Scatter Measurements Research Articles

Simultaneous scatter rejection and correction method using 2D antiscatter grids for CBCT.

While two-dimensional antiscatter grids (2D grid) reduce scatter intensity substantially in Cone Beam Computed Tomography (CBCT), a small fraction of scattered radiation is transmitted through the 2D grid to the detector. Residual scatter limits the accuracy of CT numbers and interferes with the correction of grid's septal shadows, or footprint, in projections. If grid's septal shadows are not adequately suppressed in projections, it will lead to ring artifacts in CBCT images. In this work, we present a new method to correct residual scatter transmitted through the grid by employing the 2D grid itself as a residual scatter measurement device. Our method, referred as grid-based scatter sampling (GSS), exploits the spatial modulation of primary x-ray fluence by 2D grid's septal shadows. The shape of the signal modulation pattern varies as a function of residual scatter intensity registered by detector pixels. Such a variation in signal pattern was employed to measure residual scatter intensity in each projection, and subsequently, residual scatter was subtracted from each projection. To validate the GSS method, CBCT imaging experiments were conducted using a 2D antiscatter grid prototype in a linac mounted CBCT system. The effect of GSS method on the ring artifact reduction was quantified by measuring noise in CBCT images. In addition, the nonuniformity of Hounsfield Units (HU) and HU accuracy was measured in both head and pelvis-sized phantoms. In qualitative evaluations, GSS method successfully reduced ring artifacts caused by 2D grid's footprint. Image noise reduced by 23% due to reduction of ring artifacts. HU nonuniformity in water-equivalent sections was reduced from 20 HU to 10 HU, and streak artifacts between high density inserts were reduced. The phantom size dependent variations in HU was also reduced after application of GSS method. Without GSS method, HU of density inserts reduced by 9% on the average when phantom size was increased from head to pelvis. With GSS method, HU values reduced only by 5% under the same conditions. In summary, GSS method complements the 2D grid's scatter suppression performance, by correcting the scatter transmitted through the grid. This approach does not require additional scatter-measurement hardware, such as beam-stop arrays, since the grid itself is employed as the scatter measurement device. By suppressing residual scatter in projections, our proposed method successfully reduced artifacts caused by 2D grid's footprint, and further improved CT number accuracy.

Non-parametric machine learning methods for interpolation of spatially varying non-stationary and non-Gaussian geotechnical properties

Spatial interpolation has been frequently encountered in earth sciences and engineering. A reasonable appraisal of subsurface heterogeneity plays a significant role in planning, risk assessment and decision making for geotechnical practice. Geostatistics is commonly used to interpolate spatially varying properties at un-sampled locations from scatter measurements. However, successful application of classic geostatistical models requires prior characterization of spatial auto-correlation structures, which poses a great challenge for unexperienced engineers, particularly when only limited measurements are available. Data-driven machine learning methods, such as radial basis function network (RBFN), require minimal human intervention and provide effective alternatives for spatial interpolation of non-stationary and non-Gaussian data, particularly when measurements are sparse. Conventional RBFN, however, is direction independent (i.e. isotropic) and cannot quantify prediction uncertainty in spatial interpolation. In this study, an ensemble RBFN method is proposed that not only allows geotechnical anisotropy to be properly incorporated, but also quantifies uncertainty in spatial interpolation. The proposed method is illustrated using numerical examples of cone penetration test (CPT) data, which involve interpolation of a 2D CPT cross-section from limited continuous 1D CPT soundings in the vertical direction. In addition, a comparative study is performed to benchmark the proposed ensemble RBFN with two other non-parametric data-driven approaches, namely, Multiple Point Statistics (MPS) and Bayesian Compressive Sensing (BCS). The results reveal that the proposed ensemble RBFN provides a better estimation of spatial patterns and associated prediction uncertainty at un-sampled locations when a reasonable amount of data is available as input. Moreover, the prediction accuracy of all the three methods improves as the number of measurements increases, and vice versa. It is also found that BCS prediction is less sensitive to the number of measurement data and outperforms RBFN and MPS when only limited point observations are available.

Intraocular scattering as a predictor of driving performance in older adults with cataracts.

Cataracts can limit a person’s ability to perform vision-dependent tasks safely, affecting the quality of life of older people. This study examines the relationship between visual function and driving, by studying which visual parameters might be important for predicting driving performance in older drivers with and without cataracts, ascertaining whether the objective measurement of intraocular scattering should be considered in assessment procedures for older drivers. This cross-sectional study involved a total of 20 older drivers (10 patients with bilateral cataracts and 10 control subjects). All participants were examined for visual acuity, contrast sensitivity, visual discrimination capacity, and intraocular scattering. Driving performance was also tested using a driving simulator. To study the relationship between visual parameters and driving performance, a correlation analysis and regression model were used. Drivers with cataracts showed a significantly impaired (p<0.05) visual function, with an Objective Scattering Index (OSI) 3.5 times greater than the control group. Driving performance was also significantly worse (p<0.05) in drivers with cataracts, reflected by a notable deterioration in lane keeping. The correlation analysis showed significant associations between driving performance and all the visual parameters studied. Finally, the regression model revealed that the OSI was the best predictor of driving performance, accounting for 51.3% of its variance. Visual function and driving performance are markedly deteriorated when cataracts are present. Our results demonstrate that the objective scattering index (OSI) has a high predictive power when it comes to simulated driving performance in older drivers, both with and without cataracts, suggesting that scatter measurements could be important in helping better understand visual limitations in older drivers.

Microwave Modulated Scatter Active Array Distortion Sidelobes

A signal processing antenna consisting of an array of modulated scatter elements is described. Time-modulation of array elements for radiation pattern and sidelobe control was initially described more than a half-century ago. Recent decades have produced many advances and extensive documentation for both transmit and receive arrays with time modulated elements. The new article presented here includes both receive and transmit functions in an array of modulated scattering elements. This new approach to time-modulated array experiments greatly simplifies both the physical array elements and the measurement apparatus. An incoming continuous wave (CW) signal in space arrives at the array, is time-modulated by the elements, and then re-radiated in space. The only electrical connections to the individual array elements are low-frequency time-modulation signals. The measurement apparatus consists of a spectrally pure and frequency stable CW source, an audio sine wave modulator with different phases for each element, a microwave receives down-converter to a baseband IF from dc to 24 kHz, and an audio frequency power spectrum analyzer running on a laptop computer. Models that incorporate both the CW signal arrival angle and radiation angles for each modulation and distortion product are presented, along with measurements of 16 element arrays in the anechoic chamber at 2.3 GHz using time-modulation frequencies near 1 kHz. The individual array element time-modulators are nearly ideal RF switches, and the agreement between the new time-modulated scatter array models and measurements is remarkable.

Measurement of EJ-228 plastic scintillator proton light output using a coincident neutron scatter technique

The light output function of the fast plastic scintillator EJ-228 was measured using a coincident neutron scatter measurement system and quasi-monoenergetic neutron beams produced by the tandem Van de Graaff accelerator at Triangle Universities Nuclear Laboratory. The measurement of scintillator light output using neutron scatter kinematics provides a model-independent determination of the light output function with quantifiable sources of uncertainty. Consequently, light output measurements performed using this method have a key advantage compared to other techniques: the characterization of the scintillating material depends only on the material itself and not on the size of the detector. This advantage is realized if the light output is defined as the absolute neutron light output relative to the absolute electron light output and two conditions are met: (1) the scintillator volume is fully illuminated and edge effects can be neglected during energy calibration measurements and light output measurements, and (2) the light attenuation length for scintillation photons in the material is long enough that the lowest energy interaction of interest produces a statistically significant number of scintillation photons for an interaction at any position in the scintillator volume. Commonly implemented measurement and analysis techniques for light output characterizations are explored and evidence of bias is provided for characterizations performed by analyzing the full-energy deposition edge of a light output spectrum.

An experimental study to test the efficacy of Mesenchymal Stem Cells in reducing corneal scarring in an ex-vivo organ culture model

In this study, we evaluated the effect of placenta-derived Mesenchymal Stem Cells (MSCs) versus placebo in improving corneal transparency following experimental injury in an ex-vivo organ culture model of post-mortem human corneas. We also compared the influence of MSCs on the basic histopathology of the cornea and the immunohistochemistry markers of fibrotic corneal scarring. Mesenchymal Stem Cells extracted from the placenta were isolated and expanded in-vitro. Five pairs of post-mortem human corneas harvested for the corneal transplant of equal grade were included in the study. Corneas of the same pair were randomly assigned to either the case arm or the control arm. All corneas underwent a standardized superficial keratectomy, 4 mm in diameter. The case and control arm corneas received an intrastromal injection of MSCs and placebo respectively. The corneal button was maintained in an organ culture system for 28 days under the standard protocol. Laser light was passed through the corneas mounted on a self-styled modified artificial anterior chamber. Image analysis was used to quantify corneal transparency. Haematoxylin & Eosin staining and Immunohistochemistry was done for Alpha SMA (Smooth Muscle Actin). Laser scatter measurements were measured using Image Analysis (Image J Software). The difference in the mean of Full-Width Half Maximum (FWHM), Max intensity and Red pixel intensity between the cases and the controls was 101.5, 16.3 and 11.4 respectively which was found to be statistically significant (P < 0.05). Histopathology showed a disorganized Bowman's layer in the controls as compared to the cases. Alpha Smooth Muscle Actin at the injury site stained 3 + in all controls as compared to 1 + in the cases, showing a statistically significant difference (p = 0.005). Based on our findings, we consider that placenta-derived Mesenchymal Stem Cells can alter evolving corneal scarring into a more favourable outcome with better corneal transparency and lesser fibrotic corneal scarring.

High-Quality MicroCT Rock Imaging: Methodology To Measure and Correct for X-Ray Scatter

Summary Microcomputed tomography (microCT) of cores yields valuable information about rock and fluid properties at pore scale for conventional rock and at rock heterogeneity scale for unconventionals. High levels of uncorrected X-ray scatter in computed tomography (CT) data lead to strong image artifacts and erroneous Hounsfield unit (HU) values, making reconstructed images unsuitable for accurate digital rock (DR) characterization (e.g., segmentation, material decomposition, and others). MicroCT scanners do not include scatter correction techniques. To fill this gap, we developed a new methodology to measure and remove the scatter component from raw projection microCT data collected during rock core scans, and ultimately improving the image quality of scanned cores. Widely used approaches for scatter estimation, based on Monte Carlo (MC) simulations and simplified analytical models, are time-consuming and may lose accuracy when imaging complex unconventional shale cores. In this paper, we propose a more practical approach to perform scatter correction from direct scatter measurements, an approach that is based on the beam-stop array (BSA) method. The BSA method works as follows: The radiation scattered by the core sample is emitted in random directions. By placing an array of small, highly absorbing beads between the source and the core, the primary X-ray signal through the beads is blocked, but the overall object scatter signal is not affected. The observed values in the beads’ shadows on the detector are assumed to be scatter signal. Performing interpolation of the scatter signal between the shadowed by beads pixels on the detector gives an estimate of the scatter signal at every pixel on the detector. Subtracting scatter from projection data yields scatter-corrected data used for 3D CT core image reconstruction. To develop the core scatter correction methodology, we executed the following three tasks: (1) performed modeling of primary and scattered signals to optimize the BSA design (beads size and layout) and scan parameters; (2) developed and implemented an accurate scatter correction algorithm into our 3D microCT image reconstruction workflow; and (3) tested the proposed methodology using four shale core samples from the United States and the Middle East. To better assess the impact of scatter, all experiments with shale core plugs presented in this paper were conducted using source energy of 160 kVp. Our results demonstrated that in many cases, especially with higher attenuating cores, failing to correct for X-ray scatter may result in significant loss of image reconstruction accuracy. We also showed that the developed methodology allows for accurate estimation and removal of scatter from the raw (projection) CT data, enabling reconstruction of high-quality core images that are required for performing DR analysis. To assess the impact of X-ray scatter on the accuracy of DR segmentation, we compared the amount of resolved air-filled space using a stack of image slices by thresholding for the air regions. Our results showed that the amount of detected air-filled space may increase significantly when scatter correction is applied. The presented scatter correction methodology is general and can be used with any microCT scanner used by the petroleum industry to improve image quality and derive accurate HU values. This is of significant importance for quantitative characterization of highly heterogeneous rock with fine structural changes, as is the case for shale. Ultimately, this methodology should expand the operational envelope and value of microCT imaging in the exploration and production workflows.

Scatter reduction from an O-arm imaging system

Scanner detectors and gantry hardware in a typical fixed CT attenuate the primary beam. Typical isodose curves at 2 metres for a fixed CT installation show scatter air kerma distributions to be reduced by a factor of almost 100 at the 0° angle compared to the front 90° angle. However, unlike a fixed CT gantry, O-arm covering contains little beam attenuating material. Scatter measurements behind the control panel of the O-arm (the 0° angle) are low but only extend to the height of the control panel and therefore not the full height of a person. Overall, our scatter measurements for our O-arm imaging system indicated that personnel should not remain in the theatre operating room while imaging is taking place. This is a particular issue for anaesthetists during complex spinal surgery. During this type of surgery, the patient is required to do a breath hold during the cone beam CT imaging rotation and the anaesthetist is required to leave the theatre for up to two minutes. Ideally the anaesthetist could remain in the theatre to closely observe the patient. This study investigates the effect and feasibility of attaching lead drapes to the O-arm cover in the sterile theatre environment in order to determine whether scatter can be reduced to a level that will allow an anaesthetist to remain in the operating theatre during the complex O-arm procedures.

Linear accelerator bunker shielding for stereotactic radiotherapy

Shielding protocols such as NCRP 151, IAEA SRS 47 and IPEM 47 are commonly used for the design of radiotherapy facilities. Some work has been accomplished in updating the basic formula with the advent of IMRT but little consideration has been made for unflattened beams and stereotactic techniques apart from for facilities housing devices like the CyberKnife.The inevitable scenario of a stereotactic-only linear accelerator was considered in this study. The necessary shielding requirements were determined based on stereotactic data from a year’s worth treatments from one clinic as well as further measurements of leakage, scatter and use factors. These values were compared to recommendations in the literature.While tenth value layer amounts, workload and barrier widths could be kept at the status quo, major changes could be made to the parameters of leakage, scatter and use factors while still maintaining safety. Some differences could also be seen for the use of IMRT factors.Current shielding protocols were found to inadequately describe methodology for the shielding of a stereotactic-only radiotherapy linac bunker, producing overly-conservative wall thicknesses which is in disagreement with the principles of ICRP.

First Incoherent Scatter Measurements and Adaptive Suppression of Field-Aligned Irregularities by the PANSY Radar at Syowa Station, Antarctic

The Program of the Antarctic Syowa Mesosphere–Stratosphere–Troposphere/Incoherent Scatter (PANSY) radar is a large atmospheric radar located at the Antarctic Syowa Station (69.01°S, 39.59°E). The PANSY radar performed the first incoherent scatter (IS) measurements in the Antarctic region in 2015. Several specific observations were undertaken in 2017 including a 24-h observation of the ionosphere using a peripheral antenna array to suppress interference from the field-aligned irregularities (FAIs). This paper presents the preliminary results derived from the IS measurements using the PANSY radar and the adaptive signal processing techniques to suppress FAIs. The norm-constrained and directionally constrained minimization of power (NC-DCMP) algorithm was applied to the 24-h ionosphere observations by the PANSY radar with a weighting applied to the directional constraint based on the gain differences of the subarrays. When compared with the conventional nonadaptive approach, the number of usable power profiles was increased by about 24% by the gain-weighted NC-DCMP algorithm, suggesting its effectiveness for FAI clutter suppression in ionosphere observations. Furthermore, detection of FAIs using the dedicated antenna array was found valuable in assessing the reliability of estimations of electron density based on VHF-band IS radar data.

Examination of monitoring approaches for ambient air pollution: A case study for India

India faces one of the highest disease burdens from air pollution in the world, with an estimated 100% of the population living in areas with PM2.5 concentrations above the World Health Organization Guideline (10 μg/m3 annual average). With development and population growth, increases in ambient air pollution are anticipated. Combined with an aging population and increasing burden of chronic diseases, ambient air pollution will remain a concern for India well into the current century. Air quality measurements make critical contributions to the identification and prioritization of sources and locations of greatest concern, benchmarking against standards and guidelines, and in the evaluation of effectiveness of actions to reduce emissions. We compare the density of India's monitoring network with that of comparator countries and find large differences. For example, given the ~200 PM2.5 monitoring sites in operation during the 2010–2016 period, we find that India's monitor density of ~0.14 monitors/million persons (1 monitor for every 6.8 million people) is well below that of other highly populated countries such as China (1.2 monitors/million persons), the USA (3.4 monitors/million persons), Japan (0.5 monitors/million persons), Brazil (1.8) and most European countries (2–3 monitors/million persons).To address these gaps between India and monitor densities of comparator countries will require 1600–4000 monitors (1.2–3 monitors/million persons) at an estimated capital (annual operating) cost of US $212–540 ($106–270) million. Even at these densities, only relatively basic information on common air pollutants at high temporal, but limited spatial, resolution would be available. Small-scale variability in air pollution levels within urban areas would not be well-characterized, nor would there be information on chemical constituents useful for evaluating and improving simulations and forecasts, or for characterizing source contributions. As a sufficiently dense traditional network is developed over time, the potential for an integrated monitoring framework to serve as a near-term complement to a traditional network is assessed. In this design, a smaller number of traditional monitoring sites would be linked to a single advanced surface monitoring station in each of ~11 airsheds identified as a minimal number for India. These sites would combine measurement of chemical speciation of particulate matter with measurements of aerosol scatter and aerosol optical depth to link measurements with global and regional satellite-based estimates. In turn, the advanced and traditional sites could serve as calibration nodes for low-cost sensor networks designed to complement periodic mobile monitoring campaigns and/or land use regression models, to provide high spatial and temporal resolution. Such a framework could be established at a substantially reduced cost relative to that of a traditional networks, subject to specific design and complexity considerations. The same general approach may also be applicable to the many other countries with limited or no air quality monitoring and where estimates suggest air quality is a concern.

Satellite Limb Observations of Unprecedented Forest Fire Aerosol in the Stratosphere

AbstractIntense forest fires in western North America during August 2017 caused smoke plumes that reached the stratosphere. While this phenomenon has often been observed, this particular event caused increases in stratospheric aerosol extinction at higher altitudes with greater magnitude than previously observed in the satellite record. Here we use multiple satellite limb sounding observations, which provide high sensitivity to thin aerosol layers and good vertical resolution, to show that enhancements in aerosol extinction from the fires reached as high as 23 km in altitude and persisted for more than 5 months. Within 1 month, the aerosol is observed to cover latitudes from 20°N to 60°N, which is essentially the northernmost limit of the observations. At midlatitudes between 15‐ and 20‐km altitudes, the sustained level of median aerosol extinction measured at 750 nm increased by almost an order of magnitude, from approximately 10−4 km−1 to nearly 10−3 km−1. Agreement between limb scatter and occultation measurements is generally within 20% despite potential bias due to modified aerosol shape and composition.

Rapid simulation of X-ray scatter measurements for threat detection via GPU-based ray-tracing

Scatter-based X-ray imaging has increased in prominence in areas ranging from cancer diagnostics to threat detection in aviation security, due largely to the development of new algorithms and components over the last decade. However, system design and algorithm development are often hindered by an inability to generate a sufficient amount of accurate data. To further the development of scatter-based systems, we created a rapid X-ray scatter simulation tool built with a GPU-centric, parallel ray-tracing framework (NVIDIA OptiX). This tool models a full range of X-ray imaging components and describes 3D objects formed with heterogeneous media using polygon meshes. The scatter simulation algorithm we used is similar to the previously-described hybrid approach; however, instead of employing Monte Carlo techniques, we developed a purely analytical algorithm to sample the distribution of single-scatter events throughout the region of interest. The contribution of scattered photons to the measured signal is then calculated using the first-Born approximation. The accuracy of the implemented pipeline has been validated by comparing simulated against experimental data obtained with a laboratory X-ray scattering system and phantoms of varied materials. Using a single desktop computer with an NVIDIA GTX 770 GPU, we show that the scatter signal generated by an incident fan beam and recorded by a 125×125 element detector array can be simulated in on the order of a few to tens of minutes (depending on the object extent and complexity). As a point of comparison, simulations performed via CPU-based Monte Carlo tools, such as GEANT4 (i.e., the gold standard), can take up to tens of hours to achieve comparable results.

Critical Analysis of Particle Detection Artifacts in Synaptosome Flow Cytometry.

Flow cytometry and fluorescence-activated sorting are powerful techniques that hold great promise for studying heterogeneous populations of submicron particles such as synaptosomes, but many technical challenges arise in these experiments. To date, most flow cytometry studies of synaptosomes have relied on particle detection using forward scatter (FSC) measurements and size estimation with polystyrene (PS) bead standards. However, these practices have serious limitations, and special care must be taken to overcome the poor sensitivity of conventional flow cytometers in the analysis of submicron particles. Technical artifacts can confound these experiments, especially the detection of multiple particles as a single event. Here, we compared analysis of P2 crude synaptosomal preparations from murine forebrain on multiple flow cytometers using both FSC-triggered and fluorescence-triggered detection. We implemented multicolor fluorescent dye-based assays to quantify coincident particle detection and aggregation, and we assessed the false colocalization of antigens in immunostaining analyses. Our results demonstrate that fluorescence triggering and proper dilution can control for coincident particle detection, but not particle aggregation. We confirmed previous studies showing that FSC-based size estimation with PS beads underestimates biological particle size, and we identified pervasive aggregation in the FSC range analyzed in most synaptosome flow cytometry studies. We found that analyzing P2 samples in sucrose/EDTA/tris (SET) buffer reduces aggregation compared to PBS, but does not completely eliminate the presence of aggregates, especially in immunostaining experiments. Our study highlights challenges and pitfalls in synaptosome flow cytometry and provides a methodological framework for future studies.

Theory and measurements of convective Raman side scatter in inertial confinement fusion experiments.

Raman side scatter, whereby scattered light is resonant while propagating perpendicularly to a density gradient in a plasma, was identified experimentally in planar-target experiments at the National Ignition Facility at intensities orders of magnitudes below the threshold for absolute instability. We have derived a new theoretical description of convective Raman side scatter below the absolute threshold, validated by numerical simulations. We show that inertial confinement fusion experiments at full ignition scale, i.e., with mm-scale spot sizes and density scale lengths, are prone to increased coupling losses from Raman side scatter as the instability can extend from the absolute regime near the quarter-critical density to the convective regime at lower electron densities.

Limited-View X-Ray Tomography Combining Attenuation and Compton Scatter Data: Approach and Experimental Results.

X-ray inspection systems are critical in medical, non-destructive testing, and security applications, with systems typically measuring attenuation along straight-line paths connecting sources and detectors. Computed tomography (CT) systems can provide higher-quality images than single- or dual-view systems, but the need to measure many projections leads to greater system cost and complexity. Typically, off-angle Compton scattered photons are treated as noise during tomographic inversion. We seek to maximize the image quality of limited-view systems by combining attenuation data with measurements of Compton-scattered photons, exploiting the fact that the broken-ray paths followed by scattered photons provide additional geometric sampling of the scene. We describe a single-scatter forward model for Compton-scatter data measured with energy-resolving detectors, and demonstrate a reconstruction algorithm for density that combines both attenuation and scatter measurements. The experimental results suggest that including Compton-scattered data in the reconstruction process can improve image quality for density reconstruction using limited-view systems.

Stray light and ghosts in catadioptric spectrometers: incorporating grating scatter measurements into simulations and ghost sensitivity into system design

Abstract The accurate simulation of stray light is essential for the verification of the contrast requirements in optical instruments. In a spectrometer, the scattering from reflective gratings is difficult to characterize while contributing significantly to the overall system stray light and reduction of the spectrometer contrast. In addition, the multiple diffraction orders create a ghost sensitive environment, which must be considered in the design of the instrument. In this article, we present an experimental setup for, and measurement results from, the characterization of the bidirectional scattering distribution function (BSDF) of a holographic grating for a spectrometer applied in a typical earth observation mission with demanding stray light requirements. We observed distinct stray light peaks out of the diffraction plane, which are called ‘satellites.’ The main challenges in the measurement of grating BSDFs arise from the near angle limit, the determination of the instrument signature and the selection of the appropriate sampling (2D or 3D). Following the grating characterization, the next step is to introduce these measured BSDFs into stray light simulation. We have done that by fitting appropriate functions to the measured BSDF and defining them in the optical analysis software ASAP as a user-defined BSDF. Ghost analysis is done at the spectrometer level as a sensitivity analysis of the tilts of the optical elements. Due to the ghosting of higher diffraction orders of the grating, a high sensitivity to the tilts of some of the optical elements can be seen.

Kerma area product (KAP) and scatter measurements for intraoral X-ray machines using three different types of round collimation compared with rectangular beam limiter.

The objective of the study was to determine the radiation dose reduction achieved when rectangular collimation was used on various round collimators. In addition, we evaluated the tissue doses imparted to various head and neck organs. To evaluate the variation in radiation output based on the variable geometric configurations, the kerma area product (KAP) was measured using a commercially available KAP-meter with an internal ion chamber capable of detecting both radiation dose (µGy) and the primary X-ray beam area. The KAP was measured using standard 20.4, 25.7, and 31.7 cm2 round collimators with and without rectangular X-ray field restrictors. To evaluate the potential change in patient scatter radiation dose, an adult head phantom was loaded with thin strips of gafchromic film. A full mouth X-ray series was acquired with various geometric configurations. The films were quantified using a calibration factor to yield absorbed organ doses for the eyes, thyroid, and salivary glands. With the use of rectangular collimator, the KAP for a 31.7 cm2 round collimator was reduced by up to 60% while the 20.4 cm2 round collimator elicited a reduction from up to 40%. In the organ study, results of up to 81% reduction in scatter radiation dose were observed. Although, US FDA regulations allow a maximum beam size of 38.5 cm2 on the patient skin, this study suggests that the use of rectangular collimators provide clinically relevant dose reduction for patients, even when using smaller round collimation, hence the use of rectangular collimation for all intraoral radiographic procedures is highly recommended.

Optical and visual quality after small-incision lenticule extraction

To examine the relationship between corneal higher-order aberrations (HOAs), scatter, and residual refraction with visual symptoms and visual acuity after small-incision lenticule extraction (SMILE) for myopia and astigmatism. University Eye Clinic, Aarhus, Denmark. Prospective case series. Eyes had small-incision lenticule extraction for myopiaor myopic astigmatism. Examinations were performed preoperatively and 1day, 7days, 1month, and 3months postoperatively and included subjective refraction, Pentacam HR measurements, scatter measurements (objective scatter index [OSI]), and questionnaires on visual quality and symptoms. The mean preoperative spherical equivalent (SE) was -7.08 diopters (D)±1.17 (SD). At 3months, the mean uncorrected distance visual acuity (UDVA) was -0.03±0.11 logarithm ofthe minimum angle of resolution and the mean SE, -0.17±0.33 D; the OSI increased by a mean of 0.22±0.53 (P=.06) and coma by 0.1±0.1μm(P<.001); spherical aberration did not change significantly. The severity of self-reported visual symptoms decreased postoperatively. Scatter, corneal HOAs, and residual refraction were not correlated with the degree of visual symptoms. On linear regression analysis, residual refraction was a significant predictor of UDVA (1day: adjusted R2=0.16, P=.02; 3months: adjusted R2=0.55, P<.001). Scatter and corneal HOAs were not associated with postoperative UDVA. Despite statistically significant changes in scatter and corneal HOAs, the severity of self-reported visual symptoms decreased postoperatively. Residual refraction was a good predictor of postoperative UDVA. Scatter and corneal HOAs had no effect on the postoperative UDVA.

Impact of windscreen scatter on laser eye dazzle.

This study investigates the extent to which a windscreen affects the severity of laser eye dazzle (disability glare produced by a laser) experienced by a human observer. Windscreen scatter measurements were taken for a range of windscreens in a variety of conditions, showing that windscreen scatter is similar in magnitude to scatter from the human eye. Human subject experiments verified that obscuration angles caused by laser eye dazzle could be increased by the presence of a windscreen when comparing a dirty automobile windscreen to an eye-only condition with a 532-nm laser exposure. However, a light aircraft windscreen with lower scatter did not exhibit increased obscuration angles at 532 nm, and neither windscreen exhibited an increase at 635 nm. A theoretical analysis of laser eye dazzle, using measured windscreen scatter functions, has provided insight into the delicate interplay between scatter, transmission and the angular extent of dazzle. A model based on this analysis has been shown to be a useful tool to predict the impact of windscreens on laser eye dazzle, with the goal of informing future updates to the authors' laser eye dazzle safety framework.