Biomedical subjects

Mark P Ottensmeyer

Publications and source records attributed to Mark P Ottensmeyer.

4 recordsLinked to original sources

PubMed2005

Effects of perfusion on the viscoelastic characteristics of liver.

Accurate characterization of soft tissue material properties is required to enable new computer-aided medical technologies such as surgical training and planning. The current means of acquiring these properties in the in vivo and ex vivo states is fraught with problems, including limited accessibility and unknown boundary conditions in the former, and unnatural behavior in the latter. This paper presents a new testing method where a whole porcine liver is perfused under physiologic conditions and tested in an ex vivo setting. To characterize the effects of perfusion on the viscoelastic response of liver, indentation devices made force and displacement measurements across four conditions: in vivo, ex vivo perfused, ex vivo post perfused, and in vitro on an excised section. One device imposed cyclic perturbations on the liver's surface, inducing nominal strains up to 5% at frequencies from 0.1 to 200 Hz. The other device measured 300 s of the organ's creep response to applied loads, inducing nominal surface stresses of 6.9-34.7 kPa and nominal strains up to 50%. Results from empirical models indicate that the viscoelastic properties of liver change with perfusion and that two time constants on the order of 1.86 and 51.3s can characterize the liver under large strains typical of surgical manipulation across time periods up to 300 s. Unperfused conditions were stiffer and more viscous than the in vivo state, resulting in permanent strain deformation with repeated indentations. Conversely, the responses from the ex vivo perfusion condition closely approximated the in vivo response.

Animals↗

PubMed2003

Truth cube: establishing physical standards for soft tissue simulation.

Accurate real-time models of soft tissue behavior are key elements in medical simulation systems. The need for fast computation in these simulations, however, often requires simplifications that limit deformation accuracy. Validation of these simplified models remains a challenge. Currently, real-time modeling is at best validated against finite element models that have their own intrinsic limitations. This study develops a physical standard to validate real-time soft tissue deformation models. We took CT images of a cube of silicone rubber with a pattern of embedded Teflon spheres that underwent uniaxial compression and spherical indentation tests. The known material properties, geometry and controlled boundary conditions resulted in a complete set of volumetric displacement data. The results were compared to a finite element model analysis of identical situations. This work has served as a proof of concept for a robust physical standard for use in validating soft tissue models. A web site has been created to provide access to our database: http://biorobotics.harvard.edu/truthcube/ (soon to be http://www.truthcube.org).

Animals↗

PubMed2003

Independent testing of soft tissue visco-elasticity using indentation and rotary shear deformations.

Numerous techniques exist to measure the mechanical properties of soft tissues in vivo, such as mechanical stretching, indentation or shearing, as well as elastographic methods employing ultrasound or other imaging modes. Many groups have reported properties which do not necessarily correspond with each other due to differences in choice of technique, tissue model or other variations. This work deliberately makes use of the two independent modes of indentation and rotary shear, on the same material samples, employing similar modeling approximations, to attempt to determine the common, underlying material properties. This paper introduces the ROSA-2 rotary shear instrument, and presents its mechanical characteristics, as well as presenting validation experiments that were performed to verify non-slip contact with tissue. Measurements made with it are compared with those acquired with the TeMPeST l-D indentation instrument. Initial testing showed reasonably agreement when testing silicone gel samples, over a restricted range of frequencies. When testing bovine liver samples in vitro and porcine liver in vivo, significant discrepancies were found. The potential sources of these differences will be discussed, as will directions for ongoing work.

Animals↗

PubMed2002

In vivo measurement of solid organ visco-elastic properties.

To support the ongoing development of software-based surgical simulation systems, work is underway to acquire the mechanical properties of living tissue. When such simulations include force feedback, visco-elastic properties must be evaluated over a range of frequencies relevant to human perception and motor control. A minimally invasive instrument has been developed which can perform normal indentation on solid organs, and apply and measure deformations over a frequency range from DC to approximately 100Hz. Measurement performance was validated on a series of objects and materials with known properties, and the device was subsequently used in in vivo tests on porcine liver. Results of these validation tests as well as the data extracted from the in vivo experiments are presented. Testing in ongoing, and will be expanded to more completely characterize liver, as well as porcine spleen and other solid organ tissues. While these animal tissue property tests are valuable in and of themselves, they pave the way for the development of instruments and experimental protocols suitable for the measurement of human tissue properties.

Animals↗