System and method for reduced fine line artifacts in magnetic resonance imaging

Abstract

A method for imaging a subject using a magnetic resonance imaging (MR) system is presented. The method includes rotationally sampling a k-space using a plurality of radially directed blades about a center of the k-space. A first blade subset is acquired with a positive excitation pulse and a first plurality of refocusing pulses. Further, a second subset of blades is acquired with a negative excitation pulse and a second plurality of refocusing pulses. The polarity of the second subset of blades is inverted to generate a third subset of blades. The first subset of blades and the third subset of blades are then combined to generate a final k-space. Finally, a medical image of the subject is generated based on the reconstruction of the final k-space.

Description

Background Art

[0001] Embodiments disclosed herein relate to magnetic resonance imaging (MRI) systems and methods, and more particularly, to techniques for reducing fine line artifacts in MRI.

[0002] As a medical imaging mode, magnetic resonance imaging (MRI) can obtain 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 human body to be imaged is located in the static magnetic field B0, the nuclear spins associated with the hydrogen nuclei in the human tissue are polarized, so that the tissue of the part to be imaged generates a longitudinal magnetization vector at the macroscopic level. After applying a radio frequency field B1 that intersects the direction of the static magnetic field B0, the rotation direction of the protons changes, so that the tissue of the part to be imaged generates a transverse magnetization vector at the macroscopic level. After removing the radio frequency field B1, the transverse magnetization vector decays in a spiral manner until it returns to zero. A free induction decay signal is generated during the decay. The free induction decay signal can be collected as a magnetic resonance signal, and the tissue image of the part to be imaged can be reconstructed based on the collected signal.

[0003] In MRI, thin line artifacts refer to unwanted visual distortions or signal interferences that appear as thin, dark or bright lines on the image. These artifacts are usually caused by interference of desired signals with unwanted signals as a result of system imperfections.

[0004] In some cases, thin line artifacts can be reduced by increasing the gradient crusher area or using double the number of excitations (NEX2) for the same region of interest. However, the disadvantage of increasing the gradient crusher area is that it also increases the echo interval time, which leads to image blurring. On the other hand, thin line artifact reduction using the NEX2 technique doubles the scan time.

[0005] Therefore, there is a need for an improved method that overcomes the limitations of existing MRI techniques. Summary of the invention

[0006] According to an embodiment of the present technology, a method for imaging a subject using a magnetic resonance imaging (MRI) system is provided. The method includes sampling k-space in a rotational manner using a plurality of radially directed blades around the center of k-space, wherein a first subset of the blades is acquired using a positive excitation pulse and a first plurality of refocusing pulses, and wherein a second subset of the blades is acquired using a negative excitation pulse and a second plurality of refocusing pulses. The method also includes reversing the polarity of the second subset of blades to generate a third subset of blades. In addition, the first subset of blades and the third subset of blades are combined to generate a final k-space. Finally, the method includes generating a medical image of the subject based on the reconstruction of the final k-space.

[0007] According to another embodiment of the present technology, a magnetic resonance imaging (MRI) system is provided. The MRI system includes a magnet and a gradient coil assembly, the magnet is configured to generate a main magnetic field around at least a portion of a subject arranged in the MRI system, the gradient coil assembly is configured to apply at least one gradient field to the main magnetic field, the MRI system also includes a radio frequency (RF) system and a processing system, the radio frequency (RF) system is configured to apply an RF field to the subject and receive magnetic resonance signals from the subject. The processing system is programmed to sample the k-space in a rotational manner using a plurality of radially directed blades around the center of the k-space. A first subset of the blades is acquired using a positive excitation pulse and a first plurality of refocusing pulses. In addition, a second subset of the blades is acquired using a negative excitation pulse and a second plurality of refocusing pulses, the processing system is further configured to reverse the polarity of the second subset of the blades to generate a third subset of the blades, and combine the first subset of the blades and the third subset of the blades to generate a final k-space. The processing system is further configured to generate a medical image of the subject based on the reconstruction of the final k-space. BRIEF DESCRIPTION OF THE DRAWINGS

[0008] These and other features, aspects and advantages of the present invention will be better understood when the following detailed description is read with reference to the accompanying drawings, in which like characters refer to like parts throughout the several views, and in which:

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

[0010] Figure 2 is a schematic diagram of a fast spin echo (FSE) pulse sequence according to an embodiment of the present technology;

[0011] Figure 3A shows k-space data of multiple blades acquired according to a Periodically Rotated Overlapping Parallel Lines with Enhanced Reconstruction (PROPELLER) sampling scheme;

[0012] Figure 3B Shows Figure 3A a phase encoding line in one of the blades shown;

[0013] Figure 4 shows a schematic diagram depicting blade acquisition and corresponding radio frequency pulses according to an embodiment of the present technology;

[0014] Figure 5 is a schematic diagram depicting the effect of a phase cycling technique on a point spread function (PSF) according to an embodiment of the present technology;

[0015] Figure 6is a schematic diagram depicting image quality comparison according to an embodiment of the present technology; and

[0016] Figure 7 is a flow chart representing a method for generating an MR image of a subject in accordance with an embodiment of the present technology. DETAILED DESCRIPTION

[0017] One or more specific embodiments will be described below. In order to provide a concise description of these embodiments, all features of an actual implementation may not be described in the specification. It should be understood that in the development of any such actual implementation, as in any engineering or design project, many implementation-specific decisions must be made to achieve the developer's specific goals, such as complying with system-related and business-related constraints that may vary from implementation to implementation. In addition, it should be understood that such development efforts may be complex and time-consuming, but are still routine tasks for design, fabrication, and manufacturing for ordinary technicians who benefit from this disclosure.

[0018] When introducing the elements of the various embodiments of the present embodiment, the articles "a", "an", "the" and "said" are intended to mean that there are one or more of these elements. The terms "comprise", "comprising" and "having" are intended to be inclusive, and mean that there may be additional elements in addition to the listed elements. In addition, any numerical examples in the following discussion are intended to be non-limiting, and therefore additional numerical values, ranges and percentages are within the scope of the disclosed embodiments. In addition, the terms "circuit" and "circuitry" and "controller" may include a single component or multiple components, which are active and / or passive and are connected or otherwise coupled together to provide the functions.

[0019] In magnetic resonance imaging (MRI), an object or subject is placed in a magnet. When the object is in the magnetic field generated by the magnet, the magnetic moments of the 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 moment M of the nuclei. z Rotation or "tilt" from the z direction toward the lateral or xy plane. After the excitation signal B1 is terminated, the nucleus emits a signal, which is called an MR signal. In order to use the MR signal to generate an image of the object, a magnetic field gradient pulse (G x , G y and G z). Gradient pulses are used to scan through k-space, the inverse of spatial frequency or distance. There is a Fourier relationship between the acquired MR signals and the image of the object, so the image of the object can be derived by reconstructing the MR signals. The image of the object may include a two-dimensional (2D) or three-dimensional (3D) image.

[0020] To reduce motion artifacts in MR images, periodically rotated overlapping parallel lines with enhanced reconstruction (PROPELLER) imaging was introduced. PROPELLER is a fast spin echo (FSE) based application that provides motion robust imaging. In PROPELLER imaging, k-space data can be sampled from k-space data of multiple blades acquired in a rotational manner around the center of k-space. However, a challenge in all FSE applications is the artifact or unwanted signal generated by the free induction decay (FID) signal generated by the refocusing radio frequency (RF) pulses that is not sufficiently broken up. This type of artifact, known as fine lines, appears as equally spaced lines on the image.

[0021] Thin line artifacts in PROPELLER can be reduced by increasing the crusher area or acquiring the same blade more than once (real NEX using reverse excitation RF pulse polarity). The disadvantage of increasing the crusher area is that it also increases the echo spacing time, which results in image blurring. On the other hand, thin line reduction using real NEX increases the scan time by at least a factor of two. According to an embodiment of the present technology, the thin line artifact reduction problem is solved by applying RF chopping in every other blade without increasing the crusher area (which results in destructive interference in PROPELLER images) to reduce thin line artifacts without increasing the echo spacing or scanning time penalty.

[0022] Embodiments of the present disclosure will now be described by way of example with reference to the accompanying drawings, in which Figure 1A magnetic resonance imaging (MRI) apparatus 10 is shown. The MRI apparatus 10 includes a static magnetic field magnet unit 12, a gradient coil unit 13, an RF coil unit 14, an RF body or volume coil unit 15, a transmission / reception (T / R) switch 20, an RF driver unit 22, a gradient coil driver unit 23, a data acquisition unit 24, a controller unit 25, a patient examination couch or bed 26, a data processing unit 31, an operation console unit 32, and a display unit 33. In some embodiments, the RF coil unit 14 is a surface coil, which is a local coil that is usually placed near an anatomical structure of interest of a subject 16. Here, the RF body coil unit 15 is a transmission coil that transmits RF signals, and the local surface RF coil unit 14 receives MR signals. Therefore, the transmission body coil (e.g., the RF body coil unit 15) and the surface receiving coil (e.g., the RF coil unit 14) are independent but electromagnetically coupled components, and may be referred to together or individually as a radio frequency (RF) system. The MRI apparatus 10 transmits electromagnetic pulse signals to a subject 16 placed in an imaging space 18 forming a static magnetic field to perform scanning to obtain magnetic resonance signals from the subject 16. One or more images of the subject 16 can be reconstructed based on the magnetic resonance signals thus obtained by the scanning.

[0023] The static magnetic field magnet unit 12 includes, for example, an annular superconducting magnet installed in an annular vacuum container. The magnet defines a cylindrical space around the subject 16 and generates a constant main static magnetic field B0.

[0024] The MRI apparatus 10 further includes a gradient coil unit 13, which forms a gradient magnetic field in an imaging space 18 so as to provide three-dimensional position information for a magnetic resonance signal received by an RF coil array. The gradient coil unit 13 includes three gradient coil systems, each of which generates a gradient magnetic field along one of three spatial axes perpendicular to each other, and generates a gradient field in each of a frequency encoding direction, a phase encoding direction, and a slice selection direction according to an imaging condition. More specifically, the gradient coil unit 13 applies a gradient field in a slice selection direction (or a scanning direction) of the subject 16 to select a slice; and the RF body coil unit 15 or the local RF coil array can transmit an RF pulse to the selected slice of the subject 16. The gradient coil unit 13 also applies a gradient field in a phase encoding direction of the subject 16 to phase encode a magnetic resonance signal from a slice excited by an RF pulse. Then the gradient coil unit 13 applies a gradient field in a frequency encoding direction of the subject 16 to frequency encode a magnetic resonance signal from a slice excited by an RF pulse.

[0025] The RF coil unit 14 is provided, for example, to surround a region to be imaged of the subject 16. In some examples, the RF coil unit 14 may be referred to as a surface coil or a receiving coil. In a static magnetic field space or an imaging space 18 in which a static magnetic field B0 is formed by the static magnetic field magnet unit 12, the RF coil unit 15 transmits RF pulses as electromagnetic waves to the subject 16 based on a control signal from the controller unit 25, thereby generating a high-frequency magnetic field B1. This excites the proton spins in the slice to be imaged of the subject 16. The RF coil unit 14 receives the electromagnetic waves generated when the proton spins thus excited in the slice to be imaged of the subject 16 return to alignment with the initial magnetization vector as magnetic resonance signals. In some embodiments, the RF coil unit 14 may transmit RF pulses and receive MR signals. In other embodiments, the RF coil unit 14 may be used only to receive MR signals, and not to transmit RF pulses.

[0026] In one embodiment, k-space data may be sampled using a fast spin echo (FSE) pulse sequence. For example, a 90 degree excitation RF pulse may be delivered to rotate the proton magnetization 90 degrees, and then multiple 180 degree refocusing RF pulses may be delivered to reverse the direction of the proton precession, such as Figure 2 As shown. After each 180 degree RF pulse, an echo (MR signal) can be received from the RF coil. In this article, the 90 degree RF pulse is the excitation, and the duration between consecutive 90 degree RF pulses is the repetition time (TR). Data along each phase encoding line can be acquired by sampling one echo, and multiple phase encoding lines can be acquired within the TR. For example, the k-space data can be sampled using an echo planar pulse sequence or a gradient echo pulse sequence.

[0027] Figure 2 is a schematic diagram of a FSE pulse sequence 300. The pulse sequence diagram 300 can be represented by Figure 1 302 and 303 respectively show phase encoding and readout gradient waveforms, i.e., Gy and Gx. Graph 304 shows a slice selection gradient waveform Gz. Each of the gradient waveforms 302, 303, and 304 excites the corresponding physical gradient coil in the gradient coil assembly 50. In addition, graph 314 shows an RF signal waveform that excites the RF coil 56 and generates the corresponding echo signal (or MR signal) shown in graph 318. The RF signal waveform 314 includes a 90-degree excitation RF pulse 305, followed by a plurality of 180-degree refocusing RF pulses 307. It should be noted that excitation RF pulses having angles other than 90 degrees may also be used. In addition, refocusing RF pulses having angles other than 180 degrees may also be used in other embodiments.

[0028] A 90° RF excitation pulse 305 is generated in the presence of a Gz slice selection gradient pulse 306 to provide transverse magnetization in a slice through the patient. The transverse magnetization is refocused by each of a plurality of 180° refocusing RF pulses 307 to produce MR or echo signals 301 that are acquired in the presence of a Gx readout gradient pulse 308. Each MR signal 301 is individually phase encoded by a corresponding Gy phase encoding pulse 309-313.

[0029] Re-reference Figure 1 , the RF body coil unit 15 is arranged, for example, to surround the imaging space 18, and generates RF magnetic field pulses orthogonal to the main magnetic field B0 generated by the static magnetic field magnet unit 12 in the imaging space 18 to excite the nucleus. Compared with the RF coil unit 14 that can be disconnected from the MRI device 10 and replaced with another RF coil unit, the RF body coil unit 15 is fixedly attached and connected to the MRI device 10. In addition, although the local coil (such as the RF coil unit 14) can transmit or receive signals only to or from a local area of ​​the subject 16, the RF body coil unit 15 generally has a larger coverage area. For example, the RF body coil unit 15 can be used to transmit or receive signals to the whole body of the subject 16. The use of a receive-only local coil and a transmission body coil provides uniform RF excitation and good image uniformity at the expense of higher RF power deposited in the subject. For a transmit-receive local coil, the local coil provides RF excitation to the region of interest and receives MR signals, thereby reducing the RF power deposited in the subject. It should be understood that the specific use of the RF coil unit 14 and / or the RF body coil unit 15 depends on the imaging application.

[0030] The T / R switch 20 may selectively electrically connect the RF body coil unit 15 to the data acquisition unit 24 when operating in the receive mode, and may selectively electrically connect the RF body coil unit to the RF driver unit 22 when operating in the transmit mode. Similarly, the T / R switch 20 may selectively electrically connect the RF coil unit 14 to the data acquisition unit 24 when the RF coil unit 14 operates in the receive mode, and may selectively electrically connect the RF coil unit to the RF driver unit 22 when the RF coil unit operates in the transmit mode. When both the RF coil unit 14 and the RF body coil unit 15 are used for a single scan, for example, if the RF coil unit 14 is configured to receive MR signals and the RF body coil unit 15 is configured to transmit RF signals, the T / R switch 20 may direct the control signal from the RF driver unit 22 to the RF body coil unit 15 while directing the received MR signal from the RF coil unit 14 to the data acquisition unit 24. The coil of the RF body coil unit 15 may be configured to operate in a transmit-only mode or a transmit-receive mode. The coil of the local RF coil unit 14 may be configured to operate in a transmit-receive mode or a receive-only mode.

[0031] The RF driver unit 22 includes a gate modulator (not shown), an RF power amplifier (not shown), and an RF oscillator (not shown), which are used to drive an RF coil (e.g., the RF coil unit 15) and form a high-frequency magnetic field in the imaging space 18. The RF driver unit 22 modulates an RF signal received from the RF oscillator into a predetermined timing signal having a predetermined envelope based on a control signal from the controller unit 25 and using the gate modulator. The RF signal modulated by the gate modulator is amplified by the RF power amplifier and then output to the RF coil unit 15.

[0032] The gradient coil driver unit 23 drives the gradient coil unit 13 based on a control signal from the controller unit 25 and thereby generates a gradient magnetic field in the imaging space 18. The gradient coil driver unit 23 includes three driver circuits (not shown) corresponding to three gradient coil systems included in the gradient coil unit 13.

[0033] The data acquisition unit 24 includes a preamplifier (not shown), a phase detector (not shown), and an analog / digital converter (not shown) for acquiring a magnetic resonance signal received by the RF coil unit 14. In the data acquisition unit 24, the phase detector uses the output of the RF oscillator from the RF driver unit 22 as a reference signal, performs phase detection on the magnetic resonance signal received from the RF coil unit 14 and amplified by the preamplifier, and outputs the phase-detected analog magnetic resonance signal to the analog / digital converter to be converted into a digital signal. The digital signal thus obtained is output to the data processing unit 31.

[0034] The MRI apparatus 10 includes a table 26 for placing the subject 16 thereon. By moving the table 26 based on a control signal from the controller unit 25, the subject 16 can be moved inside and outside the imaging space 18.

[0035] The controller unit 25 includes a computer and a recording medium on which a program to be executed by the computer is recorded. The program, when executed by the computer, causes the various parts of the device to perform operations corresponding to a predetermined scan. The recording medium may include, for example, a ROM, a floppy disk, a hard disk, an optical disk, a magneto-optical disk, a CD-ROM, or a non-volatile memory card. The controller unit 25 is connected to the operation console unit 32 and processes an operation signal input to the operation console unit 32, and also controls the examination bed 26, the RF driver unit 22, the gradient coil driver unit 23, and the data acquisition unit 24 by outputting control signals to them. The controller unit 25 also controls the data processing unit 31 and the display unit 33 based on the operation signal received from the operation console unit 32 to obtain a desired image.

[0036] The operation console unit 32 includes a user input device such as a touch screen, a keyboard, and a mouse. The operation console unit 32 is used by the operator to input such data as an imaging protocol, and to set the area where the imaging sequence will be executed. The data on the imaging protocol and the imaging sequence execution area are output to the controller unit 25.

[0037] The data processing unit 31 includes a computer and a recording medium on which a program to be executed by the computer to perform predetermined data processing is recorded. The data processing unit 31 is connected to the controller unit 25, and performs data processing based on a control signal received from the controller unit 25. The data processing unit 31 is also connected to the data acquisition unit 24, and generates spectrum data by applying various image processing operations to the magnetic resonance signals output from the data acquisition unit 24.

[0038] The display unit 33 includes a display device, and displays an image on a display screen of the display device based on a control signal received from the controller unit 25. The display unit 33 displays, for example, images regarding input items for which an operator inputs operation data from the operation console unit 32. The display unit 33 also displays a two-dimensional (2D) slice image or a three-dimensional (3D) image of the subject 16 generated by the data processing unit 31.

[0039] FIG. 3A to FIG. 3B An exemplary sampling scheme for acquiring k-space data using the PROPELLER technique is shown. K-space is covered / sampled with multiple overlapping blades that are arranged in a rotational manner relative to the center 207 of k-space. The rotation angles between adjacent blades are the same. Each blade covers an area of ​​k-space and has a geometric center that is co-located with the center of k-space. In some embodiments, each blade covers a rectangular area of ​​k-space, and the dimensions (width and length) of the blades are the same. As an example, if k-space is sampled with N blades, adjacent blades are separated by an angle of 180 / N degrees relative to the center of k-space. In one embodiment, each blade may include multiple parallel phase encoding lines extending longitudinally across the blade.

[0040] In another embodiment, each blade may include only a single radial k-space line through the center of k-space. Such an embodiment can represent a radial MR slice acquisition using a 2D radial sampling scheme. Furthermore, in another embodiment, the technique can be extended to 3D sampling (such as star stack sampling) that uses rectilinear (Cartesian) sampling in the z direction while maintaining radial sampling in the xy plane. In such an embodiment, as will be subsequently described with respect to Figure 3A As explained, radial sampling in the xy plane is achieved by acquiring k-space blades, but each blade consists of only a single radial k-space line. In other words, in one embodiment, multiple blades (such as blades 201, 202, 203, 204, 205, 206) are part of a 2D slice, each having only a single radial line. Multiple such slices or 2D slices are then arranged in a perpendicular manner to each other (like a stack) to generate a 3D volume.

[0041] As a non-limiting example, in Figure 3A In FIG. 1 , six blades ( 201 - 206 , shown in dashed lines) are arranged in a counterclockwise rotation relative to the center 207 of k-space to cover 360 degrees of k-space. Adjacent blades are 30 degrees apart from each other. The phase encoding line of each blade is drawn as a solid line.

[0042] Collecting (or sampling) k-space data of multiple blades includes collecting k-space data of each blade in a data collection direction. The data collection direction can be a counterclockwise or clockwise direction relative to the center of the k-space. For example, the k-space data of blades 201-206 can be collected in a counterclockwise data collection direction. That is, the blades can be sampled in the order of blades 201, 202, ..., 206. In one example, the blades whose blade phase encoding axes and blade frequency encoding axes are aligned with the Ky axis and the Kx axis can be sampled first. By sampling the k-space with overlapping blades, the low-frequency components at the center of the k-space are oversampled, and the high-frequency components at the periphery of the k-space are sampled at the Nyquist rate. In this way, motion artifacts can be removed.

[0043] Figure 3B Shows Figure 3A 201. As a non-limiting example, blade 201 includes eight phase encoding lines 221-228 arranged in parallel along a blade phase encoding axis 230 of blade 201. The arrow of blade phase encoding axis 230 indicates the blade phase encoding direction. Blade frequency encoding axis 240 of blade 201 is perpendicular to blade phase encoding axis 230 and intersects blade phase encoding axis 230 at the center of the blade. The arrow of blade frequency encoding axis 240 indicates the blade frequency encoding direction. The center of blade 201 coincides with the center 207 of k-space.

[0044] The blade phase encoding lines (221-228) are indexed (e.g., numbered) sequentially from 1 to 8 along the blade phase encoding direction indicated by the arrow of the blade phase encoding axis 230. In other words, the index (value) of the phase encoding line increases in the phase encoding direction. The phase encoding lines are sampled according to a phase encoding order. The phase encoding order may be different from the phase encoding line index.

[0045] Acquiring k-space data for the blade includes acquiring data along each of the phase encoding lines of the blade. In some embodiments, a single excitation pulse is used to acquire k-space data for each blade. After excitation, as relative to Figure 2 As discussed, a series of echo signals (or echo trains) are acquired, and one phase encoding line of the blade is filled with the data acquired from each echo signal.

[0046] As previously discussed, one challenge of conventional PROPELLER technology is the artifacts that appear in the final image due to the FID signal generated by the refocusing RF pulse 307 not being adequately fragmented. This type of artifact, known as fine lines, appears as equally spaced lines on the final image. In one embodiment, fine line artifacts in PROPELLER can be reduced by increasing the crusher area 320 or by acquiring the same blade more than once by reversing the polarity of the excitation RF pulses between acquisitions (true NEX). The disadvantage of increasing the crusher area is that it also increases the echo interval time, which results in image blur. On the other hand, fine line artifact reduction using RF chopping doubles the scan time.

[0047] According to an embodiment of the present technology, the above-mentioned problem is solved by applying radio frequency (RF) chopping in every other blade without increasing the chopper area (which causes destructive interference in PROPELLER images) to reduce fine line artifacts without echo spacing or scanning time loss. In the RF chopping technique proposed in this article, the sign / polarity of the excitation pulse 305 is reversed in every other blade, as will be explained subsequently.

[0048] Figure 4 A schematic diagram 400 depicting blade acquisition and corresponding RF pulses according to an embodiment of the present technology is shown. Specifically, Figure 4 Three blades are shown: blade 1 (401), blade 2 (402) and blade 3 (403). It should be noted that for ease of explanation, only three blades are shown, however, in reality, multiple blades are collected but in Figure 4 Not shown. Figure 4 Also shown are two RF pulse sequences 404 and 406. In the illustrated embodiment, RF pulse sequence 404 corresponds to odd blade acquisitions, i.e., blades 1, 3, 5, etc., and RF pulse sequence 406 corresponds to even blade acquisitions, i.e., blades 2, 4, 6, etc. In another embodiment, blade sequence 404 may correspond to even blade acquisitions, and RF pulse sequence 406 may then correspond to odd blade acquisitions.

[0049] The RF pulse sequence 404 includes an excitation pulse 408 and a refocusing pulse 410. Similarly, the RF pulse sequence 406 includes an excitation pulse 412 and a refocusing pulse 414. The main difference between the RF pulse sequence 404 and the RF pulse sequence 406 is that their excitation pulses have opposite signs (polarities), i.e., the excitation pulse 408 and the excitation pulse 412 have opposite signs. It can be seen that the excitation pulse 408 has a positive sign, while the excitation pulse 412 has a negative sign. However, in another embodiment, these signs can be interchanged, i.e., the excitation pulse 408 can be negative, while the excitation pulse 412 can be positive. In other words, in the present technology, the acquisition of alternating blades (e.g., odd blades and even blades) is performed using an RF pulse sequence with excitation pulses having alternating signs (e.g., positive and negative signs). It should also be noted that, although the signs of the excitation pulses can be alternating, the signs of the refocusing pulses 410 and 414 for both RF pulses 404 and 406 remain the same, i.e., positive in this case.

[0050] Once all blades of k-space have been acquired using the above-described alternating excitation pulse strategy (which is also referred to as a phase cycling technique), the next thing to do is to reconstruct the image using the acquired k-space data. At reconstruction, the blade data acquired using a negative excitation pulse is multiplied by minus one (-1) to negate the effect of opposite polarity. In this technique, whenever there is a change in the polarity of the excitation pulse in the acquisition of consecutive blades, there is destructive interference, which means that the signal is spread across the entire image and is incoherent. In other words, from one blade to another, the unwanted signal changes polarity, which causes the unwanted signal to be spread across the entire image. This results in a reduction in the appearance of fine line artifacts in the final image.

[0051] Figure 5 A schematic diagram 500 is shown depicting the effect of the phase cycling technique discussed above on the point spread function (PSF). Specifically, Figure 5 Shown are a graph 502 of a point spread function, another graph 504 of the same point spread function utilizing the phase cycling technique proposed herein, and a graph 506 of cross-sectional profiles of two PSFs (ie, with and without the phase cycling technique).

[0052] As will be understood by those skilled in the art, the PSF is a representation of how an object (i.e., a signal point source) will spread in an image due to the selected imaging technique (i.e., FSE or PROPELLER, ...) and various system factors (e.g., gradient field imperfections, receiver coil characteristics, and other system-related effects). In the PSF graph 502, the representation of the signal point source appears as an expanded spot 508. In other words, in the graph 502, the signal point source appears strongest at the center and tapers off away from the center.

[0053] If the signal point source is an unwanted signal, then one would want to make the spot 508 disappear. As previously discussed, unwanted signals may appear because the free induction decay (FID) signal generated by the refocusing radio frequency (RF) pulse is not sufficiently fragmented. Therefore, the above-mentioned phase recycling technique helps to change the appearance of the spot. With the phase recycling technique, every time the polarity of the excitation pulse changes, there is destructive interference, which means that the signal point source spreads across the entire image. As can be seen from the graph 504, the spot 508 in the graph 502 appears as an artifact 510 that spreads across the entire image in the graph 504. Primarily, the central peak of the spot 508 disappears in the artifact 510, and the peak energy is distributed incoherently.

[0054] Graph 506 represents the same cross-sectional profile of two PSFs (i.e., with and without the phase cycling technique). Specifically, curve profile 512 represents the PSF without the phase cycling technique corresponding to graph 502, and curve profile 514 represents the PSF with the phase cycling technique corresponding to graph 504. It can be seen that PSF profile 512 has a very high peak at the center, while PSF profile 514 generated using the phase cycling technique has two significantly reduced (e.g., reduced by 15 to 20 times) adjacent peaks around the center. Both profiles 512 and 514 then slowly disappear (i.e., decrease to zero) as they move away from the center. Therefore, using the phase cycling technique, unwanted signals that cause fine line artifacts are reduced in the final image.

[0055] Figure 6 A schematic diagram 600 depicting image quality comparison is shown. Figure 6 , image 602 corresponds to a ground truth image of a phantom. In addition, image 604 is a reconstructed image of a phantom obtained without the phase cycling technique, and image 606 is a reconstructed image of a phantom obtained using the phase cycling technique proposed in this article. In addition, images 608, 610, and 612 are zoomed view images of images 602, 604, and 606, respectively.

[0056] As can be seen from column 604, in image 610 obtained without using the phase cycling technique, thin line artifact 614 is very prominent. However, when the image is obtained using the phase cycling technique proposed herein, thin line artifact 616 is incoherently extended across image 612. Therefore, the image quality of image 612 is significantly improved compared to image 610, and image 612 looks very similar to ground truth image 608.

[0057] Figure 7 A flowchart 700 is shown representing a method for imaging a subject according to an exemplary embodiment. The method may be performed by an MRI system such as Figure 1 The MRI system 100 shown is performed. In some embodiments, at least some operations of the method can be performed on a device separate from the MRI system, or in the cloud, or by any combination thereof. At step 702, k-space is sampled in a rotational manner using a plurality of radially directed blades around the center of k-space. Among the plurality of blades, a first subset of blades is acquired using a positive excitation pulse and a first plurality of refocusing pulses (e.g., RF pulse sequence 404). In addition, among the plurality of blades, a second subset of blades is acquired using a negative excitation pulse and a second plurality of refocusing pulses (e.g., RF pulse sequence 406).

[0058] It should also be noted that although the sign of the excitation pulses is different for the first subset of blades and the second subset of blades, the signs of the first plurality of refocusing pulses and the second plurality of refocusing pulses remain the same, e.g., positive in one embodiment. Due to the change in polarity of the excitation pulses from one blade to another, the unwanted signal changes polarity, which causes the unwanted signal to be spread across the image. This results in a reduction in the appearance of thin line artifacts in the final image.

[0059] In one embodiment, the first subset of blades includes odd blades, and the second subset of blades includes even blades. Alternatively, in another embodiment, the first subset of blades includes even blades, and the second subset of blades includes odd blades. Generally speaking, multiple radially directed blades are collected one after another. In other words, the first odd blade can be collected, and then an even blade can be collected, and the sequence continues until all blades are collected. For example, the blades can be collected in numerical order 1, 2, 3, 4, etc.

[0060] In one embodiment, a plurality of radially directed blades form a plurality of slices arranged in a perpendicular manner to each other. In another embodiment, each blade of the plurality of radially directed blades includes only a single radial k-space line. This results in each of the slices having a star shape, and when they are stacked together, the overall structure looks like a star stack sampling.

[0061] At step 704, the method may include inverting the polarity of the second subset of blades to generate a third subset of blades. Since the second subset of blades is obtained using a negative excitation pulse, it is appropriate to invert its polarity when reconstructing the final image. In one embodiment, inverting the polarity of the second subset of blades includes multiplying the second subset of blades by -1. In other words, the third subset of blades is obtained by inverting the polarity of the second subset of blades.

[0062] At step 706, the first subset of blades and the third subset of blades are combined to generate a final k-space. The final k-space may be a two-dimensional (2D) k-space or a three-dimensional (3D) k-space. Combining the first subset of blades and the third subset of blades may include placing the first subset of blades and the third subset of blades into the same k-space. This is done to create a continuous and consistent data set for the entire image. The goal is to ensure that there are no artifacts or seams in the final reconstructed image where the subsets of k-space data meet. In one embodiment, the combination process may be performed during the reconstruction step. Specifically, at step 708, a medical image of the subject is generated based on the reconstruction of the final k-space. In one embodiment, the reconstruction may be accomplished by applying a grid reconstruction or a non-uniform Fourier transform to the final k-space.

[0063] This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any included methods. The patentable scope of the invention is defined by the claims, and may include other examples that 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 do not differ from the literal language of the claims, or if they include equivalent structural elements with insignificant differences from the literal language of the claims.

Claims

1. A method for imaging a subject using a magnetic resonance imaging (MRI) system, the method comprising: sampling k-space in a rotational manner using a plurality of radially directed blades about a center of k-space, wherein a first subset of blades is acquired using a positive excitation pulse and a first plurality of refocusing pulses, and wherein a second subset of blades is acquired using a negative excitation pulse and a second plurality of refocusing pulses; inverting the polarity of the second subset of blades to generate a third subset of blades; combining the first subset of blades and the third subset of blades to generate a final k-space; A medical image of the subject is generated based on the reconstruction of the final k-space. 2 . The method according to claim 1 , wherein the plurality of radially directed blades are harvested sequentially one after the other. 3 . The method of claim 1 , wherein the first subset of blades includes an odd number of blades and the second subset of blades includes an even number of blades. 4 . The method of claim 1 , wherein the first subset of blades includes an even number of blades and the second subset of blades includes an odd number of blades. 5 . The method of claim 1 , wherein inverting the polarity of the second subset of blades comprises multiplying the second subset of blades by −1. The method of claim 1 , wherein the first plurality of refocusing pulses and the second plurality of refocusing pulses have the same polarity.

7. The method of claim 1, wherein the first plurality of refocusing pulses and the second plurality of refocusing pulses both have a positive polarity.

8. The method of claim 1, wherein the plurality of radially directed blades form a plurality of slices arranged perpendicular to one another.

9. The method of claim 8, wherein each blade of the plurality of radially directed blades comprises only a single radial k-space line.

10. The method of claim 1, wherein the final k-space is two-dimensional (2D) k-space or three-dimensional (3D) k-space.

11. A magnetic resonance imaging (MRI) system, comprising: a magnet configured to generate a main magnetic field around at least a portion of a subject disposed in the MRI system; a gradient coil assembly configured to apply at least one gradient field to the main magnetic field; a radio frequency (RF) system configured to apply an RF field to the subject and to receive magnetic resonance signals from the subject; A processing system, the processing system being programmed to: sampling k-space in a rotational manner using a plurality of radially directed blades about a center of k-space, wherein a first subset of blades is acquired using a positive excitation pulse and a first plurality of refocusing pulses, and wherein a second subset of blades is acquired using a negative excitation pulse and a second plurality of refocusing pulses; inverting the polarity of the second subset of blades to generate a third subset of blades; combining the first subset of blades and the third subset of blades to generate a final k-space; A medical image of the subject is generated based on the reconstruction of the final k-space.

12. The MRI system of claim 11, wherein the processing system is programmed to acquire the plurality of radially directed blades one after the other in a continuous manner.

13. The MRI system of claim 11, wherein the first subset of blades includes an odd number of blades and the second subset of blades includes an even number of blades.

14. The MRI system of claim 11, wherein the first subset of blades includes an even number of blades and the second subset of blades includes an odd number of blades.

15. The MRI system of claim 11, wherein the processing system is programmed to invert the polarity of the second blade subset by multiplying the second blade subset by -1.

16. The MRI system of claim 11, wherein the first plurality of refocusing pulses and the second plurality of refocusing pulses have the same polarity.

17. The MRI system of claim 11, wherein the first plurality of refocusing pulses and the second plurality of refocusing pulses each have a positive polarity.

18. The MRI system of claim 11, wherein the plurality of radially directed blades form a plurality of slices arranged perpendicular to each other.

19. The MRI system of claim 18, wherein each blade of the plurality of radially directed blades includes only a single radial k-space line.

20. The MRI system of claim 11, wherein the final k-space is two-dimensional (2D) k-space or three-dimensional (3D) k-space.

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