Patent Application
20110133729
The invention relates to magnetic induction tomography (MIT), in particular to a method and a monitoring device for conducting an RF-safe MIT scan such that an RF electromagnetic (power) exposure imposed on an examination object (especially a patient), especially in the form of a specific absorption rate (SAR), does not exceed certain limit values during MIT scanning.
Magnetic induction tomography is a non-invasive imaging technique with applications in industry and medicine. MIT is based on couplings between RF transmitters and RF receivers placed around the object of interest to be imaged. More in detail, a time varying (primary) magnetic field is applied to the object to be imaged by means of one RF transmitter or a plurality of RF transmitters (or generator or excitation coils or elements). Due to at least one of the three passive electromagnetic properties of the material to be imaged, namely its electrical conductivity, its permittivity and its magnetic permeability, eddy currents are induced within the material of the object which disturb the primary magnetic field, so that a secondary magnetic field results which is detected by means of one RF receiver or a plurality of RF receivers (or measurement or detection coils or elements) so as to generate an image of the examination object.
For examining human tissue, the primary magnetic field is applied with a radio frequency (RF) in the order of between about 100 kHz and about 10 MHz. Hence, regulatory guidelines for the specific absorption rate (SAR) of the electromagnetic (EM) power have to be considered in order to avoid a hazardous excitation and heating of the object, especially in the case of human tissue.
Apart from the frequency of the RF transmitted signals, the specific absorption rate and its spatial distribution within the object of interest depend also on many other factors like e.g. the applied MIT measurement sequence, especially the RF pulse shape, RF puls duration, duty cycle, and the RF amplitude or power of the RF transmitted signal in each RF transmission channel (each comprising one or more RF transmitter elements), as well as the frequency and the sequential and/or parallel use of the RF transmitter elements.
Furthermore, it has to be considered that the placement of the RF transmitter/receiver elements or coils around the object of interest considerably influences the coupling of the transmitted RF signals into the object of interest (i.e. the couplings between the RF coils and the object), as well as the couplings between the RF coils themselves. It was found that the values or amounts of these two RF couplings may vary in a range of between about 0.01% and 10% or more in dependence on said placement.
One aspect of the invention is to provide a method and a monitoring device for conducting an RF-safe MIT scan such that an RF electromagnetic (EM) (power) exposure imposed on an examination object during an MIT scan does not exceed a certain predetermined or prescribed limit or threshold value.
The term “RF/EM (power) exposure” is to be understood in this disclosure especially as a specific absorption rate (SAR) of an examination object, or a total RF electromagnetic power applied to the examination object, or a temperature increase or an electric current density within the examination object (in order to prevent unwanted nerve stimulation), each caused by the RF/EM field (especially the RF or MIT measurement sequence) that is transmitted by means of at least one RF transmitter element or coil to the object during an MIT scan.
The invention provides a method of conducting an RF-safe MIT scan of an object of interest, comprising the following steps:
(a) conducting an RF simulation for estimating or predicting an RF electromagnetic exposure value imposed on the object in dependence on intended MIT operating parameters, based on a model of at least one RF transmitter element or coil for applying an RF electromagnetic field to the object and on a model of the object itself;
(b) comparing the estimated or predicted RF electromagnetic exposure value with a limit or threshold value which is preset for the object such that RF safety of the object is provided, and conducting the MIT scan by means of the intended MIT operating parameters in the form of demanded MIT operating parameters, if the RF electromagnetic exposure value is below the limit or threshold value,
(c) if the RF electromagnetic exposure value is equal to or exceeds the limit or threshold value, repeating step (a) with modified intended MIT operating parameters and then repeating step (b).
Furthermore, the invention provides a monitoring device which can advantageously be used in the above-mentioned method in order to provide an additional RF safety by preventing that RF-chain faults, drifts, or any other MIT system faults cause an undesired increase in the RF exposure imposed on the object. The monitoring device comprises:
It will be appreciated that features of the invention are susceptible to being combined in any combination without departing from the scope of the invention as defined by the accompanying claims.
Further details, features, and advantages of the invention will become apparent from the following description of preferred and exemplary embodiments of the invention which are given with reference to the drawings.
As mentioned above, it is of great importance in examining human tissue that a certain predetermined or prescribed limit value of the RF exposure, especially of the specific absorption rate (SAR), is not exceeded. A direct measurement of the SAR or of the resulting temperature increase in vivo is usually not possible. Instead, according to the invention, the RF/EM exposure, especially the SAR (or at least one of the other values mentioned above), is estimated or predicted by means of a mathematical RF simulation tool using a model of the at least one RF transmitter element or coil (or a plurality of models, each for one of the plurality of RF transmitter elements or coils), which at least one RF transmitter element or coil is used for applying the RF field to the object, and using a model of the object. These models describe the properties of the at least one RF transmitter element or coil and the properties of the object, respectively, as needed by the RF simulation tool for calculating the above degree of RF/EM exposure.
More in detail, the model of the at least one RF transmitter element or coil is a model of the geometry, of the RF properties, and of the position of the at least one RF transmitter element or coil in relation to the object to be imaged. The model of the object is a model of the geometry and of the RF properties of the object.
Several such RF simulation tools are known. They are based e.g. on methods like the method of moments (MoM), the finite difference time domain method (FDTD), and other methods using an evaluation of the Maxwell equations which are commercially available. Alternatively, specially developed RF calculation tools may be used in which the special conditions of the magnetic induction tomography, such as the RF frequency range and the required three-dimensional resolution of the simulation, are considered.
Generally, data on the above relevant physical and electromagnetic properties of the RF transmitter elements or coils (usually a plurality of such elements or coils is used instead of only one element or coil) as installed in the MIT apparatus and on their positions in relation to the object of interest, and data on the above relevant physical and electromagnetic properties of the object of interest to be imaged have to be provided as input data to such RF simulation tools in order to enable the RF simulation tool to estimate or predict the RF/EM exposure (especially the SAR and its spatial distribution or an electric current density within the object of interest) in dependence on intended MIT operating parameters like an intended RF power to be applied to the RF transmitter elements or coils and an intended MIT (measurement) sequence (especially in the form of the RF pulse shape, RF pulse duration, duty cycle, magnitude of the RF signal of each RF transmitter element or coil, and the frequency of the sequential and/or parallel activation or deactivation of several of the RF transmitter elements or coils) to be applied.
These input data are provided in the form of a model of the RF transmitter coils and a model of the object of interest, which models mathematically describe or represent the above relevant electromagnetic (EM) and other relevant physical RF properties of the coils (and their positions in relation to the object of interest) and of the object itself, respectively, as mentioned above. Optionally, the motion of the object of interest may be compensated by providing the RF simulation tool with related object motion input data.
More in detail, the RF simulation tool calculates from the above input data the RF field distribution inside the object of interest generated by each of the RF transmitter elements or coils.
Then, the intended MIT sequence is fragmented into sections during which the switching or driving state (especially the simultaneous or sequential activation or deactivation of one or more) of the RF transmitter elements or coils, and the sequence parameters of the RF transmission signals (e.g. the RF pulse shape, RF pulse duration, duty cycle, magnitude and frequencies of the sinusoidal RF transmission signals) of/in each RF transmitter element or coil are at least substantially constant.
Then, the calculated RF field distribution is scaled for and by means of each section (fragment) of the MIT sequence to absolute values (i.e. on the basis of the above intended RF transmission signals for each RF transmitter element), resulting in a spatially dependent RF field distribution within the object of interest which is temporally constant for the duration of each of the sections (fragments) of the MIT sequence.
Finally, this RF field distribution is used for predicting or estimating by calculation the RF/EM exposure, especially the spatially resolved specific absorption rate of the object of interest, the total applied RF/EM power (which in a parallel transmission via a plurality of RF channels is not necessarily the same as the sum of the applied powers in all channels) and, on the basis of the relative density and the specific heat capacity of the material within the object of interest, the worst-case temperature increase.
After the above RF/EM exposure values have been calculated for each section or fragment of the MIT sequence, the total value of the RF/EM exposure for the whole MIT sequence is calculated by addition of the RF/EM exposure values of all sections or fragments.
This calculation can be performed by means of known algorithms for calculating the temporal or spatial average of the RF/EM exposure for each section or fragment of the intended MIT measurement sequence.
Furthermore, the electric current densities within the object can be calculated by means of the RF simulation tool as well on the basis of the tissue parameters within the object of interest, in order to prevent that unwanted nerve stimulations occur owing to a too high electric current density.
This can be conducted by means of known maximum-finding algorithms which require a time differential operation (d/dt) as the induced currents derive from the temporal change of the magnetic field, wherein especially the transition from one section or fragment to a next section or fragment of the MIT sequence is of interest.
At least one of these four total RF/EM exposure values (SAR, totally applied RF power, worst-case temperature increase, and electric current densities) which are relevant for an RF safe MIT scanning is then compared with related prescribed or preselected limit values which must not be exceeded. If one of these RF/EM exposure values exceeds the related limit value, other values of the above intended MIT operating parameters or MIT sequence is/are selected, and the RF simulation method is repeated until MIT operating parameters are found which according to the RF simulation method do not result in a total RF/EM exposure which exceeds the related limit values, so that an MIT scanning using these MIT operating parameters can be considered as RF safe.
In a first step 10, the RF simulation tool is started. Simultaneously or prior to this step, a first routine 20 for establishing a model of the RF transmitter elements or coils and a second routine 30 for establishing a model of the object of interest to be imaged are carried out.
The first routine 20 for establishing the model of the RF transmitter coils is carried out on the basis of the geometry of the RF transmitter coils. This geometry can be obtained from at least one of a CAD model 21 of the coils, from MIT coil data 22 (which are used for image reconstruction), and/or from a coil database 23 which is available, for example, from a magnetic resonance imaging system.
Additionally, the coil model may be extended by RF specific information such as the placement and size of lumped elements of the coils, especially of capacitors and inductances, the placement of the RF power sources, and the tuning and matching of the coils.
The second routine 30 for establishing the model of the object of interest (for example a model of the head or other body parts) is carried out on the basis of at least one of data 31 which can be obtained from other modalities (like CT, X-ray, MRI, or Electric Conductivity Imaging systems), of a model 32 which is incorporated in the MIT system (because MIT incorporates also object data for image reconstruction), and of a database 33 of object models. Depending on the required accuracy it may be sufficient to use such a database with a limited number of object models, e.g. only head models of three different sizes, or, even more simple, a suitably sized spherical model of the head.
The three relevant electromagnetic properties (electrical conductivity, permittivity, and magnetic permeability) may be taken from:
Then, the established model of the RF transmitter coils is combined with the established model of the object of interest by means of the RF simulation tool, and in a second step 50 the RF field distribution inside the object of interest is calculated on the basis of the contribution of each single RF transmitter element or coil to the total RF field distribution.
Optionally, a third routine 40 is carried out for detecting and compensating a motion of the object of interest. This detection and compensation may be performed on the basis of at least one of RF measurements 41 of the load factors of the RF transmit/receive coils, of MIT reconstruction data 42 (if available in real time), and on the basis of optical or other measurements 43, for example by means of a laser distance measurement or a visual registration of movements of the object. In this case, the RF field distribution inside the object of interest calculated in the second step 50 additionally takes into consideration the movements of the object.
Then, in a third step 60, the RF simulation tool calculates the spatially dependent RF field distribution within the object of interest by applying the absolute values of RF transmission signals 61 for each fragment (section) of the MIT sequence intended to be applied to the object (if the RF transmission signals 61 are applied simultaneously via several sources (as is usual in MIT), the phases of the individual signals and fields have to be considered accordingly).
Finally, in a fourth step 70, the RF simulation tool calculates, estimates or predicts at least one of the following RF/EM exposure values from the spatially dependent RF field distribution for each section of the MIT measurement sequence by means of the above mentioned known averaging algorithms:
In many cases limit values for the maximum electric current density within the object of interest are given in addition to the limit values for the RF/EM power deposition in order to prevent unwanted nerve stimulation. These maximum current densities can also be calculated in the fourth step 70 by means of the RF simulation tool and known maximum-finding algorithms as mentioned above taking into consideration the parameters of the tissue within the object of interest.
Then, as mentioned above, the total value of the RF/EM exposure for the whole MIT sequence is calculated in the fourth step 70 through addition of the above RF/EM exposure values of all sections or fragments.
Finally, the at least one of the above four total RF/EM exposure values can be compared to regulatory or prescribed limit values. If one of these RF exposure values, e.g. the SAR value, exceeds a related limit value, at least one of the MIT operating parameters (especially the intended RF measurement sequence) is varied and the simulation method is repeated, until the calculated RF exposure values are within the prescribed limits, so that the intended MIT operating parameters can be considered to be RF safe.
The above method of estimating, predicting or calculating at least one of the four RF exposure values and the comparison with related limit values are preferably carried out in the form of a computer program by means of a computer.
For the sake of completeness, it should be mentioned that the RF fields of the single RF transmitter elements were calculated first in the above and then the total RF field distribution was calculated by addition according to the scaling for each section of the MIT sequence, but that the method may alternatively be carried out in inverse order in that the RF field contribution of each single RF transmitter element is calculated first by scaling for each section of the MIT sequence and the resulting total RF field distribution is subsequently calculated by addition.
These alternatives are selected in dependence on the number of RF transmitter elements and the number of different sections of the MIT sequence in order to save calculation time, because in the first case the calculation of the single RF fields needs considerably more time than the scaling, whereas in the latter case the calculation for each section needs considerably more time than the addition to the resulting total RF field distribution.
The calculated RF/EM exposure values may be displayed on a user interface so that in conducting an MIT scan the user of an MIT system can adjust or input into the MIT system the MIT operating parameters which have been considered RF safe according to the above simulation method.
Furthermore, the MIT operating parameters may be submitted automatically to the related MIT apparatus or system for carrying out an RF-safe MIT scanning.
A monitoring device is provided in order to ensure that the above calculated RF-safe MIT operating parameters are correctly applied and realized in the MIT system or apparatus. Such a monitoring device can prevent that the RF exposure, especially the total RF power which is applied to the object of interest, accidentally exceeds the related RF exposure limit value, especially in the case of faults such as, for example, a broken line in an RF chain, drifts or other malfunctions of components of the MIT system, or a wrong calibration or matching of such components, such as digital to analog converters and analog RF amplifiers.
According to
The RF receiver coils Rec1, . . . Recm are connected to a measurement and A/D converter unit MU for amplification of the received RF signals and for converting these signals into digital signals. These signals are then supplied to a related circuitry M of the MIT system in order to generate an image of the object of interest OI.
As mentioned above, the intended MIT operating parameters which have been simulated and found RF safe as described above and which are now to be applied to the object of interest for scanning are supplied to the MIT system circuitry M in the form of demanded MIT operating parameters OP. On the basis of these supplied demanded MIT operating parameters OP, the MIT system circuitry M submits control signals to a signal generator S for generating digital demand RF signals for each RF chain or RF channel (i.e. for each RF transmitter element or coil Tr1, . . . Trn).
The digital demand RF signals are converted by means of a digital to analog converter D/A into an analog RF signal for each RF chain, wherein the analog RF signals represent the demanded RF transmission signals and the demanded RF measurement sequence for each RF chain.
Each RF chain comprises an RF power amplifier PA1, . . . PAn for amplifying the related analog RF signal. The output of each RF power amplifier PA1, . . . PAn is connected to a respective input of the related RF transmitter coil Tr1, . . . Trn for applying the RF transmission signal to the object of interest OI.
Each line between the output of each RF power amplifier PA1, . . . PAn and the related RF transmit coil Tr1, . . . Trn contains a directional coupler Pc1, . . . Pcn by means of which a portion of the forward RF transmission signal power fed to the related RF transmit coil and a portion of the reflected RF transmission signal power reflected at the RF transmit coil is coupled out.
These RF power portions are supplied to an analog to digital converter A/D for converting them into digital values. The RF transmit power which is actually transmitted from each RF transmit coil is measured (or calculated) on the basis of the differences between the forward and the respective reflected RF powers, while considering the coupling factor or the attenuation of the directional couplers.
The measured RF transmitted power is provided in the form of a digital signal for each channel to a comparison and termination circuit C in which it is compared with the related digital demanded RF signal for each channel, generated by and supplied from the signal generator S. If, for example, the estimated RF transmit power signal for this RF chain deviates from the demand RF signal for this chain by more than a predetermined value owing to a fault in one of the RF chains, the relevant RF chain (or all RF chains) can be switched off (stepwisely or continuously so as to avoid high temporal changes), and the RF transmissions can be terminated by means of the comparison and termination circuit C.
Furthermore, the estimated RF transmitted power signal may also be fed to the measuring unit MU in order to calibrate or adjust the gain of RF amplifiers within the measuring unit MU, which are provided for amplifying the RF signals received by the RF receiver coils.
The above described monitoring device may be additionally provided with a position-monitoring device if the RF transmitter coils are to be rotated around the object of interest. Furthermore, the monitoring device may obviously also be used if the RF transmitter and receiver coils are combined into one transceiver coil in each of the RF power channels.
The setup of the monitoring device as well as the estimation of the above four RF/EM exposure values by means of the method according to
While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive, and the invention is not limited to the disclosed embodiments. Variations to embodiments of the invention described in the foregoing are possible without departing from the scope of the invention as defined by the accompanying claims.
Variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. A single processor or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. A computer program may be stored/distributed on a suitable medium, such as an optical storage medium or a solid-state medium supplied together with or as part of other hardware, but may also be distributed in other forms, such as via the Internet or other wired or wireless telecommunication systems. Any reference signs in the claims should not be construed as limiting the scope.
| Number | Date | Country | Kind |
|---|---|---|---|
| 200810210985.5 | Aug 2008 | CN | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/IB2009/053453 | 8/7/2009 | WO | 00 | 2/11/2011 |
