CN112557978B - Multi-echo sampling method based on three-dimensional non-Cartesian trajectory - Google Patents

Abstract

The present invention provides a multi-echo sampling method based on a three-dimensional non-Cartesian trajectory for quantitative magnetic susceptibility imaging technology, which specifically includes: selecting a cylindrical k-space, selecting multiple acquisition planes in the cylindrical k-space, encoding the multiple acquisition planes; on each of the multiple acquisition planes, select a data acquisition curve; determine according to the encoding of the multiple acquisition planes and each of the acquisition curves Three-dimensional non-Cartesian trajectory; design a scan time sequence according to the three-dimensional non-Cartesian trajectory; implement resonance data acquisition according to the scan time series. Through the three-dimensional non-Cartesian trajectory, the invention increases the filling rate of the k-space, improves the data acquisition efficiency, and shortens the quantitative magnetic susceptibility imaging time.

Description

A Multi-echo Sampling Method Based on 3D Non-Cartesian Trajectories

technical field

The invention belongs to the technical field of quantitative magnetic susceptibility imaging, and in particular relates to a multi-echo sampling method based on a three-dimensional non-Cartesian trajectory.

Background technique

Quantitative Susceptibility Mapping (QSM) is a three-dimensional quantitative visualization technique for the magnetic properties of biological tissues in magnetic resonance imaging. Magnetic susceptibility is an inherent physical property of matter, which reflects the ability of matter to be magnetized in an external magnetic field. When the magnetic susceptibility source is placed in an external magnetic field, it will cause local magnetic field changes, resulting in different phase progressions of the magnetic resonance signals in multiple echoes. Quantitative magnetic susceptibility imaging technology quantitatively measures the magnetic susceptibility changes caused by physiological phenomena such as iron content, calcification, and changes in blood oxygen saturation in tissues by analyzing the image phase and using the algorithm of inverse problem solving. Brain imaging using quantitative magnetic susceptibility imaging can obtain the spatial distribution of brain iron content, which can aid in the examination of diseases such as cerebral hemorrhage, multiple sclerosis, and Parkinson's syndrome.

In existing quantitative susceptibility imaging techniques, a Cartesian trajectory is generally followed for multi-echo sampling. Specifically, for multiple gradient echo signals excited by a single pulse, a Cartesian trajectory is filled in k-space, and these echo signals are collected following the Cartesian trajectory. However, Cartesian trajectories have a low filling rate in k-space, resulting in inefficient data acquisition and long quantitative susceptibility imaging times. Taking 256 phase codes as an example, when using traditional Cartesian trajectory acquisition, only one row of data in k-space is filled after a single pulse excitation, and 256 pulse excitations are required to complete the filling of all k-space. Compared with the Cartesian trajectory, when the non-Cartesian trajectory is used for acquisition, the k-space range that can be filled after a single pulse excitation is larger, so the required pulse excitation times can be reduced, thereby improving the acquisition efficiency and shortening the scanning time.

Therefore, those skilled in the art expect to develop a multi-echo sampling method based on three-dimensional non-Cartesian trajectories, so as to improve the efficiency of data acquisition, thereby shortening the time for quantitative magnetic susceptibility imaging.

SUMMARY OF THE INVENTION

The present invention provides a multi-echo sampling method based on a three-dimensional non-Cartesian trajectory, which specifically includes the following steps:

Step 1, select a cylindrical k-space, and select a plurality of acquisition planes in the cylindrical k-space, and encode the plurality of acquisition planes;

Step 2, on each of the multiple acquisition planes, select a data acquisition curve;

Step 3, determining a three-dimensional non-Cartesian trajectory according to the encoding of the multiple acquisition planes and each of the acquisition curves;

Step 4, designing a scanning time series according to the three-dimensional non-Cartesian trajectory;

Step 5, according to the scanning time sequence, implement resonance data acquisition.

Further, in step 1, the multiple acquisition planes are parallel to each other.

Further, in step 1, the multiple acquisition planes are all perpendicular to the slice selection direction of the cylindrical k-space.

Further, in step 1, equidistant coding is used for the multiple acquisition planes to determine the layer selection direction coordinates of the acquisition planes.

Further, the equidistant coding satisfies the following relational formula:

为第i次选层编码时的采集平面的选层方向坐标，N_z为所述圆柱形k空间选层方向上坐标的最大值，i为正整数，i＝1,2,...,N_z。in,

is the layer selection direction coordinates of the acquisition plane during the i-th layer selection encoding, N _z is the maximum value of the coordinates in the layer selection direction of the cylindrical k-space, i is a positive integer, i=1, 2,..., N _z .

Further, in step 1, non-equidistant coding is used for the multiple acquisition planes to determine the layer selection direction coordinates of the acquisition planes.

Further, the non-equidistant coding satisfies the following relational expression:

为第i次选层编码时的采集平面的选层方向坐标，N_z为所述圆柱形k空间选层方向上坐标的最大值，i为正整数，i＝1,2,...,N_z。Among them, γ is the proportional coefficient, mod() is the remainder operation,

is the layer selection direction coordinates of the acquisition plane during the i-th layer selection encoding, N _z is the maximum value of the coordinates in the layer selection direction of the cylindrical k-space, i is a positive integer, i=1, 2,..., N _z .

Further, the proportional coefficient γ satisfies the following relational formula:

Further, in the step 2, the acquisition curve is a function of the acquisition coordinates on the acquisition surface and the acquisition time.

Further, the acquisition curve satisfies that the acquisition coordinates are periodically equal to the origin of the acquisition plane.

Compared with the prior art, the multi-echo sampling method based on the three-dimensional non-Cartesian trajectory provided by the present application has the technical effect of: selecting a plurality of acquisition surfaces and non-Cartesian acquisition curves in the cylindrical k-space, Compared with the Cartesian trajectory, when the non-Cartesian trajectory is used for acquisition, the k-space range that can be filled after a single pulse excitation is larger, so the required pulse excitation times can be reduced, thereby improving the acquisition efficiency and shortening the scanning time. The high filling rate of the k-space improves the data sampling efficiency, thereby shortening the scanning time and significantly reducing the time required for quantitative magnetic susceptibility imaging.

Description of drawings

1 is a schematic diagram of a sampling curve of a three-dimensional non-Cartesian trajectory in a certain sampling plane according to an embodiment of the present application;

2 is a schematic diagram of a three-dimensional non-Cartesian trajectory according to an embodiment of the present application;

FIG. 3 is a schematic diagram of a scanning time series according to an embodiment of the present application.

Detailed ways

The following describes the embodiments of the present invention with reference to the accompanying drawings, so as to make its technical content clearer and easier to understand. The present invention can be embodied in many different forms of embodiments, and the protection scope of the present invention is not limited to the embodiments mentioned herein. In this application, k-space is cylindrical space. The z-direction in the cylindrical k-space is the central axis direction, also called the kz-direction or the layer-selective coding direction.

A multi-echo sampling method based on a three-dimensional non-Cartesian trajectory provided by this application is used in quantitative magnetic susceptibility imaging technology, and specifically includes:

Step 1. Select a cylindrical k-space, and define the size of the k-space according to the quantitative magnetic susceptibility imaging scanning requirements. The parameters Nx, Ny, Nz are defined as the coordinate maxima in the x, y, z directions of the cylindrical k-space. When the parameters Nx, Ny, and Nz are determined, the cylindrical k-space is also uniquely determined. The x-y plane is a plane parallel to the top and bottom surfaces of the cylindrical k-space, and the z-axis is the central axis of the cylindrical k-space.

多个采集平面之间的间距可以是等间距的，也可以是非等间距的。In a cylindrical k-space of a certain size, multiple xy planes are selected as acquisition planes. And encode these acquisition planes. Define the z-coordinate of the acquisition plane during the i-th layer selection encoding as

The distances between the multiple acquisition planes may be equidistant or non-equidistant.

For example, when equidistant order coding is used, the following relation can be used to determine

为第i次选层编码时的采集平面的选层方向(即z方向)坐标，N_z为圆柱形k空间选层方向上坐标的最大值，i为正整数，i＝1,2,...,N_z。in,

is the coordinate of the layer selection direction (that is, the z direction) of the acquisition plane during the i-th layer selection encoding, N _z is the maximum value of the coordinates in the layer selection direction of the cylindrical k-space, i is a positive integer, i=1,2,. ..,N _z .

When using non-equidistant sequence coding, the following golden ratio sequence coding can be preferably used to determine

为第i次选层编码时的采集平面的选层方向坐标，N_z为圆柱形k空间选层方向上坐标的最大值，i为正整数，i＝1,2,...,N_z。Among them, γ is the proportional coefficient, mod() is the remainder operation,

is the layer selection direction coordinates of the acquisition plane during the i-th layer selection encoding, N _z is the maximum value of the coordinates in the cylindrical k-space layer selection direction, i is a positive integer, i=1,2,...,N _z .

For golden ratio sequential coding, γ is preferably the golden ratio number, that is:

In other similar embodiments, the layer selection coding sequence may also adopt an sequence such as random coding.

Step 2. In the equidistant or non-equidistant acquisition planes selected in step 1, construct an acquisition curve. Figure 1 shows the preferred acquisition curve in this embodiment. The acquisition curve is a spiral trajectory in the form of rose petals. Its characteristic is that the acquisition coordinates can periodically return to the center origin of the acquisition plane, so as to realize multi-echo data acquisition. The acquisition curve shown in Figure 1 has five petals, that is, five echo signals can be provided. In other similar embodiments, the acquisition curve can be set through the following relationship, and the number of petals can be determined through parameter adjustment:

k _x (t)=N _x ·sin(ω ₁ t) ·cos(ω ₂ t)

k _y (t)=N _y ·sin(ω ₁ t) ·sin(ω ₂ t)

Among them, k _x and _ky are the coordinates in the x and y directions, respectively; N _x and N _y are the cylindrical k-space x,

The maximum value of the coordinates in the y direction; ω ₁ and ω ₂ represent the oscillation frequencies of the acquisition curves of k _x and k _y . By adjusting ω ₁ and ω ₂ , the number of petals in the collection curve can be specifically changed.

Step 3: Determine a three-dimensional non-Cartesian trajectory according to the codes of the multiple acquisition planes and each acquisition curve. FIG. 2 shows the three-dimensional non-Cartesian acquisition trajectory constructed in this embodiment. In the cylindrical k-space 20, three acquisition planes are selected at equal intervals, and five-petal acquisition curves 21, 22, and 23 are set on each acquisition plane.

Step 4. Design the scanning time series according to the three-dimensional non-Cartesian trajectory. In this embodiment, according to the acquisition plane and acquisition curve selected in FIG. 3 , a multi-echo sampling sequence based on a three-dimensional non-Cartesian trajectory as shown in FIG. 3 is designed.

In Fig. 3, RF is the radio frequency excitation pulse, Gx is the magnetic field gradient applied on the x-axis, Gy is the magnetic field gradient applied on the y-axis, and Gz is the magnetic field gradient applied on the z-axis. The z-axis is the encoding direction of the selected layer. 300 is the radio frequency (Radiofrequency, RF) excitation pulse; 301 and 302 constitute the layer selection gradient; 303 is the layer selection direction encoding gradient; 304 and 305 are the readout encoding gradient; ; 309 is the next RF excitation pulse; 310 is the first echo signal time, 311 is the second echo signal time, 312 is the third echo signal time, 313 is the fourth echo signal time, 314 Denotes the fifth echo signal time, and 315 denotes the repetition time of the scan time series.

Step 5, according to the scanning time sequence designed above, implement resonance data acquisition.

The preferred embodiments of the present invention have been described in detail above. It should be understood that many modifications and changes can be made according to the concept of the present invention by those skilled in the art without creative efforts. Therefore, any technical solutions that can be obtained by those skilled in the art through logical analysis, reasoning or limited tests on the basis of the prior art according to the concept of the present invention shall fall within the protection scope determined by the claims.

Claims (6)

1. a multi-echo sampling method based on three-dimensional non-Cartesian trajectory, specifically comprising the following steps: Step 1. Select a cylindrical k-space, select multiple acquisition planes in the cylindrical k-space, and encode the multiple acquisition planes; the multiple acquisition planes are parallel to each other; the multiple acquisition planes are The planes are all perpendicular to the slice selection direction of the cylindrical k-space; the multiple acquisition planes are equidistantly encoded to determine the slice selection direction coordinates of the acquisition plane; The isometric encoding satisfies the following relation:

in,

is the layer selection direction coordinates of the acquisition plane during the i-th layer selection encoding, N _z is the maximum value of the coordinates in the layer selection direction of the cylindrical k-space, i is a positive integer, i=1, 2,..., N _z ; Step 2, on each of the multiple acquisition planes, select a data acquisition curve; Step 3, determining a three-dimensional non-Cartesian trajectory according to the encoding of the multiple acquisition planes and each of the acquisition curves; Step 4, designing a scanning time series according to the three-dimensional non-Cartesian trajectory; Step 5, according to the scanning time sequence, implement resonance data acquisition. 2. The multi-echo sampling method based on a three-dimensional non-Cartesian trajectory according to claim 1, wherein in step 1, non-equidistant coding is adopted for the plurality of acquisition planes to determine the selection of the acquisition planes. Layer orientation coordinates. 3. the multi-echo sampling method based on three-dimensional non-Cartesian trajectory as claimed in claim 2, is characterized in that, described non-equidistant coding satisfies following relational expression:

Among them, γ is the proportional coefficient, mod() is the remainder operation,

is the layer selection direction coordinates of the acquisition plane during the i-th layer selection encoding, N _z is the maximum value of the coordinates in the layer selection direction of the cylindrical k-space, i is a positive integer, i=1, 2,..., N _z . 4. The multi-echo sampling method based on three-dimensional non-Cartesian trajectory as claimed in claim 3, wherein the proportional coefficient γ satisfies the following relational expression:

5 . The multi-echo sampling method based on a three-dimensional non-Cartesian trajectory according to claim 4 , wherein, in the step 2, the acquisition curve is the difference between the acquisition coordinates on the acquisition plane and the acquisition time. 6 . function. 6 . The multi-echo sampling method based on the three-dimensional non-Cartesian trajectory according to claim 5 , wherein, it can be applied to quantitative magnetic susceptibility imaging technology. 7 .

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