等深度脉冲回波法进行非线性声参量成像的研究

Study on Nonlinearity Parameter Imaging in Pulse Reflection Mode at a Constant Depth

提出了等深度脉冲回波法对生物组织进行非线性声参量成像的原理和方法 ,建立了一套脉冲反射式C扫B/A成像系统 .利用该系统 ,对猪肝和脂肪组织的四种组合样品进行了非线性参量C 扫成像的实验 。

Acoustic nonlinea parameter B/A is a basic parameter in nonlinear acoustics.Some existing researches indicate that B/A determines the nonlinear effect of the ultrasound wave travelling in biological tissues and reflects the dynamic characteristics of the tissues.More information can be provided by this parameter than that by linear parameters such as sound velocity, attenuation and acoustic impedance.It is helpful to identify the early stage of pathological changes of the tissue.Therefore the nonlinear parameter B/A can be a new parameter to be used in medical ultrasonic imaging and diagnosis.Up to now, there are some methods of nonlinear parameter imaging and all of these methods are based on the transmission mode of acoustic wave.It is important and necessary to develop the nonlinear parameter imaging in reflection mode.In this paper, the principle and the method of nonlinear parameter imaging in reflection mode at a constant depth are proposed.By using following procedures (1) passing through the delay switch of digital oscilloscope and adjusting the imaging depth of the sample (2) segmenting the waveform, taking FFT, and obtaining the pressure amplitude of reflected second harmonics p 12A(2f 0,z) and the fundamental component p 21A(2f 0,z) respectively (3) calculating the ratio(p 12r(2f 0,z)/ p 21r(2f 0,z)), slope((p 12r(2f 0,z)/ p 21r(2f 0,z))′) and the value of B/A (4) writing the B/A value into the image file.Then the two-dimensional image of B/A, perpendicular to the z-axis, is obtained by grating C-scan method.An automatic mechanical C-scan B/A imaging system of pulse reflection mode with high precision including the Newport motion controller, a frequency synthesizer, a wideband power amplifier, a digital oscilloscope, a pulse generator, a compound PZT transducer and a computer are designed.In this system, nonlinear parameter C-scan images for four sample models made by different combinations of pocine fat and liver are carried out experimentally.The obtained images are quite satisfactory and we can distinguish liver from fat by their grey scales easily.The B/A values of the corresponding sample models are 6.9 and 9.6 respectively and coincide well with those values published in literatures.The results of this paper provide the experimental foundation for nonlinear parameter imaging to be a novel technology of medical diagnosis.