High-intensity focused ultrasound (HIFU) therapy is an emerging and versatile technique for clinical non/mini-invasive treatment. Ultrasound methods for HIFU therapy guidance, due to its readily available and cheaper advantages over MRI, that has been intensively explored as a promising technology. This thesis mainly focused on the investigation of a designed hybrid pulse sequence to perform HIFU therapy, and implement its whole journal monitoring via passive acoustic mapping (PAM) and interlaced high frame rate B-mode. For the dual ultrasound modalities, efforts have been devoted to advanced algorithms with computation efficacy to improve the contrast to noise ratio for hyperechogenicity displaying (B-mode) and the source localisation capability (PAM).
In the first contributing chapter, several parameters and factors were firstly investigated on the gel phantoms, and being monitored by interlaced B-mode and passive cavitation detection, such as the optimal duty cycle (95%) of the hybrid pulse sequence and the efficient B-mode modality via phased array. Meanwhile, the power spectrum equalized technique was proposed to decolor the re-aligned data into whiten domain for interlaced B-mode beamforming, and demonstrated an improvement in CNR and CR approximately 69% and 125% increase over the Delay Multiply and Sum (DMAS) algorithm, respectively.
In the second contributing chapter, the DC components generated from the system and passive beamforming process which can be recursively accumulated into great background noise. Therefore, the DC components subtraction was considered in the time-domain Robust Capon beamformer, aim at improving cavitation localization capability. Meanwhile, kinds of techniques were developed into PAM beamformers and illustrated with proposed methods such as eigen-subspace and scaled Wiener. Final experimental results from both BSA and ex vivo livers phantoms demonstrated an 86%-94% noise area suppression is achieved by the proposed method, compared to the time exposure acoustic (TEA) algorithm.
In the third contributing chapter, to complement experimental studies, a simulator was built based on the coated-model to provide standardised data for the quantitative analyse of PAM. Then, a pixel-wise F-number and shading weight (DFSW) strategy was developed from the soft-baffle element directivity model that was introduced into PAM beamforming. Final simulated and experimental results illustrate that the DFSW strategy could improve the PAM image quality, particularly in the near-field with approximately 6 dB enhancement.
In the final contributing chapter, a more clinically relevant HIFU system, with therapeutic and imaging transducers co-aligned was used to evaluate the polar coordinate PAM for a competent monitored field of view, whilst, a mutual convoluted strategy between interlaced B-mode and PAM was developed to improve the PAM image quality. All of these aforementioned techniques were to improve the localisation/imaging of cavitation activity from high duty cycle HIFU exposures, the promising future direction about presented jobs is to monitor the applications such as microbubble drug delivery or microbubble mediated blood-brain barrier opening .etc as the next step.
