Experimental Implementation of the Second Harmonic Inversion Imaging on an Open Ultrasonic Scanner Fanglue Lin 1 , François Varray 1 , Aymeric Guibal 2 , Christian Cachard 1 , Olivier Basset 1 1 CREATIS, Université de Lyon, CNRS UMR5220, INSERM U1044; INSA-Lyon; Université Lyon 1, Villeurbanne, France 2 Hospices Civils de Lyon, Hôpital Edouard Herriot, Service d’Imagerie Digestive, Lyon, France Abstract—Ultrasound harmonic imaging is limited by the harmonic components which are produced during wave propagation because of the native nonlinearities of the tissue. A newly proposed method, named Second Harmonic Inversion (SHI) suggests transmitting successively two pulses with the same frequency, the same amplitude and a 90° phase difference to reduce the second harmonic generated by tissue. This newly proposed SHI method is carried out on an open system equipped with a bipolar square-wave pulser and a linear probe. Measurements performed in water, on a general purpose ultrasound phantom and with a tissue mimicking phantom with circulating contrast agents are investigated. Experimental results show that SHI method can be easily implemented on an open system. Both radio frequency signals analysis and B-mode ultrasound images show that SHI method decreases significantly the native second-harmonic tissue components existing in standard harmonic images. Contrast-to-tissue ratio (CTR) of SHI image is improved by 4.6dB when compared to standard harmonic image and improved by 3.6dB when compared to PI image. SHI method enhances CTR through effective tissue generated second harmonic reduction. Moreover, the easy implementation procedure and the better specificity make SHI an interesting alternative to PI method. Keywords-harmonic suppression; second harmonic inversion; open ultrasonic scanner I. INTRODUCTION Ultrasound contrast agents (UCAs) are micro-bubbles with diameters of a few micrometers. They are currently used to improve the detection of blood flow due to the enhanced back- scattered echoes from micro-bubbles which are injected into the blood vessel. In fact, the backscattered echoes also have harmonics because of the asymmetric vibration of these micro- bubbles when exposed to the ultrasound pressures. Therefore, these harmonics can be extracted to form ultrasound harmonic images, which aim to distinguish blood pool and tissue. However it is found that the ultrasound wave propagation in tissue is also a nonlinear process which produces harmonics and reduces the contrast-to-tissue ratio (CTR) [1]. CTR is used to quantify the extent of discrimination between UCA and tissue, which is defined as: 20 log UCA P n CTR tissue P n = (1) Where UCA P n and tissue P n are backscattered pressures of n th harmonic from UCA and tissue, respectively. The following Khokhlov Zabolotskaya Kuznetsov (KZK) equation describes the complete model of nonlinear wave propagation: 2 3 2 2 2 0 3 3 3 2 2 2 2 0 00 c p p p p zt c t c t δ β ρ ∂ ∂ ∂ = Δ + + ⊥ ∂∂ ∂ ∂ (2) where p is the transmitted pressure, z is the propagating distance, t is the propagating time, 0 c is the speed of sound, δ is the attenuation of the medium, 0 ρ is the density of the medium and β is the nonlinearity coefficient of the medium. The first term of the right side of the equation means diffraction effect of the probe, the second terms stands for the absorption of the medium and the last term represents the nonlinear distortion of the medium. If the diffraction effect is ignored, the above KZK equation becomes the Burger equation, which describes the plane wave propagation. Furthermore, for a non-viscous homogeneous medium, a lossless Burger equation can be written as: 2 2 3 2 2 00 p p z c t β ρ ∂ ∂ = ∂ ∂ (3) Since 1990s, several approaches have been proposed to reduce these tissue-made harmonics. Krishnan et al. [2][3][4] used the idea of source pre-biasing to suppress the generated tissue harmonics. However, the tissue harmonic is only reduced over a narrow limited band and a limited range of axial field. Pasovic et al. [5] eliminate this disadvantage by defining a multiple frequency component second harmonic pre-biasing signal. Shen et al. [6][7] propose to transmit simultaneously a fundamental and a phase shifted third harmonic component. The difference of the two components is used to cancel the 2440 978-1-4577-1252-4/11/$26.00 ©2011 IEEE 2011 IEEE International Ultrasonics Symposium Proceedings 10.1109/ULTSYM.2011.0606