CN106175765B - Magnetic resonance imaging system and method - Google Patents
Magnetic resonance imaging system and method Download PDFInfo
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- CN106175765B CN106175765B CN201510214352.1A CN201510214352A CN106175765B CN 106175765 B CN106175765 B CN 106175765B CN 201510214352 A CN201510214352 A CN 201510214352A CN 106175765 B CN106175765 B CN 106175765B
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Abstract
The invention provides a magnetic resonance imaging system and a method, wherein the method comprises the following steps: sequentially applying an excitation radio frequency pulse and a refocusing radio frequency pulse to the radio frequency coil in each imaging period; applying a constant slice selection gradient pulse to the slice selection gradient coil for each imaging cycle; applying position encoding gradient pulses on the slice plane to the position encoding gradient coil after the application of the excitation radio frequency pulse in each imaging period; and, performing image reconstruction, comprising: acquiring magnetic resonance signals generated based on the excitation radio-frequency pulse and the refocusing radio-frequency pulse, acquiring slice information generated based on the slice selection gradient pulse, and acquiring position information on the slice generated based on the position encoding gradient pulse.
Description
Technical Field
The invention relates to the field of medical diagnosis, in particular to a magnetic resonance imaging system and a magnetic resonance imaging method.
Background
A Magnetic Resonance (MR) imaging suite system includes a radio frequency system and a gradient system. The radio frequency system comprises a transmitting system and a receiving system, wherein the transmitting system is used for transmitting radio frequency pulses with certain frequency and power to enable hydrogen protons in the detected body to generate resonance, and the receiving system is used for receiving magnetic resonance signals generated by the hydrogen protons in the detected body, and the magnetic resonance signals are used for carrying out image reconstruction on the detected part of the detected body. The gradient system is used for transmitting slice selection gradient pulses, phase encoding gradient pulses and frequency encoding gradient pulses (also called readout gradient pulses) to provide three-dimensional position information for the magnetic resonance signals to realize image reconstruction.
When MR scanning imaging is carried out, a pulse control system controls a radio frequency system and a gradient system to send out a desired pulse sequence according to a preset time sequence so as to carry out scanning imaging on a specific detection part of a detected body.
Although MR imaging systems can obtain clear images for clinical diagnosis or research, the patient is subjected to large acoustic noise during the imaging scan, and technicians propose methods for reducing the acoustic noise, such as using specially designed gradient coils, noise elimination materials, and vacuum noise elimination, but these methods require high hardware cost.
Therefore, in order to reduce the acoustic noise of the MR imaging scan, a new magnetic resonance imaging system and method need to be provided.
Disclosure of Invention
An exemplary embodiment of the present invention provides a magnetic resonance imaging method including: sequentially applying an excitation radio frequency pulse and a refocusing radio frequency pulse to the radio frequency coil in each imaging period; applying a constant slice selection gradient pulse to the slice selection gradient coil for each imaging cycle; applying position encoding gradient pulses on the slice plane to the position encoding gradient coil after the application of the excitation radio frequency pulse in each imaging period; and, performing image reconstruction, comprising: acquiring magnetic resonance signals generated based on the excitation radio-frequency pulse and the refocusing radio-frequency pulse, acquiring slice information generated based on the slice selection gradient pulse, and acquiring position information on the slice generated based on the position encoding gradient pulse.
An exemplary embodiment of the present invention also provides a magnetic resonance imaging system including a radio frequency control module, a slice selection gradient control module, a position encoding gradient control module, and an image reconstruction module. The radio frequency control module is used for sequentially applying an excitation radio frequency pulse and a refocusing radio frequency pulse to the radio frequency coil in each imaging period; the slice selection gradient control module is used for continuously sending constant slice selection gradient pulses to the slice selection gradient coil in each imaging period; the position coding gradient control module is used for applying position coding gradient pulses on the layer to the position coding gradient coil after the excitation radio frequency pulses are applied in each imaging period; the image reconstruction module is used for acquiring magnetic resonance signals generated based on the excitation radio-frequency pulse and the refocusing radio-frequency pulse, acquiring layer information generated based on the layer selection gradient pulse and acquiring position information on the layer generated based on the position encoding gradient pulse so as to reconstruct images.
Other features and aspects will become apparent from the following detailed description, the accompanying drawings, and the claims.
Drawings
The invention may be better understood by describing exemplary embodiments thereof in conjunction with the following drawings, in which:
fig. 1 is a block diagram of a magnetic resonance imaging system according to an embodiment of the present invention;
figure 2 is a timing diagram of a pulse sequence employed by the magnetic resonance imaging system of figure 1;
fig. 3 is a block diagram of a magnetic resonance imaging system according to a second embodiment of the present invention;
fig. 4 is a flowchart of a magnetic resonance imaging method according to a third embodiment of the present invention;
fig. 5 is a flowchart of a magnetic resonance imaging method according to a fourth embodiment of the present invention;
fig. 6 is a surface scan image acquired by a magnetic resonance imaging system according to a second embodiment of the present invention;
figure 7 is a timing diagram of a pulse sequence employed in a prior art magnetic resonance imaging system;
figures 8a, 8c are diagnostic images acquired by a prior art magnetic resonance imaging system; fig. 8b, 8d are 8 diagnostic images acquired by the magnetic resonance imaging system of the invention.
Detailed Description
While specific embodiments of the invention will be described below, it should be noted that in the course of the detailed description of these embodiments, in order to provide a concise and concise description, all features of an actual implementation may not be described in detail. It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation-specific decisions are made to achieve the developers' specific goals, such as compliance with system-related and business-related constraints, which may vary from one implementation to another. Moreover, it should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation-specific decisions must be made to achieve the developers' specific goals, such as compliance with system-related and business-related constraints, which may vary from one implementation to another.
Unless otherwise defined, technical or scientific terms used in the claims and the specification should have the ordinary meaning as understood by those of ordinary skill in the art to which the invention belongs. The use of "first," "second," and similar terms in the description and claims of the present application do not denote any order, quantity, or importance, but rather the terms are used to distinguish one element from another. The terms "a" or "an," and the like, do not denote a limitation of quantity, but rather denote the presence of at least one. The word "comprise" or "comprises", and the like, means that the element or item listed before "comprises" or "comprising" covers the element or item listed after "comprising" or "comprises" and its equivalent, and does not exclude other elements or items. The terms "connected" or "coupled" and the like are not restricted to physical or mechanical connections, nor are they restricted to direct or indirect connections.
Example one
Fig. 1 is a block diagram of a magnetic resonance imaging system according to an embodiment of the present invention, and as shown in fig. 1, the system includes a radio frequency control module 10, a gradient control module 20 (including a slice gradient control module and a position encoding gradient control module), and an image reconstruction module 40. The radio frequency control module 10, the slice selection gradient control module and the position encoding gradient control module are used for generating a desired pulse sequence, the pulse sequence is used for controlling a radio frequency system and a gradient system in the magnetic resonance imaging system to generate a required radio frequency field and a gradient field, and finally generating an echo chain.
The radio frequency system may specifically comprise a radio frequency coil arranged in the static magnetic field of the magnetic resonance scanner and the gradient system may comprise, for example, a slice selection gradient coil, a phase encoding gradient coil and a frequency encoding gradient coil arranged in the static magnetic field of the magnetic resonance scanner and respectively arranged along three axes of a cartesian coordinate system.
Fig. 2 is a timing diagram of a pulse sequence employed by the magnetic resonance imaging system of the present invention. As shown in fig. 1 and 2, the rf control module 10 is used for sequentially controlling the rf coil in each imaging periodAn excitation radio frequency pulse (90 ° RF pulse) and a refocusing radio frequency pulse (180 ° RF pulse) are applied. The slice selection gradient control module is used for continuously sending constant slice selection gradient pulses G to the slice selection gradient coil in each imaging periodZ。
The position encoding gradient control module is used for applying position encoding gradient pulses on the slice to the position encoding gradient coil after the application of the excitation radio frequency pulse in each imaging period.
The image reconstruction module 40 is configured to acquire magnetic resonance signals generated based on the excitation rf pulses and the refocusing rf pulses, acquire slice information generated based on the slice selection gradient pulses, and acquire position information on slices generated based on the position encoding gradient pulses to perform image reconstruction.
In the present embodiment, the "position encoding gradient coil" includes the phase encoding gradient coil and the frequency encoding gradient coil. The above-mentioned "position-encoding gradient pulses on the slice" include phase-encoding gradient pulses G applied to a phase-encoding gradient coilYAnd frequency encoding gradient pulses G applied to the frequency encoding gradient coilX. "positional information on slice" includes based on phase encoding gradient pulses GYGenerated line position information and encoding gradient pulses G based on the frequencyXThe generated column position information.
As will be appreciated by those skilled in the art, the radio frequency coil may particularly comprise a transmit coil and a receive coil, on the transmit coil of which an excitation radio frequency pulse is applied for generating an excitation radio frequency field and a slice selection gradient pulse for generating a slice selection gradient field, which jointly act on the object to resonate protons on a specific slice of the object to generate magnetic resonance signals (echo signals). A refocusing radio frequency pulse is applied to the transmit coil of the radio frequency coil to generate a radio frequency refocusing field to refocus the rapidly decaying magnetic resonance signal so that an echo signal is generated that can be measured.
Slice selection gradient pulse GzPhase encoding gradient pulse GYAnd a frequency encoding gradient pulse GXProviding three-dimensional position information to the magnetic resonance signals to realize image reconstruction, wherein the slice selection gradient pulse GzFor generating slice selection gradient fields for selecting a scan slice (a cross-sectional slice along a Z-axis, wherein the Z-axis direction coincides with the static magnetic field direction), phase encoding gradient pulses GYSum frequency encoding gradient pulse GXFor generating phase encoding gradient fields for acquiring position information on a selected slice (position information along the Y-axis and the X-axis, respectively, on the slice), in particular, for acquiring the position of the row on which each pixel is located on the slice, and frequency encoding gradient fields for acquiring the position of the column on which each pixel is located on the slice.
As shown in fig. 2, the above-mentioned "each imaging period" means between the start times of each adjacent two 90-degree RF pulses.
In the pulse sequence adopted by the magnetic resonance imaging system, constant gradient pulses are continuously arranged on the Z axis, namely the pulse direction, the width and the amplitude are unchanged. In this way, acoustic noise due to gradient field switching is reduced.
Further, in order to reduce the artifacts, in the embodiment of the present invention, the bandwidths of the excitation rf pulse and the refocusing rf pulse are (400-1000) hz, i.e. not less than 400 hz and not more than 1000 hz, which is lower than the bandwidths of the excitation rf pulse and the refocusing rf pulse of the conventional spin echo.
In order to reduce noise caused by fast switching of the gradient field, in the embodiment of the invention, the position encoding gradient pulse is a trapezoidal pulse, and compared with a rectangular pulse, the climbing speed of the position encoding gradient pulse is reduced.
Example two
Fig. 3 is a block diagram of a magnetic resonance imaging system according to a second embodiment of the present invention; as shown in fig. 3, the magnetic resonance imaging system according to the second embodiment of the present invention is similar to the magnetic resonance imaging system according to the first embodiment in structure and principle, and the differences are as follows:
first, the position encoding gradient coil is a frequency encoding gradient coil, and does not include a phase encoding gradient coil, and the position on the slice plane is encodedThe pulse is a frequency encoding gradient pulse GXAnd does not include phase encoding gradient pulses GY(ii) a The "location information on the layer" is: column position information generated based on the frequency encoding gradient pulses.
Secondly, the magnetic resonance imaging system according to the second embodiment of the present invention further includes a surface scan image acquisition module 30 for acquiring a surface scan image of the subject.
Thirdly, the image reconstruction module is further configured to acquire line position information on a slice from the surface scan image to reconstruct an image.
The surface scan image is: a surface coil is provided on the surface of an object, and a scan image is acquired based on the surface coil.
The above-mentioned "layer upper line positional information" is positional information on the Y axis of each pixel on the selected layer.
The scan image acquired based on the surface receiving coil can acquire the positional information of the row where each pixel on the slice plane is located, instead of the phase encoding gradient pulse, and therefore, in this embodiment, the phase encoding gradient pulse may not be transmitted to the phase encoding gradient coil. The specific principle is as follows:
when a surface coil is arranged on the surface of the object, in the process of controlling the scanning bed to enter the scanning cavity for scanning, the surface coil receives signals to obtain a surface coil scanning image, namely a coronal image (such as the image shown in fig. 6), and the position information of each voxel row on the slice (corresponding to the position information of the image pixel along the Y axis) can be obtained according to the coronal image. Further using frequency coded pulses GXTo acquire position information for each voxel column at the slice plane (corresponding to position information of image pixels along the X-axis), three-dimensional position information can be provided for the acquired magnetic resonance signals for image reconstruction.
EXAMPLE III
Fig. 4 is a flowchart of a magnetic resonance imaging method according to a third embodiment of the present invention; as shown in fig. 4, the method comprises the following steps S401-S404:
step S401: an excitation radio frequency pulse and a refocusing radio frequency pulse are sequentially applied to the radio frequency coil during each imaging period.
Step S402: a constant slice selection gradient pulse is continuously delivered to the slice selection gradient coil during each imaging cycle.
Step S403: during each imaging cycle, position encoding gradient pulses at the slice are applied to the position encoding gradient coil after the excitation radio frequency pulses are applied to the radio frequency coil.
Step S404: performing image reconstruction, comprising: acquiring magnetic resonance signals generated based on the excitation radio-frequency pulse and the refocusing radio-frequency pulse, acquiring slice information generated based on the slice selection gradient pulse, and acquiring position information on the slice generated based on the position encoding gradient pulse.
Further, the bandwidth of the excitation RF pulse and the refocusing RF pulse is not less than 400 Hz and not more than 1000 Hz.
Further, the position encoding gradient pulse includes a phase encoding gradient pulse and a frequency encoding gradient pulse. In step S403, "applying a position encoding gradient pulse on a slice to a position encoding gradient coil" specifically includes:
applying phase encoding gradient pulses to the phase encoding gradient coil, and,
frequency encoding gradient pulses are applied to the frequency encoding gradient coil.
The "position information on the slice" in step S404 includes row position information generated based on the phase encoding gradient pulse and column position information generated based on the frequency encoding gradient pulse.
Further, the position encoding gradient pulse is a trapezoidal pulse.
Example four
Fig. 5 is a flowchart of a magnetic resonance imaging method according to a fourth embodiment of the present invention; as shown in fig. 5, the magnetic resonance imaging method according to the fourth embodiment of the present invention is similar in principle to the magnetic resonance imaging method according to the third embodiment, except that:
first, the position encoding gradient coil is a frequency encoding gradient coil, and does not include a phase encoding gradient coil, and the position encoding pulse at the slice plane is a frequency encoding ladderDegree pulse GXAnd does not include phase encoding gradient pulses GY(ii) a The "location information on the layer" is: column position information generated based on the frequency encoding gradient pulses.
Second, the magnetic resonance imaging method according to the fourth embodiment of the present invention includes the above steps S401 to S403, and further includes step S400 and step S404'.
Step S400: a surface scan image of an object is acquired.
Step S404': acquiring magnetic resonance signals generated based on an excitation radio-frequency pulse and a refocusing radio-frequency pulse, acquiring bedding information generated based on a bedding selection gradient pulse, acquiring position information on a bedding generated based on a position coding gradient pulse, and acquiring position information of a line where pixels on the bedding are located from the surface scanning image so as to reconstruct the image.
Wherein the surface scan image is: a surface coil is provided on the surface of an object, and a scan image is acquired based on the surface coil.
The steps of the magnetic resonance imaging method according to the third and fourth embodiments of the present invention can be specifically executed by the magnetic resonance imaging system according to the first and second embodiments, respectively, and the principle and effect thereof are similar to those of the magnetic resonance imaging system described above, and are not described again.
Fig. 7 is a timing diagram of a pulse sequence used in a conventional magnetic resonance imaging system, as can be seen from a comparison of fig. 2 and 7:
in a pulse sequence adopted by an existing magnetic resonance imaging system, in each imaging period, 5 gradients are arranged on a Z axis: two slice selection gradient pulses (slice selection gradient)71, one refocusing gradient pulse (refocusing gradient)72, and two demagnetization pulses (cruser) 73. In the pulse sequence adopted by the magnetic resonance imaging system, the constant slice selection gradient pulse is continuously applied in each imaging period, so that the sound noise caused by gradient field switching is reduced.
Figures 8a, 8c are diagnostic images acquired by a prior art magnetic resonance imaging system; figures 8b and 8d are 8 diagnostic images acquired by the magnetic resonance imaging system of the present invention; by comparing fig. 8a and 8c with fig. 8b and 8d, the magnetic resonance imaging system of the present invention can obtain clearer images and less artifacts.
Some exemplary embodiments have been described above. Nevertheless, it will be understood that various modifications may be made. For example, suitable results may be achieved if the described techniques are performed in a different order and/or if components in the described systems, architectures, devices, or circuits are combined in a different manner and/or replaced or supplemented by additional components or their equivalents. Accordingly, other embodiments are within the scope of the following claims.
Claims (10)
1. A magnetic resonance imaging method, comprising:
sequentially applying an excitation radio frequency pulse and a refocusing radio frequency pulse to the radio frequency coil in each imaging period;
applying a constant slice selection gradient pulse to the slice selection gradient coil for each imaging cycle;
applying position encoding gradient pulses on a slice plane to a position encoding gradient coil after the application of the excitation radio frequency pulses within each imaging period; and the number of the first and second groups,
performing image reconstruction, comprising: acquiring magnetic resonance signals generated based on the excitation radio-frequency pulse and the refocusing radio-frequency pulse, acquiring slice information generated based on the slice selection gradient pulse, and acquiring position information on a slice generated based on the position encoding gradient pulse.
2. A method as claimed in claim 1, wherein the excitation and refocusing radio frequency pulses have a bandwidth of not less than 400 hz and not more than 1000 hz.
3. The magnetic resonance imaging method according to claim 1, wherein the applying position encoding gradient pulses on a slice to a position encoding gradient coil comprises:
applying phase encoding gradient pulses to the phase encoding gradient coil; and the number of the first and second groups,
applying frequency encoding gradient pulses to the frequency encoding gradient coil;
the "position information on the slice" includes row position information generated based on the phase encoding gradient pulse and column position information generated based on the frequency encoding gradient pulse.
4. A magnetic resonance imaging method as claimed in claim 1, characterized in that the position-encoding gradient pulses are trapezoidal pulses.
5. The magnetic resonance imaging method according to claim 1, wherein the position encoding gradient coil is a frequency encoding gradient coil, the position encoding gradient pulse is a frequency encoding gradient pulse, and the "positional information on the slice" is: column position information generated based on the frequency encoding gradient pulses; the magnetic resonance imaging method further comprises: acquiring a surface scan image of an object;
the step of "performing image reconstruction" further comprises: acquiring line position information on a layer from the surface scanning image;
wherein the surface scan image is: a surface coil is provided on a surface of an object, and a scan image is acquired based on the surface coil.
6. A magnetic resonance imaging system comprising:
the radio frequency control module is used for sequentially applying an excitation radio frequency pulse and a refocusing radio frequency pulse to the radio frequency coil in each imaging period;
the slice selection gradient control module is used for continuously sending constant slice selection gradient pulses to the slice selection gradient coil in each imaging period;
a position encoding gradient control module for applying position encoding gradient pulses on a slice to the position encoding gradient coil after the application of the excitation radio frequency pulse in each imaging period, and,
and the image reconstruction module is used for acquiring magnetic resonance signals generated based on the excitation radio-frequency pulse and the refocusing radio-frequency pulse, acquiring the layer information generated based on the layer selection gradient pulse and acquiring the position information on the layer generated based on the position coding gradient pulse so as to reconstruct images.
7. The system of claim 6, wherein the excitation RF pulse and the refocusing RF pulse have a bandwidth of no less than 400 Hz and no greater than 1000 Hz.
8. The magnetic resonance imaging system of claim 6, wherein the position encoding gradient coil includes a phase encoding gradient coil and a frequency encoding gradient coil, the position encoding gradient pulses including phase encoding gradient pulses applied to the phase encoding gradient coil and frequency encoding gradient pulses applied to the frequency encoding gradient coil; the "position information on the slice" includes row position information generated based on the phase encoding gradient pulse and column position information generated based on the frequency encoding gradient pulse.
9. The magnetic resonance imaging system of claim 6, wherein the position encoding gradient pulses are trapezoidal pulses.
10. The magnetic resonance imaging system of claim 6, wherein the position encoding gradient coil is a frequency encoding gradient coil, the position encoding gradient pulse is a frequency encoding gradient pulse, and the "position information on the slice" is: column position information generated based on the frequency encoding gradient pulses; the magnetic resonance imaging system further comprises:
a surface scan image acquisition module for acquiring a surface scan image of the subject;
the image reconstruction module is also used for acquiring line position information on a layer from the surface scanning image so as to reconstruct an image;
wherein the surface scan image is: a surface coil is provided on a surface of an object, and a scan image is acquired based on the surface coil.
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| CN107064843B (en) * | 2017-04-25 | 2019-12-03 | 上海联影医疗科技有限公司 | A kind of MR imaging method and MR imaging apparatus |
| CN109164403B (en) * | 2018-08-10 | 2019-10-25 | 中国科学院武汉物理与数学研究所 | A kind of MR imaging method based on falling-rising frequency |
| CN111257810B (en) * | 2020-01-19 | 2021-11-23 | 上海联影医疗科技股份有限公司 | Magnetic resonance signal acquisition system and method |
| CN113534032B (en) * | 2020-04-14 | 2023-01-31 | 上海联影医疗科技股份有限公司 | Magnetic resonance imaging method and system |
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