Myocardial perfusion, the blood flow to the heart muscle, can be evaluated by tracing the passage of a contrast agent using cardiac magnetic resonance (CMR) imaging. This technique, well-established for diagnosing coronary artery disease, is limited by the necessity for breath-holding, subjective assessment, and low myocardial coverage. In this thesis, we aim to address these limitations of contrast-enhanced myocardial perfusion CMR. We developed a pulse sequence and post-processing pipeline to quantify myocardial perfusion using free-breathing 3D contrast-enhanced CMR. To restrict volume acquisition to the diastolic phase, characterized by minimal cardiac motion, we employed optimal slice oversampling, maximal partial Fourier acquisition, and cartesian undersampling in spatial and temporal domains. To mitigate the effects of breathing, respiratory tracking and image registration were performed. Collaborations for the reconstruction of raw data utilizing deep learning and image registration were established. Validation in healthy volunteers demonstrates that the developed pulse sequence enables isotropic 3D acquisition (3.6 x 3.6 x 3.6 mm^3) of an arterial input function (AIF) and myocardial signal during each cardiac cycle, up to heart rates of 76 bpm. Obtained AIF images exhibit sufficient resolution for extracting the left ventricular blood pool signal and registered myocardial images are of good quality. We investigated and validated a method to convert signal intensity to T1, which is required for MBF quantification. While T1 estimates from the AIF images approximate reference values up to 500 ms well, underestimation was observed from the myocardial images and for high T1 values. ... Read more

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 ... Read more

Atrial Fibrillation (AF) is a common type of arrhythmia, characterized by rapid and irregular contraction of the atria. Irregular contraction of the atria is caused by erroneous electrical activation, originating from the tissue around the Pulmonary Veins (PV) in the Left Atrium (LA). Catheter-based treatment of AF includes ablation of the tissue around the PV.

Imaging of ablation catheters and LA-anatomy is of great importance for accurate and successful treatment of AF. In current state of the art LA-ablation procedures, static, pre-procedural acquired 3D models of the LA are used for catheter navigation and monitoring. However, due to the limited accuracy of these static 3D models there is a continuous demand for real-time 3D imaging techniques during catheter based treatment of AF.

A possible solution for real time imaging is to use trans-balloon, miniaturized transesophageal echocardiography (TEE) as imaging modality for AF ablation procedures. Trans-balloon, miniaturized TEE can provide real-time 3D ultrasound imaging of the LA.

Since trans-balloon, miniaturized TEE is not used in clinical practice yet, it is unknown whether image quality of the miniaturized TEE probe will be sufficient for imaging during AF ablation procedures. Therefore, the image quality of trans-balloon, miniaturized TEE has to be compared to image quality current state of the art TEE probes.

In this thesis, image quality of the miniaturized ultrasound probe was compared to current state of the art TEE probes, and the influence of the balloon on image quality was investigated. Image quality was assessed using standardized ultrasound image quality assessment phantoms and software. Image quality was measured using the following image quality parameters: contrast to noise ratio (CNR), spatial resolution and penetration depth of the ultrasound signal.

The results have shown that CNR of the miniaturized probe is equal to that of state of the art probes, but that spatial resolution of the miniaturized probe strongly depends on the rotation angle of the ultrasound imaging plane. Rotating the imaging plane of the miniaturized TEE probe negatively impacts spatial resolution, which was not observed for the state of the art TEE probes. The penetration depth of the ultrasound signal was significantly less compared to state of the art TEE probes.

The rotation-dependent spatial resolution of the miniaturized probe is undesired, since rotating the imaging plane is common practice in TEE. A review study involving clinical experts is recommended to judge whether image quality of the miniaturized probe with rotated imaging plane is sufficient.
Since the anatomical structures of interest during AF are close to the transducer, the limited penetration depth of the miniaturized probe will not be a major limitation. Despite aforementioned limitations, the results suggest that trans-balloon miniaturized TEE can be used for real-time ultrasound imaging during AF ablation procedures.

The research in this thesis provides a theoretical framework to measure and compare image quality of ultrasound probes, which is an important first step in the development of a novel, real-time 3D imaging technique for imaging of AF ablation procedures.

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