Diffusion-Weighted Magnetic Resonance Imaging Method and Magnetic Resonance Imaging System for Infants and Young Children

Abstract

The present invention relates to the technical field of magnetic resonance imaging, and discloses a magnetic resonance diffusion weighted imaging method and a magnetic resonance imaging system for infants and young children, including the following specific steps: using a slice selection gradient pulse to perform slice selection on the imaging tissue; applying an IR fat suppression radio frequency pulse and a slice selection gradient pulse Gs1, and when the signal of the adipose tissue recovers to zero, applying a 90° excitation pulse and a slice selection gradient pulse Gs2, at this time the tissue has the strongest signal intensity; this magnetic resonance diffusion weighted imaging method and the magnetic resonance imaging system for infants and young children, by changing the direction of the 180° slice selection gradient to be opposite to the direction of the 90° pulse slice selection gradient, thereby effectively reducing the artifacts caused by chemical shift, and adding an IR fat suppression radio frequency pulse on the basis of the existing DWI sequence, can automatically adjust the IR pulse according to the signal difference between the fat and other tissues of different patients, and then achieve effective suppression of the image motion artifacts caused by the high signal of the fat.

Description

Technical Field

[0001] The present invention relates to the technical field of magnetic resonance imaging, and particularly to a magnetic resonance diffusion weighted imaging method and a magnetic resonance imaging system for infants and young children. Background Art

[0002] Magnetic resonance imaging is one of the main imaging methods in modern medical imaging. Its basic principle is to utilize the magnetic resonance phenomenon, use radiofrequency excitation to excite hydrogen protons in the human body, use a gradient field for position encoding, then use a receiving coil to receive signals with position information, and finally reconstruct image information through Fourier transform.

[0003] Magnetic resonance diffusion weighted imaging provides tissue contrast different from that of conventional nuclear magnetic resonance imaging (MRI) images, and can provide potential and unique information on the survival and development of brain tissue. It is very sensitive in showing acute cerebral infarction and differentiating it from other acute brain lesions. At the same time, it can also provide some information on lesions such as tumors, infections, traumas, and demyelination.

[0004] In magnetic resonance diffusion weighted imaging, the diffusion sensitive gradient in the DWI sequence can be fused with any sequence pulse, including: SE, SE-EPI, FSE (TSE), GRASE. Currently, on low-field magnetic resonance, mainly SE-DWI (non-EPI acquisition) and SE-EPI-DWI are used. Due to the comprehensive influence of factors such as sequence design, signal acquisition and filling, calibration, and debugging, satisfactory images have not been obtained on low-field systems at present, and clinical diagnostic requirements cannot be met. The main disadvantages are: large artifacts, uneven signals, signal loss of some tissues, etc. SE-EPI-DWI reduces the scanning time to a certain extent due to the EPI reading method, but at the same time introduces relatively heavy susceptibility artifacts and image geometric distortion problems. Summary of the Invention

[0005] To solve the problems that currently on low-field magnetic resonance, due to the comprehensive influence of factors such as sequence design, signal acquisition and filling, calibration, and debugging, satisfactory images have not been obtained on low-field systems at present, clinical diagnostic requirements cannot be met, and there are disadvantages such as large artifacts, uneven signals, and signal loss of some tissues; SE-EPI-DWI reduces the scanning time to a certain extent due to the EPI reading method, but at the same time introduces relatively heavy susceptibility artifacts and image geometric distortion problems, the present invention is realized through the following technical solutions: A magnetic resonance diffusion weighted imaging method includes the following specific steps:

[0006] Generate radiofrequency pulses using a radiofrequency pulse generator and generate gradient pulses using a gradient pulse generator.

[0007] S1. Use a slice selection gradient pulse to perform slice selection on the imaging tissue;

[0008] S2. Apply an IR fat-suppression RF pulse and a slice selection gradient pulse Gs1. When the signal of the adipose tissue recovers to zero, apply a 90° excitation pulse and a slice selection gradient pulse Gs2, so that the tissue signal within the slice selected by the slice selection gradient flips to the XY plane, and at this time the tissue has the strongest signal intensity.

[0009] The DWI sequence in the present invention adds an IR fat-suppression RF pulse on the basis of the existing DWI sequence. It can automatically adjust the IR pulse according to the signal difference between fat and other tissues of different patients, so as to effectively suppress the image motion artifacts caused by the high signal of fat, and enhance the contrast and resolution between tissues. The parameter range of the TI of the IR pulse is 60 - 200 ms. Aiming at the inherent characteristics that the gray and white matter and fat signals of the brain tissue of infants and young children are different from those of adult tissues, the best suppression effect can be achieved, which is beneficial to reducing the strong motion artifacts caused by the high signal of fat, and thus improving the image quality as a whole.

[0010] S3. The imaging tissue undergoes transverse relaxation decay during the TE / 2 time period. During the decay process of the tissue signal, apply a diffusion-weighted gradient pulse Gd1, then apply a 180° phase rephasing pulse and a reverse slice selection gradient pulse Gs3, so that the imaging tissue signal gradually recovers during the subsequent TE / 2 time period. At the same time, apply a diffusion-weighted gradient pulse Gd2. The diffusion-weighted gradients Gd1 and Gd2 are respectively applied in three gradient directions of the slice selection gradient, frequency encoding gradient, and phase encoding gradient, and are located on both sides of the 180° RF rephasing pulse.

[0011] S4. Phase-encode the proton signal within the selected slice through the phase encoding gradient pulse Gp1.

[0012] S5. Frequency-sample the slice with phase encoding information through the frequency encoding gradient pulse Gf1 to obtain a spin echo signal.

[0013] S6. Complete the excitation and signal acquisition of multiple layers of tissue within the same scan cycle, fill the acquired signals into the K space, and obtain the scan information of the tissue image.

[0014] Generally, the displacement of water and fat is proportional to the chemical shift ppm, also proportional to the magnitude of the static magnetic field B0, and inversely proportional to the slice selection gradient field, that is: Under the same slice selection conditions, due to chemical shift, the slices of water and fat do not completely overlap. There is a displacement, which is also related to the slice thickness (slice selection gradient). The thicker the slice thickness, the greater the displacement. Although the thicker the slice thickness, the higher the signal-to-noise ratio, the partial volume effect increases, and the water-fat shift between slices is also large. Since the slice selection gradient directions of the 90° and 180° radiofrequency pulses in the traditional DWI sequence are the same, the misregistration of water and fat is retained. By changing the slice selection gradient direction of the 180° pulse to be opposite to that of the 90° pulse, the signals of water and fat tissues are subtracted, thereby better eliminating the fat signal and achieving the purpose of improving the fat suppression effect. The reverse slice selection gradient pulse (Gs3) is used to cancel the inherent chemical shift artifacts generated by the original water and fat, thereby effectively reducing the artifacts caused by chemical shift.

[0015] When a 90° pulse is applied, the slice selection gradient is in one direction, so that at a certain position, the fat tissue is misaligned relative to water. When the 180° refocusing occurs, the slice selection gradient direction is opposite, and the displacements of water and fat are also opposite. In this way, the two cancel each other out, and only the overlapping part of water and fat in the middle has a signal, while the signals of fat on both sides are canceled out, thereby achieving the purpose of being suppressed.

[0016] Furthermore, the radiofrequency pulse generator generates radiofrequency excitation pulses and phase refocusing pulses. The radiofrequency excitation pulses include IR fat-suppression radiofrequency pulses and 90° excitation pulses, and the phase refocusing pulse is a 180° phase refocusing pulse.

[0017] Furthermore, the gradient signal generator generates gradient pulses. The gradient pulses include slice selection gradient pulses Gs1, Gs2, and Gs3, where Gs3 uses a reverse gradient pulse; phase encoding gradient pulse Gp1; frequency encoding gradient pulse Gf1; diffusion-weighted gradient pulses Gd1 and Gd2. The diffusion-weighted gradients Gd1 and Gd2 are respectively applied in the three gradient directions of slice selection gradient, frequency encoding gradient, and phase encoding gradient, and are located on both sides of the 180° radiofrequency refocusing pulse.

[0018] Furthermore, the spin echo signal is a half spin echo signal. The specific acquisition steps of the half spin echo signal include:

[0019] S601. After collecting the first half spin echo signal of the first layer in the first scan period, collect the first half spin echo signal of the second layer, the first half spin echo signal of the third layer... the first half spin echo signal of the Nth layer;

[0020] S602. After collecting the second half spin echo signal of the first layer in the second scan period, collect the second half spin echo signal of the second layer, the second half spin echo signal of the third layer... the second half spin echo signal of the Nth layer;

[0021] S603. After collecting the third half spin echo signal of the first layer in the third scan period, collect the third half spin echo signal of the second layer, the third half spin echo signal of the third layer... the third half spin echo signal of the Nth layer;

[0022] S604. After collecting the Mth half spin echo signal of the first layer in the Mth scan period, collect the Mth half spin echo signal of the second layer, the Mth half spin echo signal of the third layer... the Mth half spin echo signal of the Nth layer.

[0023] The DWI sequence in the present invention adopts a half echo acquisition method, and the echo time TE is shortened. In this way, the TR time can be shortened while ensuring the same number of layers, which not only helps to improve the acquisition speed, but also is conducive to maintaining the steady state and reducing the generation of related image artifacts.

[0024] Furthermore, the K-space filling method for collecting signals is a combined filling method of half echo signal filling and half virtual filling, and the half virtual filling is simulated and filled according to the principle of frequency encoding direction symmetry.

[0025] A magnetic resonance imaging system dedicated for infants and young children includes: a magnet, gradient coils, a transceiver integrated radio frequency coil, and an equipment support;

[0026] The magnet, gradient coils, and transceiver integrated radio frequency coil are all arranged on the equipment support. The magnet is arranged on both sides of the equipment support. The transmitting unit and the receiving unit in the transceiver integrated radio frequency coil are integrally formed between the magnets, and the gradient coils are arranged between the magnet and the transceiver integrated radio frequency coil;

[0027] The adjacent wires of the gradient coils in the X / Y axis are in contact with each other. The central part of the Z-axis gradient coil of the gradient coils is a double loop, and a rubber gasket is installed between the gradient coils and the pole face of the magnet.

[0028] Furthermore, it further includes a spectrometer, a radio frequency system, a gradient system, and a control system;

[0029] The computer system issues a pulse signal instruction through the sequence control device in the control system. The pulse signal instruction controls the radio frequency pulse generator in the radio frequency system to generate radio frequency pulses, controls the gradient pulse generator in the gradient system to generate gradient pulses, drives the radio frequency power amplifier and the gradient power amplifier, and generates the required radio frequency field and gradient field through the transceiver integrated radio frequency coil and the gradient coils. The magnetic resonance signal generated by the human body is received by the transceiver integrated radio frequency coil, passed through the preamplifier in the spectrometer, and undergoes analog-to-digital conversion (A / D) and signal processing in the spectrometer, and then is received by the control system for image reconstruction and processing.

[0030] Further, the sequence control device contains multiple scanning sequence packets for different age ranges of infants and young children.

[0031] In the DWI sequence of the present invention, according to the inherent signal characteristics of the brain tissues of infants and young children in different age ranges, scanning sequence packets for different age ranges are developed, which can meet the clinical diagnosis requirements for scanning infants and young children in different age ranges, and further improve the efficiency of clinical scanning and the accuracy of image diagnosis.

[0032] Further, the spectrometer controls the radio frequency pulses in the radio frequency system according to the working instructions, including:

[0033] Superimpose a secondary phase on the spectral profile before the Shinnar-Le-Roux inverse transform, and generate radio frequency pulses that meet the Adiabatic condition through the Shinnar-Le-Roux algorithm.

[0034] Further, after the gradient pulse drives the gradient coil to generate a gradient field, it further includes:

[0035] Reduce the ramp time, fall time, and amplitude of the gradient waveform in the gradient field to reduce the peripheral nerve stimulation effect.

[0036] Compared with the prior art, the present invention has the following beneficial effects:

[0037] This magnetic resonance diffusion weighted imaging method and magnetic resonance imaging system for infants and young children change the direction of the 180° slice selection gradient to be opposite to the direction of the 90° pulse slice selection gradient, thereby effectively reducing the artifacts caused by chemical shift. An IR fat suppression radio frequency pulse is added on the basis of the existing DWI sequence, which can automatically adjust the IR pulse according to the signal difference between fat and other tissues of different patients, and then effectively suppress the image motion artifacts caused by the high signal of fat, and enhance the contrast and resolution between tissues.

[0038] This magnetic resonance diffusion weighted imaging method and magnetic resonance imaging system for infants and young children can shorten the echo time TE by adopting the half echo acquisition technique, and can shorten the TR time while ensuring the same number of slices. This not only helps to improve the acquisition speed, but also helps to maintain the steady state and reduce the generation of related image artifacts.

[0039] This magnetic resonance diffusion weighted imaging method and magnetic resonance imaging system for infants and young children fill the K space by adopting the half echo signal filling and half virtual filling methods. Without changing the number of phase encodings and frequency encodings, that is, without changing the resolution in the Kx and Ky directions, the K space filling time of the entire signal is reduced, and thus the image scanning time is reduced as a whole.

[0040] 4. The magnetic resonance diffusion weighted imaging method and the magnetic resonance imaging system for infants and young children can reduce the torque generated between wires by designing no gap between the X / Y axis wires of the gradient coil, reduce the overall vibration of the gradient coil and the internal noise generated, and at the same time can increase the number of loop coils, making the gradient field have better linearity, and can achieve a specific gradient field strength by using a smaller current, improving the efficiency of the gradient coil. In addition, using a smaller current can reduce the Lorentz force on the gradient coil, thereby reducing the vibration of the gradient coil, further weakening the impact on the mounting fixtures, and reducing the noise.

[0041] 5. The magnetic resonance diffusion weighted imaging method and the magnetic resonance imaging system for infants and young children can better improve the linearity of the Z gradient field through the double-loop design of the central part of the Z-axis gradient coil, and the transceiver integrated design of the radio frequency coil can effectively reduce the power of the radio frequency system and fully protect the soft tissues of the infant brain.

[0042] 6. The magnetic resonance diffusion weighted imaging method and the magnetic resonance imaging system for infants and young children can generate radio frequency pulses that also meet the Adiabatic condition by optimizing the SLR algorithm. By superimposing sufficient secondary phase on the spectral profile before the SLR inverse transform, a radio frequency waveform containing a radio frequency pulse with Adiabatic characteristics can be designed. The application of the secondary phase can make the radio frequency energy distribution more uniform, and at the same time reduce the radio frequency amplitude under the same radio frequency bandwidth, so the SAR value can be effectively reduced.

[0043] 7. The magnetic resonance diffusion weighted imaging method and the magnetic resonance imaging system for infants and young children reduce the cardiac and peripheral nerve stimulation effects (PNS) by optimizing the gradient waveform, reducing the rise time, fall time, amplitude of the gradient waveform, and excitation time, and reducing the dB / dt value. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Figure 1 It is a schematic diagram of the working principle of the DWI sequence of the present invention;

[0045] Figure 2 It is a schematic diagram of the acquisition of multi-layer semi-spin echo signals of the present invention;

[0046] Figure 3 It is a three-dimensional schematic diagram of the K-space signal filling of the present invention;

[0047] Figure 4 It is a two-dimensional schematic diagram of the K-space signal filling of the present invention;

[0048] Figure 5 It is a schematic diagram of the structure of the magnetic resonance imaging system of the present invention;

[0049] Figure 6 It is the wire type of the gradient coil of the present invention. Embodiment

[0050] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0051] Embodiments of the magnetic resonance diffusion weighted imaging method and the dedicated magnetic resonance imaging system for infants and young children are as follows: Embodiment

[0052] Please refer to Figures 1 - 4 , a magnetic resonance diffusion weighted imaging method, comprising the following specific steps:

[0053] The radio frequency pulse generator generates radio frequency excitation pulses and phase rephasing pulses. The radio frequency excitation pulses include IR fat suppression radio frequency pulses and 90° excitation pulses, and the phase rephasing pulse is a 180° phase rephasing pulse.

[0054] The gradient signal generator generates gradient pulses, including slice selection gradient pulses Gs1, Gs2, and Gs3. Gs3 uses a reverse gradient pulse; phase encoding gradient pulse Gp1; frequency encoding gradient pulse Gf1; diffusion weighted gradient pulses Gd1 and Gd2. The diffusion weighted gradients Gd1 and Gd2 are respectively applied in three gradient directions of the slice selection gradient, frequency encoding gradient, and phase encoding gradient, and are located on both sides of the 180° radio frequency rephasing pulse.

[0055] S1. Use the slice selection gradient pulse to perform slice selection on the imaging tissue;

[0056] S2. Apply the IR fat suppression radio frequency pulse and the slice selection gradient pulse Gs1. When the signal of the adipose tissue recovers to zero, apply the 90° excitation pulse and the slice selection gradient pulse Gs2, so that the tissue signal in the layer selected by the slice selection gradient flips to the XY plane, and at this time the tissue has the strongest signal intensity.

[0057] The DWI sequence in the present invention adds an IR fat suppression radio frequency pulse on the basis of the existing DWI sequence, which can automatically adjust the IR pulse according to the signal difference between fat and other tissues of different patients, so as to effectively suppress the image motion artifacts caused by the high signal of fat, and improve the contrast and resolution between tissues. The parameter range of the TI of the IR pulse is 60 - 200 ms. Aiming at the inherent characteristics that the gray and white matter and fat signals of the brain tissue of infants and young children are different from those of adult tissues, the best suppression effect can be achieved, which is beneficial to reducing the strong motion artifacts caused by the high signal of fat, and thus improving the image quality as a whole.

[0058] S3. The imaging tissue undergoes transverse relaxation decay within the TE / 2 time period. During the decay of the tissue signal, a diffusion-weighted gradient pulse Gd1 is applied, and then a 180° phase-rephasing pulse and an inverse slice selection gradient pulse Gs3 are applied, enabling the imaging tissue signal to gradually recover within the subsequent TE / 2 time period. Meanwhile, a diffusion-weighted gradient pulse Gd2 is applied. The diffusion-weighted gradients Gd1 and Gd2 are respectively applied in the three gradient directions of the slice selection gradient, frequency encoding gradient, and phase encoding gradient, and are located on both sides of the 180° radiofrequency rephasing pulse.

[0059] S4. Phase-encode the proton signals within the selected slice through the phase encoding gradient pulse Gp1.

[0060] S5. Perform frequency sampling on the slice with phase encoding information through the frequency encoding gradient pulse Gf1 to obtain a half-spin echo signal.

[0061] S6. Complete the excitation and signal acquisition of multiple layers of tissue within the same scan cycle, fill the acquired signals into the K-space, and obtain the scan information of the tissue image. The K-space filling method for signal acquisition is a combined filling method of half-echo signal filling and half-virtual filling. The half-virtual filling is performed according to the symmetry principle of the frequency encoding direction.

[0062] The spin echo signal is a half-spin echo signal. The specific acquisition steps of the half-spin echo signal include:

[0063] S601. After acquiring the first half-spin echo signal of the first layer within the first scan cycle, acquire the first half-spin echo signal of the second layer, the first half-spin echo signal of the third layer... the first half-spin echo signal of the Nth layer.

[0064] S602. After acquiring the second half-spin echo signal of the first layer within the second scan cycle, acquire the second half-spin echo signal of the second layer, the second half-spin echo signal of the third layer... the second half-spin echo signal of the Nth layer.

[0065] S603. After acquiring the third half-spin echo signal of the first layer within the third scan cycle, acquire the third half-spin echo signal of the second layer, the third half-spin echo signal of the third layer... the third half-spin echo signal of the Nth layer.

[0066] S604. After acquiring the Mth half-spin echo signal of the first layer within the Mth scan cycle, acquire the Mth half-spin echo signal of the second layer, the Mth half-spin echo signal of the third layer... the Mth half-spin echo signal of the Nth layer.

[0067] In the DWI sequence of the present invention, a half - echo acquisition method is adopted, and the echo time TE is shortened. In this way, the TR time can be shortened while ensuring the same number of layers, which not only helps to improve the acquisition speed, but also is beneficial to maintaining the steady state and reducing the generation of related image artifacts.

[0068] Usually, the displacement of water and fat is proportional to the chemical shift ppm, also proportional to the magnitude of the static magnetic field B0, and inversely proportional to the slice - selection gradient field, that is: . Under the same slice - selection conditions, due to the chemical shift of water and fat, the slices of water and the slices of fat tissue do not completely overlap, and there is a displacement. This displacement is also related to the slice thickness (slice - selection gradient). The thicker the slice thickness, the larger the displacement. Although the thicker the slice thickness, the higher the signal - to - noise ratio, the partial - volume effect increases, and the water - fat displacement between layers is also large. Since the slice - selection gradient directions of the 90° and 180° radio - frequency pulses in the traditional DWI sequence are the same, the misalignment of water and fat is retained. By changing the direction of the 180° slice - selection gradient to be opposite to the direction of the 90° pulse slice - selection gradient, the signals of water and fat tissue are subtracted, and then the fat signal can be better eliminated, achieving the purpose of improving the fat - suppression effect. The reverse slice - selection gradient pulse (Gs3) is used to offset the inherent chemical - shift artifacts generated by the original water and fat, thereby effectively reducing the artifacts caused by chemical shift.

[0069] When a 90° pulse is applied, the slice - selection gradient is in one direction, so that at a certain position, the fat tissue will be misaligned relative to water. When the 180° refocusing occurs, the slice - selection gradient direction is opposite, and the displacements of water and fat are also opposite. In this way, the two cancel each other out, and only the overlapping part of water and fat in the middle has a signal, while the signals of fat on both sides are canceled out, thus achieving the purpose of being suppressed. Embodiment

[0070] Please refer to Figures 1 - 6 , a magnetic resonance diffusion - weighted imaging method, including the following specific steps:

[0071] Use a radio - frequency pulse generator to generate radio - frequency pulses and a gradient pulse generator to generate gradient pulses.

[0072] S1. Use a slice - selection gradient pulse to perform slice selection on the imaging tissue;

[0073] S2. Apply an IR fat - suppression radio - frequency pulse and a slice - selection gradient pulse Gs1. When the signal of the fat tissue recovers to zero, apply a 90° excitation pulse and a slice - selection gradient pulse Gs2 so that the tissue signal within the slice selected by the slice - selection gradient flips to the XY plane, and at this time the tissue has the strongest signal intensity;

[0074] The DWI sequence in the present invention adds an IR fat-suppression RF pulse on the basis of the existing DWI sequence. It can automatically adjust the IR pulse according to the signal differences between fat and other tissues in different patients, thereby effectively suppressing the image motion artifacts caused by the high signal of fat and enhancing the contrast between tissues. The parameter range of the TI of the IR pulse is 60 - 200 ms. Aiming at the inherent characteristics that the gray and white matter and fat signals of the infant brain tissue are different from those of adult tissues, the best suppression effect can be achieved, which is beneficial to reducing the strong motion artifacts caused by the high signal of fat, and thus improving the image quality as a whole.

[0075] S3. The imaging tissue undergoes transverse relaxation decay during the TE / 2 time period. During the process of tissue signal decay, a diffusion-weighted gradient pulse Gd1 is applied, and then a 180° phase-rephasing pulse and an inverse slice selection gradient pulse Gs3 are applied, so that the imaging tissue signal gradually recovers in the subsequent TE / 2 time period. At the same time, a diffusion-weighted gradient pulse Gd2 is applied. The diffusion-weighted gradients Gd1 and Gd2 are respectively applied in the three gradient directions of the slice selection gradient, frequency encoding gradient, and phase encoding gradient, and are located on both sides of the 180° RF rephasing pulse;

[0076] S4. Phase-encode the proton signals in the selected slice through the phase encoding gradient pulse Gp1;

[0077] S5. Frequency-sample the slice with phase encoding information through the frequency encoding gradient pulse Gf1 to obtain a half-spin echo signal;

[0078] S6. Complete the excitation and signal acquisition of multiple layers of tissues within the same scan cycle, fill the acquired signals into the K space, and obtain the scan information of the tissue image.

[0079] An MRI system dedicated for infants, comprising: a magnet, gradient coils, a transceiver integrated RF coil, and an equipment support;

[0080] The magnet, gradient coils, and transceiver integrated RF coil are all arranged on the equipment support. The magnet is arranged on both sides of the equipment support. The transmitting unit and the receiving unit in the transceiver integrated RF coil are integrally formed between the magnets, and the gradient coils are arranged between the magnet and the transceiver integrated RF coil;

[0081] The RF coil adopts a transceiver integrated design. On the premise of meeting the imaging quality requirements, it can effectively reduce the power of the RF system. The maximum power of the RF system is 300 W, and the actual power required for the 90° RF pulse is about 50 - 60 W, which can fully protect the soft tissues of the infant brain.

[0082] The radio frequency coil used is a transceiver integrated head coil, which can reduce the scanning space occupied by the independent installation of the transmit coil. With the scanning space remaining unchanged, there is more space to install the gradient coil, enabling it to be farther away from the magnet pole face and closer to the B0 DSV area. By bringing the gradient coil closer to the B0 DSV area, a smaller current can be used to generate a specific gradient field strength. In addition, by moving the gradient coil away from the magnet pole face and having the gradient coil above the shim ring, eddy currents in the directions of the magnet pole face and the shim ring can be reduced, thereby improving the image quality. Moreover, by reducing eddy currents, the heat generated by the eddy currents in the magnetic poles can be decreased, preventing fluctuations in the magnetic pole temperature, reducing magnetic field drift, making the magnetic field more stable and uniform, and thus ensuring the image uniformity.

[0083] The adjacent wires of the gradient coil in the X / Y axis are in contact with each other. The wires are tightly bundled together, generating less torque between them. Although there will still be minor vibrations of the wires, due to no wire groove impacts, this will reduce the overall vibration of the gradient coil and the internal noise generated. Since there are no gaps between the wires, the number of turns of the loop coil can be increased, making the gradient field have better linearity, and a smaller current can be used to achieve a specific gradient field strength, thereby improving the efficiency of the gradient coil. Using a smaller current can reduce the Lorentz force on the gradient coil, which will reduce the vibration of the gradient coil, thus weakening the impact on the mounting fixtures and reducing the noise.

[0084] The central part of the Z-axis gradient coil of the gradient coil is a double loop. The double loop design can better improve the linearity of the Z-axis gradient field. A rubber gasket is installed between the gradient coil and the magnet pole face. The rubber gasket can absorb part of the vibration of the gradient coil, thereby reducing the intensity of the impact between the gradient coil and the magnet pole face and further reducing the noise.

[0085] It also includes a spectrometer, a radio frequency system, a gradient system, and a control system;

[0086] The computer system issues pulse signal instructions through the sequence control device in the control system. The pulse signal instructions control the radio frequency pulse generator in the radio frequency system to generate radio frequency pulses and control the gradient pulse generator in the gradient system to generate gradient pulses, driving the gradient power amplifier and the radio frequency power amplifier. The required gradient field and radio frequency field are generated through the gradient coil and the transceiver integrated radio frequency coil. The magnetic resonance signal generated by the human body is received by the transceiver integrated radio frequency coil, pre-amplified by the pre-amplifier in the spectrometer, and undergoes analog-to-digital conversion (A / D) and signal processing in the spectrometer, and then is received by the control system for image reconstruction and processing.

[0087] The sequence control device contains multiple scanning sequence packages for different age ranges of infants and young children; clinically, since infants and young children have poorer tolerance than adults, and the signal characteristics of tissue structures such as white matter and gray matter in the brain are significantly different from those of adult brain tissue structures, it is difficult to meet the requirements of clinical diagnosis if the signal characteristics of adult brain tissue are used to debug the brain images of infants and young children. The DWI sequence in the present invention develops scanning sequence packages for different age ranges of infants and young children according to the inherent signal characteristics of their brain tissue, which can meet the clinical requirements for scanning infants and young children of different age ranges, and further improve the efficiency of clinical scanning and the accuracy of image diagnosis.

[0088] The spectrometer controls the radiofrequency pulses in the radiofrequency system according to the working instructions, including:

[0089] Before the Shinnar-Le-Roux inverse transform, a quadratic phase is superimposed on the spectral profile, and radiofrequency pulses that meet the Adiabatic condition are generated by the Shinnar-Le-Roux algorithm. The application of the quadratic phase can make the radiofrequency energy distribution more uniform, and at the same time reduce the radiofrequency amplitude under the same radiofrequency bandwidth, so the SAR value can be effectively reduced.

[0090] The Adiabatic radiofrequency pulse is a special type of radiofrequency pulse that can perform amplitude and frequency modulation and is insensitive to the influence of B0 inhomogeneity and frequency offset. The Shinnar-Le-Roux (SLR) algorithm is a commonly used high-field magnetic resonance radiofrequency pulse design algorithm. This magnetic resonance optimizes the SLR algorithm to generate radiofrequency pulses that can also meet the Adiabatic condition. By superimposing sufficient quadratic phase on the spectral profile before the SLR inverse transform, the required radiofrequency waveform containing radiofrequency pulses with Adiabatic characteristics can be designed. The application of the quadratic phase can make the radiofrequency energy distribution more uniform, and at the same time reduce the radiofrequency amplitude under the same radiofrequency bandwidth, so the SAR value can be effectively reduced.

[0091] After the gradient pulse drives the gradient coil to generate a gradient field, the rise time, fall time, and amplitude of the gradient waveform in the gradient field are reduced to reduce the peripheral nerve stimulation effect.

[0092] Although the embodiments of the present invention have been shown and described, for those of ordinary skill in the art, it can be understood that various changes, modifications, substitutions, and variations can be made to these embodiments without departing from the principles and spirit of the present invention. The scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A magnetic resonance diffusion weighted imaging method, characterized in that, It includes the following specific steps: S1. Use a slice selection gradient pulse to perform slice selection on the imaging tissue; S2. Apply an IR fat-suppression RF pulse and a slice selection gradient pulse Gs1. When the signal of the adipose tissue recovers to zero, apply a 90° excitation pulse and a slice selection gradient pulse Gs2 to flip the tissue signal within the slice selected by the slice selection gradient to the XY plane, at which time the tissue has the strongest signal intensity; S3. Apply a diffusion-sensitizing gradient pulse Gd1 during the attenuation process of the tissue signal, then apply a 180° phase-rephasing pulse and a reverse slice selection gradient pulse Gs3, and then apply a diffusion-sensitizing gradient pulse Gd2; S4. Perform phase encoding on the proton signal within the selected slice through a phase encoding gradient pulse Gp1; S5. Perform frequency sampling on the slice with phase encoding information through a frequency encoding gradient pulse Gf1 to obtain a spin echo signal; S6. Complete the excitation and signal acquisition of multiple layers of tissue within the same scan cycle, fill the acquired signals into the K-space, and obtain the scan information of the tissue image.

2. The magnetic resonance diffusion weighted imaging method according to claim 1, wherein The RF pulse generator generates RF excitation pulses and phase-rephasing pulses. The RF excitation pulses include IR fat-suppression RF pulses and 90° excitation pulses, and the phase-rephasing pulse is a 180° phase-rephasing pulse.

3. The magnetic resonance diffusion weighted imaging method according to claim 2, wherein, The gradient signal generator generates gradient pulses. The gradient pulses include slice selection gradient pulses Gs1, Gs2, and Gs3, and Gs3 uses a reverse gradient pulse; a phase encoding gradient pulse Gp1; a frequency encoding gradient pulse Gf1; diffusion-weighted gradient pulses Gd1 and Gd2. The diffusion-weighted gradients Gd1 and Gd2 are respectively applied in three gradient directions of the slice selection gradient, frequency encoding gradient, and phase encoding gradient, and are located on both sides of the 180° RF rephasing pulse.

4. The magnetic resonance diffusion weighted imaging method according to claim 1, wherein, The spin echo signal is a half spin echo signal. The specific acquisition steps of the half spin echo signal include: S601. After collecting the first half spin echo signal of the first layer within the first scan cycle, collect the first half spin echo signal of the second layer, the first half spin echo signal of the third layer... the first half spin echo signal of the Nth layer; S602. After collecting the second half spin echo signal of the first layer within the second scan cycle, collect the second half spin echo signal of the second layer, the second half spin echo signal of the third layer... the second half spin echo signal of the Nth layer; S603. After collecting the third half spin echo signal of the first layer within the third scan cycle, collect the third half spin echo signal of the second layer, the third half spin echo signal of the third layer... the third half spin echo signal of the Nth layer; S604. After collecting the Mth half spin echo signal of the first layer within the Mth scan cycle, collect the Mth half spin echo signal of the second layer, the Mth half spin echo signal of the third layer... the Mth half spin echo signal of the Nth layer.

5. The magnetic resonance diffusion weighted imaging method according to claim 4, wherein, The K-space filling method of the acquired signals is a combined filling method of half echo signal filling and half virtual filling. The half virtual filling is simulated filling according to the symmetry principle of the frequency encoding direction.

6. An MRI system for infants, which adopts the magnetic resonance diffusion weighted imaging method according to any one of claims 1-5, wherein, It includes: a magnet, gradient coils, a transceiver integrated RF coil, and an equipment support; The magnet, gradient coil, and transceiver integrated RF coil are all disposed on the device support. The magnet is disposed on both sides of the device support. The transmitting unit and the receiving unit of the transceiver integrated RF coil are integrally formed between the magnets. The gradient coil is disposed between the magnet and the transceiver integrated RF coil; The adjacent wires of the gradient coil in the X / Y axis are in contact with each other. The central part of the Z-axis gradient coil of the gradient coil is a double loop. A rubber gasket is installed between the gradient coil and the pole face of the magnet.

7. The magnetic resonance imaging system for infants and young children according to claim 6, wherein, It further includes a spectrometer, an RF system, a gradient system, and a control system; The imaging system issues a pulse signal command through the sequence control device in the control system. The pulse signal command controls the RF pulse generator in the RF system to generate RF pulses, controls the gradient pulse generator in the gradient system to generate gradient pulses, drives the RF power amplifier and the gradient power amplifier, and generates the required RF field and gradient field through the transceiver integrated RF coil and the gradient coil. The magnetic resonance signal generated by the human body is received by the transceiver integrated RF coil, passed through the preamplifier in the spectrometer, and undergoes analog-to-digital conversion (A / D) and signal processing in the spectrometer, and then is received by the control system for image reconstruction and processing.

8. The magnetic resonance imaging system for infants and young children according to claim 7, wherein, The sequence control device contains multiple scan sequence packages for different age segments.

9. The magnetic resonance imaging system for infants and young children according to claim 7, wherein, The spectrometer controls the RF pulses in the RF system according to the working instructions, including: Superimposing a secondary phase on the spectral profile before the Shinnar-Le-Roux inverse transform, and generating an RF pulse that satisfies the Adiabatic condition through the Shinnar-Le-Roux algorithm.

10. The magnetic resonance imaging system for infants and young children according to claim 7, wherein, After the gradient pulse drives the gradient coil to generate a gradient field, it further includes: Reducing the rise time, fall time, and amplitude of the gradient waveform in the gradient field to reduce the peripheral nerve stimulation effect.

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