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

Abstract

In a local coil arrangement (5) for use on a patient bed (1) with a magnetic field sensor (6) for measuring the main magnetic field of a magnetic resonance device (16) in order to determine position information of the local coil arrangement (5) by comparison with a main magnetic field map, the following sequence of steps is proposed to check the correct function of the magnetic resonance sensor in a procedure: - Recording of the first measured value (12) of the magnetic field sensor (6) outside the patient recording (2) at the first measuring position of the local coil arrangement (5) at a first lying position (4) of the patient bed (1) and determination of the first position information, - Recording of a second measurement (15) of the magnetic field sensor (6) during or after completion of a movement of the patient bed (1) into the patient receptacle (2) at a second measurement position (13) of the local coil arrangement (5) at a second position (14) of the patient bed (1), which is different from the first measurement position (9) and is located in particular within the patient receptacle. - Determination of at least one reference value that describes the expected main magnetic field at the second measurement position (13), which is determined from the first position information and the displacement distance of the patient bed (1) between the first and the second bed position (4, 14) according to the position determination device (18), and - Determination of error information indicating a potential measurement error by comparing the second measured value (15) with the reference value.

Description

[0001] The invention relates to a method for operating a magnetic resonance device, comprising - a main magnet unit with a main magnet for generating a main magnetic field and a particularly cylindrical patient receptacle, wherein the field lines of the main magnetic field run in a homogeneity volume in the patient receptacle in one field direction, - a patient bed for moving a patient into the patient reception area along a z-direction corresponding to the longitudinal direction of the patient bed, wherein the patient bed is assigned a position determination device for determining a bed position along the z-direction, - at least one local coil arrangement for placement on the patient and / or the patient bed, wherein the local coil arrangement has a magnetic field sensor, in particular a Hall sensor, for measuring measured values ​​of at least the magnetic field strength of the main magnetic field, and - a control device designed to determine a first position information of the local coil arrangement describing a first measurement position by comparing a first measured value of the magnetic field sensor with a main magnetic field map, at least outside the patient recording area. The invention also relates to a magnetic resonance device, a computer program, and an electronically readable data carrier.

[0002] In magnetic resonance imaging (MRI), local coil arrays are frequently used, each with at least one local coil element capable of transmitting and / or receiving. These arrays can be freely positioned on a patient table and / or directly on a patient placed on the table. A challenge lies in positioning the patient table within the MRI scanner's patient receptacle in such a way that the local coil array is located at the isocenter. The isocenter can be defined, for example, as the center of the patient receptacle and / or the center of the homogeneous volume (these two locations often coincide). The patient table is typically moved in a direction corresponding to its longitudinal axis, which can be termed the z-direction. In cylindrical patient receptacles, the longitudinal direction of the patient table often corresponds to the field direction of the main magnetic field within the homogeneous volume.

[0003] To enable the most precise positioning possible with respect to the local coil arrangement, prior art has already proposed the use of laser positioning systems. In these systems, for example, the patient table with the local coil arrangement is first adjusted so that a laser marker indicates what is to be placed at the isocenter. The table is then automatically moved so that the marked point is positioned at the isocenter. For instance, a marking laser can be attached to a fixed position on the outside of the main magnet unit, so that the distance to the isocenter in the z-direction is known. However, marking lasers with adjustable positions have also been proposed. This type of positioning has proven to be extremely complex.

[0004] Therefore, approaches have been proposed in the prior art that use a magnetic field sensor as part of the local coil arrangement. This sensor measures the main magnetic field (B0 field), which also includes the stray field outside the patient's receptacle, at least in terms of its magnetic field strength (preferably three-dimensionally, i.e., based on measurements in three mutually orthogonal directions). Outside the patient's receptacle, i.e., where the local coil arrangement is positioned on the patient, the main magnetic field exhibits a characteristic pattern that allows for unambiguous identification. This means that, using a main magnetic field map (B0 map), the location of the local coil arrangement, at least in the z-direction, can be deduced from the measured values ​​of the magnetic field sensor, particularly one measuring three-dimensionally.This position information can easily be translated into a suitable travel path for the patient table to position the local coil arrangement at the isocenter (usually at z = 0). The positioning of a local coil arrangement using such a magnetic field strength sensor is described in more detail, for example, in DE 10 2016 203 255 A1.

[0005] For example, it is known to display a schematic representation of at least the patient table with the local coil arrangements mounted on it on a touchscreen, particularly one located on the main magnet unit adjacent to the patient reception area. By simply tapping the screen, an operator can then select a local coil arrangement, which is then automatically placed at the isocenter ("push-button," also known as "select and go"). This allows for significantly faster and more convenient positioning.

[0006] A Hall sensor, preferably a three-dimensional Hall sensor, is typically used as the magnetic field sensor. Like any other electronic component, a magnetic field sensor has a limited lifespan and can produce measurement results that deviate from the actual values ​​over time, or even cease to function correctly altogether. It is known that when a user of the magnetic resonance device exhibits a positioning malfunction, they notify customer service / a service technician. The technician performs a quality assurance diagnostic of the coil assembly and, depending on the result, decides whether the local coil assembly needs to be replaced, can be repaired, or something similar. The problem with this is that it results in a relatively large loss of time and effort for the user of the magnetic resonance device.

[0007] From DE 10 2016 203 255 A1, a method is known for determining the position of a movable device relative to a B0 field magnet along a z-coordinate. In this method, a characteristic magnetic field strength B is measured using a magnetic field strength sensor. ref of the B0 field magnet.

[0008] The invention is therefore based on the objective of providing a means of detecting changes in the measurement behavior of magnetic field sensors and thus enabling early servicing.

[0009] This problem is solved according to the invention by a method, in particular a computer-implemented method, a magnetic resonance device, a computer program, and an electronically readable data carrier according to the dependent claims. Advantageous embodiments are described in the sub-claims.

[0010] A method of the type mentioned above, for monitoring the correct function of the magnetic field sensor, comprises the following steps according to the invention: - Recording the first measurement value of the magnetic field sensor outside the patient area at the first measurement position of the local coil arrangement with the patient lying in a first position and determining the first position information, - Recording a second measurement value of the magnetic field sensor during or after completion of a movement of the patient bed into the patient reception area at a second measurement position of the local coil arrangement, different from the first measurement position, in particular located within the patient reception area, at a second lying position of the patient bed, - Determination of at least one reference value that describes the expected main magnetic field at the second measurement position, which is determined from the first position information and the displacement distance of the patient bed between the first and second bed positions according to the position determination device, and - Determination of error information indicating a potential measurement error by comparing the second measured value with the reference value, wherein in the comparison an absolute or relative deviation of the second measured value (15) from the reference value is determined, wherein if a limit value is exceeded by the deviation a measurement error is detected.

[0011] The invention enables proactive error detection, rather than simply reacting to an error, and the initiation of appropriate measures, such as a notification to the customer, preferably with a corresponding recommendation for action. This utilizes existing features of the magnetic resonance device, such as those already provided for implementing a "push-button" function, and in particular the ability to precisely determine the position of the patient bed at any time using a position detection device. Specifically, the position of the patient bed is already continuously monitored in the control unit of the magnetic resonance device using methods known in the prior art. It has now been found that a single additional measurement with the magnetic field sensor, particularly a Hall sensor, is sufficient to verify the correct functioning of the magnetic field sensor.

[0012] The magnetic field sensor allows for the measurement of the main magnetic field at at least two different locations relative to the magnet's center (isocenter), since the local coil assembly moves with the patient bed. An initial measurement of the main magnetic field is performed automatically after the local coil assembly is placed in the first measurement position. This initial measurement provides the first positional information and thus determines the location of the magnetic field sensor (first measurement position) and, consequently, the local coil assembly on the patient bed, at least in the z-direction. This allows the location of the local coil assembly to be determined as long as the patient bed is outside the patient bore (first measurement position). This position can then be used for a corresponding function, particularly the "push-button" function described earlier.

[0013] It is now proposed to perform a second measurement of the main magnetic field at a second measurement position, specifically at the position where the local coil assembly or the magnetic field sensor is located, once the patient stretcher has completed its movement along the z-direction into the patient reception area. Such a second measurement at a second measurement position, particularly at the end position of the patient stretcher's movement into the patient reception area, is quick and easy to perform, even if it is not required for the actual function, such as a "push button".

[0014] Between these two measurements of the main magnetic field lies a large spatial distance, for example, 0.3 to 2.2 m. This leads to a significant difference between the first and second measurements. If the magnetic field sensor is functioning correctly, the first measurement, combined with the displacement of the patient bed (determined by the difference between the first and second bed positions according to the position detection device), allows us to predict the expected measurement at the second measurement position (reference value). Comparing this expectation, i.e., the reference value, with the second measurement from the second main magnetic field measurement, we can determine whether the magnetic field sensor is working correctly, which is described by the error information.The error information can then be evaluated by at least one action condition to determine whether an action is required.

[0015] A particular advantage is that this functional check of the magnetic field sensor, and thus the determination of fault information, can be performed every time the position detection of the local coil arrangement is active and the patient table is moved, providing a first and at least a second measurement position. This enables continuous monitoring of the functionality and quality of the magnetic field sensor and allows for proactive action in the event of a fault. The need for a dedicated quality assurance step in the service software is eliminated.

[0016] In summary, it can be said that potential malfunctions of the magnetic field sensor can be detected earlier. The user can be proactively notified very early on, especially with a recommended course of action. This allows the user to take action before problems arise.

[0017] A three-dimensional magnetic field sensor, particularly a 3D Hall sensor, is preferably used as the magnetic field sensor (also: magnetic field strength sensor). Three-dimensional measurement of the main magnetic field allows for significantly better, more robust, and unambiguous position assignment. Specifically, measurements from three orthogonal directions are used to determine the magnetic field strength (also B0 magnitude).

[0018] The first measurement is taken when the coil assembly is still outside the patient's bed, specifically at least outside the homogeneity volume. This allows the first measurement position to be determined from the first measurement using the main magnetic field map. Furthermore, if the patient bed moves, there will be a difference between the first and second measurements. This initial bed position is also referred to as "close to home" and can be defined, for example, as the front end of the bed not extending more than 30 cm into the patient's bed. Thus, the first measurement position yields the first measurement, the first positional information, and the first bed position.

[0019] Once the patient bed has been moved into the patient receiving area, meaning that the front end of the bed is lower than 50 cm into the receiving area, the second measurement of the main magnetic field is taken, and thus the second measurement is recorded by the magnetic field sensor. At this second measurement position, the second measurement and the second position of the bed are determined.

[0020] The displacement of the patient bed in the z-direction is thus the difference between the second bed position and the first bed position; the second measurement position (the expected position of the coil arrangement after the patient bed has moved, assuming the first position information is correct) is then the sum of the first measurement position according to the first position information and the displacement.

[0021] To check whether the magnetic field sensor is functioning correctly, a reference value is now determined for the second measurement position and it is checked whether the deviation between the second measurement value and the reference value is sufficiently small.

[0022] There are several ways to determine the reference value, and of course, multiple reference values ​​can be determined and multiple comparisons can be made to achieve the most robust basis for measures and the most accurate possible documentation, if any is carried out.

[0023] A convenient embodiment of the present invention provides that a first of the at least one reference value from the main magnetic field map is determined at the determined second measurement position. This means that the comparison for such a first reference value corresponds to an alignment with the main magnetic field map, which is usually extremely reliable and can generally be assumed to be fundamentally error-free.

[0024] It is important to note that various effects, such as the presence of a test object and / or other objects, the lifetime of the magnetic resonance equipment, and / or changes in the environment, can also cause changes in the main magnetic field. Such changes in the main magnetic field are often referred to as B0 drift. To detect these changes, it is already known in the art, particularly using a simple magnetic resonance measurement, to regularly determine a current magnetic resonance frequency, i.e., the Larmor frequency in the homogeneous volume, as a calibration frequency, for example, for each magnetic resonance measurement to be performed. From this calibration frequency, the strength of the main magnetic field in the homogeneous volume can be determined using the gyromagnetic ratio.To account for the effects of drifting primary magnetic fields on the primary magnetic field map and thus avoid errors in functions utilizing the primary magnetic field map, it is already known to adjust it, for example, by correcting it by transferring the absolute or relative change to positions outside the homogeneity volume. Within the scope of the present invention, it is also preferred if the primary magnetic field map is corrected before use based on a current calibration frequency measured by a magnetic resonance measurement, which corresponds in particular to the magnetic resonance frequency (Larmor frequency in the homogeneity volume). A free induction decay (FID) measurement, for example, can be performed as the magnetic resonance measurement, as is generally known.

[0025] For a second measurement position located within the homogeneity volume, it is advantageous, preferably additionally, to provide that at least one second reference value is determined from a current calibration frequency measured by magnetic resonance (MRI), which corresponds to the MRI frequency within the homogeneity volume. If it is known that the magnetic field sensor is located within the homogeneity volume during the second measurement, a second reference value can be easily determined from the current calibration frequency and the gyromagnetic ratio. Within the homogeneity volume, the B0 value that the second measurement of the main magnetic field should yield is thus known very precisely, particularly independently of any correction of the main magnetic field map due to the calibration frequency outside the homogeneity volume, as discussed above.

[0026] It should also be noted that a second measurement in the homogeneity volume is particularly advantageous because, even with smaller positional deviations, for example due to a measurement error in the first measurement, the expected field strength of the main magnetic field remains the same, and thus a deviation can be assessed particularly well.

[0027] With particular advantage, at least at a second measurement position within the homogeneity volume, both the first and second reference values ​​can be used for evaluation and form the basis for comparison. This results in a more robust verification and additional information for assessing potential error states and temporal developments. In particular, a case distinction can be made, so that, for example, only the first reference value is used outside the homogeneity volume, while within the homogeneity volume a comparison with both the first and second reference values ​​is performed. Regarding additional information, it has been shown, for instance, that the comparison with the second reference value can allow for a better assessment of how well the magnetic field sensor is calibrated and whether the calibration is changing.

[0028] It is also conceivable to prioritize the comparisons, for example, to perform a comparison with the second reference value for plausibility checks and / or to gain further information only if the comparison with the first reference value indicates an error. It can also be stipulated that, when using both the first and second reference values, the comparison with the second reference value is only performed if the comparison result with the first measurement shows no measurement error. Thus, adjustments can be made to meet specific plausibility check, robustness, and information needs.

[0029] Of course, there are also conceivable examples of implementation in which (at least in the homogeneity amplitude) only the second reference value is used.

[0030] In general, specific embodiments of the present invention may provide for the determination of an absolute or relative deviation of the second measured value from the reference value in the comparison, whereby a measurement error is detected if a limit value is exceeded by the deviation. Depending on the reference value, a distinction can be made between considering an absolute or a relative deviation. In a specific embodiment, for example, with a nominal field strength of the main magnetic field of 1.5 T, a limit value in the range of 0.08 to 0.12 T, particularly 0.1 T, can be used for the absolute deviation, especially with respect to the first measured value. In general, suitable limit values ​​can be determined experimentally on the corresponding magnetic resonance device and may vary depending on the magnetic resonance device / nominal field strength.

[0031] In this context, it is advantageous to determine a relative deviation for the second reference value when using it. In the aforementioned specific embodiment with a nominal 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 measured value has proven useful experimentally.

[0032] As previously explained, it can be particularly advantageous to evaluate the error information based on at least one action condition, the fulfillment of which triggers a corresponding action. For example, it can be specifically provided that, in the event of an error indicating a measurement error, such as exceeding at least one limit value, a visual and / or audible warning about the measurement error is issued, particularly in conjunction with a recommended course of action. This recommended course of action could, for example, refer to a necessary repair and / or replacement of the local coil assembly. Other action conditions might, for instance, initially only result in an entry in an error memory for minor deviations.In this regard, at least one action condition can be used, for example, to monitor whether there are several consecutive entries in the error memory, in order to then trigger a warning output as an action.

[0033] However, within the scope of the present invention, it has proven particularly advantageous to log the error information. This means that each detected error is entered into a log file. This information can be used for long-term monitoring, for example, by identifying trends and the like. In particular, the error information of the present invention can be collected and evaluated together with other information acquired during field use of the magnetic resonance device in order to further develop the magnetic resonance device and / or promote new developments or determine their necessity.

[0034] As already mentioned, the initial position information can be used to implement various functions in the magnetic resonance imaging (MRI) system, as is generally known. For example, the position information can be used to generate a display showing the position of local coil arrays on the patient table. Furthermore, the control unit can also be configured to move the patient table into the patient reception area based on this initial position information.

[0035] Specifically and particularly preferably, it can be provided that, after a user selects one of the at least one local coil arrangement, this arrangement is positioned in an isocenter of the magnetic resonance imaging (MRI) system located within the homogeneous volume by moving the patient table into position based on the initial position information of this local coil arrangement. This significantly simplifies the positioning of local coil arrangements in the isocenter of the MRI system, for example, as a "push-button" function. In particular, a display can be generated that shows, especially schematically, the currently used local coil arrangements and their position on the patient table according to the respective initial position information. This display can be shown, for example, on a touchscreen or otherwise, so that a user can select one of the local coil arrangements to be placed in the isocenter.

[0036] It should be noted here that, as already mentioned, the determination of the error information can preferably be performed for each such positioning operation. In particular, it can be provided that the error information is determined for all local coil arrangements for each user-selected and positioned local coil arrangement. The determination of the error information therefore need not be limited to the selected local coil arrangement, but can apply to all local coil arrangements, especially when using the first reference value, including those that are not placed within the homogeneity volume.

[0037] In addition to the method, the invention also relates to a magnetic resonance device comprising a main magnet unit with a main magnet for generating a main magnetic field and a particularly cylindrical patient receiver, wherein the field lines of the main magnetic field run in a homogeneous volume in the patient receiver in one field direction, - a patient bed for moving a patient into the patient reception area along a z-direction corresponding to the longitudinal direction of the patient bed, wherein a position determination unit for determining a bed position along the z-direction is assigned to the patient bed, - at least one local coil arrangement for placement on the patient and / or the patient bed, wherein the local coil arrangement has a magnetic field sensor, in particular a Hall sensor, for measuring measured values ​​of at least the magnetic field strength of the main magnetic field, and - a control device which is designed to determine a first position information of the local coil arrangement describing a first measurement position by comparing a first measured value of the magnetic field sensor with a main magnetic field map at least outside the patient recording area, which is characterized in that the control device is designed to carry out a method according to the invention.

[0038] All descriptions relating to the method according to the invention can be applied analogously to the magnetic resonance device according to the invention, with which the aforementioned advantages can therefore also be obtained. In particular, the control device can comprise at least one storage medium and at least one processor. Functional units, especially for carrying out steps of the method according to the invention, can be formed by hardware and / or software.

[0039] Specifically, the control unit may, for example, have the following features: - a measuring unit for recording the first and second measured values ​​by appropriate control and reception from the magnetic field sensor, - a first measurement position determination unit for determining the first position information from the first measurement value and the main magnetic field map, - a bed position determination unit for determining the bed position using the position determination device, - a second measurement position determination unit for determining the second measurement position from the first position information and the lying positions, - a reference value determination unit for determining the reference value, and - a comparison unit for determining the error information.

[0040] Further functional units may include, for example, an action unit for checking action conditions and carrying out the corresponding actions, an output unit for displaying information, a user interaction unit for receiving user input / outputting displays, and a stretcher control unit for controlling the patient stretcher (or the actuators driving it) for moving it into and out of the patient admission area. If the control unit is also designed to control the other operations of the magnetic resonance imaging (MRI) system, in particular the admission process, then, as is generally known, a sequence unit for controlling the admission process, a reconstruction unit for determining MRI image data sets, and the like may also be provided.

[0041] A computer program according to the invention can be directly loaded into a storage medium of a control unit of a magnetic resonance device and comprises program elements which, when the computer program is executed on the control unit, cause it to perform the steps of a method according to the invention. The computer program can be stored on an electronically readable data carrier according to the invention, which therefore contains control information stored thereon, comprising at least one computer program according to the invention and designed such that, when the data carrier is used in a control unit of a magnetic resonance device, the latter is configured to perform a method according to the invention. The data carrier can, in particular, be a non-transient data carrier, for example, a CD-ROM.

[0042] Further advantages and details of the present invention will become apparent from the exemplary embodiments described below and from the drawings. These show: Fig. 1 a flowchart of an embodiment of the method according to the invention, Fig. 2 an illustration of measurement at a first measuring position, Fig. 3 an illustration of measurement at a second measuring position, Fig. 4 a magnetic resonance device according to the invention, and Fig. 5 the functional structure of a control unit of the magnetic resonance device.

[0043] Fig. Figure 1 shows a flowchart of an embodiment of the inventive method for monitoring the function of a magnetic field sensor, here a three-dimensional Hall sensor, and a local coil arrangement of a magnetic resonance imaging (MRI) device. If the local coil arrangement is positioned on a patient table and / or a patient placed on the patient table, while the patient table is in a position extended from the patient receptacle of the MRI device, the local coil arrangement is located outside the homogeneity volume of the MRI device. The patient table is movable along a z-direction corresponding to its longitudinal direction, which in this case also corresponds to the field direction of the main magnetic field (B0 field) in the homogeneity volume.The magnetic field sensor can measure the main magnetic field (B0 field) of the magnetic resonance device, and a control unit can use a main magnetic field map to determine position information from a corresponding measurement, describing the current position of the local coil arrangement, at least with respect to the z-direction. In this case, such initial position information, determined at a first measurement position of the local coil arrangement outside the homogeneity volume, can be used to implement targeted movement of the patient table such that the local coil arrangement is positioned at the isocenter of the magnetic resonance device, particularly within the framework of a "push-button" function.

[0044] However, the execution of such a target run with measurement at the first measurement position is also used here to check the function of the magnetic field sensor in a simple and cost-effective manner.

[0045] In step S1, the first measurement of the main magnetic field at the first measuring position, which is required for the function anyway, is performed. The result is a first measured value for the main magnetic field, in particular at least for its amplitude (field strength).

[0046] In step S2, the first measured value is compared with a map of the main magnetic field (corrected with respect to a currently measured calibration frequency) also outside the patient recording area in order to determine the corresponding first measurement position and thus the first position information of the local coil arrangement.

[0047] Fig. Figure 2 schematically illustrates the situation in the first measurement position. The patient bed 1 is in an extended first position 4, mostly outside the patient receptacle 2 within the main magnet unit 3. This is a so-called close-to-home position, meaning the patient bed 1 does not protrude more than 30 cm into the patient receptacle 2.

[0048] Thus, a local coil arrangement 5 with a magnetic field sensor 6, here a three-dimensional Hall sensor, arranged on the patient bed 1 or on a patient (not shown) placed on the patient bed 1, is also located outside the homogeneity volume around the isocenter 7, which in this case also defines the zero point of the z-direction 8b, which here also corresponds to the field direction 8a. The magnetic field sensor 6 is located at a first measuring position 9 at a distance 10 from the isocenter 7.

[0049] The schematic sketch also shows the profile 11 of the main magnetic field strength. At the first measurement position 9, the magnetic field sensor 6, assuming a correct measurement, thus measures a first measured value 12 of the main magnetic field. Since the profile 11 is known from the main magnetic field map, the first measurement position 9 can be determined from the first measured value 12 and is stored here as the first position information.

[0050] Returning to Fig. In step S3, the first position 4 of the patient bed 1 is also determined by a position detection device assigned to the patient bed 1. The precise, robust determination of the position 4 can be based on corresponding feedback from the actuators regarding the movement of the patient bed 1 along the z-direction 8b, on the control history, and / or dedicated sensors. The corresponding acquisition means are included in the position detection device.

[0051] In step S4, the patient bed 1 (with the fixed local coil assembly 5 mounted on it) is moved into the patient reception area 2. The reason for this movement could be, for example, the use of a function for positioning the local coil assembly 5 at the isocenter 7. To implement this function, for example as a "push-button" function, a display is generated based on the initial position information for all local coil assemblies 5 arranged on the patient bed 1, such as a schematic representation of the local coil assemblies 5 on the patient bed 1. This is displayed on a touchscreen, which could be located, for example, on a cover of the main magnet unit 3 at the front from which the patient bed 1 enters. By selecting one of the local coil assemblies 5 or...Using one of these representing control elements, a user can select the local coil arrangement 5, which, due to the known distance 10 to the isocenter 7, is moved into the isocenter 7 by appropriately controlling the actuators of the patient bed 1.

[0052] The corresponding situation that arises after the end of the journey is in Fig. Figure 3 shows a schematic representation. The magnetic field sensor 6 is now located at a second measuring position 13, which corresponds to the isocenter 7, although this is not necessarily the case. The patient bed 1 is located at a second position 14, which is offset from the first position 4 by a distance of 10.

[0053] Thus, as diagram 11 again illustrates, the local coil arrangement 5 and with it the magnetic field sensor 6 are now located in the homogeneity volume of constant magnetic field strength, so that an unambiguous position determination, i.e. a determination of a second position information, from the second measured value 15 for the main magnetic field, which is obtained in a step S5 according to Fig. 1 is recorded, which is not reasonably possible.

[0054] However, this second measurement is still carried out in order to check the function of the magnetic field sensor 6. For this purpose, in step S6, the second lying position 14 is again determined using the position determination device.

[0055] In step S7, as preparation for determining the reference value, the second measurement position 13 is determined by shifting the first measurement position 9 according to the first position information by the displacement distance of the patient bed 1 (which results directly from the difference between the first and second bed positions 4, 14). In this case, this second measurement position 13 lies within the homogeneity volume.

[0056] In step S8, at least one reference value can then be determined, with which the second measured value 15 is compared in order to verify the correct function of the magnetic field sensor 6. The reference value thus corresponds to the expectation based on the second measurement position 13, as determined in step S7.

[0057] In the present embodiment, a first reference value is determined in every case by retrieving the expected main magnetic field from the main magnetic field map (corrected according to the calibration frequency as described above) at the second measurement position 13. This first reference value is thus compared with the value expected according to the main magnetic field map. However, a case distinction is also made in step S8. Only if the second measurement position 13 is located within the homogeneity volume is a second reference value also determined from the calibration frequency, which is derived from a current measurement and corresponds to the magnetic resonance frequency in the homogeneity volume, by simple division by the gyromagnetic ratio.

[0058] In step S9, error information is determined by comparison with the available reference values. For the first reference value, an initial deviation (here absolute) is calculated, i.e., the magnitude of the difference between the first reference value and the second measured value 15. An error, i.e., a measurement error, can be detected if the initial deviation exceeds a first limit value, here, for example, a first limit value of 0.1 T for a nominal field strength of the main magnetic field of 1.5 T. Both the initial deviation and the result of the comparison with the limit value are stored in the error information.

[0059] If the second measurement position 13 is located within the homogeneity volume, a second reference value is also available, for which a second, relative, deviation is determined. This means the second deviation is calculated as the sum of the reference values ​​of the second reference value and the second measurement 15, divided by the second reference value. If this second, relative deviation is greater than a second limit value, in this example 0.07 (i.e., 7%) for the case mentioned above, another error can be detected. Both the second deviation and the comparison result are again stored in the error information.

[0060] It should be noted that embodiments are also conceivable in which only the second reference value is used at the second measurement position 13 in the homogeneity volume. Furthermore, staged comparisons, particularly with several first and / or second limit values, are also conceivable.

[0061] In step S10, the detected error information is stored in a log file in a storage medium of the control unit of the magnetic resonance device or in an external storage medium, in particular together with other operating data of the magnetic resonance device. There, it can be used for later analysis.

[0062] In step S11, the fulfillment of at least one action condition is checked. Each action condition is assigned a corresponding action. For example, an action condition can check whether one or both comparisons indicate an exceedance of the first or second limit value. This can then be followed by an action to be carried out in step S12, which may include a visual and / or audible warning output and a recommended action, in this case, replacing the local coil assembly 5. Other action conditions can, for example, check whether an entry should be made in an error log.

[0063] If no action condition is met or all assigned actions have been executed, the process returns to step S1 for the next target journey, i.e., the next use of the positioning function when using the magnetic field sensor 6. In other words, the function check of the magnetic field sensor 6 is performed for each determination of initial position information followed by a journey of the patient stretcher 1 into the patient admission area 2.

[0064] It should be noted again at this point that the second measurement position 13 does not necessarily have to be in the homogeneity volume, because a functional check of the magnetic field sensor 6 is still possible, since an expectation for the second measured value 15 also results from the main magnetic field map.

[0065] Fig. Figure 4 shows a schematic diagram of a magnetic resonance device 16 according to the invention. This comprises, as already shown in the schematic diagrams of the Fig. 2 and Fig. Figure 3 indicates a main magnet unit 3 with a cylindrical patient receptacle 2 into which the patient bed 1 can be inserted. The main magnet unit 3 also includes the main magnet 17, which is only indicated here and may be superconducting or designed as a permanent magnet.

[0066] The position determination device 18 for determining the lying position of the patient bed 1 is also indicated. On a touchscreen 20 attached to the front 19 of a cover of the main magnet unit 3, for example, a representation of the patient bed 1 with the local coil arrangements 5 arranged on it can be displayed, in order to make local coil arrangements 5 selectable for positioning in the isocenter 7 by interaction with the representation and / or control elements.

[0067] The operation of the magnetic resonance device 16 is controlled by a control unit 21. The functional structure of the control unit 21, which is used to carry out the procedure according to Fig. 1 is trained, with reference to this procedure shows Fig. 5 more precisely.

[0068] Accordingly, the control unit 21 initially comprises a storage medium 22 in which various pieces of information, including information about the procedure, can be stored. Furthermore, the control unit 21 comprises a measuring unit 23 for recording the first and second measured values ​​12, 15 according to steps S1 and S5, a first measuring position determination unit 24 for determining the first position information from the first measured value 12 and the main magnetic field map according to step S2, a lying position determination unit 25 for determining the first and second lying positions 4, 14 according to steps S3 and S6, a second measuring position determination unit 26 for determining the second measuring position 13 from the first position information and the lying positions 4, 14 according to step S7, a reference value determination unit 27 for determining at least one reference value according to step S8, and a comparison unit 28 for determining the error information according to step S9.This is also designed to store the error information in the log file according to step S10; alternatively, a logging unit can also be provided.

[0069] Furthermore, as part of the control unit 21, a control unit 29 is also included for checking control conditions and for carrying out the corresponding measures in accordance with steps S11 and S12, and a user interaction unit 30 for receiving user input and for outputting displays and / or instructions. Finally, a stretcher control unit 31 is also provided for controlling the patient stretcher 1 (or the actuators driving it) for movement into and out of the patient admission area 2, in particular also in accordance with step S4.

[0070] Since the control unit 21 is also designed to control the other operation of the magnetic resonance device 16, corresponding additional functional units, which are not shown here, are also provided.

[0071] Although the invention has been illustrated and described in detail by the preferred embodiment, the invention is 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.

[0072] Regardless of the grammatical gender of a particular term, persons with male, female or other gender identities are included.

Claims

[1] Method for operating a magnetic resonance device (16), comprising - a main magnet unit (3) with a main magnet (17) for generating a main magnetic field and a particularly cylindrical patient receptacle (2), wherein the field lines of the main magnetic field run in a homogeneity volume in the patient receptacle (2) in a field direction (8a), - a patient bed (1) for moving a patient into the patient receptacle (2) along a z-direction (8b) corresponding to the longitudinal direction of the patient bed (2), wherein the patient bed (1) is assigned a position-determining device (18) for determining a bed position along the z-direction (8b), - at least one local coil arrangement (5) for placement on the patient and / or the patient bed (1), wherein the local coil arrangement (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) which is designed to determine a first position information item of the local coil arrangement (5) describing a first measuring position (9) by comparing a first measured value (12) of the magnetic field sensor (6) with a main magnetic field map at least outside the patient receptacle (2), characterized by that the procedure includes: - recording the first measured value (12) of the magnetic field sensor (6) outside the patient receptacle (2) at the first measuring position of the local coil arrangement (5) at a first lying position (4) of the patient bed (1) and determining the first position information, - recording a second measured value (15) of the magnetic field sensor (6) during or after completion of a movement of the patient couch (1) into the patient receptacle (2) at a second measuring position (13) of the local coil arrangement (5) different from the first measuring position (9), in particular located within the patient receptacle, at a second lying position (14) of the patient couch (1), - determining at least one reference value which describes the expected main magnetic field at the second measuring position (13), which is determined from the first position information and the displacement distance of the patient bed (1) between the first and the second bed position (4, 14) according to the position determination device (18), and - Determining error information indicating a potential measurement error by comparing the second measured value (15) with the reference value, wherein an absolute or relative deviation of the second measured value (15) from the reference value is determined in the comparison, wherein a measurement error is detected if a limit value is exceeded by the deviation. [2] Method according to claim 1, characterized by that a first of the at least one reference value is determined from the main magnetic field map at the determined second measuring position (13). [3] Method according to claim 1 or 2, characterized by that the main magnetic field map is corrected before use using a current calibration frequency measured by a magnetic resonance measurement. [4] Method according to one of the preceding claims, characterized bythat, with the second measuring position (13) located within the homogeneity volume, at least a second of the at least one reference value is determined from a current calibration frequency or the current calibration frequency measured by a magnetic resonance measurement, which corresponds to the magnetic resonance frequency in the homogeneity volume. [5] Method according to one of the preceding claims, characterized by that when using the second reference value, a relative deviation is determined for the second reference value. [6] Method according to one of the preceding claims, characterized by that in the case of error information indicating a measurement error, an optical and / or acoustic indication of the measurement error is output, in particular together with a recommendation for action, and / or the error information is logged. [7] Method according to one of the preceding claims, characterized bythat after a user-side selection of one of the at least one local coil arrangement (5), this is arranged in an isocenter (7) of the magnetic resonance device (1), which is located in the homogeneity volume, by moving the patient bed (1) on the basis of the first position information of this local coil arrangement (5). [8] Method according to claim 7, characterized by that the error information for each user-side selection and positioning of a local coil arrangement (5) is determined for all local coil arrangements (5). [9] Magnetic resonance device (16), comprising - a main magnet unit (3) with a main magnet (17) for generating a main magnetic field and a particularly cylindrical patient receptacle (2), wherein the field lines of the main magnetic field run in a homogeneity volume in the patient receptacle (2) in a field direction (8a), - a patient bed (1) for moving a patient into the patient receptacle (2) along a z-direction (8b) corresponding to the longitudinal direction of the patient bed (2), wherein the patient bed (1) is assigned a position-determining device (18) for determining a bed position along the z-direction (8b), - at least one local coil arrangement (5) for placement on the patient and / or the patient bed (1), wherein the local coil arrangement (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) which is designed to determine a first position information item of the local coil arrangement (5) describing a first measuring position (9) by comparing a first measured value (12) of the magnetic field sensor (6) with a main magnetic field map at least outside the patient receptacle (2), characterized bythat the control device (21) is designed to carry out a method according to one of the preceding claims. [10] Computer program which, when executed on a control device (21) of a magnetic resonance device (16), causes the device (16) to carry out the steps of a method according to one of claims 1 to 8. [11] An electronically readable data carrier on which a computer program according to claim 10 is stored.

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