Magnetic resonance imaging techniques

Abstract

A method for magnetic resonance imaging of a subject using a magnetic resonance device, comprising the following steps: - Defining an examination volume for magnetic resonance imaging in a body of the subject, - Determining a local shim volume comprising a sub-region of the examination volume, - Calculating shim settings, wherein tissue of the subject present in the local shim volume is included in the calculation of the shim settings, and - Acquiring magnetic resonance image data of the examination volume using the magnetic resonance device, wherein the acquisition of the magnetic resonance image data is performed using the calculated shim settings, and wherein a local shim coil is controlled according to the shim settings during the acquisition of the magnetic resonance image data.wherein the local shim volume is determined by positioning the local shim coil in relation to the body of the object under investigation or by limiting the current of the local shim coil.

Description

The invention relates to a method for magnetic resonance imaging, a magnetic resonance device and a computer program product. In a magnetic resonance imaging (MRI) scanner, the body of the subject, especially a patient, is typically exposed to a relatively strong main magnetic field, for example, 1.5, 3, or 7 Tesla, using a main magnet. Additionally, gradient pulses are generated using a gradient coil unit. High-frequency pulses, such as excitation pulses, are then emitted via a radio frequency antenna unit using suitable antenna elements. This causes the nuclear spins of certain atoms, resonantly excited by these radio frequency pulses, to be tilted by a defined angle relative to the magnetic field lines of the main magnetic field. During the relaxation of the nuclear spins, radio frequency signals, known as magnetic resonance signals, are emitted. These signals are received by suitable radio frequency antennas and then further processed.The desired image data can then be reconstructed from the raw data acquired in this way. For a specific measurement, a particular magnetic resonance sequence, also called a pulse sequence, must be transmitted. This sequence consists of a series of high-frequency pulses, such as excitation pulses and refocusing pulses, as well as coordinated gradient pulses transmitted along different gradient axes in various spatial directions. Readout windows are set accordingly, defining the time periods in which the induced magnetic resonance signals are acquired. In magnetic resonance imaging (MRI) using a magnetic resonance scanner, the homogeneity of the main magnetic field within the examination volume is of great importance. Even small deviations in homogeneity can lead to large deviations in the frequency distribution of the nuclear spins, resulting in the acquisition of low-quality MRI image data. Shim units are used to improve homogeneity within the volume under investigation. When a magnetic resonance imaging (MRI) scanner is installed at its designated location, ambient fields can restrict the homogeneity of the main magnetic field, particularly around an isocenter of the MRI scanner. Therefore, during the installation and commissioning of an MRI scanner, often in conjunction with measurements, the shim unit is adjusted to achieve optimal homogeneity. Thus, baseline shim settings are calculated during the installation and commissioning of the MRI scanner. However, another source of inhomogeneity is the object being examined itself. For example, if a person is placed in the magnetic resonance imaging (MRI) scanner, the body's matter disrupts the homogeneity once again. To address this problem, it is known to use an adjustable shim unit. In particular, shim coils are known for this purpose, which, driven by different shim currents, generate different compensating magnetic fields to improve homogeneity. To shim these disturbances of the object under investigation, it is common practice to first measure the field distribution using the MRI scanner itself, after the person being examined has been placed in a patient acquisition area of ​​the MRI scanner. This measurement is taken using the baseline shim settings obtained during the installation and commissioning of the MRI scanner. Subsequently, based on the baseline shim settings, shim settings are determined by a control unit, taking into account the measured field distribution. The shim unit is then controlled using these settings to achieve optimal homogeneity. The following are examples of prior art: DE 10 2014 204 451 A1, DE 10 2007 009 203 A1, DE 10 2008 044 844 A1 and DE 11 2013 003 860 T5. The invention is based on the objective of enabling an improved calculation of shim settings for magnetic resonance imaging. This objective is achieved by the features of the independent claims. Advantageous embodiments are described in the dependent claims. The inventive method for magnetic resonance imaging of an examination object using a magnetic resonance device comprises the following process steps: - Defining an examination volume for magnetic resonance imaging in a body of the examination object, - Determining a local shim volume which comprises a sub-region of the examination volume, - Calculating shim settings, wherein tissue of the body of the examination object present in the local shim volume is included in the calculation of the shim settings, and - Acquiring magnetic resonance image data of the examination volume using the magnetic resonance device, wherein the acquisition of the magnetic resonance image data is carried out using the calculated shim settings. The subject of the examination can be a patient, a healthy volunteer, an animal, or a phantom. The acquired magnetic resonance imaging (MRI) data is made available, meaning it is displayed to a user on a display unit and / or stored in a database. The examination volume, also called the acquisition volume (field of view, FOV), represents the volume that is captured in the acquired MRI data. The examination volume is typically defined by a user, for example, on an overview image (localizer). Alternatively, or additionally, the examination volume can also be defined automatically, for example, based on a selected protocol. The local shim volume represents, in particular, the volume to be shimmed, for which the shim settings are to be calculated. The local shim volume can represent that sub-region of the examination volume which, with respect to inhomogeneities in the main magnetic field, significantly influences the magnetic resonance imaging or the image quality of the acquired magnetic resonance image data. The fact that the local shim volume comprises a sub-region of the examination volume means, in particular, that the local shim volume does not completely cover the examination volume. The local shim volume can be entirely contained within the examination volume, in which case the local shim volume is smaller than the examination volume. In certain cases, parts of the local shim volume may also lie outside the examination volume. In addition to the local shim volume, a global shim volume, which covers the entire examination area, may also exist.Separate global shim settings can then be calculated for the global shim volume, which are set separately for the global shim volume during the acquisition of the magnetic resonance image data. The calculation of shim settings based on the tissue of the subject's body present within the local shim volume can involve measuring magnetic properties, such as the field strength of a primary magnetic field (B0 field strength), of the tissue within the local shim volume. A B0 field map, encompassing or limited to the local shim volume, can be measured. The shim settings can then be calculated based on the field distribution stored in the B0 field map. Of course, other methods for calculating the shim settings based on the tissue present in the local shim volume that would be considered appropriate by a person skilled in the art are also conceivable. The shim settings can include settings for controlling the electrical shim coils of the magnetic resonance imaging (MRI) system. The fact that the MRI data is acquired using the calculated shim settings can, in particular, mean that the shim coils are controlled according to these settings during the MRI data acquisition process. Different shim coils can be controlled based on the calculated shim settings than those described in the previous section. In a preferred application, the shim settings, which are calculated based on the tissue present in the local shim volume, are configured to control local shim coils, which are positioned particularly close to the subject of the examination. The shim settings can, in particular, define a current distribution in the shim coils, possibly with a time-dependent characteristic.A shim control unit can then apply currents to the shim coils during the acquisition of magnetic resonance imaging (MRI) data, using the currents defined by the shim settings. Frequency adjustment can also be performed prior to MRI acquisition based on the shim settings. The calculated shim settings can be particularly well-suited to the local shim volume. In this way, using the shim settings during the acquisition of magnetic resonance imaging data can lead to a particularly high homogeneity of the main magnetic field within the local shim volume, or to a particularly advantageous compensation of inhomogeneities in the main magnetic field within the local shim volume. One embodiment provides that a local shim coil is controlled based on the shim settings during the acquisition of the magnetic resonance image data. A local shim coil is arranged in the immediate vicinity of a surface, particularly a body surface, of the object under investigation. For this purpose, the local shim coil is advantageously integrated into a local radio frequency coil, which is positioned on the area of ​​the object under investigation for acquiring the magnetic resonance image data. The local shim coil typically comprises one or more conductor loops, which are subjected to a direct current, determined by the shim settings, during the acquisition of the magnetic resonance image data. In this way, the local shim coils can contribute a defined B0 field to homogenize the main magnetic field. Advantageously, the geometry of the conductor loops and the current flowing through them are selected such that the magnetic field disturbances generated by the object under investigation can be compensated as effectively as possible on a local basis. The local shim coil can thus be designed to compensate for local inhomogeneities in the main magnetic field, particularly within a defined area of ​​the object. Since the local shim coil is positioned close to the location of the magnetic field disturbances, it can compensate for magnetic field disturbances that are typically not compensable with conventional global shim coils integrated into the magnetic resonance imaging (MRI) unit. However, the use of local shim coils can lead to undesired magnetic field gradients, particularly in a direction perpendicular to the local shim coil, in addition to the desired field distribution. These undesired magnetic field gradients can necessitate different shim currents or shim settings for compensating the main magnetic field, depending on the distance of the volume being shimmed from the local shim coil. Consequently, the local shim coil typically cannot achieve a homogenization of the magnetic field across the entire body of the object under investigation. The local shim volume can be positioned in close proximity to the local shim coil and / or the local high-frequency coil into which the local shim coil is integrated. In this way, the local shim volume can be advantageously matched to the local shim coil, for example, to a specific position of the local shim coil and / or a current limit of the local shim coil. The local shim volume can, for instance, encompass the defined area of ​​the subject in which the main magnetic field is to be homogenized by means of the local shim coil. Due to the advantageous positioning of the local shim volume, the shim settings can then also be configured to be particularly beneficial for controlling the local shim coil during the acquisition of magnetic resonance imaging data. One embodiment provides that only the tissue of the body of the subject present in the local shim volume is included in the calculation of the shim settings, according to which the local shim coil is controlled. In addition to the local shim settings, global shim settings can also be calculated for controlling global shim coils integrated into a magnet unit of the MRI scanner. Tissue from the patient's body within a global shim volume larger than the local shim volume can be included in the calculation of the global shim settings. However, for calculating the shim settings to control the local shim coil, it is advantageous to consider only the tissue from the local shim volume, as the local shim volume can be particularly well-suited to the local shim coil. The local shim volume can be determined separately from the global shim volume, making it especially suitable for determining the shim settings. One embodiment provides that the local shim volume is determined by positioning the local shim coil in relation to the body of the object under investigation. In this way, the position, orientation, and / or size of the local shim volume can be determined based on the positioning of the local shim coil relative to the body of the object under investigation. This determination is advantageously performed semi-automatically or automatically. In particular, the distance of the local shim volume from the local shim coil, for example, the distance of an edge and / or center of the shim volume from the positioning of the local shim coil, can be suitably selected. The determination of the local shim volume based on the positioning of the local shim coil can incorporate information such as the rate attenuation of the shim field with increasing distance from the local shim coil. Furthermore, a current limiting effect of the local shim coil, described in more detail in the following section, can be considered when determining the distance of the local shim volume from the local shim coil. If the local shim volume is positioned too far from the local shim coil, the B0 field within the local shim volume may no longer be adequately homogenized by the local shim coil. Such positioning of the local shim volume can, in fact, lead to an undesirable, excessive influence of the local shim field in the immediate vicinity of the local shim coil, particularly outside the local shim volume. To align the local shim volume with the positioning of the local shim coil, positional information characterizing the local shim coil's position relative to the body of the object under investigation can first be acquired. This positional information can be stored in a database or coil file based on prior knowledge, particularly in the case of a fixed local shim coil position. For variable local shim coil positioning, the positional information can be acquired using methods familiar to those skilled in the art, for example, based on preliminary measurement data obtained using a magnetic resonance imaging (MRI) device. One embodiment provides that the local shim volume is determined by means of a current limiting of the local shim coil. The current limit of the local shim coil is determined in particular by the design of the conductors of the local shim coil and / or by the boundaries of a power supply to which the local shim coil is connected. The position, orientation, and / or size of the local shim volume can be selected such that the current limit of the local shim coil is maintained during homogenization of the B0 field within the local shim volume. In particular, the distance of the local shim volume from the local shim coil can be chosen such that a current applied by the power supply of the local shim coil for shimming within the local shim volume remains within the boundaries of the power supply. In this way, the local shim volume can be particularly advantageously matched to the parameters of the local shim coil. One embodiment provides that an anatomical structure of the body of the object under investigation is present in the examination volume, wherein a pre-measurement is carried out using the magnetic resonance device before the magnetic resonance measurement data is acquired, wherein a position of the anatomical structure is identified based on the magnetic resonance pre-measurement data acquired in the pre-measurement and the determination of the local shim volume is carried out using the identified position of the anatomical structure. As described in the preceding sections, the local shim volume can serve as the basis for calculating the shim settings for controlling a local shim coil. Matching the shim settings to the anatomical structure of the patient is particularly useful when using a local shim coil, especially when the coil is positioned close to the anatomical structure. Of course, it is also conceivable that the local shim volume determined in this way could serve as the basis for calculating shim settings for controlling global shim coils. The anatomical structure is, in particular, a structure within the body of the subject that is of special interest in the magnetic resonance imaging data. Simultaneously, the primary magnetic field in the region of the anatomical structure should be homogenized by means of shim settings. This can be achieved particularly advantageously by adjusting the local shim volume to the anatomical structure. The pre-measurement for acquiring the magnetic resonance (MRI) pre-measurement data can be completed, in particular, before the start of acquiring the MRI image data. The MRI pre-measurement data can be acquired specifically for determining the local shim volume. Alternatively or additionally, medical MRI pre-measurement data intended for user review can also be acquired. Alternatively or additionally, the measurement data acquired during an auto-align measurement, a pre-scan normalize measurement, or a coil sensitivity measurement can be used as MRI pre-measurement data. The MRI pre-measurement data acquired during an auto-align measurement can be particularly advantageous for determining the location of the anatomical structure, since the coordinates of the anatomical structure, such as the spine, can be automatically acquired during the auto-align measurement.As described in one of the following sections, the magnetic resonance pre-measurement data acquired in the pre-measurement are particularly preferably used directly to determine the shim settings. Identifying the location of the anatomical structure may involve identifying its position, orientation, and / or size within the pre-measurement magnetic resonance data. The local shim volume, specifically its position, orientation, and / or size, can then be determined based on this identified position, orientation, and / or size. Advantageously, the local shim volume can be determined to encompass the entire anatomical structure and / or a large portion thereof. Alternatively, the local shim volume can be determined to be limited to the size of the anatomical structure. One embodiment provides that the shim settings are calculated using the magnetic resonance pre-measurement data. For this purpose, the magnetic resonance (MRI) pre-measurement data can be structured as a three-dimensional measurement dataset. Based on this MRI pre-measurement data, a B0 field map is then determined, particularly using a method known to those skilled in the art. This map characterizes the distribution of the main magnetic field within a measurement area of ​​the pre-measurement. The B0 field map can then serve as the basis for calculating the shim settings, especially in the area of ​​the local shim volume. A double-echo steady state (DESS) magnetic resonance sequence can be particularly advantageously used to acquire the MRI pre-measurement data. The described procedure allows the magnetic resonance (MRI) pre-measurement data to fulfill a particularly advantageous dual function. The MRI pre-measurement data can be used to determine the location of the anatomical structure, which serves as the basis for determining the local shim volume. Simultaneously, the MRI pre-measurement data can be directly incorporated into the calculation of the shim settings, for example, by generating a B0 field map from the MRI pre-measurement data. This advantageously eliminates the need for an additional measurement to determine the location of the anatomical structure, thus saving measurement time. One embodiment provides that the anatomical structure is a spine of the subject under investigation, wherein projection data are created from the magnetic resonance pre-measurement data and the position of the spine is identified based on the projection data. The projection data can be generated from the magnetic resonance (MRI) pre-measurement data in such a way that the projection data have a smaller dimension than the MRI pre-measurement data. Specifically, the projection data are two-dimensional, while the MRI pre-measurement data are three-dimensional. Generating the projection data from the MRI pre-measurement data can be done using a method known to those skilled in the art. Particularly advantageously, as will be described in more detail in one of the following sections, a radon transformation can be employed. Using the projection data, the position of the spine in the pre-measurement magnetic resonance imaging data can be determined particularly easily or automatically. An automated process for determining a suitable local shim volume tailored to the spine is thus conceivable. One embodiment provides that the creation of the projection data from the magnetic resonance pre-measurement data includes a radon transformation of the magnetic resonance pre-measurement data, by means of which the magnetic resonance pre-measurement data are converted into a sinogram, whereby the position of the spine in the sinogram is identified. This approach is based on the consideration that the position of the spine can be identified particularly easily in the sinogram. Local intensity maxima in the sinogram correspond, in particular, to elongated straight line segments. These elongated straight line segments typically have their maximum length in the region of the spine. Thus, by determining the local maxima in the sinogram, the position of the spine can be identified particularly easily. Before converting the magnetic resonance (MRI) pre-measurement data into a sinogram, it can be advantageous to reformat the measurement layers of the MRI pre-measurement data into sagittal or coronal oriented images, which can then be radon-transformed. Furthermore, it can be beneficial to filter the MRI pre-measurement data before radon transformation. The use of a Canny filter (Canny edge detector), known to those skilled in the art, has proven particularly advantageous, as it can highlight or monochromatize specific image areas in the MRI pre-measurement data where the brightness difference between adjacent pixels is particularly high.Filtering the magnetic resonance pre-measurement data can enable a particularly simple determination of the spine in the sinogram, which is generated from the filtered magnetic resonance pre-measurement data using radon transformation. The described procedure enables a particularly advantageous automatic determination of the spinal column's position in the pre-measurement magnetic resonance imaging (MRI) data. This allows the local shim volume to be determined with particular benefit based on the spinal column's position, and the shim settings can be especially suitable for homogenizing the main magnetic field in the spinal region. The magnetic resonance device according to the invention comprises an image data acquisition unit and a computing unit, which includes a setting unit, a determination unit and a calculation unit, wherein the magnetic resonance device is configured to carry out a method according to the invention. The magnetic resonance imaging (MRI) device according to the invention is configured to perform an inventive method for magnetic resonance imaging of a subject. The definition unit is configured to define an examination volume for MRI imaging within a body of the subject. The determination unit is configured to determine a local shim volume, which represents a sub-region of the examination volume. The calculation unit is configured to calculate shim settings, taking into account tissue of the subject's body present within the local shim volume. The image acquisition unit is configured to acquire MRI image data of the examination volume using the MRI device, acquiring the MRI image data using the calculated shim settings. The components of the computing unit of the magnetic resonance device according to the invention, namely the control unit, the determination unit, and the computation unit, can be predominantly implemented as software components. However, these components can also be partially implemented as software-supported hardware components, such as FPGAs or the like, particularly when very fast calculations are required. Likewise, the necessary interfaces, for example, when it is only a matter of transferring data from other software components, can be implemented as software interfaces. Alternatively, they can be implemented as hardware interfaces controlled by suitable software. Of course, it is also conceivable that several of the aforementioned components are combined into a single software component or a software-supported hardware component. The computer program product according to the invention can be directly loaded into the memory of a programmable processing unit of a magnetic resonance imaging (MRI) device and includes program code means for executing a method according to the invention when the computer program product is executed in the processing unit of the MRI device. The computer program product particularly comprises a computer program. This allows the method according to the invention to be executed quickly, identically, and robustly. The computer program product is configured so that it can execute the method steps according to the invention by means of the processing unit. The processing unit must have the necessary prerequisites, such as sufficient main memory, a suitable graphics card, or a suitable logic unit, so that the respective method steps can be executed efficiently.The computer program product is, for example, stored on a computer-readable medium or on a network or server, from where it can be loaded into the processor of a local computing unit, which may be directly connected to the magnetic resonance device or be designed as part of the magnetic resonance device. Furthermore, control information of the computer program product can be stored on an electronically readable data carrier. The control information of the electronically readable data carrier can be designed such that, when the data carrier is used in a computing unit of the magnetic resonance device, it executes a method according to the invention. Examples of electronically readable data carriers are a DVD, a magnetic tape, or a USB flash drive on which electronically readable control information, in particular software (see above), is stored.If this control information (software) is read from the data carrier and stored in a control and / or processing unit of the magnetic resonance device, all embodiments of the methods described above can be carried out according to the invention. Thus, the invention can also be based on the aforementioned computer-readable medium and / or the aforementioned electronically readable data carrier. The advantages of the magnetic resonance device and the computer program product according to the invention essentially correspond to the advantages of the method according to the invention, which have been described in detail above. Features, advantages, or alternative embodiments mentioned herein can also be applied to the other claimed items and vice versa. In other words, the claims can also be further developed with the features described or claimed in connection with a method. The corresponding functional features of the method are thereby implemented by corresponding physical modules, in particular by hardware modules. The invention will now be described and explained in more detail with reference to the exemplary embodiments shown in the figures. Figure 1 shows a schematic representation of a magnetic resonance device according to the invention, Figure 2 shows a flowchart of a first embodiment of a method according to the invention, Figure 3 shows a flowchart of a second embodiment of a method according to the invention, and Figure 4 shows a possibility for identifying the position of the spine in the magnetic resonance pre-measurement data. Fig. 1 schematically depicts a magnetic resonance imaging (MRI) device 11 according to the invention. The MRI device 11 comprises a detector unit formed by a magnetic unit 13 with a main magnet 17 for generating a strong and, in particular, constant main magnetic field 18. The MRI device 11 also has a cylindrical patient reception area 14 for receiving a test subject 15, in this case a patient, wherein the patient reception area 14 is cylindrically enclosed in a circumferential direction by the magnetic unit 13. The patient 15 can be moved into the patient reception area 14 by means of a patient positioning device 16 of the MRI device 11. For this purpose, the patient positioning device 16 has a table that is movably arranged within the MRI device 11. The magnetic unit 13 is shielded externally by means of a housing 31 of the MRI device. The magnet unit 13 further comprises a gradient coil unit 19 for generating magnetic field gradients, which are used for spatial encoding during imaging. The gradient coil unit 19 is controlled by a gradient control unit 28. The magnet unit 13 also includes a high-frequency antenna unit 20, which in the illustrated case is configured as a body coil permanently integrated into the magnetic resonance device 10, and a high-frequency antenna control unit 29 for exciting a polarization that arises in the main magnetic field 18 generated by the main magnet 17. The high-frequency antenna unit 20 is controlled by the high-frequency antenna control unit 29 and transmits high-frequency magnetic resonance sequences into an examination space, which is essentially formed by the patient acquisition area 14.The high-frequency antenna unit 20 is further equipped to receive magnetic resonance signals, especially from patient 15. The magnetic resonance imaging (MRI) device 11 includes a processing unit 24 for controlling the main magnet 17, the gradient control unit 28, and the high-frequency antenna control unit 29. The processing unit 24 centrally controls the MRI device 11, for example, by performing a predetermined imaging gradient echo sequence. Control information, such as imaging parameters, as well as reconstructed MRI images, can be made available to a user on a display unit 25 of the MRI device 11. The MRI device 11 also includes an input unit 26, which allows a user to enter information and / or parameters during a measurement procedure. The processing unit 24 can comprise the gradient control unit 28, the high-frequency antenna control unit 29, the display unit 25, and / or the input unit 26. In the case shown, the calculation unit 24 comprises a definition unit 33, a determination unit 34 and a calculation unit 35. The magnetic resonance imaging (MRI) device 11 further comprises an image data acquisition unit 32. In this case, the image data acquisition unit 32 is formed by the magnet unit 13 together with the high-frequency antenna control unit 29 and the gradient control unit 28. The MRI device 11, together with the image data acquisition unit 32 and the processing unit 24, is thus designed to carry out a magnetic resonance imaging method according to the invention. In the case shown in Fig. 1, the magnetic resonance imaging (MRI) device 11 has a local high-frequency coil 30 designed to receive magnetic resonance signals. The local high-frequency coil 30 is also encompassed by the image data acquisition unit 32. For a magnetic resonance examination, the local high-frequency coil 30 is applied by medical personnel to a body region of the subject 15 to be examined. In the present embodiment, the local high-frequency coil 30 is formed by a body antenna unit. In principle, it is also conceivable to configure the local high-frequency coil 30 as a knee antenna unit, back antenna unit, head antenna unit, etc. It is also conceivable that more than one local high-frequency coil 30 is positioned on the subject 15. The local high-frequency coil 30 has a local shim coil 36, which in the example shown is integrated into a housing of the local high-frequency coil 30. Of course, other positioning of the local shim coil 36 is also conceivable. The local shim coil 36 can be controlled during the acquisition of the magnetic resonance image data by the image data acquisition unit 32 based on shim settings calculated by the processing unit 35. The depicted magnetic resonance device 11 can, of course, include other components that magnetic resonance devices 11 typically possess. Furthermore, the general operating principle of a magnetic resonance device 11 is known to those skilled in the art, so a detailed description of the other components is omitted. Fig. 2 shows a flowchart of a first embodiment of a method according to the invention for magnetic resonance imaging of the object under investigation 15 using the magnetic resonance device 11. In a first process step 40, an examination volume for magnetic resonance imaging is defined in a body of the object under investigation using the definition unit 33 of the computing unit 24. In a further process step 41, a local shim volume is determined using the determination unit 34 of the computation unit 24, wherein the local shim volume comprises a sub-area of ​​the investigation volume. In a further process step 42, shim settings are calculated using the calculation unit 35 of the computation unit 24, whereby tissue of the body of the object under investigation present in the local shim volume is included in the calculation of the shim settings. In a further process step 43, magnetic resonance image data of the examination volume is acquired using the image data acquisition unit 32 of the magnetic resonance device 11, whereby the acquisition of the magnetic resonance image data is carried out using the calculated shim settings. Fig. 3 shows a flowchart of a second embodiment of a method according to the invention for magnetic resonance imaging of the object under investigation 15 using the magnetic resonance device 11. The following description is essentially limited to the differences from the embodiment shown in Fig. 2, whereby reference is made to the description of the embodiment shown in Fig. 2 with regard to process steps that remain essentially the same. Process steps that remain essentially the same are generally numbered with the same reference numerals. The embodiment of the method according to the invention shown in Fig. 3 essentially comprises the process steps 40, 41, 42, and 43 of the first embodiment of the method according to Fig. 2. In addition, the embodiment of the method according to the invention shown in Fig. 3 includes further process steps and sub-steps. An alternative process sequence to Fig. 3 is also conceivable, which includes only some of the additional process steps and / or sub-steps shown in Fig. 3. Naturally, an alternative process sequence to Fig. 3 can also include additional process steps and / or sub-steps. Figure 3 illustrates two possible procedures for determining the local shim volume, referred to below as a first and a second procedure, which can be used separately or in combination. The first procedure includes the additional steps 44 and 45 introduced in Figure 3 compared to Figure 2. The second procedure includes the additional steps 46 and 47 introduced in Figure 3 compared to Figure 2. The first method for determining the local shim volume involves controlling a local shim coil 36 based on the shim settings during the acquisition of the magnetic resonance image data in a further procedure step 43-1. This is also possible in the second method described in one of the following sections, but is not mandatory. It may be advantageous, however, to consider only the tissue of the body of the subject 15 present in the local shim volume when calculating the shim settings used to control the local shim coil. To calculate the shim settings for the local shim coil 36, the position of the local shim coil 36 relative to the body of the test object 15 can be determined in the subsequent process step 44, and then the local shim volume can be determined based on the position of the local shim coil 36 in the subsequent process step 41. Alternatively or additionally, it is also conceivable that the current limit of the local shim coil is determined in the subsequent process step 45, based on which the local shim volume can then be determined in the subsequent process step. The second method for determining the local shim volume involves performing a preliminary measurement using the magnetic resonance device 11 in the subsequent process step 46, prior to acquiring the magnetic resonance measurement data. In the subsequent process step 47, the location of an anatomical structure within the body of the subject 15 can then be identified using this preliminary magnetic resonance measurement data. The local shim volume can then be determined in the subsequent process step 41 using the identified location of the anatomical structure. Particularly advantageously, the shim settings can also be calculated in the subsequent process step 42 using the preliminary magnetic resonance measurement data. If the anatomical structure is the spine of the subject 15, a particularly advantageous method for identifying the position of the spine in the subsequent procedure step 47 involves creating projection data from the pre-measurement magnetic resonance data and identifying the position of the spine based on this projection data. For this purpose, in a first sub-step 47-1 of the subsequent procedure step 47, a radon transformation of the pre-measurement magnetic resonance data can be performed, by means of which the pre-measurement magnetic resonance data are converted into a sinogram. In a second sub-step 47-2 of the subsequent procedure step 47, the position of the spine can then be identified in the sinogram. Figure 4 illustrates this particularly advantageous method for identifying the position of the spine in the magnetic resonance pre-measurement data (see step 47 of Figure 3) step by step. Of course, Figure 4 only illustrates one example of how the position of the spine can be determined automatically. As a starting point, step 51 provides magnetic resonance pre-measurement data in a sagittal orientation, which in the case shown depicts the head and part of the spine of the subject 15. In step 52, a Canny filter (Canny edge detector) is then applied to the magnetic resonance pre-measurement data to highlight or monochromatize certain image areas in the magnetic resonance pre-measurement data where the brightness difference between neighboring pixels is particularly high. In step 53, projection data is created from the pre-measurement magnetic resonance image using a Radon transformation. As shown in step 53, the projection data is now available as a sinogram. Using this sinogram, the position of the spine can then be determined particularly easily in the following steps. In steps 54 and 55, local intensity maxima are identified in the sinogram. In the case shown in Fig. 4, the identification of the local intensity maxima takes place in two calculation steps; of course, a different number of calculation steps is also conceivable. The local intensity maxima in the sinogram correspond in particular to elongated straight line segments, which typically have their maximum length in the region of the spine. In step 56, the sinogram, from which the local intensity maxima have been extracted, undergoes a reverse transformation using an inverse Radon transformation. It is evident that the position of the spine is now particularly prominent in step 56. The described procedure thus enables a particularly advantageous automatic determination of the spine's position in the pre-measurement magnetic resonance data. In this way, the local shim volume can be determined particularly effectively based on the spine's position, and the shim settings can be especially suitable for homogenizing the main magnetic field in the spinal region. The process steps of the method according to the invention, illustrated in Figures 2-4, are executed by the computing unit. For this purpose, the computing unit comprises the necessary software and / or computer programs, which are stored in a memory unit of the computing unit. The software and / or computer programs include program elements designed to execute the method according to the invention when the computer program and / or the software is executed in the computing unit by means of a processor unit of the computing unit. Although the invention has been illustrated and described in detail by the preferred embodiments, the invention is nevertheless not limited by the disclosed examples and other variations can be derived by the person skilled in the art without leaving the scope of protection of the invention.

Claims

A method for magnetic resonance imaging of a subject using a magnetic resonance device, comprising the following steps: - Defining an examination volume for magnetic resonance imaging within a body of the subject, - Determining a local shim volume comprising a sub-region of the examination volume, - Calculating shim settings, wherein tissue of the subject's body present within the local shim volume is incorporated into the calculation of the shim settings, and - Acquiring magnetic resonance image data of the examination volume using the magnetic resonance device, wherein the acquisition of the magnetic resonance image data is performed using the calculated shim settings, and wherein a local shim coil is controlled according to the shim settings during the acquisition of the magnetic resonance image data.wherein the local shim volume is determined by positioning the local shim coil in relation to the body of the object under investigation or by limiting the current of the local shim coil. Method according to claim 1, wherein only the tissue of the body of the subject present in the local shim volume is included in the calculation of the shim settings by which the local shim coil is controlled. Method according to one of the preceding claims, wherein an anatomical structure of the body of the object under investigation is present in the examination volume, wherein a pre-measurement is carried out using the magnetic resonance device before the acquisition of the magnetic resonance measurement data, wherein a position of the anatomical structure is identified on the basis of the magnetic resonance pre-measurement data acquired in the pre-measurement and the determination of the local shim volume is carried out using the identified position of the anatomical structure. Method according to claim 3, wherein the shim settings are calculated using the magnetic resonance pre-measurement data. Method according to one of claims 3-4, wherein the anatomical structure is a spine of the subject under investigation, wherein projection data are created from the magnetic resonance pre-measurement data and the position of the spine is identified on the basis of the projection data. Method according to claim 5, wherein the creation of the projection data from the magnetic resonance pre-measurement data comprises a radon transformation of the magnetic resonance pre-measurement data by means of which the magnetic resonance pre-measurement data are converted into a sinogram, wherein the position of the spine is identified in the sinogram. Magnetic resonance imaging device comprising an image data acquisition unit and a computing unit comprising a setting unit, a determination unit and a calculation unit, wherein the magnetic resonance imaging device is configured to perform a method according to one of the preceding claims. A computer program product that can be directly loaded into a memory of a programmable computing unit of a magnetic resonance device, comprising program code means for executing a method according to any one of claims 1-6 when the computer program product is executed in the computing unit of the magnetic resonance device.

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