A magnetic resonance phase imaging method based on adiabatic radio frequency pulses

Abstract

The present invention discloses a magnetic resonance phase imaging method based on adiabatic radio frequency pulses, which determines the duration, amplitude, frequency and frequency bias of the reverse adiabatic pulse; flips the magnetization vector to the transverse XY plane, superimposes a negative frequency bias on the reverse adiabatic pulse, applies the reverse adiabatic pulse, performs MRI sampling, and obtains a first phase signal; flips the magnetization vector to the transverse XY plane, superimposes a positive frequency bias on the frequency of the reverse adiabatic pulse, applies the reverse adiabatic pulse, performs MRI sampling, and obtains a second phase signal; calculates the phase change, and obtains the field distribution according to the phase change calculation. The present invention improves the phase detection sensitivity, and can be further applied to magnetic resonance field distribution imaging, temperature imaging, magnetic susceptibility weighted imaging, etc., and has important practical value.

Description

Technical Field

[0001] The present invention belongs to the field of magnetic resonance imaging pulse sequence methods, and in particular relates to a magnetic resonance phase imaging method based on adiabatic radio frequency pulses. The present invention is applicable to magnetic resonance field distribution imaging, temperature imaging, magnetic susceptibility weighted imaging and the like. Background Art

[0002] Magnetic resonance imaging (MRI) can detect living tissues non-invasively without ionizing radiation, and obtain high-resolution, high-contrast living image information. It is an important imaging tool for studying the structure, function and metabolism of biological tissues. The data obtained by magnetic resonance sampling is complex data, which can be reconstructed into signal intensity images and phase images at the same time. Currently, most MRI images used for diagnosis are based on magnetic resonance signal intensity images, which reflect the nuclear spin density, relaxation time and other information of the tissue, while the phase image is usually ignored.

[0003] However, the phase information reflects the precession frequency information of the spin nucleus. Different precession frequencies result in different phases of the evolution of the transverse magnetization vector within the echo time, so the phase change can be understood as the integral of the precession angular frequency of the spin nucleus over time. The precession frequency of the spin nucleus is usually affected by field distribution, temperature, and tissue magnetic susceptibility, so the phase imaging direction is widely used in field distribution imaging, temperature imaging, magnetic susceptibility weighted imaging, etc.

[0004] Robinson S et al. [B0 mapping with multi-channel RF coils at high field [J]. Magnetic resonance in medicine, 2011, 66 (4): 976-988.] used phase imaging to detect the B0 field. The linear relationship between the precession frequency and the B0 field can be used to calculate the B0 field distribution through the phase image. This method can also be used for B0 field correction in chemical exchange saturation transfer magnetic resonance imaging.

[0005] Ishihara Y et al. [A precise and fast temperature mapping using waterproton chemical shift. Magnetic Resonance in Medicine, 1995, 34(6): 814-823.] proposed the application of phase imaging to magnetic resonance temperature measurement by using the linear relationship between the temperature range and the spin nuclear precession frequency. They proved that the phase change has good temperature dependence and the measurement speed of the phase imaging method is improved compared with the intensity imaging method. This method can be used for real-time monitoring of body temperature changes in tumor thermal therapy.

[0006] Haacke EM et al. [Susceptibility weighted imaging (SWI). Magnetic Resonance in Medicine, 2004, 52(3): 612-618.] used phase imaging to detect differences in tissue magnetic susceptibility and developed it into the susceptibility weighted imaging (SWI) method. The antimagnetic or paramagnetic properties of tissues cause local magnetic field changes, resulting in corresponding phase changes. The phase difference is large at the tissue interface with large differences in magnetic susceptibility, resulting in strong SWI contrast. This method can effectively detect vascular distribution and local bleeding, etc.

[0007] At present, phase imaging is mainly based on the dual-echo time gradient echo method, using sequences such as Fast Low Angle Shot (FLASH). For example, in field distribution measurement, the FLASH sequence is used to collect two phase images with different echo times, and the two phase images are subtracted to obtain the phase change of each voxel. The phase change size satisfies: Δphase = γ × ΔB0 × (TE1-TE2), where Δphase is the phase change, γ is the gyromagnetic ratio of the spin nucleus, ΔB0 is the field distribution minus the central field strength, and TE1 and TE2 are the corresponding two echo times. Using this relationship, the field distribution can be calculated. In the phase imaging method based on the FLASH sequence, the phase change is positively correlated with the echo time difference, and the phase contrast can only be further increased by increasing the echo time difference. The phase change of temperature imaging and magnetic susceptibility weighted imaging is also limited by this.

[0008] The present invention proposes a magnetic resonance phase imaging method based on adiabatic radio frequency pulses, which is a phase imaging method that utilizes inverted adiabatic pulses to modulate phase changes. The obtained phase change is related to the frequency bias of the adiabatic radio frequency pulse. A larger phase change than a conventional gradient echo sequence can be obtained within the same echo time. It can be understood as a phase-amplified magnetic resonance phase imaging method. The method designs a magnetic resonance imaging pulse sequence based on inverted adiabatic pulses and a new phase calculation method, and has the potential to be applied to phase imaging methods such as field distribution imaging, temperature imaging, and magnetic susceptibility weighted imaging. Summary of the invention

[0009] The purpose of the present invention is to address the above-mentioned problems existing in the prior art and provide a magnetic resonance phase imaging method based on adiabatic radio frequency pulses, which uses adiabatic pulse frequency bias to modulate phase changes.

[0010] The above-mentioned purpose of the present invention is achieved by the following technical means:

[0011] Adiabatic pulse is a special RF pulse whose amplitude and frequency are modulated by time. In the rotating coordinate system, the magnetization vector moves along the effective B1 field, i.e., B eff Field precession. Reversal of adiabatic pulse B eff The field turns 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 time, a reversal adiabatic pulse is applied, and the transverse magnetization vector will be flipped 180°. During the flipping process, the frequency of the reversal adiabatic pulse is modulated by time. When the precession frequency of the transverse magnetization vector of a certain voxel resonates with the reversal adiabatic pulse frequency, the magnetization vector is flipped 90° until the reversal adiabatic pulse frequency is far away from the precession frequency of the transverse magnetization vector and the 180° flip is completed. Therefore, the magnetization vector whose precession frequency of the transverse magnetization vector resonates with the adiabatic pulse frequency first will be flipped first, and a phase difference will be generated with the transverse magnetization vector that reaches resonance later. Therefore, the frequency bias of the adiabatic pulse can modulate the phase change. When the duration, amplitude and frequency modulation of the adiabatic pulse are determined, the phase difference is determined by the field distribution and the frequency bias of the adiabatic pulse, as expressed as follows:

[0012] Δphase= Const ×γ×ΔB0 ×△offset Formula (1)

[0013] Where Δphase is the phase change. Const is a constant determined by the duration of the adiabatic pulse and the sweep frequency range of the adiabatic pulse. γ is the gyromagnetic ratio of the spin nucleus, ΔB0 is the field distribution minus the central field strength, and △offset is the phase offset range of the adiabatic pulse.

[0014] Based on the above principle, a new magnetic resonance imaging pulse sequence is designed. In terms of timing design, a conventional pulse is first used to flip the magnetization vector to the transverse XY plane, and then an adiabatic pulse and a reverse adiabatic pulse are applied to modulate the phase of the transverse magnetization vector, and finally a phase signal is obtained by sampling.

[0015] A magnetic resonance phase imaging method based on adiabatic radio frequency pulses comprises the following steps:

[0016] Step 1, determining the duration, amplitude, frequency and frequency offset of the adiabatic pulse, wherein the frequency offset includes a positive frequency offset and a negative frequency offset;

[0017] Step 2, flipping the magnetization vector to the transverse XY plane, superimposing a negative frequency bias on the frequency of the adiabatic pulse, applying the adiabatic pulse, performing MRI sampling, and obtaining a first phase signal;

[0018] Step 3, flipping the magnetization vector to the transverse XY plane, superimposing a positive frequency bias on the frequency of the adiabatic pulse, applying an adiabatic pulse, performing MRI sampling, and obtaining a second phase signal;

[0019] Step 4: The first phase signal and the second phase signal are reconstructed respectively, and then subtracted after being warped to obtain a phase change Δphase.

[0020] Step 5: According to the obtained phase change Δphase, the field distribution ΔB0 is calculated based on the following formula.

[0021] Δphase=Const×γ×ΔB0×Δoffset

[0022] Among them, Δphase is the phase change, Const is a constant, γ is the gyromagnetic ratio of the spin nucleus, ΔB0 is the field distribution minus the central field strength, and △offset is the frequency offset range of the adiabatic pulse.

[0023] As described above in step 5, Const=2×Tp / BW, wherein Tp is the duration of the adiabatic pulse, and BW is the frequency sweep range of the adiabatic pulse.

[0024] Compared with the existing methods, the present invention has the following beneficial effects:

[0025] 1. The present invention designs a new magnetic resonance imaging pulse sequence and a new phase calculation method, which can modulate the phase change by frequency bias of adiabatic pulses;

[0026] 2. The present invention obtains a larger phase change than conventional methods within the same echo time, thereby improving the phase detection sensitivity;

[0027] 3. The present invention can be flexibly applied to a variety of phase imaging methods. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 is a flow chart of the method of the present invention;

[0029] Figure 2 is the amplitude and frequency modulation of the adiabatic pulse used in the present invention;

[0030] Figure 3 is a pulse timing diagram implemented in the present invention, including a conventional pulse for flipping the magnetization vector to the transverse XY plane, an adiabatic pulse for phase modulation, and an MRI sampling module, etc., wherein ω rf is the absolute value of the frequency offset;

[0031] Figure 4 is a schematic diagram of the principle of the adiabatic pulse used in the present invention to produce phase change;

[0032] Figure 51 is a phase change diagram obtained by using different frequency biases of adiabatic pulses in the present invention, wherein the three diagrams from left to right are phase change diagrams obtained by frequency biases of ±5000 Hz, ±4000 Hz, and ±3000 Hz respectively;

[0033] Figure 6 It is a comparison diagram of the phase change diagram obtained by the present invention and the phase change diagram obtained by the conventional FLASH method, wherein the left diagram is the phase change diagram obtained by the present invention at a frequency bias of ±5000 Hz, and the right diagram is the phase change diagram obtained by the conventional FLASH method. DETAILED DESCRIPTION

[0034] To facilitate those skilled in the art to understand and implement the method of the present invention, the method of the present invention is further described in detail below in conjunction with embodiments. It should be understood that the embodiments described herein are only used to illustrate and explain the present invention and are not used to limit the present invention.

[0035] Example:

[0036] This example was conducted on a Bruker 400M magnetic resonance spectrometer (AVANCE 400), using a 1% agarose gel sample filled in a 10 mm nuclear magnetic sample tube for testing in this example. A saddle coil with an inner diameter of 10 mm was used for transmission and reception, and the sample temperature was maintained at 298 K. Under this condition, a conventional FLASH sequence and a pulse sequence designed in the present invention were used for phase imaging comparison. This example is only used to demonstrate the method of the present invention, which can be used for samples and can also be used for in vivo detection, and is not limited by the type of magnetic resonance spectrometer and coil.

[0037] This embodiment includes the following steps:

[0038] Step 1: Figure 1 As shown in the flow chart, first determine the duration, amplitude, frequency and frequency offset of the adiabatic pulse. This embodiment uses an adiabatic pulse with a duration of 20ms. The amplitude and frequency of the adiabatic pulse are modulated based on the tanh / tan equation. The maximum amplitude is 16μT. The frequency of the adiabatic pulse is superimposed with a frequency offset. The frequency offset includes a positive frequency offset and a negative frequency offset, so that the frequency sweep range of the adiabatic pulse is -6000 to 6000Hz (such as Figure 2 As shown). According to the duration, amplitude, frequency, and frequency offset of the adiabatic pulse, the constant Const in formula (1) is determined. In this embodiment, Const = 2 × Tp / BW, where Tp is the duration of the adiabatic pulse, i.e., 20 ms, and BW is the sweep frequency range of the adiabatic pulse, i.e., 12000 Hz.

[0039] Step 2, design a pulse sequence. In terms of timing, first use a conventional pulse to flip the magnetization vector to the transverse XY plane, superimpose a negative frequency offset on the adiabatic pulse determined in step 1, then apply the adiabatic pulse with the superimposed negative frequency offset, and finally perform MRI sampling. Use the designed pulse sequence (pulse sequence such as Figure 3 The adiabatic pulse is set to a negative frequency offset (this embodiment uses negative frequency offsets of -3000 Hz, -4000 Hz and -5000 Hz respectively), and the first phase signal is obtained by sampling. The sampling parameters of this embodiment are: TR time 1s, TE time 25ms, repetition number 1, sampling matrix 128×128, single layer, and layer thickness 2mm.

[0040] Step 3, using the designed pulse sequence and adiabatic pulse, first use a conventional pulse to flip the magnetization vector to the transverse XY plane in terms of timing, superimpose a positive frequency offset on the adiabatic pulse determined in step 1, then apply the adiabatic pulse after superimposing the positive frequency offset, and finally perform MRI sampling. In this step, the frequency offset of the adiabatic pulse is set to a positive frequency offset (this embodiment uses positive frequency offsets of 3000Hz, 4000Hz and 5000Hz respectively), and the positive frequency offset is the same as the absolute value of the negative frequency offset set in step 2. The second phase signal is obtained by sampling, and the sampling parameters are consistent with step 2. The principle of phase modulation is as follows Figure 4 As shown, within the frequency sweep range of the adiabatic pulse, the phase is proportional to the frequency offset of the adiabatic pulse, so the phase change is proportional to the frequency offset difference.

[0041] Step 4: Use the FLASH sequence as a control for the method of the present invention. Sampling parameters: TR time 1s, repetition number 1, sampling matrix 128×128, single layer, layer thickness 2mm. The echo time of the long TE experiment is set to 25ms, and the third phase signal is obtained by sampling; the echo time of the short TE experiment is set to 5ms, and the fourth phase signal is obtained by sampling.

[0042] Step 5, reconstruct the first to fourth phase signals obtained by sampling in steps 1, 2, 3, and 4, construct the K space in phase encoding order, obtain complex image domain data after fast Fourier transform, take the argument of the complex value corresponding to each voxel of the image domain data, calculate the phase value corresponding to each voxel, and reconstruct the reconstructed first to fourth phase signals respectively. Because the argument exceeds 2π radians, folding will occur, and the phase de-folding algorithm is used to de-fold the reconstructed first to fourth phase signals.

[0043] Step 6: Subtract the first phase signal from the second phase signal to obtain the phase change Δphase ( Figure 5). The third phase signal is subtracted from the fourth phase signal to obtain the phase change measured by the FLASH method. The phase change Δphase measured by the present invention using the frequency offset ±5000Hz is about 1.6 times the result of the phase change measured by the FLASH method. Figure 6 As shown, it is shown that the phase imaging method of the present invention greatly improves the phase detection sensitivity compared with the conventional FLASH method.

[0044] Step 7: Obtain the phase change Δphase according to step 6, and calculate the field distribution ΔB0 based on formula (1).

[0045] Δphase= Const ×γ×ΔB0 ×△offset Formula (1)

[0046] Where Δphase is the phase change. Const is a constant determined by the duration of the adiabatic pulse and the frequency sweep range of the adiabatic pulse. γ is the gyromagnetic ratio of the spin nucleus, ΔB0 is the field distribution minus the central field strength, and △offset is the frequency offset range of the adiabatic pulse.

[0047] It should be noted that the specific embodiments described in the present invention are merely examples of the spirit of the present invention. Those skilled in the art may make various modifications or additions to the specific embodiments described or replace them in similar ways, but they will not deviate from the spirit of the present invention or exceed the scope defined by the attached claims.

Claims

1. A magnetic resonance phase imaging method based on adiabatic radio frequency pulses, characterized in that: The following steps are involved: Step 1, determining the duration, amplitude, frequency and frequency offset of the adiabatic pulse, wherein the frequency offset includes a positive frequency offset and a negative frequency offset; Step 2, flipping the magnetization vector to the transverse XY plane, superimposing a negative frequency bias on the frequency of the adiabatic pulse, applying the adiabatic pulse, performing MRI sampling, and obtaining a first phase signal; Step 3, flipping the magnetization vector to the transverse XY plane, superimposing a positive frequency bias on the frequency of the adiabatic pulse, applying an adiabatic pulse, performing MRI sampling, and obtaining a second phase signal; Step 4: The first phase signal and the second phase signal are reconstructed respectively, and then subtracted after being warped to obtain a phase change Δphase. Step 5: According to the obtained phase change Δphase, the field distribution ΔB0 is calculated based on the following formula: Δphase=Const×γ×ΔB0×Δoffset Among them, Δphase is the phase change, Const is a constant, γ is the gyromagnetic ratio of the spin nucleus, ΔB0 is the field distribution minus the central field strength, and △offset is the frequency offset range of the adiabatic pulse.

2. The method for magnetic resonance phase imaging based on adiabatic radio frequency pulses according to claim 1, characterized in that: In step 5, Const=2×Tp / BW, wherein Tp is the duration of the adiabatic pulse, and BW is the frequency sweep range of the adiabatic pulse.

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