CN102908141B - Inverse tangent method for reconstructing isopotential line back projection electrical impedance tomography image - Google Patents

Abstract

The invention discloses an inverse tangent method for reconstructing an isopotential line back projection electrical impedance tomography image. The method comprises the following steps of: sticking electrical impedance measuring electrodes to the circumferential surface of a body to be measured at equal intervals; selecting two adjacent electrodes, applying current excitation to a measured target through the two electrodes, and acquiring boundary response voltage of an object to be measured between remaining two adjacent electrodes simultaneously; switching a pair of adjacent electrodes in sequence for applying excitation to a target, and acquiring boundary response voltage of the object to be measured between remaining two adjacent electrodes simultaneously; saving the boundary voltage/excitation current as a frame of data, and defining as a reference frame boundary voltage; measuring repeatedly according to a set time interval to obtain the boundary voltage/excitation current of another frame, and defining as current frame boundary voltage; performing inverse tangent processing on a difference between the current frame boundary voltage and the reference frame boundary voltage by adopting an inverse tangent method; and substituting processed data into an isopotential line back projection algorithm for performing electrical impedance dynamic image reconstruction, and displaying.

Description

A kind of equipotential line back projection electrical impedance imaging image rebuilding method of arc tangent compression

Technical field

The present invention relates to a kind of projection electrical impedance imaging image rebuilding method, particularly a kind of equipotential line back projection electrical impedance imaging image rebuilding method of arc tangent compression.

Background technology

Electrical impedance tomography (Electrical impedance tomography, EIT) be by applying the signal of telecommunication of one or more frequencies around at a certain section of human body, and measure it in the response signal on section border, according to the relation of response signal and driving signal, utilize specific electrical impedance algorithm for reconstructing, and then distribution and the variation thereof of calculating resistivity on this section, the distribution of this resistivity or its variation can show in the mode of two-dimensional digital image, thereby provides a kind of new image check means for clinical diagnose.

Electrical impedance algorithm for reconstructing is the core technology of EIT, and electrical impedance algorithm for reconstructing mainly contains two classes: dynamic electric impedance algorithm for reconstructing and Static Electro impedance algorithm for reconstructing.Static Electro impedance algorithm for reconstructing is the absolute value that the resistivity of calculating section inside according to many groups border transfer impedance of recording is divided face, and its algorithm complex is high, and very high to hardware system required precision.And dynamic electric impedance algorithm for reconstructing is the picture that is varied to resistivity distribution on section, due to obtain be a relative quantity (twice or repeatedly between variation), thereby greatly reduce the requirement to hardware system error level and precision, this imaging algorithm is applied in actual EIT system.

Because the object of electrical impedance algorithm for reconstructing is to calculate section internal driving according to the border mutual impedance of section to distribute, it to be processed to as if boundary voltage He electricity circle electric current.Conventionally EIT system can uniformly-spaced be placed a plurality of electrodes around at imaging section, respectively in order to load driver signal and measurement response signal, in imaging process, there is pair of electrodes to add driving signal simultaneously, pair of electrodes is measured response signal, driving/measurement scheme that EIT system generally adopts adjacent electrode driving/adjacent electrode to measure.This driving/the measurement scheme adopting just because of EIT system, between the boundary response signal that causes recording, have a very large excursion, away from response signal between the electrode of drive electrode by much smaller than near response signal between the electrode of drive electrode, in 32 electrode single-frequency EIT systems, the ratio of peak response signal and minimum response signal is greater than 100, and along with this ratio of increase of border electrode number also can be larger, this phenomenon is referred to as the dynamic range of response signal, the dynamic range of boundary response signal is excessive is not only a very large obstacle to prime measuring system, to electrical impedance algorithm for reconstructing, be also a huge problem simultaneously, it can cause algorithm for reconstructing very insensitive to the central area of target section, thereby the electrical impedance of can not observing central area on final image changes.

Equipotential line Inverse Projection is a kind of dynamic electric impedance algorithm for reconstructing, its last imaging reflects the variation of the resistivity distribution in target section during twice measurement, classical equipotential line Inverse Projection is based on following hypothesis: 1, two interelectrode distance of current drives are less than section zone radius, thereby field domain is equivalent to the electric field of a pair of electric dipole generation the viewing area in; 2, measure the variation of discontinuity surface internal resistance rate for twice much smaller than the absolute value of background resistivity, i.e. it is a little disturbance that this variation distributes for field domain internal resistance rate.

Equipotential line impedance back projection is actually the imaging to target area distribution of impedance disturbance: if the target area that is σ (x, y) in distribution of impedance loads one group of drive current around, obtaining one group of response voltage is V ₀, distribution of impedance occurs after a less variation (disturbance), σ (x, y) → σ (x, y)+Δ σ (x, y), and recording response voltage is V ₁, by carrying out back projection after two groups of voltage difference normalization, can obtain the disturbance of distribution of impedance.

Isoelectric level collimation method is a kind of impedance reconstructing method of approximately linear, and its main thought derives from early stage x-ray CT imaging technique.After every a pair of drive electrode load driver electric current, in target area, form a specific Electric Field Distribution, such electric field that two drive electrodes are formed is equivalent to the electric field that an electric dipole produces, electric dipole is positioned at two electrode centers, by electric dipole, to each measurement electrode, there are a series of equipotential lines like this, region between adjacent equipotential line is back projection district, the voltage difference recording before and after impedance disturbance is superimposed to this region, the voltage difference back projection of all projections is returned behind corresponding projected area, just can obtain the disturbance image (variation) of region electrical impedance distribution.

If measuring for the first time one group of boundary response voltmeter is shown:

V _ref

The boundary response voltmeter measuring after the internal resistance rate generation disturbance of region is shown:

V _d

One group of voltage difference normalization between twice measurement is expressed as V later _n:

$v_n=\frac{v_d-v_{\mathrm{ref}}}{v_{\mathrm{ref}}}$

In actual EIT system, V _refand V _dbody has very large dynamic range, and peak signal and minimum signal ratio are generally greater than 100, after its difference normalization, and V _nwill there is larger dynamic range, the ratio of its peak signal and minimum signal, wherein, v _nmax: the peak signal in one group of boundary voltage, v _nmin: the minimum signal in one group of boundary voltage; Thereby aggravated dynamic range, it is responsive that result makes the electrical impedance disturbance of central area can not show a candle to the disturbance of borderline region electrical impedance, very likely in the impedance disturbance that cannot observe central area on image.

In multifrequency EIT system, sideband signal will have larger dynamic range simultaneously, and when rebuilding imaginary part, phase shift distribution, the now use of equipotential line Inverse Projection in a kind of like this dynamic range will be deeply restricted.

Summary of the invention

The object of the invention is to solve the larger problem of EIT sideband signal dynamic range, a kind of equipotential line back projection electrical impedance imaging image rebuilding method of dynamic arc tangent compression is provided.

For achieving the above object, the technical solution used in the present invention is:

1) first 16 impedance bioelectrical measurement electrodes are uniformly-spaced pasted on to body peripheral surface to be measured;

2) impedance bioelectrical measurement electrode is connected with EIT acquisition system;

3) select wherein two adjacent electrodes, by these two electrode pair measured targets, apply current excitation, at all the other, between adjacent electrode, gather between two the boundary response voltage of object under test simultaneously;

4) switch successively a pair of adjacent electrode target is applied to excitation, and simultaneously at all the other, between adjacent electrode, gather between two the boundary response voltage of object under test, repeat this process, until all electrodes were all used as exciting electrode;

5) above-mentioned 16 groups of boundary voltage/exciting currents are preserved as frame data, be defined as reference frame boundary voltage, be designated as V _ref;

6) according to the interval repeating step 3 of setting), 4) measurement, obtain 16 groups of boundary voltage/exciting currents of an other frame, be defined as present frame boundary voltage, be designated as V _d;

7) adopt arc tangent method to carry out arc tangent processing to the difference of present frame boundary voltage and reference frame boundary voltage: arctg (v _d-v _ref);

8) finally the data substitution equipotential line backprojection algorithm after above-mentioned processing is carried out to the reconstruction of electrical impedance dynamic image, and show.

For equipotential line Inverse Projection in the excessive situation of sideband signal dynamic range ( ) cannot use this problem, the present invention proposes the dynamic electric impedance fault imaging algorithm based on arc tangent back projection.For the consideration that improves dynamic range of signals, introduced arctan function and processed that will to intend carrying out the boundary voltage of back projection poor, use:

arctg(v _d-v _ref)

Replace carry out back projection.The dynamic electric impedance fault imaging algorithm of utilization based on arc tangent back projection carried out the real part imaging under many drive currents frequency EIT, and picture quality has had showing and improves.Meanwhile, also obtained with the not getable imaginary impedance picture of original equipotential line Inverse Projection.

The specific embodiment

The equipotential line back projection electrical impedance imaging image rebuilding method of arc tangent compression of the present invention is as follows:

1) first 16 impedance bioelectrical measurement electrodes are uniformly-spaced pasted on to body peripheral surface to be measured;

2) impedance bioelectrical measurement electrode is connected with EIT acquisition system;

3) select wherein two adjacent electrodes, by these two electrode pair measured targets, apply current excitation, at all the other, between adjacent electrode, gather between two the boundary response voltage of object under test simultaneously;

4) switch successively a pair of adjacent electrode target is applied to excitation, and simultaneously at all the other, between adjacent electrode, gather between two the boundary response voltage of object under test, repeat this process, until all electrodes were all used as exciting electrode;

5) above-mentioned 16 groups of boundary voltage/exciting currents are preserved as frame data, be defined as reference frame boundary voltage, be designated as V _ref;

6) according to the interval repeating step 3 of setting), 4) measurement, obtain 16 groups of boundary voltage/exciting currents of an other frame, be defined as present frame boundary voltage, be designated as V _d;

7) adopt arc tangent method to carry out arc tangent processing to the difference of present frame boundary voltage and reference frame boundary voltage: arctg (v _d-v _ref);

8) finally the data substitution equipotential line backprojection algorithm after above-mentioned processing is carried out to the reconstruction of electrical impedance dynamic image, and show.

The present invention adopts the method for arc tangent to have following characteristics:

1, first, analyze the character of arctan function:

When the independent variable x of arctan function f (x)=arctg (x) changes in open interval (∞+∞), its codomain is in open interval interior variation, and between its domain of definition and codomain, be mapping relations one to one.Through after such functional transformation, the excursion of the dependent variable f of function (x) has become a finite interval compared with the excursion of its independent variable x from infinite interval.

Observe arctan function feature in shape.Such function, it changes in different intervals is inhomogeneous: approaching central area (domain of definition is between zero left and right region), its dependent variable changes and showing with independent variable; And away from central area (more approaching ± ∞), the variation occurring that its dependent variable changes with independent variable is more not obvious.

2, the improvement of arctan function to back projection's parameter

Due to the These characteristics of arctan function, thereby Hui Dui back projection parameter is brought very large impact: if by the variation v of boundary voltage _di-v _refafter asking for arc tangent, as back projection, measure, obtain:

arctg(v _d-v _ref)

With the original variable for back projection compare, back projection's variable has had following main some improvement:

1) arctg (v _d-v _ref) codomain scope obtained the compression showing, (∞+∞) boil down to open interval from open interval

2) in v _refmay be very little, sometimes approximately level off to 0, this can cause and in back projection's variable, produce some values very large " singular point ", thereby final reconstructed results is brought to very large interference, and projection variable does not have this problem;

3), because equipotential line Inverse Projection is to be based upon on the less hypothesis of internal resistance rate disturbance, the boundary voltage that brings thus changes and back projection's variable of deriving be also a disturbance among a small circle, otherwise can run counter to theoretical basis.And border voltage difference is carried out after arc tangent conversion, little variation shows significantly, and large variation is effectively compressed.

Claims (1)

1. the equipotential line back projection electrical impedance imaging image rebuilding method that arc tangent is compressed, is characterized in that:

1) first 16 impedance bioelectrical measurement electrodes are uniformly-spaced pasted on to body peripheral surface to be measured;

2) impedance bioelectrical measurement electrode is connected with EIT acquisition system;

3) select wherein two adjacent electrodes, by these two electrode pair measured targets, apply current excitation, at all the other, between adjacent electrode, gather between two the boundary response voltage of object under test simultaneously;

4) switch successively a pair of adjacent electrode target is applied to excitation, and simultaneously at all the other, between adjacent electrode, gather between two the boundary response voltage of object under test, repeat this process, until all electrodes were all used as exciting electrode;

5) above-mentioned 16 groups of boundary voltages and exciting current are preserved as frame data, be defined as reference frame boundary voltage, be designated as V _ref;

6) according to the interval repeating step 3 of setting), 4) measurement, obtain 16 groups of boundary voltages of an other frame and exciting current, be defined as present frame boundary voltage, be designated as V _d;

7) adopt arc tangent method to carry out arc tangent processing to the difference of present frame boundary voltage and reference frame boundary voltage: arctg (v _d-v _ref);

8) finally the data substitution equipotential line backprojection algorithm after above-mentioned processing is carried out to the reconstruction of electrical impedance dynamic image, and show.