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Section: New Results
Medical robotics
Non-rigid target tracking in ultrasound images combining dense information and physically-based model
Participants : Lucas Royer, Alexandre Krupa.
This study concerns the real-time tracking of deformable targets within a sequence of ultrasound (US) images. The proposed approach combines dense information with a physically-based model and has therefore the advantage of not using any fiducial marker. The physical model is represented by a mass-spring damper system driven by external and internal forces. The external forces are obtained by maximizing an image similarity metric between a reference target and the deformed target along the time. The internal forces of the mass-spring damper system constrain the deformation to be physically plausible and therefore efficiently reduce the sensitivity to the speckle noise. This approach was first validated from simulated and real sequences of 2D US images [49] . It was then extended for deformable target tracking in a sequence of 3D ultrasound volumes and tested on a robotic setup used to apply deformation on an organic phantom [48] . The performance of this deformable 3D target tracking approach was evaluated with visual assessment combined with robotic odometry ground truth. This method was also tested and compared with respect to state-of-the-art techniques by using 3D image databases provided by MICCAI CLUST'14 and CLUST'15 challenges [47] (MICCAI Challenge on Liver Ultrasound Tracking). It was awarded by the organizers of the CLUST challenges as being the best method for accurate target tracking in 3D ultrasound sequences. We recently improved our approach in order to increase its robustness to the presence of ultrasound shadows, local illumination changes and image occlusions.
3D steering of flexible needle by ultrasound visual servoing
Participants : Pierre Chatelain, Jason Chevrie, Marie Babel, Alexandre Krupa.
The objective of this work is to provide robotic assistance during needle insertion procedures such as biopsy or ablation of localized tumor. In previous work, we designed a control approach based on a duty cycling technique for steering a beveled-tip flexible needle actuated by a robotic arm in such a way to control the needle curvature in 3D space and reach a desired target by visual servoing. In this preliminary work, the control approach was validated by using visual features extracted from 2 images provided by 2 orthogonal cameras observing a translucent gelatin phantom where the needle was inserted. This year, we have pursued our work towards this needle steering robotic assistance by developing a new algorithm able to track in real-time a flexible needle in a sequence of 3D ultrasound images (volumes). The flexible needle modeled as a polynomial curve is tracked during the automatic insertion using particle filtering. This new tracking algorithm enables real-time closed-loop needle control with 3D ultrasound feedback. The target to reach was manually defined by the user in the US image and can be on-line tracked thanks to the template tracking algorithm proposed in [21] based on ultrasound dense visual servoing [7] . Experimental results of an automatic needle tip positioning in a home-made gelatine phantom demonstrate the feasibility of 3D ultrasound-guided needle steering for reaching a desired target by ultrasound visual servoing [33] . Recently a new control law for needle steering that uses both direct manipulation of the needle base and the duty cycling method has been studied. It is based on a 3D model of a beveled tip needle using virtual springs that characterize the needle mechanical interaction with soft tissue. From this model, a measure of the controllability of the needle tip degrees of freedom was proposed in order to mix the control between the direct base manipulation and the duty cycling technique. Preliminary simulations show that this hybrid control allows better targeting capabilities in terms of larger needle workspace and reduced needle bending.
Optimization of ultrasound image quality by visual servoing
Participants : Pierre Chatelain, Alexandre Krupa.
This study focuses on a new ultrasound-based visual servoing approach that optimizes the positioning of an ultrasound probe manipulated by a robotic arm in order to improve the quality of the acquired ultrasound images. To this end, we use the recent framework of ultrasound confidence map, developed in the Chair for Computer Aided Medical Procedures and Augmented Reality of Prof. Nassir Navab, which aims at estimating the per-pixel quality of the ultrasound signal based on a model of sound propagation in soft tissues. More specifically, we treat the ultrasound confidence maps as a new modality and designed a visual servoing control law for image quality optimization. We illustrated our approach with the application of robotic tele-echography where the in-plane rotation of a 2D probe is visually servoed by the confidence map and the other degrees of freedom are teleoperated by the user. Experiments performed on both an ultrasound examination training phantom and ex vivo tissue samples validated this new concept [32] . Currently, we consider the confidence-driven servoing of other degrees of freedom, in particular out-of-plane motions that were controlled in our previous works from image moments [6] , which could provide finer control of the image quality.
Visual servoing based on ultrasound elastography
Participants : Pedro Alfonso Patlan Rosales, Alexandre Krupa.
This study concerns the use of the ultrasound elastography as a new image modality for the control of the motion of an ultrasound probe actuated by a robotic manipulator. Elastography imaging is performed by applying continuous stress variation on soft tissues in order to estimate a strain map of the observed tissues. It is obtained by estimating, from the RF (radio-frequency) signal along each scan line of the probe transducer, the echo time delays between pre- and post-compressed tissue. Usually, this continuous stress variation is performed manually by the user who manipulates the US probe and it results therefore in a user-dependent quality of the elastography image. To improve the US elastography imaging, we recently developed an assistant robotic palpation system that automatically moves an ultrasound probe in such a way to optimize ultrasound elastography. The main originality of this preliminary work concerns the use of the elastography modality directly as input of the robot controller thanks to an innovative ultrasound elastography-based visual servoing approach.
Visual servoing using shearlet transform
Participants : Lesley-Ann Duflot, Alexandre Krupa.
Similar to wavelet transform, shearlet transform is usually used in the field of signal or image compression. At the best of our knowledge these image representations were never used directly as feedback of a closed-loop control scheme. The objective of this work is to study the feasibility of using the coefficients of shearlet transform of the observed ultrasound image directly as the visual features of an image-based visual servoing. In this study we estimated numerically the interaction matrix that links the time variation of the coarsest coefficients of the shearlet to the motion of the ultrasound probe. This shearlet-based visual servoing was experimentally tested for automatically positioning a 2D US probe, held by a robot, on a desired section of an abdominal phantom. The first results demonstrated promising performances.