Showing papers by "Francis E. Kennedy published in 2002"

In vivo quantification of retraction deformation modeling for updated image-guidance during neurosurgery

Abstract: The use of coregistered preoperative anatomical scans to provide navigational information in the operating room has greatly benefited the field of neurosurgery. Nonetheless, it has been widely acknowledged that significant errors between the operating field and the preoperative images are generated as surgery progresses. Quantification of tissue shift can be accomplished with volumetric intraoperative imaging; however, more functional, lower cost alternative solutions to this challenge are desirable. We are developing the strategy of exploiting a computational model driven by sparse data obtained from intraoperative ultrasound and cortical surface tracking to warp preoperative images to reflect the current state of the operating field. This paper presents an initial quantification of the predictive capability of the current model to computationally capture tissue deformation during retraction in the porcine brain. Performance validation is achieved through comparisons of displacement and pressure predictions to experimental measurements obtained from computed tomographic images and pressure sensor recordings. Group results are based upon a generalized set of boundary conditions for four subjects that, on average, account for at least 75% of tissue motion generated during interhemispheric retraction. Individualized boundary conditions can improve the degree of data-model match by 10% or more but warrant further study. Overall, the level of quantitative agreement achieved in these experiments is encouraging for updating preoperative images to reflect tissue deformation resulting from retraction, especially since model improvements are likely as a result of the intraoperative constraints that can be applied through sparse data collection.

Microstructure, mechanical properties and wear of Ni-Al-Fe alloys

Abstract: The purpose of this paper is to describe the microstructure, mechanical properties and wear behavior of NiAl-based alloys both with and without a ductile b.c.c. phase. The materials studied had equiatomic amounts of Ni and Al, with four different additions of iron – 10, 20, 30 and 44 at.%. The microstructures were examined using both optical microscopy and transmission electron microscopy. The alloys containing 10 and 20% iron were single phase with an ordered b.c.c. (or B2 structure), whilst those containing 30 and 44% Fe were two-phase with a B2 matrix containing fine b.c.c. precipitates. The addition of iron did not improve the room temperature tensile ductility significantly, although some increase in both the compressive yield strength and the hardness was noted as the iron content increased. Wear tests, which were conducted in an argon atmosphere to minimize environmental effects, showed a higher wear rate for the two-phase alloys despite their slightly higher hardnesses, presumably due to shear of the ductile phases present in these alloys.

Evidence of the Anisotropic Nature of the Mechanical Properties of Breast Tissue

Abstract: This paper presents evidence of a newly discovered mechanical property of breast tissue that might be important in breast cancer detection Breast cancer remains the most commonly detected cancer in women Magnetic resonance (MR) elastography, a recent innovation in magnetic resonance imaging, is being developed by several groups of researchers worldwide to help diagnose breast cancer earlier and more accurately in the effort to reduce the mortality and morbidity from this disease Elastography measures the stiffness of tissue in vivo Breast cancer appears to be stiffer than normal tissue in MR elastography just as cancer feels harder to a woman or physician in the physical examination except that MR elastography is quantitative and more sensitive than the physical examination

Incorporation of diffusion tensor anisotropy in brain deformation models for updating preoperative images to improve image-guidance

Abstract: Image-guided neurosurgery requires a method for compensating for the loss in registration accuracy as the surgery progresses due to tissue deformation subsequent to craniotomy at the start of a case. Modeling methods may be able to update the preoperative scans by estimating the tissue movement as surgery progresses. For these techniques to be useful, knowledge of the intrinsic mechanical and hydrodynamical tissue property parameters will be important. Preoperative DTI may be able to supply patient-specific data on model property parameters influenced by anisotropy in the flow field patterns. In this paper, we show simulation results on a model constructed from an actual clinical exam where DTI data was available. Specifically, we describe a simple technique for extracting the tensor map embodied in the DTI images and applying it within the model during tissue deformation calculations. The computed pressure field shows signs of localized disturbances which are congruent with zones having a high degree of anisotropy.

Mechanical Properties Characterization of Abdominal Aortic Aneurysm Tissue Using Biaxial Testing

Abstract: An abdominal aortic aneurysm (AAA) is an abnormal, localized enlargement of the aorta. If untreated, a AAA will continue to enlarge in size and eventually rupture. Currently, AAA diameter is used as the principal indicator of impending rupture. However, this method it is not totally reliable. In an effort to improve the estimation of rupture risk, some researchers are currently studying the mechanical wall stresses of AAAs using patient-specific medical imaging techniques and finite element modeling [1,2]. The accuracy of these models depends significantly on the constitutive law used to describe the mechanical properties of the AAA tissue. To date, only isotropic constitutive laws have been used in these models.Copyright © 2002 by ASME

Determination of the zero-pressure configuration of cardiovascular structures from in-vivo configuration

Abstract: In the biomechanical analyses of human cardiovascular structures, it is often necessary to start with a zero-pressure configuration and apply the relevant boundary conditions. However, imaging modalities can only provide the in-vivo configuration of a cardiovascular structure for a patient, where it is really under non-zero pressure. Presently, the in-vivo configuration is just assumed to be the zero-pressure configuration for biomechanical analyses. In all likelihood, the zero-pressure geometry is smaller than the in-vivo geometry. In this work, we have developed a methodology by which a patient-specific in-vivo configuration of a blood vessel or ventricle may be shrunk to determine its zero-pressure configuration. We demonstrate this by applying the methodology to the case of a patient-specific 3D model of an abdominal aortic aneurysm and compare the results to that obtained by the conventional approach.