The design and validation of a magnetic resonance imaging–compatible device for obtaining mechanical properties of plantar soft tissue via gated acquisition

Abstract

Changes in the mechanical properties of the plantar soft tissue in people with diabetes may contribute to the formation of plantar ulcers. Such ulcers have been shown to be in the causal pathway for lower extremity amputation. The hydraulic plantar soft tissue reducer (HyPSTER) was designed to measure in vivo, rate-dependent plantar soft tissue compressive force and three-dimensional deformations to help understand, predict, and prevent ulcer formation. These patient-specific values can then be used in an inverse finite element analysis to determine tissue moduli, and subsequently used in a foot model to show regions of high stress under a wide variety of loading conditions. The HyPSTER uses an actuator to drive a magnetic resonance imaging-compatible hydraulic loading platform. Pressure and actuator position were synchronized with gated magnetic resonance imaging acquisition. Achievable loading rates were slower than those found in normal walking because of a water-hammer effect (pressure wave ringing) in the hydraulic system when the actuator direction was changed rapidly. The subsequent verification tests were, therefore, performed at 0.2 Hz. The unloaded displacement accuracy of the system was within 0.31%. Compliance, presumably in the system's plastic components, caused a displacement loss of 5.7 mm during a 20-mm actuator test at 1354 N. This was accounted for with a target to actual calibration curve. The positional accuracy of the HyPSTER during loaded displacement verification tests from 3 to 9 mm against a silicone backstop was 95.9% with a precision of 98.7%. The HyPSTER generated minimal artifact in the magnetic resonance imaging scanner. Careful analysis of the synchronization of the HyPSTER and the magnetic resonance imaging scanner was performed. With some limitations, the HyPSTER provided key functionality in measuring dynamic, patient-specific plantar soft tissue mechanical properties.

A preliminary study of patient-specific mechanical properties of diabetic and healthy plantar soft tissue from gated magnetic resonance imaging

“…The design and verification of the HyPSTR have been described previously in detail. 4 It consists of a LabVIEW controlled metal actuator and data acquisition circuit board, paired with a water-filled plastic hydraulic system that is both MRI compatible and capable of applying body weight forces on a subject's foot ( Figure 1). Displacement between the actuator and platen was transmitted via vacuum-rated tubing.…”

“…Because of the calcaneal shift, the required motor displacement was generally higher than calculated from the calibration curves, which were determined with a rigid backstop and a silicone block. 4 The loading platen was advanced to make visual contact with the subject's foot and then cycled 10 times to precondition the plantar tissue. Previous HyPSTR verification tests showed a hydraulic pressure ringing, or ''water hammer,'' with platen speeds greater than 0.2 Hz.…”

“…Previous HyPSTR verification tests showed a hydraulic pressure ringing, or ''water hammer,'' with platen speeds greater than 0.2 Hz. 4 A stable pressure signal was needed to calculate the applied force on the platen, thus all tests were completed with 0.2 Hz cycles even though the motor was capable of a 6-Hz rate for the given piston travel.…”

“…We have discussed the sources and solutions for all identifiable timing inconsistencies elsewhere. 4,18 For the scope of this study, it is sufficient to know that timing errors were minimized through careful procedures and real-time displacement feedback during the imaging, and then accounted for with custom functions in MATLAB post-processing.…”

“…3 The hydraulic plantar soft tissue reducer (HyPSTR) is a tool to gain understanding of the variables that define high stress in the foot. 4 The first set of studies to quantify plantar tissue stiffness reported deformation in relation to an external reference frame during compression with a hand-held device or a linear/pendulum impact tester. [5][6][7][8] These studies lacked details of internal foot mechanics but are very useful for stiffness comparisons when the heel pad is approximated as a uniform material.…”