Table 2.

Design specifications

Concept	A needle (cannula and stylet) positioner and rigid guide for MRI-guided percutaneous injection into the ventral horn of the spinal cord, which requires the needle to follow a trajectory through the gaps between vertebrae2, 3 into the spinal cord.
Size constraint	The procedure is performed in a 60–70 cm-cylinder MRI bore, where a large portion of the volume is occupied by the patient torso and abdomen, limiting the size and workspace of the robot9, 12.
Materials	Fabrication from MR-safe materials, preferably those which do not create image-degrading magnetic susceptibility artifacts in order ensure safety and utility.
Needle	A 16-gauge cannula is necessary to allow a needle from a microinjection system to be used, as well as ensuring sufficient strength and stiffness to pierce muscle and lamina.
Insertion	Needle insertions are performed by hand in order to obtain realistic force feedback.
Method of attachment	The method with which the device is mounted to the patient must be minimally invasive. The device should require minimal adjustment after it has been positioned on the patient.
Clinical requirement	Must be disposable or able to be sterilized for future in-vivo trials.
Procedure length	Use of device should shorten procedure (typical length of direct injection procedure is 4 hours).
Function	Allow physician to perform multiple insertions along an approximately 10 cm length of the spinal cord without adjusting fixation to the patient, which reduces procedure time.
Range of motion	Allow manipulation of the needle orientation and position by providing: 2-DOF needle translations (90 mm in left-to-right direction, and 140 mm in head-to-foot direction relative to patient orientation) 2-DOF needle rotations (±30° left-to-right rotation, ±35° head-to-foot rotation relative to patient orientation) Provide rigid support for needle during manual 1-DOF needle insertion.
Targeting accuracy (angle)	2°
Targeting accuracy (position)	2.5 mm