Positive Energy Functional for Massless Scalars in Rotating Black Hole Backgrounds of Maximal Ungauged Supergravity.

We outline a proof of the stability of a massless neutral scalar field ψ in the background of a wide class of four dimensional asymptotically flat rotating and "electrically charged" solutions of supergravity, and the low energy limit of string theory, known as STU metrics. Despite their complexity, we find it possible to circumvent the difficulties presented by the existence of ergo regions and the related phenomenon of superradiance in the original metrics by following a strategy due to Whiting, and passing to an auxiliary metric admitting an everywhere lightlike Killing field and constructing a scalar field ψ (related to a possible unstable mode ψ by a nonlocal transformation) which satisfies the massless wave equation with respect to the auxiliary metric. By contrast with the case for ψ, the associated energy density of ψ is not only conserved but is also non-negative.

Assessment of technical and biological parameters of volumetric quantitative computed tomography of the foot: a phantom study.

SUMMARY: Few studies exist for bone densitometry of the whole foot. A phantom study demonstrated the sources of error and necessary controls for accurate quantitative computed tomography of the foot. A loss in bone mineral density (BMD) in the small foot bones may be an early indicator of diabetic foot complications. INTRODUCTION: Volumetric quantitative computed tomography (vQCT) facilitates the assessment of pedal bone osteopenia, which, in the presence of peripheral neuropathy, may well be an early sign of diabetic foot deformity. To date, sources and magnitudes of error in foot vQCT measurements have not been reported. METHODS: Foot phantoms were scanned using a 64-slice CT scanner. Energy (in kilovoltage peak), table height, phantom size and orientation, location of "bone" inserts, insert material, location of calibration phantom, and reconstruction kernel were systematically varied during scan acquisition. RESULTS: Energy (in kilovoltage peak) and distance from the isocenter (table height) resulted in relative attenuation changes from -5% to 22% and -5% to 0%, respectively, and average BMD changes from -0.9% to 0.0% and -1.1% to 0.3%, respectively, compared to a baseline 120-kVp scan performed at the isocenter. BMD compared to manufacturer-specified values ranged, on average, from -2.2% to 0.9%. Phantom size and location of bone-equivalent material inserts resulted in relative attenuation changes of -1.2% to 1.4% compared to the medium-sized phantom. CONCLUSION: This study demonstrated that variations in kilovoltage peak and table height can be controlled using a calibration phantom scanned at the same energy and height as a foot phantom; however, error due to soft tissue thickness and location of bones within a foot cannot be controlled using a calibration phantom alone.

SU-E-I-23: Effective Estimation of Focal Spot Isocenter for Sinograms Obtained with Flying Focal Spot Scans for Filtered Back Projection.

PURPOSE: To validate the determination of parameters required to preprocess sinograms obtained by fan beam CT with flying focal spot (FFS) scans, such that a combined data set could be used with existing filtered back projection (FBP) reconstruction software based on single focal spot (SFS) geometry. METHODS: The preprocessing step requires determination of the isocenter for each FFS in the target reconstruction geometry. A Philips Brilliance Big Bore CT was used to obtain FFS sinograms, which double samples the number of gantry positions relative to SFS sinograms, having the same number of detector measurements for each gantry position. Mass weighting of sinograms of a Teflon rod scan and curve fitting were used to determine isocenters for focal spots. FFS sinograms were reformatted before FBP such that each pair of adjacent gantry-step measurements were combined by interleaving detector readings. The interleaved measurements were assigned to the average gantry angle. Image transition sharpness was compared for 33 images of the Teflon rod reconstructed with 1/8 focal spot deflection difference to test the isocenter Result. SFS and FFS Sinograms of a high- resolution bar pattern phantom were reconstructed by an in-house reconstruction code package for resolution comparison. In-house reconstructed images of the Teflon rod were compared with scanner images for verifying the effectiveness of this sinogram reformatting method. RESULTS: Image profile was the sharpest when the Teflon rod sinogram was reconstructed with isocenter parameters found by the curve fitting experiment. It was comparable to the sharpness of scanner image profile when the in-house image was reconstructed with resolution of 1.0mm FWHM. CONCLUSIONS: The good clinical resolution of images reconstructed with investigated FFS isocenters suggests that the experiment of estimating focal spot centers and reformatting sinograms is effective for utilizing reconstruction software that does not support FFS geometry. Supported by grant R01 CA149305 and a grant from Varian Medical Systems.

Near-field radiative heat transfer between macroscopic planar surfaces.

Near-field radiation allows heat to propagate across a small vacuum gap at rates several orders of magnitude above that of far-field, blackbody radiation. Although heat transfer via near-field effects has been discussed for many years, experimental verification of this theory has been very limited. We have measured the heat transfer between two macroscopic sapphire plates, finding an increase in agreement with expectations from theory. These experiments, conducted near 300 K, have measured the heat transfer as a function of separation over mm to µm and as a function of temperature differences between 2.5 and 30 K. The experiments demonstrate that evanescence can be put to work to transfer heat from an object without actually touching it.

Cochlear nucleus neurons redistribute synaptic AMPA and glycine receptors in response to monaural conductive hearing loss.

Neurons restore their function in response to external or internal perturbations and maintain neuronal or network stability through a homeostatic scaling mechanism. Homeostatic responses at synapses along the auditory system would be important for adaptation to normal and abnormal fluctuations in the sensory environment. We investigated at the electron microscopic level and after postembedding immunogold labeling whether projection neurons in the cochlear nucleus responded to modifications of auditory nerve activity. After unilaterally reducing the level of auditory inputs by approximately 20 dB by monaural earplugging, auditory nerve synapses on bushy cells somata and basal dendrites of fusiform cells of the ventral and dorsal cochlear nucleus, respectively, upregulated GluR3 AMPA receptor subunit, while inhibitory synapses decreased the expression of GlyRalpha1 subunit. These changes in expression levels were fully reversible once the earplug was removed, indicating that activity affects the trafficking of receptors at synapses. Excitatory synapses on apical dendrites of fusiform cells (parallel fibers) with different synaptic AMPA receptor subunit composition, were not affected by sound attenuation, as the expression levels of AMPA receptor subunits were the same as in normal hearing littermates. GlyRalpha1 subunit expression at inhibitory synapses on apical dendrites of fusiform cells was also found unaffected. Furthermore, fusiform and bushy cells of the contralateral side to the earplugging upregulated the GluR3 subunit at auditory nerve synapses. These results show that cochlear nucleus neurons innervated by the auditory nerve, are able to respond to small changes in sound levels by redistributing specific AMPA and glycine receptor subunits.

Phase effects in the diffraction of light: beyond the grating equation.

Diffraction gratings affect the absolute phase of light in a way that is not obvious from the usual derivation of optical paths using the grating equation. For example, consider light which encounters first one and then the second of two parallel gratings. If one grating is moved parallel to its surface, the phase of the light diffracted from the grating pair is shifted by 2pi each time the grating is moved by one grating constant, even though the geometric path length is not altered by the motion. This additional phase shift must be included when incorporating diffraction gratings in interferometers.

The evolution of the cortico-cerebellar complex in primates: anatomical connections predict patterns of correlated evolution.

Investigations into the evolution of the primate brain have tended to neglect the role of connectivity in determining which brain structures have changed in size, focusing instead on changes in the size of the whole brain or of individual brain structures, such as the neocortex, in isolation. We show that the primate cerebellum, neocortex, vestibular nuclei and relays between them exhibit correlated volumetric evolution, even after removing the effects of change in other structures. The patterns of correlated evolution among individual nuclei correspond to their known patterns of connectivity. These results support the idea that the brain evolved by mosaic size change in arrays of functionally connected structures. Furthermore, they suggest that the much discussed expansion of the primate neocortex should be re-evaluated in the light of conjoint cerebellar expansion.

Cochlear implants: three-dimensional localization by means of coregistration of CT and conventional radiographs.

With use of radiopaque implanted objects as internal fiducial markers, the authors developed and evaluated a technique for coregistering computed tomographic (CT) and computed radiographic images to help determine three-dimensional location information for implant electrodes in the cochlea in phantoms and patients. Three-dimensional positional data from CT were assigned on a radiograph, which permitted identification of individual cochlear electrode locations that were not depicted at CT.

Deblurring subject to nonnegativity constraints when known functions are present with application to object-constrained computerized tomography.

The reconstruction of tomographic images is often treated as a linear deblurring problem. When a high-density, man-made metal object is present somewhere in the image field, it is a deblurring problem in which the unknown function has a component that is known except for some location and orientation parameters. We first address general linear deblurring problems in which a known function having unknown parameters is present. We then show how the resulting iterative solution can be applied to tomographic imaging in the presence of man-made foreign objects, and we apply the result, in particular, to X-ray computed tomography imaging used in support of brachytherapy treatment of advanced cervical cancer.

CT data storage reduction by means of compressing projection data instead of images: feasibility study.

The authors developed and evaluated a technique of compressing raw projection data at computed tomography (CT). Raw projection data acquired at CT were compressed and decompressed and then used for image reconstruction. For comparison, original images were compressed by comparable ratios. Projection data files were more compressible than image files. Projection data compression is a promising, efficient method to reduce data file size and thus to facilitate retrospective image reconstruction.

Influence of image acquisition parameters on CT artifacts and polyp depiction in spiral CT colonography: in vitro evaluation.

PURPOSE: To quantify the effects of spiral computed tomographic (CT) acquisition parameters on the magnitude of three-dimensional (3D) rippling artifacts and polyp depiction. MATERIALS AND METHODS: An in vitro colon phantom was constructed with air-filled acrylic cylinders that contained synthetic polyps of 3-13 mm. The phantom was submerged in fluid and positioned at four angles of inclination relative to the z axis. Image data were acquired at collimation and pitch combinations of 3 mm and 1.67 and 5 mm and 1.6, respectively. Rippling artifacts were quantified by measuring the longitudinal variation of in-plane phantom edge width, and the influence of these artifacts on the depiction of pedunculated and sessile polyps was assessed qualitatively. RESULTS: The in-plane magnitude of the rippling artifact was a function of the angle of inclination relative to the longitudinal axis and the table increment. The through-plane periodicity of the artifact was equal to one-half the table increment. CONCLUSION: The table increment and angle of inclination of the surface of the object relative to the z axis determine the periodicity and magnitude of the rippling artifact at 3D spiral CT colonography. Although the depiction of small pedunculated polyps was not compromised, some sessile polyps were degraded by the artifact.

Assessment of visually lossless irreversible image compression: comparison of three methods by using an image-comparison workstation.

PURPOSE: To determine the degree of irreversible image compression detectable in conservative viewing conditions. MATERIALS AND METHODS: An image-comparison workstation, which alternately displayed two registered and magnified versions of an image, was used to study observer detection of image degradation introduced by irreversible compression. Five observers evaluated 20 16-bit posteroanterior digital chest radiographs compressed with Joint Photographic Experts Group (JPEG) or wavelet-based trellis-coded quantization (WTCQ) algorithms at compression ratios of 8:1-128:1 and x2 magnification by using (a) traditional two-alternative forced choice; (b) original-revealed two-alternative forced choice, in which the noncompressed image is identified to the observer; and (c) a resolution-metric method of matching test images to degraded reference images. RESULTS: The visually lossless threshold was between 8:1 and 16:1 for four observers. JPEG compression resulted in performance as good as that with WTCQ compression at these ratios. The original-revealed forced-choice method was faster and as sensitive as the two-alternative forced-choice method. The resolution-metric results were robust and provided information on performance above visually lossless levels. CONCLUSION: The image-comparison workstation is a versatile tool for comparative assessment of image quality. At x2 magnification, images compressed with either JPEG or WTCQ algorithms were indistinguishable from unaltered original images for most observers at compression ratios between 8:1 and 16:1, indicating that 10:1 compression is acceptable for primary image interpretation.

Dental photostimulable phosphor radiography.

Photostimulable phosphor radiography (PPR), a mature industry in medical imaging, became available in dentistry in 1994. At least three dental PPR systems have been made available commercially since that time. Although dental PPR offers a number of advantages over film-based methods, some aspects of dental PPR need improvement. Further research is needed to more fully compare and contrast this modality with film-based methods.

X-ray CT metal artifact reduction using wavelets: an application for imaging total hip prostheses.

Traditional computed tomography (CT) reconstructions of total joint prostheses are limited by metal artifacts from corrupted projection data. Published metal artifact reduction methods are based on the assumption that severe attenuation of X-rays by prostheses renders corresponding portions of projection data unavailable, hence the "missing" data are either avoided (in iterative reconstruction) or interpolated (in filtered backprojection with data completion; typically, with filling data "gaps" via linear functions). In this paper, we propose a wavelet-based multiresolution analysis method for metal artifact reduction, in which information is extracted from corrupted projection data. The wavelet method improves image quality by a successive interpolation in the wavelet domain. Theoretical analysis and experimental results demonstrate that the metal artifacts due to both photon starving and beam hardening can be effectively suppressed using our method. As compared to the filtered backprojection after linear interpolation, the wavelet-based reconstruction is significantly more accurate for depiction of anatomical structures, especially in the immediate neighborhood of the prostheses. This superior imaging precision is highly advantageous in geometric modeling for fitting hip prostheses.

Health surveillance of Glasgow medical undergraduates pursuing elective studies abroad (1992-1998).

BACKGROUND: An integral part of the training for many UK medical undergraduates involves a period of elective study abroad. There is concern about the health risks this poses to the students, and uncertainty regarding the responsibility this places on medical schools. METHODS: Annually since 1992, medical undergraduates at Glasgow University have been asked to complete and return a confidential questionnaire on return from their elective studies. This records personal demographic details, the countries visited, and information about illnesses experienced. Analyses were conducted on the students' health experiences, lifestyle, the health precautions taken, and the climates experienced. RESULTS: Global statistics were compiled on 750 respondents. A subset of 267 completed a more extensive, post-1996, questionnaire enabling detailed study of comparative illness rates. A majority took pretravel health advice, visited only one country, stayed for 1 to 2 months, and experienced a tropical climate. Forty-five percent reported symptoms of illness, and alimentary symptoms predominated (77% of those ill). Higher illness rates were reported in those who experienced a hot desert or tropical climate compared with those who did not. There was correlation between taking professional pretravel health advice and exposure to a more hazardous climate. CONCLUSIONS: The attack rate for medical students on electives compares favorably to that for package holidaymakers; similarly the attack rate for students staying in the tropics compared with other travelers. A preexisting health problem did not predispose to a higher attack rate. Attack rates can be minimized by avoiding climatically extreme locations. This surveillance provides a focus of interest to the students, insight on minimizing avoidable health problems, evidences social responsibility by the Medical Faculty, and has the potential for expansion to other medical schools. Current Scottish medical school policies on HIV risk management would be strengthened by a more coordinated approach.

The effect of cimetidine on the pharmacokinetics of epirubicin in patients with advanced breast cancer: preliminary evidence of a potentially common drug interaction.

Epirubicin is known to be metabolized in the liver. Therefore, drugs such as cimetidine, which inhibit the cytochrome P-450 enzyme system or reduce liver blood flow, may reduce the plasma clearance of epirubicin. In a small study, epirubicin 100 mg/m2 every 3 weeks was administered intravenously to eight patients, who also received oral cimetidine (400 mg b.d. for 7 days starting 5 days before chemotherapy) with either the first or second cycles. Epirubicin pharmacokinetics and liver blood flow (idocyanine green clearance) were assessed at each course. The areas under the plasma concentration time curves (AUCs) were used to compare the systemic exposure to epirubicin and its metabolites with each course. The estimated median percentage increase (95% confidence interval CI) in the AUC with cimetidine were: epirubicin 50% (95% CI -18 to 193, epirubicinol 41% (95% CI 1 to 92). Despite the small numbers studied, the increase in the active metabolite epirubicinol was significant (P < 0.05). These changes in exposure were not explained by reduced cytochrome P-450 activity as the 7-deoxy-doxorubicinol aglycone AUC was not reduced (357% increase: 95% CI 17 to 719) or by a decrease in liver blood flow (17% increase: 95% CI -39 to 104). Cimetidine is likely to be coprescribed or self-administered with epirubicin and therefore clinicians should be aware of this potential interaction.

Understanding the relative sensitivity of radiographic screens to scattered radiation.

This study compared the relative response of various screen-film and computed radiography (CR) systems to diagnostic radiation exposure. An analytic model was developed to calculate the total energy deposition within the depth of screen and the readout signal generated from this energy for the x-ray detection system. The model was used to predict the relative sensitivity of several screen-film and CR systems to scattered radiation as a function of various parameters, such as x-ray spectra, phantom thickness, phosphor composition, screen thickness, screen configuration (single front screen, single back screen, screen pair), and readout conditions. In addition, measurements of the scatter degradation factor (SDF) for different screen systems by using the beam stop technique with water phantoms were made to verify the model results. Theoretically calculated values of SDF were in good agreement with experimental data. These results are consistent with the common observation that rare-earth screens generally produce better image quality than calcium tungstate screens and the CR screen.

Analysis of animal pharmacokinetic data: performance of the one point per animal design.

A simulation study was carried out to determine the impact of various design factors on the accuracy and precision with which population pharmacokinetic parameters are estimated in preclinical pharmacokinetic studies. A drug given by intravenous bolus injection and having mono-exponential disposition characteristics was assumed. The factors investigated were (i) number of animals sampled at specified times with one observation taken per animal, (ii) error in observed concentration measurements, and (iii) doubling the number of observations per animal while varying the number of animals. Data were analyzed with the NONMEM program, and the least number of animals per time point (where each animal supplied one concentration-time point) required for accurate and precise parameter estimation was determined. The one observation per animal design yielded biased and imprecise estimates of variability, and residual variability could not be estimated. Increasing the error in the concentration measurement led to a significant deterioration in the accuracy and precision with which variability was estimated. Obtaining a second sample from each animal practically eliminated bias and facilitated the partitioning of interanimal variability and residual intraanimal variability, by introducing information about the latter. Doubling the total number of observations per animal required using half (i.e., 50) the total number of animals required for accurate and precise parameter estimation with the one sample per animal design.

Experimental design and efficient parameter estimation in preclinical pharmacokinetic studies.

Monte Carlo simulation technique used to evaluate the effect of the arrangement of concentrations on the efficiency of estimation of population pharmacokinetic parameters in the preclinical setting is described. Although the simulations were restricted to the one compartment model with intravenous bolus input, they provide the basis of discussing some structural aspects involved in designing a destructive ("quantic") preclinical population pharmacokinetic study with a fixed sample size as is usually the case in such studies. The efficiency of parameter estimation obtained with sampling strategies based on the three and four time point designs were evaluated in terms of the percent prediction error, design number, individual and joint confidence intervals coverage for parameter estimates approaches, and correlation analysis. The data sets contained random terms for both inter- and residual intra-animal variability. The results showed that the typical population parameter estimates for clearance and volume were efficiently (accurately and precisely) estimated for both designs, while interanimal variability (the only random effect parameter that could be estimated) was inefficiently (inaccurately and imprecisely) estimated with most sampling schedules of the two designs. The exact location of the third and fourth time point for the three and four time point designs, respectively, was not critical to the efficiency of overall estimation of all population parameters of the model. However, some individual population pharmacokinetic parameters were sensitive to the location of these times.

Evaluation of a Bayesian approach to the pharmacokinetic interpretation of cyclosporin concentrations in renal allograft recipients.

The utility of a Bayesian parameter estimation program in the interpretation of cyclosporin concentrations was investigated in a group of 32 patients following renal transplantation. The program was evaluated by comparing concentrations predicted from individual estimates of pharmacokinetic parameters with measured concentrations. A one-compartment model incorporating an exponential, time-related change in clearance was used for data collected in the first 4 weeks after transplantation, and predictions of concentrations measured during weeks 5-14 were made using three schemes: a changing clearance model using all data from week 1 onward; a changing clearance model using data from week 4 onward; and a nonchanging clearance model using data from week 4 onward. Results demonstrated that predictions made by the Bayesian program were unreliable during the first 4 weeks of therapy, but that there was a progressive improvement as time after transplantation increased. The changing clearance model was superior to the constant clearance model and its performance was not compromised by including data from the first 4 weeks of therapy. Although the Bayesian approach may help with the interpretation of blood cyclosporin concentrations during maintenance therapy, the large variability in the pharmacokinetics of orally administered cyclosporin limits the usefulness of the approach in the early weeks following transplantation.