Force and Position Control System for Freehand Ultrasound

A hand-held force-controlled ultrasound probe has been developed for medical imaging applications. The probe-patient contact force can be held constant to improve image stability, swept through a range, or cycled. The mechanical portion of the device consists of a ball screw linear actuator driven by a servo motor, along with a load cell, accelerometer, and limit switches. The performance of the system was assessed in terms of the frequency response to simulated sonographer hand motion and in hand-held image feature tracking during simulated patient motion. The system was found to attenuate contact force variation by 97% at 0.1 Hz, 83% at 1 Hz, and 33% 10 Hz, a range that spans the typical human hand tremor frequency spectrum. In studies with 15 human operators, the device applied the target contact force with ten times less variation than in conventional ultrasound imaging. An ergonomic, human-in-the-loop, imaging-workflow enhancing control scheme, which combines both force- and position-control, permits smooth making and breaking of probe-patient contact, and helps the operator keep the probe centered within its range of motion. By controlling ultrasound probe contact force and consequently the amount of tissue deformation, the system enhances the repeatability, usability, and diagnostic capabilities of ultrasound imaging.