Showing papers by "Septimiu E. Salcudean published in 2004"

Method for imaging the mechanical properties of tissue

Abstract: An imaging system comprises a device to excite mechanical waves in elastic tissue, a device for measuring the resulting tissue motion at a plurality of locations interior to the tissue at a number of time instances, a computing device to calculate the mechanical properties of tissue from the measurements, and a display to show the properties according to their location. A parameter identification method for calculating the mechanical properties is based on fitting a lumped dynamic parametric model of the tissue dynamics to their measurements. Alternatively, the mechanical properties are calculated from transfer functions computed from measurements at adjacent locations in the tissue. The excitation can be produced by mechanical vibrators, medical needles or structures supporting the patient. The measurements may be performed by a conventional ultrasound imaging system and the resulting properties displayed as semi-transparent overlays on the ultrasound images.

Real-Time Dosimetry for Prostate Brachytherapy Using TRUS and Fluoroscopy

Abstract: A new approach for computing real-time dosimetry (RTD) for prostate brachytherapy is presented. Transrectal ultrasound (TRUS) and fluoroscopic images are fused to compute dosimetry. The frontal plane coordinates of the seeds are found in the fluoroscopic images and the remaining coordinate is computed from the TRUS. Our approach is verified on a phantom and tested on clinical data. A method for tracking intraoperative seed motion using TRUS is also presented.

Haptic rendering of rigid body collisions

Abstract: This paper addresses the haptic rendering of rigid body collisions. A new method is proposed in which collision rendering is achieved in two steps. First, the haptic simulation uses a contact model whose stiffness is infinite during collisions and finite during sustained contact. This model is combined with a passive collision resolution scheme to compute collision impulses when new contacts arise. Second, the impulses are applied to the user's hand by a controller that coordinates forces and positions between the virtual environment and the haptic interface. Haptic rendering of rigid body collisions imparts forces that generate large hand accelerations when new contacts arise without requiring increased contact stiffness and damping. Experiments with a planar rigid virtual world validate the proposed approach.

Impulsive forces for haptic rendering of rigid contacts

Abstract: This paper presents a haptic rendering method that enables users to feel collisions while they interact with a multi-rigid-body virtual environment through a virtual tool. The virtual tool can be a rigid object or a linkage. Linkage collisions are rendered by extending a collision resolution method proposed for single bodies to articulated structures using a configuration-space representation of dynamics. The configuration-space collision resolution scheme is incorporated into a local model of interaction and used to compute impulsive forces upon contact between the virtual tool and other virtual objects. A four channel teleoperation controller optimized for transparency is used to apply the provably passive impulsive forces to the user’s hand. Experiments with a planar rigid virtual world validate the passivity of the impulsive forces.

Haptic simulation of linear elastic media with fluid pockets

Abstract: A fast technique for simulating fluid pockets enclosed in an elastic body has been developed using the finite element method. By treating fluid pressure as a force boundary condition, the relationship between the volume and pressure of a fluid cavity can be enforced with an iterative solver. This computational approach has been shown to agree with experimental data taken from a gelatin phantom that contains a small fluid pocket. Combining linear methods and condensation techniques with this iterative solver, fast simulation of elastic bodies that include fluid pockets can be achieved. For example, an extension of a two dimensional needle insertion simulation can be carried out at 512Hz for a 24 node incompressible fluid pocket.

Optimal selection of manipulator impedance for contact tasks

Abstract: This paper addresses position and force control of robotic manipulators that are in contact with environments that exhibit mechanical impedances covering a large continuous range, from very soft to very stiff. For robot programming, automation, teleoperation and haptics, such environments enforce a trade-off between position and force control, which can be accommodated by impedance controllers. In this work, we extend the concept of duality and consider impedance matching in order to optimise a combined position and force trajectory error metric. The analysis of numerical optimisation results provides clear guidance on the choice of target impedance parameters, based on environment and manipulator dynamics.