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Discretely Actuated Steerable Cannula (Elif Ayvali)

Motivation: Several percutaneous needle-based and intravascular procedures require a needle or a catheter to guide diagnostic or therapeutic tools to the target location. In almost all instances, these probes (can be needle or cannula) need to be steered to the target location by a variety of maneuvers at the distal end of the probe. For example, prostate biopsy and breast biopsy procedures employ needles with a bevel or a symmetric tip for a specific procedure. Often, these needles need to be re-introduced or withdrawn slightly to correct for the error in targeting due to needle and soft-tissue interaction. We aim to develop a discretely actuated steerable cannula with multiple degrees-of-freedom to steer the probe by discrete actuation along the probe length. We are interested in using the cannula to introduce both diagnostic and therapeutic tools, which may otherwise be difficult to deliver to the appropriate location.

Project Highlights

• The probe is composed of straight segments connected by shape memory alloy (SMA) actuators. SMA wires were annealed through a customized training process to an arc shape and mounted on the outer surface of the probe to generate local bending upon thermal actuation. The two ends of the SMA wires are connected on the outer surface of the links to enable bending of the probe when the SMA is actuated. Resistive heating is used to heat up the SMA wires.
• We have developed a setup to characterize an SMA actuator that is trained to have an arc shape upon thermal actuation. The apparatus consists of a rotary encoder and a pin attached a fixed distance away from the center of the encoder. As the SMA wire transforms into an arc shape, it rotates the pin. From the geometry of the setup, the encoder reading can be related to the radius of curvature of the SMA wire (hence the strain in the SMA wire). There is a cable connected to the SMA wire, and it can be routed around the screw and connected to a mass via pulley to apply constant stress or it can be connected to the extension spring to apply variable loading.
• We propose a combined vision-based and temperature-based approach to control the bending angle at each joint. A pulse width modulation-based control scheme was implemented to control all SMA actuators simultaneously to enable multiple joint motions using a single power supply. Stereo vision is used as the main feedback for position control of the cannula since we are eventually interested in image guided control of the cannula using an imaging modality such as CT, MRI or ultrasound. Micron Tracker (Claron Inc.) is used for stereo imaging and Pyramidal Lucas-Kanade optical flow algorithm is used to find the pose of the cannula by tracking the markers that are placed on the links. SMA actuator has also been fully characterized to use temperature feedback to control the strain in the SMA actuators in case of poor image feedback quality from the imaging modality.

Relevant Publications:

1. E. Ayvali, M. Ho, and J. P. Desai, “A novel discretely actuated steerable probe for percutaneous procedures”, In Proc. 12th International Symposium on Experimental Robotics (ISER 2010), Springer Tracts in Advanced Robotics, 79, pp. 115-123, 2012.
2. E. Ayvali, C. P. Liang, M. Ho, Y. Chen, and J. P. Desai, “Towards a discretely actuated steerable cannula for diagnostic and therapeutic procedures”, The International Journal of Robotics Research, April 2012 vol. 31 no. 5588-603.
3. E. Ayvali, and J. P. Desai, “Towards a discretely actuated steerable cannula”, In IEEE International Conference on Robotics and Automation (ICRA 2012), Saint Paul, MN, May 14-18, 2012.
4. E. Ayvali, and J. P. Desai, “Motion planning for the discretely actuated steerable cannula”, In IEEE International Conference on Biomedical Robotics and Biomechatronics (BIOROB 2012), June 24-27, Rome, Italy.