Clinical feasibility of x-ray based pose estimation of a transthoracic echo probe using attached fiducials

Co-registered display of x-ray fluoroscopy (XRF) and echocardiography during structural heart interventions can provide visualization of both catheter-based devices and soft tissue anatomy. For transesophageal echocardiography (TEE), registration can be achieved by estimating the probe pose in the x-ray image. This work investigated the potential clinical requirements for a similar approach using a transthoracic echocardiography (TTE) probe with attached x-ray-visible fiducials. Clinically, the limited number of acoustic windows for TTE dictates probe positioning on the chest, and the interventional task drives the positioning of the C-arm gantry of the x-ray system. A fiducial apparatus must be compatible with these positions and allow for accurate 3D probe pose estimation. TTE imaging of the aortic and mitral valves was performed on eight healthy subjects to determine typical 3D probe positioning in parasternal and apical acoustic windows. This data was incorporated into software that allowed for the simulation of different 3D configurations of fiducials relative to the probe, patient and x-ray system. Three candidate fiducial designs were identified, each consisting of two 40-mm diameter rings with 16 3-mm diameter spheres. X-ray imaging was simulated for C-arm angles of 30° RAO, PA, and 30° LAO, each with cranial-caudal angles typical of a TAVR procedure. Subjectively graded TTE image quality was highest for the parasternal long axis window. A fiducial configuration for the parasternal long window was identified which yielded median 3D TRE ranging from 0.44 mm to 1.04 mm in simulations. An experimental prototype of this design produced a measured 3D TRE of 1.25±0.19 mm.