Spin-echo MR imaging using spiral acquisition

Abstract

The present invention relates to a method for MR imaging of a subject 10 placed within an examination volume of an MR device 1. The object of the present invention is to enable spiral MR images with a reduced level of ringing artifacts due to strong local main magnetic field inhomogeneities. The method includes the steps of generating spin echoes by subjecting the subject to an imaging sequence having an RF excitation pulse 31 followed by an RF refocusing pulse 32, where a modulated readout magnetic field gradient 34 is applied after the RF refocusing pulse 32; acquiring MR signal data by recording the spin echoes along a spiral trajectory in k-space, where the waveform of the readout magnetic field gradient 34 defining the spiral trajectory starts before the spin echo center 33; and reconstructing an MR image from the acquired MR signal data. The present invention further relates to the MR device 1 and a computer program for the 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 further relates to an MR device and to a computer program running on an MR device. [Background technology]

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

[0003] According to a typical MR method, the subject, e.g., the body of a patient to be examined, is placed in a strong, uniform magnetic field, the direction of which defines the axis (usually the z-axis) of the coordinate system on which the measurements are based. The magnetic field generates different energy levels for individual nuclear spins, depending on the field strength (spin resonance), which can be excited by the application of an alternating electromagnetic field (RF field) of a defined frequency (the so-called Larmor frequency or MR frequency). From a macroscopic perspective, the distribution of individual nuclear spins generates an overall magnetization, which can be deflected out of equilibrium by the application of an electromagnetic pulse (RF pulse) of an appropriate frequency, resulting in a precession of the magnetization around the z-axis. The precession describes the surface of a cone whose opening angle 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 in a plane transverse to the z-axis (flip angle 90°).

[0004] After the RF pulse ends, the magnetization returns to its original equilibrium state, where the magnetization in the z direction reestablishes with a first time constant T1 (spin-lattice or longitudinal relaxation time), while the magnetization in the direction perpendicular to the z direction relaxes with a second time constant T2 (spin-spin or transverse relaxation time). The magnetization fluctuations can be detected by a receive RF coil positioned within the examination volume of the MR system and oriented so that the magnetization fluctuations are measured in a direction perpendicular to the z axis. The decay of the transverse magnetization, for example, after the application of a 90° pulse, involves the transition of nuclear spins (caused by inhomogeneities in the local magnetic field) from an ordered state with the same phase to a state in which all phase angles are uniformly distributed (dephasing). The dephasing can be compensated for, for example, by a refocusing pulse (e.g., a 180° pulse). This generates an echo signal (spin echo) in the receive coil.

[0005] To achieve spatial resolution within the body, magnetic field gradients extending along the three principal axes are superimposed on a uniform magnetic field, resulting in a linear spatial dependence of the spin resonance frequency. The signals picked up in the receive coils then contain different frequency components that can be associated with different locations within the body. The signal data obtained through the receive coils corresponds to the spatial frequency domain and is called k-space data. The k-space data set is converted into an MR image by an image reconstruction algorithm. Summary of the Invention [Problem to be solved by the invention]

[0006] Spiral imaging is a fast MR imaging technique that benefits from efficient k-space coverage and low sensitivity to motion and flow artifacts. Spiral k-space trajectories allow efficient and time-flexible sampling of k-space because shorter paths are required to cover the desired k-space region, and signal acquisition can begin at the center of k-space. However, spiral imaging techniques are vulnerable to amplitude inhomogeneities in the main magnetic field B, which cause blurring and degrade image quality.

[0007] Methods for deblurring spiral MR imaging are known in the art. For example, a B0 map is acquired and MR signal data is corrected for B0 inhomogeneity effects based on the B0 map (see, for example, Ahunbay et al., "Rapid method for de-blurring spiral MR images", Magn. Reson. Med. 2000, vol. 44, pp. 491-494; Sutton et al., "Fast, iterative image reconstruction for MRI in the presence of field inhomogeneities", IEEE Trans. Med. Imaging. 2003, vol. 22, pp. 178-188; Nayak et al., "Efficient off-resonance correction for spiral imaging", Magn. Reson. Med. 2001, vol. 45, pp. 521-524).

[0008] However, even after applying the above-mentioned types of de-blurring methods, artifacts often remain in image regions with very strong susceptibility-induced magnetic field gradients. In the case of spiral k-space trajectories, such artifacts appear in reconstructed, deblurred MR images as characteristic ringing artifacts, which can overlap or obscure anatomical details of interest. For example, images obtained by spiral MR imaging of the brain are typically contaminated by off-resonance signal contributions from the sagittal sinus and nasal cavity. The reason for such residual artifacts is that, in the case of strong local magnetic field gradients, the shape of the spiral k-space trajectory deviates significantly from the theoretical spiral shape of each voxel. This is illustrated in the two-dimensional k-space diagrams of Figures 2A and 2B. Figure 2A shows an "ideal" spiral k-space trajectory obtained in the presence of a perfectly uniform main magnetic field B0. Spin echoes are generated by applying an RF excitation pulse followed by an RF refocusing pulse, followed by a modulated readout magnetic field gradient (typically in the x and y directions). The waveform of the modulated readout magnetic field gradient defines the spiral trajectory along which spin echo signals are recorded, sampled outward from the center of k-space toward its periphery. In the examples of Figures 2A and 2B, the temporal starting point of the trajectory is the spin echo center, i.e., the moment when the nuclear spins are perfectly in phase after refocusing. In Figure 2B, although the same imaging sequence and readout magnetic field gradient waveform are used as in Figure 2A, B is inhomogeneous with a strong gradient in the x direction, and the corresponding voxel locations "see" distorted k-space trajectories that deviate significantly from the ideal spiral shape. The effect of the distortion of the k-space spiral in Figure 2B caused by the local magnetic field gradient is a pileup of samples in the k-space region near the k-space center (indicated by the arrow in Figure 2B). It is this pile-up of samples that causes the characteristic ringing artifacts.In regions with particularly high local magnetic field gradients, these artifacts cannot be adequately corrected in practice because the B0 map used as the basis for the deblurring process never perfectly matches the actual magnetic field distribution. As a result, the respective k-space region remains over-enhanced even after deblurring, and ringing artifacts are still present.

[0009] From the above, it can be readily appreciated that there is a need for improved MR imaging techniques. It is an object of the present invention to address the above-mentioned limitations and enable spiral MR images with reduced levels of artifacts. [Means for solving the problem]

[0010] According to the present invention, a method for MR imaging of an object disposed within an examination volume of an MR device is disclosed, comprising the steps of generating spin echoes by subjecting the object to an imaging sequence having an RF excitation pulse followed by an RF refocusing pulse, wherein a modulated readout magnetic field gradient is applied after the RF refocusing pulse, acquiring MR signal data by recording the spin echoes along a spiral trajectory in k-space, wherein a waveform of the readout magnetic field gradient defining the spiral trajectory is initiated before the spin echo center, and reconstructing an MR image from the acquired MR signal data.

[0011] The gist of the present invention is that, instead of starting spiral acquisition at the echo center, i.e., when all nuclear spins are in phase, as is conventionally done, the starting point of the gradient waveform defining the spiral k-space trajectory is shifted to a time sufficiently earlier than the echo center. The result of the present approach is that, in the presence of a local magnetic field gradient from the center of k-space toward the k-space periphery, the k-space location of the first recorded MR signal data (ideally, the location where the center of the k-space spiral is assumed to be) is shifted. While magnetic field inhomogeneity still causes distortion of the spiral trajectory and signal sample pileup is not prevented by the present approach, the k-space location of the pileup is shifted from the k-space center to a k-space region with less important image information, resulting in less noticeable ringing artifacts in the reconstructed MR image.

[0012] In a preferred embodiment, the start of the readout gradient waveform is shifted relative to the spin echo center by one-quarter to one-half, preferably about one-third, of the total acquisition time of the spiral trajectory. In this way, the location of the signal pileup is shifted from the k-space center toward the k-space periphery by a sufficient amount so that ringing artifacts become significantly less noticeable or are no longer perceptible. In a practical embodiment, the readout gradient waveform can be initiated 2-15 ms before the spin echo center to achieve the desired effect.

[0013] In a possible embodiment, the reconstructed MR image is diffusion-weighted. For this purpose, diffusion-weighted magnetic field gradients are applied before and after the RF refocusing pulse, with the duration of the diffusion-weighted magnetic field gradient applied before the RF refocusing pulse being longer than the duration of the diffusion-weighted magnetic field gradient applied after the RF refocusing pulse. In this way, it can be achieved that the time shift proposed by the present invention does not result in an increase in the echo time.

[0014] In yet another practical embodiment, the reconstruction of the MR image includes deblurring based on the B map. Known deblurring methods for spiral MR imaging, including separate acquisition of the B map, can be used to correct the MR signal data for B inhomogeneity effects. Such a deblurring process is advantageous for compensating for distortions of the spiral k-space trajectory caused by local magnetic field gradients, regardless of the occurrence of signal pileup.

[0015] The inventive method described above can be performed by an MR apparatus having at least one main magnet coil for generating a homogeneous static magnetic field in an examination volume, multiple gradient coils for generating switched magnetic field gradients in different spatial directions in the examination volume, at least one RF coil for generating RF pulses in the examination volume and / or receiving MR signals from a subject arranged in the examination volume, a control unit for controlling the temporal succession of RF pulses and switched magnetic field gradients, and a reconstruction unit for reconstructing MR images from the received MR signals. The inventive method can be realized, for example, by a corresponding program in the reconstruction unit and / or the control unit of the MR apparatus.

[0016] The method of the present invention can be advantageously implemented in most MR devices currently in clinical use. For this purpose, it is only necessary to utilize a computer program that controls the MR device to perform the above-described method steps of the present invention. The computer program may be present on a data carrier or in a data network, so as to be downloaded for installation in the control unit of the MR device.

[0017] The magnetic resonance imaging system is configured to realize the reconstruction of the set of magnetic resonance images in that reconstruction software is installed on the computing system or the computing system has access to a remote reconstruction facility. The reconstruction software may be installed on a remote server at a medical institution, for example, accessible to a data network, in that the reconstruction software is available in the "cloud." In these remote configurations, the computing system is provided with the functionality to realize the reconstruction of the set of magnetic resonance images in a remotely located reconstruction function.

[0018] The accompanying drawings disclose preferred embodiments of the present invention, but it is to be understood that the drawings are designed for purposes of illustration only and not as a definition of the limits of the invention. [Brief explanation of the drawings]

[0019] [Figure 1] 1 is a diagram showing an MR apparatus for carrying out the method of the present invention. [Figure 2A] K-space diagram showing spiral sampling without main field inhomogeneity. [Figure 2B] k-space diagram showing spiral sampling with main field inhomogeneity. [Figure 3] FIG. 2 is a diagram illustrating an imaging sequence according to the present invention. [Figure 4] FIG. 2 is a k-space diagram illustrating the spiral acquisition scheme of the present invention in a second embodiment. [Figure 5] 1 shows a T1-weighted MR brain image (left image) with noticeable ringing artifacts and a T1-weighted MR brain image (right image) with reduced levels of artifacts using the method of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0020] Referring to Figure 1, there is shown a schematic representation of an MR system 1. The system has superconducting or resistive main magnet coils 2 such that a substantially uniform and temporally constant main magnetic field is generated along the z-axis through an examination volume.

[0021] The magnetic resonance generation and manipulation system applies a series of RF pulses and switching 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, and the like to perform MR imaging.

[0022] More specifically, gradient pulse amplifiers 3 apply current pulses to selected ones 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 whole-body volume RF coil 9 via transmit / receive switch 8 to transmit RF pulses into the examination volume. A typical MR imaging sequence consists of packets having multiple short-duration RF pulse segments that, together with any applied magnetic field gradients, achieve selected manipulation of nuclear magnetic resonance. The RF pulses are used to saturate resonance, excite resonance, invert magnetization, refocus resonance, or manipulate resonance, as well as to select portions of a body 10 located within the examination volume. MR signals are also picked up by the whole-body volume RF coil 9.

[0023] To generate MR images of a limited region of the body 10, a set of local array RF coils 11, 12, 13 are positioned adjacent to the region selected for imaging. The array coils 11, 12, 13 can be used to receive MR signals caused by body coil RF transmissions.

[0024] The resulting MR signals are picked up by a whole-body volume RF coil 9 and / or array RF coils 11, 12, 13 and demodulated by a receiver 14, preferably having a preamplifier (not shown). The receiver 14 is connected to the RF coils 9, 11, 12, and 13 via a transmit / receive switch 8.

[0025] A host computer 15 controls the gradient pulse amplifier 3 and the transmitter 7 to generate multiple MR imaging sequences, such as spin-echo imaging, and acquire MR signals along spiral k-space trajectories according to the present invention. For a selected sequence, the receiver 14 receives single or multiple MR data along each k-space trajectory. A data acquisition system 16 performs analog-to-digital conversion of the received signals and converts each MR signal into a digital format suitable for further processing. In modern MR devices, the data acquisition system 16 is a separate computer dedicated to acquiring raw image data.

[0026] Ultimately, the digital raw image data is reconstructed into an image representation by a reconstruction processor 17, which applies interpolation or re-gridding of the data from the spiral acquisitions before a Fourier transform or other appropriate reconstruction algorithm. The MR image may represent a planar slice across the patient, an array of parallel planar slices, a three-dimensional volume, etc. The image is then stored in an image memory, which can be accessed to convert slices, projections, or other portions of the image representation into an appropriate format for visualization, for example, via a video monitor 18, which provides a human-readable display of the resulting MR image.

[0027] The MR apparatus 1 is configured, for example, by suitable programming of a host computer 15 and a reconstruction processor 17 to carry out the imaging method of the invention as described above and below.

[0028] With continued reference to FIG. 1, and further reference to FIGS. 3-5, one embodiment of the imaging approach of the present invention will now be described.

[0029] FIG. 3 illustrates a schematic diagram of an imaging sequence according to the present invention. The imaging sequence is a spin echo sequence beginning with an RF excitation pulse 31. After a delay of duration TE / 2 (TE is the echo time), an RF refocusing pulse 32 is emitted. This generates a spin echo (not shown) whose spin echo center 33 is located at t=TE (t=0 at the time of the RF excitation pulse 31). A modulated readout magnetic field gradient 34 is applied after the RF refocusing pulse 32. For simplicity, FIG. 3 shows only one component of the readout magnetic field gradient waveform, e.g., generated in the x-direction. To obtain the desired spiral k-space trajectory, another modulated readout magnetic field gradient component (not shown) is generated in the perpendicular direction (y-direction). According to the present invention, the waveform of the readout magnetic field gradient 34, which defines the spiral k-space trajectory, is initiated before the spin echo center 33. The starting point 35 of the waveform of the readout magnetic field gradient 34 is shifted approximately 2 to 15 ms relative to the spin echo center 33. Diffusion-weighted magnetic field gradients 36, 37 are applied before and after the RF refocusing pulse 32, with the duration of the diffusion-weighted magnetic field gradient 36 applied before the RF refocusing pulse 32 being longer than the duration of the diffusion-weighted magnetic field gradient 37 applied after the RF refocusing pulse 33. This allows a diffusion-weighted MR image to be reconstructed from MR signal data recorded along a spiral k-space trajectory. The deviation from the standard Stejskal-Tanner method (which uses symmetric diffusion-weighted gradients) has the effect that a shift of the onset 35 to a position before the echo center 33 does not result in an increase in the echo time TE.

[0030] The effect of the inventive approach is illustrated in FIG. 4. The k-space location of the first MR signal data recorded (assuming the center point of the k-space spiral in the ideal case) is shifted in the presence of susceptibility-induced magnetic field gradients from the center of k-space toward the periphery of k-space. Magnetic field inhomogeneities still cause the same distortion of the spiral trajectory, as can be seen by comparison with FIG. 2B. Furthermore, signal sample pileups (marked by arrows in FIGS. 2B and 4, respectively) are not prevented by the inventive approach, but the k-space locations of the pileups are shifted away from the k-space center, into k-space regions where less significant image information is present. As a result, ringing artifacts become less noticeable.

[0031] This can be seen in Figure 5. The MR signal data for the left MR brain image was acquired conventionally using a spiral k-space trajectory acquisition starting exactly at the spin echo center. The ringing artifacts near the nasal cavity are marked with arrows. The echo time was 31 ms. The inventive approach was applied to the acquisition of the MR data for the right image in Figure 5. The same imaging parameters were used as for the left image, except that the start of the spiral acquisition was shifted in time to 10 ms before the echo center. As can be seen, the ringing artifacts are much less noticeable than in the left image. A conventional deblurring process based on separately acquired B0 maps was applied during the reconstruction of both images.

Claims

1. 1. A method of MR imaging of an object disposed in an examination volume of an MR device, comprising: generating spin echoes by subjecting the subject to an imaging sequence including an RF excitation pulse followed by an RF refocusing pulse, wherein a modulated readout magnetic field gradient is applied after the RF refocusing pulse; acquiring MR signal data by recording spin echoes along a spiral trajectory in k-space in the presence of an inhomogeneous main magnetic field having strong local magnetic field gradient regions, wherein the waveform of the readout magnetic field gradient defining the spiral trajectory begins before the spin echo center so as to shift the k-space location of pile-ups caused by the local magnetic field gradients by a sufficient amount from the k-space center toward the k-space periphery, and single diffusion-weighted magnetic field gradients are applied before and after the RF refocus pulse to diffusion-weight the acquired MR signals, the duration of the single diffusion-weighted magnetic field gradient applied before the RF refocus pulse being longer than the duration of the single diffusion-weighted magnetic field gradient applied after the RF refocus pulse; reconstructing a diffusion-weighted MR image from the acquired MR signal data; A method having the following.

2. 2. The method of claim 1, wherein the start of the readout magnetic field gradient waveform is shifted by one-quarter to one-half, or one-third of the total acquisition time of the spiral trajectory relative to the spin echo center.

3. 3. The method according to claim 1, wherein the waveform of the readout magnetic field gradient that defines the spiral trajectory is initiated 2 to 15 ms before the center of the spin echo.

4. The method of claim 1 , wherein the reconstruction of the MR image comprises performing deblurring based on a B0 map.

5. at least one main magnet coil for generating a uniform static magnetic field within the examination volume; a plurality of gradient coils for generating gradients in different spatial directions within the examination volume; at least one RF coil for generating RF pulses in the examination volume and / or receiving MR signals from the subject positioned within the examination volume; a control unit for controlling the time-sequence of RF pulses and switched magnetic field gradients; a reconstruction unit for reconstructing an MR image from the received MR signals; 1. An MR device comprising: generating spin echoes by generating an imaging sequence including an RF excitation pulse followed by an RF refocusing pulse, wherein a modulated readout magnetic field gradient is applied after the RF refocusing pulse; acquiring MR signal data by recording spin echoes along a spiral trajectory in k-space in the presence of an inhomogeneous main magnetic field having strong local magnetic field gradient regions, wherein the waveform of the readout magnetic field gradient defining the spiral trajectory begins before the spin echo center so as to shift the k-space location of pile-ups caused by the local magnetic field gradients by a sufficient amount from the k-space center toward the k-space periphery, and single diffusion-weighted magnetic field gradients are applied before and after the RF refocus pulse to diffusion-weight the acquired MR signals, the duration of the single diffusion-weighted magnetic field gradient applied before the RF refocus pulse being longer than the duration of the single diffusion-weighted magnetic field gradient applied after the RF refocus pulse; reconstructing a diffusion-weighted MR image from the acquired MR signal data; An MR device that performs the above.

6. A computer program executed on an MR device, generating spin echoes by applying an imaging sequence including an RF excitation pulse followed by an RF refocusing pulse, wherein a modulated readout magnetic field gradient is applied after the RF refocusing pulse; acquiring MR signal data by recording spin echoes along a spiral trajectory in k-space in the presence of an inhomogeneous main magnetic field having strong local magnetic field gradient regions, wherein a waveform of a readout magnetic field gradient defining the spiral trajectory begins before a spin echo center so as to shift a k-space location of pile-ups caused by the local magnetic field gradients from the k-space center toward the k-space periphery by a sufficient amount, and single diffusion-weighted magnetic field gradients are applied before and after the RF refocus pulse to diffusion-weight the acquired MR signals, the duration of the single diffusion-weighted magnetic field gradient applied before the RF refocus pulse being longer than the duration of the single diffusion-weighted magnetic field gradient applied after the RF refocus pulse; reconstructing a diffusion-weighted MR image from the acquired MR signal data; A computer program having instructions to cause the computer to execute a program.

Images