Determining the location of devices in an MRT system

Abstract

To determine a position of a device in an MRT system.SOLUTION: According to a method for determining a position of a device inside an imaging volume 3 of an MRT system 1, an imaging volume is encircled by a field magnet and a gradient coil 2 for creating a static basic magnetic field along a vertical axis z, and the device has a first conductor loop 4 formed in a loop plane. An AC magnetic field inside the imaging volume is created by the gradient coil. At least one measurement value is determined by at least one first conductor loop, and the measurement value depends on induction voltage inducted by a constituent of the AC magnetic field orthogonal to the vertical axis z in at least one conductor loop. A position of the device inside the imaging volume is at least partially determined as at least one measurement value and a function of a prescribed magnetic model with respect to the gradient coil.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The invention relates to a method for determining the position of a device inside an imaging volume of an MRT system, the imaging volume being surrounded by field magnets for the generation of a static basic magnetic field along a longitudinal axis and by gradient coils of the MRT system. The invention further relates to a corresponding MRT system. [Background technology]

[0002] A magnetic resonance imaging system, abbreviated as MRT system, is an imaging device that uses a strong external magnetic field to align the nuclear spins of an object under examination and then apply RF excitation pulses to cause them to precess about the corresponding alignment. The precession, or transition of the spins from an excited state to a state with lower energy, generates an electromagnetic alternating magnetic field in response, which can be detected as an MR signal via a receiving antenna.

[0003] Magnetic field gradients can be used to apply a position encoding onto the signals, which can then be assigned to volume elements of the examination object, and the received signals can then be evaluated, for example to create a pictorial representation of the examination object.

[0004] In many MRT applications it is advantageous to know the location, especially the position and / or orientation, of a device within an imaging volume defined by gradient coils, for example with respect to the patient's anatomy. Since the patient's position within the imaging volume can be defined or determined, for example by visual markers, it is desirable to determine the location of the device as accurately as possible. The device may comprise, for example, a local MR receiving coil that is placed directly on the patient, such as a head coil, a knee coil, etc. However, it may also comprise devices for the treatment of the patient, such as catheters, surgical instruments, biopsy needles, robotic arms, etc.

[0005] Patient movement during MRT examinations is a known problem, especially in time-consuming MRT examinations. Patient movement can alter the measured signal and cause image artifacts, which can hinder or obscure the recognition of important features, i.e., especially radiological findings. Especially against this background, the determination of the device's position is advantageous.

[0006] One method for recognizing the position of the receiving coil uses a Hall sensor integrated into the coil electronics that measures the local strength of the static magnetic field. Outside the imaging volume, the static magnetic field is highly inhomogeneous and has a strong static gradient field. The measurements of the Hall sensor can be used to establish the position of the receiving coil as it and the patient move from the patient table into the imaging volume. However, as soon as the receiving coil is located within an imaging space characterized by a highly homogeneous magnetic field, the signal of the Hall sensor remains essentially constant, even if the coil position changes within the imaging volume. As a result, changes in the position of the receiving coil due to patient movement may remain undetected.

[0007] In other methods, a camera is used to recognize the position of the receiving coil on the patient's body before the patient brings the patient table into the imaging volume. A further camera in the imaging volume can recognize the movement of the receiving coil during the examination. However, this makes a clear line of sight between the camera and the receiving coil an absolute requirement, which may be hindered, for example, by covers, other accessories, support elements or dielectric pads for improving the RF environment, or by the dimensions of the patient's extremities. Summary of the Invention [Problem to be solved by the invention]

[0008] It is an object of the present invention to reliably determine the position of a device within the imaging volume of an MRT system.

[0009] This object is achieved by the subject matter of the respective independent claims. Advantageous developments and preferred forms of embodiment are the subject matter of the dependent claims.

[0010] The invention is based on the knowledge that gradient coils generate a magnetic field inside the gradient coil and thus inside the imaging volume, which not only contains a component parallel to the basic magnetic field, but always also contains a significant component perpendicular to it, which components are detected according to the invention by at least one conductor loop and used to determine the position of the conductor loop and thus the position of the device. [Means for solving the problem]

[0011] According to one aspect of the invention, a method for determining a position of a device within an imaging volume of an MRT system is specified. The MRT system of the technical field has a field magnet for creating a static basic magnetic field along a longitudinal axis of the MRT system, as well as a gradient coil surrounding the imaging volume. The device has at least one first conductor loop formed in a first loop plane. An alternating magnetic field is generated in the imaging volume by the gradient coil, and a first measurement is determined by the at least one first conductor loop. The first measurement is dependent on a first induced voltage induced in the at least one first conductor loop by a first component of the alternating magnetic field perpendicular to the longitudinal axis. The position of the device within the imaging volume is determined, at least in part, as a function of the at least one first measurement and a predefined magnetic field model for the gradient coil.

[0012] In particular, the imaging volume includes a region within the MR scanner of the MRT system, in particular within the so-called patient tunnel, also called the lumen, which is essentially defined by the gradient coils and, if necessary, by the RF transmit coils arranged radially within the gradient coils to transmit the radio frequency alternating magnetic field. The imaging volume is therefore defined as a volumetric region in which the examination object can in principle be imaged, in particular when the RF transmit coils are also used as receive coils. If a local MR receive coil is used, this is in particular located within the imaging volume. This means that the local MR receive coil does not define the imaging volume, but if necessary further defines an imaging volume inside the local MR receive coil.

[0013] At least one conductor loop may be, for example, part of a local MR receiving coil, in other words the device may then correspond to a local MR receiving coil. However, in other forms of embodiment the device is embodied as a dedicated sensor device, in particular independent of a possible local MR receiving coil. Determining the position of the device in this case makes it possible to estimate the position of a further examination object, for example of a patient, or of a part of the patient's body, or of a medical tool, if its relative position with respect to the device is correspondingly predetermined or defined.

[0014] In order to at least partially determine the position of the device, the position of the at least one conductor loop is determined, in particular at least partially, depending on the at least one measurement value and the magnetic field model. The position of the at least one conductor loop can be the same as the position of the device or the position of the device can be derived from the position of the at least one conductor loop.

[0015] Also, the at least one conductor loop may have a finite extent in a direction perpendicular to the first loop plane. The fact that the at least one conductor loop is formed in the first loop plane may be interpreted as all conductor loops of the at least one first conductor loop being parallel to each other and in each case parallel to the loop plane.

[0016] The position of the at least one conductor loop and accordingly the position of the device can be determined in a given coordinate system, for example a fixed point coordinate system, with respect to the gradient coil. The longitudinal axis of the MRT system can be interpreted, for example, as the z direction of this coordinate system and the corresponding directions perpendicular to the z direction as the x and y axes of the coordinate system. However, other reference systems can also be selected, if these are advantageous for further use. The term position can in particular be understood as a combination of a three-dimensional position and a three-dimensional orientation in the corresponding reference system. In other words, the position can also be called a pose. For example, the position can be given by the three-dimensional coordinates of a reference point of the device and three orientation angles of a specific reference direction of the device. In order to completely determine the position, in particular the three-dimensional position and the three-dimensional orientation of the device must be determined.

[0017] The fact that the position of the device is at least partially determined by the method of the invention can be understood in such a way that the position is completely or incompletely determined. An incomplete determination of the position can be understood, for example, as only individual coordinates of a three-dimensional position or individual angles of a three-dimensional direction being determined, but not all three coordinates of the position and all three angles of the direction. As an alternative, the partial determination of the position of the device can consist of one or more limitations of six degrees of freedom, i.e. three coordinates and three directions, which are explicitly or implicitly determined, where the limitations are beyond the placement of at least one first conductor loop in the imaging volume.

[0018] For example, at least a partial determination of the position thus makes it possible to determine at what distance the reference point of the device is located from the center of the imaging volume, i.e. to the longitudinal axis, and at this distance, in what angular range the corresponding orientation means is located, etc. Whether the position is completely or partially determined and how much information is determined with respect to the position of the device depends on what additional information is available and what it is, as well as on at least one measured value and a magnetic field model for position determination. However, it should be pointed out that even without such additional information, a partial determination of the location is possible solely on the basis of at least one measured value and a magnetic field model. However, in principle, further information is required to completely determine the position, which can be determined, for example, as a result of construction limitations or possibly by further sensor systems such as Hall sensors, further conductor loops, cameras, etc.

[0019] The magnetic field model can include the magnetic field generated by the gradient coil inside the imaging volume, for example, in a spatially resolved form and in all three spatial directions, or in any event in the three spatial dimensions. For example, also in a spatially resolved form and in a time resolved form. Thus, the magnetic field model depends on the geometrical and electrical properties of the gradient coil and possibly on the activation of the gradient coil. The magnetic field model can be determined, for example, by measuring the magnetic field inside the imaging volume and / or by simulation and / or other calculations, and can be stored on the MRT system, on at least one evaluation unit of the specific MRT system.

[0020] The gradient coils of the MRT system are designed in such a way that inside the imaging volume, mainly gradient magnetic fields of the longitudinal magnetic field can be generated. The gradient magnetic fields correspond to a change in magnetic field strength in one of three spatial directions. The MRT system in this case has, as a rule, three such gradient coils, of which the magnetic field strength changes in each case in different spatial directions. However, in theory, only for infinitely large coils, for example, are magnetic fields oriented exclusively along the longitudinal axis possible. In each practical implementation of the gradient coils, the magnetic field generated by said coils in the imaging volume always has components in all three spatial directions. A pronounced symmetrical point in the exact center of the imaging volume can possibly be an exception, but this is only of theoretical significance. This knowledge and its use for the determination of the position of the device in the imaging volume of the MRT system are the basis of the present invention.

[0021] In particular, as a result of the situation outlined above, independently of the orientation of the loop plane with respect to the longitudinal axis, it is always possible to measure the corresponding induced voltage or the corresponding induced current, and together with the simultaneous knowledge of the magnetic field model, it is possible to make an estimate as to the position of at least one conductor loop. In this way, the complexity of the means for determining the position of the device, i.e., in particular, the device itself, can be reduced.

[0022] In particular, the invention allows the local MR receiving coil itself to be used to determine the position of the MR receiving coil, despite the fact that the conductor loops of the local MR receiving coil are in principle parallel or not perpendicular to the basic magnetic field. This allows a particularly advantageous synergistic combination of the underlying function of the local MR receiving coil, namely the detection of MR signals from the examination object, with the additional function of determining the position. It should be pointed out that in this case the receiving coil cannot only acquire the high-frequency alternating magnetic field of the transmitting coil or the nuclear spin resonance signals reacting to these, but also the signals of the gradient coil, which may have a much lower frequency, which must be created during the MRT examination in any event to create an alternating magnetic field. It is therefore not absolutely necessary that the alternating magnetic field of the gradient coil is created exclusively for the position determination. This is indeed possible, but it is advantageous to be able to use the gradient pulses generated in any event.

[0023] In case the device comprises a position sensor and is embodied as a dedicated sensor system independent of the local MR receiving coil, the position sensor can be embodied more simply according to the invention, for example by a position sensor having only at least one first conductor loop, but additionally no further conductor loops perpendicular to the loop plane, but this does not exclude a number of such position sensors provided at different positions in the imaging volume for the most complete possible position determination of the device.

[0024] If the position of the device is known or at least narrowed down, for example, the user of the MRT system can automatically be informed whether the position deviates from the desired position or from the optimal position. Above and below this, algorithms can be employed which automatically select one or more scan parameters, such as for example the acceleration factor R, the phase encoding direction, etc., as a function of the at least partially determined position of the device, so that a given position or example of the device for a patient can be used in an optimal way. As an alternative or in addition, the image reconstruction algorithm can also use the at least partially determined position in turn to improve the resulting image quality, for example through a more accurate estimation of the coil sensitivity of the local MR receiving coils.

[0025] According to at least one embodiment of the method of the invention, an MR image can be produced as a function of MR signals from an examined object in an imaging volume.

[0026] During the creation of the MR image, the at least partially determined position of the device can be taken into account automatically or manually. For example, the MR recording can be repeated or partially repeated if the position does not correspond to the given requirements or to the given expectations. In order to create the MR image, more motion compensation algorithms can be automatically executed depending on the at least partially determined position.

[0027] To create the MR signals, known MR sequences can be applied, where, for example, a global RF transmit coil surrounding the imaging volume emits corresponding RF pulses into the imaging volume and a gradient coil creates a sequence of gradient magnetic fields in the imaging volume, which causes nuclear spin resonance in the examination object, and the generated radio frequency signals can be detected by the RF transmit coil (if it is used as a receive coil) and / or by one or more local MR receive coils.

[0028] According to at least one aspect of this embodiment, MR signals from an examination object are detected in an imaging volume by at least one first conductor loop and an MR image is produced as a function of the MR signals.

[0029] In other words, the at least one first conductor loop is used not only for producing the at least one measurement value and therefore for at least partially determining the position of the device, but also as a proper local MR receiving coil, for which the device or the at least one conductor loop is in particular part of the local MR receiving coil.

[0030] In principle, the MR signal is also detected via a corresponding induced voltage in at least one conductor loop. The induced voltage resulting from the MR signal and the alternating magnetic field of the gradient coil, employed for the determination of at least one measurement value, can be separated from each other, in time or otherwise. For example, the fact that the frequency of the signal generated by the transmission coil, and accordingly the frequency of the MR signal, is a multiple greater than the frequency of the alternating magnetic field generated by the gradient coil, can be used. Thus, frequency filtering can be performed in order to separate from each other the detection of the MR signal, in particular from the detection of the alternating magnetic field generated by the gradient coil.

[0031] The frequency of the MR signal in this case corresponds in particular to the corresponding Larmor frequency of the nuclei used for imaging. This lies, for example, in the frequency range of 1 MHz to 500 MHz, depending on the basic magnetic field strength of the field magnet of the MRT system. On the other hand, the pulsed gradient fields of the gradient coils, i.e. the pulsed gradient fields of the alternating magnetic field created by the gradient coils, have frequencies in the range of several kHz or several tens of kHz.

[0032] According to at least one embodiment in which the MR signal is detected by at least one first conductor loop to determine the at least one first measured value, the MR signal or a further induced voltage or a corresponding signal resulting from the MR signal is suppressed, in particular by a filter circuit.

[0033] In this way what is achieved is that at least one measurement simply reflects the magnetic field of the gradient coil and not the MR signal.

[0034] On the other hand, for the determination or detection of MR signals, induced voltages resulting from the alternating magnetic fields of the gradient coils can be suppressed, for example, by a filter circuit or further filter circuits.

[0035] In this way, the detection of MR signals can be prevented from being affected by the alternating magnetic field of the gradient coil.

[0036] For example, the at least one first measurement value and the MR signal may be detected by a plurality of measurement or receiving channels, and the filter circuit and / or the further filter circuit are implemented in the corresponding measurement channels.

[0037] According to at least one embodiment, the device is positioned within the imaging volume in such a way that the loop plane is parallel to the longitudinal axis, in particular at least approximately parallel to the longitudinal axis.

[0038] In this case, the loop plane can be considered to be at least approximately parallel to the longitudinal axis when, for example, the angle between the normal direction, which is perpendicular to the loop plane, and the longitudinal axis is at least approximately 90°, i.e., for example, greater than 60° and less than 120°, or, for example, greater than 70° and less than 110°, or, for example, greater than 80° and less than 100°.

[0039] This may occur in particular if the arrangement of local MR receive coils corresponds to or is part of such a plane.

[0040] According to at least one form of embodiment, the device has at least one second conductor loop extending in a second loop plane different from the first loop plane. The at least one second measurement is determined by the at least one second conductor loop as being dependent on a second induced voltage induced by a second component of the AC field perpendicular to the longitudinal axis in the at least one second conductor loop. A position of the device is determined, at least in part, as a function of the at least one measurement, the at least one second measurement, and a magnetic field model for the gradient coil.

[0041] For example, in such an embodiment, the at least one first conductor loop and the at least one second conductor loop may be part of a local MR receive coil.

[0042] In particular, the apparatus is positioned within the imaging volume such that the second loop plane is at least approximately parallel to the longitudinal axis, and in particular such that the first loop plane is likewise at least approximately parallel to the longitudinal axis.

[0043] For example, the position of the at least one first conductor loop relative to the at least one second conductor loop may be known or predetermined as a fixed position. Thus, for at least partial determination of the position of the device, on the one hand, the position of the at least one first conductor loop and the position of the at least one second conductor loop may be determined as described. In this way, the position of the device can be more accurately determined and thereby more completely narrowed down by the predetermined or known relative positions relative to each other.

[0044] In a corresponding development, multiple further conductor loops can be used in a similar manner to the at least one first and at least one second conductor loop to enable a determination of the position of the device that is as accurate or complete as possible, where the further conductor loops can, for example, be part of a local MR receiving coil.

[0045] According to at least one form of embodiment, the device has at least one third conductor loop extending in a third loop plane. At least one third measurement value is determined by the at least one third conductor loop, the measurement value being dependent on a third induced voltage, the third induced voltage being induced by a third component of an AC field perpendicular to a longitudinal axis in the at least one third conductor loop. A first position of the at least one first conductor loop within the imaging volume is determined at least in part as a function of the at least one first measurement value and the magnetic field model, and a third position of the at least one third conductor loop within the imaging volume is determined at least in part as a function of the at least one third measurement value and the magnetic field model. A relative position of the at least one third conductor loop with respect to the at least one first conductor loop is determined as a function of the first position and as a function of the third position.

[0046] In such an embodiment, the relative position of the third conductor loop and the first conductor loop in particular is not known in advance or is not precisely known, but can be determined or approximately determined in the described manner. Such a form of embodiment is likewise particularly advantageous when the local MR receiving coil comprises at least one first conductor loop and at least one third conductor loop and is embodied, for example, as a flexible surface coil. Such a flexible surface coil may, if necessary, adapt itself to the surface of the patient or the like, so that the individual conductor loops do not have a known orientation relative to one another. In this way, the position or form of the surface of the flexible surface coil may be determined.

[0047] According to a further aspect of the invention, an MRT system is also specified, which comprises a field magnet for generating a static basic magnetic field along a longitudinal axis and a gradient coil, the field magnet and the gradient coil surrounding an imaging volume of the MRT system. The MRT system comprises an apparatus with at least one first conductor loop, the at least one first conductor loop operating in a first loop plane. The MRT system comprises a control unit configured to activate the gradient coil to create an alternating magnetic field in the imaging volume. The MRT system comprises a measurement unit connected to the at least one first conductor loop. The measurement unit is configured to determine at least one first measurement value in response to a first induced voltage induced by a component of the alternating field perpendicular to the longitudinal axis in the at least one first conductor loop. The MRT system comprises at least one evaluation unit, which is configured to determine a position of the apparatus within the imaging volume at least in part as a function of the at least one first measurement value and a predefined magnetic field model for the gradient coil.

[0048] In different forms of the control unit, the measuring unit and / or the at least one evaluation unit can be provided separately from one another or can be partially or completely combined.

[0049] According to at least one form of embodiment, the MRT system has a local MR receive coil arrangement including the device.

[0050] According to at least one form of embodiment, the local receive coil arrangement is embodied as a flexible surface coil array.

[0051] The at least one first conductor loop then corresponds to a surface coil of the surface coil array.

[0052] In accordance with at least one embodiment, the MRT system includes a device for treatment of a patient, the at least one first conductor loop and the device having a predetermined spatial position relative to one another.

[0053] The elements may include, for example, biopsy needles, catheters, surgical instruments, robotic arms, and the like.

[0054] According to at least one embodiment, the device comprises a tuning capacitance, in particular a tuning capacitor arranged between a first terminal of the at least one first conductor loop and a second terminal of the at least one first conductor loop, respectively, and an inductive element arranged electrically in parallel with the tuning capacitance.

[0055] Such a form of the device is particularly advantageous when the device is part of or is the same as a local MR receive coil array.

[0056] The induced voltage is in particular present between a first terminal and a second terminal of at least one conductor loop.

[0057] The tuning capacitance can be realized as a tuning capacitor, i.e. a corresponding electronic component, or as a parasitic capacitance between the conductor segments of the at least one first conductor loop.

[0058] For example, the device may have multiple tuning capacitances disposed between a first terminal and a second terminal, where the device in particular has, for each tuning capacitance, a corresponding assigned inductive component electrically connected in parallel to the tuning capacitance.

[0059] Without the inductive component, at least one conductor loop would be non-conductive, especially for direct currents, or would have a very high impedance for low-frequency alternating currents, due to the tuning capacitance. Through the inductive element, the conductivity at low frequencies is increased. With regard to radio-frequency MR signals, on the one hand, the inductance of the inductive element effectively acts as a resistance, so that as a result of its parallel connection to the tuning capacitance, there is no significant effect during the detection of the MR signal.

[0060] In a different form of embodiment, the device can also have a detuning capacitor, in particular a detuning capacitor, which is arranged between the first terminal and the second terminal, respectively. In this case, the device has a further inductive element, which is electrically connected to the tuning capacitance and the container. Thus, the detuning capacitance also has no or little effect at low frequencies, while the further inductive element has no or no significant effect at high frequencies.

[0061] According to at least one embodiment, the measurement unit has an amplifier, which is connected, in particular directly or indirectly, to the first terminal and to the second terminal and is configured to provide at least one measured value at an output of the amplifier connected to the at least one evaluation unit.

[0062] In particular, the amplifier has first and second inputs, the first input being connected to a first terminal and the second input being connected to a second terminal.

[0063] According to at least one embodiment, the measurement unit or device has a further amplifier, which is connected to the first and second terminals as well and is configured with an output of a further amplifier, which is connected to at least one evaluation unit as well to provide the MR signal or a measurement signal dependent on the MR signal. The amplifier and the further amplifier can therefore in particular be corresponding parts of a first and a second measurement channel for detecting different currents acquired in at least one conductor loop.

[0064] According to at least one embodiment, the measurement unit comprises a filter circuit arranged between the first terminal and the first input of the amplifier and between the second terminal and the second input of the amplifier, the filter circuit being designed to suppress MR signals acquired by the at least one conductor loop.

[0065] The filter circuit has, among other things, a first input connected to the first terminal and a first output connected to the first input of the amplifier, and, among other things, a second input connected to the second terminal and a second output connected to the second input of the amplifier.

[0066] The filter circuit can be designed, for example, as a low-pass filter or a band-pass filter, in any case, tuned to essentially pass frequencies corresponding to the gradient coil, basic magnetic field, or gradient coil activation, of the alternating magnetic field of the gradient coil, while it essentially suppresses frequencies corresponding to the MR signals.

[0067] For example, the device can have a further filter circuit arranged between the first terminal and a first input of the further amplifier and between the second terminal and a second input of the further amplifier, the further filter circuit designed to suppress signals generated by the alternating magnetic field of the gradient coil in the at least one first conductor loop.

[0068] The second filter circuit can be designed, for example, as a high-pass filter or as a further band-pass filter. The further filter circuit is therefore in particular configured complementary to the filter circuit. In particular, a first terminal of at least one conductor loop can be connected to a first input of the further filter circuit and a second terminal of at least one conductor loop can be connected to a second input of the further filter circuit. A first output of the further filter circuit can be connected, for example, to a first input of a further amplifier and a second output of the further filter circuit can be connected to a second input of the further amplifier.

[0069] Further aspects of the embodiments of the MRT system of the present invention follow directly from the different aspects of the method of the present invention, and vice versa. Specific individual features and corresponding descriptions of the different aspects of the embodiments of the method of the present invention can be transferred analogously to the corresponding aspects of the embodiments of the MRT system of the present invention, and vice versa. In particular, the MRT system of the present invention is embodied or programmed to implement the method of the present invention. In particular, the MRT system of the present invention implements the method of the present invention.

[0070] According to yet another aspect of the present invention, an apparatus for determining a position of an MRT system is also provided, the apparatus having at least one first conductor loop formed in a first loop plane, the apparatus further having a measurement unit connected to the at least one conductor loop and configured to determine at least one first measurement value in response to a first induced voltage induced in the at least one first conductor loop.

[0071] According to at least one embodiment of the device, it has tuned capacitive and inductive components, as previously described.

[0072] Further aspects of the embodiment of the apparatus of the present invention are in accordance with other aspects than the embodiment of the MRT system of the present invention and the method of the present invention.

[0073] A computation unit may in particular be understood as a data processing device including processing circuits, and may therefore in particular process data for performing computational operations, which may include operations for performing indexed accesses to data structures such as LUTs (Look Up Tables).

[0074] The computing unit may in particular include one or more computers, one or more microcontrollers and / or one or more integrated circuits, such as one or more application-specific integrated circuits on a chip (SoC), one or more field programmable gate arrays (FPGAs) and / or one or more systems. The computing unit may also include one or more processing devices, such as one or more microprocessors, one or more central processing units, one or more graphic processing units (GPUs) and / or one or more signal processing devices, in particular one or more digital signal processing devices (DSPs). The computing unit may also include computers or other physical or virtual networks of said units.

[0075] In different forms of embodiment, the computing unit includes one or more hardware and / or software interfaces and / or one or more memory units.

[0076] The memory unit may be, for example, a dynamic random access memory or a static random access memory or a non-volatile data memory, for example, a read-only memory, a programmable read-only memory, a programmable read-only memory, a programmable read-only memory, an erasable read-only memory, an electrically erasable read-only memory, a flash memory or a flash EEPROM, a ferroelectric random access memory, a ferroelectric random access memory, a magnetic storage random access memory, a magnetic storage random access memory or a phase change random access memory (PCRAM).

[0077] At least one evaluation unit, control unit and / or measurement unit of the MRT system of the present invention may include one or more computational units according to this understanding, and may include at least one evaluation unit, control unit and / or measurement unit.

[0078] Within the framework of the present disclosure, as far as the components of the MRT system of the present invention, in particular the control unit, the measuring unit or at least one evaluation unit of the MRT system, are concerned, they are configured and designed to perform or realize a particular function, achieve a particular effect or serve a particular purpose, which is understandable through the corresponding adaptation, programming, physical embodiment, etc. of the component, beyond the principle or theoretical usefulness or the suitability of the component for this function, effect or purpose, and are in a concrete position to actually perform or realize the function, achieve the effect or serve the purpose.

[0079] A connection between two electrical or electronic components may be understood in such a manner that an electrical connection exists between the components or may be established by the actuation of one or more switching elements, unless expressly stated otherwise. In particular, unless otherwise stated, the components may be directly or indirectly connected to one another. In such cases, a direct connection may be understood to mean that, apart from any one or more switching elements, there are no further electrical or electronic components disposed between the components, whereas an indirect connection may be understood to mean that, in addition to any one or more switching elements, there are one or more electrical or electronic components, such as resistors, capacitors, coils, etc., disposed between the components.

[0080] Further features of the invention will become apparent from the claims, the drawings and the description of the drawings. The features and combinations given above in the specification and the features and combinations given below in the description of the drawings and / or in the drawings can be included not only in the combinations specified in each case, but also in other combinations of the invention. Certain versions and combinations of features can be constituted by inventions that do not have all the features of the claims originally described. Beyond this, versions and combinations of the invention can include combinations of the invention that go beyond or deviate from the combinations of the invention described in the references of the claims.

[0081] The present invention will be explained in more detail below with the aid of specific exemplary embodiments and associated schematic drawings, in which identical elements or elements with identical functions are provided with the same reference characters, and the description of the same elements or elements with the same functions is not necessarily repeated with respect to the figures. [Brief description of the drawings]

[0082] [Figure 1] FIG. 1 is a schematic diagram of an example of an embodiment of an MRT system of the present invention. [Diagram 2] FIG. 2 is a schematic diagram of a magnetic field. [Diagram 3] FIG. 13 is a schematic diagram of an apparatus according to yet another example embodiment of the MRT system of the present invention. [Figure 4] FIG. 13 is a schematic diagram of an apparatus according to yet another example embodiment of the MRT system of the present invention. [Diagram 5] FIG. 13 is a schematic diagram of an apparatus according to yet another example embodiment of the MRT system of the present invention. [Figure 6] FIG. 13 is a schematic diagram of an apparatus according to yet another example embodiment of the MRT system of the present invention. [Figure 7] FIG. 13 is a schematic diagram of an apparatus according to yet another example embodiment of the MRT system of the present invention. [Figure 8] FIG. 13 is a schematic diagram of an apparatus according to yet another example embodiment of the MRT system of the present invention. [Figure 9] FIG. 13 is a schematic diagram of an apparatus according to yet another example embodiment of the MRT system of the present invention. [Figure 10] 1 shows a schematic diagram of a possible basic form of a local MR receive coil. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0083] FIG. 1 shows an example of an embodiment of an MRT system 1 according to the present invention.

[0084] The MRT system 1 also comprises a field magnet (not shown), which generates a static magnetic field for alignment of the nuclear spins of a sample, e.g. a patient, in an imaging volume 3 in the z-direction, which can be referred to as the longitudinal axis of the MRT system. The imaging volume 3 is characterized by having a highly homogeneous static magnetic field in the z-direction. The field magnet can comprise, for example, a superconducting magnet, which can provide a magnetic field with a magnetic flux density of up to 3 T or even more. However, for smaller magnetic field strengths, permanent magnets or electromagnets with resistive coils can also be used.

[0085] Furthermore, the MRT system 1 comprises a gradient coil 2 and a control unit 5 for activating the gradient coil 2, which is designed for spatial differentiation of the imaging volume acquired within the imaging volume 3 so as to superimpose a static magnetic field with a magnetic field whose amount may vary according to the position along all three spatial directions x, y, z. The gradient coil 2 may for example be embodied as a coil of a resistive conductor.

[0086] The MRT system 1 may for example comprise a body coil 30 as a transmitting antenna, which is designed to radiate into the imaging volume 3 a radio frequency signal which is supplied via a signal line.

[0087] The control unit 5 can also supply the gradient coil 2 and the body coil 30 with different signals. The control unit 5 can for example comprise a gradient control device which is designed to supply the gradient coil 2 via supply lines with a time-adjusted variable current in order to provide the desired magnetic gradient fields in the imaging volume 3.

[0088] The control unit 5 may also have a radio frequency unit, which is designed to generate radio frequency or excitation pulses with a predetermined time waveform, amplitude and spectral power distribution for exciting the magnetic resonance of the nuclear spins in the patient. In such a case, pulse powers in the range of several kilowatts may be used. The excitation pulses may be emitted into the patient via the body coil 30 or via one or more local emission antennas. The control unit 5 may also include a controller that may communicate with the gradient controller and the radio frequency unit via a signal bus.

[0089] The body coil 30 can also be used in some embodiments to receive resonance signals, also referred to as MR signals, emitted by the patient and output them via a signal line, and the body coil 30 in this form of embodiment can thus serve as both a receiving antenna and a transmitting antenna.

[0090] Optionally, a local MR receiving coil (not shown) of the MRT system 1 can be placed in the immediate vicinity of the patient and can be coupled via a connection line to the measurement unit 6, which can also be part of the control unit 5. Depending on the embodiment, the local coil can act as a receiving antenna as an alternative or in addition to the body coil 30.

[0091] The MRT system 1 further comprises an evaluation unit 7, which is connected to the control unit 5 and in particular to the radio frequency unit. The evaluation unit 7 can evaluate the MR signals and on the basis of which an MR image can be reconstructed according to known methods. The control unit 5 can also be part of the evaluation unit 7.

[0092] The MRT system 1 comprises an apparatus with at least one conductor loop 4, the loop surface of which operates in a loop plane and is in particular arranged within the imaging volume 3 such that the loop plane is oriented essentially parallel to the z-direction.

[0093] As mentioned above, the gradient coils 2 are activated by the control unit 5 to generate an alternating magnetic field in the imaging volume 3. This alternating magnetic field generally has field components in all three spatial directions x, y, z. As a result, an induced voltage is produced in at least one conductor loop 4, provided that the loop plane is oriented essentially parallel to the z direction.

[0094] A measurement unit 6 is connected to the at least one conductor loop 4 and is configured to determine at least one measured value as a function of the induced voltage. An evaluation unit 7 is configured to at least partially determine a position of the apparatus inside the imaging volume 3 as a function of the at least one measured value for the gradient coil 2 and a predefined magnetic field model.

[0095] In an embodiment with a local MR receive coil, this may include the device or at least one conductor loop 4. The position of the device therefore corresponds to the position of the local MR receive coil. As an alternative, the device may be used as a self-contained position sensor, which may, for example, at least partially determine the position of a medical device (not shown) within the imaging volume 3, particularly when the relative position of the medical device with respect to the at least one conductor loop 4 is known.

[0096] An example of a magnetic field model is shown diagrammatically in FIG. 2. Contrary to the usual simplifying assumptions, the gradient magnetic field generated by the gradient coil 2 in the imaging volume 3 is not aligned exactly parallel to the z-direction of the static magnetic field. Instead, the gradient coil 2 generates additional magnetic field components aligned orthogonal to z, i.e. along the x or y axis, whose amplitude is also comparable to the z-component. Since the alternating magnetic field components along the x or y axis are much smaller than the static basic magnetic field, these components can be neglected for regular MRT images. For this reason, the alternating magnetic field components along the x and y axes have not been considered in the past, even for other possible applications. Since they, unlike the static basic magnetic field, are time-dependent, they contribute significantly to the induction in the conductor loop 4 and can therefore be used for the position determination according to the invention.

[0097] FIG. 2 shows, as an example, the Cartesian components of the magnetic field as acquired inside a gradient coil 2. This gradient coil is operated in static mode, i.e. with constant current, when the basic magnetic field of the field magnet is not present. At each sampling point, three magnetic field values ​​corresponding to the three orthogonal magnetic field components Bx, By, Bz are measured with a vector magnetometer attached to a robot arm and, in turn, arranged at 480 spatial positions, which are distributed over the surface of a sphere. On the basis of these measurements, and with the help of a calibrated magnetic field model, the magnetic field at any given location in the imaging volume 3 can be calculated. At least one conductor loop 4, which is arranged in the imaging volume 3 of the MRT system 1, thus acquires the signals induced by the pulsed gradient magnetic field, even if the loop plane is essentially oriented parallel to the z-direction.

[0098] Throughout the present invention, it is not absolutely necessary to use an additional sensor for determining the position of the local MR receiving coil: instead, the conductor loop of the already existing local MR receiving coil can be employed both for the detection of the weak radio frequency MRT signal and for the detection of the signal induced by the gradient pulse in the low frequency band.

[0099] The voltage induced in a conductor loop when the magnetic flux through the area enclosed by the conductor loop is changing is produced by the integration of the change of the magnetic vector field B over the surface A enclosed by the loop, i.e. by application of Faraday's law of induction.

number

[0100] Shown in Figures 3 to 5 are schematic implementations of different examples of embodiments of the MRT system 1 of the invention, e.g. the MRT system 1 from Figure 1, of an apparatus with at least one conductor loop 4.

[0101] Through these devices, the MRT system 1 can simultaneously receive radio frequency MR signals and low frequency signals for position determination induced by pulsed gradient magnetic fields.

[0102] A multi-channel MR receiving coil can be embodied as a two-dimensional flexible array of a number of receiving elements, for example from 2 to 32 or even 64 receiving elements. Such receiving elements are shown in figures 3 to 5. The receiving element comprises at least one conductor loop 4, designed for example as at least one copper loop, and tuning capacitors 9a, 9b, arranged for example between a first terminal 21a and a second terminal 21b of the at least one conductor loop 4. Detuning means 10 can also be provided, comprising for example a detuning capacitor 12 arranged in parallel with a detuning inductance 13 and a diode 14, and a series circuit.

[0103] Said amplifier circuit 18 may further be provided, which is connected at its input side to the terminals 21a, 21b via the matching circuit 16 and at its output side to an analog-to-digital converter 19, which may be linked via a data bus 20 to the evaluation unit 7 or to a computer. The tuning capacitors 9a, 9b may for example be distributed along the at least one conductor loop 4 in order to reduce the electric field, which may otherwise occur over long line conductor segments and lead to dielectric losses and thus to a reduced signal-to-noise ratio. The capacitance of the tuning capacitors 9a, 9b is tuned to resonate with the inductance of the at least one conductor loop 4, in particular at the Larmor resonance frequency of the MRT system 1, which may have a high frequency, for example in the range of 1 MHz to 500 MHz, depending on the electric field strength.

[0104] In parallel to the tuning capacitors 9a, 9b and in each case the detuning capacitor 12 there are inductive elements 11a, 11b, 11c, so that low-frequency signals induced by the gradient fields in the range of several kHz can be obtained. The inductivity of the inductive elements 11a, 11b, 11c is selected such that for the induced radio-frequency MR signals they have a high impedance and correspond practically to an open circuit. In contrast, the electrical impedance of the inductive elements 11a, 11b, 11c at low frequencies is essentially equivalent to a short circuit closing at least one conductor loop 4 for the signals induced by the pulsed gradient fields. The values ​​of these inductions, depending on the Larmor frequency, are for example in the range from several hundred μH to several mH.

[0105] In one form of embodiment, the signal preamplifier 17 can be connected on the input side via a filter circuit 15, which can be embodied, for example, as a low-pass filter at terminals 21a, 21b and on the output side to a further input of an analog-to-digital converter 19 or to a further analog-to-digital converter (not shown). The signal induced by the pulsed gradient and acquired by the at least one conductor loop 4 is then read out via a data bus 20 and can be further used by a signal processing algorithm to extract information about the position of the at least one conductor loop 4. In the same way, the position of the further receiving elements and thus the configuration of the described flexible multi-channel MR receiving coil can be determined.

[0106] Figure 4 shows a schematic diagram of the receiving element of the device of a further form of embodiment of the inventive MRT system 1 for a newer type of MR receiving coil. This MR receiving coil uses a distributed tuned capacitance 9 instead of a discrete capacitor, which is formed by a parasitic capacitance between the conductor segments of at least one conductor loop 4. In this case, the separate conductor segments are effectively shorted by inductive elements 11a, 11b, 11c for lower frequencies, so that the conductor segments form a double loop. Also shown in Figure 4 is the supply voltage 8 for the preamplifier circuit 18.

[0107] 5 shows diagrammatically a receiving element of an apparatus in a further form of embodiment of the MRT system 1 of the invention. The receiving element is based on the receiving element shown in FIG.

[0108] The receiving element of Fig. 5 performs, in a correspondingly shaped embodiment, a function called local shimming. For this purpose, for example, the evaluation unit 7, or for example another computer connected to the receiving element, can adapt the direct current through at least one conductor loop 4 in order to compensate for local inhomogeneities of the static magnetic field. The desired digital value of the direct current is transferred, for example, via a further data bus 25 to a further digital-to-analog converter 24, the output of which outputs a corresponding signal to a constant current driver 23. The constant current driver 23 can transmit the signal to the at least one conductor loop 4, for example via a further low-pass filter 22.

[0109] The further low-pass filter 22 in this case is designed in particular to transmit a DC current value from the constant current driver 23 to the at least one conductor loop 4, and in this way blocks the low-frequency AC signals induced by the pulsed gradient field as well as the radio-frequency MR signals. The filter circuit 15 can then be embodied, for example, as a band-pass filter, which is intended to pass the AC signals induced by the pulsed gradient field and also suppress the DC component and the radio-frequency MR signals. In the same way, the receiving element with the distributed tuning capacitance 9 can be adapted as in FIG. 4.

[0110] The spatial position of the examination object in the imaging volume 3 can be determined, for example, by processing signals dependent on quadrature coils, which are attached to the examination object as a function of the voltage induced by the pulsed gradient fields. One method can start from an initial guess and then iteratively adapt the examination object position and alignment until a certain convergence criterion is met. Another method calibrates the translation matrix in a pre-training step, in which the test object is moved in steps, simultaneously acquiring an imaging volume for each step and measuring the gradient activity. These methods can also be used with the aid of the inventive method of the inventive MRT system 1 or the device of the inventive MRT system 1 to achieve the described advantages of the invention.

[0111] Thus, further methods can be used to recognize the shape and position of the local flexible MR receive coil 28 and to recognize patient motion, as shown diagrammatically in Figures 6 to 9, making use of the advantages of the invention already described.

[0112] The flexible MR receive coils 28, for example, have a relatively large number of receive elements with corresponding conductor loops, which when attached to the body of the patient 29, can change their shape to track the contours of the body of the patient 29, as shown in Figures 6 to 9, and can also move due to the respiratory movements or heartbeat of the patient 29.

[0113] Shown in Fig. 6 is a flexible MR receive coil 28 with multiple receive elements, which have corresponding conductor loops 4, which may for example be embedded in a flexible plastic material 26 and connected to a controller 27. Fig. 7 shows a schematic flexible MR receive coil 28 for imaging the head of a patient 29, Fig. 8 shows a flexible MR receive coil 28 for imaging the knee of a patient 29, and Fig. 9 shows a flexible MR receive coil 28 for imaging the abdomen of a patient 29.

[0114] To mathematically describe the flexible MR receive coil 28 with high accuracy, mathematical models of quadrilateral surfaces can be used. Quadrilateral surfaces include spheres, ellipsoids, cylinders, particularly cylinders or elliptical cylinders, elliptical paraboloids, paraboloidal cylinders, cones, hyperboloids, double-layered hyperboloids, hyperboloidal paraboloids, single-layered hyperboloids, one- or two-layered hyperboloids, etc., and are shown diagrammatically in FIG. 10 from top left to bottom right. Mathematically, a quartic surface is a graph of a quadratic equation in three variables x, y, and z. The general form of the equation is expressed as follows:

number

[0115] At this juncture, it should be pointed out that several steps of the method can also be further optimized. For a flexible MR receive coil 28 as shown in FIG. 6, the initial shape can be restricted to correspond, for example, to the surface of a cylinder or parabolic cylinder oriented along an axis. A parabolic cylinder that is symmetric along the x-axis can be expressed by the very simple formula:

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[0116] For other MR receive coils 28, one or more hyperbolic paraboloid shapes may be more suitable. In this simplified formula:

number

[0117] This type of pre-optimization speeds up the conversion speed of the iterative algorithm in that it reduces the number of coefficients A to J and sets a starting point closer to the final solution. Similar considerations apply to the coil offset and coil rotation: the numerical range within which these parameters may vary can now be limited and in this way the iterative algorithm is forced to stay close to the final solution.

[0118] The invention may also be advantageously applied to wireless coils that combine an on-coil analog-to-digital converter with wireless digital transmission.

[0119] The above mentioned methods are varied by known methods for recognizing patient motion, such as by Hall sensors, 2D or 3D video cameras or MR mobile navigators, and can be combined to further refine and improve the results.

Claims

1. A method for determining a position of a device within an imaging volume (3) of an MRT system (1), comprising: The imaging volume (3) is surrounded by field magnets for generating a static basic magnetic field along a longitudinal axis (z), gradient coils (2) of the MRT system (1) and a body coil (30) for radiating radio frequency signals into the imaging volume (3), The device comprises a local receiving coil for receiving MR signals from an examination object and at least one first conductor loop (4) formed in a first loop plane, the first conductor loop being part of the local receiving coil; The gradient coil (2) generates an alternating magnetic field in the imaging volume (3), At least one first measurement value is determined by the at least one first conductor loop (4), the at least one first measurement value being dependent on a first induced voltage, the first induced voltage being induced by a first component of an alternating magnetic field perpendicular to the longitudinal axis (z) in the at least one first conductor loop (4), a position of the device within the imaging volume (3) is determined at least in part as a function of at least one first measurement and a predefined magnetic field model; The method is characterized in that the MR signals in the imaging volume (3) are detected by the local receiving coil including the at least one first conductor loop (4) and an MR image of the examination object is created as a function of the MR signals.

2. The method of claim 1 , wherein the MR signal is suppressed to determine the at least one first measurement.

3. 3. The method of claim 1 or 2, wherein the device is positioned within the imaging volume (3) such that the first loop plane is at least approximately parallel to the longitudinal axis (z).

4. the device having at least one second conductor loop formed in a second loop plane; At least one second measurement is determined by the at least one second conductor loop, the at least one second measurement being dependent on a second induced voltage induced by a second component of an alternating magnetic field perpendicular to the longitudinal axis (z) in the at least one second conductor loop; 3. The method of claim 1, wherein the position of the device is determined at least in part as a function of the at least first measurement, the at least one second measurement, and the magnetic field model for the gradient coil (2).

5. The device is configured such that the second loop surface is at least approximately parallel to the longitudinal axis (z). The method of claim 4, wherein the imaging volume (3) is arranged such that

6. the device having at least one third conductor loop formed in a third loop plane; At least one third measurement is determined by the at least one third conductor loop, the at least one third measurement being dependent on a third induced voltage induced by a third component of an alternating magnetic field perpendicular to the longitudinal axis (z) in the at least one third conductor loop; a first position of the at least one first conductor loop (4) within the imaging volume (3) is determined at least in part as a function of the at least one first measurement value and the magnetic field model; a third position of the at least one third conductor loop in the imaging volume (3) is determined at least in part as a function of the at least one third measurement value and the magnetic field model; a relative position of the at least one third conductor loop with respect to the at least one first conductor loop (4) is determined as a function of the first position and the third position; The method according to claim 1 or 2.

7. 1. An MRT system (1), comprising: a field magnet for generating a static basic magnetic field along a longitudinal axis (z) and gradient coils (2), said field magnet and said gradient coils (2) surrounding an imaging volume (3) of said MRT system (1); a body coil (30) for radiating radio frequency signals into the imaging volume (3) and surrounding the imaging volume (3); - a device comprising a local receiving coil for receiving MR signals from an examination object and at least one first conductor loop (4) formed in a first loop plane, said first conductor loop being part of said local receiving coil; a control unit (5) configured to operate the gradient coil (2) to generate an alternating magnetic field in the imaging volume (3); a measurement unit (6) configured to be connected to the at least one first conductor loop (4) and to determine at least one first measurement value as a function of a first induced voltage induced by a component of an AC field perpendicular to the longitudinal axis (z) in the at least one first conductor loop (4); at least one evaluation unit (7) configured to determine a position of the device inside at least one imaging volume (3) at least partially as a function of at least one first measurement value and a predetermined magnetic field model for the gradient coil (2), The at least one evaluation unit (7) is configured to generate an MR image in response to MR signals from an examination object within the imaging volume (3) received by the local receiving coil including the at least one first conductor loop (4).

8. 8. The MRT system (1) according to claim 7, comprising a local MR receive coil arrangement (28) housing the device.

9. The MRT system (1) according to claim 8, wherein the local MR receive coil arrangement (28) is embodied as a flexible surface coil array.

10. a device for treating a patient (29), 8. The MRT system (1) of claim 7, wherein the at least one first conductor loop (4) and the device have a predetermined spatial position relative to each other.

11. The apparatus, a tuning capacitance (9, 9a, 9b) arranged between a first terminal (21a) of the at least one first conductor loop (4) and a second terminal (21b) of the at least one first conductor loop (4); 11. The MRT system (1) according to any one of claims 7 to 10, comprising an inductive element (11a, 11b, 11c) arranged electrically in parallel with the tuning capacitance (9, 9a, 9b).

12. The measuring unit (6) an amplifier (17) connected to the first terminal (21a) and the second terminal (21b); 12. The MRT system (1) according to claim 11, configured to provide the at least one first measured value at an output of the amplifier (17) connected to the at least one evaluation unit (7).

13. 13. The MRT system (1) of claim 12, wherein the measurement unit (6) comprises a filter circuit (15) arranged between the first terminal (21 a) and a first input of the amplifier (17) and between the second terminal (21 b) and a second input of the amplifier (17) and configured to suppress MR signals acquired by the at least one first conductor loop.

Images