Patent Application
20110133731
The invention relates to magnetic induction tomography, particularly to specific coil arrangements for a magnetic induction tomography scanner.
Magnetic induction tomography (MIT) is a noninvasive and contactless imaging technique with applications in industry and medical imaging. In contrast to other electrical imaging techniques, MIT does not require direct contact of the sensors with the object of interest for imaging.
MIT is used to reconstruct the spatial distribution of the passive electrical properties inside the object of interest, for example, conductivity σ, permittivity ε and permeability μ. In MIT, sinusoidal electric current, normally between a few kHz up to several MHz, is applied to a transmitting coil, generating a time varying magnetic field. This is normally called the primary magnetic field. Due to the conducting object of interest, for example a biological tissue, the primary field produces eddy currents in the object of interest. These eddy currents generate the secondary magnetic field. The combination of these magnetic fields induces voltages in the receiving coils. Using several transmitting coils and repeating the measurements, sets of measurement data are taken and used to visualize changes in time of the electromagnetic properties of the object. MIT is sensitive to all three passive electromagnetic properties: electrical conductivity, permittivity and magnetic permeability. As a result, for example, the conductivity contribution in the object of interest can be reconstructed. In particular, MIT is suitable for examination of biological tissue, because of the magnetic permeability value of such tissue μR≈1.
Prior art patent application WO2007072343 discloses a magnetic induction tomography system for studying the electromagnetic properties of an object. The system comprises: one or more generator coils adapted for generating a primary magnetic field, said primary magnetic field inducing an eddy current in the object; one or more sensor coils adapted for sensing a secondary magnetic field, said secondary magnetic field being generated as a result of said eddy current; and means for providing a relative movement between one or more generator coils and/or one or more sensor coils, on the one hand, and the object to be studied, on the other hand.
However, the sensitivity in the centre of the object of interest is not very good with existing MIT technology. This is due to the fact that the transmitting coils and the measurement coils for detection are positioned around the object of interest while the fields are not focused in the center of the object of interest and therefore the sensitivity near the surface of the object is higher than that in the center of the object. That becomes a problem when information about the central part of the object is of interest.
According to one embodiment of the invention, a device that improves the sensitivity of MIT in a central part of an object of interest is provided. The device comprises:
It is advantageous for the pair of transmitting coils and the pair of measurement coils to be respective Helmholtz coils.
By replacing conventional transmitting coils and measurement coils with Helmholtz coils or coils arranged substantially close to Helmholtz coils, fairly homogenous but still localized sensitivity distribution inside the object of interest can be achieved.
It is also advantageous for the pair of transmitting coils and the pair of measurement coils to be arranged along the axis. The distance between the pair of transmitting coils and the pair of measurement coils is determined such that the maximum current density of eddy current in the object of interest generated by the pair of transmitting coils and the distribution of the maximum sensitivity of the pair of measurement coils are overlapped.
By overlapping the maximum current density of eddy current and the distribution of maximum sensitivity of the pair of measurement coils, the sensitivity in the center of the object of interest is maximized.
According to another embodiment of the invention, this invention further provides a method that improves the sensitivity of MIT in a central part of an object of interest. The method comprises the steps of:
Detailed explanations and other aspects of the invention will be given below.
The above and other objects and features of the present invention will become more apparent from the following detailed description considered in connection with the accompanying drawings, in which:
a and 2b depict a distribution of current density of the eddy current generated by the transmitting coils in accordance with the invention.
a and 3b depict a distribution of sensitivity of the measurement coils in accordance with the invention.
a, 4b, 4c and 4d depict how to position the transmitting coils and the measurement coils in accordance with the invention.
a and 5b depict the coil arrangement with resulting sensitivity line used for measurement in accordance with the invention.
a and 6b depict how to obtain multiple sets of measurements in accordance with the invention.
a and 8b further depict another exemplary embodiment of the device in accordance with the invention.
The same reference numerals are used to denote similar parts throughout the figures.
According to the invention, the device 100 comprises a transmitting coil arrangement, which comprises a pair of transmitting coils 112, 114 that are positioned symmetrically along a common axis A, e.g. the two transmitting coils are placed at two sides of an object of interest 101. The object of interest 101 is an object to be measured, for example, the head of a human being, or any other conductive material.
The transmitting coils 112, 114 are intended for carry substantially equal electrical current flowing in the same direction to generate a primary magnetic field. As shown in
In an embodiment, the pair of transmitting coils 112, 114 can be connected to ensure the electrical currents are substantially equal and flowing in the same direction.
The device further comprises a measurement coil arrangement, which comprises a pair of measurement coils 122, 124 that are connected. The two measurement coils are positioned symmetrically along axis A, similar to the transmitting coils.
The measurement coils 122, 124 are arranged for measuring signals induced by the secondary magnetic fields to generate a set of measurement data for image reconstruction. As the secondary magnetic field generated by the eddy current, it carries information about the inside of the object of interest, for example, the conductivity distribution of a tissue of a human head or any other conductive material.
The signals induced by the secondary magnetic field are induced voltages. As the voltage induced by the secondary magnetic field is very small relative to the voltage induced by the primary magnetic field, it is difficult to extract the voltage induced by the secondary magnetic field directly, given the strong background magnetic field.
Among a number of measurement techniques discussed in prior art documents, one approach is to measure the voltage change from a reference measurement. The measured voltage change indicates the change of the secondary magnetic field generated by the eddy current and thus can be used for difference imaging to visualize the change of the conductivity distribution in the object of interest.
The device further comprises a processor 140 for reconstructing images based on the set of measurement data. The image reconstruction may follow the method of conductivity calculations and image reconstruction that is described in the prior art document “Image reconstruction approaches for Philips magnetic induction tomography”, M. Vauhkonen, M. Hamsch and C. H. Igney, ICEBI 2007, IFMBE Proceedings 17, pp. 468-471, 2007. The image reconstruction, e.g. the calculation of conductivity distribution in the object of interest can be advantageously implemented by a software program embedded in the processor.
It is advantageous for the pair of transmitting coils 112, 114 and the pair of measurement coils 122, 124 to be respective Helmholtz coils.
It is well known that Helmholtz coils consist of two identical circular magnetic coils that are placed symmetrically one on each side of the experimental area, i.e. the object of interest, along a common axis, and separated by a distance h equal to the radius R of the coils. Each coil carries an equal electrical current flowing in the same direction. Optionally, the Helmholtz coils can be electrically connected so that their currents flow in the same direction (the connection may be either serial or parallel).
It is should be noticed that the size and the shape of the object of interest depicted in
a and 2b depicts a distribution of current density of the eddy current generated by the transmitting coils in accordance with the invention.
A pair of circular coils 112, 114 (Helmholtz coils), working as transmitting coils, are positioned symmetrically along axis A, e.g. are placed at two sides of the object of interest 101, assuming the object of interest 101 is a homogeneous tissue block with constant conductivity. When the two transmitting coils are fed with substantially equal electrical current flowing in the same direction, two thin linear areas are generated in the object of interest that represent maximum current density of the eddy current produced by the transmitting coils. The linear areas go through the object of 101 between the coils and are indicated by lines 201, 202.
a and 3b depict a distribution of sensitivity of the measurement coils in accordance with the invention.
A pair of circular coils 122, 124 (Helmholtz coils) working as measurement coils, is positioned in symmetrically along axis A, e.g. they are placed at two sides of the tissue block. The linear areas indicated by lines 305, 306 are formed that represent maximum sensitivity areas of the measurement coils, e.g., the measurement coils have high sensitivity in the area along the lines 305, 306.
a,
4
b, 4c and 4d depict how to position the transmitting coils and the measurement coils in accordance with the invention.
As shown in
When transmitting coils 112, 114 and measurement coils 122, 124 are placed inwards, e.g. along axis A in the arrowhead directions as shown in
a and 5b depict the coil arrangement with resulting sensitivity line used for measurement.
There is only one linear area, e.g., line 508, which represents the overlap of the expected maximum sensitivity of the measurement coils and the maximum current density of eddy current generated by the transmitting coils, and thus is of concern for efficient measurement of the secondary magnetic field generated by the eddy current. So for measurement, the measurement data mainly comprises the information of signals from this area.
As shown in
It is also appreciated by those skilled in the art that the device in accordance with the invention may comprise means (not shown in the figures) for providing such a relative movement between the coils arrangement and the object of interest.
It is appreciated by those skilled in the art that the transmitting arrangement and/or the measurement arrangement may comprise a plurality of Helmholtz coils to speed up the measurement procedure.
As shown in
As shown in
It is appreciated by those skilled in the art that the scanners described in
According to the invention, the method of magnetic induction tomography comprises a step 910 of generating a primary magnetic field by providing an excitation signal to a transmitting coil arrangement. The transmitting coil arrangement comprises a pair of transmitting coils 112, 114 intended for carrying substantially equal electrical current flowing in the same direction. The two transmitting coils are positioned symmetrically along a common axis A. The primary magnetic field induces an eddy current in an object of interest that generates a secondary magnetic field.
The method further comprises a step 920 of measuring signals induced by the secondary magnetic fields to generate a set of measurement data by using a measurement coil arrangement. The measurement coil arrangement comprises a pair of measurement coils 122, 124 that are connected and positioned symmetrically along axis A. In an embodiment, the pair of transmitting coils 112, 114 and the pair of measurement coils 122, 124 are respective Helmholtz coils.
The method further comprises a step 930 of reconstructing an image representing conductivity distribution of the object of interest based on the set of measurement data obtained in step 920.
It is advantageous for the method to further comprise a step 902 of positioning the pair of transmitting coils and the pair of measurement coils along the axis A and a step 905 of determining the distance between the pair of transmitting coils and the pair of measurement coils such that the distribution of the maximum current density of eddy current in the object of interest generated by the pair of transmitting coils and the distribution of the maximum sensitivity of the pair of measurement coils have an overlapped area.
It is advantageous for the method to further comprise a step 925 of providing a relative movement between the coils arrangement and the object of interest so as to collect a plurality of sets of measurement data for image reconstruction.
It should be noted that the above-mentioned embodiments illustrate rather than limit the invention and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference symbols placed between parentheses shall not be construed as limiting the claim. Use of the verb “comprise” and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. Use of the indefinite article “a” or “an” preceding an element does not exclude the presence of a plurality of such elements. The invention can be implemented by means of hardware comprising several distinct elements and by means of a suitably programmed computer. In the device claims enumerating several units, several of these units can be embodied by one and the same item of hardware or software. Use of the words “first”, “second” and “third”, etc. does not indicate any ordering. These words are to be interpreted as names.
| Number | Date | Country | Kind |
|---|---|---|---|
| 200810211035.4 | Aug 2008 | CN | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/IB2009/053460 | 8/7/2009 | WO | 00 | 2/14/2011 |
