Non linear imaging

Nonlinear imaging is now widely used in medicine because of the high contrast enhancement produced by the technique, which allows physicians to produce better diagnosis. However, absolute measurement of the local nonlinear parameter is not of easy access as exact fundamental and harmonic pressure on the front face of the transducer has to be known. The method developed at the GREMAN [1] uses an in-situ calibration method based on the principle of reciprocity, thus achieving pressure evaluation from voltage and current measurements. By using a Gaussian basis functions to solve the KZK equation, the acoustic field can be calculated at the fundamental and harmonic frequencies. The feasibility of the technique was first demonstrated in water and alcohol and then applied to the measurement of the nonlinear parameter of materials in evolution (such as a gel during its formation) [2].
The method has been further developed to include the use of a focused transducer and also to measure the nonlinear parameter with a medium interleaved between the transducer and the sample. This way, the local measurement of the nonlinear parameter of the sample placed in the focused area is possible. Measurements performed on a silicon oil/water two-phase medium show clear transition and demonstrate the potential of the technique for C-scan images. This was then coupled with an XY scanning system to produce images of the non linear parameter of a sample formed by an aqueous gel slab placed in denatured alcohol [3].



In these media, the nonlinear parameters obtained by the technique are found in good agreement with those previously published in the literature.

[1] F. Vander Meulen and L. Haumesser. “Evaluation of b/a nonlinear parameter using an acoustic self-calibrated pulse-echo method.” App. Phys. Let. 92(21), 2008.
[2] Guillaume G Robin, François F Vander Meulen, Nicolas N Wilkie-Chancellier, Loïc L Martinez, Lionel L Haumesser, Jérôme J Fortineau, Pascal P Griesmar, Marc M Lethiecq and Guy G Feuillard , "Ultrasonic self-calibrated method applied to monitoring of sol-gel transition" Ultrasonics 52(5):622-7, 2012.
[3] François Vander Meulen, Guillaume Robin, Jérôme Fortineau, Guy Feuillard. “Nonlinear parametric imaging using an ultrasonic focused transducer.”