System and method for separating water and fat signals in spin lock magnetic resonance imaging
阅读说明：本技术 自旋锁磁共振成像中分离水和脂肪信号的系统和方法 (System and method for separating water and fat signals in spin lock magnetic resonance imaging ) 是由 陈蔚天 于 2019-05-28 设计创作，主要内容包括：MRI图像中脂肪和水信号的分离可以通过一种技术实现,该技术包括使用包含绝热脉冲的自旋锁RF脉冲序列和使用Dixon方法进行水/脂肪分离。自旋锁RF脉冲序列可以是绝热连续波恒幅自旋锁(ACCSL)脉冲序列。数据采集可以使用任何与Dixon方法兼容的采集方法。数据采集完成后,可以生成源图像并进行分析(例如,使用Dixon方法),生成单独的水和脂肪图像。基于自旋锁的成像生物标志物(如T1ρ)的空间分布可以从水图像和/或脂肪图像中确定。(Separation of fat and water signals in MRI images can be achieved by a technique that includes the use of spin-lock RF pulse sequences containing adiabatic pulses and water/fat separation using the Dixon method. The spin lock RF pulse sequence may be an adiabatic continuous wave constant amplitude spin lock (ACCSL) pulse sequence. The data acquisition may use any acquisition method compatible with the Dixon method. After data acquisition is complete, source images can be generated and analyzed (e.g., using the Dixon method) to generate separate water and fat images. The spatial distribution of spin lock based imaging biomarkers (e.g., T1 ρ) may be determined from water images and/or fat images.)
1. A method of separating water and fat signals in an image of a region of interest of an individual using a Magnetic Resonance Imaging (MRI) device, the method comprising:
applying a magnetization preparation sequence comprising an adiabatic continuous wave constant amplitude spin lock (ACCSL) pulse sequence;
implementing an acquisition sequence to acquire a data set, wherein the acquisition sequence comprises a Dixon acquisition method;
generating a source image from the data set; and
the source images are analyzed using a Dixon analysis method to generate water and fat images.
2. The method of claim 1, further comprising:
determining a spatial distribution of spin lock based imaging biomarkers for each of a plurality of locations within the region of interest based at least on the water image.
3. The method of claim 2, wherein the spin lock based imaging biomarker is T1 p.
4. The method of claim 1, wherein the ACCSL pulse sequence comprises an adiabatic half-channel (AHP), a constant amplitude spin lock Radio Frequency (RF) pulse having a spin lock time, and an inverse AHP, wherein the RF amplitudes of the AHP and inverse AHP are equal to the spin lock amplitude.
5. The method of claim 1, wherein the Dixon collection method is a three-point Dixon method.
6. The method of claim 1, wherein analyzing the data set to generate a water image and a fat image comprises modeling the source image as:
wherein s (r, TE, TSL) is the source image element obtained at position r in terms of the spin lock time TSL and the echo time TE; rhow(r, TSL) and ρf(r, TSL) are water image and fat image, respectively; Ψ (r) is a field map; f. ofnChemical shift of nth fat peak, amplitude coefficient βn(TSL) is the relative amplitude of the nth fat peak.
7. The method of claim 6, wherein each amplitude coefficient βn(TSL) is approximated to be independent of the TSL constant.
8. The method of claim 6, further comprising:
implementing a pre-calibration procedure to determine each amplitude coefficient βn(TSL) value.
9. A Magnetic Resonance Imaging (MRI) system comprising:
an MRI apparatus having a magnet, a gradient coil, and one or more Radio Frequency (RF) coils; and
a computer communicably connected to the MRI device, the computer having a processor, a memory, and a user interface, the processor configured to:
applying a magnetization preparation sequence comprising an adiabatic continuous wave constant amplitude spin lock (ACCSL) pulse sequence;
performing an acquisition sequence to acquire a dataset, wherein the acquisition sequence comprises a Dixon acquisition method;
generating a source image from the data set; and
and analyzing the source image by using a Dixon analysis method to generate a water image and a fat image.
10. The system of claim 9, wherein the processor is further configured to:
based at least on the water image, a spatial distribution of spin lock based imaging biomarkers is determined for each of a plurality of locations within a region of interest.
11. The system of claim 10, wherein the spin lock based imaging biomarker is T1 p.
12. The system of claim 9, wherein the ACCSL pulse sequence comprises an adiabatic half-channel (AHP), a constant amplitude spin lock Radio Frequency (RF) pulse having a spin lock time, and an inverse AHP, wherein the RF amplitudes of the AHP and inverse AHP are equal to the spin lock amplitude.
13. The system of claim 9, wherein the Dixon collection method is a three-point Dixon method.
14. The system of claim 9, wherein the processor is further configured such that analyzing the dataset to generate water and fat images comprises modeling the source images as:
wherein s (r, TE, TSL) is the source image element obtained at position r in terms of the spin lock time TSL and the echo time TE; rhow(r, TSL) and ρf(r, TSL) are water image and fat image, respectively; Ψ (r) is a field map; f. ofnChemical shift of nth fat peak, amplitude coefficient βn(TSL) is the relative amplitude of the nth fat peak.
15. The system of claim 14, wherein each amplitude coefficient βn(TSL) is approximated to be independent of the TSL constant.
16. The system of claim 14, wherein the processor is further configured to:
a pre-calibration routine is performed to determine each amplitude coefficient βn(TSL) value.
17. The system of claim 10, wherein the region of interest comprises tissue of a patient.
18. A computer-readable medium storing a plurality of instructions, wherein the plurality of instructions, when executed by a processor, perform operations comprising:
applying a magnetization preparation sequence comprising an adiabatic continuous wave constant amplitude spin lock (ACCSL) pulse sequence;
implementing an acquisition sequence to acquire a data set, wherein the acquisition sequence comprises a Dixon acquisition method;
generating a source image from the data set; and
the source images are analyzed using a Dixon analysis method to generate water and fat images.
19. The computer-readable medium of claim 18, wherein the operations further comprise:
determining a spatial distribution of spin lock based imaging biomarkers for each of a plurality of locations within the region of interest based at least on the water image.
20. The computer readable medium of claim 19, wherein the spin lock based imaging biomarker is T1 ρ.
21. The computer readable medium of claim 18, wherein the ACCSL pulse sequence comprises an adiabatic half-channel (AHP), a constant amplitude spin lock Radio Frequency (RF) pulse having a spin lock time, and an inverse AHP, wherein the RF amplitudes of the AHP and inverse AHP are equal to the spin lock amplitude.
22. The computer-readable medium of claim 18, wherein the Dixon collection method is a three-point Dixon method.
23. The computer readable medium of claim 18, wherein analyzing the dataset to generate a water image and a fat image comprises modeling the source images as:
wherein s (r, TE, TSL) is the source image element obtained at position r in terms of the spin lock time TSL and the echo time TE; rhow(r, TSL) and ρf(r, TSL) are water image and fat image, respectively; Ψ (r) is a field map; f. ofnChemical shift of nth fat peak, amplitude coefficient βn(TSL) is the relative amplitude of the nth fat peak.
24. The computer of claim 23Readable medium, wherein each amplitude coefficient βn(TSL) is approximated to be independent of the TSL constant.
25. The computer-readable medium of claim 23, wherein the operations further comprise:
implementing a pre-calibration procedure to determine each amplitude coefficient βn(TSL) value.
Background
The present application relates generally to Magnetic Resonance Imaging (MRI) technology and, in particular, to a technique for separating water and fat signals in spin lock MRI. For example, these techniques may be used for T1 p imaging and quantification as well as other imaging biomarkers.
Magnetic Resonance Imaging (MRI) is a non-invasive imaging technique that can assess the composition and state of a variety of tissues. During MRI, the patient is subjected to a strong longitudinal magnetic field (B0) which causes the patient to moveThe nuclear spins of the inner atoms align, producing a net magnetization vector. A radio frequency pulse (RF) having a magnetic field component (B1) traverses the longitudinal magnetic field, applying an isotope (typically, an isotope) tuned to be of interest1H) Of larmor frequency. These pulses can flip the spins to a higher energy state, producing a transverse component of the magnetization vector. When these spins return to the ground state, responsive magnetic resonance signals from the patient's body can be detected. From these signals, the magnetization characteristics can be measured.
Spin-lock techniques in MRI typically involve the application of long RF pulses (called "spin-lock" pulses) to lock the magnetization around the effective magnetic field. These techniques can be used to quantify various imaging biomarkers that can reveal useful information about the macromolecular content in tissue. For example, the spin lattice relaxation time in the rotating coordinate system (T1 ρ) characterizes the decay (or relaxation) rate during spin-lock. Since many diseases start to alter the macromolecular content of tissues at a very early stage, spin-lock MRI offers the possibility of early detection of diseases. In addition, spin lock MRI may be used to monitor the effectiveness of treatment at the macromolecular level.
Conventional spin lock MRI techniques are very sensitive to the presence of fat, in part because protons in the fat molecules generate chemical shifts that can lead to spin lock failure. For tissues including infiltrating adipose tissue, this can lead to false results and quantification errors. To reduce such artifacts and errors, spectrally selective RF pulses (e.g., prior to spin lock pulses) have been applied to suppress fat signals. However, this approach has limitations, partly because it is susceptible to B0 magnetic field inhomogeneity (which is common in modern MRI systems), and partly because fat has multiple chemical shift components, each with a different chemical shift, and spectrally selective RF pulses cannot suppress all of these pulses.
Therefore, there is a need for improved fat suppression techniques in spin lock MRI.
Summary of The Invention
Some embodiments of the present application relate to the separation of fat and water signals in spin lock MRI. In some embodiments, separation of fat and water signals can be achieved by the following techniques: the technique involves the use of a spin lock RF pulse sequence incorporating adiabatic pulses and the use of the Dixon method for water/fat separation. For example, the spin lock RF pulse train may be an adiabatic continuous wave constant amplitude spin lock (ACCSL) pulse train, which may include an adiabatic half-channel (AHP), a constant amplitude spin lock RF pulse with a spin lock time, and an inverse AHP, where the AHP and inverse AHP have RF amplitudes equal to the spin lock amplitude. The data acquisition may use any acquisition method compatible with the Dixon method. After data acquisition, a source image may be generated (e.g., using conventional techniques for generating images from MRI data). The source images can be analyzed to generate separate water and fat images. In some embodiments, the spatial distribution of spin lock based imaging biomarkers (e.g., T1 ρ) may be quantitatively determined, for example, from water images. To the extent that the water image does not contribute to the fat signal, the reliability of the quantification can be improved.
The following detailed description and the accompanying drawings provide a further understanding of the nature and advantages of the claimed invention.