CN107024670B - Correction method and device for magnetic resonance system - Google Patents
Correction method and device for magnetic resonance system Download PDFInfo
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Abstract
The invention discloses a correction method of a magnetic resonance system, wherein the magnetic resonance system comprises a radio frequency transmitting module, a gradient module and a radio frequency receiving module, and the correction method specifically comprises the following steps: executing a first pulse sequence, acquiring a first group of echo signals and acquiring the frequency domain phase of the first group of echo signals, wherein the first pulse sequence is a spin echo sequence; executing a second pulse sequence, acquiring a second group of echo signals and acquiring the frequency domain phase of the second group of echo signals, wherein the second pulse sequence is a non-layer-selection spin echo sequence; acquiring system relative delay according to the frequency domain phases of the first group of echo signals and the second group of echo signals, wherein the system relative delay comprises the relative delay of a radio frequency transmitting module and a gradient module and the relative delay of a radio frequency receiving module and the gradient module; and performing compensation correction on the magnetic resonance system according to the relative time delay of the system. The invention can accurately acquire the relative time delay of the magnetic resonance system through the phase operation of the echo signals. In addition, the invention also provides a correction device of the magnetic resonance system.
Description
[ technical field ] A method for producing a semiconductor device
The invention relates to the technical field of magnetic resonance imaging, in particular to a correction method and a correction device of a magnetic resonance system.
[ background of the invention ]
Magnetic Resonance Imaging (MRI) is a novel medical Imaging technique which mainly utilizes the nuclear Magnetic Resonance phenomenon of certain atomic nuclei in human tissues, processes the obtained radio-frequency signals through an electronic computer and reconstructs an image of a certain layer of a human body. The strength of the magnetic resonance signal is generally influenced by various factors such as the proton density of the tissue, the T1 value, the T2 value and the like, and the imaging quality can be improved by adjusting imaging parameters such as a radio frequency pulse, a gradient field, signal acquisition time and the like. Generally, the settings of relevant parameters such as radio frequency pulse, gradient field, signal acquisition time and the like and the arrangement of the relevant parameters in time sequence are collectively called as an MRI pulse sequence and respectively correspond to a radio frequency transmitting module, a gradient module and a radio frequency receiving module of the system.
In order to obtain an accurate scan structure, precise control of the individual modules is required. However, in an actual hardware system, different delays exist in the release of the radio frequency module, the gradient module and the receiving module in the magnetic resonance imaging sequence, so that a time difference exists between an actually occurring physical time sequence and an ideal time sequence in the magnetic resonance imaging sequence, for example, a filtering circuit or an amplifying circuit and a transmission channel existing in each module may cause the gradient module to generate a gradient link delay, the radio frequency transmitting module to generate a transmitting link delay, and the radio frequency receiving module to generate a receiving link delay, which affects the accuracy of the magnetic resonance imaging sequence time sequence, thereby affecting the signal-to-noise ratio and the contrast of an image, and even causing various artifacts. Therefore, it is necessary to calibrate the delays of the various modules of the system to improve MRI image quality.
The most direct method in the prior art is to measure the time delay of each subsystem (radio frequency transmitting module, gradient module and radio frequency receiving module), measure the time delay generated by each link command issuing and the actual electromagnetic field by using peripheral equipment, and then compensate the time delay of each sub-module to enable the system to synchronously work on time. However, this method is complicated, costly and extremely prone to introduce measurement system errors. The other time delay compensation method calculates the relative time delay of each subsystem by using the acquired magnetic resonance signals, and comprises the following specific processes: and obtaining an echo signal by using a Spin Echo (SE) or gradient echo (GRE) sequence, then fitting and calculating an actual time point of an echo center, and subtracting a theoretical time point from the actual time point to obtain the delay quantity of the gradient field in the system. However, the accuracy of the signal acquired by this method is easily affected by non-idealities, for example, in the case of B0 field non-uniformity, the results of the GRE sequence and SE sequence are greatly affected; the sampling interval of the ADC is not infinitesimally small when data is acquired, and the accuracy can only be controlled at the level of the sampling interval by calculating the position of the maximum value, and the accuracy is to be further improved. With the above analysis, it is very difficult to accurately measure the absolute delay of each subsystem link, so there is a need for an improvement on the existing method for measuring or compensating the delay of the magnetic resonance system.
[ summary of the invention ]
The invention aims to provide a correction method and a correction device of a magnetic resonance system, which can accurately correct the relative time delay among modules of the system and improve the imaging quality.
The technical scheme adopted by the invention for solving the technical problems is as follows: a correction method of a magnetic resonance system, the magnetic resonance system comprises a radio frequency transmitting module, a gradient module and a radio frequency receiving module, and the correction method specifically comprises the following steps:
executing a first pulse sequence, acquiring a first group of echo signals and acquiring the frequency domain phase of the first group of echo signals, wherein the first pulse sequence is a spin echo sequence;
executing a second pulse sequence, acquiring a second group of echo signals and acquiring the frequency domain phase of the second group of echo signals, wherein the second pulse sequence is a non-layer-selection spin echo sequence;
acquiring system relative delay according to the frequency domain phases of the first group of echo signals and the second group of echo signals, wherein the system relative delay comprises the relative delay of a radio frequency transmitting module and a gradient module and the relative delay of a radio frequency receiving module and the gradient module;
and performing compensation correction on the magnetic resonance system according to the relative time delay of the system.
Further, the first pulse sequence comprises three radio frequency pulses of 90 °, 180 °, two slice selection gradients, two readout gradients and one readout pre-interspersed phase gradient, and the second pulse sequence comprises three radio frequency pulses of 90 °, 180 °, two readout gradients and one readout pre-interspersed phase gradient.
Further, the relative delay between the radio frequency transmitting module and the gradient module is proportional to the difference between the slopes of the frequency domain phases of the first group of echo signals and the second group of echo signals, the relative delay between the radio frequency receiving module and the gradient module is proportional to the slope of the frequency domain phases of the second group of echo signals, and the slope of the frequency domain phases of the echo signals is the first derivative of the frequency domain phases of the echo signals with respect to the off-center distance of the imaging voxels.
Further, the gradient fields in the first pulse sequence or the second pulse sequence are positive along the X-axis.
Further, the specific process of performing compensation correction on the magnetic resonance system according to the relative delay of the system comprises: and selecting the gradient module as reference, and respectively correcting the time delay of the radio frequency transmitting module and the radio frequency receiving module relative to the gradient module according to the relative time delay of the system.
Further, the specific process of performing compensation correction on the magnetic resonance system according to the relative delay of the system comprises: and acquiring the relative time delay of the radio frequency transmitting module and the radio frequency receiving module according to the relative time delay of the system, and respectively correcting the time delay of the radio frequency transmitting module and the time delay of the gradient module relative to the radio frequency receiving module by taking the radio frequency receiving module as a reference.
Further, the first set of echo signals and/or the second set of echo signals do not contain free induction decay signals.
Further, when the gradient field is along the X axis, the Y axis and the Z axis, the relative delay between the radio frequency receiving module and the gradient module or the relative delay between the radio frequency transmitting module and the gradient module are calculated respectively, the relative delay of the gradient field of the gradient module along the X axis, the Y axis and the Z axis is obtained, and the gradient module is corrected according to the relative delay.
Further, the specific process of performing compensation correction on the magnetic resonance system according to the relative delay of the system comprises:
obtaining gradient link delay corresponding to the gradient module; respectively acquiring transmitting link delay corresponding to a radio frequency transmitting module and receiving link delay corresponding to a radio frequency receiving module according to the relative delay of the system and the gradient link delay; and performing compensation correction on the magnetic resonance system according to the gradient link delay, the transmitting link delay and the receiving link delay.
The present invention also provides a calibration apparatus for a magnetic resonance system, comprising:
the device comprises a detection unit, a data processing unit and a processing unit, wherein the detection unit is used for executing a scanning sequence and acquiring echo signals, the scanning sequence comprises a first pulse sequence and a second pulse sequence, the first pulse sequence is a spin echo sequence and is used for generating a first group of echo signals, and the second pulse sequence is a non-layer-selection spin echo sequence and is used for generating a second group of echo signals;
a relative delay acquiring unit, configured to acquire a slope of a frequency domain phase of the echo signal according to the first group of echo signals and the second group of echo signals, and acquire a relative delay between the radio frequency transmitting module and the gradient module of the magnetic resonance system and a relative delay between the radio frequency receiving module and the gradient module according to the slope of the frequency domain phase of the echo signal;
a link delay obtaining unit, configured to obtain a gradient link delay corresponding to the gradient module, and obtain a transmit link delay corresponding to the radio frequency transmitting module and a receive link delay corresponding to the radio frequency receiving module according to the relative delay and the gradient link delay;
and the correction module is used for compensating and correcting the scanning parameters of the magnetic resonance system according to the gradient link delay, the transmitting link delay or the receiving link delay.
Compared with the prior art, the invention has the advantages that: according to the method, relative time delay exists between a radio frequency module and a gradient module of a magnetic resonance system and between a receiving module and the gradient module, and the relative time delay among the modules can cause phase deviation of an echo signal in a linear relation with the relative time delay, so that the magnetic resonance signal is collected and the phase of the signal in a frequency domain space is obtained, the relative time delay among the modules is directly obtained through phase operation of the echo signal, detection errors caused by sampling intervals can be effectively avoided, the measurement result is more accurate, and the influence of factors such as B0 field nonuniformity is not easy to affect.
[ description of the drawings ]
FIG. 1 is a schematic diagram of a magnetic resonance system;
FIG. 2 is a flow chart of a calibration method for a magnetic resonance system according to the present invention;
FIG. 3 is a diagram illustrating a first pulse sequence structure according to an embodiment of the present invention;
FIG. 4 is a diagram illustrating a second pulse sequence structure according to an embodiment of the present invention;
FIG. 5 is a schematic diagram of phases of echo signals obtained according to a scanning sequence with different delays;
fig. 6 is a diagram illustrating the relationship between the slope of the phase of the echo signal obtained from fig. 5 and the corresponding delay function.
[ detailed description ] embodiments
In order to make the aforementioned objects, features and advantages of the present invention comprehensible, specific embodiments accompanied with figures and examples are described in detail below.
As shown in fig. 1, the magnetic resonance system hardware mainly includes: the magnetic spectrometer system comprises a magnet module 100, a gradient module 200, a radio frequency module 300, a spectrometer system 400, a computer system 500 and other auxiliary systems, wherein the magnet module 100 is used for generating a main magnetic field, and the gradient module 200 mainly comprises a gradient current Amplifier (AMP) and a gradient coil; the rf module 300 mainly includes an rf transmitting module and an rf receiving module; the spectrometer system 400 mainly comprises a pulse sequence generator, a gradient waveform generator, a transmitter and a receiver, etc., and the computer system 500 is used for controlling the system operation and final imaging, and the general processes of the imaging are as follows: the computer system 500 stores and transmits a scan sequence (scan sequence) command to be executed, the pulse sequence generator controls the gradient waveform generator and the transmitter according to the scan sequence command, the gradient waveform generator outputs a gradient pulse signal with a predetermined timing sequence and waveform, the signal passes through Gx, Gy and Gz gradient current amplifiers and then passes through three independent channels Gx, Gy and Gz in the gradient module 200, each gradient amplifier excites a corresponding gradient coil in the gradient coil set to generate a gradient field for generating a corresponding spatial encoding signal, so as to spatially position the magnetic resonance signal; the pulse sequencer in the spectrometer system 400 also executes a scan sequence, outputs data including timing, intensity, shape, etc. of the rf pulses transmitted by the rf, and timing of the rf reception and the length of the data acquisition window to the transmitter, and at the same time, the transmitter transmits the corresponding rf pulses to the body transmitting coil in the rf module 300 to generate a B1 field, signals emitted from the excited nuclei in the patient's body under the B1 field are sensed by the receiving coil in the rf module 300, and then transmitted to the preamplifier through the transmit/receive switch, and the amplified magnetic resonance signals are subjected to digital processing such as demodulation, filtering, AD conversion, etc., and then transmitted to the storage module of the computer system 500. After the storage module acquires a set of raw k-space data, the scan is finished. The original k-space data is rearranged into separate k-space data sets corresponding to each image to be reconstructed, and each k-space data set is input to an array processor for image reconstruction and then combined with the magnetic resonance signals to form a set of image data. In the imaging process, the setting and the time sequence arrangement of relevant parameters such as radio frequency pulse, gradient field, signal acquisition time and the like become an MRI pulse sequence. In order to improve the electrical performance, a filter is usually adopted in the design of the waveform generator or the amplifier, however, the use of the filter inevitably causes the waveform signal in the transmission channel to be delayed, and in addition, the self-induction from the cable or other components also causes certain system delay. Therefore, in an actual hardware system, the gradient module 200 may have a certain gradient link delay, the radio frequency transmitting module may have a certain transmitting link delay, the radio frequency receiving module or the signal collecting module may also have a certain receiving link delay, and the link delay of each module finally causes inaccurate physical timing sequence of the imaging sequence or system delay.
The invention provides a calibration method of a magnetic resonance system, which detects the relative time delay of a gradient module and a radio frequency transmitting module as well as the relative time delay of the gradient module and a radio frequency receiving module, and mainly comprises the following steps as shown in figure 2:
and S10, executing a first pulse sequence, acquiring a first group of echo signals and acquiring the frequency domain phase of the first group of echo signals, wherein the first pulse sequence is a spin echo sequence. Fig. 3 is a schematic structural diagram of a first pulse sequence, which includes a 90 ° rf pulse, two subsequent 180 ° rf pulses, two slice selection gradients Gs, two readout gradients Gr, and a readout pre-dispersed phase gradient Gr/2, where the directions of the gradient fields are the same and are all applied to the same gradient axis, the gradient fields may be along any one of the X axis (pointing to the left side of the anatomical location of the human body), the Y axis (pointing to the front side of the anatomical location of the human body), or the Z axis (long axis of the human body), and the gradient fields are respectively along the X axis gradient field (Gx), the Y axis gradient field (Gy), and the Z axis gradient field (Gz). The gradient field used in this embodiment is Gx, the gradient axis is x-axis, and the above-mentioned pulse sequence can be used to obtain a first group of echo signals within the time range of applying the readout gradient, where the first group of echo signals includes echo signals 1 and 2, where echo signal 1 is generated within the range of the first readout gradient GrThe generator is influenced by the time delay between the radio frequency transmitting module and the gradient module and the time delay between the radio frequency receiving module and the gradient module; the echo signal 2 is generated in the range of the second readout gradient Gr, and due to the use of two 180 ° rf pulses, the phase shift of the echo signal caused by the delay between most of the mr system rf transmitting module and the gradient module can be cancelled out, that is, the phase shift of the echo signal caused by the delay is negligible. In addition, the intensity S of the echo signal 1 is added to the influence of the pre-dispersed phase gradient on the echo signalE1And the intensity S of the echo signal 2E2Can be expressed as:
wherein FOV is an imaging visual field, rho represents the signal density in a unit volume, x represents the off-center distance of an imaging voxel, gamma is the gyromagnetic ratio, t represents the acquisition time of an echo signal, and Gs and Gr represent the intensities of a slice selection gradient and a readout gradient respectively. T isD1For relative delay of the RF transmitting module and the gradient module of the magnetic resonance system, the existence of the magnetic resonance signal and T in the echo signal at each position along the gradient axisD1Linearly related phase shift 2Gs · x · γ · TD1，TD2The relative time delay of the radio frequency receiving module and the gradient module of the magnetic resonance system, the existence of the magnetic resonance signal and T in each position along the gradient field axis direction in the echo signalD2Linearly dependent phase shift Gr · x · γ · TD2If Δ M is a possible variation in the integrated area of the pre-dispersed phase gradient Gr/2, a phase variation of-x · γ · Δ M occurs in the echo signal 1, and a phase variation of + x · γ · Δ M occurs in the echo 2. The phase of the echo signals in the frequency domain can be obtained by performing Fourier transform on the first group of echo signals.
And S20, executing a second pulse sequence, collecting a second group of echo signals and acquiring the frequency domain phase of the second group of echo signals, wherein the second pulse sequence is a non-layer-selection spin echo sequence. Such asFig. 4 shows a schematic diagram of a second pulse sequence comprising a 90 ° rf pulse followed by two 180 ° rf pulses, two readout gradients Gr and a readout pre-dispersed phase gradient Gr/2, the gradient fields being applied in the same direction as the gradient fields applied in the first pulse sequence, but being different from the first pulse sequence in that the second pulse sequence does not comprise two slice selection gradients. And obtaining a second group of echo signals within the time range of applying the readout gradient by adopting the pulse sequence, wherein the second group of echo signals also comprises echo signals 3 and 4, the echo signal 3 is generated within the range of the first readout gradient Gr, the echo signal 4 is generated within the range of the second readout gradient Gr, and the two echo signals are influenced by the relative delay between a receiving module and a gradient module of the magnetic resonance system and the readout of the predispersion phase gradient. Intensity S of echo signal 3E3And the intensity S of the echo signal 4E4Can be expressed as:
wherein FOV is an imaging visual field, rho represents the signal density in a unit volume, x represents the off-center distance of an imaging voxel, gamma is a gyromagnetic ratio, t represents the acquisition time of an echo signal, and Gr represents the strength of a readout gradient. T isD2In the relative delay between the receiving module and the gradient module of the magnetic resonance system, Δ M is the possible deviation of the integrated area of the pre-dispersed phase gradient Gr/2. Similarly, the phase of the echo signals in the frequency domain may be obtained by performing a Fourier transform on the second set of echo signals.
According to the analysis, a certain corresponding relation exists between the acquired echo signals and the relative delay of each module, in order to verify the correctness of the assumption, the invention sets a series of scanning sequences to act on a magnetic resonance system, the relative delay of a gradient module and a radio frequency emission module is respectively set to be-8 mu s, -4 mu s, 0 mu s, 4 mu s and 8 mu s in time sequence of each scanning sequence, and other scanning parameters are the same or the same as the ideal time sequence. In the magnetic resonance systemThe system signal receiving end can acquire a series of echo signals, transform the echo signals to a frequency domain and acquire the frequency domain phase of each echo signal as shown in the figure
And the relation with the off-center distance x of the imaging voxel is shown schematically. As shown in FIG. 5, the abscissa represents the off-center distance x (mm) of the imaging voxel, and the ordinate represents the phase of the echo signal in the frequency domain
(°), the phase of the echo signal belonging to the same scanning sequence and x are fitted to be in a linear relation, and the echo signals of different scanning sequences (different delays) and x meet different linear relations (show different slopes). Further, according to fig. 5, a relationship between a slope of a frequency domain phase of the echo signal and a corresponding delay can be obtained, taking a delay (μ s) of the scanning sequence as an abscissa and a phase slope (degree/mm) of the echo signal in the frequency domain obtained by the corresponding scanning sequence as an ordinate, and when relative delays of the gradient module and the radio frequency transmission module are-8 μ s, -4 μ s, 0 μ s, 4 μ s, and 8 μ s, respectively, scattered points formed by phase slopes of the corresponding echo signal can be obtained, and fitting is performed, such that the slope of the frequency domain phase of the echo signal and the corresponding delay conform to a linear function relationship as shown in fig. 6.
Thus, the relative time delay of the magnetic resonance system can be obtained by: detecting any two scanning sequences to obtain two groups of echo signals, wherein the relative delay parameters of the systems of the two scanning sequences are different and are known values; acquiring the phases of the two groups of echo signals in the frequency domain, and acquiring the slope of the phase of the frequency domain of the echo signals according to the phases of the two groups of echo signals in the frequency domain; obtaining a linear function relation between the relative delay of the system and the slope of the frequency domain phase of the echo signal according to the slopes of the frequency domain phases of the two groups of echo signals; executing a current scanning sequence to obtain a current echo signal, wherein the system delay corresponding to the current echo sequence is unknown; converting the current echo signal to a frequency domain, and acquiring the slope of the current echo signal in the frequency domain; the relative delay of the current system can be accurately obtained according to the linear function relationship between the relative delay of the system and the slope of the frequency domain phase of the echo signal and the slope of the current echo signal in the frequency domain; compensation timing parameters can then be set on the software side or in the spectrometer system based on the current system relative delay.
And S30, acquiring the relative delay of the system according to the frequency domain phases of the first group of echo signals and the second group of echo signals, wherein the relative delay of the system comprises the relative delay of the radio frequency transmitting module and the gradient module and the relative delay of the radio frequency receiving module and the gradient module. The specific process is as follows: according to the phase positions of the first group of echo signals and the second group of echo signals in the frequency domain, acquiring the functional relation between the phase position of the frequency domain and the off-center distance x of the imaging voxel
Wherein sigma1Representing the zero order offset, the slope of the frequency domain phase of the echo signal 1 (first derivative of the frequency domain phase with respect to x)
Is (2 Gs.T)D1+Gr·TD2- Δ M) γ; frequency domain phase of echo signal 2 as a function of x
Wherein sigma2Representing the zero order offset and the slope of the frequency domain phase of the echo signal 2 (first derivative of the frequency domain phase with respect to x)
Is (Gr. T)D2+ Δ M) γ. Fourier transform is carried out on the echo signals 3 and 4 obtained by collection to obtain the functional relation between the frequency domain phase and x
Wherein sigma3Representing the zero order offset and the slope of the frequency domain phase of the echo signal 3 (first derivative of the frequency domain phase with respect to x)
Is (Gr. T)D2- Δ M) γ; frequency domain phase of echo signal 4 as a function of x
Wherein sigma4Representing the zero order offset and the slope of the frequency domain phase of the echo signal 4 (first derivative of the frequency domain phase with respect to x)
Is (Gr. T)D2+ Δ M) γ. From the above information, it is possible to obtain: relative time delay between radio frequency emission module and gradient module
The relative delay of the radio frequency transmitting module and the gradient module is in direct proportion to the difference between the slope of the frequency domain phase of the first group of echo signals and the slope of the frequency domain phase of the second group of echo signals; relative time delay between radio frequency receiving module and gradient module
Namely, the relative delay of the radio frequency receiving module and the gradient module is proportional to the slope of the frequency domain phase of the second group of echo signals.
In the embodiment of the invention, the above process is adopted to respectively obtain the time delay between the radio frequency transmitting module and the gradient module of the gradient field along the X axis and the time delay between the radio frequency receiving module and the gradient module of the gradient field along the X axis, theoretically, the relative time delay between the gradient module of the gradient field along the X axis and the radio frequency transmitting module can be preferentially corrected, and then the relative time delay between the gradient module of the gradient field along the X axis and the radio frequency receiving module can be corrected. Similar to the above process, the relative time delay between the rf transmitting module, the rf receiving module and the corresponding gradient module in the magnetic resonance system with the gradient field along the Y-axis or the Z-axis can also be obtained separately. In addition, the relative delay of the gradient modules along three different gradient field directions can be obtained according to the structure, and the gradient modules are corrected according to the relative delay.
In another embodiment, the RF receiving module is selected as a reference according to the relative delay T between the RF receiving module and the gradient moduleD2Adjusting the timing of the gradient module, wherein the gradient field direction of the gradient module is along the X-axis, Y-axis or Z-axis. Furthermore, the relative time delay T between the RF transmit module and the gradient module in the MR system according to the previous embodimentD1And the relative time delay T between the radio frequency receiving module and the gradient moduleD2The relative time delay T between the radio frequency transmitting module and the radio frequency receiving module can be obtainedD3＝TD1-TD2And then, taking the radio frequency receiving module as a reference to adjust the corresponding parameters of the radio frequency transmitting module.
After the radio frequency pulse excitation, the magnetization vector M0 in the thermal equilibrium state is partially or completely overturned to the transverse plane of the vertical magnetic field, the spins are not completely positioned on the expected resonance frequency due to the factors of local magnetic field nonuniformity, chemical shift and the like, and the off-resonance phenomenon enables the transverse magnetic vectors not to be positioned in the same direction any more along with the time, so that the vector sum of the transverse magnetic vectors is reduced, the signal intensity is reduced, and the acquired spin echo signal is easily influenced by a Free Induction Decay (FID) signal.
In another embodiment of the invention, to reduce GsThe effect of uncertainty in magnitude on the system delay measurement, the system delay being the mean of the system delays measured in the positive and negative gradient directions. The method specifically comprises the following steps: respectively acquiring a first group of echo signals and a second group of echo signals by adopting a first pulse sequence and a second pulse sequence, wherein the first pulse sequence comprises three radio frequency pulses of 90 degrees, 180 degrees and 180 degrees, two layer selection gradients Gs, two readout gradients Gr and a single pre-dispersed phase gradient Gr/2, the second pulse sequence comprises three radio frequency pulses of 90 degrees, 180 degrees and 180 degrees, two readout gradients Gr and a readout pre-dispersed phase gradient Gr/2, the directions of all gradients are positive directions, and the magnetic resonance system delay in the positive gradient direction can be obtained according to the method; the three radio frequency pulses of the third pulse sequence comprise three radio frequency pulses of 90 DEG, 180 DEG and 180 DEG, two slice selection gradients Gs, two readout gradients Gr and one readout pre-dispersed phase gradient Gr/2, the fourth pulse sequence comprises three radio frequency pulses of 90 DEG, 180 DEG and 180 DEG, two readout gradients Gr and one readout pre-dispersed phase gradient Gr/2, and the third pulse sequence comprises a third pulse sequence and a fourth pulse sequenceThe direction of the gradient in the three-pulse sequence is opposite to the direction of the gradient in the first sequence, namely the direction of the negative gradient; wherein the third pulse sequence generates a third set of echo signals, the fourth set of pulse sequences can generate a fourth set of echo signals, and the system delay measured when the gradient field is in the negative direction along the X axis can be obtained according to the method; further, the average value of the system time delays measured in the directions of the positive and negative gradient fields is taken as a final measurement result. By adopting the method, the interference of non-ideal factors such as non-uniform main magnetic field on the measurement result is effectively reduced.
And S40, correspondingly compensating the magnetic resonance imaging system according to the relative system delay. According to the detection result, the relative advance time of the three modules is logically preset to compensate the time delay of the magnetic resonance system, in one embodiment, the time delay of the radio frequency transmitting module relative to the gradient module can be corrected according to the relative time delay between the gradient module and the radio frequency transmitting module by taking the gradient module as a reference, and then the time delay of the radio frequency receiving module relative to the gradient module can be corrected according to the mutual time delay between the gradient module and the radio frequency receiving module by taking the gradient module as a reference. The operation can effectively ensure the accuracy of the magnetic resonance signal generated by the magnetic resonance system, improve the signal-to-noise ratio and effectively eliminate the artifact caused by the echo phase fluctuation. In another embodiment, three directions of a gradient field along an X axis, a Y axis or a Z axis can be respectively selected to obtain system relative delays in the three directions, relative delays of a radio frequency transmitting module and a radio frequency receiving module are obtained through the relative delays of the radio frequency transmitting module and the gradient module and the relative delays of the radio frequency receiving module and the gradient module, and delay parameters of the gradient module relative to the radio frequency receiving module and delay parameters of the radio frequency receiving module relative to the radio frequency receiving module in the three directions of the gradient field are respectively corrected by taking the radio frequency receiving module as a reference. It should be noted that the correction of the delay parameter can be specifically accomplished by adjusting the timing of the scan sequence generated in the spectrometer system.
In another embodiment of the present invention, correcting a magnetic resonance system based on system relative delays may be accomplished by:
acquiring gradient link delay corresponding to a gradient module, wherein the gradient link delay can be specifically acquired according to the corresponding relation between the gradient amplitude and the gradient delay, and can also be acquired according to the time when the echo signal peak point appears;
according to the relative delay of the system and the gradient link delay, the transmitting link delay corresponding to the radio frequency transmitting module and the receiving link delay corresponding to the radio frequency receiving module can be respectively obtained;
and performing compensation correction on the magnetic resonance system according to the gradient link delay, the transmitting link delay and the receiving link delay, and specifically, respectively adjusting scanning parameters such as the transmitting time sequence of the radio-frequency pulse, the transmitting time sequence of the gradient pulse and the time for receiving signals in the spectrometer system.
The present invention also provides a calibration apparatus for a magnetic resonance system, comprising:
a detection unit 100, configured to execute a scan sequence and acquire echo signals, where the scan sequence includes a first pulse sequence and a second pulse sequence, and the first pulse sequence is a spin echo sequence and is used to generate a first set of echo signals; the second pulse sequence is a non-layer-selection spin echo sequence and is used for generating a second group of echo signals;
a relative delay obtaining unit 200, connected to the detecting unit 100, for obtaining a slope of a frequency domain phase of the echo signal according to the first group of echo signals and the second group of echo signals, and obtaining a relative delay between the radio frequency transmitting module and the gradient module of the magnetic resonance system and a relative delay between the radio frequency receiving module and the gradient module according to the slope of the frequency domain phase of the echo signal;
a link delay obtaining unit 300, connected to the relative delay obtaining unit 200, for obtaining the gradient link delay corresponding to the gradient module, and obtaining the transmitting link delay corresponding to the radio frequency transmitting module and the receiving link delay corresponding to the radio frequency receiving module according to the relative delay
And the correcting unit 400 is connected with the link delay acquiring unit 300 and is used for performing compensation correction on the scanning parameters of the magnetic resonance system according to the gradient link delay, the transmitting link delay or the receiving link delay.
The above description is only for the purpose of illustrating the preferred embodiments of the present invention and is not to be construed as limiting the invention, and any modifications, equivalents, improvements and the like made within the spirit and principle of the present invention should be included in the scope of the present invention.
Claims (10)
1. A correction method of a magnetic resonance system, the magnetic resonance system comprises a radio frequency transmitting module, a gradient module and a radio frequency receiving module, and the correction method specifically comprises the following steps:
executing a first pulse sequence, acquiring a first group of echo signals and acquiring the frequency domain phase of the first group of echo signals, wherein the first pulse sequence is a spin echo sequence;
executing a second pulse sequence, acquiring a second group of echo signals and acquiring the frequency domain phase of the second group of echo signals, wherein the second pulse sequence is a non-layer-selection spin echo sequence;
acquiring system relative delay according to the frequency domain phases of the first group of echo signals and the second group of echo signals, wherein the system relative delay comprises the relative delay of a radio frequency transmitting module and a gradient module and the relative delay of a radio frequency receiving module and the gradient module;
and performing compensation correction on the magnetic resonance system according to the relative time delay of the system.
2. The method of claim 1, wherein the first pulse sequence comprises a 90 ° rf pulse followed by two 180 ° rf pulses, two slice gradients, two readout gradients, and one readout pre-interspersed phase gradient, and wherein the second pulse sequence comprises a 90 ° rf pulse followed by two 180 ° rf pulses, two readout gradients, and one readout pre-interspersed phase gradient.
3. The calibration method of claim 2, wherein the relative delay between the radio frequency emission module and the gradient module is proportional to the difference between the slopes of the frequency domain phases of the first group of echo signals and the second group of echo signals, the relative delay between the radio frequency reception module and the gradient module is proportional to the slope of the frequency domain phases of the second group of echo signals, the slope of the frequency domain phases of the first group of echo signals is the first derivative of the frequency domain phases of the first group of echo signals with respect to the off-center distance of the imaging voxel, and the slope of the frequency domain phases of the second group of echo signals is the first derivative of the frequency domain phases of the second group of echo signals with respect to the off-center distance of the imaging voxel.
4. A method of calibrating a magnetic resonance system according to claim 2, characterized in that the gradient fields in the first or second pulse sequence are positive along the X-axis.
5. The method for calibrating a magnetic resonance system according to claim 1, wherein the specific process of performing compensation calibration on the magnetic resonance system according to the relative delay of the system comprises: and selecting the gradient module as reference, and respectively correcting the time delay of the radio frequency transmitting module and the radio frequency receiving module relative to the gradient module according to the relative time delay of the system.
6. The method for calibrating a magnetic resonance system according to claim 1, wherein the specific process of performing compensation calibration on the magnetic resonance system according to the relative delay of the system comprises: and acquiring the relative time delay of the radio frequency transmitting module and the radio frequency receiving module according to the relative time delay of the system, and respectively correcting the time delay of the radio frequency transmitting module and the time delay of the gradient module relative to the radio frequency receiving module by taking the radio frequency receiving module as a reference.
7. A method of calibrating a magnetic resonance system according to claim 1, characterized in that the first set of echo signals and/or the second set of echo signals do not contain free induction decay signals.
8. The calibration method of claim 1, wherein the relative delay between the RF receiving module and the gradient module or the relative delay between the RF transmitting module and the gradient module when the gradient field is along the X-axis, the Y-axis and the Z-axis are calculated respectively.
9. The method for calibrating a magnetic resonance system according to claim 1, wherein the specific process of performing compensation calibration on the magnetic resonance system according to the relative delay of the system comprises:
obtaining gradient link delay corresponding to the gradient module; respectively acquiring transmitting link delay corresponding to a radio frequency transmitting module and receiving link delay corresponding to a radio frequency receiving module according to the relative delay of the system and the gradient link delay; and performing compensation correction on the magnetic resonance system according to the gradient link delay, the transmitting link delay and the receiving link delay.
10. A calibration apparatus for a magnetic resonance system, comprising:
the device comprises a detection unit, a data processing unit and a processing unit, wherein the detection unit is used for executing a scanning sequence and acquiring echo signals, the scanning sequence comprises a first pulse sequence and a second pulse sequence, the first pulse sequence is a spin echo sequence and is used for generating a first group of echo signals, and the second pulse sequence is a non-layer-selection spin echo sequence and is used for generating a second group of echo signals;
a relative delay acquiring unit, configured to acquire a slope of a frequency domain phase of the first group of echo signals and a slope of a frequency domain phase of the second group of echo signals according to the first group of echo signals and the second group of echo signals, respectively, and acquire a system relative delay according to the slope of the frequency domain phase of the first group of echo signals and the slope of the frequency domain phase of the second group of echo signals, where the system relative delay includes a relative delay between the radio frequency transmitting module and the gradient module of the magnetic resonance system and a relative delay between the radio frequency receiving module and the gradient module;
a link delay obtaining unit, configured to obtain a gradient link delay corresponding to the gradient module, and obtain a transmit link delay corresponding to the radio frequency transmitting module and a receive link delay corresponding to the radio frequency receiving module according to the system relative delay and the gradient link delay;
and the correction module is used for compensating and correcting the scanning parameters of the magnetic resonance system according to the gradient link delay, the transmitting link delay or the receiving link delay.
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