MR imaging with fat suppression by using selective inversion

Abstract

The invention relates to magnetic resonance (MR) imaging, comprising subjecting a portion of a subject (10) to a preparation sequence (PRE) comprising a spectrally selective adiabatic inversion radio frequency (RF) pulse (21) with a flip angle (a) to selectively invert nuclear magnetization associated with fat protons; subjecting the portion to an imaging sequence (IM) comprising an excitation RF pulse (28) to generate MR signals (27) after a delay time (T D ) determined such that a first spectral species of fat protons affected by the inversion RF pulse and a second spectral species of fat protons not affected by the inversion RF pulse contribute substantially to cancel each other out to the MR signals; acquiring the MR signals; and reconstructing a MR image from the MR signals, wherein contributions from both the first spectral species and the second spectral species of fat protons are suppressed in the MR image.

Description

Technical Field

[0001] This invention relates to the field of magnetic resonance (MR) imaging. Specifically, it relates to a method for performing MR imaging on an object placed in a main magnetic field B0. The invention also relates to an MR system and a computer program to run on the MR system. Background Technology

[0002] Currently, imaging MR methods that utilize the interaction between magnetic fields and nuclear spins to form two-dimensional or three-dimensional images are widely used, especially in the field of medical diagnostics, because they are superior to other imaging methods in many aspects of imaging soft tissues, as they do not require ionizing radiation and are generally non-invasive.

[0003] A key challenge in MR imaging is tissue differentiation, particularly when distinguishing pathological tissue from surrounding fat, which can obscure crucial details. To address this, dedicated MR imaging techniques for fat suppression have been developed, among which spectral presaturation with inversion recovery (SPIR) and spectral attenuation inversion recovery (SPAIR) are prominent. These techniques enhance tissue contrast by selectively suppressing the contribution of fat protons to the generated and acquired MR signals.

[0004] SPIR is a technique that combines spectral presaturation with inversion recovery to suppress fat signals in MR images. SPIR is performed by first applying a spectrally selectively inverted radio frequency (RF) pulse specifically tuned to the resonant frequency of fat protons. This RF pulse achieves a selective partial inversion of the nuclear magnetization associated with fat protons without significantly affecting the nuclear magnetization of water protons or other tissues. The spectrally selectively inverted RF pulse has a flip angle that determines the degree of magnetization inversion. This is followed by a delay time, allowing the nuclear magnetization of fat protons to recover to its equilibrium state. By carefully determining the flip angle and delay time, the start of the subsequent imaging sequence is made to coincide with the zero point of fat nuclear magnetization (i.e., the moment when the longitudinal nuclear magnetization of fat protons is zero), thus minimizing the contribution of the MR signal from fat protons in the resulting MR image. Contributions from other tissues to the MR signal are not selectively inverted and therefore are not suppressed in the MR image. Their respective longitudinal nuclear magnetizations are in equilibrium at the start of the imaging sequence. SPIR is particularly useful in regions where fat and other tissues are closely mixed, such as in the breast, liver, and musculoskeletal system, providing images more suitable for diagnostic purposes.

[0005] SPAIR is an improvement on the aforementioned SPIR technique. Similar to SPIR, SPAIR involves applying a spectrally selective RF inversion pulse. However, it is designed to completely invert the magnetization of fat (i.e., the RF inversion pulse has a flip angle of 180°). SPAIR employs an adiabatic inversion RF pulse that completely inverts the magnetization of fat even when the B1 field is spatially inhomogeneous, making the technique less sensitive to variations in B1. SPAIR is highly valuable in abdominal, pelvic, and orthopedic imaging, where consistent fat suppression across a large field of view is crucial for accurate diagnosis.

[0006] By selectively targeting and inhibiting MR signals from fat protons while preserving or enhancing MR signals from other tissues, SPIR and SPAIR greatly aid in the diagnosis and assessment of a wide range of medical conditions, from detecting tumors and lesions to evaluating musculoskeletal injuries.

[0007] Chemical shift-selective fat suppression methods like SPIR and SPAIR are effective in eliminating the dominant spectral component of the fat signal, which originates from a large number of methylene protons, as well as other (aliphatic) fatty protons with chemical shifts sufficiently distant from the water proton resonance. However, a significant fat signal also exists due to olefin protons (which constitute 5-10% of all fatty protons). The chemical shifts of olefin protons are close to those of water protons. Therefore, known SPIR and SPAIR techniques only partially suppress the MR signal contribution from fat. Residual contributions from adipose tissue in the reconstructed MR images can still obscure critical details, thus impairing medical diagnosis.

[0008] US 2011 / 267060 A1 discloses a magnetic resonance imaging (MRI) apparatus including a magnetic field generation module and a control module for controlling a receiving module according to a predetermined pulse sequence, the predetermined pulse sequence including an unnecessary material suppression sequence unit for eliminating signals from unnecessary material (which is not the measurement target) and a main imaging sequence unit for measuring MRI signals for creating an image of a subject. The unnecessary material suppression sequence unit generates at least two or more high-frequency magnetic field pulses such that the longitudinal magnetization of the unnecessary material becomes spatially uniform in the imaging space upon application of a first high-frequency magnetic field pulse in the main imaging sequence unit. The MRI apparatus further includes: an adjustment module for adjusting the flip angle of the two or more high-frequency magnetic field pulses; and a control module for applying the two or more high-frequency magnetic field pulses at the flip angle adjusted by the adjustment module.

[0009] The paper "Optimization of chemical shift selective suppression of fat" by Kuroda K. et al. (published in *Magnetic Resonance in Medicine*, Vol. 40, No. 4, 1998, pp. 505-510) discloses a strategy for optimizing the flip angle of chemical shift selective fat suppression. This involves mathematical models for fat suppression in perturbed gradient echo acquisition, spin echo, and RARE, which incorporate steady-state conditions and multiple spectral components of fat. It has been found that the optimal suppression flip angle is more than 10° larger than that determined using a single fat component model, due to contributions from unflipped components located outside the suppression band (e.g., olefin protons and glycerol protons). Summary of the Invention

[0010] As can be readily understood from the foregoing, improved MR imaging techniques are needed. Therefore, the object of this invention is to achieve MR imaging with more complete fat signal suppression.

[0011] According to the present invention, a method for performing MR imaging on an object placed in a main magnetic field B0 is disclosed.

[0012] The method of the present invention includes: The object is subjected to a preparatory sequence of spectrally selectively reversed RF pulses with a flip angle (α) to selectively reverse the nuclear magnetization associated with fatty protons within the object; After a delay time, the object is subjected to an imaging sequence comprising at least one excitation RF pulse and a switched magnetic field gradient to generate an MR signal, wherein the delay time and / or flip angle are determined such that the contributions of a first spectral species of fatty protons affected by the reverse RF pulse and a second spectral species of fatty protons unaffected by the reverse RF pulse to the MR signal substantially cancel each other out. Acquiring MR signals from the object; and MR images are reconstructed from the acquired MR signals, or in other words, MR images are generated, in which the contributions of both the first and second spectra of fatty protons are suppressed in the MR images due to selective inversion caused by the delay time and / or flip angle determined by the preparation sequence.

[0013] The method of this invention employs a preparation sequence including spectrally selectively reversed RF pulses to at least partially reverse the nuclear magnetization associated with fatty protons according to the flip angle. Optionally, the reversed RF pulses are followed by a perturbation magnetic field gradient pulse to dephase any transverse magnetization.

[0014] In conventional SPIR and SPAIR techniques as described above, the delay time and / or flip angle are selected to zero the longitudinal NMR of fatty protons affected by the spectrally selectively reversed RF pulse. This invention differs from this conventional method by determining the delay time and / or flip angle in such a way that the contributions of the first spectral species (primarily derived from methylene protons) of fatty protons reversed by the reversed RF pulse and the second spectral species (primarily derived from olefin protons) of fatty protons unaffected by the reversed RF pulse to the MR signal substantially cancel each other out at the moment when the delay time (the duration of the time interval between the reversed RF pulse and the start of the imaging sequence) has elapsed. According to this invention, the term "substantially cancels each other out" should be understood as: the difference between the longitudinal NMR of the first and second spectral species of fatty protons is less than 5% of the portion of the NMR associated with fatty protons that has been reversed by the spectrally selectively reversed RF pulse, preferably less than 2.5%, more preferably less than 1%, and most preferably less than 0.5%.

[0015] It is important to note that the expression "affected by the inverted RF pulse" used in this paper refers to the nuclear magnetization of the first spectral species of fatty protons undergoing a flip angle, which largely corresponds to the nominal angle α of the inverted RF pulse. The expression "unaffected by the inverted RF pulse" does not preclude any influence of the inverted RF pulse on the nuclear magnetization of the second spectral species of fatty protons. Depending on the spectral characteristics of the inverted RF pulse, in practice, some residual influence on the nuclear magnetization of protons whose resonances lie outside the designed bandwidth of the inverted RF pulse cannot be completely avoided. At least as a good approximation, the effective flip angle of the inverted RF pulse used for the nuclear magnetization of the second spectral species of fatty protons is close to zero.

[0016] Therefore, this invention achieves complete suppression of signal contributions from fat by employing the basic principles of known SPIR and SPAIR techniques. However, this invention proposes integrating the multi-peak spectrum of fat protons into the determination of the flip angle and / or delay time to balance the inverted and non-inverted signal components from fat protons in subsequent imaging sequences, such that they cancel each other out due to their opposite phase. In principle, any conventional imaging sequence can be used to image water protons (unaffected by the preparation sequence).

[0017] In a possible embodiment, the preparation sequence uses an adiabatic inversion RF pulse. This corresponds to the SPAIR technique. The adiabatic inversion RF pulse is designed to completely invert the magnetization of fat, i.e., the first spectral species of fat protons falling within the bandwidth of the spectrally selective inversion RF pulse. Therefore, the flip angle of the RF inversion pulse is fixed at 180°. According to the method of the invention, only the delay time can be used as a tuning parameter to achieve mutual cancellation of the signal contribution from the first spectral species and the signal contribution from the second spectral species.

[0018] The first spectrum species may include at least one of the following: a methyl proton resonating at -3.8 ppm relative to a water proton, a methylene proton resonating at -3.4 ppm relative to a water proton, a β-carboxyl proton resonating at -3.1 ppm relative to a water proton, an α-carboxyl proton resonating at -2.5 ppm relative to a water proton, an α-olefin proton resonating at -2.7 ppm relative to a water proton, and a diacyl proton resonating at -1.9 ppm relative to a water proton.

[0019] The second spectrum species may include at least one of the following: olefin protons resonating at +0.6 ppm relative to water protons, and glycerol protons resonating at +0.5 ppm and -0.5 ppm relative to water protons.

[0020] The flip angle α of the spectrally selectively reversed RF pulse determines the degree of magnetization reversal, i.e., the amount of negative longitudinal magnetization associated with the first spectral species of adipose protons after the reversed RF pulse. The nuclear magnetization of water protons and other adipose protons (second spectral species) (mainly olefin protons with chemical shifts close to those of water protons) is unaffected by the reversed RF pulse.

[0021] To suppress the fatty signal as completely as possible, it may be useful, optionally, to consider the different effective flip angles associated with the individual spectral peaks of the first and second spectral species of fatty protons when determining the delay time and / or flip angle of the inverted RF pulse. The respective effective flip angles of the protons of the first and second spectral species are related to the nominal flip angle by the spectral profile of the spectrally selective inverted RF pulse; in other words, since the inverted RF pulse is spectrally selective, the effective flip angles of the different proton species vary due to their different resonant frequencies.

[0022] In one embodiment, the different longitudinal relaxation times of the first and second spectral species are considered when determining the delay time and / or flip angle. Protons of the first and second spectral species undergo longitudinal relaxation during the delay time. To achieve the desired balance between these two spectral species at the start of the imaging sequence, the possible different longitudinal relaxation times of the protons of these two spectral species should be considered.

[0023] In some embodiments, the imaging sequence is a spin echo sequence. In this case, particularly with long echo times, the different transverse relaxation times of the protons of the two spectral species become correlated and may interfere with the desired balance. This can lead to incomplete fatty suppression. This can be compensated for by also considering the echo times of the spin echo sequence and the different transverse relaxation times of the protons of the first and second spectral species when determining the delay time and / or flip angle.

[0024] In other embodiments, the imaging sequence is a gradient echo sequence. In this case, the echo time of the gradient echo sequence and the phase relationship between protons of the first spectral species and protons of the second spectral species can be considered when determining the delay time and / or flip angle in order to achieve optimized fat suppression.

[0025] The method of the present invention described herein can be performed using an MR system comprising: at least one main magnet coil for generating a uniform, stable magnetic field B0 within an examination volume; a plurality of gradient coils for generating switched magnetic field gradients in different spatial directions within the examination volume; at least one whole-body RF coil for generating RF pulses within the examination volume and / or for receiving MR signals from the body of a patient located within the examination volume; a control unit for controlling the time series of the RF pulses and the switched magnetic field gradients; and a reconstruction unit for reconstructing MR images from the received MR signals. The method of the present invention can be implemented by correspondingly programming the reconstruction unit and / or the control unit of the MR system.

[0026] The method of the present invention can be advantageously performed in most MR systems currently used in clinical practice. For this purpose, only a computer program is needed to control the MR system to perform the above-described method steps of the present invention. The computer program can reside on a data carrier or in a data network for download and installation in the control unit of the MR system. Processing of the acquired MR signals used to reconstruct MR images can also be performed on a computer separate from the MR system used to acquire the MR signals.

[0027] This summary is provided to present, in a simplified form, the selection of concepts further described below in the detailed description. This summary is not intended to identify key or essential features of the claimed subject matter, nor is it intended to limit the scope of the claimed subject matter. A broader presentation of the features, details, uses, and advantages of the methods and systems as defined in the claims is provided in the following written description of various embodiments of this disclosure, and these presentations are illustrated in the accompanying drawings. Attached Figure Description

[0028] The accompanying drawings disclose preferred embodiments of the invention. However, it should be understood that the drawings are for illustrative purposes only and are not intended to limit the scope of the invention. In the drawings: Figure 1 A block diagram of an MR system for performing the method of the present invention is shown; Figure 2 The method of the present invention is schematically illustrated in the form of a sequence diagram. Detailed Implementation

[0029] To facilitate and understand the principles of this disclosure, reference will now be made to embodiments illustrated in the accompanying drawings, and these embodiments will be described using specific language. However, it should be understood that this document is not intended to limit the scope of this disclosure. Any changes and further modifications to the described apparatus, systems, and methods, as well as any further applications of the principles of this disclosure, are those that would commonly occur to those skilled in the art related to this disclosure and are therefore fully contemplated and included in this disclosure. In particular, it is fully contemplated that features, components, and / or steps described with respect to one embodiment may be combined with features, components, and / or steps described with respect to other embodiments of this disclosure. However, for the sake of brevity, numerous iterations of these combinations will not be described separately. Features described with respect to methods may be implemented in a corresponding manner in systems, computer-implemented methods, and / or computer program products.

[0030] refer to Figure 1 The MR system 1 is shown in block diagram form. The device includes a superconducting or resistive main magnet coil 2, which generates a substantially uniform, time-constant main magnetic field B0 along the z-axis through the examination volume. The device also includes a set of shimming coils 2' (first-order, second-order, and, where applicable, third-order), wherein the current flowing through each shimming coil of the set 2' is controllable to minimize deviations in B0 within the examination volume.

[0031] The magnetic resonance generation and manipulation system applies a series of RF pulses and switched magnetic field gradients to invert, excite, saturate, refocus, and spatially encode magnetic resonance to perform MR imaging.

[0032] More specifically, gradient pulse amplifier 3 applies current pulses along the x, y, and z axes of the examination volume to selected whole-body gradient coils in whole-body gradient coils 4, 5, and 6. Digital RF frequency transmitter 7 transmits RF pulses or pulse packets to RF body coil 9 via transmit / receive switch 8 to direct RF pulses into the examination volume. A typical MR imaging sequence consists of packets of short-duration RF pulse segments that, together with any applied magnetic field gradient, achieve selective manipulation of MRI. Specifically, the RF pulse selection is located on a portion of body 10 within the examination volume. The MR signal is also picked up by RF body coil 9.

[0033] To generate an MR image of a limited area of ​​body 10, a set of locally arrayed RF coils 11, 12, and 13 are placed adjacent to the area selected for imaging. The arrayed coils 11, 12, and 13 can be used to receive MR signals induced by the emission of the RF body coils.

[0034] The resulting MR signal is picked up by RF body coil 9 and / or by array RF coils 11, 12, 13, and demodulated by receiver 14, which preferably includes a preamplifier (not shown). Receiver 14 is connected to RF coils 9, 11, 12, and 13 via transmit / receive switch 8.

[0035] The host computer 15 (in other words, the control unit) controls the shimming coil 2', the gradient pulse amplifier 3, and the transmitter 7 to generate the imaging sequence of the present invention. For the selected sequence, the receiver 14 rapidly and continuously receives signal data from one or more k-space lines after each RF excitation pulse. The data acquisition system 16 performs analog-to-digital conversion on the received signals and converts each k-space line into a digital format suitable for further processing. In modern MR systems, the data acquisition system 16 is a separate computer dedicated to acquiring raw image data.

[0036] Finally, the reconstruction processor 17 reconstructs the original digital image data into an image representation, applying Fourier transform or other suitable reconstruction algorithms (e.g., SENSE). MR images can be represented by planar slices of the patient, arrays of parallel planar slices, three-dimensional volumes, etc. The images are then stored in an image memory, accessible in the image memory to convert slices, projections, or other portions of the image representation into a format suitable for visualization (e.g., via video monitor 18 or a display screen), providing a user-readable display of the resulting MR image.

[0037] The host computer 15 and the reconstruction processor 17 are configured, through corresponding programming, to perform the methods of the present invention described above and below.

[0038] Any combination of the main computer 15, the data acquisition system 16, and the reconstruction processor 17 can be integrated into a single computing system.

[0039] The computing system may include memory, which may include volatile and non-volatile memory storage modules and components. In some embodiments, the memory may be based on or may rely on cloud-based data stored in logical pools on different commodity storage servers deployed locally or in logical pools on different commodity storage servers in a data center managed by a third-party cloud provider. In some embodiments, image reconstruction may also be cloud-based reconstruction, wherein the raw digital image data is processed in the cloud and the reconstructed image is transmitted back to the system for output on display 18. In such embodiments, the raw digital data is transmitted to the cloud, and the cloud-based image reconstruction function replaces or may be a supplement to the reconstruction processor 17. Furthermore, the computing system may be a distributed computing system, which consists of multiple software components on multiple computers operating as a single computing system. The computers in the distributed computing system may be physically close and connected via a local network, or they may be geographically distant and connected via a wide area network.

[0040] Continue to refer to Figure 1 And further reference Figure 2 The embodiments of the method of the present invention are explained.

[0041] Figure 2 The schematic diagram illustrates a modified SPIR or SPAIR technique according to the present invention. This diagram shows the generation of a magnetic field gradient (GX) as a function of time t for the generation and switching of a radio frequency (RF) pulse. For simplicity, the diagram only shows the generation of the magnetic field gradient in one spatial direction (x-direction). In general, magnetic field gradients are generated in all three spatial directions.

[0042] The method begins by subjecting subject 10 to a preparation sequence. In the specification, subject 10 can be understood as a subject or a part of a subject's body, wherein the subject may be a patient. Figure 2 The PRE indicates the time interval used to apply the preparation sequence. The preparation sequence PRE includes a spectrally selective inversion RF pulse with a flip angle α to selectively invert the nuclear magnetization associated with fatty protons within object 10, followed by a perturbation magnetic field gradient pulse 22 to dephase any transverse nuclear magnetization.

[0043] Figure 2 The lower part of the schematic diagram illustrates longitudinal nuclear magnetization. M z The temporal evolution. Downward-pointing arrow 23 indicates the nuclear magnetization associated with fatty protons. M zThe portion of the magnetization has been inverted by the spectrally selectively inverted RF pulse 21. These fatty protons (first spectral species) are primarily methylene protons and other (aliphatic) fatty protons. The flip angle α of the spectrally selectively inverted RF pulse 21 determines the degree of magnetization inversion (i.e., the negative longitudinal magnetization associated with the first spectral species of fatty protons after the inverted RF pulse 21). M z The amount of water protons and other fatty protons (second-spectrum species) (mainly olefin protons with chemical shifts close to those of water protons) are not affected by the inverted RF pulse 21.

[0044] After the inverted RF pulse, after a delay time T D This allows the nuclear magnetization of the first spectral species of fatty protons to return to its equilibrium state. This is indicated by curve 24. Arrow 25 indicates the time delay T. D The remaining negative longitudinal magnetization of the first spectral species at the past moment. The longitudinal nuclear magnetization of the second (unreversed) spectral species of adipose protons at the same moment is indicated by the dashed arrow 26 pointing in the positive direction. From... Figure 2 As can be seen from the image, the longitudinal nuclear magnetization of the first (arrow 25) spectral species and the second (dashed arrow 26) spectral species of fatty protons at a delay time T D They cancel each other out after they have passed. This is achieved by appropriately determining the flip angle α and / or the delay time T. D This is achieved by carefully determining the flip angle α and / or the delay time T. D The total longitudinal NMR of fat (the superposition of the NMR of the first and second spectral species of fat protons) is zeroed at the start of the imaging sequence IM. Due to this cancellation, the contributions of the first and second spectral species to the MR signal acquired during the imaging sequence IM have equal amplitude and opposite phase, resulting in effective and complete suppression of the fat signal from both spectral species in the MR image reconstructed from the acquired MR signal 27. As in SPIR, the contribution of water protons to the MR signal is not selectively reversed and therefore not suppressed in the MR image. Their respective longitudinal NMRs are in equilibrium at the start of the imaging sequence IM.

[0045] Corresponding to the SPAIR technique, an adiabatic inversion RF pulse 21 can optionally be used. The adiabatic inversion RF pulse 21 can completely reverse the magnetization of the first spectral species of fatty protons even when the RF field B1 is spatially non-uniform, making the technique less sensitive to changes in B1. In this case, the flip angle α is 180°. Delay time T D It is the only parameter that can be used for tuning to achieve a balance of nuclear magnetization of the two spectral species of fatty protons according to the present invention.

[0046] The imaging sequence IM can, in principle, be any imaging sequence. For simplicity, Figure 2 The schematic diagram only shows the first excitation RF pulse 28 and the readout magnetic field gradient pulse 29 of the imaging sequence IM. However, as will be apparent to those skilled in the art, the imaging sequence may or will include multiple RF pulses 28 and switched magnetic field gradients 29 (in different spatial directions) to generate multiple frequency-coded and phase-coded MR signals 27 and sample a predetermined region of k-space as required by the corresponding imaging task.

[0047] The following describes in more detail the application of the present invention to the flip angle α and the delay time T. D Determination: Longitudinal magnetization of fatty protons using conventional SPIR technology M z The effect is described by the following formula: (Formula 1) in M z (0) - () represents the fraction of longitudinal magnetization before the application of the SPIR inverted RF pulse at t=0, and T1 represents the longitudinal relaxation time constant of fat protons. This formula assumes a single-peak spectrum of fat and an infinitely short (partial) inverted RF pulse. In conventional SPIR, the angle α of the inverted RF pulse is chosen such that when the excitation RF pulse for subsequent imaging sequences is applied, exist t = T D The time has passed zero. Therefore, it is given by the following formula: (Formula 2) As mentioned above, SPAIR relies on a fully inverted RF pulse with a fixed α = 180°, and the selection of the delay time makes... exist t = T D The time has passed zero. Therefore, the delay is given by the following formula: (Formula 3) Neither SPIR nor SPAIR designs take into account the multi-peak spectrum of fat protons, resulting in only partial suppression of the MR signal from fat. This invention aims to completely suppress the MR signal from fat using SPIR and SPAIR, and proposes integrating the multi-peak spectrum of fat into the design for flip angle α and / or delay time T. D In the calculation, the amplitudes of the MR signal components from fatty protons (affected by the reverse RF pulse and unaffected by the reverse RF pulse, respectively) are balanced so that they cancel each other out due to their opposite phases.

[0048] Considering the multi-peak spectrum of adipose protons, formula (1) is extended to: (Formula 4) Where n indexes each peak, and w n Represents the corresponding fraction of longitudinal magnetization.

[0049] For the sake of simplicity, this is omitted. T 1 The difference yields: (Formula 5) However, the difference in α is significant due to the spectral selectivity of the partially and fully inverted RF pulses, respectively. In the practical approximation, α is practically 0 for aliphatic proton peaks close to the water proton (second spectral species). For other aliphatic proton peaks (which are referred to as the first spectral species and include the main peak originating from methylene protons at 3.4 ppm relative to water protons), α is primarily nominal α. However, for aliphatic proton peaks originating from diacyl protons at -2.0 ppm relative to water protons, α can take an intermediate value, which can vary with the spectral profile of the inverted RF pulse and the local main field inhomogeneity Δ. B 0 Significant changes. Again, for simplicity, this will be ignored in the following text, given that the area of ​​the peak is usually very small.

[0050] Assume the flip angle α is either 0 or nominal α, corresponding to the fraction of longitudinal magnetization, respectively. w 0 and w α Formula (5) simplifies to: (Formula 6) Require exist t = T D When time passes zero, we get: (Formula 7) Its alternative formula (2).

[0051] Given a fixed α = 180° (for SPAIR), T is solved alternatively. D ,get: (Formula 8) And utilize w 0+ w α = 1, resulting in: (Formula 9) In both cases, the MR signal components from the first and second spectral species of fatty protons differ due to their opposite phase (cos(...)). α <0) and matching amplitude ( t = T D And thus offset.

[0052] Although It depends only on the RF pulses of the subsequent imaging sequence, but It is also affected by the inversion RF pulse of the preparation sequence.

[0053] The difference between formula (2) and formula (9) is illustrated by the following example: By using T D =10.6 ms and single-shot fast spin echo (TSE) imaging with low-high profile order, using SPIR to image the components containing w Imaging was performed on a phantom of sunflower seed oil with a concentration of 0 = 0.14 (i.e., the second spectrum species of fatty protons account for 14% of the longitudinal magnetization). T1 = 292 ms was set, and α = 92° and α = 102° were selected according to formulas (2) and (9), respectively.

[0054] The signal strength of sunflower oil obtained using SPIR at α = 92° is 14% of the signal strength obtained without SPIR. This matches the expected result based on theoretical considerations of the composition of sunflower oil. The signal strength obtained using SPIR at 102° is reduced to 2%, which is close to the ideal suppression effect, especially considering the noise bias in the amplitude image, even when the signal is practically zero, a positive average signal strength is obtained.

[0055] In this example, the echo time (TE) is intentionally chosen to be short. If a longer echo time were chosen, the differences in the transverse relaxation times (T2) of the various peaks of the fatty protons would become relevant. In particular, the T2 of the first spectrum species of fatty protons is generally longer than that of the second spectrum species. This can be taken into account by appropriately weighting the two terms in Equation (6), resulting in: (Formula 10) The optimal α for SPIR is relatively low. For T in SPIR... D This can include similar T2 correction.

[0056] Similarly, other effects (such as J coupling) can be considered.

[0057] In short, the present invention relates to setting in formula (4) And solve for α in the case of SPIR, or solve for T in the case of SPIR where α = 180°.D .

[0058] Unlike TSE imaging, in fast field echo (TFE) imaging, the individual peaks of fatty protons are not necessarily in phase again. Generally, this makes complete annihilation of the MR signal components from the first and second spectral species of fatty protons impossible. However, it still allows for reduction of residual fatty signal. For example, using a TE with the MR signal components of the first and second spectral species out of phase, with an α value effectively below 90°, would be preferred. The MR signal components of the first and second spectral species will then remain aligned after SPIR, but precess at different frequencies after the excitation RF pulse of the imaging sequence, which will ultimately result in the desired out-of-phase at the TE.

[0059] Although various embodiments of the claimed subject matter have been described above with a degree of specificity or with reference to one or more individual embodiments, those skilled in the art can make many changes to the disclosed embodiments without departing from the spirit or scope of the claimed subject matter.

[0060] Other embodiments are also contemplated. All content contained in the above description and shown in the accompanying drawings is intended to be interpreted as illustrative of particular embodiments only and not as limiting. Changes in detail or structure may be made without departing from the essential elements of the subject matter as defined in the foregoing claims.

Claims

1. A method for imaging an object (10), comprising: At least a portion of the object (10) is subjected to a preparation sequence (PRE) including a spectrally selective adiabatic reversal radio frequency (RF) pulse (21) with a flip angle (α) to selectively reverse the nuclear magnetization associated with fatty protons within the at least a portion of the object (10). During the delay time (T) D Afterward, the at least portion of the object (10) is subjected to an imaging sequence (IM) comprising at least one excitation RF pulse (28) and a switched magnetic field gradient (29) to generate a magnetic resonance MR signal (27), wherein the delay time (T) D The flip angle (α) is determined such that the contributions of the first spectral species of fatty protons affected by the inverted RF pulse (21) and the second spectral species of fatty protons unaffected by the inverted RF pulse (21) to the MR signal substantially cancel each other out. The MR signal (27) is acquired from at least a portion of the object (10); and MR images are reconstructed based on the acquired MR signals (27), wherein the determination of the delay time (T) is due to the preparation sequence (PRE) combined with the determined delay time. D The selective inversion caused by the flip angle (α) and / or the flip angle (α) suppresses the contributions of both the first spectral species from fatty protons and the second spectral species from fatty protons in the MR image.

2. The method according to claim 1, wherein, The flip angle α is 180°.

3. The method according to claim 1 or 2, wherein, The second spectrum species includes at least one of the following: olefin protons resonating at +0.6 ppm relative to water protons, and glycerol protons resonating at +0.5 ppm and -0.5 ppm relative to water protons.

4. The method according to any one of claims 1 to 3, wherein, The first spectrum species includes at least one of the following: a methyl proton resonating at -3.8 ppm relative to a water proton, a methylene proton resonating at -3.4 ppm relative to a water proton, a β-carboxyl proton resonating at -3.1 ppm relative to a water proton, an α-carboxyl proton resonating at -2.5 ppm relative to a water proton, an α-olefin proton resonating at -2.7 ppm relative to a water proton, and a diacyl proton resonating at -1.9 ppm relative to a water proton.

5. The method according to any one of claims 1 to 4, wherein, In determining the delay time (T) D When considering the different effective flip angles of the first and second spectra of fatty protons, and / or the flip angle (α), the different effective flip angles are taken into account.

6. The method according to claim 5, wherein, The effective flip angles of the protons of the first and second spectral species of adipose protons are correlated with the flip angle (α) by the spectral profile of the spectral selective inversion RF pulse (21).

7. The method according to any one of claims 1 to 6, wherein, In determining the delay time (T) D When considering the different longitudinal relaxation times of protons in the first and second spectral species, and / or the flip angle (α), the different longitudinal relaxation times of protons in the first and second spectral species are taken into account.

8. The method according to any one of claims 1 to 7, wherein, The imaging sequence is a spin echo sequence.

9. The method according to claim 8, wherein, In determining the delay time (T) D When considering the echo time of the spin echo sequence and the different transverse relaxation times of the protons of the first and second spectral species, and / or the flip angle (α), the following should be taken into account.

10. The method according to any one of claims 1 to 7, wherein, The imaging sequence is a gradient echo sequence.

11. The method according to claim 10, wherein, In determining the delay time (T) D When considering the echo time of the gradient echo sequence and the phase relationship between the protons of the first spectral species and the second spectral species, and / or the flip angle (α), the following factors are taken into account.

12. A magnetic resonance system (1), comprising: A computing system configured to perform the method according to any one of claims 1 to 11; as well as The display screen (18) is configured to display the reconstructed MR image.

13. The system according to claim 12, further comprising: At least one main magnet coil (2) is used to generate a static magnetic field within the inspection volume; Multiple gradient coils (4, 5, 6) for generating switched magnetic field gradients in different spatial directions within the examination volume; at least one radio frequency (RF) coil (9) for generating RF pulses within the examination volume and / or for receiving magnetic resonance (MR) signals from at least a portion of the object (10) located in the examination volume.

14. The system according to claim 12 or 13, wherein, The computing system includes: Control unit (15), which controls the time series of RF pulses and switched magnetic field gradients; and The reconstruction unit (17) is used to generate an MR image based on the received MR signal.

15. A computer program comprising machine-executable instructions, wherein, The execution of the machine-executable instructions causes the computing system to perform the method according to any one of claims 1 to 11.

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