Method for operating magnetic resonance device, magnetic resonance device, computer program and electronically readable data carrier

Abstract

A local coil arrangement (5) for use on an examination couch (1) has a magnetic field sensor (6) for measuring a main magnetic field of a magnetic resonance device (16) in order to determine position information of the local coil arrangement (5) by alignment with a main magnetic field map, in which the magnetic field sensor (6) is configured to measure the main magnetic field of the magnetic resonance device (16) in order to examine whether the function of the magnetic resonance sensor is correct. The method according to the invention is performed.

Description

Field of the Invention

[0001] The present invention relates to a method for operating a magnetic resonance apparatus, the magnetic resonance apparatus having:

[0002] - a main magnet unit having a main magnet for generating a main magnetic field and a patient accommodation, in particular cylindrical, wherein the field lines in the homogeneous volume in the patient accommodation extend in the field direction;

[0003] - an examination couch for moving a patient into the patient accommodation along a z-direction corresponding to the longitudinal direction of the examination couch, wherein the examination couch is associated with a position determination device for determining the couch position along the z-direction;

[0004] - at least one local coil device for placement on the patient and / or the examination couch, the local coil device having a magnetic field sensor, in particular a Hall sensor, for measuring a measured value of the magnetic field strength of at least the main magnetic field, and

[0005] - a control device configured to determine first position information describing a first measurement position of the local coil device by aligning a first measured value of the magnetic field sensor with at least a main magnetic field map outside the patient accommodation.

[0006] Furthermore, the present invention relates to a magnetic resonance apparatus, a computer program, and an electronically readable data carrier. Background Art

[0007] In magnetic resonance imaging, local coil devices having at least one local coil element that can be used for transmission and / or reception are also often used. The local coil devices can be freely arranged on the examination couch and / or directly on a patient placed on the examination couch. A challenge is then to move the examination couch into the patient accommodation of the magnetic resonance apparatus such that the local coil device is placed in the isocenter. The isocenter can be defined, for example, as the center of the patient accommodation and / or the center of the homogeneous volume (the two locations often coincide). The examination couch is usually moved in a direction corresponding to its longitudinal direction, which can be referred to as the z-direction. In a cylindrical patient accommodation, the longitudinal direction of the examination couch usually corresponds to the field direction of the main magnetic field in the homogeneous volume.

[0008] In order to achieve as precise a positioning as possible in the case of local coil devices, it has been proposed in the prior art to use a laser positioning device. For example, first, the examination table with the local coil device should be adjusted such that the laser marking shows what is to be placed in the isocenter, and then the table is automatically moved so that the marked point is placed in the isocenter. For example, for this purpose, the marking laser can be fastened externally at a fixed position to the main magnet unit such that the spacing from the isocenter in the z direction is known. However, a marking laser with a variable position has also been proposed. This positioning method has proven to be extremely time-consuming.

[0009] Therefore, in the prior art, a method of using a magnetic field sensor as part of the local coil device has also been proposed at this time. The magnetic field sensor measures the main magnetic field (B0 field), which also includes the stray field outside the patient accommodation here, and at least measures its magnetic field strength (preferably three-dimensionally, i.e., based on measurements in three mutually orthogonal directions). Outside the patient accommodation, i.e., where the local coil device is positioned at the patient, the main magnetic field has a characteristic variation curve that allows a one-to-one correlation in particular. This means that from the measured values of the magnetic field sensor, which are preferably three-dimensionally measured, the position of the local coil device can be inferred at least in the z direction using the main magnetic field map (B0 map). This position information can be easily converted into a suitable movement section of the examination table in order to place the local coil device in the isocenter (usually in the case of z = 0). The position determination of the local coil device with the aid of such a magnetic field strength sensor is described in more detail, for example, in DE 10 2016 203 255 A1.

[0010] For this purpose, for example, it is known to display at least a schematic diagram of the examination table together with the local coil device arranged thereon on a touch screen, which is arranged in particular adjacent to the patient accommodation at the main magnet unit. Then, the operator can select the local coil device with a simple click, and the local coil device is then automatically placed in the isocenter ("Push-Button", also known as "Select and Go"). Thereby, a significantly faster and more comfortable positioning is possible.

[0011] Here, a Hall sensor, preferably a three-dimensional Hall sensor, is generally used as the magnetic field sensor. Like any other electronic component, the magnetic field sensor also has a limited service life and may provide measurement results that deviate from the actual value over time or even stop working properly altogether. It is known in this regard that if the magnetic resonance device shows incorrect behavior in terms of positioning, the user of the magnetic resonance device notifies the after-sales service personnel / maintenance technician. The after-sales service personnel / maintenance technician perform a quality assurance diagnosis of the coil device and decide based on the results whether the local coil device must be replaced, whether repair is feasible, etc. The problem here is that for the user of the magnetic resonance device, a relatively large time loss and expense are incurred. Summary of the Invention

[0012] Therefore, the object underlying the present invention is to provide the feasibility of identifying a change in the measurement behavior of the magnetic field sensor for early maintenance.

[0013] According to the present invention, the object is achieved by a method implemented especially by a computer, a magnetic resonance device, a computer program, and an electronically readable data carrier. Advantageous refinements are derived from the following description.

[0014] According to the present invention, in order to monitor the correct operation of the magnetic field sensor, a method of the type mentioned at the beginning has the following steps:

[0015] - In the case of a first bed position of the examination table, a first measured value of the magnetic field sensor is recorded at a first measurement position of the local coil device outside the patient accommodation, and first position information is determined.

[0016] - In the case of a second bed position of the examination table, during or after the examination table travels into the patient accommodation, a second measured value of the magnetic field sensor is recorded at a second measurement position of the local coil device that is different from the first measurement position, especially within the patient accommodation.

[0017] - Determine at least one reference value that describes the expected main magnetic field at the second measurement position, and determine the second measurement position from the first position information and the movement section of the examination table between the first bed position and the second bed position according to the position determination means.

[0018] - Determine error information indicating a potential measurement error by comparing the second measured value with the reference value.

[0019] The present invention can achieve, instead of reacting to errors, proactively detecting errors and triggering corresponding measures, such as a prompt for the customer, preferably together with a corresponding action recommendation. Here, the always existing mechanism of the magnetic resonance device, for example, provided for implementing the "button" function, is utilized, and in particular, the feasibility of accurately determining the position of the examination table at any time by means of a position determination device is also utilized. In particular, the position of the examination table is always continuously tracked in a manner known in the prior art in the control device of the magnetic resonance device. Now it has been determined that performing only a single additional measurement by means of a magnetic field sensor, in particular a Hall sensor, is sufficient to check whether the function of the magnetic field sensor is correct.

[0020] Since there is the feasibility by means of the magnetic field sensor to perform measurements of the main magnetic field at at least two different positions relative to the center of the magnet (isocenter), because the local coil device moves with the examination table. After the local coil device is placed in the first measurement position, a first measurement of the main magnetic field is always performed in order to obtain first position information from the corresponding first measurement value of the main magnetic field, thereby also determining the magnetic field sensor (first measurement position), and thus where the local coil device is located on the examination table, at least in the z direction. As long as the examination table is outside the patient accommodation (bore) (first measurement position), the location of the local coil device can be determined in this way. The local coil device can be used for corresponding functions, in particular the "button" function described at the beginning, as is common.

[0021] Now it is proposed to perform a second measurement of the main magnetic field at the second measurement position, in particular at the position where the local coil device or the magnetic field sensor is located when the examination table has completed its travel along the z direction into the patient accommodation. This recording of the second measurement value at the second measurement position, in particular at the end position of the travel of the examination table into the patient accommodation, can be performed quickly and without problems, even if this recording is not required for the actual function, such as the "button".

[0022] There is a large spatial distance, for example, between 0.3 m and 2.2 m, between the two measurements of the main magnetic field. This distance causes a significant difference between the first measurement value and the second measurement value. If the magnetic field sensor is working correctly and properly, it is possible to predict from the first measurement, by obtaining the first position information together with the travel section of the examination table (which can be obtained from the difference between the second bed position and the first bed position according to the position determination device), what measurement value (reference value) is expected at the second measurement position. If now the expected value, i.e., the reference value, is compared with the second measurement value as the result of the second measurement of the main magnetic field, it can be deduced therefrom whether the magnetic field sensor is working correctly, which is described by an error message. Then, the error message can be evaluated by means of at least one measure condition as to whether a measure is required.

[0023] Particularly advantageously, whenever the position identification of the local coil device is activated and a movement of the examination table occurs that provides a first measurement position and at least one second measurement position, the examination of the function of the magnetic field sensor can be carried out, thereby obtaining error information. Thus, a permanent check of the functionality and quality of the magnetic field sensor and a proactive action in case of errors are feasible. No dedicated quality assurance steps are required in the service software.

[0024] Thus, generally speaking, possible error behavior of the magnetic field sensor can be identified earlier. It is already possible to very early proactively provide the user with an indication, in particular together with action recommendations. Thus, the user can actively participate before they are necessarily annoyed by the problems that occur.

[0025] Preferably, a three-dimensional measurement magnetic field sensor, in particular a 3D Hall sensor, is used as the magnetic field sensor (also called: magnetic field strength sensor). When measuring the main magnetic field three-dimensionally, significantly better, more robust and one-to-one position associations are feasible. In particular, for obtaining the magnetic field strength (also called the B0 magnitude), measurements from three orthogonal directions are used.

[0026] Here, when the coil device is still outside the patient accommodation, in particular at least outside the homogeneous volume, the recording of the first measurement value is carried out. Thus, it is feasible to obtain the first measurement position from the first measurement value with the help of the main magnetic field map. In addition, when the examination table moves, there is a difference between the first measurement value and the second measurement value. The corresponding first table position is also called "Close to Home", and can be defined, for example, as the depth at which the front end of the examination table extends into the patient accommodation not exceeding 30 cm. Thus, the first measurement value, the first position information and the first table position are obtained at the first measurement position.

[0027] If the examination table has been moved into the patient accommodation, i.e., in particular the front end of the examination table is deeper than 50 cm in the patient accommodation, a second measurement of the main magnetic field is carried out, so that the second measurement value is recorded with the help of the magnetic field sensor. Thus, the second measurement value and the second table position are obtained at the second measurement position.

[0028] Thus, the moving section of the examination table in the z direction is obtained as the difference between the second table position and the first table position, and the second measurement position (which is equal to the expected position of the coil device after the examination table has traveled, assuming the first position information is correct) is then obtained as the sum of the first measurement position according to the first position information and the moving section.

[0029] To check whether the magnetic field sensor is working correctly, a reference value is now obtained for the second measurement position, and it is checked whether the deviation between the second measurement value and the reference value is small enough.

[0030] There are various possibilities for correctly determining reference values. Of course, it is also possible to determine multiple reference values and perform multiple comparisons in order to achieve as robust a basis as possible for the measures and as precise a logging as possible, if such logging is carried out.

[0031] A purposeful improvement of the invention provides that a first reference value of at least one reference value is determined from the main magnetic field map at the second measurement position determined. This means that for such a first reference value, the comparison corresponds to the alignment with the main magnetic field map, which is usually extremely reliable and generally can be considered to be error-free in principle.

[0032] Here, of course, it should be noted that due to different effects, such as the presence of the examination object and / or other objects, the service life of the magnetic resonance device and / or changes in the environment, changes may also occur in the main magnetic field. Such a change in the main magnetic field is often also referred to as B0 drift. In order to be able to detect such a change in the main magnetic field, it is already known in the prior art that, in particular by means of simple magnetic resonance measurements, regularly, for example for each magnetic resonance measurement to be carried out, the current magnetic resonance frequency, i.e., the Larmor frequency in the homogeneous volume is determined as the calibration frequency, from which the intensity of the main magnetic field in the homogeneous volume can be determined by means of the gyromagnetic ratio. In order to take into account the influence of the drifting main magnetic field also on the main magnetic field map and thus avoid errors in the use of the functions of the main magnetic field map, it is already known to adjust, and thus correct, for example, by transferring the absolute change or relative change to positions outside the homogeneous volume, the main magnetic field map. It is also preferred within the scope of the present invention to correct the main magnetic field map before its use according to the current calibration frequency measured by means of a magnetic resonance measurement, which calibration frequency in particular corresponds to the magnetic resonance frequency (Larmor frequency in the homogeneous volume). As a magnetic resonance measurement, for example, an FID measurement (Free Induction Decay) can be carried out, as is known in principle.

[0033] In the case of a second measurement position within the homogeneous volume, it can be purposefully and preferably additionally provided that at least one second reference value of at least one reference value is determined from the current calibration frequency measured by means of a magnetic resonance measurement or the current calibration frequency, which current calibration frequency corresponds to the magnetic resonance frequency in the homogeneous volume. If it is known that the magnetic field sensor is in the homogeneous volume during the second measurement, the second reference value can be easily determined from the current calibration frequency and the gyromagnetic ratio. Thus, in the homogeneous volume, it is very precisely known which B0 value the second measurement of the main magnetic field should provide, in particular also independently of the above-discussed correction of the main magnetic field map due to the calibration frequency outside the homogeneous volume.

[0034] It should also be noted here that the second measurement in the homogeneous volume is also particularly preferred as follows: Even in the case of a small positional deviation, for example due to an incorrect measurement of the first measurement value, the expected field strength of the main magnetic field remains the same, so that the deviation can be evaluated particularly well.

[0035] Particularly advantageously, at least in the case of the second measurement position in the homogeneous volume, not only the first reference value but also the second reference value can be used for evaluation and form the basis for comparison. In this way, a more robust check and additional information for evaluating potential error states and temporal developments are obtained. Here, in particular, a situation distinction can be made, such that, for example, only the first reference value is used outside the homogeneous volume, while inside the homogeneous volume, a comparison is made not only with the first reference value but also with the second reference value. In terms of additional information, it has been shown, for example, that the comparison with the second reference value can allow an improved evaluation as to how well the magnetic field sensor is calibrated and whether the calibration has changed.

[0036] It is also conceivable to determine the priority of the comparison, for example, to compare with the second reference value only when the comparison with the first reference value shows an error situation, for a plausibility check and / or to obtain other information. It can also be proposed that when using the first reference value and the second reference value, the comparison with the second reference value is made only when the comparison result with the first measurement value does not show a measurement error. Thus, it can be adapted to specific plausibility, robustness, and information requirements.

[0037] Of course, embodiments are also conceivable in which only the second reference value is used (at least in the homogeneous volume).

[0038] Generally, in a specific embodiment of the present invention, it can be proposed that in the comparison, the absolute or relative deviation of the second measurement value from the reference value is determined, and a measurement error is identified when the deviation exceeds a limit value. Here, it can be distinguished whether to consider the absolute deviation or the relative deviation based on the reference value. In a specific embodiment, for example, in the case of a rated field strength of 1.5 T for the main magnetic field, a limit value in the range of 0.08 T to 0.12 T, in particular 0.1 T, can be used for the absolute deviation, especially with respect to the first measurement value. Generally, suitable limit values can be determined experimentally at the corresponding magnetic resonance device and can be different depending on the magnetic resonance device / rated field strength.

[0039] Advantageously, in this context, when using the second reference value, the relative deviation is determined for the second reference value. In the specific embodiment already mentioned in the case of a rated field strength of 1.5 T for the main magnetic field, a limit value in the range of 0.05 to 0.09 (i.e., 5% to 9%) for the relative deviation with respect to the second measurement value has proven to be suitable based on experiments.

[0040] As already described, it can be particularly advantageous to provide that error messages are evaluated by means of at least one measure condition and that, when the measure condition is met, the associated measure is carried out. For example, it can be specifically provided that, in the case of an error message indicating a measurement error, for example when at least one limit value is exceeded, an optical and / or acoustic indication output for the measurement error is output, in particular together with an action recommendation. The action recommendation can, for example, relate to a repair to be carried out and / or a replacement to be carried out on the local coil device. Other measure conditions, for example in the case of a smaller deviation, initially only cause an entry into the error memory. In this regard, it can also be monitored, for example by means of at least one measure condition, whether there are a plurality of successive entries into the error memory in order to subsequently also trigger an indication output as a measure.

[0041] However, within the scope of the present invention, it has proven to be particularly advantageous to record error messages. This means that each error message obtained is entered into a log file. The information can be used for long-term monitoring, for example by determining trends and the like. In particular, the error messages of the present invention can be collected and evaluated together with other information recorded when using the magnetic resonance device on site in order to further develop the magnetic resonance device and / or to facilitate new developments or to determine the need for such developments.

[0042] As already mentioned, the first position information can be used to implement different functions in a magnetic resonance device, as is generally known. For example, the position information can be incorporated into obtaining a view showing the local coil device on the examination couch together with its position. In addition, the control device can also be configured to manipulate the examination couch in relation to the first position information in order to move it into the patient receiving section.

[0043] Specifically and particularly preferably, it can be provided that, when one of the at least one local coil device is selected on the user side, based on the first position information of the local coil device, the local coil device is set in the isocenter of the magnetic resonance device by moving the examination couch, the isocenter being in a homogeneous volume. In this way, the positioning of the local coil device in the isocenter of the magnetic resonance device is significantly simplified, for example as a "button" function. In particular, a view can be generated which schematically shows the currently used local coil device and its position in accordance with the corresponding first position information on the examination couch. The view can, for example, be output on a touch screen or in some other way such that the user can select one of the local coil devices as the local coil device to be placed in the isocenter.

[0044] It should also be noted here that, as already mentioned, the determination of the error information can preferably be carried out for each such positioning process. In particular, it can be proposed that the error information for each user-side selection and the positioning of the local coil device be determined for all local coil devices. Thus, the determination of the error information need not be limited to the selected local coil device, but can relate to all local coil devices, especially when using the first reference value, and can also relate to local coil devices not placed within the homogeneous volume.

[0045] In addition to the method, the invention also relates to a magnetic resonance device having:

[0046] - a main magnet unit having a main magnet for generating a main magnetic field and a patient receiving part, in particular a cylindrical one, wherein the field lines of the main magnetic field in the homogeneous volume in the patient receiving part extend in the field direction,

[0047] - an examination couch for moving a patient into the patient receiving part along the z-direction corresponding to the longitudinal direction of the examination couch, wherein the examination couch is associated with position determination means for determining the couch position along the z-direction,

[0048] - at least one local coil device for placement on the patient and / or the examination couch, wherein the local coil device has a magnetic field sensor, in particular a Hall sensor, for measuring the measured value of the magnetic field strength of at least the main magnetic field, and

[0049] - a control device configured to determine first position information describing a first measurement position of the local coil device by aligning a first measured value of the magnetic field sensor with at least a main magnetic field map outside the patient receiving part,

[0050] The magnetic resonance device is characterized in that the control device is configured to execute the method according to the invention.

[0051] All embodiments of the method according to the invention can be similarly transferred to the magnetic resonance device according to the invention, and with the aid of the magnetic resonance device, the advantages already mentioned can thus also be obtained. In particular, the control device can have at least one storage means and at least one processor. Functional units, in particular functional units for executing the steps of the method according to the invention, can be formed by hardware and / or software.

[0052] Specifically, the control device can for example have:

[0053] - a measurement unit for recording a first measured value and a second measured value by corresponding manipulation and reception of the magnetic field sensor,

[0054] - A first measurement position obtaining unit, which is configured to obtain first position information from a first measurement value and a main magnetic field map.

[0055] - A bed position obtaining unit, which is configured to obtain a bed position by means of a position obtaining device.

[0056] - A second measurement position obtaining unit, which is configured to obtain a second measurement position from the first position information and the bed position.

[0057] - A reference value obtaining unit, which is configured to obtain a reference value, and

[0058] - A comparison unit, which is configured to obtain error information.

[0059] Other functional units may be, for example, a measure unit for checking measure conditions and executing corresponding measures, an output unit for outputting indications, a user interaction unit for receiving user input / output diagrams, and a bed control unit for controlling the examination bed (or the actuator driving the examination bed) to move into or out of the patient accommodation part. If the control device is also configured to control other operations of the magnetic resonance device, especially the recording operation, a sequence unit for controlling the recording operation, a reconstruction unit for obtaining a magnetic resonance image data set, etc. may also be provided, as is known in principle.

[0060] A computer program according to the present invention can be directly loaded into the storage mechanism of the control device of the magnetic resonance device and has a program mechanism. When the computer program is executed on the control device, the program mechanism causes the control device to execute the steps of the method according to the present invention. The computer program can be stored on an electronically readable data carrier according to the present invention. Therefore, the data carrier has control information stored thereon, and the control information includes at least one computer program according to the present invention and is designed such that when the data carrier is used in the control device of the magnetic resonance device, the control device is configured to execute the method according to the present invention. The data carrier may in particular be a non-transitory data carrier, such as a CD-ROM. Description of the Drawings

[0061] Other advantages and details of the present invention result from the embodiments described below and from the drawings. Shown here are:

[0062] Figure 1 A flowchart showing an embodiment of the method according to the present invention.

[0063] Figure 2 A diagrammatic illustration for performing a measurement at the first measurement position.

[0064] Figure 3Shows an illustrative illustration for performing measurements at a second measurement position,

[0065] Figure 4 shows a magnetic resonance apparatus according to the invention, and

[0066] Figure 5 shows the functional configuration of the control device of the magnetic resonance apparatus. Detailed Description

[0067] Figure 1 Shows a flowchart of an embodiment of a method for monitoring the function of a magnetic field sensor of a local coil device of a magnetic resonance apparatus, a Hall sensor for three-dimensional measurement here. Thus, if the local coil device is arranged on the examination table and / or on a patient placed on the examination table, and the examination table is arranged in a position moved out of the patient accommodation of the magnetic resonance apparatus, then the local coil device is outside the homogeneous volume of the magnetic resonance apparatus. Here, the examination table can be moved in the z-direction corresponding to its longitudinal direction, and the z-direction currently also corresponds to the field direction of the main magnetic field (B0 field) in the homogeneous volume. The magnetic field sensor can measure the main magnetic field (B0 field) of the magnetic resonance apparatus, and the control device can obtain position information from the corresponding measurement values by means of a main magnetic field map, and the position information describes at least the current position of the local coil device with respect to the z-direction. Currently, the first position information obtained at the first measurement position of the local coil device outside the homogeneous volume can be used to implement a targeted movement of the examination table, so that the local coil device is positioned at the isocenter of the magnetic resonance apparatus, especially within the scope of the "button" function.

[0068] However, performing such a targeted movement by means of the measurement at the first measurement position is currently also used to check the function of the magnetic field sensor in a simple and low-cost manner and method.

[0069] For this purpose, in step S1, first, a first measurement of the main magnetic field required for the function is performed at the first measurement position. The result is a first measurement value for the main magnetic field, especially at least for the amplitude (field strength) of the main magnetic field.

[0070] In step S2, the first measurement value is calibrated with a map (Karte) of the main magnetic field (corrected with respect to the currently measured calibration frequency) that is also outside the patient accommodation, in order to obtain the corresponding first measurement position, and thus obtain the first position information of the local coil device.

[0071] For this purpose, Figure 2 Schematically shows the situation in the first measurement position. In the first moved bed position 4, the examination table 1 is mostly outside the patient accommodation 2 in the main magnet unit 3. Here, it relates to a so-called near-in-situ position, or the examination table 1 extends into the patient accommodation 2 by no more than 30 cm.

[0072] Therefore, the local coil device 5 with the magnetic field sensor 6, the Hall sensor measured three-dimensionally here, which is arranged on the examination table 1 or on a patient (not shown) placed on the examination table 1, is also arranged outside the homogeneous volume around the isocenter 7, and the isocenter 7 currently also defines the zero point of the z-direction 8b corresponding to the field direction 8a here. The magnetic field sensor 6 is here at the first measurement position 9 at a distance 10 from the isocenter 7.

[0073] The curve 11 of the change in the intensity of the main magnetic field is also shown superposed on the schematic diagram. Therefore, for a correct measurement, at the first measurement position 9, the magnetic field sensor 6 measures the first measurement value 12 of the main magnetic field. After the curve 11 is known from the main magnetic field map, the first measurement position 9 can be determined from the first measurement value 12 and stored here as the first position information.

[0074] Back to Figure 1 , in step S3, the first table position 4 is also determined by the position acquisition device associated with the examination table 1. The precise and robust acquisition of the table position 4 can be based here on the corresponding feedback, control history, and / or dedicated sensors of the actuator for moving the examination table 1 along the z-direction 8b. The corresponding detection mechanism is included in the position acquisition device.

[0075] In step S4, now the examination table 1 (together with the local coil device 5 fixedly arranged thereon) is moved into the patient accommodation 2. For example, the reason for this movement can be the use of the function for positioning the local coil device 5 in the isocenter 7. Here, currently, in order to implement the function, for example as a "button" function, a diagram is generated based on the first position information about all the local coil devices 5 arranged on the examination table 1, for example, a schematic diagram of the local coil device 5 on the examination table 1. The diagram is output on the touch screen, which can be arranged, for example, at the covering of the main magnet unit 3 at the end side from which the examination table 1 is moved in. The user can select the local coil device 5 by selecting one of the local coil devices 5 or the operating element representing the local coil device 5, and due to the known distance 10 from the isocenter 7, the local coil device 5 is moved into the isocenter 7 by correspondingly controlling the actuator of the examination table 1.

[0076] The corresponding situation obtained after the movement is schematically shown in Figure 3 . It can be seen that the magnetic field sensor 6 is now at the second measurement position 13, which corresponds to the isocenter 7, however, this does not necessarily have to be the case. For this purpose, the examination table 1 is at the second table position 14, which is displaced by the distance 10 relative to the first table position 4.

[0077] Thus, as also indicated by the variation curve 11, the local coil device 5 and therewith the magnetic field sensor 6 are now in a homogeneous volume with a constant magnetic field strength, such that it is not meaningfully feasible to determine the position one-to-one, i.e., to obtain the second position information, from the second measured value 15 recorded in step S5 for the main magnetic field according to Figure 1 However, the second measurement is still currently carried out in order to check the functionality of the magnetic field sensor 6. For this purpose, the second bed position 14 is again determined in step S6 by means of the position determination means.

[0078] In step S7, as a preparation for obtaining the reference value, the second measurement position 13 is obtained in such a way that the first measurement position 9 is moved according to the first position information by the moving section of the examination bed 1 (the moving section is directly obtained from the difference between the first bed position and the second bed position 4, 14). Currently, the second measurement position 13 is within the homogeneous volume.

[0079] Subsequently, in step S8, at least one reference value can be determined and the second measured value 15 is compared with the at least one reference value in order to check whether the functionality of the magnetic field sensor 6 is correct. Thus, the reference value corresponds here to the expectation based on the second measurement position 13, as obtained in step S7.

[0080] In the present embodiment, in any case, the first reference value is obtained by calling the expected main magnetic field from the main magnetic field map at the second measurement position 13 (corrected based on the calibration frequency as described above). Thus, for the first reference value, a comparison with the expectation according to the main magnetic field map is proposed. However, a case distinction is also made in step S8. Only when the second measurement position 13 is within the homogeneous volume, the second reference value is obtained by simply dividing the calibration frequency, which results from the current measurement and corresponds to the magnetic resonance frequency in the homogeneous volume, by the gyromagnetic ratio.

[0081] Subsequently, in step S9, the error information is obtained by comparison with the available reference values. For this purpose, a first deviation, here an absolute deviation, is determined for the first reference value, thus obtaining the magnitude of the difference between the first reference value and the second measured value 15. If the first deviation is greater than the first limit value, here exemplarily a first limit value of 0.1 T for the rated field strength of the 1.5 T main magnetic field, an error condition, thus an incorrect measurement, can be determined. Both the first deviation and the result of the comparison with the limit value are stored in the error information.

[0082]

[0083] ​If the second measurement position 13 is within a homogeneous volume, there is also a second reference value, and a second deviation, here a relative deviation, is determined for the second reference value. This means that the second deviation is determined by dividing the magnitude of the difference between the second reference value and the second measurement value 15 by the second reference value. If the second relative deviation is greater than a second limit value, here exemplarily 0.07 (i.e., 7%) for the case mentioned above, an error situation can be determined again. Not only the second deviation but also the comparison result is stored again in the error message.

[0084] It should be noted here that embodiments are also conceivable in which the second reference value is used alone only in the case of the second measurement position 13 within the homogeneous volume. In addition, cross-comparisons (gestaffelte Vergleiche) are also conceivable, in particular cross-comparisons with a plurality of first limit values and / or second limit values.

[0085] In step S10, the determined error message is stored in a log file in a storage mechanism of the control device of the magnetic resonance apparatus or in an external storage mechanism, in particular together with other operating data of the magnetic resonance apparatus. The error message can be provided there for subsequent evaluation.

[0086] Subsequently, in step S11, it is checked whether at least one measure condition is met. Each measure condition is associated with a measure. For example, the measure condition can check whether one or both comparisons in the comparison show an exceedance of the first limit value or the second limit value. Subsequently, as a measure to be performed in step S12, an optical and / or acoustic indication output can be associated, and an action recommendation, here the replacement of the local coil device 5, also belongs to the indication output. Other measure conditions can for example check whether an entry into the error memory should be made.

[0087] If the measure condition is not met or all associated measures have been implemented, it returns again to step S1 for the next target travel, i.e., for the next utilization of the positioning function when using the magnetic field sensor 6. In other words, for each determination of the first positioning information, the function of the magnetic field sensor 6 is checked, and subsequently the examination table 1 is advanced into the patient accommodation 2.

[0088] It should be pointed out again at this point that the second measurement position 13 does not necessarily have to be within the homogeneous volume, because after the expectation for the second measurement value 15 has also been derived from the main magnetic field map, the function check of the magnetic field sensor 6 is still feasible.

[0089] Figure 4 A schematic diagram of the principle of a magnetic resonance apparatus 16 according to the invention is shown. As already in Figure 2 and Figure 3As illustrated in the schematic diagram of the principle, the magnetic resonance apparatus 16 includes a main magnet unit 3 with a cylindrical patient accommodation 2 into which the examination table 1 can be moved. The main magnet unit 2 also includes a main magnet 17, which is only indicated here and is superconducting or configured as a permanent magnet.

[0090] In addition, a position detection device 18 for detecting the position of the examination table 1 is also indicated. On a touch screen 20 arranged at the end side 19 of the covering of the main magnet unit 3, for example, a diagram of the examination table 1 together with the local coil device 5 arranged on the examination table can be output, so that the local coil device 5 can be made selectable for positioning in the isocenter 7 by interacting with the diagram and / or operating elements.

[0091] The operation of the magnetic resonance apparatus 16 is controlled by a control device 21. Figure 5 With reference to the Figure 1 method, the functional structure of the control device 21 configured to execute the method is shown more precisely.

[0092] Accordingly, the control device 21 first includes a storage mechanism 22 in which different information can be stored, and different information of the method can also be stored. In addition, the control device 21 includes: a measurement unit 23 for recording a first measurement value and a second measurement value 12, 15 according to steps S1 and S5; a first measurement position detection unit 24 for obtaining first position information from the first measurement value 12 and the main magnetic field map according to step S2; a bed position detection unit 25 for obtaining a first bed position and a second bed position 4, 14 according to steps S3 and S6; a second measurement position detection unit 26 for obtaining a second measurement position 13 from the first position information and the bed positions 4, 14 according to step S7; a reference value detection unit 27 for obtaining at least one reference value according to step S8; and a comparison unit 28 for obtaining error information according to step S9. The control device 21 is also configured to store the error information in a log file according to step S10; alternatively, a logging unit can also be provided.

[0093] In addition, also included as part of the control device 21 are: a measure unit 29 for checking measure conditions and executing corresponding measures according to steps S11 and S12; and a user interaction unit 30 for receiving user input and outputting diagrams and / or instructions. Finally, a bed control unit 31 is also provided, which is used to control the examination table 1 (or the actuator driving the examination table 1) to move into and out of the patient accommodation 2, in particular also according to step S4.

[0094] After the control device 21 is also configured to control other operations of the magnetic resonance apparatus 16, corresponding other functional units (not shown here) are also provided.

[0095] Although the details of the present invention have been described in detail by way of preferred embodiments, the present invention is not limited by the disclosed examples and other variant solutions can be derived therefrom by those skilled in the art without departing from the scope of protection of the present invention.

[0096] Regardless of the grammatical gender of a particular term, persons of either male or female gender identity are included.

Claims

1. A method for operating a magnetic resonance apparatus (16), the magnetic resonance apparatus (16) having: - a main magnet unit (3) having a main magnet (17) for generating a main magnetic field and a patient accommodation (2), in particular cylindrical, wherein the field lines of the main magnetic field in a homogeneous volume in the patient accommodation (2) extend in a field direction (8a), - an examination table (1) for moving a patient into the patient accommodation (2) along a z-direction (8b) corresponding to the longitudinal direction of the examination table (2), wherein the examination table (1) is associated with a position acquisition device (18) for acquiring the table position along the z-direction (8b), - at least one local coil device (5) for placement on the patient and / or the examination table (1), wherein the local coil device (5) has a magnetic field sensor (6) for measuring measured values of at least the magnetic field strength of the main magnetic field, and - a control device (21) configured to obtain first position information describing a first measurement position (9) of the local coil device (5) by aligning a first measured value (12) of the magnetic field sensor (6) with a main magnetic field map at least outside the patient accommodation (2), It is characterized in that The method includes: - at a first table position (4) of the examination table (1), recording a first measured value (12) of the magnetic field sensor (6) at a first measurement position of the local coil device (5) outside the patient accommodation (2) and obtaining the first position information, - at a second table position (14) of the examination table (1), during or after the examination table (1) has moved into the patient accommodation (2), recording a second measured value (15) of the magnetic field sensor (6) at a second measurement position (13) of the local coil device (5) that is different from the first measurement position (9), in particular within the patient accommodation, - obtaining at least one reference value that describes the expected main magnetic field at the second measurement position (13), obtaining the second measurement position (13) from the first position information and the movement section of the examination table (1) between the first and the second table positions (4, 14) according to the position acquisition device (18), and - obtaining error information indicating a potential measurement error by comparing the second measured value (15) with the reference value.

2. The method according to claim 1, characterized in that a first reference value among the at least one reference value is obtained from the main magnetic field map at the obtained second measurement position (13).

3. The method according to claim 1 or 2, characterized in that before using the main magnetic field map, the main magnetic field map is corrected according to a currently measured calibration frequency measured by magnetic resonance measurement.

4. The method according to any one of the above claims, characterized in that In the case of a second measurement position (13) within the homogeneous volume, at least one second reference value of the at least one reference value is determined from the current calibration frequency measured by magnetic resonance measurement or the current calibration frequency, the calibration frequency corresponding to the magnetic resonance frequency in the homogeneous volume.

5. The method according to any one of the preceding claims, characterized in that in the comparison, an absolute or relative deviation of the second measurement value (15) from the reference value is determined, and a measurement error is identified when the deviation exceeds a limit value.

6. The method according to claim 5, characterized in that when using the second reference value, a relative deviation is determined for the second reference value.

7. The method according to any one of the preceding claims, characterized in that in the case of displaying an error message for the measurement error, an optical output and / or an indication output for the measurement error is output, in particular jointly with a recommended action, and / or the error message is recorded.

8. The method according to any one of the preceding claims, characterized in that after a local coil device of the at least one local coil device (5) is selected on the user side, based on the first position information of the local coil device (5), the local coil device is arranged in the isocenter (7) of the magnetic resonance device (1) by moving the examination table (1) into the isocenter (7) being in the homogeneous volume.

9. The method according to claim 8, characterized in that the error message is determined for each user-side selection and positioning of the local coil device (5) for all local coil devices (5).

10. A magnetic resonance device (16), the magnetic resonance device (16) having: - a main magnet unit (3), the main magnet unit (3) having a main magnet (17) for generating a main magnetic field and a patient receiving part (2) which is in particular cylindrical, wherein the field lines of the main magnetic field in the homogeneous volume in the patient receiving part (2) extend in the field direction (8a), - an examination table (1), the examination table (1) for moving a patient into the patient receiving part (2) along the z-direction (8b) corresponding to the longitudinal direction of the examination table (2), wherein the examination table (1) is associated with a position determination device (18) for determining the table position along the z-direction (8b), - at least one local coil device (5), the local coil device (5) for being placed on the patient and / or the examination table (1), wherein the local coil device (5) has a magnetic field sensor (6) for measuring a measurement value of at least the magnetic field strength of the main magnetic field, and - a control device (21), the control device (21) being configured to determine first position information describing a first measurement position (9) of the local coil device (5) by calibrating a first measurement value (12) of the magnetic field sensor (6) with a main magnetic field map at least outside the patient receiving part (2), characterized in that The control device (21) is configured to perform the method according to any one of the preceding claims.

11. A computer program which, when executed on a control device (21) of a magnetic resonance apparatus (16), causes the control device to perform the steps of the method according to any one of claims 1 to 9.

12. An electronically readable data carrier on which the computer program according to claim 11 is stored.

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