Improved PETRA procedure

Abstract

Within the scope of a method according to the invention for creating measurement data from an imaging region of a measurement volume of a magnetic resonance system using a PETRA method, a pulse duration of at least one RF excitation pulse radiated during a single-shot acquisition method of the PETRA method is extended compared to a pulse duration of RF excitation pulses radiated during a radial acquisition method of the PETRA method, whereby computational operations for correcting interfering folding can be simplified.

Description

[0001] The invention relates to an improvement of a PETRA process.

[0002] Magnetic resonance imaging (MR) is a well-known technique for generating images of the interior of an object under examination. Simply put, the object under examination is positioned in a magnetic resonance scanner in a comparatively strong, static, homogeneous basic magnetic field, also known as a B0 field, with field strengths ranging from 0.2 Tesla to 7 Tesla and more, so that its nuclear spins are oriented along the basic magnetic field. To trigger nuclear magnetic resonances, which can be measured as signals, high-frequency excitation pulses (RF pulses) are radiated into the object under examination. The triggered nuclear magnetic resonances are measured as so-called k-space data using coils designed for reception, and based on these, MR images are reconstructed or spectroscopy data is determined. The alternating magnetic field generated by the excitation pulses radiated by at least one transmitting coil is also referred to as the B1 field.To spatially encode the measurement data, rapidly switched magnetic gradient fields, called gradients for short, are superimposed on the basic magnetic field. A scheme used, which describes a temporal sequence of RF pulses to be delivered and gradients to be switched, is referred to as a pulse sequence (scheme), or simply a 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, for example, using a multidimensional Fourier transform.

[0003] It is not possible to use MR sequences to display materials or tissues whose T2* time, the effective decay of the transverse magnetization of this material or tissue, is significantly shorter than the shortest possible echo times within these sequences, since a corresponding signal from these materials or tissues has already decayed at the time of the acquisition.

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

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

[0006] One such MR technique is the UTE sequence (“Ult-short Echo Time”), as described, among other things, in the article by Sonia Nielles-Vallespin “3D radial projection technique with ult-rashort 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- or slice-selective excitation, data acquisition begins at the same time as the gradients for spatial encoding are ramped up. The k-space trajectory sampled in this way after excitation runs radially outwards from the center of k-space. Therefore, before the image data can be reconstructed from the raw data recorded in k-space using Fourier transformation, these raw data must first be converted to a Cartesian k-space grid, e.g., by regridding.

[0007] Other MR techniques that allow particularly short echo times are zTE (“zero echo time”) and PETRA (“pointwise encoding time reduction with radial acquisition”) or WASPI (“water- and fat-suppressed proton projection MRI”) sequences and 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 spokes whose gradients, which are switched for spatial encoding, are already fully ramped up at the time of excitation of the spins in an object under investigation, which saves valuable encoding time.However, this also creates a region in the k-space center that cannot be sampled by these radial spokes. Compared to UTE methods, zTE and PETRA methods are more robust, as eddy currents or unwanted, small time shifts from switched gradients or radiated RF pulses have no or at most a negligible influence on the measurements.

[0008] An example of a part of a pulse sequence for such a recording of measurement data along radial spokes, as used in zTE methods and PETRA methods, is shown in Fig. 1. The upper row, “Tx / Rx,” shows the applied RF excitation pulses RF1 and the readout time windows ADC, during which the measurement data is acquired. The middle row, “G1,” shows the gradients switched in an encoding direction, which have each reached their desired strength for the subsequent acquisition of measurement data at the time of the application of an RF excitation pulse RF1. The lower row, “k-sp,” shows the corresponding k-space points sampled along the k-space trajectory defined by the applied gradient field to acquire the measurement data. Measured k-space points are shown as black points, and k-space points located before the start of the readout time window, and therefore not read out, are shown as “empty” points. The fact that k-space points are not read out is due to the fact that, as described, a constant gradient field is already applied before the RF excitation pulse RF is applied.This would require the central k-space point (k0) to be measured simultaneously with the radiation of the RF excitation pulse RF1, which is technically not possible. Only after a minimum required switching time Ts after the end of the RF excitation pulse RF1, which depends on the hardware of the magnetic resonance system used, can the acquisition of measurement data begin in the readout time window ADC, which still results in the shortest possible echo time TE. The first k-space point k* read out in the readout time window ADC has the smallest distance from the k-space center k0 among the readout k-space points. The last k-space point k. maxhas the maximum distance from the k-space center k0 among the readout k-space points. The duration of the readout time window ADC (acquisition time) is determined by the strength of the applied gradient field G1 and the required resolution or the desired image area (FOV) or the matrix of the image to be created from the measured data.

[0009] A corresponding sampling scheme of k-space is shown in Fig. 2, where a radial k-space spoke corresponds to a k-space trajectory along which ADC measurement data are acquired in a readout time window. In the region B1, measurement data are acquired along radial spokes in different coding directions until, for example, a desired sampling density in k-space is reached. The radius of the central region B2, in which no measurement data are acquired along the described radial spokes because the absolute value of a distance of a k-space point k is smaller than k*, depends on the k-space moment accumulated after excitation until the acquisition of the measurement data, and thus on the echo time TE, in which the gradient is switched with a constant strength, and the strength of the switched gradient field G. The longer this echo time TE and the stronger the gradient strength, and thus the higher the readout bandwidth, the more k-space points are not measured on the radial k-space trajectories.

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

[0011] In PETRA procedures, measurement data from the non-radially scanned area B2 can be acquired using an MR single-point acquisition method, 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” method 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 Fig. 2 is roughly schematically represented by filled points in area B2, which represent individual recorded k-space points.

[0012] The MR single-point acquisition method used, which can be performed, for example, in a separate measurement section from the acquisition of the radial measurement data, measures individual k-space points at a time TE after irradiation of an RF excitation pulse RF2 in a respective readout time window ADC, whereby the respective strength of gradients G2 switched for spatial encoding is adjusted in such a way that the desired k-space point in k-space is precisely hit. The gradients G2 are changed for each k-space point using the magnetic resonance system as described in Fig. 3a and Fig. Figure 3b shows a schematic representation of a RASP single-point acquisition method.

[0013] The disadvantage of pre-activating gradients during excitation in PETRA, zTE, and WASPI techniques is that it limits the excitation options. While UTE techniques also allow for 2D or other flexible 3D excitation pulses, zTE- or PETRA-type techniques require excitation with the shortest possible rectangular pulse, also called a "hard pulse." As described in the article by Grodzki et al., "Correcting slice selectivity in hard pulse sequences," Proc. Intl. Soc. Mag. Reson. Med. 20, p. 2479, 2012, despite the use of such spatially non-selective hard pulses as RF excitation pulses, unwanted slice excitation can occur precisely because of the gradients already switched during the radiation of the RF excitation pulses, in which, depending on the scanned k-space trajectory, a different spectral pulse profile dependent on the switched gradients, for example a sinc-shaped one, folds into the image.This perturbation can be described as the folding of a pulse profile P(x,k), which depends on both the position space (x) and the k-space (k), into the magnetization distribution f(x). The result is a k-space F'(k) perturbed by a superposition of the pulse profile P(x,k). F′(k)=∑xf(x)P(x,k)eikx.

[0014] Fig. Figure 4 replicates a figure from the aforementioned article by Grodzki et al. and schematically shows the full width half maximum (FWHM) of exemplary pulse profiles of used RF excitation pulses in the image along a line through k-space within a PETRA sequence. For k-space points with a distance |k| from the k-space center that is greater than the distance of k* from the k-space center (k ≥ k*), and thus the radial part of the acquisition of the measurement data, the magnitude of the gradient strength G(k) is always the same. Thus, any disturbance resulting from superposition with the respective pulse profile is also the same. Between -k* and k*, and thus in the part of the acquisition of the measurement data that is performed using an MR single-point acquisition method, the gradient strength G(k) is proportional to the distance |k| of the respective k-space point k from the k-space center. Thus, the gradient strength G(k) is lower for |k| < k* than for |k| ≥ k*.At low gradient strength G(k), the half-width FWHM increases, causing zero crossings of the pulse profile to slide further outwards and thus significantly reducing the disturbing influence of the pulse profile on the reconstructed MR image.

[0015] The correction method described in the aforementioned article by Grodzki et al. to eliminate the disturbances caused by the unwanted slice selection is based on a matrix inversion that calculates the aforementioned complex and k-space-dependent convolution using the Fourier transform of the image reconstruction. However, the computation required for this correction method can be quite time-consuming – especially in the case of high gradient strengths, for example, when the strength of the first gradient G1 is more than 15 mT / m at an isotropic resolution of 1 mm and a matrix size of approximately 256 3, - and on the other hand, such a matrix inversion can increase noise (especially outside the first minimum of the slice profile) to such an extent that evaluation is no longer possible there.

[0016] The invention is based on the object of correcting disturbances in PETRA methods caused by a described unwanted layer selection and thus enabling improved PETRA methods which in particular do not require computationally intensive mathematical operations.

[0017] The object is achieved by a method for creating measurement data from an imaging region of an examination object located in a measurement volume of a magnetic resonance system according to claim 1, a magnetic resonance system according to claim 11, a computer program according to claim 12, and an electronically readable data carrier according to claim 13.

[0018] A method according to the invention for creating measurement data from an imaging area of ​​an examination object located in a measurement volume of a magnetic resonance system comprises the steps: - Reading out a k-space corresponding to the imaging area, comprising the substeps: a) Ramping up a first gradient until it has reached a first strength in a desired coding direction, by means of a gradient unit of a magnetic resonance system, b) radiating a first RF excitation pulse while the first gradient has the first strength, by means of a radio-frequency transmit / receive control of the magnetic resonance system, c) Recording first echo signals generated by the first RF excitation pulse by means of the radio-frequency transmit / receive control after a switching time after irradiation of a first RF excitation pulse, d) storing the recorded first echo signals as first measurement data along the k-space trajectory specified by the first strength of the switched first gradient and the desired encoding direction, wherein steps a) to d) are repeatedly carried out with constant first gradients switched in different desired coding directions until the k-space corresponding to the imaging area is read out in a first range dependent on the switching time, wherein the different coding directions are composed of at least two phase coding directions, - reading out a second region of the k-space corresponding to the imaging area, which is at least partially not covered by the first region of the k-space and which comprises at least the k-space center, using a single-shot acquisition method in which, after repeated irradiation of a second RF excitation pulse, excited second echo signals are acquired by switching two gradients for coding for each k-space point of the second region and stored as second measurement data until all desired k-space points of the second region have been acquired, - Reconstructing image data from the recorded first measurement data and the recorded second measurement data, wherein a pulse duration of at least one of the second RF excitation pulses is extended compared to a pulse duration of first RF excitation pulses for recording second echo signals of at least one k-space point in the second region, wherein a pulse duration of an extended second RF excitation pulse (RF2) is determined as a function of the switching time Ts.

[0019] The method according to the invention can thus be characterized as a method for creating measurement data from an imaging region in a measurement volume of a magnetic resonance system using a PETRA method, in which a pulse duration of at least one RF excitation pulse radiated during a single-shot acquisition method of the PETRA method is extended compared to a pulse duration of RF excitation pulses radiated during a radial acquisition method of the PETRA method.

[0020] By extending a pulse duration of at least one second RF excitation pulse (i.e., extending the duration of at least one second RF excitation pulse) compared to the pulse duration of first RF excitation pulses (i.e., the duration of the first RF excitation pulses), a pulse profile of an extended second RF excitation pulse is changed, whereby undesired folding of the pulse profiles is likewise changed. This can, for example, achieve the adjustment of a dependence of a pulse profile of the first RF excitation pulses on a k-space position of first measurement data and a dependence of the pulse profile of an extended second RF excitation pulse on a k-space position of associated second measurement data, whereby computational operations for correcting the interfering folding are simplified.In particular, it can be achieved that a dependence of the pulse profiles on the k-space position of the respectively acquired measurement data is reduced, up to and including completely eliminated or at least becomes negligible, whereby instead of the previously required computationally intensive matrix inversion, a simple division by the pulse profile P(x), which now only depends on the spatial space (x), is sufficient to correct the folding. In this way, the reconstruction of image data can be significantly simplified and accelerated. A noise increase in regions outside of the first zero crossings of pulse profiles of used RF excitation pulses can also be remedied by an inventive extension of the pulse duration of second RF excitation pulses, so that the quality of the obtained image data is increased and overall there remain significantly fewer restrictions due to the undesired slice selection.

[0021] The process can run without any action or knowledge on the part of the user, so no requirements are necessary for the user.

[0022] The method is particularly important for systems with strong gradients, such as the Cima.X, since with strong gradients the proportion of single points increases, which extremely slows down or even crashes the reconstruction with matrix inversion, as has been necessary up to now according to the state of the art.

[0023] A magnetic resonance system according to the invention comprises a magnet unit, a gradient unit, a radio frequency unit and a control device designed to carry out a method according to the invention, having a pulse duration adaptation unit.

[0024] 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 comprises instructions that, when the program is executed by a control device, e.g., a control device of a magnetic resonance imaging system, cause this control device to execute a method according to the invention. The control device can be configured in the form of a computer.

[0025] The computer program can also be in the form of a computer program product which can be loaded directly into a memory of a control device, with program code means for carrying out a method according to the invention when the computer program product is executed in a computing unit of the computing system.

[0026] 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 the control device to carry out a method according to the invention.

[0027] The computer-readable storage medium can be designed as an electronically readable data carrier which comprises electronically readable control information stored thereon, which control information comprises at least one computer program according to the invention and is designed such that it carries out a method according to the invention when the data carrier is used in a control device of a magnetic resonance system.

[0028] The advantages and details given with regard to the method also apply analogously to the magnetic resonance system, the computer program product and the electronically readable data carrier.

[0029] Further advantages and details of the present invention will become apparent from the exemplary embodiments described below and from the drawings. The examples listed do not represent a limitation of the invention. They show: Fig. 1 a schematic representation of part of a pulse sequence scheme for acquiring measurement data, Fig. 2 a schematically illustrated exemplary k-space sampling scheme, Fig. 3a and Fig. 3b schematically illustrated parts of a pulse sequence scheme for acquiring measurement data according to a RASP method, Fig. 4 schematic half-width exemplary pulse profiles, Fig. 5 a schematic flow diagram of a method according to the invention for generating measurement data from an imaging area of ​​an examination object located in a measurement volume of a magnetic resonance system, Fig. 6 a schematically illustrated magnetic resonance system according to the invention.

[0030] Fig. 5 is a schematic flow diagram of a method according to the invention for creating measurement data from an imaging region of an examination object located in a measurement volume of a magnetic resonance system.

[0031] In a first area B1 of the k-space corresponding to the imaging area, first measurement data MDS1 are recorded (block 101).

[0032] A first gradient G1 is ramped up by means of a gradient unit of the magnetic resonance system until it has reached a first strength in a desired coding direction (block 101.1).

[0033] While the first gradient G1 has the first strength, ie while the first gradient G1 is constantly applied with the first strength, a first RF excitation pulse RF1 is radiated by means of a radio frequency transmit / receive control of the magnetic resonance system (block 101.3).

[0034] First echo signals generated by the first RF excitation pulse RF1 are recorded by the radio-frequency transmit / receive control of the magnetic resonance system after a switching time Ts after irradiation of a first RF excitation pulse RF1 and stored as first measurement data MDS1 along the k-space trajectory specified by the first strength of the switched gradient and the desired coding (block 101.5).

[0035] Blocks 101.1 to 101.5 are repeatedly executed with constant first gradients G1 switched in various desired coding directions until the k-space corresponding to the imaging region is read out in the first range B1 dependent on the switching time Ts, wherein the various coding directions are composed of at least two phase coding directions. For this purpose, a query 100 can check whether first measurement data MDS1 have already been acquired in all desired coding directions and, if not ("n"), initiate a further acquisition of first measurement data in another desired coding direction.

[0036] A second region B2 of the k-space corresponding to the imaging area, which is at least partially not covered by the first region B1 of the k-space, and which includes at least the k-space center, is acquired using a single-shot acquisition method, wherein excited second echo signals for each k-space point are acquired after excitation by a second RF excitation pulse RF2 and stored as second measurement data MDS2 (block 103).

[0037] In this case, second gradients G2 are switched by means of the gradient unit of the magnetic resonance system for encoding second echo signals to be recorded (block 103.1).

[0038] To excite the second echo signals to be recorded, second RF excitation pulses RF2 are radiated (block 103.3), wherein a pulse duration of at least one of the second RF excitation pulses is extended compared to a pulse duration of first RF excitation pulses for recording second echo signals of at least one k-space point in the second region B2.

[0039] After each second RF excitation pulse RF2 is radiated, excited second echo signals are acquired using the radio-frequency transmit / receive control of the magnetic resonance system and stored as second measurement data for one k-space point per radiated second RF excitation pulse RF2 (block 103.5). The acquisition of second echo signals occurs, in particular, after an echo time TE corresponding to the switching time Ts after the respective second RF excitation pulse RF2 is radiated, so that first measurement data MDS1 and second measurement data MDS2 are acquired with the same minimum echo times.

[0040] The extension of the pulse duration of the at least one second RF excitation pulse RF2 can be carried out in such a way that a dependency of a pulse profile of the first RF excitation pulses RF1 on a k-space position of first measurement data and a dependency of the pulse profile of an extended second RF excitation pulse on a k-space position of associated second measurement data is adjusted, in particular that a dependency of a half-width of a pulse profile of the first RF excitation pulses RF1 on a k-space position of first measurement data and a dependency of a half-width of a pulse profile of an extended second RF excitation pulse on a k-space position of associated second measurement data is adjusted.

[0041] First RF excitation pulses can and usually are always selected to be uniform, in particular with the same pulse duration (duration of irradiation of an RF excitation pulse). Furthermore, first RF excitation pulses RF1 are each irradiated while switching first gradients G1, each of which has the first strength (albeit in different encoding directions). Therefore, first RF excitation pulses have identical pulse profiles for all irradiated first RF excitation pulses RF1, which thus do not depend on the k-space position of the respective first measurement data MDS1.

[0042] However, as described above, second gradients G2 have different strengths that are smaller than the first strength of the first gradient G1, and which cause the pulse profiles of second RF excitation pulses to depend on the k-space position of the respective associated second measurement data MDS2.

[0043] The invention is based on the finding that by extending the pulse duration of a second RF excitation pulse RF2, a pulse profile of the extended second RF excitation pulse can be changed in such a way that the desired alignment of the dependency of a pulse profile of a first RF excitation pulse on a k-space position of first measurement data, which were recorded as first echo signals after excitation by the first RF excitation pulse RF1, with the dependency of a pulse profile of an extended second RF excitation pulse on the k-space position of second measurement data, which were recorded as second echo signals after excitation by the extended second RF excitation pulse RF2, and thus even an elimination of the dependency of the pulse profiles of extended second RF excitation pulses RF2 on a k-space position of the respectively associated second measurement data MDS2 can be achieved.

[0044] This can be explained as follows: Although an extension of the pulse duration of a second RF excitation pulse generally (with otherwise unchanged parameters) results in a sharpening of the (spectral) profile of the second RF excitation pulse and thus a reduction of the half-width of the profile of the second RF excitation pulse, the (as already described in particular with reference to Fig. 4), compared to the first strength of the first gradient G1, switched second gradients G2 have an opposing effect, namely a broadening of the profile of a second RF excitation pulse compared to a profile of a first RF excitation pulse. This broadening of the profile of the second RF excitation pulse RF2 for |k| < k* can be canceled out or at least reduced by extending a pulse duration of a second RF excitation pulse RF2, so that a dependence of a pulse profile of a second RF excitation pulse RF2 on a k-space position k of the associated second measurement data MDS2 can be reduced or even eliminated.

[0045] This allows the computational effort required to correct disturbing convolutions of the pulse profiles in a recorded k-space disturbed by the convolutions to be significantly reduced.

[0046] Here, a pulse duration of an extended second RF excitation pulse RF2 is determined as a function of the switching time Ts. As described above, the switching time Ts determines the shortest possible (and thus smallest) echo time TE at which first measurement data MDS1 can be acquired and thus also determines the smallest value k* of a k-space position at which first measurement data MDS1 can be acquired. The value k* dependent on the switching time Ts thus corresponds to the smallest distance from the k-space center k0 at which first measurement data MDS1 can be acquired and is thus the smallest k-space position at which a dependence of a pulse profile of a first RF excitation pulse RF1 on a k-space position k of associated first measurement data can be determined, and thus the dependence on k of a pulse profile of a first RF excitation pulse RF1 to which an adjustment of a dependence on k of a pulse profile of a second RF excitation pulse RF2 is desired.

[0047] The pulse duration of an extended second RF excitation pulse RF2 can be extended inversely proportional to the respective strength of the second gradients G2 switched to acquire the respective k-space points acquired as second measurement data MDS2 with the extended second RF excitation pulse RF2. Since, as described above, the strength of the switched second gradients G2 is proportional to the k-space position k of the associated second measurement data, such an extension of second RF excitation pulses RF2 exerts an effect on the pulse profile of an extended second RF excitation pulse RF2 that cancels out the effect of the changing strengths of the gradients G2.

[0048] In particular, the extension of a pulse duration of a second RF excitation pulse RF2 can thus be achieved by a factor k* / |k|. If a pulse duration of a second RF excitation pulse RF2 is extended by a factor k* / |k|, the zero crossings of the pulse profiles of the first excitation pulses RF1 and second RF excitation pulses RF2 are the same for all k-space positions k, and a dependence of the pulse profiles on the k-space position k is eliminated. Fig. 4, corresponding half-widths of pulse profiles of such extended second RF excitation pulses RF2 (ie for |k| < k*) would be equal to the half-widths of pulse profiles of first RF excitation pulses RF1 (ie for |k| ≥ k*), so that the half-widths of the pulse profiles of first RF excitation pulses RF1 and second RF excitation pulses RF2 are equal for all k-space positions k.

[0049] A k-space F'(k) perturbed by a convolution with a pulse profile P(x) that only depends on the position x can be expressed as: F′(k)=∑xf(x)P(x)eikx.

[0050] Equation [2] corresponds to a simple convolution of k-space with the pulse profile P(x). Such an involution can be removed with a simple division in image space, which can be performed with little computational effort.

[0051] As a special case, a second RF excitation pulse RF2, which is radiated for the acquisition of second measurement data MDS2 in the k-space center k0 (k-space position k=0), can be considered, since for an encoding of the k-space center k0, the strength of the switched gradients is zero, and thus there is no superposition (by folding) with the corresponding pulse profile of this second RF excitation pulse RF2.

[0052] Therefore, a pulse duration of a second RF excitation pulse RF2 for recording second echo signals in the k-space center can be arbitrarily selected. In particular, a pulse duration of a second RF excitation pulse RF2 for recording second echo signals in the k-space center can be selected to be equal to the pulse duration of the first RF excitation pulse RF1, so that such a second RF excitation pulse can be equal to a first RF excitation pulse RF1.

[0053] It is conceivable that specifications within the framework of the pulse sequence scheme used, particularly regarding the timing of the pulse sequence, do not permit any arbitrary extension of the RF excitation pulses used, but rather that a maximum duration Dm is specified, which an irradiated RF excitation pulse should not exceed. Such a maximum duration Dm can depend, in particular, on a desired echo time, after which second echo signals are to be recorded after irradiating a second RF excitation pulse and acquired as second measurement data.

[0054] If an extended pulse duration of a second RF excitation pulse RF2, determined as a function of the switching time Ts and inversely proportional to the respective strength of the second gradient switched after the second RF excitation pulse to record a respective k-space point, exceeds a predetermined maximum duration Dm, the extended pulse duration of this second RF excitation pulse RF2 can be set to the maximum duration Dm. Such cases only occur for k-space positions in absolute proximity to the k-space center, ie for |k*-k| < k min (where kmin is the k-space position from which a pulse duration extended inversely proportional to the k-space position exceeds the maximum duration Dm). An extended pulse duration of second RF excitation pulses RF2 can be applied from this k-space point k minto the k-space center k0 are kept constant at the value of the maximum duration. This is possible in a first approximation without problems because a) only a few k-space points are affected and b) the strengths of second gradients G2 for encoding second measurement data MDS2 for k-space positions smaller k min are extremely small, and therefore any remaining folding is also negligible. k min can therefore also be chosen as the k-space position from which remaining foldings become negligible.

[0055] Blocks 103.1 to 103.5 are repeatedly performed with different second gradients G2, each encoding a desired k-space point, until second echo signals are recorded for all desired k-space points and stored as second measurement data MDS2. For this purpose, a query 100' can check whether second measurement data MDS2 have already been recorded for all desired k-space points and, if not ("n"), initiate another recording of second measurement data MDS2 for another desired k-space point.

[0056] At least one k-space point is acquired in the second region B2. At least 100 k-space points can be acquired in the second region B2, particularly with greater strengths of the switched gradients G1, for example, more than 20 mT / m. Particularly with a high resolution, for example, a resolution of less than one millimeter, several thousand k-space points can be acquired. As the number of k-space points for which second measurement data MDS2 are acquired in the second region B2 increases, so does the complexity of the matrix inversion previously required to correct for disturbances caused by unwanted slice selection. The method according to the invention significantly reduces the computational effort required for the correction.The number of k-space points recorded in the second area B2 increases (with the same coverage) with a maximum (magnitude) strength of second gradients G2 (and thus with increasing radius k* of the second area B2), since a larger strength of the first gradient G1 leads to a larger k* and thus to a larger radius of the second area B2.

[0057] An extension of a pulse duration can be performed for at least two second RF excitation pulses RF2 associated with different k-space points. The achievable simplification of the correction of the folding caused by the undesired slice selection is greater the more second RF excitation pulses RF2 experience an extension of their pulse duration. Therefore, pulse durations of second RF excitation pulses RF2 can be provided for as many as possible, in particular all k-space positions k with k0 < k < k min to which second measurement data MDS2 are recorded.

[0058] Image data BD is reconstructed from the acquired first measurement data MDS1 and the acquired second measurement data MDS2 using a control device of the magnetic resonance system (block 107). Reconstructing the image data BD from the acquired first measurement data MDS1 and the acquired second measurement data MDS2 can include dividing the image data BD by a location-dependent pulse profile of the first and second RF excitation pulses RF1, RF2 to correct for interference caused by undesired slice selection.

[0059] When reconstructing the image data BD from the recorded first measurement data MDS1 and the recorded second measurement data MDS2, a division by a (largely only) location-dependent pulse profile of the first and second RF excitation pulses is carried out. By extending the pulse duration of the second RF excitation pulses RF2 according to the invention, it can be achieved that

[0060] Fig. 6 schematically illustrates a magnetic resonance system 1 according to the invention. This comprises a magnet unit 3 for generating the basic 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 device 9 designed to carry out a method according to the invention.

[0061] In the Fig. 6, these subunits of the magnetic resonance system 1 are shown only in a rough schematic. The radio-frequency unit 7 can consist of several subunits and, for example, comprise several coils. In particular, the radio-frequency unit 7 can comprise a body coil that is permanently integrated into the magnetic resonance system 1 and, for example, can comprise two antenna elements 7.1 and 7.2.

[0062] To examine an object U, for example a patient or a phantom, it can be placed on a couch L in the magnetic resonance system 1 in its measurement volume. The slice S i represents an exemplary target volume of an imaging area of ​​the object under investigation, from which echo signals can be recorded and recorded as measurement data.

[0063] The control device 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.

[0064] The radio-frequency unit 7, together with its radio-frequency transmit / receive control 7', is responsible for generating and transmitting (transmitting) a radio-frequency alternating field for manipulating the spins in a region to be manipulated (for example, in slices S to be measured) of the object U under investigation. The center frequency of the radio-frequency alternating field, also referred to as the B1 field, is generally set as close as possible to the resonant frequency of the spins to be manipulated. Deviations from the center frequency to the resonant frequency are referred to as off-resonance. To generate the B1 field, controlled currents are applied to the RF coils in the radio-frequency unit 7 by means of the radio-frequency transmit / receive control 7'.

[0065] Furthermore, the control device 9 comprises a pulse duration adjustment unit 15 for extending the pulse durations of second RF excitation pulses according to the invention. The control device 9 is designed overall to carry out a method according to the invention.

[0066] A computing unit 13 included in the control device 9 is configured to perform all the computational operations required for the necessary measurements and determinations. Intermediate results and results required for this purpose or determined in this process can be stored in a memory unit S of the control device 9. The units shown are not necessarily to be understood as physically separate units, but merely represent a subdivision into meaningful units, which, however, can also be realized, for example, in fewer or even in only a single physical unit.

[0067] Via an input / output device I / O of the magnetic resonance system 1, control commands can be sent to the magnetic resonance system, e.g. by a user, and / or results of the control device 9, such as image data, can be displayed.

[0068] A method described herein may also be in the form of a computer program comprising instructions that execute the described method on a control device 9. Likewise, a computer-readable storage medium may be present comprising instructions that, when executed by a control device 9 of a magnetic resonance system 1, cause the system 1 to execute the described method.

[0069] Regardless of the grammatical gender of a particular term, persons with male, female or other gender identities are included.

Claims

[1] Method for generating measurement data (MDS1, MDS2) from an imaging area of ​​a test object (U) located in a measurement volume of a magnetic resonance system (1) comprising the steps: - Reading out a k-space corresponding to the imaging domain, including the substeps: a) Starting up a first gradient (G1) until it reaches a first strength in a desired coding direction, using a gradient unit (5) of a magnetic resonance system (1), b) Irradiation of a first RF excitation pulse (RF1) while the first gradient (G1) has the first strength, by means of a high-frequency transmit / receive control (7') of the magnetic resonance system (1), c) Receiving first echo signals generated by the first RF excitation pulse (RF1) using the high-frequency transmit / receive control (7') after a switching time (Ts) following the transmission of a first RF excitation pulse (RF1), d) Storing the recorded first echo signals as first measurement data (MDS1) along the k-space trajectory defined by the first strength of the switched first gradient (G1) and the desired coding direction, wherein steps a) to d) are repeated with constant first gradients (G1) switched in different desired coding directions until the k-space corresponding to the imaging domain is read out in a first region (B1) dependent on the switching time (Ts), wherein the different coding directions are composed of at least two phase coding directions, - Reading out a second region (B2) of the k-space corresponding to the imaging area, which is at least partially not covered by the first region (B1) of the k-space and which includes at least the k-space center, using a single-shot recording method in which, after repeated irradiation of a second RF excitation pulse (RF2), excited second echo signals are recorded by switching second gradients (G2) for encoding one k-space point of the second region (B2) each and stored as second measurement data (MDS2) until all desired k-space points of the second region (B2) have been recorded. - Reconstructing image data (BD) from the recorded first measurement data (MDS1) and the recorded second measurement data (MDS2), wherein a pulse duration of at least one of the second RF excitation pulses (RF2) is extended compared to a pulse duration of first RF excitation pulses (RF1) for recording second echo signals of at least one k-space point in the second area (B2), wherein a pulse duration of an extended second RF excitation pulse (RF2) is determined as a function of the switching time Ts. [2] Method according to claim 1, wherein the pulse duration of an extended second RF excitation pulse (RF2) is extended such that a dependence of a pulse profile of the first RF excitation pulses (RF1) on a k-space position of first measurement data (MDS1) and a dependence of the pulse profile of an extended second RF excitation pulse (RF2) on a k-space position of associated second measurement data (MDS2) is aligned. [3] Method according to one of the preceding claims, wherein the pulse duration of an extended second RF excitation pulse (RF2) is extended inversely proportional to the respective strength of the second gradient (G2) switched to record the respective k-space points recorded as second measurement data (MDS2) with the extended second RF excitation pulse (RF2). [4] Method according to one of the preceding claims, wherein a pulse duration of a second RF excitation pulse (RF2) for recording second echo signals in the k-space center is arbitrary. [5] Method according to any one of claims 1 to 3, wherein the pulse duration of a second RF excitation pulse (RF1) for recording second echo signals in the k-space center is equal to the pulse duration of first RF excitation pulses (RF1). [6] Method according to one of the preceding claims, wherein the pulse duration of an extended second RF excitation pulse (RF2) does not exceed a predetermined maximum duration (Dm). [7] Method according to one of the preceding claims, wherein a pulse duration of an extended second RF excitation pulse (RF2), which, after a determination as a function of the switching time Ts and inversely proportional to a respective strength of the second gradient (G2) switched after the second RF excitation pulse to record a respective k-space point, exceeds a maximum duration (Dm) dependent on a desired echo time after which second echo signals are recorded after irradiation of a second RF excitation pulse (RF2) and acquired as second measurement data (MDS2), is set to the maximum duration (Dm). [8] Method according to one of the preceding claims, wherein at least 100, in particular at least 200 or at least 1000, k-space points are recorded in the second area (B2) and / or the strength of the first gradient (G1) used is at least 15 mT / m, in particular at least 20 mT / m. [9] Method according to one of the preceding claims, wherein a reconstruction of the image data (BD) from the recorded first measurement data (MDS1) and the recorded second measurement data (MDS2) comprises a division by a location-dependent pulse profile of the first and second RF excitation pulses (RF1, RF2). [10] 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 a pulse duration matching unit (15), wherein the control unit (9) is configured to perform a method according to any one of claims 1 to 9 on the magnetic resonance system (1). [11] Computer program comprising commands which, when the program is executed by a control device (9) of a magnetic resonance system (1), cause it to execute the method according to any one of claims 1 to 9. [12] Computer-readable storage medium comprising instructions which, when executed by a control device (9) of a magnetic resonance system (1), cause it to execute the method according to any one of claims 1 to 9.

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