and feeding back to the MR control unit 24 in time. If the safety index exceeds the threshold, the MR control unit 24 stops the MR scanning of the MR scanning device 22, otherwise, continues the scanning.

FIG. 6 shows an example of the segmentation method of sequence 2. Sequence 2 may need to scan multiple layers of images. Each layer of image is reconstructed from a set of k-space data. Each set of k-space data is generated by scanning multiple TR units. Each TR unit scans to generate a row or row in k-space. few rows of data. Sequence 3 can be interspersed between layers, that is, each layer or every several layers is interspersed with a sequence 3 scan. More generally, the multi-layer image scanning of sequence 2 is composed of several TR units. Therefore, the sequence 3 can also be interspersed between lines of the k-space data of a layer of image. The interleaving mode of sequence 3 can be adjusted and set according to the relationship between _ni TR and T _slice . Referring to Fig. 7, taking the axial scan of the whole head as an example, when n _i TR > T _slice , you can select the temperature measurement sequence 3 scanning of the k-space data obtained in the imaging sequence interspersed with the layer selection for temperature measurement Way. Suppose n _i TR = nT _slice , n is a natural number. Stop the imaging scan of sequence 2 every time the imaging image of n layers is obtained, and perform a scan of temperature measurement sequence 3 on the temperature measurement layer. Stop after an n-layer imaging image, and then scan the temperature measurement sequence 3 once for the temperature measurement layer, and continue in turn.

Referring to Fig. 8, when n _i TR<T _slice , or when n _i TR is not an integer multiple of T _slice , it is necessary to select the method of temperature measurement sequence 3 scans interspersed between k-space data lines for temperature measurement layer selection. Assuming that the K-space of the imaging sequence has a total of P rows, the data collected after each phase encoding corresponds to a row of K-space, that is, a row of K-space data is collected at TR time. After collecting j lines in the K space of a certain layer of the imaging sequence, suspend the scanning of the imaging sequence 2, transfer the data that has been collected in the K space to the storage unit, and then start scanning the temperature measurement sequence 3 for the temperature measurement layer, The collected data is transmitted to the data processing unit 26 in real time to obtain the temperature distribution. After the scanning of the temperature measurement sequence 3 is completed, the scanning of the imaging sequence 2 will continue from the j+1th line, and the collected data will continue to be saved to the storage unit, and the subsequent scanning process is similar. During the above scanning process, the MR control unit 24 transmits the collected data to the data processing unit 26 in real time.

In the step S12, the data processing unit 26 calculates the temperature distribution at this time according to the temperature measurement image data from the MR control unit 24, and then provides a safety index for evaluating safety at this time, which is different from the previous one. The set threshold is compared, and timely feedback is given to the MR control unit 24 .

Specifically, in the step S12, the safety assessment according to the scanning results of the temperature measurement sequence 3 includes the following steps:

Step S121, determining the evaluation area;

Step S122, determining the temperature rise distribution of the evaluation area;

Step S123, calculating the security index; and

Step S124, comparing the security index with a security threshold.

In the step S121 , the data processing unit 26 determines a security assessment area according to the data of sequence 1 or sequence 3 . Generally, the evaluation area should be selected as close as possible to the surface of the implant with severe fever, such as the contact surface of the deep brain electrical stimulation electrode. Due to the different physical properties of implants and biological tissues, especially the different magnetic susceptibility coefficients of metal parts, they will be magnetized by the static magnetic field in the magnetic resonance environment, resulting in distortion of the surrounding magnetic field, resulting in distortion of the image signal around the implant. Artifacts. Usually it is difficult to extract useful information from the signal in this part of the area. Therefore, the selection of the evaluation area usually involves the determination of the artifact area of the active implant. In general, the induced electric field generated by the interaction of the active implant with the radio frequency magnetic field of magnetic resonance is strongest at the tip surface of the elongated conductor structure, resulting in the highest temperature rise, which gradually decreases with heat conduction to the surroundings . For example, the temperature rise is more likely to occur at the contact points of the deep brain electrical stimulation electrodes. Assessing safety therefore requires determining the assessment area around the implant artifact, as close as possible to the highest temperature rise on the implant surface, and being able to extract temperature information from the sequence 3 data.

Preferably, the evaluation area is selected to be a certain distance away from the edge of the artifact. Preferably, this distance is 1-6mm. The artifact edge 42 can be detected by the threshold method. The internal signal strength of the artifact is I0, and the signal strength of the surrounding area is I1. A certain value I2 between I0 and I1 is set as the threshold value. If it is higher than this threshold value, it is considered as an artifact outside the assessment area. Preferably, (I2-I0)/(I1-I0) is 0.3-0.5. The artifact edge 42 can also be determined using an edge detection algorithm, and the determination process is shown in FIG. 9 . Preferably, the artifact edge 42 can be determined using a canny algorithm, a sober algorithm, or a Roberts algorithm. The area other than the artifact edge 42 is selected as the evaluation area. Further preferably, the artifact edge 42 belongs to the transition area between the metal artifact area 40 and the tissue signal 44 , and a classification algorithm is used to determine the type of the pixel point of the artifact edge 42 , and if it belongs to the tissue signal 44 , it is included in the evaluation area. Preferably, the Bayesian classification algorithm can be used to classify the artifact edge 42, determine the category of the pixels covered by the artifact edge 42, and organize the signal 44 or the artifact area 40, so that the artifact area can be separated from the image 40 sure came out.

In the step S122, determining the distribution of temperature rise in the assessment area includes the following steps: after each scan of the sequence 3, the data processing unit 26 receives a set of data, and the temperature-related information of the assessment area can be obtained through processing. Starting from the second sequence 3 scan, the results of each sequence 3 scan can be differentiated from the previous results to obtain the temperature rise distribution. In particular, the difference between each scan and the scan result of the first sequence 3 is obtained to obtain the temperature rise distribution relative to the state before the sequence 2 scan.

The temperature-related information depends on different magnetic resonance temperature measurement methods. Preferably, the temperature distribution map can be obtained by using an MR temperature measurement method based on proton resonance frequency shift. The temperature measurement process includes, before implementing the imaging sequence scan, first conduct a scan of the temperature measurement sequence 3, and use the obtained phase map as the reference phase map The phase map obtained by the kth acquisition is denoted as The temperature change distribution ΔT _map at the kth acquisition can be obtained according to formula (3),

where the phase difference

Since in actual MRI images, the value range of the phase is usually [-π, π], the phase will jump at the edge, resulting in the so-called phase warping. Therefore, the above-mentioned method of directly subtracting the phase difference may cause relatively large errors. In order to avoid phase wrapping, the phase difference can be calculated by the following formula (4)

in It is the complex exponential form of the two-sweep phase signal, and Im() and Re() represent the imaginary part and real part of the complex number respectively. Expand the above calculation formula to get the following formula (5)

By substituting it into the temperature change calculation formula, the temperature change at the Kth scan can be calculated.

The static magnetic field generated by the MRI scanner may drift, resulting in a phase change, resulting in inaccurate temperature distribution obtained in the above steps. Therefore, preferably, temperature changes caused by field drift need to be corrected. The static magnetic field drift has a distribution in space, and this distribution can be approximated by a polynomial. General correction needs to select at least 3 positions for 1st order plane correction. In particular, for the situation where the distribution of the measurement area relative to the static magnetic field is small and the first-order item has little influence, the zero-order correction can be performed directly by subtracting the mean value c, and at this time at least one point is selected. As shown in FIG. 10 , in the temperature distribution diagram, each field drift correction container 34 selects several points corresponding to the central area 46 of the image. Another method of correcting for field drift does not rely on a field drift correction container. 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 distribution map), and the reference area should contain at least one pixel. The present invention 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 here may also include the image area corresponding to the field drift 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 (tissue signal includes amplitude signal, phase signal and temperature signal), ensuring that the reference area is representative . For the first-order calibration, select ≥ 3 points, store the position information and temperature change information of each point in the matrix A(i, j, ΔT), and use the linear interpolation method to obtain the pseudo temperature change distribution map caused by field drift . The calculation process can be solved by solving the problem:

Among them, the first column of [ij 1] _n×3 is A(:,1), the second column is A(:,2), and the third column is all 1s. Solve to get the fitting plane z(i,j)=a i+b j+c in the sense of least squares, subtract z from the original temperature change distribution map, and then get the corrected actual temperature distribution ΔT _correction , That is, formula (7),

ΔT _correction (i, j) = ΔT _map (i, j) - z (i, j) (7).

For zero-order calibration, 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 calculate the field drift caused by The pseudo temperature change z, the calculation process is as follows (8):

Where n is the number of points selected in all reference areas, that is, the number of elements in the quantity B(i). Subtract z from the original temperature change distribution graph to obtain the corrected temperature distribution (9):

ΔT _correction (i,j)=ΔT _map (i,j)-z (9)

In the step S123, the safety indicator may be a temperature rise value, or a thermal cumulative dose value, which is usually characterized by cumulative equivalent minutes at 43 degrees Celsius (CEM43, Cumulative Equivalent Minutes@43 degrees Celsius). The safety index may be the maximum value of the evaluation area, that is, the maximum value in step S122 is selected. It can also be, but not limited to, the maximum value of the surface of the implant (such as an electrode) estimated at this time, the maximum value of the evaluation area after a certain moment of calculation, and the maximum value of the surface of an implant (such as an electrode) after a certain moment of calculation. Wait.

The calculation method of the safety index includes the empirical table or empirical formula obtained from the experiment, or the heat transfer law of temperature rise, obtained by approximate fitting method or numerical analysis method.

Since the signal-to-noise ratio in the artifact area 40 is very low, the temperature data in the artifact area 40 is unreliable, and the temperature change in the artifact area 40 needs to be obtained by calculating the temperature outside the artifact area 40 . Generally, the temperature information of pixels within a certain range 42 outside the edge of the artifact is selected as the boundary condition, and the position information and temperature change information of each pixel are respectively stored in the matrix r=(r ₁ ,r ₂ ,...r _m ) and p _k =(T _k1 ,T _k2 ,...,T _km ), where r _m represents the position information of the mth pixel, and T _km represents the temperature change of the mth pixel measured for the kth time . Store the temperature data obtained from the kth measurement combined with the previous k-1 measurements into a matrix P,

Considering the biological heat transfer model shown in Equation (1), ignoring the metabolic heat production, assuming a homogeneous medium and taking the blood temperature _Tb as the benchmark, the distribution relation (11) for temperature rise can be obtained:

This formula has homogeneous characteristics, that is, the temperature rise ΔT at a certain space point and a certain time point is proportional to the heat source Q. However, in the nuclear magnetic radio frequency heating problem involved in the present invention, the heat source Q is proportional to the square of the induced electric field E, and the electric field E is proportional to the current density J. It can be known that the temperature rise ΔT is also proportional to the square of the current density J. Using this feature, combined with the spatial and temporal distribution of heat transfer, the temperature rise at other times and other regions can be calculated based on the temperature rise at a certain moment and in a certain region.

Specifically, a numerical model of electromagnetic field and heat transfer is established, and the electric field distribution under different heating modes can be obtained by using the current density J at the conductive part of the active implant-tissue interface as a parameter, and then the law of heat transfer and diffusion can be obtained. In this embodiment, the current density J ₀ , for example, 1000A/m ² , is used as the standard thermal diffusion model to calculate the position at r=(r ₁ ,r ₂ ,...r _m ), corresponding to k times of scanning time temperature change matrix st_P,

It can be understood that st_P(i, j) represents the temperature change value at the position r _j corresponding to the i-th scan time in the standard diffusion model. According to the formula ΔT=a·J ² can be obtained,

Here, ΔT ₁ ＝P, ΔT ₀ ＝st_P, λ can be obtained in the sense of least squares, namely formula (14)

The minimum value of , the extreme point can be obtained by making the derivative of the above formula equal to zero the value of

Will Bringing it into the thermal diffusion simulation model st_P(i,j) results in the thermal diffusion model corresponding to the experiment, from which the temperature change curve at the highest point of temperature rise can be extracted. The temperature change curve at the highest point of temperature rise is the tissue interface temperature change curve.

Further, according to the tissue interface temperature change curve, the corresponding safety indicators within the scan time range, such as the maximum temperature rise ΔT _max and heat accumulation, can be obtained. It can be understood that thermal damage depends not only on the temperature, but also on the duration of the temperature, which is the so-called heat accumulation. The more commonly used thermal accumulation model is CEM ₄₃ , and its calculation formula (16) is,

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

In the step S124, the calculated safety index is compared with a preset safety threshold. For example, compare the calculated heat accumulation CEM ₄₃ with the preset threshold threshold_CEM ₄₃ , or compare the maximum temperature rise ΔT _max with the preset maximum temperature rise threshold threshold_ΔT _max , or compare the two at the same time. If any one exceeds the threshold, the The data processing unit 26 sends a danger warning to the MR control unit 24 in time, and automatically stops the scanning of the MR scanning device 22 .

A number of embodiments of the present invention have been given above, and it can be understood that various changes, substitutions, and changes can be made without departing from the spirit and scope of the present disclosure, and these embodiments are also included in the present invention. within the scope of protection.

Claims (10)

1. A method for monitoring the temperature of tissue surrounding an active implant, the method being based on magnetic resonance thermometry and employing a magnetic resonance imaging system; the magnetic resonance imaging system is used to generate at least one or a sequence 2 for other purposes and a sequence 3 for measuring temperature distribution; the method comprises the following steps: Step S11, use sequence 2 to scan, and intersperse the scan of temperature measurement sequence 3 in sequence 2, so as to divide sequence 2 into multiple parts, each part is composed of several units, and each unit is within a TR time , which includes a series of specific radio frequency pulses and temporal changes of the gradient magnetic field, and can collect a set of data to form part of the sequence 2 image k-space; and In step S12, a safety assessment is performed according to the scanning results of the temperature measurement sequence 3, and it is judged whether the safety index of the tissue temperature around the active implant exceeds a safety threshold. 2. The method for monitoring the tissue temperature around the active implant according to claim 1, characterized in that, the method of scanning by using sequence 2 and interspersing the scanning of temperature measurement sequence 3 in sequence 2 comprises: 2 is divided into i parts, and each part contains n ₁ , n ₂ ,..., n _i TR units, wherein, TR is the interval time between two excitation pulses in sequence 2; the scan of sequence 3 is interspersed between the beginning and end of sequence 2 and each part, and each part of sequence 2 is connected with the previous and the following The time intervals between sequences 3 are Δt _1a , Δt _1b , Δt _2a , Δt _2b , . . . , Δt _ia , Δt _ib , respectively. 3. The method for monitoring tissue temperature around an active implant according to claim 2, wherein the time intervals Δt _1a , Δt _1b , Δt _2a , Δt _2b , . . . , Δt _ia , Δt _ib are all zero. 4. The method for monitoring the temperature of the surrounding tissues of the active implant according to claim 1, characterized in that the step S11 further comprises: before the sequence 2 scans, firstly use the temperature measurement sequence 3 to measure the temperature selection layer once Temperature scanning to obtain the initial temperature or temperature-related information. 5. The method for monitoring tissue temperature around the active implant according to claim 1, characterized in that, said step S12 comprises the following steps: Step S121, determining the evaluation area; Step S122, determining the temperature rise distribution of the evaluation area; Step S123, calculating the maximum temperature rise ΔT _max of the evaluation area or the heat accumulation CEM ₄₃ of the evaluation area as the safety index; and Step S124, comparing the security index with a security threshold. 6 . The method for monitoring tissue temperature around the active implant according to claim 5 , wherein the step S121 comprises: using an edge detection algorithm to determine the edge of the artifact, and taking the outside of the edge as the evaluation area. 7. The method for monitoring the temperature of the tissue around the active implant according to claim 5, characterized in that, in the step S122, determining the temperature rise distribution in the evaluation area comprises: each time the sequence 3 is scanned, the magnetic resonance imaging system The data processing unit receives a set of data. Starting from the second sequence 3 scan, the result of each sequence 3 scan is differentiated from the previous result to obtain the temperature rise distribution. 8. The method for monitoring tissue temperature around the active implant according to claim 5, characterized in that, the step S124 comprises: comparing the calculated thermal accumulation CEM ₄₃ with a preset threshold threshold_CEM ₄₃ , or comparing the calculated The highest temperature rise ΔT _max is compared with the preset maximum temperature rise threshold threshold_ΔT _max , or both are compared at the same time; if any one exceeds the threshold, the data processing unit of the magnetic resonance imaging system sends a danger warning to the MR control unit in time, and automatically Stop the scanning of the MR scanning device. 9. The method for monitoring tissue temperature around an active implant according to claim 1, wherein the magnetic resonance imaging system further comprises a sequence 1 for positioning or other scanning purposes; Before step S11, step S10 is further included, using sequence 1 to perform positioning scanning to determine the selected layer for temperature measurement and the selected layer for imaging. 10. A magnetic resonance imaging system comprising: an MR scanning device for generating at least one sequence 2 for clinical examination or scientific research or other purposes and one sequence 3 for measuring temperature distribution; An MR control unit, the MR control unit is used to control the MR scanning device to scan using sequence 2 and sequence 3; and A data processing unit, the data processing unit is used to process the scanning results of the temperature measurement sequence 3, It is characterized in that the magnetic resonance imaging system has the function of monitoring the temperature of the tissue around the active implant, and the method for monitoring the temperature of the tissue around the active implant by the magnetic resonance imaging system is as in any one of claims 1 to 9 the method described.