CN114820838B - Magnetic resonance temperature imaging method for correcting susceptibility errors - Google Patents

Abstract

The application discloses a magnetic resonance temperature imaging method for correcting susceptibility errors, which uses a gradient echo sequence containing i different echo times to acquire magnetic resonance image data of a target part, selects at least one phase diagram corresponding to the echo time as a first group of phase diagrams, and selects other phase diagrams corresponding to the echo time as a second group of phase diagrams; calculating a first group of phase difference images or temperature difference images by using the first group of phase images, and calculating a second group of phase difference images or temperature difference images by using the second group of phase images; comparing whether the absolute value of the difference value between the phase difference or the temperature difference in the second group of phase difference diagrams or the temperature difference diagrams in the corresponding pixels and the phase difference or the temperature difference in the first group of phase difference diagrams or the temperature difference diagrams exceeds a preset threshold value, and if the absolute value exceeds the preset threshold value, calibrating the phase difference or the temperature difference in the second group of phase difference diagrams or the temperature difference diagrams.

Description

Magnetic resonance temperature imaging method for correcting susceptibility errors

technical field

The present application relates to the field of medical imaging, more specifically, to a magnetic resonance temperature imaging method for correcting magnetic susceptibility errors.

Background technique

The use of magnetic resonance temperature imaging (Magnetic Resonance Temperature Imaging, MRI) to monitor the temperature change of the target site plays a very important role in thermal ablation therapy (such as laser interstitial hyperthermia, confocal ultrasound and other minimally invasive or non-invasive surgery), However, in the existing method, as the temperature of the part to be ablated rises rapidly, the magnetic susceptibility of the part will change rapidly, resulting in a magnetic susceptibility error, so that the temperature of the ablation center part cannot be obtained or a large error occurs. How to eliminate the temperature increase caused by The magnetic susceptibility error is a problem that needs to be solved.

Contents of the invention

In order to solve the above technical problems, in the first aspect, the application provides a magnetic resonance temperature imaging method for correcting magnetic susceptibility errors, which includes:

Using a gradient echo sequence containing i different echo times to acquire magnetic resonance image data of the target site, where i is a positive integer greater than or equal to 2, the magnetic resonance image data includes a phase map corresponding to the echo time,

Selecting the phase diagram corresponding to at least one echo time as the first group of phase diagrams, and the phase diagrams corresponding to other echo times as the second group of phase diagrams;

Using the first group of phase diagrams to calculate a first group of phase difference diagrams or temperature difference diagrams, and using the second group of phase diagrams to calculate a second group of phase difference diagrams or temperature difference diagrams;

Comparing whether the absolute value of the difference between the phase difference or temperature difference in the second group of phase difference maps or temperature difference maps in the corresponding pixel and the phase difference or temperature difference in the first group of phase difference maps or temperature difference maps exceeds a predetermined value A threshold is set, and if the preset threshold is exceeded, the phase difference or temperature difference in the second group of phase difference diagrams or temperature difference diagrams is calibrated.

Using the first set of phase difference maps or temperature difference maps to obtain a first temperature map, using the second set of phase difference maps or temperature difference maps to obtain a second temperature map, and then calculating the temperature map.

Wherein, the threshold value can be freely selected, for example, the threshold value used can be 1, 2, 3, 4, 5, 6, 7, 8 or 9, etc.; there can be multiple methods for correction, for example, the temperature value of the first temperature map can be used to replace The temperature value of the second temperature map, or use the temperature value of the adjacent pixel in the second temperature map to replace the temperature value of the pixel in the second temperature map, or calculate the temperature value based on the temperature value of the adjacent pixel and the temperature value of the first temperature map Combine an approximate temperature to replace the temperature value of the second temperature map.

Optionally, in the method of the present invention, calculating the temperature map includes the step of weighting at least one first temperature map and at least one second temperature map, and the weighting is to weight the values on different temperature maps on the pixels to form a new Temperature map; further, the weighting can be the weighting of various weights, such as average weighting, or it can be a temperature map corresponding to a single echo time, that is, the weighting coefficient of this temperature map is 1, and the weighting coefficient of other phase temperature maps is 0.

Optionally, in the method of the present invention, at least one corresponding echo time in the first group of phase diagrams does not exceed 18ms, 17ms, 16ms, 15ms, 14ms, 13ms, 12ms, 11ms, 10ms, 9ms, 8ms, 7ms, 6ms, 5ms or 4ms. .

Optionally, in the method of the present invention, the echo time corresponding to the first group of phase diagrams is shorter than the echo time corresponding to the second group of phase diagrams to which it is compared. Further, the first group of phase maps only contains the phase maps corresponding to the minimum echo time.

Optionally, the method of the present invention further includes a step of correcting the phase error caused by motion, which is simulated by linear least squares at each pixel using at least two sets of phase difference maps or temperature difference maps corresponding to different echo times. Combined to remove the phase error caused by motion.

In a second aspect, the present invention also provides a storage medium, which is characterized in that a program code is stored on the storage medium, and when the program code is executed, the magnetic resonance temperature imaging method of the present invention is implemented.

In the third aspect, the present invention also provides a temperature imager, which is characterized in that it includes: a host, the host includes a processor and can receive magnetic resonance image data, the processor is loaded with program code, and the program code methods for carrying out the invention.

In the fourth aspect, the present invention also provides a laser hyperthermia system, which includes the temperature imager in the third aspect, capable of implementing the method of the present invention.

Description of drawings

In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the following will briefly introduce the drawings that need to be used in the description of the embodiments or the prior art. Obviously, the accompanying drawings in the following description are only It is an embodiment of the present application, and those skilled in the art can also obtain other drawings according to the provided drawings without creative work.

Figure 1 is the magnitude and phase images obtained in the ex vivo environment;

Fig. 2 provides the temperature map after magnetic susceptibility correction for an embodiment of the present application;

FIG. 3 is a comparison before and after correction of magnetic susceptibility at the same pixel position provided by another embodiment of the present application.

FIG. 4 provides a comparison of temperature maps of different frames for another embodiment of the present application, showing temperature maps of different processing methods and stages.

Detailed ways

Magnetic resonance temperature imaging can guide a variety of energy delivery treatments, such as laser interstitial hyperthermia, focused ultrasound therapy, radiofrequency ablation, etc., to monitor the target tissue temperature and treatment effect. Through research, the inventor found that the main sources of errors in the obtained temperature map are phase errors caused by phase unwrapping misalignment, magnetic susceptibility errors, and phase errors caused by motion. As the input energy dose is changed, the magnetic susceptibility distortion leads to a reduction in image amplitude and a corresponding error in the image phase, thus corrupting the reconstructed temperature map in and around the heating center. Errors in reconstructing the temperature map can lead to misestimation of the ablation area, which can lead to variations in treatment efficacy as well as thermal damage to critical tissues. Therefore, accurate temperature imaging is critical for the efficacy and safety of therapy, especially when applied to ablated regions of brain tissue.

Thermometry based on the shift of the proton resonance frequency is based on the principle that the resonance frequency of the hydrogen proton changes with the temperature in the water molecule. For hydrated tissues, the variation of the local magnetic field with temperature can be described as:

where α is the proton resonance frequency coefficient that varies with temperature. The corresponding resonance frequency change of water protons affected by temperature can be expressed as:

Δf=αγB ₀ ·ΔT; (2)

Among them, ΔT represents the temperature change, Δf represents the resonance frequency change, γ represents the gyromagnetic ratio, and _B0 represents the static magnetic field strength.

Changes in resonance frequency due to temperature changes can be observed in the phase of complex magnetic resonance imaging. For a given interval time TE of the gradient echo sequence, the relative temperature change ΔT can be calculated according to the phase difference Δφ, and the equation can be expressed as:

Gradient recall echo pulse sequence, referred to as gradient echo sequence, is the most commonly used sequence in thermometry based on proton resonance frequency shift. According to formula (3), the longer the gradient echo sequence, the larger the phase difference may be caused by the same temperature change, indicating that higher temperature sensitivity can be obtained.

In Fig. 1, as the echo time of the gradient echo sequence increases, both the phase contrast and the phase wrapping increase, indicating a higher temperature sensitivity and more phase unwrapping procedures at later echo times. The amplitude, phase and temperature of the first to fourth echoes obtained in (ex vivo, porcine brain) by the gradient echo sequence containing 4 different echo times used in the examples of the present application. Using conventional algorithms to calculate the temperature map (lower row) per TE (echo time) setting, intense laser heating causes signal loss due to magnetic susceptibility changes, which in turn lead to phase and temperature anomalies in pixels around the heated center.

The local magnetic field of water protons should also consider the magnetic susceptibility x ₀ , the formula (1) becomes:

表示由磁化率引起的局部磁场变化。in,

Indicates the local magnetic field change due to magnetic susceptibility.

The inventors found that laser heating causes significant magnetization artifacts in GRE imaging around the laser tip. Referring to FIG. 1 , heating centers with sharp temperature changes (as indicated by the arrows in FIG. 1 ) show severe signal loss on the order of longer echo times.

The magnetic susceptibility artifact caused by laser heating, especially the magnetic susceptibility artifact in the image corresponding to the gradient echo sequence with a long echo time, is an important cause of the error. Referring to Figure 1, in ex vivo or in vivo experiments, phase errors around the heating center translate into pseudo-cold temperatures on magnetic resonance thermography. Ideally, during imaging, a gradient pulse sequence with the shortest possible echo time is used to minimize susceptibility artifacts. However, the gradient pulse sequence with longer echo time can provide better temperature sensitivity and signal-to-noise ratio, and how to achieve the coordination of sensitivity and signal-to-noise ratio is a problem that needs to be solved.

In order to take into account temperature sensitivity, signal-to-noise ratio and low error, an embodiment of the present application provides a magnetic resonance temperature imaging method for correcting magnetic susceptibility errors, including the following steps:

Using a gradient echo sequence containing i different echo times to acquire magnetic resonance image data of the target site, where i is a positive integer greater than or equal to 2, the magnetic resonance image data includes a phase map corresponding to the echo time,

Selecting the phase diagram corresponding to at least one echo time as the first group of phase diagrams, and the phase diagrams corresponding to other echo times as the second group of phase diagrams;

Using the first group of phase diagrams to calculate a first group of phase difference diagrams or temperature difference diagrams, and using the second group of phase diagrams to calculate a second group of phase difference diagrams or temperature difference diagrams;

Comparing whether the absolute value of the difference between the phase difference or temperature difference in the second group of phase difference maps or temperature difference maps in the corresponding pixel and the phase difference or temperature difference in the first group of phase difference maps or temperature difference maps exceeds a predetermined value A threshold is set, and if the preset threshold is exceeded, the phase difference or temperature difference in the second group of phase difference diagrams or temperature difference diagrams is calibrated.

Using the first set of phase difference maps or temperature difference maps to obtain a first temperature map, using the second set of phase difference maps or temperature difference maps to obtain a second temperature map, and then calculating the temperature map.

The threshold value can be selected according to actual needs. For example, in the temperature difference diagram, the threshold used can be 1, 2, 3, 4, 5, 6, 7, 8 or 9, etc. The second group of phase difference diagrams or temperature difference There are many ways to calibrate the phase difference or temperature difference in the graph. For example, the temperature value of the first temperature graph can be used to replace the temperature value of the second temperature graph, or it can be based on the temperature values of adjacent pixels and the temperature value of the first temperature graph. The temperature values are fitted to an approximate temperature instead of the temperature values of the second temperature map, or may be substituted or estimated using temperature values of neighboring pixels.

The magnetic resonance temperature imaging method provided in the embodiment of the present application will be verified below in combination with specific experiments.

In vivo experiments in Doberman pinschers have been approved by the Institutional Review Board of Tsinghua University. An adult Doberman pinscher received laser interstitial hyperthermia. The heating process was monitored on a 3T MR scanner (Ingenia, Philips Healthcare, Best, The Netherlands) using a multi-echo time gradient echo sequence with 32 receiver coils, flip angle = 30°, TE = 6/12/18/ 24ms, TR=22ms, matrix=176×176, FOV=200×200mm ² , slice thickness=5mm, 3s/image. Use the sequence corresponding to 6ms as the first group of sequences, and the sequence corresponding to the remaining echo times as the second group of sequences, the absolute value of the difference between the phase difference in the second group of temperature difference diagrams and the temperature difference in the first group of phase difference diagrams When it exceeds 5 degrees Celsius, use the value of the first set of temperature map to replace the value of the second set of temperature map in the pixel.

See Figure 2 to Figure 4 for the experimental results.

Figure 2 shows a temperature diagram of a section of the whole, which eliminates the abnormal temperature of the heating center (black spots appearing) caused by the sudden change of magnetic susceptibility.

Fig. 3 is the change of temperature with time for a pixel, where the susceptibility error in the sequence of 12, 18, 24 ms echo time is corrected by using the sequence of 6 ms echo time.

Figure 4 shows the comparison of the results of different methods. The first row of results shows the traditional unwrapped temperature map. Different frame numbers reflect different positions. The second row is the temperature result after unwrapping correction. The third line is the demerit after the magnetic susceptibility correction of the present invention is carried out. It can be seen that the temperature distortion of the center has been corrected; the fourth line is the result of weighted average of the temperature maps obtained by using the sequence of different echo times. Graph transitions are smoother.

The features recorded in the various embodiments in this specification can be replaced or combined with each other. What each embodiment focuses on is the difference from other embodiments. The same and similar parts of the various embodiments can be referred to each other.

The above 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 general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the application. Therefore, the present application will not 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 (8)

1. A magnetic resonance temperature imaging method for correcting susceptibility errors, comprising:

acquiring magnetic resonance image data of a target region using a gradient echo sequence comprising i different echo times, i being a positive integer greater than or equal to 2, the magnetic resonance image data comprising a phase map corresponding to the echo times,

selecting at least one phase diagram corresponding to echo time as a first group of phase diagrams, wherein phase diagrams corresponding to other echo time are second group of phase diagrams, the echo time corresponding to at least one of the first group of phase diagrams is not more than 18ms, and the echo time corresponding to the first group of phase diagrams is smaller than the echo time corresponding to the second group of phase diagrams compared with the first group of phase diagrams;

calculating a first group of phase difference images or temperature difference images by using the first group of phase images, and calculating a second group of phase difference images or temperature difference images by using the second group of phase images;

comparing whether the absolute value of the difference value between the phase difference or the temperature difference in the second group of phase difference diagrams or the temperature difference diagrams in the corresponding pixels and the phase difference or the temperature difference in the first group of phase difference diagrams or the temperature difference diagrams exceeds a preset threshold value, and if the absolute value exceeds the preset threshold value, calibrating the phase difference or the temperature difference in the second group of phase difference diagrams or the temperature difference diagrams;

obtaining a first temperature map by using the first group of phase difference maps or the temperature difference maps, obtaining a second temperature map by using the second group of phase difference maps or the temperature difference maps, and then calculating the temperature map.

2. The method of claim 1, wherein calculating the temperature map includes the step of weighting at least one first temperature map and at least one second temperature map by weighting values on different temperature maps on pixels to form a new temperature map.

3. The method of claim 2, wherein the weighting is an average weighting.

4. The method of claim 1, wherein the first set of phase maps comprises only the phase map corresponding to the minimum echo time.

5. The method according to any one of claims 1 to 4, further comprising the step of correcting the motion induced phase error by removing the motion induced phase error using a linear least squares fit of at least two sets of phase difference maps or temperature difference maps corresponding to different echo times at each pixel.

6. A storage medium having stored thereon program code which when executed implements the magnetic resonance temperature imaging method of any one of claims 1 to 5.

7. A temperature imager, comprising: a host computer comprising a processor and being capable of receiving magnetic resonance image data, the processor being loaded with program code for performing the method of any one of claims 1 to 5.

8. A laser hyperthermia system comprising the temperature imager of claim 7, capable of performing the method of any of claims 1-5.

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