Method for optimizing the flip angle of a magnetic resonance variable flip-angle sequence, CEST imaging method, medium, and apparatus.

Abstract

This invention discloses a method for optimizing the flip angle of a variable magnetic resonance flip-angle sequence, a CEST imaging method, a medium, and an apparatus. This invention models the signal-to-noise ratio (SNR) enhancement problem as a flip-angle optimization problem and constructs an overall target function consisting of an SNR maximization target term and a resolution penalty term. The optimal solution of this overall target function can be found using a method that finds the derivative with respect to the flip angle, thereby obtaining the optimal flip angle that maximizes the SNR. Because the objective function of this invention includes a resolution penalty term, the variable flip-angle CEST can optimize SNR while simultaneously maintaining resolution. Compared to conventional filtering methods, this invention avoids noise amplification due to filtering and also eliminates the need for manual pre-window function setting.

Description

Technical Field

[0001] The present invention belongs to the field of magnetic resonance technology, and particularly relates to a method for optimizing the flip angle of a magnetic resonance variable flip angle sequence, a CEST imaging method, a medium, and an apparatus.

Background Art

[0002] Chemical exchange saturation transfer (CEST) imaging is a magnetic resonance molecular imaging technology that can detect various metabolites in the human body, such as glucose, free proteins, polypeptides, etc. The signal-to-noise ratio (SNR) of CEST imaging is extremely important for imaging quality. Since CEST imaging performs imaging depending on the attenuation of the water signal, its original SNR is low. In typical CEST imaging applications such as brain tumors, the contrast between normal tissues and lesions is only 2% - 3%, and the low SNR further reduces the contrast between the two.

[0003] Furthermore, for CEST imaging to be clinically valuable, its imaging speed and spatial coverage need to be close to that of existing structural imaging techniques. Gradient echo-based sequences, such as gradient echo (GRE) and echo planar imaging (EPI), offer fast imaging speeds but are susceptible to magnetic sensitivity and prone to artifacts at tissue-air boundaries. The researchers proposed SPACE-CEST, a whole-brain CEST imaging sequence using variable flip angle (Sampling perfection with application optimized contrasts by using different flip angle evolutions, SPACE). This sequence is based on turbo spin echo (TSE) and is unaffected by magnetic sensitivity. By reducing the flip angle (FA), the echo train length (ETL) is effectively extended, shortening acquisition time while covering the entire brain. However, the flip angle used in the constant flip angle 120° (CFA 120°) readout mode employed in SPACE-CEST sequences is not optimized, resulting in low SNR efficiency. Furthermore, the high flip angle used increases the specific absorption rate (SAR).

[0004] In existing techniques, researchers have achieved improved signal-to-noise ratio (SNR) by combining variable flip angle and k-space filtering. This method has a predetermined target window, defines SNR as a function of the flip angle and the target window, and maximizes SNR by changing the flip angle through iterative optimization. However, this method involves filtering the k-space signal, and for low signals in the echo chain, the filtering multiplies them by a relatively large coefficient, thus amplified both the signal and the noise. Furthermore, the selection of the target window also affects the SNR. [Overview of the project]

[0005] The objective of the present invention is to solve the problem of low imaging SNR due to the unoptimized flip angle used in existing whole-brain CEST imaging methods such as variable flip-angle CEST, and to provide a method for optimizing the flip angle of a magnetic resonance variable flip-angle sequence, a CEST imaging method, a medium, and an apparatus.

[0006] The specific technical solution employed in this invention is as follows: In a first embodiment, the present invention provides a method for optimizing the flip angle of a magnetic resonance variable flip angle sequence, which includes the following: For a magnetic resonance variable flip-angle sequence with an echo chain length of N-1, the flip-angle optimization problem for its N-1 refocus pulses is modeled with the following objective function. JPEG2026516293000002.jpg89170

[0007] As a preferred embodiment of the first embodiment, the iterative process for each round of solving the objective function is as follows: JPEG2026516293000003.jpg252170JPEG2026516293000004.jpg80170

[0008] In a preferred embodiment of the first embodiment described above, in S23, when optimizing the set of flip angles to be optimized based on the overall derivative, a gradient descent method or a conjugate gradient descent method is employed.

[0009] JPEG2026516293000005.jpg53170

[0010] In a preferred embodiment of the first embodiment described above, the ratio of the weighting coefficients a / b is between 100 and 2000.

[0011] In a second aspect, the present invention provides a magnetic resonance CEST imaging method. The imaging sequence used in this method includes a CEST saturation module, a fat suppression module, and a variable flip angle readout module. Of these, the echo chain length of the variable flip angle sequence in the variable flip angle readout module is N-1, and the flip angles of the N-1 refocus pulses are obtained by the flip angle optimization method for the magnetic resonance variable flip angle sequence described in any one of the first aspects.

[0012] In a third aspect, the present invention provides a computer-readable storage medium. The storage medium stores a computer program, and when the computer program is executed by a processor, the magnetic resonance variable flip angle sequence flip angle optimization method described in any of the first aspects is realized.

[0013] In a fourth aspect, the present invention provides a computer electronic device including a storage device and a processor. The aforementioned storage device is used to store computer programs. When the processor executes the computer program, it implements the magnetic resonance variable flip angle sequence flip angle optimization method described in any of the first embodiments.

[0014] In a fifth aspect, the present invention provides a variable flip-angle magnetic resonance imaging apparatus, comprising a magnetic resonance scanner and a control unit. A computer program is stored in the control unit. When the computer program is executed, it is used to implement the magnetic resonance variable flip-angle sequence flip-angle optimization method described in any one of the first aspects described above. The magnetic resonance scanner is used to perform magnetic resonance CEST imaging according to the flip-angle sequence obtained by optimization and to acquire a CEST image.

[0015] The present invention has the following beneficial effects compared to existing technologies. This invention models the SNR enhancement problem in magnetic resonance programs as a flip angle optimization problem for a variable flip angle sequence. This optimization problem has an analytical derivative with respect to the flip angle, allowing for efficient problem solving. Simultaneously, since this optimization problem includes a resolution penalty term, resolution can be maintained while the variable flip angle CEST optimizes the SNR. Compared to conventional filtering methods, this invention avoids noise amplification due to filtering and also eliminates the need for manual pre-window function setting. [Brief explanation of the drawing]

[0016] [Figure 1] Figure 1 shows the timing diagram of an SNR-boosted variable flip-angle CEST imaging sequence, with an echo chain length (ETL) of N-1. Of these, the N-1 variable flip-angle readouts are the parts that require optimization. [Figure 2] Figure 2 is a schematic diagram of the EPG (Electronic Program Guide) state. [Figure 3] Figure 3 shows the flip angle, signal curve, and point spread function obtained by optimization under different settings. (A~C) SNR only considered, (D~F) Resolution only considered, (G~I) SNR and resolution considered simultaneously. [Figure 4] Figure 4 shows a localized magnification of a layer of images collected with a flip angle optimized for resolution only, a localized magnification of a layer of images collected with CFA120°, and the quantitative degree of blurring of images from different layers. [Modes for carrying out the invention]

[0017] The present invention will be described in more detail below in conjunction with the drawings and specific implementation methods. The technical features of each implementation method in the present invention can be combined in corresponding ways, provided that they are not mutually contradictory. To make the above-mentioned objectives, features, and advantages of the present invention clearer and easier to understand, specific embodiments of the present invention will be described in detail below with reference to the drawings. Many specific details are provided in the following description to fully understand the present invention. However, the present invention can be implemented in many other ways different from those described herein, and those skilled in the art can make similar improvements without contradicting the spirit of the invention; therefore, the present invention is not limited by the specific embodiments disclosed below. The technical features in each embodiment of the present invention can be combined in corresponding ways, provided they are not mutually contradictory.

[0018] The concept of this invention is to model the signal-to-noise ratio (SNR) enhancement problem as a flip angle optimization problem and construct an overall target function consisting of an SNR maximization target term and a resolution penalty term. Here, the resolution penalty term adopts a functional form of division of two terms, and the optimal solution of this overall target function is found using a method of finding the derivative with respect to the flip angle, thereby obtaining the optimal flip angle that maximizes the SNR.

[0019] To facilitate understanding, before describing the specific technical implementation of the present invention, we will first explain the principle of the present invention in detail. 1. Modeling SNR enhancement as an optimization problem This invention provides an SNR-enhanced variable flip-angle CEST imaging method, which is called SNR-boosted variable flip-angle CEST. This method models SNR as a function of the flip angle and maximizes SNR by optimizing this function. At the same time, it imposes constraints on the signal to reduce image blurring due to signal modulation. Figure 1 shows the timing diagram of the SNR-boosted variable flip-angle CEST sequence. It includes three parts: CEST saturation, fat suppression, and signal readout, and this invention optimizes the signal readout portion. For a uniform object, its i-th echo in k-space can be expressed as follows: JPEG2026516293000006.jpg217170

[0020] 2 Solving the optimization problem The following focuses on solving the above optimization problem using EPG. 2.1 Introduction to the EPG framework JPEG2026516293000007.jpg135170

[0021] 2.2 Splitting the optimization problem and solving the derivative of the sub-problem JPEG2026516293000008.jpg98170

[0022] The following uses the adjoint state method (ASM) to find the derivative of g with respect to the final flip angle α , , , , ,

[0023] , ,

[0024] of. JPEG2026516293000009.jpg250170JPEG2026516293000010.jpg248170JPEG2026516293000011.jpg172170Based on the above theoretical considerations, in the following, the present invention shows the specific implementation manner and technical effects of the flip angle optimization method of the magnetic resonance variable flip angle sequence through specific embodiments.

Embodiment

[0023] In this embodiment, the specific procedure of the flip angle optimization method of the magnetic resonance variable flip angle sequence is as follows. S1. For a magnetic resonance variable flip angle sequence with an echo train length of N - 1, model the flip angle optimization problem of its N - 1 refocusing pulses as the following objective function. JPEG2026516293000012.jpg250170JPEG2026516293000013.jpg250170

[0024] Based on the optimal solution of the flip angle set obtained by the optimization solution described above, it can be used as the flip angle setting value for N-1 refocus pulses in the variable flip angle sequence in the imaging sequence. This forms an imaging sequence for magnetic resonance CEST imaging and is used for magnetic resonance imaging. As shown in Figure 1, this imaging sequence includes a CEST saturation module, a fat suppression module, and a variable flip angle readout module, the echo chain length of the variable flip angle sequence in the variable flip angle readout module is N-1, and the flip angles of the N-1 refocus pulses are obtained by the flip angle optimization method for the magnetic resonance variable flip angle sequence described above. The CEST saturation module and fat suppression module in the sequence belong to existing technology, and in this embodiment, you can refer to Figure 1 for specifics, so they will not be explained again here.

[0025] To demonstrate the effects of the two optimization terms in the objective function of the present invention, this embodiment performs flip angle optimization for three cases: (1) Optimization targeting only SNR, i.e., setting coefficient a to 1 and coefficient b to 0. (2) Optimization targeting only resolution, i.e., setting coefficient a to 0 and coefficient b to 1. (3) Optimization targeting both SNR and resolution simultaneously, i.e., setting coefficient a to 1 and coefficient b to 0.001. In the optimization process of this embodiment, the physiological parameters are T1=1000ms, T2=100ms, and the sequence parameters are echo spacing (tesp)=4.06ms, echo chain length ETL=140. The initial values ​​and upper and lower limits of the flip angles to be optimized are set as follows: the initial value of all flip angles in the entire echo chain is 10°, the upper limit of all flip angles is 180°, and the lower limit of all flip angles is 0°. Figures 3A, 3B, and 3C show the results of optimization targeting only SNR. The signal curve obtained by optimization is similar to that of CFA120°, but the flip angle used is much smaller. Figures 3D, 3E, and 3F show the results of optimization targeting only resolution. The signal curve obtained by optimization remains almost constant throughout the entire echo chain, and the corresponding point spread function closely matches that of the ideal case. Figures 3G, 3H, and 3I show the results of optimization targeting both SNR and resolution simultaneously. The signal obtained by optimization is 4.7% higher than that of CFA120°, and the full width at half maximum (FWHM) of the simultaneously obtained point spread function is 6.1% smaller than that of CFA 120°. Figure 4B shows images collected at a flip angle obtained by optimizing only the resolution. Compared to the images collected with CFA120° in Figure 4A, there is no obvious blurring in the phase encoding direction. The quantitative blur level in Figure 4C further shows that the CFA120° image is more blurred, indicating that the image is more blurred.

[0026] Furthermore, the flip angle optimization method for a magnetic resonance variable flip angle sequence provided in the above-described embodiment may essentially exist in the form of a program processing flow. Therefore, similarly, based on the same concept of the invention, another preferred embodiment of the present invention further provides a computer electronic device corresponding to the flip angle optimization method for a magnetic resonance variable flip angle sequence provided in the above-described embodiment. It includes a memory device and a processor. The aforementioned storage device is used to store computer programs. When the processor executes the computer program, it implements the flip angle optimization method for the magnetic resonance variable flip angle sequence described above.

[0027] Similarly, based on the same concept of the invention, another preferred embodiment of the present invention further provides a computer-readable storage medium corresponding to the flip angle optimization method for a magnetic resonance variable flip angle sequence provided in the above-described embodiment. The storage medium stores a computer program, and when the computer program is executed by a processor, the aforementioned flip angle optimization method for a magnetic resonance variable flip angle sequence is realized. To make it clear, the above-mentioned storage devices and storage media can include Random Access Memory (RAM) and Non-Volatile Memory (NVM), such as at least one disk storage device. At the same time, the storage media can also be various other media capable of storing program code, such as USB memory, removable hard disks, magnetic disks, or optical disks. Of course, with the widespread application of cloud servers, the aforementioned software programs are also installed on cloud platforms to provide corresponding services, so computer-readable storage media are not limited to local hardware formats. To make it clear, the aforementioned processors can include general-purpose processors, i.e., Central Processing Units (CPUs), Network Processors (NPs), etc., and may also be Digital Signal Processing (DSPs), Application Specific Integrated Circuits (ASICs), Field-Programmable Gate Arrays (FPGAs), or other programmable logic devices, discrete gates or transistor logic devices, or discrete hardware components. It should be noted that, for the sake of convenience and brevity, technicians in the relevant technical field can refer to the corresponding processes in the embodiments of the aforementioned methods for the specific operating processes of the apparatus described above, and therefore will not be described again here. In each embodiment provided by this application, the division of steps or modules in the apparatus and methods is merely a logical functional division, and other division methods may be possible in actual implementation. For example, multiple modules or steps can be combined or integrated, and a single module or step can also be divided.

[0028] Furthermore, the logical instructions in the aforementioned memory device may be implemented in the form of software function units, which, when sold or used as independent products, can be stored on a single computer-readable storage medium. Based on this understanding, the essence of the technical solution of the present invention, or its contribution to the prior art, or a portion of the technical solution, may be embodied in the form of a software product, which is stored on a single storage medium and includes a plurality of instructions for a single computer device (which may be a personal computer, server, or network device, etc.) to perform all or part of the steps of the methods described in each embodiment of the present invention. Similarly, based on the same concept of the invention, another preferred embodiment of the present invention further provides a variable flip-angle magnetic resonance imaging apparatus corresponding to the flip-angle optimization method for a variable flip-angle magnetic resonance sequence provided in the above-described embodiment, comprising a magnetic resonance scanner and a control unit. A computer program is stored in the control unit, and when the computer program is executed, it is used to implement the flip angle optimization method for the magnetic resonance variable flip angle sequence described in any one of the first embodiments described above. The aforementioned magnetic resonance scanner is used to perform magnetic resonance CEST imaging according to the flip angle sequence obtained through optimization, and to acquire CEST images. It should be noted that the magnetic resonance scanner can be any magnetic resonance scanner capable of performing CEST imaging, its structure belongs to existing technology, mature commercially available products can be used, and specific model numbers are not limited. Furthermore, in addition to the computer program mentioned above, the control unit of the magnetic resonance imaging apparatus should also include the imaging sequence and other software programs necessary to perform CEST imaging. Of course, the control unit described above can be an independent control unit, or it can be a control unit originally installed in the magnetic resonance scanner. In other words, the flip angle optimization method for the variable flip angle sequence of magnetic resonance described above can be integrated into the control unit of the magnetic resonance imaging device in the form of a data processing program, thereby allowing the magnetic resonance scanner to directly optimize the variable flip angle sequence in the imaging sequence online without the need for an additional control unit, and to perform magnetic resonance CEST imaging according to the flip angle sequence obtained through optimization.

[0029] The embodiments described above represent only one preferred approach to the present invention, but are not intended to limit it. A person ordinarily skilled in the art can make various further changes and modifications without departing from the spirit and scope of the invention. Therefore, all technical solutions obtained by equivalent substitution or equivalent transformation fall within the scope of the present invention.

Claims

1. A method for optimizing the flip angle of a magnetic resonance variable flip angle sequence, S1, For a magnetic resonance variable flip-angle sequence with an echo chain length of N-1, the flip-angle optimization problem for its N-1 refocus pulses is modeled as follows: S2, A method for optimizing the flip angle of a magnetic resonance variable flip angle sequence, characterized in that the objective function is represented in an extended phase diagram (EPG), an iterative solution is performed to obtain the optimal solution of the optimized flip angle set, and this is used as the flip angle setting value for N-1 refocus pulses in the magnetic resonance variable flip angle sequence.

2. The iterative process for each round of solving the aforementioned objective function is as follows:

3. The flip angle optimization method for a magnetic resonance variable flip angle sequence according to claim 1, characterized in that, in S23 above, when optimizing the set of flip angles to be optimized based on the overall derivative, a gradient descent method or a conjugate gradient descent method is employed.

4.

5. The flip angle optimization method for a magnetic resonance variable flip angle sequence according to claim 1, characterized in that the ratio of the weight coefficients a / b is 100 to 2000.

6. Magnetic resonance imaging (CEST) method, A magnetic resonance CEST imaging method characterized in that the imaging sequence used in the above method includes a CEST saturation module, a fat suppression module, and a variable flip angle readout module, wherein the echo chain length of the variable flip angle sequence in the variable flip angle readout module is N-1, and the flip angles of N-1 refocus pulses are obtained by the magnetic resonance variable flip angle sequence flip angle optimization method described in any one of claims 1 to 5.

7. A computer-readable storage medium, A computer-readable storage medium, wherein a computer program is stored in the storage medium, and when the computer program is executed by a processor, a method for optimizing the flip angle of a variable magnetic resonance flip angle sequence described in any one of claims 1 to 5 is realized.

8. A computer electronic device, including a memory device and a processor, The aforementioned storage device is used to store computer programs. The computer electronic device is characterized in that, when the processor executes the computer program, it implements the method for optimizing the flip angle of a magnetic resonance variable flip angle sequence according to any one of claims 1 to 5.

9. A variable flip-angle magnetic resonance imaging apparatus, comprising a magnetic resonance scanner and a control unit, A computer program is stored in the control unit, and when the computer program is executed, it is used to realize the flip angle optimization method for a magnetic resonance variable flip angle sequence described in any one of claims 1 to 5. A variable flip-angle magnetic resonance imaging apparatus, characterized in that the magnetic resonance scanner is used to perform magnetic resonance CEST imaging according to a flip-angle sequence obtained by optimization and to acquire a CEST image.

Images