A comparative study of optimal fundamental, second-and superharmonicimaging

A number of ultrasound methods are available for medical imaging.Fundamental imaging uses the echoes from the same spectral band as thetransmitted pulse. Tissue harmonic imaging (THI) utilizes frequencies atmultiple(s) of the fundamental and effectively suppresses reverberations, andoff-axis and near-field artifacts. Two types of THI comprise second-andsuperharmonic imaging (SHI). The former uses the second harmonic of the echoesand the latter combines the third to fifth harmonics. Clinical research showedthat the optimal transmit frequency for fundamental and second harmonic cardiacimaging is 3.5 and 1.8 MHz respectively. As the level of the harmonics isdetermined by a balance of nonlinear propagation and attenuation, the optimalfrequency for SHI is expected to be lower. The first goal of this study was toinvestigate the optimal transmit frequency for SHI by simulating the entireimaging chain based on an adapted SONAR equation. Two simulation cases areexamined: the first uses cardiac tissue properties and the second is based on amix of 50% cardiac tissue and 50% blood. Using the SONAR equation thesignal-to-noise ratio (SNR) for the second to fifth harmonics was computed up to15 cm for 1-2.5 MHz transmit frequencies. The transducer's transmit and receivetransfer was modeled, as well as its noise. The adaptation included nonlinearforward propagation calculated with axisymmetric KZK, the backpropagation waslinear. The highest frequency yielding a 30 dB dynamic range at the requiredimaging depth was assumed optimal. The second goal of this study was to comparethe beams produced by optimal fundamental, second and SHI for cardiacapplications. To this end we used a 3D KZK implementation for rectangularapertures. The optimal transmit frequency for SHI was 1.0-1.2 MHz at 13 cm usingcardiac tissue properties, this increased to 1.7 MHz if the properties of thecardiac tissue/blood mix were used. The 6 dB lateral beam width of the optimalfundamental, second-and SHI at 10 cm was 1.2, 1 and 0.7 cm respectively. Thenormalized intensity 1 cm off the beam axis was 14, 20 and 25 dB for thefundamental, second harmonic and superharmonic respectively. The optimaltransmit frequency for cardiac SHI is 1.0-1.7 MHz providing a feasible dynamicrange. The lateral resolution of SHI in the far field is higher compared tofundamental and second harmonic imaging.

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