Evaluation of strain averaging area and strain estimation errors in a spheroidal left ventricular model using synthetic image data and speckle tracking

Abstract

Background In majority of studies on speckle tracking echocardiography (STE) the strain estimates are averaged over large areas of the left ventricle. This may impair the diagnostic capability of the STE in the case of e.g. local changes of the cardiac contractility. This work attempts to evaluate, how far one can reduce the averaging area, without sacrificing the estimation accuracy that could be important from the clinical point of view. Methods Synthetic radio frequency (RF) data of a spheroidal left ventricular (LV) model were generated using FIELD II package and meshes obtained from finite element method (FEM) simulation. The apical two chamber (A2C) view and the mid parasternal short axis view (pSAXM) were simulated. The sector encompassed the entire cross-section (full view) of the LV model or its part (partial view). The wall segments obtained according to the American Heart Association (AHA17) were divided into subsegments of area decreasing down to 3 mm2. Longitudinal, circumferential and radial strain estimates, obtained using a hierarchical block-matching method, were averaged over these subsegments. Estimation accuracy was assessed using several error measures, making most use of the prediction of the maximal relative error of the strain estimate obtained using the FEM derived reference. Three limits of this predicted maximal error were studied, namely 16.7%, 33% and 66%. The smallest averaging area resulting in the strain estimation error below one of these limits was considered the smallest allowable averaging area (SAAA) of the strain estimation. Results In all AHA17 segments, using the A2C projection, the SAAA ensuring maximal longitudinal strain estimates error below 33% was below 3 mm2, except for the segment no 17 where it was above 278 mm2. The SAAA ensuring maximal circumferential strain estimates error below 33% depended on the AHA17 segment position within the imaging sector and view type and ranged from below 3–287 mm2. The SAAA ensuring maximal radial strain estimates error below 33% obtained in the pSAXM projection was not less than 287 mm2. The SAAA values obtained using other maximal error limits differ from SAAA values observed for the 33% error limit only in limited number of cases. SAAA decreased when using maximal error limit equal to 66% in these cases. The use of the partial view (narrow sector) resulted in a decrease of the SAAA. Conclusions The SAAA varies strongly between strain components. In a vast part of the LV model wall in the A2C view the longitudinal strain could be estimated using SAAA below 3 mm2, which is smaller than the averaging area currently used in clinic, thus with a higher resolution. The SAAA of the circumferential strain estimation strongly depends on the position of the region of interest and the parameters of the acquisition. The SAAA of the radial strain estimation takes the highest values. The use of a narrow sector could increase diagnostic capabilities of 2D STE.

Parametrization of subsegmental infarcts using high spatial resolution 2DSTE and synthetic ultrasonic data

“…The LV model used here was based on a physical LV phantom and has been fully described elsewhere. 29 Briefly, the 3D model had the shape of a half spheroid at the end-systolic (ES) phase (Figure 1). The semi-axes of the internal surface of the model at the ES phase were 72 mm and 15.75 mm.…”

“…For several years, we have been working on methods to identify small subsegmental or non-transmural infarcts using 2DSTE and synthetic ultrasonic data obtained in models of the left ventricle (LV). [27][28][29][30] With the objective to achieve higher resolution and, therefore, more precise infarct localization, we aim to use smaller strain averaging areas than those currently used in cardiology. We recently evaluated the performance of the 2DSTE and assessed the exchange between the strain averaging area and the strain estimation error.…”

“…We recently evaluated the performance of the 2DSTE and assessed the exchange between the strain averaging area and the strain estimation error. 29 We averaged strain estimates over subsegments of the LV model wall with areas ranging from 280 mm 2 down to 3 mm 2 . The smallest averaging area resulting in an estimation error not exceeding an acceptable limit may be considered a measure of the spatial resolution of strain estimation.…”

“…Our subsequent study focused on assessing the accuracy of the segmentation of simulated infarcts in a model of the LV 28 , using the approach outlined in the previous paragraph 29 . There have been numerous studies of the STE accuracy.…”

“…The objective of the present study is to evaluate the parametrization of simulated infarcts in LV models, featuring varying size and transmurality, using the model deformation and strain estimation methodology proposed previously. 28, 29 The results of these experiments are supposed to indicate whether the main limitation of the simulated infarct parametrization accuracy is the strain estimation accuracy or the spatial strain distribution in the vicinity of the infarct. The proposed parametrization includes estimating the infarct cross-section surface, estimating the reduction of the strain within the infarcted area and within the entire segment of the LV wall concerned, as well as estimating the transmural extent of the infarct.…”