Accurate in-plane and out-of-plane ultrasound-based tracking of the discretely actuated steerable cannula

Discretely actuated steerable cannula is a multi-degree-of-freedom hollow needle (cannula) that could potentially be used in needle-based procedures to deliver therapeutic and diagnostic tools to a target region through its hollow inner core. Needle-based procedures are commonly performed using intra-operative image guidance. 2D ultrasound is one of the most commonly used imaging modalities in clinics. It is portable, inexpensive and free of ionizing radiation. The success of the needle-based procedures depends on accurate detection of the needle. The accuracy of the out-of-plane detection, where the ultrasound transducer is placed at a right angle to the long-axis of the needle, depends on the ultrasound beam width. Finite width of the ultrasound beam and uniform cross-section of the needle introduce errors in tracking. The accuracy of in-plane tracking depends on the tracking algorithm used and the spatial resolution of the ultrasound transducer. This work presents a method to quantify the finite ultrasound beam width and the spatial accuracy of the transducer. An out-of-plane detection method was implemented to locate the tip of the discretely actuated steerable cannula. An in-plane tracking algorithm based on optical flow was also developed to obtain the shape of a planar cannula. The algorithms and the methods developed in this work are general and they can be extended to needles having straight, curved and arbitrary shapes.