Improved acquisition of magnetic resonance data to avoid cropping effects

Abstract

In a method according to the invention, non-linearity data describing a gradient unit of the magnetic resonance system are loaded, at least one planning image of an object of investigation located in a measurement volume of the magnetic resonance system is displayed in which a desired field of view is selected, a measurement protocol to be used for recording the measurement data is loaded, and at least one loss factor is determined based on the non-linearity data and on the basis of the desired field of view, wherein the at least one loss factor indicates an expected foreshortening of an image reconstructed on the basis of measurement data recorded with the loaded measurement protocol, as well as filter parameters for a frequency filter for filtering frequencies of echo signals received during the recording of measurement data with the measurement protocol based on the loss factor and on the basis of the desired field of view.and an oversampling factor is determined based on the filter parameters of the frequency filter, where the oversampling factor specifies the oversampling to be applied when acquiring measurement data. Using the filter parameters and oversampling factor thus optimized, oversampled measurement data is acquired when executing the loaded measurement protocol.

Description

[0001] The invention relates to an improved recording of magnetic resonance data to avoid cropping effects.

[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] Often, k-space is scanned line by line along k-space rows running in the readout direction on a Cartesian grid, thus filling a corresponding k-space matrix line by line with values, where, for example, a k-space row is scanned after an excitation. If the field of view (FOV) captured in this way has insufficient coverage, inflection artifacts can occur.

[0004] Such inversion artifacts, also simply called inversions, can occur during the acquisition of measurement data because the generated echo signals are periodic signals that are recorded as measurement data at discrete k-space points. Therefore, only a certain number of frequencies can be uniquely captured (Nyquist's theorem). For example, if measurement data is acquired for 1000 k-space points per second, frequencies from -500 Hz to +500 Hz can be captured. If there are echo signals with higher or lower frequencies, these also appear in the interval -500 Hz to +500 Hz, but shifted by a multiple of 1000 Hz. For example, a signal with a frequency of 600 Hz would be captured as a signal with -400 Hz in a described measurement. This phenomenon is generally referred to as aliasing.

[0005] To prevent such infolding and thus aliasing artifacts, different approaches can be used for different encoding directions.

[0006] If the object under investigation extends beyond a selected field of view in the phase-encoding direction, only oversampling can help. In this process, the field of view in the image space is chosen to be larger than the object under investigation (resulting in more k-space positions in the affected encoding direction, and thus, for example, more k-space lines being sampled in the phase-encoding direction). Later, the resulting image is cropped back to a smaller field of view containing the object under investigation.

[0007] If the object under investigation extends beyond a selected field of view in the direction of readout, an alternative approach can be used: A bandpass filter can be applied to the received echo signal before the measurement data is recorded. This filter suppresses excessively high and low frequencies, i.e., frequencies that lie outside the field of view defined by the selected field of view. In simplified terms, the bandpass filter can be visualized as a trapezoidal filter in the frequency domain, as schematically depicted in Fig. Figure 1 shows a filter that allows all frequencies to pass through unaffected in a central plateau c of the trapezoid a, and completely suppresses all frequencies outside the trapezoid a. The legs of the trapezoid a represent transition bands b, in which the filter alternates between passing and blocking.

[0008] Normally, the plateau c is chosen such that, by extending it on both sides by an equally wide frequency range d, the edges of the plateau c lie reliably outside the field of view freqFOV (c = freqFOV + 2*d; d > 0), so that attenuation within the transition bands b has no effect on a filtered signal within the field of view. Real bandpass filters do not have a perfectly trapezoidal shape in the frequency band; rather, the "corners" are more "rounded," so d must be chosen large enough to ensure that the filter does not actually filter a signal within the field of view freqFOV.

[0009] The frequency range d is chosen to be as small as possible, because the width of the spectrum of frequencies still present in a filtered signal is freqFOV + 2*d + 2 * b, and this should be kept as small as possible.

[0010] Despite the application of such a bandpass filter, the frequency spectrum in a filtered signal is still larger than the frequency spectrum originating from the field of view (FOV) due to the transition bands (b). This means the FOV spectrum is larger than the FOV spectrum. Therefore, oversampling can be performed in the readout direction in addition to the described bandpass filter. It has become standard practice to oversample by a factor of 2, i.e., to acquire twice as many data points per unit of time as would actually be necessary for a chosen Nyquist field of view. The factor of 2 is primarily due to the fact that even-numbered factors were easiest to implement with the electronics available at the time such a method was introduced.It has therefore become common practice to use a fixed oversampling in the readout direction, always with a factor of 2, and a fixed band filter, whose transition bands b are chosen at intervals of always equal frequency ranges d to the edge frequencies of the field of view freqFOV, to counteract inflections.

[0011] It should be noted that oversampling in the readout direction does not incur any additional time, but it does in the phase-encoding direction, since in the phase-encoding direction each additional k-space point to be sampled requires a new acquisition of measurement data, e.g., along another k-space line. Therefore, a bandpass filter cannot be used in the phase-encoding direction, precisely because in this encoding direction a received echo signal is determined in many individual measurements.

[0012] Data acquired through oversampling is always discarded in image space to ensure that the desired data from the field of view is cleanly separated from unused data outside the field of view. This is not possible in k-space, as desired and unused image data overlap at all measurement points. Since a k-space acquired with a factor of 2 oversampling also requires twice the computing power, such data acquired through oversampling to avoid inflection but not used in image space is typically discarded very early on, before more computationally intensive steps of final image data reconstruction, such as those involved in parallel imaging or coil combinations, etc., are performed.

[0013] The gradient fields used for spatial coding during the recording of measurement data using magnetic resonance should ideally be generated in such a way that the most accurate linearity, i.e. a linear field behavior, is given, while the fundamental magnetic field should be as homogeneous as possible.

[0014] However, imperfections in the gradient fields can occur, particularly due to the inherent properties of the gradient coils within a gradient unit, leading to deviations from ideal linearity. Such deviations, and the associated non-linearities of the gradient fields, typically occur at the edges of a magnetic resonance (MRI) measurement volume. They can also be intentionally allowed during the development of the gradient coils, for example, to make the coils particularly thin, quiet, and / or cost-effective. These non-linearities can result in gradient field slopes that are steeper or shallower than would be expected under the assumption of linearity at a given location within the MRI measurement volume containing the gradient unit.However, when planning the acquisition of measurement data within a field of view, linear gradients are assumed. This results in a slightly different spatial area being encoded than the planned field of view.

[0015] Therefore, such non-linearities of a gradient unit can lead to distortions, e.g., expansions and / or compressions, in spatial encoding, which usually occur in outer regions of MR images reconstructed from such acquired measurement data, and which depend on the gradient unit used (especially the type of gradient coils) or its distortion characteristics and also on the measurement protocol used when acquiring the measurement data (especially the pulse sequence used).

[0016] Distortion correction (also known as distortion correction) eliminates non-linearities by means of image-based shifts of the corresponding pixels, using stored distortion information, particularly a distortion map. For example, it is known in the art to perform a calibration measurement on a phantom whose geometry is known, at least with respect to some marker points, in order to determine a distortion map with respect to the non-linearity of the gradient fields of the gradient unit for a specific magnetic resonance system. Such a distortion map can describe distortion coefficients that specify how each pixel must be shifted to correct the distortion caused by the non-linearity of the gradients.Such directory corrections are applicable to both two-dimensional (2D) MR image datasets and three-dimensional (3D) MR image datasets.

[0017] For all areas in a directory-corrected image where the distortion correction shifts pixels inwards, areas appear at the image's edge where no measurement data is present, as this data would have to originate from outside the measured image. This effect of seemingly cut-off edge areas in the image is called cropping.

[0018] The planning (location and position of the area of ​​interest of an object under investigation, also called field of view, FOV) of MRI measurements is usually carried out using directory-corrected planning images, also called MR (reference) images or localizers. Planning on directory-corrected MR images has many advantages, such as the fact that distortions caused by the non-linearities of the gradient unit are not visible in the directory-corrected MR image, so that distances, for example, can be determined without distortion. Furthermore, if non-directory-corrected MR images were used for planning, a new MR (reference) image would have to be acquired after a (frequently necessary) movement of the MRI table between planning and the measurement in order to show the distortions valid for the new position of the table.

[0019] The problem here is that, as described, depending on the direction of the pixel shift performed during the directory correction, the originally defined field of view, usually rectangular or cuboidal, can be reduced. This occurs because the measurement data is acquired during the scan using the non-linear gradient fields of the gradient unit, and therefore may cover a different area of ​​the object under investigation than would be expected based on the planning, especially on pre-corrected images. Thus, due to distortion in certain areas, particularly at the edges of the MRI images, no measurement data may be acquired during the planned scan. Consequently, these areas are not imaged, as the acquired measurement data contains no information for them.

[0020] This problem occurs particularly frequently in off-center measurements, i.e., recordings of measurement data outside the center of the homogeneity area of ​​the basic magnetic field of the magnetic resonance system used and thus in the edge areas of the homogeneity volume of the basic magnetic field, but also, for example, in more modern, more open and freely designed magnetic resonance systems, so that corpping effects occur.

[0021] Such cropping effects generally cannot be corrected even if oversampling has already been performed to counteract aliasing effects, because the oversampled data is discarded before image reconstruction due to the otherwise excessive computational effort, and therefore, as described, is missing in the affected peripheral areas. It is therefore common practice for the user to select a larger field of view than actually necessary when performing off-center measurements, e.g., at the edge of the bore of the MRI scanner, so that the areas potentially not visible due to cropping effects contain no or no relevant anatomy. However, sufficient user experience is required to select a field of view large enough, but not larger than necessary, to display all relevant information with sufficient resolution and within an acceptable measurement time.

[0022] The article by A. Stadler et al., “Artifacts in body MR imaging: their appearance and how to eliminate them”, Eur Rad. 17: pp. 1242-1255, 2007, describes various artifacts that occur in MR imaging and ways to eliminate them.

[0023] The invention is based on the objective of avoiding cropping effects and enabling the representation of all desired areas, especially in off-center measurements, so that high-quality MR images with few artifacts can be obtained with as little computational effort and as short a measurement time as possible.

[0024] The problem is solved by a method for improved acquisition of measurement data from an imaging area of ​​an object of investigation located in a measurement volume of a magnetic resonance system according to claim 1, a magnetic resonance system according to claim 13, a computer program according to claim 14, and an electronically readable data carrier according to claim 15.

[0025] An inventive method for improved acquisition of measurement data from an imaging area of ​​a test object located in a measurement volume of a magnetic resonance system comprises the following steps: - Loading non-linearity data describing a non-linearity of a gradient unit of the magnetic resonance system, - Displays, e.g. on a user interface of the magnetic resonance system, of at least one planning image of an object under investigation located in a measurement volume of the magnetic resonance system, - Selecting a desired field of view in which at least one planning image is displayed, - Loading a measurement protocol to be used for recording the measurement data, which includes gradients to be switched and RF pulses to be irradiated, which trigger echo signals that are received to record the measurement data, - Determine at least one loss factor based on the non-linearity data and on the basis of the desired field of view, wherein the at least one loss factor indicates an expected shortening of an image reconstructed on the basis of measurement data recorded with the loaded measurement protocol, - Determining filter parameters for a frequency filter to filter frequencies of echo signals received during the recording of measurement data with the measurement protocol, based on the loss factor and on the basis of the desired field of view, - Determining an oversampling factor based on certain filter parameters of the frequency filter, where the oversampling factor specifies an oversampling (according to Nyquist) to be applied when recording measurement data, - Acquiring oversampled measurement data by performing the loaded measurement protocol in the selected field of view using the filter parameters and the oversampling factor.

[0026] By selecting filter parameters according to the invention for a frequency filter to filter frequencies of echo signals received during the acquisition of measurement data with the measurement protocol, based on a loss factor determined for the magnetic resonance system used and on the basis of the desired field of view, a frequency filter optimized for the intended acquisition of measurement data is determined. An oversampling factor determined on the basis of such optimized filter parameters is itself also optimized for the intended acquisition of measurement data. By using such an optimized frequency filter and such an optimized oversampling factor, it can be ensured that a desired field of view is completely acquired during the acquisition of measurement data, despite non-linearities of a gradient unit of the magnetic resonance system used, thereby avoiding aliasing artifacts.The method thus enables the avoidance of cropping effects and aliasing artifacts without requiring any action from the user, e.g., to select an oversampling factor, whereby the required acquisition times for the measurement data and reconstruction times for image data from the acquired measurement data are not increased or are only slightly increased compared to previous methods.

[0027] Non-linearity data describing the non-linearity of a gradient unit of the magnetic resonance system can be loaded in the form of a distortion map.

[0028] The distortion map, at least with regard to the imperfections of the gradient fields, can be considered specific to magnetic resonance imaging (MRI) systems. This means that a distortion map, determined, for example, through calibration, is known and stored in a memory medium of the control unit. Thus, knowledge about the actual spatial displacements that occur is also known within the context of distortion correction. One of the underlying ideas of the invention is to utilize this knowledge of the distortion map prior to a magnetic resonance recording in order to determine optimized filter parameters for a frequency filter and an optimized oversampling factor, with which measurement data is acquired.

[0029] 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 a filter parameter determination unit.

[0030] 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.

[0031] 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 the computing unit of the computing system.

[0032] An electronically readable data carrier according to the invention comprises electronically readable control information stored thereon, 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 device of a magnetic resonance system, it carries out a method according to the invention.

[0033] 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.

[0034] 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 schematic simplified representation of a frequency filter, Fig. 2 a schematic flowchart of a method according to the invention, Fig. 3 a schematically illustrated magnetic resonance system according to the invention.

[0035] Fig. Figure 2 is a schematic flowchart of a method according to the invention for improved acquisition of measurement data MD from an imaging area of ​​a test object U located in a measurement volume of a magnetic resonance system 1.

[0036] Nonlinearity data VK describing the nonlinearity of a gradient unit 5 of the magnetic resonance system 1 is loaded (block 101). As mentioned above, the nonlinearity data VK can be loaded, for example, in the form of a known distortion map. Such distortion maps VK are usually determined at least during the installation of a magnetic resonance system and are stored, for example, in a memory within the system.

[0037] At least one planning image PB of an object U located in a measurement volume of magnetic resonance imaging (MRI) system 1 is loaded (Block 103). Such planning images PB are generally acquired before the start of MRI data acquisition to ensure that the measurement data is acquired in a desired area of ​​interest within the object under investigation. The display of at least one planning image PB can, for example, be performed at a user interface (I / O) of MRI system 1. The planning image PB can also be directory-corrected using the loaded non-linearity data (VK).

[0038] A desired field of view (FOV) is selected in at least one planning image (PB), e.g., at a user interface (I / O) of magnetic resonance imaging (MRI) system 1 (Block 105). This is a common procedure, usually performed by a user. The method proposed here is particularly recommended when a desired field of view (FOV) is not central but "off-center" in the planning image (PB), and thus an off-center measurement is to be performed, in which cropping effects would occur without the procedure described here.

[0039] A measurement protocol MP, used for acquiring the measurement data MD, is loaded (Block 107). This protocol includes switching gradients and RF pulses to be applied, which trigger echo signals that are to be received for acquiring the measurement data MD. In particular, the measurement protocol specifies a pulse sequence to be used for acquiring the measurement data and thus defines how the measurement data are to be acquired and what contrasts are to be achieved.

[0040] Based on the non-linearity data VK and on the basis of the desired field of view FOV, at least one loss factor VF is determined, where the at least one loss factor VF indicates an expected shortening of an image reconstructed on the basis of measurement data MD recorded with the loaded measurement protocol MP (Block 109).

[0041] A specific loss factor VF can, in particular, indicate the expected shortening of an image reconstructed from recorded measurement data MD in the readout direction. In the readout direction, as described above, a frequency filter and / or oversampling can be advantageously used to counteract artifacts.

[0042] The loss factor VF can be at least one: a "left loss factor," indicating expected foreshortening on the left side of the image, and / or a "right loss factor," indicating expected foreshortening on the right side of the image. In general, local loss factors VF can be determined from the non-linearity data VK according to the local non-linearities present in the non-linearity data VK. The peripheral regions of images acquired using MRI are usually most affected by gradient non-linearities, so loss factors can be determined for at least the left and / or right edge of the image. Depending on the field of view (FOV), only a left or right loss factor may be determined, for example, if foreshortening is expected only at the left or right edge of the image due to the prevailing gradient non-linearities.

[0043] Based on the at least one loss factor VF and on the basis of the desired field of view FOV, filter parameters (FP) for a frequency filter for filtering frequencies of echo signals received during the recording of measurement data MD with the measurement protocol MP are determined (Block 111).

[0044] The basis for the frequency filter can be a according to Fig. The trapezoidal bandpass filter described in section 1 is used, such that the filter parameters define, in particular, a stopband, a passband, and transition bands of the frequency filter. In the present method, the passband is defined as a field-of-view range (freq) that depends on the desired field of view (FOV). FOVand include at least one extension range d determined based on the loss factor VF. This extension range d depends on a respective loss factor VF, which is determined based on the non-linearity data VK of the magnetic resonance system 1 and the field of view (FOV). It is therefore specific to both the planned acquisition of measurement data MD and the magnetic resonance system 1 used, and is thus particularly effective against cropping effects. The transition regions are generally dependent on the filter hardware used and are selected to be as small as possible. The stopband is then defined as those frequencies that do not lie in a transition region or in the passband.

[0045] An extension range d can be determined, for example, such that the extension range d lies directly adjacent to the field of view area, i.e., that the frequencies of the extension range d are directly adjacent to the frequencies of the field of view area freq FOV connect, and a width of the extension area d corresponding to the loss factor VF, a multiple of the field of view area freq FOV This corresponds to the following: If, for example, the determination of the loss factor results in a reduction of 25%, i.e., that 25% of the image would be lost due to the non-linearities without further measures, then the width of the extension area d is set to 25% of the field of view area freq. FOV The passband of the frequency filter can then be set on both sides of the field of view. FOV to be widened by the width of the extension area d, so that the passage area to the left and right of the field of view area freq FOVThe crossover is widened by the width of an extension range d. This ensures that sufficient signal is available in the passband for subsequent distortion correction.

[0046] However, it is also conceivable that a separate extension area d on the left, based on a "left" loss factor VF, and a separate extension area d on the right, based on a "right" loss factor VF, each could be located next to the field of view area freq. FOV be determined so that the passage area then corresponds to the left extension area d, the field of view area freq FOV and includes the right extension area d. An extension area d associated with an edge of a field of view (FOV) at the edge of the bore of the magnetic resonance instrument will be larger than an extension area d associated with an edge of a field of view (FOV) closer to the center of the bore of the magnetic resonance instrument, the latter being negligibly small.

[0047] An oversampling factor OF, which specifies the oversampling to be applied when acquiring measurement data (according to Nyquist), is determined based on at least one specific filter parameter FP of the frequency filter (Block 111). The oversampling factor OF thus determines the oversampling to be applied. The oversampling, and therefore the oversampling factor, should be as large as necessary to cover, for example, the entire bandwidth of a frequency filter, i.e., its passband and any possible transition regions, but kept as small as possible to avoid unnecessary data acquisition. With modern analog-to-digital converters or other filtering devices in a magnetic resonance system 1, any value greater than two for oversampling factors OF can be set without problems to avoid aliasing artifacts.

[0048] From the optimized filter parameters of the frequency filter, the oversampling factor OF can also be determined, for example, by calculating the oversampling factor OF as the quotient of the sum of the widths of a passband of the frequency filter and the widths of the transition bands of the frequency filter as the dividend, and the width of a field-of-view area dependent on the desired field of view as the divisor. In this way, unlike previous rigid methods with fixed broadening ranges d, it is ensured that sufficient, but not excessive, measurement data is acquired through oversampling according to the oversampling factor OF.

[0049] Oversampled measurement data MD are acquired by performing the loaded measurement protocol MP in the selected field of view FOV using the filter parameters FP and the oversampling factor OV (Block 113).

[0050] A first part of the oversampled measurement data MD, determined based on the filter parameters FP of the frequency filter, for example, measurement data from frequency ranges of the transition regions b of the frequency filter, can be discarded (Block 115), leaving a reduced set of measurement data MDv1 (MDv1). In this way, measurement data that is not necessary for further processing steps and that still remains despite applying the frequency filter can be removed, thus simplifying and accelerating further processing without compromising the quality of the ultimately obtained image data.

[0051] From a reduced set of measurement data MDv1, initial image data BD1 can be reconstructed, in particular by a one-dimensional Fourier transform in the readout direction of already individual recorded k-space lines (Block 117). Such initial image data BD1 can be directory-corrected using the loaded non-linearity data VK, so that the initial image data BD1 are directory-corrected. Since the initial image data BD1 are already reconstructed from a reduced set of measurement data MDv1, their directory correction requires less effort. The directory correction allows distortions in the image data caused by the non-linearities of the gradients to be corrected.

[0052] In the possibly directory-corrected first image data BD1, only data recorded due to the oversampling factor OF used can be discarded, so that reduced first image data BD1' remain (Block 119).

[0053] Such a discarding of "excess" recorded data can be carried out as described above by identifying in the image space those data points that can be discarded because they were only recorded by oversampling in the extension regions d to avoid inflections. In this way, measurement data recorded in the extension regions d solely for artifact avoidance can be discarded in the image space, resulting in reduced initial image data BD1'.

[0054] The reduced (possibly directory-corrected) initial image data BD1' can be transformed back into the frequency domain of the measurement data, resulting in back-transformed reduced measurement data MD' (Block 121). Due to the distortions of measurement data performed in Blocks 115 and 119, the back-transformed reduced measurement data MD' no longer contains measurement data recorded solely to avoid negative effects such as cropping, involution, and aliasing artifacts, which would otherwise significantly slow down and / or degrade more complex and computationally intensive reconstructions.

[0055] From the back-transformed reduced measurement data MD', an image BD of the imaging area can be reconstructed that is free of cropping effects and aliasing artifacts.

[0056] Fig. Figure 3 schematically represents 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 for carrying out a method according to the invention.

[0057] In the Fig. Figure 3 shows only a rough schematic representation of these subunits of the magnetic resonance system 1. In particular, the high-frequency unit 7 can consist of several subunits, for example, several coils such as the schematically shown coils 7.1 and 7.2, or more coils, which can be designed either only for transmitting high-frequency signals, only for receiving the triggered high-frequency signals, or for both.

[0058] 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 (MRI) system 1 within its measurement volume. The layers or slabs S1 and S2 represent exemplary (partial) target volumes of the test object from which echo signals are to be recorded and acquired as measurement data.

[0059] 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.

[0060] 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'.

[0061] A pulse sequence unit 13, comprised of the control unit 9, is designed to perform all the necessary calculations for the required measurements and determinations, including the calculation of RF pulses to be emitted and gradients to be switched. Intermediate results and final results required for this purpose, or determined in the process, can be stored in a memory 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 implemented, for example, in fewer or even just a single physical unit.

[0062] Furthermore, the control unit 9 comprises a filter parameter determination unit 15, with which, according to the invention, filter parameters for an optimized frequency filter for recording measurement data can be determined, based on which, e.g., by means of the pulse sequence unit 13, an optimized oversampling factor for recording measurement data can be determined. The control unit 9 is configured overall to carry out a method according to the invention.

[0063] Via a user interface I / O of the magnetic resonance system 1 for input and output of data and information, control commands can be sent to the magnetic resonance system by a user, for example, and / or results of the control unit 9, such as image data, can be displayed.

[0064] A method described herein may also be in the form of a computer program product comprising a program that implements the described method on a control unit 9 when executed on the control unit 9. Likewise, an electronically readable data carrier 26 containing electronically readable control information may be present, comprising at least one such computer program product described above and designed such that, when the data carrier 26 is used in a control unit 9 of a magnetic resonance system 1, it performs the described method.

[0065] Regardless of the grammatical gender of a particular term, persons with male, female or other gender identities are included.

Claims

[1] Method for improved acquisition of measurement data (MD) from an imaging area of ​​a test object located in a measurement volume of a magnetic resonance system (1), comprising the steps: - Loading non-linearity data (VK) describing a non-linearity of a gradient unit (5) of the magnetic resonance instrument (1), - Displays, e.g. at a user interface (I / O) of the magnetic resonance system (1), of at least one planning image (PB) of an object under investigation (U) located in a measurement volume of the magnetic resonance system (1), - Selecting a desired field of view (FOV) in which at least one planning image (PB) is displayed, - Loading a measurement protocol (MP) to be used for recording the measurement data (MD), which includes gradients to be switched and RF pulses to be irradiated, which trigger echo signals that are received for recording the measurement data (MD), - Determine at least one loss factor (VF) based on the non-linearity data (VK) and on the basis of the desired field of view (FOV), wherein the at least one loss factor (VF) indicates an expected shortening of an image (BD) reconstructed on the basis of measurement data (MD) acquired with the loaded measurement protocol (MP), - Determining filter parameters (FP) for a frequency filter to filter frequencies of echo signals received during the recording of measurement data (MD) with the measurement protocol (MP) based on the loss factor (VF) and on the basis of the desired field of view (FOV), - Determining an oversampling factor (OF) based on certain filter parameters (FP) of the frequency filter, wherein the oversampling factor (OF) specifies an oversampling (according to Nyquist) to be applied when recording measurement data (MD), - Acquiring oversampled measurement data (MD) by performing the loaded measurement protocol (MP) in the selected field of view (FOV) using the filter parameters (FP) and the oversampling factor (OF). [2] Method according to claim 1, wherein the non-linearity data (VK) describing a non-linearity of a gradient unit (5) of the magnetic resonance system (1) are loaded in the form of a distortion map. [3] Method according to one of the preceding claims, wherein a certain loss factor (VF) indicates an expected shortening of an image reconstructed on the basis of recorded measurement data in the readout direction. [4] A method according to any of the preceding claims, wherein the filter parameters (FP) define a stopband, a passband and transition bands of the frequency filter, wherein the passband is a field of view range (freq) dependent on the desired field of view (FOV). FOV) and includes at least one extension range (d) determined on the basis of the loss factor (VF). [5] Method according to claim 4, wherein an extension area (d) is determined such that the extension area (d) is directly adjacent to the field of view area (freq FOV ) lies and a width of the extension area (d) is a multiple of the field of view area (freq) corresponding to the loss factor (VF). FOV ) corresponds. [6] Method according to one of claims 4 or 5, wherein an extension area (d) is determined to the left and right of the field of vision area. [7] Method according to any of the preceding claims, wherein the oversampling factor (OF) is defined as the quotient of the sum of the widths of a passband (c) of the frequency filter and the widths of the transition bands (b) of the frequency filter as dividend and the width of a field-of-view area (freq) dependent on the desired field of view. FOV) is determined as the divisor. [8] Method according to one of the preceding claims, wherein a first part of the oversampled measurement data (MD) determined on the basis of the filter parameters (FP) is discarded, leaving a reduced set of measurement data (MDv1), and wherein first image data (BD1) are reconstructed from the reduced set of measurement data (MDv1), in particular by a one-dimensional Fourier transform in the readout direction. [9] Method according to claim 8, wherein the first image data (BD1) are directory-corrected using the loaded non-linearity data (VK) such that the first image data (BD1) are directory-corrected. [10] Method according to one of claims 8 or 9, wherein in the possibly directory-corrected first image data (BD1) only data recorded because of the oversampling factor (OF) used are discarded, so that reduced first image data (BD1') remain, which are converted back into the frequency domain of the measurement data, so that one obtains back-transformed reduced measurement data (MD'), on the basis of which an image (BD) of the imaging area is reconstructed. [11] Method according to one of the preceding claims, wherein a desired field of view (FOV) is not centrally located in the planning image (PB). [12] Method according to any of the preceding claims, wherein the at least one loss factor (VF) is a left loss factor indicating an expected foreshortening on the left side of the image, and / or a right loss factor indicating an expected foreshortening on the right side of the image. [13] 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 filter parameter determination unit (15), wherein the control unit (9) is configured to perform a method according to any one of claims 1 to 12 on the magnetic resonance system (1). [14] Computer program which can be directly loaded into a memory of a control unit (9) of a magnetic resonance system (1), comprising program means to execute the steps of a method according to any one of claims 1 to 12 when the program is executed in the control unit (9) of the magnetic resonance system (1). [15] Electronically readable data carrier with electronically readable control information stored thereon, which includes at least one computer program according to claim 14 and is designed such that, when the data carrier is used in a control unit (9) of a magnetic resonance system (1), it performs a method according to one of claims 1 to 12.

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