(J-391) Characterization of Hydraulic Actuation System for Active Positioning in Magnetic Resonance Elastography

Introduction:: Intervertebral disks (IVD) are the source of flexibility in the spine. Degenerative Disc Disease (DDD) is a condition that impacts the stiffness property of the IVD, specifically targeting the nucleus pulpous, a hydrated and viscous gel that provides the shock-absorbing effect and flexibility in the spine [1]. The Pfirrmann degeneration scoring relies on T2-weighted imaging rather than quantifying the stiffness of disks for the evaluation of DDD [2].

Magnetic Resonance Elastography (MRE) is an imaging technique capable of quantifying DDD. A mechanical MRE driver induces shear waves into the medium and the harmonic displacements are captured in the phase of the MR signal. The phase images are processed into a stiffness map, or elastogram, enabling physicians to identify changes in tissue structures and better quantify changes in the IVDs.

The AUTOSPINe is a 4-degree-of-freedom, planar, Magnetic Resonance Imaging-compatible robot that can position the mechanical driver used for the spinal MRE procedures (Fig.1) [3]. However, the current AUTOSPINe suffers from two clinical problems: the current positioning relies on fragile and expensive piezo-electric actuators for positioning the MRE driver, and the size and placement of the robot are incompatible with different body types within the MRI bore. These challenges motivated the development of new AUTOSPINe-inspired robots and control systems. This work presents two image-guided, driver-positioning robots and the characterization of a low-cost long-distance hydraulic positioning system.

Materials and Methods:: To move the robots, a leading syringe (LS) and end effector syringe (EES) are connected via a tube. A stepper motor (SM) will push down on the LS, forcing the displacement of the EES (Fig.2). The SM is controlled via a MATLAB graphical user interface where a motor, direction, and step count of actuation are specified (Fig.3).

The robots are fabricated from machined acetal resin and polylactic components to meet MRI-compatibility design constraints. In both robots, two EES control the position of the driver within a single plane. Each plane consists of two unique sleds, a base sled and an MRE driver sled (DS), actuated by the EES. To visualize the position of the MRE driver within the procedure, a passive fiducial made from a silicone ring is attached to the bottom of the DS. Positioning of the driver is enabled by a ball joint centered in the middle of the DS.



Two different experiments conducted an evaluation of the system: 1) The first experiment aimed to investigate the hysteresis of the system by actuating the SM by increments of 100 steps and measuring the EES displacement. The experiment was repeated for the inverse motion. 2) The second experiment verified the repeatability of the movement of the system. The SM is incremented and decremented by multiples of 100 steps until the EES is displaced. Afterward, the step count that produced the motion was repeated and the displacement of the EES was measured using a caliper (Fig. 4).




Results, Conclusions, and Discussions:: The results of the first experiment produced the following hysteresis relationship between the displacement of the syringe against the step increase (Fig.5) . The forward motion required a step input between 700-800 steps to obtain a significant displacement. Subsequently, the inverse actuation required between 500-600 steps to move the robot. Overall, the movement resulted in a nine percent error between the starting and ending position of the actuation. The second experiment found the minimum singular input to move the robot was 500 steps. Eight trials were subsequently conducted and both the starting position and displacement were measured (Fig.6). The following standard deviations were calculated: 1.236 mm when moving in the forward direction and .389 mm when starting in the inverse direction.


Evaluation of the low-cost long-distance hydraulic system was conducted through two different experiments. A larger step count is an indicator of greater force applied to the LS. The first experiment indicated the forward direction required greater force compared to the inverse direction. When coupled with the repeatability experiments, the inverse direction saw greater precision than the forward direction, indicated by the smaller standard deviation. While smaller step inputs are unable to drive the robot, further classification of larger step inputs will enable the robot to be driven to different positions. Overall, the system is deemed to be usable as the standard deviation of the motion is less than five percent of the total length (29.34 mm).

The evaluation of a hydraulic control system used to control two MRE robots was presented. The results indicated that while there is a hysteresis relationship within the motion control system, the system is consistent at large-step inputs. Future work will be focused on classifying a wide range of step inputs and using the robot to target specific positions on IVD phantoms.


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