The application of photoacoustic (PA) phenomena to medical imaging has been investigated for more than a decade. To implement this modality, one may choose between two types of laser sources, pulsed or continuous wave (CW). This selection affects all features of the imaging technique. Nowadays pulsed lasers are the state-of-the-art technique in the PA research. In this work, various features of the alternative frequency-domain (FD) PA were investigated. An axially symmetric transfer function model of PA wave generation and a Krimholtz-Leedom-Matthaei (KLM) transducer model were developed and used to analyze the experimental results. The controllable parameters of the FD-PA were optimized to improve the signal-to-noise ratio (SNR), contrast, axial resolution and depth detectivity. For example, it was shown that employing the optimal chirp bandwidth can enhance the SNR by more than 10 dB. In addition to the image produced by the cross-correlation amplitude, the phase of the correlation signal was used as a separate channel. A statistical method was introduced to generate an image from this phase channel, and also to filter the PA amplitude channel.
A study was also performed to compare FD PA and the prevalent pulsed method. Various features of both methods were examined experimentally using a dual-mode PA system and under the condition of maximum permissible exposure (MPE). The SNRs of both methods were evaluated theoretically and experimentally. It was shown that at low frequencies, both modalities generate comparable SNRs, and at high frequencies pulsed PA produces superior SNRs and depth detetivity. However, by increasing the laser power and decreasing the chirp duration within the safety limits, the SNR and depth detectivity of the FD-PA method are enhanced considerably. The main cause to achieve lower experimental SNRs than theoretical predictions for pulsed PA response was shown to be the oscillating baseline, which can be partially eliminated by filtering.
| Identifer | oai:union.ndltd.org:TORONTO/oai:tspace.library.utoronto.ca:1807/31825 |
| Date | 10 January 2012 |
| Creators | Lashkari, Bahman |
| Contributors | Mandelis, Andreas |
| Source Sets | University of Toronto |
| Language | en_ca |
| Detected Language | English |
| Type | Thesis |
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