By Mehdi Eskandari and Mark Monaghan
Mehdi Eskandari and Mark Monaghan explore the use of transoesophageal echocardiography (TOE) and fluoroscopy during transcatheter aortic valve implantation (TAVI), focusing on how fusion imaging may help to improve safety and accuracy during these procedures.
TAVI has become an established and accepted therapeutic option for inoperable patients with symptomatic severe aortic stenosis or those at prohibited risk for surgery. Use of intra-procedural TOE during these procedures remains an area of debate and while there has been a trend towards local anaesthesia and fluoroscopy-only guided TAVI, many centres still use TOE for intraprocedural guidance and early detection of complications. Furthermore although fluoroscopy affords high frame rates and offers excellent catheter and device visualisation, it lacks 3D orientation and is unable to characterise soft tissue. Positioning of the aortic valve implant during TAVI is mainly based on aortic root angiography and valve calcification. However, a better appreciation of relationship between catheter, guide wires, devices and anatomical structure would improve procedural precision.
The EchoNavigator system (Philips Healthcare) enables the overlaying of echo images onto the X-ray field view, resulting in combining the precise device visibility of fluoroscopy with real-time 2D or 3D soft tissue images. After registration of the probe with X-ray, in which a digitally reconstructed projection of the probe is performed, a specific algorithm repeatedly and rapidly repositions the reconstructed projection by tracking the TOE probe position and updates the echo image as X-ray C-arm moves.1 The system is automatic with no impact on procedural or imaging workflow. The EchoNavigator screen has a four image lay out including a corresponding X-ray view, which displays the live image from fluoroscopy screen and three echo screens: a real-time echo image that can be used to mark area of interest; a free echo image that can be rotated and cropped; and an echo image in the same orientation of C-arm. The echo image with C-arm orientation can be fused on the X-ray view so that the interventionist can see the relation between the device and soft tissue in real time. The colour intensity of the overlaid 2D or 3D echo image on X-ray view can also be adjusted for optimal visualisation of both echo and X-ray image.
Use of EchoNavigator in TAVI
Proper positioning of the prosthetic aortic valve is crucial to prevent complications such as valve embolisation, coronary obstruction and paravalvular leak. The first step in achieving this goal is to find the optimal valve view—ie. the plane perpendicular to native valve annulus. Aortic root angiography has been the conventional method which, especially in high- risk TAVI patients, carries risk of contrast induced kidney injury. In our centre we have been recently using EchoNavigator to obtain the optimal valve view which potentially can reduce contrast use. This is done by marking the annulus on two perpendicular echo planes using X-plane imaging of the aortic valve on the EchoNavigator real time echo view. This then automatically projects the annulus on EchoNavigator C-arm view. The EchoNavigator C-arm view can be used to visualise different fluoroscopy C-arm orientations which allows identifying optimal valve view. Based on our limited yet very successful experience, the EchoNavigator has the potential to be solely used for finding the valve view. Using an integrated annotation tool, other area of interests can also be marked and will be automatically identified on the EchoNavigator X-ray view. For example, when crossing the aortic valve is challenging, we place a mark on valve orifice that can be used as a road map. Another important application of EchoNavigator in our centre is fusion of real-time 3D echo (with the same fluoroscopic orientation) on EchoNavigator X-ray screen so that the position of prosthetic valve in relation to leaflets, aortic root and left ventricular outflow tract can be seen and assessed simultaneously. This helps with precise valve positioning and deployment and increases interventionist confidence.
Now that there is a trend towards to using TAVI for lower risk patients, especially in the setting of a less calcified valve, the role of fusion imaging may be even more important.
The rapid increase in the number of minimally invasive structural heart interventions being performed—and being used in cases that are increasingly more complex—means that multimodality and integrative imaging approach is required. Fusion imaging can play a pivotal role in improving safety and accuracy, with the ultimate goal of achieving a better outcome for patients. The use of EchoNavigator in a wide range of structural heart interventions, such as trans-septal puncture, transcatheter mitral valve repair, atrial septal defect closure, left atrium appendage closure and paravalvular leak closure has been reported. However, studies are needed to further assess its accuracy and whether using the system will lead to an improvement in patient outcomes.
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- Arujuna et al. IEEE J Transl Eng Health Med 2014.