Improved acquisition of measurement data using a magnetic resonance imaging system with ultra-short echo times

Abstract

An inventive method for recording measurement data of the object under investigation using a magnetic resonance system with a sequence of ultrashort echo times comprises the following steps: - Loading the half-spokes in k-space to be sampled for the desired recordings with the associated gradients to be switched for spatial coding on three axes of a gradient unit of the magnetic resonance system, - Determining a sequence of recordings of measurement data to be taken successively along each half-spoke with a distribution of gradients to be switched on the axes optimized with respect to the load on the axes, - Performing the recording of the measurement data along the half-spokes to be scanned according to the specified sequence. By optimizing the sequence in which half-spokes of a test object are successively measured using ultrashort echo times, it is possible to allow higher gradient strengths (amplitudes) for the gradients used for spatial encoding than previously possible. In particular, gradient strengths exceeding 70%, for example 80%, 90%, or even more, of the maximum gradient strength Gmax possible on one axis of the gradient unit for recording the half-spokes to be sampled can be permitted. The method according to the invention thus enables shorter readout time windows in sequences with ultrashort echo times. This improves image quality and sharpness, and therefore the resolution of substances in the test object with a very rapidly decaying signal.

Description

[0001] The invention relates to an improved acquisition of measurement data with a magnetic resonance system with ultrashort echo times, in particular an improvement of the acquisition of measurement data by means of a sequence that scans the k-space along half-spokes, such as a PETRA sequence, a zTE sequence, a WASPI sequence or a UTE sequence.

[0002] Magnetic resonance imaging (MRI) is a well-known technique used to generate images of the interior of an object. In simplified terms, the object is positioned in a magnetic resonance imaging (MRI) scanner within a relatively strong, static, homogeneous background magnetic field, also known as the B0 field, with field strengths ranging from 0.2 Tesla to 7 Tesla and higher. This causes the object's nuclear spins to align with the background magnetic field. To trigger measurable nuclear spin resonances, high-frequency excitation pulses (RF pulses) are applied to the object. The resulting nuclear spin resonances are measured as so-called k-space data using specially designed coils, and MR images or spectroscopic data are then derived from these measurements. The alternating magnetic field generated by the excitation pulses applied via at least one transmitting coil is also referred to as the B1 field.To spatially encode the measurement data, rapidly switched magnetic gradient fields, or gradients for short, are superimposed on the underlying magnetic field. A scheme used that describes a temporal sequence of RF pulses to be applied and gradients to be switched is called a pulse sequence (or simply sequence). The recorded measurement data are digitized and stored as complex numerical values ​​in a k-space matrix. A corresponding MR image can be reconstructed from the k-space matrix containing these values, for example, using a multidimensional Fourier transform.

[0003] It is not possible to visualize substances or tissues using MR sequences if their T2* time, the effective decay of the transverse magnetization of this substance or tissue, is significantly shorter than the shortest possible echo times within these sequences, because then a corresponding signal from these substances or tissues has already decayed at the time of acquisition.

[0004] Therefore, using conventional sequences, such as a (T)SE sequence (“(Turbo) Spin Echo”) or a GRE sequence (“Gradient Echo”), it is not possible to image substances or tissues, such as bones, tendons, ligaments, teeth or ice, which have T2* times of significantly less than 500 microseconds (µs).

[0005] However, MRI techniques are already known that allow very short echo times (TE) (e.g., TE < 500 µs or, with appropriate hardware, even TE < 50 µs), which are in the range of the corresponding decay time. With these techniques, it is possible, for example, to visualize bones, tendons, ligaments, teeth, or ice in an MRI image, even though the T2* time of such substances or tissues is in the range of 30–80 µs.

[0006] These MRI techniques include, for example, the UTE sequence (“Ultrashort Echo Time”), as described, among other places, in the article by Sonia Nielles-Vallespin, “3D radial projection technique with ultrashort echo times for sodium MRI: Clinical applications in human brain and skeletal muscle,” Magn. Res. Med. 2007; 57; pp. 74–81. In this sequence type, after a waiting period following non-selective or slice-selective excitation, data acquisition begins simultaneously with the ramp-up of the gradients for spatial encoding. The k-space trajectory sampled in this way runs radially outward from the k-space center, thus along a half-spoke. Therefore, before reconstructing the image data from the raw k-space data using Fourier transformation, this raw data must first be converted to a Cartesian k-space grid, for example, by regridding.

[0007] Other MRI techniques that allow for particularly short echo times include zTE (zero echo time), PETRA (pointwise encoding time reduction with radial acquisition), and WASPI (water- and fat-suppressed proton projection MRI) sequences. These are described, for example, in the article by Weiger et al., "MRI with Zero Echo Time: Hard versus Sweep Pulse Excitation," Magnetic Resonance in Medicine 66: pp. 379-389, 2011, in US8878533B2 (PETRA), and in the article by Wu et al., "Density of Organic Matrix of Native Mineralized Bone Measured by Water- and Fat-Suppressed Proton Projection MRI," Magn. Reson. Med. 50: pp. 59-68, 2003. In both methods, measurement data are recorded in k-space along radial half-spokes, whose gradients switched for spatial coding are already fully ramped up at the time of excitation of the spins in a test object, which saves valuable coding time.However, this also creates a region in the k-space center that cannot be scanned by these radial half-spokes. Compared to UTE methods, zTE and PETRA methods are more robust because eddy currents or unintentional, small time shifts of switched gradients or irradiated RF pulses have no or at most a negligible influence on the measurements.

[0008] An example of part of a pulse sequence for such an acquisition A of measurement data along radial half-spokes, as used in zTE and PETRA methods, is shown in Fig. Figure 1 shows the following. The top row, "Tx / Rx," displays the applied RF excitation pulses RF1 and the readout time windows ADC, during which the acquisition A of the measurement data takes place. The middle row, "G1," shows the gradients switched in one coding direction, each of which has reached its desired strength for the subsequent acquisition A of measurement data at the time of the application of an RF excitation pulse RF1. The bottom row, "k-sp," shows the corresponding k-space points sampled along the k-space trajectory defined by the applied gradient field for the acquisition A of the measurement data. Measured k-space points are shown as black dots, and k-space points that occurred before the start of the readout time window, and were therefore not read out, are shown as "empty" dots. The fact that k-space points are not read out is due to the constant gradient field being applied before the application of the RF excitation pulse RF, as described above.This would mean that the central k-space point (k0) would be measured simultaneously with the application of the RF excitation pulse RF1, which is technically impossible. Only after a minimum switching time Ts, dependent on the hardware of the magnetic resonance system used, following the end of the RF excitation pulse RF1, can the acquisition of measurement data A begin within the readout time window ADC, thus still resulting in the shortest possible echo time TE. The first k-space point k* read out within the readout time window ADC has the minimum distance from the k-space center k0 among all the readout k-space points. The last k-space point read out within the readout time window ADC is k. maxThe maximum distance between the k-space points read out is k0, the maximum distance from the k-space center. The duration of the readout time window ADC (acquisition time) is given by the strength of the applied gradient field G1 and the required resolution or the desired image area (field of view, FOV) or the matrix of the image to be created from the measurement data.

[0009] A corresponding sampling scheme of k-space is given in Fig. Figure 2 shows a radial half-spoke in k-space corresponding to a k-space trajectory along which ADC measurement data are acquired within a readout time window. In region B1, measurement data are acquired along radial half-spokes in various encoding directions until, for example, a desired sampling density in k-space is achieved. The radius of the central region B2, in which no measurement data are acquired along the described radial half-spokes because the magnitude of a distance k from a k-space point is less than k*, depends on the k-space moment accumulated after excitation until the acquisition A of the measurement data, and thus on the echo time TE during which the gradient is switched with constant strength, and on the strength of the switched gradient field G. Fig. Figure 2 shows only one plane in k-space. The sampling scheme is often performed in all three dimensions, with the half-spokes sampled in the recordings A of the measurement data in k-space ranging from the surface of a sphere with radius k* to the surface of a sphere with radius k. max get lost.

[0010] MR data from this non-radially scanned region B2 can be algebraically reconstructed from the measurement data of the radial half-spokes using zTE methods. In WASPI methods, a small number of additional measurements are performed along secondary radial k-space trajectories, with reduced gradient strengths to acquire measurement data closer to the k-space center.

[0011] In PETRA procedures, measurement data from the non-radially scanned area B2 can be acquired using a single-point MR imaging technique, e.g., RASP (“rapid single point”) as described in the article by Heid et al., “Rapid Single Point (RASP) imaging”, Proc. Intl. Soc. Mag. Reson. Med., p. 684, 1995, or a “single-point SPRITE” technique as described in the article by Balcom et al., “Single-Point Ramped Imaging with T1 Enhancement (SPRITE)”, J. Magn. Reson. A 123(1): pp. 131–134, 1996, particularly on a Cartesian grid. This is described in Fig. 2 roughly schematically represented by filled points in area B2, which represent individual recorded k-space points.

[0012] In addition to an extremely short echo time (TE), a fast readout of the acquired measurement data, i.e., a short readout time window, is crucial for achieving the highest possible resolution and thus generating the sharpest possible image data. Unlike the extremely short echo time (TE), which ensures that as much signal as possible is present in the k-space center, a short readout time window results in more signal and therefore more information in an outer region of k-space. More signal in an outer region of the sampled k-space leads to a higher signal strength of the rapidly decaying signals. Higher signal strength allows for the generation of sharper images. It has been calculated that optimal resolution can be achieved when the readout time window is of the same order of magnitude as the T2 time of the tissue being imaged in the subject.

[0013] To use a short readout window, measurements must be taken with very high readout bandwidths. Using high readout bandwidths reduces the overall signal-to-noise ratio (SNR). However, this can be compensated for, for example, by averaging techniques, in which measurement data is acquired multiple times and an average value is calculated. Since the shorter readout window allows for shorter repetition times (TR), the overall measurement time is not necessarily longer. Additionally or alternatively, techniques such as denoising or other deep learning-based methods can be used to compensate for the reduced overall SNR.

[0014] Furthermore, a high readout bandwidth, depending on the desired resolution, also requires high gradient strengths during the readout time window. Theoretically, for optimal resolution of rapidly decaying signals, gradient strengths of >100 mT / m would be needed. However, such high gradient strengths are not achievable with all common magnetic resonance imaging (MRI) systems.

[0015] To ensure that measurements with ultrashort echo times do not exceed the hardware-related limitations of a magnetic resonance system, in particular its gradient power amplifier (GPA) and other parts of its gradient unit as well as its cooling system, the available gradient gnomon values ​​that can be switched to a maximum within a sequence for ultrashort echo times are therefore usually limited on the individual axes (x,y,z) of the gradient unit to values ​​far below a maximum amplitude GMax possible on one axis, e.g. to 60-70% of GMax.

[0016] However, sequences with ultrashort echo times are designed in such a way that gradients must be switched continuously, and only the distribution of the gradients to be switched across the axes changes from repetition to repetition, not the total power G^2 = Gx^2+Gy^2+Gz^2. Therefore, playing back these sequences is very challenging for the GPA and the entire gradient unit.

[0017] The invention is therefore based on the objective of enabling a measurement with a sequence with an ultrashort echo time and with the largest possible bandwidth.

[0018] The problem is solved by a procedure for according to claim 1, a control device according to claim 10, a computer program according to claim 11, and an electronically readable data carrier according to claim 12.

[0019] An inventive method for recording measurement data of the object under investigation using a magnetic resonance system with a sequence of ultrashort echo times comprises the following steps: - Loading the half-spokes in k-space to be sampled for the desired recordings with the associated gradients to be switched for spatial coding on three axes of a gradient unit of the magnetic resonance system, - Determining a sequence of recordings of measurement data to be taken successively along each half-spoke with a distribution of gradients to be switched on the axes optimized with respect to the load on the axes, - Performing the recording of the measurement data along the half-spokes to be scanned according to the specified sequence.

[0020] By optimizing the sequence in which half-spokes of a test object are successively measured using ultrashort echo times, it is possible to allow higher gradient strengths (amplitudes) for the gradients used for spatial encoding than previously possible. In particular, gradient strengths exceeding 70%, for example 80%, 90%, or even more, of the maximum gradient strength Gmax possible on one axis of the gradient unit for recording the half-spokes to be sampled can be permitted. The method according to the invention thus enables shorter readout time windows in sequences with ultrashort echo times. This improves image quality and sharpness, and therefore the resolution of substances in the test object with a very rapidly decaying signal.

[0021] A magnetic resonance system according to the invention comprises a magnet unit, a gradient unit, a radio frequency unit and a control device designed for carrying out a method according to the invention with an optimization unit.

[0022] A computer program according to the invention implements a method according to the invention on a control device when it is executed on the control device. For example, the computer program includes instructions that, when the program is executed by a control device, e.g., a control device of a magnetic resonance system, cause this control device to execute a method according to the invention. The control device can be in the form of a computer.

[0023] The computer program can also be in the form of a computer program product that can be directly loaded into a memory of a control device, with program code means to execute a method according to the invention, when the computer program product is executed in a computing unit of a computing system of the control device.

[0024] A computer-readable storage medium according to the invention comprises instructions which, when executed by a control device, e.g. a control device of a magnetic resonance system, cause it to execute a method according to the invention.

[0025] The computer-readable storage medium can be designed as an electronically readable data carrier which includes electronically readable control information stored on it, which includes at least one computer program according to the invention and is designed in such a way that, when the data carrier is used in a control unit of a magnetic resonance system, it carries out a method according to the invention.

[0026] The advantages and explanations given regarding the procedure also apply analogously to the magnetic resonance system, the computer program product and the electronically readable data carrier.

[0027] Further advantages and details of the present invention will become apparent from the exemplary embodiments described below and from the drawings. The examples shown do not constitute a limitation of the invention. They show: Fig. 1 a schematically represented part of a pulse sequence scheme for recording measurement data, Fig. 2 a schematically represented exemplary k-space scanning scheme, Fig. 3 a schematic flowchart of a method according to the invention for recording measurement data of the object under investigation using a magnetic resonance system with a sequence with ultrashort echo times, Fig. 4 a schematically illustrated magnetic resonance system according to the invention.

[0028] Fig. Figure 3 is a schematic flowchart of a method according to the invention for recording measurement data MD of the object under investigation U using a magnetic resonance system 1 with a sequence with ultrashort echo times.

[0029] The half-spokes HS1, HS2, ..., HSn to be scanned in k-space for the desired recordings A are combined with the associated gradients G to be switched for spatial coding on three axes x, y, z of a gradient unit 5 of the magnetic resonance system. x , G y , G x loaded (Block 101).

[0030] A sequence R of recordings A of measurement data MD taken successively along each half-spoke HS1, HS2, ..., HSn, with a distribution of gradients G to be switched on the axes x, y, z optimized with respect to a load on the axes x, y, z x , G y , G x is determined (Block 105). The optimized distribution can, in particular, be as uniform as possible distribution of the gradients G to be switched. x , G y , G x loads caused on the x, y, z axes reach the x, y, z axes.

[0031] Determining the sequence R can involve randomly selecting half-spokes HS1, HS2, ..., HSn of the half-spokes HS1, HS2, ..., HSn to be sampled, which are then recorded sequentially. Currently, the half-spokes HS1, HS2, ..., HSn to be sampled are sampled sequentially according to a sorted sequence. For example, for recordings A of measurement data MD in all three dimensions of k-space, different angles are successively ascended in two spatial directions of k-space, and in the third spatial direction of k-space, the gradients to be switched for the half-spokes to be sampled are successively sorted in increments from a maximum gradient strength in the negative orientation in the third spatial direction to the maximum gradient strength in the positive orientation in the third spatial direction.For an axis of a gradient unit used to generate gradients in such a third spatial direction, this means that high values ​​for the gradient strengths of the gradients to be switched must be achieved and maintained both at the beginning and at the end of such a sorted sequence over a longer period during which multiple measurement data acquisitions are carried out. This places an enormous load on the affected axis of the gradient unit.

[0032] Even a random selection of a sequence R of recordings A of measurement data MD, as suggested above, along each half-spoke HS1, HS2, ..., HSn, can better distribute the load of the gradient unit 5 across the different axes x, y, z of the gradient unit 5.

[0033] Determining the sequence R can also include inserting pauses P during which no gradients are applied on at least one axis x, y, z of the gradient unit 5, at least for a cooling period encompassed by the pause P. Inserting such pauses P ensures that the gradient unit 5 is not overloaded.

[0034] Additionally or alternatively, it is possible to determine the order R under the condition that a gradient strength of a gradient to be switched on at least one, in particular each, of the axes x, y, z during a first acquisition of measurement data of a half-spoke is not a gradient strength that is the next largest or next smallest of all gradient strengths of the gradient strengths to be switched than a gradient strength of a gradient to be switched on the same axis in a subsequent acquisition of measurement data of a half-spoke of the half-spokes to be sampled following the first acquisition of measurement data.

[0035] In this way, the load on a gradient unit 5 during the acquisition of measurement data MD along the sampled half-spokes HS1, HS2, ..., HSn can be better distributed across the x, y, z axes of gradient unit 5. For example, if a half-spoke randomly selected as the next half-spoke does not meet the condition, it can be randomly selected again. This can be repeated until the condition is met or a termination criterion is reached. A pause P can also be inserted, if necessary, such that the half-spokes HS1, HS2, ..., HSn that violate the condition and are to be sampled consecutively in sequence R are separated from each other by the pause P.

[0036] Determining the sequence R can involve dividing k-space into at least two, for example four or eight, and in particular 16, segments (Block 103), each of which is a contiguous segment of the sphere spanned in k-space by the half-spokes to be sampled. The half-spokes HS1, HS2, ..., HSn to be sampled can then be divided into groups G1, G2, ..., Gn according to the segment in which they lie. The half-spokes of a group G1, G2, ..., Gn to be sampled can then be recorded sequentially within the determined sequence R. In this way, it can be prevented that the gradient strengths of the gradients to be switched in successive recordings A of measurement data MD of the half-spokes HS1, HS2, ..., HSn to be sampled are subject to large fluctuations and / or frequent inversions in their orientation.By dividing the data into segments in this way, it can be prevented that, over a large number of repetitions of recordings A of measurement data MD along each of the half-spokes HS1, HDd2, ..., HSn to be sampled, an axis of a gradient unit and also a polarity corresponding to the orientation of the gradient to be switched occurs too frequently in successive recordings A of measurement data MD of half-spokes HS1, HS2, ..., HSn to be sampled.

[0037] Determining the sequence R within a group G1, G2, ..., Gn can involve randomly selecting half-spokes HS1, HS2, ..., HSn of the half-spokes HS1, HS2, ..., HSn of the group G1, G2, ..., Gn to be sampled, which are then recorded sequentially. By randomly selecting a sequence R of recordings A of measurement data MD within a group G1, G2, ..., Gn, to be recorded sequentially along each half-spoke HS1, HS2, ..., HSn, the load on the gradient unit 5 can be better distributed across the various axes x, y, z of the gradient unit 5 assigned to the segment of the group G1, G2, ..., Gn.

[0038] Determining the order R within a group can also be done under the condition that a gradient strength of a gradient to be switched on at least one, in particular each, of the axes x, y, z during a first acquisition of measurement data of a half-spoke of the group is not a gradient strength that is the next largest or next smallest of all gradient strengths of the gradient strengths to be switched in the group than a gradient strength of a gradient to be switched on the same axis in a subsequent acquisition of measurement data of a half-spoke of the half-spokes to be sampled of the group following the first acquisition of measurement data of the group.

[0039] In this way, the load on a gradient unit 5 during the acquisition of measurement data MD along the sampled half-spokes of a group can be better distributed across the x, y, z axes of gradient unit 5 assigned to the segment of the group. For example, if a half-spoke randomly selected as the next half-spoke of the group does not meet the condition, it can be randomly selected again. This can be repeated until the condition is met or a termination criterion is reached. A pause P can also be inserted, if necessary, such that the half-spokes of the group that violate the condition and are to be sampled consecutively in sequence R are separated from each other by the pause P.

[0040] Before a pause P inserted between two acquisitions A of measurement data MD along the half-spokes HS1, HS2, ..., HSn to be sampled, gradients switched can be reduced during the pause P and / or increased after the pause P for a subsequent acquisition A of measurement data MD of the half-spokes HS1, HS2, ..., HSn to be sampled, until they reach a gradient strength required for the following acquisition A of measurement data MD. In this way, the duration for changing the gradient strength of the voltages applied to the gradient unit 5 can be used to control it.

[0041] Before each recording A of measurement data MD of a half-spoke HS1, HS2, ..., HSn, as above with reference to Fig. As illustrated by the example of a PETRA sequence, at least one RF pulse RF1 is injected into the object under investigation U. At least one such RF pulse RF1 can also be injected during a pause P, so that the rhythm of injected RF pulses RF1 is not interrupted by the pause P, and a steady state in the magnetization of the spins in the object under investigation is maintained. If a pause P of a longer duration than a repetition time TR is to be inserted, several RF pulses RF1 can also be injected during the course of the pause P.

[0042] The temperature T of gradient unit 5 can be monitored (Block 102). This can be done in a known manner. If the monitoring of the temperature T of gradient unit 5 shows that the temperature T of gradient unit 5 reaches an upper threshold ST (Block 104), a pause P can also be inserted to cool the gradient unit 5.

[0043] By monitoring the temperature T of the gradient unit 5 in this way, it can be ensured that the gradient unit 5 does not overheat.

[0044] The recordings A of the measurement data MD along the spokes HS1, HS2, ..., HSn to be scanned are carried out according to the specified sequence R (Block 107).

[0045] Based on the recorded measurement data MD, image data BD can be created, e.g. using a Fourier transform (Block 109).

[0046] Fig. Figure 4 schematically depicts a magnetic resonance system 1 according to the invention. This system comprises a magnet unit 3 for generating the fundamental magnetic field, a gradient unit 5 for generating the gradient fields, a radio frequency unit 7 for irradiating and receiving radio frequency signals, and a control unit 9 designed for carrying out a method according to the invention. The representation of the control unit 9 in Fig. Figure 2 is purely exemplary. A control device 9 according to the invention for carrying out the method according to the invention can also be designed independently of a magnetic resonance system 1 without restriction.

[0047] In the Fig.Figure 4 shows only a rough schematic representation of these subunits of the magnetic resonance system 1. The high-frequency unit 7 can consist of several subunits and, for example, comprise several coils. In particular, the high-frequency unit 7 can comprise a body coil that is permanently integrated into the magnetic resonance system 1 and, in turn, can comprise two or more antenna elements. Furthermore, the high-frequency unit 7 can comprise one or more different local coils 7.1 and 7.2, which can be designed either only for transmitting high-frequency signals or only for receiving the triggered high-frequency signals, or for both, and which themselves can comprise several antenna elements and associated coil channels.

[0048] To examine a test object U, for example a patient or a phantom, it can be placed on a table L in the magnetic resonance imaging system 1 within its measuring volume. The layer S irepresents an exemplary target volume of the object under investigation, in which a desired image area can be selected, and from which echo signals can be recorded and captured as measurement data.

[0049] The control unit 9 serves to control the magnetic resonance system 1 and can, in particular, control the gradient unit 5 by means of a gradient controller 5' and the radio frequency unit 7 by means of a radio frequency transmit / receive controller 7'. The radio frequency unit 7 can comprise several channels on which signals can be transmitted or received.

[0050] The high-frequency unit 7, together with its high-frequency transmit / receive control 7', is responsible for generating and transmitting a high-frequency alternating field to manipulate the spins in a region to be manipulated (for example, in layers S to be measured) of the object under investigation U. The center frequency of the high-frequency alternating field, also referred to as the B1 field, is generally set as close as possible to the resonance frequency of the spins to be manipulated. Deviations from the center frequency to the resonance frequency are referred to as off-resonance. To generate the B1 field, controlled currents are applied to the RF coils in the high-frequency unit 7 by means of the high-frequency transmit / receive control 7'.

[0051] Furthermore, the control unit 9 comprises an optimization unit 15 for determining, according to the invention, an optimized sequence of successively scanned half-spokes. The control unit 9 is designed overall to carry out a method according to the invention.

[0052] A computing unit 13, encompassed by the control unit 9, is designed to perform all the necessary calculations for the required measurements and determinations. Intermediate results and final results required for this purpose, or determined in the process, can be stored in a storage unit S of the control unit 9. The units shown here are not necessarily to be understood as physically separate units, but merely represent a subdivision into conceptual units, which can also be realized, for example, in fewer or even just a single physical unit.

[0053] Via an input / output device (I / O) of the magnetic resonance system 1, control commands can be sent to the magnetic resonance system by a user, for example, and / or results from the control device 9, such as image data, can be displayed.

[0054] The method described herein may also be in the form of a computer program comprising instructions that execute the described method on a control unit 9. Likewise, a computer-readable storage medium may be present, comprising instructions that, when executed by a control unit 9 of a magnetic resonance system 1, cause it to execute the described method.

[0055] Regardless of the grammatical gender of a particular term, persons with male, female or other gender identities are included. QUOTES INCLUDED IN THE DESCRIPTION

[0000] This list of documents cited by the applicant was automatically generated and is included solely for the reader's convenience. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions. Cited patent literature

[0000] US 8878533B2

[0007] Cited non-patent literature

[0000] Sonia Nielles-Vallespin “3D radial projection technique with ultrashort echo times for sodium MRI: Clinical applications in human brain and skeletal muscle”, Magn. Res. Med. 2007; 57; pp. 74-81

[0006] Weiger et al., “MRI with Zero Echo Time: Hard versus Sweep Pulse Excitation” Magnetic Resonance in Medicine 66: pp. 379-389, 2011

[0007] Wu et al., „Density of Organic Matrix of Native Mineralized Bone Measured by Water- and Fat-Suppressed Proton Projection MRI“, Magn. Reson. Med. 50:S. 59-68, 2003

[0007] von Heid et al., „Rapid Single Point (RASP) imaging“, Proc. Intl. Soc. Mag. Reson. Med., S. 684, 1995

[0011] von Balcom et al., „Single-Point Ramped Imaging with T1 Enhancement (SPRITE)“, J. Magn. Reson. A 123(1): S. 131-134, 1996

[0011]

Claims

[1] Method for recording (A) measurement data (MD) of the object under investigation (U) using a magnetic resonance system (1) with an ultrashort echo time sequence comprising the steps: - Loading the half-spokes (HS1, HS2, ..., HSn) to be sampled in k-space for the desired recordings (A) with the associated gradients to be switched for spatial coding on three axes of a gradient unit (5) of the magnetic resonance system (1), - Determining a sequence (R) of recordings (A) of measurement data (MD) to be taken successively along each half-spoke (HS1, HS2, ..., HSn) with a distribution of gradients to be switched on the axes optimized with respect to a load on the axes, - Performing the recording (A) of the measurement data (MD) along the half-spokes to be scanned (HS1, HS2, ..., HSn) according to the specified sequence (R). [2] Method according to claim 1, wherein determining the sequence (R) comprises randomly selecting half-spokes (HS1, HS2, ..., HSn) of the half-spokes (HS1, HS2, ..., HSn) to be scanned, which are recorded one after the other. [3] Method according to one of the preceding claims, wherein the determination of the sequence (R) is carried out under the condition that a gradient strength of a gradient to be switched on at least one, in particular each, of the axes during a first acquisition (A) of measurement data (MD) of a half-spoke is not a gradient strength that is the next largest or next smallest of all gradient strengths of the gradient strengths to be switched than a gradient strength of a gradient to be switched on the same axis in a subsequent acquisition (A) of measurement data (MD) of a half-spoke (HS1, HS2, ..., HSn) of the half-spokes (HS1, HS2, ..., HSn) following the first acquisition (A) of measurement data (MD). [4] Method according to one of the preceding claims, wherein determining the sequence (R) comprises dividing the k-space into at least two, for example four or eight, in particular 16, segments, and the half-spokes to be scanned are divided into groups (G1, G2, ..., Gn) according to a segment in which they are located, and wherein the half-spokes to be scanned (HS1, HS2, ..., HSn) of a group are recorded successively in the determined sequence (R). [5] Method according to claim 4, wherein determining the sequence (R) within a group comprises randomly selecting half-spokes (HS1, HS2, ..., HSn) of the half-spokes (HS1, HS2, ..., HSn) of the group to be scanned, which are recorded one after the other. [6] A method according to one of claims 4 or 5, wherein the determination of the sequence (R) within a group (G1, G2, ..., Gn) is carried out under the condition that a gradient strength of a gradient to be switched on at least one, in particular each, of the axes during a first acquisition (A) of measurement data (MD) of a half-spoke (HS1, HS2, ..., HSn) of the group (G1, G2, ..., Gn) is not a next larger or next smaller gradient strength of all gradient strengths of the gradient strengths to be switched in the group (G1, G2, ..., Gn) than a gradient strength of a gradient on the same axis in a subsequent acquisition (A) of measurement data (MD) of a half-spoke (HS1, HS2, ..., HSn) of the half-spokes (HS1, HS2, ..., ...) to be sampled following the first acquisition (A) of measurement data (MD) of the group (G1, G2, ..., Gn). HSn) of the group to switching gradients. [7] Method according to one of the preceding claims, wherein determining the sequence (R) comprises inserting pauses (P) in which no gradients are switched on at least one axis, at least for a cooling period encompassed by the pause (P). [8] Method according to claim 7, wherein gradients switched before a pause (P) are reduced during the pause (P) and / or gradients to be switched after the pause (P) for a subsequent acquisition (A) of measurement data (MD) are increased during the pause (P) after the cooling period until they have reached a gradient strength required for the subsequent acquisition (A) of measurement data (MD). [9] Method according to one of claims 7 or 8, wherein at least one RF pulse (RF1) is irradiated into the object under investigation (U) before each recording (A) of measurement data (MD) of a half-spoke (HS1, HS2, ..., HSn), and wherein at least one such RF pulse (RF1) is also irradiated during a pause (P), so that a rhythm of irradiated RF pulses (RF1) is not interrupted by the pause (P). [10] Method according to any one of claims 7 to 9, wherein a monitoring of a temperature (T) of the gradient unit (5) is carried out and, when the temperature (T) reaches an upper threshold (ST), a pause (P) is inserted. [11] Magnetic resonance system (1) comprising a magnet unit (3), a gradient unit (5), a radio frequency unit (7) and a control unit (9) with a radio frequency transmit / receive control (7') and with an optimization unit (15), wherein the control unit (9) is configured to perform a method according to any one of claims 1 to 10 on the magnetic resonance system (1). [12] Computer program comprising instructions which, when the program is executed by a control device (9), cause it to execute the method according to any one of claims 1 to 10. [13] Computer-readable storage medium comprising instructions which, when executed by a control device (9), cause it to execute the method according to any one of claims 1 to 10.

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