Exploiting Ultrasound Harmonics

Ultrasound imaging is an inexpensive method which provides an accurate diagnosis tool. This thesis provides elements to characterise the acoustic pressure generated by ultrasound transducers as well as signal processing method that could improve modern echography. The first measurement method to assess transducers characteristics consists in deducing the absolute amplitude of the acoustic pressure from a distortion measurement. Using this method, the acoustic pressure was determined within 20% of the calibrated hydrophone measurement. Another measurement method of harmonic acoustic beams was developed based on Schlieren photography. For some patients, ultrasound imaging presents difficulties. Super harmonic imaging (SHI) improves further the compromise between penetration and resolution. A description of the strategy to fulfil the bandwidth requirements of such an exigent method is provided. The detailed piezomaterial selection is depicted as well as the optimization of the array configuration for transthoracic and abdominal applications. Furthermore, two signal processing techniques are described to improve the SHI point spread function. The first technique consists in a dual pulse method and second uses chirp compression. Both methods permit to recover a good axial resolution. Those two methods present different performances. Finally, for contrast echography, two methods are described in order to further enhance the contrast to tissue ratio. The first one uses source pre-biasing technique. Improvement of contrast to tissue ratio (CTR) yields to 7.4 dB. The second technique consists in applying a power modulation scheme on a filtered population of bubbles. An improvement of 13 dB in CTR compared was observed.

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