P4360Personalised shape models of the left ventricle from 3D echocardiography: an initial comparison to cardiac magnetic resonance imaging
Authors: Zhao D , Gilbert K , McDougal C R , Wang V Y , Houle H , Doughty R , Young A A , Nash M P 
Published: October 2019

Abstract

Background
The heart constantly adapts to maintain cardiac output. In the longer term, this process (remodeling) can manifest as changes in ventricular volume, sphericity, and/or wall thickness, amongst several other morphological indices. Previous studies have shown the significance of remodeling in evaluations of survival, and as a determinant of the clinical course of heart failure. Yet surprisingly, diagnostic measures, typically of left ventricular (LV) mass and ejection fraction, neglect much of the shape information that is available through imaging. A recent UK Biobank study revealed that morphometric atlases show more compelling associations with cardiovascular risk factors, than do LV mass and volumes. While it has been possible to construct shape models from cardiac magnetic resonance imaging (MRI), such a framework is still under development for echocardiography (echo).

Purpose
Despite MRI being long regarded as the gold standard, it is greatly limited by high costs, long scan times and incompatibility with ferromagnetic cardiac devices. In contrast, echo has presented as a convenient alternative, whilst also offering good temporal resolution. The advancements of 3D echo now provide adequate spatial resolution and thus elicit the possibility of conducting more complex analyses on this modality. With the ability to extract LV geometry directly from 3D echo acquisitions, we sought to create dynamic, 3D patient-specific models–and subsequently compare these results to those derived from MRI.

Methods
As part of an ongoing study, 8 volunteers with no known cardiovascular problems (nor family history thereof), were recruited for non-invasive imaging. Cine MRI and 3D echo of the LV were acquired within a 2 hour session. A Siemens Avanto Fit 1.5 T MRI scanner and Siemens ACUSON SC2000 Ultrasound System with a 4Z1c Transducer were used. 3D models of the LV were generated independently from echo (EchobuildR 2.7 prototype software, Siemens Ultrasound) and MRI acquisitions (Cardiac Image Modeller v8.1), and registered to fiducial landmarks (apex, base plane, right ventricular inserts) and myocardial contours.

Results
Euclidian distances between 1682 corresponding points sampled from the surface of echo/MRI models were calculated, and used as a discrepancy measure (Figure). Across the 8 cases, we found an average root mean square deviation (RMSD) of 5.71 mm at end-systole and 6.03 mm at end-diastole. The maximum RMSD for a single model was 9.47 mm (case 8, ES).

Conclusion
We demonstrate that it is possible to create shape models from 3D echo examinations for comparison with MRI. As more cases are collected, we will devise methods to objectively quantify the mismatch that may arise between models derived from the two modalities. The establishment of such a framework would not only provide previously unavailable measures of shape and function, but in turn leverage the significantly wider clinical reach of echocardiography.