CN104473640A - Electric conductivity rebuilding method for magnetocaloric acoustical imaging - Google Patents

Abstract

The invention relates to an electric conductivity rebuilding method for magnetocaloric acoustical imaging. The electric conductivity rebuilding method is based on a magnetocaloric acoustical imaging principle. An exciting coil is used for exerting MHz current excitation on a conducting object, joule heat is generated in the conducting object, and further, ultrasonic signals are generated. An ultrasonic transducer is used for receiving the ultrasonic signals, the received ultrasonic signals are processed and collected, and then, an electric conductivity image of the conducting object is obtained by adopting an electric conductivity image rebuilding algorithm. The electric conductivity rebuilding method comprises the concrete steps that 1, firstly, high-signal-to-noise-ratio magnetocaloric acoustical signals are obtained; 2, the obtained magnetocaloric acoustical signals are used for rebuilding to obtain thermal sound source distribution of the conducting object; 3, the thermal sound source distribution and a one-order magnetic vector space component are used, and a non-linear finite element solution method is adopted for rebuilding a scalar potential space component; 4, the rebuilt scalar potential space component is used for rebuilding the electric conductivity.

Description

A kind of electrical conductivity method for reconstructing of magnetic thermal acoustic imaging

Technical field

The present invention relates to a kind of method for reconstructing of conductivity imaging, particularly a kind of electrical conductivity method for reconstructing of magnetic thermal acoustic imaging.

Background technology

Sensitivity and the spatial resolution of current traditional electrical impedance imaging technique are not high, and main because electrical impedance imaging adopts electromagnetic wave that frequency is lower as excitation usually, because wavelength is far longer than imaging body, cause electromagnetic exploration contrast high, but resolution is low.Undoubtedly, single Chang Douyouqi physical limitation.Therefore multiple physical field imaging technique receives increasing concern, act on biological tissue by a kind of physical field, be converted to another kind of physical field and detect, provide resolution by a kind of physical field, another kind of physical field provides contrast, improves while realizing contrast and resolution.Electromagnetic field and the ultrasonic multiple physical field imaging technique combined consider the high-resolution characteristic of electromagnetic field to the high-contrast of tissue electrical conductivity and ultrasonic listening just, become the study hotspot of people, magnetic thermal acoustic imaging comes into one's own as a kind of novel multiple physical field imaging technique for nearest 1 year.

Magnetic thermal acoustic imaging is the novel electrical impedance imaging method proposed first in 2013 by Nanyang Technological University, by applying the alternating magnetic field lower than 20MHz to conductive body, induction field is produced in conductive body inside, and then generation Joule heat, excite thermoelastic acoustical signal, detect acoustical signal and carry out imaging.The method is a kind of using alternating magnetic field as driving source, based on the difference of biological tissue's inside Joule heat absorbance, using the ultrasonic harmless Biologic Medical Image technology as information carrier.Compared with microwave thermoacoustic imaging technology, the frequency of driving source reduces, and can be deep into the more depths of electric conductor, make the hot acoustic image of magnetic expand to the deep layer of tissue.Be divided into two processes by the ultrasonic signal measured to the reconstruction of electrical conductivity, first rebuild hot sound source by the ultrasonic signal measured and distribute, then utilize hot sound source to distribute and rebuild distribution of conductivity, current pertinent literature and patent have only rebuild hot sound source (S=σ E ²(σ), E is the spatial component of electric field intensity here), and do not mention the reconstruction of conductivityσ.Obviously, electric field strength E is relevant with the distribution of conductivityσ, and from hot sound source S, reconstruct conductivityσ is very difficult.

Summary of the invention

The object of the invention is to overcome the deficiency that existing magnetic thermal acoustic imaging method cannot provide distribution of conductivity, propose a kind of method utilizing the distribution of hot sound source to rebuild distribution of conductivity.The present invention can rebuild the electrical conductivity of conductive body accurately.

The present invention is based on magnetic thermal acoustic imaging principle.The image-forming principle of magnetic thermal acoustic imaging is: utilize excitation coil to apply MHz current excitation to conductive body, in conductive body, produce Joule heat, and then produce ultrasonic signal.Utilize ultrasonic transducer to receive ultrasonic signal, the ultrasonic signal received is carried out to process and the collection of ultrasonic signal, after being amplified filtered ultrasonic signal, adopt conductivity imaging algorithm for reconstructing to obtain the conductivity imaging of conductive body.

The conductivity imaging method for reconstructing of magnetic thermal acoustic imaging of the present invention comprises four steps: the first step, first obtains the hot acoustical signal of magnetic of high s/n ratio; Second step utilizes the magnetic hot acoustical signal reconstruction obtained to obtain the hot sound source distribution of conductive body; 3rd step utilizes hot sound source to distribute and a magnetic vector spatial component, adopts nonlinear finite element method for solving to rebuild electric scalar potential spatial component; 4th step utilizes the electric scalar potential spatial component rebuild to rebuild electrical conductivity.

Concrete process of reconstruction is described below:

The first step: the hot acoustical signal of magnetic obtaining high s/n ratio

Pulse excitation source is encouraged conductive body by a pair last of the twelve Earthly Branches nurse hertz excitation coil, conductive body is due to faradic effect generation Joule heat, and then produce hot acoustical signal, hot acoustical signal is coupled in ultrasonic transducer by couplant, and couplant can be deionized water also can be insulating oil.After being processed by the enlarge leadingly, filtering, secondary amplification etc. of ultrasonic signal processing, acquisition subsystem after ultrasonic transducer receives ultrasonic signal, store.The scanning monitor of ultrasonic transducer is to the circular scanning of conductive body under the control of control circuit, and all ultrasonic signals then utilizing scanning to obtain carry out image reconstruction.Described control circuit realizes the control to ultrasonic transducer scanning monitor, pulsed current excitation source, image reconstruction and ultrasonic signal processing, acquisition subsystem.

Second step: obtain the distribution of conducting objects body heat sound source

The acoustic pressure wave equation of known magnetic thermal acoustic imaging:

${\▿}^{2}p(r,t)-\frac{1}{{c_s}^2}\frac{{\∂}^2}{\∂t^2}p(r,t)=-\frac{\mathrm{\β}}{C_P}S\left(r\right){\mathrm{\δ}}^{\′}\left(t\right)---\left(1\right)$

Wherein, r is hot sound source position coordinate, and p (r, t) is the acoustic pressure that hot sound source produces, c _sfor the ultrasonic signal acoustic speed of propagation in media as well that hot sound source produces, C _pfor the specific heat capacity of conductive body, β is the thermal coefficient of expansion of conductive body, and δ (t) is Dirac function, laplacian, t is the time, S is hot sound source;

First a certain fault plane z=z of conductive body is chosen ₀, make ultrasonic transducer carry out circular scanning on this fault plane, gather ultrasonic signal, utilize the acoustic pressure wave equation (1) of magnetic thermal acoustic imaging, equation (1) is solved an equation, solves z=z ₀hot sound source S (x, y, z on fault plane ₀) distribution, then move ultrasonic transducer and circular scanning is carried out to another fault plane, gather ultrasonic signal, equally equation (1) is solved an equation, solve the hot sound source distribution on another fault plane, by mobile ultrasonic transducer, adopt same method of solving an equation, solve the hot sound source distribution in the whole region of conductive body;

Hot sound source S is expressed as:

S＝σE ²＝σE·E (2)

Wherein σ is the electrical conductivity of conductive body, and E is electric field intensity;

3rd step: solve electric scalar potential spatial component

According to current continuity theorem, the divergence of electric current is zero, by:

$\▿\·\mathrm{\σ}(\▿\mathrm{\φ}+A_1)=0---\left(3\right)$

Wherein, A ₁be the spatial component of a magnetic vector potential, a magnetic vector potential and conductive body have nothing to do, and are the magnetic vector potentials that driving source produces in a vacuum, utilize Biot Savart law to calculate the spatial component A of a magnetic vector potential ₁, φ is the spatial component of electric scalar potential, for divergence symbol, it is the gradient of the spatial component φ of electric scalar potential;

Because the electrical conductivity of biological tissue is lower, the spatial component of electric field intensity can be expressed as:

$E\≈-\▿\mathrm{\φ}{-A}_1---\left(4\right)$

Then formula (2) can be write as further:

$\mathrm{\σ}=\frac{S}{E\·E}=\frac{S}{(\▿\mathrm{\φ}{+A}_1)\·(\▿\mathrm{\φ}{+A}_1)}---\left(5\right)$

Formula (5) is substituted into formula (3):

$\▿\·\frac{S}{(\▿\mathrm{\φ}{+A}_1)\·(\▿\mathrm{\φ}{+A}_1)}(\▿\mathrm{\φ}{+A}_1)=0---\left(6\right)$

By the spatial component A of hot sound source distribution S (x, y, z) and a magnetic vector potential ₁after substituting into formula (6), consider electric insulation boundary condition, carry out finite element method and solve, the spatial component φ obtaining electric scalar potential can be rebuild;

4th step: solve electrical conductivity

The spatial component φ of electric scalar potential is substituted into formula (5), can conductivityσ be rebuild.

Accompanying drawing explanation

The hot acoustical signal acquisition device intention of magnetic involved by Fig. 1 method for reconstructing of the present invention;

In figure: 1 pulse excitation source 2 control circuit 3 excitation coil 4 conductive body 5 couplant 6 ultrasonic transducer 7 ultrasonic transducer scanning monitor 8 ultrasonic signal processing, acquisition subsystem 9 host computer.

Detailed description of the invention

The present invention is further illustrated below in conjunction with the drawings and specific embodiments.

The hot acoustical signal acquisition device of magnetic involved by method for reconstructing of the present invention mainly comprises excitation system, detection system, control system and conductive body 4 totally four parts.Described excitation system comprises pulse excitation source 1 and excitation coil 3, pulse excitation source 1 and being isolated by air between excitation coil 3 and couplant 5, be electrically connected between pulse excitation source 1 with excitation coil 3, excitation coil 3 can be the helmholtz coil occurred in pairs, also can be single coil, for ensureing the uniformity of imaging region electromagnetic excitation, be preferably helmholtz coil; Described detection system comprises ultrasonic transducer 6, ultrasonic signal processing, acquisition subsystem 8 and host computer 9, is coupled between ultrasonic transducer 6 and conductive body 4 by couplant, and ultrasonic transducer 6 is electrically connected with between ultrasonic signal processing, acquisition subsystem 8; Host computer 9 realizes the storage of image data and the reconstruction of image; Described control system comprises control circuit 2 and ultrasonic transducer scanning monitor 7, and control circuit 2 is electrically connected pulse excitation source 1, ultrasonic transducer scanning monitor 7 and host computer 9.The image-forming principle of magnetic thermal acoustic imaging is: utilize and apply pulsed current excitation with a pair last of the twelve Earthly Branches nurse hertz excitation coil, 3 pairs of conductive bodies 4, under the excitation in pulse excitation source 1, produces Joule heat in conductive body 4, and then produces ultrasonic signal.Ultrasonic transducer 6 is utilized to receive ultrasonic signal, the ultrasonic signal received is carried out to process and the collection of ultrasonic signal, after being amplified filtered ultrasonic signal, utilize host computer 9 to store, and the conductivity imaging adopting image reconstruction algorithm to obtain conductive body show on host computer 9.

Conductivity imaging process of reconstruction comprises four steps: the first step, first obtains the hot acoustical signal of magnetic of high s/n ratio; Second step utilizes and obtains the hot acoustical signal of magnetic rebuilds the hot sound source distribution obtaining conductive body; 3rd step utilizes hot sound source to distribute and a magnetic vector spatial component adopts nonlinear finite element method for solving to rebuild electric scalar potential spatial component; 4th step utilizes the electric scalar potential spatial component rebuild to rebuild electrical conductivity.Concrete process of reconstruction is described below:

The first step: obtain the hot acoustical signal of magnetic

As shown in Figure 1, pulse excitation source 1 is encouraged by a pair last of the twelve Earthly Branches nurse hertz excitation coil, 3 pairs of conductive bodies 4, and conductive body 4 due to faradic effect generation Joule heat, and then produces hot acoustical signal; Described hot acoustical signal is coupled in ultrasonic transducer 6 by couplant 5; After being amplified by the enlarge leadingly of ultrasonic signal processing, acquisition subsystem 8, filtering, secondary after ultrasonic transducer 6 receives ultrasonic signal, store; The scanning monitor 7 of ultrasonic transducer 6 under the control of control circuit 2 to the circular scanning of conductive body 4, then the ultrasonic signal utilizing scanning to obtain carries out image reconstruction, control circuit 2 realizes ultrasonic transducer scanning monitor 7, pulsed current excitation source 1, image reconstruction, and ultrasonic signal processing, acquisition subsystem 8 control;

Second step: obtain the distribution of conducting objects body heat sound source

The acoustic pressure wave equation of known magnetic thermal acoustic imaging:

${\▿}^{2}p(r,t)-\frac{1}{{c_s}^2}\frac{{\∂}^2}{\∂t^2}p(r,t)=-\frac{\mathrm{\β}}{C_P}S\left(r\right){\mathrm{\δ}}^{\′}\left(t\right)---\left(1\right)$

Wherein, r is hot sound source position coordinate, and p (r, t) is the acoustic pressure that hot sound source produces, c _sfor the ultrasonic signal acoustic speed of propagation in media as well that hot sound source produces, C _pfor the specific heat capacity of conductive body, β is the thermal coefficient of expansion of conductive body, and δ (t) is Dirac function, laplacian, t is the time, S is hot sound source;

First a certain fault plane z=z of conductive body is chosen ₀, make ultrasonic transducer carry out circular scanning on this fault plane, gather ultrasonic signal, utilize equation (1), equation (1) is solved an equation, solves z=z ₀hot sound source S (x, y, z on fault plane ₀) distribution, then move ultrasonic transducer and carry out circular scanning on another fault plane, gather ultrasonic signal, equally equation (1) is solved an equation, solve the hot sound source distribution on another fault plane, by mobile ultrasonic transducer, adopt same method of solving an equation, solve the hot sound source distribution in the whole region of conductive body;

Hot sound source S is expressed as:

S＝σE ²＝σE·E (2)

Wherein σ is the electrical conductivity of conductive body, and E is electric field intensity;

3rd step: solve electric scalar potential spatial component according to current continuity theorem, by:

$\▿\·\mathrm{\σ}(\▿\mathrm{\φ}+A_1)=0---\left(3\right)$

Wherein, A ₁be the spatial component of a magnetic vector potential, a magnetic vector potential and conductive body have nothing to do, and are the magnetic vector potentials that driving source produces in a vacuum, utilize Biot Savart law to calculate the spatial component A of a magnetic vector potential ₁, φ is the spatial component of electric scalar potential;

Because the electrical conductivity of biological tissue is lower, the spatial component of electric field intensity is expressed as:

$E\≈-\▿\mathrm{\φ}{-A}_1---\left(4\right)$

Then formula (2) can be write as further:

$\mathrm{\σ}=\frac{S}{E\·E}=\frac{S}{(\▿\mathrm{\φ}{+A}_1)\·(\▿\mathrm{\φ}{+A}_1)}---\left(5\right)$

Formula (5) is substituted into formula (3):

$\▿\·\frac{S}{(\▿\mathrm{\φ}{+A}_1)\·(\▿\mathrm{\φ}{+A}_1)}(\▿\mathrm{\φ}{+A}_1)=0---\left(6\right)$

By the spatial component A of hot sound source distribution S (x, y, z) and a magnetic vector potential ₁after substituting into formula (6), consider electric insulation boundary condition, carry out finite element method and solve, the spatial component φ obtaining electric scalar potential can be rebuild;

4th step: solve electrical conductivity

The spatial component φ of electric scalar potential is substituted into formula (5), can conductivityσ be rebuild.

Claims (1)

1. an electrical conductivity method for reconstructing for magnetic thermal acoustic imaging, is characterized in that: the method for reconstructing of described magnetic thermal acoustic imaging comprises the following steps:

The first step: obtain the hot acoustical signal of magnetic

Pulse excitation source (1) is encouraged conductive body (4) by a pair last of the twelve Earthly Branches nurse hertz excitation coil (3), and conductive body (4) due to faradic effect generation Joule heat, and then produces hot acoustical signal; Described hot acoustical signal is coupled in ultrasonic transducer (6) by couplant (5); After being amplified by the enlarge leadingly of ultrasonic signal processing, acquisition subsystem (8), filtering, secondary after ultrasonic transducer (6) receives ultrasonic signal, store; The scanning monitor (7) of ultrasonic transducer (6) under the control of control circuit (2) to the circular scanning of conductive body (4), then the ultrasonic signal utilizing scanning to obtain carries out image reconstruction, control circuit (2) realize to ultrasonic transducer scanning monitor (7), pulsed current excitation source (1), image reconstruction, and ultrasonic signal processing, acquisition subsystem (8) control;

Second step: obtain the distribution of conducting objects body heat sound source

The acoustic pressure wave equation of known magnetic thermal acoustic imaging:

${\▿}^{2}p(r,t)-\frac{1}{{c_s}^2}\frac{{\∂}^2}{\∂t^2}p(r,t)=-\frac{\mathrm{\β}}{C_P}S\left(r\right){\mathrm{\δ}}^{\′}\left(t\right)$

Wherein, r is hot sound source position coordinate, and p (r, t) is the acoustic pressure that hot sound source produces, c _sfor the ultrasonic signal acoustic speed of propagation in media as well that hot sound source produces, C _pfor the specific heat capacity of conductive body, β is the thermal coefficient of expansion of conductive body, and δ (t) is Dirac function, laplacian, t is the time, S is hot sound source;

First a certain fault plane z=z of conductive body is chosen ₀, make ultrasonic transducer carry out circular scanning on this fault plane, gather ultrasonic signal, utilize equation (1), equation (1) is solved an equation, solves z=z ₀hot sound source S (x, y, z on fault plane ₀) distribution, then move ultrasonic transducer and carry out circular scanning on another fault plane, gather ultrasonic signal, equally equation (1) is solved an equation, solve the hot sound source distribution on another fault plane, by mobile ultrasonic transducer, adopt same method of solving an equation, solve the hot sound source distribution in the whole region of conductive body;

Hot sound source S is expressed as:

S＝σE ²＝σE·E (2)

Wherein, σ is the electrical conductivity of conductive body, and E is electric field intensity;

3rd step: solve electric scalar potential spatial component

According to current continuity theorem, the divergence of electric current is zero, by:

$\▿\·\mathrm{\σ}(\▿\mathrm{\φ}+A_1)=0---\left(3\right)$

Wherein, A ₁be the spatial component of a magnetic vector potential, a magnetic vector potential and conductive body have nothing to do, and are the magnetic vector potentials that driving source produces in a vacuum, utilize Biot Savart law to calculate the spatial component A of a magnetic vector potential ₁, φ is the spatial component of electric scalar potential, for divergence symbol, it is the gradient of the spatial component φ of electric scalar potential;

Because the electrical conductivity of biological tissue is lower, the spatial component of electric field intensity is expressed as:

$E\≈-\▿\mathrm{\φ}-A_1---\left(4\right)$

Then formula (2) can be write as further:

$\mathrm{\σ}=\frac{S}{E\·E}=\frac{S}{(\▿\mathrm{\φ}+A_1)\·(\mathrm{\Δ\φ}+A_1)}---\left(5\right)$

Formula (5) is substituted into formula (3)

$\▿\·\frac{S}{(\▿\mathrm{\φ}+A_1)\·(\▿\mathrm{\φ}+A_1)}(\▿\mathrm{\φ}+A_1)=0---\left(6\right)$

By the spatial component A of hot sound source distribution S (x, y, z) and a magnetic vector potential ₁after substituting into formula (6), consider electric insulation boundary condition, carry out finite element method and solve, the spatial component φ obtaining electric scalar potential can be rebuild;

4th step: solve electrical conductivity

The spatial component φ of electric scalar potential is substituted into formula (5), can conductivityσ be rebuild.