A coarse to fine architecture decouples the motion and simplifies the kinematics, while guaranteeing high targeting accuracy. As shown in Fig. 2 top, the clinical 18Gauge needles for prostate brachytherapy have an inner stylet and hollow sheath. Radioactive seeds are pre-loaded with 5.5mm spacers between them before staring the surgery. During the insertion, one hand holds the cannula and the other hand brace against stylet hub to prevent relative motion.
After insertion, the sheath is withdrawn over the stylet while leaving the seeds in place. To mimic the physician preload needle type brachytherapy procedure, the needle driver provides 1-DOF cannula rotation about its axis with 1-DOF translational insertion. Another 1-DOF of translational stylet motion is implemented to coordinate the motion with respect to the cannula. The rotation motion of the cannula may be used for bevel-based steering to limit deflection [12] or may be used for active cannula [13]. 2) Operation in confined space: when the patient lies in the scanner bore with semilithotomy position, the lateral space between the legs is around 8cm. To fit into this space, the width of the needle driver module has a wedge shape with 6cm front width (10cm long) and 10cm back width (25cm long). 3) Sterilization: only the plastic tip guide, collet, nut and guide sleeve have direct contact with the needle and are
removable and sterilizable