CN115267627B - Magnetic resonance magnetic field measurement method and device based on jump echo coding - Google Patents
Magnetic resonance magnetic field measurement method and device based on jump echo coding Download PDFInfo
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Abstract
The invention discloses a magnetic resonance magnetic field measurement method and device based on a jump echo code. First, no separate application of the imaging tissue in two scansPositive and negative transformed currents synchronized with refocusing pulses until the end of the first echo signal sample for each repetition time, such that the current-generated magnetic field-induced phase changes can continue to accumulate until the end of the echo sample; in the jump echo coding, the first n echo signal samples are skipped, so that the first echo signal is subjected to long enough external current passing time to further improve the signal to noise ratio of the result; after the current is finished, a rapid spin echo sequence mode is adopted to collect signals, and an acceleration factor m is set; finally, the phase results of the two different initial current scans are differenced, and the phase difference and the current are used to generate a magnetic field (B z ) The linear relation between the two is calculated to obtain B z As a result. The invention realizes the improvement of B under the condition of not losing the signal to noise ratio z The measurement speed of the magnetic field is ten times.
Description
Technical Field
The application relates to the technical field of magnetic resonance, in particular to the field of magnetic resonance MRCDI and MREIT imaging magnetic field measurement.
Background
Magnetic resonance current density imaging (Magnetic Resonance Current Density Imaging, MRCDI) and magnetic resonance electrical impedance imaging (Magnetic Resonance Electrical Impedance Tomography, MREIT) are two emerging imaging methods for measuring internal electrical characteristic parameters of tissue, wherein the MREIT technique allows non-invasive acquisition of high spatial resolution conductivity distribution inside tissue by combining injection of external currents; the MRCDI technology can obtain the current density distribution of external current stimulation in the tissue, and can provide guidance for the specific implementation of nerve regulation and control technologies such as transcranial electric stimulation (tDCS).
The technique combines the application of current through two electrodes outside the imaged object, using magnetic resonance phase information to apply a magnetic field (B 0 ) Principle of sensitivity of the magnitude of the directional magnetic field, measuring the spatial distribution of the magnetic field generated by the current inside the imaging volume with respect to the main magnetic field (B 0 ) Component of parallel direction (B z ) Further utilize B z Solving information for calculating the current density or conductivity distribution inside the imaging volume.
At present, B z The magnetic field is measured mainly by Spin Echo (SE) sequence, continuous accumulation of signal phase is realized by applying direct current with direction change synchronous with refocusing pulse, phase data of scanning results of two different currents (such as 1mA and 2mA,2mA and-2 mA) are used for making difference, and phase difference is used for obtaining phase difference and B z Calculating the linear relation of B z Is a distribution of (a). However, since SE sequences are inefficient at filling k-space when acquiring signals, B is present z The measurement of the total brain is still not efficient enough, and the spatial distribution information of the total brain is difficult to acquire in a short time, so that the total brain is difficult to apply in actual demands. The rapid magnetic field measurement sequence based on the jump echo code of the invention not only can effectively measure B z Meanwhile, the measurement efficiency is greatly improved, and the required time is reduced, so that the possibility is provided for the practical application of MRCDI and MREIT aiming at clinical demands.
Disclosure of Invention
For the prior art B z The invention aims to improve the prior art and provides a method for measuring B more efficiently z Methods and apparatus for measurement. The invention is to adopt a fast spin echo (TSE) mode to collect data, the turbo factor is an acceleration factor (m), and compared with the original Spin Echo (SE) sequence, the collection speed of the sequence can be improved by nearly ten times. However, the echo signal exhibits T due to the spin echo sequence 2 With larger echo spacing, a larger acceleration factor would make the signal of each TR post-echo too small, thereby reducing the resolution of the final imaging, and therefore a smaller echo spacing is necessary. In the background of the problem, the duration of the external current of the spin echo sequence method is consistent with the echo spacing, so that when the echo spacing is smaller, the shorter current duration leads to B z The phase change caused is smaller, and B is reduced z Signal to noise ratio of the measurement.
Based on the method and the device, the magnetic resonance magnetic field measurement method and the device based on the jump wave coding are provided, the rapid measurement of the magnetic field generated by external current is realized, meanwhile, the measurement signal-to-noise ratio and the measurement effect are ensured, and the magnetic resonance MRCDI and MREIT imaging performance is improved.
The invention is realized by the following technical scheme:
in a first aspect, the present invention provides a magnetic resonance magnetic field measurement method based on a jump echo code, which specifically includes:
scanning target tissue twice by using the same imaging sequence, applying a first stimulating current of positive-negative conversion synchronous with refocusing pulse to the imaging tissue in the first scanning process and obtaining a first scanning result, and applying a second stimulating current of positive-negative conversion synchronous with refocusing pulse to the imaging tissue in the second scanning process and obtaining a second scanning result; calculating a phase difference between the first scan result and the second scan result, and using the phase difference and B z The linear relationship between the magnetic fields gives B z A magnetic field;
the imaging sequence divides all 180-degree refocusing pulses in each repetition period in the rapid spin echo sequence into a jump-back wave coding module and a rapid spin echo sampling module, wherein the first n 180-degree refocusing pulses belong to the jump-back wave coding, the rest 180-degree refocusing pulses belong to the rapid spin echo sampling part, and n is a positive integer not less than 1; in the jump echo coding module, 180-degree refocusing pulses in the module and layer gradient codes corresponding to the 180-degree refocusing pulses in the module are consistent with the fast spin echo sequence, but phase gradient codes and frequency gradient codes after each 180-degree refocusing pulse in the module are cancelled; the rapid spin echo sampling module samples signals according to a rapid spin echo mode and fills the signals into k space, and the first frequency gradient code in each repeated period is the first frequency gradient code in the rapid spin echo sampling module;
in each repetition period, the first stimulating current is continuously from the beginning of the first 90-degree radio frequency pulse to the end of the first frequency gradient coding, and is only set to zero during the application period of each 90-degree and 180-degree radio frequency pulse, the current in one current continuous section between two adjacent radio frequency pulses is always the same as the positive and negative directions, and the current in any two adjacent current continuous sections is the same but opposite to the positive and negative directions; the first stimulation current and the second stimulation current have the same number of current duration sections in each repetition period and are in one-to-one correspondence with each current duration section, and the starting and ending time of each corresponding current duration section is identical but the currents are different.
As a preference of the above-mentioned first aspect, the imaging sequence is realized in particular as follows:
in each repetition period of the imaging sequence, firstly, applying a 90-degree radio frequency pulse and simultaneously applying a layer selection coding gradient, then, applying a pre-refocusing frequency gradient to enable spins to restore to be in phase at an echo center, and then, executing a jump echo coding module and a fast spin echo sampling module; in the jump echo coding module, n 180-degree refocusing pulses are applied at intervals, and layer selection gradient coding is performed while each 180-degree refocusing pulse is applied; in the rapid spin echo sampling module, m 180-degree refocusing pulses are applied at intervals, layer selection gradient coding is carried out while each 180-degree refocusing pulse is applied, phase gradient coding is carried out after layer selection gradient coding, frequency gradient coding is carried out after phase gradient coding, and k-space data are obtained through echo signal sampling while frequency gradient coding; the two sides of n+m layer selection gradient codes applied in the jump echo coding module and the quick spin echo sampling module are provided with phase disturbance gradients, wherein the area of the phase disturbance gradient at the two sides of the first layer selection gradient code is set to be a numerical value capable of causing 4 pi phase dispersion, any two adjacent layer selection gradient codes in the n+m layer selection gradient codes apply phase disturbance gradients with different areas, and the area of the phase disturbance gradient at the two sides of any one layer selection gradient code in the n+m layer selection gradient codes is not lower than the area of the phase disturbance gradient at the two sides of the first layer selection gradient code; finally, reconstructing an amplitude image and a phase image of each scanning through Fourier transform calculation on the k-space number obtained by sampling; n and m are each a positive integer of not less than 1;
as a preferable aspect of the first aspect, the disturbing gradient area relationship on both sides of any two adjacent slice selection gradient codes in the n+m slice selection gradient codes is preferably 2 or 1/2 times.
As the first mentioned aboveIn a preferred aspect, the phase difference and B are used z The linear relationship between the magnetic fields gives B z The calculation formula of the magnetic field is as follows:
B z (x,y)＝ΔΦ(x,y)/(2γ·Tc·(I 1 -I 2 ))
wherein: b (B) z (x, y) represents B generated per unit external current at point (x, y) z Magnetic field, unit: nT/mA, ΔΦ (x, y) represents the phase difference between the first and second scan results at point (x, y), γ is the gyromagnetic ratio of hydrogen atoms, tc is the duration of the stimulus current in one repetition period, I 1 And I 2 The first stimulating current and the second stimulating current are respectively distinguished from each other.
As a preferred aspect of the above first aspect, the duration Tc of the stimulation current in the one repetition period is calculated as:
T c ＝(ESP-τ π )·(n+1)-0.5τ π/2
wherein: ESP is the time interval of adjacent 180 refocusing pulses, τ π 、τ π/2 The time of action of a single 180 refocusing pulse and a single 90 radio frequency pulse, respectively.
As a preferable aspect of the first aspect, n is preferably 1 to 3.
As a preferable aspect of the first aspect, the n is further preferably 1.
As a preference of the above first aspect, the m is preferably not more than 10.
As a preferable aspect of the first aspect, the m is more preferably 5.
As a preference of the first aspect, in each repetition period, the start-stop time and the magnitude of the current duration corresponding to each group of the first stimulation current and the second stimulation current are identical, but the positive and negative directions of the currents are opposite.
As a preference of the above first aspect, the first current duration is preferably a maximum value within a safe range of the external current allowed to be applied by the target tissue, of the first and second stimulation currents.
In a second aspect, the present invention provides a magnetic resonance magnetic field measurement apparatus based on a jump echo code for implementing the magnetic resonance magnetic field measurement method according to any one of the first aspect, which comprises a magnetic resonance device, an external electrical stimulation device and a calculation module;
the external electrical stimulation device is used for applying the first stimulation current and the second stimulation current to target tissue in the scanning process of the magnetic resonance device;
the magnetic resonance apparatus is configured to perform the imaging sequence and obtain a first scan result and a second scan result;
the calculation module is used for calculating the phase difference between the first scanning result and the second scanning result and utilizing the phase difference and B z The linear relationship between the magnetic fields gives B z A magnetic field.
As a preference of the above second aspect, the magnetic resonance device, during execution of the imaging sequence to scan the target tissue, sends out a first synchronization signal to the external electrical stimulation device before applying the 90 ° radio frequency pulses, sends out a second synchronization signal to the external electrical stimulation device before applying each 180 ° refocusing pulse of the echo skip coding module, and sends out a third synchronization signal to the external electrical stimulation device before applying the first 180 ° refocusing pulse of the echo signal sampling module;
the external electric stimulation equipment starts to apply a first current continuous section to the target tissue according to the set time delay after receiving the first synchronous signal;
after the external electric stimulation equipment receives the second synchronous signal each time, applying the next current continuous section with the positive and negative directions opposite to those of the previous current continuous section to the target tissue according to the set time delay;
and after receiving the third synchronous signal, the external electric stimulation equipment applies a last current continuous section with the positive and negative directions opposite to those of the last current continuous section to the target tissue according to the set time delay.
As a preferable aspect of the second aspect, the external electrical stimulation device receives the first synchronization signal and then applies the same current in the first current duration but in opposite positive and negative directions during the two scans of the magnetic resonance device.
As a preferred aspect of the second aspect, the magnitude of the first-segment current duration applied by the external electrical stimulation device after receiving the first synchronization signal during the two scans of the magnetic resonance device is further preferably a maximum value within a safe range of the external current allowed to be applied by the target tissue.
Compared with the prior art, the invention has the following beneficial effects:
the invention reasonably skips the sampling of the partial echo in the jump echo coding, so that the measured echo signal experiences long enough external current action time, improves the phase accumulation induced by external current, improves the quality of the measured magnetic field, and solves the problem of low signal-to-noise ratio caused by short echo interval in the traditional spin echo sequence-based measurement method. Meanwhile, the imaging time is greatly reduced by a rapid spin echo sampling mode, the imaging performance of the magnetic resonance MRCDI and MREIT is integrally improved, and the imaging time meets the limit of clinical application, so that the invention has very important clinical application value.
Drawings
Figure 1 is a block diagram of a magnetic resonance magnetic field measurement sequence based on a jump echo encoding.
FIG. 2 is a B of a conventional multi-echo spin echo (MESE) sequence weighted scan water model experiment based on a jump echo encoded magnetic resonance magnetic field measurement sequence z And (5) image comparison and quantitative analysis.
FIG. 3 is a broad coverage B of a magnetic resonance magnetic field measurement sequence scanning water model experiment based on a jump back wave encoding z Image effects.
FIG. 4 is a broad coverage B of a magnetic resonance magnetic field measurement sequence scan pork experiment based on a jump-back wave encoding z Image effects.
Detailed Description
In order that the above objects, features and advantages of the invention will be readily understood, a more particular description of the invention will be rendered by reference to the appended drawings. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. The present invention may be embodied in many other forms than described herein and similarly modified by those skilled in the art without departing from the spirit of the invention, whereby the invention is not limited to the specific embodiments disclosed below. The technical features of the embodiments of the invention can be combined correspondingly on the premise of no mutual conflict.
In the description of the present invention, it should be understood that the terms "first" and "second" are used solely for the purpose of distinguishing between the descriptions and not necessarily for the purpose of indicating or implying a relative importance or implicitly indicating the number of features indicated. Thus, a feature defining "a first" or "a second" may explicitly or implicitly include at least one such feature.
Referring to fig. 1, the present invention provides a magnetic resonance magnetic field measurement method based on a jump-back wave code, which can be actually expressed as a magnetic resonance magnetic field measurement sequence based on the jump-back wave code, wherein the magnetic resonance magnetic field measurement sequence is implemented by coupling an external stimulation current applied by an external electrical stimulation device based on an imaging sequence of the magnetic resonance device. The specific points of improvement and design principles of the magnetic resonance field measurement sequence are described in detail below.
Inventive measurement B z In the magnetic field, the target tissue is scanned twice with a magnetic resonance apparatus based on an imaging sequence that is modified based on a conventional fast spin echo (TSE) sequence. In the two scans, the external electric stimulation device applies positive and negative converted currents with the same magnitude and opposite initial directions and synchronous with refocusing pulse to the imaging tissue respectively, the currents last until the sampling of the first echo signal in each repetition time TR is finished, so as to ensure that the magnetic field induced phase change generated by the currents can be accumulated continuously before the end of the echo sampling, and the phases of the two scanning results are phi respectively + (x, y) and Φ - (x,y)。
Each repetition period of the fast spin echo sequence contains a 90 deg. rf pulse followed by a number of refocusing pulses (i.e., 180 deg. rf pulses) applied after the 90 deg. rf pulse. In the invention, all 180-degree refocusing pulses in each repetition period in a rapid spin echo sequence are divided into a jump echo coding module and a rapid spin echo sampling module, wherein the first n 180-degree refocusing pulses belong to jump echo coding, the rest 180-degree refocusing pulses (the number is recorded as m) belong to a rapid spin echo sampling part, and n is a positive integer not less than 1. In the jump back wave coding module, 180 degrees refocusing pulse in the module and layer selection gradient coding corresponding to the jump back wave coding module are consistent with TSE sequences, but the difference is that phase gradient coding and frequency gradient coding after each 180 degrees refocusing pulse in the jump back wave coding module are cancelled; while the fast spin echo sampling module samples the signal in a TSE sequence and fills in k-space. The first frequency gradient code in the whole repetition period is the first frequency gradient code in the fast spin echo sampling module because the frequency gradient code after 180 degrees refocusing pulse in the jump echo coding module in each repetition period is cancelled.
In the skip echo coding of the imaging sequence, n represents the number of 180 ° refocusing pulses applied in the skip echo coding module, which is a skip echo parameter that needs to be set reasonably for skipping the sampling of the first n echo signals, so that the first echo signal taken experiences a sufficiently long external current action time. In order to enable the current applied by the external electric stimulation device to continuously and linearly accumulate in each echo, the influence of the stimulated echo caused by imperfect refocusing pulse needs to be eliminated in each echo, and the invention adopts a mode of changing the adjacent disturbance phase gradient area.
In addition, the number m of 180 DEG refocusing pulses in the fast spin echo sampling module is the acceleration factor of the imaging sequence. After the current is finished, signals are acquired by adopting a conventional rapid spin echo (TSE) sequence mode, and each TR acquires m echo signals to fill m rows of k space, so that the imaging speed is improved by m times.
After two scans are performed under the external stimulus current based on the imaging sequence, the phase difference delta phi (x, y) of each point position on the image is obtained by differencing the two phase results of the scans, and delta phi (x, y) and B are utilized z Linear switch betweenSolving for B z For the calculation of MREIT or MRCDI imaging.
Therefore, the magnetic resonance magnetic field measuring method based on the jump wave code prolongs the action time of external current and improves B through the jump wave code z Is effective in the following. Meanwhile, the signal is acquired by using a rapid spin echo mode, the imaging speed can be improved by 10 times, and the B is efficiently, stably and rapidly realized z The measurement of the magnetic field improves the magnetic resonance MRCDI and MRCDI imaging performance.
Since in the present invention the magnetic resonance imaging device and the external electrical stimulation device are two different device architectures, as a synchronization signal needs to be relied upon between the two to achieve the synergy of the applied external current and the imaging sequence.
In the present invention, the external stimulation current applied by the external electrical stimulation device may be implemented as follows:
before all radio frequency pulses before the first sampled echo of each TR are applied, the sequence programming causes the magnetic resonance machine to give a synchronization pulse signal of specific width according to three different current command (start, flip, end) requirements, which is received by the external electrical stimulation device, which programs the applied stimulation current by identifying the pulse width. The three instruction requirements are respectively: the "start" synchronization signal is applied before the 90 ° rf pulse, the "end" synchronization signal is applied before the 180 ° rf pulse before the echo signal is sampled, and the "flip" synchronization signal is applied before all the 180 ° rf pulses between the two. The application of the external current needs to take into account the specific timing relationship between the synchronization signal and the radio frequency pulses at the time of programming. It should be noted in particular that in order to reduce the complex effect of the external current on the result during the application of the radio frequency pulses, the application of the external current should be zeroed during the radio frequency pulses. The specific pattern of current application can be seen in fig. 1.
Conventional fast spin echo sequences typically acquire data starting from the first echo signal, filling all echo signals into k-space. But at B z In the context of measurement problems, the application of an external stimulus current to the end of the first echo sample as required for a conventional fast spin echo sequenceIn other words, a long echo interval (ESP) causes a great loss of the echo signal at the back, greatly affects the resolution of imaging in the case of a large acceleration factor (m), and weakens the acceleration effect in the case of a small acceleration factor (m); at the same time, a short echo interval (ESP) reduces the time of application of the current, resulting in an un-complementary accumulation of phase, resulting in a signal that is difficult to measure with a higher signal-to-noise ratio. Therefore, under the condition of selecting a small echo interval (ESP) and a large acceleration factor (m), the invention designs a sequence for skipping the first n echo samples, namely the time of the first n echoes is only used for applying a current accumulation phase, and the echo signals are acquired after the current is ended. In this case, the action time of the external stimulus current in one repetition period can be expressed by the following formula:
T c ＝(ESP-τ π )·n+1)-0.5τ π/2 formula (1)
Wherein ESP is the time interval of adjacent 180 refocusing pulses, τ π 、τ π/2 Respectively 180 deg. refocusing pulse and single 90 deg. radio frequency pulse. The specific relation between the sampling of the echo signal and the application of the external current can be seen in fig. 1.
In addition, in the magnetic resonance magnetic field measurement sequence, the mode of ensuring continuous linear accumulation of current among continuous echoes by the jump echo coding module is improved according to the following method:
the method for eliminating the excitation echo (stimulated echo) component in the echo by changing the adjacent disturbance phase gradient area comprises the following specific steps: the area of the first group of disturbing phase gradients is set to a value which can cause the phase dispersion corresponding to 4 pi, and the areas of the rest disturbing phase gradients are 2 or 1/2 times that of the former group, but the areas of the disturbing phase gradients are not smaller than that of the first group of disturbing phase gradients. The sequence designed according to the method can only keep the main echo signal in the echo, eliminate the excitation echo component, and enable the phase caused by the current generation magnetic field to be linearly accumulated in the continuous echo signal.
However, the disturbing gradient areas of adjacent layer selection gradient codes do not necessarily satisfy a relationship of 2 times or 1/2 times, but are merely a preferable mode, and it is only necessary to apply disturbing gradients having different areas to any two adjacent layer selection gradient codes theoretically.
In addition, in the magnetic resonance magnetic field measurement sequence of the present invention, echo data sampling and k-space filling are improved according to the following method:
unlike conventional single echo Spin Echo (SE) sequence based B z Each TR in the measurement sequence only collects one echo signal to fill k space, the designed sequence adopts a rapid spin echo (TSE) sequence to sample signals, m is an acceleration factor, each TR collects m echo signals to fill m rows of k space, and the scanning time is shortened by m times.
In the magnetic resonance magnetic field measurement sequence of the invention, B z The calculation of the magnetic field may be as follows:
in the scan result of two opposite starting current directions, the signal at each point (x, y) on the image can be expressed as:
where ρ (x, y) is the signal density, TE is the echo time, T 2 For the imaging volume relaxation time, δ (x, y) is the system intrinsic phase, γ is the hydrogen atom gyromagnetic ratio, γ=26.75x10 7 rad/(T.s), time of action of current T c ＝(ESP-τ π )·n+1)-0.5τ π/2 . Thus, the phase difference of the two scan results can be expressed as:
thus, find B z (x, y) can be calculated by the following formula:
B z (x,y)＝ΔΦ(x,y)/(2γ·Tc·(I 1 -I 2 ) Formula (4)
Wherein: i 1 And I 2 The stimulation currents respectively applied in the two scanning processes are recorded as a first stimulation current and a second stimulation current, I 1 And I 2 To distinguish between positive and negative, i.e. to distinguish between opposite current flows from positive and negative valuesDividing into two parts.
Based on the above description of the specific improvement points and design principles of the magnetic resonance magnetic field measurement sequence shown in fig. 1, in a preferred embodiment of the present invention, a magnetic resonance magnetic field measurement method based on the jump echo code is further provided, and the specific method flow is as follows:
scanning target tissue twice by using the same imaging sequence, applying a first stimulating current of positive-negative conversion synchronous with refocusing pulse to the imaging tissue in the first scanning process and obtaining a first scanning result, and applying a second stimulating current of positive-negative conversion synchronous with refocusing pulse to the imaging tissue in the second scanning process and obtaining a second scanning result; calculating a phase difference between the first scan result and the second scan result, and using the phase difference and B z The linear relationship between the magnetic fields gives B z A magnetic field.
The sequence formed by coupling the imaging sequence and the stimulating current (the first stimulating current or the second stimulating current) applied synchronously in time sequence is shown in fig. 1, and specifically comprises the following steps:
in each repetition period of the imaging sequence, a 90 ° rf pulse is first applied and a slice-select encoding gradient is applied simultaneously, then a pre-refocusing frequency gradient is applied to bring the spins back into phase at the echo center, and then a jump echo encoding and fast spin echo sampling are performed. In the jump echo coding module, n 180 DEG refocusing pulses are applied at intervals, and layer selection gradient coding is performed while each 180 DEG refocusing pulse is applied; in the fast spin echo sampling module, m 180-degree refocusing pulses are applied at intervals, layer selection gradient encoding is performed while each 180-degree refocusing pulse is applied, phase gradient encoding is performed after layer selection gradient encoding, frequency gradient encoding is performed after phase gradient encoding, and k-space data are obtained through echo signal sampling while frequency gradient encoding. And in order that the phase induced by the external current can be retained in the reconstructed phase image, the component of the excitation echo caused by the imperfect refocusing pulse needs to be eliminated in the sampled echo, and the problem can be solved by reasonably setting the disturbance phase gradient in the imaging sequence, specifically: the method comprises the steps that disturbance gradients are arranged on two sides of n+m layer selection gradient codes applied in an echo jump coding module and a fast spin echo sampling module, wherein the disturbance gradient areas on two sides of a first layer selection gradient code are set to be values capable of causing 4 pi phase dispersion, in the n+m layer selection gradient codes, the disturbance gradients with different application areas are applied to any two adjacent layer selection gradient codes (preferably, the disturbance gradient areas applied by the two adjacent layer selection gradient codes are in a 2-time or 1/2-time relation), and the disturbance gradient area on two sides of any one layer selection gradient code in the n+m layer selection gradient codes is not lower than the disturbance gradient area on two sides of the first layer selection gradient code; n and m are each a positive integer of not less than 1. And finally, calculating and reconstructing an amplitude image and a phase image of each scanning by carrying out Fourier transformation on the k-space number obtained by sampling the imaging sequence.
In each repetition period, the first stimulation current is continuously from the first 90-degree radio frequency pulse to the end of the first frequency gradient coding, and is set to zero only during the application period of each radio frequency pulse (including the 90-degree radio frequency pulse and the 180-degree radio frequency pulse, namely the refocusing pulse), the current in one current duration section between two adjacent radio frequency pulses is always the same as the positive and negative directions, and the current in any two adjacent current duration sections is the same but opposite to the positive and negative directions; the first stimulation current and the second stimulation current have the same number of current duration sections in each repetition period and are in one-to-one correspondence with each current duration section, and the starting and ending time of each corresponding current duration section is identical but the currents are different.
As a specific implementation of the embodiment of the present invention, as described above, the phase difference and B are used z The linear relationship between the magnetic fields gives B z The specific calculation formula of the magnetic field is shown as formula (4). In B of z (x, y) represents B at point (x, y) z A magnetic field, ΔΦ (x, y), representing the phase difference between the first and second scan results at points (x, y), γ being the gyromagnetic ratio of hydrogen atoms, tc being the duration of the stimulus current in one repetition period, I 1 And I 2 The first stimulating current and the second stimulating current are respectively distinguished from each other. Wherein Tc is calculatedThe foregoing formula (1) may be employed.
In addition, parameters n and m in the present invention need to be optimally adjusted according to the actual situation. Among them, n is preferably 1 to 3, more preferably 1.m is preferably not more than 10, more preferably 5.
In each repetition period, the start-stop time of each corresponding current duration period of the first stimulation current and the second stimulation current is basically the same but the currents may be different, and the currents may be different (for example, currents of 1mA and 2mA are respectively applied) or the directions of the currents may be different (for example, currents of 2mA and-2 mA are respectively applied). However, as a specific implementation manner of the embodiment of the present invention, the start-stop time and the magnitude of each corresponding current duration of each group of the first stimulation current and the second stimulation current are identical, but the positive and negative directions of the currents are opposite.
In another embodiment of the present invention, based on the same inventive concept, in order to implement the magnetic resonance magnetic field measurement method based on echo jump coding, the present invention further provides a magnetic resonance magnetic field measurement device based on echo jump coding, where the device includes a magnetic resonance apparatus, an external electrical stimulation apparatus, and a calculation module;
the external electrical stimulation device is used for applying the first stimulation current and the second stimulation current to target tissue in the scanning process of the magnetic resonance device;
the magnetic resonance apparatus is configured to perform the imaging sequence and obtain a first scan result and a second scan result;
the calculation module is used for calculating the phase difference between the first scanning result and the second scanning result and utilizing the phase difference and B z The linear relationship between the magnetic fields gives B z A magnetic field.
It should be noted that the magnetic resonance device, during the execution of the imaging sequence to scan the target tissue, sends a first synchronization signal representing "start" to the external electrical stimulation device before applying the 90 ° radio frequency pulses, sends a second synchronization signal representing "flip" to the external electrical stimulation device before applying each 180 ° refocusing pulse of the echo skip coding module, and sends a third synchronization signal representing "end" to the external electrical stimulation device before applying the first 180 ° refocusing pulse of the echo signal sampling module;
the external electric stimulation equipment starts to apply a first section of current duration to the target tissue according to the set time delay after receiving the first synchronous signal,
after the external electric stimulation equipment receives the second synchronous signal each time, applying the next current continuous section with the positive and negative directions opposite to those of the previous current continuous section to the target tissue according to the set time delay;
and after receiving the third synchronous signal, the external electric stimulation equipment applies a last current continuous section with the positive and negative directions opposite to those of the last current continuous section to the target tissue according to the set time delay.
As previously mentioned, in principle, the first stimulation current and the second stimulation current may be identical in terms of the start-stop time of each corresponding current duration of each group and different in terms of the current, but are preferably set such that the start-stop time and the magnitude of each corresponding current duration of each group are identical, but the current is opposite in positive and negative directions.
It should be specifically noted that, after each synchronization signal is received by the external electrical stimulation device, the specific electrical stimulation time delay to be executed needs to be determined according to the actual starting time of the current duration period from the sending of the synchronization signal to the corresponding sequence shown in fig. 1, so as to accurately apply the corresponding external stimulation current to the target tissue according to the sequence shown in fig. 1. And in the case that the target tissue can bear, the larger and better the applied external current, the size of the first-section current continuous section in the first stimulation current and the second stimulation current is preferably the maximum value in the safety range of the applied external current allowed by the target tissue.
It should be noted that, the positive and negative directions of the first stimulation current and the second stimulation current may be controlled by setting the positive and negative directions of the first current continuous section, that is, the positive and negative directions of the first current continuous section applied by the external electrical stimulation device after receiving the first synchronization signal in the two scanning processes of the magnetic resonance device are opposite. For example, the first stimulation current may be applied in accordance with the external current sequence shown in fig. 1, i.e., the current direction of the first segment current duration is forward; while the second stimulation current may be applied in the reverse direction of the external current sequence shown in fig. 1, i.e. the current direction of the first current duration is negative. Because the other current continuous sections are sequentially alternated in positive and negative, the current directions of any one group of current continuous sections corresponding to the first stimulation current and the second stimulation current are opposite as long as the current directions of the current continuous sections of the first section are opposite.
Those skilled in the art will appreciate that the computing modules involved in the present invention may be implemented by circuitry, other hardware, or executable program code, as long as the corresponding functions are enabled. If code is employed, the code may be stored in a memory device and executed by corresponding elements in the computing device. The implementation of the present invention is not limited to any specific combination of hardware and software. The calculation module in the above-described apparatus may also be integrated in an internal calculation unit of the magnetic resonance imaging device or may be in the form of a separate data processing device, without limitation.
In addition, the magnetic resonance imaging equipment and the external electric stimulation equipment can be commercially available products or homemade equipment, and can be selected according to actual user requirements.
The following shows specific technical effects thereof based on the above-described method in combination with examples so that those skilled in the art can better understand the spirit of the present invention.
Examples:
the magnetic resonance magnetic field measurement method and the magnetic resonance magnetic field measurement device based on the jump echo code are tested in a magnetic resonance magnetic field measurement experiment of a spherical water model and pork tissue, and are compared with the magnetic field measurement result of a conventional multi-echo spin echo sequence weighting mode. The specific scheme of the magnetic resonance magnetic field measuring method and the magnetic resonance magnetic field measuring device based on the jump echo coding is as described above, the imaging sequence applied during the first scanning and the first stimulation current applied synchronously are shown in the figure 1, and the interference phase gradient area applied by the two adjacent layer selection gradient codes is in a 2-time or 1/2-time relation; the imaging sequence applied in the second scanning is the same as that in the first scanning, but the starting and ending time and the magnitude of the current duration periods corresponding to each group of the second stimulation current and the first stimulation current are identical, and the positive and negative directions of the currents are opposite. The specific imaging sequences and stimulation currents are not described in detail herein, and only specific parameters herein are described below. In the present embodiment, the skip echo parameter n=1, and the acceleration factor m=5.
The experimental results in this example are shown in fig. 2, fig. 3, and fig. 4:
as can be seen from fig. 2, in the magnetic resonance magnetic field measurement experiment on the water model, the magnetic resonance magnetic field measurement method based on the jump echo code has high consistency with the magnetic field measurement result of the conventional multi-echo spin echo sequence weighting mode, meanwhile, the measurement time is shortened by 5 times, and the quantitative analysis further illustrates the effect of the proposed sequence on the magnetic field measurement problem (regression result r=0.98, s=1.02, p <0.001; and Bland-Altman Plot result shows 97% points in the credible interval), thereby illustrating the effectiveness of the invention.
It can be seen from fig. 3 and fig. 4 that the magnetic resonance magnetic field measurement method based on the jump echo code can provide a wide range of imaging coverage capability (z is different layers) in clinically allowable measurement time both on the water model and biological tissue, and no obvious artifact is found in the magnetic field measurement result and the amplitude result, which further proves the effectiveness of the invention.
The above embodiment is only a preferred embodiment of the present invention, but it is not intended to limit the present invention. Various changes and modifications may be made by one of ordinary skill in the pertinent art without departing from the spirit and scope of the present invention. Therefore, all the technical schemes obtained by adopting the equivalent substitution or equivalent transformation are within the protection scope of the invention.
Claims (11)
1. A magnetic resonance magnetic field measuring method based on jump echo coding is characterized in that:
using the same imaging sequenceThe method comprises the steps of scanning target tissues twice, applying a first stimulating current of positive-negative conversion synchronous with refocusing pulses to imaging tissues in a first scanning process and obtaining a first scanning result, and applying a second stimulating current of positive-negative conversion synchronous with refocusing pulses to imaging tissues in a second scanning process and obtaining a second scanning result; calculating a phase difference between the first scan result and the second scan result, and using the phase difference and the second scan result
The linear relationship between the magnetic fields gives +.>
A magnetic field;
the imaging sequence divides all 180-degree refocusing pulses in each repetition period in the rapid spin echo sequence into a jump-back wave coding module and a rapid spin echo sampling module, wherein the first n 180-degree refocusing pulses belong to the jump-back wave coding, the rest 180-degree refocusing pulses belong to the rapid spin echo sampling part, and n is a positive integer not less than 1; in the jump echo coding module, 180-degree refocusing pulses in the module and layer gradient codes corresponding to the 180-degree refocusing pulses in the module are consistent with the fast spin echo sequence, but phase gradient codes and frequency gradient codes after each 180-degree refocusing pulse in the module are cancelled; the rapid spin echo sampling module samples signals according to a rapid spin echo mode and fills the signals into k space, and the first frequency gradient code in each repeated period is the first frequency gradient code in the rapid spin echo sampling module;
in each repetition period, the first stimulating current is continuously from the beginning of the first 90-degree radio frequency pulse to the end of the first frequency gradient coding, and is only set to zero during the application period of each 90-degree and 180-degree radio frequency pulse, the current in one current continuous section between two adjacent radio frequency pulses is always the same as the positive and negative directions, and the current in any two adjacent current continuous sections is the same but opposite to the positive and negative directions; the first stimulation current and the second stimulation current have the same number of current duration sections in each repetition period and are in one-to-one correspondence with each current duration section, and the starting and ending time of each corresponding current duration section is identical but the currents are different.
2. The magnetic resonance magnetic field measurement method based on the jump echo coding as claimed in claim 1, wherein: the imaging sequence is realized in the following way:
in each repetition period of the imaging sequence, firstly, applying a 90-degree radio frequency pulse and simultaneously applying a layer selection coding gradient, then, applying a pre-refocusing frequency gradient to enable spins to restore to be in phase at an echo center, and then, executing a jump echo coding module and a fast spin echo sampling module; in the jump echo coding module, n 180-degree refocusing pulses are applied at intervals, and layer selection gradient coding is performed while each 180-degree refocusing pulse is applied; in the rapid spin echo sampling module, m 180-degree refocusing pulses are applied at intervals, layer selection gradient coding is carried out while each 180-degree refocusing pulse is applied, phase gradient coding is carried out after layer selection gradient coding, frequency gradient coding is carried out after phase gradient coding, and k-space data are obtained through echo signal sampling while frequency gradient coding; the two sides of n+m layer selection gradient codes applied in the jump echo coding module and the quick spin echo sampling module are provided with phase disturbing gradients, wherein the area of the phase disturbing gradient at the two sides of the first layer selection gradient code is set to be a numerical value capable of causing 4 pi phase dispersion, the two sides of any two adjacent layer selection gradient codes in the n+m layer selection gradient codes are applied with phase disturbing gradients with different areas, and the area of the phase disturbing gradient at the two sides of any one layer selection gradient code in the n+m layer selection gradient codes is not lower than the area of the phase disturbing gradient at the two sides of the first layer selection gradient code; finally, reconstructing an amplitude image and a phase image of each scanning through Fourier transform calculation on the k-space number obtained by sampling; n and m are each a positive integer of not less than 1.
3. The magnetic resonance magnetic field measurement method based on the jump echo coding as claimed in claim 2, wherein: the area relation of the disturbing phase gradients at two sides of any two adjacent layer selection gradient codes in the n+m layer selection gradient codes is preferably 2 or 1/2 times.
4. The magnetic resonance magnetic field measurement method based on the jump echo coding as claimed in claim 1, wherein: the phase difference and the phase difference are utilized
The linear relationship between the magnetic fields gives +.>
The calculation formula of the magnetic field is as follows:
wherein:
representation dot->
The unit external current is generated +.>
Magnetic field, unit: nT/mA, < >>
Representing that said first scan result and said second scan result are at the spot +.>
Phase difference of->
Is hydrogen gyromagnetic ratio, +.>
For the duration of the stimulus current in one repetition period,/for the duration of the stimulus current in one repetition period>
And->
The first stimulating current and the second stimulating current are respectively distinguished from each other.
5. The magnetic resonance magnetic field measurement method based on the jump echo coding as claimed in claim 4, wherein: duration of the stimulus current in the one repetition period
The calculation formula of (2) is as follows:
wherein: ESP is the time interval between adjacent 180 refocusing pulses,
、/>
the time of action of a single 180 refocusing pulse and a single 90 radio frequency pulse, respectively.
6. The magnetic resonance magnetic field measurement method based on the jump echo coding as claimed in claim 2, wherein: n is 1-3; the m is not more than 10.
7. The magnetic resonance magnetic field measurement method based on the jump echo coding as set forth in claim 6, wherein: n is 1; and m is 5.
8. The magnetic resonance magnetic field measurement method based on the jump echo coding as claimed in claim 1, wherein: in each repetition period, the starting and ending time and the magnitude of each corresponding current duration of each group of the first stimulation current and the second stimulation current are identical, but the positive direction and the negative direction of the current are opposite.
9. The magnetic resonance magnetic field measurement method based on the jump echo coding as claimed in claim 1, wherein: the first current duration is preferably the maximum value within the safe range of the external current allowed to be applied by the target tissue, among the first and second stimulation currents.
10. A magnetic resonance magnetic field measurement apparatus based on a jump echo code for implementing the magnetic resonance magnetic field measurement method according to any one of claims 1 to 9, characterized by comprising a magnetic resonance device, an external electrical stimulation device and a calculation module;
the external electrical stimulation device is used for applying the first stimulation current and the second stimulation current to target tissue in the scanning process of the magnetic resonance device;
the magnetic resonance apparatus is configured to perform the imaging sequence and obtain a first scan result and a second scan result;
the calculation module is used for calculating the phase difference between the first scanning result and the second scanning result and utilizing the phase difference and the second scanning result
The linear relationship between the magnetic fields gives +.>
A magnetic field.
11. The apparatus of claim 10, wherein the magnetic resonance device, during execution of the imaging sequence to scan a target tissue, sends a first synchronization signal to the external electrical stimulation device prior to application of the 90 ° radio frequency pulses, sends a second synchronization signal to the external electrical stimulation device prior to application of each 180 ° refocusing pulse of the skip echo encoding module, and sends a third synchronization signal to the external electrical stimulation device prior to application of the first 180 ° refocusing pulse of the echo signal sampling module;
the external electric stimulation equipment starts to apply a first current continuous section to the target tissue according to the set time delay after receiving the first synchronous signal;
after the external electric stimulation equipment receives the second synchronous signal each time, applying the next current continuous section with the positive and negative directions opposite to those of the previous current continuous section to the target tissue according to the set time delay;
and after receiving the third synchronous signal, the external electric stimulation equipment applies a last current continuous section with the positive and negative directions opposite to those of the last current continuous section to the target tissue according to the set time delay.
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