Magneto-acoustic imaging by continuous-wave excitation

Magneto-acoustic-electrical tomography combining maximum length sequence-coded excitation and liquid metal image contrast agent

Magneto-acoustic-electrical tomography (MAET) combines the advantages of ultrasound and electrical impedance functional tomography methods and has important research value in the early diagnosis of tumors and biocurrent monitoring. In this study, we propose a MAET method combining liquid metal image enhancement and maximum length sequence (MLS)-coded excitation to improve the signal-to-noise ratio (SNR) of MAET. The results indicate that liquid metal can achieve image enhancement of tissue MAET. When 7-, 15-, 31- and 63-bit MLS-coded excitations were applied to the samples, the SNR was improved by 28, 34, 40 and 45 dB, respectively. For a similar SNR improvement, the total used time under 31-bit MLS-coded excitation can be shortened to 23% under single-pulse excitation. The experimental results prove the feasibility of applying the MLS code to rotational MAET and reveal the significance of liquid metal as an image intensifier to improve the quality of MAET.

A Preliminary Study on Non-Invasive Detection of Electrical Stimulation Current Based on Magneto-Acoustic Effect

Electrical stimulation is widely used in nerve regulation and treatment. The detection of the distribution of stimulation current in tissues is of great significance to improve the accuracy of electrical stimulation, but the current technical means are still difficult to achieve the non-invasive detection of stimulation current. This study proposes a non-invasive detection method of electrical stimulation current based on the magneto-acoustic (MA) effect, which has the advantages of high spatial resolution and high spatial contrast. In this study, continuous sine waves with the frequency of 20kHz are used to stimulate samples. The MA signal generated by the stimulation current in the samples in a stable magnetic field is detected by lock-in amplifier, and the two-dimensional distribution of sound source is extracted. The current density distribution of samples is simulated by the method of finite element analysis, and the simulation results are verified by experiments. The results show that under the electrical stimulation of the order of 0.01A, the location measurement of two-dimensional surface sound source with millimeter accuracy can be achieved non-invasively in isolated pork and pig brain, and the measurement accuracy of weak MA signal can reach 10^-7Pa. In short, the stimulation current detection method based on MA effect can achieve the non-invasive and high-precision detection of electrical stimulation current distribution, which is of great significance to improve the stimulation accuracy and study the neural regulation mechanism of electrical stimulation.

Novel reconstruction algorithm of magnetoacoustic tomography based on ring transducer array for acoustic speed inhomogeneous tissues

PURPOSE

Magnetoacoustic tomography with magnetic induction (MAT-MI) is a technique that utilizes the acoustic signals induced by magnetic stimulation to reconstruct the electrical impedance distribution in biological tissues. Most algorithms ignored the fact that acoustic properties in human tissues are heterogeneous, which lead to distortion and blurring of small reconstructed objects. In this study, a novel algorithm is proposed for exact reconstruction of the sound source distribution in acoustic heterogeneous tissues.

METHODS

Based on the ring transducer array, we develop an algorithm which combines algebraic reconstruction technique (ART) and time reversal method. Different to existing reconstruction methods, the ultrasonic transmission tomography (UTT) and the MAT-MI can be completed in same system, which decreases the system complexity. The sound velocity distribution is reconstructed with the ART so that the propagation time of the magnetoacoustic signals in the heterogeneous tissue is corrected. And then, the sound source image is reconstructed based on the time reversal method from new sound pressure data. Both numerical simulations and phantom experiments are established to validate the proposed method.

RESULTS

Compared with the results without consideration of the variation on acoustic speed, sound sources reconstructed by our method are more consistent with the model in terms of size and shape.

CONCLUSIONS

The novel algorithm can be used to reconstruct the high-accuracy image of MAT-MI sound source in the sound velocity inhomogeneous media. In addition, this method is applicable to scenarios that the prior knowledge of the imaging targets is unknown. The signal acquisition time of MAT-MI in acoustically heterogeneity media is greatly reduced due to the introduction of ring transducer array into the imaging system. Therefore, our method will promote the application of MAT-MI in noninvasive early diagnosis of tumor for preclinical study.

A preliminary in vivo study of a method for measuring magneto-acoustic sonic source under electrical stimulation

BACKGROUND

Stimulating current distribution in the tissue is unknown due to the complex distribution.

OBJECTIVE

A preliminary in vivo measurement of the magneto-acoustic (MA) signal of the human finger is performed in this study. The approach for locating the magneto-acoustic source of the stimulating current is studied.

METHODS

We use a lock-in amplifier to measure the MA signal under continuous wave electrical stimulation. The phase of the MA signal is used to extract the location of the sonic source. The experimental system is designed to measure the MA signal under electrical stimulation.

RESULTS

Preliminary experiments results show that the amplitude precision is improved to less than 1 μPa. The sonic source is located with millimetre precision.

CONCLUSIONS

We propose a new MA source-locating method with high measurement and location precision. This method will be significant to the study of the imaging and monitoring of the current distribution of electrical stimulation with high precision.

M-sequence-coded excitation for magneto-acoustic imaging

Magneto-acoustic imaging is a novel functional imaging method to electrical characteristics of tissue. It provides valuable tools for diagnosing early stage tumor and monitoring bioelectrical current. Common single short-pulse excitation limits SNR due to the short-pulse duration and low power of magneto-acoustic signal. In this study, we propose M-sequence-coded excitation and pulse compression approach to improve SNR of magneto-acoustic imaging. Simulations on the magneto-acoustic signal under different bit lengths M-sequence-coded excitation are performed. Experiments on the samples made of pork and graphite slices are done to validate the proposed coded excitation method. The SNR and sidelobe levels were investigated. The results showed when 7, 15, 31, 63, 127 bits M-sequence-coded excitations were applied onto the samples, SNR was improved by 17.4 dB, 24.2 dB, 30.6 dB, 37.6 dB, and 40.1 dB, respectively. For a similar SNR improvement, the total used time under coded excitation can be shortened to 9.4% under the single pulse excitation. The result indicates the M-sequence-coded excitation approach is effective to improve the magneto-acoustic signal SNR and shorten the imaging time. Graphical abstract SNR of the magneto-acoustic signal is significantly improved by the coded excitation than the pulse excitation, the reconstructed image of the front and back boundary of the pork can be seen clearly under the 7, 15, 31, 63, 127 bit M-sequence-coded excitations.