Ana Valria Greco de Sousa - anagreco@uol.com.br
Brazilian Navy Institute of Research
Rua Ipiru 2
Rio de Janeiro, Brazil, 21931 095
Wagner C.A. Pereira
Biomedical Engineering Program
Federal University of Rio de Janeiro
Joo C. Machado
Biomedical Engineering Program
Federal University of Rio de Janeiro
Ultrasound (US) is commonly used in many areas of scientific research. The development of different types of transducers and the advance of signal processing techniques allow detailed studies of structures with various dimensions and depths, considering the frequency-resolution relation (the idea that it is possible to work with higher frequencies and get better resolution). Ultrasound's main advantages rely on the fact that it is a non-invasive and non-destructive method. Accurate measurement of acoustic parameters, such as wave velocity (the velocity with which an US wave is propagated), attenuation and dispersion has important applications in theoretical acoustics, non-destructive testing and US tissue characterization.
In Medicine, US is part of diagnostic routine. However, commercial equipments generate a qualitative image, based only on the amplitude of the received echoes, presented in a gray scale (B-scan). A parametric image would give more reliability to US diagnosis, as it would turn possible to distinguish solid tumors from benign cysts.
In 1995, the Ultrasound Laboratory of the Federal University of Rio de Janeiro-COPPE obtained the preliminary experimental results on the subject, investigating the determination of multilayered media wave velocity and thickness. These initial studies showed that geometrical acoustics models had a good potential, but were highly dependent on alignment.
This work presents a method to obtain simultaneously the wave velocity and layer thickness in a stratified media, in order to generate a parametrical image. The influence of refraction is also measured and included in the propagation model. In biomedical ultrasound, this certainly would help diagnostic procedures as it discriminates different tissues, in opposition to conventional US-such as benign cysts from malignant tissue. It can also be applied to determine the degree of corrosion on metal ducts and delamination on ships and aircraft painting.
The propagation model is based on geometrical acoustics and uses two transducers to obtain experimental data (256 echoes per layer): one active (3.4 MHz) and a hydrophone, which is moved laterally through 15 positions and rotated. A two-layered phantom (6,2 mm alcohol and 6,0 mm acrylic) is immersed in a water tank and insonified. The echoes are captured and processed using a cross-correlation based method. The layers parameters (wave velocity and thickness) have an accuracy of 1% rms and 2% for precision for water; 4% and 9% for alcohol; and 12% and 15% for the acrylic, respectively. The experimental results show that the model can be successfully applied to both in vitro data and non-destructive testing.