Magnetic resonance imaging system and method

Abstract

This disclosure provides a method for generating an image of an object using an MRI system, the method comprising providing a readout-coded imaging shearing (SIREN) gradient pulse in a phase gradient signal waveform. The phase gradient signal waveform is applied to the phase gradient coil of the MRI system. The application of the SIREN gradient pulse provides the object's SIREN k-space, which has SIREN k-space lines with shear angles. MR image spatial data from this SIREN k-space is then obtained by applying reconstruction techniques. Finally, the image of the object is generated by transforming the SIREN MR image spatial data into regular image spatial data based on the shear angle using a decoding algorithm.

Description

Background Technology

[0001] The embodiments disclosed in this invention relate to medical imaging technology, and more specifically to a method for acquiring magnetic resonance imaging (MRI) data and a magnetic resonance imaging system.

[0002] As a medical imaging modality, magnetic resonance imaging (MRI) can acquire images of the human body without using X-rays or other ionizing radiation. MRI uses a magnet with a strong magnetic field to generate a static magnetic field B0. When the part of the body to be imaged is placed in the static magnetic field B0, the nuclear spins associated with hydrogen nuclei in the tissue are polarized, resulting in a longitudinal magnetization vector at the macroscopic level. After applying a radio frequency field B1 intersecting the direction of the static magnetic field B0, the spin direction of the protons changes, resulting in a transverse magnetization vector at the macroscopic level. After the radio frequency field B1 is removed, this transverse magnetization vector decays in a spiral manner until it returns to zero. A free-inductance decay signal is generated during this decay. This free-inductance decay signal can be acquired as an MRI signal, and an image of the tissue to be imaged can be reconstructed based on the acquired signal.

[0003] MRI can include one or more types of artifacts that reduce image sharpness and resolution. The presence of artifacts in medical images can affect diagnostic quality. Phase aliasing, or phase rollover, is one such artifact that occurs in MR images when the field of view (FOV) in the phase encoding direction is smaller than the body portion being imaged. Typically, objects outside one side of the phase FOV are projected onto the other side of the phase FOV in the image. This type of artifact can be corrected by increasing the phase FOV. However, increasing the phase FOV also increases the total MRI scan time.

[0004] Therefore, an improved magnetic resonance imaging system and method are needed. Summary of the Invention

[0005] According to embodiments of the present invention, a method for generating an image of an object using a magnetic resonance imaging (MRI) system is provided. The method includes providing a readout-coded shearing (SIREN) gradient pulse in a phase gradient signal waveform applied to a phase gradient coil of the MRI system. The method further includes obtaining the SIREN k-space of the object, wherein the k-space lines of the SIREN k-space have shear angles. MR image spatial data is then obtained from the SIREN k-space. The method further includes generating an image of the object by transforming the SIREN MR image spatial data into regular image spatial data based on the shear angle using a decoding algorithm.

[0006] According to another embodiment of the invention, an MRI system is provided, the MRI system having a magnet configured to generate a polarization magnetic field around at least a portion of an object disposed in the MRI system. The MRI system includes a gradient coil assembly comprising a readout gradient coil, a phase gradient coil, and a slice selection gradient coil, the gradient coil assembly being configured to apply at least one gradient field to the polarization magnetic field. Further, a radio frequency (RF) system is configured to apply an RF field to the object and receive magnetic resonance signals from the object. The MRI system also includes a processing system programmed to provide readout-coded imaging shearing (SIREN) gradient pulses in a phase gradient signal waveform, wherein the phase gradient signal waveform is applied to the phase gradient coil. The processing system is also programmed to obtain the SIREN k-space of the object, wherein the k-space lines of the SIREN k-space have shear angles. MR image spatial data is then obtained from the SIREN k-space. The processing system is further programmed to generate an image of the object by transforming the SIREN MR image spatial data into regular image spatial data based on the shear angle using a decoding algorithm.

[0007] According to another embodiment of the present invention, a method for generating an image of an object using a magnetic resonance imaging (MRI) system is provided. The method includes providing a readout-coded shearing (SIREN) gradient pulse in a phase gradient signal waveform applied to a phase gradient coil of the MRI system. The method further includes obtaining SIREN k-space data of the object, wherein the k-space lines of the SIREN k-space data have shear angles. The method further includes transforming the SIREN k-space data of the object into regular domain k-space data based on the shear angle using a decoding algorithm. Finally, the method includes generating an image of the object from the regular domain k-space data. Attached Figure Description

[0008] These and other features, aspects, and advantages of the invention will be better understood when the following detailed description is read with reference to the accompanying drawings, in which the same reference numerals denote the same parts throughout the drawings, wherein:

[0009] Figure 1 This is a schematic diagram of an exemplary magnetic resonance imaging (MRI) system according to an embodiment of the present invention;

[0010] Figure 2 This is a schematic diagram of the k-space trajectory of an MRI system;

[0011] Figure 3 This is a schematic diagram of a pulse sequence from a conventional MRI system.

[0012] Figure 4This is a schematic diagram of a pulse sequence of an MRI system according to an embodiment of the present invention;

[0013] Figure 5 It is used in the implementation scheme of the present invention. Figure 4 A schematic diagram of the k-space trajectory obtained from the pulse sequence diagram;

[0014] Figure 6 This is a schematic diagram illustrating a comparison between the regular Cartesian image space and the SIREN image space according to embodiments of the present invention;

[0015] Figure 7 This is a schematic diagram illustrating the phase aliasing effect in both regular Cartesian image space and SIREN image space according to an embodiment of the present invention;

[0016] Figure 8 This is a schematic diagram depicting an embodiment of the present invention, which transforms the SIREN image space into a decoded SIREN image space;

[0017] Figure 9 This is a schematic diagram depicting the image segmentation effect caused by transforming the SIREN image space into a decoded SIREN image space according to an embodiment of the present invention;

[0018] Figure 10 This is a schematic diagram illustrating the effect of shear angle variation on phase aliasing effect according to an embodiment of the present invention;

[0019] Figure 11 This describes a SIREN decoding pair based on an embodiment of the technology according to the present invention. Figure 10 A schematic diagram illustrating the influence of the images in the diagram;

[0020] Figure 12 This is a flowchart illustrating a method for generating MR images of an object using an MRI system according to an embodiment of the present invention; and

[0021] Figure 13 This is a flowchart of another method for generating MR images of an object using an MRI system according to an embodiment of the present invention. Detailed Implementation

[0022] One or more specific implementations will be described below. To provide a concise description of these implementations, not all characteristics of the actual implementation may be described in this specification. It should be understood 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 developer's specific objectives, such as complying with system-related and business-related constraints that may differ from implementation to implementation. Furthermore, it should be understood that such development efforts may be complex and time-consuming, but remain routine tasks of design, fabrication, and manufacturing for those skilled in the art who benefit from this disclosure.

[0023] When describing elements of various embodiments of this invention, the articles “a,” “an,” “the,” and “the” are intended to refer to one or more of these elements. The terms “comprising,” “including,” and “having” are intended to be inclusive and mean that additional elements may be present in addition to those listed. Furthermore, any numerical examples in the following discussion are intended to be non-limiting, and therefore any additional values, ranges, and percentages are within the scope of the disclosed embodiments. Additionally, the terms “circuit,” “circuit system,” and “controller” can include single or multiple components that are active and / or passive and are connected or otherwise coupled together to provide the described functionality.

[0024] In magnetic resonance imaging (MRI), an object is placed in a magnet. When the object is in a magnetic field generated by the magnet, the magnetic moments of nuclei (such as protons) attempt to align with the magnetic field, but precess around the magnetic field in a random order at the Larmor frequency of the nuclei. The magnetic field of the magnet is called B0 and extends in the longitudinal or z-direction. During the acquisition of MR images, a magnetic field in the xy-plane and close to the Larmor frequency (called the excitation field B1) is generated by a radio frequency (RF) coil and can be used to align the net magnetic moments of the nuclei M0 with the magnetic field. z Rotate or "tilt" from the z-direction towards the lateral or xy-plane. After the excitation signal B1 terminates, the nucleus emits a signal, which is called the MR signal. To generate an image of the object using the MR signal, a magnetic field gradient pulse (G) is used. x G y and G z Gradient pulses are used to scan the inverse of a space or distance through k-space, spatial frequency, or space. A Fourier relationship exists between the acquired MR signal and the image of the object, therefore the image of the object can be derived by reconstructing the MR signal. The image of the object can be a two-dimensional (2D) or three-dimensional (3D) image.

[0025] Embodiments of this disclosure will now be described by way of example with reference to the accompanying drawings, wherein Figure 1This is a schematic diagram of a magnetic resonance imaging (MRI) system 10. Operation of the system 10 can be controlled from an operator console 12, which includes an input device 13, a control panel 14, and a display screen 16. The input device 13 may be a mouse, joystick, keyboard, trackball, touchscreen, light bar, voice controller, and / or other input device. The input device 13 can be used for interactive geometry specification. The console 12 communicates with a computer system 20 via a link 18, which enables the operator to control the generation and display of images on the display screen 16. The link 18 can be a wireless or wired connection. The computer system 20 may include modules that communicate with each other via a backplane 20a. Modules of the computer system 20 may include, for example, an image processor module 22, a central processing unit (CPU) module 24, and a memory module 26, which may include, for example, a frame buffer for storing image data arrays. The computer system 20 may be linked to archive media devices, permanent or backup storage, or a network for storing image data and programs, and communicates with the MRI system controls 32 via a high-speed signal link 34. The MRI system control 32 can be separate from or integrated with the computer system 20. The computer system 20 and the MRI system control 32 together form the "MRI controller" 33 or the "controller".

[0026] In an exemplary embodiment, the MRI system control 32 includes modules connected to a base plate 32a. These modules include a CPU module 36 and a pulse generator module 38. The CPU module 36 is connected to the operator console 12 via a data link 40. The MRI system control 32 receives commands from the operator via the data link 40 to indicate the scan sequence to be executed. The CPU module 36 operates the system components to execute the desired scan sequence and generates data indicating the timing, intensity, and shape of the generated RF pulses, as well as the timing and length of the data acquisition window. The CPU module 36 is connected to components operated by the MRI controller 32, including the pulse generator module 38, which controls the gradient amplifier 42, the physiological acquisition controller (PAC) 44, and the scan chamber interface circuitry 46.

[0027] In one example, CPU module 36 receives subject data from physiological acquisition controller 44, which receives signals from sensors connected to the subject, such as ECG signals received from electrodes attached to the patient. As used herein, the subject is a person (or patient), animal, or body membrane. CPU module 36 receives signals from sensors associated with the patient and magnet system status via scan chamber interface circuitry 46. Scan chamber interface circuitry 46 also enables MRI controller 33 to command patient positioning system 48 to move the patient to the desired location for scanning.

[0028] A whole-body RF coil 56 is used to transmit waveforms toward the subject's anatomical structures. The whole-body RF coil 56 can be a body coil. The RF coil can also be a local coil, which can be placed closer to the subject's anatomical structures than a body coil. The RF coil 56 can also be a surface coil. An RF coil containing RF receiver channels can be used to receive signals from the subject's anatomical structures. A typical surface coil will have eight receiver channels; however, different numbers of channels are possible. It is known that using a combination of both the body coil 56 and the surface coil can provide better image quality.

[0029] Pulse generator module 38 is operable to gradient amplifier 42 to achieve the desired timing and shape of gradient pulses generated during scanning. The gradient waveform generated by pulse generator module 38 can be applied to gradient amplifier system 42 with Gx, Gy, and Gz amplifiers. Each gradient amplifier excites a corresponding physical gradient coil in gradient coil assembly 50 to generate a magnetic field gradient for spatial encoding of the acquired signal. Specifically, Gx corresponds to the flux / frequency encoding gradient, Gy corresponds to the phase encoding gradient, and Gz corresponds to the slice selection gradient. Gradient coil assembly 50 may form part of magnet assembly 52, which also includes polarized magnet 54 (in operation, the polarized magnet provides a longitudinal magnetic field B0 throughout the target space 55 surrounded by magnet assembly 52) and whole-body RF coil 56 (in operation, this coil provides a transverse magnetic field B1, which is substantially perpendicular to B0 throughout the target space 55). Transceiver module 58 in MRI system control 32 generates pulses that can be amplified by RF amplifier 60, which is coupled to RF coil 56 via transmit / receive switch 62. The resulting signal emitted by the stimulated nuclei in the subject's anatomical structures can be sensed by a receiving coil (not shown) and provided to a preamplifier 64 via a transmit / receive switch 62. The amplified MR signal is demodulated, filtered, and digitized in the receiver section of transceiver 58. The transmit / receive switch 62 is controlled by a signal from the pulse generator module 38 to electrically connect the RF amplifier 60 to the coil 56 during transmit mode and to connect the preamplifier 64 to the receiving coil during receive mode.

[0030] The MR signal generated by the excitation of the target is digitized via transceiver module 58. MR system control 32 then processes the digitized signal via Fourier transform to generate k-space data, which is transmitted via MRI system control 32 to memory module 66 or other computer-readable medium. "Computer-readable medium" may include, for example, structures configured such that electrical, optical, or magnetic states can be perceptibly and reproducibly fixed by a conventional computer (e.g., text or images printed on paper or displayed on a screen, optical disc, or other optical storage medium; "flash memory," EEPROM, SDRAM, or other electrical storage media; floppy disks or other magnetic disks, magnetic tape, or other magnetic storage media).

[0031] The scan is complete when an array of raw k-space data is acquired in a computer-readable medium 66. For each image to be reconstructed, the raw k-space data is rearranged into separate k-space data arrays, and each of these k-space data arrays is input to an array processor 68, which operates to reconstruct the data into an array of image data using a reconstruction algorithm such as Fourier transform. When complete k-space data is obtained, it represents the entire volume of the subject's body, and the k-space thus obtained can be referred to as the reference k-space. Similarly, when only partial k-space data is obtained, the image can be referred to as partial k-space. The image data is transmitted to the computer system 20 via data link 34 and stored in memory. In response to a command received from the operator console 12, the image data may be archived in a long-term storage device, or it may be further processed by the image processor 22 and transmitted to the operator console 12 and displayed on the monitor 16.

[0032] The MR signal is represented by a complex number, where each position in the k-space is represented by a complex number, and the I and Q orthogonal MR signals are real and imaginary components, respectively. A composite MR image can be reconstructed based on the I orthogonal MR signals and the Q orthogonal MR signals using procedures such as the Fourier transform of the k-space MR data. The composite MR image is an MR image with each pixel represented by a complex number, which also has real and imaginary components. The magnitude M of the received MR signal can be determined as the square root of the sum of the squares of the I orthogonal and Q orthogonal components of the received MR signal, as shown in equation (3) below:

[0033]

[0034] Furthermore, the phase φ of the received MR signal can also be determined by the following equation (2):

[0035]

[0036] As previously discussed, phase aliasing artifacts occur in MR images when the field of view (FOV) is smaller than the body portion being imaged. Typically, objects outside one side of the FOV are projected onto the other side of the FOV in the image. According to one embodiment of the invention, pulse generator module 38 generates a shearing (hereinafter referred to as "SIREN") gradient pulse to the phase gradient (Gy) in readout-coded imaging, which causes shearing in the readout-coded direction. Due to this shearing, the phase aliasing direction changes from the original phase direction. Therefore, when objects outside the FOV are projected onto the other side of the FOV, they are moved out of the region of interest, as will be explained in subsequent paragraphs. Finally, the remaining region of interest can be decoded back to its original position to acquire an image with reduced phase aliasing artifacts.

[0037] Figure 2 This is a schematic diagram of a k-space trajectory 200 according to an embodiment of the present invention. The k-space 200 is also referred to as a regular Cartesian k-space, and is defined by the maximum kx or ky value K. x，最大 and K x，最大 The maximum kx or ky value is defined by the maximum frequency or phase-encoded gradient. k-space 200 is defined as follows: Generally, k-space 200 represents the raw data obtained directly from the MR signal, where the kx-ky values ​​in k-space correspond to the spatial frequencies of the MR image. In one implementation, multiple excitation (NEX) images are acquired for each row of 202k spatial data.

[0038] Figure 3 This is a schematic diagram depicting a pulse sequence plot 300 of an MRI system. The pulse sequence plot 300 can be derived from... Figure 1 The different modules of the MRI system control 32 are generated. Figure 3 In the diagram, curve 302 illustrates the RF signal waveform that excites the RF coil 56. Curve 308 illustrates the readout gradient signal waveform Gx, curve 316 illustrates the phase gradient signal waveform Gy, and curve 322 illustrates the slice-selected gradient signal waveform Gz. Each gradient signal waveform in curves 308, 316, and 322 excites the corresponding physical gradient coil in the gradient coil assembly 50. RF signal waveform 302 includes a 90° RF pulse 304 followed by a 180° RF pulse 306. This combination of 90° and 180° RF pulses is repeated at each time interval TR (repetition time) and is referred to as a spin echo. The k-space line 202 of k-space 200 is filled with each repetition. As those skilled in the art will understand, the 180° RF pulse compensates for the loss of transverse magnetization caused by magnetic field inhomogeneities. It should be noted that pulse sequence diagram 300 is merely one example representing a regular Cartesian acquisition. However, in other embodiments, different pulse sequence diagrams, such as gradient echo sequences or diffusion-weighted sequence diagrams, may also be used.

[0039] The readout gradient signal waveform 308 shows a dephase lobe 310 with amplitude R1, a main readout lobe 312 with amplitude R2, and a spoiler lobe 314 with amplitude R3. As those skilled in the art will understand, the dephase lobe 310 causes the spins to become dephased according to their position within the gradient. The main readout lobe 312 brings these spins back into phase to produce an echo, and the spoiler lobe 314 removes any residual transverse magnetization before the next excitation pulse.

[0040] Phase gradient signal waveform 316 illustrates a first phase encoding gradient 318 (with peak amplitude P1) and a second phase encoding gradient 320 (with peak amplitude P2). The first phase encoding gradient 318, with different amplitudes, is applied for equal duration (indicated by horizontal shading) for each repetition time of the pulse sequence to provide different degrees of phase encoding at each repetition. Generally, the amplitude of the first phase encoding gradient 318 determines which line 202 of k-space 200 is filled. If the amplitude is positive, the k-space line 202 in the upper half of k-space 200 is filled. Conversely, if the amplitude of the first phase encoding gradient 318 is negative, the k-space line 202 in the lower half of k-space 200 is filled. The second phase encoding gradient 320, referred to as the rewinder gradient, is applied with the opposite polarity (as with gradient 318) at the end of each cycle. The purpose of these rewinder gradients is to ensure the stability of the MR signal phase during each repetition interval, i.e., to produce a net zero phase shift and contribute to the development of coherent transverse magnetization.

[0041] The slice selection gradient signal waveform 322 has an upward lobe 324 (with amplitude S1) referred to as the main slice selection gradient and a downward lobe 326 (with amplitude S2) referred to as the slice rephase lobe. The slice rephase lobe 326 is applied exactly after the 90° RF pulse 304. As those skilled in the art will understand, the slice rephase lobe 326 helps correct the phase dispersion of the transverse magnetization that accompanies the application of the main slice selection gradient 324.

[0042] Figure 4 This is a schematic diagram depicting a pulse sequence diagram 400 of an MRI system according to an embodiment of the present invention. The pulse sequence diagram 400 can be derived from... Figure 1 The different modules of the MRI system control 32 are generated. Figure 4 The implementation scheme shown uses the above Figure 3 The component numbers are similar to those of the corresponding parts in the illustrated embodiment. (And...) Figure 3 similar, Figure 4The pulse sequence diagram 400 includes an RF signal waveform 302, a readout gradient signal waveform 308Gx, and a slice selection gradient signal waveform 322Gz. However, Figure 3 The phase gradient signal waveform 316 in the image has been replaced with... Figure 4 A new phase gradient signal waveform 410 GHz is generated. Each gradient signal waveform in gradient signal waveforms 308, 410, and 322 excites a corresponding physical gradient coil in gradient coil assembly 50. Specifically, gradient coil assembly 50 includes a readout gradient coil, a phase gradient coil, and a slice selection gradient coil. Gradient signal waveforms 308, 410, and 322 excite the readout gradient coil, phase gradient coil, and slice selection gradient coil, respectively. RF signal waveform 302 includes a 90° RF pulse 304 followed by a 180° RF pulse 306. This combination of 90° and 180° RF pulses is repeated at each time interval TR (repetition time). The k-space lines are filled using each repetition. As those skilled in the art will understand, in one embodiment, partial sampling of the k-space may also be used. In partial k-space, instead of filling all the lines of the k-space, some lines of the k-space are skipped.

[0043] Phase gradient signal waveform 410 illustrates the SIREN dephaser gradient 412, the first phase encoding gradient 414, the SIREN gradient 416, and the second phase encoding gradient 418. Specifically, the SIREN dephaser gradient 412 is applied before the first phase encoding gradient 414, and the SIREN gradient is applied between the first phase encoding gradient 414 and the second phase encoding gradient 418. Similar to gradients 318 and 320, the first phase encoding gradient 414 and the second phase encoding gradient 418 are applied to fill the k-space and create a net zero phase shift, respectively. Furthermore, the SIREN dephaser gradient 412 is applied simultaneously with the dephase lobe 310, and the SIREN gradient 416 is applied simultaneously with the main readout lobe 312. The effect of applying the SIREN gradient 416 is that it causes... Figure 5 The shearing in the readout coding direction is visible. Furthermore, due to the SIREN gradient 416, phase aliasing artifacts in the image can be reduced, as will be explained below. It should be noted that this invention is not limited to... Figure 4 The pulse sequence map 400. In another embodiment, different pulse sequence maps used to obtain MR images can also be used with the SIREN gradient 416. For example, the pulse sequence map can be a diffusion-weighted sequence, an inversion recovery sequence, a gradient echo sequence, etc.

[0044] Figure 5This is a schematic diagram of a k-space trajectory 500 according to an embodiment of the present invention. Compared to k-space 200, in k-space 500, the k-space line 502 is inclined relative to the x-axis by a shear angle θ. This shear angle is caused by the SIREN gradient 416 applied to the y- or phase gradient coil of the gradient coil assembly 50. Therefore, k-space 500 is also referred to as SIREN k-space data. The value of the shear angle θ depends on the amplitude R2 of the main readout lobe 312 and the amplitude P4 of the SIREN gradient 416. In one embodiment, the shear angle is given as follows:

[0045]

[0046] Figure 6 This is a schematic diagram 600 depicting a comparison between a regular Cartesian image space 602 and a SIREN image space 604. Those skilled in the art will understand that image spaces 602 and 604 are obtained after applying a Fourier transform to the corresponding k-spaces 200 and 500. The regular Cartesian image space 602 is the result of pulse sequence diagram 300, and the SIREN image space 604 is the result of pulse sequence diagram 400. It should be noted that phase aliasing is not shown in either the regular Cartesian image space 602 or the SIREN image space 604. Phase aliasing will be... Figure 7 As shown in the diagram, the object image 606 corresponding to the regular Cartesian image space 602 is a straight image, while the object image 608 corresponding to the SIREN image space 604 is a tilted image with a shear angle θ. Due to this shear angle, the (x,y) coordinates in image 606 will be shifted to the coordinates (x+y*tanθ,y) in image 608. In other words, the y coordinates will be the same, but the x coordinates will be shifted by a factor of y*tanθ in image 606. It should be noted that due to this coordinate shift, points 610 and 612 fall outside image space 604 and are therefore cut off from the final image. However, these points are outside the image of interest 608 and therefore do not cause any problems. Further, in one embodiment, to make image 608 straight, a mathematical transformation or decoding / de-shearing algorithm is applied to the SIREN image space data 604 to transform the coordinates from (x+y*tanθ,y) to (x,y). In another implementation, instead of using a decoding algorithm to transform the image, the SIREN k-space 500 itself can be first transformed into regular k-space data (also known as straight k-space data), and then this regular k-space data can be used to generate a regular image (or straight image) of the object using the inverse Fourier transform (IFT).

[0047] Figure 7This is a schematic diagram 700 depicting the phase aliasing effect in both the regular Cartesian image space 702 and the SIREN image space 704. Phase aliasing artifacts occur in MR images when the field of view (FOV) is smaller than the body portion being imaged. In this case, objects outside one side of the FOV are projected onto the other side of that FOV in the image. Figure 7 In the diagram, the regular Cartesian FOV is represented by box 706, while the SIREN FOV is represented by box 712. As can be seen, due to phase aliasing, the object 708 falling outside the FOV 706 is projected onto point 710 in the image space, resulting in image artifacts. This phase aliasing effect can be avoided by increasing the FOV up to the object point 708. However, the scan time used for imaging increases with increasing FOV.

[0048] Similar to regular Cartesian methods, for SIREN acquisition, object 714, outside FOV 712, is also projected onto point 716. However, in SIREN acquisition, the image is cropped by an angle θ, as described above. Therefore, even though object 714 is projected on the other side of FOV 712, projection 716 falls outside the region of interest or the main image space 718. Therefore, when the decropping algorithm is applied to the SIREN image space 704, there are no artifacts due to phase aliasing in the decropped image.

[0049] Figure 8This is a schematic diagram 800 depicting the transformation of SIREN image space 802 into decoded SIREN image space 804. After applying a decoding / decropping algorithm to SIREN image space 802 to obtain an uncropped image, decoded SIREN image space 804 is obtained. In one embodiment, the decropping algorithm is a mathematical transformation factor. As previously discussed, in SIREN image space 802, an object 806 outside the FOV is projected onto a point 808 falling outside the region of interest or main image space 810. Now, when the decropping algorithm is applied, image space 802 is decropped to generate decoded SIREN image space 804. In decoded SIREN image space 804, all x-coordinates are shifted to the left by a distance equal to y*tanθ. Therefore, even the projected object 808 in image space 802 moves the same distance to reach point 810, which is still outside the region of interest or main image space 812. It should be noted that the shear angle θ determines where the projection of object 806 will fall in image space 802 or 804. For example, if the shear angle is zero, there is no shear factor, and the projection of object 806 will then fall on the main image as shown in regular Cartesian image space 702, resulting in artifacts in the final image. Furthermore, it should be noted that when the decropping algorithm is applied to SIREN image space 802, a portion of the image can be cut off, as shown regarding... Figure 6 The explanation given by points 610 and 612.

[0050] Figure 9 This is a schematic diagram 900 depicting the image clipping effect resulting from transforming the SIREN image space 902 to the decoded SIREN image space 904. After the transformation, shadow corners 906 and 908 are primarily clipped from the decoded SIREN image space 904. As can be seen, if any part of the image of interest 910 does not fall under the shadow portions 906 and 908, there is no problem even if the shadow portions 906 and 908 are removed from the final image. However, in some cases, it may happen that a portion of the main image 910 falls within the shadow portions and may be clipped. In such cases, it may be necessary to reduce the shear angle, which can introduce a small amount of phase aliasing. Therefore, the choice of shear angle depends on the medical application, i.e., it depends on which part of the body is being imaged. Alternatively, in some embodiments, the FOV can be increased in the frequency or x-direction, which covers a larger area of ​​the image and avoids image clipping. By increasing the FOV in the x-direction, the scan time can be increased. However, in this case, the increase in scan time may not be as significant as if the FOV were increased in the y- or phase directions. In another embodiment, a mask can be applied to the area that will be cropped in the final image. Applying a mask to the cropped area will reduce the system's processing time.

[0051] Figure 10 This is a schematic diagram 1000 illustrating the effect of shear angle variation on phase aliasing according to an embodiment of the present invention. Specifically, Figure 10 Six different images A, B, C, D, E, and F of object 1002 are shown, corresponding to six different shearing factors (tanθ) of 0, 0.2, 0.4, 0.6, 0.8, and 1.0. Object 1002 has a portion 1004 below the field of view (FOV) and another portion 1006 above portion 1004 but outside the FOV. However, due to phase aliasing, portion 1006 is projected / overlaid on the bottom side of portion 1004. As the shearing factor increases from 0 to 1, portion 1006 begins to shift away from portion 1004. It can be seen that in images D, E, and F, portion 1006 is completely outside the region of interest 1004. However, at the same time, portion 1004 is also sheared according to the shearing factor.

[0052] Furthermore, it should be noted that in image A, there exists an artifact 1010 due to the annefact effect. As those skilled in the art will understand, MR signals generated in regions with nonlinear gradient fields (such as at corners or boundaries) cause the annefact effect in MR images. Using the SIREN technique proposed herein, the annefact artifact 1010 also shifts out of the region of interest, i.e., part 1006, as the shearing factor increases from 0 to 1.

[0053] Figure 11 This describes a SIREN decoding pair based on an embodiment of the technology according to the present invention. Figure 10 A schematic diagram 1100 illustrates the influence of the image in the diagram. Specifically, Figure 11 Three images, D, E, and F, are shown, along with their SIREN decoded versions G, H, and I after a decropping algorithm is applied. Images D, E, and F correspond to clipping factors (tanθ) of 0.6, 0.8, and 1.0, respectively. As can be seen from images G, H, and I, the application of the decropping algorithm prevents images D, E, and F from being clipped. However, a portion of each image is simultaneously clipped during this process. For example, due to SIREN decoding, two corners, 1102 and 1104, in all images G, H, and I are clipped. The size of the clipped corners depends on the clipping factor tanθ. For instance, among the three images, image G, corresponding to a clipping factor tanθ = 0.6, has the smallest corner region clipping, while image I, corresponding to a clipping factor tanθ = 1.0, has the largest corner region clipping. Therefore, the clipping factor, or clipping angle θ, is determined based on the medical application (i.e., which body part is being imaged) and the field of view.

[0054] In one implementation, once the shear angle θ is known, the amplitude P4 of the SIREN gradient 416 can be determined as:

[0055] P4=R2tanθ (2)

[0056] Where R2 is the amplitude of the main readout lobe or pulse 312. Furthermore, the area below the SIREN phase depletion gradient pulse 412 (i.e., the SIREN phase depletion area) is set to be equal to half the area below the SIREN gradient 416 (i.e., the SIREN area). Based on the SIREN phase depletion area (Ad), the amplitude P3 of the SIREN phase depletion can be determined as:

[0057]

[0058] Where d1 and d2 are the base and peak pulse durations of the SIREN phase depletion gradient pulse 412, and are equal to the base and peak pulse durations of the two-phase lobe or pulse 310.

[0059] Figure 12 This is a flowchart of a method 1200 for generating an MR image of an object using an MRI system 10 according to an embodiment of the present invention. At step 1202, the method includes providing a readout-coded imaging shearing (SIREN) gradient pulse 416 in the phase gradient signal waveform 410 of the MRI system. Generally, an MRI system includes three gradient signal waveforms: a readout gradient signal waveform 308, a slice selection gradient signal waveform 322, and a phase gradient signal waveform 410, such as... Figure 4 As shown. The readout gradient signal waveform 308 includes a two-phase lobe or pulse 310, a main readout pulse 312, and a spoiler pulse 314. In addition to the SIREN gradient pulse 416, the phase gradient signal waveform 410 also includes a SIREN phase decoupling gradient 412, a first phase encoding gradient 414, and a second phase encoding gradient 418. The application of the SIREN gradient 416 results in a SIREN MR k space 500 of the object obtained at step 1204. The SIREN MR k space 500 has as shown Figure 5 The shear angle θ is shown.

[0060] At step 1206, the method includes obtaining MR image spatial data. In one embodiment, a reconstruction technique is applied to the SIREN MR k space 500 to obtain, as shown... Figure 8The MR image spatial data 802 is shown. Reconstruction techniques may include inverse Fourier transform. The shear angle θ from the SIREN MR k-space 500 is also reflected in the MR image spatial data 802. Therefore, at step 1208, the MR image spatial data (also referred to as "SIREN image spatial data") is transformed into regular domain image spatial data 804 (also referred to as "decoded image spatial data") based on the shear angle θ to generate an image of the object. In one embodiment, a mathematical transformation or decoding / de-shearing algorithm is applied to transform the coordinates of the MR image spatial data 802 from (x+y*tanθ,y) to (x,y) to generate regular domain image spatial data 804 or an image of the object with reduced artifacts.

[0061] Figure 13 This is a flowchart of another method 1300 for generating an MR image of an object using an MRI system 10 according to an embodiment of the present invention. The main difference between method 1200 and method 1300 is that, in method 1300, the SIREN MR k-space itself is transformed to a regular domain k-space, rather than transforming the MR image spatial data to regular domain image spatial data. Therefore, at step 1302, method 1300 includes providing a readout-coded shearing (SIREN) gradient pulse 416 in the phase gradient signal waveform 410 of the MRI system. In addition to the SIREN gradient pulse 416, the phase gradient signal waveform 410 also includes a SIREN phase decoupling gradient 412, a first phase coding gradient 414, and a second phase coding gradient 418. The application of the SIREN gradient 416 results in a SIREN MR k-space 500 of the object obtained at step 1304. The SIREN MR k-space 500 has as follows Figure 5 The shear angle θ is shown.

[0062] At step 1306, the method includes transforming the SIREN k-space data into regular domain k-space data (also referred to as "decoded k-space data") based on a shear angle. In one embodiment, a mathematical transformation or decoding / de-shearing algorithm is applied to the SIREN k-space data to transform it into regular domain k-space data. Finally, at step 1308, an image of the object is generated based on the regular domain k-space data. In one embodiment, a reconstruction technique is applied to the regular domain k-space data to obtain an MR image of the object.

[0063] One advantage of this invention is that it reduces or eliminates phase aliasing artifacts and peripheral signal artifacts in images. Furthermore, this technique can be applied to both 2D and 3D image acquisition of an object. When a 3D image of the object is desired, an additional SIREN gradient pulse is also provided in the slice selection gradient signal waveform 322. Moreover, using this invention, it is not necessary to increase the field of view (FOV) to reduce phase aliasing artifacts, and therefore this reduces scan time.

[0064] This written description uses examples to disclose the invention, including the best mode, and also enables those skilled in the art to practice the invention, including making and using any device or system and performing any included methods. The scope of the invention is defined by the claims and may include other examples that would occur to those skilled in the art. Such other examples are intended to fall within the scope of the claims if they have structural elements that are not indistinguishable from the literal language of the claims, or if they include equivalent structural elements that have minor differences from the literal language of the claims.

Claims

1. A method for generating an image of an object using an MRI system, the method comprising: The SIREN gradient pulse in readout-coded imaging is provided in the phase gradient signal waveform applied to the phase gradient coil of the MRI system; Obtain the SIREN k-space of the object, wherein the k-space line of the SIREN k-space has a shear angle, the shear angle being the tilt angle of the k-space line in the SIREN k-space; Obtain MR image spatial data from the SIREN k space; as well as The image of the object is generated by transforming the MR image spatial data into regular image spatial data using a decoding algorithm based on the shear angle.

2. The method of claim 1, further comprising applying a readout gradient signal waveform to a readout gradient coil of the MRI system, wherein the readout gradient signal waveform includes a phase-out pulse and a main readout pulse.

3. The method of claim 2, wherein the phase gradient signal waveform includes a first phase-coded gradient pulse and a second phase-coded gradient pulse, and wherein the SIREN gradient pulse is applied between the first phase-coded gradient pulse and the second phase-coded gradient pulse.

4. The method of claim 2, wherein the amplitude of the SIREN gradient pulse is determined based on the shear angle and the amplitude of the main readout pulse.

5. The method of claim 2, wherein the SIREN gradient pulse is applied simultaneously with the main readout pulse.

6. The method of claim 1, further comprising providing a SIREN phase decelerator gradient pulse prior to the SIREN gradient pulse in the phase gradient signal waveform applied to the phase gradient coil of the MRI system.

7. The method of claim 6, wherein the area under the SIREN phase depletion gradient pulse is half the area under the SIREN gradient pulse.

8. The method of claim 1, wherein the shear angle is determined based on a medical imaging application, a field of view, or a combination thereof.

9. The method of claim 1, wherein obtaining MR image spatial data from the SIREN k space comprises applying an inverse Fourier transform to the SIREN k space.

10. The method according to claim 1, wherein the decoding algorithm shifts the x-axis coordinate of the MR image spatial data by a factor y*tanθ, where y is the corresponding y-axis coordinate of the MR image spatial data, and θ is the shear angle.

11. The method of claim 1, wherein when the image of the object has a truncated region, the method includes using a larger field of view in the y-axis of the MR image spatial data.

12. The method of claim 1, wherein when the image of the object is a 3D image, the method includes providing an additional SIREN gradient pulse in the slice selection gradient signal waveform of the MRI system.

13. An MRI system, the MRI system comprising: A magnet configured to generate a polarized magnetic field around at least a portion of an object arranged in the MRI system; A gradient coil assembly, comprising a readout gradient coil, a phase gradient coil, and a slice selection gradient coil, the gradient coil assembly being configured to apply at least one gradient field to the polarization magnetic field; An RF system configured to apply an RF field to the object and receive magnetic resonance signals from the object; Processing system, the processing system being programmed to: A SIREN gradient pulse in readout-coded imaging is provided in a phase gradient signal waveform, wherein the phase gradient signal waveform is applied to the phase gradient coil; Obtain the SIREN k-space of the object, wherein the k-space line of the SIREN k-space has a shear angle, the shear angle being the tilt angle of the k-space line in the SIREN k-space; Obtain MR image spatial data from the SIREN k space; as well as The image of the object is generated by transforming the MR image spatial data into regular image spatial data using a decoding algorithm based on the shear angle.

14. The MRI system of claim 13, wherein the processing system is programmed to apply a readout gradient signal waveform to the readout gradient coil of the MRI system, wherein the readout gradient signal waveform includes a phase-out pulse and a main readout pulse.

15. The MRI system of claim 14, wherein the phase gradient signal waveform includes a first phase-coded gradient pulse and a second phase-coded gradient pulse, and wherein the SIREN gradient pulse is applied between the first phase-coded gradient pulse and the second phase-coded gradient pulse.

16. The MRI system of claim 14, wherein the amplitude of the SIREN gradient pulse is determined based on the shear angle and the amplitude of the main readout pulse.

17. The MRI system of claim 13, wherein the processing system is programmed to obtain the MR image spatial data from the SIREN k space by applying an inverse Fourier transform to the SIREN k space.

18. The MRI system of claim 13, wherein the decoding algorithm shifts the x-axis coordinate of the MR image spatial data by a factor y*tanθ, where y is the corresponding y-axis coordinate of the MR image spatial data and θ is the shear angle.

19. The MRI system of claim 13, wherein when the image of the object is a 3D image, the processing system is programmed to provide additional SIREN gradient pulses in the slice selection gradient signal waveform of the MRI system.

20. A method for generating an image of an object using an MRI system, the method comprising: The SIREN gradient pulse in readout-coded imaging is provided in the phase gradient signal waveform applied to the phase gradient coil of the MRI system; Obtain the SIREN k-space data of the object, wherein the k-space lines of the SIREN k-space data have a shear angle, the shear angle being the tilt angle of the k-space lines in the SIREN k-space. Based on the shear angle, a decoding algorithm is used to transform the SIREN k-space data of the object into regular domain k-space data. as well as The image of the object is generated from the k-space data of the rule domain.

Images