Showing papers by "State University of New York System published in 1998"

Synthesis of Large Arrays of Well-Aligned Carbon Nanotubes on Glass

Abstract: Free-standing aligned carbon nanotubes have previously been grown above 700°C on mesoporous silica embedded with iron nanoparticles. Here, carbon nanotubes aligned over areas up to several square centimeters were grown on nickel-coated glass below 666°C by plasma-enhanced hot filament chemical vapor deposition. Acetylene gas was used as the carbon source and ammonia gas was used as a catalyst and dilution gas. Nanotubes with controllable diameters from 20 to 400 nanometers and lengths from 0.1 to 50 micrometers were obtained. Using this method, large panels of aligned carbon nanotubes can be made under conditions that are suitable for device fabrication.

Rotating Nuclear Rings and Extreme Starbursts in Ultraluminous Galaxies

Abstract: New CO interferometer data show that the molecular gas in infrared ultraluminous galaxies is in rotating nuclear disks or rings. The CO maps yield disk radii, kinematic major axes, rotation speeds, enclosed dynamical masses, and gas masses. The CO brightness temperatures, the double-peaked CO line profiles, the limits on thermal continuum flux from dust, and the constraint that the gas mass must be less than the dynamical mass all indicate that the CO lines are subthermally excited and moderately opaque (τ = 4 to 10). We fit kinematic models in which most of the CO flux comes from a moderate-density warm intercloud medium, rather than from self-gravitating clouds. Typical ring radii are 300 to 800 pc. We derive gas masses not from a standard CO-to-mass ratio, but from a model of radiative transfer through subthermally excited CO in the molecular disks. This model yields gas masses of ~5 × 109 M☉, ~5 times lower than the standard method, and a ratio M/L ≈ 0.8 M☉ (K km s-1 pc2)-1. In the nuclear disks, we derive a ratio of gas to dynamical mass of Mgas/Mdyn ≈ 1/6, and a maximum ratio of gas to total mass surface density, μ/μtot, of 1/3. For the galaxies VII Zw 31, Arp 193, and IRAS 10565+2448, the CO position-velocity diagrams provide good evidence for rotating molecular rings with a central gap. In addition to the rotating central rings or disks, a new class of star formation region is identified, which we call an extreme starburst. These have a characteristic sizes of only 100 pc, with about 109 M☉ of gas and an IR luminosity of ≈ 3 × 1011 L☉ from recently formed OB stars. Four extreme starbursts are identified in the 3 closest galaxies in the sample, including Arp 220, Arp 193, and Mrk 273. These are the most prodigious star formation events in the local universe, each representing about 1000 times as many OB stars as 30 Doradus. In Mrk 231, the CO (2-1) velocity diagram along the line of nodes shows a 12 diameter inner disk and a 3'' diameter outer disk. The narrow CO line width, the single-peak line profile, the equality of the major and minor axes, and the observed velocity gradients all imply that the molecular disk is nearly face-on, yielding low optical and UV extinction to the active galactic nucleus (AGN). Such a geometry means that the molecular disk cannot be heated by the AGN; the far-infrared (FIR) luminosity of Mrk 231 is powered by a starburst, not the AGN. In Mrk 273, the CO (1-0) maps show long streamers of radius 5 kpc (7'') with velocity gradients north-south, and a nuclear disk of radius 400 pc (06) with velocity gradients east-west. The nuclear disk contains a bright CO core of radius 120 pc (02). In Arp 220, the CO and 1.3 mm continuum maps show the two "nuclei" embedded in a central ring or disk at P.A. 50° and a fainter structure extending 7'' (3 kpc) to the east, normal to the nuclear disk. Models of the CO and dust flux indicate that the two K-band sources contain high-density gas, with n(H2) = 2 × 104 cm-3. There is no evidence that these sources really are the premerger nuclei. They are more likely to be compact extreme starburst regions, containing 109 M☉ of dense molecular gas and new stars, but no old stars. Most of the HCN emission arises in the two nuclei. The luminosity-to-mass ratios for the CO sources in Arp 220 are compatible with the early phases of compact starbursts. There is a large mass of molecular gas currently forming stars with plenty of ionizing photons, and no obvious AGN. The entire bolometric luminosity of Arp 220 comes from starbursts, not an AGN. The CO maps show that the gas in ultraluminous IR galaxies is in extended disks that cannot intercept all the power of central AGNs, even if they exist. We conclude that in ultraluminous IR galaxies—even in Mrk 231, which hosts a quasar—the FIR luminosity is powered by extreme starbursts in the molecular rings or disks, not by dust-enshrouded quasars.

Efficient collision detection using bounding volume hierarchies of k-DOPs

Abstract: Collision detection is of paramount importance for many applications in computer graphics and visualization. Typically, the input to a collision detection algorithm is a large number of geometric objects comprising an environment, together with a set of objects moving within the environment. In addition to determining accurately the contacts that occur between pairs of objects, one needs also to do so at real-time rates. Applications such as haptic force feedback can require over 1000 collision queries per second. We develop and analyze a method, based on bounding-volume hierarchies, for efficient collision detection for objects moving within highly complex environments. Our choice of bounding volume is to use a discrete orientation polytope (k-DOP), a convex polytope whose facets are determined by halfspaces whose outward normals come from a small fixed set of k orientations. We compare a variety of methods for constructing hierarchies (BV-trees) of bounding k-DOPs. Further, we propose algorithms for maintaining an effective BV-tree of k-DOPs for moving objects, as they rotate, and for performing fast collision detection using BV-trees of the moving objects and of the environment. Our algorithms have been implemented and tested. We provide experimental evidence showing that our approach yields substantially faster collision detection than previous methods.

Supramolecular structure of the Salmonella typhimurium type III protein secretion system.

Abstract: The type III secretion system of Salmonella typhimurium directs the translocation of proteins into host cells. Evolutionarily related to the flagellar assembly machinery, this system is also present in other pathogenic bacteria, but its organization is unknown. Electron microscopy revealed supramolecular structures spanning the inner and outer membranes of flagellated and nonflagellated strains; such structures were not detected in strains carrying null mutations in components of the type III apparatus. Isolated structures were found to contain at least three proteins of this secretion system. Thus, the type III apparatus of S. typhimurium, and presumably other bacteria, exists as a supramolecular structure in the bacterial envelope.

Signatures of the Tricritical Point in QCD

Abstract: Several approaches to QCD with two {ital massless} quarks at finite temperature T and baryon chemical potential {mu} suggest the existence of a tricritical point on the boundary of the phase with spontaneously broken chiral symmetry. In QCD with {ital massive} quarks there is then a critical point at the end of a first order transition line. We discuss possible experimental signatures of this point, which provide information about its location and properties. We propose a combination of event-by-event observables, including suppressed fluctuations in T and {mu} and, simultaneously, enhanced fluctuations in the multiplicity of soft pions. {copyright} {ital 1998} {ital The American Physical Society }

Outcomes in patients with acute non-Q-wave myocardial infarction randomly assigned to an invasive as compared with a conservative management strategy. Veterans Affairs Non-Q-Wave Infarction Strategies in Hospital (VANQWISH) Trial Investigators

Abstract: Background Non–Q-wave myocardial infarction is usually managed according to an “invasive” strategy (i.e., one of routine coronary angiography followed by myocardial revascularization). Methods We randomly assigned 920 patients to either “invasive” management (462 patients) or “conservative” management, defined as medical therapy and noninvasive testing, with subsequent invasive management if indicated by the development of spontaneous or inducible ischemia (458 patients), within 72 hours of the onset of a non–Q-wave infarction. Death or nonfatal infarction made up the combined primary end point. Results During an average follow-up of 23 months, 152 events (80 deaths and 72 nonfatal infarctions) occurred in 138 patients who had been randomly assigned to the invasive strategy, and 139 events (59 deaths and 80 nonfatal infarctions) in 123 patients assigned to the conservative strategy (P=0.35). Patients assigned to the invasive strategy had worse clinical outcomes during the first year of follow-up. The numb...

A prokaryotic-like mode of cytoplasmic eukaryotic ribosome binding to the initiation codon during internal translation initation of hepatitis C and classical swine fever virus RNAs

Abstract: Initiation of translation of hepatitis C virus and classical swine fever virus mRNAs results from internal ribosomal entry. We reconstituted internal ribosomal entry in vitro from purified translation components and monitored assembly of 48S ribosomal preinitiation complexes by toe-printing. Ribosomal subunits (40S) formed stable binary complexes on both mRNAs. The complex structure of these RNAs determined the correct positioning of the initiation codon in the ribosomal "P" site in binary complexes. Ribosomal binding and positioning on these mRNAs did not require the initiation factors eIF3, eIF4A, eIF4B, and eIF4F and translation of these mRNAs was not inhibited by a trans-dominant eIF4A mutant. Addition of Met-tRNAiMet, eIF2, and GTP to these binary ribosomal complexes resulted in formation of 48S preinitiation complexes. The striking similarities between this eukaryotic initiation mechanism and the mechanism of translation initiation in prokaryotes are discussed.

On the actions that one nerve cell can have on another: Distinguishing “drivers” from “modulators”

Abstract: When one nerve cell acts on another, its postsynaptic effect can vary greatly. In sensory systems, inputs from “drivers” can be differentiated from those of “modulators.” The driver can be identified as the transmitter of receptive field properties; the modulator can be identified as altering the probability of certain aspects of that transmission. Where receptive fields are not available, the distinction is more difficult and currently is undefined. We use the visual pathways, particularly the thalamic geniculate relay for which much relevant evidence is available, to explore ways in which drivers can be distinguished from modulators. The extent to which the distinction may apply first to other parts of the thalamus and then, possibly, to other parts of the brain is considered. We suggest the following distinctions: Cross-correlograms from driver inputs have sharper peaks than those from modulators; there are likely to be few drivers but many modulators for any one cell; and drivers are likely to act only through ionotropic receptors having a fast postsynaptic effect whereas modulators also are likely to activate metabotropic receptors having a slow and prolonged postsynaptic effect.

Diquark Bose condensates in high density matter and instantons

Abstract: Instantons lead to strong correlations between up and down quarks with spin zero and antisymmetric color wave functions. In cold and dense matter, n{sub b}{gt}n{sub c}{approx_equal}1 fm {sup {minus}3} and T{lt}T{sub c}{approximately}50 thinspthinspMeV, these pairs Bose condense, replacing the usual {l_angle}{bar q}q{r_angle} condensate and restoring chiral symmetry. At high density, the ground state is a color superconductor in which diquarks play the role of Cooper pairs. An interesting toy model is provided by QCD with two colors: it has a particle-antiparticle symmetry which relates {l_angle}{bar q}q{r_angle} and {l_angle}qq{r_angle} condensates. {copyright} {ital 1998} {ital The American Physical Society}

Increased NMDA current and spine density in mice lacking the NMDA receptor subunit NR3A.

Abstract: The NMDA (N -methyl-D-aspartate) subclass of glutamate receptor1 is essential for the synaptic plasticity thought to underlie learning and memory2,3,4 and for synaptic refinement during development5,6. It is currently believed that the NMDA receptor (NMDAR) is a heteromultimeric channel comprising the ubiquitous NR1 subunit and at least one regionally localized NR2 subunit7,8,9,10,11. Here we report the characterization of a regulatory NMDAR subunit, NR3A (formerly termed NMDAR-L or χ-1), which is expressed primarily during brain development12,13. NR3Aco-immunoprecipitates with receptor subunits NR1 and NR2 in cerebrocortical extracts. In single-channel recordings from Xenopus oocytes, addition of NR3A to NR1 and NR2 leads to the appearance of a smaller unitary conductance. Genetic knockout of NR3A in mice results in enhanced NMDA responses and increased dendritic spines in early postnatal cerebrocortical neurons. These data suggest that NR3A is involved in the development of synaptic elements by modulating NMDAR activity.

The functional neuroanatomy of tinnitus : Evidence for limbic system links and neural plasticity

Abstract: We used PET to map brain regions responding to changes in tinnitus loudness in four patients who could alter tinnitus loudness by performing voluntary oral facial movements (OFMs). Cerebral blood flow was measured in four patients and six controls at rest, during the OFM, and during stimulation with pure tones. OFM-induced loudness changes affected the auditory cortex contralateral to the ear in which tinnitus was perceived, whereas unilateral cochlear stimulation caused bilateral effects, suggesting a retrocochlear origin for their tinnitus. Patients, compared with controls, showed evidence for more widespread activation by the tones and aberrant links between the limbic and auditory systems. These abnormal patterns provide evidence for cortical plasticity that may account for tinnitus and associated symptoms. Although audiologic symptoms and examinations of these patients were typical, the unusual ability to modulate tinnitus loudness with an OFM suggests some caution may be warranted in generalizing these findings.

Role of Actin in Anchoring Postsynaptic Receptors in Cultured Hippocampal Neurons: Differential Attachment of NMDA versus AMPA Receptors

Abstract: We used actin-perturbing agents and detergent extraction of primary hippocampal cultures to test directly the role of the actin cytoskeleton in localizing GABAA receptors, AMPA- and NMDA-type glutamate receptors, and potential anchoring proteins at postsynaptic sites. Excitatory postsynaptic sites on dendritic spines contained a high concentration of F-actin that was resistant to cytochalasin D but could be depolymerized using the novel compound latrunculin A. Depolymerization of F-actin led to a 40% decrease in both the number of synaptic NMDA receptor (NMDAR1) clusters and the number of AMPA receptor (GluR1)-labeled spines. The nonsynaptic NMDA receptors appeared to remain clustered and to coalesce in cell bodies. α-Actinin-2, which binds both actin and NMDA receptors, dissociated from the receptor clusters, but PSD-95 remained associated with both the synaptic and nonsynaptic receptor clusters, consistent with a proposed cross-linking function. AMPA receptors behaved differently; on GABAergic neurons, the clusters redistributed to nonsynaptic sites, whereas on pyramidal neurons, many of the clusters appeared to disperse. Furthermore, in control neurons, AMPA receptors were detergent extractable from pyramidal cell spines, whereas AMPA receptors on GABAergic neurons and NMDA receptors were unextractable. GABAA receptors were not dependent on F-actin for the maintenance or synaptic localization of clusters. These results indicate fundamental differences in the mechanisms of receptor anchoring at postsynaptic sites, both regarding the anchoring of a single receptor (the AMPA receptor) in pyramidal cells versus GABAergic interneurons and regarding the anchoring of different receptors (AMPA vs NMDA receptors) at a single class of postsynaptic sites on pyramidal cell dendritic spines.

Tumor Necrosis Factor-α in Sera of Obese Patients: Fall with Weight Loss

Abstract: In view of the recent demonstration that obesity in animals and humans is associated with an increase in tumor necrosis factor-alpha (TNFalpha) expression, that this expression falls with weight loss, and that TNFalpha may specifically inhibit insulin action, the possibility that TNFalpha may be a mediator of insulin resistance has been raised. We have undertaken this study to investigate whether serum TNFalpha concentrations are elevated in obese subjects, whether they fall after weight loss, and whether this fall parallels the fall in insulin release after glucose challenge. Obese patients (age range: 25-54, weight mean +/- SD: 96.4 +/- 13.8 kg, body mass index: 35.7 +/- 5.6 kg/m2) were started on a diet program. The mean weight fell to 84.5 +/- 11.3 (P < 0.0001) and body mass index to 31.3 +/- 4.9 (P < 0.0001). Plasma TNFalpha concentrations were markedly elevated in the obese (3.45 +/- 0.16 pg/mL), when compared with controls (0.72 +/- 0.28 pg/mL), and fell significantly (2.63 +/- 1.40 pg/mL) after weight loss (P < 0.02). The magnitude of insulin release after glucose (75 g) challenge (area under the curve) also fell significantly (P < 0.01) after weight loss. The magnitude of weight loss and fall in TNFalpha were related to basal body weight (r = 0.57, P < 0.001) and basal TNFalpha (r = 0.55, P < 0.001) concentrations, respectively, but not to each other or to the glucose-induced insulin release (area under the curve). We conclude that obesity is associated with increased plasma TNFalpha concentrations, which fall with weight loss. Because circulating TNFalpha may mediate insulin resistance in the obese, a fall in TNFalpha concentrations may contribute to the restoration of insulin resistance after weight loss, Thus, TNFalpha may be an important circulating cytokine, which may provide a potentially reversible mechanism for mediating insulin resistance.

Product Placement in Movies: The Effect of Prominence and Mode on Audience Recall

Abstract: A promotional strategy of growing interest is the placement of branded products in movies. An experiment compared the recall effectiveness of common product-placement strategies with each other and with advertising. Prominent placements elicited higher recall than did advertisements, which, in turn, outperformed subtle placements. The explicit mention of a product in the audio script (without a visual depiction) led to better recall than a subtle visual placement (without audio reinforcement). However, the addition of a complementary audio message did not significantly enhance the recall of a product that already enjoyed prominent visual display.

The Differentiation of Self Inventory: Development and Initial Validation

Abstract: Despite the importance of Bowen theory (M.Bowen, 1976,1978; M. E. KerrB Nichols & Schwartz, 1998). Indeed, Bowen theory provides a foundation for the field of family therapy that renders i t distinct from the multitude of theoretica l approaches to individual psychotherapy. At present, many of Bowen's (1976, 1978) concepts (e.g., differentiation of self, interlocking triangles , o r reactiv e emotiona l distancing ) pervade th e famil y system s literature . Despit e th e vas t attention Bowen theory has received from clinicians and theorists alike, there have been, to date, few programmatic attempts t o tes t it s validit y wit h respec t t o personalit y functioning or quality of interpersonal relations or to changes as a result of psychotherapy. To begin filling this notable gap, we developed the Differentiation o f Self Inventory, a self-report instrument for adults (ages 25+). In this article, we present psychometric support for the measure, validation studies to date, and implications for theory, research, and practice.

Eukaryotic ribosomes require initiation factors 1 and 1A to locate initiation codons

Abstract: The scanning model of translation initiation is a coherent description of how eukaryotic ribosomes reach the initiation codon after being recruited to the capped 5' end of messenger RNA. Five eukaryotic initiation factors (eIF 2, 3, 4A, 4B and 4F) with established functions have been assumed to be sufficient to mediate this process. Here we report that eIF1 and eIF1A are also both essential for translation initiation. In their absence, 43S ribosomal preinitiation complexes incubated with ATP, eIF4A, eIF4B and eIF4F bind exclusively to the cap-proximal region but are unable to reach the initiation codon. Individually, eIF1A enhances formation of this cap-proximal complex, and eIF1 weakly promotes formation of a 48S ribosomal complex at the initiation codon. These proteins act synergistically to mediate assembly of ribosomal initiation complexes at the initiation codon and dissociate aberrant complexes from the mRNA.

Psychiatric Illness and Family Stigma

Abstract: Considerable research has documented the stigmatization of people with mental illnesses and its negative consequences. Recently it has been shown that stigma may also seriously affect families of psychiatric patients, but little empirical research has addressed this problem. We examine perceptions of and reactions to stigma among 156 parents and spouses of a population-based sample of first-admission psychiatric patients. While most family members did not perceive themselves as being avoided by others because of their relative's hospitalization, half reported concealing the hospitalization at least to some degree. Both the characteristics of the mental illness (the stigmatizing mark) and the social characteristics of the family were significantly related to levels of family stigma. Family members were more likely to conceal the mental illness if they did not live with their ill relative, if the relative was female, and if the relative had less severe positive symptoms. Family members with more education and whose relative had experienced an episode of illness within the past 6 months reported greater avoidance by others.

Orthogonal Column Latin Hypercubes and Their Application in Computer Experiments

Abstract: Latin hypercubes have been frequently used in conducting computer experiments. In this paper, a class of orthogonal Latin hypercubes that preserves orthogonality among columns is proposed. Applying an orthogonal Latin hypercube design to a computer experiment benefits the data analysis in two ways. First, it retains the orthogonality of traditional experimental designs. The estimates of linear effects of all factors are uncorrelated not only with each other, but also with the estimates of all quadratic effects and bilinear interactions. Second, it can facilitate nonparametric fitting procedures, because one can select good space-filling designs within the class of orthogonal Latin hypercubes according to selection criteria.

On applications of geometric morphometrics to studies of ontogeny and phylogeny.

Abstract: In press − − do not cite 2 The field of geometric morphometrics is relatively new (see Bookstein, 1991, Rohlf and Marcus, 1993) and has shown very rapid progress over the last few years. As might be expected during a period of rapid development, there can be technical problems in some of the pioneering studies as biologists attempt to apply the new tools. It is only now becoming clear how the new techniques should be combined in order to carry out comprehensive analyses of real datasets. Bookstein (1996b) gave a list of recommendations for such applications and Bookstein (1996a) gave several comprehensive examples of morphometric analyses. However, these accounts do not address some of the types of applications that are of particular interest in systematic biology. Bookstein (1994) pointed out problems with using geometric morphometric methods in the usual character-based cladistic studies. He emphasized that morphometrics cannot supply homologous shape characters. The purpose of this note is to comment some recent applications of morphometric methods in systematic biology. This note is particularly concerned with the morphometric methods used by Zelditch et al. These methods were also used by Burke et al. (1996). For convenience, these studies will be referred to using the acronym Z&F. In these studies the authors investigated the use of partial warps (Bookstein, 1991) as variables in ontogenetic and taxonomic studies. They were impressed by their observation that differences in partial warps scores (Rohlf, 1993b) corresponded to shape differences that could be localized on the bodies of the organisms. They also found differences in these variables between developmental stages and between species. They concluded that partial warps could be interpreted and used as traditional taxonomic characters and would be useful in evolutionary studies. Lynch et al. (1996) tried to interpret the partial warps they obtained in their study but they were cautious about using them in the ways advocated by Z&F. They suggested that extensive simulation studies needed to be done to validate the Z&F approach. Naylor (1996) investigated their approach using data based on a simulated phylogeny. Even though his simulated phylogeny was based on a sequence of simple morphological changes and had no homoplasy, the results showed high levels of homoplasy and did not recover the morphological changes used to create the phylogeny. The present paper is concerned with theoretical problems rather than empirical problems requiring validation through simulations. The fundamental problem is that Z&F …

Protein-primed RNA synthesis by purified poliovirus RNA polymerase

Abstract: A small protein, VPg, is covalently linked to the 5′ end of the plus-stranded poliovirus genomic RNA1,2,3. Poliovirus messenger RNA, identical in nucleotide sequence to genomic RNA, is not capped at its 5′ end by the methylated structure that is common to most eukaryotic mRNAs. These discoveries presented two problems. First, as cap structures are usually required for translation of mRNA into protein, how does this uncapped viral RNA act as a template for translation? Second, what is the function of VPg? The identification of the internal ribosomal-entry site, which allows the entry of ribosomes into viral mRNA independently of the 5′ mRNA end, has solved the first conundrum4,5,6. Here we describe the resolution of the second problem. VPg is linked to the genomic RNA through the 5′-terminal uridylic acid of the RNA. We show that VPg can be uridylylated by the poliovirus RNA polymerase 3Dpol. Uridylylated VPg can then prime the transcription of polyadenylate RNA by 3Dpol to produce VPg-linked poly(U). Initiation of transcription of the poliovirus genome from the polyadenylated 3′ end therefore depends on VPg.

EEG-based communication: improved accuracy by response verification

Abstract: Humans can learn to control the amplitude of electroencephalographic (EEG) activity in specific frequency bands over sensorimotor cortex and use it to move a cursor to a target on a computer screen. EEG-based communication could provide a new augmentative communication channel for individuals with motor disabilities. In the present system, each dimension of cursor movement is controlled by a linear equation. While the intercept in the equation is continually updated, it does not perfectly eliminate the impact of spontaneous variations in EEG amplitude. This imperfection reduces the accuracy of cursor movement. The authors evaluated a response verification (RV) procedure in which each outcome is determined by two opposite trials (e.g., one top-target trial and one bottom-target trial). Success, or failure, on both is required for a definitive outcome. The RV procedure reduces errors due to imperfection in intercept selection. Accuracy for opposite-trial pairs exceeds that predicted from the accuracies of individual trials, and greatly exceeds that for same-trial pairs. The RV procedure should be particularly valuable when the first trial has >2 possible targets, because the second trial need only confirm or deny the outcome of the first, and it should be applicable to nonlinear as well as to linear algorithms.

Patterned Protein Layers on Solid Substrates by Thin Stamp Microcontact Printing

Abstract: Microcontact printing (μCP) is a new method of molecularly patterning surfaces on a micrometer scale. In this paper, we present the extension of microcontact printing to producing patterned layers of proteins on solid substrates. μCP avoids the use of strong acids and bases necessary in photolithographic patterning, allowing its use for patterning of proteins and other biological layers. We also describe the methods of thin stamp microcontact printing that allow printing of isolated features previously unattainable by microcontact printing. A solution of polylysine in borate-buffered saline was printed onto a glass coverslip, yielding micrometer scale features over an area of 4 cm2.

Comparison of finite-difference transport and diffusion calculations for photon migration in homogeneous and heterogeneous tissues

Abstract: We analyse the limits of the diffusion approximation to the time-independent equation of radiative transfer for homogeneous and heterogeneous biological media. Analytical calculations and finite-difference simulations based on diffusion theory are compared with discrete-ordinate, finite-difference transport calculations. The influence of the ratio of absorption and transport scattering coefficient (mu(a)/mu'(s)) on the accuracy of the diffusion approximation are quantified and different definitions for the diffusion coefficient, D, are discussed. We also address effects caused by void-like heterogeneities in which absorption and scattering are very small compared with the surrounding medium. Based on results for simple homogeneous and heterogeneous systems, we analyse diffusion and transport calculation of light propagation in the human brain. For these simulations we convert density maps obtained from magnetic resonance imaging (MRI) to optical-parameter maps (mu(a) and mu'(s)) of the brain. We show that diffusion theory fails to describe accurately light propagation in highly absorbing regions, such as haematoma, and void-like spaces, such as the ventricles and the subarachnoid space.