CN115372873A - K space trajectory correction system and method applied to miscellaneous nuclear imaging - Google Patents

Abstract

The invention discloses a k-space trajectory correction system and a k-space trajectory correction method applied to heteronuclear imaging, and belongs to the technical field of imaging. The track correction system comprises a sampling sequence and signal processing system, a spectrometer control system and a magnet and radio frequency system, wherein the sampling sequence and signal processing system is connected with the spectrometer control system, and the spectrometer control system is connected with the magnet and the radio frequency system through an amplifier and an auxiliary control part. The k space trajectory correction method applied to the heteronuclear imaging is used for correcting target k space trajectory deviation caused by insufficient gradient hardware or incompletely eliminated eddy current effect, so that the influence of image artifacts is reduced or eliminated. The method does not need a separate sequence for measuring the k-space trajectory, the scanning time is hardly increased, and the measured gradient waveform, namely the imaging gradient used in actual imaging, can avoid the influence of insufficient stability of a gradient system.

Description

A k-space trajectory correction system and method applied to heteronucleus imaging

technical field

The invention relates to the field of imaging technology, in particular to a k-space trajectory correction system and method applied to heteronucleus imaging.

Background technique

The traditional magnetic resonance imaging system uses ¹ H nucleus as the observation nucleus, and mainly obtains anatomical images with different tissue contrasts, which can reflect limited metabolic and functional information. Heteronuclear imaging can effectively expand metabolic and functional information by observing a variety of non-proton magnetic nuclei (such as ¹⁹ F, ²³ Na, ³¹ P), break through the limitations of traditional magnetic resonance, and dig out a large amount of important information contained in weak biological signals. Bioinformatics.

However, heteronuclear imaging still faces considerable challenges in practical applications. As we all know, magnetic resonance imaging collects data in k-space, and then obtains images through Fourier transform. The deformation of the k-space trajectory will cause artifacts such as deformation and curling on the image. In order to alleviate the deformation of the k-space trajectory, modern magnetic resonance equipment mainly relies on gradient self-shielding coils and gradient pre-emphasis techniques. For the traditional Cartesian k-space acquisition, because the acquisition method itself is not sensitive to trajectory deformation, it can achieve a good correction effect in scanning. In heteronucleus imaging, the magnetic resonance physical properties of heteronuclei are quite different from those of traditional ¹ H nuclei, and non-Cartesian scanning sequences such as Radial and Spiral are preferred in many cases. Non-Cartesian acquisition can bring benefits in many ways, such as reducing the motion sensitivity of the scan, acquiring signals from ultrafast relaxing tissues, and having natural compatibility with compressed sensing techniques, etc. High requirements, insufficient gradient hardware or eddy current effects not fully eliminated during the system calibration phase can cause deviations in the target k-space trajectory and cause image artifacts. Therefore, in MRI heteronucleus imaging, it is necessary to correct the k-space trajectory of the non-Cartesian acquisition sequence applied to heteronucleus imaging, in order to reduce the eddy current effect due to insufficient gradient hardware or not completely eliminated, thereby improving image quality .

One way to compensate for these trajectory deformations is to estimate the k-space trajectory. This method regards the gradient system as a linear time-invariant system, and measures the frequency response function (Gradient ImpulseResponse Function, GIRF) of the gradient system by a certain method, and then The actual k-space trajectory is estimated and used in image reconstruction. Or similarly, GIRF can also be used to estimate the predistortion gradient waveform required to generate an ideal k-space trajectory, and compensate it during the acquisition process. Another compensation method is to measure the actual k-space trajectory using the ¹ H nuclear signal with a high SNR before collecting the non-Cartesian scan data of the heteronuclei, and then apply this information to the image reconstruction of the heteronucleus, thus obtaining Improved image quality.

The above-mentioned first compensation method needs to measure the frequency response function of the gradient system. The measurement process itself has a certain complexity, and with the passage of time and some operations in the machine maintenance process, the frequency response function will often change slightly. In practice, the accuracy of this method is difficult to guarantee.

The second compensation method described above requires a separate sequence of trajectory measurements first, followed by non-Cartesian data acquisition, and finally image reconstruction using the measured trajectories. This technology can generally obtain a relatively stable compensation effect, but it prolongs the scanning time; in addition, since the trajectory data and image data are collected separately, there is a certain delay in time, and what is actually measured is not the gradient waveform applied during imaging. When the stability of the gradient system itself is insufficient, this method will also be affected, and there is a certain risk of measurement failure.

Contents of the invention

The purpose of the present invention is to provide a k-space trajectory correction system and method applied to heteronucleus imaging, so as to solve the problems raised in the above-mentioned background technology.

To achieve the above object, the present invention provides the following technical solutions:

A k-space trajectory correction system applied to heteronucleus imaging, including a sampling sequence and signal processing system, a spectrometer control system, a magnet and a radio frequency system, the sampling sequence and signal processing system are connected to the spectrometer control system, and the spectrometer control system passes through an amplifier And the auxiliary control part connects the magnet with the radio frequency system.

As a further technical solution of the present invention: the amplifier includes a heteronuclide RF amplifier, ^{a 1} H nuclide RF amplifier, ^{a 1} H nuclide RF preamplifier, a heteronuclide RF preamplifier and a gradient amplifier.

As a further technical solution of the present invention: the sampling sequence and signal processing system includes a sampling sequence and image processing module and a main control system.

As a further technical solution of the present invention: the magnet and radio frequency system adopts a ¹ H and heteronuclear radio frequency transmitting and receiving integrated coil composed of ¹ H coils and heteronuclear coils.

As a further technical solution of the present invention: the spectrometer control system includes a broadband multi-core signal excitation control system, a receiver, a gradient and radio frequency power system, and a gradient control and system main control module.

Further, when the k-space trajectory correction system applied to heteronucleus imaging is working, the sampling sequence and signal processing system will send the control instructions, imaging sequences and parameters of ¹ H and heteronuclei to the spectrometer control system; the spectrometer control The broadband multi-core signal of the system excites and controls the receiving parameters of the system. Through frequency synthesis, waveform generation and quadrature modulation, etc., a radio frequency pulse small signal with a specific frequency, bandwidth, phase and amplitude is generated, and then amplified by the radio frequency power amplifier to generate a small signal at ¹ H and heteronuclei The radio frequency magnetic field is generated in the transmitting part of the radio frequency transmitting and receiving integrated coil, which excites the ¹ H and heteronuclei of the imaging object to resonate. At the same time, the gradient waveform of the spectrometer control system generates part of the receiving parameters, and then calculates the gradient waveform in the processing sequence, and The gradient waveform signal is output, and after being amplified by the gradient power amplifier, the gradient coil in the magnet is driven to generate a gradient magnetic field; the resonance signal generated by ¹ H and the heteronucleus passes through the receiving part of the integrated radio frequency transmitting and receiving coil of ¹ H and the heteronucleus to generate a high-frequency modulation signal , which is amplified by ¹ H and heteronuclear preamplifiers and then sent to the receiver of the spectrometer system. The signal is filtered, amplified, demodulated, collected and transmitted by the receiver to form a magnetic resonance signal that can be collected; finally , the collected ¹ H/hetero-NMR signal is transmitted back to the sampling sequence and signal processing system, and the required magnetic resonance image is obtained after data processing.

A k-space trajectory correction method applied to heteronucleus imaging, using the above-mentioned system, comprising the following steps:

Step 1: Apply a layer-selective excitation unit on the ¹ H channel before the heteronucleus imaging sequence unit;

Step 2: In the heteronucleus imaging sequence unit, first transmit radio frequency pulses in the heteronucleus channel to excite the heteronuclei to generate signals, and then enable the receiving link of the ¹ H/heteronucleus channel in the signal receiving stage to collect ¹ H/heteronuclei nuclear signal; the collected ¹ H signal is processed to generate measured k-space trajectory data; the collected heteronuclear signal is recorded as heteronuclear signal k-space data;

Step 3: Using the k-space trajectory data generated in step 2, reconstruct the k-space composed of heteronucleus signals in step 2 to obtain a trajectory-corrected heteronuclei image.

As a further technical solution of the present invention: the layer selection excitation unit on the ¹ H channel includes a ¹ H selective excitation pulse and a corresponding layer selection gradient to selectively excite an off-center layer.

As a further technical solution of the present invention: the layer selection gradient direction in the layer selection excitation unit on the ¹ H channel should be the same as the readout gradient direction in the heteronucleus imaging sequence unit.

Preferably, in the k-space trajectory correction method applied to heteronucleus imaging, only several k-space trajectories in the readout gradient direction can be selectively measured, and then a complete k-space can be obtained by the least square method and data interpolation track.

Preferably, in the k-space trajectory correction method applied to heteronucleus imaging, in the signal receiving stage, if there is a dephasing gradient before the read gradient in the heteronucleus imaging sequence unit, ^{the 1} H channel data needs to be dephased at the beginning of the gradient Gathering begins when applied. In the phase of heteronucleus signal acquisition, ^{the 1} H/heteronuclei signal needs to be acquired synchronously.

Compared with the prior art, the beneficial effect of the present invention is: the k-space trajectory correction system and method applied to heteronucleus imaging proposed by the present invention are used to correct the target caused by the lack of gradient hardware or the eddy current effect that is not completely eliminated. k-space trajectory deviation, thereby reducing or eliminating the influence of image artifacts. This method does not need a separate sequence for measuring the k-space trajectory, hardly increases the scan time, and the measured gradient waveform is the imaging gradient used in actual imaging, which can avoid the influence of insufficient stability of the gradient system. The correction system and method can be conveniently applied to various heteronuclei and imaging sequences of various heteronuclei.

Description of drawings

Fig. 1 System block diagram of k-space trajectory correction applied to heteronucleus imaging.

Figure 2 is a 3D UTE sequence diagram.

Fig. 3 is a diagram of the k-space trajectory correction pulse sequence applied to heteronucleus imaging of ^{the 1} H and ²³ Na parallel imaging system used for 3D UTE.

Detailed ways

The following will clearly and completely describe the technical solutions in the embodiments of the present invention with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only some, not all, embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by persons of ordinary skill in the art without making creative efforts belong to the protection scope of the present invention.

Embodiment 1: Please refer to FIG. 1, a k-space trajectory correction system applied to heteronucleus imaging, including a sampling sequence and signal processing system, a spectrometer control system, a magnet and a radio frequency system, and the sampling sequence and signal processing system are connected to the spectrometer Control system, the spectrometer control system connects the magnet and the radio frequency system through the amplifier and the auxiliary control part.

Amplifiers include heteronuclide RF amplifiers, ¹ H nuclide RF amplifiers, ¹ H nuclide RF preamplifiers, heteronuclide RF preamplifiers and gradient amplifiers. The sampling sequence and signal processing system includes a sampling sequence and image processing module and a main control system. The magnet and radio frequency system adopts the ¹ H and heteronuclear radio frequency transmitting and receiving integrated coil composed of ¹ H coil and heteronuclear coil. The spectrometer control system includes broadband multi-core signal excitation control system, receiver, gradient and radio frequency power system, gradient control and system main control module.

When the system is working, the sampling sequence and signal processing system will send the ¹ H and heteronucleus control instructions, imaging sequence and parameters to the spectrometer control system; , waveform generation and quadrature modulation, etc., to generate small RF pulse signals of specific frequency, bandwidth, phase, and amplitude, and then amplified by the RF power amplifier to generate a radio frequency magnetic field in the transmitting part of the ¹ H and heteronuclear radio frequency transmitting and receiving integrated coil. ^{The 1} H and heteronuclei of the imaging object are excited to resonate. At the same time, the gradient waveform of the spectrometer control system generates part of the receiving parameters, and then calculates the gradient waveform in the processing sequence, and outputs the gradient waveform signal, which is amplified by the gradient power amplifier and then drives the magnet. The gradient coil produces a gradient magnetic field; the resonance signal generated by ¹ H and the heteronucleus passes through the receiving part of the ¹ H and heteronucleus radio frequency transmitting and receiving integrated coil to generate a high-frequency modulation signal, which is amplified by ^{the 1} H and heteronucleus preamplifier and sent to The receiver of the spectrometer system, the signal is filtered, amplified, demodulated, collected and transmitted by the receiver to form a magnetic resonance signal that can be collected; finally, the collected ¹ H/hetero-NMR signal is transmitted back to the sampler The sequence and signal processing system obtains the required magnetic resonance images after data processing.

This design also discloses a k-space trajectory correction method applied to heteronucleus imaging, using the above-mentioned system, including the following steps:

Step 1: Apply a layer-selective excitation unit on the ¹ H channel before the heteronucleus imaging sequence unit;

Step 2: In the heteronucleus imaging sequence unit, first transmit radio frequency pulses in the heteronucleus channel to excite the heteronuclei to generate signals, and then enable the receiving link of the ¹ H/heteronucleus channel in the signal receiving stage to collect ¹ H/heteronuclei nuclear signal; the collected ¹ H signal is processed to generate measured k-space trajectory data; the collected heteronuclear signal is recorded as heteronuclear signal k-space data;

Step 3: Using the k-space trajectory data generated in step 2, reconstruct the k-space composed of heteronucleus signals described in step 2 to obtain a trajectory-corrected heteronucleus image.

Embodiment 2: On the basis of Embodiment 1, the acquisition of ²³ Na 3D-UTE signals is taken as an example for illustration.

The basic sequence diagram of 3D-UTE is shown in Figure 2. After the excitation pulse, the three physical gradient axes output three spatial encoding gradients, and their gradient values are:

Among them, G _r is the reading gradient size, G _x , G _y , G _z are the gradient values on the three physical gradient axes, ψ and θ are the polar angle and azimuth angle in the spherical coordinate system, respectively. During the 3D-UTE sequence when the ADC is turned on, that is, from the encoding gradient edge, the receiving system starts to acquire k-space data. This non-Cartesian acquisition places high demands on trajectory correction. In practice, a common practice is to use the ¹ H signal to specifically determine its k-space trajectory in advance.

The k-space trajectory correction method applied to heteronucleus imaging according to the present invention is applied to the UTE sequence, and its sequence diagram is shown in FIG. 3 . Compared with Figure 2, in Figure 3, the UTE read gradient is no longer expressed as the waveform on the gradient axis in the three physical coordinate systems of X, Y, and Z, but directly expressed as a gradient in the logical coordinate system. In the logical coordinate system, the read gradient waveform of UTE remains unchanged, but the applied direction rotates in k-space, which can be realized by changing the rotation matrix of the gradient system. In addition, compared with the basic 3D-UTE sequence, the sequence in Figure 3 adds a ¹ H RF pulse before the ²³ Na RF pulse, and at the same time, a gradient is added in the direction of the UTE readout gradient. This gradient is used as the slice selection gradient and slice refocusing gradient of the ¹ H channel, which can selectively excite a thin slice off-center. When the ADC is on, the receiving system collects the ¹ H/ ²³ Na signal synchronously, and ^{the 23} Na signal is the k-space data collected by the UTE sequence, and the phase expression of the ¹ H signal satisfies the following formula:

为磁场不均匀性等其他系统因素引起的自旋相位。Among them, γ is the magnetic rotation ratio of ¹ H, D _r is the off-center distance of the selected thin layer, t is the time variable, r is the space variable, G _r (t) is the 3D-UTE read gradient waveform, k _r (t ) is the measured k-space trajectory. The integral of the first term on the right side of the formula is the spin phase caused by the 3D-UTE read gradient, which can be seen to be proportional to the k-space trajectory; the second term

is the spin phase caused by other system factors such as magnetic field inhomogeneity.

In order to eliminate the influence of system factors on the ¹ H signal phase, each readout gradient direction can have a corresponding reverse readout gradient direction in the trajectory design stage of the UTE. As shown in Figure 3, each UTE reading gradient can find another reading gradient opposite to it, and repeat the above signal acquisition process, then the phase expression of the ¹ H signal is shown in formula (3):

Observing the formulas (2) and (3), the phase difference of the two measured MR signals can eliminate the phase caused by the system factors and obtain the k-space trajectory:

k _r (t) = ΔΦ _r (t)/2D _r (4)

Such a set of ¹ H signals can determine the k-space trajectory in the direction of the read gradient, which is denoted as kr here.

When the k-space trajectories in m different directions are measured, kx, ky, and kz can be estimated (that is, the k-space trajectories when the gradient directions are respectively X, Y, and Z three physical gradient axes):

Among them, T is the coefficient matrix determined by the direction of the measured reading gradient, n is the number of sampling points of the trajectory data, and m is the number of directions collected. For the 3D-UTE sequence, as long as m≥3, kx, ky, and kz can be estimated, and then a complete k-space trajectory can be synthesized through a certain interpolation algorithm. In the simplest way, when the reading gradient is selected to coincide with the three physical axes of X, Y, and Z respectively, when ^{the 1} H signal is used to collect trajectory data, formula (6) is an identity matrix, and the measured trajectory kr ₁ , kr ₂ , kr ₃ are Corresponding to kx, ky, kz respectively. When m>3, the accuracy of the measurement data can be improved, and the least square method can be used to obtain:

Since it is only necessary to measure k-space trajectories in several directions, and for ²³ Na, the gradient applied during the ¹ H RF pulse is equivalent to the damage gradient, which can be merged with the damage gradient of the UTE sequence itself. This time is usually only a few milliseconds , so this approach adds little to the scan time. In addition, since the gradient waveform of trajectory measurement is the gradient used in the actual acquisition of k-space data, this method can also avoid the influence of insufficient stability of the gradient system.

It will be apparent to those skilled in the art that the invention is not limited to the details of the above-described exemplary embodiments, but that the invention can be embodied in other specific forms without departing from the spirit or essential characteristics of the invention. Accordingly, the embodiments should be regarded in all points of view as exemplary and not restrictive, the scope of the invention being defined by the appended claims rather than the foregoing description, and it is therefore intended that the scope of the invention be defined by the appended claims rather than by the foregoing description. All changes within the meaning and range of equivalents of the elements are embraced in the present invention. Any reference sign in a claim should not be construed as limiting the claim concerned.

In addition, it should be understood that although this specification is described according to implementation modes, not each implementation mode only contains an independent technical solution, and this description in the specification is only for clarity, and those skilled in the art should take the specification as a whole , the technical solutions in the various embodiments can also be properly combined to form other implementations that can be understood by those skilled in the art.

Claims (10)

1. A k-space trajectory correction system applied to heteronuclear imaging is characterized by comprising a sampling sequence and signal processing system, a spectrometer control system and a magnet and radio frequency system, wherein the sampling sequence and signal processing system is connected with the spectrometer control system, and the spectrometer control system is connected with the magnet and the radio frequency system through an amplifier and an auxiliary control part.

2. The k-space trajectory correction system for heteronuclear imaging of claim 1, wherein the amplifier comprises a heteronuclear RF amplifier, ¹ An H nuclear species RF amplifier, ¹ An H-species RF preamplifier, a heteronuclear RF preamplifier, and a gradient amplifier.

3. The k-space trajectory correction system for use in heteronuclear imaging of claim 1, wherein the sampling sequence and signal processing system comprises a sampling sequence and image processing module and a host system.

4. The k-space trajectory correction system for use in heteronuclear imaging as defined in claim 1, wherein the magnet and radio frequency system employs a radio frequency transmit receive integrated coil.

5. The system of claim 1, wherein the spectrometer control system comprises a broadband multi-kernel signal excitation control system, a receiver, a gradient and radio frequency power system, and a gradient control and system master control module.

6. A k-space trajectory correction method for use in heteronuclear imaging, using the system of any of claims 1-5, comprising a new pulse sequence unit and corresponding data processing, characterized by:

step 1. Before the heteronuclear imaging sequence unit, in ¹ Applying a layer selection excitation unit to the H channel;

step 2, in the heteronuclear imaging sequence unit, firstly, the radio frequency pulse is emitted in the heteronuclear channel to excite the heteronuclear to generate signals, and then in the signal receiving stage, the signals are respectively enabled ¹ Receive chain of H/miscellaneous core channel, acquisition ¹ H/heteronuclear signal; the acquired heteronuclear signal is the heteronuclear signalk-space data; collected to obtain ¹ The H signal is processed to generate measured k space trajectory data;

and 3, reconstructing a k space formed by the heteronuclear signals by using the k space track data to obtain a track corrected heteronuclear image.

7. The method of claim 6, wherein the k-space trajectory correction method is applied to heteronuclear imaging ¹ A layer-selective excitation unit on the H channel, comprising ¹ H selective excitation pulse and corresponding slice selection gradient selectively excite an off-center layer.

8. The method of claim 6, wherein the k-space trajectory correction method is applied to heteronuclear imaging ¹ The gradient direction of the selection layer in the selection layer excitation unit on the H channel is the same as the readout gradient direction in the heteronuclear imaging sequence unit.

9. The method of claim 6, wherein k-space trajectory data in a plurality of readout gradient directions are selectively measured, and then a complete k-space trajectory is obtained by least squares and data interpolation.

10. The method of k-space trajectory correction for use in heteronuclear imaging as defined in claim 6 wherein, in the signal reception phase, if a dephasing gradient precedes the read gradients in the units of the heteronuclear imaging sequence, ¹ h channel data needs to be collected when dephasing gradient begins to be applied; in the phase of acquisition of the heteronuclear signals, ¹ the H/heteronuclear signals need to be acquired synchronously.

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