Real-time motion tracking and correction is technically feasible on a helical tomotherapy system. In one experiment, dose differences due to respiratory motion were greatly reduced. Dose differences due to nonrespiratory motion were also reduced, although not as much as in the respiratory case due to less frequent tracking updates. In both cases, beam-on time was not increased by motion correction, since the system tracks and corrects for motion simultaneously with treatment delivery.

A multileaf collimator (MLC) optimized for SBRT delivery with the CyberKnife ® Robotic Radiosurgery System (Accuray Incorporated, Sunnyvale, CA, USA) is described. The MLC is exchangeable with the alternate fixed and variable circular aperture collimator systems. The non-coplanar workspace is effectively equivalent for all three collimation types. The same range of tracking options, including real-time respiratory motion tracking, and the same tolerance on beam pointing accuracy (0.95 mm) is maintained with all three collimation types. The MLC includes 52 flat-sided leaves, each of which is 90 mm tall and projects 3.85 mm width at the nominal treatment distance of 800 mm SAD. The design allows 100% overtravel and unrestricted interdigitation. Leaf position is determined by primary motor encoders and is checked with a secondary optical camera system. Maximum leakage, including inter-leaf and under the closed position leaf-tip gap was measured on five units to be 0.44%, while mean leakage and transmission ranged from 0.22%-0.25%. Leaf positioning accuracy measured over the full range of leaf positions, all robot and MLC orientations, and including variation with leaf motion direction and accumulated leaf motion after initialization had a mean error <0.2 mm, with 2%-98% range of ±0.5 mm (projected at 800 mm SAD) on three units tested. The only factor found to effect leaf positioning accuracy was sag under gravity, which systematically altered leaf positions by 0.1 mm. Tilting the leaves to reduce inter-leaf leakage results in 0.5 mm asymmetry in leaf-side penumbra at 100 mm depth, and a partial leaf-edge transmission pattern analogous to the tongue and groove effect observed with interlocking leaves.

In this work we present an inverse finite-element modeling framework for constitutive modeling and parameter estimation of soft tissues using full-field volumetric deformation data obtained from 3D ultrasound. The finite-element model is coupled to full-field visual measurements by regularization springs attached at nodal locations. The free ends of the springs are displaced according to the locally estimated tissue motion and the normalized potential energy stored in all springs serves as a measure of model-experiment agreement for material parameter optimization. We demonstrate good accuracy of estimated parameters and consistent convergence properties on synthetically generated data. We present constitutive model selection and parameter estimation for perfused porcine liver in indentation and demonstrate that a quasilinear viscoelastic model with shear modulus relaxation offers good model-experiment agreement in terms of indenter displacement (0.19 mm RMS error) and tissue displacement field (0.97 mm RMS error).

We describe a modeling methodology intended as a preliminary step in the identification of appropriate constitutive frameworks for the time-dependent response of biological tissues. The modeling approach comprises a customizable rheological network of viscous and elastic elements governed by user-defined 1D constitutive relationships. The model parameters are identified by iterative nonlinear optimization, minimizing the error between experimental and model-predicted structural (load-displacement) tissue response under a specific mode of deformation. We demonstrate the use of this methodology by determining the minimal rheological arrangement, constitutive relationships, and model parameters for the structural response of various soft tissues, including ex-vivo perfused porcine liver in indentation, ex-vivo porcine brain cortical tissue in indentation, and ex-vivo human cervical tissue in unconfined compression. Our results indicate that the identified rheological configurations provide good agreement with experimental data, including multiple constant strain-rate load/unload tests and stress-relaxation tests. Our experience suggests that the described modeling framework is an efficient tool for exploring a wide array of constitutive relationships and rheological arrangements, which can subsequently serve as a basis for 3D constitutive model development and finite-element implementations. The proposed approach can also be employed as a self-contained tool to obtain simplified 1D phenomenological models of the structural response of biological tissue to single-axis manipulations for applications in haptic technologies.