An optimized CAIPI encoding gradient calculation method

Abstract

The application relates to an optimized CAIPI encoding gradient calculation method and relates to the technical field of image processing, and comprises the following steps: step one, calculating the magnetic resonance signal phase difference between two layers after a linear gradient magnetic field is applied in a selected layer direction; step two, performing a modulo operation on the magnetic resonance signal phase difference between the two layers according to a 2pi period to obtain a remainder after the magnetic resonance signal phase difference is modulated, and taking the remainder as an equivalent phase value; and step three, obtaining an optimized phase difference and the area of the CAIPI encoding gradient according to the equivalent phase value. The phase of the index has a periodic characteristic, the value range of the phase is shortened, the CAIPI gradient area is optimized, the hardware load of the gradient system is reduced, and the problem that the value range of the CAIPI encoding gradient area to be applied is increased, the value range cannot be reduced in a single echo sequence, and a large cumulative phase difference is introduced, thereby causing signal attenuation is solved.

Description

Technical Field

[0001] This invention relates to the field of image processing technology, specifically to an optimized CAIPI coding gradient calculation method. Background Technology

[0002] In the field of image processing technology, CAIPI technology is used. Specifically, CAIPI technology refers to controlled aliasing artifacts, which is used in multi-layer simultaneous excitation technology to excite data from multiple layers at the same time. Subsequently, different image domain shifts are required for different data to improve the image quality of parallel reconstruction.

[0003] This image domain translation is along the phase-encoding direction, and therefore also uses the phase-encoding direction field of view (FOV). y The ratio r CAIPI It is measured by the fact that adjacent layers have r. CAIPI ×FOV y To achieve the desired image domain spacing, the j-th phase-coded data in the i-th layer must undergo phase modulation. The specific modulation method used is: ...Formula 1; Where i0 and j0 are arbitrary constants, this phase modulation is generally achieved by applying a gradient magnetic field in the layer selection direction before acquiring the j-th phase-encoded data. This gradient magnetic field is the CAIPI encoding gradient, specifically, the area of ​​the gradient magnetic field is A. CAIPI ; The formula for expressing the area of ​​the gradient magnetic field is: ...Formula 2; Δz is the spacing (in mm) between adjacent layers in multilayer excitation.

[0004] The main drawback of the above method for applying the gradient magnetic field is that as the number of phase codes increases, the range of values ​​for the CAIPI coded gradient area to be applied also increases. Assuming there are a total of N... y If there are 1 phase code, then the range of values ​​for the CAIPI coding gradient area of ​​all phase codes is: This range of values ​​cannot be reduced in a single-echo sequence and will introduce a large cumulative phase difference, leading to signal attenuation. Furthermore, an excessively large gradient area will increase the load on the gradient system, distort the output gradient waveform, and consequently affect image quality. Summary of the Invention

[0005] The purpose of this invention is to provide an optimized method for calculating CAIPI encoding gradients.

[0006] The technical problem solved by this invention is: in the prior art, as the number of phase codes increases, the range of values ​​for the area of ​​the CAIPI coding gradient to be applied increases, and this range cannot be reduced in a single echo sequence, and will introduce a large cumulative phase difference, thereby causing signal attenuation.

[0007] This invention can be achieved through the following technical solution: an optimized CAIPI encoding gradient calculation method, comprising the following steps: Step 1: Calculate the phase difference of the magnetic resonance signal between the two layers after applying a linear gradient magnetic field in the selected layer direction; Step 2: The phase difference of the magnetic resonance signals between the two layers is calculated according to 2... The periodic modulo operation is performed to obtain the remainder after taking the modulo of the phase difference of the magnetic resonance signal, and the remainder is used as the equivalent phase value; Step 3: Obtain the optimized phase difference and the area of ​​the CAIPI encoding gradient based on the equivalent phase value.

[0008] A further technical improvement of the present invention is that it also includes the following steps: Step 4: Based on the image off-center distance obtained by acquisition and calculation during multi-layer data excitation, the phase accumulation under the influence of the optimized CAIPI coding gradient is calculated. Step 5: Based on the accumulated phase, obtain the compensated phase value, and then compensate the phase according to the compensated phase value.

[0009] A further technical improvement of the present invention is that the method of phase compensation includes acquiring image data obtained under theoretical conditions, reconstructing the image based on the acquired data, and multiplying the complex data by the corresponding compensation phase value during reconstruction.

[0010] A further technical improvement of the present invention is that the phase compensation method includes adjusting the bias phase of the digital-to-analog converter used to receive data according to the compensation phase value when acquiring image data obtained under theoretical conditions.

[0011] A further technical improvement of this invention lies in the following: the method for calculating the phase difference of the magnetic resonance signal includes: The area A of the linear gradient magnetic field under the theoretical model is collected, and the interval between the two layers under the theoretical model is collected. The phase difference of the magnetic resonance signal was obtained. for: .

[0012] A further technical improvement of the present invention is that the method for obtaining the equivalent phase value includes: Phase difference of magnetic resonance signal under theoretical conditions Optimized magnetic resonance phase difference under theoretical conditions was collected. , ; j represents the position of the phase code. Indicates the phase-encoded direction of the field of view The ratio, MOD represents the modulo operation; Based on the optimized magnetic resonance phase difference The area of ​​the optimized CAIPI encoding gradient is calculated. ; .

[0013] A further technical improvement of the present invention is that the optimized phase accumulation acquisition method under the influence of CAIPI encoding gradient includes: Image off-center distance under theoretical conditions during multi-layer data excitation The layer intervals that are simultaneously excited by collecting multi-layer data under theoretical conditions z, where i represents the number of positions in the excitation layer and j represents the position of the phase encoder, yields the phase accumulation. ; ; This represents the optimized magnetic resonance phase difference.

[0014] A further technical improvement of the present invention lies in: compensating for the phase value The data collection methods include: ; Indicates the phase-encoded direction of the field of view The proportion.

[0015] Compared with the prior art, the present invention has the following beneficial effects: 1. This application uses a theoretical model to calculate the phase difference of the magnetic resonance signal between two layers after applying a linear gradient magnetic field in the selected layer direction, and then applies the obtained magnetic resonance signal phase difference according to 2 The periodic modulo operation yields the remainder after taking the modulus of the phase difference of the magnetic resonance signal. This remainder is used as the equivalent phase value. Based on the equivalent phase value, the optimized phase difference and the area of ​​the CAIPI encoded gradient are obtained. By leveraging the periodic nature of the exponential phase, the value range of the phase is shortened, thereby optimizing the CAIPI gradient area, reducing the hardware load of the gradient system, and solving the problem that the range of values ​​for the applied CAIPI encoded gradient area increases, which cannot be reduced in a single echo sequence and introduces a large cumulative phase difference, leading to signal attenuation.

[0016] 2. This application collects the image off-center distance when multi-layer data is excited under theoretical conditions, calculates the phase accumulation under the influence of the optimized CAIPI coding gradient based on the obtained image off-center distance, and then obtains the compensated phase value based on the obtained phase accumulation. The phase is compensated based on the compensated phase value, which can solve the problem of artifacts. That is, the phase compensation method of off-center imaging can solve the phase error problem introduced by the optimized CAIPI coding gradient, stably and effectively correct the introduced phase error, and avoid artifacts caused by off-center excitation. Attached Figure Description

[0017] To facilitate understanding by those skilled in the art, the present invention will be further described below with reference to the accompanying drawings.

[0018] Figure 1 This is a system principle block diagram of the present invention; Figure 2 This is a schematic diagram of the CAIPI encoding gradient before optimization in this invention; Figure 3 This is a schematic diagram of the optimized CAIPI encoding gradient of the present invention; Figure 4 This is a schematic diagram of phase-affected imaging during off-center imaging according to the present invention; Figure 5 This is a schematic diagram illustrating the phase effect on the off-center distance during off-center imaging according to the present invention; Figure 6 The bias phase value of the ADC after off-center phase compensation in the structure of this invention is given. Detailed Implementation

[0019] To further illustrate the technical means and effects of the present invention in achieving its intended purpose, the following detailed description of the specific implementation methods, structures, features, and effects of the present invention, in conjunction with the accompanying drawings and preferred embodiments, is provided below.

[0020] The following explanations are provided based on the technical terms used in this application: Phase coding: One type of coding used for spatial positioning in magnetic resonance imaging. In one phase coding, a phase modulation mode is applied and data is acquired. Multiple sets of signals with different phase modulation modes are acquired through multiple phase codings, and the spatial organization signal is solved by Fourier transform.

[0021] Phase coding direction: One direction in a two-dimensional magnetic resonance image (the other direction is the readout coding direction).

[0022] Phase encoding number N y The number of phase codes can generally be approximated as the number of pixels in that direction in the reconstructed image.

[0023] Field of view (FOV): The effective area of ​​an MRI scan, typically 200-400 mm, to ensure coverage of the scanned area (such as the brain, knees, abdomen, etc.).

[0024] Gradient magnetic field: The magnetic field that enables spatial positioning and phase modulation of magnetic resonance. Its intensity varies linearly along the spatial coordinates, and its waveform in the magnetic resonance time sequence is generally trapezoidal.

[0025] CAIPI (Controlled Aliasing in Parallel Imaging) is an accelerated imaging technique that improves image reconstruction quality during accelerated imaging by allowing multi-layer data to have controllable image domain undersampling artifacts.

[0026] CAIPI Encoding Gradient: The gradient magnetic field used to achieve the CAIPI effect. It encodes the signal through a certain phase modulation, thereby causing different levels of data to have different degrees of image domain translation.

[0027] Please see Figure 1-6 As shown, an optimized CAIPI encoding gradient calculation method includes the following steps: Step one: First, this application performs simulation calculations based on a theoretical model. After applying a linear gradient magnetic field with an area of ​​A in the selected layer direction, the phase difference of the magnetic resonance signal between two layers with a spacing of Δz is calculated. for: ...Formula 3; Among them G z This represents the gradient magnetic field applied along the z-axis (where the gradient magnetic field refers to a magnetic field whose intensity changes linearly along a certain direction).

[0028] To satisfy the CAIPI phase encoding condition shown in Formula 1, the phase difference between adjacent layers at the j-th phase encoding point... The area A of the applied CAIPI encoding gradient CAIPI (j) can be written in the following form: ...Formula 4; Formula 4 includes the method for calculating the area of ​​the CAIPI coding gradient in Formula 2, and this area increases with the increase of the phase coding number j; Step two, by utilizing the properties of the exponential function, the aforementioned phase difference is calculated according to... The modulo operation is performed periodically, and the remainder obtained after the modulo operation is the equivalent phase value after the range of values ​​is reduced. Therefore, the optimized phase difference proposed in this application Area of ​​the CAIPI encoded gradient as follows: ...Formula 5; Where MOD(a,b) represents |(ab×floor(a / b))|, that is, it means The modulo operation, where floor() represents the rounding operation towards negative infinity, can optimize the CAIPI encoding gradient, reduce the amplitude of the CAIPI gradient waveform of multiple phase encodings, and maintain the encoding effect of CAIPI.

[0029] Step 3: In order to solve the problem of off-center imaging after applying the optimized CAIPI encoding gradient (i.e., the scanning plane is not in the center of the magnet), additional phase compensation is performed on the acquired data to avoid artifacts.

[0030] Assuming the distance of the off-center scan is The layer spacing of multiple simultaneous excitations is Then the first one is activated at the same time The first layer Phase accumulation of phase-encoded data under the influence of the optimized CAIPI encoding gradient in Formula 5 for: ...Formula 6; Formula 6 is based on the off-center distance The generation of an additional phase may violate the CAIPI condition in Equation 1. When =0, there is no additional effect. Layer interval When it is an integer multiple of , it will cause the interlayer to shift by a distance along the phase coding direction; in Not the number of layers When the value is an integer multiple of the phase, it will disrupt the linearity of phase modulation between phase codes, thereby causing image artifacts; For example, in the appendix Figure 5 The three scenarios shown depict three layers of data being excited simultaneously, and the location of the excitation center layer is compared in each case. The results before and after phase compensation at the location.

[0031] Step four: This application employs two specific methods to perform phase compensation during off-center imaging.

[0032] To address the off-center phase error caused by the optimized CAIPI encoding gradient in Formula 5, phase compensation is required for each phase-encoded data. The phase compensation is calculated as follows: ...Formula 7; in This indicates the phase value that needs to be compensated.

[0033] There are two specific compensation methods. The first method is to multiply the complex data by the corresponding phase value during the reconstruction process after data acquisition. The second method is to adjust the bias phase of the digital-to-analog converter (ADC) used to receive data during data acquisition to achieve the effect of automatic phase compensation.

[0034] like Figure 6 The figure shows the ADC bias phase values ​​for each phase encoding stage after applying off-center phase compensation, in degrees (where the bias phase is not specified). Figure 6 The horizontal axis in the equation represents the time axis, which is the duration of the digital-to-analog converter (ADC) used for signal acquisition. The vertical axis represents the phase, in degrees, which corresponds to the phase modulation value of the ADC under different phase encodings. (A horizontal line represents the ADC phase modulation value under one phase encoding.) Since the phase compensation involved in Equation 7 has a certain periodicity, even with 128 phase encodings, the value of Equation 7 is much smaller than 128 because there are only 16 phase offsets.

[0035] Through the above technical solutions, this application adopts an optimized CAIPI gradient area calculation method, which can effectively reduce the hardware load of the gradient system, enabling CAIPI imaging to be realized even in low-performance gradient systems with low gradient switching rates and low peak output voltages. At the same time, the phase compensation method of off-center imaging described in this application can solve the phase error problem caused by the optimized CAIPI coded gradient, stably and effectively correct this introduced phase error, and avoid artifacts caused by off-center excitation.

[0036] In use, this invention first calculates the phase difference of the magnetic resonance signal between two layers after applying a linear gradient magnetic field in the selected layer direction, based on the calculation method of the CAIPI encoded gradient and the theoretical model. for: To optimize the gradient area, this invention utilizes the properties of the exponential function. Because the phase of the exponential function has... Due to its periodic characteristics, the phase difference mentioned above can be used for signal encoding according to... The modulo operation is performed periodically, and the remainder obtained after modulo is the equivalent phase value after narrowing the value range. At this point, the optimized phase difference proposed in this application is obtained. Area of ​​the CAIPI encoded gradient as follows: Subsequently, after applying the optimized CAIPI encoded gradient, additional phase compensation is required for the acquired data during off-center imaging (i.e., the scanning plane is not at the center of the magnet); otherwise, artifacts will occur. Specifically, the assumed off-center scanning distance is... The layer spacing of multiple simultaneous excitations is Then the first one is activated at the same time The first layer Phase accumulation of phase-encoded data under the influence of optimized CAIPI encoding gradient for: Subsequently, based on phase accumulation The calculation method for phase compensation is obtained, namely... Phase compensation is achieved by obtaining the numerical values.

[0037] This application utilizes the periodicity of the phase to optimize the area of ​​the CAIPI encoding gradient. While ensuring that the CAIPI phase modulation conditions are met, its area is reduced. When the phase encoding number is 128, this application reduces the area of ​​the CAIPI encoding gradient to less than 16% of the original, reducing the load on the gradient system and avoiding problems such as eddy currents and waveform distortion that may be caused when applying a strong gradient magnetic field, thus ensuring imaging quality.

[0038] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some modifications or alterations to the above-disclosed technical content to create equivalent embodiments without departing from the scope of the present invention. Any simple modifications, equivalent changes and alterations made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the scope of the present invention.

Claims

1. An optimized method for calculating the CAIPI encoding gradient, characterized in that: Includes the following steps: Step 1: Calculate the phase difference of the magnetic resonance signal between the two layers after applying a linear gradient magnetic field in the selected layer direction; Step 2: The phase difference of the magnetic resonance signals between the two layers is calculated according to 2... The periodic modulo operation is performed to obtain the remainder after taking the modulo of the phase difference of the magnetic resonance signal, and the remainder is used as the equivalent phase value; Step 3: Obtain the optimized phase difference and the area of ​​the CAIPI encoded gradient based on the equivalent phase value; Methods for calculating the phase difference of magnetic resonance signals include: The area A of the linear gradient magnetic field under the theoretical model is collected, and the interval between the two layers under the theoretical model is collected. The phase difference of the magnetic resonance signal was obtained. for: ; The methods for obtaining the equivalent phase value include: Phase difference of magnetic resonance signal under theoretical conditions Optimized magnetic resonance phase difference under theoretical conditions was collected. , ； j represents the position of the phase code. Indicates the phase-encoded direction of the field of view The ratio, MOD represents the modulo operation; Based on the optimized magnetic resonance phase difference The area of ​​the optimized CAIPI encoding gradient is calculated. ; 。 2. The optimized CAIPI encoding gradient calculation method according to claim 1, characterized in that, It also includes the following steps: Step 4: Based on the image off-center distance obtained by acquisition and calculation during multi-layer data excitation, the phase accumulation under the influence of the optimized CAIPI coding gradient is calculated. Step 5: Based on the accumulated phase, obtain the compensated phase value, and then compensate the phase according to the compensated phase value.

3. The optimized CAIPI encoding gradient calculation method according to claim 2, characterized in that, Phase compensation methods include acquiring image data obtained under theoretical conditions, reconstructing the image based on the acquired data, and multiplying the complex data by the corresponding compensation phase value during reconstruction.

4. The optimized CAIPI encoding gradient calculation method according to claim 2, characterized in that, Phase compensation methods include adjusting the bias phase of the digital-to-analog converter used to receive data based on the compensated phase value when acquiring image data obtained under theoretical conditions.

5. The optimized CAIPI encoding gradient calculation method according to claim 1, characterized in that, The optimized phase accumulation acquisition method under the influence of CAIPI encoded gradient includes: Image off-center distance under theoretical acquisition conditions during multi-layer data excitation The layer intervals that are simultaneously excited by collecting multi-layer data under theoretical conditions. z, where i represents the number of positions in the excitation layer and j represents the position of the phase encoder, yields the phase accumulation. ; ； This represents the optimized magnetic resonance phase difference.

6. The optimized CAIPI encoding gradient calculation method according to claim 5, characterized in that, Compensation phase value The data collection methods include: ； Indicates the phase-encoded direction of the field of view The proportion.

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