Method to estimate the deviation from ideal uniaxial compression during freehand elastography

Quasi-static ultrasound elastography was introduced in the early 1990s to provide a way to visualize the mechanical properties of target tissue. Most commonly, only the axial strain is imaged and referred to as an Axial Strain Elastogram (ASE) or elastogram for simplicity. It has been shown that one can image the axial-shear strain distributions as well in addition to ASE. The image of the axial-shear strain is referred to as an axial-shear strain elastogram (ASSE). It has also been shown that the presence or absence of non-zero axial-shear strain values inside the inclusion (referred to as fill-in) along with contrasting margin at its boundary may serve as a potential feature from ASSE that can aid in non-invasive breast lesion classification. However, during freehand elastography, deviations from uniaxial compression often occur typically appearing in several of the frames of a cine-loop obtained during compression. It was shown recently that accounting for such deviations would be important for reliable interpretation of the "fill-in" observed in ASSE. In this article, we describe a method to estimate the angle of iso-displacement contour at a given depth and use this as a measure to quantify the deviation from the desired uniaxial compression during freehand elastography. We validate the estimated angle obtained from the axial-displacement map against the designed values in simulation and tissue-mimicking phantom experiments. The potential of the angle estimate to detect unreliable ASSE frames among the freehand-acquired data cine-loop is demonstrated using example cases of in vivo breast lesion data. Based on the results, we conclude that the angle of the iso-displacement contour from the axial-displacement map can be used as a metric to qualify an ASSE frame as reliable to interpret or not. Importantly, this metric can be obtained in real time and thus can provide operator feedback to guide and improve in vivo freehand elastography data acquisition quality.