3D reconstruction of the deformed breast surface from autonomously acquired ultrasound images

Recent statistics show that about 26.2% of the women that suffered from cancer in the year 2020 were diagnosed with breast cancer, making it the most prevalent forms of cancer in women in the Netherlands. The first step usually performed to detect cancerous lesions in the breast is done with the help of a mammogram. However, a  mammogram fails to detect lesions in women with dense breast tissues. This is where the MRI and Ultrasound Robotic Assisted Biopsy (MURAB) has its main advantage. The MURAB project aims to combine the precision and sensitivity of the pre-operative Magnetic Resonance (MR) imaging with safe, real-time intraoperative Ultrasound (US) images, to further target small lesions in dense breast tissues. The ultrasound images are acquired autonomously with the help of a linear ultrasound transducer that is attached to a 7DOF robotic manipulator. The manipulator is controlled such that the scanning trajectory is based on US feedback and is always maintained tangentially to the breast surface. Additionally, the control algorithm also ensures that at any given time, there is 70% contact between the transducer surface of the US probe and the breast surface.

The obtained MR and US images are further fused together by placing the MR volume on top of the US volume, for 3D volume reconstruction. However, in order to ensure accurate fusion between the two volumes, there are two main requirements. Firstly, although both the volumes are taken in the prone position, they are taken separately and are thus in different workspaces. To solve this, a transformation of the volume is needed and can be done with the help of a surface model of the breast. Secondly, in MRI, the
breast is undeformed, however, in US imaging, the breast surface is deformed due to the probe contact force. For these purposes, a camera scan is an essential step that locates the breast surface in the US workspace and further calculates the deformation in the breast shape between the MR and US workspace. Due to the patient’s position, other deformations such as breathing or heartbeat deformations are considered to be negligible.

As mentioned in the introduction, the robotic manipulator is controlled such that 70% contact is ensured between the US probe and the breast surface. This is done in order to minimize the tissue deformation that occurs due to contact forces. This contact is centred along the centre scan line, thus ensuring an equal amount of no acoustic coupling on both sides of the transducer. Literature states several solutions such as a stereo camera, Kinect, monocular camera or laser scanners (to mention a few) for the 3D reconstruction of a surface. However, during this assignment, the transition area between acoustic coupling and no acoustic coupling will be used to obtain a 3D reconstructed surface of the deformed breast. Successful reconstruction of the breast surface using US images will in turn eliminate the need for a camera scanner, thus reducing the number of sensors required and further improving and simplifying the MURAB workflow.