Magnetic resonance thermometry imaging method and use thereof

Abstract

The application provides a magnetic resonance thermometry imaging method. The method is based on the temperature dependence of proton resonance frequency, and a phase self-calibration echo is added in a thermometry imaging sequence; a phase image is phase unwrapped in a spatial dimension by using existing technology, and a thermometry phase image is phase unwrapped in a time dimension by using a self-calibration phase image. The method provided by the application can solve the reference benchmark failure problem of magnetic resonance thermometry imaging, does not lose thermometry accuracy, and does not increase additional thermometry tools and intraoperative waiting time. The method provided by the application can also be combined with an extended echo time TE to improve thermometry accuracy. The method provided by the application can be used for magnetic resonance guided thermal ablation treatment.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of magnetic resonance imaging, and particularly relates to a magnetic resonance thermometry imaging method and application thereof. BACKGROUND

[0002] Compared with other thermometry techniques, the magnetic resonance thermometry imaging technique can perform multi-slice multi-angle imaging of internal tissues of a human body at any angle, has the advantages of high thermometry precision, high spatial resolution and coverage, high soft tissue contrast, no bone artifacts, non-invasiveness, no ionizing radiation, etc., and neither the patient nor the doctor will suffer radiation damage in the continuous scanning process, so the technique is particularly suitable for continuously and quantitatively monitoring the distribution and changes of the temperature of internal tissues of a human body in thermal ablation treatment. With the progress of related techniques, magnetic resonance guided thermal ablation treatment is being applied more and more widely in tumor treatment and treatment of other diseases.

[0003] Thermal ablation is a method for treating tumors, the principle of which is to control the temperature of the tissue in the target treatment area at 47ºC or above for a period of time to kill tumor cells. Thermal ablation treatment of tumors has the advantages of minimally invasive / non-invasive, less blood loss, etc., and can improve the immune function of the body by affecting the tumor microenvironment, thereby improving the anti-tumor ability, so thermal ablation plays an irreplaceable role in tumor treatment. The common thermal ablation methods at present include high-intensity focused ultrasound ablation (HIFU), radiofrequency ablation (RFA) and microwave ablation (MWA), etc.

[0004] In thermal ablation treatment, it is crucial to obtain the temperature distribution of the target treatment area and the surrounding tissues in real time and non-invasively to ensure the safety and effectiveness of the treatment. Magnetic resonance thermometry is mainly based on the temperature dependence of magnetic resonance parameters such as relaxation time, diffusion coefficient, proton resonance frequency and magnetization transfer. In magnetic resonance guided thermal ablation treatment, the most widely used magnetic resonance thermometry imaging is currently based on the temperature dependence of proton resonance frequency. The principle is that the proton resonance frequency has a linear relationship with temperature (-0.01 ppm / ºC) in a large temperature range (-15ºC~100ºC), the magnetic resonance phase map can be obtained using a gradient echo imaging sequence, then the phase difference between the magnetic resonance phase maps at different times is calculated, and finally the changes in the temperature distribution are calculated according to the echo time TE and the main magnetic field strength (hereinafter referred to as the main magnetic field strength) B0 of the magnetic resonance imaging system.

[0005] In practical application, a frame of 3D magnetic resonance phase image or multi-slice 2D magnetic resonance phase image (hereinafter referred to as magnetic resonance phase image) is collected as a reference phase image before the implementation of thermal ablation. The reference phase image corresponds to the basic temperature (hereinafter referred to as basic temperature) distribution of the treatment target and the surrounding tissue. During the thermal ablation treatment, the magnetic resonance phase image obtained at a certain time is taken as a temperature measurement phase image, and the reference phase image is subtracted to obtain a phase difference image. Then, the change of the temperature distribution relative to the basic temperature distribution is calculated according to the echo time TE and the main magnetic field strength B0 of the imaging sequence. Finally, the temperature distribution at the certain time is obtained according to the basic temperature distribution.

[0006] According to the properties of inverse trigonometric functions, the phase value of each pixel in the magnetic resonance phase image ranges between -π and π. Therefore, before displaying the phase image or the phase difference image between two frames of images, phase unwrapping in the spatial dimension is required to eliminate the phase jumps between adjacent pixels in the image.

[0007] During the thermal ablation treatment, a temperature measurement phase image is usually collected every few seconds to tens of seconds according to the needs of treatment and temperature monitoring. When the temperature changes slowly, the phase difference between adjacent two frames of images will not exceed half a phase period (one phase period is 2π). In the continuous monitoring process, the temperature measurement phase image is continuously collected, and by recording the phase difference between adjacent two frames of images, it can be determined how many phase periods the current temperature measurement phase image has crossed relative to the reference phase image, so as to correctly calculate the current temperature distribution. That is, in the magnetic resonance guided thermal ablation treatment, in addition to the phase unwrapping in the spatial dimension, the phase unwrapping in the time dimension is also required for the magnetic resonance phase images obtained at different times.

[0008] In practical application, due to various factors in the process of thermal ablation treatment and temperature monitoring, there are often situations that require to pause the temperature measurement imaging. During the period of pausing the temperature measurement imaging, if the temperature change of the tissue exceeds a certain range (for example, the tissue rapidly heats up after applying the heat dose, or there is a large blood vessel around the tissue, and the temperature of the tissue rapidly drops after stopping heating), the phase difference between the two frames of temperature measurement phase images before and after pausing the temperature measurement imaging will exceed half or one or even multiple phase periods. Since it is not possible to know how many phase periods the phase difference is, the temperature measurement imaging cannot be restarted based on the reference phase image, resulting in the failure to correctly calculate the temperature distribution corresponding to the temperature measurement phase image, i.e., the reference reference is invalid.

[0009] There are three solutions to the reference failure problem. The first solution is to wait for the temperature of the tissue to return to the baseline temperature and reacquire the reference phase map. The disadvantage of this solution is that it prolongs the intraoperative waiting time, which can affect the implementation of the treatment plan and the treatment effect. The second solution is to use other temperature measurement tools as an auxiliary (such as optical fiber temperature meter, temperature measurement probe, etc.) for minimally invasive temperature measurement. The disadvantage of this solution is that it increases the additional tools and operations, thereby increasing the intraoperative workload and operation complexity. In addition, since the auxiliary temperature measurement tool is usually invasive, the second solution is difficult to use for non-invasive treatment. The third solution is to shorten the echo time TE of the imaging sequence. For the same temperature change, the shorter the TE, the smaller the phase change, and the less likely the phase wrapping occurs. That is, shortening the TE can avoid reference failure. However, in magnetic resonance thermometry imaging based on the temperature dependence of proton resonance frequency, the echo time TE is proportional to the thermometry accuracy, and shortening the TE will reduce the thermometry accuracy. Therefore, the third solution is only suitable for cases where the thermometry accuracy requirement is not high or the temperature change range is small.

[0010] Chinese Patent Publication No. CN 101507603 B discloses a high time resolution magnetic resonance temperature imaging method and device, belonging to the technical field of magnetic resonance temperature imaging. The high time resolution magnetic resonance temperature imaging method includes: using a radial sequence to perform magnetic resonance coil-based scanning on a region to be diagnosed, to obtain k-space data corresponding to the region to be diagnosed; grouping the k-space data, the radial sequence spokes, and the radial sequence weights based on time frames, to obtain at least two groups of grouped data; reconstructing the at least two groups of grouped data based on a sensitivity map of the magnetic resonance coil, to obtain multiple target time frame images; the multiple target time frame images have no undersampling artifacts; and obtaining a target temperature change image based on the temperature difference between any two adjacent target time frame images. The high time resolution magnetic resonance temperature imaging method can obtain the continuous temperature change process of the region to be diagnosed, and improves the time resolution of the temperature change process.

[0011] International Publication No. WO2019126934A1 discloses a local shim system and a shim method for magnetic resonance imaging, including the following steps: acquiring B0 field map information (301) using two-dimensional gradient echo; calculating and evaluating the uniformity of B0 (302); optimizing the current of each channel shim coil (303); determining whether the minimum standard deviation value of Δf is obtained (304); outputting the optimal current combination value, and setting the optimal current value of each channel shim coil on the current control software (305); and testing and evaluating the uniformity of B0 to achieve the shim target (306).

[0012] Chinese patent application number CN202410086540.X discloses a high temporal resolution real-time magnetic resonance temperature imaging method, relating to the technical field of temperature imaging; the method comprises: constructing a temperature prediction equation and determining each parameter; predicting the temperature of each subsequent image and obtaining predicted K-space data; performing a variable density sampling strategy to obtain actual under-sampled K-space data; performing data padding on the actual under-sampled K-space data to obtain updated K-space data; performing phase difference calculation based on the PRF method to obtain a plurality of temperature difference images and output as a temperature dynamic image. The present application uses the predicted K-space data to perform data padding on the K-space data obtained by under-sampling scanning to obtain complete K-space data; not only greatly saves time at the data acquisition level, but also eliminates artifacts caused by under-sampling, while at the data processing level, it does not need as much time as the compressed sensing technology, ensuring the real-time nature of the data and greatly improving the temporal resolution of the treatment temperature change process.

[0013] Chinese patent publication number CN 111568390B discloses a method for reducing system temperature error in real-time magnetic resonance temperature imaging: placing a plurality of reference water films around the imaging object, and measuring the temperature of the plurality of reference water films in real time through a magnetic resonance compatible temperature optical fiber; by collecting magnetic resonance phase images before and during heating, calculating the phase difference of each reference water film temperature measurement point, and the actual temperature change of each reference water film temperature measurement point, the main magnetic field deviation of each reference water film temperature measurement point is calculated; according to the real-time main magnetic field deviation of the reference water film temperature measurement point, estimate the overall planar distribution of the main magnetic field deviation; according to the overall planar distribution of the main magnetic field deviation, the temperature calculated directly from the phase image for each pixel point of the magnetic resonance imaging is corrected to obtain the actual temperature. The method provided by the present application accurately estimates the main magnetic field deviation value and spatial distribution in the magnetic resonance temperature imaging process, thereby reducing the temperature measurement error in the magnetic resonance temperature imaging.

[0014] In addition, Chinese patent publication number CN 113842210 A discloses a vertebra tumor microwave ablation surgery simulation method and device, relating to the technical field of medical devices and simulation, wherein the method comprises: acquiring a magnetic resonance image of the patient in the current body state during surgery, performing image segmentation on the magnetic resonance image, and establishing an individual model of the patient based on the segmented image; registering the individual model according to the preoperative simulation model to obtain an accurate model of the patient in the current body state; inserting a microwave probe according to the target needle insertion position and heating, collecting actual temperature data during heating, and correcting the microwave probe model using the actual temperature data; performing temperature simulation on the corrected microwave probe model at the target needle insertion position of the accurate model, and performing thermal damage assessment based on the temperature simulation result to correct the target needle insertion position and the target heating time, and generating a surgery simulation scheme for the ablation region. This method can improve the accuracy and reliability of surgical simulation and increase the success rate of vertebra tumor microwave thermal ablation surgery. SUMMARY

[0015] The present application aims at the deficiencies of the prior art, and provides a magnetic resonance thermometry imaging method.

[0016] Still another object of the present application is to provide an application of the magnetic resonance thermometry imaging method.

[0017] To achieve the above object, the present application adopts the following technical scheme: a magnetic resonance thermometry imaging method, based on the temperature dependence of proton resonance frequency, adding a phase self-calibration echo in a thermometry imaging sequence; performing phase unwrapping in a spatial dimension on a phase image, using a self-calibration phase image to perform phase unwrapping in a time dimension on a thermometry phase image; calculating a temperature distribution using the thermometry phase image unwrapped in the time dimension and the spatial dimension; comprising the following steps:

[0018] (1) adding a phase self-calibration echo in a thermometry imaging sequence:

[0019] In the thermometry imaging sequence, a phase self-calibration echo is added, the phase self-calibration echo has the same composition and operation mode as a thermometry imaging echo, and the positive and negative polarities of the converging gradients corresponding to the two echoes are opposite; the thermometry imaging echo has a time TE, and the phase self-calibration echo has a time TE', the difference between the two is ; the phase self-calibration echo is before the thermometry imaging echo or after the thermometry imaging echo ; The absolute value of the difference is less than a time threshold t max and greater than a minimum time value t min allowed by a magnetic resonance imaging instrument, The time threshold t max is determined by the manufacturing technology of the magnetic resonance imaging instrument, and is

[0020]

[0021] In the formula, γ represents the gyromagnetic ratio of the nucleus, B0 represents the main magnetic field strength, represents the maximum value of the temperature change occurring in the temperature measurement interval between two adjacent times; in particular it can be used to represent the maximum value of the temperature change that can occur in the temperature measurement interval between two adjacent times in thermal ablation technology;

[0022] (2) executing a thermometry imaging sequence to obtain a thermometry phase image and a self-calibration phase image:

[0023] Before the thermal ablation technique is implemented, a temperature measurement imaging sequence is executed to acquire a frame of temperature measurement phase image and a frame of self-calibration phase image as a reference temperature measurement phase image and a reference self-calibration phase image corresponding to a basic temperature distribution; during the thermal ablation technique, each time the temperature measurement imaging sequence is executed, a frame of temperature measurement phase image and a frame of self-calibration phase image are acquired corresponding to a temperature distribution at that time;

[0024] (3) The existing technology can be used to respectively perform spatial dimension phase unwrapping on the reference temperature measurement phase image, the reference self-calibration phase image, the temperature measurement phase image and the self-calibration phase image, so as to eliminate phase jumps between adjacent pixels in the images;

[0025] (4) The self-calibration phase image is used to perform time dimension phase unwrapping on the temperature measurement phase image:

[0026] If the phase value of a certain pixel in the reference temperature measurement phase image is , the phase value of the pixel in the reference self-calibration phase image is , the phase difference between the reference self-calibration phase image and the reference temperature measurement phase image is ; if the phase value of the certain pixel in the temperature measurement phase image is , the phase value of the pixel in the self-calibration phase image is , the phase difference between the self-calibration phase image and the temperature measurement phase image is ; the number of phase periods n that the pixel crosses between the temperature measurement phase image and the reference temperature measurement phase image is calculated by using the following formula, is the phase change accumulation, according to the relationship among phase, frequency and time, , the ratio of to TE is equal to the ratio of ( - ) to ΔTE. That is, the frequency calculated by using and TE is equal to the frequency calculated by using ( - ) and ΔTE; then the phase value of the pixel in the temperature measurement phase image after unwrapping is calculated by using n ;

[0027]

[0028] wherein fix( ) represents rounding the value in the parentheses to zero;

[0029] The above operation is performed on each pixel in the above phase images;

[0030] (5) The temperature distribution is calculated by using the temperature measurement phase image after time dimension and spatial dimension unwrapping:

[0031] After time dimension and spatial dimension unwrapping, if the phase value of a certain pixel in the temperature measurement phase image is In the reference temperature measurement phase diagram, the phase value is , the basic temperature is , and the temperature T corresponding to the pixel is calculated by using the following formula m :

[0032]

[0033] wherein, ;

[0034] The above operation is performed on each pixel in the phase diagram to obtain a temperature distribution.

[0035] The principle of the present application is that, based on the temperature dependence of the proton resonance frequency, a phase self-calibration echo is added in the temperature measurement imaging sequence; the phase image is phase unwrapped in the spatial dimension by using the existing technology, the temperature measurement phase image is phase unwrapped in the time dimension by using the self-calibration phase image; and the temperature distribution is calculated by using the temperature measurement phase image unwrapped in the time dimension and the spatial dimension.

[0036] The method of the present application can solve the reference benchmark failure problem of magnetic resonance temperature measurement imaging, and does not lose the temperature measurement accuracy and does not increase the additional temperature measurement tool and intraoperative waiting time; the method proposed by the present application can also improve the temperature measurement accuracy in combination with the extension of the echo time TE.

[0037] On the basis of the above scheme, the order of steps (3) and (4) is exchanged.

[0038] Preferably, based on the linear relationship between the proton resonance frequency and the temperature in the temperature range of -15℃ to 100℃, which is -0.01ppm / ℃, a magnetic resonance phase image is obtained by using a gradient echo imaging sequence, then the phase difference between the magnetic resonance phase images at different times is calculated, and then the temperature distribution change is calculated according to the echo time TE of the imaging sequence and the main magnetic field strength B0; that is, in the temperature measurement imaging process, the phase is obtained first, then the phase difference , the proton resonance change amount Δf and the temperature change amount ΔT are used to calculate ΔT, and then the current temperature Tm is calculated according to the basic temperature Tb, wherein,

[0039]

[0040] The temperature measurement accuracy is proportional to the temperature measurement imaging echo time TE, if TE is long, then a slight change in temperature can cause a significant change in the actual phase measurement value, that is, the phase difference increases, ≤ half a phase period.

[0041] Further, the phase self-calibration echo time TE' is different from the temperature measurement imaging echo time TE by ΔTE ; the phase difference The change is proportional to the temperature change ΔT The change is proportional to the temperature change ΔT≤ 65℃, the phase difference between adjacent two frames does not change more than half a phase period, in combination with the relationship between the change of (E) and (E) , the time dimension of (E) is unwound.

[0042] The application provides an application of the above-mentioned magnetic resonance thermometry imaging method in a magnetic resonance guided thermal ablation device. It is suitable for magnetic resonance guided thermal ablation technology.

[0043] The application has the advantages that the method can solve the reference benchmark failure problem of magnetic resonance thermometry imaging, does not lose the thermometry precision, and does not increase the additional thermometry tool and intraoperative waiting time. The method can also be combined with the extension of echo time TE to improve the thermometry precision. The method is particularly suitable for magnetic resonance guided thermal ablation treatment. BRIEF DESCRIPTION OF DRAWINGS

[0044] Figure 1 For the thermometry imaging sequence, the phase self-calibration echo is after the thermometry imaging echo;

[0045] Figure 2 For the thermometry imaging sequence, the phase self-calibration echo is before the thermometry imaging echo;

[0046] Figure 3 For the flowchart of the application;

[0047] MARKS OF THE DRAWINGS:

[0048] TR—pulse sequence repetition time; TE—thermometry imaging echo time; TE’—phase self-calibration echo time;

[0049] 101, 102—radio frequency excitation pulse;

[0050] 201, 202—selective layer gradient pulse;

[0051] 301—phase encoding gradient pulse; 400—dispersion phase gradient pulse;

[0052] 401, 402—convergent gradient pulse;

[0053] 501—thermometry imaging echo; 502—phase self-calibration echo. DETAILED DESCRIPTION

[0054] Magnetic resonance thermometry based on the temperature dependence of proton resonance frequency works on the principle that the proton resonance frequency has a linear relationship with temperature (-0.01 ppm / ℃) over a wide temperature range (-15ºC to 100ºC). A magnetic resonance phase map can be obtained using a gradient echo imaging sequence. Then, the phase difference between the phase maps at different times is calculated, and the temperature distribution change is calculated based on the echo time TE and the main magnetic field strength B0 of the imaging sequence. In other words, the phase is first obtained during thermometry imaging, and then the temperature distribution is calculated based on the phase difference. ΔT is calculated using the proton resonance change Δf and the temperature change ΔT, and then the current temperature Tm is calculated based on the baseline temperature Tb. The phase difference is also considered. for:

[0055]

[0056] The proton resonance change Δf is:

[0057]

[0058] The current temperature Tm is:

[0059]

[0060] Based on the above principle, it can be seen that the temperature measurement accuracy is directly proportional to the temperature imaging echo time (TE). That is, if the TE is long, even a small change in temperature can cause a significant change in the actual measured value (phase). Taking temperature imaging using a 3T magnetic resonance imaging system as an example, the proton resonance frequency (… The frequency is approximately 128 MHz. If a temperature measurement accuracy of 1°C is required, considering the impact of image noise, the TE should be no less than 50 ms, and the phase difference corresponding to a 1°C temperature change should be no less than 0.128π. Under these conditions, if the temperature difference between two adjacent temperature measurement phase maps... If the temperature exceeds 10 ºC, the phase difference between corresponding pixels in the phase maps of the two frames will be significant. More than half a phase period; if Above 20ºC, More than one phase period. If the unwinding exceeds half a phase period or more, it will cause phase unwinding failure in the time dimension. That is, it is impossible to correctly unwind the phase map of the next frame using phase continuity, and therefore it is impossible to calculate the temperature distribution using the reference phase, i.e., the reference reference fails. For example, based on the incorrectly unwound phase, an actual temperature difference of 10 ºC between two adjacent frames will be calculated as -5 ºC; an actual temperature difference of 20 ºC will be calculated as 5 ºC.

[0061] Under the above conditions, if TE is shortened to 15ms, even Above 20ºC It will still not exceed half a phase period, so reference failure will not occur, but the temperature measurement accuracy will decrease to 3ºC. Further shortening the TE (transition time) between two adjacent frames... Larger values ​​can be allowed without causing phase winding and reference failure, but temperature measurement accuracy will further decrease. Therefore, the reference failure problem cannot be solved directly by shortening TE.

[0062] See Figure 1 , Figure 2 This invention adds a phase self-calibration echo 502 to the imaging sequence. Phase self-calibration echo time. Phase difference with temperature imaging echo time TE During temperature changes Under the same conditions, The smaller the value, the smaller the phase difference between the corresponding pixels in the self-calibration phase map and the temperature measurement phase map. The smaller the phase difference () Changes in temperature The temperature is directly proportional to the baseline temperature of human tissue, which is typically above 35°C, while the temperature for thermal ablation treatment is below 100°C. Therefore, even with a long pause in the temperature measurement imaging process, the temperature difference between two adjacent frames will not exceed 65°C. According to the laws of heat conduction, the closer to the baseline temperature, the slower the temperature change; therefore, the temperature difference before and after pausing temperature measurement during treatment is generally between 20°C and 40°C. It is evident that as long as… If the value is reasonable, it can ensure that the maximum temperature change is achieved ( The change between two adjacent frames does not exceed half a phase period. Then, using ( The constraint is that the change in () does not exceed half a period, combined with () The change in ) and The relationship, on Perform unwinding in the time dimension.

[0063] The present invention will now be described in detail with reference to specific embodiments.

[0064] Example 1

[0065] See Figures 1-3 The magnetic resonance thermometry imaging method provided by this invention includes the following specific steps:

[0066] (1) Add phase self-calibration echo to the temperature imaging sequence:

[0067] In the temperature imaging sequence, a phase self-calibration echo 502 is added, which is the same in composition and operation mode as the temperature imaging echo 501; the positive and negative polarities of the converging gradients 402 and 401 corresponding to the two echoes are opposite. The temperature imaging echo time is TE, and the phase self-calibration echo time is TE', and the difference between the two is ; the phase self-calibration echo 502 can be before ( ) or after ( ) the temperature imaging echo 501; The absolute value of the phase change amount max of the phase self-calibration echo 502 is less than an echo time threshold t max and greater than the minimum value allowed by the magnetic resonance imaging instrument , and the echo time threshold t min is:

[0068]

[0069] In the formula, t min is 1ms, which is determined by the manufacturing technology of the magnetic resonance imaging instrument; represents the gyromagnetic ratio of the nucleus, B0 represents the main magnetic field strength, and taking the temperature imaging with a 3T magnetic resonance imaging system as an example, the proton resonance frequency ≈128MHz; represents the maximum value that the temperature change may occur within the interval between two adjacent temperature measurements, and for thermal ablation technology, the value range is 15℃~65℃;

[0070] Preferably, in step (1),

[0071] .

[0072] If the situation is complex in actual application, the temperature change between two adjacent frames is severe, and it is difficult to accurately estimate ΔTmax, then it is set to 65℃, and the absolute value of ΔTE is 5ms or 1ms

[0073] If the main magnetic field strength B0 is larger, such as 5T or 7T, the same corresponding The preferred absolute value is smaller; on the contrary, if the main magnetic field strength B0 is smaller, such as 1T or 1.5T, the same corresponding The preferred absolute value is larger.

[0074] For a magnetic resonance imaging system with a main magnetic field strength B0 of 7T, the proton resonance frequency ≈298MHz; t min is 1ms; when The absolute value of the phase change amount between two adjacent frames satisfies that it does not exceed half a phase period.

[0075] (2) execute the temperature imaging sequence to obtain a temperature phase map and a self-calibration phase map:

[0076] Before implementing the thermal ablation technique, execute the temperature imaging sequence to collect a frame of temperature phase map and self-calibration phase map as a reference temperature phase map and a reference self-calibration phase map, which correspond to the basic temperature distribution. During the thermal ablation treatment, each time the temperature imaging sequence is executed, a frame of temperature phase map and self-calibration phase map are obtained, which correspond to the temperature distribution at that time.

[0077] (3) using the prior art, respectively, the reference temperature phase map, the reference self-calibration phase map, the temperature phase map and the self-calibration phase map are phase unwrapped in the spatial dimension to eliminate the phase jump between adjacent pixels in the image.

[0078] (4) using the self-calibration phase map to perform phase unwrapping in the time dimension for the temperature phase map:

[0079] If the phase value of a certain pixel in the reference temperature phase map is , the phase value in the reference self-calibration phase map is , the phase value in the temperature phase map is , and the phase value in the self-calibration phase map is , then the number of phase periods n that the pixel spans between the temperature phase map and the reference temperature phase map is calculated using the following formula, and then n is used to calculate the phase value of the pixel in the temperature phase map after unwrapping .

[0080]

[0081] fix( ) represents rounding the value inside the parentheses to zero.

[0082] The above operation is performed on each pixel in the above phase map.

[0083] (5) using the temperature phase map after time dimension and spatial dimension unwrapping to calculate the temperature distribution:

[0084] After time dimension and spatial dimension unwrapping, if the phase value of a certain pixel in the temperature phase map is , the phase value in the reference temperature phase map is , and the basic temperature is , then the temperature corresponding to the pixel is calculated using the following formula is

[0085]

[0086] The above operation is performed on each pixel in the above phase map to obtain the temperature distribution.

[0087] The order of the above step (3) and step (4) can be exchanged.

[0088] Embodiment 2

[0089] The method provided by the application can also combine to prolong the echo time TE to improve the temperature measurement accuracy.

[0090] Taking temperature measurement imaging by using a 1.5T magnetic resonance imaging system as an example, the proton resonance frequency (f0) is about 64MHz. If the temperature measurement accuracy is required to be 1ºC, considering the influence of image noise, the TE needs to be set to be not less than 100ms, and the phase difference corresponding to the temperature change of 1ºC is about not less than 0.128π. If the prior art is used, when the temperature difference corresponding to adjacent two frames of temperature measurement images exceeds 10ºC, the phase difference corresponding to the temperature change is about 1.28π (exceeding half a phase period π). However, the temperature difference of 10ºC between adjacent two frames is common in thermal ablation treatment. At this time, the TE needs to be shortened to meet the requirement of the temperature difference between adjacent two frames, but the temperature measurement accuracy is reduced. If the TE is shortened to meet the requirement of the temperature difference between adjacent two frames without losing the temperature measurement accuracy, the phase difference corresponding to the temperature change of 1ºC becomes smaller, which will make the measurement phase or the calculation phase difference more sensitive to noise, that is, the signal-to-noise ratio of the phase image is reduced, and then the temperature measurement accuracy is affected. Therefore, by using the prior art, a trade-off needs to be made between meeting the requirement of the temperature difference between adjacent two frames and maintaining the temperature measurement accuracy.

[0091] By using the method provided by the application, the TE can be prolonged to meet the requirement of the temperature measurement accuracy, and the temperature measurement imaging echo and the phase self-calibration echo are used. If the echo time difference between the two is small enough, the difference between adjacent two frames of temperature measurement phase images can be avoided to exceed half a phase period, so that the requirement of the temperature difference between adjacent two frames can be met in a larger range. That is, by using the method provided by the application, the TE and the echo time difference between the temperature measurement imaging echo and the phase self-calibration echo are set respectively, so that the requirements of the temperature difference between adjacent two frames, the temperature measurement accuracy and the temperature measurement accuracy can be met at the same time.

[0092]

[0093] The specific steps are referred to Embodiment 1.​​​

Claims

1. A method of magnetic resonance thermometry imaging, characterized by, Based on the proton resonance frequency temperature dependence, a phase self-calibration echo is added in the temperature imaging sequence; the phase images are phase unwrapped in spatial dimension, the self-calibration phase images are used to phase unwrap the temperature phase images in time dimension; the temperature distribution is calculated using the temperature phase images unwrapped in time and spatial dimensions; comprising the following steps: (1) adding a phase self-calibration echo in the temperature imaging sequence: In a temperature imaging sequence, a phase self-calibration echo is added, which has the same composition and operating mode as the temperature imaging echo, and the positive and negative polarities of the converging gradients corresponding to the two echoes are opposite; the temperature imaging echo time is TE, and the phase self-calibration echo time is TE', and the difference between the two is ; the phase self-calibration echo is before the temperature imaging echo or after the temperature imaging echo ; The absolute value of the difference is less than a time threshold value t max and greater than a minimum value t min allowed by the magnetic resonance imaging instrument. ， where γ represents the gyromagnetic ratio of the nucleus, B0represents the main magnetic field strength, represents the maximum value of the temperature change occurring within the interval between two successive temperature measurements; (2) executing the temperature imaging sequence to obtain temperature phase images and self-calibration phase images: A frame of temperature phase images and self-calibration phase images are collected as reference temperature phase images and reference self-calibration phase images, corresponding to the basic temperature distribution; each time the temperature imaging sequence is executed, a frame of temperature phase images and self-calibration phase images are obtained, corresponding to the temperature distribution at that time; (3) phase unwrapping the reference temperature phase images, reference self-calibration phase images, temperature phase images and self-calibration phase images in spatial dimension respectively to eliminate the phase jumps between adjacent pixels in the images; (4) phase unwrapping the temperature phase images in time dimension using the self-calibration phase images: If the phase value of a certain pixel in the reference thermometry phase map is , the phase value of the pixel in the reference self-calibration phase map is , then the phase difference between the reference self-calibration phase map and the reference thermometry phase map is ; if the phase value of the certain pixel in the thermometry phase map is , the phase value of the pixel in the self-calibration phase map is , then the phase difference between the self-calibration phase map and the thermometry phase map is ; the number of phase periods n that the pixel spans between the thermometry phase map and the reference thermometry phase map is calculated by the following formula, is the cumulative phase change, according to the relationship between phase, frequency and time, , the ratio of to TE is equal to , the ratio of to ΔTE; that is, the frequency calculated by and TE is equal to the frequency calculated by and ΔTE; then the phase value of the pixel in the thermometry phase map after unwrapping is calculated by n ; ， Wherein, fix() represents rounding the value in the parentheses to zero; The above operations are performed on each pixel in the phase images; (5) calculating the temperature distribution using the temperature phase images unwrapped in time and spatial dimensions: After unwrapping in the time dimension and the space dimension, if the phase value of a pixel in the temperature measurement phase map is , the phase value in the reference temperature measurement phase map is , and the base temperature is , then the temperature T corresponding to the pixel is calculated by using the following formula m : ， wherein The above operation is performed for each pixel in the above phase map to obtain a temperature distribution.

2. The magnetic resonance thermometry imaging method of claim 1, wherein, The order of steps (3) and (4) is exchanged.

3. The magnetic resonance thermometry imaging method of claim 1, wherein, Based on the proton resonance frequency linearly related to temperature at a rate of -0.01ppm / ℃ in the temperature range of -15℃~100℃, a magnetic resonance phase map is obtained by using a gradient echo imaging sequence, then a phase difference between magnetic resonance phase maps at different times is calculated, and then a temperature distribution change is calculated according to an echo time TE of the imaging sequence and a main magnetic field strength B0; that is, in the temperature imaging process, a phase is first obtained, then a current temperature Tm is calculated according to a basic temperature Tb and a proton resonance change amount Δf and a temperature change amount ΔT, wherein, the basic temperature Tb is obtained by using a temperature calibration sequence, and the temperature calibration sequence is a sequence for calibrating a temperature change of the magnetic resonance phase map. , the proton resonance change amount Δf and the temperature change amount ΔT, and then the current temperature Tm is calculated according to the basic temperature Tb. ， The temperature measurement accuracy is proportional to the temperature imaging echo time TE. If TE is long, a slight change in temperature can cause a significant change in the actual phase measurement value, i.e. a phase difference increased, ≤ half a phase period.

4. The magnetic resonance thermometry imaging method of claim 1, wherein, The phase self-calibration echo time TE' and the temperature imaging echo time TE differ by a certain factor. ΔTE Phase difference Changes and temperature variations ΔT Proportional, when ΔT≤ 65°C, phase difference between two adjacent frames The change between them does not exceed half a phase period, combined with ( The change in ) and The relationship, on Perform unwinding in the time dimension.

5. The magnetic resonance thermometry imaging method of any one of claims 1 to 4, characterized in that, A 3T magnetic resonance imaging system is used for temperature imaging, according to the following steps: (1) adding a phase self-calibration echo in the temperature imaging sequence: In the temperature imaging sequence, a phase self-calibration echo (502) is added, which is the same in composition and operation mode as the temperature imaging echo (501); the positive and negative polarities of the converging gradients (402 and 401) corresponding to the two echoes are opposite; the temperature imaging echo time is TE, and the phase self-calibration echo time is , ; wherein the phase self-calibration echo (502) is before (or after ) the temperature imaging echo (501) ; The absolute value of the echo time threshold t max is less than the minimum value t min allowed by the magnetic resonance imaging instrument, and greater than the minimum value t max allowed by the magnetic resonance imaging instrument, and the echo time threshold t max is: , where t min is 1 ms, determined by the magnetic resonance imaging instrument manufacturing technology; represents the atomic nucleus gyromagnetic ratio, B0 represents the main magnetic field strength, the proton resonance frequency ≈ 128 MHz; represents the maximum value that the temperature change can appear in the interval between two adjacent temperature measurements, the value range is 15℃~65℃; (2) executing the temperature imaging sequence to obtain temperature phase images and self-calibration phase images: A frame of temperature phase images and self-calibration phase images are collected as reference temperature phase images and reference self-calibration phase images, corresponding to the basic temperature distribution; each time the temperature imaging sequence is executed, a frame of temperature phase images and self-calibration phase images are obtained, corresponding to the temperature distribution at that time; (3) using the prior art, phase unwrapping the reference temperature phase images, reference self-calibration phase images, temperature phase images and self-calibration phase images in spatial dimension respectively to eliminate the phase jumps between adjacent pixels in the images; (4) phase unwrapping the temperature phase images in time dimension using the self-calibration phase images: If the phase value of a pixel in the reference thermometry phase map is , and the phase value in the self-calibration phase map is , and the phase value in the thermometry phase map is , and the phase value in the self-calibration phase map is , then the number of phase periods n that the pixel spans between the thermometry phase map and the reference thermometry phase map is calculated using Equations 2-6, and then the phase value of the pixel in the thermometry phase map after unwrapping is calculated using n . The above operations are performed on each pixel in the phase images; (5) calculating the temperature distribution using the temperature phase images unwrapped in time and spatial dimensions: After the unwrapping in the time dimension and the space dimension, if the phase value of a pixel in the temperature measurement phase map is , the phase value in the reference temperature measurement phase map is , and the base temperature is , then the temperature corresponding to the pixel is calculated by using formula 7 ; The above operations are performed on each pixel in the phase images to obtain the temperature distribution.

6. The magnetic resonance thermometry imaging method of claim 5, wherein, In step (1), ， If the situation is complex in actual application, the temperature changes sharply between adjacent frames, and it is difficult to accurately estimate ΔTmax, it is set to 65ºC, and the absolute value of ΔTE is 5ms or 1ms, which is related to the main magnetic field strength B0.

7. The magnetic resonance thermometry imaging method of claim 5, wherein, If the main magnetic field strength B0 is 5T or 7T, the same The preferred absolute value of the corresponding is smaller; conversely, if the main magnetic field strength B0 is 1T or 1.5T, the same The preferred absolute value of the corresponding is greater.

8. The magnetic resonance thermometry imaging method of claim 6, wherein, For a magnetic resonance imaging system with a main magnetic field strength B0 of 7T, the proton resonance frequency ; t min is 1ms; when , the absolute value of the phase change between two adjacent frames satisfies no more than half a phase period.

9. The magnetic resonance thermometry imaging method of any one of claims 1 to 4, wherein, A 1.5T magnetic resonance imaging system is used for temperature imaging, according to the following steps: Proton resonance frequency If the temperature measurement accuracy is required to be 1°C, considering the influence of image noise, the TE is set to be not less than 100 ms, and the phase difference corresponding to a temperature change of 1°C is not less than 0.128π. TE and to meet the requirements of temperature difference between two adjacent frames, temperature measurement accuracy and temperature measurement accuracy at the same time.

10. The use of the magnetic resonance temperature imaging method according to any one of claims 1 to 9 in a magnetic resonance guided thermal ablation device.

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