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At least 613 records · Page 34Linked to original sources
PubMed1998

Interaction of the Na+-K+ pump and Na+-Ca2+ exchange via [Na+]i in a restricted space of guinea-pig ventricular cells.

1. The whole-cell Na+-K+ pump current (INa-K) and Na+-Ca2+ exchange current (INa-Ca) were recorded in guinea-pig ventricular myocytes to study the interaction between the two Na+ transport mechanisms. 2. INa-K was isolated as an external K+-induced current, and INa-Ca as an external Ca2+- induced or Ni2+-sensitive current. The experimental protocol used for one ion carrier did not affect the other. 3. The amplitude of INa-K decreased to 54 +/- 17 % of the initial peak during continuous application of K+ with 20 mM Na+ in the pipette. The outward INa-Ca, which was intermittently activated by brief applications of Ca2+, decreased during activation of INa-K, and recovered after cessation of INa-K activation. These findings revealed a dynamic interaction between INa-K and INa-Ca via a depletion of Na+ under the sarcolemma. 4. To estimate changes in Na+ concentration ([Na+]i) under the sarcolemma, the reversal potential (Vrev) of INa-Ca was measured. Unexpectedly, Vrev hardly changed during activation of INa-K. However, when INa-Ca was blocked by Ni2+ at the same time that INa-K was activated, Vrev changed markedly, maximally by +100 mV, immediately after the removal of Ni2+ and K+. 5. Subsarcolemmal [Na+]i was calculated from the Vrev of INa-Ca on the assumption that the subsarcolemmal Ca2+ concentration ([Ca2+]i) was fixed with EGTA, and mean [Na+]i was calculated from both the time integral of INa-K and the cell volume. The subsarcolemmal [Na+]i was about seven times greater than the mean [Na+]i. 6. The interaction between the Na+-K+ pump and Na+-Ca2+ exchange was well simulated by a diffusion model, in which Na+ diffusion was restricted to one-seventh (14 %) of the total cell volume.

Animals↗
PubMed2003

Performance of the CELSS Antarctic Analog Project (CAAP) crop production system.

Regenerative life support systems potentially offer a level of self-sufficiency and a decrease in logistics and associated costs in support of space exploration and habitation missions. Current state-of-the-art in plant-based, regenerative life support requires resources in excess of allocation proposed for candidate mission scenarios. Feasibility thresholds have been identified for candidate exploration missions. The goal of this paper is to review recent advances in performance achieved in the CELSS Antarctic Analog Project (CAAP) in light of the likely resource constraints. A prototype CAAP crop production chamber has been constructed and operated at the Ames Research Center. The chamber includes a number of unique hardware and software components focused on attempts to increase production efficiency, increase energy efficiency, and control the flow of energy and mass through the system. Both single crop, batch production and continuous cultivation of mixed crops production studies have been completed. The crop productivity as well as engineering performance of the chamber are described. For each scenario, energy required and partitioned for lighting, cooling, pumping, fans, etc. is quantified. Crop production and the resulting lighting efficiency and energy conversion efficiencies are presented. In the mixed-crop scenario, with 27 different crops under cultivation, 17 m2 of crop area provided a mean of 515 g edible biomass per day (85% of the approximate 620 g required for one person). Enhanced engineering and crop production performance achieved with the CAAP chamber, compared with current state-of-the-art, places plant-based life support systems at the threshold of feasibility.

Agriculture↗
PubMed2006

Energy partitioning schemes.

The paper gives an overview, generalization and systematization of the different energy decomposition schemes we have devised in the last few years by using both the 3-D analysis (the atoms are represented by different parts of the physical space) and the Hilbert space analysis in terms of the basis orbitals assigned to the individual atoms. The so called "atomic decomposition of identity" provides us the most general formalism for analyzing different physical quantities in terms of individual atoms or pairs of atoms. (The "atomic decomposition of identity" means that we present the identity operator as a sum of operators assigned to the individual atoms.) By proper definitions of the atomic operators, both Hilbert-space and the different 3-D decomposition schemes can be treated on an equal footing. Several different but closely related energy decomposition schemes have been proposed for the Hilbert space analysis. They differ by exact or approximate treatment of the three- and four-center integrals and by considering the kinetic energy as a part of the atomic Hamiltonian or as having genuine two-center components, too. (Also, some finite basis correction terms may be treated in different manners.) The exact schemes are obtained by using the "atomic decomposition of identity". In the approximate schemes a projective integral approximation is also introduced, thus the energy components contain only one- and two-center integrals. The diatomic energy contributions have also been decomposed into terms of different physical nature (electrostatic, exchange etc.) The 3-D analysis may be performed either in terms of disjunct atomic domains, as in the case of the AIM formalism, or by using the so called "fuzzy atoms" which do not have sharp boundaries but exhibit a continuous transition from one to another. The different schemes give different numbers, but each is capable of reflecting the most important intramolecular interactions as well as the secondary ones--e.g. intramolecular interactions of type C-H[...]O.

Algorithms↗
PubMedDate not supplied

Temperature-dependent versus constant-rate blood perfusion modelling in ferromagnetic thermoseed hyperthermia: results with a model of the human prostate.

Finite-element solutions to the Pennes bioheat equation are obtained with a model of a tumour-containing, human prostate and surrounding normal tissues. Simulations of ferromagnetic hyperthermia treatments are conducted on the tissue model in which the prostate is implanted with an irregularly spaced array of thermoseeds. Several combinations of thermoseed temperatures with different Curie points are investigated. Non-uniform, constant-rate blood perfusion models are studied and compared with temperature-dependent descriptions of blood perfusion. Blood perfusions in the temperature-dependent models initially increase with tissue temperature and then decrease at higher temperatures. Simulations with temperature-dependent versus constant-rate blood perfusion models reveal significant differences in temperature distributions in and surrounding the tumour-containing prostate. Results from the simulations include differences (between temperature-dependent and constant-rate models) in (1) the percentage of normal tissue volume and tumour volume at temperatures > 42 degrees C, and (2) temperature descriptors in the tumour (subscript t) and normal (subscript n) tissues including Tmax.t, Tmin.t and Tmax.n. Isotherms and grey-scale contours in the tumour and surrounding normal tissues are presented for four simulations that model a combination of high-temperature thermoseeds. Several simulations show that Tmin.t is between 1.7 and 2.6 degrees C higher and Tmax.n is between 2.1 and 3.3 degrees C higher with a temperature-dependent versus a comparable constant-rate blood perfusion model. The same simulations reveal that the percentages of tumour volume at temperatures > 42 degrees C are between 0 and 68% higher with the temperature-dependent versus the constant-rate perfusion model over all seed combinations studied. In summary, a numerical method is presented which makes it possible to investigate temperature-dependent, continuous functions of blood perfusion in simulations of hyperthermia treatments. Simulations with this numerical method reveal that the use of constant-rate instead of temperature-dependent blood perfusion models can be a conservative approach in treatment planning of ferromagnetic hyperthermia.

Body Temperature Regulation↗
PubMedDate not supplied

COLUMBUS Orbital Facility and Automated Transfer Vehicle: a challenge for agency & industry.

Long term continuous operation of the COLUMBUS Orbital Facility (COF) flight- and ground segment requires continuous mission control and operations support capability to ensure proper operation and configuration of the COF systems in support of ongoing science and technology payloads. The ISS logistics scenario will be supported by the Automated Transfer Vehicle (ATV). These operational needs require the built-up of a new ground infrastructure in Europe and USA, enabling an efficient operations for preparation, planning and mission execution. The challenge for the European space community consists in the development and operation of a user friendly operational environment but keeping costs within budgetary constraints. Results of detailed definition studies performed by both agency and industry for the ground infrastructure indicate solutions to those technical and programmatic requirements by using of existing centers and facilities, re-use of C/D phase products (Hardware, Software) and COTS equipment to avoid costly new developments, using engineering expertise of the industrial personnel from flight element phase C/D. The concept for operations execution defines the task sharing between Operations Control Facilities (OCF), Operations Support Facilities and User Operations Sites. Operations support consists of on-line engineering support, off-line engineering support, payload integration, logistics support and crew training support performed by industry. DASA RI has made internal investments in organizational concepts for mission operations as well as in mission technologies and tools based on the standard COLUMBUS Ground Software (CGS) toolset and on knowledge based systems to enable an efficient industrial operations support. These tools are available as prototypes being evaluated in a simulated operational environment.

Astronauts↗
PubMed2000

Quantitative ultrasound imaging of the calcaneus: precision and variations during a 120-Day bed rest.

This study reports on the precision and variation of quantitative ultrasound (US) parameters [broadband ultrasonic attenuation (BUA) or slope of the frequency-dependent attenuation in dB/MHz and speed of sound (SOS m/second)] after 120 days of continuous bed rest in six normal male volunteers. Quantitative US was measured at the calcaneus using a new US bone imaging scanner. The measurements were carried out on both heels at approximately 2-week intervals. The short-term precision was 0.31% for SOS and 2.8% for BUA. The long-term precision was 0.58% for SOS, 4.7% for BUA. A significant decrease of SOS values of -26 m/second (P < 0.0001) for the right heel and -17 m/second (P < 0.05) for the left heel was found at the group level. In terms of percentage change this represents -1.7% for the right heel and -1.1% for the left heel. These percentage decrements were 3.5-5.5 times that of the short-term precision and 2-3 times that of the long-term precision of the technique. At the individual level, the decrease of SOS was statistically significant (P < 0.05) or marginally significant (P < 0.1) for four out of 6 subjects. For 2 other subjects, similar trends were observed, but without reaching statistical significance. BUA did not change significantly during follow-up. These results are consistent with previous findings on changes of ultrasonic properties from the calcaneus during aging, pregnancy, or therapy, showing that calcaneus SOS is a valuable index of bone loss. These preliminary data suggest that prolonged exposure to simulated weightlessness may lead to a lower SOS, which then could be used for the follow-up of bone demineralization occurring during long-term space flights.

Adult↗
PubMed1998

Effects of glucose deprivation, chemical hypoxia, and simulated ischemia on Na+ homeostasis in rat spinal cord astrocytes.

A steep inwardly directed Na+ gradient is essential for glial functions such as glutamate reuptake and regulation of intracellular ion concentrations. We investigated the effects of glucose deprivation, chemical hypoxia, and simulated ischemia on intracellular Na+ concentration ([Na+]i) in cultured spinal cord astrocytes using fluorescence ratio imaging with sodium-binding benzofuran isophthalate (SBFI) AM. Glucose removal or chemical hypoxia (induced by 10 mM NaN3) for 60 min increased [Na+]i from a baseline of 8.3 to 11 mM. Combined glycolytic and respiratory blockage by NaN3 and 0 glucose saline caused [Na+]i to increase by 20 mM, similar to the [Na+]i increases elicited by blocking the Na+/K+-ATPase with ouabain. Recovery from large [Na+]i increases (>15 mM) induced by the glutamatergic agonist kainate was attenuated during glucose deprivation or NaN3 application and was blocked in NaN3 and 0 glucose. To mimic in vivo ischemia, we exposed astrocytes to NaN3 and 0 glucose saline containing L-lactate and glutamate with increased [K+] and decreased [Na+], [Ca2+], and pH. This induced an [Na+]i decrease followed by an [Na+]i rise and a further [Na+]i increase after reperfusion with standard saline. Similar multiphasic [Na+]i changes were observed after NaN3 and 0 glucose saline with only reduced [Na+]e. Our results suggest that the ability to maintain a low [Na+]i enables spinal cord astrocytes to continue uptake of K+ and/or glutamate at the onset of energy failure. With prolonged energy failure, however, astrocytic [Na+]i rises; with loss of their steep transmembrane Na+ gradient, astrocytes may aggravate metabolic insults by carrier reversal and release of acid, K+, and/or glutamate into the extracellular space.

Animals↗
PubMed2007

Augmented generalized SENSE reconstruction to correct for rigid body motion.

The correction of motion artifacts continues to be a significant problem in MRI. In the case of uncooperative patients, such as children, or patients who are unable to remain stationary, the accurate determination and correction of motion artifacts becomes a very important prerequisite for achieving good image quality. The application of conventional motion-correction strategies often produces inconsistencies in k-space data. As a result, significant residual artifacts can persist. In this work a formalism is introduced for parallel imaging in the presence of motion. The proposed method can improve overall image quality because it diminishes k-space inconsistencies by exploiting the complementary image encoding capacity of individual receiver coils. Specifically, an augmented version of an iterative SENSE reconstruction is used as a means of synthesizing the missing data in k-space. Motion is determined from low-resolution navigator images that are coregistered by an automatic registration routine. Navigator data can be derived from self-navigating k-space trajectories or in combination with other navigation schemes that estimate patient motion. This correction method is demonstrated by interleaved spiral images collected from volunteers. Conventional spiral scans and scans corrected with proposed techniques are shown, and the results illustrate the capacity of this new correction approach.

Algorithms↗
PubMed2003

Steady-state free precession with myocardial tagging: CSPAMM in a single breathhold.

A method is presented that combines steady-state free precession (SSFP) cine imaging with myocardial tagging. Before the tagging preparation at each ECG-R wave, the steady-state magnetization is stored as longitudinal magnetization by an alpha/2 flip-back pulse. Imaging is continued immediately after tagging preparation, using linearly increasing startup angles (LISA) with a rampup over 10 pulses. Interleaved segmented k-space ordering is used to prevent artifacts from the increasing signal during the LISA rampup. First, this LISA-SSFP method was evaluated regarding ghost artifacts from the steady-state interruption by comparing LISA with an alpha/2 startup method. Next, LISA-SSFP was compared with spoiled gradient echo (SGRE) imaging, regarding tag contrast-to-noise ratio and tag persistence. The measurements were performed in phantoms and in six subjects applying breathhold cine imaging with tagging (temporal resolution 51 ms). The results show that ghost artifacts are negligible for the LISA method. Compared to the SGRE reference, LISA-SSFP was two times faster, with a slightly better tag contrast-to-noise. Additionally, the tags persisted 126 ms longer with LISA-SSFP than with SGRE imaging. The high efficiency of LISA-SSFP enables the acquisition of complementary tagged (CSPAMM) images in a single breathhold.

Artifacts↗
PubMed2006

Invariant visual object recognition: a model, with lighting invariance.

How are invariant representations of objects formed in the visual cortex? We describe a neurophysiological and computational approach which focusses on a feature hierarchy model in which invariant representations can be built by self-organizing learning based on the statistics of the visual input. The model can use temporal continuity in an associative synaptic learning rule with a short term memory trace, and/or it can use spatial continuity in Continuous Transformation learning. The model of visual processing in the ventral cortical stream can build representations of objects that are invariant with respect to translation, view, size, and in this paper we show also lighting. The model has been extended to provide an account of invariant representations in the dorsal visual system of the global motion produced by objects such as looming, rotation, and object-based movement. The model has been extended to incorporate top-down feedback connections to model the control of attention by biased competition in for example spatial and object search tasks. The model has also been extended to account for how the visual system can select single objects in complex visual scenes, and how multiple objects can be represented in a scene.

Animals↗
PubMed2004

The MELISSA pilot plant facility as as integration test-bed for advanced life support systems.

The different advances in the Micro Ecological Life Support System Alternative project (MELISSA), fostered and coordinated by the European Space Agency, as well as in other associated technologies, are integrated and demonstrated in the MELISSA Pilot Plant laboratory. During the first period of operation, the definition of the different compartments at an individual basis has been achieved, and the complete facility is being re-designed to face a new period of integration of all these compartments. The final objective is to demonstrate the potentiality of biological systems such as MELISSA as life support systems. The facility will also serve as a test bed to study the robustness and stability of the continuous operation of a complex biological system. This includes testing of the associated instrumentation and control for a safe operation, characterization of the chemical and microbial safety of the system, as well as tracking the genetic stability of the microbial strains used. The new period is envisaged as a contribution to the further development of more complete biological life support systems for long-term manned missions, that should be better defined from the knowledge to be gained from this integration phase. This contribution summarizes the current status of the Pilot Plant and the planned steps for the new period.

Biodegradation, Environmental↗
PubMed2001

The beta-beta-alpha fold: explorations in sequence space.

The computational redesign of the second zinc finger of Zif268 to produce a 28 residue peptide (FSD-1) that assumes a betabetaalpha fold without metal binding was recently reported. In order to explore the tolerance of this metal-free fold towards sequence variability, six additional peptides resulting from the ORBIT computational protein design process were synthesized and characterized. The experimental stabilities of five of these peptides are strongly correlated with the energies calculated by ORBIT. However, when a peptide with a mutation in the beta-turn is examined, the calculated stability does not accurately predict the experimentally determined stability. The NMR solution structure of a peptide incorporating this mutation (FSD-EY) reveals that the register between the beta-strands is different from the model structure used to select and score the sequences. FSD-EY has a type I' turn instead of the target EbaaagbE turn (rubredoxin knuckle). Two additional peptides that have improved side-chain to backbone hydrogen bonding and turn propensity for the target turn were characterized. Both are of stability comparable to that of FSD-1. These results demonstrate the robustness of the ORBIT protein design methods and underscore the need for continued improvements in negative design.

Algorithms↗
PubMed2000

Spatial frequency maps in cat visual cortex.

Neurons in the primary visual cortex (V1) respond preferentially to stimuli with distinct orientations and spatial frequencies. Although the organization of orientation selectivity has been thoroughly described, the arrangement of spatial frequency (SF) preference in V1 is controversial. Several layouts have been suggested, including laminar, columnar, clustered, pinwheel, and binary (high and low SF domains). We have reexamined the cortical organization of SF preference by imaging intrinsic cortical signals induced by stimuli of various orientations and SFs. SF preference maps, produced from optimally oriented stimuli, were verified using targeted microelectrode recordings. We found that a wide range of SFs is represented independently and mostly continuously within V1. Domains with SF preferences at the extremes of the SF continuum were separated by no more than (3/4) mm (conforming to the hypercolumn description of cortical organization) and were often found at pinwheel center singularities in the cortical map of orientation preference. The organization of cortical maps permits nearly all combinations of orientation and SF preference to be represented in V1, and the overall arrangement of SF preference in V1 suggests that SF-specific adaptation effects, found in psychophysical experiments, may be explained by local interactions within a given SF domain. By reanalyzing our data using a different definition of SF preference than is used in electrophysiological and psychophysical studies, we can reproduce the different SF organizations suggested by earlier studies.

Algorithms↗
PubMed2002

Interaction between water flow and spatial distribution of microbial growth in a two-dimensional flow field in saturated porous media.

Bacterial growth and its interaction with water flow was investigated in a two-dimensional flow field in a saturated porous medium. A flow cell (56 x 44 x 1 cm) was filled with glass beads and operated under a continuous flow of a mineral medium containing nitrate as electron acceptor. A glucose solution was injected through an injection port, simulating a point source contamination. Visible light transmission was used to observe the distribution of the growing biomass and water flow during the experiment. At the end of the experiment (on day 31), porous medium samples were destructively collected and analyzed for abundance of total and active bacterial cells, bacterial cell volume and concentration of polysaccharides and proteins. Microbial growth was observed in two stripes along the length of the flow cell, starting at the glucose injection port, where highest biomass concentrations were obtained. The spatial distribution of biomass indicated that microbial activity was limited by transverse mixing between glucose and nitrate media, as only in the mixing zone between the media high biological activities were achieved. The ability of the biomass to change the flow pattern in the flow cell was observed, indicating that the biomass was locally reducing the hydraulic conductivity of the porous medium. This bioclogging effect became evident when the injection of the glucose solution was turned off and water flow still bypassed the area around the glucose injection port, preserving the flow pattern as it was during the injection of the glucose solution. As flow bypass was possible in this system, the average hydraulic properties of the flow cell were not affected by the produced biomass. Even in the vicinity of the injection port, the total volume of the bacterial cells remained below 0.01% of the pore space and was unlikely to be responsible for the bioclogging. However, the bacteria produced large amounts of extracellular polymeric substances (EPS), which likely caused the observed bioclogging effects.

Benzenesulfonates↗
PubMed1999

GLLS for optimally sampled continuous dynamic system modeling: theory and algorithm.

The original generalized linear least squares (GLLS) algorithm was developed for non-uniformly sampled biomedical system parameter estimation using finely sampled instantaneous measurements (D. Feng, S.C. Huang, Z. Wang, D. Ho, An unbiased parametric imaging algorithm for non-uniformly sampled biomedical system parameter estimation, IEEE Trans. Med. Imag. 15 (1996) 512-518). This algorithm is particularly useful for image-wide generation of parametric images with positron emission tomography (PET), as it is computationally efficient and statistically reliable (D. Feng, D. Ho, Chen, K., L.C. Wu, J.K. Wang, R.S. Liu, S.H. Yeh, An evaluation of the algorithms for determining local cerebral metabolic rates of glucose using positron emission tomography dynamic data, IEEE Trans. Med. Imag. 14 (1995) 697-710). However, when dynamic PET image data are sampled according to the optimal image sampling schedule (OISS) to reduce memory and storage space (X. Li, D. Feng, K. Chen, Optimal image sampling schedule: A new effective way to reduce dynamic image storage space and functional image processing time, IEEE Trans. Med. Imag. 15 (1996) 710-718), only a few temporal image frames are recorded (e.g. only four images are recorded for the four parameter fluoro-deoxy-glucose (FDG) model). These image frames are recorded in terms of accumulated radio-activity counts and as a result, the direct application of GLLS is not reliable as instantaneous measurement samples can no longer be approximated by averaging of accumulated measurements over the sampling intervals. In this paper, we extend GLLS to OISS-GLLS which deals with the fewer accumulated measurement samples obtained from OISS dynamic systems. The theory and algorithm of this new technique are formulated and studied extensively. To investigate statistical reliability and computational efficiency of OISS-GLLS, a simulation study using dynamic PET data was performed. OISS-GLLS using 4-measurement samples was compared to the non-linear least squares (NLS) method using 22-measurement samples, GLLS using 22-measurement samples and OISS-NLS using 4-measurement samples. Results demonstrated that OISS-GLLS was able to achieve parameter estimates of equivalent accuracy and reliability in comparison to NLS or GLLS using finely sampled measurements (22-measurement samples), or OISS-NLS using optimally sampled measurements (4-measurement samples). Further more, as fewer measurement samples are used in OISS-GLLS, this algorithm is computationally faster than NLS or GLLS. Therefore, OISS-GLLS is well-suited for image-wide parameter estimation when PET image data are recorded according to the optimal image sampling schedule.

Algorithms↗
PubMedDate not supplied

Vertical cross contamination of trichloroethylene in a borehole in fractured sandstone.

Boreholes drilled through contaminated zones in fractured rock create the potential for vertical movement of contaminated ground water between fractures. The usual assumption is that purging eliminates cross contamination; however, the results of a field study conducted in a trichloroethylene (TCE) plume in fractured sandstone with a mean matrix porosity of 13% demonstrates that matrix-diffusion effects can be strong and persistent. A deep borehole was drilled to 110 m below ground surface (mbgs) near a shallow bedrock well containing high TCE concentrations. The borehole was cored continuously to collect closely spaced samples of rock for analysis of TCE concentrations. Geophysical logging and flowmetering were conducted in the open borehole, and a removable multilevel monitoring system was installed to provide hydraulic-head and ground water samples from discrete fracture zones. The borehole was later reamed to complete a well screened from 89 to 100 mbgs; persistent TCE concentrations at this depth ranged from 2100 to 33,000 microg/L. Rock-core analyses, combined with the other types of borehole information, show that nearly all of this deep contamination was due to the lingering effects of the downward flow of dissolved TCE from shallower depths during the few days of open-hole conditions that existed prior to installation of the multilevel system. This study demonstrates that transfer of contaminant mass to the matrix by diffusion can cause severe cross contamination effects in sedimentary rocks, but these effects generally are not identified from information normally obtained in fractured-rock investigations, resulting in potential misinterpretation of site conditions.

California↗
PubMed2004

Optimization of a vector quantization codebook for objective evaluation of surgical skill.

Surgical robotic systems and virtual reality simulators have introduced an unprecedented precision of measurement for both tool-tissue and tool-surgeon interaction; thus holding promise for more objective analyses of surgical skill. Integrative or averaged metrics such as path length, time-to-task, success/failure percentages, etc., have often been employed towards this end but these fail to address the processes associated with a surgical task as a dynamic phenomena. Stochastic tools such as Markov modeling using a 'white-box' approach have proven amenable to this type of analysis. While such an approach reveals the internal structure of the of the surgical task as a process, it requires a task decomposition based on expert knowledge, which may result in a relatively large/complex model. In this work, a 'black box' approach is developed with generalized cross-procedural applications., the model is characterized by a compact topology, abstract state definitions, and optimized codebook size. Data sets of isolated tasks were extracted from the Blue DRAGON database consisting of 30 surgical subjects stratified into six training levels. Vector quantization (VQ) was employed on the entire database, thus synthesizing a lexicon of discrete, task-independent surgical tool/tissue interactions. VQ has successfully established a dictionary of 63 surgical code words and displayed non-temporal skill discrimination. VQ allows for a more cross-procedural analysis without relying on a thorough study of the procedure, links the results of the black-box approach to observable phenomena, and reduces the computational cost of the analysis by discretizing a complex, continuous data space.

Algorithms↗
PubMed1991

The scanning acoustic microprobe: I. Analysis and synthesis of a spherically symmetric point spread function.

The scanning acoustic microprobe is a novel system which probes and characterizes, from a limited acoustic window, the fine-scale structural features of an object, point-by-point, using a multiplicity of acoustic pulses all aimed and focused at that point. Spherically symmetric, three-dimensional Gaussian pulses are synthesized to measure the backscatter diffraction pattern of the least-resolvable volume of scatterers centered at the point in question. The size and distribution of the scattering volume is forced to be constant, independent of frequency and angle. This method is analytically simple, compared with other pulse-echo techniques, and is applicable to scatterers ranging continuously in size from Rayleigh scatterers to specular reflectors. This is the first of a number of papers describing the development and application of systems based on these concepts. The analytical principles will be described herein for examination of one point at a time. In a companion paper appearing in this issue [F. E. Barber, J. Acoust. Soc. Am. 90, 11-19 (1991)], application to measurement and characterization of a discrete, flat, circular "piston" will be presented. Application to human tissue imaging and tissue characterization will be described in a subsequent third paper. The primary features detected experimentally are the strength of nondirective patterns, and the strength, orientation, and directivity of angle-dependent echo functions associated with planar or layered structures. Fine-scale structural features of a scattering center are obtained either by pattern recognition in k(data) space or inverse Fourier transformation. It is shown that when the bandwidth criteria are met to produce a spherically symmetric point spread function, scattering phenomena are completely described by only two parameters, namely the center frequency of the pulse-echo system and the characteristic diameter of the Gaussian point spread function.

Computer Simulation↗