CN113534032B - Magnetic resonance imaging method and system - Google Patents
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Abstract
The embodiment of the invention discloses a magnetic resonance imaging method and a magnetic resonance imaging system. The method comprises the following steps: simultaneously exciting multiple layers of a scanned object by using a preset imaging sequence, and acquiring multiple layers of aliasing magnetic resonance signals of the scanned object, wherein the preset imaging sequence is provided with a preset gradient peak at a preset time point of a slice selection gradient so as to realize interlayer preset field-of-view shift of the multiple layers of aliasing magnetic resonance signals on an image domain, and the preset time point is between an excitation pulse and a first refocusing pulse in a radio frequency pulse and between any two refocusing pulses; and performing interlayer de-aliasing and image reconstruction on the multilayer aliasing magnetic resonance signals to generate magnetic resonance images of all layers of the scanning object. By the technical scheme, the de-aliasing of the multi-layer aliasing signals of simultaneous multi-layer excitation imaging is realized, and the de-aliasing efficiency and the image signal-to-noise ratio are improved.
Description
Technical Field
The embodiments of the present invention relate to magnetic resonance technology, and in particular, to a magnetic resonance imaging method and system.
Background
In order to improve the data acquisition efficiency of magnetic resonance, a Simultaneous Multi-Slice (SMS) technique is known in the current magnetic resonance imaging technology, and the SMS technique allows several slices of a scanned object to be excited at the same time. However, the acquired data obtained by the imaging technology is subjected to multi-slice aliasing, a magnetic resonance image directly reconstructed by using the acquired data is also subjected to multi-slice aliasing, the signal-to-noise ratio of the image is low, and the clinical requirement cannot be met.
Disclosure of Invention
The embodiment of the invention provides a magnetic resonance imaging method and a magnetic resonance imaging system, which are used for realizing the de-aliasing of multi-layer aliasing signals for simultaneous multi-layer excitation imaging and improving the de-aliasing efficiency and the image signal-to-noise ratio.
In a first aspect, an embodiment of the present invention provides a magnetic resonance imaging method, including:
simultaneously exciting multiple layers of a scanned object by using a preset imaging sequence, and acquiring multiple layers of aliasing magnetic resonance signals of the scanned object, wherein the preset imaging sequence is provided with a preset gradient peak at a preset time point of a slice selection gradient so as to realize interlayer preset field-of-view shift of the multiple layers of aliasing magnetic resonance signals on an image domain, and the preset time point is between an excitation pulse and a first refocusing pulse in a radio frequency pulse and between any two refocusing pulses;
and performing interlayer de-aliasing and image reconstruction on the multilayer aliasing magnetic resonance signals to generate magnetic resonance images of all layers of the scanning object.
In a second aspect, an embodiment of the present invention further provides a magnetic resonance imaging method, including:
determining a region to be detected of a scanning object in a scanning field of view, wherein the region to be detected comprises a plurality of slices;
acquiring the distance between at least two adjacent sheets in the multi-sheet;
simultaneously exciting the at least two adjacent slices by using a spin echo sequence, and successively applying a first preset gradient peak and a second preset gradient peak with different gradient moments on a slice selection gradient in the execution process of the spin echo sequence, wherein the gradient moments of the first preset gradient peak and the second preset gradient peak are determined according to the distance between the at least two adjacent slices, and the application of the first preset gradient peak and the second preset gradient peak causes the shift of the scanning visual field;
acquiring a multi-slice aliased magnetic resonance signal of the at least two adjacent slices by using a receiving coil, and generating a magnetic resonance image of each slice of the scanning object according to the multi-slice aliased magnetic resonance signal.
In a third aspect, an embodiment of the present invention further provides a magnetic resonance imaging system, including:
an MRI scanner, and a processor communicatively coupled to the MRI scanner;
the MRI scanning device is used for simultaneously exciting multiple layers of a scanned object by utilizing a preset imaging sequence and acquiring multiple layers of aliasing magnetic resonance signals of the scanned object, wherein the preset imaging sequence is provided with a preset gradient peak at a preset time point of a slice selection gradient so as to realize interlayer preset field-of-view shift of the multiple layers of aliasing magnetic resonance signals on an image domain, and the preset time point is between an excitation pulse and a first refocusing pulse in a radio frequency pulse and between any two refocusing pulses;
the processor is configured to perform inter-layer de-aliasing and image reconstruction on the multi-layer aliased magnetic resonance signals to generate magnetic resonance images of each layer of the scan object.
In a fourth aspect, an embodiment of the present invention further provides a magnetic resonance imaging system, including:
an MRI scanner, and a processor communicatively coupled to the MRI scanner;
the MRI scanner is configured to:
determining a region to be detected of a scanning object in a scanning field of view, wherein the region to be detected comprises a plurality of slices;
acquiring the distance between at least two adjacent slice layers in the multiple slice layers;
simultaneously exciting the at least two adjacent slices by using a spin echo sequence, and successively applying a first preset gradient peak and a second preset gradient peak with different gradient moments on a slice selection gradient in the execution process of the spin echo sequence, wherein the gradient moments of the first preset gradient peak and the second preset gradient peak are determined according to the distance between the at least two adjacent slices, and the application of the first preset gradient peak and the second preset gradient peak causes the shift of the scanning visual field;
acquiring a plurality of layers of aliased magnetic resonance signals of the at least two adjacent slices by using a receiving coil;
the processor is configured to generate a magnetic resonance image of each slice of the scan subject from the multi-slice aliased magnetic resonance signals.
The method comprises the steps of simultaneously exciting multiple layers of a scanned object by utilizing a preset imaging sequence, and collecting multiple layers of aliasing magnetic resonance signals of the scanned object, wherein the preset imaging sequence is provided with a preset gradient peak at a preset time point of a layer selection gradient so as to realize interlayer preset field offset of the multiple layers of aliasing magnetic resonance signals on an image domain, and the preset time point is positioned between an excitation pulse and a first refocusing pulse in a radio frequency pulse and between any two refocusing pulses; and performing interlayer de-aliasing and image reconstruction on the multilayer aliasing magnetic resonance signals to generate magnetic resonance images of all layers of the scanning object. The method and the device have the advantages that in the magnetic resonance scanning process, the preset gradient peak with different gradient distances is applied to the layer selection gradient of the preset imaging sequence to cause the interlayer preset field-of-view offset, so that the interlayer sensitivity difference is increased, the multi-layer aliasing magnetic resonance signals are subjected to de-aliasing, and the aliasing knowing efficiency and the signal-to-noise ratio of the magnetic resonance images are improved.
Drawings
Fig. 1 is a flow chart of a magnetic resonance imaging method in accordance with a first embodiment of the present invention;
fig. 2 is a flowchart of a preset imaging sequence generation method in a magnetic resonance imaging method according to a second embodiment of the present invention;
FIG. 3 is a diagram illustrating a preset imaging sequence 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 schematic structural diagram of a magnetic resonance imaging system in a fourth embodiment of the present invention.
Detailed Description
The present invention will be described in further detail with reference to the accompanying drawings and examples. It is to be understood that the specific embodiments described herein are merely illustrative of the invention and are not limiting of the invention. It should be further noted that, for the convenience of description, only some of the structures related to the present invention are shown in the drawings, not all of the structures.
Example one
The magnetic resonance imaging method provided by the embodiment can be suitable for magnetic resonance imaging with simultaneous multi-layer excitation. The method may be performed by a magnetic resonance imaging system, which may be implemented in software and/or hardware. For example, the procedures of sequence design, scan control and signal processing can be implemented by software, but the magnetic resonance scan of the scanned object is implemented by a magnetic resonance scanning apparatus. Referring to fig. 1, the method of the present embodiment specifically includes the following steps:
s110, simultaneously exciting multiple layers of a scanned object by using a preset imaging sequence, and collecting multiple layers of aliasing magnetic resonance signals of the scanned object, wherein the preset imaging sequence is provided with a preset gradient peak at a preset time point of a layer selection gradient so as to realize interlayer preset field-of-view shift of the multiple layers of aliasing magnetic resonance signals on an image domain, and the preset time point is between an excitation pulse and a first refocusing pulse in a radio frequency pulse and between any two refocusing pulses.
The preset imaging sequence refers to a preset pulse sequence for acquiring magnetic resonance data, and may be, for example, a fast spin echo sequence FSE. The preset gradient spikes (gradient blips) are gradient spikes having a preset gradient pitch and a preset gradient waveform. The inter-layer preset field-of-view shift refers to a preset field-of-view shift (FOV shift) between layers, or an adjacent layer image shift.
In particular, to increase the magnetic resonance scanning speed, a Simultaneous Multi-Slice (SMS) scanning technique of exciting multiple slices of the scanning object to simultaneously acquire magnetic resonance signals may be employed. However, the magnetic resonance signals obtained by the simultaneous multi-slice excitation scan (i.e. the multi-slice aliased magnetic resonance signals) are the magnetic resonance signals of multiple slices mixed together, and they need to be de-aliased (the signals of different slices are separated) to obtain the magnetic resonance signals of each slice. The basis for inter-layer de-aliasing in magnetic resonance technology is that the coil sensitivities of the receiving coils at different layers are different, and there is enough coil sensitivity difference between layers. However, in actual scanning, due to the small distance between two adjacent layers or the small number of receiving coils arranged in the layer selection direction, the difference in sensitivity matrix between layers (i.e., the difference in sensitivity between layers) is small, and thus aliasing cannot be solved well.
In the embodiment of the invention, in order to improve the de-aliasing efficiency and the image signal to noise ratio of the subsequent reconstructed image, the magnetic resonance scanning sequence is specially designed, namely, preset gradient peaks with different gradient distances are added at different positions on the layer selection gradient of the imaging sequence to form a preset imaging sequence. The preset gradient peaks can realize interlayer preset field-of-view shift between layers of the magnetic resonance signals obtained by subsequent scanning in an image domain, and the interlayer preset field-of-view shift can ensure that enough interlayer sensitivity difference exists between the layers, so that the interlayer sensitivity difference can be utilized to well de-alias the multi-layer aliasing magnetic resonance signals obtained by scanning subsequently.
The addition position of the preset gradient peak corresponds to a preset time point between the excitation pulse and the first refocusing pulse or between any two refocusing pulses in the RF pulse RF. For example, the preset time point is set before the signal acquisition window, and the odd phase encoding positions in the phase encoding direction may be sampled. The preset time point is set after the signal acquisition window, and the even phase encoding position in the phase encoding direction can be sampled. After a preset imaging sequence is designed, the preset imaging sequence is utilized to simultaneously excite the multiple layers of the scanning object, and a receiving coil is utilized to acquire magnetic resonance signals, so that multiple layers of aliasing magnetic resonance signals are obtained.
And S120, performing interlayer de-aliasing and image reconstruction on the multilayer aliasing magnetic resonance signals to generate magnetic resonance images of all layers of the scanned object.
Specifically, the inter-layer sensitivity difference corresponding to the inter-layer preset field offset is used for performing de-aliasing processing on the multi-layer aliasing magnetic resonance signal, and a de-aliased single-layer magnetic resonance signal is obtained. Then, the single-layer magnetic resonance signals are subjected to image reconstruction by using an image reconstruction algorithm, so that the magnetic resonance images of all the layers can be obtained.
Illustratively, inter-slice antialiasing and image reconstruction of multi-slice aliased magnetic resonance signals, generating an antialiased image of each slice of the scan object comprises: filling the multi-layer aliasing magnetic resonance signals into a K space, and acquiring aliasing K space data; performing interlayer de-aliasing on the aliased K space data based on interlayer sensitivity difference corresponding to interlayer preset field offset to generate de-aliased K space data of each layer of the scanning object; and carrying out image reconstruction on the antialiasing K space data of each layer to obtain magnetic resonance images of each layer of the scanning object.
Specifically, the process of obtaining the magnetic resonance image may be: firstly, a plurality of layers of aliasing magnetic resonance signals are filled into a K space after certain conversion (including phase encoding and frequency encoding), and K space data with aliasing signals (namely aliasing K space data) is obtained. Then, inter-layer de-aliasing is performed on the aliased K-space data using the inter-layer sensitivity difference, and de-aliased K-space data (i.e., de-aliased K-space data) of each layer is obtained. And then, according to the selected image reconstruction algorithm, performing corresponding reconstruction processing on the de-aliasing K space data of each layer to generate a magnetic resonance image of each layer. Illustratively, inter-layer de-aliasing of data over aliased K-space using inter-layer sensitivity differences may employ a sub-calibration parallel acquisition (Split-slice GRAPPA) algorithm, a parallel reconstruction algorithm (SENSE), or the like.
In the technical scheme of this embodiment, a preset imaging sequence is used to simultaneously excite multiple slices of a scanned object, and multiple slices of aliased magnetic resonance signals of the scanned object are acquired, wherein the preset imaging sequence is provided with a preset gradient peak at a preset time point of a slice selection gradient to realize interlayer preset field offset of the multiple slices of aliased magnetic resonance signals on an image domain, and the preset time point is located between an excitation pulse and a first refocusing pulse in a radio frequency pulse and between any two refocusing pulses; and performing interlayer de-aliasing and image reconstruction on the multilayer aliasing magnetic resonance signals to generate magnetic resonance images of all layers of the scanning object. The method and the device have the advantages that in the magnetic resonance scanning process, the preset gradient peak with different gradient distances is applied to the layer selection gradient of the preset imaging sequence to cause the interlayer preset field-of-view offset, so that the interlayer sensitivity difference is increased, the multi-layer aliasing magnetic resonance signals are subjected to de-aliasing, and the aliasing knowing efficiency and the signal-to-noise ratio of the magnetic resonance images are improved.
On the basis of the technical scheme, the method for simultaneously exciting the multiple layers of the scanning object by utilizing the preset imaging sequence and acquiring the multiple layers of aliasing magnetic resonance signals of the scanning object comprises the following steps: based on an undersampling mode, a preset imaging sequence is utilized to simultaneously excite multiple layers of a scanned object, and multiple layers of aliasing magnetic resonance signals of the scanned object are acquired.
Specifically, in order to further increase the magnetic resonance scanning speed, when acquiring a plurality of layers of aliasing magnetic resonance signals, the embodiment of the present invention may perform intra-layer scanning acceleration by combining an undersampled scanning mode on the basis of multi-layer simultaneous excitation scanning, so as to obtain the undersampled multi-layer aliasing magnetic resonance signals in each layer.
Accordingly, when each slice of the multi-slice aliased magnetic resonance signals is under-sampled, each slice of the obtained antialiased K-space data by antialiasing the multi-slice aliased magnetic resonance signals is the under-sampled antialiased K-space data.
On the basis of the technical scheme, image reconstruction is carried out on each layer of the de-aliasing K space data, and the magnetic resonance image of each layer of the scanning object is obtained by the following steps: acquiring a coil sensitivity distribution map of a receiving coil; restoring the antialiasing K space data of each layer of undersampled data according to the coil sensitivity distribution diagram to generate full sampling K space data of the corresponding undersampled antialiasing K space data; and carrying out image reconstruction according to the fully sampled K space data to obtain magnetic resonance images of all layers of the scanned object.
In particular for intra-slice undersampled magnetic resonance signals, the undersampled K-space data needs to be filled in first if a magnetic resonance image reconstruction is to be performed. To fill the K-space data accurately, the coil sensitivity profiles of the receive coils can be utilized. This is because the receiving coil of magnetic resonance is a phased array coil, which is formed by combining a plurality of sub-coils in a certain array, and the sensitivity of each sub-coil constitutes a coil sensitivity array, in which the higher the coil sensitivity is, the stronger the signal intensity in the K-space data is. Therefore, the coil sensitivity distribution map and the under-sampled aliasing-removing K-space data can be utilized to calculate the data which is not sampled in the K-space, and the data is filled to the corresponding position in the K-space, so as to obtain the full-sampling K-space data corresponding to the under-sampled aliasing-removing K-space data of the layer. And then, carrying out image reconstruction on the full sampling K space data to obtain a magnetic resonance image of the layer.
The coil sensitivity profiles may be pre-existing data or may be obtained by adding a low-resolution reference scan before the main scan. For example, exciting the detection area, and obtaining a low-resolution full-FOV reference image of the detection area by using the body coil of the magnet; the images of a plurality of receiving coils of the detection area are acquired by the receiving coils respectively, and then the images of the receiving coils are divided by the reference image one by one to obtain the coil sensitivity distribution diagram of the receiving coils. The advantage of this kind of setting can further improve magnetic resonance scanning speed, saves scanning time, can improve the image SNR again.
Example two
The present embodiment further optimizes the "preset imaging sequence" on the basis of the first embodiment. Wherein explanations of the same or corresponding terms as those of the above embodiments are omitted. Referring to fig. 2, the preset imaging sequence generation method in the magnetic resonance imaging method provided in this embodiment includes:
s210, determining a first preset gradient peak and a second preset gradient peak according to the interlayer preset field offset.
Specifically, the inter-layer preset field-of-view offset is a phase offset of a scanned image in a phase encoding direction, which is required to be realized by the preset imaging sequence, so that gradient distances of different preset gradient peaks added to the imaging sequence, that is, a first preset gradient peak and a second preset gradient peak, can be calculated according to the inter-layer preset field-of-view offset. A first preset gradient spike is arranged after an excitation pulse in the radio frequency pulse and before a refocusing pulse, the first preset gradient spike is applied to the slice selection gradient direction, and phase modulation is carried out on data lines of a K space so as to cause image field of view shift of a scanned object.
Exemplarily, S210 includes: determining a gradient zero order moment according to the interlayer preset field offset, and determining a first preset gradient peak and a second preset gradient peak according to the gradient zero order moment.
Specifically, the interlayer preset field offset and the gradient zero-order distance have the following relationship:
wherein M represents gradient zero step;
indicating a phase value corresponding to a predetermined field of view shift between layers, e.g. a predetermined field of view shift of FOV/2, then
Is pi; gamma denotes the gyromagnetic ratio, gamma =42.58 · 2 · pi · 10 6 ·T -1 (ii) a d denotes the layer spacing, i.e. the width of the gap between two adjacent scan slices, which may also be referred to as a distance factor, e.g. 10mm, 20mm, 50mm or other values.
According to equation (1), the gradient zero order moment can be calculated from the interlayer preset field offset and the interlayer distance. Then, 0.5M and M are taken as the first preset gradient peak and the second preset gradient peak, respectively. In other embodiments, when the predetermined field offset is changed, the phase value corresponding to the predetermined field offset between layers is also changed, for example, the predetermined field offset is FOV/3, then
Is 2 pi/3; the preset field of view is offset by FOV/4, then
The value is pi/2, i.e. the preset visual field offset in the embodiment of the present application is not particularly limited, and can be determined according to the setting of the physician.
S220, according to the predetermined phase encoding mode, a first predetermined time point is determined between the excitation pulse and the first refocusing pulse of the rf pulse, and a second predetermined time point is determined between any two refocusing pulses of the rf pulse.
The preset phase encoding mode is a phase encoding mode when the K space is filled with data, and may be, for example, a random encoding mode, an odd (even) phase encoding mode, or the like.
Specifically, the adding position of the preset gradient peak on the slice selection gradient is related to the phase encoding mode of the scanning signal in the K space, so that when the preset phase encoding modes are different, the adding positions of the first preset gradient peak and the second preset gradient peak on the slice selection gradient in the preset imaging sequence are also correspondingly different. Referring to fig. 3, the pre-set imaging sequence includes slice selection gradients, frequency and phase encoding gradients, radio frequency pulses, which may include a radio frequency pulse with a flip angle of 90 degrees (referred to as an excitation pulse 303) followed by one or more radio frequency pulses with a flip angle of 180 degrees (referred to as an echo pulse 304), and a signal acquisition window (ADC). On the slice selection gradient, a plurality of gradient spikes are applied. The first predetermined gradient spike 301 is at a first predetermined point in time P0 corresponding to between the excitation pulse 303 and the first refocusing pulse 304 of the rf pulse. The second predetermined gradient spike 302 is at a second predetermined point in time corresponding to between every two refocusing pulses 304 of the rf pulse. And the specific location of the second predetermined gradient spike between the two refocusing pulses (i.e., the second predetermined point in time) depends on the predetermined phase encoding mode.
Illustratively, determining the second preset time point between any two refocusing pulses of the radio frequency pulse according to the preset phase encoding mode includes: and determining a second preset time point between every two refocusing pulses according to a preset phase coding mode between the previous refocusing pulse of the radio frequency pulse and the signal acquisition window or between the signal acquisition window and the next refocusing pulse of the radio frequency pulse.
Specifically, based on the requirement of the non-overlapping sequence design of the phase encoding and the readout encoding, between any two echo pulses, according to the difference of the preset phase encoding modes, a time point may be selected as the second preset time point between the previous echo pulse of the two echo pulses and the signal acquisition window 305 (for example, P11, P21, or P31), and between the signal acquisition window 305 and the next echo pulse of the two echo pulses (for P11, P21, P31, P12, P22, P32).
Illustratively, determining the second predetermined time point between a previous refocusing pulse of the rf pulse and the signal acquisition window or between the signal acquisition window and a subsequent refocusing pulse of the rf pulse according to the predetermined phase encoding pattern comprises: when the preset phase encoding mode is a random encoding mode, a second preset time point is randomly determined between a previous refocusing pulse of the radio frequency pulse and the signal acquisition window or between the signal acquisition window and a subsequent refocusing pulse of the radio frequency pulse.
Specifically, when the preset phase encoding mode is a random encoding mode, the second preset time point may be randomly determined as a position between two refocusing pulses, before or after the signal acquisition window, for example, the second preset time point in fig. 3 may be randomly determined as P11 or P12, P21 or P22, P31 or P32.
Illustratively, determining the second predetermined time point between a previous refocusing pulse of the rf pulse and the signal acquisition window or between the signal acquisition window and a subsequent refocusing pulse of the rf pulse according to the predetermined phase encoding pattern comprises: when the preset phase encoding mode is an odd phase encoding mode, determining a second preset time point between a previous refocusing pulse of the radio frequency pulse and the signal acquisition window; and when the preset phase coding mode is an even phase coding mode, determining a second preset time point between the signal acquisition window and a backward echo pulse of the radio frequency pulse.
Specifically, when the preset phase encoding mode is the odd encoding mode, the second preset time point is determined as the position between the previous refocusing pulse and the signal acquisition window 305, for example, the second preset time points are determined as P11, P21, P31 in fig. 3. When the predetermined phase-encoding mode is the even-encoding mode, the second predetermined time point is determined as the position between the signal acquisition window 305 and the subsequent echo pulse, for example, the second predetermined time points are determined as P12, P22, and P32 in fig. 3.
And S230, in the layer selection gradient, adding a first preset gradient peak to a first preset time point, and adding a second preset gradient peak to each second preset time point.
Specifically, on the slice selection gradient of the imaging sequence (e.g., FSE), a first preset gradient peak of 0.5M is added to a first preset time point, and a second preset gradient peak M is added to each second preset time point, so that the preset imaging sequence is generated.
According to the technical scheme of the embodiment, a first preset gradient peak and a second preset gradient peak are determined according to interlayer preset field offset; determining a first preset time point between an excitation pulse and a first refocusing pulse of a radio frequency pulse and determining a second preset time point between any two refocusing pulses of the radio frequency pulse according to a preset phase encoding mode; in the aspect selection gradient, a first preset gradient peak is added to a first preset time point, and second preset gradient peaks are added to second preset time points. The method and the device have the advantages that the first preset gradient peak and the second preset gradient peak with different gradient distances are added to different positions in the gradient direction of the selected layer of the imaging sequence, the preset imaging sequence is generated, and the de-aliasing efficiency of the subsequent multi-layer aliasing magnetic resonance signals is improved.
EXAMPLE III
The magnetic resonance imaging method provided by the embodiment can be suitable for magnetic resonance imaging with simultaneous multi-slice excitation. The method may be performed by a magnetic resonance imaging system, which may be implemented in software and/or hardware. For example, the procedures of sequence design, scan control and signal processing can be implemented by software, but the magnetic resonance scan of the scanned object is implemented by a magnetic resonance scanning apparatus. Wherein explanations of the same or corresponding terms as those of the above embodiments are omitted.
Referring to fig. 4, the method of the present embodiment specifically includes the following steps:
s310, determining a region to be detected of the scanning object in the scanning field of view, wherein the region to be detected comprises a plurality of layers of slices.
Specifically, before the magnetic resonance scan is performed, an area to be detected within the scan field of view is determined, and the area to be detected comprises two or more scan slices, so that the magnetic resonance scan with simultaneous multi-slice excitation can be performed later. In an embodiment, the region to be detected in the scan field of view may be selected by a positioning box set by a doctor, or determined by an automatic identification algorithm through a computer device, and the obtaining manner of the region to be detected in this embodiment is not particularly limited. Further, the region to be examined usually contains multiple layers, and the number of scanning layers and the layer thickness can be determined by the doctor before the scanning is performed.
S320, obtaining the distance between at least two adjacent sheets in the multi-sheet.
Specifically, after the region to be detected is determined, the distance between every two adjacent slices (i.e., the inter-slice distance d) needs to be determined according to the set scanning parameters. The layer spacing refers to the gap width of two adjacent layers, for example 10mm, 20mm, 50mm or other values, and may also be referred to as a distance factor.
S330, a spin echo sequence is utilized to simultaneously excite at least two adjacent slices, and a first preset gradient peak and a second preset gradient peak with different gradient moments are successively applied to a slice selection gradient in the execution process of the spin echo sequence, wherein the gradient moments of the first preset gradient peak and the second preset gradient peak are determined according to the distance between the at least two adjacent slices, and the application of the first preset gradient peak and the second preset gradient peak causes the shift of a scanning visual field.
In particular, a spin echo sequence is used to simultaneously excite the scanning of at least two adjacent slices in the region to be examined. Due to the design of the scanning sequence, in the execution process of the spin echo sequence, a first preset gradient peak and a second preset gradient peak with different gradient distances are applied to the slice selection gradient of the scanning sequence in sequence. The application of the two gradient peaks at different gradient distances leads to a relative displacement of the image in the phase encoding direction, i.e. to an inter-layer preset field-of-view shift in the image domain.
Exemplary, determining the first preset gradient peak and the second preset gradient peak according to the spacing of at least two adjacent slices comprises: and determining a gradient zero order moment according to the distance between at least two adjacent sheets, and determining a first preset gradient peak and a second preset gradient peak according to the gradient zero order moment. Specifically, the first preset gradient peak 0.5M and the second preset gradient peak M may be calculated and obtained according to formula (1) according to the phase value and the interlayer distance corresponding to the interlayer preset field offset.
S340, acquiring multi-layer aliasing magnetic resonance signals of at least two adjacent slices by using the receiving coil, and generating a magnetic resonance image of each slice of the scanned object according to the multi-layer aliasing magnetic resonance signals.
Specifically, a receiving coil is used for collecting multi-layer aliasing magnetic resonance signals, and the inter-layer sensitivity difference corresponding to the inter-layer preset field offset is used for de-aliasing the multi-layer aliasing magnetic resonance signals. And then, carrying out image reconstruction on the unmixed and overlapped magnetic resonance signals by using an image reconstruction algorithm to obtain a magnetic resonance image of each slice of the scanned object.
Optionally, applying a first preset gradient peak and a second preset gradient peak with different gradient moments on the slice selection gradient comprises:
determining a first preset time point between an excitation pulse and a first refocusing pulse of a radio frequency pulse of a spin echo sequence and determining a second preset time point between any two refocusing pulses of the radio frequency pulse according to a preset phase encoding mode;
in the aspect selection gradient, a first preset gradient peak is added to a first preset time point, and second preset gradient peaks are added to second preset time points.
Optionally, determining a second preset time point between any two refocusing pulses of the rf pulse according to the preset phase encoding mode includes:
and determining a second preset time point between every two refocusing pulses according to a preset phase coding mode between the previous refocusing pulse of the radio frequency pulse and the signal acquisition window or between the signal acquisition window and the next refocusing pulse of the radio frequency pulse.
Optionally, according to the predetermined phase encoding mode, determining the second predetermined time point between a previous refocusing pulse of the rf pulse and the signal acquisition window, or between the signal acquisition window and a subsequent refocusing pulse of the rf pulse comprises:
when the preset phase encoding mode is a random encoding mode, a second preset time point is randomly determined between a previous refocusing pulse of the radio frequency pulse and the signal acquisition window or between the signal acquisition window and a subsequent refocusing pulse of the radio frequency pulse.
Optionally, according to the predetermined phase encoding mode, determining the second predetermined time point between a previous refocusing pulse of the rf pulse and the signal acquisition window, or between the signal acquisition window and a subsequent refocusing pulse of the rf pulse comprises:
when the preset phase encoding mode is an odd phase encoding mode, determining a second preset time point between a previous refocusing pulse of the radio frequency pulse and the signal acquisition window;
and when the preset phase coding mode is an even phase coding mode, determining a second preset time point between the signal acquisition window and a backward echo pulse of the radio frequency pulse.
Optionally, generating a magnetic resonance image of each slice of the scanned object from the multi-slice aliased magnetic resonance signals comprises:
filling the multi-layer aliasing magnetic resonance signals into a K space, and acquiring aliasing K space data;
performing interlayer de-aliasing on the aliased K space data based on interlayer sensitivity difference corresponding to interlayer preset field offset to generate de-aliased K space data of each layer of the scanning object;
and carrying out image reconstruction on the antialiasing K space data of each layer to obtain magnetic resonance images of each layer of the scanning object.
Optionally, simultaneously exciting at least two adjacent slices with the spin echo sequence comprises:
based on an undersampling mode, simultaneously exciting at least two adjacent slices by using a spin echo sequence;
accordingly, the unaliased K-space data is undersampled unaliased K-space data.
Further, performing image reconstruction on each slice of the antialiased K-space data to obtain a magnetic resonance image of each slice of the scan object includes:
acquiring a coil sensitivity distribution map of a receiving coil;
restoring the antialiasing K space data of each layer of undersampled data according to the coil sensitivity distribution diagram to generate full sampling K space data of the corresponding undersampled antialiasing K space data;
and carrying out image reconstruction according to the fully sampled K space data to obtain magnetic resonance images of all layers of the scanned object.
According to the technical scheme of the embodiment, the region to be detected of the scanned object is determined in the scanning field of view, and the region to be detected comprises a plurality of slices; acquiring the distance between at least two adjacent sheets in the multi-sheet; the method comprises the steps that a spin echo sequence is utilized to excite at least two adjacent slices simultaneously, and a first preset gradient peak and a second preset gradient peak with different gradient moments are applied to a slice selection gradient in sequence in the execution process of the spin echo sequence, wherein the gradient moments of the first preset gradient peak and the second preset gradient peak are determined according to the distance between the at least two adjacent slices, and the application of the first preset gradient peak and the second preset gradient peak causes the shift of a scanning view field; a receive coil is used to acquire a plurality of slices of aliased magnetic resonance signals of at least two adjacent slices and a magnetic resonance image of each slice of the scanned object is generated from the plurality of slices of aliased magnetic resonance signals. The method and the device have the advantages that in the magnetic resonance scanning process, the preset gradient peak with different gradient distances is applied to the layer selection gradient of the preset imaging sequence to cause the interlayer preset field-of-view offset, so that the interlayer sensitivity difference is increased, the multi-layer aliasing magnetic resonance signals are subjected to de-aliasing, and the aliasing knowing efficiency and the signal-to-noise ratio of the magnetic resonance images are improved.
Example four
The present embodiment provides a magnetic resonance imaging system 500, referring to fig. 5, the system specifically includes: an MRI scanning device 510, and a processor 520 communicatively coupled to the MRI scanning device 510;
the MRI scanning apparatus 510 is configured to simultaneously excite multiple slices of a scanned object by using a preset imaging sequence, and acquire multiple slices of aliased magnetic resonance signals of the scanned object, where the preset imaging sequence is provided with a preset gradient peak at a preset time point of a slice selection gradient to implement interlayer preset field offset of the multiple slices of aliased magnetic resonance signals on an image domain, and the preset time point is located between an excitation pulse and a first refocusing pulse in the radio frequency pulse and between any two refocusing pulses;
a processor 520 for performing inter-layer de-aliasing and image reconstruction on the multi-layer aliased magnetic resonance signals to generate magnetic resonance images of the layers of the scanned object.
Of course, it will be understood by those skilled in the art that the processor 520 may also implement the solution of the magnetic resonance imaging method provided by any embodiment of the present invention.
The magnetic resonance imaging system 500 shown in fig. 5 is only an example and should not impose any limitations on the functionality or scope of use of embodiments of the present invention. As shown in fig. 5, the magnetic resonance imaging system 500 further includes an output device 530 based on the above technical solution.
The processor 520 may monitor or control the MRI scanning device 510 and the output device 530 simultaneously. Processor 520 may include one or a combination of Central Processing Unit (CPU), application-Specific Integrated Circuit (ASIC), application Specific Instruction Processor (ASIP), graphics Processing Unit (GPU), physical Processing Unit (PPU), digital Signal Processor (DSP), field-Programmable Gate Array (FPGA), ARM Processor, and the like.
An output device 530, such as a display, may display a magnetic resonance image of the region of interest. Further, the output device 530 can also display the height, weight, age, imaging part of the subject, and the operating state of the MRI scanner 510. The output device 530 may be one or a combination of Cathode Ray Tube (CRT) output device, liquid crystal output device (LCD), organic light emitting output device (OLED), plasma output device, and the like.
The magnetic resonance imaging system 500 may be connected to a Local Area Network (LAN), wide Area Network (WAN), public Network, private Network, public Switched Telephone Network (PSTN), the internet, wireless Network, virtual Network, or any combination thereof.
The MRI scanner 510 includes an MR signal acquisition module, an MR control module, and an MR data storage module. Wherein the MR signal acquisition module comprises a magnet unit and a radio frequency unit. The magnet unit mainly comprises a main magnet generating a B0 main magnetic field and gradient components generating gradients. The main magnet contained in the magnet unit may be a permanent magnet or a superconducting magnet, the gradient assembly mainly includes a gradient current Amplifier (AMP), a gradient coil, and may further include three independent channels Gx, gy, gz, each gradient amplifier excites a corresponding one of the gradient coils in the gradient coil set to generate a gradient field for generating a corresponding spatial encoding signal to spatially locate the magnetic resonance signal. The radio frequency unit mainly comprises a radio frequency transmitting coil and a radio frequency receiving coil, the radio frequency transmitting coil is used for transmitting radio frequency pulse signals to a detected person or a human body, the radio frequency receiving coil is used for receiving magnetic resonance signals collected from the human body, and the radio frequency coils forming the radio frequency unit can be divided into a body coil and a local coil according to different functions. In one embodiment, the type of body coil or local coil may be a birdcage coil, a solenoid coil, a saddle coil, a Helmholtz coil, an array coil, a loop coil, or the like. In one embodiment, the local coils are arranged as array coils, and the array coils can be arranged in a 4-channel mode, an 8-channel mode, or a 16-channel mode. The magnet unit and the radio frequency unit can form an open low-field magnetic resonance device or a closed superconducting magnetic resonance device.
The MR control module may monitor the MR signal acquisition module, the MR data processing module, which comprise the magnet unit and the radio frequency unit. Specifically, the MR control module may receive information or pulse parameters sent by the MR signal acquisition module; in addition, the MR control module can also control the processing of the MR data processing module. In one embodiment, the MR control module is further connected to a controller including a pulse sequence generator, a gradient waveform generator, a transmitter, a receiver, etc. for controlling the magnetic field module to execute a corresponding scan sequence after receiving a command from a console.
Illustratively, the specific process of generating MR data by the MRI scanner 510 of the present invention includes: a main magnet generates a B0 main magnetic field, and atomic nuclei in a body of a detected person generate precession frequency under the action of the main magnetic field, wherein the precession frequency is in direct proportion to the strength of the main magnetic field; the MR control module stores and sends a command of a scanning sequence (scan sequence) to be executed, the pulse sequence generator controls the gradient waveform generator and the transmitter according to the scanning sequence command, the gradient waveform generator outputs a gradient pulse signal with a preset time sequence and waveform, the signal passes through Gx, gy and Gz gradient current amplifiers and then passes through three independent channels Gx, gy and Gz in the gradient assembly, each gradient amplifier excites a corresponding gradient coil in the gradient coil group to generate a gradient field for generating a corresponding spatial coding signal so as to spatially position a magnetic resonance signal; the pulse sequence generator also executes a scanning sequence, outputs data including timing, strength, shape and the like of radio frequency transmitted radio frequency pulses and timing of radio frequency receiving and the length of a data acquisition window to the transmitter, simultaneously the transmitter sends corresponding radio frequency pulses to a body transmitting coil in the radio frequency unit to generate a B1 field, signals emitted by atomic nuclei excited in a patient body under the action of the B1 field are sensed by a receiving coil in the radio frequency unit, then the signals are transmitted to the MR data processing module through the transmitting/receiving switch, and the signals are subjected to digital processing such as amplification, demodulation, filtering, AD conversion and the like and then transmitted to the MR data storage module. And when the MR data storage module acquires a group of original K space data, ending the scanning. The original K-space data is rearranged into individual K-space data sets corresponding to each image to be reconstructed, each K-space data set is input to an array processor, and the image data is reconstructed and combined with magnetic resonance signals to form a set of image data.
According to the magnetic resonance imaging system, the preset gradient peak with different gradient distances is applied to the layer selection gradient of the preset imaging sequence in the magnetic resonance scanning process, so that the interlayer preset field offset is caused, the interlayer sensitivity difference is increased, the multi-layer aliasing magnetic resonance signals are subjected to de-aliasing, and the aliasing understanding efficiency and the signal-to-noise ratio of the magnetic resonance image are improved.
Embodiments of the present invention also provide another magnetic resonance imaging system, which includes: an MRI scanner, and a processor communicatively coupled to the MRI scanner;
an MRI scanning device is used to:
determining a region to be detected of a scanned object in a scanning field of view, wherein the region to be detected comprises a plurality of layers of sheets;
acquiring the distance between at least two adjacent slice layers in the multiple slice layers;
simultaneously exciting at least two adjacent slices by using a spin echo sequence, and successively applying a first preset gradient peak and a second preset gradient peak with different gradient moments on a slice selection gradient in the execution process of the spin echo sequence, wherein the gradient moments of the first preset gradient peak and the second preset gradient peak are determined according to the distance between the at least two adjacent slices, and the application of the first preset gradient peak and the second preset gradient peak causes the shift of a scanning field;
acquiring a multi-slice aliasing magnetic resonance signal of at least two adjacent slices by using a receiving coil;
a processor for generating a magnetic resonance image of each slice of the scanned object from the multi-slice aliased magnetic resonance signals.
Of course, it will be understood by those skilled in the art that the processor may also implement the technical solution of the magnetic resonance imaging method provided by any embodiment of the present invention. The hardware structure and function of the magnetic resonance imaging system can be explained with reference to the fourth embodiment.
It is to be noted that the foregoing is only illustrative of the preferred embodiments of the present invention and the technical principles employed. It will be understood by those skilled in the art that the present invention is not limited to the particular embodiments illustrated herein, but is capable of various obvious changes, rearrangements and substitutions as will now become apparent to those skilled in the art without departing from the scope of the invention. Therefore, although the present invention has been described in greater detail by the above embodiments, the present invention is not limited to the above embodiments, and may include other equivalent embodiments without departing from the spirit of the present invention, and the scope of the present invention is determined by the scope of the appended claims.
Claims (10)
1. A magnetic resonance imaging method, comprising:
simultaneously exciting multiple layers of a scanned object by using a preset imaging sequence, and acquiring multiple layers of aliasing magnetic resonance signals of the scanned object, wherein the preset imaging sequence is provided with a preset gradient peak at a preset time point of a slice selection gradient so as to realize interlayer preset field-of-view shift of the multiple layers of aliasing magnetic resonance signals on an image domain, and the preset time point is between an excitation pulse and a first refocusing pulse in a radio frequency pulse and between any two refocusing pulses;
and performing interlayer de-aliasing and image reconstruction on the multilayer aliasing magnetic resonance signals to generate magnetic resonance images of all layers of the scanning object.
2. The method according to claim 1, characterized in that the preset imaging sequence is pre-generated by:
determining a first preset gradient peak and a second preset gradient peak according to the interlayer preset field offset;
determining a first preset time point between an excitation pulse and a first refocusing pulse of the radio frequency pulse and determining a second preset time point between any two refocusing pulses of the radio frequency pulse according to a preset phase encoding mode;
in the aspect selection gradient, adding the first preset gradient peak to the first preset time point, and adding the second preset gradient peak to each of the second preset time points.
3. The method of claim 2, wherein determining a second predetermined time point between any two refocusing pulses of the rf pulse according to a predetermined phase encoding mode comprises:
and determining the second preset time point between every two refocusing pulses according to the preset phase coding mode, between the previous refocusing pulse of the radio frequency pulse and the signal acquisition window, or between the signal acquisition window and the subsequent refocusing pulse of the radio frequency pulse.
4. The method of claim 3, wherein determining the second predetermined time point between a previous echo pulse of the RF pulse and a signal acquisition window or between the signal acquisition window and a subsequent echo pulse of the RF pulse according to the predetermined phase encoding pattern comprises:
when the preset phase encoding mode is an odd phase encoding mode, determining a second preset time point between a previous refocusing pulse of the radio frequency pulse and a signal acquisition window;
and when the preset phase coding mode is an even phase coding mode, determining the second preset time point between the signal acquisition window and a backward echo pulse of the radio frequency pulse.
5. The method of claim 1, wherein performing inter-slice antialiasing and image reconstruction on the multi-slice aliased magnetic resonance signals to generate unaliased images of the scan object slices comprises:
filling the multilayer aliasing magnetic resonance signals into a K space, and acquiring aliasing K space data;
performing interlayer de-aliasing on the aliased K space data based on the interlayer sensitivity difference corresponding to the interlayer preset field offset to generate de-aliased K space data of each layer of the scanning object;
and carrying out image reconstruction on the de-aliasing K space data of each layer to obtain magnetic resonance images of each layer of the scanning object.
6. The method of claim 5, wherein simultaneously exciting multiple slices of the scan object with a preset imaging sequence, acquiring multiple slices of aliased magnetic resonance signals of the scan object comprises:
based on an undersampling mode, simultaneously exciting multiple layers of a scanning object by utilizing a preset imaging sequence, and acquiring multiple layers of aliasing magnetic resonance signals of the scanning object;
accordingly, the unaliased K-space data is undersampled unaliased K-space data.
7. The method of claim 6, wherein performing image reconstruction of each slice of the de-aliased K-space data to obtain magnetic resonance images of each slice of the scan object comprises:
acquiring a coil sensitivity distribution map of a receiving coil;
restoring the aliasing-removing K space data of each layer of undersampled data according to the coil sensitivity distribution map to generate fully sampled K space data of the corresponding undersampled aliasing-removing K space data;
and carrying out image reconstruction according to the fully sampled K space data to obtain magnetic resonance images of all layers of the scanning object.
8. A magnetic resonance imaging method, comprising:
determining a region to be detected of a scanning object in a scanning field of view, wherein the region to be detected comprises a plurality of slices;
acquiring the distance between at least two adjacent sheets in the multi-sheet;
simultaneously exciting the at least two adjacent slices by using a spin echo sequence, and successively applying a first preset gradient peak and a second preset gradient peak with different gradient moments on a slice selection gradient in the execution process of the spin echo sequence, wherein the gradient moments of the first preset gradient peak and the second preset gradient peak are determined according to the distance between the at least two adjacent slices, and the application of the first preset gradient peak and the second preset gradient peak causes the shift of the scanning visual field;
acquiring a plurality of layers of aliased magnetic resonance signals of the at least two adjacent slices by using a receiving coil, and generating a magnetic resonance image of each slice of the scanning object according to the aliased magnetic resonance signals.
9. A magnetic resonance imaging system, comprising: an MRI scanner, and a processor communicatively coupled to the MRI scanner;
the MRI scanning device is used for simultaneously exciting multiple layers of a scanned object by utilizing a preset imaging sequence and acquiring multiple layers of aliasing magnetic resonance signals of the scanned object, wherein the preset imaging sequence is provided with a preset gradient peak at a preset time point of a slice selection gradient so as to realize interlayer preset field-of-view shift of the multiple layers of aliasing magnetic resonance signals on an image domain, and the preset time point is between an excitation pulse and a first refocusing pulse in a radio frequency pulse and between any two refocusing pulses;
the processor is configured to perform inter-layer de-aliasing and image reconstruction on the multi-layer aliased magnetic resonance signals to generate magnetic resonance images of each layer of the scan object.
10. A magnetic resonance imaging system, comprising: an MRI scanner, and a processor communicatively coupled to the MRI scanner;
the MRI scanner is configured to:
determining a region to be detected of a scanning object in a scanning field of view, wherein the region to be detected comprises a plurality of layers of sheets;
acquiring the distance between at least two adjacent sheets in the multi-sheet;
simultaneously exciting the at least two adjacent slices by using a spin echo sequence, and successively applying a first preset gradient peak and a second preset gradient peak with different gradient moments on a slice selection gradient in the execution process of the spin echo sequence, wherein the gradient moments of the first preset gradient peak and the second preset gradient peak are determined according to the distance between the at least two adjacent slices, and the application of the first preset gradient peak and the second preset gradient peak causes the shift of the scanning visual field;
acquiring a plurality of layers of aliased magnetic resonance signals of the at least two adjacent slices by using a receiving coil;
the processor is configured to generate a magnetic resonance image of each slice of the scanned object from the multi-slice aliased magnetic resonance signals.
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