CN114814689B - Magnetic resonance temperature imaging method - Google Patents
Magnetic resonance temperature imaging method Download PDFInfo
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Abstract
The application discloses a magnetic resonance temperature imaging method and a related system, wherein, the method uses a gradient echo sequence containing i different echo times to acquire magnetic resonance image data (i is a positive integer greater than or equal to 2) of a target part, and selects at least two groups of phase diagrams corresponding to the different echo times to acquire a plurality of corresponding temperature difference diagrams; obtaining a temperature map according to the temperature difference map; the magnetic resonance temperature imaging method reduces errors of phase unwrapping, can reduce susceptibility errors and motion interference errors, improves robustness and accuracy, has small operand, and can provide a nearly real-time temperature map.
Description
Technical Field
The present application relates to the field of image processing technology, and more particularly, to a magnetic resonance temperature imaging method.
Background
The magnetic resonance temperature imaging (Magnetic Resonance Temperature Imaging, MRTI) can realize noninvasive, real-time and in-vivo monitoring of temperature distribution and change in the tested object, and has important application in the monitoring process of minimally invasive and noninvasive hyperthermia, such as magnetic resonance interstitial hyperthermia, focused ultrasound treatment and the like.
One of the current magnetic resonance temperature imaging methods is a temperature measurement method based on proton resonance frequency (Proton Resonance Frequency, PRF) displacement, and in the practical process, the temperature measurement method based on proton resonance frequency displacement is found to be greatly influenced by objective environmental factors such as magnetic resonance coil magnetic field uniformity, tissue magnetic susceptibility distribution non-uniformity, tissue motion caused by respiration/blood flow pulsation and the like, common errors caused by the induction include phase unwrapping errors, errors caused by magnetic susceptibility rapid change, errors caused by motion and the like, and the problem of great difference between a finally acquired temperature map and actual temperature is easily caused, so that the temperature map loses reference significance.
Disclosure of Invention
In order to solve or alleviate one or more of the above technical problems, the present application provides a magnetic resonance temperature imaging method and a related device, so as to achieve the purposes of reducing errors of a finally obtained temperature map and improving accuracy of the temperature map.
In a first aspect, an embodiment of the present application provides a magnetic resonance temperature imaging method, including:
acquiring magnetic resonance image data of a target part by using a gradient echo sequence containing i different echo times, wherein i is a positive integer greater than or equal to 2, and the magnetic resonance image data comprises a phase map corresponding to the echo times;
selecting at least two groups of phase diagrams corresponding to different echo times to obtain a plurality of corresponding temperature difference diagrams;
and obtaining a temperature map according to the temperature difference map.
Further, the step of obtaining the temperature difference map in the method of the present invention includes:
subtracting the phase diagram of the reference time from the phase diagram of any time to obtain a phase diagram of the time, selecting the phase diagram corresponding to the minimum echo time as a reference phase diagram, and calibrating the phase diagrams to be calibrated corresponding to other echo times based on the reference phase diagram to obtain calibrated phase diagrams;
a temperature difference map is calculated using the reference phase difference map and the calibrated phase difference map.
Further, obtaining a calibrated phase difference map includes the steps of:
according to the proportional relation between the phase difference diagram and the echo time, calculating to obtain an estimated value of the phase difference diagram to be calibrated based on the echo time and the reference phase difference diagram;
and unwrapping the phase difference diagram to be calibrated according to the phase periodicity by using the estimated value to obtain a calibrated phase difference diagram.
Optionally, in the method of the present invention, the echo time corresponding to the reference phase difference map does not exceed 18ms,17ms,16ms,15ms,14ms,13ms,12ms,11ms,10ms,9ms,8ms,7ms,6ms,5ms or 4ms.
Optionally, the method of the present invention further comprises the step of eliminating phase drift caused by the magnetic resonance system, which is performed on the phase difference map or the temperature difference map.
Optionally, the method of the present invention further comprises a step of susceptibility correction, the step of susceptibility correction being performed on a phase difference map or a temperature difference map;
the step of correcting magnetic susceptibility on the temperature difference map includes:
obtaining a first temperature map according to the reference phase difference map, and obtaining a corresponding second temperature map according to the calibrated phase difference map;
judging whether the absolute value of the difference value between the temperature value corresponding to each pixel in the second temperature map and the temperature value corresponding to the corresponding pixel in the first temperature map exceeds a preset temperature threshold value, and if so, correcting the temperature value corresponding to the corresponding pixel in the second temperature map;
the step of performing susceptibility correction on the phase difference map includes:
and judging whether the absolute value of the difference value of the phase difference value corresponding to each pixel in the calibrated phase difference graph and the phase difference value corresponding to the corresponding pixel in the reference phase graph exceeds a preset phase difference threshold value, and if so, correcting the phase difference corresponding to the corresponding pixel in the calibrated phase difference graph.
Optionally, the method of the present invention further comprises the step of correcting motion-induced phase errors performed on the phase difference map or the temperature map;
the step of correcting the motion-induced phase error performed on the phase difference map includes:
a linear least squares fit at each pixel of the phase difference plot removes motion-induced phase errors;
the step of correcting the motion-induced phase error performed on the temperature difference map includes:
the motion-induced phase error is removed at each pixel of the temperature difference map by a linear least squares fit.
Optionally, the method of the present invention further comprises the step of obtaining a weighted temperature map, which calculates a temperature difference map for that moment using the reference phase difference map and the calibrated phase difference map:
and calculating a temperature difference map by using the reference phase difference map and the calibrated phase difference map, and weighting to obtain a weighted temperature map of the target to be measured.
In another aspect, the present invention also provides a magnetic resonance temperature imaging system, comprising: a memory and a processor;
the memory is used for storing program code, and the processor is used for calling the program code, and the program code is used for executing the magnetic resonance temperature imaging method.
Furthermore, the magnetic resonance temperature imaging method is not a traceability algorithm or an iterative algorithm, has small operand, can provide a nearly real-time temperature map, and has higher reference significance.
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In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the drawings that are required to be used in the embodiments or the description of the prior art will be briefly described below, and it is obvious that the drawings in the following description are only embodiments of the present application, and that other drawings may be obtained according to the provided drawings without inventive effort to a person skilled in the art.
Figure 1 is a diagram of amplitude, phase and temperature of magnetic resonance data acquired in an ex vivo and in vivo environment of the prior art;
fig. 2 is a schematic flow chart of a magnetic resonance temperature imaging method according to an embodiment of the present application;
FIG. 3 is a phase diagram, a phase difference diagram, and a temperature diagram obtained as provided by one embodiment of the present application;
fig. 4 is a flow chart of a magnetic resonance temperature imaging method according to another embodiment of the present application;
FIG. 5 is a schematic diagram of an experimental apparatus provided in one embodiment of the present application;
FIG. 6 is a partial enlargement of a laser interstitial thermotherapy temperature map of an in vitro pork experiment according to one embodiment of the present application;
FIG. 7 is a graph showing temperature over time during an experiment of a tissue mimic (FIG. 7 (a)) or an ex vivo pork (FIG. 7 (b)) provided in one embodiment of the present application;
FIG. 8 is a representative temperature plot of dog 01 in an in vivo experiment provided in one embodiment of the present application;
Detailed Description
The magnetic resonance temperature imaging can guide various energy delivery type treatment means, such as laser interstitial thermotherapy, focused ultrasound therapy, radio frequency ablation and the like, to monitor the temperature of target tissues and the treatment effect. The inventor finds that the main sources of errors in the acquired temperature map are phase errors caused by phase unwrapping dislocation, magnetic susceptibility errors and phase errors caused by motion through research. As the input energy dose changes, the susceptibility distortion can lead to a decrease in image amplitude and corresponding errors in image phase, thereby destroying the heating center and its surrounding reconstructed temperature map. Errors in reconstructing the temperature map may lead to erroneous estimates of the ablation region, which may lead to variations in the therapeutic effect and thermal damage to critical tissue. Thus, accurate temperature imaging is critical to the effectiveness and safety of the treatment, especially when applied to the ablated region of brain tissue.
The temperature measurement method based on proton resonance frequency shift is based on the following principle: the resonant frequency of hydrogen protons varies with the temperature in water molecules. For aqueous tissue, the local magnetic field variation with temperature can be described as:
wherein, alpha is the proton resonance frequency coefficient changing with temperature, and the temperature is 0.008-0.015 ppm/DEG C. The corresponding resonance frequency change of the temperature-affected water protons can be expressed as:
Δf＝αγB 0 ·ΔT； (2)
wherein DeltaT represents temperature change, deltaf represents resonance frequency change, gamma represents gyromagnetic ratio, and B 0 Representing the static magnetic field strength.
Changes in resonance frequency due to temperature changes can be observed in the phases of complex magnetic resonance imaging. For a given interval TE of the gradient echo sequence, the relative temperature change Δt can be calculated from the phase difference ΔΦ, which equation can be expressed as:
the gradient recall echo (Gradient recalled echo, GRE) pulse sequence, abbreviated as gradient echo sequence, is a sequence used in a temperature measurement method based on proton resonance frequency displacement, and includes sequences corresponding to different echo times, for example, may include echo sequences corresponding to i echo times, where i is a positive integer not less than 2. As can be seen from the formula (3), the longer the echo time in the gradient echo sequence, the greater the phase difference may be caused by the same temperature change, and the higher the temperature sensitivity may be obtained.
Referring to fig. 1, there are several common problems in the prior art with temperature maps obtained using echo sequences of a single echo time, both phase contrast and phase wrapping increase with increasing echo time of the gradient echo sequence, indicating that with sequences of longer echo times, temperature sensitivity is higher and phase wrapping is more. In fig. 1, the amplitudes (upper row) and phases (second row) of the first to fourth echoes obtained by the gradient echo sequence containing 4 different echo times used in the embodiments of the present application are in (a) (ex vivo, pig brain) and (b) (in vivo). A temperature map (lower row) is calculated from each TE (echo time) setting using a conventional PRF algorithm. More phase wrapping occurs over longer echo times, with a corresponding increase in image contrast. Intense heating can result in signal loss due to changes in susceptibility and can also translate into phase and temperature errors for pixels around the heating center. In vivo experiments, cerebrospinal fluid (Cerebrospinal Fluid, CSF) motion can cause incorrectly high temperatures on magnetic resonance temperature imaging, which is more pronounced in the previous echo, because shorter TEs are less tolerant to similar phase errors introduced.
In particular, inter-scan motion is a problem in thermometry based on proton resonance frequency shift measurement due to the motion of cerebrospinal fluid (CSF) in the brain. The amplitude and phase signals of the cerebrospinal fluid are often changed over the pulse gradient echo sequence by the normal dynamic motion of the cerebrospinal fluid, which may confound the temperature estimation. Cerebrospinal fluid movement may also cause pixel movement in the ventricles and surrounding ventricles, resulting in error in the phase contrast map. As shown in fig. 1b, the in vivo temperature map shows false high temperatures in the third brain chamber due to cerebrospinal fluid movement. The temperature errors are more pronounced on pulse gradient echo sequences of shorter echo times, because they are less tolerant of the phase shift intensities introduced by cerebrospinal fluid flow according to equation (3).
In fact, the local magnetic field of the water protons should also take into account the magnetic susceptibility x 0 Equation (1) becomes:
wherein, the liquid crystal display device comprises a liquid crystal display device,indicating the local magnetic field change caused by susceptibility.
Laser heating can cause significant magnetization artifacts in GRE imaging around the laser tip. Still referring to fig. 1, the heating center (shown by the arrow in fig. 1 (a)) with sharp temperature changes shows severe signal loss over a longer echo time amplitude. Intra-voxel spin phase shift (intra-voxel spin dephasing) is caused by local magnetic field inhomogeneities caused by temperature and susceptibility changes.
Magnetization artifacts caused by heating, in particular in images corresponding to gradient echo sequences of longer echo times, are an important cause of errors. Still referring to fig. 1, in an ex vivo or in vivo experiment, the phase error around the heating center translates into artificial low temperature on magnetic resonance thermal imaging. In general, in gradient echo magnetic resonance imaging, it is recommended to use a magnetic resonance system with as short an echo time delay as possible to minimize susceptibility artifacts. However, a longer echo time gradient pulse sequence may provide better temperature sensitivity and signal-to-noise ratio, which is currently a dilemma of choice.
In order to achieve the aim of achieving temperature sensitivity, signal-to-noise ratio and low error, the embodiment of the application provides a magnetic resonance temperature imaging method, which comprises the following steps:
acquiring magnetic resonance image data of a target part by using a gradient echo sequence containing i different echo times, wherein i is a positive integer greater than or equal to 2, and the magnetic resonance image data comprises a phase map corresponding to the echo times;
selecting at least two groups of phase diagrams corresponding to different echo times to obtain corresponding temperature difference diagrams;
and obtaining a temperature map according to the temperature difference map.
Wherein the step of obtaining the temperature difference map includes:
subtracting the phase diagram of the reference time from the phase diagram of any time to obtain a phase diagram of the time, selecting the phase diagram corresponding to the minimum echo time as a reference phase diagram, and calibrating the phase diagrams to be calibrated corresponding to other echo times based on the reference phase diagram to obtain calibrated phase diagrams;
a temperature difference map is calculated using the reference phase difference map and the calibrated phase difference map.
The echo time of the gradient echo sequence is in direct proportion to the size of the magnetic susceptibility artifact, so that the phase diagram obtained by the gradient echo sequence corresponding to the minimum echo time is least influenced by the magnetic susceptibility change caused by heating, the image data of the gradient echo sequence still keeps the correct phase, and therefore the phase diagram corresponding to the minimum echo time can be selected as a reference phase diagram to calibrate other phase diagrams, so that the error of the finally obtained temperature diagram is reduced, and the accuracy of the obtained temperature diagram is improved.
Furthermore, the magnetic resonance temperature imaging method is not a traceability algorithm or an iterative algorithm, has small operand, can provide a nearly real-time temperature map, and has higher reference significance.
One specific example of the magnetic resonance temperature imaging method of the present invention, as shown in fig. 2, includes:
s101: scanning a target to be detected by using a gradient echo sequence containing i different echo times to obtain i groups of phase diagrams corresponding to the echo times at different moments, wherein i is a positive integer greater than or equal to 2;
in step S101, the minimum value and the maximum value of the echo time in the gradient echo sequence may be determined according to actual requirements, and in general, in order to reduce the susceptibility artifact caused by the susceptibility change as much as possible, the minimum value of the echo time in the gradient echo sequence may be the minimum value that can be obtained by the magnetic resonance temperature imaging device, and the maximum value of the echo time in the gradient echo sequence generally does not exceed the upper limit of the range of the echo time for imaging the target to be detected. For example, for head imaging, the echo time of the optional gradient time sequence is in the range of 3-30 ms, and the specific echo time included in the gradient echo sequence is in the range of the echo time.
The gradient echo sequence information containing i different echo times can be read or received from a server or other storage devices, or can be obtained in real time according to the setting of a staff.
Referring to fig. 3, and specifically to fig. 3 (a), the acquired phase diagram is shown in fig. 3 (a).
S102: selecting at least two groups of phase diagrams corresponding to different echo times at any moment to obtain a phase difference diagram corresponding to the moment, see fig. 3 (b), wherein phase wrapping is shown; then, selecting a phase difference diagram corresponding to the minimum echo time as a reference phase difference diagram, and calibrating the phase difference diagrams to be calibrated corresponding to other echo times based on the reference phase difference diagram to obtain a calibrated phase difference diagram, wherein the result is shown in fig. 3 (c), and the phase wrapping is eliminated;
s103, a temperature map is then obtained from the phase difference map, see fig. 3 (d), in which errors due to magnetic susceptibility are always shown.
Possible implementations of the steps of the magnetic resonance temperature imaging method provided in the embodiments of the present application are described below.
Based on the above embodiments, in one embodiment of the present application, the specific steps for obtaining the temperature difference map include:
obtaining a phase difference map of any time by subtracting a phase map of a reference time from the phase map of any time, wherein the reference time is any time before energy (such as heat energy, light energy, radio frequency ablation and cryoablation) is transmitted to the target tissue, and preferably is a time immediately before energy transmission, such as a time about to transmit energy;
optionally, the echo time of the phase unwrapping with reference to the phase difference map has a value less than or equal to 12ms, preferably not more than 10ms, more preferably not more than 6ms.
The step of selecting the phase difference diagram corresponding to the minimum echo time at the moment as the reference phase difference diagram and calibrating the phase difference diagram to be calibrated corresponding to other echo time comprises the following steps:
using the reference phase difference diagram and the phase difference diagram corresponding to the echo time to be calibrated, and calculating to obtain an estimated value of the phase difference diagram corresponding to the echo time to be calibrated based on the phase difference of the echo time and the reference phase difference diagram according to the proportional relation between the phase difference and the echo time; and then unwrapping the phase difference to be calibrated according to the phase periodicity by using the estimated value to obtain the calibrated phase difference.
On the basis of the above embodiment, in another embodiment of the present application, the magnetic resonance temperature imaging method further includes:
s104, eliminating phase drift caused by the magnetic resonance system, wherein the phase drift is performed on a phase difference diagram or a temperature diagram.
The step of eliminating the phase shift caused by the magnetic resonance system on the phase difference graph comprises the following steps:
selecting a plurality of thermal reference points (Region of Interest, ROI), wherein by subtracting the average phase difference of the thermal reference points from each phase difference map, the regions with stable and unchanged physical temperature and uniform tissues can be used as the thermal reference points;
the step of eliminating phase drift caused by the magnetic resonance system on the temperature difference map comprises the steps of:
the average temperature difference of any of the thermal reference points is subtracted from the temperature difference map for correction.
On the basis of the above embodiment, in another embodiment of the present application, the magnetic resonance temperature imaging method further includes:
s105: a step of correcting magnetic susceptibility, which is performed on a phase difference map or a temperature map;
the step of performing susceptibility correction on the temperature map includes:
obtaining a first temperature map according to the reference phase difference map, and obtaining a corresponding second temperature map according to the calibrated phase difference map;
optionally, determining a preset area in the first temperature map and each of the second temperature maps;
judging whether the absolute value of the difference value between the temperature value corresponding to each pixel in the second temperature map and the temperature value corresponding to the corresponding pixel in the preset area in the first temperature map exceeds a preset temperature threshold value, and if so, correcting the temperature value corresponding to the corresponding pixel in the second temperature map; there are various ways of correcting, for example, the temperature value of the first temperature map may be used to replace the temperature value of the second temperature map, or the temperature value of an adjacent pixel in the second temperature map may be used to replace the temperature value of the pixel in the second temperature map, or an approximate temperature may be fitted to replace the temperature value of the second temperature map based on the temperature value of the adjacent pixel and the temperature value of the first temperature map;
the step of performing susceptibility correction on the phase difference map includes:
determining a preset area in the reference phase difference diagram and the calibrated phase difference diagram;
and judging whether the absolute value of the difference value of the phase difference value corresponding to each pixel in the calibrated phase difference graph and the phase difference value corresponding to the corresponding pixel in the preset area in the reference phase graph exceeds a preset phase difference threshold value, and if so, correcting the phase difference in the calibrated phase difference graph.
On the basis of the above embodiment, in yet another embodiment of the present application, the magnetic resonance temperature imaging method further includes:
s106: a step of correcting a phase error caused by the motion performed on the phase difference map or the temperature map;
the step of correcting the motion-induced phase error performed on the phase difference map includes:
removing motion-induced phase errors by using a linear least squares fit of the reference phase difference map and the calibrated phase difference map at each pixel;
the step of correcting the motion induced phase error performed on the temperature map comprises:
obtaining a first temperature map according to the reference phase difference map, and obtaining a corresponding second temperature map according to the phase difference map;
the motion-induced phase error is removed by a linear least squares fit at each pixel using the first temperature map and the second temperature map.
Still referring to fig. 4 (c), fig. 4 (c) shows the phase difference (first row) and the relative temperature change (second row) as a function of time without (left) and with (right) Motion Error correction. For shorter echo times, the phase error Δφ (x, y) bias Larger temperature deviations may be introduced but may be properly eliminated after the linear least squares fit.
As described above, steps S104, S105 and S106 may be performed on the phase difference map level or on the temperature map. I.e. the step of eliminating the phase shift caused by the magnetic resonance system is performed on a phase difference map and/or a temperature map, and the step of susceptibility correction is performed on a phase difference map and/or a temperature map. The step of correcting the motion induced phase error is performed on a phase difference map and/or a temperature map.
Still referring to fig. 3, fig. 3 (c) shows a corrected phase difference map, fig. 3 (d) shows a temperature map, fig. 3 (e) shows a susceptibility corrected image, and fig. 3 (f) shows a motion error corrected image.
On the basis of the above embodiments, in still another embodiment of the present application, the calculating the temperature map at the time using the reference phase difference map and the calibrated phase difference map includes:
s1031: the temperature is calculated using the reference phase difference map and the calibrated phase difference map, and the calculated temperature is weighted to obtain a temperature map of the object to be measured, and the weighting may be performed on both the temperature difference map and the temperature map, and the weighting may be various weighting methods, such as average weighting, or may be a temperature map corresponding to a single echo time, that is, the weighting coefficient of the temperature map is 1, and the weighting coefficients of other phase temperature maps are 0.
After step S1031, the method may further include:
s107: and carrying out multiple interpolation processing on the temperature map of the target to be detected, and calculating an ablation area boundary by using the temperature map of the target to be detected after the interpolation processing.
The purpose of performing multiple interpolation processing on the temperature map of the target to be measured is to obtain a smoother ablation region boundary, and the specific number of times of difference processing can be 2 or 3 times.
In calculating the boundary of the ablation region, the following formula is specifically used:
wherein E is a The activation energy is represented by A being a frequency factor, R being a universal gas constant, T (τ) being a function of temperature (. Degree. C.) and time τ, T being the current time. Pixels with omega values exceeding a set threshold (e.g. 1) are considered ablated.
The magnetic resonance temperature imaging method provided in the embodiment of the present application is verified below in conjunction with specific examples.
A laser ablation system comprising a 10w, 480 nm diode laser and a cooled laser applicator system was used to heat the tissue simulant (gel mold). The phase image is atA 3T MR scanner (Ingenia, philips Healthcare, best, netherlands) uses 16 receive coils acquired using a multi-echo time gradient echo sequence: flip angle = 30 °, TE = 6/12/18/24ms, tr = 22ms, matrix = 176 x 176, fov = 200x200mm 2 Slice thickness = 5mm,3 s/image.
As shown in fig. 5, two MR compatible fiber optic temperature probes were also inserted into the tissue simulant with the probe tip positioned near the ablating fiber to obtain the gel temperature at each point. Since the fiber optic probe is affected by the ablating fiber during heating, the thermometer monitors only the cooling phase. In fig. 5. Specifically, fig. 5 shows the insertion of an ablation fiber and two fiber optic temperature probes in a tissue simulator, with a gel filled reference tube fixed around for an insulated reference.
In vitro experiments of pork and pig brain were performed using the same scan parameters as the tissue mimic experiments. Two experiments were performed on each type of tissue (gel, pork, pig brain), one of which was heated by several laser cycles and the other was continuously heated and cooled. The root mean square error between the MR measured temperature and the fiber measured temperature is calculated as a measure of temperature accuracy.
Du Bingou in vivo experiments have been approved by the ethical review Committee of the university of Qinghai. Nine adults Du Bingou received the hyperthermia of laser interstitial hyperthermia. The heating process was monitored on a 3T MR scanner (Ingenia, philips Healthcare, best, netherlands) with 32 receiver head coils using a multi-echo time gradient echo sequence.
Still referring to fig. 3 and 4, fig. 3 and 4 illustrate one example of a magnetic resonance temperature imaging method provided by embodiments of the present application. Figure 3 shows a specific example step of magnetic resonance temperature imaging. FIG. 3 (a), at one time obtained during laser hyperthermia, first a coil combination phase image is obtained by a multiple TE echo sequence; fig. 3 (b), then a phase difference map is obtained, and white arrows indicate phase wrapping occurring on the phase map around the heating center; fig. 3 (c) shows a phase difference diagram after phase unwrapping and B0 drift correction. Fig. 3 (d) is a temperature map calculated from fig. 3 (c) according to the PRF offset method. White arrows highlight susceptibility induced errors. Fig. 3 (e) temperature diagram after susceptibility correction. The white arrows show the errors caused by the residual cerebrospinal fluid movements (motion). Fig. 3 (f) temperature diagram of motion correction.
In fig. 4, one exemplary method flow for a representative pixel includes: step 1, a phase difference diagram (upper left) and a phase diagram (upper right) obtained by unwrapping estimated by referring to the phase difference diagram are obtained, and as shown by black arrows, the phase difference diagrams corresponding to 18ms and 24ms echo time are wrapped under the condition of rapid temperature change. Step 2, a phase unwrapping map is acquired, step 3, static magnetic field strength (B 0 ) Drift correction, B0 drift correction is to reduce system fluctuations, step 4, phase error correction due to susceptibility, and temperature error due to susceptibility changes (black arrows) over longer echo Times (TE) is corrected using the shortest echo Time (TE). Step 5, phase error correction caused by motion. The first and second lines are time-varying phase differences and corresponding temperature variations over time. For multiple echo times, the motion-induced phase error (black arrow) is nearly the same, thus resulting in more pronounced temperature errors at shorter TEs. The result of correcting the motion error shows a smoother phase and temperature profile.
Die body and ex vivo experimental results:
fig. 6 shows a representative temperature profile of an ex vivo pork experiment during laser interstitial thermotherapy. Six representative images (# 50 represents the 50 th frame, #146 represents the 146 th frame), and so on) are selected from 300 frames (3 s/frame) acquired during the thermal cycle. The first row and the second row are respectively using a conventional phase unwrapping method and the multi-echo time based phase unwrapping method proposed in the embodiments of the present application. With the prior art phase unwrapping method, the pixels on the temperature map would be severely damaged due to the change in susceptibility caused by the laser heat, and cannot be recovered even without using the laser. The technical principle is as follows: the phase unwrapping method of the prior art is applied to the time dimension for phase jump detection, and if the phase difference map of the current frame is unwrapped by mistake, all the subsequent frames are affected. On the other hand, the magnetic resonance temperature imaging method is performed on the basis of multiple echo dimensions, so that interference from previous frames is avoided. The third row is a single echo time temperature plot with phase unwrapping and susceptibility correction, with the damaged pixels around the heating center having recovered correctly. The last line is the multi-echo time data combination result using the magnetic resonance temperature imaging method provided by the embodiments of the present application. The resulting magnetic resonance thermal imaging is shown to be more uniform in temperature at the hot spot.
Fig. 7 shows the temperature over time during the experiments of the tissue simulant (fig. 7 (a)) or the ex vivo pork (fig. 7 (b)), calculated from two thermometry fiber measurements (red bars) and the method provided in the examples of the present application, respectively (dashed black bars). In the case of multiple heating (fig. 7 (a)) or single heating (fig. 7 (b)), the temperature-time behavior of the Proton Resonance Frequency (PRF) calculation is very matched to the temperature-time measured by the thermometric optical fiber during the cool down phase. Table 2 lists Root Mean Square Error (RMSE) values between MR calculated values and thermometry fiber measurements, which represent the temperature accuracy of the proposed algorithm. Experiment 1 performed several laser cycles of heating, while experiment 2 was a continuous heating and cooling phase. The results show that in most cases the root mean square error of the gel, pork or pig brain tissue is less than 0.5 ℃.
Table 2. The method provided by the examples of the present application compares between the temperature measured by the fiber and the temperature calculated by the MR.
Abbreviations: RMSE, root mean square error; experiment, experiment L (R), left (right) fiber optic temperature probe.
Fig. 8 shows a representative temperature profile of dog 01 in an in vivo experiment. It should be noted that the ablation zone is located near the third ventricle and the lateral ventricle. 100 frames (3 s/frame) of images acquired during laser ablation are selected to be superimposed on post-ablation T2w magnetic resonance thermal imaging. From top to bottom are temperature maps calculated by prior art algorithms from single echo Time (TE) data (te=6 ms and te=24 ms), respectively, and from multiple (joint) TE echo sequences using the proposed algorithm of the present invention. The first line (te=6 ms) shows the third ventricle and artificial high temperatures in the ventricle, indicating that the short TE calculated temperature is severely affected by CSF flow artifacts. CSF-induced third brain indoor artifact (indicated by white arrow) is still present in the second row (te=24 ms), but well suppressed by the proposed multi-TE echo sequence algorithm. The second row shows that a longer TE (te=24 ms) can provide smoother boundaries and better temperature SNR than a shorter TE (te=6 ms), but as described above, pixels around the heating center can be damaged due to changes in susceptibility. On the other hand, we propose a method that integrates the information of multiple echoes, so that the obtained temperature map simultaneously eliminates CSF-induced errors and susceptibility-induced errors, showing a more uniform and symmetrical heating region.
The above experimental results show that the magnetic resonance temperature imaging method provided by the embodiment of the present application can be used to correct errors caused by magnetic susceptibility in the proton resonance frequency temperature map caused by heating the laser itself. We first propose the application of a multi-echo time gradient echo pulse sequence to magnetic resonance thermal imaging by proton resonance frequency shift method. Instead of a single echo sequence, a multi-gradient echo sequence can provide more information without additional scan time and provide a new approach for phase unwrapping and artifact cancellation.
Shorter echo times can tolerate susceptibility artifacts but are sensitive to noise, while longer echo times have better temperature sensitivity and signal-to-noise ratio but are greatly affected by susceptibility artifacts. The magnetic resonance temperature imaging method provided by the invention combines the advantages of different echoes, and obtains better temperature map measurement results. Moreover, the magnetic resonance temperature imaging method can remarkably improve the robustness and the signal to noise ratio of the magnetic resonance thermal imaging, thereby avoiding damage to healthy tissues caused by misestimation of low temperature.
The methods of the present invention also provide excellent cerebrospinal fluid flow motif inhibition and can provide accurate temperature measurements in and around the ventricle. Compensating for errors caused by cerebrospinal fluid movements is clinically important for laser interstitial hyperthermia treatment of periventricular brain lesions. Furthermore, the proposed algorithm is online compatible, does not require iterative calculations, and is therefore well suited for magnetic resonance thermal imaging, since a very near real-time temperature map is required.
The following describes a magnetic resonance temperature imaging system provided in an embodiment of the present application, and the magnetic resonance temperature imaging system described below may be referred to in correspondence with the magnetic resonance temperature imaging method described above.
Accordingly, an embodiment of the present application further provides a magnetic resonance temperature imaging system, which is characterized by including: a memory and a processor;
the memory is used for storing program code, and the processor is used for calling the program code, and the program code is used for executing the magnetic resonance temperature imaging method.
In summary, the embodiments of the present application provide a magnetic resonance temperature imaging method and a related device, where the magnetic resonance temperature imaging method obtains i groups of phase diagrams based on a gradient echo sequence containing i different echo times, selects at least two groups of phase diagrams corresponding to different echo times to obtain a plurality of corresponding phase diagrams, and obtains a temperature diagram according to the temperature difference diagrams. The inventor finds that the echo time of the gradient echo sequence is in a direct proportion relation with the size of the magnetic susceptibility artifact, so that the phase diagram obtained by the gradient echo sequence corresponding to the smaller echo time is least influenced by the magnetic susceptibility change caused by heating, and the image data of the phase diagram still keeps the correct phase, so that the temperature diagram can be obtained based on i groups of phase diagrams and phase difference diagrams obtained by the gradient echo sequence containing i different echo times, thereby reducing the error of the finally obtained temperature diagram and improving the accuracy of the obtained temperature diagram.
Furthermore, the magnetic resonance temperature imaging method is not a traceability algorithm or an iterative algorithm, has small operand, can provide a nearly real-time temperature map, and has higher reference significance.
The following description of the embodiments of the present application will be made clearly and fully with reference to the accompanying drawings, in which it is evident that the embodiments described are only some, but not all, of the embodiments of the present application. All other embodiments, which can be made by one of ordinary skill in the art without undue burden from the present disclosure, are within the scope of the present disclosure.
Features described in the embodiments in this specification may be replaced or combined with each other, and each embodiment is mainly described in the differences from the other embodiments, and identical and similar parts between the embodiments are all enough to be referred to each other.
The previous description of the disclosed embodiments is provided to enable any person skilled in the art to make or use the present application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without departing from the spirit or scope of the application. Thus, the present application is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.
Claims (9)
1. A method of magnetic resonance temperature imaging, comprising:
acquiring magnetic resonance image data of a target part by using a gradient echo sequence containing i different echo times, wherein i is a positive integer greater than or equal to 2, and the magnetic resonance image data comprises a phase map corresponding to the echo times;
selecting phase diagrams of at least two groups of sequences corresponding to different echo times in the gradient echo sequence at different moments to obtain a phase diagram, wherein for the sequence of any echo time, the phase diagram of any moment in the sequence is used for subtracting the phase diagram of the reference moment to obtain the phase diagram of the moment, the phase diagram corresponding to the minimum echo time is selected at any moment as a reference phase diagram, and the phase diagrams corresponding to other echo times at the moment are calibrated based on the reference phase diagram to obtain calibrated phase diagrams; then calculating a temperature difference map using the reference phase difference map and the calibrated phase difference map;
and obtaining a temperature map according to the temperature difference map.
2. The method of claim 1, wherein the obtaining a calibrated phase difference map comprises the steps of:
according to the proportional relation between the phase difference diagram and the echo time, calculating to obtain an estimated value of the phase difference diagram to be calibrated based on the echo time and the reference phase difference diagram;
and unwrapping the phase difference diagram to be calibrated according to the phase periodicity by using the estimated value to obtain a calibrated phase difference diagram.
3. The method of claim 2, wherein the echo time corresponding to the reference phase difference map is no more than 18ms.
4. The method of claim 1, further comprising the step of canceling the phase shift caused by the magnetic resonance system, the step of canceling the phase shift caused by the magnetic resonance system being performed on a phase difference map or a temperature difference map.
5. The method of claim 4, wherein the step of eliminating phase drift caused by the magnetic resonance system on the phase difference map comprises:
selecting a plurality of thermal reference points by subtracting an average phase difference of the thermal reference points from each phase difference map;
the step of eliminating phase drift caused by the magnetic resonance system on the temperature difference map comprises the steps of:
the average temperature difference of the thermal reference points is subtracted from the temperature difference map for correction.
6. The method according to any one of claims 1 to 5, further comprising a step of susceptibility correction, the step of susceptibility correction being performed on a phase difference map or a temperature difference map;
the step of correcting magnetic susceptibility on the temperature difference map includes:
obtaining a first temperature map according to the reference phase difference map, and obtaining a corresponding second temperature map according to the calibrated phase difference map;
judging whether the absolute value of the difference value between the temperature value corresponding to each pixel in the second temperature map and the temperature value corresponding to the corresponding pixel in the first temperature map exceeds a preset temperature threshold value, and if so, correcting the temperature value corresponding to the corresponding pixel in the second temperature map;
the step of performing susceptibility correction on the phase difference map includes:
and judging whether the absolute value of the difference value of the phase difference value corresponding to each pixel in the calibrated phase difference graph and the phase difference value corresponding to the corresponding pixel in the reference phase graph exceeds a preset phase difference threshold value, and if so, correcting the phase difference corresponding to the corresponding pixel in the calibrated phase difference graph.
7. The method according to any one of claims 1 to 6, further comprising the step of correcting a motion-induced phase error performed on the phase difference map or the temperature map;
the step of correcting the motion-induced phase error performed on the phase difference map includes:
a linear least squares fit at each pixel of the phase difference plot removes motion-induced phase errors;
the step of correcting the motion-induced phase error performed on the temperature difference map includes:
the motion-induced phase error is removed at each pixel of the temperature difference map by a linear least squares fit.
8. The method according to any one of claims 1 to 7, wherein the calculating a temperature difference map at that time using the reference phase difference map and the calibrated phase difference map comprises:
and calculating a temperature difference map by using the reference phase difference map and the calibrated phase difference map, and weighting to obtain a temperature map of the target to be measured.
9. A magnetic resonance temperature imaging system, comprising: a memory and a processor;
the memory is for storing program code, the processor is for invoking the program code, the program code is for performing the magnetic resonance temperature imaging method of any one of claims 1 to 8.
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