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.