A comparative study of optimal fundamental, second- and superharmonic imaging

摘要

A number of ultrasound methods are available for medical imaging. Fundamental imaging uses the echoes from the same spectral band as the transmitted pulse. Tissue harmonic imaging (THI) utilizes frequencies at multiple(s) of the fundamental and effectively suppresses reverberations, and off-axis and near-field artifacts. Two types of THI comprise second- and superharmonic imaging (SHI). The former uses the second harmonic of the echoes and the latter combines the third to fifth harmonics. Clinical research showed that the optimal transmit frequency for fundamental and second harmonic cardiac imaging is 3.5 and 1.8 MHz respectively. As the level of the harmonics is determined by a balance of nonlinear propagation and attenuation, the optimal frequency for SHI is expected to be lower. The first goal of this study was to investigate the optimal transmit frequency for SHI by simulating the entire imaging chain based on an adapted SONAR equation. Two simulation cases are examined: the first uses cardiac tissue properties and the second is based on a mix of 50% cardiac tissue and 50% blood. Using the SONAR equation the signal-to-noise ratio (SNR) for the second to fifth harmonics was computed up to 15 cm for 1–2.5 MHz transmit frequencies. The transducer''s transmit and receive transfer was modeled, as well as its noise. The adaptation included nonlinear forward propagation calculated with axisymmetric KZK, the backpropagation was linear. The highest frequency yielding a 30 dB dynamic range at the required imaging depth was assumed optimal. The second goal of this study was to compare the beams produced by optimal fundamental, second — and SHI for cardiac applications. To this end we used a 3D KZK implementation for rectangular apertures. The optimal transmit frequency for SHI was 1.0–1.2 MHz at 13 cm using cardiac tissue properties, this increased to 1.7 MHz if the properties of the cardiac tissue/blood mix were used. The −6 dB lateral bea--m width of the optimal fundamental, second- and SHI at 10 cm was 1.2, 1 and 0.7 cm respectively. The normalized intensity 1 cm off the beam axis was −14, −20 and −25 dB for the fundamental, second harmonic and superharmonic respectively. The optimal transmit frequency for cardiac SHI is 1.0–1.7 MHz providing a feasible dynamic range. The lateral resolution of SHI in the far field is higher compared to fundamental and second harmonic imaging.