Background: In the range of acoustic pressures used in ultrasound imaging, the waves propagating in tissue are distorted due to the nonlinear properties of the tissue. The adipose tissue exhibits up to 2 times greater nonlinear behavior than other tissue. In general, the nonlinear property of the media is addressed by using the B/A nonlinearity parameter measured in transmission mode and requiring wide band reception systems.
Method: We propose a new simple approach for estimating the non-linear properties of tissue. The proposed method involves the use of several waves transmissions with significantly different acoustic pressures and recording the echoes only in the fundamental frequency band. The set of images of "linear" tissues (B/A=0) will differ only in amplitude proportional to the amplitude of the transmitted wave. Therefore, the ratios of the echoes’ amplitudes recorded for the sequence of low and high pressure transmission, after compensating for different transmit acoustic pressures, should be close to one. If there are areas in the imaging space with B/A¹0, the amplitude ratios will differ from one and this value should increase with increasing nonlinearity coefficient of the imaged tissue.
The research was carried out in three steps. In the first step of the evaluation, the amplitude of the first and higher harmonics in water and in vegetable oil was measured for various sound pressures. Sound pressures were measured using a needle hydrophone at a depth of 1 cm up to 6 cm from the face of the ultrasonic head. In the second part the echoes from the thread phantom in water and sunflower oil were analyzed. Finally, we studied the backscattered ultrasound measurements from the sample of beef liver with oil mimicking fatty tissue.
Results: We have introduced a simple non-linearity index (NLI) in the form of the quotient of the backscattered signals amplitudes EP1 and EP2for two different transmitted pressures, after compensating for the transmitted pressures, NLI=EP1/EP2.