WO1989009564A1 - Tomography - Google Patents
Tomography Download PDFInfo
- Publication number
- WO1989009564A1 WO1989009564A1 PCT/GB1989/000363 GB8900363W WO8909564A1 WO 1989009564 A1 WO1989009564 A1 WO 1989009564A1 GB 8900363 W GB8900363 W GB 8900363W WO 8909564 A1 WO8909564 A1 WO 8909564A1
- Authority
- WO
- WIPO (PCT)
- Prior art keywords
- electrodes
- external shape
- potential
- image
- pairs
- Prior art date
Links
- 238000003325 tomography Methods 0.000 title description 3
- 238000004070 electrodeposition Methods 0.000 claims abstract description 17
- 238000000034 method Methods 0.000 claims abstract description 17
- 238000005259 measurement Methods 0.000 claims abstract description 9
- 230000003247 decreasing effect Effects 0.000 claims description 3
- 238000009826 distribution Methods 0.000 description 5
- 230000001419 dependent effect Effects 0.000 description 2
- 230000000694 effects Effects 0.000 description 1
- 230000000007 visual effect Effects 0.000 description 1
Classifications
-
- A—HUMAN NECESSITIES
- A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
- A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
- A61B5/00—Measuring for diagnostic purposes; Identification of persons
- A61B5/05—Detecting, measuring or recording for diagnosis by means of electric currents or magnetic fields; Measuring using microwaves or radio waves
- A61B5/053—Measuring electrical impedance or conductance of a portion of the body
- A61B5/0536—Impedance imaging, e.g. by tomography
Definitions
- This invention relates to tomography of the type described in GB-PS 2119520B in which tomographic images of a body are constructed by placing a plurality of surface electrodes at spaced intervals on the body, causing currents to flow in the body, and measuring the potentials between pairs of electrodes, calculating the potential in each case on the assumption that the body consists of one uniform medium, plotting the i sopotenti als corresponding to the calculated results to create a uniform image of the body, obtaining the ratio between the measured potential and the calculated potential in each case, and modifying the image in accordance with the respective ratios by increasing the impedance along an i ⁇ opotential in proportion to a ratio greater than unity or decreasing the impedance in proportion to a ratio less than unity.
- VDU visual display unit
- An additional method is provided for indicating a change of state in the body in GB-PS 2160323B, and this method includes the step of determining ratios between initial and subsequent measured actual potentials between electrodes.
- An object of the present invention is the determination of the external shape of a body preparatory to obtaining to ographic images of it.
- Another object is to construct a tomographic image that accurately depicts the internal structure of the body.
- a method of determining the external shape of a body comprises placing a plurality of electrodes adjacent the surface at spaced intervals round the body, causing currents to flow in the body by applying an electrical potential between pairs of electrodes in turn, measuring potentials between other pairs of electrodes, relating the measured potentials to the distances between the corresponding points of current introduction and of potential measurement, and determining electrode positions consistent with these distances, which electrode positions define the external shape of the body.
- the external shape of the body can be plotted, e.g., on a VDU and or as a print-out, by means of a computer, on the assumption that the body consists of one uniform medium and plotting the image of electrode positions corresponding to the measured values.
- V ijkl I.F(_ri,_rj,_rk,_r ⁇ ,b(_r) ,c(_r)) where ri r-i rk r rl are t * ⁇ e positions of the electrodes, b (r_) is the function representing the boundary and c(_r) is the internal impedance distribution. Since the Vijkl are on --y weakly dependent on c(r) this can be replaced by c, a uniform impedance throughout the object. Similarly since the dependence of V ⁇ on b .) is wea ker than the dependence on electrode position, these can be combined into a single function F 1 of the electrode position. in the absence of a boundary this function can be obtained from simple geometric considerations alone. In the presence of a boundary this function will deviate from this ideal form in a way that may be modelled empirically. In summary
- the position vector s r. of the N electrodes may be computed and hence the boundary determined.
- the fact that there are at least N(N-4)/2 independent measurements and only 2 electrode coordinates to compute means that the problem is usefully over-determined leading to a robust solution for the electrode positions.
- these computed electrode positions are not sensitive to the internal impedance distribution; data from non-uniform and uniform internal impedance distributions will generate almost the same electrode positions.
- ° ⁇ V ijkl corresponding to a 'reference' distribution may be generated and used to calculate images of absolute impedance in conjunction with the measured 'data 1 set.
- More accurate estimates of boundary shape may be achieved by (a) a more complex version of F' than that given in Equation 2, such as
- a tomographic image that accurately depicts the internal structure of the body can be constructed by calculating the potential between pairs of electrodes on the assumption that the body consists of one uniform medium, plotting the isopotential s corresponding to the calculated results to form an image, obtaining the ratio between the measured potential and the calculated potential in each case, and modifying the image in accordance with the respective ratios by increasing the impedance along an isopotential in proportion to a ratio greater than unity or decreasing the impedance in proportion to a ratio less than unity, as described in GB-PS 2119520B.
- a typical print-out image is shown in Figure 4 of the accompanying drawings.
- the method can be adapted for indicating a change of state in the body by determining ratios between initial and subsequent measured actual potentials between electrodes.
Abstract
A method of determining the external shape of a body comprises placing a plurality of electrodes adjacent the surface at spaced intervals round the body, causing currents to flow in the body by applying an electrical potential between pairs of electrodes in turn, measuring potentials between other pairs of electrodes, relating the measured potentials to the distances between the corresponding points of current introduction and of potential measurement, and determining electrode positions consistent with these distances, which electrode positions define the external shape of the body.
Description
TOMOGRAPHY
This invention relates to tomography of the type described in GB-PS 2119520B in which tomographic images of a body are constructed by placing a plurality of surface electrodes at spaced intervals on the body, causing currents to flow in the body, and measuring the potentials between pairs of electrodes, calculating the potential in each case on the assumption that the body consists of one uniform medium, plotting the i sopotenti als corresponding to the calculated results to create a uniform image of the body, obtaining the ratio between the measured potential and the calculated potential in each case, and modifying the image in accordance with the respective ratios by increasing the impedance along an iεopotential in proportion to a ratio greater than unity or decreasing the impedance in proportion to a ratio less than unity.
The calculations of potentials and the obtaining of ratios have been carried out using a computer, and the plotting of the i so potential s have been carried out by a visual display unit (VDU) and/or a print-out run off the computer.
An additional method is provided for indicating a change of state in the body in GB-PS 2160323B, and this method includes the step of determining ratios between initial and subsequent measured actual potentials between electrodes.
Both methods are described in US-PS 4617939 and both methods assume that the cross-sectional shape of a body is circular and thus produce circular images, which may not be
satisfactory if the true cross- sectional shape is far from circular, as is frequently the case.
An object of the present invention is the determination of the external shape of a body preparatory to obtaining to ographic images of it.
Another object is to construct a tomographic image that accurately depicts the internal structure of the body.
According to the present invention, a method of determining the external shape of a body comprises placing a plurality of electrodes adjacent the surface at spaced intervals round the body, causing currents to flow in the body by applying an electrical potential between pairs of electrodes in turn, measuring potentials between other pairs of electrodes, relating the measured potentials to the distances between the corresponding points of current introduction and of potential measurement, and determining electrode positions consistent with these distances, which electrode positions define the external shape of the body.
The external shape of the body can be plotted, e.g., on a VDU and or as a print-out, by means of a computer, on the assumption that the body consists of one uniform medium and plotting the image of electrode positions corresponding to the measured values.
The method of determining the external shape of a body will now be described with reference to the diagrammatic
Figures 1 to 3 of the accompanying drawings.
The voltage measured between a pair of electrodes connected to a body surface when a current is applied between a second pair of electrodes applied to the body surface
depends on (a) the relative positions of the electrodes on the surface; (b) the shape of the body surface and (c) the internal impedance distribution within the body. (a) is the dominant factor followed by (b) and then (c). However the use of a calculated or measured reference set allows the contributions from (a) and (b) to be largely eliminated from the image reconstruction calculation; the differences in the sets of measurements between the 'reference' set and the •data' set are dependent almost solely on the changes in internal impedance. Because the effect of boundary shape is supressed the shape of the boundary for the reconstructed images is by default circular.
The following describes how a single set of measurements ('data') can be used to calculate the shape of the boundary and generate a computed set of reference measurements.
THEORY
If N electrodes defining a plane are connected to the body surface then a measurement set consists of N(N-3)/2 independent measurements if drive electrodes are adjacent to each other or N(N-4)/2 otherwise. The voltages measured between a pair of electrodes (k,l) when a current I is applied between a second pair of electrodes (i,j) is given by (Figure 1)
Vijkl = I.F(_ri,_rj,_rk,_rι,b(_r) ,c(_r)) where ri r-i rkrrl are t*ιe positions of the electrodes, b (r_) is the function representing the boundary and c(_r) is the internal impedance distribution. Since the Vijkl are on--y
weakly dependent on c(r) this can be replaced by c, a uniform impedance throughout the object. Similarly since the dependence of V^^ on b .) is weaker than the dependence on electrode position, these can be combined into a single function F1 of the electrode position. in the absence of a boundary this function can be obtained from simple geometric considerations alone. In the presence of a boundary this function will deviate from this ideal form in a way that may be modelled empirically. In summary
Vijkl - ICP' irJZjrXkril) (1)
Given the set of measured V and a model of the function F1 the position vector s r. of the N electrodes may be computed and hence the boundary determined. In the absence of an exact model of F1 the fact that there are at least N(N-4)/2 independent measurements and only 2 electrode coordinates to compute means that the problem is usefully over-determined leading to a robust solution for the electrode positions. For a similar reason these computed electrode positions are not sensitive to the internal impedance distribution; data from non-uniform and uniform internal impedance distributions will generate almost the same electrode positions. By inserting the electrode positions back into Equation 1 values
°^ Vijkl corresponding to a 'reference' distribution may be generated and used to calculate images of absolute impedance in conjunction with the measured 'data1 set.
REAL ISA ION
If the voltages are always measured between adjacent electrodes, current is also always applied between adjacent electrodes, the spacing between adjacent electrodes is
constant and this spacing is small compared to the dimensions of the body then empirically the function F may be represented by
F» (d) = 1 ikn or, representing
vi,i+l,k,k+l by Vik
Vik Ic dikn where di •**•-**■* t*tιe distance between the midpoint between the electrodes i and i + 1 and the midpoint between electrodes k and k + 1 (Figure 2). A value of n = 3.7 has proved effective.
The starting point for determining the boundary shape is a circular boundary with equally spaced electrodes. Two pairs of electrodes are taken (typically those placed at the ends of the maximum diameter of the body) and the coordinates of these electrode pairs fixed. The distance between them is chosen, (this fixes the scale of the boundary) and the constant C in vik = §-7 (2) d.. ■3-
1.. is computed from this distance and the measured value of V-^. Changing the scale of this reference d simply changes the value of the constant C. For the remaining pairs of 'drive' and 'receive' electrode pairs the distances between them are
Consider a 'drive' pair initially situated on the circular boundary and a 'receive' pair also on the boundary.
The value of d is computed and this distance is measured out along the line joining the 'drive' and 'receive' pairs from the 'drive' pair. The point at the end of this distance is now taken to be a new estimate of the position of the 'receive' pair. This process is repeated for all combinations of 'drive' and 'receive' electrode pairs. For each electrode pair there are N-3 position estimates relative to all other pairs seen as 'drive* electrode pairs and the position estimates are averaged. These position estimates now define a new boundary, no longer circular. The process of position estimation is now repeated until no further change in the position estimates occurs. Estimation of the electrode position (and boundary shape) is now complete (Figure 3). Faster convergence may be obtained if a starting boundary shape (e.g. an ellipse) closer to that of the required boundary shape than a circle is chosen. Once the boundary shape has been defined an appropriate form of the function F' in equation 1 may be determined from the measured values of Vi;jkl and the calculated electrode positions and used to calculate reference values.
More accurate estimates of boundary shape may be achieved by (a) a more complex version of F' than that given in Equation 2, such as
1.0 and m = 2.6,
or (b) choice of a simple form for F1 but restriction of its
use to a pre-εelected subset of drive and receive pairs.
A tomographic image that accurately depicts the internal structure of the body can be constructed by calculating the potential between pairs of electrodes on the assumption that the body consists of one uniform medium, plotting the isopotential s corresponding to the calculated results to form an image, obtaining the ratio between the measured potential and the calculated potential in each case, and modifying the image in accordance with the respective ratios by increasing the impedance along an isopotential in proportion to a ratio greater than unity or decreasing the impedance in proportion to a ratio less than unity, as described in GB-PS 2119520B. A typical print-out image is shown in Figure 4 of the accompanying drawings.
Again, as described in GB-PS 2160323B and US-PS 4617939, the method can be adapted for indicating a change of state in the body by determining ratios between initial and subsequent measured actual potentials between electrodes.
Claims
1. A method of determining the external shape of a body comprising placing a plurality of electrodes adjacent the surface at spaced intervals round the body, causing currents to flow in the body by applying an electrical potential between pairs of electrodes in turn, measuring potentials between other pairs of electrodes, relating the measured potentials to the distances between the corresponding points of current introduction and of potential measurement, and determining electrode positions consistent with these distances, which electrode positions define the external shape of the body.
2. A method as in Claim 1, wherein the external shape of the body is plotted on a VDU by means of a computer, on the assumption that the body consists of one uniform medium and plotting the image of electrode positions corresponding to the measured values.
3. A method as in Claim 1 or Claim 2, wherein the external shape of the body is plotted as a print-out by means of a computer, on the assumption that the body consists of one uniform medium and plotting the image of electrode positions corresponding to the measured values.
4. A method of constructing a tomographic image that accurately depicts the internal structure of a body comprising the method of determining the external shape of the body as in Claim 2 or Claim 3 and then calculating the potential between pairs of electrodes on the assumption that the body consists of one uniform medium, plotting the isopotential s corresponding to the calculated results to form an image, obtaining the ratio between the measured potential and the calculated potential in each case, and modifying the image in accordance with the respective ratios by increasing the impedance along an isopotential in proportion to a ratio greater than unity or decreasing the impedance in proportion to a ratio less than unity.
5. A method of indicating a change of state in a body comprising constructing tomographic images by the method of Claim 4 determined by ratios between initial and subsequent measured potentials between electrodes respectively before and after a change in the state of the body.
Priority Applications (1)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
GB9023905A GB2236397B (en) | 1988-04-15 | 1990-11-02 | Tomography |
Applications Claiming Priority (2)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
GB8808969.3 | 1988-04-15 | ||
GB888808969A GB8808969D0 (en) | 1988-04-15 | 1988-04-15 | Tomography |
Publications (1)
Publication Number | Publication Date |
---|---|
WO1989009564A1 true WO1989009564A1 (en) | 1989-10-19 |
Family
ID=10635274
Family Applications (1)
Application Number | Title | Priority Date | Filing Date |
---|---|---|---|
PCT/GB1989/000363 WO1989009564A1 (en) | 1988-04-15 | 1989-04-10 | Tomography |
Country Status (5)
Country | Link |
---|---|
AU (1) | AU628334B2 (en) |
FR (1) | FR2630235B1 (en) |
GB (2) | GB8808969D0 (en) |
IT (1) | IT1229015B (en) |
WO (1) | WO1989009564A1 (en) |
Cited By (2)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
US5575292A (en) * | 1991-06-27 | 1996-11-19 | British Technology Group Limited | Applied potential tomography |
GB2580164A (en) * | 2018-12-21 | 2020-07-15 | Imperial College Sci Tech & Medicine | A sensor |
Families Citing this family (1)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
US7984567B2 (en) * | 2008-10-07 | 2011-07-26 | Christ Bill Bertakis | Apparatus for cleaning simulated hair articles |
Citations (3)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
GB2138148A (en) * | 1983-04-13 | 1984-10-17 | Denis Nigel Smith | Method and apparatus for deriving currents and potentials representative of the impedance of zones of a body |
US4486835A (en) * | 1981-05-13 | 1984-12-04 | Yeda Research And Development Co. Ltd. | Apparatus and techniques for electric tomography |
US4617939A (en) * | 1982-04-30 | 1986-10-21 | The University Of Sheffield | Tomography |
-
1988
- 1988-04-15 GB GB888808969A patent/GB8808969D0/en active Pending
-
1989
- 1989-04-10 AU AU34465/89A patent/AU628334B2/en not_active Ceased
- 1989-04-10 WO PCT/GB1989/000363 patent/WO1989009564A1/en unknown
- 1989-04-14 FR FR898905236A patent/FR2630235B1/en not_active Expired - Fee Related
- 1989-04-14 IT IT8920139A patent/IT1229015B/en active
-
1990
- 1990-11-02 GB GB9023905A patent/GB2236397B/en not_active Expired - Fee Related
Patent Citations (3)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
US4486835A (en) * | 1981-05-13 | 1984-12-04 | Yeda Research And Development Co. Ltd. | Apparatus and techniques for electric tomography |
US4617939A (en) * | 1982-04-30 | 1986-10-21 | The University Of Sheffield | Tomography |
GB2138148A (en) * | 1983-04-13 | 1984-10-17 | Denis Nigel Smith | Method and apparatus for deriving currents and potentials representative of the impedance of zones of a body |
Cited By (2)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
US5575292A (en) * | 1991-06-27 | 1996-11-19 | British Technology Group Limited | Applied potential tomography |
GB2580164A (en) * | 2018-12-21 | 2020-07-15 | Imperial College Sci Tech & Medicine | A sensor |
Also Published As
Publication number | Publication date |
---|---|
GB9023905D0 (en) | 1991-01-09 |
GB2236397B (en) | 1992-01-15 |
AU3446589A (en) | 1989-11-03 |
IT1229015B (en) | 1991-07-12 |
GB2236397A (en) | 1991-04-03 |
FR2630235A1 (en) | 1989-10-20 |
GB8808969D0 (en) | 1988-05-18 |
FR2630235B1 (en) | 1994-09-02 |
IT8920139A0 (en) | 1989-04-14 |
AU628334B2 (en) | 1992-09-17 |
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