Classification of electrical impedance imaging techniques:
Electrical impedance imaging techniques are broadly divided into the following three categories based on the different signal acquisition methods and excitation sources:
1. Injective electrical impedance imaging
This is the traditional electrical impedance imaging method, which usually adopts the current input to measure the voltage output or adopts the voltage input to measure the current output. Since the electrodes are placed on the skin, and the skin's stratum corneum has a high impedance, in order to minimize the effect of skin impedance, the method of measuring voltage with current input is usually used.
The EIT system is roughly composed of three parts: signal injection and extraction, A/D conversion and computer interface, and image reconstruction and display.
Of these, the main components for signal injection and extraction are the electrodes. Because the measured signal is very weak in the EIT system plus the electrode is the front end of the whole imaging system, the performance of the electrode is crucial to the performance of the whole system and the imaging quality. Therefore, a good electrode must have the following characteristics:
1) favorable for injecting current and extracting voltage;
2) low contact impedance with the skin surface;
3) easy to form electrode arrays;
4) non-toxic to the skin with no side effects.
In order to meet the above requirements, most of the electrodes used in EIT systems are made of silver, copper, stainless steel and other materials. In addition, most EIT systems now use 16 electrodes, and there are also 32 and 64 electrode systems. Increasing the number of electrodes can increase the resolution and imaging quality of the system, but the amount of data processing will increase, and the imaging algorithm will be more complex.
The function of the control circuit and current source system is to generate the input current and control the switching of the current input between the electrodes.The current of the EIT system uses tens of KHZ alternating current. Therefore, the key circuit in this section is the amplitude-stabilized oscillating constant current source.
EIT system oscillation frequency is determined by the following factors:
1) Skin impedance decreases as the frequency of the injected signal increases, and the impedance at 100KHZ is about 220 ohms.
2) The impedance of human tissues is mainly resistive under the action of currents with a frequency of 100KHZ or less;
3) AC signals are less harmful to the human body than DC signals.
4) Lower input impedance and smaller distribution capacitance.
Total requirements, the EIT input current frequency is mostly tens of KHZ.
Requirements of the voltage measurement circuit:
Because the EIT injected current is very small, so the skin meter distribution voltage to be measured is very small, and it must be amplified before it can be measured. In order to meet the system error requirements, the voltage measurement circuit to meet the requirements of certain indicators, for example: CMRR to reach 120dB, the system should achieve 0.1% accuracy. This in the system design to pay special attention.
The image reconstruction algorithm is an important part of the EIT system. There are many kinds of algorithms applied to the EIT system, according to the way divided into two main categories: dynamic imaging and static imaging. Dynamic imaging utilizes two different moments of measurement data, through the image reconstruction algorithm to obtain the difference between these two moments of electrical impedance distribution, so as to construct a pair of S differential image. Dynamic imaging is an early development of image reconstruction algorithms, mainly inverse projection algorithm, the advantage is that many of the noise in the measurement data can be eliminated in the phase subtraction to the elimination of the image reconstruction algorithm is not too high on the requirements of the data acquisition system, easy to realize, and in addition, it is generally smaller computational volume, the disadvantage is that the scope of application is narrow. Static imaging reconstruction algorithm development is a little later, but because of its wide range of applications and relatively good imaging effect, by the general attention, has become the mainstream of EIT imaging reconstruction algorithm. Nowadays, the popular algorithms are Newton-Raphson class algorithms, in addition, the perturbation algorithm, the proposed Newton class algorithm has also been developed. The disadvantages of static type imaging are: large amount of computation, poor noise performance, how to solve this problem, has become the focus and difficulty of EIT technology research.
2. Induced current electrical impedance imaging
Induced current electrical impedance imaging (ICEIT) is a new EIT technique, which places a number of excitation coils around the periphery of the measured target, and applies a time-varying current to it to generate an alternating magnetic field, which can be detected in the space. The time-varying current is applied to the target to generate an alternating magnetic field, which in turn generates an induced current in the target. The voltage difference between neighboring electrodes on the target surface is measured and this data is used for image reconstruction of the conductivity of the target area. ICEIT has the following advantages over conventional injection EIT:
1) the current within the imaging target is not limited by the current density at the electrodes, making it possible to use larger currents to improve the signal-to-noise ratio;
2) the electrode design can be optimized because the surrounding electrodes measure only the output voltage and are not used for current drive;
3) the electrode design can be optimized by changing the shape and position of the coil. By changing the shape and position of the coil, the spatial magnetic field is changed, thus changing the current distribution within the target and extracting the details of a certain part;
4) When there is a shielding layer outside the imaging target, selecting the appropriate frequency of the drive current, the shielding layer can be made to have little effect on the induced current density division, so that a more ideal imaging result may be obtained.
5) It is suitable for diagnosing intracranial lesions and exploring the evolution of brain diseases, and has certain advantages in distinguishing different types of brain edema;
6) It is non-contact, non-invasive, and the system cost is relatively low;
First of all, the position of the excitation coil is fixed, and the constant magnetic field generated by the excitation coil induces a reference signal in the compensation coil, and at the same time, this field passes through the volume conductor and induces a signal to be measured in the detection coil. At the same time, the magnetic field passes through the volume conductor and induces the signal to be measured in the detection coil. The function of the compensation coil is:
1) to offset the main magnetic field induced by the excitation coil directly on the detection coil by symmetry, so that the test coil theoretically has only the eddy current induced magnetic field on the volume conductor;
2) to provide a reference signal directly for the phase detection circuit. The signal to be measured (10mv-50mv) first buffer, in the primary amplification and secondary amplification sent to one end of the discriminator circuit; and the reference signal (1-5v) first buffer and then add a limit, sent to the other end of the discriminator circuit. The output of the phase-discriminating circuit is Im (db/B). This phase is fed into the A/D board as a measured value. After that, turn the inner slip ring to change the position of the excitation coil to get other sets of measurements. Finally, the sampling is finished and the data is sent to the computer for processing and displaying the results.
3. Multi-frequency Electrical Impedance Tomography
Multi-frequency Electrical Impedance Tomography (MFEIT) is a new imaging technique developed on the basis of the EIT technique, which measures impedance information in human tissues under multiple frequency excitations and processes this information to achieve better results. It measures the impedance information in human tissues under multiple frequency excitations and processes this information to better distinguish between different tissues or different states of tissues. Since MFEIT extracts the complex impedance information of human tissues at multiple frequencies, it can be used to highlight the tissues of interest by selecting the frequencies purposely, and even estimate the parameters of the tissue impedance model by using the impedance information at multiple frequencies, so as to make the final image more informative and to obtain a higher imaging quality. Therefore, the MFEIT technique is a further development of the FIT technique and has good application prospects.
The main technology used in multi-frequency EIT system
Compared with the conventional EIT technology, the most important feature of the MFEIT hardware system is the broadening of its operating bandwidth and the increase in the number of measured frequencies. As the bandwidth widens, some problems that are not particularly prominent in conventional EIT systems begin to sharpen, the most prominent of which is stray capacitance. At higher frequencies, large stray capacitance not only reduces the output impedance of the current source and the input impedance of the voltage measurement circuit, but also directly affects the accuracy of the measurement of the impedance imaginary part of the MFEIT for this problem, pay attention to the control of the signal-to-noise ratio of the foreground at the same time, around the frequency of the multi-frequency system combinations, the shortening of the acquisition time, the demodulation of signals and the problem of noise suppression, the proposed measures to solve the respective problems.
As a new imaging technique, multi-frequency impedance imaging extracts the complex impedance information at multiple frequencies, which is the final image contains more information and obtains higher imaging quality. As one of the main research contents of this technology, the key problem to be solved by the hardware system is how to meet the requirements of high accuracy and high speed of the EIT data measurement system under the case of operating bandwidth extension.