A new combined high-frequency ultrasound-impedance planimetry measuring system for the quantification of organ wall biomechanics in vivo.

The aim was to develop a method for the quantification of the stress-strain distribution of tubular organs in vivo using the porcine duodenum as an experimental model. We placed four electrodes for impedance planimetry and a 20 MHz ultrasound transducer inside an inflatable balloon mounted on a 6 mm-diameter probe for intraluminal use. By means of the ultrasound transducer and the impedance planimetric system, we measured the wall thickness and luminal cross-sectional area of the duodenum. We calculated the luminal radius on the basis of the latter. We validated ultrasonic measurements of wall thicknesses by comparing the former to microscopic measurements of the wall thickness of unstrained duodenal specimens in vitro. Also, we tested whether the magnitude of the applied balloon pressure affected ultrasonic measurements in vitro. The luminal and the outer radius increased non-linearly by a factor of three, rising steeply at low balloon pressures, moderately at higher pressures. The wall thickness decreased as an almost linear function of the applied balloon pressure, resulting in a 50% decrease. The stress-strain distribution calculated on the basis of the balloon pressures, the wall thicknesses and the luminal radii were non-linear. The wall reached a maximum circumferential strain of 1.71 at 6 kPa, corresponding to a stress of 64.8 kPa. Our system enabled us to quantify the stress-strain distribution of the porcine duodenum in vivo, and it may become a valuable tool for future biomechanical investigations of tubular organs in health and disease.