Automated strain measurements with AutoStrain LV/RV/LA

TOMTEC, a Philips technology, has a long history of providing strain measurements with its software applications, which are well recognized and accepted by clinical users and supported by hundreds of journal publications. AutoStrain is the first TOMTEC application integrated on the Philips EPIQ CVx ultrasound system. AutoStrain, powered by Auto View Recognition, Auto Contour Placement and speckle tracking, enables a robust, reproducible one button GLS measurement, making it an efficient tool for routine clinical use.

Deformation imaging to assess left atrial (LA) and right ventricular (RV) function is a newer clinical area. AutoStrain LA and AutoStrain RV follow the Strain Task Force standardization³ document. Based on its fast and easy workflow, AutoStrain makes these measurements available for clinical routine practice.

The AutoStrain application utilizes two automation technologies: Auto View Recognition and Auto Contour Placement. While the implementation of these automation tools drives simple, fast workflow for robust and reproducible GLS measurements, the user retains the ability to edit and override the automation to ensure good clinical practice.

Auto View Recognition automatically identifies which selected image is apical 4 chamber (A4C), apical 2 chamber (A2C) and apical 3 chamber (A3C), and automatically assigns the labels to the selected images. The label is shown on the image as a schematic overlay as shown (Figure 1). The algorithm has been validated on more than 6,000 clinical images with a success rate of 99%. This means that only 1 out of 100 cases will require manual intervention.

A specialized contour detection module for the respective apical view is applied to each of the three image sequences and operates in three steps. First, a complete R-R cycle – end-diastole (ED) start to ED end – is selected from each sequence. Second, in the ED start frame of that cycle, the left ventricle (LV) is automatically located. Third, a view-specific deformable endocardial contour model is aligned to the individual image content (Figure 2). The same approach is applied for RV Auto Contour Placement while the initial LA contour is placed in the end-systolic (ES) frame (Figure 2).

Once the endocardial border is automatically placed in ED (LV, RV) or in ES (LA), it follows the cardiac motion using speckle tracking over the entire cardiac cycle. In Tracking Revision and Analysis Workflow step, the user has all the measurements displayed along with the ED and ES borders next to a dynamic display of all views to easily verify that the borders are correctly placed and tracked. If border editing is needed, it is highly recommended to start editing in ED. Editing the border in ED triggers new speckle tracking of the border throughout the cardiac cycle. When editing the ES border, the edits are propagated to the neighboring frames but the ED border remains untouched.

Longitudinal strain is measured at the endocardial border as indicated by the green line. Instantaneous endocardial strain is visualized by color-coding close to the endocardial border.

The ED frame is always the first frame in the selected cardiac cycle. The ES time point is automatically defined as the time point of global peak strain. It can be adjusted according to aortic valve closure (AVC) time in the AVC layout.

GLS is calculated as global shortening of the endocardial border. It is defined as a peak value and thus independent from AVC as shown (Figure 3).

Based on the deformation of the green endocardial contour, longitudinal strain is computed for the free wall (RVFWSL), the global 4-chamber contour (RV4CSL) and the three free wall segments.

The LA cardiac cycle consists of three phases: reservoir phase goes from ED to ES, conduit phase ends at the time point right before atrial contraction (AC) – also denoted as PreA time point – and contraction phase completes the cardiac cycle.

Validation of AutoStrain LV has been done in comparison to the well-accepted TOMTEC 2D CPA application, with 225 clips analyzed with 2D CPA and re-evaluated with AutoStrain LV using consistent border definitions and manual corrections where necessary in both packages.

For AutoStrain LA validation, LA reservoir strain with reference frame at ED (LASr_ED) has been compared to 2D CPA using 71 clips. AutoStrain RV compared RV global 4 chamber longitudinal strain (RV4CSL) in 75 clips with the corresponding 2D CPA measurements.

As deformation within the myocardium varies regionally, GLS as a measure of myocardial deformation shows interdependence with initial contour placement. To assess reproducibility of the AutoStrain tracking, the influence of manual adjustments of the initial endocardial contour was evaluated. Based on 225 user-reviewed initial contours, manual refinements have been simulated by slight modification of all original contour point positions (within ±2 pixels distance). Then resulting GLS measurements were compared against the original readings.