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Hand-held resonance sensor for tissue stiffness measurements—a theoretical and experimental analysis

A piezoelectric transducer in a feedback circuit operating in a resonance state is the basis of a resonance sensor. Upon contact with a soft object a change in the resonance frequency reflects the acoustic impedance. Together with force measurement it is possible to obtain the elastic stiffness of the object. The aim of this study was to evaluate the concept of a hand-held resonance sensor for tissue stiffness measurement. A time derivative analysis of the force and the frequency change showed that a stiffness-sensitive parameter was independent of the impression speed. Soft tissue phantoms of gelatin were used in an experimental validation of the theory. A force indentation method was used as a reference method for assessing the gelatin's elastic stiffness. Results from the hand-held measurements showed that the stiffness parameter accurately measured the elastic stiffness of the gelatin (R2 = 0.94, p < 0.05). The stiffness parameter was weakly (on average R2 = 0.15) and non-significantly (p > 0.05, 14 out of 17) dependent on an impression speed parameter. On average, a small amount of the total variance was explained by the impression speed. In conclusion, soft tissue stiffness can be objectively measured with free-hand measurement with a resonance sensor. This study contributes a theoretical analysis and an experimental demonstration of the concept of a hand-held resonance sensor for stiffness measurements.