RU2785553C2 - Magnetic resonance imaging with spiral data collection - Google Patents

Abstract

FIELD: diagnostics.

SUBSTANCE: group of inventions relates to magnetic resonance imaging of an object. A method for magnetic resonance imaging of an object includes exposure of the object to an imaging sequence containing at least one radio-frequency excitation impulse and sinusoidally modulated magnetic field gradients, obtainment of magnetic resonance signals along two or more spiral trajectories of k space, set by sinusoidal modulation of magnetic field gradients, wherein beginnings of spiral trajectories of k space are displaced relatively to each other, and restoration of a magnetic resonance image based on received magnetic resonance signals.

EFFECT: provision of effective spiral magnetic resonance imaging even in cases of strong field inhomogeneity B₀.

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Description

The field of technology to which the invention belongs

The invention relates to the field of magnetic resonance (MR) tomography. The invention relates in particular to a method for magnetic resonance imaging of an object. The invention also relates to a magnetic resonance imaging (MRI) device and a computer program to be executed on the magnetic resonance imaging device.

State of the art

Magnetic resonance imaging methods that use the interaction between magnetic fields and nuclear spins to form two-dimensional or three-dimensional images are currently widely used, in particular in the field of medical diagnostics, since they are superior in many respects to other imaging methods for soft tissue imaging, do not require ionizing radiation and are generally non-invasive.

In accordance with the magnetic resonance method in general, an object, such as the body of the patient being examined, is placed in a strong, uniform magnetic field, the direction of which at the same time determines the axis (usually the z-axis) of the coordinate system on which the measurement is based. The magnetic field creates different energy levels for individual nuclear spins depending on the strength of the magnetic field, which can be excited (spin resonance) by applying an alternating electromagnetic field (radio frequency (RF) field) of a given frequency (the so-called Larmor frequency or magnetic resonance frequency ). From a macroscopic point of view, the distribution of individual nuclear spins creates an overall magnetization that can be brought out of equilibrium by the application of an electromagnetic pulse of the appropriate frequency (radio frequency (RF) pulse), so that the magnetization performs a precessional motion around the z-axis. The precessional motion describes the surface of a cone, the aperture angle of which is called the flip angle (or the so-called flip angle, from the English flip angle). The value of the flip angle depends on the strength and duration of the applied electromagnetic pulse. In the case of the so-called 90° momentum, the spins deviate from the z-axis into the transverse plane (the flip angle is 90°).

After the end of the RF pulse, the magnetization returns to the initial state of equilibrium, in which the magnetization in the z direction increases again with the first time constant T ₁ (spin-lattice or longitudinal relaxation time), and in the direction perpendicular to the z direction, the magnetization relaxes with the second time constant T ₂ ( spin-spin or transverse relaxation time). The change in magnetization can be detected using RF receiving coils that are positioned and oriented within the volume of the MRI device to be examined in such a way that the change in magnetization is measured in the direction perpendicular to the z-axis. The drop in the transverse magnetization is accompanied, for example, after the application of a 90° pulse, by the transition of nuclear spins (induced by local inhomogeneities of the 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, with a refocusing pulse (eg a 180° pulse). In this case, an echo signal (spin echo) is created in the receiving coils.

To realize spatial resolution in the body, constant magnetic field gradients along three main axes are superimposed on a uniform magnetic field, which leads to a linear spatial dependence of the spin resonance frequency. The signal picked up by the pickup coils thus contains components of different frequencies that can be associated with different locations in the body. The signal data received through the pickup coils corresponds to the spatial frequency domain and is referred to as k-space data. The k-space dataset is converted into an MR image using an image reconstruction algorithm.

Spiral imaging is a fast magnetic resonance imaging (tomography) that has the advantages of efficient k-space coverage and low sensitivity to flow artifacts. However, the disadvantage is the vulnerability to inhomogeneities in the amplitude of the main magnetic field In ₀ that causes blurring and degradation of image quality. The degree of inhomogeneity of the field B ₀ is scaled with the strength of the main magnetic field.

Methods for blurring for spiral magnetic resonance imaging are known in the art. It is known, for example, to obtain a series of maps of the B ₀ field using an inward/outward spiral double echo sequence with different echo times (see, e.g., Ahunbay et al., "Rapid method for deblurring spiral MR images" blurring of 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 inhomogeneties" images 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"("Effective off-resonance correction for spiral imaging"). imaging"), Magn. Reson. Med. 2001, vol. 45, pp. 521-524).

In the case of strong inhomogeneities of the magnetic field, the shape of the helical trajectories of k-space deviates to a large extent from the theoretical shape of the helix. This is shown in the 2D k-space diagrams in FIG. 2. In FIG. 2a shows an "ideal" k-space helical trajectory that would be obtained using sinusoidal magnetic field gradients in the k _x and k _y directions in the presence of a perfectly uniform main magnetic field B ₀ . In FIG. 2b, the B ₀ field is non-uniform with a strong gradient in the x direction such that the rendered object "sees" a k-space trajectory that deviates substantially from the ideal helix shape. The consequence of this effect is that practically no usable signal data is sampled from the central k-space; k _x =k _y =0 is not even included in the distorted sampling trajectory. Completely missing parts of the k-space data may occur in areas with a large inhomogeneity of the field B ₀ . Similar effects can arise in the case of imperfect magnetic field gradients.

Disclosure of the essence of the invention

Based on the foregoing, it is easy to understand that there is a need for an improved MRI technique. The aim of the invention is to overcome the above limitations and to provide efficient spiral magnetic resonance imaging even in situations of strong inhomogeneity of the field B ₀ .

In accordance with the invention, a method is disclosed for magnetic resonance imaging of an object located in the investigated volume of a magnetic resonance imaging device. The method includes:

exposing the object to an imaging sequence comprising at least one RF excitation pulse and sinusoidally modulated magnetic field gradients,

receiving MR signals along two or more k-space helical trajectories defined by sinusoidal modulation of magnetic field gradients, wherein the beginnings of the k-space helical trajectories are displaced relative to each other, and

- reconstruction of the MR image based on the received MR signals.

To prevent loss of parts of the central k-space from the helical sample in non-uniform B ₀ fields, the invention proposes to use two or more helical k-space trajectories to obtain MR signals. Since the beginnings of the k-space spiral paths are offset from each other, the central points of the spirals do not coincide, and a better coverage of k-space can be achieved in the central region of k-space in which the k-space spiral paths overlap each other, even if the spirals strongly overlap. distorted by the inhomogeneity of the field B ₀ or imperfections in the magnetic field gradient. Preferably, the origins of all k-space helical trajectories are offset from the origin of k-space, spaced at equal distances, and symmetrically located around the origin of k-space. Thus, optimal sampling in the region around the center of k-space can be achieved in essentially all situations of non-uniformity or imperfections in the B ₀ field gradient encountered in practice.

As defined within the scope of the invention, the k-space helical trajectory is defined as a curve that turns around the origin with a constant increase or decrease in distance. This definition covers (among other things) a spiral in 3D k-space, such as a conical spiral that winds around an axis, as well as a curve in a plane that winds around a fixed center point at an ever increasing or decreasing distance from the center point, i.e. a spiral in 2D k-space, where the center point is the beginning of the spiral.

In a preferred embodiment, the B ₀ field map is derived based on the MR signals received from the central region of k-space. The MR image can then be reconstructed with B ₀ field non-uniformity correction based on the derived B ₀ field map.

In accordance with another preferred embodiment, the subject is exposed to a preparation sequence prior to the rendering sequence, wherein the B ₀ field map is derived from the MR signals obtained during the preparation sequence. The number and/or arrangement of k-space helical trajectories in k-space can also be automatically selected depending on the map of the B ₀ field to achieve optimal coverage of the central k-space. To avoid unnecessary scan time, the degree of unevenness of the B ₀ field can be estimated automatically using an appropriate algorithm that decides whether two or more helical trajectories should be applied, or it is sufficient to use only one k-space helical trajectory, as in conventional helical imaging.

In another preferred embodiment, MR signals are obtained in accordance with the "stack of spirals" scheme (eng. stack-of-spirals). In the well-known 3D "stack of helix" data acquisition scheme, several spatially non-selective or block-selective RF excitations are used, each of which is accompanied by the acquisition of one or more MR signals. Several k-space helixes are placed in different positions along the k _z direction, standard Cartesian phase encoding is carried out in the k _z direction. The result is a cylindrical covering of k-space, consisting of a stack of disks ("a stack of spirals"). Technically, this is implemented by generating magnetic field gradients in the planar directions of the k-space spirals using sinusoidal modulation of the gradient amplitudes. According to the invention, k-space helical trajectories are obtained which are offset from each other. The spiral stack includes several spirals that are not centered around the k _z axis (k _x =k _y =0) in this embodiment. The offset from the k _z axis can vary from one k-space helix to another k-space helix in the stack.

MR signals can be obtained, for example, in the form of free induction decay signals (FID, from the English. free induction decay) immediately after the emission of the corresponding RF excitation pulse. MR signals may also be obtained as spin echoes or as gradient echoes.

According to a preferred embodiment of the invention, MR signals are acquired at two or more different echo times, with the MR image reconstruction step separating the signal contributions of water protons and fat protons. In this embodiment, the method according to the invention is used in combination with Dixon water-fat separation. With Dixon magnetic resonance imaging or Dixon water-fat magnetic resonance imaging, the separation of water and fat is achieved by calculating the contributions of water and fat from two or more corresponding spirals obtained at different echo times. In general, such a separation is possible due to the well-known difference in hydrogen precession frequencies in fat and water. In its simplest form, water and fat images are generated by adding or subtracting "in-phase" and "out-of-phase" datasets. However, in recent years, several methods of Dixon-type MRI have been proposed. In addition to various strategies for separating water and fat, known techniques are generally characterized by the number of echoes (or "points") obtained and the limitations they place on the echo times used. Conventional two- and three-point methods require in-phase and anti-phase echo times, in which the water and fat signals are parallel and anti-parallel in the complex plane, respectively. Three-point modes have been gradually generalized to provide flexible echo times. A map of the B ₀ field can also be extracted from Dixon datasets, since accurate knowledge of the field distribution is required to properly separate water and fat in the presence of B ₀ field heterogeneity. According to the invention, k-space sampling is carried out two or more times in regions where two or more k-space helical trajectories overlap, wherein the k-space helical acquisition time point can be chosen such that a given position in k-space is passed two or more times, each time with a different (effective) echo time. This allows the aforementioned Dixon technique to be applied to separate signal contributions from water protons and fat protons.

The method disclosed herein in accordance with the invention can be implemented using a magnetic resonance imaging (MRI) device, including at least one main magnetic coil for generating a uniform static magnetic field inside the study volume, several gradient coils for generating switched gradients magnetic field in different spatial directions inside the test volume, at least one RF coil for generating RF pulses inside the test volume and / or for receiving MR signals from an object located in the test volume, a control unit for controlling the time sequence of RF pulses and switched magnetic gradients fields, a reconstruction unit for reconstructing an MR image based on the received MR signals. The method according to the invention can be implemented, for example, by appropriate programming of the recovery unit and/or the control unit of the MRI device.

The method according to the invention can be successfully implemented in most MRI devices currently used in clinical practice. To do this, it is simply necessary to use a computer program with which the MRI device is controlled in such a way that it performs the above-described steps of the method according to the invention. The computer program may be present either on a storage medium or on a data network for the purpose of being downloaded for installation in the control unit of the MRI device.

Brief description of the drawings

The accompanying drawings disclose preferred embodiments of the present invention. However, it should be understood that the drawings are for illustrative purposes only and not to define the scope of the invention.

In FIG. 1 shows an MRI device for carrying out the method according to the invention;

In FIG. 2 is a k-space diagram illustrating an "ideal" k-space helical trajectory (a) and a distorted k-space helix (b) in the presence of a main magnetic field inhomogeneity;

In FIG. 3 is a k-space diagram illustrating the approach of the invention.

Implementation of the invention

In FIG. 1 schematically shows a magnetic resonance imaging (MRI) device 1. The device contains superconducting or resistive main magnetic coils 2, so that along the z-axis, a substantially uniform, time-constant main magnetic field is generated through the volume of interest.

The magnetic resonance generation and control system supplies a series of RF pulses and switched magnetic field gradients to invert or excite nuclear magnetic spins, induce magnetic resonance, refocus magnetic resonance, control magnetic resonance, spatially and otherwise encode magnetic resonance, saturate spins, and the like to perform MR imaging (tomography).

More specifically, the gradient pulse amplifier 3 applies current pulses to selected whole body gradient coils 4, 5 and 6 along the x, y and z axes of the volume of interest. The digital RF transmitter 7 transmits the RF pulses or bursts through the transmit/receive switch 8 to the whole body RF volumetric coil 9 to transmit the RF pulses into the volume of interest. A typical MR imaging sequence consists of a burst of short duration RF pulse segments that, when combined with each other and any applied magnetic field gradients, achieve the selected nuclear magnetic resonance control. The RF pulses are used to saturate, excite the resonance, invert the magnetization, refocus the resonance, or control the resonance and select the part of the body 10 located in the volume of interest. The whole body volumetric RF coil 9 also picks up MR signals.

To form MR images of limited areas of the body 10, a set of RF coils 11, 12, 13 of the local array are placed adjacent to the area selected for imaging. Coils 11, 12, 13 of the array can be used to receive MR signals induced by RF transmissions from the body coils.

The received MR signals are picked up by the whole body RF surround coil 9 and/or by the array RF coils 11, 12, 13 and demodulated by the 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.

The host computer 15 controls the gradient pulse amplifier 3 and the transmitter 7 to generate any of a variety of MR imaging sequences such as planar echo imaging (EPI), volumetric echo imaging, gradient and spin echo imaging, fast spin echo (TSE) imaging, etc. P. to obtain MR signals along k-space helical trajectories in accordance with the invention. For the selected sequence, receiver 14 acquires one or more MR data along respective k-space trajectories in rapid succession after each RF excitation pulse. The data acquisition system 16 performs A/D conversion of the received signals and converts each MP signal into a digital format suitable for further processing. In modern MRI devices, the data acquisition system 16 is a separate computer that is specialized in collecting raw image data.

Ultimately, the digital raw image data is reconstructed into an image representation by a reconstruction processor 17 that applies a Fourier transform or other appropriate reconstruction algorithms. The MR image may be a plane section of the patient, an array of parallel planar slices, a three-dimensional volume, and the like. The image is then stored in an image memory where it can be accessed to convert slices, projections, or other portions of the image representation into a suitable format for rendering, such as with a video monitor 18, which provides a human-readable display of the resulting MR image.

The MRI device 1 is organized, for example, by appropriately programming the host computer 15 and the restoration processor 17, to perform the imaging method according to the invention, as disclosed herein above and below.

In FIG. 1 and also in FIG. 3 shows an embodiment of an imaging approach in accordance with the invention.

In FIG. 3 shows a spiral data acquisition strategy in accordance with the invention. The invention proposes to use multiple helical k-space trajectories to obtain MR signals in order to prevent the loss of parts of the central k-space from the helical sample in non-uniform B ₀ fields (as in Fig. 2b). In the embodiment shown in FIG. 3, three helixes 31, 32, 33 of k-space with non-coincident central points (beginnings) are used for two-dimensional scanning, with a symmetrical arrangement around the center of k-space (k _x =k _y =0). For 3D scanning, it is necessary to use four 3D spirals, the axes of which are arranged, for example, in a tetrahedral manner. In the embodiment shown in FIG. 3, it is ensured that the signal data is actually acquired (or sampled) in the triangle formed by the three center points of the spirals 31, 32, 33, which allows reconstruction of the area of high B ₀ field non-uniformity at least with low resolution.

Claims (25)

1. The method of magnetic resonance imaging of an object (10) located in the investigated volume of the device (1) of magnetic resonance imaging, including: - exposing the object (10) to an imaging sequence comprising at least one RF excitation pulse and sinusoidally modulated magnetic field gradients, - obtaining magnetic resonance signals along two or more spiral trajectories (31, 32, 33) of the k-space, and the beginnings of the spiral trajectories (31, 32, 33) of the k-space are displaced relative to each other, - moreover, the displaced spiral trajectories (31, 32, 33) of the k-space overlap in the region around the beginning of the k-space, and - reconstruction of the magnetic resonance image based on the obtained magnetic resonance signals. 2. The method according to claim 1, wherein the beginnings of the helical paths (31, 32, 33) of the k-space are offset from the beginning of the k-space. 3. The method according to claim 1 or 2, wherein the beginnings of the helical paths (31, 32, 33) of the k-space are located at equal distances from the beginning of the k-space. 4. The method according to claim 3, wherein the beginnings of the helical paths (31, 32, 33) of the k-space are symmetrically arranged around the beginning of the k-space. 5. The method of claim. 1, in which, based on the magnetic resonance signals received from the area around the beginning of the k-space, display a map of the field B ₀ . 6. The method of claim. 5, in which the magnetic resonance image is restored with correction of the B ₀ field inhomogeneity based on the derived map of the B ₀ field. 7. The method according to any one of paragraphs. 1-4, in which the object (10) is exposed to a preparatory sequence before the imaging sequence, wherein the B ₀ field map is derived based on the magnetic resonance signals received during the preparatory sequence. 8. The method according to claim 7, wherein the number and/or location of k-space helical paths (31, 32, 33) in k-space are selected automatically depending on the field map B ₀ . 9. The method according to any one of paragraphs. 1-8, in which magnetic resonance signals are obtained in accordance with the "stack of spirals" scheme (stack-of-spirals) from several parallel slices of k-space located at adjacent positions along the direction perpendicular to the slices of k-space. 10. The method according to any one of paragraphs. 1-9, in which the magnetic resonance signals are received at two or more different echo times, while at the stage of the reconstruction of the magnetic resonance image, the separation of signal contributions from water protons and fat protons is carried out. 11. The method according to any one of paragraphs. 1-10, wherein k-space sampling is carried out in three dimensions along two or more k-space helical paths (31, 32, 33). 12. A device for magnetic resonance imaging, including at least one main magnetic coil (2) for generating a uniform static magnetic field inside the studied volume, several gradient coils (4, 5, 6) for generating switched magnetic field gradients in various spatial directions inside the test volume, at least one radio frequency coil (9) for generating radio frequency pulses inside the test volume and / or for receiving magnetic resonance signals from an object (10) located in the test volume, a control unit (15) for controlling the time sequence radiofrequency pulses and switched magnetic field gradients, as well as a restoration unit (17) for restoring a magnetic resonance image based on the obtained magnetic resonance signals, and the magnetic resonance imaging device (1) is configured to perform the following steps: - exposing the object (10) to an imaging sequence comprising at least one RF excitation pulse and sinusoidally modulated magnetic field gradients, - obtaining magnetic resonance signals along two or more spiral trajectories (31, 32, 33) of k-space, and the beginnings of the spiral trajectories of k-space are displaced relative to each other, - moreover, the displaced spiral trajectories (31, 32, 33) of the k-space overlap in the region around the beginning of the k-space, and - reconstruction of the magnetic resonance image based on the obtained magnetic resonance signals. 13. A computer-readable medium containing a computer program for execution on a magnetic resonance imaging device, containing instructions for: - generating an imaging sequence containing at least one RF excitation pulse and sinusoidally modulated magnetic field gradients, - obtaining magnetic resonance signals along two or more spiral trajectories (31, 32, 33) of k-space, and the beginnings of the spiral trajectories of k-space are displaced relative to each other, - moreover, the displaced spiral trajectories (31, 32, 33) of the k-space overlap in the region around the beginning of the k-space, and - reconstruction of the magnetic resonance image based on the obtained magnetic resonance signals.

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