Magnetic resonance angiography imaging method and system

Abstract

The invention relates to a method of MR imaging of at least a portion of a body (10) of a patient placed in an examination volume of an MR system (1). It is an object of the invention to provide an improved fast gradient echo imaging technique with the ability to obtain angiographic information. The method of the invention comprises the steps of subjecting the body portion to a magnetization preparation RF pulse (MP); subjecting the body portion to a gradient echo imaging sequence comprising a series of RF excitations (EP) after a preparation delay (TD), wherein a phase encoding gradient (PE) and a refocusing gradient (RO) and at least one bipolar velocity encoding gradient (VE) pair are applied in repetition time intervals (TR) between successive RF excitations (EP) to sample a predefined region of k-space and generate gradient echoes in each repetition time interval (TR); acquiring echo signals from the body portion; reconstructing a morphological MR image of the body portion from the acquired echo signals; and deriving phase contrast angiographic information from the phase imparted on the echo signals by the velocity encoding gradients (VE). Furthermore, the invention also relates to an MR system (1) and a computer program.

Description

Technical Field

[0001] This invention relates to the field of magnetic resonance (MR) imaging. Specifically, it relates to a method for MR imaging of at least a portion of a patient's body placed within the examination volume of an MR system. The invention also relates to an MR system and a computer program running on that MR system. Background Technology

[0002] Currently, MR image formation 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. This is because these MR image formation methods are superior to other imaging methods in many ways for soft tissue imaging, do not require ionizing radiation, and are generally non-invasive.

[0003] In MR imaging, gradient echo imaging is widely used in a wide range of applications, from angiography to perfusion and further to functional MR imaging. Compared to spin echo techniques, the very short repetition time of gradient echo methods enables very fast 2D and 3D imaging.

[0004] The so-called Fast Gradient Echo Sequence (RAGE) consists of a series of RF excitation pulses with specific flip angles, wherein a phase-encoded magnetic field gradient and a refocusing magnetic field gradient are applied in repetition time (TR) intervals between successive RF excitations to sample a predefined region in k-space and generate a gradient echo in each repetition time interval. The echo time (TE) is the time from each RF excitation pulse to the formation of the subsequent gradient echo.

[0005] To enhance the inherent contrast of gradient echo imaging, magnetization preparation schemes are often used. Fast gradient echo imaging with magnetization preparation is commonly referred to as MP-RAGE. The magnetization preparation used in MP-RAGE can be performed (e.g., by applying a spatially non-selective inverted RF pulse) to obtain T1-weighted contrast. The magnetization preparation RF pulse precedes the actual fast gradient echo sequence, with a time delay between the magnetization preparation RF pulse and the start of gradient echo readout. This delay (along with the flip angle of the RF excitation pulse) determines the image contrast.

[0006] In particular, 3D MP-RAGE provides excellent tissue contrast, high spatial resolution, and complete T1-weighted coverage when imaging the brain at clinically feasible scan times. MP-RAGE is one of the most widely used sequences in clinical practice.

[0007] Phase-contrast (PC) MR imaging is a specialized technique primarily used to visualize and quantify fluid flow within the body, such as blood flow in blood vessels or cerebrospinal fluid flow in the central nervous system. This technique is particularly valuable in the diagnosis and study of vascular and cardiovascular diseases, as well as certain neurological disorders.

[0008] PCMR imaging utilizes the phase information of magnetic resonance signals. This phase information encodes details about the location and movement of nuclear spins within tissue. In PCMR imaging, magnetic resonance signals are acquired using bipolar velocity-encoded magnetic field gradient pairs applied across the region of interest (e.g., a blood vessel or cerebrospinal fluid region). The bipolar gradient pair shifts the phase of the magnetic resonance signal proportionally to the velocity of the moving fluid. The resulting phase-contrast image contains information about the velocity and direction of fluid flow within the imaged region. The amount of phase shift is proportional to the velocity of the flowing fluid, and the direction of the shift indicates the flow direction. By knowing specific imaging parameters and calibration factors, the flow velocity can be quantitatively measured.

[0009] Typically, in known PC MR imaging schemes, two MR image datasets are acquired. One MR image dataset is acquired using a bipolar velocity-coded gradient applied across the region of interest. The other MR image dataset is acquired without the bipolar velocity-coded gradient, serving as a reference or baseline. The phase information from this reference dataset is subtracted from the phase information of the image dataset acquired with the bipolar velocity-coded gradient applied. This subtraction process highlights the phase shift caused by fluid flow.

[0010] Phase-contrast angiography (PCA) retains the potential of a comprehensive vascular MR imaging technique based on an interpreted phase-contrast protocol that provides vascular images without the use of gadolinium-based contrast agents. However, PCA imaging has not yet been widely adopted in clinical practice due to its particularly long imaging time. On the other hand, recent advances in acceleration technologies, such as compressed sensing and deep learning-based reconstruction, have enabled significant reductions in the required scan time. Summary of the Invention

[0011] In view of this background, the object of the present invention is to provide an improved fast gradient echo imaging technique with the ability to acquire angiographic information.

[0012] According to the present invention, a method for performing MR imaging on at least a portion of a patient's body placed within the examination volume of an MR system is disclosed. The method includes the following steps:

[0013] To prepare for RF pulses by magnetizing parts of the body;

[0014] After a preparation delay, the body part is subjected to a gradient echo imaging sequence comprising a series of RF excitations, wherein phase-coded gradients and refocusing gradients, as well as at least one pair of bipolar velocity-coded gradients, are applied in repetition time intervals between successive RF excitations to sample a predefined region in k-space and generate gradient echoes in each repetition time interval.

[0015] Echo signals are acquired from the body parts;

[0016] Based on the acquired echo signals, morphological MR images of the body parts are reconstructed; and

[0017] Phase-contrast angiography information is derived from the phase applied to the echo signal by the velocity-encoded gradient.

[0018] According to the present invention, a conventional MP-RAGE imaging sequence is combined with a PC imaging scheme. For this purpose, at least one bipolar velocity-encoded gradient pair is incorporated into each TR interval, which applies a velocity-dependent phase to the echo signal. This allows for dual evaluation of the acquired echo signal. Morphological MR images are reconstructed as in conventional MP-RAGE imaging, and additionally, phase-contrast angiography information is obtained by evaluating the phase of the echo signal (automatic co-registration with the morphological MR image).

[0019] The term "morphological image" refers to an image that primarily provides information about the shape, structure, and anatomical content of the imaged body part. Such images offer contrast between different types of tissue based on their inherent physical properties (e.g., T1, T2, proton density, etc.). Morphological MR images are essential for visualizing the internal structure of body parts.

[0020] The term "phase-contrast angiography information" refers to information relating to blood flow within vessels contained within the imaged anatomy. This includes information about the direction and velocity of blood flow, which can be used to evaluate vascular anatomy, identify stenosis, and detect different types of blood flow abnormalities. In neuroimaging, phase-contrast angiography information enables the assessment of cerebral blood flow and conditions such as arteriovenous malformations (AVMs) or aneurysms.

[0021] Therefore, this invention enables the capture of more anatomical, physiological, and functional data to improve diagnostic utility. The simultaneous MP-RAGE and PCA acquisition provided by this invention combines the benefits of both technologies in a single acquisition within a clinically feasible scan time.

[0022] In this embodiment, the magnetization preparation RF pulse is a spatially non-selective inversion pulse. In this way, (3D)T1-weighted morphological MR images and neurovascular information can be obtained. The cerebrovascular system and blood flow are altered in various diseases. Phase-contrast angiography information can be effectively used as a luminescence imaging tool to identify aneurysms, arteriovenous malformations, and vascular stenosis in the cerebrovascular system.

[0023] In another embodiment, deriving phase-contrast angiography information includes reconstructing a phase-contrast angiography MR image based on the acquired echo signals. The image contrast of the phase-contrast angiography MR image is determined by the flow velocity at different image locations. A weighted subtraction of the morphological MR image and the phase-contrast angiography MR image can be calculated to obtain an MR image of the body portion with a dark blood appearance, without requiring co-registration of different images. By utilizing weighted subtraction, image intensity in the lumen of the blood vessels where blood flow occurs is removed. This is useful, for example, for vascular wall imaging and for improving the confidence of post-angiographic diagnosis of brain metastases. Therefore, the present invention provides advantageous imaging options due to its non-invasive nature (no contrast agent), imaging capability for vessels with complex geometries, high spatial resolution, and ability to correlate morphological image information with angiographic image information.

[0024] The method of the present invention can be further extended to acquire time-resolved 3D flow imaging, thereby allowing morphological and functional 4D neurovascular flow analysis.

[0025] In another embodiment, the gradient echo imaging sequence performs k-space undersampling to accelerate the sampling of the predefined region. k-space undersampling allows for faster image acquisition while maintaining acceptable image quality. This technique is crucial for achieving clinically feasible scan times. k-space undersampling involves acquiring only a portion of the total k-space data points required to create a complete high-resolution image, according to the Nyquist theorem. This is typically achieved by intentionally skipping or reducing the amount of phase coding. Advanced reconstruction algorithms can be used to obtain diagnostically useful and artifact-free images from the undersampled MR signal data.

[0026] Morphological MR images and / or phase-contrast angiography MR images can be reconstructed from undersampled echo signal data using techniques such as compressed sensing, deep learning-based reconstruction, or other advanced reconstruction techniques. Compressed sensing is a mathematical framework that leverages the sparsity or compressibility of MR images in certain domains (e.g., wavelet domain) to achieve high-quality MR image reconstruction from significantly undersampled k-space data. Deep learning reconstruction techniques use artificial neural networks (specifically convolutional neural networks (CNNs)) to reconstruct MR images from undersampled k-space data.

[0027] Parallel imaging can also be used to acquire and reconstruct the morphological MR images and / or the phase-contrast angiography MR images. In parallel imaging, magnetic resonance signals are acquired simultaneously via multiple receiving coils or antennas with different spatial sensitivity distributions across the imaged region. By taking into account these distributions (e.g., SENSE (one of the most widely used parallel imaging techniques)), MR images can be reconstructed very definitively even from significantly undersampled MR signal data. GRAPPA is another popular parallel imaging technique. It uses an automatic calibration line in k-space to estimate missing data points in the undersampled MR signal data. These estimates are used to reconstruct the complete MR image. A major advantage of parallel imaging is its ability to acquire MR images much faster than conventional MR imaging techniques.

[0028] 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 substantially uniform static magnetic field B0 within an examination volume; a plurality of gradient coils for generating switching magnetic field gradients in different spatial directions within the examination volume; at least one body RF coil for generating RF pulses within the examination volume and / or for receiving MR signals from the body of a patient positioned within the examination volume; a control unit for controlling the timing of the RF pulses and the switching magnetic field gradients; and a reconstruction unit for reconstructing MR images based on the received MR signals. The method of the present invention can be implemented through corresponding programming of the reconstruction unit and / or the control unit of the MR system.

[0029] The method of the present invention can be advantageously performed on most MR systems currently used in clinical practice. Therefore, only a computer program is needed to control the MR system to perform the steps of the method described above. 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. Attached Figure Description

[0030] 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:

[0031] Figure 1 An MR system for performing the method of the present invention is shown;

[0032] Figure 2 A schematic (simplified) pulse sequence diagram of the imaging sequence according to the present invention is shown;

[0033] Figure 3 Exemplary brain images are shown to illustrate the capability of the methods of the present invention;

[0034] Figure 4 Exemplary angiography images are shown to illustrate the capability of the method of the present invention. Detailed Implementation

[0035] To facilitate an understanding of the principles of this disclosure, reference will now be made to embodiments illustrated in the accompanying drawings, and specific language will be used to describe this disclosure. 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 devices, systems, and methods described in this disclosure, as well as any further applications of the principles of this disclosure, will be fully contemplated and included within this disclosure, as will be expected by those skilled in the art to which this disclosure pertains. In particular, it is fully contemplated that features, components, and / or steps described with respect to one embodiment can be combined with features, components, and / or steps described with respect to other embodiments of this disclosure. However, for the sake of brevity, various iterations of these combinations will not be described separately. The features described regarding the system can be implemented in a corresponding manner in a computer-implemented method and / or computer program product.

[0036] refer to Figure 1 The MR system 1 is shown as a block diagram. The system includes a superconducting or resistive master magnet coil 2, which creates a substantially uniform, time-constant master magnetic field B0 along the z-axis through the examination volume. The device also includes a set of (1st, 2nd, and possibly 3rd order) shimming coils 2', wherein the current flowing through each shimming coil of the set 2' is controllable to minimize deviations in B0 within the examination volume.

[0037] Magnetic resonance generation and manipulation systems employ a series of RF pulses and switched magnetic field gradients to invert, excite, refocus, and / or spatially and otherwise encode magnetic resonance to perform MR imaging.

[0038] More specifically, the gradient pulse amplifier 3 applies current pulses to selected gradient coils in the whole-body gradient coils 4, 5, and 6 along the x, y, and z axes of the examination volume. A digital RF frequency transmitter 7 transmits RF pulses or pulse packets to the RF body coil 9 via a transmit / receive switch 8 to deliver RF pulses into the examination volume. A typical MR imaging sequence consists of packets of short-duration RF pulse segments, which, together with any applied magnetic field gradient, achieve selective manipulation of the magnetic resonance. The RF pulses are specifically used to select portions of the body 10 located within the examination volume. The MR signal is also picked up by the RF body coil 9.

[0039] To generate an MR image of a limited area of ​​body 10, a set of local RF array coils 11, 12, and 13 is placed adjacent to the area selected for imaging. The array coils 11, 12, and 13 can be used to receive MR signals induced by the emission of the RF body coils.

[0040] The resulting MR signal is picked up by RF body coil 9 and / or RF array 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.

[0041] The host computer 15 controls the shimming coil 2', gradient pulse amplifier 3, and transmitter 7 to generate the imaging sequence of the present invention. For a selected sequence, the receiver 14 rapidly and sequentially receives one or more MR data lines after each RF excitation. The data acquisition system 16 performs analog-to-digital conversion on the received signals and converts each MR data 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.

[0042] Finally, the original digital image data is reconstructed into an image representation by reconstruction processor 17, which applies Fourier transform or other suitable reconstruction algorithms (e.g., SENSE or GRAPPA). MR images can be represented by planar slices of the patient, arrays of parallel planar slices, 3D volumes, etc. The images are then stored in image memory, where they can be accessed to convert slices, projections, or other portions of the image representation into appropriate formats for visualization (e.g., via a display screen or video monitor 18, which provides a human-readable display of the resulting MR images).

[0043] The host computer 15 and the reconstruction processor 17 are configured, through corresponding programming, to perform the methods of the present invention described herein. Although the host computer 15, the reconstruction processor 17, and the data acquisition system 16 are... Figure 1 They are represented as different entities, but their functions can be performed by a computing system that includes one or more computing systems or computing cores located at one or more locations.

[0044] According to the present invention, a (3D) MP-RAGE imaging sequence combined with velocity-encoded bipolar gradient pairs is proposed. This is in Figure 2 The sequence diagram in the figure is shown. Figure 2 This diagram illustrates the switching of magnetic field gradients along the frequency encoding direction M, phase encoding direction P, and slab selection direction S during an arbitrarily chosen repetition time interval TR in a fast gradient echo imaging sequence. The figure shows the initial magnetization preparation RF pulse MP, followed by the gradient echo imaging sequence after a delay time TD. Figure 2Two successive excitation RF pulses EP are illustrated in the presence of a thick-slice selected gradient SS. Different phase-encoded gradients are applied in directions P and S during successive repetition time intervals TR of the gradient echo sequence to sample a predefined region in k-space. A refocusing and readout gradient RO is applied in each repetition time interval TR to generate the gradient echo. The gradient echo is received and digitally sampled as an echo signal during the acquisition interval ACQ. Furthermore, a pair of bipolar velocity-encoded gradients VE is applied in the repetition time intervals TR to impart phase information to the gradient echo and thus to the acquired echo signal. The bipolar gradient VE pair shifts the phase of the echo signal proportionally to the velocity of the moving nuclear spins (i.e., water protons in flowing blood). This phase information allows for the derivation of phase-contrast angiography information (i.e., information about the velocity and direction of blood flow within the imaged area) from the acquired echo signal, in addition to the morphological MR image reconstructed in a conventional manner from the acquired echo signal. Therefore, the illustrated imaging scheme combines MP-RAGE imaging with PCA. In a conventional PCA, two signal datasets can be acquired: one with a bipolar velocity coding gradient VE, and the other without a bipolar velocity coding gradient VE, to obtain a reference. Figure 2 In this process, velocity-coded bipolar gradient pairs are applied in the frequency-coded direction M. However, velocity coding can be applied in different spatial directions, preferably in all three spatial directions M, P, and S, to obtain the flow velocity vector at each location in the imaged region.

[0045] Figure 2 The MR imaging scheme shown can be combined with known advanced acceleration techniques (parallel imaging, compressed sensing, deep learning-based reconstruction, etc.) to acquire the echo signal data required to obtain 3D T1-weighted morphological images and neurovascular information (PCA) within a clinically feasible scan time.

[0046] The cerebrovascular system and blood flow change in various diseases, and PCA has been effectively used as a luminescence imaging tool for identifying aneurysms, arteriovenous malformations, and vascular stenosis in the cerebrovascular system. By using this luminescence imaging information from PCA, the combined MP-RAGE / PCA method of this invention can be used as a black-blood imaging technique for vascular wall imaging without the need for co-registration. This is in Figure 3 The middle image shows, Figure 3This diagram shows a comparison of conventionally acquired ADNI (“Alzheimer’s Disease Neuroimaging Initiation”) 3D MP-RAGE brain images (left column) with morphological MR images acquired using the MP-RAGE / PCA method of this invention (middle column), and a weighted subtraction result (right column) of the morphological MR images shown in the middle column with phase-contrast angiography MR images derived from the phase information of the echo signals. The comparison... Figure 3 The middle and right columns show that MR images with a dark blood appearance were obtained through weighted digital subtraction (right column). White arrows highlight the bright vessels in the image shown in the middle column and the same vessels with dark blood in the image in the right column. This indicates that the method of this invention provides advantageous neurovascular imaging options due to its non-invasive nature, imaging capability for vessels with complex geometries, high spatial resolution, and good correlation with other imaging features. Gray arrows show post-contrast enhancement results in MP-RAGE and MP-RAGE / PCA.

[0047] Figure 3 A comparison is shown between maximum intensity projection (MIP) brain images obtained by conventional PCA imaging (left column) and maximum intensity projection (MIP) brain images obtained by deriving phase information from echo signals acquired using the method according to the invention. It can be seen that the images are virtually identical.

[0048] Figure 4 Exemplary angiographic images illustrating the capability of the methods of the present invention are shown, wherein the left column represents PCA and the right column represents the corresponding MP-RAGE / PCA images.

[0049] It should be understood that one or more embodiments of the foregoing embodiments of the present invention may be combined, as long as the combined embodiments are not mutually exclusive.

[0050] As those skilled in the art will understand, aspects of the present invention can be embodied as an apparatus, method, or computer program product. Therefore, aspects of the present invention can take the form of a completely hardware embodiment, a completely software embodiment (including firmware, resident software, microcode, etc.), or an embodiment combining software and hardware aspects, which are generally referred to herein as “modules” or “systems.” Furthermore, aspects of the present invention can take the form of a computer program product embodied in one or more computer-readable media having computer-executable code thereon.

Claims

1. A method for performing MR imaging on at least a portion of a patient's body (10) placed in an examination volume of a magnetic resonance MR system (1), the method comprising the steps of: The body parts are magnetized in preparation for radio frequency (RF) pulses (MP). After a preparation delay (TD), the body part is subjected to a gradient echo imaging sequence comprising a series of RF excitations (EP), wherein phase-coded gradients (PE) and refocusing gradients (RO) and at least one pair of bipolar velocity-coded gradients (VE) are applied in repetition time intervals (TR) between successive RF excitations (EP) to sample a predefined region in k-space and generate gradient echoes in each repetition time interval (TR). Echo signals are acquired from the body parts; Based on the acquired echo signals, morphological MR images of the body parts are reconstructed; and Phase-contrast angiography information is derived from the phase applied to the echo signal by the velocity-encoded gradient (VE).

2. The method according to claim 1, wherein, The magnetization preparation RF pulse (MP) is a spatially non-selective inversion pulse.

3. The method according to claim 1, wherein, Exporting phase-contrast angiography information includes reconstructing phase-contrast angiography MR images based on the acquired echo signals.

4. The method according to claim 3, wherein, The weighted subtraction of the morphological MR image and the phase-contrast angiography MR image is calculated to obtain a black-blood MR image of the body part.

5. The method according to any one of claims 1 to 4, wherein, The gradient echo imaging sequence performs undersampling in the k-space to accelerate the sampling of the predefined region.

6. The method according to claim 5, wherein, Parallel imaging is used to acquire and reconstruct the morphological MR images and / or the phase-contrast angiography MR images.

7. The method according to claim 5 or 6, wherein, The morphological MR images and / or the phase-contrast angiography MR images are reconstructed using compressed sensing or deep learning-based reconstruction.

8. The method according to any one of claims 1 to 7, wherein, The morphological MR image is a T1 contrast-enhanced MR image.

9. The method according to any one of the preceding claims, wherein, The echo signal acquired from the body part is based on a pulse sequence conforming to MP-RAGE fast gradient imaging with magnetization preparation, and the echo signal acquired from the body part also includes the at least one bipolar velocity-coded gradient (VE) pair.

10. The method of claim 9, further comprising displaying on a monitor (18) an echo signal acquired from the body based on an enhanced MP-RAGE pulse sequence using a bipolar velocity coding gradient (VE).

11. A computational system for controlling the timing sequence of radio frequency (RF) pulses and switched magnetic field gradients in a magnetic resonance (MR) system, the computational system being configured to control the MR system to perform the following operations: The body parts are magnetized in preparation for RF pulses (MP). After a preparation delay (TD), the body portion is subjected to a gradient echo imaging sequence comprising a series of RF excitations (EP), wherein, Phase-coded gradients (PE) and refocusing gradients (RO), along with at least one pair of bipolar velocity-coded gradients (VE), are applied in repetition time intervals (TR) between successive RF excitations (EP) to sample a predefined region in k-space and generate gradient echoes in each repetition time interval (TR). Echo signals are acquired from the body parts; The computing system is also configured to perform the following operations: Based on the acquired echo signals, morphological MR images of the body parts are reconstructed; and Phase-contrast angiography information is derived from the phase applied to the echo signal by the velocity-encoded gradient (VE).

12. A magnetic resonance (MR) system, comprising: At least one main magnet coil (2) is used to generate a substantially uniform and stable magnetic field within the inspection volume; Multiple gradient coils (4, 5, 6) are used to generate switching magnetic field gradients in different spatial directions within the inspection volume; At least one RF coil (9) is used to generate RF pulses within the examination volume and / or to receive MR signals from a body part (10) located in the examination volume; as well as The computing system according to claim 11.

13. The system according to claim 11 or 12, wherein, The echo signal acquired from the body part is based on a pulse sequence conforming to MP-RAGE fast gradient imaging with magnetization preparation, and the echo signal acquired from the body part also includes the at least one bipolar velocity-coded gradient (VE) pair.

14. The system of claim 13 further includes a monitor (18) configured to display a representation based on echo signals acquired from the body using a bipolar velocity coding gradient (VE) to enhance the MP-RAGE pulse sequence.

15. A computer program product comprising instructions executed by a computing system of a magnetic resonance MR system (1), wherein, Execution of the instructions causes the MR system (1) to perform the method according to any one of claims 1 to 10.

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