Efficient self-refocusing zero echo time MR imaging

Abstract

The present invention relates to a method for MR imaging of an object located in an examination volume of an MR device 1. It is an object of the present invention to enable efficient and quiet ZTE imaging using self-refocusing. The method of the present invention comprises the steps of: specifying a set of radial k-space spokes to cover a spherical k-space volume; selecting a predetermined number of subsets of spokes from the specified set such that the connection of the spokes contained in each subset forms a closed trajectory in k-space, the selection of the subsets comprising optimizing a cost function; subjecting the object 10 to a zero echo time imaging sequence, wherein each subset of spokes is acquired as a sequence of gradient echo signals; and reconstructing an MR image from the acquired spokes. Furthermore, the present invention relates to an MR device and a computer program for the MR device.

Description

[Technical field]

[0001] The present invention relates to the field of magnetic resonance (MR) imaging. The present invention relates to a method of MR imaging with three-dimensional radial sampling of k-space. The present invention further relates to an MR device and a computer program running on the MR device. [Background technology]

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

[0003] Generally, according to the MR method, the body of the patient to be examined is placed in a strong and uniform magnetic field (B0 field), the direction of which at the same time 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 the individual nuclear magnetic spins depending on the magnetic field strength, which can be excited (magnetic resonance) by application of an alternating electromagnetic field (RF field, also called B1 field) of a defined frequency (Larmor frequency, or MR frequency). From a macroscopic point of view, the magnetic field extends perpendicular to the z-axis, while the distribution of the individual nuclear magnetic spins generates a total magnetization that can be thrown out of equilibrium by application of an electromagnetic pulse (RF pulse) of a suitable frequency, and thus the magnetization performs a precession around the z-axis. The precession defines the surface of a cone, the angle of opening of which is called the flip angle. The magnitude of the flip angle depends on the strength and duration of the applied RF pulse. For a flip angle of 90°, the nuclear magnetic spins are deflected from the z-axis to a transverse plane.

[0004] After the end of the RF pulse, the magnetization relaxes back to its original equilibrium state, in which the magnetization in the z direction is again increased 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). The change in magnetization can be detected by one or more receive RF coils arranged and oriented in the examination volume of the MR device in such a way that the change in magnetization is measured in a direction perpendicular to the z axis. The decay of the transverse magnetization is followed by a transition of the nuclear magnetic spins (induced by the inhomogeneity of the local magnetic field) from an ordered state of in-phase to a state in which all phase angles are uniformly distributed (dephasing) after the application of an RF pulse with, for example, a flip angle of 90°. The dephasing can be compensated by a refocusing RF pulse (for example, an RF pulse with a flip angle of 180°). This generates an echo signal (spin echo) in the receive coil.

[0005] To achieve spatial resolution within the patient's body, linear magnetic field gradients extending along the three principal axes are superimposed on the uniform magnetic field, resulting in a linear spatial dependence of the resonance frequency of the nuclear magnetic spins. Thus, the signal picked up in the receiving coil contains components of different frequencies that can be associated with different locations within the patient's body. The MR signals received via the RF coil correspond to a spatial frequency domain called k-space. Data is typically collected in k-space along multiple lines acquired with different phase encodings. Each line is digitized by collecting a certain number of samples. The k-space data set is converted into an MR image by Fourier transformation or other suitable reconstruction algorithm.

[0006] MR imaging of tissues with very short transverse relaxation times, such as bone or lung, is becoming increasingly important. Nearly all known methods for this purpose essentially use three-dimensional (3D) radial k-space sampling. In the so-called zero echo time (ZTE) technique, the readout field gradient is set up before the excitation of magnetic resonance using a high-bandwidth and therefore short hard RF pulse. With this approach, frequency encoding begins immediately upon excitation of magnetic resonance. The acquisition of free induction decay (FID) signals along the radial k-space spokes begins immediately after the RF pulse, resulting in an effectively zero echo time (TE). After acquisition, only a minimal time is required to set up the next readout field gradient before the next RF pulse can be applied, thus allowing very short repetition times (TR). The direction of the readout field gradient is gradually changed for each repetition, with each acquired spoke starting from the k-space center, until a spherical volume in k-space has been sampled to the required extent. Without the need to switch off the readout field gradient during the TR, ZTE imaging can be performed virtually silently.

[0007] Recently, self-refocusing ZTE imaging has been proposed (see, for example, US Patent Application Publication No. 2017 / 0307703(A1)). In self-refocusing ZTE imaging, gradient echo refocusing is added to conventional ZTE imaging. As in conventional ZTE imaging, the amplitude of the readout magnetic field gradient is kept constant and only directional updates are applied between iterations until the entire spherical volume in k-space is sampled, resulting in quiet three-dimensional radial imaging. RF excitation is further achieved by short RF pulses in the presence of the readout magnetic field gradient. The self-refocusing ZTE imaging sequence is then organized into a number of (two or more) segments, each segment being divided into a number of loops. Each loop includes the acquisition of a number of radial k-space spokes. RF excitation is performed only in the first loop (FID acquisition loop) and not in the subsequent second and further loops (gradient echo acquisition loops). The radial k-space spokes of each loop cumulatively form a closed orbit in k-space. In this manner, the second and further loops form gradient echoes of the initial FID excited in the first loop. The TE of these gradient echoes is TE=(e-1)*N*TR where e is the loop counter and N is the number of radial k-space spokes acquired in each loop. The TR is typically short (on the order of 1 ms). While the amplitude of the readout field gradient is still kept constant across the acquisition, the directional updates applied between repetitions are typically larger than in conventional ZTE imaging. However, for a self-refocusing ZTE imaging sequence to remain quiet, these directional updates at any one time must be as small as possible.

[0008] To sample the required spherical k-space volume, known self-refocusing ZTE techniques typically rely on splitting the rotation of radial k-space spokes into two rotations around orthogonal k-space axes. First, a certain number of segments (also called interleaving) are acquired from a planar circular area in k-space that represents a cross-section of the entire spherical k-space volume. Within the circular area, the connected spokes form a closed (polygonal) k-space trajectory to allow gradient echo acquisition. Next, to acquire further segments from the rotated circular area, the circular area is rotated around an axis in the plane of the circular area. This continues until the entire spherical k-space volume is covered. With this approach, the number of spokes required to acquire the entire spherical k-space volume is large, but the spokes are non-uniformly distributed, with unnecessary oversampling in some areas. Summary of the Invention [Problem to be solved by the invention]

[0009] From the above, it can be readily appreciated that there is a need for improved methods of ZTE imaging. It is an object of the present invention to enable efficient and quiet ZTE imaging using self-refocusing. [Means for solving the problem]

[0010] According to the present invention, a method for MR imaging of an object located in an examination volume of an MR device is disclosed. The method of the present invention comprises the steps of: - specifying a set of radial k-space spokes to cover a spherical k-space volume; - a step of selection of a predetermined number of subsets of spokes from the designated set such that the concatenation of the spokes contained in each of the subsets forms a closed trajectory in k-space, the selection of the subsets comprising optimizing a cost function; - subjecting the object to a zero echo time imaging sequence, where each of a subset of spokes is acquired as a sequence of gradient echo signals; - reconstructing MR images from the acquired spokes; has.

[0011] According to the invention, a set of radial k-space spokes is first specified (step a) to cover a spherical k-space volume. The spherical k-space volume is specified by the desired field of view (FOV). The radial k-space spokes start at the k-space origin as described above and end at the surface of the sphere. The minimum density of the spokes' endpoints on the sphere is specified by the desired (acquired) spatial resolution of the final reconstructed MR image. Many possible approaches are considered to specify the distribution of the endpoints. The initial specification of a set of radial k-space spokes has the advantage over previously known approaches that the number of spokes can be closer to the minimum number of spokes required for a particular FOV and spatial resolution, thus shortening the scan time. Furthermore, the distribution of the endpoints can be more uniform, thus increasing the signal-to-noise ratio (SNR).

[0012] As a next step (step b), each of the subsets comprising a predefined number of spokes is selected from the specified set of radial k-space spokes. The predefined number corresponds to the number of spokes acquired in one segment of the ZTE imaging sequence. The selection is performed in such a way that the concatenation of the spokes contained in each of the subsets forms a closed trajectory in k-space, which is necessary for the generation of gradient echo signals as described above. Furthermore, a cost function is taken into account in the selection process. The cost function preferably depends on the relative orientation of each of the spokes of the subset, which basically determines the acoustic noise generated during the acquisition of these spokes. The gist of the invention is that each of the spokes of the subset (from the specified set of radial k-space spokes) is selected such that (i) echo generation occurs and (ii) acoustic noise is minimized. In principle, any prior knowledge information about the acoustic noise generated by gradient switching in a particular MR device or a particular type of MR device is incorporated in the cost function, so that the generation of acoustic noise during the acquisition of a set of radial k-space spokes is penalized.

[0013] The actual acquisition of the set of radial k-space spokes using a ZTE imaging sequence is then performed in step c).

[0014] Finally, in step d), an MR image is reconstructed from the acquired radial k-space spokes.

[0015] An essential aspect of the invention is, in other words, to minimize the maximum angle between subsequently acquired spokes within the same segment, while ensuring that the connection of the spokes of each subset / segment forms a closed orbit in k-space, and to maximize the uniformity of the distribution of the end points of all spokes on the surface of the covered spherical k-space volume. That is, according to the invention, a subset of spokes is selected based on a cost function that drives the selection to favor smaller maximum angles between subsequent spokes, smaller variations in the distribution of the end points, under the constraint that the connection of the spokes of each subset forms a closed orbit in k-space.

[0016] Ideally, the selection of a predetermined number of spokes for each of the subsets is made based on a global optimum of the cost function for the entire acquisition. However, it is often preferable to derive a local optimum that approximates the global optimum sufficiently well in a reasonable amount of computation time. Greedy selection techniques, as are naturally known in the art, may be used for this purpose, which are applied to successively select individual subsets of spokes and / or individual spokes or pairs of spokes for each of these subsets.

[0017] In a preferred embodiment of the invention, the selection of the subsets includes specifying the sequence in which the spokes included in each of the subsets are acquired. The acoustic noise introduced by setting the readout field gradient between acquisitions of individual radial k-space spokes depends on the relative orientation of subsequently acquired spokes in the same segment. Thus, the selection of the subsets must include not only the selection of the spokes acquired in each segment, but also their sequence.

[0018] In a preferred embodiment of the method of the present invention, the set of radial k-space spokes consists of pairs of spokes with anti-parallel orientation. Based on this, the subsets can be selected to consist of pairs of spokes with anti-parallel orientation. This ensures that the concatenation of the spokes included in each of the subsets forms a closed trajectory in k-space, and thus a gradient echo is generated.

[0019] Gradient echo as well as FID signals are acquired and used to improve MR image reconstruction, for example to identify spatial variations in spin density and / or the main magnetic field. Furthermore, they can be used to reduce noise and / or T2 * Furthermore, correction for motion occurring between acquisitions of the individual segments can be based on the FID signal, for example by using the FID signal as a navigator for motion detection or motion compensation.

[0020] The inventive method as described so far can be performed by an MR device comprising at least one main electromagnetic coil for generating a uniform constant magnetic field in the examination volume, a number of 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 for receiving MR signals from a patient's body located in the examination volume, a control unit for controlling the temporal succession of the RF pulses and the switched magnetic field gradients, and a reconstruction unit. The inventive method is preferably realized by corresponding programming of the reconstruction unit and / or the control unit of the MR device.

[0021] The method of the invention can be advantageously performed by many current clinical MR devices. For this purpose, it is only necessary to use a computer program, which is controlled by the MR device so that the MR device performs the steps of the above-mentioned method of the invention. The computer program is present on a data storage medium or in a data network, to be downloaded for installation in the control unit of the MR device.

[0022] The accompanying drawings disclose preferred embodiments of the present invention. It is to be understood, however, that the drawings are given for illustrative purposes only and are not intended as a definition of the limits of the invention. In the drawings: [Brief description of the drawings]

[0023] [Figure 1] FIG. 2 shows a schematic diagram of an MR device for carrying out the method of the present invention. [Diagram 2] FIG. 1 illustrates the designation of a set of radial k-space spokes in accordance with the present invention. [Diagram 3] FIG. 13 shows the relative orientation angles between spokes successively obtained according to the present invention. [Figure 4] 13A-13C are further diagrams illustrating the relative orientation angles between successively obtained spokes according to another embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0024] With reference to Fig. 1, an MR device 1 that can be used to implement the method of the present invention is shown. The device comprises a superconducting or resistive main electromagnetic coil 2, such that a substantially uniform and time-constant main magnetic field B0 is generated along the z-axis through the examination volume. The device further comprises a set of (first, second and - if applicable - third) shim coils 2', the current flow through the individual shim coils of the set 2' being controllable with the aim of minimizing B0 deviations within the examination volume.

[0025] The magnetic resonance generation and manipulation system applies a series of RF pulses and switched magnetic field gradients to excite, invert or saturate nuclear magnetic spins, to stimulate, refocus and manipulate magnetic resonance, to spatially and otherwise encode magnetic resonance, etc., to perform MR imaging.

[0026] 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 transmitter 7 transmits RF pulses via a transmit / receive switch 8 to the body RF coil 9 to transmit the RF pulses within the examination volume. A typical MR imaging sequence consists of short duration RF pulses that are used together with any applied magnetic field gradients to achieve selected manipulation of nuclear magnetic resonance with a selection of a portion of a body 10 located in the examination volume. MR signals are further extracted by the body RF coil 9.

[0027] For the generation of MR images of limited regions of the body 10 by parallel imaging, a set of local array RF coils 11, 12, 13 are placed contiguous with the region to be imaged. The array coils 11, 12, 13 can be used to receive MR signals induced by RF transmission using the body RF coil.

[0028] The resulting MR signals are sampled by the body RF coil 9 and / or by the array RF coils 11, 12, 13 and demodulated by a receiver 14, which preferably includes a preamplifier (not shown). The receiver 14 is connected to the RF coils 9, 11, 12 and 13 via a transmit / receive switch 8.

[0029] A host computer 15 controls the current flow through the shim coils 2' as well as the gradient pulse amplifiers 3 and the RF transmitter 7 to generate the ZTE imaging sequence according to the invention. The receiver 14 receives the MR signals from the individual radial k-space spokes after the RF excitation pulses in rapid succession. The data acquisition system 16 performs an analog-to-digital conversion of the received MR signals and converts them into a digital format suitable for further processing. In modern MR devices, the data acquisition system 16 is a separate computer dedicated to the acquisition of raw image data.

[0030] Finally, the digital raw image data is reconstructed into an image representation by a reconstruction processor 17, which applies appropriate reconstruction algorithms. The MR image represents a three-dimensional volume. The image is then stored in an image memory, where it is accessed to convert the projections or other parts 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.

[0031] The essence of the substantially silent ZTE technique applied according to the invention is that an RF excitation pulse is transmitted while a frequency encoding readout gradient is switched on. The readout gradient is not intended to be a slice selection gradient, which means that the RF excitation pulse needs to be short (typically on the order of 1 μs or 10 μs) to achieve a sufficient excitation bandwidth. Alternatively, an RF excitation pulse using a frequency sweep is applied. The reading of the FID signal is performed during an interval immediately following the RF excitation pulse in the presence of the readout gradient. These intervals are even more preferably short (typically on the order of 100 μs or 1 ms). The readout gradient has a strength and a direction that both remain substantially constant over each excitation / read cycle in order to acquire MR signals from one radial k-space spoke. After each excitation / read cycle, the direction is changed. This is repeated until the spherical volume is completely covered with sufficient density for a complete sampling of the k-space.

[0032] According to the present invention, self-refocusing ZTE imaging is realized by a gradient echo refocusing mechanism. The pulse sequence is organized into a number of (two or more) segments, and each segment is divided into a number of loops. Each loop has the acquisition of a number of radial k-space spokes. RF excitation is only enabled for the first loop (FID acquisition loop) and then switched off for the subsequent second and further loops (gradient echo acquisition loops). The radial k-space spokes of each loop form a closed orbit in k-space. In this way, the later loops form gradient echoes of the initial FIDs excited in the initial loop. For details of the self-refocusing ZTE imaging sequence used by the present invention, please refer to US Patent Application Publication No. 2017 / 0307703(A1).

[0033] The present invention first proposes that a collection of radial k-space spokes be specified to cover the spherical k-space volume to be acquired. In a possible embodiment, the set of spokes is specified as follows:

[0034] A sufficient number N P are defined on the surface of the k-space sphere, e.g.

number

[0035] A sufficient number of endpoints, N E are spaced equidistant along each parallel object, e.g.

number

[0036] N P Choose odd values ​​for N EBy choosing an even value for , the existence of pairs of anti-parallel radial spokes is ensured. One such predefined set of radial k-space spoke endpoints is plotted in the diagram of FIG.

[0037] In the next step, pairs of antiparallel spokes are selected from the set specified in the optimization, which ensures that the k-space trajectory resulting from the concatenation of a subset of radial k-space spokes corresponding to an acquisition segment is self-reconvergent.

[0038] In the optimization, the following operations are repeated until all radial k-space spokes are assigned to one subset.

[0039] One previously unselected spoke is selected, either systematically or randomly, from the specified set.

[0040] The cost function is this spoke and N S / 2-1 are evaluated for all permutations of the unselected spokes, where N S represents the number of spokes in each subset.

[0041] By applying a greedy selection technique, the optimal permutation (eg, the one for which the cost function takes its minimum value) is selected and each spoke and their anti-parallel counterpart is assigned to the current subset.

[0042] As a cost function, for example, the squared relative orientation angle between subsequent spokes may be used to achieve a minimum level of acoustic noise during ZTE acquisition.

[0043] Of course, known greedy selection techniques are designed to make a locally optimal selection for each subset with the goal of deriving a global optimum for the entire specified set. Although greedy optimization techniques do not actually derive a true global optimum solution, greedy optimization techniques obtain local optimum solutions for the subsets that at least approximate the global optimum solution in a reasonable amount of computation time. The global optimum solution minimizes the level of acoustic noise not only for each subset or acquisition segment, but globally for the entire specified set or the entire acquisition.

[0044] However, any known heuristic algorithm for an efficient but approximate search for an optimal permutation may be applied instead of the greedy selection technique. In particular, the search is stopped whenever a predefined maximum value of the relative orientation angle between subsequent spokes in a segment is reached.

[0045] Representative results obtained using the above process are shown in Figure 3. In this example, eight spokes per subset were selected, with an optimal angle between subsequent spokes of 45°. By using a greedy selection technique, the actual angle between subsequent spokes is close to the optimal value for the majority of the spokes. Only towards the end of the process does the selection of the remaining unselected spokes become very limited, resulting in angles between subsequent spokes significantly larger than the optimal value.

[0046] When a predefined maximum of the relative orientation angle between subsequent spokes in a subset can no longer be reached, spokes are added, at the cost of a small increase in the number of spokes required and a corresponding small increase in scan time. This is illustrated in Figure 4, where reacquisition of spokes is allowed, i.e., some spokes are assigned to a subset that have already been selected for a previous subset. This reselection of spokes is allowed when the relative orientation angle is greater than 50°, resulting in a 13% increase in the total number of spokes.

Claims

1. 1. A method of MR imaging of an object located in an examination volume of an MR device, said method comprising: specifying a set of radial k-space spokes to cover a spherical k-space volume; selecting subsets, each of the subsets including a predetermined number of the radial k-space spokes from a designated set, such that a concatenation of the radial k-space spokes included in each of the subsets forms a closed orbit in k-space, and the selecting of the subsets is based on minimizing a maximum angle between subsequently acquired radial k-space spokes within the same subset while ensuring that the concatenation of the radial k-space spokes of each of the subsets forms a closed orbit in k-space; subjecting the object to a zero echo time imaging sequence, wherein each of the subset of radial k-space spokes is acquired by implementing a free induction decay (FID) acquisition loop that acquires an FID signal, and a gradient echo acquisition loop; reconstructing an MR image from the acquired radial k-space spokes; The method comprising:

2. and selecting the subsets includes identifying a sequence in which the radial k-space spokes included in each of the subsets are acquired. The method of claim 1.

3. the set of radial k-space spokes comprises pairs of radial k-space spokes having anti-parallel orientations. The method according to claim 1 or 2.

4. the subset being selected to consist of pairs of the radial k-space spokes having anti-parallel orientations; The method according to claim 3.

5. a greedy selection technique is used in successive selection of each of the subsets of radial k-space spokes; 5. The method according to any one of claims 1 to 4.

6. a greedy selection technique is used in successive selection of individual radial k-space spokes or pairs of radial k-space spokes for each of the subsets; 6. The method according to any one of claims 1 to 5.

7. 1. An MR device comprising at least one main electromagnetic coil for generating a uniform static magnetic field in an examination volume, a number of 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 for receiving MR signals from an object located in the examination volume, a control unit for controlling the temporal succession of the RF pulses and the switched magnetic field gradients, and a reconstruction unit, wherein the MR device comprises: specifying a set of radial k-space spokes to cover a spherical k-space volume; selecting a predetermined number of the radial k-space spokes from the designated set such that a concatenation of the radial k-space spokes included in each of the subsets forms a closed orbit in k-space, the step of selecting the subsets being based on minimizing a maximum angle between subsequently acquired radial k-space spokes within the same subset while ensuring that the concatenation of the radial k-space spokes of each of the subsets forms a closed orbit in k-space; subjecting the object to a zero echo time imaging sequence, wherein each of the subset of radial k-space spokes is acquired by implementing a free induction decay (FID) acquisition loop that acquires an FID signal, and a gradient echo acquisition loop; reconstructing an MR image from the acquired radial k-space spokes; To carry out MR device.

8. A computer program executing on an MR device, the computer program comprising: specifying a set of radial k-space spokes to cover a spherical k-space volume; selecting a predetermined number of the radial k-space spokes from the designated set such that a connection of the radial k-space spokes included in each of the subsets forms a closed orbit in k-space, wherein the step of selecting the subsets is based on a cost function that preferably minimizes the maximum angle between subsequently acquired radial k-space spokes within the same subset while ensuring that the connection of the radial k-space spokes of each of the subsets forms a closed orbit in k-space; generating a zero echo time imaging sequence, wherein each of the subsets of radial k-space spokes is acquired by implementing a free induction decay (FID) acquisition loop that acquires an FID signal and a gradient echo acquisition loop; reconstructing an MR image from the acquired radial k-space spokes; Includes instructions for Computer program.

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