Transcatheter interventions for structural heart disease demand real-time visualization of catheter devices and their relationship to cardiac anatomy. Co-registration of x-ray fluoroscopy with echocardiography has been proposed to provide the necessary device and soft tissue visualization for these procedures. Development of real-time 3D x-ray/echo registration systems with device tracking has been hampered by the lack of a suitable test model. This study presents a phantom that is compatible with x-ray, CT, transthoracic (TTE), and transesophageal echo (TEE) for testing the feasibility and accuracy of new registration solutions. The phantom consists of a 20.3-cm diameter, 15-cm tall cylindrical shell with acoustic windows for TTE and an access port for a TEE probe. The interior contains 24 dual-modality targets, 5-mm in diameter, suspended in a three-turn helix occupying a volume that is similar to an adult heart. An ultrasound-compatible, tissue-mimicking slurry medium fills the remainder of the phantom. The dual-modality targets are agar based with barium sulfate (BaSO4) powder and glass beads added to generate contrast in both x-ray and ultrasound. Appropriate concentrations of these additives were determined experimentally with contrast measurements in x-ray, CT, and ultrasound. Selected concentrations were 150 mg/mL BaSO4 and 100 mg/mL of 53-63 μm diameter glass beads. Average target contrast (± SD) was 16% ± 2% in x-ray fluoroscopy (90 kV) and 1805 ± 99 HU in CT (100 kV). In ultrasound, target CNR was 4.30 ± 0.62 in 2D B-mode and 4.03 ±1.06 in 4D-mode images acquired at a center frequency of 2.8 MHz.
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.