Assessing the influence of a magnetic resonance imaging sequence on at least one component in or on an examination tunnel and protecting the component

Abstract

A computer-implemented method for estimating the influence of a magnetic resonance imaging (MRI) sequence on at least one component in or on an examination tunnel of a MRI system, in particular a combined MRI / Positron Emission Tomography (PEM) system, wherein the method comprises the following steps: inputting at least one gradient profile of the MRI sequence into a temperature model; using the temperature model: calculating a temperature increase in the at least one component caused by the application of the MRI sequence, wherein the temperature model is designed to perform the calculation of the temperature increase based on an estimation of eddy currents induced by the at least one gradient profile in the at least one component;Output the calculated temperature increase and / or the temperature reached as a result of the temperature increase from the temperature model.

Description

[0001] The invention relates to a method for estimating an influence of a magnetic resonance imaging sequence on at least one component in or on an examination tunnel of a magnetic resonance imaging system, a method for protecting at least one component in or on an examination tunnel of a magnetic resonance imaging system, a corresponding computer program and a magnetic resonance imaging system.

[0002] Regardless of the grammatical gender of a particular term, it includes persons with male, female or other gender identities.

[0003] Due to the strong magnetic fields, and particularly the large magnetic field changes, that occur in the examination tunnel of magnetic resonance imaging (MRI) systems, there is a risk of damaging components within the examination tunnel. This problem can arise, in particular, when another imaging system is combined with the MRI system. For example, it is common to combine positron emission tomography (PET) systems with MRI systems. In such combined MR-PET systems, PET detector cassettes (DEAs) can be provided for the time-resolved spatial localization of gamma radiation in the examination tunnel of the MRI system. Due to the influence of an MRI scan, high temperatures, for example temperatures of up to 400°C, can occur in the DEAs. Such temperatures are capable of destroying the DEAs.However, even significantly lower temperatures, which may not directly destroy the DEAs, can impair their functionality.

[0004] From DE 10 2020 211 844 A1 a method for compensating for a heating effect of an imaging sequence of a second imaging modality on a detector of a first modality of a combined imaging device is known.

[0005] The numerical analysis of the influence of eddy currents is known from Bossavit, Alain, "On the numerical analysis of eddy-current problems." Computer methods in applied mechanics and engineering, 1981, vol. 27, no. 3, pp. 303-318. DOI: https: / / doi.org / 10.1016 / 0045-7825(81)90135-3.

[0006] It is an object of the present invention to provide a possibility with which the safety of one or more components in an examination channel can be increased and / or with which the influence of an MRI scan on the components can be determined.

[0007] This object is achieved by a method according to claim 1, a method according to claim 7, a computer program according to claim 8 and a magnetic resonance imaging system according to claim 9. Further features and advantages emerge from the dependent claims, the description and the attached figures.

[0008] According to a first aspect of the invention, a computer-implemented method is provided for estimating the influence of a magnetic resonance imaging sequence on at least one component in or on an examination tunnel of a magnetic resonance imaging system, in particular a combined magnetic resonance imaging / positron emission tomography system. The method comprises the following steps: (a) inputting at least one gradient of the magnetic resonance imaging sequence into a temperature model; (b) with the temperature model: calculating a temperature increase in the at least one component caused by an application of the magnetic resonance imaging sequence, wherein the temperature model is configured to calculate the temperature increase based on an estimate of eddy currents induced by the at least one gradient in the at least one component; (c) outputting the calculated temperature increase and / or a temperature reached by the temperature increase from the temperature model.

[0009] The magnetic resonance imaging (MRI) system can be a combined system in which a magnetic resonance imaging scanner is combined with another imaging modality. For example, the magnetic resonance imaging system can be a combined magnetic resonance imaging / positron emission tomography (MR-PET) system. The examination tunnel can, in particular, be an MR tube. The examination tunnel can also be referred to as an MR bore. The at least one component can preferably be a component that can be heated by the influence of magnetic fields. Heating by the influence of magnetic fields can, in particular, occur through magnetically induced eddy currents. The at least one component is preferably a component that can be heated due to eddy currents. The at least one component can, in particular, comprise electrically conductive portions.For example, the at least one component can be or comprise a component of the further imaging modality. However, other components in the examination tunnel can also be monitored according to the invention. The at least one component can comprise part of the MRI system itself. For example, the at least one component can comprise part of a motion monitor, part of an entertainment and / or information system (e.g., for a patient in the examination tunnel), and / or a body coil of the MRI. In MR-PET systems, PET detector cassettes (DEAs) are usually located inside the examination tunnel. The at least one component can be or comprise a component of the PET. In particular, the at least one component can comprise at least one PET detector cassette. The PET detector cassettes can be provided for a time-resolved spatial location of gamma radiation.The PET detector cassettes can be arranged in a ring in the examination tunnel. The orientation of the ring-shaped arrangement of the PET detector cassettes can correspond to a ring-shaped cross-section of the examination tunnel. The PET detector cassettes are preferably arranged between a whole-body coil and a gradient coil of the MRI system. While the method according to the invention can generally be used for various components, the method can be particularly advantageous for protecting the PET detector cassettes. The PET detector cassettes can be damaged or even destroyed at excessively high temperatures. This can impair or completely eliminate the functionality of the PET detector cassettes.

[0010] Depending on the magnetic resonance imaging (MRI) sequence used, current may flow to the at least one component based on induced eddy currents. The eddy currents may, in particular, be due to a change in the gradient signals of the individual gradient coils of the MRI system. A change in the gradient signals typically corresponds to the slew rate of the gradient signals. The eddy currents may lead to an increase in the temperature of the at least one component. The influence of the MRI sequence thus includes, in particular, an influence on the temperature of the at least one component. Large-area conductive components or component parts, for example, large-area copper ground planes of DEA electronic boards, may be particularly susceptible to such heating.The temperature increase can, in particular, arise from the superposition of different gradient fields from different gradient coils (in particular the x-, y-, and z-coils). In principle, it is conceivable, for example, to test scan protocols with magnetic resonance imaging sequences in advance in order to reduce the risk for at least one component. This would allow a combined MRI system to have preset protocols that assume sufficient safety for at least one component. However, within the scope of the invention, it was recognized that deviations from such preset protocols can occur. This can lead to new combinations of gradient fields, which could potentially lead to increased heating.For example, a slice can be tilted according to a preset protocol in a change to the protocol, whereby the proportion of the various gradients can be changed and thus possibly also the resulting overall influence on the at least one component. Advantageously, the method according to the invention can already detect a potentially occurring temperature increase in advance of a measurement, even if a previously unknown MRI sequence is to be tried out. Advantageously, the method according to the invention can also simplify the development of an MRI system by determining in advance whether relevant MRI sequences could cause a critical temperature increase. For example, "virtual engineering" can be enabled by optimizing hardware components in advance based on the intended MRI sequences.For example, it can also be clarified at an early stage whether clinically relevant or merely technical special sequences can lead to an increased temperature. Technical special sequences can be special sequences which are not required for normal clinical use but which can nevertheless be carried out in principle. While it is usually assumed during development that such special sequences will not be used, they can nevertheless be (unforeseen) used, e.g. for testing purposes. Advantageously, the method according to the invention can also be used in these cases at an early stage, i.e. in particular before the actual start of the special sequence, to determine whether a temperature increase in at least one component is to be expected.The information obtained by the method can be used, for example, to warn a user, to suggest a change to the MRI sequence to the user or to execute it automatically, and / or to automatically prevent the MRI sequence from being executed if an expected temperature increase is detected.

[0011] Within the scope of the invention, the term “gradient curve” is to be understood broadly. It generally relates to the curve of at least one gradient coil provided according to the MRI sequence. For example, the gradient curve can be the resulting magnetic field resulting from an activation of the gradient coil provided in the MRI sequence (e.g., locally or as a spatial distribution). It is also conceivable for the gradient curve to be input as current supply to at least one gradient coil, from which the gradient field can be derived. The gradient curve can optionally also be described by the change in the gradient field, for example by specifying the (time-dependent) slew rate of the respective gradient coil or the respective gradient. The gradient curve can be defined by a gradient pulse. For example, the gradient curve can comprise a periodically recurring gradient pulse.

[0012] Within the scope of the method according to the invention, the gradient profile is entered into a temperature model. The temperature model can be a physical model. The temperature model can, for example, comprise a computer algorithm. The temperature model is designed in particular to calculate a temperature increase in the at least one component based on an estimate of eddy currents induced by the at least one gradient profile in the at least one component. According to the invention, the temperature model is used to calculate a temperature increase caused by applying the magnetic resonance imaging sequence. The calculation can comprise an approximate estimate. Advantageously, local heating can also be determined by calculating induced eddy currents.It can be considered that eddy currents occur with varying intensity at different locations on at least one component. This can also lead to localized heating of the component. For example, the component may heat up only slightly overall, but significantly more locally. This can, in principle, lead to localized heat-related damage.

[0013] Finally, the calculated temperature increase is output. This should generally be understood broadly. The temperature increase can be output in the form of an absolute temperature, which can be understood in relation to a reference temperature. The absolute temperature can be the reached temperature. The “reached temperature” is to be understood in particular to mean the temperature of the at least one component that it reaches or can be expected to reach as a result of the temperature increase. The reference temperature can, for example, be the temperature of the at least one component in a resting state of the MRI system. The reference temperature can, for example, be a room temperature. The temperature increase can, for example, be output by the temperature model to another instance. The another instance can, for example, be a monitoring algorithm.The monitoring algorithm can be configured to compare the temperature increase with a maximum permissible temperature increase or a calculated temperature with a maximum permissible temperature. The monitoring algorithm can be configured to initiate a measure if the maximum permissible temperature or temperature increase is exceeded. The measure can, for example, be the output of an alarm, user information, or a control signal. The user information can, for example, relate to the expected excessive temperature of at least one component and / or a suggested change to the MRI sequence. The control signal can, for example, include preventing the execution of the MRI sequence and / or changing the MRI sequence.

[0014] According to one embodiment, the temperature model for calculating the temperature increase takes into account both at least one absorbed heat contribution and at least one emitted heat contribution of the at least one component. The at least one absorbed heat contribution can in particular comprise heat induced by the eddy currents. The at least one emitted heat contribution can, for example, be an aspect of the heat emission that is responsible for the greatest heat loss. The at least one emitted heat contribution can, for example, comprise heat emitted via heat conduction and / or heat emitted via thermal radiation. Advantageously, this embodiment can be used to balance the heat emitted and the absorbed heat. In particular, by taking into account not only the absorbed heat but also heat loss in the form of emitted heat, a more precise estimate of the temperature development orTemperature increase can be achieved. For example, the energy balance can be defined by an equation of the form ΔQ=ΔQ++ΔQ− where ΔQ + an absorbed heat and ΔQ - is heat released. The heat absorbed and / or the heat released can be time-dependent.

[0015] For example, if we assume that the warming ΔQ + depends on an induced power P(t), which in turn depends on a change in the gradient fields Ġ, which can be defined by a slew rate S of the gradients, the contribution for absorbed heat ΔQ+=P(t)Δt∼S2(t)Δt=G˙2(t)Δt The absorbed heat ΔQ + can be calculated, for example, for a reference pulse of the magnetic resonance imaging sequence. The reference pulse can be, for example, a sawtooth pulse. The absorbed heat ΔQ +can be calculated, for example, for several positions in at least one component. The absorbed heat ΔQ + can be calculated separately for each gradient axis (e.g., for each of the three gradient axes x, y, z). G or Ġ can be a gradient vector. The gradient vector can be, for example, a general 3D vector. According to the heat input, a gradient vector can be calculated, for example, for several positions in the at least one component. The gradient vector can be calculated separately for each gradient axis (e.g., for each of the three gradient axes x, y, z). For a sequence with superimposed gradient pulses, a vector sum can be formed, for example. Based on the superimposed gradient pulses, a resulting total heat input (e.g., Qx++Qy++Qzx+ ) can be calculated.

[0016] Assuming that the heat released ΔQ -by a heat emission with an emission coefficient χ, the emitted heat can be determined, for example, by ΔQ−=χ(T−T0)Δt where T is the temperature and T0 is the ambient temperature, where T=Qc with the heat capacity c.

[0017] According to one embodiment, the temperature model calculates the temperature increase based on solving a differential equation, wherein the differential equation relates to the change in temperature of the at least one component and includes at least one heat contribution, preferably at least two heat contributions. The differential equation can be based on an energy balance equation. For example, based on the above-mentioned energy balance (Equation 1), the differential equation can have the form T˙(t)=k1ΔQ+Δt−k2ΔQ−Δt where T(t) is the change in temperature over time t and k1 and k2 are constants. Using equation 2 and equation 3 and a temperature decay constant τ=cχ the differential equation (equation 4) can be written to the concrete form T˙(t)=1cG˙2(t)−1τ(T(t)−T0) By solving this differential equation 5, heat development in at least one component can be estimated. The differential equation can be solved numerically, for example. The differential equation can be solved, for example, by an algorithm designed for solving differential equations, in particular for solving differential equations numerically. The algorithm can be part of the temperature model.

[0018] According to the invention, the temperature model for calculating the temperature increase determines at least one radial vector that is characteristic of an induced eddy current and is perpendicular to a surface of the at least one component. The radial vector can, in particular, characterize a change in the magnetic flux perpendicular to the surface. In particular, the radial vector can indicate a measure of the change in the magnetic flux at the location of the at least one component. If it is assumed that the at least one component is arranged or installed at a fixed location, it can be assumed, for example, that a distance of a specific gradient coil from the at least one component and / or a (geometric) arrangement of the at least one component relative to the gradient coil is always the same.Accordingly, it can be assumed, for example, that a relative gradient field at the location of the at least one component is always the same. The radial vector can be characteristic of the relative gradient field. The radial vector can have a value that is characteristic of the relative gradient field. The value can, in particular, correspond to an absolute value of the radial vector, wherein the value can preferably be signed. In other words, the value can be both negative and positive. The sign of the value can correspond to the direction of the gradient field change parallel to the radial vector. Depending on the orientation of the field change, the radial vector can thus point in one of two opposite directions. Advantageously, the value of the radial vector can be determined once.For example, the value of the radial vector may have been determined in advance for at least one component based on the geometry of one or more gradient coils. If there are multiple components, a value of a radial vector may have been determined in advance for each component based on the geometry of one or more gradient coils. For example, the value of the radial vector or the values ​​of the radial vectors may have been determined by simulation. The values ​​for different components can be stored, for example, as a table. It can be provided that the value of the radial vector is representative of the entire at least one component. For example, the value can be a value summed over the entire range of the at least one component that is relevant for eddy current induction.The relevant area can in particular comprise contiguous conductive ground surfaces (in which eddy currents can be induced, which are classified as critical). The classification can be carried out, for example, by individual assessment by an expert and / or by simulation / measurement and a threshold value. The threshold value can be, for example, a temperature threshold value or an induced current threshold value. The value can be determined, for example, by simulation, calculation or measurement. For example, the value can be recalculated by exemplary measurement or simulation of a temperature determination. A relative gradient field in this context is to be understood as relative to another location, for example relative to a location of another component. An absolute gradient field or a change in the absolute gradient field at the location of the at least one component can, for example, still be determined by the respective MRI sequence, i.e.in particular depend on the respective change in the gradient field according to the MRI sequence. In this context, an absolute gradient field can be the actual gradient field, taking into account the geometry and the MRI sequence (i.e., e.g., the current passed through the gradient coil to generate the gradient field). For example, the gradient field can have a relative magnitude of 0.5 at a first location at the at least one component and a different relative magnitude of 2.8 at a further location (e.g., the location of a further component). Accordingly, for example, the radial vector at the location of the at least one component can have the value 0.5 and the value of another radial vector at the further location of a further component can be 2.8.Depending on the respective MRI sequence, in this example, a value can be determined (for example, by multiplying the value 0.5 by a factor that depends on the MRI sequence) that is representative of the actual value of the field change at the location of at least one component. The change in the magnetic flux is typically proportional to the induced voltage, which causes a current or an eddy current in the component, and to the change in the magnetic field. The radial vector can correspond to the changing gradient field and / or have a fixed relationship to the gradient field. For example, the radial vector can be proportional to the changing gradient field. The heat transfer or heat contribution can be derived from the radial vector. For example, it can be assumed that an induced voltage and / or a changing gradient field is proportional to the square root of the heat transfer.Multiple radial vectors can be provided, each for one of multiple components and / or each for one of multiple positions on a component. A radial vector can describe a relative contribution of a gradient or a gradient field generated by the gradient to an eddy current induced in a component. Preferably, the radial vector can be perpendicular to a surface of the at least one component or to a plane of the at least one component in which an eddy current is induced. The surface can, in particular, be the largest surface of the at least one component. For example, the surface can be a ground plane of a circuit board, in particular an electronic circuit board. The ground plane can, for example, be a copper ground plane. The circuit board can, for example, be an electronic circuit board of a PET detector cassette or a DEA electronic circuit board.The radial vector can be an efficient and reliable way to predict heat generation based on eddy currents. For example, when determining heat generation, spatial directions that are not perpendicular to the component's surface and therefore do not induce significant eddy currents can be ignored.

[0019] According to one embodiment, the magnetic resonance imaging sequence comprises a plurality of gradient coils, in particular at least two or three gradient coils, or the control of a plurality of gradient coils, each with at least one gradient profile, wherein at least one radial vector is provided for each of the gradient coils. In the case of a plurality of components, at least one radial vector can be provided for each of the plurality of components for each of the gradient coils. In particular, each radial vector can indicate a measure of the change in the magnetic flux at the location of the at least one component caused by the respective gradient coil. Each of the radial vectors can be characteristic of the relative gradient field of the respective corresponding gradient coil. Each radial vector can have a value that is characteristic of the respective relative gradient field. Advantageously, the value of the radial vectors can have been determined once.For example, the value of the radial vectors can be determined based on the geometry of the gradient coils for at least one component, in particular by simulation. If there are multiple components, a value of a radial vector per gradient coil can be determined for each component based on the geometry of the gradient coils. The values ​​for different components and / or different gradients can be stored as a table, for example. The values ​​of the table can be determined, for example, by simulation. The table with the values ​​can be structured, for example, as follows (Table I): Table I Component # Value for x-gradient U x Value for y-gradient U y Value for z-gradient U z 1 0,5 5,0 -0,6 2 -0,3 -4,5 0,5 3 -2,1 -5,0 0,5 etc. etc. etc. etc.

[0020] The values ​​of the radial vector can be converted into the heat input using a factor determined by calibration and / or using at least one adjustment dependent on the MRI sequence (e.g., another factor). For example, the gradient field caused by an x-gradient at the location of the at least one component can have a relative magnitude of 0.5, the gradient field caused by a y-gradient at the location of the at least one component can have a relative magnitude of 5.0, and the gradient field caused by a z-gradient at the location of the at least one component can have a relative magnitude of -0.6. The overall influence can then be determined, for example, by superimposing the fields caused by the various gradient fields. Because the radial vectors are all perpendicular to the surface, they are aligned parallel to one another.Therefore, determining the superposition is relatively simple and can be determined, for example, by simple addition. By using the radial vectors perpendicular to the surface of the component, a simple addition of the vector magnitudes can advantageously be used instead of vector addition. Depending on the respective MRI sequence, a value can be determined that is representative of the actual value of the field change at the location of the at least one component. For example, the magnetic flux change or the change in the gradient field can be determined from the value of each radial vector for each gradient field together with at least one value characteristic of the course of the respective gradient field according to the MRI sequence. In the example mentioned, the value of the first radial vector (e.g., 0.5) can be correlated with a characteristic value of the x-gradient, the value of the second radial vector (e.g.,5.0) is multiplied by a characteristic value of the y-gradient, and the value of the third radial vector (e.g. -0.6) is multiplied by a characteristic value of the z-gradient. This embodiment can be particularly advantageous if the at least one component is located particularly close to the gradient coils. For example, PET detector cassettes are typically located very close to the gradient coils. If the at least one component is located particularly close to the gradient coils, the near-field development of the gradient field can be relatively inhomogeneous. For example, it has been found that the near-field development at PET detector cassettes with X and Y saddle coils can be very inhomogeneous, particularly below the winding centers of the coils. The winding centers can also be referred to as the eyes of the coils.With such proximity of the at least one component to the gradient coils, the gradient fields on the at least one component are not aligned perpendicular to one another and may also be unevenly distributed. For example, the gradient fields may be unevenly distributed across the positions of PET detector cassettes arranged in a ring. Forming a vector sum may be numerically unstable, which may complicate the calculation or prediction of temperature development. This embodiment with radial vectors can be particularly advantageous here. Based on the radial vectors, a heat input or the absorbed heat at the at least one component can be determined as a signed induced voltage, which depends on the respective gradient perpendicular to a surface of the component (e.g., perpendicular to a DEA board).In this case, the radial vectors of all gradient contributions can advantageously point in the same perpendicular direction (namely, in the direction of the radial vector). Thus, a simple vector summation can be used to calculate a total heat input. The representation perpendicular to the surface is particularly advantageous for flat components, such as circuit boards or electronic boards. For flat components, it can typically be assumed that only a radial component perpendicular to the flat surface contributes significantly to the heating by induction.

[0021] According to one embodiment, the heat contribution from the induced eddy currents is calculated by weighting the contributions of the gradient fields to an induction voltage with a respective slew rate of the gradient coils. With only one radial vector, the contribution of the gradient field to the induction voltage can be weighted with the slew rate of the gradient coil. The slew rate can be the slew rate provided according to the MRI sequence. Advantageously, a change in a magnetic flux or the gradient field can be proportional to the value of the radial vector and the slew rate. The absolute change in the gradient field Ġ can be determined by multiplying the value of the radial vector by the slew rate. According to equation 2, the heat input ΔQ + proportional to the change in the squared gradient field Ġ 2(t) or the squared induced voltage. The contributions of several gradient coils or gradient fields can be determined by weighted addition of the values ​​of the radial vectors with the respective slew rate of the gradient coils. For example, the values ​​of three radial vectors (U x , U y , U z ) each with a slew rate (S y , S y , S z ) of three gradient coils (x, y, z) by the following addition to determine the heat input (ΔQ + ) over a certain period of time (t sample ) be used: ΔQ+∼(sx×Ux+sy×Uy+sz×Uz)2×tsample in particular Ġ 2 (t sample ) proportional to or equal to (s x × U x + s y × U y + s z × U z ) 2 × t sampleAccordingly, the values ​​of the radial vectors can be unique (e.g. based on the geometry of the gradient coils, on the position of at least one component and / or on a simulation) and applicable for any field change (which can be determined by the slew rates s x , s y , s z included in the calculation). The respective slew rate may depend on the MRI sequence used. The slew rate values ​​(s x , s y , s z ) do not necessarily have to be the actual slew rate, but can also be the slew rate, for example. The values ​​of the radial vectors (U x , U y , U z) can correspond to a reference pulse, whereby the values ​​can be further calculated based on the slew rate. In particular, the slew rates used to determine the values ​​can be taken into account as a reference value in the weighting. Accordingly, the slew rate values ​​can be a conversion factor from the reference pulse to the actual pulse. Depending on the respective MRI sequence, some slew rate values ​​can, for example, also be zero. It is also conceivable that the calculated sum (depending on the slew rates) becomes zero if the radial vectors are partly positive and partly negative. This corresponds to a destructive superposition of the magnetic fields at the location of at least one component. For example, if there are several components, the heat input can be zero for at least one component, but not for one or more other components. The slew rates can be time-dependent.At least one calibration factor may be provided, particularly in addition to the weighting with the slew rates. The calibration factor may have been determined by a calibration measurement. For example, the calibration factor may be a heat capacity constant or depend on a heat capacity constant.

[0022] According to one embodiment, the temperature model is tailored to the at least one component, in particular by at least one calibration measurement. For example, the temperature model can comprise information about at least one property of the at least one component. The at least one property can in particular relate to a temperature behavior of the at least one component. For example, the at least one property can relate to the ability of the at least one component to absorb and / or release heat. The at least one property can be a material-dependent and / or a geometry-dependent property. Advantageously, the temperature model can thus take one or more characteristics of the at least one component into account and achieve greater accuracy.

[0023] According to one embodiment, the temperature model comprises at least one characteristic physical constant relating to the temperature profile of the at least one component. The characteristic physical constant can in particular be one or more of: a heat capacity constant of the at least one component, a thermal time constant of the at least one component, and a delay in the occurrence of an induced eddy current relative to the set gradient profile. The physical constant can have been determined by a calibration measurement. The thermal time constant can be a cooling time constant. The cooling time constant can be characteristic of how quickly the at least one component cools or at what rate it releases heat.The thermal time constant may depend, for example, on the heat radiation behavior of the at least one component or on heat dissipation from the at least one component. The heat dissipation may also depend, in particular, on the environment or the installation of the at least one component.

[0024] The thermal time constant can be determined by a calibration measurement. For example, an MRI sequence can be activated and then switched off during a calibration measurement. After the MRI sequence is switched off, the temperature profile of the component can be further recorded and fitted to an e-function. An e-function is a typical temperature profile of a component after the MRI sequence is switched off. The decay constant of the e-function can correspond to the thermal time constant. Advantageously, this makes it possible to determine a thermal time constant that is independent of the respective MRI sequence itself, i.e., that can be used for multiple MRI sequences.

[0025] A heat capacity constant is to be understood broadly within the scope of this invention. The heat capacity constant can be a heat capacity or a quantity linked to the heat capacity. For example, the heat capacity constant can be a heat capacity, a specific heat capacity, or a molar heat capacity. The heat capacity can link an absorbed heat contribution to a temperature increase of the component.

[0026] The heat capacity constant can be determined by a calibration measurement. For example, an MRI sequence can be performed until thermal equilibrium is established and then simulated using the temperature model. The temperature of at least one component can be measured, for example, using a temperature sensor. Calibration can be performed by adjusting the temperature of thermal equilibrium in the simulation to the temperature of the measurement using the temperature model. Optionally, the heat capacity constant can be determined via an initial gradient of the component's temperature when applying an MRI sequence. The initial gradient can be determined using the mass m of the component, the power P, and the time t over which this power acts (see equation 2): dT=(P×t) / (c×m) which means the specific heat capacity c c=(P×t) / (dT×m) or the heat capacity C = c × m C=c×m=(P×t) / dT can be determined.

[0027] Preferably, the thermal time constant can also be used to determine the heat capacity constant, in particular to correctly calculate thermal equilibrium. For example, a combined calibration of the thermal time constant and heat capacity constant can be provided. With the combined calibration, for example, an MRI sequence can be performed until thermal equilibrium is reached (e.g., until the temperature of the component no longer changes significantly), simulated with the temperature model, and the temperature profile recorded. Subsequently, the thermal time constant and then the heat capacity constant can be determined.

[0028] Due to the inductance of at least one component, an induced eddy current and thus also heating can occur with a delay. In some cases, this delay can be negligible. In some cases, however, it can be important to determine the temperature profile (particularly influenced by heating and cooling) as precisely as possible. This can be the case, for example, if an MRI sequence leads to a temperature increase that brings at least one component very close to a potentially critical temperature. In this case or other cases, it can be very useful to determine as precisely as possible when a heat contribution is absorbed. Accordingly, a delay in the occurrence of an induced eddy current can be calibrated relative to the gradient profile. The delay can be determined, for example, by comparing measurement and simulation with the temperature model.

[0029] Another aspect of the invention is a computer-implemented method for protecting at least one component in or on an examination tunnel of a magnetic resonance imaging system. The magnetic resonance imaging system can be a magnetic resonance imaging system combined with another imaging modality. The magnetic resonance imaging system can, in particular, be a combined magnetic resonance imaging / positron emission tomography system. The method comprises the following steps: - Receiving a magnetic resonance imaging sequence for execution with the magnetic resonance imaging system; - applying the method for estimating an influence of a magnetic resonance imaging sequence on at least one component in or on an examination tunnel of a magnetic resonance imaging system as described herein; - Check whether the calculated temperature increase exceeds a predetermined temperature threshold; - If the temperature threshold is exceeded: Automated adaptation of the magnetic resonance imaging sequence and / or issuing a suggestion for adapting the magnetic resonance imaging sequence to a user.

[0030] All advantages and features of the method for estimating the influence of a magnetic resonance imaging sequence on at least one component in or on an examination tunnel can be transferred analogously to the method for protecting at least one component in or on an examination tunnel, and vice versa. Advantageously, this aspect can be used to protect the at least one component from damage by determining, even before the magnetic resonance imaging sequence (MRI sequence) is carried out, whether the component's temperature may be too high. This can also be advantageous compared to monitoring the temperature during the measurement (e.g., using temperature sensors), because it can prevent the method from being started in the first place. This can, for example, prevent a risky temperature development from the outset.In addition, the unnecessary starting of a sequence that would then have to be aborted can be prevented, thus saving time and potentially costs. This procedure can also be advantageous for avoiding potentially stressful or health-threatening examinations on patients (for example, when using radioactive PET contrast agents), which then have to be aborted. This method can be particularly advantageous when using newly developed MRI sequences, because it can be determined before testing whether the respective MRI sequence can be performed without causing unsafe temperatures in at least one component. For example, modifications to the MRI sequence can be suggested to a user. A modification could include limiting the slew rate, applying a VERSE pulse, increasing the TR, changing the gradient setting from "high" to "normal" or "whisper," and / or limiting parameters, etc.

[0031] A further aspect of the invention is a computer program comprising a temperature model as described herein. The computer program can, in particular, comprise instructions which, when executed by a computer, cause the computer to carry out the steps of a method as described herein. All advantages and features of the method for estimating the influence of a magnetic resonance imaging sequence on at least one component in or on an examination tunnel and of the method for protecting at least one component in or on an examination tunnel can be transferred analogously to the computer program and vice versa. The computer program can, for example, be stored on a computer-readable storage medium, in particular a non-volatile storage medium. The storage medium can, for example, be a hard disk, an SSD, a flash memory, an online server, etc.

[0032] Another aspect of the invention is a method for calibrating a temperature model, in particular a temperature model as described herein. The method for calibrating a temperature model comprises the following steps: - Calculating a temperature profile of a component arranged in an examination tunnel of a magnetic resonance imaging system during and after the application of a magnetic resonance imaging sequence using the temperature model; - Performing the magnetic resonance imaging sequence and sensory measuring of the temperature profile during and after performing the magnetic resonance imaging sequence; - Comparing the calculated and measured temperature curve to determine a thermal time constant and / or a heat capacity constant of the component; - Fitting the temperature model with the determined thermal time constant and the determined heat capacity.

[0033] The thermal time constant can be determined, for example, by comparing a calculated and measured temperature drop after the end of the magnetic resonance imaging sequence. The heat capacity constant can be determined, for example, by comparing a calculated and measured thermal equilibrium and / or a calculated and measured initial temperature rise during the magnetic resonance imaging sequence. All advantages and features of the method for estimating the influence of a magnetic resonance imaging sequence on at least one component in or on an examination tunnel, the method for protecting at least one component in or on an examination tunnel, and the computer program can be transferred analogously to the method for calibrating a temperature model, and vice versa.In particular, the advantages, steps and features of calibration explained in the context of the other procedures can also be applied to the calibration procedure.

[0034] A further aspect of the invention is a magnetic resonance imaging system with a control module comprising a computer program as described herein. The magnetic resonance imaging system can be configured, in particular, to carry out a method as described herein. All advantages and features of the method for estimating the influence of a magnetic resonance imaging sequence on at least one component in or on an examination tunnel, the method for protecting at least one component in or on an examination tunnel, the computer program, and the method for calibrating a temperature model can be transferred analogously to the magnetic resonance imaging system, and vice versa.

[0035] Embodiments are described below with reference to the attached figures. Fig. 1 shows a flowchart of a computer-implemented method for estimating an influence of a magnetic resonance imaging sequence on at least one component in or on an examination tunnel of a magnetic resonance imaging system according to an embodiment of the invention, Fig. 2 shows several components of a system according to the invention and the orientation of two exemplary radial vectors, Fig. 3 shows a flowchart of a computer-implemented method for protecting at least one component in or on an examination tunnel of a magnetic resonance imaging system according to an embodiment of the invention, Fig. 4 shows a flowchart of a method for calibrating a temperature model according to an embodiment of the invention, Fig.5 shows a temperature profile calculated with a temperature model according to an embodiment of the invention, Fig. 6 shows a temperature curve of a measurement corresponding to Fig. 5 corresponds to the calculated temperature curve shown, Fig. 7 shows a gradient curve, a corresponding curve of the slew rate of a gradient and a resulting curve of a heat input of a component and Fig. 8 shows a magnetic resonance imaging system according to an embodiment of the invention.

[0036] Fig.1 shows a flowchart of a computer-implemented method for estimating the influence of a magnetic resonance imaging sequence on at least one component in or on an examination tunnel of a magnetic resonance imaging system according to an embodiment of the invention. The magnetic resonance imaging system can, in particular, be a combined magnetic resonance imaging / positron emission tomography system.

[0037] In a first step 102, at least one gradient profile of the magnetic resonance imaging sequence is input into a temperature model. The temperature model is preferably tailored to the at least one component. For example, the temperature model can comprise at least one characteristic physical constant relating to the temperature profile of the at least one component. The at least one characteristic physical constant can be, for example, a heat capacity constant of the at least one component and / or a thermal time constant of the at least one component and / or a delay in the occurrence of an induced eddy current relative to the gradient profile. For example, the temperature model can be tailored to the at least one component by one or more calibration measurements.

[0038] In a further step 103, the temperature model is used to calculate a temperature increase in the at least one component caused by application of the magnetic resonance imaging sequence. The temperature model is designed such that it calculates the temperature increase based on an estimate of eddy currents induced by the at least one gradient in the at least one component. To calculate the temperature increase, at least one heat contribution absorbed by the at least one component is used. The absorbed heat contribution can, in particular, be heat induced by the eddy currents. Preferably, at least one heat contribution released by the at least one component is also taken into account for the calculation. Heat can, for example, be released through thermal conduction and / or thermal radiation.For example, the temperature model can calculate the temperature increase based on solving a differential equation that relates to the change in temperature of the at least one component and includes one or more heat contributions. For example, a differential equation according to equation 5 can be used, wherein the differential equation relates to the change in temperature of the at least one component and includes at least one heat contribution, preferably at least two heat contributions. Preferably, only the portions of the gradient fields that are perpendicular to a surface of the component can be used for this purpose. This can be done by a radial vector R that is perpendicular to a surface of the respective component and that represents the relative contribution of the gradient field at the respective location or component in the direction of the radial vector R. xThe current I induced by the respective gradient field is then proportional to the radial vector R x and the respective slew rate. Fig. Figure 2 schematically shows several components, e.g., an arrangement of DEA electronic boards of a PET system, and the orientation of two exemplary radial vectors, each of which can be used to determine an induced current and thus also an induced heat. The contribution of several gradient coils to the induced current can be determined by determining the relative contributions or values ​​(e.g., U x , U y , U z ) of the gradient coils, each represented by a radial vector R x, y, z (which is perpendicular to the surface and can be positive or negative), weighted with the slew rate of the respective gradient coils (see e.g. equation 6).

[0039] In a further step 104, the calculated temperature increase and / or a temperature reached by the temperature increase is output from the temperature model.

[0040] Fig. 3 shows a flowchart of a computer-implemented method for protecting at least one component in or on an examination tunnel of a magnetic resonance imaging system according to an embodiment of the invention. The magnetic resonance imaging system can, in particular, be a combined magnetic resonance imaging / positron emission tomography system. In a first step 201, a magnetic resonance imaging sequence intended for execution with the magnetic resonance imaging system is received. The next steps 202-204 can correspond to steps 102-104 of the method described in Fig.1. In a further step 205, a check is carried out to determine whether the calculated temperature increase exceeds a predetermined temperature threshold. This can be done, for example, by a simple threshold adjustment. Different thresholds can optionally be provided for different components and / or for different locations on one or more components. In a further step 206, if the temperature threshold is exceeded, the magnetic resonance imaging sequence is automatically adjusted. Alternatively or additionally, in this step 206, a suggestion for adjusting the magnetic resonance imaging sequence can be output to a user. In particular, an adjustment can be suggested which does not result in the temperature threshold being exceeded, e.g., by adjusting at least one parameter of the sequence (e.g., a duration).

[0041] Fig.Figure 4 shows a method for calibrating a temperature model. The temperature model may in particular be a temperature model that (after calibration) is calibrated in a method as in Fig. 1 or in Fig. 3. In a first step 311 of the method, a temperature profile of a component arranged in an examination tunnel of a magnetic resonance imaging system is calculated. The temperature profile is calculated during and after the application of a magnetic resonance imaging sequence using the temperature model. Fig. Figure 5 shows such a temperature profile, which was calculated using a temperature model. In a further step 312, the magnetic resonance imaging sequence is executed, and the actual temperature profile is measured using sensors, e.g., temperature sensors, during and after the execution of the magnetic resonance imaging sequence. The temperature profile of a corresponding measurement is shown in Fig.6. In both Fig. 5 and Fig. 6, the time of switching off the MR sequence is clearly visible due to the temperature drop that occurs. The time of switching off is also shown in Fig.6 is marked by a dashed line. In a further step 313, the calculated and measured temperature decay after the end of the magnetic resonance imaging sequence are compared to determine a thermal time constant of the component, which is characteristic of a general temperature decay of the component. The time constant can be determined from the falling edge, e.g., using a conventional decay term. In addition, the calculated and measured thermal equilibrium during the magnetic resonance imaging sequence is compared to determine a heat capacity of the component. The thermal equilibrium can be assumed (approximately) in the (almost) horizontal temperature curve. Alternatively, the heat capacity can also be determined from the gradient of the temperature curve at the beginning.The time constant and the heat constant can, for example, be determined in such a way that their value in the temperature model is adjusted so that the measured and the calculated temperature curves are almost identical. In order to be able to determine the thermal equilibrium as accurately as possible, the time constant can first be determined, which can then be used to determine the equilibrium. The heat capacity can, in particular, be determined based on the level of thermal equilibrium and the time constant of the temperature drop. In a further step 314, the temperature model is adjusted using the determined thermal time constant and the determined heat capacity. In the simulated temperature curve of the temperature model shown, the model was already calibrated, so that the measured and the simulated temperature curves are almost the same. The comparison of the . Fig. 5 and Fig.6 shows that the temperature model can be used to simulate a temperature curve very accurately.

[0042] Fig.Figure 7 shows an example of a gradient curve (top), a corresponding curve of the slew rate of a gradient (middle), and a curve of heat input (supply of heat) in a component determined from this (bottom). The slew rate can be used, for example, as specified in equation 6, to calculate the heat input. The plot of the gradient curve shows an actual gradient curve (dashed line) and a gradient curve which corresponds to a delay in the gradient curve caused by induction (solid line). This offset of the actual gradient curve and the set gradient curve can be determined, for example, by a simulation, and the temperature model can be corrected accordingly using this offset (which results from the difference between the dashed and the solid line). For example, a time constant of the induced eddy currents can be determined by a simulation.The time constant can depend, in particular, on the geometry and conductivity of the component. The time constant can be incorporated into the model to obtain the actual, delayed "gradient" curve. This delayed curve then corresponds, in particular, to the eddy current curve.

[0043] Fig. 8 shows a magnetic resonance imaging system with a control module 3, which comprises a computer program which is configured to carry out a method according to the invention, e.g. as in Fig. 1 or in Fig. 3. The magnetic resonance imaging system may include a PET system, so that DEA electronic boards as shown in Fig. 3 (here concealed by the outer shell of the examination tunnel). Some or all of the DEA electronic boards may be the components monitored or protected by the procedure.

Claims

[1] Computer-implemented method for estimating an influence of a magnetic resonance imaging sequence on at least one component in or on an examination tunnel of a magnetic resonance imaging system, in particular a combined magnetic resonance imaging / positron emission tomography system, the method comprising the following steps: (a) inputting at least one gradient of the magnetic resonance imaging sequence into a temperature model; (b) with the temperature model: calculating a temperature increase in the at least one component caused by an application of the magnetic resonance imaging sequence, wherein the temperature model is designed such that it carries out the calculation of the temperature increase on the basis of an estimation of eddy currents induced by the at least one gradient profile in the at least one component, wherein the temperature model determines at least one radial vector for calculating the temperature increase which is characteristic of an induced eddy current and is perpendicular to a surface of the at least one component; (c) Outputting the calculated temperature increase and / or a temperature achieved by the temperature increase from the temperature model. [2] Method according to claim 1, wherein the temperature model for calculating the temperature increase takes into account both at least one absorbed heat contribution, including in particular heat induced by the eddy currents, and at least one emitted heat contribution of the at least one component. [3] Method according to one of the preceding claims, wherein the temperature model performs the calculation of the temperature increase based on the solution of a differential equation, wherein the differential equation relates to the change in temperature of the at least one component and comprises at least one heat contribution, preferably at least two heat contributions. [4] Method according to one of the preceding claims, wherein the magnetic resonance imaging sequence comprises a plurality of gradient coils, in particular at least two or three gradient coils, each with at least one gradient curve, wherein at least one radial vector is provided for each of the gradient coils. [5] Method according to claim 4, wherein the heat contribution by the induced eddy currents is calculated by weighting the contributions of the gradient fields to an induced voltage with a respective slew rate of the gradient coils. [6] Method according to one of the preceding claims, wherein the temperature model comprises at least one characteristic physical constant relating to the temperature profile of the at least one component, in particular comprising a heat capacity constant of the at least one component and / or a thermal time constant of the at least one component and / or a delay in the occurrence of an induced eddy current relative to the set gradient profile. [7] Computer-implemented method for protecting at least one component in or on an examination tunnel of a magnetic resonance imaging system, in particular a combined magnetic resonance imaging / positron emission tomography system, the method comprising the following steps: - Receiving a magnetic resonance imaging sequence for execution with the magnetic resonance imaging system; - Applying the method according to one of the preceding claims; - Check whether the calculated temperature increase exceeds a predetermined temperature threshold; - If the temperature threshold is exceeded, automated adjustment of the magnetic resonance imaging sequence and / or issuing a suggestion for adapting the magnetic resonance imaging sequence to a user. [8] A computer program comprising instructions which, when executed by a computer, cause the computer to carry out the steps of the method according to any one of the preceding claims. [9] Magnetic resonance imaging system with a control module comprising a computer program according to claim 8.

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