Experimental methods for soft-tissue characterisation

		Aspiration device: Finite element model for the extraction of the mechanical parameters Aspiration device: Ex-vivo experiments on human uterus just after resection Aspiration device: In vivo experiments during open surgery for the determination of the mechanical properties of human liver. Torsional Resonator Device: Experimental set-up Torsional Resonator Device: Working principle Torsional Resonator Device: SH-waves propagation in a tissue sample

Aspiration experiment

The aspiration device has been designed for in vivo applications addressing issues associated with safety, sterilisability, space limitation and fast data acquisition. The device is based upon the pipette aspiration technique. It consists of a tube in which the internal pressure can be controlled according to a desired pressure law. Experiments are performed by (i) gently pushing the tube against the tissue to ensure a good initial contact, (ii) creating a (time variable) vacuum inside the tube so that the tissue is sucked in through the aspiration hole. Assuming isotropic, homogeneous tissue, a complete description of the deformed tissue can be given by simply monitoring the side-view profile of the tissue during its deformation with a digital camera. Experiments have been performed on organs obtained from adult cadavers during post-mortem examination in Central Manchester and Manchester Childrens' Hospital NHS Trust. A 3-D model based on a continuum mechanics approach provides a phenomenological model for the tissue. Such models can be applied to simulate any geometry and any loading condition for the material under investigation, and can therefore be applied for several different purposes.

Ongoing work focuses on in-vivo testing during open surgery and on the creation of a database of the mechanical properties of various human organs. Human uterine cervices of menopausal woman have been tested during vaginal and abdominal hysterectomy (in vivo and ex vivo) in collaboration with the Medical University Graz, the reproducibility of the mechanical data from the in vivo experiment has been considered and differences between ex vivo and in vivo behavior have been outlined. A new miniaturized version of the aspiration device has been developed and will be applied soon in a clinical application, aiming at analyzing the mechanical response of normal cervical tissue at different gestational ages, and identifying the mechanical properties that characterize pathologic conditions such as cervical insufficiency leading to preterm delivery.

Human "diseased" livers are tested (in vivo and ex vivo) during abdominal surgery (University Hospital of Zurich), biopsies are taken and the mechanical properties of the material are related to the pathological conditions. Human "healthy" livers are tested (only in vivo) during abdominal surgery (Cantonal Hospital of Chur) in order to extract a proper mechanical constitutive model of a healthy living liver.

Torsional Oscillator

A novel technique for testing the mechanical properties of soft tissues based on dynamic testing has been developed. The material parameters are determined from the damping and resonance characteristics of a vibrating system. The mechanical parameters are measured in the linear viscoelastic region, i.e. at low strain and stresses. The frequency-dependent behaviour of these parameters is studied at high frequencies. The experiment is designed to enable future in-vivo measurements of material properties.

The SH-wave propagation is solved in the soft tissue using an analytical model, enabling real-time extraction of the material parameters. Results on bovine internal organs ex-vivo (liver, kidney, uterus, spleen) have confirmed the validity of the proposed technique, showing a strong frequency-dependent behaviour of the viscoelastic parameters.

An international collaboration on physical standards for validation of soft tissue experimental techniques was started (CIMIT group, Boston), comparing the results obtained from several techniques on synthetic phantoms.