Systems and methods for imaging tissue

Abstract

To reliably detect a change in local magnetic susceptibility in a tissue.SOLUTION: A system and a method for detecting a part in a tissue having a change in local magnetic susceptibility using a magnetic resonance imaging (MRI) device include: a step for transmitting a first spin echo pulse sequence to the tissue, the first spin echo pulse sequence including a refocusing pulse of a first number and a first TE value; a step for transmitting a second spin echo pulse sequence to the tissue, the second spin echo pulse sequence including a refocusing pulse of a second number and a second TE value; a step for acquiring a first image and a second image; a step for determining one or more positions in the second image having signal intensity different from signal intensity at the same one or more positions in the first image; and a step for identifying a part of the tissue having local magnetic susceptibility that has changed on the basis of the determined one or more positions in the second image.SELECTED DRAWING: None

Description

Technical Field

[0001] Cross - Reference to Related Applications This application claims the benefit of U.S. Provisional No. 62 / 687,513, filed Jun. 20, 2018, and No. 62 / 723,703, filed Aug. 28, 2018, and incorporates them herein by reference in their entirety.

[0002] Field of the Invention The present invention generally relates to imaging tissue using a magnetic resonance imaging (MRI) device. More particularly, the present invention relates to systems and methods for detecting local susceptibility variations within tissue using an MRI device.

Background Art

[0003] Massive cerebral hemorrhage can be paramagnetic and can have a clearly different magnetic susceptibility from the surrounding tissue. When taking an MRI image of a brain with hemorrhage, the hemorrhage typically appears darker (e.g., lower intensity) in an image (e.g., a T2* - weighted image) obtained with a gradient - echo (GRE) sequence that has a sufficiently long time (e.g., 10 - 40 ms) between signal excitation and the generation of the detected echo signal (TE) compared to non - hemorrhagic regions of the brain, which is due to the presence of a local internal magnetic - field gradient at the location of the hemorrhage. However, the mere appearance of low intensity in an MRI image is typically not sufficient to declare hemorrhage because other factors can cause low intensity in an MRI image. For example, tissue having a short T2 relaxation time (e.g., 100 ms or less) may also appear low intensity.

[0004] Current methods for determining whether a low-intensity region in an MRI image is due to bleeding typically involve susceptibility-weighted imaging (SWI). The site of bleeding can be identified by identifying local phase changes that can indicate paramagnetic centers. The location of bleeding can be identified by signs (e.g., negative phase), magnitude (e.g., exceeding a predetermined value), and / or spatial frequency (e.g., changes occurring over a short distance). SWI can include combining the magnitude of MRI images obtained using a GRE sequence and a phase map.

[0005] One difficulty with SWI for identifying cerebral hemorrhage can include the fact that the local phase shift caused by bleeding can be very small (e.g., a phase shift on the order of 10 degrees) compared to the phase shift caused by the inhomogeneous field of the main magnet. Thus, in MRI acquired with an inhomogeneous magnetic field, the inhomogeneity of the main magnetic field can hide local gradients and make it difficult to detect bleeding. Accordingly, it may be desirable to reliably detect local changes in magnetic susceptibility within tissue (e.g., bleeding within the brain). Summary of the Invention Means for Solving the Problems

[0006] An embodiment includes detecting a portion within a tissue having a change in local susceptibility using a magnetic resonance imaging (MRI) device. For example, the MRI device includes transmitting a first spin-echo pulse sequence to the tissue, the first spin-echo pulse sequence including a first number of refocusing pulses and a first echo time (TE) value, acquiring a first image of the tissue by the MRI device, transmitting a second spin-echo pulse sequence to the tissue by the MRI device, the second spin-echo pulse sequence including a second number of refocusing pulses and a second TE value, acquiring a second image of the tissue by the MRI device, determining one or more locations within the second image of the tissue having a signal intensity different from the signal intensity at the same one or more locations within the first image of the tissue, and identifying a portion of the tissue having a changed local susceptibility based on the determined one or more locations within the second image of the tissue.

[0007] In some embodiments, the first number of refocusing pulses is different from the second number of refocusing pulses. In some embodiments, the first TE value is different from the second TE value. In some embodiments, identifying a portion of the tissue can include identifying a location within the tissue where the effective TE of the first pulse sequence and the second pulse sequence is the same.

[0008] In some embodiments, identifying a portion of the tissue can include applying a correction matrix to the first image of the tissue and the second image of the tissue, the correction matrix being based on at least two calibration images acquired using the MRI device. In some embodiments, the first number of refocusing pulses is less than the second number of refocusing pulses. In some embodiments, the second number of refocusing pulses is less than the first number of refocusing pulses. In some embodiments, one or more calibration images can be acquired from a phantom without an internal sensitivity gradient.

[0009] In some embodiments, a correction matrix can be generated based on one or more calibration images, and the generated correction matrix can be applied to at least one of a first image of the tissue and a second image of the tissue. In one embodiment, the tissue is the brain. In some embodiments, the MRI is a permanent magnet MRI. In some embodiments, the identified portion of the tissue can be transmitted to a display.

[0010] In some embodiments, the signal intensity difference between the first image of the tissue and the second image of the tissue can be caused by the intensity of the local magnetic susceptibility gradient. In some embodiments, an image mask can be generated, the first image of the tissue and the second image of the tissue can be weighted by the relaxation time T2, and the image mask can be overlaid on the weighted first image of the tissue and the second image of the tissue.

[0011] In some embodiments, at least one of the first spin echo pulse sequence and the second spin echo pulse sequence can be a three-dimensional sequence. In some embodiments, a fast spin echo sequence can be applied to the tissue to obtain a reading value of the tissue. In some embodiments, a plurality of spin echo pulse sequences can be applied during a predetermined period before applying the fast spin echo sequence.

[0012] An embodiment includes, for example, a system for detecting a portion within a tissue having a change in local magnetic susceptibility, including a magnetic resonance imaging (MRI) device, for transmitting a first spin echo pulse sequence to the tissue, the first spin echo pulse sequence including a first number of refocusing pulses and a first echo time (TE) value, acquiring a first image of the tissue, transmitting a second spin echo pulse sequence to the tissue, the second spin echo pulse sequence including a second number of refocusing pulses and a second TE value, acquiring a second image of the tissue, determining one or more positions within the second image of the tissue having signal intensities different from the signal intensities at the same one or more positions within the first image of the tissue, and identifying a portion of the tissue having a changed local magnetic susceptibility based on the determined one or more positions within the second image of the tissue.

[0013] An embodiment is a computer program product including instructions that, when executed by a computer, cause the computer to instruct an magnetic resonance imaging (MRI) device to detect a portion within a tissue having a change in local magnetic susceptibility, the instructions including, for example, causing the MRI device to transmit a first spin echo pulse sequence to the tissue, the first spin echo pulse sequence including a first number of refocusing pulses and a first echo time (TE) value, causing the MRI device to obtain a first image of the tissue, causing the MRI device to transmit a second spin echo pulse sequence to the tissue, the second spin echo pulse sequence including a second number of refocusing pulses and a second TE value, causing the MRI device to obtain a second image of the tissue, determining one or more positions within the second image of the tissue having signal intensities different from the signal intensities at the same one or more positions within the first image of the tissue, and identifying a portion of the tissue having various local magnetic susceptibilities based on the determined one or more positions within the second image of the tissue.

[0014] Non-limiting examples of embodiments of the present disclosure are described below with reference to the figures attached to this specification, which are listed after this paragraph. The dimensions of the features shown in the figures are selected for convenience and clarity of presentation and are not necessarily shown to scale.

[0015] The subject matter regarded as the invention is particularly pointed out and distinctly claimed in the concluding portion of the specification. However, the invention can be understood by reference to the following detailed description when read in conjunction with the accompanying drawings, with respect to both its construction, its method of operation, and both its objects, features and advantages. Embodiments of the invention are shown by way of example and not limitation in the figures of the accompanying drawings, and like reference numerals indicate corresponding, similar, or like elements.

Brief Description of the Drawings

[0016]

Figure 1

Figure 2A

Figure 2B

Figure 3A

Figure 3B

Figure 3C

Figure 4

Figure 5

Figure 6

[0017] It will be understood that, for the sake of simplicity and clarity, the elements shown in the figures are not necessarily drawn to scale. For example, the dimensions of some elements may be exaggerated relative to other elements for clarity. Further, reference numerals may be repeated within the figures where appropriate to indicate corresponding or similar elements.

DETAILED DESCRIPTION OF THE INVENTION

[0018] In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. However, one of ordinary skill in the art will understand that the present invention may be practiced without these specific details. In other instances, well-known methods, procedures, and components, modules, units, and / or circuits are not described in detail so as not to obscure the present invention. Some features or elements described with respect to one embodiment may be combined with features or elements described with respect to other embodiments. For clarity, consideration of the same or similar features or elements may not be repeated.

[0019] Embodiments of the present invention are not limited in this regard. For example, considerations using terms such as "processing", "computing", "calculating", "determining", "establishing", "analyzing", "checking", etc. refer to operations and / or processes (multiple possible) of manipulating and / or converting data represented as physical quantities (e.g., electronic) in a computer register and / or memory of a computer, computing platform, computing system, or other electronic computing device, into other data similarly represented as physical quantities in the computer register and / or memory, or other non-transitory storage media that may store instructions capable of executing the operations and / or processes. Embodiments of the present invention are not limited in this regard, but the terms "plurality" and "a plurality" as used herein may include, for example, "multiple" or "two or more". The terms "plurality" and "a plurality" may be used throughout the specification to describe two or more components, devices, elements, units, parameters, etc. The term "set" as used herein may include one or more items. Unless explicitly stated otherwise, embodiments of the methods described herein are not restricted to a particular order or arrangement. Further, some of the described method embodiments or elements thereof may exist or be executed simultaneously, at the same time, or concurrently.

[0020] Advantages of the present invention can include the ability to detect local susceptibility changes within an organization, such as local susceptibility caused by bleeding in the brain.

[0021] When obtaining a magnetic resonance image of an object, spin-echo decay may be present within the object in the presence of a non-uniform magnetic field (e.g., B0 field). The relaxation time T2 can indicate the decay of transverse magnetization (e.g., signal intensity versus echo time (TE)) in a spin-echo (SE) pulse sequence. The decay can be exponential, as shown in Equation No. 1 below.

Number

[0022] In a spin echo pulse sequence, the signal can be generated by an excitation pulse (e.g., a 90° flip angle), followed by a series of n (n≧1) refocusing pulses (e.g., a 180° flip angle). For simplicity, in one embodiment, it can be assumed that the refocusing pulse has a 180° inversion angle. In this example, each refocusing pulse can generate an echo signal. The echo signal can have an intensity according to formula number 1 each time an echo peak occurs. The echo time (TE) can be the time between the excitation pulse and the peak of the echo signal.

[0023] Referring to FIG. 1, it schematically shows a pulse sequence for spin echo MRI according to some embodiments of the present invention. An excitation pulse 110 can be generated by an MRI apparatus 100, followed by a refocusing pulse 120. When the refocusing pulse 120 is generated, the MRI can detect a signal 130. The refocusing pulse 120 can be repeated "N" times, where "N" is an integer greater than 1. The echo time (TE) can be the time between the excitation pulse and the detected signal. Also, the pulse sequence of FIG. 1 can form the basis of a single SE pulse sequence, a plurality of SE pulse sequences, and / or a fast SE (FSE) pulse sequence. For an FSE pulse sequence, each echo can have a different phase encoding (as opposed to an SE pulse sequence where all echoes experience the same phase encoding gradient), and a single MRI image can be reconstructed from all the echoes. The contrast in FSE can be determined by one echo at the center of "k-space" (e.g., an echo acquired with zero phase encoding).

[0024] The numerical value of the relaxation time T2 can depend on the homogeneity of the magnetic field B0 and / or the details of the specific pulse train used to measure T2. The phenomenon of spin echo can depend on the fact that the respective resonance frequencies of the nuclei within the observation volume between the signal excitation pulse and the signal refocusing pulse can be the same as the frequency between the refocusing pulse and the formation of the echo. However, this condition may not hold, for example, when the nuclei move within a non-uniform B0 field during two time intervals (e.g., the time intervals are the periods before and after the refocusing pulse). In the presence of nuclei that move (e.g., due to diffusion and / or flow), signal refocusing can be incomplete and / or the amplitude of the echo signal can be lower than expected considering only the value of T2 that occurs without the moving nuclei (as may be suggested by Equation No. 1).

[0025] Taking into account the diffusion of nuclear spins in a non-uniform B0 field, the equation describing the decay of the echo, which would be as shown in Equation No. 1, can be modified as follows.

Number

[0026] Another consideration regarding MRI measurements can be the type of nuclear effect, molecular environment, and / or temperature on the relaxation time T2. As can be seen from Equation No. 2, measuring T2 by generating a single echo signal at varying times can result in inaccuracies in T2 and / or difficulties in obtaining T2 measurements. In some embodiments, a series (e.g., or "train") of echo signals can be generated that can reduce the effect of the second term in Equation No. 2 by repeating the application of refocusing pulses (e.g., at equal intervals). In some embodiments, repeating the application of refocusing pulses can be performed according to a "Carr-Purcell-Meiboom-Gill" (CPMG) echo train pulse sequence, for example, to measure the transverse or spin-spin T2 relaxation time of any nucleus. In these embodiments, multiple echoes can be detected at varying values of TE. In some embodiments, "N" refocusing pulses can be applied to detect and / or measure, for example, n echo signals as shown in FIG. 1.

[0027] In some embodiments, the contribution of the second term in Equation No. 2 can be reduced, for example, as shown in Equation No. 4 below, by increasing the number of refocusing pulses: The time interval between consecutive refocusing pulses and refocused echo signals is represented by IED (inter-echo-delay). The time between the excitation pulse and the first refocusing pulse is equal to IED / 2. The echo time TE for each echo in the train can be as follows: TE = n(IED) Equation No. 3

[0028] The attenuation of the echo signal is given by the following equation:

Number

[0029] As can be seen from Equation No. 4, the relative contribution of the second term (which may be due to diffusion in the local gradient) can be reduced by increasing the number of refocusing pulses "n" within a given TE (this may be equivalent to shortening the IED). In some embodiments where the magnitude of the second term is negligible compared to the magnitude of the first term, the relaxation time T2 can be accurately (or substantially accurately) measured from the decay of the echo train signal, even when the field B0 is strongly inhomogeneous and the TE is long. In some embodiments, T2 can be measured using a single signal excitation.

[0030] As described above, for example, in order to detect cerebral hemorrhage, it may be desirable to generate contrast in the MRI image based on the presence of a strong internal magnetic field gradient. Obtaining an MRI image that depends solely (or substantially solely) on the strength of the local gradient (represented by "g") without being affected (or substantially affected) by any other factors such as intensity (T1, T2, etc.), and / or generating a mathematical MRI image mask that can be overlaid on the T2-weighted MRI image to highlight and / or emphasize the internal susceptibility gradient. As verified by Equation No. 4, the amount of change in the IED (which can be considered equivalent to the amount of change in "n") can be used to embed different levels of sensitivity to the internal gradient in the signal intensity of the MRI image.

[0031] In some embodiments, two sets of MRI images are acquired, in which case all acquisition parameters (including TE) are the same (or substantially the same), and only the number of refocusing pulses (n) is different. In these embodiments, the only difference (or the only substantial difference) between the two sets of MRI images is due to the strength of the local susceptibility gradient.

[0032] According to some embodiments, an MRI device is used to detect specific relevant parts within a tissue having a change in local magnetic susceptibility, for example, to detect cerebral hemorrhage. Referring to FIGS. 2A - 2B, a flowchart of a method for detecting a part within a tissue having a change in local magnetic susceptibility using an MRI device according to some embodiments of the present invention is shown.

[0033] This method includes transmitting a first spin - echo pulse sequence to the tissue by the MRI device, and the first spin - echo pulse sequence includes a first number of refocusing pulses and a first echo time (TE) value (step 210).

[0034] Also, this method includes obtaining a first image of the tissue by the MRI device (step 220).

[0035] This method also includes transmitting a second spin - echo pulse sequence to the tissue by the MRI device, and the second spin - echo pulse sequence includes a second number of refocusing pulses and a second echo time (TE) value (step 230).

[0036] Also, this method includes obtaining a second image of the tissue by the MRI device (step 240). In some embodiments, the first number of refocusing pulses is different from the second number of refocusing pulses. In some embodiments, the first TE value is different from the second TE value.

[0037] This method also includes determining one or more positions within the second image of the tissue having signal intensities different from the signal intensities at the same one or more positions within the first image of the tissue (step 250). The determination can be made by the MRI device and / or a computer processing device.

[0038] This method also includes identifying portions of tissue having various local susceptibilities based on one or more determined positions within a second image of the tissue (step 260). Identification of portions of tissue having local susceptibility changes can be performed by detection of algebraic post-processing of the images. The identification can be performed by an MRI device and / or a computer processing device.

[0039] In various embodiments, the first spin echo pulse sequence and / or the second spin echo pulse sequence is a three-dimensional sequence having an acquisition matrix of 64x64x28, a slice thickness equal to 1.5 mm, a field of view (FOV) equal to 32 mm, a TR equal to 400 ms, and a TE equal to 22.4 ms.

[0040] In various embodiments, the first number of refocusing pulses is between 1 and "n", where "n" is an integer greater than 1. In various embodiments, the second number of refocusing pulses is between 1 and "m", where m is an integer greater than 1. In some embodiments, "n" and "m" are different. In some embodiments, "n" is greater than "m". In some embodiments, "m" is greater than "n".

[0041] In some embodiments, an indicator of the amount of local susceptibility change detected within the tissue is transmitted to a display for viewing, for example, by an operator of the MRI device.

[0042] In various embodiments, any number of scans (e.g., a first spin-echo sequence, a second spin-echo sequence, a third spin-echo sequence, ..., "n" spin-echo sequences, where "n" is an integer greater than 1) can be present. In these embodiments, for each spin-echo sequence within the "n" spin-echo sequences, the number of refocusing pulses can be any integer value. In these embodiments, for each spin-echo sequence within the "n" spin-echo sequences, the number of refocusing pulses can be different for each of the "n" spin-echo sequences. In these embodiments, for each spin-echo sequence within the "n" spin-echo sequences, the TE value can be different for each of the n spin-echo sequences. In these embodiments, the strength of the local susceptibility gradient can be determined based on non-least squares fitting, exponential fitting to create a map of "effective" T2 or R2 values, principal component analysis (PCA), and / or any mathematical analysis known to be appropriate in the art.

[0043] In some scenarios, since the radiofrequency fields during the first and second pulse sequences may lack uniformity, this method can include acquiring a calibration MRI image for a purpose without an internal sensitivity gradient at a significant level (e.g., a phantom), and using the calibration MRI image to generate a correction matrix that can be applied to each of the MRI images received during the first and second pulse sequences. The correction matrix can depend on the particular RF coil and the number of refocusing pulses used during imaging, but typically does not depend on other parameters such as the field of view, acquisition matrix, TR, and / or TE. Calibration can be performed once for each coil over a range of values for the number of refocusing pulses. The amount of change in the radiofrequency field is typically not sample-dependent such that calibration cannot be repeated for each subject being scanned.

[0044] Referring to FIGS. 3A - 3B, examples of MRI images of tissue (e.g., sample tissue) obtained with a first pulse sequence and a second pulse sequence scanned on an MRI apparatus (e.g., an MRI apparatus manufactured by aspect imaging) according to some embodiments of the present invention are shown. The sample tissue is a phantom placed in a test tube with a length of 37 mm and an inner diameter of 10 mm, and a small amount of magnetite gel with a concentration of 44 μg of iron oxide powder per 1 ml of gel is used. The phase map derived from GRE on the sample can show that the bulk susceptibility difference (Δχ) between the magnetite gel and the background gel is 1.0 ppm. The value of 1.0 ppm can be seen as the upper limit of the estimated value for a specific hemorrhagic lesion, for example, when compared with the Δχ values for hemorrhages in the range of 0.1 - 1.5 ppm.

[0045] The MRI apparatus may be a permanent magnet MRI apparatus having a magnetic field strength of 1.0 tesla and having a direction of the vertical B0 magnetic field (e.g., perpendicular to the long axis of the cylindrical sample tube). The transmit / receive RF coil is a solenoid with an inner diameter of 35 mm.

[0046] In FIGS. 3A and 3B, the MRI images are obtained based on first and second pulse sequences with a 3D sequence, an acquisition matrix of 64x64x28, a slice thickness equal to 1.5 mm, a field of view (FOV) equal to 32 mm, a TR equal to 400 ms, and a TE equal to 22.4 ms. In the case of FIG. 3A, the number of refocusing pulses is "n" = 4, and in the case of FIG. 3B, the number of refocusing pulses is "m" = 1. In the first pulse sequence of FIG. 3A, the inter - echo delay (IED) is 5.6 ms. FIG. 3A is from the fourth echo of the echo train of the first pulse sequence, so its TE = 5.6×4 = 22.4 ms, which is the same as the TE of the second pulse sequence with "m" = 1.

[0047] As shown in FIGS. 3A and 3B, the signal intensity of the magnetite gel (with a substantial internal gradient) is much lower on the "n" = 1 MRI image (FIG. 3A) compared to the "n" = 4 MRI image (FIG. 3B), while the intensity of the surrounding gel (with a much weaker gradient) is substantially the same. In this particular example, the signal intensity is quantitatively comparable between the two scans. Thus, an MRI image showing the normalized difference (dn) between the two scans can be defined as follows.

Number

[0048] Refer to FIG. 3C, which shows a desired local susceptibility specificity MRI image determined from the MRI images of FIGS. 3A and 3B according to some embodiments of the present invention. In FIG. 3C, only regions with a sufficiently strong internal gradient show a dn pixel value (e.g., the intensity at a pixel of the MRI image) greater than 0, while all other regions have an MRI image intensity close to 0 (e.g., within the range of experimental uncertainty), regardless of their tissue type, relaxation time, etc.

[0049] FIG. 3C shows an example of an MRI image that is completely equivalent to an ideally filtered conventional susceptibility weighted imaging (SWI) phase map, and SWI typically involves the distinction of showing blood vessels and bleeding as low intensity.

[0050] The graph in FIG. 3C is determined by (I4 - I1) / I4, where I4 is the intensity of the MRI image in FIG. 3A and I1 is the intensity of the MRI image from 3B.

[0051] In some embodiments, MRI can weight the first MRI image of the tissue and the second MRI image of the tissue at relaxation time T2 by generating an MRI image mask and overlaying the MRI image mask on the weighted first MRI image of the tissue and the second MRI image of the tissue.

[0052] Refer to FIGS. 4A and 4B, which show examples of MRI images of tissue (e.g., a sample) obtained with a first pulse sequence and a second pulse sequence according to some embodiments of the present invention. The sample includes a phantom placed within the MRI such that it is located in a region of very poor B0 homogeneity. In this example, the phantom consists of a tube with a length of 10 cm and an inner diameter of 16 mm. The magnetite gel contains iron oxide powder at a concentration of 22 μg per 1 ml of gel.

[0053] In FIG. 4A, the MRI image is obtained with a first pulse sequence of a two-dimensional fast spin echo sequence, and in FIG. 4B, the MRI image is obtained with a second pulse sequence of a two-dimensional spin echo sequence.

[0054] The MRI images of FIGS. 4A and 4B are obtained with first and second pulse sequences where the slice thickness is equal to 2 mm, the field of view (FOV) is equal to 50×100 mm, the acquisition matrix = 128X140, the TR is equal to 2000 ms, and the TE is equal to 40 ms. The first pulse sequence includes an echo train length = 16 and an IED = 5.0 ms, and the second pulse sequence includes an echo train length = 1 and an IED = 40 ms. The first pulse sequence has a number of refocusing pulses “n” = 8, and the second pulse sequence has a number of refocusing pulses “m” = 1. The phase encoding pattern in the first pulse sequence is set such that the center of “k-space” is collected at the 8th echo, resulting in an effective TE of 39.9 ms. In some embodiments, the identification of a portion of the tissue can include identifying a location within the tissue where the effective TEs of the first and second pulse sequences are the same.

[0055] The magnetite gel appears with a clearly different contrast on two MRI images, and its intensity is much lower in Figure 4B compared to Figure 4A. Figure 4B shows the lower part of the MRI image (the area enclosed by the white rectangle), which appears with distortion due to the inhomogeneity of the B0 field in this region of the magnet (e.g., caused by higher-order background gradients). The appearance of this region (e.g., from the perspective of geometric shape and intensity) is substantially the same in Figures 4A and 4B. This indicates that the scale of the background gradient is not large enough to cause a significant effect and that the presence of the magnetite gel can be detected. In some embodiments, the magnetite gel can be detected without applying a high-pass filter that may be required for post-processing of conventional SWI results.

[0056] As will be apparent to those skilled in the art, the present invention can be implemented within any type of MRI device. The MRI device can be any MRI device known in the art. The MRI device can be a permanent magnet MRI. The MRI device can have a magnetic field strength of 1.0 tesla. The MRI device can have a B0 magnetic field in the vertical direction (e.g., perpendicular to the long axis of a cylindrical sample tube). The MRI device can be an MRI scanner from Aspect Imaging (Shoham, Israel). The transmit / receive RF coil can be a solenoid. The transmit / receive RF coil can be a solenoid with an inner diameter of 35 mm.

[0057] Referring to Figures 5A and 5B, examples of MRI images of tissue (e.g., a sample) obtained with a first pulse sequence and a second pulse sequence according to some embodiments of the present invention are shown. The sample includes a magnetite gel phantom containing 2.95 μg of iron oxide powder per ml of gel, and the Δχ with respect to the surrounding background gel is 0.14 ppm, which is approximately equal to the expected susceptibility difference between venous blood and the surrounding tissue. The effective TE for the MRI image is 28 ms.

[0058] Figure 5A shows an MRI image for a specific slice from the first echo (n = 1) of a three-dimensional multi-echo SE (MESE) acquisition using the following acquisition parameters: 64×64×13 acquisition matrix, TR = 400 ms, sampling dwell time = 12 μs, FOV = 45 mm, slice thickness = 3.0 mm, IED = 28 ms, total imaging time = 5.5 minutes. Figure 5B shows an MRI image for a second three-dimensional MESE acquisition using the same parameters except for this acquisition IED = 5.6 ms. The MRI image shows the same specific slide of Figure 5A reconstructed from the fifth echo (“n” = 5).

[0059] The magnetite gel (indicated by the white arrow) shows that the intensity on the MRI image of Figure 5A is decreased compared to the intensity of Figure 5B and is hardly distinguishable. This difference is weaker than the differences seen in Figures 3A and 3B, which are due to the thinner presence of iron oxide particles in the samples of Figures 5A and 5B, for example.

[0060] In contrast to the results shown in Figures 3A and 3B, the MRI images of Figures 5A and 5B reveal that the intensity of the background gel is not equal to the scans shown in Figures 5A and 5B of the drawing. As pointed out by the black arrow, there are regions in the sample where the intensity of Figure 5B is lower than that of Figure 5A. This can be due to, for example, refocusing pulse imperfections caused by a non-uniform B1 field. Such imperfections can be expected to have a stronger effect on the echoes detected after a larger number of refocusing pulses (in this case, five refocusing pulses in Figure 5B versus a single refocusing pulse in Figure 5A). In some embodiments, the intensity differences between Figures 5A and 5B that are not present in Figures 3A and 3B can be due to the fact that Figures 3A and 3B show MRI images of regions that are smaller and closer to the center of the magnet of the MRI device, where the B1 magnetic field is relatively uniform, than the regions shown in Figures 5A and 5B. The MRI images of Figures 5A and 5B cover the entire length of the sample tube, where the edges approach the edges of the RF coil length and stronger variations in B1 are expected.

[0061] In some embodiments, the method can include being performed during in vivo application. Some things desired for in vivo application can include good spatial resolution, thin and continuous slices (which may require a 3D acquisition protocol), and / or reasonably short scan times, for example, slice thicknesses on the order of 2 - 3 mm, spatial resolution on the order of 1 mm, and scan times on the order of 5 - 10 minutes. Conventional SWI can use GRE sequences that satisfy these requirements without being too difficult. SE sequences (especially their 3D versions) can typically take much longer to acquire MRI images. One approach to overcome this problem can include the application of various methods of performance time reduction, such as the use of multiple receive coils and / or compressed sensing. In some embodiments, the generated pulse sequence can enable rapid (e.g., 5 - 10 minute) acquisition for both the required n = 1 and n = n' scans.

[0062] Referring to FIG. 6, a pulse sequence for spin - echo MRI for the implementation of a susceptibility - weighted imaging spin - echo according to some embodiments of the present invention is shown. The pulse sequence can include a preparation part and a read - out part. The preparation part can include an excitation pulse 610 and a signal 630 (A0) that can follow a refocusing pulse 620. The signal 630 can have high sensitivity to local internal gradients when n = 1 and much lower sensitivity to such gradients when n = n'. The read - out part can include a centered - out phase - encoding and a rapid segmented FSE sequence using a minimum IED and TE (e.g., the minimum IED and TE can be 5 ms).

[0063] In the preparation part (e.g., between n = 1 and n = n' scans), the transverse magnetization is prepared with a T2 weighting having either low sensitivity (n = n') or high sensitivity (n = 1) to the internal gradient. The preparation part can be further refocused by an FSE train (e.g., "N" repetitions) to create 2D or 3D MRI images. In some embodiments, for example, to preserve the contrast generated by the preparation, the FSE sequence can be collected using center-out phase encoding and the shortest possible effective TE value. This sequence can be applied in 3D mode, in which case the IEDs in both the preparation part and the read part can be minimized by using non-selective rectangular RF pulses.

[0064] The sequence shown in FIG. 6 can have the following advantages: First, it can enable a relatively short imaging time even in 3D mode (the acquisition can be combined with multiple coils and compressed sensing for further savings in scan time), second, the fact that the read part of the sequence can be the same for n = 1 and n = n' acquisitions, which can ensure that regions with small internal sensitivity gradients can have the same absolute intensity for both acquisitions.

[0065] Unless explicitly stated otherwise, embodiments of the methods described herein are not restricted to a particular order or sequence. Further, some of the described method embodiments or elements thereof can occur or be performed simultaneously, at the same time, or concurrently.

[0066] Various embodiments are presented. Each of these embodiments may, of course, include features from the other embodiments presented, and embodiments not specifically described may include various features described herein.

Description of Reference Numerals

[0067] 100 MRI apparatus 110 Excitation Pulse 120 Refocusing Pulse 130 Signal 610 Excitation Pulse 620 Refocusing Pulse 630 Signal

Claims

Claim 1 A method for detecting a part within a tissue having a change in local magnetic susceptibility using a magnetic resonance imaging (MRI) apparatus, wherein the MRI apparatus has a non-uniform magnetic field, and as a result, when a conventional susceptibility-weighted imaging is performed using the MRI apparatus and the phase shift caused by the change in local magnetic susceptibility is smaller than the phase shift caused by the non-uniform magnetic field, the conventional susceptibility-weighted imaging becomes unreliable for detecting such a change in local magnetic susceptibility. The method includes the following steps: Transmitting, by the MRI apparatus, a first spin-echo pulse sequence to the tissue, wherein the first spin-echo pulse sequence includes a first number of refocusing pulses and a first echo time (TE) value; Obtaining, by the MRI apparatus, a first image of the tissue; Transmitting, by the MRI apparatus, a second spin-echo pulse sequence to the tissue, wherein the second spin-echo pulse sequence includes a second number of refocusing pulses and a second TE value; Obtaining, by the MRI apparatus, a second image of the tissue; Determining one or more positions in the second image of the tissue having signal intensities different from the signal intensities at the same one or more positions in the first image of the tissue; Identifying a part of the tissue having various local magnetic susceptibilities based on the determined one or more positions in the second image of the tissue, wherein the first number of refocusing pulses is different from the second number of refocusing pulses, but all other acquisition parameters including the first TE value and the second TE value are the same or substantially the same. Claim 2 Identifying the part of the tissue further includes identifying positions within the tissue where the effective TE of the first spin-echo pulse sequence and the second spin-echo pulse sequence is the same, according to the method of Claim 1. Claim 3 Identifying the part of the tissue further includes applying a correction matrix to the first image of the tissue and the second image of the tissue, and the correction matrix is based on at least two calibration images taken using the MRI apparatus, according to the method of Claim 1. Claim 4 The method according to claim 3, wherein the first number of the refocusing pulses is less than the second number of the refocusing pulses.

5. The method according to claim 3, wherein the second number of the refocusing pulses is less than the first number of the refocusing pulses.

6. The method according to claim 1, further comprising the step of obtaining one or more calibration images from a phantom without an internal susceptibility gradient.

7. The following steps: generating a correction matrix based on the one or more calibration images; and applying the generated correction matrix to at least one of the first image of the tissue and the second image of the tissue The method according to claim 6, further comprising.

8. The method according to claim 3, wherein the tissue is the brain.

9. The method according to claim 3, wherein the MRI is a permanent magnet MRI.

10. The method according to claim 3, further comprising the step of transmitting the identified portion of the tissue to a display.

11. The method according to claim 3, wherein the signal intensity difference between the first image of the tissue and the second image of the tissue is caused by the intensity of the local susceptibility gradient.

12. The following steps: generating an image mask; weighting the first image of the tissue and the second image of the tissue with a relaxation time T2; and overlaying the image mask on the weighted first image of the tissue and the second image of the tissue The method according to claim 3, further comprising.

13. The method according to claim 3, wherein at least one of the first spin echo pulse sequence and the second spin echo pulse sequence is a three-dimensional sequence.

14. The method according to claim 3, further comprising the step of applying a fast spin echo sequence to the tissue to obtain a readout of the tissue.

15. The method according to claim 14, further comprising the step of applying a plurality of spin echo pulse sequences during a predetermined period before applying the fast spin echo sequence.

16. A system for detecting a portion within a tissue having a change in local susceptibility, the system including a magnetic resonance imaging (MRI) device, the MRI device having a non-uniform magnetic field, such that when conventional susceptibility-weighted imaging is performed using the MRI device and the phase shift caused by the change in local susceptibility is smaller than the phase shift caused by the non-uniform magnetic field, the conventional susceptibility-weighted imaging becomes unreliable for detecting such a change in local susceptibility, the MRI device: Transmit a first spin-echo pulse sequence to the tissue, the first spin-echo pulse sequence including a first number of refocusing pulses and a first echo time (TE) value; Obtain a first image of the tissue; Transmit a second spin-echo pulse sequence to the tissue, the second spin-echo pulse sequence including a second number of refocusing pulses and a second TE value; Obtain a second image of the tissue; Determine one or more positions in the second image of the tissue having signal intensities different from the signal intensities at the same one or more positions in the first image of the tissue; Based on the determined one or more positions in the second image of the tissue, identify a portion of the tissue having various local susceptibilities, and is configured to The system wherein the first number of refocusing pulses is different from the second number of refocusing pulses, but all other acquisition parameters including the first TE value and the second TE value are the same or substantially the same. Claim 17 A computer program product for detecting a portion within a tissue having a change in local susceptibility, the computer program product, when executed by a computer, causing the computer to: Transmitting a first spin echo pulse sequence to one or more components of magnetic resonance imaging (MRI) of the tissue, the first spin echo pulse sequence including a first number of refocusing pulses and a first echo time (TE) value, the MRI apparatus having a non-uniform magnetic field, such that when conventional susceptibility weighted imaging is performed using the MRI apparatus and the phase shift caused by a local susceptibility change is smaller than the phase shift caused by the non-uniform magnetic field, the conventional susceptibility weighted imaging becomes unreliable for detecting such local susceptibility changes; transmitting the first spin echo pulse sequence; Obtaining a first image of the tissue; Transmitting a second spin echo pulse sequence to the tissue, the second spin echo pulse sequence including a second number of refocusing pulses and a second TE value; transmitting the second spin echo pulse sequence; Obtaining a second image of the tissue; Determining one or more positions in the second image of the tissue having signal intensities different from the signal intensities at the same one or more positions in the first image of the tissue; Identifying a portion of the tissue having various local susceptibilities based on the determined one or more positions in the second image of the tissue; Including instructions to cause to be performed; A computer program product, wherein the first number of refocusing pulses is different from the second number of refocusing pulses, but all other acquisition parameters including the first TE value and the second TE value are the same or substantially the same.

Images