Deformation-Aware Robotic 3D Ultrasound

Tissue deformation in ultrasound (US) imaging leads to geometrical errors when measuring tissues due to the pressure exerted by probes. Such deformation has an even larger effect on 3D US volumes as the correct compounding is limited by the inconsistent location and geometry. This work proposes a patient-specified stiffness-based method to correct the tissue deformations in robotic 3D US acquisitions. To obtain the patient-specified model, robotic palpation is performed at sampling positions on the tissue. The contact force, US images and probe poses of the palpation procedure are recorded. The contact force and the probe poses are used to estimate the nonlinear tissue stiffness. The images are fed to an optical flow algorithm to compute the pixel displacement. Then the pixel-wise tissue deformation under different forces is characterized by a coupled quadratic regression. To correct the deformation at unseen positions on the trajectory for building 3D volumes, an interpolation is performed based on the stiffness values computed at the sampling positions. With the stiffness and recorded force, the tissue displacement could be corrected. The method was validated on two blood vessel phantoms with different stiffness. The results demonstrate that the method can effectively correct the force-induced deformation and finally generate 3D tissue geometries.

I. Introduction

Ultrasound (US) is a broadly utilized diagnostic imaging modality for examinations of internal organs. It is also commonly used to obtain the location and geometric information of disease intraoperatively as US imaging is highly available, non-invasive and radiation-free. However, to obtain optimal acoustic coupling of a US transducer and thus achieve good visibility of target anatomies, a certain pressure is required to be applied to the imaged anatomy. Due to the exerted pressure, the shape distortion of visualized tissue structures is inevitable, particularly for soft tissues such as superficial blood vessels (Fig. 1). The shape of the cephalic vein continues to compress when the contact force increases. The vein loses its complete lumen when the force increases to . As a result, the distortion can severely obfuscate the geometrical measurements of subsurface targets, e.g., measuring blood vessel diameter for diagnosing vascular stenosis.