Intravascular Ultrasound and Photoacoustic Image Reconstruction

Abstract

Introduction: Coronary atherosclerosis can form large lipid-rich plaques inside the arteries, which are prone to rupture. Rupture can lead to thrombus formation and subsequent obstruction of the blood flow, which is the most common cause of acute coronary events like myocardial infarction. Current imaging modalities lack the ability to image important factors in the diagnosis of these vulnerable plaques or have a limited image quality. A novel imaging modality, intravascular ultrasound (IVUS) combined with intravascular photoacoustics (IVPA), is a good candidate for accurate lipid imaging in atherosclerotic coronary arteries. IVPA enables specific lipid imaging alongside the artery morphology image provided by IVUS. Kaminari Medical is developing the first rotating IVUS and IVPA catheter for this purpose. Objective: The objective of this research project is to develop an image reconstruction method with improved lateral resolution (LR) and Signal-to-Noise Ratio (SNR) compared to the conventional image reconstruction method. Methods and Materials: A literature review was executed to find the state-of-the-art image reconstruction method for IVUS and IVPA. A virtual source synthetic aperture (VSSA) beamforming method with Coherence Factor Weighting (CFW) was selected and implemented to achieve the desired image quality improvement. The principle of the VSSA is that the signals captured at adjacent transducer positions are delayed and summed to use all available image information. The delays are calculated with respect to the virtual sources, which are placed at the natural focus of the beams under the assumption that this yields the best alignment after delaying the signals. A pixel-based implementation of VSSA with CFW was developed on IVUS data which directly reconstructs the image pixel values. Subsequently, the algorithm is optimized to achieve the best image quality for IVUS and IVPA data acquired by the Kaminari Medical catheter. Results: The implementation of the algorithm based on the literature did not show the image quality as expected, most likely due to the invalid assumption that the virtual source should lie at the natural focus of the ultrasound beam. Therefore, the virtual source depth and the opening angle of the beam are optimized to find the parameter combination that achieves the best image quality. For both IVUS and IVPA, narrow beam shapes and a virtual source behind the transducer, thus using diverging beam shapes, should be used to obtain the best LR and SNR. The optimized VSSA leads to an increased SNR by 20.3% and 77.7% for IVUS and IVPA, respectively. The LR is increased by 7% for IVPA but shows a LR reduction for the IVUS data. Conclusion: The optimized VSSA meets the objective of improving the LR and SNR to a large extent. However, it is recommended to focus future work on developing a substitutional weighting method for the CFW to also improve the LR for the IVUS data