System and method for magnetic resonance imaging with variable echo time

Abstract

A method for imaging a subject using an MRI system includes determining a plurality of echo times and then acquiring a plurality of variable echo time signals corresponding to the plurality of echo times at a plurality of k-space locations. In the method, a water k-space data signal and a fat k-space data signal for each of a plurality of k-space locations are generated based on a plurality of variable echo time signals corresponding to each of the plurality of k-space locations. Finally, the method includes generating a medical image of the subject based on the water k-space data signals and the fat k-space data signals at the plurality of k-space locations, respectively.

Description

Background Art

[0001] Embodiments disclosed herein relate to magnetic resonance imaging (MRI) systems and methods, and more particularly, to techniques for enhancing the quality and accuracy of MRI imaging.

[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 general, MRI provides detailed anatomical and functional information about various tissues in the human body. However, due to the low signal intensity and short T2 relaxation time associated with bone tissue, traditional MRI techniques often face challenges when imaging bone structures. Therefore, bone imaging has historically been limited in its resolution and contrast, hampering its clinical usefulness for accurate diagnosis and assessment of bone-related pathologies.

[0004] Efforts have been made to improve bone MRI imaging by employing specialized pulse sequences, contrast agents, and hardware modifications. Some techniques focus on optimizing pulse sequences to enhance bone signals while suppressing unwanted signals from surrounding tissues.

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

[0006] According to one embodiment of the present technology, a method for imaging a subject using a magnetic resonance imaging (MRI) system is provided. The method includes determining multiple echo times and acquiring multiple variable echo time signals corresponding to the multiple echo times at multiple k-space positions. The method also includes generating a water k-space data signal and a fat k-space data signal for each k-space position in the multiple k-space positions based on the multiple variable echo time signals corresponding to each k-space position in the multiple k-space positions. Finally, the method includes generating a medical image of the subject based on the water k-space data signal and the fat k-space data signal.

[0007] According to another embodiment of the present technology, a magnetic resonance imaging (MRI) system is provided, which includes a magnet configured to generate a polarized magnetic field around at least a portion of a subject arranged in the MRI system. The MRI system also includes a gradient coil assembly, which includes a readout gradient coil, a phase gradient coil, and a slice selection gradient coil, and the gradient coil assembly is configured to apply at least one gradient field to the polarized magnetic field. The MRI system also includes: a radio frequency (RF) system, which is configured to apply an RF field to the subject and receive magnetic resonance signals from the subject; and a processing system. The processing system is programmed to determine multiple echo times and acquire multiple variable echo time signals corresponding to multiple echo times at multiple k-space positions. The processing system is also programmed to generate a water k-space data signal and a fat k-space data signal for each of the multiple k-space positions based on the multiple variable echo time signals corresponding to each of the multiple k-space positions. The processing system is also programmed to generate a medical image of the subject based on the water k-space data signals and the fat k-space data signals at a plurality of k-space locations, respectively. 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 depicting images acquired with zero echo time (ZTE) and liver volume accelerated acquisition dynamic (LAVA Flex) imaging techniques;

[0011] Figure 3 is a schematic diagram illustrating the effect of the fat-water chemical shift effect in the pelvis in the ZTE imaging method;

[0012] Figure 4 is a schematic diagram depicting variable echo time acquisition according to an embodiment of the present technology;

[0013] Figure 5 is a schematic diagram of experimental results of images reconstructed using k-space data acquired at multiple variable echo times according to an embodiment of the present technology;

[0014] Figure 6 is a schematic diagram of an image reconstructed using a variable echo time method according to an embodiment of the present technology; and

[0015] 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

[0016] 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.

[0017] 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.

[0018] 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.

[0019] Embodiments of the present disclosure will now be described by way of example with reference to the accompanying drawings, in which Figure 1 1 is a schematic diagram of a magnetic resonance imaging (MRI) system 10. The 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 can be a mouse, a joystick, a keyboard, a trackball, a touch-activated screen, a light stick, a voice controller, and / or other input devices. The input device 13 can be used for interactive geometric shape designation. The console 12 communicates with a computer system 20 via a link 18, which enables an operator to control the generation and display of images on a display screen 16. The link 18 can be a wireless or wired connection. The computer system 20 can include modules that communicate with each other via a backplane 20A. The modules of the computer system 20 can include, for example, an image processor module 22, a central processing unit (CPU) module 24, and a memory module 26, which can include, for example, a frame buffer for storing an array of image data. The computer system 20 can be linked to an archival media device, a permanent or backup memory, or a network for storing image data and programs, and communicates with an MRI system control 32 via a high-speed signal link 34. The MRI system control 32 may be separate from or integrated with the computer system 20. The computer system 20 and the MRI system control 32 together form an "MRI controller" 33 or "controller."

[0020] In an exemplary embodiment, the MRI system control 32 includes modules connected by a backplane 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 performed. The CPU module 36 operates the system components to implement the desired scan sequence and generates data indicating the timing, intensity and shape of the generated RF pulses and the timing and length of the data acquisition window. The CPU module 36 is connected to the components operated by the MRI controller 32, including the pulse generator module 38 that controls the gradient amplifier 42, the physiological acquisition controller (PAC) 44, and the scan room interface circuit 46.

[0021] In one example, the CPU module 36 receives subject data from a physiological acquisition controller 44, which receives signals from sensors connected to the object, such as ECG signals received from electrodes attached to the patient. As used herein, an object or subject is a person (or patient), an animal, or a phantom. The CPU module 36 receives signals from sensors associated with the condition of the patient and the magnet system via a scan room interface circuit 46. The scan room interface circuit 46 also enables the MRI controller 33 to command a patient positioning system 48 to move the patient to a desired position for scanning.

[0022] The whole body RF coil 56 is used to transmit the waveform toward the subject's anatomical structure. 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 structure than the 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 structure. A typical surface coil will have eight receiving channels; however, a different number of channels is possible. It is known to use a combination of both the body coil 56 and the surface coil to provide better image quality.

[0023] The pulse generator module 38 can operate the gradient amplifier 42 to achieve the desired timing and shape of the gradient pulses generated during the scan. The gradient waveform generated by the pulse generator module 38 can be applied to the gradient amplifier system 42 having Gx, Gy and Gz amplifiers. Each gradient amplifier excites a corresponding physical gradient coil in the gradient coil assembly 50 to generate a magnetic field gradient for spatially encoding the acquired signal. Specifically, Gx corresponds to a flow / frequency encoding gradient, Gy corresponds to a phase encoding gradient, and Gz corresponds to a slice selection gradient. The gradient coil assembly 50 can form a part of a magnet assembly 52, which also includes a polarizing magnet 54 (in operation, the polarizing magnet provides a longitudinal magnetic field B0 throughout a target space 55 surrounded by the magnet assembly 52) and a whole-body RF coil 56 (in operation, the coil provides a transverse magnetic field B1, which is approximately perpendicular to B0 throughout the target space 55). The transceiver module 58 in the MRI system control 32 generates pulses that can be amplified by an RF amplifier 60, which is coupled to the RF coil 56 through a transmit / receive switch 62. The resulting signals emitted by the excited nuclei in the subject's anatomy may be sensed by a receive coil (not shown) and provided to a preamplifier 64 via a transmit / receive switch 62. The amplified MR signals are demodulated, filtered, and digitized in the receiver portion of the 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 a transmit mode and to connect the preamplifier 64 to the receive coil during a receive mode.

[0024] The MR signals generated by the excitation of the target are digitized by the transceiver module 58. The digitized signals are then processed by the MRI system control 32 via Fourier transform to generate k-space data, which is transmitted to the memory module 66 or other computer-readable medium via the MRI system control 32. "Computer-readable medium" may include, for example, a structure configured so that an electrical, optical, or magnetic state can be fixed in a manner that is perceptible and reproducible by a conventional computer (e.g., text or images printed on paper or displayed on a screen, a compact disk or other optical storage medium, "flash" memory, EEPROM, SDRAM or other electrical storage medium; a floppy disk or other magnetic disk, a tape or other magnetic storage medium).

[0025] The scan is complete when an array of raw k-space data is acquired in the computer readable medium 66. For each image to be reconstructed, the raw k-space data is rearranged into a separate k-space data array, and each of these k-space data arrays is input to the array processor 68, which operates to reconstruct the data into an array of image data using a reconstruction algorithm such as a Fourier transform. When complete k-space data is obtained, it represents the entire volume of the subject's body, and the k-space so obtained may be referred to as reference k-space. Similarly, when only partial k-space data is obtained, the image may be referred to as partial k-space. The image data is transmitted to the computer system 20 via the data link 34 and stored in the memory. In response to commands received from the operator console 12, the image data may be archived in a long-term storage device, or may be further processed by the image processor 22 and transmitted to the operator console 12 and presented on the display 16.

[0026] The MR signal is represented by a complex number, where each position at k-space is represented by a complex number, where the I and Q quadrature MR signals are real and imaginary components. A composite MR image can be reconstructed based on the I quadrature MR signal and the Q quadrature MR signal using a process such as Fourier transform of the k-space MR data. A composite MR image is an MR image having each pixel represented by a complex number, which also has a real component and an imaginary component. The magnitude M of the received MR signal can be determined as the square root of the sum of the squares of the I quadrature component and the Q quadrature component of the received MR signal, as shown in equation (3) below:

[0027]

[0028] And the phase φ of the received MR signal can also be determined as follows:

[0029]

[0030] Generally speaking, MRI is able to generate images with various contrasts that highlight different tissue properties and features. These contrasts provide valuable information about tissue composition, relaxation time and other physiological parameters. By acquiring multiple contrasts, clinicians and researchers can gain a more comprehensive understanding of the imaged tissue, thereby helping diagnosis, characterization and treatment planning. Some techniques for such multi-contrast images include T1-weighted imaging, T2-weighted imaging, gradient echo imaging and free induction decay (FID) zero echo time (ZTE) imaging. Each of these images has different applications.

[0031] For bone imaging applications, many clinicians use ZTE imaging techniques. ZTE imaging techniques generate both magnitude and phase images of the patient. In ZTE imaging, image encoding begins immediately upon RF excitation, effectively resulting in a nominal zero echo time (TE). This unique feature enables ZTE to capture signals from tissues with very short T2* relaxation times, such as bone and tendons, which would otherwise be challenging to visualize using conventional imaging methods.

[0032] Liver Volume Accelerated Acquisition (LAVA) Dynamic (Flex) imaging is another technique that clinicians use for bone imaging, especially for sacroiliac joint imaging. LAVA Flex is a three-dimensional (3D), fast perturbation gradient echo (FSPGR) imaging technique that generates water-only, fat-only, in-phase, and out-of-phase echoes in a single acquisition. The echo time (TE) in LAVA Flex technology is generally longer than that of ZTE technology.

[0033] Figure 2 1 is a schematic diagram 100 depicting images acquired using ZTE and LAVA Flex imaging techniques. Specifically, images 102 and 104 are acquired using ZTE techniques, while images 106 and 108 are acquired using LAVA Flex imaging techniques. Image 102 is a ZTE magnitude image, and image 104 is a ZTE phase image. Another image 106 is a LAVA Flex in-phase image, and image 108 is a LAVA Flex water image.

[0034] As can be seen from images 102 and 104, bone structures appear to have low signal intensity or appear dark, while soft tissue structures appear bright and have high signal intensity. It should be noted that sometimes for ZTE bone imaging, an inverted grayscale color map is used to highlight cortical bone structures. In general, echo time affects the signal intensity detected from bone and tissue. Soft tissues (including muscles and organs) have longer T2* relaxation times compared to bones and tendons. As will be appreciated by those skilled in the art, the T2* relaxation time is a parameter in MRI that describes the rate at which an MRI signal decays or loses cohesion due to various factors, including local magnetic field inhomogeneities and interactions between adjacent nuclei (such as protons) within a tissue. The signal intensity (S) as a function of echo time (TE) and T2* is scaled according to the following formula:

[0035]

[0036] Among them, Mt ss is the steady-state transverse magnetization. From the above formula, it can be seen that the long T2* signal component will decay slowly, while the short T2* signal component will decay very quickly. Soft tissue has a long T2*, so it appears bright in images 102, 104. In contrast, bone has a very short T2* (and also has a low proton density PD), and therefore appears dark in images 102, 104.

[0037] Furthermore, it can be seen from images 106, 108 that in LAVA Flex imaging, bone structures appear even darker and more significantly surrounded by uniformly presented bright soft tissue. It should be noted that the LAVA Flex imaging method is not possible with a zero echo time, instead the method typically acquires in-phase and out-of-phase gradient echo (GRE) images that are then further resolved into fat and water components. In one embodiment, the LAVA Flex method may use fat-water in-phase TE images (i.e., 4.6 ms at 1.5T and 2.3 ms at 3T). Furthermore, in this method, proton density (PD) weighting may be used to minimize soft tissue T1 contrast that may otherwise interfere with bone appearance.

[0038] from Figure 2It can be seen that in ZTE imaging, i.e., images 102 and 104 acquired at TE=0, the bones appear sharp, but slightly too bright in certain locations (due to minimal T2* signal attenuation), resulting in areas of relatively weak bone contrast. On the other hand, in LAVA Flex in-phase images, i.e., image 106 acquired at fat-water in-phase echo times (i.e., TE=2.3ms at 3T and TE=4.6ms at 1.5T), the bones also appear sharp, but appear too dark in certain locations (due to enhanced T2* signal attenuation), and in addition, the soft tissue also appears less uniform due to significant fat-water chemical shift interference (particularly around the fat-water tissue interface). Therefore, according to an embodiment of the present technology, echo time is used as a flexible parameter to tune and adjust the optimal bone appearance. In one embodiment, echo time is a flexible parameter that depends on the subject's anatomy or user preference for bone delineation. Acquiring multiple TEs enables in-phase reconstruction (clearing fat-water chemical shift effects) as well as fat-water separation.

[0039] Another problem that arises in the above-mentioned ZTE and LAVA Flex imaging methods is the problem of chemical shift. Chemical shift is a phenomenon in MRI caused by small differences in the resonance frequency of atomic nuclei, especially water and fat molecules. It affects the phase of the MR signal, causing image artifacts, but also provides the opportunity to distinguish soft tissue into fat and water components.

[0040] Figure 3 A schematic diagram 200 illustrating the effect of the fat-water chemical shift effect in the pelvis in a ZTE imaging method is shown. Figure 3 The bottom row 206 in FIG. 2 shows sampling time curves. Specifically, the horizontal axis 220 refers to time in milliseconds, while the vertical axis 222 refers to the k-space sampling position. As will be understood by those skilled in the art, sampling time refers to the period of time between RF excitation and data acquisition. In addition, the top row 202 shows pelvic images in linear grayscale, while the middle row 204 shows the same pelvic images in inverter grayscale. Specifically, row 202 shows three images 208, 210 and 212 corresponding to imaging bandwidths of ±125 kHz (curve 228), ±62.5 kHz (curve 226) and ±31.25 kHz (curve 224), respectively. Images 214, 216 and 218 in the middle row 204 also correspond to bandwidths of ±125 kHz, ±62.5 kHz and ±31.25 kHz, respectively, but have inverted grayscale.

[0041] The bottom row 206 also depicts the effective sampling time (i.e., tn=n*Δt) depending on the 3D center-out radial sampling position and the imaging bandwidth (Δt=1 / BW). For the highest imaging bandwidth (i.e., BW=±125kHz, curve 228) with the shortest readout duration of only 0.38ms, the out-of-phase effect is minimal (i.e., fat and water are largely in-phase), and soft tissue homogeneity appears excellent. In contrast, for the lowest imaging bandwidth (i.e., BW=±31.25kHz, curve 224), the readout duration is 1.54ms and is therefore longer than the out-of-phase TE (i.e., 1.15ms at 3T), resulting in severe out-of-phase interference (i.e., fat and water are out-of-phase), mainly affecting tissue boundaries.

[0042] In one embodiment of the present technology, in order to correct chemical shift artifacts, each radial k-space spoke is acquired multiple times at multiple echo times (i.e., variable echo times). For example, each k-space line in the k-space line acquired by the radial k-space is acquired multiple times at multiple echo times. This process is referred to as variable echo time acquisition in this article. The multiple echo time periods can be 2, 3 or more different echo times, so the k-space line can be acquired 2, 3 or more times accordingly. In one embodiment, the number of multiple echo time periods and the time of these multiple echo time periods are selected depending on the desired image contrast and tissue characteristics. In addition, the k-space data acquisition can be sequential or interleaved. When it is sequential, all data of one echo time (e.g., TE1) can be acquired first, and then all data for another echo time (e.g., TE2) can be acquired, and so on, if there are more echo times TE3, TE4, etc. On the other hand, in the case where the k-space data acquisition is interleaved, partial data of one echo time TE1 can also be acquired first, and then partial data for another echo time TE2 (as well as TE3, TE4, etc.) can be acquired. Likewise, this is followed by partial data acquisition for the first echo time TE1, followed by partial data acquisition for TE2, until all data in k-space are acquired.

[0043] Figure 4 4 is a schematic diagram 400 illustrating variable echo time acquisition using three different TEs according to an embodiment of the present technology. Specifically, Figure 4Curves 402, 404 and 406 are shown, which show an RF pulse 408, followed by readout gradients 412, 414 and 416, respectively. It can be seen that the echo times of the gradient signals 412, 414 and 416 are different. For example, the echo time of the gradient signal 412 is TE1, the echo time of the gradient signal 414 is TE2, and the echo time of the gradient signal 416 is TE3. Data acquisition can include Cartesian or non-Cartesian (e.g., radial, spiral, star stack, ...) sampling, but should remain the same and only shift between echo times (e.g., TE1, TE2, TE3) and be acquired in a sequential or interleaved manner.

[0044] Figure 4 Also shown is a graph 422 having a horizontal axis 420 representing the k-space readout point and a vertical axis 424 representing the sampling time. As can be seen from the graph 422, at times TE1+t corresponding to three different echo time readout lines 426, 428, and 430, respectively, n TE2+t n and TE3+t n Each k-space readout point k is acquired three times n In addition, three echo time readout lines 426, 428 and 430 correspond to three different echo times TE1, TE2 and TE3, respectively. n k-space data collected at m ), that is, data m (k n ) can be expressed as:

[0045]

[0046] where d 3 r represents the differential volume element image in three-dimensional space 水 (r) and images 脂肪 (r) are the unknown water and fat MRI data in the spatial domain, respectively. In other words, these two terms represent the water and fat components of the imaged tissue at a specific spatial location or position r. TEm represents the echo time, t n Indicates the time increment or the readout point data obtained after the echo time k n The time required, i is an imaginary unit, and ω 水 And ω 脂肪 represent the chemical shift resonance frequencies of water and fat respectively. Explained by the imaging volume d 3 The position r in r is relative to the readout point k in k-space n The phase shift introduced.

[0047] After solving formula (1), we can get:

[0048]

[0049] Among them, data 水 (k n ) and data 脂肪 (k n ) represent the unknown water and fat MRI data in the k-space domain respectively. If there are M acquired echo times, the matrix formula for the M variables TE can be expressed as

[0050]

[0051] in

[0052] In the above formula 3, the vector on the left hand side (LHS) is the acquired data, and the vector on the right hand side (RHS) associated with the fat and water data is the unknown term. The variable CS refers to the chemical shift matrix defined in formula 4 and is based on the echo time and angular frequency associated with the water resonance and fat resonance. It should be noted that if only 3 echo times are used, the LHS matrix in formula 3 will only be a 3-row 1-column matrix. By solving formula 3, the MRI data in k-space for the water signal and the fat signal can be determined. For example, the Moore Penrose pseudo-inverse can be used to solve formula (3) + or other forms of (regularized) matrix inversion:

[0053]

[0054] It should be noted that because equation (5) accounts for chemical shift (CS), the k-space data determined for water and fat signals will not be affected by chemical shift artifacts. After obtaining water- and fat-resolved k-space data, the water and fat image components can then be reconstructed separately. Therefore, by combining all of the multiple variable echo time signal data 1 (k n ) and data M (k n ), a water and fat image of the subject can be obtained without chemical shift artifacts. Optionally, the chemical shift of fat can be more accurately modeled using a discrete distribution of chemical shifts rather than just a single resonance. In addition, other tissues or implants (e.g., silicon) can also be included in the chemical shift model.

[0055] It should be noted that, although the present technology is explained above with respect to a three-dimensional (3D) radial k-space scenario, the present technology is equally applicable to 2D and 3D MRI acquisitions using Cartesian or non-Cartesian (e.g., radial, spiral, star stack, ...) sampling, and to different types of pulse sequences, such as ultrashort TE (UTE), gradient echo (GRE), spin echo (SE), diffusion imaging, or any other similar pulse sequence technique.

[0056] Figure 5 5 is a schematic diagram 500 of experimental results comparing images reconstructed using conventional ultra-short echo time (UTE) techniques and multiple variable in-phase echo times according to embodiments of the present technology. Specifically, column 502 corresponds to images reconstructed using conventional UTE techniques. In addition, columns 504, 506, and 508 correspond to in-phase images acquired with variable echo times corresponding to average echo times of 0.4 ms, 0.6 ms, and 0.8 ms, respectively. Generally speaking, three echo times of 0.2 ms, 0.4 ms, and 0.6 ms were used when acquiring the images in column 504. For column 506, the three echo times used were 0.4 ms, 0.6 ms, and 0.8 ms, while for column 508, the echo times used were 0.6 ms, 0.8 ms, and 1 ms. In addition, rows 510, 512, and 514 correspond to axial, coronal, and sagittal cross-sections of the knee. It should be noted that these images in columns 504, 506, and 508 are reconstructed based on the acquired MR data (ie, the LHS matrix of equation (3)), where all readout points k corresponding to the complete k-space are n was used to generate these images.

[0057] from Figure 5 It can be seen that for all images in columns 504, 506, and 508, there is a perfect in-phase appearance independent of the echo time TE. In addition, with longer TE, short T2* structures (such as bone 516, tendon 520, ligament 522), the plastic coil housing 518 becomes darker and more prominent. In other words, as the average echo time in columns 504 to 508 increases from 0.4 ms to 0.8 ms, structures with short T2* signal lifetimes decay rapidly, resulting in dark signals for example, bone, tendon, and plastic coil housing. In-phase images with different effective TEs (i.e., 504, 506, 508) can also be used for quantitative T2* fitting.

[0058] Figure 6 600 is a schematic diagram of an image reconstructed using a variable echo time method according to an embodiment of the present technology. The images in columns 602, 604, 606, and 608 represent water, fat, perfect in-phase, and perfect out-of-phase images. In addition, rows 610, 612, and 614 correspond to axial, coronal, and sagittal cross-sections of the knee. Specifically, in Figure 5 The data acquired for three echo times of 0.4 ms, 0.6 ms and 0.8 ms in the experiment were used to obtain Figure 6 For example, using Formula 5 and data acquired for three echo times, water and fat signal k-space data are obtained. The water and fat signal k-space data are then reconstructed to obtain the images in columns 602 and 604.

[0059] The in-phase images in column 606 are obtained by adding the water and fat images in columns 602 and 604, respectively. In addition, the out-of-phase images in column 608 are obtained by subtracting the water and fat images in columns 602 and 604, respectively. Figure 6 As can be seen, the soft tissue is free of chemical shift artifacts. Thus, the trabecular bone structure 616 in these images is clearly visible, which is not usually the case in conventional MRI due to chemical shift artifacts.

[0060] Figure 7 700 is a flow chart of a method for generating an MR image of a subject using an MRI system 10 according to an embodiment of the present technology. According to an embodiment of the present technology, the method 700 can be specifically implemented in coordination with other components of an MRI system for magnetic resonance imaging with variable echo times on an MRI controller 33. In step 702, the method includes determining a plurality of echo times. For example, a plurality of echo times can be determined based on desired image contrast, imaged tissue characteristics, clinical applications, and signal-to-noise ratio. However, each echo time selected in the plurality of echo times is different from another echo time. In one embodiment, a main echo time is first determined, and another echo time is determined near the main echo time. For example, if the main echo time is 0.2ms, a user can decide to use a total of 2 echo times around the main echo time, such as 0.1ms and 0.3ms. In another example, a user can decide to use three echo times 0.1ms, 0.2ms, 0.3ms. In one embodiment, the number of echo times in the plurality of echo times (ie, whether 2, 3, 4 or more echo times are used) may depend on chemical shift effects, scan time requirements, and image quality requirements.

[0061] At step 704, the method includes acquiring a plurality of variable echo time signals corresponding to a plurality of echo times at a plurality of k-space positions. The variable echo time signal refers to the readout signal acquisition. For example, if three echo times are selected, then for each k-space position k n, there will be 3 readout signals or echo time signal acquisitions Data 1 (kn), Data 2 (kn) and Data 3 (kn). It should be noted that although the echo times are different, the other pulse sequence parameters (i.e., flip angle, repetition time, imaging bandwidth, etc.) will remain the same to perform these measurements. In addition, the method is also applicable to pulse sequence techniques such as ultra-short echo time (UTE) imaging techniques as well as 2D gradient echo (GRE) imaging techniques, 3D GRE imaging techniques, 2D spin echo (SE) imaging techniques, and 3D SE imaging techniques.

[0062] In one embodiment, these measurements or acquisitions (e.g., data 1 (kn), data 2 (kn), and data 3 (kn)) can be sequential or interleaved. For the sequential case, if there are a larger number of echo times TE3, TE4, etc., the variable echo time signals of all k-space positions of one echo time (e.g., TE1) will be acquired first, and then the variable echo time signals of all k-space positions for another echo time (e.g., TE2) will be acquired, etc. On the other hand, when the k-space data acquisition is interleaved, the variable echo time signals of a partial number of k-space positions of one echo time TE1 can be acquired first, and then the variable echo signals of a partial number of k-space positions for another echo time TE2 (as well as TE3, TE4, etc.) can be acquired. Similarly, the variable echo signals of another partial number of k-space positions of the first echo time TE1 are acquired, and then the variable echo signals of another partial number of k-space positions of TE2 are acquired, until all data of all k-space positions are acquired.

[0063] In step 706, the method includes generating a water k-space data signal data for each k-space position kn in the plurality of k-space positions based on a plurality of variable echo time signals corresponding to each k-space position kn in the plurality of k-space positions kn, namely, data 1 (kn), data 2 (kn), and data 3 (kn). 水 (kn) and fat k-space data signal data 脂肪 (kn). Generating the water k-space data signal and the fat k-space data signal also involves compensating for chemical shift by multiplying the plurality of variable echo time signals by the inverse of a chemical shift (CS) matrix, as shown in Equation 5. The chemical shift factor is based on the echo time and angular frequency associated with the water resonance and fat resonance of the tissue to be imaged.

[0064] Furthermore, in step 708, the method includes generating a medical image of the subject based on the water k-space data signal and the fat k-space data signal at the plurality of k-space positions, respectively. The medical image may include a water image, a fat image, an in-phase image, an out-of-phase image, or a combination thereof. In one embodiment, the step includes combining the water k-space data signal at each of the plurality of k-space positions to generate a water k-space, and combining the fat k-space data signal at each of the plurality of k-space positions to generate a fat k-space. The water k-space and the fat k-space are then reconstructed to generate a water image and a fat image of the subject, respectively.

[0065] The method may further include generating a perfect in-phase image and a perfect out-of-phase image of the subject. In one embodiment, the perfect in-phase image is obtained by adding a water image and a fat image of the subject. Additionally, the perfect out-of-phase image may be obtained by subtracting the fat image from the water image of the subject. The method may further include an optional step of contrast inversion of the obtained in-phase image to enhance bone visualization.

[0066] In one embodiment, the variable echo time MRI technique proposed herein can also be used to measure the T2* relaxation time of tissues, which indicates how quickly certain types of signals gradually disappear after applying an MRI pulse. This is important for characterizing different tissues. This can be achieved by analyzing the exponential decay of the T2* signal at the different echo times used to determine the T2* relaxation time. By measuring the exponential decay at various time intervals, a mathematical model can be fitted to the data of each pixel and a T2* relaxation time (T2*) map can be created, which highlights the change in relaxation time. In addition, a magnetic susceptibility map can be generated to capture phase effects related to tissue characteristics. In other words, T2* examines the magnitude effect by quantifying the effective transverse relaxation time, while magnetic susceptibility focuses on the phase effect by quantifying magnetic susceptibility. The latter allows us to distinguish tissues with different magnetic properties. Creating water, fat, and in-phase images of different echo times enables the extraction of T2* and quantitative magnetic susceptibility mapping (QSM) techniques, which provide more insights into tissue characteristics and magnetic susceptibility changes.

[0067] One of the advantages of the present technique is that it allows reconstruction of perfect in-phase images without chemical shift interference artifacts for arbitrary echo times TE (not just discrete in-phase echo times), and thereby provides the user with parameters (i.e., TE) for adjusting the appearance of cortical bone according to the bone anatomy of interest and / or user preferences. The technique can also be extended to quantitative T2* and / or susceptibility mapping. It is also expected that this method will improve synthetic CT conversion, depiction of interventional devices (needles, catheters, electrodes, gold seeds, etc.), depiction of calcifications, and other ultra-short echo time (UTE) MR imaging applications.

[0068] 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: Determine multiple echo times; acquiring a plurality of variable echo time signals corresponding to the plurality of echo times at a plurality of k-space positions; generating a water k-space data signal and a fat k-space data signal for each of the plurality of k-space positions based on the plurality of variable echo time signals corresponding to each of the plurality of k-space positions; as well as A medical image of the subject is generated based on the water k-space data signals and the fat k-space data signals at the plurality of k-space positions, respectively. 2 . The method of claim 1 , wherein the plurality of echo times are determined based on desired image contrast, characteristics of the tissue being imaged, clinical application, and signal-to-noise ratio. 3 . The method of claim 1 , wherein each echo time selected from the plurality of echo times is different from another echo time. 4 . The method of claim 1 , wherein acquiring the plurality of variable echo time signals of the plurality of echo times comprises keeping other pulse sequence parameters the same, wherein the other pulse sequence parameters comprise flip angle, repetition time, and imaging bandwidth.

5. The method according to claim 1 further includes using a pulse sequence technology, wherein the pulse sequence technology includes ultra-short echo time (UTE) imaging technology, 2D gradient echo (GRE) imaging technology, 3D GRE imaging technology, 2D spin echo (SE) imaging technology or 3D SE imaging technology. The method of claim 1 , wherein acquiring the plurality of variable echo time signals comprises sequential acquisition or interleaved acquisition.

7. The method of claim 1 , wherein generating the water k-space data signal and the fat k-space data signal for each of the plurality of k-space locations comprises compensating for chemical shift by multiplying the plurality of variable echo time signals by an inverse of a chemical shift matrix.

8. The method of claim 7, wherein the chemical shift matrix is ​​based on echo times and angular frequencies associated with water resonances and fat resonances of the tissue to be imaged.

9. The method of claim 1, wherein the medical image comprises a fat image, a water image, a perfect in-phase image, a perfect out-of-phase image, or a combination thereof.

10. The method of claim 1, further comprising generating a relaxation time (T2*) map and a magnetic susceptibility map by analyzing an exponential decay of a T2* signal over the plurality of echo times used.

11. A magnetic resonance imaging (MRI) system, the MRI system comprising: a magnet configured to generate a polarized magnetic field around at least a portion of a subject disposed in the MRI system; a gradient coil assembly, the 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 polarizing 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: Determine multiple echo times; acquiring a plurality of variable echo time signals corresponding to the plurality of echo times at a plurality of k-space positions; generating a water k-space data signal and a fat k-space data signal for each of the plurality of k-space positions based on the plurality of variable echo time signals corresponding to each of the plurality of k-space positions; as well as A medical image of the subject is generated based on the water k-space data signals and the fat k-space data signals at the plurality of k-space positions, respectively.

12. The MRI system of claim 11, wherein the processing system is programmed to determine the plurality of echo times based on desired image contrast, characteristics of the tissue being imaged, clinical application, and signal-to-noise ratio.

13. The MRI system of claim 11, wherein each echo time selected from the plurality of echo times is different from another echo time.

14. The MRI system of claim 11, wherein the processing system is programmed to acquire the plurality of variable echo time signals for the plurality of echo times by keeping other pulse sequence parameters the same, wherein the other pulse sequence parameters include flip angle, repetition time, and imaging bandwidth.

15. The MRI system of claim 11, wherein the processing system is programmed to acquire the plurality of variable echo time signals of the plurality of echo times in a sequential manner or in an interleaved manner.

16. The MRI system of claim 11, wherein the processing system is programmed to compensate for chemical shift by multiplying the plurality of variable echo time signals by an inverse of a chemical shift factor, thereby generating the water k-space data signal and the fat k-space data signal for each of the plurality of k-space positions.

17. The MRI system of claim 16, wherein the chemical shift factor is based on echo times and angular frequencies associated with water resonance and fat resonance of the tissue to be imaged.

18. The MRI system of claim 11, wherein the medical image comprises a fat image, a water image, a perfect in-phase image, a perfect out-of-phase image, or a combination thereof.

19. The MRI system of claim 11, wherein the processing system is further programmed to generate a relaxation time (T2*) map and a magnetic susceptibility map by analyzing an exponential decay of a T2* signal over the plurality of echo times used.

20. The MRI system of claim 11, wherein the processing system is further programmed to use a pulse sequence technique comprising an ultra-short echo time (UTE) imaging technique, a 2D gradient echo (GRE) imaging technique, a 3D GRE imaging technique, a 2D spin echo (SE) imaging technique, or a 3D SE imaging technique.

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