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

Abstract

The present invention discloses a k-space trajectory correction system and method applied to heteronuclear imaging, belonging to the field of imaging technology. The trajectory correction system includes a sampling sequence and signal processing system, a spectrometer control system, and a magnet and radio frequency system. The sampling sequence and signal processing system are connected to the spectrometer control system, and the spectrometer control system is connected to the magnet and radio frequency system through an amplifier and an auxiliary control part. The k-space trajectory correction method applied to heteronuclear imaging proposed by the present invention is used to correct the target k-space trajectory deviation caused by the insufficiency of gradient hardware or the uncompletely eliminated eddy current effect, so as to reduce or eliminate the influence of image artifacts. This method does not require a separate sequence for measuring the k-space trajectory, hardly increases the scanning time, and the measured gradient waveform is the imaging gradient actually used in imaging, which can avoid the influence of insufficient gradient system stability.

Description

Technical Field

[0001] The present invention relates to the field of imaging technology, and in particular to a k-space trajectory correction system and method applied to heteronuclear imaging. Background Art

[0002] Conventional MRI systems are based on 1 H nuclei are observation nuclei, which mainly obtain anatomical images of various tissue contrasts, and can reflect limited metabolic and functional information. 19 F, 23 Na, 31 The observation of P) can effectively expand metabolic and functional information, break the limitations of traditional magnetic resonance imaging, and tap into the massive amount of important biological information contained in weak biological signals.

[0003] However, hybrid nucleus 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 deformation, curling and other artifacts on the image. In order to reduce the deformation of the k-space trajectory, modern magnetic resonance equipment mainly relies on gradient self-shielding coils and gradient pre-emphasis technology. For traditional Cartesian k-space acquisition, since the acquisition method itself is insensitive to trajectory deformation, good correction effects can be achieved during scanning. In hybrid nucleus imaging, the magnetic resonance physical properties of hybrid nuclei are different from those of traditional 1 H-nuclei vary greatly, and non-Cartesian scanning sequences such as Radial and Spiral are preferred in many cases. Non-Cartesian acquisition can bring benefits in many aspects, such as reducing the motion sensitivity of the scan, obtaining signals from ultrafast relaxing tissues, and having natural compatibility with compressed sensing technology, etc. However, non-Cartesian imaging sequences have high requirements for gradient hardware. Insufficient gradient hardware or eddy current effects that are not completely eliminated during the system correction stage will cause deviations in the target K-space trajectory and cause image artifacts. Therefore, in magnetic resonance mixed-nucleus imaging, k-space trajectory correction for non-Cartesian acquisition sequences applied to mixed-nucleus imaging is essential, in the hope of reducing the eddy current effects caused by insufficient or incompletely eliminated gradient hardware, thereby improving image quality.

[0004] One way to compensate for these trajectory deformations is to estimate the k-space trajectory. This method treats the gradient system as a linear time-invariant system, measures the frequency response function of the gradient system (Gradient Impulse Response Function, GIRF) through a certain method, then estimates the actual k-space trajectory, and applies it to image reconstruction. Similarly, the pre-distortion gradient waveform required to generate an ideal k-space trajectory can also be estimated using GIRF and compensated during the acquisition process. Another compensation method is to measure the actual k-space trajectory using the high signal-to-noise ratio 1 H nuclear signal before acquiring non-Cartesian scan data of the heteronuclear, and then apply this information to the image reconstruction of the heteronuclear to obtain improved image quality.

[0005] The first compensation method mentioned above requires measuring the frequency response function of the gradient system, and the measurement process itself is somewhat complex. Moreover, with the passage of time and some operations during machine maintenance, the frequency response function often changes slightly, and it is difficult to guarantee the accuracy of this method in practice.

[0006] The second compensation method mentioned above requires a separate sequence to first measure the trajectory, then acquire non-Cartesian data, and finally perform image reconstruction using the measured trajectory. This technique generally can obtain relatively stable compensation effects, but it prolongs the scanning time; in addition, since the trajectory data and image data are acquired separately, there is a certain time delay, and the actually measured gradient waveform is not the one applied during imaging. If the gradient system itself lacks stability, this method will also be affected, and there is a certain risk of measurement failure. Summary of the Invention

[0007] The purpose of the present invention is to provide a k-space trajectory correction system and method for heteronuclear imaging to solve the problems raised in the above background technology.

[0008] To achieve the above purpose, the present invention provides the following technical solutions:

[0009] A k-space trajectory correction system for heteronuclear imaging includes a sampling sequence and signal processing system, a spectrometer control system, and a magnet and RF system. The sampling sequence and signal processing system are connected to the spectrometer control system, and the spectrometer control system is connected to the magnet and RF system through an amplifier and an auxiliary control part.

[0010] As a further technical solution of the present invention: The amplifier includes a heteronuclear RF amplifier, 1 H nuclear RF amplifier, 1 H nuclear RF preamplifier, heteronuclear RF preamplifier, and gradient amplifier.

[0011] As a further technical solution of the present invention: the sampling sequence and signal processing system include a sampling sequence, an image processing module, and a main control system.

[0012] As a further technical solution of the present invention: the magnet and RF system adopt an 1 H coil and a heteronuclear coil to form an 1 H and heteronuclear RF transmit-receive integrated coil.

[0013] As a further technical solution of the present invention: the spectrometer control system includes a broadband multi-nuclear signal excitation control system, a receiver, a gradient and RF power system, and a gradient control and system main control module.

[0014] Further, when the k-space trajectory correction system applied to heteronuclear imaging works, the sampling sequence and signal processing system will 1 send control instructions, imaging sequences, parameters, etc. of H and heteronuclei to the spectrometer control system; the broadband multi-nuclear signal excitation control system of the spectrometer control system receives the parameters, and through frequency synthesis, waveform generation, quadrature modulation, etc., generates RF pulse small signals with specific frequencies, bandwidths, phases, and amplitudes, and then after being amplified by the RF power amplifier, generates an RF magnetic field in the transmission part of the H and heteronuclear RF transmit-receive integrated coil to excite the 1 H and heteronuclei in the imaging object to resonate. At the same time, the gradient waveform generation part of the spectrometer control system receives the parameters, then calculates and processes the gradient waveforms in the sequence, and outputs the gradient waveform signals. After being amplified by the gradient power amplifier, it drives the gradient coils in the magnet to generate a gradient magnetic field; 1 The resonance signals generated by H and heteronuclei pass through the 1 receiving part of the H and heteronuclear RF transmit-receive integrated coil to generate high-frequency modulation signals, which are amplified by the 1 H and heteronuclear preamplifier and sent to the receiver of the spectrometer system. After the signals are filtered, amplified, signal demodulated, signal acquired, and signal transmitted by the receiver, magnetic resonance signals that can be acquired are formed; finally, the 1 acquired H / heteronuclear magnetic resonance signals are transmitted back to the sampling sequence and signal processing system, and after data processing, the required magnetic resonance images are obtained. 1 H / heteronuclear magnetic resonance signals are transmitted back to the sampling sequence and signal processing system, and after data processing, the required magnetic resonance images are obtained.

[0015] A k-space trajectory correction method applied to heteronuclear imaging, using the above system, includes the following steps:

[0016] Step 1: Before the heteronuclear imaging sequence unit, apply a slice excitation unit in the 1 H channel;

[0017] Step 2: In the heteronuclear imaging sequence unit, first transmit an RF pulse in the heteronuclear channel to excite the heteronuclei to generate signals, and then in the signal reception stage, enable 1The receiving link of the H / heteronuclear channel acquires 1 H / heteronuclear signals; the acquired 1 H signals are processed to generate measured k-space trajectory data; the acquired heteronuclear signals are denoted as heteronuclear signal k-space data;

[0018] Step 3: Using the k-space trajectory data generated in Step 2, reconstruct the k-space composed of the heteronuclear signals in Step 2 to obtain a heteronuclear image with trajectory correction.

[0019] As a further technical solution of the present invention: the 1 Slice excitation unit on the H channel includes a 1 H selective excitation pulse and corresponding slice gradient to selectively excite an off-center slice.

[0020] As a further technical solution of the present invention: the 1 Slice gradient direction in the slice excitation unit on the H channel should be the same as the readout gradient direction in the heteronuclear imaging sequence unit.

[0021] Preferably, in the k-space trajectory correction method applied to heteronuclear imaging, only several k-space trajectories in the readout gradient directions can be selectively measured, and then the complete k-space trajectory can be obtained through the least squares method and data interpolation.

[0022] Preferably, in the k-space trajectory correction method applied to heteronuclear imaging, during the signal reception stage, if there is a dephasing gradient before the read gradient in the heteronuclear imaging sequence unit, 1 H channel data needs to be acquired when the dephasing gradient starts to be applied. During the heteronuclear signal acquisition stage, 1 H / heteronuclear signals need to be acquired synchronously.

[0023] Compared with the prior art, the beneficial effects of the present invention are: the k-space trajectory correction system and method applied to heteronuclear imaging proposed by the present invention are used to correct the target k-space trajectory deviation caused by the deficiency of gradient hardware or the eddy current effect that has not been completely eliminated, thereby reducing or eliminating the influence of image artifacts. This method does not require a separate sequence to measure the k-space trajectory, hardly increases the scanning time, and the measured gradient waveform is the imaging gradient actually used in imaging, which can avoid the influence of insufficient gradient system stability. This correction system and method can be conveniently applied to various heteronuclei and various heteronuclear imaging sequences. Description of the Drawings

[0024] Figure 1 System framework diagram of k-space trajectory correction applied to heteronuclear imaging.

[0025] Figure 2 It is a 3D UTE sequence diagram.

[0026] Figure 3 For 1 H and 23 The k-space trajectory correction pulse sequence diagram for the application of the H and Na parallel imaging system to 3D UTE in heteronuclear imaging. Specific embodiments

[0027] 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.

[0028] Embodiment 1: Please refer to Figure 1 , a k-space trajectory correction system for heteronuclear imaging, including a sampling sequence and signal processing system, a spectrometer control system, and a magnet and RF system. The sampling sequence and signal processing system are connected to the spectrometer control system, and the spectrometer control system is connected to the magnet and RF system through an amplifier and an auxiliary control part.

[0029] The amplifier includes a heteronuclear RF amplifier, 1 H nuclide RF amplifier, 1 H nuclide RF preamplifier, heteronuclear RF preamplifier, and gradient amplifier. The sampling sequence and signal processing system include a sampling sequence and image processing module and a main control system. The magnet and RF system uses an H and heteronuclear RF transmit-receive integrated coil composed of an 1 H coil and a heteronuclear coil. 1 The spectrometer control system includes a broadband multi-nuclear signal excitation control system, a receiver, a gradient and RF power system, and a gradient control and system main control module.

[0030] When the system is working, the sampling sequence and signal processing system send control instructions, imaging sequences, and parameters of 1 H and heteronuclei to the spectrometer control system; the broadband multi-nuclear signal excitation control system of the spectrometer control system receives the parameters, generates a small RF pulse signal with a specific frequency, bandwidth, phase, and amplitude through frequency synthesis, waveform generation, and quadrature modulation, and then after being amplified by the RF power amplifier, generates an RF magnetic field in the transmission part of the 1 H and heteronuclear RF transmit-receive integrated coil to excite the 1 H and heteronuclei in the imaging object to resonate. At the same time, the gradient waveform generation part of the spectrometer control system receives the parameters, then calculates and processes the gradient waveforms in the sequence, and outputs the gradient waveform signals. After being amplified by the gradient power amplifier, it drives the gradient coils in the magnet to generate a gradient magnetic field; 1 The resonance signals generated by1 The receiving part of the H and heteronuclear radiofrequency transmit-receive integrated coil generates a high-frequency modulated signal, which is 1 amplified by the H and heteronuclear preamplifiers and then sent to the receiver of the spectrometer system. After the signal is filtered, amplified, signal demodulated, signal acquired, and signal transmitted by the receiver, a magnetic resonance signal that can be acquired is formed; finally, the 1 H / heteronuclear magnetic resonance signal is transmitted back to the sampling sequence and signal processing system, and the required magnetic resonance image is obtained after data processing.

[0031] The present design also discloses a k-space trajectory correction method applied to heteronuclear imaging. Using the above system, it includes the following steps:

[0032] Step 1: Before the heteronuclear imaging sequence unit, a slice excitation unit is applied in the 1 H channel.

[0033] Step 2: In the heteronuclear imaging sequence unit, first, a radiofrequency pulse is transmitted in the heteronuclear channel to excite the heteronuclei to generate a signal. Then, in the signal reception stage, the receiving links of the 1 H / heteronuclear channels are enabled respectively, and the 1 H / heteronuclear signals are acquired; the acquired 1 H signal is processed to generate measured k-space trajectory data; the acquired heteronuclear signal is denoted as heteronuclear signal k-space data.

[0034] Step 3: Using the k-space trajectory data generated in Step 2, the k-space composed of the heteronuclear signals described in Step 2 is reconstructed to obtain a trajectory-corrected heteronuclear image.

[0035] Example 2: On the basis of Example 1, it is illustrated by taking the acquisition of 23 3D-UTE signal of Na as an example.

[0036] The basic sequence diagram of 3D-UTE is as Figure 2 shown. After the excitation pulse, three physical gradient axes output three-way spatial encoding gradients, and their gradient values are respectively:

[0037]

[0038] Among them, G r is the read gradient magnitude, and G x , G y , G z are the gradient value magnitudes on the three physical gradient axes respectively, and ψ and θ are the polar angle and azimuth angle in the spherical coordinate system. During the ADC opening period of the 3D-UTE sequence, that is, from the encoding gradient edge, the receiving system starts to acquire k-space data. This non-Cartesian acquisition has high requirements for trajectory correction. In practice, the common method is to use1 The k-space trajectory of the H signal is pre-determined specifically.

[0039] Apply the k-space trajectory correction method for heteronuclear imaging described in the present invention to the UTE sequence, and its sequence diagram is as Figure 3 shown. Compared with Figure 2 , Figure 3 In [reference], the read gradient of UTE is no longer represented as waveforms on the gradient axes in the three physical coordinate systems of X, Y, and Z, but is directly expressed as a path of 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 the k-space, which can be achieved by changing the rotation matrix of the gradient system. In addition, compared with the basic 3D-UTE sequence, Figure 3 the sequence in 23 adds a path of 1 H radio frequency pulse before the radio frequency pulse of 1 Na, and at the same time, adds a gradient in the readout gradient direction of UTE. This gradient serves as 1 the slice selection gradient and slice rephasing gradient of the 1 H channel, and can selectively excite an off-center thin slice. During the opening of ADC, the receiving system synchronously acquires 1 H / 23 Na signals, 23 the signal of 1 Na is the k-space data acquired by the UTE sequence, while the phase expression of the 1 H signal satisfies the following formula:

[0040]

[0041] where γ is the gyromagnetic ratio of 1 H, D r is the off-center distance of the selected thin slice, t is the time variable, r is the spatial variable, G r (t) is the 3D-UTE read gradient waveform, and k r (t) is the measured k-space trajectory. The first integral term on the right side of the formula is the spin phase caused by the 3D-UTE read gradient, and it can be seen that it is proportional to the k-space trajectory; the second term is the spin phase caused by other system factors such as magnetic field inhomogeneity.

[0042] To eliminate the influence of system factors on the 1 phase of the 1 H signal, in the trajectory design stage of UTE, each read gradient direction can have a corresponding reverse read gradient direction. As Figure 3 shown, for each UTE read gradient, another reverse read gradient can be found. Repeating the above signal acquisition process, then the phase expression of the 1 H signal is as shown in Equation (3):

[0043]

[0044] Observing Formulas (2) and (3), taking the difference of the phases of the MR signals measured twice, the phase caused by system factors can be eliminated to obtain the k-space trajectory:

[0045] k r (t) = ΔΦ r (t) / 2D r (4)

[0046] Such a set of 1 H signals can determine the k-space trajectory in the read gradient direction, which is denoted as kr here.

[0047] When the k-space trajectories in m different directions are measured, kx, ky, kz (i.e., the k-space trajectories when the read gradient directions are the three physical gradient axes X, Y, Z respectively) can be estimated:

[0048]

[0049]

[0050] Among them, T is the coefficient matrix determined by the measured read gradient direction, n is the number of sampling points of the trajectory data, and m is the number of collected directions. For the 3D-UTE sequence, as long as m ≥ 3, kx, ky, kz can be estimated, and then the complete k-space trajectory can be synthesized through a certain interpolation algorithm. Most simply, when the read gradient is selected to coincide with the three physical axes X, Y, Z respectively, when using 1 H signal to collect trajectory data, Equation (6) is the identity matrix, and the measured trajectories kr1, kr2, kr3 respectively correspond to kx, ky, kz. When m > 3, the accuracy of the measurement data can be improved, and the least squares method can be used to obtain:

[0051]

[0052] Since only the k-space trajectories in several directions need to be measured, and for 23 Na, 1 the gradient applied during the

[0053] For those skilled in the art, it is obvious that the present invention is not limited to the details of the above-described exemplary embodiments, and the present invention can be implemented in other specific forms without departing from the spirit or basic characteristics of the present invention. Therefore, from any point of view, the embodiments should be regarded as exemplary and non-limiting. The scope of the present invention is defined by the appended claims rather than the above description. Therefore, all changes falling within the meaning and scope of the equivalent elements of the claims are intended to be embraced within the present invention. Any reference signs in the claims should not be construed as limiting the claims involved.

[0054] In addition, it should be understood that although this specification is described in terms of embodiments, not every embodiment contains only one independent technical solution. This narrative manner of the specification is only for clarity. Those skilled in the art should regard the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

Claims

1. A k-space trajectory correction method for heteronuclear imaging, which uses a k-space trajectory correction system, includes a new pulse sequence unit and a corresponding data processing process, and is characterized in that: Step 1. Before the heteronuclear imaging sequence unit, apply a slice excitation unit to the 1 H channel; Step 2. In the heteronuclear imaging sequence unit, first, a radio frequency pulse is emitted in the heteronuclear channel to excite the heteronuclei to generate signals. Then, in the signal reception stage, the reception links of the 1 1 H / heteronuclear channels are enabled respectively to collect 1 H / heteronuclear signals; the collected heteronuclear signals are the k-space data of the heteronuclear signals; the collected 1 H signals are processed to generate the measured k-space trajectory data; Step 3. Using the k-space trajectory data, reconstruct the k-space composed of the heteronuclear signals to obtain a heteronuclear image with trajectory correction; The 1 slice selection excitation unit on the H channel includes an 1 H selective excitation pulse and corresponding slice selection gradient, and selectively excites an off-center slice; The 1 The slice selection gradient direction in the slice selection excitation unit on the H channel should be the same as the readout gradient direction in the heteronuclear imaging sequence unit; Selectively measure the k-space trajectory data in several readout gradient directions, and then obtain a complete k-space trajectory through the least squares method and data interpolation; In the signal reception stage, if there is a dephasing gradient before the read gradient in the heteronuclear imaging sequence unit, 1 the data of the H channel needs to be collected when the dephasing gradient starts to be applied; in the heteronuclear signal acquisition stage, 1 the H / heteronuclear signals need to be acquired synchronously; The k-space trajectory correction system includes a sampling sequence and signal processing system, a spectrometer control system, and a magnet and RF system. The sampling sequence and signal processing system are connected to the spectrometer control system, and the spectrometer control system is connected to the magnet and RF system through an amplifier and an auxiliary control section. The amplifier includes a heteronuclear RF amplifier, 1 an H-nuclide RF amplifier, 1 an H-nuclide RF preamplifier, a heteronuclear RF preamplifier, and a gradient amplifier. The spectrometer control system includes a broadband multi-nuclear signal excitation control system, a receiver, a gradient and RF power system, and a gradient control and system main control module. The sampling sequence and signal processing system includes a sampling sequence and image processing module and a main control system. The magnet and RF system uses a radio frequency transmitting and receiving integrated coil.

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