CN116413649B - A magnetic resonance phase imaging method based on adiabatic radio frequency pulses - Google Patents
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Abstract
The invention discloses a magnetic resonance phase imaging method based on adiabatic radio frequency pulses, which comprises the steps of determining duration, amplitude, frequency and frequency bias of an inversion adiabatic pulse, overturning a magnetization vector to a transverse XY plane, superposing negative frequency bias on the inversion adiabatic pulse, applying the inversion adiabatic pulse, performing MRI sampling to obtain a first phase signal, overturning the magnetization vector to the transverse XY plane, superposing positive frequency bias on the frequency of the inversion adiabatic pulse, applying the inversion adiabatic pulse, performing MRI sampling to obtain a second phase signal, calculating phase change, and calculating according to the phase change to obtain field distribution. The invention improves the phase detection sensitivity, can be further applied to the aspects of magnetic resonance field distribution imaging, temperature imaging, magnetic sensitivity weighted imaging and the like, and has important practical value.
Description
Technical Field
The invention belongs to the field of magnetic resonance imaging pulse sequence methods, and particularly relates to a magnetic resonance phase imaging method based on adiabatic radio frequency pulses.
Background
The magnetic resonance imaging (Magnetic Resonance Imaging, MRI) can detect living tissues in a non-invasive way without ionizing radiation, obtains living image information with high resolution and high contrast, and is an important imaging tool for researching the structure, the function and the metabolism of the living tissues. The data obtained by magnetic resonance sampling are complex data, and a signal intensity image and a phase image can be simultaneously reconstructed. Most of the MRI images used for diagnosis are based on the magnetic resonance signal intensity diagram, and the information such as nuclear spin density, relaxation time and the like of the reaction tissue is usually ignored.
However, the phase information reflects precession frequency information of the spin nuclei. The precession frequency is different and the phase of the evolution of the transverse magnetization vector in echo time is different, so the phase change can be understood as the integral of the spin nuclear precession angular frequency over time. The precession frequency of the spin nuclei is generally influenced by field distribution, temperature and tissue magnetic sensitivity, so that the phase imaging direction is widely applied to the aspects of field distribution imaging, temperature imaging, magnetic sensitivity weighted imaging and the like.
Robinson S et al [B0 mapping with multi-channel RF coils at high field[J].Magnetic resonance in medicine,2011,66(4):976-988.] uses phase imaging for B 0 field detection. Using the linear relationship of precession frequency and B 0 field, the B 0 field distribution can be calculated from the phase image. The method can also be used for B 0 field alignment of chemical exchange saturation transfer magnetic resonance imaging.
ISHIHARA Y et al [Aprecise and fast temperature mapping using water proton chemical shift.Magnetic Resonance in Medicine,1995,34(6):814-823.] propose that phase imaging is applied to magnetic resonance temperature measurement by utilizing the linear relation between a temperature interval and a spin nuclear precession frequency, so that good temperature dependence of phase change is proved, the measurement speed of a phase imaging method is improved compared with that of an intensity imaging method, and the method can be used for monitoring the body temperature change of tumor hyperthermia in real time and the like.
Haacke E M et al [ Susceptibility WEIGHTED IMAGING (SWI) Magnetic Resonance IN MEDICINE,2004,52 (3): 612-618 ] detects tissue magnetic Susceptibility differences using phase imaging, and develops into a magnetic Susceptibility Weighted Imaging (SWI) method, where the anti-magnetic or paramagnetic properties of the tissue cause local magnetic field changes, resulting in corresponding phase changes, and large phase differences at tissue interfaces with large magnetic Susceptibility differences, resulting in stronger SWI contrast, which can effectively detect vascularity, local bleeding, and the like.
At present, the phase imaging is mainly based on a double-echo time gradient echo method, and a rapid small-angle excitation (Fast Low Angle Shot, FLASH) sequence and the like are used. For example, in field distribution measurement, a FLASH sequence is used to acquire two phase images of different echo times, and the two phase images are subtracted to obtain the phase change of each voxel. The magnitude of the phase change satisfies Δphase=γ×Δb 0×(TE1-TE2), where Δphase is the phase change, γ is the gyromagnetic ratio of the spin nuclei, Δb 0 is the field distribution minus the center field strength, and TE 1 and TE 2 are the corresponding two echo times, respectively. Using this relationship, the field distribution can be calculated. The phase imaging method based on FLASH sequence has positive correlation between phase change and echo time difference, and can only further increase phase contrast by increasing echo time difference, and the phase change of temperature imaging and magnetic sensitive weighted imaging is limited by the method.
The invention provides a magnetic resonance phase imaging method based on adiabatic radio frequency pulse, which is a phase imaging method utilizing inversion adiabatic pulse modulation phase change, the obtained phase change is related to frequency bias of the adiabatic radio frequency pulse, and phase change larger than that of a conventional gradient echo sequence can be obtained in the same echo time.
Disclosure of Invention
The present invention aims to solve the above-mentioned problems occurring in the prior art, and to provide a magnetic resonance phase imaging method based on adiabatic radio frequency pulses, which uses adiabatic pulse frequency offset to modulate phase changes.
The above object of the present invention is achieved by the following technical means:
An adiabatic pulse is a special radio frequency pulse whose amplitude and frequency are modulated in time. In a rotating coordinate system, the magnetization vector precesses along the effective B1 field, i.e., the B eff field. The B eff field of the inverted adiabatic pulse is turned from the Z-axis direction to the-Z-axis direction. When the magnetization vector is flipped to the transverse XY plane, a transverse magnetization vector is generated. At this point, the transverse magnetization vector will be flipped 180 ° by applying an inversion adiabatic pulse. In the turning process, the frequency of the reverse adiabatic pulse is modulated by time, and when the transverse magnetization vector precession frequency of a certain voxel and the reverse adiabatic pulse frequency reach resonance, the magnetization vector is turned by 90 degrees until the reverse adiabatic pulse frequency is far away from the transverse magnetization vector precession frequency, and 180-degree turning is completed. Therefore, the magnetization vector whose transverse magnetization vector precession frequency and adiabatic pulse frequency reach resonance first will be inverted first, and a phase difference is generated with the transverse magnetization vector which reaches resonance later. Thus, the frequency bias of the adiabatic pulses may modulate the phase change. When the duration, amplitude and frequency modulation of the adiabatic pulses are determined, the phase difference is determined by the field distribution together with the frequency bias of the adiabatic pulses, as follows:
Δphase=const x y x delta B 0 x delta offset equation (1)
Wherein Δphase is the phase change. Const is a constant and is determined by the duration of the adiabatic pulse and the swept range of the adiabatic pulse. Gamma is the gyromagnetic ratio of the spin nuclei, Δb 0 is the field distribution minus the central field strength, and Δoffset is the phase offset range of the adiabatic pulse.
According to the above principle, a new magnetic resonance imaging pulse sequence is designed. In time sequence design, the magnetization vector is turned to the transverse XY plane by using a conventional pulse, then the phase of the transverse magnetization vector is modulated by applying adiabatic pulse and inverse adiabatic pulse, and finally the phase signal is obtained by sampling.
A magnetic resonance phase imaging method based on adiabatic radio frequency pulses, comprising the steps of:
Step 1, determining the duration, amplitude, frequency and frequency offset of adiabatic pulses, wherein the frequency offset comprises a positive frequency offset and a negative frequency offset;
step 2, turning the magnetization vector to a transverse XY plane, superposing negative frequency bias on the frequency of adiabatic pulse, applying the adiabatic pulse, and performing MRI sampling to obtain a first phase signal;
Step 3, overturning the magnetization vector to a transverse XY plane, superposing positive frequency bias on the frequency of adiabatic pulse, applying the adiabatic pulse, and performing MRI sampling to obtain a second phase signal;
Step4, reconstructing the first phase signal and the second phase signal respectively, performing convolution treatment, and subtracting to obtain phase change delta phase,
And 5, calculating the obtained field distribution delta B 0 based on the following formula according to the obtained phase change delta phase.
Δphase=Const×γ×ΔB0×△offset
Where Δphase is the phase change, const is a constant, γ is the gyromagnetic ratio of the spin nuclei, Δb 0 is the field distribution minus the center field strength, and Δoffset is the frequency offset range of the adiabatic pulse.
In step 5, as described above, const=2×tp/BW, where Tp is the duration of the adiabatic pulse and BW is the sweep range of the adiabatic pulse.
Compared with the prior art, the method has the following beneficial effects:
1. The invention designs a new magnetic resonance imaging pulse sequence and a new phase calculation method, and the phase change can be modulated by the frequency bias of adiabatic pulses;
2. the invention obtains larger phase change than the conventional method in the same echo time, and improves the phase detection sensitivity;
3. the invention can be flexibly applied to various phase imaging methods.
Drawings
FIG. 1 is a flow chart of the method of the present invention;
FIG. 2 is an amplitude versus frequency modulation of adiabatic pulses used in the present invention;
FIG. 3 is a pulse timing diagram of an implementation of the present invention including a conventional pulse to flip the magnetization vector to the transverse XY plane, an adiabatic pulse for phase modulation, and an MRI sampling module, etc., where ω rf is the absolute value of the frequency offset;
FIG. 4 is a schematic diagram of the phase change produced by the adiabatic pulses used in the present invention;
FIG. 5 is a graph of phase change using different frequency offsets of adiabatic pulses according to the present invention, wherein the three graphs from left to right are respectively a + -5000 Hz frequency offset, a + -4000 Hz frequency offset, and a + -3000 Hz frequency offset;
Fig. 6 is a comparison of the phase change pattern obtained by the present invention with the phase change pattern obtained by the conventional FLASH method, wherein the left graph is the phase change pattern obtained by the ± 5000Hz frequency offset of the present invention, and the right graph is the phase change pattern obtained by the conventional FLASH method.
Detailed Description
The method of the present invention will be described in further detail below in conjunction with examples to facilitate the understanding and practice of the method of the present invention by those of ordinary skill in the art, and it is to be understood that the examples described herein are for the purpose of illustration and explanation only and are not intended to limit the present invention thereto.
Examples:
This example was performed on a Bruker 400M magnetic resonance spectrometer (AVANCE 400) using a 1% agarose gel sample filled in a 10mm nuclear magnetic sample tube. The transmission and reception were performed using saddle coils with an inner diameter of 10mm, and the sample temperature was maintained at 298K. Under the condition, the conventional FLASH sequence is respectively used for phase imaging comparison with the pulse sequence designed by the invention. This example is only used to demonstrate the method of the invention, which can be used for samples as well as for living body detection, without being limited by the magnetic resonance spectrometer and the coil type.
The embodiment comprises the following steps:
Step 1, as shown in the flow chart of fig. 1, the duration, amplitude, frequency and frequency offset of the adiabatic pulse are first determined. The present embodiment employs adiabatic pulses of 20ms duration, the amplitude and frequency of which are modulated based on the tanh/tan equation, with a maximum amplitude of 16 μT, the frequency of the adiabatic pulses being superimposed with a frequency bias comprising a positive frequency bias and a negative frequency bias such that the frequency of the adiabatic pulses is swept in the range-6000 to 6000Hz (as shown in FIG. 2). The constant Const in equation (1) is determined based on the duration, amplitude, frequency offset of the adiabatic pulse. In this embodiment, const=2×tp/BW, where Tp is the duration of the adiabatic pulse, i.e. 20ms, and BW is the swept range of the adiabatic pulse, i.e. 12000Hz.
And 2, designing a pulse sequence, namely turning the magnetization vector to a transverse XY plane by using conventional pulses in time sequence, superposing negative frequency bias on the adiabatic pulse determined in the step 1, then applying the adiabatic pulse with the superposed negative frequency bias, and finally performing MRI sampling. The first phase signal is obtained by sampling using a designed pulse sequence (pulse sequence is shown in fig. 3) and adiabatic pulses, with the adiabatic pulse frequency offset set to a negative frequency offset (this embodiment employs negative frequency offsets of-3000 Hz, -4000Hz and-5000 Hz, respectively). The sampling parameters of this example are TR time 1s, TE time 25ms, repetition number 1, sampling matrix 128×128, monolayer, layer thickness 2mm.
Step 3, using the designed pulse sequence and adiabatic pulse, turning the magnetization vector to the transverse XY plane by using the conventional pulse in time sequence, superposing the positive frequency bias on the adiabatic pulse determined in the step 1, then applying the adiabatic pulse after superposing the positive frequency bias, and finally performing MRI sampling, setting the frequency offset of the adiabatic pulse to be positive frequency offset (3000 Hz, 4000Hz and 5000Hz positive frequency offset are adopted in the embodiment), wherein the positive frequency offset is the same as the negative frequency offset absolute value set in the step 2, sampling to obtain a second phase signal, and the sampling parameters are consistent with the step 2. The phase modulation principle is shown in fig. 4, and in the sweep frequency range of the adiabatic pulse, the phase is proportional to the frequency offset of the adiabatic pulse, and thus the phase change is proportional to the frequency offset difference.
And 4, using a FLASH sequence as a control of the method. The sampling parameters are TR time 1s, the repetition number 1, the sampling matrix 128×128, the monolayer and the layer thickness 2mm. The long TE experiment echo time is set to 25ms, the third phase signal is obtained by sampling, the short TE experiment echo time is set to 5ms, and the fourth phase signal is obtained by sampling.
And 5, reconstructing the first to fourth phase signals obtained by sampling in the steps 1,2, 3 and 4, constructing a K space according to a phase coding sequence, obtaining complex image domain data after fast Fourier transformation, taking the argument of the complex value corresponding to each voxel of the image domain data, and calculating to obtain a phase value corresponding to each voxel, thereby respectively reconstructing the reconstructed first to fourth phase signals. The radial angle exceeds 2 pi radians to be folded, and the phase deconvolution algorithm is used for deconvolution of the reconstructed first to fourth phase signals.
And 6, subtracting the first phase signal from the second phase signal to obtain the phase change delta phase (figure 5) measured by the method. And subtracting the third phase signal from the fourth phase signal to obtain the phase change measured by the FLASH method. The phase change delta phase measured by using the frequency offset +/-5000 Hz is about 1.6 times of the phase change measured by the FLASH method, as shown in figure 6, so that the phase imaging method of the invention greatly improves the phase detection sensitivity compared with the conventional FLASH method.
And 7, obtaining phase change delta phase according to the step 6, and calculating based on the formula (1) to obtain field distribution delta B 0.
Δphase=const x y x delta B 0 x delta offset equation (1)
Wherein Δphase is the phase change. Const is a constant and is determined by the duration of the adiabatic pulse and the swept range of the adiabatic pulse. Gamma is the gyromagnetic ratio of the spin nuclei, Δb 0 is the field distribution minus the central field strength, and Δoffset is the frequency offset range of the adiabatic pulse.
It should be noted that the specific embodiments described in this application are merely illustrative of the spirit of the invention. Those skilled in the art may make various modifications or additions to the described embodiments or substitutions thereof without departing from the spirit of the invention or its scope as defined in the accompanying claims.
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| EP3557275A1 (en) * | 2018-04-16 | 2019-10-23 | Siemens Healthcare GmbH | Diffusion-weighted magnetic resonance imaging with flow-compensated magnetic field gradients |
| CN110780249A (en) * | 2019-11-21 | 2020-02-11 | 中国科学院武汉物理与数学研究所 | Magnetic resonance imaging method using adiabatic radio frequency pulses to measure radio frequency B1 field distribution |
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| CN110780249A (en) * | 2019-11-21 | 2020-02-11 | 中国科学院武汉物理与数学研究所 | Magnetic resonance imaging method using adiabatic radio frequency pulses to measure radio frequency B1 field distribution |
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