Multi-echo MR imaging using spiral acquisition

Abstract

The invention relates to a method for MR imaging of an object (10) positioned in an examination volume of an MR device (1). The object of the invention is to achieve fast spiral MR imaging with a defined T2 contrast. The method of the invention comprises the following steps: generating a plurality of spin echoes by subjecting the object (10) to one or more shots of an imaging sequence, each shot comprising an RF excitation pulse (21) followed by a plurality of RF refocusing pulses (22), wherein a modulated readout magnetic field gradient (23, 24) is applied in each interval between consecutive RF refocusing pulses (22); acquiring MR signal data, wherein each spin echo is recorded in k-space along a spiral trajectory (31-33, 41-43) winding around a k-space origin at a continuously varying radial distance, wherein the trajectory (31, 41) of at least one spin echo has a different rate of change of the radial distance than the trajectories (32, 33, 42, 43) of the other spin echoes, at least in a central k-space region; and reconstructing an MR image based on the acquired MR signal data. The invention also relates to an MR device (1) and a computer program for an MR device (1).

Description

Technical Field

[0001] The present invention relates to the field of magnetic resonance (MR) imaging. The present invention relates to a method for MR imaging of an object. The present invention also relates to an MR device and a computer program to be run on an MR device. Background Art

[0002] MR imaging methods that utilize the interaction between magnetic fields and nuclear spins to form two- or three-dimensional images have become widely used, particularly in the field of medical diagnosis, because they are superior in many aspects to other imaging methods for soft tissue imaging, do not require ionizing radiation, and are generally noninvasive.

[0003] Typically, according to the MR method, an object (e.g., the body of a patient to be examined) is placed in a strong and uniform magnetic field, the direction of which simultaneously defines the axis of the coordinate system on which the measurement is based (usually the z-axis). The magnetic field generates different energy levels for individual nuclear spins, depending on the magnetic field strength, which can be excited (spin resonance) by applying an electromagnetic alternating field (RF field) of a defined frequency (the so-called Larmor frequency or MR frequency). From a macroscopic point of view, the distribution of individual nuclear spins produces an overall magnetization that can be deflected from its equilibrium state by applying electromagnetic pulses (RF pulses) of appropriate frequency, causing it to precess about the z-axis. This precession motion describes the surface of a cone, the angle of which is called the flip angle. The magnitude of the flip angle depends on the strength and duration of the applied electromagnetic pulse. In the case of a so-called 90° pulse, the spins are deflected from the z-axis into the transverse plane (flip angle 90°).

[0004] After the RF pulse terminates, the magnetization relaxes back to its original equilibrium state, where the magnetization in the z-direction is reestablished with a first time constant T1 (spin lattice or longitudinal relaxation time), and the magnetization in the direction perpendicular to the z-direction relaxes with a second time constant T2 (spin-spin or transverse relaxation time). Changes in the magnetization can be detected using receive RF coils, which are arranged and oriented within the examination volume of the MR system so that magnetization changes are measured perpendicular to the z-axis. After applying a 90° pulse, for example, the decay of the transverse magnetization is accompanied by a transition of nuclear spins from an ordered state with identical phases to a state where all phase angles are uniformly distributed (dephased) (caused by local magnetic field inhomogeneities). Dephasing can be compensated, for example, with the aid of a refocusing pulse (e.g., a 180° pulse). This generates an echo signal (spin echo) in the receive coils.

[0005] To achieve spatial resolution in the body, constant magnetic field gradients extending along the three main axes are superimposed on the uniform magnetic field, so that the spin resonance frequency has a linear spatial dependence. The signal picked up in the receiving coil then contains components of different frequencies that can be associated with different locations in the body. The signal data obtained via the receiving coil corresponds to the spatial frequency domain and is called k-space data. K-space data typically includes multiple lines acquired with different phase encodings. Each line is digitized by collecting a large number of samples. The k-space data set is converted into an MR image with the help of an image reconstruction algorithm.

[0006] Spiral imaging is a fast MR imaging technique that offers the benefits of efficient k-space coverage and low sensitivity to flow artifacts. However, spiral imaging is susceptible to inhomogeneities in the amplitude of the main magnetic field, B0, which can cause blurring and degrade image quality, particularly at high main magnetic field strengths. In spiral imaging, MR signals are acquired in k-space along a spiral trajectory that winds around the origin of k-space with increasing ("spiraling out") or decreasing ("spiraling in") radial distance from the k-space origin.

[0007] Due to its efficient k-space coverage, spiral imaging can be advantageously combined with fast spin echo (FSE) imaging to further reduce the total scan time (because fewer excitations, i.e., "shots," of the imaging sequence are required). FSE uses a series of spin echoes generated as a train following a single RF excitation pulse. In FSE imaging, each echo is formed by a (180°) refocusing RF pulse. Due to transverse relaxation (T2) from echo to echo in the train (as the number of echoes increases), the echo signal amplitude decreases. In combination with spiral imaging, each spin echo is recorded along a spiral trajectory in k-space. The trajectories of the spin echoes of the entire echo train form a set of staggered spirals in k-space to achieve appropriate sampling of k-space, thereby enabling MR images to be reconstructed for the desired field of view and resolution. Acquisition of staggered k-space spirals has the additional advantage that the acquisition time of each stagger can be kept small, so that off-resonance ambiguity is significantly reduced.

[0008] Because the spin echoes are recorded at different acquisition times after RF excitation, the MR signal data consisting of all recorded echoes is a mixture of different T2 weightings. Each spiral k-space trajectory of a spin echo train covers the entire k-space, including the region near the k-space origin that dominates the contrast of the reconstructed MR images. As a result, the reconstructed MR images do not have a defined T2 weighting. This is a major disadvantage of the described FSE technique using (interleaved) spiral acquisition. Summary of the Invention

[0009] From the above it is readily apparent that there is a need for improved MR imaging techniques.It is an object of the present invention to address the above limitations and to enable fast spiral MR imaging with accurate and robust control of image contrast, in particular with a defined T2 contrast.

[0010] According to the present invention, a method for MR imaging of an object positioned in an examination volume of an MR device is disclosed. The method comprises the following steps:

[0011] generating a plurality of spin echoes by subjecting the object to one or more shots of an imaging sequence, each shot comprising an RF excitation pulse followed by a plurality of RF refocusing pulses, wherein a modulated readout magnetic field gradient is applied in each interval between successive RF refocusing pulses,

[0012] Acquiring MR signal data, wherein each spin echo is recorded in k-space along a spiral trajectory that winds around the k-space origin at a continuously varying radial distance,

[0013] wherein the trajectory of at least one spin echo has a different rate of change of the radial distance compared to the trajectories of the other spin echoes, at least in a central k-space region, and

[0014] An MR image is reconstructed based on the acquired MR signal data.

[0015] Similar to the prior art, the trajectories associated with the individual echoes are preferably chosen to form a set of (eg Archimedean) interleaved spirals in k-space to achieve the required sampling of the k-space region of interest.

[0016] The spiral trajectory can start or end at the k-space origin at a continuously increasing or decreasing radial distance from the k-space origin, respectively. In other words, the method of the invention can be used with both spirally inward and spirally outward acquisitions.

[0017] According to the present invention, at least one (preferably exactly one) spin echo is selected to have a different (particularly smaller) rate of increase or decrease with radial distance (i.e., the speed with which the trajectory advances from the k-space origin or approaches the k-space origin from a more distant starting position in k-space) compared to the trajectories of the other spin echoes. This allows the central k-space region, which dominates the contrast of the reconstructed MR image, to be more densely sampled during the specific acquisition time of the selected spin echo. For example, the acquisition time for all spiral arms can be approximately equal. In another example, all but one or a few spiral arms can have a higher (much higher) radial velocity in the central k-space region. This effectively causes all but one or a few spiral arms to begin sampling outside the central k-space region. The present invention enables a single spiral arm covering the central k-space region to accurately and robustly control the desired image contrast. According to the present invention, the central k-space region is sampled using the selected spin echo to obtain MR images with well-defined T2 weighting. To achieve this, for example, during the MR image reconstruction step, selected spin echoes whose k-space trajectories exhibit different (particularly smaller) rates of increase / decrease of radial distance can be weighted differently (particularly more strongly) than other spin echoes. The weighting of the acquired MR signal data can depend on the number of echoes and the k-space position. To tune the desired T2 weighting, the number of echoes in the echo train can be appropriately selected for at least one spin echo to exhibit deviating rates of change of radial distance.

[0018] The method of the present invention can also be advantageously used to obtain T1-weighted MR images. To this end, the imaging sequence can include appropriate measures to generate T1 weighting, such as a corresponding magnetization preparation sequence. In an embodiment of the present invention, the trajectory of the at least one spin echo covers only the central k-space region, while the trajectories of the other spin echoes also cover peripheral k-space regions that extend at a greater radial distance from the k-space origin than the central k-space region. In this way, the selected spin echo is focused on the central k-space region to achieve the desired contrast. The other spin echoes provide data from the peripheral k-space regions, acquired with different T2 weightings depending on their respective acquisition times. This has only a minimal impact on the contrast of the reconstructed MR image, as the data from the central k-space region that dominates the image contrast is available for the selected spin echo with the defined T2 weighting.

[0019] In another embodiment of the present invention, the rate of increase of the radial distance of the trajectories of all spin echoes except the at least one spin echo in the central k-space region is greater than the rate of increase of the radial distance in the peripheral k-space region. This embodiment provides for the situation in which all spin echoes except the at least one spin echo have a particularly low sampling density in the center of k-space. Signal data from the central k-space region is provided solely by the at least one spin echo, which has a lower rate of increase of radial distance in the central k-space region than the other spin echoes. In this way, MR images with well-defined T2 weighting can again be acquired. This embodiment has the advantage that the k-space sampling density can be kept more or less constant throughout k-space.

[0020] The method of the present invention described so far can be performed with the aid of an MR system comprising: at least one main magnet coil for generating a uniform static magnetic field 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 RF coil for generating RF pulses within the examination volume and / or receiving MR signals from an object positioned within the examination volume; a control unit for controlling the temporal progression of the RF pulses and the switched magnetic field gradients; and a reconstruction unit for reconstructing an MR image from the received MR signals. The method of the present invention can be implemented, for example, by correspondingly programming the reconstruction unit and / or the control unit of the MR system.

[0021] The method of the present invention can be advantageously implemented in most MR devices currently in clinical use. To this end, it is only necessary to utilize a computer program to control the MR device so that it performs the aforementioned method steps of the present invention. This computer program can be stored on a data carrier or over a data network so that it can be downloaded and installed in the control unit of the MR device. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] The accompanying drawings disclose preferred embodiments of the present invention. However, it should be understood that the drawings are designed for illustrative purposes only and are not intended to limit the present invention. In the drawings:

[0023] Figure 1 An MR device for carrying out the method of the present invention is shown;

[0024] Figure 2 Schematically illustrates a modified spiral FSE imaging sequence of the present invention;

[0025] Figure 3 shows a diagram schematically illustrating k-space of a spiral acquisition scheme of the present invention in a first embodiment;

[0026] Figure 4 A diagram of k-space schematically illustrating the spiral acquisition scheme of the present invention in a second embodiment is shown. DETAILED DESCRIPTION

[0027] refer to Figure 1 , schematically shows an MR device 1. The device comprises superconducting or resistive main magnet coils 2, such that a substantially uniform, temporally constant main magnetic field is created along a z-axis through an examination volume.

[0028] Magnetic resonance generation and manipulation systems apply a series of RF pulses and switched magnetic field gradients to invert or excite nuclear magnetic spins, induce magnetic resonance, refocus magnetic resonance, manipulate magnetic resonance, spatially and otherwise encode magnetic resonance, saturate spins, etc. to perform MR imaging.

[0029] More specifically, a gradient pulse amplifier 3 applies current pulses to a selected one of 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 a whole-body volume RF coil 9 via a transmit / receive switch 8 to transmit RF pulses into the examination volume. A typical MR imaging sequence comprises packets of short-duration RF pulse segments that are acquired in conjunction with one another and any applied magnetic field gradients to achieve selected manipulation of nuclear magnetic resonance. RF pulses are used to saturate resonance, excite resonance, invert magnetization, refocus resonance, or manipulate resonance, as well as to select a portion of a body 10 positioned within the examination volume. MR signals are also picked up by the whole-body volume RF coil 9.

[0030] To generate an MR image of a limited region of the body 10, a set of local array RF coils 11, 12, 13 are placed adjacent the region selected for imaging.The array coils 11, 12, 13 can be used to receive MR signals induced by body coil RF transmissions.

[0031] The resulting MR signals are picked up by the whole body volume RF coil 9 and / or array RF coils 11, 12, 13 and demodulated by a receiver 14, preferably including a preamplifier (not shown). The receiver 14 is connected to the RF coils 9, 11, 12 and 13 via a transmit / receive switch 8.

[0032] A host computer 15 controls the gradient pulse amplifier 3 and transmitter 7 to generate any of a number of MR imaging sequences. For the selected sequence, the receiver 14 receives single or multiple MR signals in rapid succession following each RF excitation pulse. A data acquisition system 16 performs analog-to-digital conversion on the received signals and converts each MR signal into a digital format for further processing. In modern MR devices, the data acquisition system 16 is a separate computer dedicated to acquiring raw image data.

[0033] Ultimately, the digital raw image data is reconstructed into an image representation by a reconstruction processor 17, which applies a Fourier transform or other appropriate reconstruction algorithm. The MR image may represent a planar slice through the patient, an array of parallel planar slices, a three-dimensional volume, etc. The image is then stored in an image memory where it can be accessed to convert the slice, projection, or other portion of the image representation into a suitable format for visualization, for example, via a video monitor 18, which provides a human-readable display of the resulting MR image.

[0034] The MR device 1 is arranged (eg by appropriate programming of the host computer 15 and the reconstruction processor 17) to perform the imaging method of the invention as described above and below.

[0035] Continue to refer Figure 1 And further reference Figure 2-4 , an embodiment of the imaging method of the present invention is explained.

[0036] Figure 2 The imaging sequence used according to the invention is schematically illustrated.For simplicity, only the RF pulses (black blocks) and the readout magnetic field gradient in the x-direction (oscillating signal) are shown. Figure 2 A portion of a shot of an imaging sequence that is a modified FSE sequence is shown. The shot starts with a 90° RF excitation pulse 21, followed by a plurality of 180° RF refocusing pulses 22. In order to acquire the train of generated spin echoes in the intervals between successive refocusing pulses 22 along a spiral k-space trajectory, sinusoidally modulated readout magnetic field gradients 23, 24 are applied. It can be seen that the amplitude of the gradient oscillations increases continuously in each interval, which means that the acquisition starts at the k-space origin and winds around the k-space origin with increasing radial distance. The magnetic field gradient in the y direction (not depicted) oscillates in the same way, but with a phase offset of 90°. Compared to the magnetic field gradient 23, the magnetic field gradient 24 oscillates at a higher frequency, while the maximum gradient amplitude reached over the acquisition interval is smaller.

[0037] exist Figure 3The resulting spiral k-space sampling is illustrated in FIG. The k-space trajectory 31 of the spin echo associated with the readout magnetic field gradient 24 has a smaller rate of increase in radial distance than the trajectories of the other spin echoes associated with spiral trajectories 32 and 33. At the same time, the number of wraps of trajectory 31 in the central k-space region is higher than that of the other trajectories that cover the entire k-space (i.e., both the central k-space region and the peripheral k-space region, which extends at a greater radial distance from the k-space origin than the central k-space region). Thus, trajectory 31 samples only the central k-space region, but at a higher density than each of trajectories 32 and 33. The complete set of interleaved k-space spirals completely samples k-space at the required density corresponding to the desired field of view. Figure 3 Six interlaced spiral trajectories corresponding to six spin echo signals are shown, but for greater clarity, only trajectories 31, 32, and 33 are designated by reference numerals. According to the present invention, trajectory 31 is used to sample the central k-space region in order to reconstruct an MR image with well-defined T2 weighting from the acquired signal data. To this end, the recorded spin echo associated with trajectory 31 is given greater weight in the reconstruction than the other spin echoes.

[0038] exist Figure 4 In the embodiment shown, the interleaved spiral trajectories 42, 43 of all but one spin echo have a greater rate of increase in radial distance in the central k-space region than in the peripheral k-space regions. This results in a significantly lower sampling density of the trajectories 42, 43 in the central k-space region, with only the trajectory 41 sampling the central k-space region at a high density. In this way, it is possible to reconstruct an MR image with a well-defined T2 weighting, since the signal data in the central k-space region, which dominates the image contrast, is associated with a single spin echo. This embodiment has the following advantages: Figure 3 In the embodiment shown, the k-space sampling density can be kept more or less constant across the entire k-space, while the central k-space region is oversampled.

Claims

1. A method for MR imaging of an object (10) positioned in an examination volume of an MR device (1), the method comprising: A plurality of spin echoes are generated by subjecting the object (10) to one or more shots of an imaging sequence, each shot comprising an RF excitation pulse (21) followed by a plurality of RF refocusing pulses (22), wherein a modulated readout magnetic field gradient (23, 24) is applied in each interval between successive RF refocusing pulses (22), Acquiring MR signal data, wherein each spin echo is recorded in k-space along a spiral trajectory (31-33, 41-43) which winds around a k-space origin at a continuously varying radial distance, wherein the trajectory (31, 41) of at least one spin echo has a different rate of change of the radial distance than the trajectories (32, 33, 42, 43) of the other spin echoes, at least in a central k-space region, and An MR image is reconstructed based on the acquired MR signal data.

2. The method according to claim 1, wherein The trajectories ( 31 - 33 , 41 - 43 ) form a set of interleaved spirals in k-space.

3. The method according to claim 1 or 2, wherein: The trajectory (31, 41) of the at least one spin echo has a higher winding number in the central k-space region than the trajectories (32, 33, 42, 43) of the other spin echoes.

4. The method according to claim 3, wherein: The trajectory (31, 41) of the at least one spin echo has a lower rate of change of the radial distance in the central k-space region than the trajectories of the other spin echoes.

5. The method according to claim 3, wherein: The trajectory (31) of the at least one spin echo covers only the central k-space region, whereas the trajectories (32, 33) of the other spin echoes also cover peripheral k-space regions, which extend at a greater radial distance from the k-space origin than the central k-space region.

6. The method according to claim 5, wherein: The trajectories (42, 43) of all spin echoes except the at least one spin echo have a rate of increase of the radial distance in the central k-space region that is greater than the rate of increase of the radial distance in the peripheral k-space region.

7. The method according to any one of claims 1 to 2, wherein: The reconstructed MR images are T2-weighted.

8. The method according to any one of claims 1 to 2, wherein: The reconstructed MR images are T1-weighted.

9. The method according to any one of claims 1 to 2, wherein: In the step of reconstructing the MR image, at least one recorded spin echo (31, 41) of the k-space trajectory having a different rate of change of the radial distance is weighted differently than the other spin echoes.

10. An MR device, comprising: at least one main magnet coil (2) for generating a uniform static magnetic field within an examination volume; A plurality of gradient coils (4, 5, 6) for generating switched magnetic field gradients in different spatial directions within the examination volume; at least one RF coil (9) for generating RF pulses within the examination volume and / or receiving MR signals from an object (10) positioned in the examination volume; a control unit (15) for controlling the temporal succession of RF pulses and the switched magnetic field gradients; and a reconstruction unit (17) for reconstructing an MR image from the received MR signals, wherein the MR device (1) is arranged to perform the following steps: A plurality of spin echoes are generated by subjecting the object (10) to one or more shots of an imaging sequence, each shot comprising an RF excitation pulse (21) followed by a plurality of RF refocusing pulses (22), wherein a modulated readout magnetic field gradient (23, 24) is applied in each interval between successive RF refocusing pulses (22), Acquiring MR signal data, wherein each spin echo is recorded in k-space along a spiral trajectory (31-33, 41-43) which winds around a k-space origin at a varying radial distance, wherein the trajectory (31, 41) of at least one spin echo has a different rate of change of the radial distance than the trajectories (32, 33, 42, 43) of the other spin echoes, at least in a central k-space region, and An MR image is reconstructed based on the acquired MR signal data.

11. A computer program product comprising a computer program running on an MR device, the computer program comprising instructions for: One or more shots of an imaging sequence are applied, each shot comprising an RF excitation pulse (21) followed by a plurality of RF refocusing pulses (22), wherein A modulated readout magnetic field gradient (23, 24) is applied in each interval between successive RF refocusing pulses (22), Acquiring MR signal data, wherein each of a plurality of spin echoes is recorded in k-space along a spiral trajectory (31-33, 41-43) which winds around a k-space origin at a varying radial distance, wherein the trajectory (31, 41) of at least one spin echo has a different rate of change of the radial distance than the trajectories (32, 33, 42, 43) of the other spin echoes, at least in a central k-space region, and An MR image is reconstructed based on the acquired MR signal data.

Images