Showing papers by "Francis E. Kennedy published in 2003"
TL;DR: For AAAs under observation, peak AAA wall stress seems superior to diameter in differentiating patients who will experience catastrophic outcome, and with proportional hazards analysis, peak wall stress and gender were the only significant independent predictors of rupture.
674 citations
TL;DR: Initial in vivo experiences with a subzone‐based, steady‐state MR elastography (MRE) method are described to shed light on some of the strengths and weaknesses of existing clinical MRE approaches and to indicate important areas of future research.
Abstract: Purpose: To describe initial in vivo experiences with a subzone-based, steady-state MR elastography (MRE) method. This sparse collection of in vivo results is intended to shed light on some of the strengths and weaknesses of existing clinical MRE approaches and to indicate important areas of future research. Materials and Methods: Elastic property reconstruction results are compared with data compiled from the limited existing body of published studies in breast elasticity. Mechanical parameter distributions are also investigated in terms of their implications for the nature of biological soft tissue. Additionally, a derivation of the statistical variance of the elastic parameter reconstruction is given and the resulting confidence intervals (CIs) for different parameter solutions are examined. Results: By comparison with existing estimates of the elastic properties of breast tissue, the subzone-based, steadystate MRE method is seen to produce reasonable estimates for the mechanical properties of in vivo tissue.
245 citations
TL;DR: To evaluate the effect of the various cross-linking processes on physical and mechanical properties of ultrahigh molecular weight polyethylene, commercially available cross-linked polyethylenes from six orthopaedic manufacturers were tested.
Abstract: Cross-linked polyethylenes are being marketed by orthopaedic manufacturers to address the problem of osteolysis caused by polyethylene particulate wear debris. Wear testing of these cross-linked polyethylenes in hip simulators has shown dramatic reduction in wear rate compared with standard ultrahigh molecular weight polyethylene, either gamma irradiated in air or nitrogen - or ethylene oxide-sterilized. However, this reduction in wear rate is not without cost. The cross-linking processes can result in materials with lower mechanical properties than standard ultrahigh molecular weight polyethylene. To evaluate the effect of the various cross-linking processes on physical and mechanical properties of ultrahigh molecular weight polyethylene, commercially available cross-linked polyethylenes from six orthopaedic manufacturers were tested. This study was the culmination of collaboration with these manufacturers, who provided cross-linked polyethylene for this study, wear characteristics of the material they provided, and review of the physical and mechanical properties measure for their polyethylene. Cross-linked materials were evaluated as received and after an accelerated aging protocol. Free radical identity and concentration, oxidation, crystallinity, melt temperature, ultimate tensile strength, elongation at break, tensile stress at yield, and toughness are reported for each material. By comparing these physical and mechanical properties, surgeons can evaluate the trade-off that results from developing materials with substantially lower wear rates.
160 citations
TL;DR: An objective contrast-detail analysis was performed to assess the low contrast detectability of a clinical prototype harmonic magnetic resonance elastographic imaging system and it is demonstrated that lesions as small as 5 mm can be detected with a minimum modulus contrast of 14 dB.
Abstract: An objective contrast-detail analysis was performed in this study to assess the low contrast detectability of a clinical prototype harmonic magnetic resonance elastographic imaging system. Elastographic imaging was performed on gelatin phantoms containing spherical inclusions of varying size and modulus contrast. The results demonstrate that lesions as small as 5 mm can be detected with a minimum modulus contrast of 14 dB. However, the shear modulus of such small lesions was not accurately recovered. In general, the shear modulus of larger focal lesions was accurately (i.e., within 25% of the true value) recovered. The minimum modulus contrast needed to detect focal lesions was observed to decrease with increasing lesion size.
70 citations
TL;DR: In this paper, wear and friction studies on ultra-thin (~2 nm) film of perfluoropolyether (PFPE) coated on glass substrate magnetic hard disks were performed.
47 citations
TL;DR: The findings indicate the potential accuracy in coregistered freehand US displacement tracking in brain tissue and suggest that the resulting information can be used to drive a modeling re-registration strategy to comparable levels of agreement.
Abstract: Brain shift during open cranial surgery presents a challenge for maintaining registration with image-guidance systems. Ultrasound (US) is a convenient intraoperative imaging modality that may be a useful tool in detecting tissue shift and updating preoperative images based on intraoperative measurements of brain deformation. We have quantitatively evaluated the ability of spatially tracked freehand US to detect displacement of implanted markers in a series of three in vivo porcine experiments, where both US and computed tomography (CT) image acquisitions were obtained before and after deforming the brain. Marker displacements ranged from 0.5 to 8.5 mm. Comparisons between CT and US measurements showed a mean target localization error of 1.5 mm, and a mean vector error for displacement of 1.1 mm. Mean error in the magnitude of displacement was 0.6 mm. For one of the animals studied, the US data was used in conjunction with a biomechanical model to nonrigidly re-register a baseline CT to the deformed brain. The mean error between the actual and deformed CT's was found to be on average 1.2 and 1.9 mm at the marker locations depending on the extent of the deformation induced. These findings indicate the potential accuracy in coregistered freehand US displacement tracking in brain tissue and suggest that the resulting information can be used to drive a modeling re-registration strategy to comparable levels of agreement.
42 citations
TL;DR: In this paper, the tribological behavior of hard carbon films on steel substrates has been studied under moderately high contact stress conditions, and it was found that wear of the carbon coatings was approximately linearly proportional to contact force.
31 citations
TL;DR: In this article, two types of subsurface-originated failure modes are investigated: macroscopic contact fatigue and microscopic pitting, and the cracks responsible for both modes of failure initiate in polyethylene material that has been embrittled by oxidation; this oxidation is an undesirable outcome of the gamma irradiation (in air) used to sterilize the tibial bearings.
Abstract: Tibial hearings of total knee replacements are generally made from ultra-high molecular weight polyethylene (UHMWPE). Failure of those hearings has been found to result in many cases from subsurface crack initiation and propagation. Two types of subsurface-originated failure modes are investigated in this study: macroscopic contact fatigue and microscopic pitting. The cracks responsible for both modes of failure initiate in polyethylene material that has been embrittled by oxidation; this oxidation is an undesirable outcome of the gamma irradiation (in air) used to sterilize the tibial bearings. Accelerated aging was used in this work to achieve oxidation levels similar to those found in retrieved bearings. Tribotesting of the aged bearing materials was carried out under simulated service conditions using a rolling/sliding tester. Macroscopic fatigue cracks and microscopic surface pitting developed in UHMWPE specimens tested on the rolling/sliding tester, and the damage was similar to that found in retrie...
22 citations
TL;DR: A strategy for integrating sparse displacement data acquired during surgery with the computational model using an efficient and accurate approach and has the advantage of incorporating intraoperatively acquired measurements of true displacements into the model to ensure a more accurate estimation of tissue motion.
19 citations
30 May 2003
TL;DR: An inverse method has been developed using sparse data and model constraints to generate estimates of brain motion that combines estimates of statistical error in forcing conditions with a least squares minimization of the model-data misfit to directly estimate the full displacement solution.
Abstract: Model-based approaches to correct for brain shift in image-guided neurosurgery systems have shown promising results. Despite the initial success of such methods, the complex mechanical behavior of the brain under surgical loads makes it likely that model predictions could be improved with the incorporation of real-time measurements of tissue shift in the OR. To this end, an inverse method has been developed using sparse data and model constraints to generate estimates of brain motion. Based on methodology from ocean circulation modeling, this computational scheme combines estimates of statistical error in forcing conditions with a least squares minimization of the model-data misfit to directly estimate the full displacement solution. The method is tested on a 2D simulation based on clinical data in which ultrasound images were co-registered to the preoperative MR stack. Calculations from the 2D forward model are used as the 'gold standard' to which the inverse scheme is compared. Initial results are promising, though further study is needed to ascertain its value in 3D shift estimates.
7 citations
01 Jan 2003
TL;DR: Surgery to repair AAAs also carries mortality risks, so surgeons desire a reliable tool to evaluate therisk of rupture versus the risk of surgery.
Abstract: Abdominal Aortic Aneurysms (AAAs) are localized enlargements of the aorta. If untreated, AAAs will grow irreversibly until rupture occurs. Ruptured AAAs are usually fatal and are a leading cause of death in the United States, killing 15,000 per year (National Center for Health Statistics, 2001). Surgery to repair AAAs also carries mortality risks, so surgeons desire a reliable tool to evaluate the risk of rupture versus the risk of surgery.Copyright © 2003 by ASME
01 Jan 2003
TL;DR: The reconstruction of elastic properties from the imaged displacement field is called magnetic resonance elastography (MRE), and is the subject of this paper.
Abstract: In-vivo measurement of the elastic properties of soft tissue have been made using a variety of direct techniques, such as indentation probes and rotary shear actuators, but they are unable to access much of the soft tissue of interest. Indirect ultrasonic methods for imaging elastic properties of soft tissue were first introduced about 15 years ago, see Ophir (1991). Although the results of ultrasonic elastography studies have been quite promising, they may not be suited for applications requiring accurate quantification of soft tissue properties. An alternative to ultrasound, magnetic resonance imaging, has the advantage of enabling precise measurement of all three components of tissue displacement. The reconstruction of elastic properties from the imaged displacement field is called magnetic resonance elastography (MRE), and is the subject of this paper.Copyright © 2003 by ASME