CN111714097B - Bimodal magnetic resonance temperature measurement method based on multi-gradient echo sequence - Google Patents
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Abstract
The invention discloses a bimodal magnetic resonance temperature measurement method based on a multi-gradient echo sequence: acquiring magnetic resonance signals before and during heating through a multi-gradient echo magnetic resonance sequence, and simultaneously reserving a phase diagram and an amplitude diagram of each echo; according to the phase diagrams of different echoes before and during heating, calculating the temperature change before and after heating of each pixel point on the phase diagrams
Calculating the signal intensity of each pixel point TE when TE is 0 according to amplitude maps of different echoes in heating; calculating the temperature change delta T (M) of each pixel point based on the amplitude value according to the signal intensity of the amplitude diagram before heating and the signal intensity of each pixel point TE when the signal intensity is 0; according to
And Δ T (M) determines the final actual temperature T of each pixel point or issues an error warning. According to the temperature measurement method, the amplitude diagram and the phase diagram in multi-gradient-echo magnetic resonance temperature imaging are synchronously used, so that bimodal temperature imaging is realized, and the accuracy of magnetic resonance temperature imaging is further improved.
Description
Technical Field
The invention relates to a magnetic resonance imaging technology, in particular to a bimodal magnetic resonance temperature measurement method based on a multi-gradient echo sequence.
Background
The real-time magnetic resonance temperature imaging can realize noninvasive, rapid and accurate temperature measurement on human tissues, is mainly applied to real-time temperature detection of minimally invasive surgeries such as tumor tissue thermal ablation and the like, and judges whether the temperature reaches an ideal value and temperature spatial distribution, thereby helping to judge the effect of the minimally invasive surgeries and early warning risks in the surgeries such as local over-temperature and the like.
Gradient echo magnetic resonance sequences (GRE, or other deformation sequences) are the mainstream technique of magnetic resonance temperature imaging methods, mainly used for proton resonance frequency shift (PRF) based magnetic resonance temperature imaging. Proton resonance frequency shift magnetic resonance temperature imaging is the mainstream imaging technology of magnetic resonance temperature imaging at present because of better space-time resolution, higher sensitivity and accuracy under high field and near linear sensitivity to higher temperature. The method mainly acquires space phase images through a gradient echo magnetic resonance sequence, acquires phase images before and during heating respectively, and calculates the difference (phase difference image for short) between the phase images to obtain a temperature image. For example, chinese patent publication No. CN102258362a discloses a method for reducing magnetic resonance temperature measurement errors, which is used in a high-intensity focused ultrasound apparatus for magnetic resonance imaging monitoring, and the method includes: before the high-intensity focused ultrasound equipment heats a heating area, acquiring a magnetic resonance phase image as a reference image; during or after the high-intensity focused ultrasound equipment is heated, acquiring another magnetic resonance phase image as a heating image; calculating the temperature change of a heating area according to the heating image and the reference image; the method further comprises the following steps: measuring the magnetic field change caused by the position change of an ultrasonic transducer of the high-intensity focused ultrasound equipment, and compensating the temperature change according to the magnetic field change. For example, chinese patent publication No. CN107468251a discloses a method for correcting low-field magnetic resonance temperature imaging phase drift, which is applied to monitoring the area of a heating region and an accurate temperature change value, and the method includes: before a microwave ablation instrument ablates a target region, a magnetic resonance image is acquired by using a GRE sequence or an SPGR sequence and is used as a reference image; acquiring another magnetic resonance image during or after MW ablation as a heating image; selecting a simulated heating area and an unheated area in the heating image; fitting out phase change caused by non-temperature change in the heating area by using a first-order polynomial model of a weighted least square method according to the phase shift of the unheated area; calculating a temperature difference value according to the phase difference value; and obtaining the area of the heating area according to the phase difference diagram and the temperature difference diagram.
In recent years, a multi-gradient-echo magnetic resonance sequence (the traditional sequence is a single echo) is introduced into magnetic resonance temperature imaging, and the phase diagrams of a plurality of echoes are collected, so that the accuracy of the magnetic resonance temperature imaging is improved to a certain extent. The main disadvantage of phase-based magnetic resonance temperature imaging is that the phase difference map is simultaneously influenced by non-temperature factors, such as the offset of the main magnetic field, and so on, with a certain risk of measurement deviation. Meanwhile, a phase diagram and an amplitude diagram can be obtained simultaneously by a multi-gradient echo magnetic resonance sequence, while the amplitude diagram is usually abandoned in the traditional magnetic resonance temperature imaging, only the phase diagram is used, and the temperature information in the amplitude diagram is not fully utilized.
Disclosure of Invention
The invention aims to disclose a bimodal magnetic resonance temperature measurement method based on a multi-gradient echo sequence, which realizes bimodal temperature imaging by synchronous use of an amplitude diagram and a phase diagram in multi-gradient echo magnetic resonance temperature imaging, and further improves the accuracy of the magnetic resonance temperature imaging.
The invention provides the following technical scheme:
a bimodal magnetic resonance temperature measurement method based on a multi-gradient echo sequence comprises the following steps:
(1) Acquiring magnetic resonance signals before and during heating through a multi-gradient echo magnetic resonance sequence, and simultaneously reserving a phase diagram and an amplitude diagram of each echo;
(2) According to the phase diagrams of different echoes before and during heating, calculating the temperature change before and after heating of each pixel point on the phase diagrams
(3) Calculating the signal intensity M when each pixel TE is 0 according to amplitude maps of different echoes in heating;
(4) Calculating the temperature change delta T (M) of each pixel point based on the amplitude value according to the signal intensity of the amplitude diagram before heating and the signal intensity M in the step (3);
(5) According to step (2)
And in step (4)
And judging the final temperature change delta T of each pixel point or sending out error warning.
In the step (1), before heating, scanning a multi-gradient echo magnetic resonance sequence for 5-10 frames, wherein the number of echoes is N, and N is more than or equal to 2; and averaging the phase diagram and the amplitude diagram of each echo obtained by the multi-gradient-echo magnetic resonance sequence scanning before heating to obtain the phase diagram and the amplitude diagram of each echo before heating.
Preferably, in step (1), the sequence repetition Time (TR) should be as long as possible, e.g. more than 200ms, and the flip angle should be as small as possible, e.g. less than 5 degrees, in order to eliminate the T1 effect.
Preferably, in step (1), if two echoes are used, the first echo time should be as short as possible, e.g. within 2ms, and the second echo time should be as close as possible to the time constant T2 * Thereby improving the estimation accuracy of the signal strength M of step (3).
In the step (2), the temperature change before and after each pixel point on the phase diagram is heated is calculated
The method comprises the following steps:
(2-1) according to the formula
Calculating the temperature change Delta T (i) of each echo, wherein gamma represents the gyromagnetic ratio of hydrogen proton, B 0 Showing the static magnetic field intensity, TE (i) showing the echo time of the ith echo, alpha showing the hydrogen proton temperature frequency coefficient,
phase difference before and after heating of each echo of each pixel point;
(2-2) by
Obtaining the temperature change before and after heating of each pixel point based on the phase diagram
Preferably, in step (2), a phase correction technique may be employed to improve the accuracy of the phase estimation.
In step (3), the method for calculating the signal intensity M when each pixel TE is 0 includes: according to the formula
Fitting the above equation using a nonlinear least squares sum method to obtain M, where S (TE (i)) isThe signal strength of the pixel point in the amplitude map of the ith echo,
is the significant transverse relaxation time.
In step (4), the temperature change Δ T (M) is based on
Wherein, T 0 Is the actual temperature of the tissue before heating, M 0 Signal intensity of the amplitude plot before heating.
In step (5), if
The absolute value of the difference between the sum delta T (M) and the sum delta T (M) is less than 5 degrees, and finally the sum delta T is adopted
The final actual tissue temperature T is T = DeltaT + T 0 Otherwise, a measurement error warning is issued.
Wherein, in the step (5),
and the acceptance range of the difference value of the sum delta T (M) can be flexibly adjusted according to the requirement of experiment precision.
Wherein, in step (5), if the final demand is that the temperature reaches a certain range rather than an accurate value, it may be set
And Δ T (M) must be in this range at the same time to meet the temperature demand.
Compared with the prior art, the invention has the main advantages that:
(1) In the prior art, only a phase diagram of multi-gradient echoes is often adopted, an amplitude diagram is abandoned, and temperature information in magnetic resonance signals is wasted. The method estimates the temperature information from the signal amplitude by reasonably calculating the amplitude diagram.
(2) The method adopts the same sequence to generate temperature images of two modes, thereby realizing the purposes of mutual correction and improving the temperature measurement accuracy.
Drawings
FIG. 1 is a flow chart of magnetic resonance temperature bimodal imaging data acquisition and real-time temperature calculation based on multiple gradient echoes;
FIG. 2 is a flow chart of temperature change before and after heating calculated based on a multi-gradient echo sequence amplitude plot;
FIG. 3 is a logic diagram of a bimodal thermographic co-fusion.
Detailed Description
The invention is described in further detail below with reference to the figures and embodiments (e.g., head imaging).
1. As shown in fig. 1, a scout image, a necessary structural image, and the like are scanned first.
2. As shown in FIG. 1, before heating, a multi-gradient-echo magnetic resonance sequence is scanned in advance for 5-10 frames, the number of echoes is N, and N is required to be more than or equal to 2.
3. As shown in fig. 1, the phase map and the amplitude map of each echo obtained by the multi-gradient echo magnetic resonance sequence scan before heating are averaged to obtain the phase map and the amplitude map of each echo before heating.
4. As shown in fig. 1, the heating apparatus is turned on and the collection of the multi-gradient-echo magnetic resonance sequence in the heating is started.
5. As shown in fig. 1, phase and amplitude maps of a multi-gradient echo magnetic resonance scan in heating are transmitted in real time.
6. As shown in FIG. 1, the phase difference before and after heating of each echo of each pixel point is calculated
Where i is the ith echo, starting from 1 to N,
to be the phase of the ith echo before heating,
the phase of the ith echo after heating.
7. As shown in fig. 1According to the formula
Calculating the temperature change Delta T (i) of each echo, wherein gamma represents the gyromagnetic ratio of hydrogen proton, B 0 Showing the static magnetic field intensity, TE (i) showing the echo time of the ith echo, and α showing the hydrogen proton temperature frequency coefficient.
8. As shown in fig. 1, by
Obtaining the temperature change before and after heating of each pixel point based on the phase diagram
9. As shown in fig. 2, according to the formula
Fitting the formula by adopting a nonlinear least square sum method to obtain the signal intensity M of each pixel point before heating 0 . Wherein S (TE (i)) is the signal intensity of the amplitude map of the ith echo of the pixel point before heating,
is a significant transverse relaxation time of the material,
and M 0 Are fitting parameters.
10. As shown in fig. 2, according to the formula
And fitting the formula by adopting a nonlinear least square sum method to obtain the signal intensity M of each pixel point in heating. Wherein S (TE (i)) is the signal intensity of the amplitude map of the ith echo in heating of the pixel point,
is a significant transverse relaxation time of the material,
and M 0 Are fitting parameters.
11. As shown in fig. 2, according to the formula
Calculating a temperature change before and after heating Δ T (M) based on an amplitude map, wherein T 0 Is the actual temperature of the tissue (in Kelvin) before heating, M and M 0 The signal intensities during and before heating calculated in steps 10-11, respectively.
12. As shown in fig. 3, for determining a region of interest
Whether or not it is satisfied.
13. As shown in FIG. 3, the temperature change of the area is determined as satisfied by step 12
The actual temperature T is T = DeltaT + T 0 。
14. As shown in fig. 3, if step 12 is not satisfied, a measurement error and termination scan command is issued.
15. As shown in fig. 3, it is determined whether the temperature change Δ T of the area reaches the termination scan request as shown in step 13.
16. As shown in fig. 3, a terminate scan command is issued, as satisfied by step 15.
17. If the instruction of terminating the scanning is not received, the multi-gradient echo magnetic resonance sequence of the next frame is scanned continuously as shown in fig. 1 and 3.
18. As shown in fig. 1, steps 5-17 are repeated.
The above-mentioned embodiments are intended to illustrate the technical solutions and advantages of the present invention, and it should be understood that the above-mentioned embodiments are only the most preferred embodiments of the present invention, and are not intended to limit the present invention, and any modifications, additions, equivalents, etc. made within the scope of the principles of the present invention should be included in the scope of the present invention.
Claims (7)
1. A bimodal magnetic resonance temperature measurement method based on a multi-gradient echo sequence is characterized by comprising the following steps:
(1) Acquiring magnetic resonance signals before and during heating through a multi-gradient echo magnetic resonance sequence, and simultaneously reserving a phase diagram and an amplitude diagram of each echo;
(2) According to the phase diagrams of different echoes before and during heating, calculating the temperature change before and after heating of each pixel point on the phase diagrams
(3) According to the amplitude diagram of different echoes during heating, according to the formula
Calculating the signal intensity of each pixel point TE when the signal intensity is 0, fitting the formula by adopting a nonlinear least square sum method, wherein M is the signal intensity of each pixel point TE when the signal intensity is 0, S (TE (i)) is the signal intensity of the amplitude map of the ith echo of the pixel point,
is the significant transverse relaxation time;
(4) Calculating temperature change delta T (M) of each pixel point based on the amplitude value according to the signal intensity of the amplitude diagram before heating and the signal intensity in the step (3);
(5) According to step (2)
And the delta T (M) in the step (4) judges the final actual temperature T of each pixel point or sends out error warning.
2. The bimodal magnetic resonance temperature measurement method based on the multi-gradient echo sequence as claimed in claim 1, wherein in the step (1), before heating, the multi-gradient echo magnetic resonance sequence is scanned for 5-10 frames, the number of echoes is N, and N is more than or equal to 2; and averaging the phase map and the amplitude map of each echo obtained by the multi-gradient echo magnetic resonance sequence scanning before heating to obtain the phase map and the amplitude map of each echo before heating.
3. The dual-modality magnetic resonance thermometry method based on multi-gradient echo sequence of claim 1, wherein in step (1), the sequence repetition time TR > 200ms and the flip angle < 5 degrees.
4. The dual-modality MR thermometry method based on multi-gradient echo sequence of claim 1, wherein in step (1) two echoes are used, the first echo time being within 2ms and the second echo time being close to the time constant T2 * 。
5. The dual-modality MR thermometry method based on multi-gradient echo sequence of claim 1, wherein in step (2), the temperature change before and after heating of each pixel point on the phase map is calculated
The method comprises the following steps:
(2-1) according to the formula
Calculating the temperature change Delta T () of each echo, wherein gamma represents the gyromagnetic ratio of hydrogen proton, B 0 Showing the static magnetic field intensity, TE (i) showing the echo time of the ith echo, alpha showing the hydrogen proton temperature frequency coefficient,
phase difference before and after heating of each echo of each pixel point;
(2-2) by
Obtaining the temperature change before and after heating of each pixel point based on the phase diagram
6. The dual-modality magnetic resonance thermometry method based on multiple gradient echo sequence of claim 1, wherein in step (4), the temperature change Δ T (M) is based on
Wherein, T 0 Is the actual temperature of the tissue before heating, M 0 Signal intensity of the amplitude plot before heating.
7. The dual-modality MR thermometry method based on multi-gradient echo sequence of claim 1, wherein in step (5), if
The absolute value of the difference between the sum delta T (M) and the sum delta T (M) is less than 5 degrees, and finally the sum delta T is adopted
The final temperature T is T = Δ T + T 0 Otherwise, a measurement error warning is issued.
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