Method and system for improving image contrast in high speed driven balanced inversion recovery imaging - Patents.com

Abstract

To improve an MRI signal generated by magnetization when imaging an object in an examination volume.SOLUTION: An MRI method comprises the steps of: performing, by an MRI device, a pulse sequence comprising three consecutive RF elements, namely, a first element that is an inversion recovery pulse sequence, a second element that is a pulse sequence starting at the inversion time with an excitation RF pulse, at least one image-encoding gradient following it, and at least one data sampling, and a third element following the second element; acquiring, by the MRI device, an MRI signal generated by the object in each TR and sampled during an image reading period; and reconstructing, from the MRI signal, an image of the object. The third element is a modified driven equilibrium pulse sequence configured for converting a transverse magnetization component of the magnetization of interest into positive longitudinal magnetization, in a state aligned with the positive z-axis and a B0-field direction.SELECTED DRAWING: Figure 6A

Description

[Technical field]

[0001] The present invention is in the field of magnetic resonance imaging (MRI) systems and methods, and more particularly, the present invention relates to a method and system for improving image contrast in fast driven balanced inversion recovery imaging in MRI.

[0002] More specifically, the present invention addresses the problem of long acquisition times required for fluid-sensitive fast spin-echo MRI or turbo spin-echo-based MRI (FSE-MRI or TSE-MRI) with robust fat signal suppression by short tau inversion (short-TI) recovery (STIR, [1,2]).

[0003] The central problem is the slow recovery of the longitudinal fluid magnetization, i.e. the long time required for regeneration after the spectrally non-selective STIR inversion preparation pulse and the turbo spin-echo readout radio frequency (RF) pulse. The characteristic recovery time TI of low-viscosity fluids is in the range of several seconds at the field strengths of clinical scanners (e.g. 1.5-7 Tesla). Furthermore, even if the fat TI time is relatively short, e.g. about 380 ms at 3 Tesla, there is still a relatively long inversion recovery time (TI) to wait for the zero crossing of the fat magnetization at the start of the image readout after the application of the inversion pulse.

[0004] In 2D sequences, the above problems can be mitigated by alternating slice selection inversion in one slice and imaging readout in the other slice. However, such an approach is not possible in the corresponding 3D image acquisition, and therefore would take too long for widespread clinical use, for example for musculoskeletal fluid detection. For example, studies have shown that imaging small anatomical structures, most often nerve targets, with reduced field of view and very short repetition times (TR), requires long scan times even when strong positive fluid contrast is not primarily required [3]. Nevertheless, 3D STIR sequences are considered useful for the evaluation of the musculoskeletal system, for example for the evaluation of inflammation around metal implants in the hip, knee or shoulder joints.

[0005] At present, no suitable solution exists: 2D STIR acquisitions typically take a relatively long time and have a lower spatial resolution than corresponding, e.g., proton density weighted (PDw) TSE scans, and 3D STIR sequences take a long time that is often inapplicable in standard clinical joint imaging and can only be used for specialized applications where detectable sensitivity of body fluids is not a primary objective, e.g., MR neurography (neurovisualization) of the cervical or lumbar plexus.

[0006] It has also been proposed to accelerate the recovery of longitudinal fluid magnetization using "non-driven equilibrium" DE, i.e., pulse sequence elements, to enhance the fluid signal in images acquired with short TR sequences without inversion recovery preparation. This technique was first performed in NMR spectroscopy [4]-[9] and has since been applied to MRI (see [9]). Such sequences containing DE pulse sequence elements are referred to by terms such as "fast recovery fast spin echo", "restore TSE", or "driven" sequences. The basic idea is summarized in Figure 3, where the upper sequence representation A shows a basic TSE sequence and the lower sequence representation B shows a basic TSE sequence with a subsequent "driven equilibrium" pulse sequence element.

[0007] The fate of fluid magnetization (long T1, long T2-T2, which are transverse relaxations), tissue magnetization (medium T1, medium T2, e.g. muscle tissue) and fat magnetization (short T1, long “TSE-” T2) during the course of each sequence is outlined under each sequence diagram. Before the application of the next excitation pulse, a significantly larger longitudinal fluid magnetization (time point 5 in the lower sequence representation B versus time point 4 in the upper sequence representation A) can be observed in the lower sequence representation B at the end of the TR period. This means that a larger signal will occur during the next TR period with the next TSE train. The magnitude enhancement is achieved by a driven counterbalance element that refocuses the remaining transverse magnetization at the end of the imaging echo train and converts it into longitudinal magnetization by a 90° “flip-back” pulse. The phase of the pulse here should be different (180° ("-x")) from the phase of the initial excitation pulse if the phase of the refocusing pulse differs by 90° ("+ / -y") from the phase of the initial excitation pulse ("+x"). The RF pulses applied between time 1 and time 4 in the lower sequence representation B can both be considered as a composite 0° pulse for on-resonance magnetization with sufficiently long relaxation times such as low viscosity body fluids. In the figure, the following abbreviations are used: Inv: inversion pulse, TI: inversion (recovery) time, Exc: excitation pulse, Ref: refocusing pulse, ES: echo interval, Acq: acquisition, ET: echo train, ETL: echo train length, eq: equilibrium, TF: turbo factor, n PhE : number of phase encoding steps, TR: repetition time.

[0008] In this case, examples of images from 3D acquisitions with and without the DE element are shown in Fig. 4, where in the left image A or right image B, respectively, 3D fast / turbo spin-echo images (TR / TE 211 / 60 ms) are shown acquired with and without the driven equilibrium pulse sequence element accompanying the imaging sequence. Despite the short TR time (211 ms) (for a fluid-sensitive spin-echo sequence), a stronger signal and a brighter appearance of CSF is obtained in the right image B (see arrow 41) compared to the left image A. It should also be noted that in the images obtained from these sequences, a bright subcutaneous fat signal can be observed without magnetization preparation. The images in Fig. 4 are taken from reference

[10] .

[0009] The situation is different in STIR imaging, which seeks to minimize fat signal and enhance magnetization with long T1 (and typically long T2) times, such as in low-viscosity body fluids. In this case, the imaging sequence is preceded by a 180° inversion pulse, followed by a recovery period TI. The recovery time is approximately 380-400 ms for a field strength of 3 Tesla, during which the fastest relaxing fat signal component T1 reaches its zero crossing TI. zc (Assuming that a complete 180° inversion pulse is applied when the longitudinal fat magnetization reaches its maximum value, TI zc = T1*ln(2)= T1*0.693). In practice, shorter TI times are often chosen in STIR imaging to aid image readout: (i) acquisition is faster and (ii) residual fat signal, e.g., in bone marrow, is included.

[0010] Since fluid magnetization has a significantly longer T1 time (in the range of an order of magnitude) than fat, if this fluid magnetization had a significant magnitude along the positive z-axis before the application of the inversion pulse, it will be aligned along the negative z-axis at the time the excitation pulse is applied. That is, a conventional DE element such as that shown in FIG. 3 will flip the fluid magnetization back to form a negative z-axis, which in turn will delay the recovery of the longitudinal fluid magnetization. Thus, the longitudinal fluid magnetization in dynamic equilibrium will be smaller and will be converted into an attenuated fluid signal instead of an enhanced signal.

[0011] This is shown in more detail in FIG. 5, where a basic STIR-TSE sequence with unmodified or prior art "driven balance" pulse sequence elements is shown in schematic form similar to the lower sequence representation B of FIG. 3. Since the fluid magnetization is aligned along the negative z-axis before the excitation pulse (time 3), and the RF pulse between time 3 and time 6 acts as a composite 0° pulse, the fluid magnetization will also be aligned with the negative z-axis at time 6, far away from its fully relaxed maximum thermodynamic equilibrium value. In the case of fluid magnetization, the RF pulse applied between time 1 and time 6 approaches a composite 180° rotation, which, if repeated in succession with a sufficiently short TR, reduces the magnitude of the fluid magnetization in dynamic equilibrium. Instead of enhancing the fluid signal, the unmodified driven balance elements actually reduce the strength of the fluid signal, as shown in FIG. 5.

[0012] Although the above introduction focuses on the problem of fluid-sensitive imaging, the present invention is in a more general context and aims to find a system and method that can improve image contrast for a magnetization of interest while maintaining suitability for clinical applications. In particular, the system and method according to the invention can provide fluid-sensitive MRI images with improved contrast, preferably with attenuation or suppression of fat signals, suitable for clinical applications. By suitable for clinical applications, it is to be understood that the above system and method can reduce imaging time while maintaining high signal strength for the magnetization of interest (e.g. fluid magnetization).

[0013] The above mentioned object is achieved according to the invention by a system and method for improving image contrast in fast driven balanced inversion recovery imaging, in particular in multi-echo imaging, in accordance with the objects of the independent claims. The respective dependent claims present further advantages of the invention.

[0014] The invention relates to an MRI method for improving an MRI signal for a magnetization of interest, i.e. for improving the image contrast for said magnetization of interest, when imaging an object placed in an examination volume of an MRI system, said method comprising the following steps: - performing, by the MRI system, an MRI pulse sequence comprising three successive radio frequency ("RF") elements, a first element being an inversion recovery pulse sequence characterized by an inversion (recovery) time TI, and a second element being an excitation RF pulse followed by at least one image encoding gradient and at least one data sampling starting at the inversion time TI. In particular, the data sampling comprises radial sampling of the MRI signal at an ultrashort echo time. Alternatively, the data sampling is a sampling of the MRI signal occurring during the formation of one or more gradient echoes, or during the formation of one or more spin echoes, or during the formation of a combination of gradient echoes and spin echoes. For example, the excitation RF pulse can be followed by a refocusing RF pulse, an imaging encoding gradient and a data sampling. According to another example, the excitation RF pulse can be followed by a train of refocusing RF pulses, an imaging encoding gradient and a data sampling. The second element is then followed by a third element, which is a modified driven-equilibrium (hereinafter "mDE") pulse sequence configured to achieve conversion of the transverse magnetization component of the magnetization of interest to a positive longitudinal magnetization aligned with the positive z-axis and B0 field, which, in contrast, is converted to a negative longitudinal magnetization aligned along the opposite negative z-axis with respect to the B0 field direction by application of a transverse driven-equilibrium pulse sequence. In particular, the mDE pulse sequence according to the present invention is terminated by a phase-shifted flip-back pulse characterized by a 90° flip angle relative to the refocusing pulse (+y), preferably a 90° phase shift (+x), or a non-phase shift relative to the excitation RF pulse (+x) (i.e., the same phase as the excitation RF pulse (+x)), or a 180° phase shift relative to the prior art DE flip-back pulse (+x), where execution of said MRI pulse sequence is cyclically repeated by the MRI system according to a repetition time TR.In other words, the mDE pulse sequence according to the invention is a phase-shifted DE ("phase-shifted" compared to prior art DE) pulse sequence configured to convert the residual transverse magnetization components of the magnetization of interest, in particular the fluid magnetization, into positive longitudinal magnetization in order to increase the longitudinal magnetization before the next said MRI pulse sequence is initiated and applied, i.e. after the 180° inversion pulse of the inversion recovery pulse sequence, an inversion recovery pulse sequence configured to exhibit a 180° phase shift compared to the situation without the inversion recovery pulse sequence. Preferably, the MRI pulse sequence according to the invention is a modified driven equilibrium pulse sequence element comprising a STIR-TSE or STIR-SE sequence followed by said phase-shifted flip-back pulse. Advantageously, by applying an mDE pulse sequence after the second element of the MRI pulse sequence, as compared to applying a DE pulse sequence known in the art after the second element (whereby the MRI pulse sequence acts as a 180° composite pulse), the mDE pulse sequence acts as a composite 0° pulse on the longitudinal magnetization of interest (i.e., the longitudinal magnetization component of the magnetization of interest), e.g., longitudinal fluid magnetization. - further comprising a step of acquiring, by the MRI system, MRI signals applied to the object by the MRI system during each repetition time TR, generated by the object and sampled by an image readout block. It is to be particularly understood that by acquiring MRI signals, MRI signal data sampled by the image readout block are collected for image reconstruction. The MRI signals generated by the object include in particular MRI signals generated by the magnetization of interest. - further comprising the step of reconstructing an image of said object from said MRI signals.

[0015] The invention also relates to an MRI system or device adapted for imaging a biological object and adapted to carry out the steps of the method described above.

[0016] The foregoing has outlined rather broadly the features and technical advantages of the present disclosure so that those skilled in the art may better understand the detailed description that follows.

[0017] Additional features and advantages of the disclosure which form the subject of the claims will be described hereinafter. Those skilled in the art will appreciate that the conception and specific embodiments disclosed may be readily used as a basis for modifying or designing other structures for carrying out the same purposes of the present disclosure.

[0018] For a more complete understanding of the present disclosure and its advantages, reference is now made to the following description taken in conjunction with the accompanying drawings, in which like reference characters represent like objects, and in which: [Brief description of the drawings]

[0019] [Figure 1] 1 is a schematic diagram of an MRI system according to the present invention; [Diagram 2] 2 is a flow chart illustrating a method according to the present invention. [Diagram 3] FIG. 1 is a schematic diagram showing a basic TSE sequence (upper sequence representation A) and a basic TSE sequence with a "driven balanced" pulse sequence element (lower sequence representation B). [Figure 4] FIG. 1 shows a 3D fast / turbo spin echo image (TR / TE 211 / 60ms) acquired without a driven balanced pulse sequence element (left image A) and with a driven balanced pulse sequence element associated with the imaging sequence (right image B). [Diagram 5] FIG. 4 is a schematic diagram showing a basic STIR-TSE sequence with unadjusted “driven balanced” pulse sequence elements as shown in FIG. 3B. [Figure 6A] FIG. 1 is a schematic diagram illustrating a preferred embodiment of an MRI pulse sequence according to the present invention. [Figure 6B]FIG. 6B is a schematic diagram showing the fate of fluid magnetization (dotted arrows), tissue magnetization (straight arrows), and fat magnetization (dotted arrows) at times t1-t7 in FIG. 6A. [Figure 7] FIG. 13 shows a comparison of 2D STIR images acquired without DE, with DE and with and without the method according to the invention. [Figure 8] FIG. 2 shows a comparison of 3D STIR images acquired with and without the method according to the invention. [Figure 9] FIG. 2 is a schematic diagram illustrating another preferred embodiment of an MRI pulse sequence according to the present invention.

[0020] 1 shows a preferred embodiment of an MRI system 100 according to the invention, i.e. an MRI system 100 capable of improving image contrast in fast driven balanced inversion recovery multi-echo imaging, in particular improving fat signal suppression, when implementing the method according to the invention. The MRI system 100 comprises, in particular, various coils 101 and respective coil controllers configured to generate magnetic fields and RF pulses for acquiring MRI signals from a biological object 106. For example, the MRI system 100 may include: an axial magnetic field coil controller for controlling an axial magnetic field coil and an axial magnetic field B0 generated by said axial magnetic field coil; a gradient coil controller configured to control a gradient coil and a magnetic field gradient G generated by said gradient coil; an RF coil controller configured to control an RF coil and an RF magnetic field B1 generated by said RF coil; a receiving coil, which may be the same as the RF coil, configured to detect changes in the magnetization of a biological object 106 to be imaged and to communicate said changes to a receiving coil controller, a receiving coil controller, which may be the same as the RF coil controller, for outputting MRI signals, and a processing unit 102, comprising a processor and a memory 103, for processing the output MRI signals in order to reconstruct an image of said biological object; a display 104 for displaying the reconstructed image; Equipped with.

[0021] Unlike the prior art MRI systems having the MRI pulse sequences shown in Figures 3 and 5 and already discussed in the introduction of the invention, the MRI system 100 according to the invention is configured to perform a novel pulse sequence. A preferred embodiment of said novel pulse sequence is shown diagrammatically in Figure 6A and further characterized in Figure 6B. Another preferred embodiment of said novel pulse sequence is also shown diagrammatically in Figure 9, where the fate of fluid, tissue and fat magnetization at times t1 to t7 is further characterized by Figure 6B. To perform the method according to the invention, the MRI system 100 may include a non-transitory computer-readable medium, e.g. a memory, encoded with instructions for cooperating an axial magnetic field coil controller, a gradient coil controller, an RF coil controller and a receive coil controller when executed to realize a method for imaging an object 106 according to the invention. The method, now described in more detail with reference to the preferred embodiment of Figures 6A and 6B, comprises in particular the following steps:

[0022] The MRI system 100, e.g. its RF coil controller, is configured to execute an MRI pulse sequence 600 in step 201, in which a pulse train repeated for each TR period mimics a composite 0° pulse for the magnetization of interest, typically a fluid magnetization with T1 and T2 values. Said MRI pulse sequence 600, which is represented diagrammatically in FIG. 6A for the case of a spin echo sequence and in FIG. 9 for the case of a gradient echo sequence, comprises an RF pulse train as a function of time. According to the invention, the MRI pulse sequence 600 comprises three consecutive RF elements, namely a first element 610, a second element 620 and a third element 630. The first element 610 is an inversion recovery pulse sequence, e.g. a STIR pulse sequence. It comprises a 180° inversion pulse INV. The second element 620 is an image encoding pulse sequence, e.g. a TSE pulse sequence. It starts with an excitation RF pulse EXC. The time separating the 180° inversion pulse INV and the excitation RF pulse EXC (typically a 90° RF pulse) is the inversion time TI. In the case of FIG. 6A, the excitation RF pulse EXC is then followed by a refocusing RF pulse REF, an imaging encoding gradient and a train of data sampling, as known from the prior art. The echo interval ES represents the time between echoes in the multiple echo sequence of the second element 620, ES / 2 (i.e. 1 / 2 of the echo interval time) is the time separating the excitation RF pulse EXC from the refocusing RF pulse REF, the echo train length ETL or turbo factor TF represents the number of echoes in the second element 620, and the number of phase encoding steps is N PhE In particular, according to the present invention, the minimum number of echoes (ETL or TF) is equal to 1. The time interval between each repetition of the MRI pulse sequence 600 is the repetition time TR. The signal acquisition ACQ as a function of time is also shown diagrammatically in FIG. 6A, where the peak of the stimulated signal S occurs at a time ES / 2 after the refocusing RF pulse REF.

[0023] Unlike existing DE modules, the third element 630 is an mDE pulse sequence module in that it ends with a +90° flip-back pulse 632 along the +x-axis. In particular, the mDE pulse sequence module includes a first RF pulse 631 applied at a time ΔT after the last echo signal of the echo train (i.e., the time delay or period separating the first RF pulse 631 and the last echo signal of the echo train is equal to ΔT; note that this is different from the gradient echo case presented in FIG. 9, as will be described later), and the +90° flip-back pulse 632 is applied at a time ΔT after the 180° RF pulse 631 (i.e., the time delay or period separating the first RF pulse 631 and the +90° flip-back pulse 632 is equal to ΔT), i.e., when a spin echo or RF echo of the magnetization of interest occurs, to flip back the magnetization of interest to a positive longitudinal orientation along the positive z-axis. In particular, the time period ΔT is minimized, i.e., preferably tends to ½ES, and more preferably has a minimum value equal to ½ES (i.e., ΔT=½ES). This is better explained by considering FIG 6B, which shows the fate of fluid magnetization (long T1, long T2), tissue magnetization (medium T1, medium T2, e.g., muscle tissue), and fat magnetization (short T1, long “TSE T2”) throughout the course of the MRI pulse sequence 600, i.e., as a function of time points t1-t7 in FIG 6A.

[0024] As will be understood by those skilled in the art, in the present invention, the phase difference between the excitation RF pulse EXC, the refocusing RF pulse REF, and the flip-back pulse 632 is significant. In particular, as long as there is a 90° phase difference between all of the excitation RF pulse EXC(x) and the refocusing RF pulse REF(+ / -y), a 90° flip-back pulse having the same phase as the excitation RF pulse EXC (i.e., +x) will be used in the mDE pulse sequence, whereas in conventional DE, a 180° phase difference between the 90° DE pulse and the 90° RF pulse is used. For example, if the 90° excitation RF pulse has a 180° (-x) phase and the refocusing 180° RF pulse REF has a 90° (+y) phase, the flip-back pulse must be a 90° pulse having a 180° (-x) phase.

[0025] In FIG. 6B, the temporal evolution of the components of fluid magnetization (dotted arrows), tissue magnetization (solid arrows) and fat magnetization (dotted arrows) is represented diagrammatically with respect to a Cartesian coordinate system with reference axes (XYZ), the z-axis being aligned with the main static magnetic field B0, said magnetization components being represented for seven successive different time points t1 to t7 shown in FIG. 6A.

[0026] At time t1, i.e. just before the application of the inversion pulse INV, the fluid, fat and tissue magnetization components are aligned along the z-direction, i.e. parallel to the main static magnetic field B0. Just before the application of the first inversion pulse in the first TR period, the initial longitudinal magnetization corresponds to the magnetization (M0) of all biological materials of the object 106 in thermodynamic equilibrium. In subsequent TR periods, depending on the value of TR related to the T1 time of the material, the magnitude of these magnetizations may decrease in subsequent TR periods until reaching a steady value in the later TR period. However, at time t1, the longitudinal magnetization component remains the magnetization of interest.

[0027] At time t2, i.e. immediately after the inversion pulse INV is applied, the longitudinal magnetization of the fluids, tissues and fat, i.e. the initial magnetization, is flipped according to an angle of 180° so as to point in the opposite direction to the direction of the main magnetic field B0, thereby aligning it with the negative z-axis.

[0028] At time t3, i.e., at the end of the inversion time TI but before the excitation RF pulse EXC is applied, the tissue, fluid and fat magnetization components are still aligned with the negative z-axis but decrease at their own rates according to T1 relaxation, resulting in tissue magnetization being greater than fat magnetization and fluid magnetization being greater than tissue magnetization. Application of the excitation pulse EXC flips the longitudinal magnetization into the XY plane, resulting in transverse magnetization.

[0029] At time t4, the transverse magnetization is shown diagrammatically for fluid, tissue and fat. As depicted in the graph, fat magnetization is very small or zero.

[0030] During the echo train, T2 relaxation occurs, which is shown diagrammatically by the graph at t5, i.e. immediately after the last echo of the echo train at that time, where, depending on the echo train length in relation to the T2 time of the tissue, the fluid magnetization has a maximum magnitude, while the transverse tissue magnetization is significantly attenuated and the fat magnetization has an amplitude of nearly zero.

[0031] The application of the +90° flip-back pulse 632 according to the invention, which has the same phase as the excitation pulse EXC, results in the transformation of the transverse magnetization (corresponding to the graph at t5) into a positive longitudinal magnetization, as shown in the graph at t6, aligned with B0. Thus, the mDE element generates a longitudinal fluid magnetization at time t6, so that after relaxation between times t6 and t7, at time t7 it has a larger magnitude than it would have if the mDE element were not present. This is clearly shown by comparing the magnetization at time t6 in FIG. 5 with the magnetization at time t6 in FIG. 6B, where the +90° flip-back pulse 632 can transform the transverse magnetization into a positive longitudinal magnetization, whereas in the conventional DE shown in FIG. 5, said magnetization is transformed into a negative longitudinal magnetization (negative z-axis), resulting in an increased time for the longitudinal magnetization associated with the signal of interest to reach a positive minimum value that allows the generation of a minimum signal during the next pulse sequence. Conversely, mDE according to the present invention can reduce the minimum recovery time required for the longitudinal magnetization to reach a minimum magnitude, thereby enabling the generation of a minimum signal during the next pulse sequence shot. Thus, mDE according to the present invention can increase the signal from the tissue of interest without decreasing it. For example, said mDE can ensure that the fluid magnetization is flipped back to the positive z-axis at the end of the STIR-prepared turbo spin echo train, thus providing a good starting point for recovery.

[0032] Another preferred embodiment of the novel pulse sequence according to the invention is shown in FIG. 9. In the case of FIG. 9, the spin echo sequence appearing after the excitation RF pulse EXC in the second element 620 shown in FIG. 6A is replaced by a gradient echo sequence. As already mentioned, the invention works for echo sequences with a single (spin or gradient) echo as well as echo sequences with multiple (spin and / or gradient) echoes. In FIG. 9, the case of multiple gradient echoes included in the second element 620 is shown, where a third element 630 is configured to refocus the transverse magnetization and drive its equilibrium. As shown in FIG. 9, a first 180° RF pulse 631 is applied at a time ΔT′ after the excitation RF pulse EXC, and then a +90° flip-back pulse 632 is applied at a time ΔT′ after the first 180° RF pulse 631, similar to the case of the spin echo sequence in FIG. 6A. In other words, after the last induction signal S (see signal acquisition ACQ in Figure 9) has been acquired, the third module 630 starts with the first RF pulse 631, which is at the midpoint in time of the period separating the time when the 90° flip-back pulse 632 is applied and the last time when the magnetization (component) of interest is refocused, which corresponds to time t4 in the case of a gradient echo sequence (corresponding to time t5 shown in Figure 6A).

[0033] The MRI system 100 is configured to acquire MRI signals generated by the object 106, step 202. As is known in the art, the MRI signals may be applied to the object 106 by the MRI system 100 during each repetition time TR and sampled by an image readout block.

[0034] The MRI system 100 or computing system is configured to reconstruct an image of the object 106 from the MRI signals in step 203, where MRI signal data is collected for the image reconstruction, typically by repeating steps 201 and 202 in a loop. In practice, as known in the art, the MRI system 100 or computing system may be configured to repeat steps 201 and 202 for a selected number of TR periods such that a number of MRI signals are acquired and then available for use in the image reconstruction of the object 106 by the MRI system 100 or computing system.

[0035] The advantages of the present invention are illustrated in Figures 7 and 8, in which an example of the fluid signal enhancement effect of the MRI pulse sequence 600 according to the present invention is shown in the specific case of 2D STIR imaging and 3D STIR imaging in the lumbar spine. 2D STIR images of the lumbar spine (see Figure 7) were acquired at 3 Tesla with a significantly short TR time of 2340 ms without a DE flip-back pulse (A), with a conventional flip-back pulse optimized for a non-magnetization prepared sequence (B), and with mDE according to the present invention (C). It can be clearly observed that the signal intensity and signal brightness of the CSF in the spinal canal is relatively low in image (A), and that in image (B) these signal intensity and signal brightness are further reduced, whereas in image (C) they are clearly higher. In particular, to observe at least 95% of the maximum fluid signal in STIR imaging without driven balance, a TR time at least three times longer than the T1 time of the fluid magnetization, i.e., certainly longer than about 3.5 seconds, is desirable, which is often too long for clinical applications and a compromise between smaller signal and shorter TR time is often required.

[0036] 8 shows 3D STIR images of the lumbar spine acquired at 3 Tesla without a driven balanced flip-back pulse (A) and with mDE according to the invention (B) at an even shorter TR time of 1500 ms. Again, a clearly higher CSF signal intensity can be observed in image (B) compared to image (A).

[0037] In conclusion, the method according to the invention allows for an improvement of the MRI signal from the magnetization of interest, i.e. the MRI signal generated by the magnetization of interest. To achieve this improvement, a new MRI pulse sequence 600 including mDE is proposed, which is particularly suitable for fluid imaging, whereby a clear enhancement of the fluid signal is obtained even when using short TR times to accelerate image acquisition. This is particularly beneficial for 3D STIR imaging, where alternating inversion preparation and data acquisition TSE pulse trains is not possible in 2D STIR imaging. Moreover, the invention allows for faster 2D STIR imaging, while still providing good fluid sensitivity, especially when a small number of 2D slices are measured. 3D STIR sequences are particularly useful for the evaluation of the musculoskeletal system, for example for the evaluation of inflammation around metal implants in the hip, knee or shoulder joint.

[0038] References list [1]Bydder, GM and IR Young, MR imaging: clinical use of the inversion recovery sequence, J Comput Assist Tomogr, 1985. 9(4): p.659-675. [2]Bydder, GM, JV Hajnal, and IR Young, MRI: use of the inversion recovery pulse sequence. Clin Radiol, 1998. 53(3): p.159-176. [3]Pravata, E., et al., Dedicated 3D-T2-STIR-ZOOMit Imaging Improves Demyelinating Lesion Detection in the Anterior Visual Pathways of Patients with Multiple Sclerosis. American Journal of Neuroradiology, 2021. 42(6): p.1061. [4]Becker, E.D., J. A. Feretti, and T.C. Farrar, Driven Equilibrium Fourier Transform Spectroscopy. A New Method for Nuclear Magnetic Resonance Signal Enhancement. Journal of the American Chemical Society, 1969. 91(27): p.7784-7785. [5]Waugh, J.S., Sensitivity in Fourier transform NMR spectroscopy of slowly relaxing systems. Journal of Molecular Spectroscopy, 1970. 35(2): p.298-305. [6]Waldstein, P. and W.E.W. Jr., Driven Equilibrium Methods for Enhancement of Nuclear Transients. Review of Scientific Instruments, 1971. 42(4): p.437-440. [7]Wallace, W.E., Theory and Optimization of the Pulsed NMR Driven Equilibrium Technique. The Journal of Chemical Physics, 1971. 54(3): p.1425-1427. [8]Jones, D.E., Fourier transform nuclear magnetic resonance. II. Driven equilibrium fourier transform and spin-echo fourier transform. Journal of Magnetic Resonance (1969), 1972. 6(2): p.183-190. [9]Van Uijen, C.M.J. and J.H. Den Boef, Driven-equilibrium radiofrequency pulses in NMR imaging. Magnetic Resonance in Medicine, 1984. 1(4): p.502-507.

[10] Elias R. Melhem, Ryuta Itoh, and Paul J. M. Folkers, Cervical spine: three-dimensional fast spin-echo MR imaging-improved recovery of longitudinal magnetization with driven equilibrium pulse. Radiology, 2001. 218(1): p.283-8.

Claims

1. 1. A magnetic resonance imaging (hereinafter "MRI") method for improving an MRI signal produced by a magnetization of interest when imaging an object (106) disposed within an examination volume of an MRI system (100), the method comprising: - executing (201) with said MRI system (100) an MRI pulse sequence (600) comprising three successive radio frequency (hereinafter "RF") elements, namely a first element (610) which is an inversion recovery pulse sequence characterized by an inversion time (TI), a second element (620) which is an image encoding pulse sequence starting at said inversion time (TI) having an excitation RF pulse (EXC) followed by at least one image encoding gradient and at least one data sampling, and a third element (630) which follows said second element, wherein at the end of the inversion time (TI) but before the excitation RF pulse (EXC) is applied, said magnetization of interest is still aligned with the negative z-axis; - acquiring (202) by the MRI system (100) MRI signals applied by the MRI system (100) to the object (106) during each repetition time (TR), generated by the object (106) and sampled by an image readout block; - reconstructing (203) an image of the object (106) from the MRI signals; In a method of MRI comprising: The third element (630) is a modified driven equilibrium (hereinafter "mDE") pulse sequence configured to achieve a transformation of the transverse magnetization component of the magnetization of interest into a positive longitudinal magnetization aligned with the positive z-axis and B0 field directions.

1. An MRI method comprising:

2. The MRI method of claim 1 , wherein the mDE pulse sequence terminates with a phase-shifted flip-back pulse (632) characterized by a +90° flip angle.

3. 3. The MRI method of claim 2, wherein the phase-shifted flip-back pulse (632) is characterized by a 90° phase shift (+x) relative to a refocusing RF pulse (+y), or is characterized by a no phase shift relative to the excitation RF pulse (+x), or is characterized by a 180° phase shift relative to a flip-back pulse (-x) of a driven equilibrium pulse sequence (DE).

4. The MRI method of claim 1 , wherein the first element is a STIR pulse sequence.

5. The MRI method of claim 1 , wherein the second element (620) includes single or multiple data sampling.

6. The MRI method of claim 1 , wherein the second element (620) is a sequence configured to generate one or more gradient echoes or spin echoes.

7. The MRI method of claim 1 , wherein the second element (620) is a sequence configured to generate a combination of gradient echo and spin echo.

8. The MRI method of claim 1 , wherein the second element is a TSE pulse sequence.

9. 3. The MRI method of claim 2, wherein the mDE pulse sequence includes a first RF pulse (631) that is midway in time between the time interval separating the time at which a +90° flip-back pulse (632) is applied and the last time at which magnetization is refocused.

10. The mDE pulse sequence includes a first RF pulse (631) that is a 180° RF pulse applied at a time equal to ½ the echo interval (ES) after the last echo signal of the echo train; The +90° flip-back pulse (632) is applied at a time equal to ½ the echo interval (ES) after the first RF pulse (631).

10. The MRI method according to claim 9.

11. An MRI system (100) configured to perform an MRI method according to any one of claims 1 to 10.

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