Nicholas Fraser - Academia.edu (original) (raw)

Papers by Nicholas Fraser

Medical Physics, 2016

PURPOSE To predict pediatric patient dose from diagnostic CT scans using Monte Carlo simulation o... more PURPOSE To predict pediatric patient dose from diagnostic CT scans using Monte Carlo simulation of realistic reference phantoms of various ages, weights, and heights. METHODS A series of deformable pediatric reference phantoms using Non-Uniform Rational B-Splines (NURBS) was developed for a large range of ages, percentiles, and reference anatomy. Individual bones were modeled using age-dependent factors, and red marrow was modeled as functions of age and spatial distribution based on Cristy1. Organ and effective doses for the phantom series were calculated using Monte Carlo simulation of chest, abdominopelvic, and chest-abdomen-pelvis CT exams. Non-linear regression was performed to determine the relationship between dose-length-product (DLP)-normalized organ and effective doses and phantom diameter. Patient-specific voxel computational phantoms were also created by manual segmentation of previously acquired CT images for 40 pediatric patients (0.7 to 17 years). Organ and effective doses were determined by Monte Carlo simulation of these patient-specific phantoms. Each patient was matched to the closest pediatric reference phantom based primarily on age and diameter for all major organs within the torso. RESULTS A total of 80 NURBS phantoms were created ranging from newborn to 15 years with height/weight percentiles from 10 to 90%. Organ and effective dose normalized by DLP correlated strongly with exponentially decreasing average phantom diameter (R2 > 0.95 for most organs). A similar relationship was determined for the patient-specific voxel phantoms. Differences between patient-phantom matched organ-dose values ranged from 0.37 to 2.39 mGy (2.87% to 22.1%). CONCLUSION Dose estimation using NURBS-based pediatric reference phantoms offers the ability to predict patient dose before and after CT examinations, and physicians and scientists can use this information in their analysis of dose prescriptions for particular subjects and study types. This may lead to practices that minimize radiation dose while still achieving high quality images and, ultimately, improved patient care. NIH/NCI 1 R01 CA155400-01A1.

Physica B: Condensed Matter, 2018

Pediatric radiology, Jan 10, 2017

The estimation of organ doses and effective doses for children receiving CT examinations is of hi... more The estimation of organ doses and effective doses for children receiving CT examinations is of high interest. Newer, more realistic anthropomorphic body models can provide information on individual organ doses and improved estimates of effective dose. Previously developed body models representing 50th-percentile individuals at reference ages (newborn, 1, 5, 10 and 15 years) were modified to represent 10th, 25th, 75th and 90th height percentiles for both genders and an expanded range of ages (3, 8 and 13 years). We calculated doses for 80 pediatric reference phantoms from simulated chest-abdomen-pelvis exams on a model of a Philips Brilliance 64 CT scanner. Individual organ and effective doses were normalized to dose-length product (DLP) and fit as a function of body diameter. We calculated organ and effective doses for 80 reference phantoms and plotted them against body diameter. The data were well fit with an exponential function. We found DLP-normalized organ dose to correlate str...

Health physics, 2015

The authors have created a radiation transport code using the GEANT4 Monte Carlo toolkit to simul... more The authors have created a radiation transport code using the GEANT4 Monte Carlo toolkit to simulate pediatric patients undergoing CT examinations. The focus of this paper is to validate their simulation with real-world physical dosimetry measurements using two independent techniques. Exposure measurements were made with a standard 100-mm CT pencil ionization chamber, and absorbed doses were also measured using optically stimulated luminescent (OSL) dosimeters. Measurements were made in air with a standard 16-cm acrylic head phantom and with a standard 32-cm acrylic body phantom. Physical dose measurements determined from the ionization chamber in air for 100 and 120 kVp beam energies were used to derive photon-fluence calibration factors. Both ion chamber and OSL measurement results provide useful comparisons in the validation of the Monte Carlo simulations. It was found that simulated and measured CTDI values were within an overall average of 6% of each other.

Earth and Environmental Science Transactions of the Royal Society of Edinburgh, 2012

ABSTRACTThe Post Quarry, within the lower part of the type section of the Upper Triassic Cooper C... more ABSTRACTThe Post Quarry, within the lower part of the type section of the Upper Triassic Cooper Canyon Formation in southern Garza County, western Texas, contains a remarkably diverse vertebrate assemblage. The Post Quarry has produced: the small temnospondylRileymillerus cosgriffi; the metoposauridApachesaurus gregorii; possible dicynodonts and eucynodonts; a clevosaurid sphenodontian; non-archosauriform archosauromorphs (Trilophosaurus dornorum, simiosaurians, and possiblyMalerisaurus); the phytosaurLeptosuchus; several aetosaurs (Calyptosuchus wellesi,Typothorax coccinarum,Paratypothorax, andDesmatosuchus smalli); the poposauroidShuvosaurus inexpectatus(“Chatterjeea elegans”); the rauisuchidPostosuchus kirkpatricki; an early crocodylomorph; several dinosauromorphs (the lagerpetidDromomeron gregorii, the silesauridTechnosaurus smalli, a herrerasaurid, and an early neotheropod); and several enigmatic small diapsids. Revised lithostratigraphic correlations of the lower Cooper Canyon...

Journal of Vertebrate Paleontology, 2008

580 offering. Certainly the Fraser book offers much more in terms of information, yet the similar... more 580 offering. Certainly the Fraser book offers much more in terms of information, yet the similarity of titles can be confusing. Despite some minor criticisms, I would highly recommend “Dawn of the Dinosaurs: Life in the Triassic” to the interested professional and avocational paleontologist as well as the lay reader, although I would caution that the work requires a solid prior paleontological background. Nicholas Fraser should be commended for providing such a detailed synthesis that is well-organized, well-written, and full of ...

Journal of Paleontology, 2004

ABSTRACT

Insect Systematics & Evolution, 2005

The Mesozoic family Pseudopolycentropodidae presently consists of seven described species from th... more The Mesozoic family Pseudopolycentropodidae presently consists of seven described species from the mid-Triassic to the Late Jurassic of Europe and Asia. Pseudopolycentropus prolatipennis Whalley, from the Early Jurassic of England, is revised based on re-examination of the type. Four new species are described herein that add significant distributional and stratigraphic extensions to the family. Pseudopolycentropodes virginicus Grimaldi and Fraser, gen. n., sp. n. from the Late Triassic (Carnian) of Virginia USA is the first species of the family from the Western Hemisphere. Pseudopolycentropus daohugouensis Zhang, sp. n. from the Late Jurassic of China is very similar to P. latipennis Martynov, 1927 from the Late Jurassic of Kazakhstan. Four specimens belonging to two very similar species in mid-Cretaceous amber from northern Burma (Myanmar), Parapolycentropus burmiticus Grimaldi and Rasnitsyn, gen. n., sp. n. and P. paraburmiticus Grimaldi and Rasnistyn, sp. n., are the only specim...

arXiv (Cornell University), May 21, 2018

Deep learning as a means to inferencing has proliferated thanks to its versatility and ability to... more Deep learning as a means to inferencing has proliferated thanks to its versatility and ability to approach or exceed human-level accuracy. These computational models have seemingly insatiable appetites for computational resources not only while training, but also when deployed at scales ranging from data centers all the way down to embedded devices. As such, increasing consideration is being made to maximize the computational efficiency given limited hardware and energy resources and, as a result, inferencing with reduced precision has emerged as a viable alternative to the IEEE 754 Standard for Floating-Point Arithmetic. We propose a quantization scheme that allows inferencing to be carried out using arithmetic that is fundamentally more efficient when compared to even half-precision floating-point. Our quantization procedure is significant in that we determine our quantization scheme parameters by calibrating against its reference floating-point model using a single inference batch rather than (re)training and achieve end-to-end post quantization accuracies comparable to the reference model.

ArXiv, 2018

I declare that this thesis is the result of my own work and that the work contained herein is my ... more I declare that this thesis is the result of my own work and that the work contained herein is my own except where explicitly stated otherwise in the text before each chapter. This work has been carried out by myself in the laboratory of Associate Professor Colin Jackson. To the best of my knowledge, the work presented in this thesis has not been previously submitted, in part or whole, to any university or institution for any degree, diploma, or other qualification.

Nature Ecology & Evolution, 2016

The end-Devonian to mid-Mississippian time interval has long been known for its depauperate palae... more The end-Devonian to mid-Mississippian time interval has long been known for its depauperate palaeontological record, especially for tetrapods. This interval encapsulates the time of increasing terrestriality among tetrapods, but only two Tournaisian localities previously produced tetrapod fossils. Here we describe five new Tournaisian tetrapods from two localities in their environmental context. A phylogenetic analysis retrieved three taxa as stem tetrapods, interspersed among Devonian and Carboniferous forms, and two as stem amphibians, suggesting a deep split among crown tetrapods. The new taxa suggest that tetrapod diversification was well established by the Tournaisian. Sedimentary evidence indicates that tetrapod fossils are usually associated with sandy siltstones overlying wetland palaeosols. Tetrapods were probably living on vegetated surfaces subsequently flooded. We show that atmospheric oxygen levels were stable across the Devonian/Carboniferous boundary, and did not inhibit the evolution of terrestriality. This wealth of tetrapods from new Tournaisian localities highlights the potential for discoveries elsewhere. The term "Romer's Gap" was coined 1,2 for a hiatus in the fossil record of tetrapods from the end-Devonian to the Mid-Mississippian (Viséan), an interval of approximately 25 million years (Myr) 3. Following the end-Devonian, the earliest terrestrial tetrapod fauna was known from the late Asbian (late Viséan) locality of East Kirkton near Bathgate, Scotland 4,5. By that time, tetrapods were ecologically diverse, exploited a wide range of 45 niches, and were terrestrially capable. With five or fewer digits, some had gracile limbs 46 6,7 , unlike the polydactylous aquatic or semi-aquatic fish-like tetrapods of the Late

Quantitative Science Studies

In 2014, a union of German research organizations established Projekt DEAL, a national-level proj... more In 2014, a union of German research organizations established Projekt DEAL, a national-level project to negotiate licensing agreements with large scientific publishers. Negotiations between DEAL and Elsevier began in 2016, and broke down without a successful agreement in 2018; during this time, around 200 German research institutions canceled their license agreements with Elsevier, leading Elsevier to restrict journal access at those institutions. We investigated the effect on researchers’ publishing and citing behaviors from a bibliometric perspective, using a data set of ∼400,000 articles published by researchers at DEAL institutions during 2012–2020. We further investigated these effects with respect to the timing of contract cancellations, research disciplines, collaboration patterns, and article open-access status. We find evidence for a decrease in Elsevier’s market share of articles from DEAL institutions, with the largest year-on-year market share decreases occurring from 20...

The scientific publishing market suffers from injustice in several regards. Interestingly, traces... more The scientific publishing market suffers from injustice in several regards. Interestingly, traces of injustice can be found in both open-access (OA) and closed-access (CA) publishing-models.<br> The OA-model makes scientific literature freely accessible for all scholars - but, the costs of publishing have to be assumed by somebody (e.g. the authors via APCs). This, however, may not be possible for authors who do not benefit from financial support for OA-publishing. In succession, such authors cannot publish in the OA-model (instead they must turn to the CA-model) and cannot profit from the citation advantage that was shown for many OA-articles [1]. Researchers (at institutions) who are not able to maintain subscriptions to CA-publications suffer from the CA-model as it builds a barrier to access the required resources. This may result in falling behind the state of the art in the field which along with it may negatively affect future publication success in outlets of high repu...

Pediatric Radiology, 2015

Background-Organ dose is essential for accurate estimates of patient dose from CT. Objective-To d... more Background-Organ dose is essential for accurate estimates of patient dose from CT. Objective-To determine organ doses from a broad range of pediatric patients undergoing diagnostic chest-abdomen-pelvis CT and investigate how these relate to patient size. Materials and methods-We used a previously validated Monte Carlo simulation model of a Philips Brilliance 64 multi-detector CT scanner (Philips Healthcare, Best, The Netherlands) to calculate organ doses for 40 pediatric patients (M:F=21:19; range 0.6-17 years). Organ volumes and positions were determined from the images using standard segmentation techniques. Nonlinear regression was performed to determine the relationship between volume CT dose index (CTDI vol)-normalized organ doses and abdominopelvic diameter. We then compared results with values obtained from independent studies. Results-We found that CTDI vol-normalized organ dose correlated strongly with exponentially decreasing abdominopelvic diameter (R 2 >0.8 for most organs). A similar relationship was determined for effective dose when normalized by dose-length product (R 2 =0.95). Our results agreed with previous studies within 12% using similar scan parameters (i.e. bowtie filter size, beam collimation); however results varied up to 25% when compared to studies using different bowtie filters. Conclusion-Our study determined that organ doses can be estimated from measurements of patient size, namely body diameter, and CTDI vol prior to CT examination. This information provides an improved method for patient dose estimation.

arXiv (Cornell University), Oct 25, 2019

The term Open Access not only describes a certain model of scholarly publishing-namely in digital... more The term Open Access not only describes a certain model of scholarly publishing-namely in digital format freely accessible to readers-but often also implies that free availability of research results is desirable, and hence has a normative character. Together with the large variety of presently used definitions of different Open Access types, this normativity hinders a systematic investigation of the development of open availability of scholarly literature. In this paper, we propose a non-normative definition of Open Access and its usage as a neutral, descriptive term in bibliometric studies and research on science. To this end, we first specify what normative figures are commonly associated with the term Open Access and then develop a neutral definition. We further identify distinguishing characteristics of openly accessible literature, called dimensions, and derive a classification scheme into Open Access categories based on these dimensions. Additionally, we present an operationalisation method to assign scientific publications to the respective categories in practice. Here, we describe useful data sources, which can be employed to gather the information needed for the classification of scholarly works according to the presented classification scheme. 27 www.openarchives.org/OAI/openarchivesprotocol.html. 28 This approach has been implemented by Jahn (2017) in the Hybrid OA Dashboard.