Transvascular intracardiac applications of a miniaturized phased-array ultrasonic endoscope. Initial experience with intracardiac imaging in piglets. BACKGROUND. Recent advances in miniaturization of phased-array and mechanical ultrasound devices have resulted in exploration of alternative approaches to cardiac and vascular imaging in the form of transesophageal or intravascular imaging. Preliminary efforts in adapting phased-array endoscopes designed for transesophageal use to a transvascular approach have used full-sized phased-array devices introduced directly into the right atrium in open-chested animals. The purpose of this study was to assess the feasibility of using a custom-made, very small phased-array endoscope for intracardiac imaging introduced intravascularly through a jugular venous approach in young piglets. METHODS AND RESULTS. Experimental atrial septal defects created in four piglets (3-4 weeks old) had been closed with a buttoned atrial septal defect closure device consisting of an occluder in the left atrium and a counteroccluder in the right atrium. Five to 15 days after atrial septal defect closure, the piglets were returned to the experimental laboratory, where a 6.3-mm, 17-element, 5-MHz phased-array probe mounted on a 4-mm endoscope was introduced through a cutdown incision of the external jugular vein and advanced to the right atrium. From the right atrium all four cardiac chambers, their inflows and outflows, and all four valves were well imaged with minimal superior and inferior rotation. High-resolution imaging of the atrial septum defined with anatomical accuracy, later verified by autopsy, the exact placement of both the occluder and counteroccluder in the left and right sides of the atrial septal defects and the absence of any shunting across the atrial septum in any of the four animals. CONCLUSIONS. Our efforts indicate that transvascular passage of small phased-array probes can be easily accomplished and is a promising technique for detailed visualization of cardiac structures. This approach may provide an alternative to transesophageal echocardiography, particularly for guiding interventional procedures such as placement of transcatheter closure devices in pediatric patients. 