3F-5 Development of a Phased Array for Tissue and Contrast Super Harmonic Imaging

Abstract

For several years, the standard in ultrasound imaging has been second harmonic imaging Recently, a new imaging modality, dubbed super harmonic imaging (SHI), has been proposed. SHI takes advantage of the higher - third to fifth - harmonics produced by either nonlinear propagation or contrast agents. Tissue SHI shows a better suppression of near field artefacts and improved lateral and axial resolutions resulting in images with improved clarity compared to second harmonic imaging. If used with contrast agents SHI produces a higher contrast-to-tissue ratio. To enable SHI with a high dynamic range an array sufficiently sensitive at the frequency up to its fifth harmonic is necessary (bandwidth > 130%). We present the results of custom built test arrays aiming specifically on receive sensitivity and SNR. The initial piezomaterial selection was done using the KLM model. From the selected materials test arrays were built (element size 13 times 0.2 mm2, resonance frequency 4 MHz, no matching layer, backing 5.3 MRayl). A calibrated source generated a pressure pulse, while the test array was located in the far field of the source. From the received pressure wave the element transfer functions and SNR were calculated, after compensation for diffraction and spatial averaging. The receive transfer function and SNR were evaluated on a per element basis. The most sensitive test array had an average peak receive sensitivity of 21 muV/Pa and could detect a long sinusoidal burst with amplitude 1 Pa with 22 dB SNR Using these results a conservative estimate predicts a dynamic range for SHI of 55 dB. These results suggest that in vivo tissue and contrast SHI could be feasible using the current array configuration, which is based on interleaved low and high frequency elements.