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PubMed2004

Microgravity simulation: physical and psychological workload evaluation tests in an underwater environment.

As weightlessness is not completely reproducible on Earth, usability evaluation of space systems is often simulated through tests in an aquatic environment. A Neutral Buoyancy Facility test programme was organized in a special pool to simulate Extra-Vehicular Activities on the Columbus module of the future International Space Station with the aim of assessing various aspects of crew interface design. This study was designed to evaluate workload using visibility, accessibility and operability tests. Diving workload was determined through basic physiological measurements, such as pulmonary ventilation and heart rate during underwater operations. As anxiety can influence physiological processes, and consequently also the workload evaluation determined through these parameters, we developed an evaluation methodology to investigate the anxiety level based on a specific questionnaire submitted to all subjects before and after the dives. Heart rate increased in underwater work to a value approximately 50% larger than the value obtained in the resting condition while sitting outside the pool. This increase in heart rate was accompanied by an increase in pulmonary ventilation of 200% larger than the value recorded in the rest condition while sitting outside the water. The extent of these increases was notable in all the test subjects, who varied in age and stature. Recorded values of workload, heart rate and pulmonary ventilation were evaluated on the basis of Christensen's (Arbeitsphysiol. 14 (1950) 251) and Wells' (J. Appl. Physiol. 10 (1957) 51) classifications. Through this analysis it was possible to determine that the workload, indicated by performance on our neutral buoyancy tests, corresponds to moderate physiological work. For test subjects, anxiety related to underwater performance was light. Among the causes of anxiety all the subjects indicated the lack of confidence with neutral buoyancy tests and a feeling of lack of safety, typical of aquatic environments. We can conclude that context did not produce considerable psychological effects, and consequently that the psychological load did not influence heart rate and pulmonary ventilation values that can therefore be directly related to task workload.

Adult↗
PubMed2005

Virtual MRI: a PC-based simulation of a clinical MR scanner.

RATIONALE AND OBJECTIVE: The aim of this project was to simulate the features and functions of a clinical or real-world MR scanner on a personal computer by means of a computer program. The users should be able to change all relevant settings of the virtual scanner and adapt them to the expected pathology. MATERIALS AND METHODS: The algorithms of the simulation are based on parameter images of the three physical basic properties T1, T2, and proton density. From this, the synthetic images are calculated pixel by pixel on the basis of the well-known formulas of the pulse sequences chosen and modified by the user. The graphical user interface is oriented to a real-world MR scanner. The software is programmed in pure Java and is freely available under the GPL license. RESULTS: Besides spin echo pulse sequence, 6 other pulse sequence classes are implemented. Parameters like repetition time and echo time can be adjusted. The choice of parameters like matrix size, slice-thickness, and number of acquisitions has an impact on the signal-to-noise ratio of the images. In a first step, the simulation calculates the signal intensity in k-space. Wraparound and motion artifacts are simulated by modifying the data of k-space. In a last step, a 2D-Fourier transform of k-space data is performed. As the image calculation takes only a few seconds, an interactive manner of working is possible. CONCLUSION: The simulation has been used in the education of medical students and interns for more than 1 year and has gained widespread acceptance.

Algorithms↗
PubMed2001

Simulated shuttle egress: role of helmet visor position during approach and landing.

BACKGROUND: We previously reported that carbon dioxide (CO2) rapidly accumulates in the helmet of the NASA Launch and Entry Suit (LES) during a simulated egress from the Space Shuttle following 6 min of visor-closed seated rest to simulate approach and landing. The purpose of this study was to determine if CO2 accumulation and walking time in the LES would be improved by helmet visor-open rather than visor-closed seated rest prior to the performance of the simulated egress. METHODS: Wearing the LES, 12 male subjects performed 4 laboratory egress simulations consisting of 6-min seated rest, 2-min stand, and 5-min walk at 1.56 m x s(-1) (3.5 mph). During seated rest, subjects sat either with the visor open, breathing room air until the visor was closed on standing, or with the visor closed for the duration of the simulation. For all visor-closed operations 100%, O2 was supplied. The G-suit was either deflated (0.0 psi) or inflated to 1.5 psi. Inspired CO2 and walking time were measured. Data were analyzed at the end of seated rest, standing, and after 5 min of walking at 0.0 psi or after 2 min of walking at 1.5 psi (>90% of data available). RESULTS: Walk time was not different following visor-open (0.0 psi: 5.0 +/- 0.0; 1.5 psi: 3.4 +/- 0.3 min) or visor-closed (0.0 psi: 4.8 +/- 0.2; 1.5 psi: 3.5 +/- 0.4 min) seated rest at either G-suit pressure. Inspired CO2 levels were not different between the two conditions during walking at 5 min at 0.0 psi (p = 0.50; Open: 4.39 +/- 0.14; Closed: 4.48 +/- 0.18%) or at 2 min at 1.5 psi (p = 0.53; Open: 3.59 +/- 0.12; Closed: 3.65 + 0.21%). CONCLUSIONS: Visor position during seated rest immediately preceding the egress walk had no effect on inspired CO2 or walking time.

Adult↗
PubMed2001

Recurrence of acute disseminated encephalomyelitis at the previously affected brain site.

BACKGROUND: Acute disseminated encephalomyelitis (ADEM) is a usually monophasic demyelinating disorder of the central nervous system. Recurrences pose a diagnostic challenge because they can be overlooked or suggest an alternative diagnosis. OBJECTIVE: To examine the frequency, nature, and outcome of recurrent ADEM. DESIGN: Review of the medical records of patients diagnosed in our institution as having ADEM between January 1, 1983, and May 31, 1998. Recurrences were defined as appearance of new symptoms and signs at least 1 month after the previous episode. RESULTS: Five (24%) of 21 patients with ADEM developed recurrent disease episodes. In all, diagnosis was confirmed by brain biopsy. One patient had 4 disease episodes, 2 had 3, and the other 2 each had 2. Recurrence appeared 1.5 to 32 months after initial presentation and involved the same brain territory in 6 of 9 recurrences in 3 of 5 patients. In 2 patients, recurrences included neuropsychiatric signs. A good response to corticosteroid therapy was observed in 10 of 13 of treated ADEM attacks: in 3 of the 4 treated initial events and in 7 of 9 recurrences. CONCLUSIONS: Recurrent ADEM may be more prevalent than previously recognized. Patients who relapse tend to have more than 1 recurrence that usually involves, clinically and radiologically, a brain territory that was affected before and can simulate a space-occupying lesion that requires histologic diagnosis. Neuropsychiatric features may be the main presentation of a relapse. Since recurrent ADEM is a corticosteroid-responsive condition, awareness and early diagnosis are mandatory.

Adolescent↗
PubMed1987

Evaluation of mask-bag ventilation in resuscitation of infants.

Performance of mask-bag ventilation was evaluated on an infant resuscitation mannequin to resolve uncertainty regarding the proficiency of pediatric resuscitation personnel in this technique and to determine whether the type of resuscitation bag used would affect performance. Performance using a self-inflatable resuscitation bag was generally adequate. Forty-six of 50 operators achieved an adequate minute ventilation, and 48 of 50 operators achieved a mean tidal volume exceeding that of the mask plus simulated physiologic dead space. Wide variation with a tendency to hyperventilate and to use excessive pressures indicate the need for improved standard training methods. Technical difficulties with an anesthesia bag impaired performance, suggesting that only self-inflatable bags should be used for mask-bag ventilation during pediatric resuscitation, unless the staff's proficiency with anesthesia bags is clearly demonstrated.

Equipment Design↗
PubMed2003

High-resolution diffusion-weighted 3D MRI, using diffusion-weighted driven-equilibrium (DW-DE) and multishot segmented 3D-SSFP without navigator echoes.

In this work we report on the development of a novel technique for high-resolution diffusion-weighted (DW) MRI based upon 3D steady-state free precession (3D-SSFP). First the 3D-SSFP acquisition was segmented (each segment consisting of a series of RF pulses and gradient-recalled echoes), and then DW-driven equilibrium (DE) was inserted between each segment. The in-plane imaging matrix was typically 256 x 192 or 256 x 160, which resulted in high-resolution DW images. The DW-DE segmented SSFP signal was contaminated by the non-DW magnetization, which recovered and contributed signal during the readout train (T(1) contamination). Center-out slice encoding was used to place the greatest diffusion weighting at the center of k-space. A numerical simulation and supporting experiments were performed to evaluate the relationship of the transverse magnetization to imaging parameters, such as the b-value, echo-train length (ETL), echo-train (group) repetition time (TR(g)), and RF excitation TR (Delta t). Both the numerical simulation and the experiments suggested that the effect of T(1) contamination would be reduced with a longer TR(g), smaller b-value, shorter ETL, and center-out slice phase encoding. Phase errors caused by microscopic motions during the diffusion gradients were converted into amplitude errors by the tip-up pulse at the end of the diffusion-weighting segment. As a result, small bulk motions, such as CSF pulsation, did not cause motion-related ghosting artifacts, which would be typical in images from other multishot DWI techniques. This technique can be used for high-resolution DWI of nonbrain anatomies.

Artifacts↗
PubMed1995

Differential influence of extracellular and intracellular pH on K+ accumulation in ischaemic mammalian cardiac tissue.

The separate and the combined influence of lowering extracellular and intracellular pH on the extracellular accumulation of K+ ions in ischaemic mammalian cardiac tissue was investigated. Isolated guinea-pig papillary muscles were superfused in vitro while micro-electrode measurements of the transmembrane potentials and of the extracellular pH and K+ activity and of the intracellular pH and Na+ activity were performed. Muscles were reversibly subjected to spaced episodes of simulated ischaemia, and ischaemic K+ accumulation was measured. During normal superfusion, acidification of the intracellular pH (pHi approximately 6.8) effected by transient exposure to NH+4 containing superfusate was associated with an amiloride-sensitive rise of the intracellular Na+ activity (aiNa) from 5.1 +/- 0.5 to 19.5 +/- 2.4 mM. This increase was associated with a transient hyperpolarization of the membrane potential and shortening of the action potential duration from 190 +/- 7 to 111 +/- 16 ms. When a similar intracellular acidosis was induced 3 to 5 min before imposing ischaemia, extracellular K+ accumulation was increased compared to ischaemia alone. Surface K+ activity (asK) measured after 10 min of ischaemia was 9.6 +/- 0.8 mM following intracellular acidification v 6.4 +/- 0.4 mM following control superfusion. When the extracellular pH (pHo) was decreased to 6.85 prior to the ischaemic insult, extracellular K+ accumulation was not different from that observed during ischaemia after control superfusion (5.1 +/- 0.5 v 4.7 +/- 0.3 mM, respectively, after 10 min of ischaemia). When combined intracellular and extracellular acidosis was produced before ischaemia, the enhancing influence of lowered pHi on K+ accumulation was not observed. In these acid loading conditions, aiNa rose to the same level as observed following NH+4 withdrawal without simultaneous external acidification (26.9 +/- 2.0 mM v 23.9 +/- 2.0 mM, respectively). The results indicate that decreasing pHi before ischaemia accelerates the ischaemic increase of extracellular K+. On the other hand, although prior extracellular acidification by itself does not directly influence ischaemic K+ accumulation, it is in some way protective by reducing the intracellular proton-stimulated K+ efflux. This reduction seems not to be due to decreased cellular Na+ loading.

Animals↗
PubMed1997

Using the Basis of a Knowledge Space for Determining the Fringe of a Knowledge State

Doignon and Falmagne have developed the concept of the fringe of a knowledge state based upon the neighborhood of knowledge states. This concept is useful for probabilistic assessment of knowledge and for the suggestion of training problems in an intelligent tutoring system. In this paper, a generalized definition of the fringe of knowledge states is introduced. An efficient procedure is presented which computes the fringe of knowledge states using the basis of a knowledge space. In a simulation study, this procedure is compared with the approach described by Doignon and Falmagne. Copyright 1997 Academic Press

Journal Article↗
PubMed1996

State-space analysis of a myocybernetic model of the lower urinary tract.

To study the control of the lower urinary tract, the state space of the myocybernetic model by Bastiaanssen et al. (1996) is analysed. This model is able to respond to input signals from a neural network and includes descriptions of the muscle dynamics of both the detrusor in the bladder wall and the urethral sphincter. The equilibrium states of the model for constant input signals were found by evaluation of the roots of calculated flow curves. Two types of equilibrium states could be distinguished: (i) the inflow and the outflow of the bladder are both equal to zero and (ii) the bladder in- and outflow are both equal to a prescribed small constant flow from the ureters into the bladder. The first type of equilibrium features a very high bladder pressure, which in vivo could result in a reflux of urine into the ureters. The second type shows a constant loss of urine. For different combinations of constant input signals, several stable equilibrium states of both types were found. The neural controller should avoid these states so that the lower urinary tract fulfils either its storage or its voiding function. Therefore, the trajectory through the state space of a simulated normal filling and micturition event was evaluated here. It appeared that equilibrium states were avoided by rapid changes of the input signals. The behaviour of the model outside the normal trajectory is compared with neurologic urinary tract disorders. Several pathological behaviours are in qualitative agreement with the model predictions.

Computer Simulation↗
PubMed1991

Color categorization and color constancy in a neural network model of V4.

We develop a neural network model that instantiates color constancy and color categorization in a single unified framework. Previous models achieve similar effects but ignore important biological constraints. Color constancy in this model is achieved by a new application of the double opponent cells found in the "blobs" of the visual cortex. Color categorization emerges naturally, as a consequence of processing chromatic stimuli as vectors in a four-dimensional color space. A computer simulation of this model is subjected to the classic psychophysical tests that first uncovered these phenomena, and its response matches psychophysical results very closely.

Animals↗
PubMed1985

Aneurysmal bone cysts in the cranial vault and base of skull.

Three cases of aneurysmal bone cyst of the skull are reported. The localization in the base of the skull (two cases) is extremely rare and simulates a space-occupying intracerebral lesion. The clinical and microscopic findings, and their significance for the differential diagnosis from malignant brain tumors, are described.

Adult↗
PubMed1995

A design methodology for short, whole-body, shielded gradient coils for MRI.

A series of designs is presented for restricted length, whole-body, shielded gradient coils. By using the real space optimization technique, simulated annealing (SA), it is possible to produce viable gradient sets with a length-to-diameter ratio (LDR) of just 1.0. Radially remote return paths for the transverse coils aid in producing such short coils. While the linear regions of such coils cannot be as large as longer coils, they produce homogeneous linear regions suitable for use in whole-body imaging. The coil sets are well shielded even at such small LDRs.

Magnetic Resonance Imaging↗
PubMedDate not supplied

Spectrum of CT and sonographic appearance of fatty infiltration of the liver.

This report presents the common and unusual appearances of fatty infiltration of the liver (FIL) on ultrasound (US) and computerized tomography (CT) from pathological, proven cases. Potential diagnostic pitfalls and methods of avoiding these pitfalls will be emphasized. Generalized and the geographic pattern of FIL are well recognized. Focal FIL and focal sparring of the liver have a number of atypical appearances. Focal sparring has several distinct patterns (pseudotumor, glove pattern and possible metastasis), which may be confused with other pathologies. Likewise, focal FIL may simulate a space-occupied mass. Among the more helpful features used to distinguish focal FIL from other abnormalities include its lack of mass effect, low density on CT combined with increased echogenicity on US.

Diagnosis, Differential↗
PubMed2005

Spatiotemporal clustering of fMRI time series in the spectral domain.

We propose a new method for the analysis of functional magnetic resonance images (fMRI). The decision that a voxel v0 is activated is based not solely on the value of the fMRI signal at v0, but rather on the comparison of all time series s(v)(t) in a small neighborhood Nv0 around v0. Our approach explicitly takes into account the intrinsic spatiotemporal correlations that exist in the data. We focus on experimental designs with periodic stimuli, and therefore we can capture most of the features of the BOLD signal with a low dimensional subspace in the frequency domain. The presence of activated time series can be detected by partitioning the time series in this low dimensional space. Experiments with simulated data, and experimental fMRI data, demonstrate that our approach can outperform standard methods of analysis, such as the t-test.

Brain↗
PubMed2006

k-space inherited parallel acquisition (KIPA): application on dynamic magnetic resonance imaging thermometry.

In this study, a novel method for dynamic parallel image acquisition and reconstruction is presented. In this method, called k-space inherited parallel acquisition (KIPA), localized reconstruction coefficients are used to achieve higher reduction factors, and lower noise and artifact levels compared to that of generalized autocalibrating partially parallel acquisition (GRAPPA) reconstruction. In KIPA, the full k-space for the first frame and the partial k-space for later frames are required to reconstruct a whole series of images. Reconstruction coefficients calculated for different segments of k-space from the first frame data set are used to estimate missing k-space lines in corresponding k-space segments of other frames. The local determination of KIPA reconstruction coefficients is essential to adjusting them according to the local signal-to-noise ratio characteristics of k-space data. The proposed algorithm is applicable to dynamic imaging with arbitrary k-space sampling trajectories. Simulations of magnetic resonance thermometry using the KIPA method with a reduction factor of 6 and using dynamic imaging studies of human subjects with reduction factors of 4 and 6 have been performed to prove the feasibility of our method and to show apparent improvement in image quality in comparison with GRAPPA for dynamic imaging.

Algorithms↗
PubMed2006

Vector-based spatial-temporal minimum L1-norm solution for MEG.

Minimum L1-norm solutions have been used by many investigators to analyze MEG responses because they provide high spatial resolution images. However, conventional minimum L1-norm approaches suffer from instability in spatial construction, and poor smoothness of the reconstructed source time-courses. Activity commonly "jumps" from one grid point to (usually) the neighboring grid points. Equivalently, the time-course of one specific grid point can show substantial "spiky-looking" discontinuity. In the present study, we present a new vector-based spatial-temporal analysis using a L1-minimum-norm (VESTAL). This approach is based on a principle of MEG physics: the magnetic waveforms in sensor-space are linear functions of the source time-courses in the imaging-space. Our computer simulations showed that VESTAL provides good reconstruction of the source amplitude and orientation, with high stability and resolution in both the spatial and temporal domains. "Spiky-looking" discontinuity was not observed in the source time-courses. Importantly, the simulations also showed that VESTAL can resolve sources that are 100% correlated. We then examined the performance of VESTAL in the analysis of human median-nerve MEG responses. The results demonstrated that this method easily distinguishes sources very spatially close to each other, including individual primary somatosensory areas (BA 1, 2, 3b), primary motor area (BA 4), and other regions in the somatosensory system (e.g., BA 5, 7, SII, SMA, and temporal-parietal junction) with high temporal stability and resolution. VESTAL's potential for obtaining information on source extent was also examined.

Adult↗
PubMedDate not supplied

The NASA Performance Assessment Workstation: cognitive performance during head-down bed rest.

The NASA Performance Assessment Workstation was used to assess cognitive performance changes in eight males subjected to seventeen days of 6 degrees head-down bed rest. PAWS uses six performance tasks to assess directed and divided attention, spatial, mathematical, and memory skills, and tracking ability. Subjective scales assess overall fatigue and mood state. Subjects completed training trials, practice trials, bed rest trials, and recovery trials. The last eight practice trials and all bed rest trials were performed with subjects lying face-down on a gurney. In general, there was no apparent cumulative effect of bed rest. Following a short period of performance stabilization, a slight but steady trend of performance improvement was observed across all trials. For most tasks, this trend of performance improvement was enhanced during recovery. No statistically significant differences in performance were observed when comparing bed rest with the control period. Additionally, fatigue scores showed little change across all periods.

Adult↗
PubMed1995

Cosmic dust analog simulation in a microgravity environment: the STARDUST program.

We have undertaken a project called STARDUST which is a collaboration with Italian and American investigators. The goals of this program are to study the condensation and coagulation of refractory materials from the vapor and to study the properties of the resulting grains as analogs to cosmic dust particles. To reduce thermal convective currents and to develop valuable experience in designing an experiment for the Gas-Grain Simulation Facility aboard Space Station Freedom we have built and flown a new chamber to study these processes under periods of microgravity available on NASA's KC-135 Research Aircraft. Preliminary results from flights with magnesium and zinc are discussed.

Cosmic Dust↗