Parameter Estimation And Parametric Imaging In Ultrasonic Backscatter Method For Evaluating The Status Of Cancellous Bone

Osteoporosis (OP) has been becoming a serious public issue. Cancellous bonewith this chronic bone disease has low bone mineral density (BMD) and deterioratedbone microstructure. Compared with DXA, QCT and other diagnostic methods,ultrasound diagnostic technique offers advantages including lack of ionizing radiation,portability, speediness and low cost. Conventional ultrasound diagnostic technique isultrasonic transmission method, in which two transducers are used to acquireultrasonic transmission signals at human calcaneous bone and then ultrasonicparameters (broadband ultrasound attenuation (BUA) and speed of sound (SOS)) arecalculated to represent BMD. The novel ultrasound technique is ultrasonic backscattermethod, which uses only one transducer, thus enabling direct analysis of somecommon fracture sites such as spine and wrist other than the cancaneus. In addition,ultrasonic backscatter can provide additional microstructure information. Thus,currently, researchers have been becoming interested in investigating the potential ofultrasonic backscatter for osteoporosis diagnosis.This thesis was organized with the thought of starting from ultrasonic backscattersignal analysis to bone's micro-structural parameter estimation, and ending withultrasonic backscatter parametric imaging of cancellous bone. The contents aresummarized as follows:1. Based on the introduction of ultrasonic backscatter theoretical model ofcancellous bone, single-cylindrical phantom experiments were conducted to analyzethe frequency-dependence of ultrasonic backscatter, thus verifying the ability ofsingle-cylindrical model to explain ultrasonic backscatter. This can help providetheoretical foundation for the concept that ultrasonic backscatter signals can offeradditional microstructure information of cancellous bone.2. In cancellous bone, trabeculae are considered as major scatterers, and the meanspace of these trabeculae (Mean trabecular Bone Spacing, MTBS) is an importantparameter to characterize cancellous bone. This thesis conducted some researches ondirectly extracting MTBS information from ultrasonic backscatter signals usingseveral signal processing algorithms.(1) A simulation system is proposed to generate ultrasonic backscatter simulatedsignals for cancellous bone. In the system, the characterization of quasi-periodicmicrostructures of cancellous bone can almost be represented by four parameters: mean trabecular bone spacing (MTBS), regular+diffuse scattering to white noise SNR(SNR_wn), standard deviation of regular spacing (JITTER), and amplitude ratio ofdiffuse scatterers to regular ones (A_d).(2) Two algorithms are presented to estimate MTBS from ultrasonic backscattersignals: a frequency-domain based technique named improved AR cepstrum and atime-domain based technique named Simplified Inverse Filter Tracking (SIFT)algorithm. Comparisons were conducted to test the performance of the two proposedmethods using the backscatter data from simulation system, phantom, bovinetrabeculae in vitro, and human calcaneous bone in vivo. The simulated data weregenerated from the proposed simulation system, and the real backscatter data fromphantom and bone samples were acquired by a backscatter measurement system.Experimental results demonstrated that the improved AR cepstrum has a larger MTBSestimation range, and more robustness to the randomness in trabecular spacing,diffuse scattering and noise than the conventional AR cepstrum. On the other hand,the proposed SIFT algorithm outperformed the AR cepstrum technique in all casesand compared well with the quadratic transformation (QT) technique. In addition tostronger robustness, SIFT algorithm possesses better stability than thefrequency-based methods such as AR cepstrum and QT technique.3. Acoustic properties are not consistent at different positions of cancellous bonedue to its high anisotropy, while ultrasonic parametric imaging can provide adirect-view of the distribution of bone microstructure information. Thus, researcheswere conducted on ultrasonic parametric imaging of cancellous bone in this thesis.(1) Ultrasonic parametric imaging techniques were proposed based on ultrasonicbackscatter method. Four parameters were estimated from backscattered signals,including acoustic characteristic impedance (Z_b), apparent backscatter coefficient(BC), apparent integrated backscatter coefficient (AIB) and spectrum maximum shift(SMS), and parametric images with these four parameters were constructed.(2) Correlations were studied between the four parameters and bonemicro-structural parameters obtained from aμ-CT, which are mean trabecularthickness (Tb.Th), mean trabecular spacing (Tb.Sp, the terminology of MTBS used asμ-CT result), bone volume/total volume ratio (BV/TV), bone surface/volume ratio(BS/BV), and bone material density (BD). Results demonstrated that Z_b, BC, AIB,SMS parametric images provide the physical property distributions of cancellous bonein different angles: ①Parameter Z_b has no strong correlation with bone microstructure (Tb.Th,r=-0.233; Tb.Sp, r=0.257), but a mediate correlation with bone material density (BD)(r=0.448, p＜0.05). The Z_b parametric image can show the physical properties relatedto the upper surface of cancellous bone at different locations.②Parameter BC and AIB have negative correlations with Tb.Sp (r=-0.513～-0.596, p＜0.01) and increase with Tb.Th (r=0.265～0.339), but the correlation withTb.Th is not significant as well as the correlations with BV/TV and BS/BV. The BCand AIB parametric images can offer the distribution of backscatter strengths of innerbone microstructure.③Parameter SMS has mediate and significant correlations with all theparameters obtained fromμ-CT (Tb.Th, r=-0.699, p＜0.01; Tb.Sp, r=0.477, p＜0.05;BV/TV, r=-0.675, p＜0.01; BS/BV, r=0.663, p＜0.01; BD, r=0.663, p＜0.01). The SMSparametric image displays the distribution of ultrasonic energy lost when ultrasoundpenetrates into and propagates inside bone microstructure.④The spatial variation of BC and AIB values was significantly (p＜0.01) relatedwith bone micro-structural parameters (Tb.Th, r=-0.550～-0.645; Tb.Sp, r=0.405～0.627; BV/TV, r=-0.572～-0.668; BS/BV, r=0.513～0.640), and the spatial variation ofSMS has the similar result (Tb.Th, r=-0.720; Tb.Sp, r=0.771; BV/TV, r=-0.754;BS/BV, r=0.802), which indicate that the spatial variation of parameters BC, AIB andSMS may help characterize bone quality and osteoporosis diagnosis.Research results in this thesis provide theoretical support for ultrasonicbackscatter in evaluating the status of cancellous bone and diagnosing osteoporosis.