## 4 Discussion  
Spatial compounding has been demonstrated for many applications to improve field of view and image quality. Incorporating image information into the weighting function \begin{table}
\begin{tabular}{l c} \hline \hline
**Excised Valve** & **Chordae Measurement Absolute Difference (mm)** \\ \hline Valve 1 & \(0.7\pm 0.6\) \\ Valve 2 & \(0.6\pm 0.6\) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Volume comparison metrics between compounded echo and CT.  
Figure 8: Visualizations of compounded TEE data from five different TEE volumes. (**Left**), a volume rendered view. (**Middle**), a commissure-commissure slice. (**Right**), an AP slice. Figure 7: Side-by-side volume rendered comparisons from multiple view points of the CT data (**top**) and compounded echo (**bottom**). shows clear improvement over prior blending approaches. The combined distance and oriented symmetry weighting improves image quality and helps eliminate blending artefacts where the separate image acquisitions did not entirely agree. Compared to our previous results for this application, the registration strategy incorporating simultaneous group registration as the initial step improved robustness and eliminated the effect of initialization order, helping to prevent registration failure where the volumes do not reach alignment. For the application of mitral valve procedure planning, we show that spatially compounded 3D echocardiography volumes are able to capture the complex structures in the left ventricle. Utilizing spatial compounding reduces image noise and provides a single volume containing the mitral valve, chordae tendineae, and papillary muscles, enabling clinicians to work from a single volume, instead of reconciling multiple separate volumes together. We demonstrate on porcine models that our spatial compounding method using a 3D TEE probe can reproduce the structures captured by a CT scan with high geometrical accuracy. Although the chordae appear thicker in the compounded volume, the separate individual chordae can still be identified from leaflet to papillary muscle. We found that the length of the chordae can be accurately measured from the compounded volume, as the thickening artefact does not affect the measurement of the length of the chordae. The workflow described here can be integrated into the clinical standard of care, requiring only 4-5 acquisitions with approximately 80% overlap. Standard practice currently includes diagnostic 3D TEE for patients undergoing mitral valve procedures and transgastric images are already acquired as part of this process in the form of 2D long-axis views in an attempt to capture the chordae in their entirety. This compounding technique enables the transgastric images to be acquired as a series of 3D volumes instead of the traditional 2D views, while still maintaining visibility of the entire chordae structures. Our workflow makes it possible for clinicians to map almost the entire chordal structure in 3D from the leaflet to the anchoring point in the left ventricle, greatly improving the surgeon's ability to optimize lengths for the introduced neochordae. Another instance where detailed, compounded 3D echocardiography has potential is the early diagnosis of endocarditis, where individual 3D image volume analysis can often remain ambiguous [16]. The volumes produced by applying 3D spatial compounding to TEE imaging capture the entire valve complex. Currently, many procedures require additional imaging in the form of cardiac CT/MRI to accurately perform diagnoses or plan for procedures [17]. Spatially compounded multi-view echo has the potential to add to the clinical imaging workflow by providing similar levels of information to cardiac CT at high frame rates, low cost and no radiation exposure to patients. Further validation of the clinical utility of this method will aim to demonstrate the effectiveness of the compounded echo, in particular related to cases in which cardiac CT is currently necessary.