Medical ultrasound imaging with the nonlinear second harmonic component (SHC) provides improved spatial resolution and reduced reverberation artifacts. However, the signal-to-noise ratio (SNR) of the SHC is low compared to the fundamental frequency component. In this study, chirp coded signals are explored as an exci- tation technique in ultrasound harmonic imaging. The objective is to increase the SNR of the SHC and improve the axial resolution of ultrasound harmonic imaging using a single transmission event. In ultrasound harmonic imaging with chirp coded excitation, a harmonic matched filter (HMF) is typically used on the received signal to perform pulse compression of the SHC to restore axial resolution. Designing the HMF is a problematic issue as it requires optimal window selection. In the compressed signal, the sidelobe level may increase and the mainlobe width widen under a mismatched condition, resulting in loss of axial resolution. An alternate method of pulse compression using the fractional Fourier transform (FrFT) is presented. It is demonstrated that the FrFT can perform pulse compression of the SHC similar to the HMF with improved axial resolution and comparable peak sidelobe level. The use of nonlinear frequency modulated (NLFM) signals are explored in ultra- sound harmonic imaging. The objective is to optimise the SNR gain and to reduce the peak sidelobe level in the compressed second harmonic chirp signal. Results are presented which show a reduction of peak sidelobe level in the compressed NLFM signal with a comparable axial mainlobe width when compared with the compressed linear frequency modulated (LFM) signal. Chirp coded excitation is also proposed in the area of superharmonic imaging (SHI). It is demonstrated that the chirp coded signals can potentially enhance the SNR and axial resolution of the SHI when compared with conventional tone-burst excitation. Finally, the subharmonic response from contrast microbubbles is measured using LFM, NLFM and tone-burst excitations. Results are presented which indicate that the subharmonic response for wide bandwidth NLFM excitation is higher than the LFM excitation. Both LFM and NLFM excitations provide better subharmonic power than the tone-burst excitation.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:555857 |
| Date | January 2010 |
| Creators | Arif, Muhammad |
| Publisher | University of Leeds |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
Page generated in 0.0021 seconds