| Microstructures of the normal soft tissues are organized in characteristic patterns.However,temperature and pathological changes in the tissues could alter their spatial organization and characteristics.Owing to the inhomogeneity of biological tissues,ultrasound RF echo signals reflected from the tissues in the ultrasound propagation process carry the microstructure information of the tissues,in which the resolvable scatterer spacing(SS)is a parameter to describe the characteristics of the microstructures and ultrasonic scattering of biological tissues for the discrimination of the normal and diseased livers,breast tissues characterization and monitoring of the microwave-induced thermal lesions,and it plays an important role in the quantitative diagnosis of diseases and tissue characterization.There are several methods proposed for the mean scatterer spacing(MSS)estimation from ultrasound RF echo signals,in which the spectral analysis methods assume the strict periodicity of the regular structures in tissues,and the estimation accuracy is limited by the data length and time–frequency resolution.Other EEMD and WT-based methods utilize the peaks in the decomposed components that are closely related to the locations of the regular scatterers to estimate the MSS.Because of the difficulty in the selection of the appropriate components,these two methods are limited to obtain objective and accurate MSS estimation results.In addition,the previous studies on MSS estimation are all based on fundamental RF echo signals(FRSs),and the nonlinear variation in different tissue types or between the normal and pathological tissues,and heated tissues in the thermal lesions induced by microwave ablation was not considered.According to the above limitations,a new method by locating optimally matched Gabor atoms(LOMGA)from ultrasound RF echo signals is proposed to improve the MSS estimation,and the accuracy performance of the MSS estimation is evaluated based on the simulation and the microwave-induced thermal lesions experiments.The MSS estimation is further used for the study on the discrimination of the benign and malignant breast tumors.The main studies and innovations are as follows:1.To evaluate the accuracy performance of the proposed methods for MSS estimation,the realistic tissue models incorporating the regular scatterers and diffuse scatterers are generated,and the ultrasound RF echo signals are simulated.First,four regular degrees of the scatterers with the Gamma distribution and the diffuse scatterers with the uniform distribution are spatially modeled.Then,the coherent and diffuse components are simulated based on the regular and diffuse scatterer distributions with the Field II.Finally,the ultrasound RF echo signals are generated by combining the two components with preset coherent-to-diffuse ratios(CDRs).The results show that the simulation can provide a reliable data source to evaluate the accuracy performance of the MSS estimation with the proposed methods.2.A new LOMGA method is proposed to improve the MSS estimation from the ultrasound RF echo signals.First,generate the redundant Gabor atom set,and a series of Gabor atoms are obtained from the signals with a matching pursuit algorithm.Then,the optimum atoms highly correlated with the coherent components are automatically selected according to the second-order difference(SOD)of the reconstructed signal-toresidue ratio(RSRR).Finally,the distances between the locations of adjacent atoms are applied to estimate the MSSs.The simulation results demonstrate that the LOMGA method can provide more accurate and stable MSS estimation compared with WT method.The microwave ablation results indicate that the LOMGA method provides a better and stable discrimination between normal and coagulated tissues.3.Based on the tissue nonlinearity,a harmonic-based MSS estimation is proposed for improving the assessment of microwave-induced thermal lesions.First,the secondharmonic RF echo signals(SHRSs)are separated from the ultrasound RF echo signals using the Butterworth filter.Then,the Gabor atoms correlated with the coherent components are identified with the LOMGA method from the SHRSs,and the concise atoms are selected from the identified atoms with the spacing threshold(ST).Finally,MSSs are estimated with the distances of the adjacent concise atoms for the thermal lesion evaluation.The experimental results show that the SHRS-based increments in the intensity variation and the coherent components are larger than the FRS-based increments.Additionally,the SHRS-based MSS exhibits larger variations and smaller deviations than the FRS-based MSS with the increasing ablation time.In conclusion,the SHRS-based MSSs are more sensitive and robust to variations in the ablated tissues than the FRSbased MSSs,which can improve the assessment of microwave-induced thermal lesions.4.The MSS estimation with the LOMGA method is also proposed for the discrimination of the benign and malignant breast lesions.First,100 breast lesions are selected.Two individual orthogonal scans(longitudinal and transverse)from the pathological breast regions were performed with the Ultrasonix Sonix Touch Research ultrasound scanner,and the corresponding B-mode images are extracted.Second,based on the breast imaging reporting and data system(BI-RADS)categories,the regions of interest(ROIs)of the 48 benign and 52 malignant breast lesions,as well as the histologically assessed 31 benign and 25 malignant breast lesions are selected to collect the raw ultrasound RF echo signals.Then,the MSSs are estimated from the collected raw breast ultrasound RF echo signals with the LOMGA and WT methods,and the means and standard deviations(MSDs)of the MSSs are calculated for comparison.Finally,to validate the effectiveness of the discrimination of the benign and malignant breast lesions with the MSS estimation,the 1D scatter plots for the LOMGA-based and WT-based MSS estimates are shown,one-way analysis of variance(ANOVA)tests are performed,and pvalues in the ANOVA tests and the areas under the ROC curves(AUCs)are calculated for further comparison and evaluation.The results indicate that the LOMGA method has a better discrimination of the benign and malignant breast lesions compared with WT method.In conclusion,the ultrasound RF echo signals based on the regular and diffuse scatterer distributions are simulated to provide the realistic data source for evaluation of MSS estimation with the proposed methods.Then,a LOMGA method is proposed to improve the MSS estimation from ultrasound RF echo signals,and a harmonic-based MSS estimation is further presented for improving the assessment of microwave-induced thermal lesions.Finally,the LOMGA-based MSS estimation is also proposed for the discrimination of the benign and malignant breast lesions.The related studies are advantageous for improving the quantitative diagnosis of diseases and tissue characterization. |
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