Optimization on artifacts in photoacoustic images based on spectrum analyses and signal extraction.

Photoacoustic (PA) imaging is a promising technology for functional imaging of biological tissues, offering optical contrast and acoustic penetration depth. However, the presence of signal aliasing from multiple PA sources within the same imaging object can introduce artifacts and significantly impact the quality of the PA tomographic images. In this study, an optimized method is proposed to suppress these artifacts and enhance image quality effectively. By leveraging signal time-frequency spectrum, signals from each PA source can be extracted. Subsequently, the images are reconstructed using these extracted signals and fused together to obtain an optimized image. To verify this proposed method, PA imaging experiments were conducted on two phantoms and two in vitro samples and the distribution relative error and root mean square error of the images obtained through conventional and optimized methods were calculated. The results demonstrate that the proposed method successfully suppresses the artifacts and substantially improves the image quality.

Optimized Reconstruction Procedure of Photoacoustic Imaging for Reflection Artifacts Reduction.

Photoacoustic (PA) imaging technology is of some value in medical diagnoses such as breast cancer detection, vasculature imaging, and surgery navigating. While as most imaging objects are bounded, the received RF signals consist of the direct-arrived signals (DAS) from the PA sources and the boundary-reflected signals (BRS). The undesired BRS will severely impair the quality during the image reconstruction. They will bring in many artifacts and confuse the actual shape and location of the PA sources. We improved the reconstruction procedure by removing the BRS before the regular reconstruction process to suppress those artifacts. To verify our proposed method, we compared the results of the conventional and optimized procedures experimentally. In terms of qualitative observation, the reconstructed images by the optimized procedure illustrate fewer artifacts and more accurate shapes of the PA sources. To quantitatively evaluate the traditional and the optimized imaging procedure, we calculated the Distribution Relative Error (DRE) between each experiment result and its standard drawing of the phantoms. For both phantoms and the ex-vivo sample, the DREs of reconstruction result by the optimized reconstruction procedure decrease significantly. The results suggest that the optimized reconstruction process can effectively suppress the reflection artifacts and improve the shape accuracy of the PA sources.

Skeletal Muscle Fatigue State Evaluation with Ultrasound Image Entropy.

Muscle fatigue often occurs over a long period of exercise, and it can increase the risk of muscle injury. Evaluating the state of muscle fatigue can avoid unnecessary overtraining and injury of the muscle. Ultrasound imaging can non-invasively visualize muscle tissue in real-time. Image entropy is commonly used to characterize the texture of an image. In this study, we evaluated changes in the ultrasound image entropy (USIE) during the fatigue process. Twelve volunteers performed static sustained contractions of biceps brachii at four different intensities (20%, 30%, 40%, and 50% of maximal voluntary contraction torque). The ultrasound images and surface electromyography (sEMG) signals were acquired during exercise to fatigue. We found that (1) the root-mean-square of the sEMG signal increased, the USIE decreased significantly with time during the sustained contractions; (2) the maximum endurance time (MET) and the decline percentage of USIE were significantly different (p < .05) among the four contraction intensities; (3) the decline slope of USIE of the same volunteer was basically the same at different contraction intensities. The USIE could be a new method for the evaluation of skeletal muscle fatigue state.

Generating an Adjustable Focused Field With an Annular Shape Using a Cylindrical Acoustic Transducer Array.

We present the focusing structure of a cylindrical acoustic transducer array consisting of many annular piezoelectric wafer elements operating in the radial vibration mode. Using Huygens' principle, we calculated the delay parameters associated with the excitation signal of each element. Given the respective delay rules, the array transducer produces an adjustable acoustic focused field in the form of a 3-D circular ring. From a theoretical analysis, we designed and fabricated an array transducer with 64 elements and measured its actual field distribution. Simulation and actual experimental results show that the proposed circular cylindrical array transducer controls the annular acoustic focused field well. The sound field intensity of the annular focus region is related to the number of excited array elements, and the radial and axial positions of the annular focus region obey the delay rules of the excitation signal. These acoustic field control methods may be applied in ultrasound detection when scanning a circular sound field.

Focusing phenomenon based on the coupling effect of acoustic waveguide.

We investigated the coupling effect in a pair of parallel acoustic cladded waveguides and extended the effect to obtain energy focusing in a specially structured waveguide of nested pipes. For the structure composed of two parallel waveguides, we simulated and verified that when an acoustic input wave propagates in one waveguide, the wave couples with the other and alternates between the two parallel waveguides with periodic amplitude and a well-defined coupling length. This length is related to the frequency of the input wave and the structure of the waveguide. Moreover, we fabricated a specially structured waveguide composed of four concentric circular pipes. In both simulations and experiments, we further demonstrated that the acoustic wave transmitted through designated ports of this fabricated waveguide structure can be focused onto the central waveguide with a well-defined focusing length, and that the focusing length is related to the wave frequency. Similar to its optics counterpart, not only can the coupling effect between the acoustic cladded waveguides be used in energy focusing in the nested waveguide structure, but it can also be used in other acoustic wave controlling devices, such as the frequency-selective device, the power switch for an acoustic wave, and the highly efficient pure-mode transducer.