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Biomedical subjects

Mandayam A Srinivasan

Publications and source records attributed to Mandayam A Srinivasan.

9 recordsLinked to original sources
PubMed2006

Continuous shared control for stabilizing reaching and grasping with brain-machine interfaces.

Research on brain-machine interfaces (BMI's) is directed toward enabling paralyzed individuals to manipulate their environment through slave robots. Even for able-bodied individuals, using a robot to reach and grasp objects in unstructured environments can be a difficult telemanipulation task. Controlling the slave directly with neural signals instead of a hand-master adds further challenges, such as uncertainty about the intended trajectory coupled with a low update rate for the command signal. To address these challenges, a continuous shared control (CSC) paradigm is introduced for BMI where robot sensors produce reflex-like reactions to augment brain-controlled trajectories. To test the merits of this approach, CSC was implemented on a 3-degree-of-freedom robot with a gripper bearing three co-located range sensors. The robot was commanded to follow eighty-three reach-and-grasp trajectories estimated previously from the outputs of a population of neurons recorded from the brain of a monkey. Five different levels of sensor-based reflexes were tested. Weighting brain commands 70% and sensor commands 30% produced the best task performance, better than brain signals alone by more than seven-fold. Such a marked performance improvement in this test case suggests that some level of machine autonomy will be an important component of successful BMI systems in general.

Arm↗
PubMed2006

In vivo mechanical behavior of intra-abdominal organs.

For realistic surgical simulation in a virtual environment, in vivo material properties of biological tissues are required for simulating the deformations and the reaction forces from the tool-tissue interactions. In this paper, the in vivo static and dynamic mechanical behavior of the liver and lower esophagus of pigs were presented both in linear and nonlinear regions under compressive and shear indentations. A robotic device was programmed to function as a mechanical stimulator with a 2-mm flat-tipped cylindrical probe attached to its tip. A series of ramp and hold stimuli, as well as sinusoidal indentation stimuli, were delivered to the organs and reaction forces were measured. The conditions for these indentation stimuli were designed such that they were similar to conditions in an operating room. Experiments were also carried out on the organs for ex vivo and in vitro conditions. Results show that the breathing and pulse rate significantly affect the measured force responses of the organs. From the obtained force-displacement relationships, steady-state impedances as well as dynamic impedances of both organs were calculated. The results also show that in vivo steady-state impedance of the lower esophagus is significantly higher than that of the liver. The in vivo steady-state response of the liver, however, exhibits a greater degree of nonlinearity than that of the lower esophagus. The in vivo steady-state response of the lower esophagus in the three orthogonal directions also indicates that the lower esophagus is not significantly anisotropic. The impedance of both organs under sinusoidal indentations (0-5 Hz) are fairly similar each other. Magnitudes of the impedance over the stimulus frequencies are fairly constant. The impedance phase angles decrease over the range of stimulus frequencies applied. Comparison of the measurements obtained from the in vivo, ex vivo, and in vitro experiments shows that the mechanical properties of the biological tissues change significantly after the death of the animal. The tissues generally become stiffer and exhibit greater nonlinearity. The degree of change in their mechanical properties is dependent on the amount of time after the death of the animal. These data can be further utilized in the computing of the material parameters of tissue models for laparoscopic surgery simulators as well as open surgery simulators.

Abdomen↗
PubMed2005

Characterization of viscoelastic soft tissue properties from in vivo animal experiments and inverse FE parameter estimation.

Soft tissue characterization and modeling based on living tissues has been investigated in order to provide a more realistic behavior in a virtual reality based surgical simulation. In this paper, we characterize the nonlinear viscoelastic properties of intra-abdominal organs using the data from in vivo animal experiments and inverse FE parameter estimation algorithm. In the assumptions of quasi-linear-viscoelastic theory, we estimated the viscoelastic and hyerelastic material parameters to provide a physically based simulation of tissue deformations. To calibrate the parameters to the experimental results, we developed a three dimensional FE model to simulate the forces at the indenter and an optimization program that updates new parameters and runs the simulation iteratively. We can successfully reduce the time and computation resources by decoupling the viscoelastic part and nonlinear elastic part in a tissue model. The comparison between simulation and experimental behavior of pig intra abdominal soft tissue are presented to provide a validness of the tissue model using our approach.

Animals↗
PubMed2004

Virtual-reality-based laparoscopic surgical training: the role of simulation fidelity in haptic feedback.

Although there have been significant advances in the development of virtual-reality-based surgical simulations, there remain fundamental questions concerning the fidelity required for effective surgical training. A dual-station experimental platform was built for the purpose of investigating these fidelity requirements. Analogous laparoscopic surgical tasks were implemented on a virtual station and a real station, with the virtual station modeling the real environment with various degrees of fidelity. After measuring the subjects' initial performance on the real station, different groups of subjects were trained on the virtual station under a variety of conditions and finally tested on the real station. Experiments involved bimanual pushing and cutting tasks on a nonlinear elastic object. The results showed that force feedback results in significantly improved training transfer as compared to training without force feedback. The training effectiveness of a linear approximation model was approximately the same as that of a more accurate nonlinear model.

Analysis of Variance↗
PubMed2003

Quantitative ultrasonic methods for characterization of skin lesions in vivo.

Quantitative ultrasonic methods were studied for characterizing skin lesions in vivo using contact dermatitis as an example. The parameters studied include skin thickness, echogenicity, attenuation coefficient slope and parameters related to echo statistics (signal-to-noise ratio and shape parameters of Weibull, K and generalized gamma distributions). Data were collected using a high-frequency ultrasound (US) system (center frequency = 33 MHz). To compensate for depth-dependent diffraction effects, correction curves as a function of the distance between the transducer and the tissue were first empirically obtained. Diffraction-corrected quantitative parameters were then compared between healthy and affected skin of volunteers, who underwent patch testing for allergic and irritant contact dermatitis. A significant increase in skin thickness, decrease in echogenicity of the upper dermis and decrease in attenuation coefficient slope were found at the affected sites compared to those of healthy skin. However, no differences in parameters related to the echo statistics of the mid-dermis were found. These results indicate that a combination of quantitative ultrasonic parameters have the potential for extracting information for characterizing skin conditions.

Adult↗
PubMed2003

Robust deconvolution of high-frequency ultrasound images using higher-order spectral analysis and wavelets.

Deconvolution of high-frequency (30-40 MHz) ultrasonic images of human skin was studied in vivo. Separate one-dimensional (1-D) functions for the axial and lateral profiles were first estimated using higher-order spectral methods. Subsequently, deconvolution was implemented us ing a regularized inverse Wiener filtering of the wavelet and scaling coefficients that were obtained after a wavelet decomposition of the RF signals. Deconvolution was first performed in the axial direction, then in the lateral direction. The methods were applied to data obtained from the skin of 16 volunteers using three different transducers. Significant improvements in both the axial and lateral resolutions were obtained in all the cases. Features such as hair follicles in the dermis and fingerprints on the surface of the finger were more clearly displayed in the processed images compared to the original images. The results indicate that the deconvolution method using higher-order spectral methods and wavelet analysis could significantly improve the quality of high-frequency ultrasonic skin images.

Adult↗
PubMed2003

3-D finite-element models of human and monkey fingertips to investigate the mechanics of tactile sense.

The biomechanics of skin and underlying tissues plays a fundamental role in the human sense of touch. It governs the mechanics of contact between the skin and an object, the transmission of the mechanical signals through the skin, and their transduction into neural signals by the mechanoreceptors. To better understand the mechanics of touch, it is necessary to establish quantitative relationships between the loads imposed on the skin by an object, the state of stresses/strains at mechanoreceptor locations, and the resulting neural response. Towards this goal, 3-D finite-element models of human and monkey fingertips with realistic external geometries were developed. By computing fingertip model deformations under line loads, it was shown that a multi-layered model was necessary to match previously obtained in vivo data on skin surface displacements. An optimal ratio of elastic moduli of the layers was determined through numerical experiments whose results were matched with empirical data. Numerical values of the elastic moduli of the skin layers were obtained by matching computed results with empirically determined force-displacement relationships for a variety of indentors. Finally, as an example of the relevance of the model to the study of tactile neural response, the multilayered 3-D finite-element model was shown to be able to predict the responses of the slowly adapting type I (SA-I) mechanoreceptors to indentations by complex object shapes.

Animals↗
PubMed2002

Statistics of envelope of high-frequency ultrasonic backscatter from human skin in vivo.

The statistics of envelope of high-frequency ultrasonic backscatter signals from in vivo normal human dermis and subcutaneous fat were studied. The capability of six probability distributions (Rayleigh, Rician, K, Nakagami, Weibull, and Generalized Gamma) to model empirical envelope data was studied using the Kolmogorov-Smirnov (KS) goodness of fit statistic. The parameters of all the distributions were obtained using the maximum likelihood method. It was found that the Generalized Gamma distribution with two shape parameters provided the best fit among all the distributions in terms of the KS goodness of fit. The K and Weibull distributions also modeled the envelope statistics well. The Rayleigh and Rician distributions provided poorer fits. The parameters of the Generalized Gamma distribution, however, showed a larger variability than those of the other distributions. The intersubject variability in the estimated parameters of all the distributions was found to be comparable to the intrasubject variability. Fat was seen to exhibit significantly more pre-Rayleigh behavior compared to the dermis. The parameters of the Generalized Gamma distribution also showed significant differences between the dermis at the forearm and fingertip regions.

Adipose Tissue↗