160 results on '"McGill, George"'
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2. TOPOGRAPHY, SURFACE PROPERTIES, AND TECTONIC EVOLUTION
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McGILL, GEORGE E., primary, WARNER, JEFFREY L., additional, MALIN, MICHAEL C., additional, ARVIDSON, RAYMOND E., additional, ELIASON, ERIC, additional, NOZETTE, STEWART, additional, and REASENBERG, ROBERT D., additional
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- 2022
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3. Utility of 68Ga-DOTATATE PET-MRI for Gamma Knife® stereotactic radiosurgery treatment planning for meningioma
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Ratnayake, Gishan, primary, Huo, Michael, additional, Mehta, Akash, additional, Ramachandran, Prabhakar, additional, Pinkham, Mark B, additional, Law, Phillip, additional, Watkins, Trevor, additional, Olson, Sarah, additional, Hall, Bruce, additional, Brown, Simon, additional, Lusk, Ryan, additional, Jones, Catherine, additional, O’Mahoney, Eoin, additional, McGill, George, additional, and Foote, Matthew C, additional
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- 2023
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4. Utility of 68Ga-DOTATATE PET-MRI for Gamma Knife® stereotactic radiosurgery treatment planning for meningioma.
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Ratnayake, Gishan, Huo, Michael, Mehta, Akash, Ramachandran, Prabhakar, Pinkham, Mark B, Law, Phillip, Watkins, Trevor, Olson, Sarah, Hall, Bruce, Brown, Simon, Lusk, Ryan, Jones, Catherine, O'Mahoney, Eoin, McGill, George, and Foote, Matthew C
- Subjects
STEREOTACTIC radiosurgery ,COHEN'S kappa coefficient (Statistics) ,MENINGIOMA - Abstract
Objectives To investigate the impact of adding
68 Ga-DOTATATE PET/MRI to standard MRI for target volume delineation in Gamma Knife® stereotactic radiosurgery (GKSRS) for meningioma. Methods Seventeen patients with 18 lesions undergoing GKSRS for WHO grade 1 meningioma were enrolled in a prospective study. All patients underwent pre-treatment68 Ga-DOTATATE PET/MRI examination in addition to standard procedures. Five clinicians independently contoured the gross tumour volume (GTV) based on standard MRI (GTVMRI ) and PET/MRI (GTVPET/MRI ) on separate occasions. Interobserver agreement was evaluated using Cohen's Kappa statistic (CKS), Dice similarity coefficient (DC), and Hausdorff distance (HD). Statistical analysis was performed with paired t -test and Wilcoxon signed rank test. Results The addition of PET/MRI significantly increased GTV contour volume (mean GTVPET/MRI 3.59 cm3 versus mean GTVMRI 3.18 cm3 , P = .008). Using the treating clinician's pre-treatment GTVMRI as the reference, median CKS (87.2 vs 77.5, P = .006) and DC (87.2 vs 77.4, P = .006) were significantly lower, and median HD (25.2 vs 31.0, P = .001) was significantly higher with the addition of PET/MRI. No significant difference was observed in interobserver contouring reproducibility between GTVMRI and GTVPET/MRI . Conclusion The addition of68 Ga-DOTATATE PET/MRI for target volume delineation in GKSRS for meningioma is associated with an increase in GTV volume and greater interobserver variation. PET/MRI did not affect interobserver contouring reproducibility. Advances in Knowledge This study provides novel insights into the impact of68 Ga-DOTATATE PET/MRI on GTV delineation and interobserver agreement in meningioma GKSRS, highlighting its potential for improving GKSRS treatment accuracy. [ABSTRACT FROM AUTHOR]- Published
- 2024
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5. Results from a phantom based multi-centre paediatric computed tomography dose survey
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Thiele, David L., Aliuddin, Sarah-Ellyana, Irvine, Mike, and McGill, George P.
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- 2016
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6. A 3D printed phantom to assess MRI geometric distortion
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Ramachandran, Prabhakar, Noble, Christopher, Langton, Christian, Perrett, Ben, Cox, Joshua, Chapman, Mike, McGill, George, Ramachandran, Prabhakar, Noble, Christopher, Langton, Christian, Perrett, Ben, Cox, Joshua, Chapman, Mike, and McGill, George
- Abstract
Geometric distortions in magnetic resonance can introduce significant uncertainties into applications such as radiotherapy treatment planning and need to be assessed as part of a comprehensive quality assurance program. We report the design, fabrication, and imaging of a custom 3D printed unibody MR distortion phantom along with quantitative image analysis. Methods: The internal cavity of the phantom is an orthogonal three-dimensional planar lattice, composed of 3 mm diameter rods spaced equidistantly at a 20 mm centre-centre offset repeating along the X, Y, and Z axes. The phantom featured an overall length of 308.5 mm, a width of 246 mm, and a height of 264 mm with lines on the external surface for phantom positioning matched to external lasers. The MR phantom was 3D printed in Nylon-12 using an advancement on traditional selective laser sintering (SLS) (HP Jet Fusion 3D - 4200 machine). The phantom was scanned on a Toshiba Aquilion CT scanner to check the integrity of the 3D print and correct for any resultant issues. The phantom was then filled with NiSO4 solution and scanned on a 3T PET-MR Siemens scanner for selected T1 and T2 sequences, from which distortion vectors were generated and analysed using in-house software written in Python. Results: All deviations of the node positions from the print design were less than 1 mm, with an average displacement of 0.228 mm. The majority of the deviations were smaller than the 0.692 mm pixel size for this dataset. Conclusion: A customised 3D printed MRI-phantom was successfully printed and tested for assessing geometric distortion on MRI scanners. 3D printed phantoms can be considered for clinics wishing to assess geometric distortions under specific conditions, but require resources for design, fabrication, commissioning, and verification.
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- 2021
7. A 3D printed phantom to assess MRI geometric distortion
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Ramachandran, Prabhakar, primary, Noble, Christopher, additional, Langton, Christian, additional, Perrett, Ben, additional, Cox, Joshua, additional, Chapman, Mike, additional, and McGill, George, additional
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- 2021
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8. Tectonic and stratigraphic implications of the relative ages of venusian plains and wrinkle ridges
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McGill, George E.
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Tectonics (Geology) -- Models ,Tectonics (Geology) -- Analysis ,Venus (Planet) -- Origin ,Venus (Planet) -- Natural history ,Planet formation -- Analysis ,Planet formation -- Models ,Venus (Planet) -- Atmosphere ,Venus (Planet) -- Analysis ,Venus (Planet) -- Models ,Astronomy ,Earth sciences - Abstract
A major ongoing controversy concerns the style of crustal evolution on Venus. At one extreme is a directional model that proposes a sequence of depositional and deformational events that occur at specific times in the evolution of the crust and that are global in extent. At the other extreme is a model that argues for different ages of these events in different places on the planet. A test of the directional model is here focused on whether wrinkle ridges formed at a single time in the recorded crustal history of Venus. Where sets of wrinkle ridges intersect it commonly is possible to determine that one set is older than the other. Also, the deformation responsible for wrinkle ridges is, in places, clearly progressive with respect to stratigraphic material units. These observations are not consistent with a specific single time for the formation of wrinkle ridges within the stratigraphic sequence. Within an area including about 1/3 of the surface of Venus 15% of craters that are younger than regional plains are older than wrinkle ridges, 85% are younger than wrinkle ridges. Taking 750 myr as a reasonable mean age for the regional plains, this implies that the mean age of wrinkle ridges is 110 myr younger than the mean age of plains. Solomon et al. (1999, Science 286, 87) propose that the emplacement of a large volume of plains lava would lead to a major atmospheric temperature increase. Their model predicts thermal stresses in the lithosphere that, at shallow depth, would reach peak compressive stresses in about 100 myr, a number very similar to the time lag between plains emplacement and wrinkle ridge formation indicated by the crater data. The thermal compressive stresses responsible for wrinkle ridges would be maintained at a level sufficient to deform basalt for at least 100 myr and possibly for as long as 350 myr. These time intervals are not really short compared to the mean age of the plains. Finally. because wrinkle ridges are demonstrably younger than the plains they deform, they cannot be related to the processes that formed the plains and thus should not be used to define a 'plains with wrinkle ridges' unit. Keywords: Venus, surface; Tectonics
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- 2004
9. Planetary structural mapping
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Tanaka, Kenneth L., primary, Anderson, Robert, additional, Dohm, James M., additional, Hansen, Vicki L., additional, McGill, George E., additional, Pappalardo, Robert T., additional, Schultz, Richard A., additional, and Watters, Thomas R., additional
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- 2009
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10. Venus tectonics
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McGill, George E., primary, Stofan, Ellen R., additional, and Smrekar, Suzanne E., additional
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- 2009
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11. Examining the UK Covid‐19 mortality paradox: Pandemic preparedness, healthcare expenditure, and the nursing workforce
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Stribling, Julian, primary, Clifton, Andrew, additional, McGill, George, additional, and Vries, Kay, additional
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- 2020
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12. Discrepancies between positron emission tomography/magnetic resonance imaging and positron emission tomography/computed tomography in a cohort of oncological patients
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Oh, Geon, primary, O’Mahoney, Eoin, additional, Jeavons, Susanne, additional, Law, Phillip, additional, Ngai, Stanley, additional, McGill, George, additional, Yu, Chris, additional, and Miles, Kenneth A, additional
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- 2020
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13. Surface evolution of Venus
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Basilevsky, Alexander T., primary and McGill, George E., additional
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- 2007
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14. Geology of the Lachesis Tessera Quadrangle (V-18), Venus
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McGowan, Eileen M and McGill, George G
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Geosciences (General) - Abstract
The Lachesis Tessera Quadrangle (V-18) lies between 25deg and 50deg north, 300deg and 330deg east. Most of the quadrangle consists of "regional plains" (1) of Sedna and Guinevere Planitiae. A first draft of the geology has been completed, and the tentative number of mapped units by terrain type is: tesserae - 2; plains - 4; ridge belts - 1; fracture belts - 1 (plus embayed fragments of possible additional belts); coronae - 5; central volcanoes - 2; shield flows - 2; paterae - 1; impact craters - 13; undifferentiated flows - 1; bright materials - 1.
- Published
- 2010
15. Geology of the Lachesis Tessera Quadrangle (V-18), Venus
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McGill, George E
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Geophysics - Abstract
The Lachesis Tessera Quadrangle (V-18) lies between 25deg and 50deg north, 300deg and 330deg east. Most of the quadrangle consists of "regional plains" (1) of Sedna and Guinevere Planitiae. A first draft of the geology has been completed, and the tentative number of mapped units by terrain type is: Tesserae - 2; plains - 4; ridge belts - 1; fracture belts - 1 (plus embayed fragments of possible additional belts); coronae - 3; central volcanoes - 1; shield flows - 2; paterae - 1; impact craters - 1; undifferentiated flows - 1; bright materials - 1. By far the areally most extensive materials are regional plains. These are mapped as two units, based on radar backscatter ("radar brightness"). The brighter unit appears to be younger than the darker unit. This inference is based on the common presence within the lighter unit of circular or nearly circular inliers of material with radar backscatter characteristic of the darker unit. The circular inliers are most likely low shield volcanoes, which are commonly present on the darker unit, that were only partially covered by the brighter unit. Clear cut examples of wrinkle ridges and fractures superposed on the darker unit but truncated by the brighter unit have not been found to date. These relationships indicate that the brighter unit is superposed on the darker unit, but that the difference in age between them is very small. Because they are so widespread, the regional plains are a convenient relative age time "marker." The number of impact craters superposed on these plains is too small to measure age differences (2), and thus we cannot estimate how much time elapsed between the emplacement of the darker and brighter regional plains units. More local plains units are defined by significantly lower radar backscatter or by a texture that is mottled at scores to hundreds of kilometers scale. A plains-like unit with a homogenous, bright diffuse backscatter is present as scattered exposures in the eastern part of the quadrangle. These exposures have been mapped as "bright material," but it is not clear at present if this is a valid unit or if it is part of the brighter regional plains unit. Tessera terrain is primarily found along the western border of the quadrangle, where Lachesis Tessera refers to the southern exposures, and Zirka Tessera refers to northern exposures. A second tessera unit has been mapped with the symbol "t?." This unit appears to be deformed by the requisite 2 sets of closely spaced structures, but it is so extensively flooded by regional plains materials that the structural fabric is partially obscured. Tessera terrain is present in the adjacent V-17 quadrangle, where both Lachesis Tessera and Zirka Tessera are areally more extensive than in V-18. Ridge and fracture belts are both present, but not as extensive as is the case in, for example, the Pandrosos Dorsa (3) and Lavinia Planitia (4) quadrangles. As is commonly the case, it is difficult to determine if the materials of these belts are older or younger than regional plains. A recent study using radar properties (5) demonstrated that at least most ridge belts appear to be older than regional plains. The materials of fracture belts probably are also older than regional plains, but the fractures themselves can be both older and younger than regional plains (e.g., 3).
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- 2008
16. Planetary Geology and Geophysics Program
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McGill, George E
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Lunar And Planetary Science And Exploration - Abstract
Geological mapping and topical studies, primarily in the southern Acidalia Planitia/Cydonia Mensae region of Mars is presented. The overall objective was to understand geologic processes and crustal history in the northern lowland in order to assess the probability that an ocean once existed in this region. The major deliverable is a block of 6 1:500,000 scale geologic maps that will be published in 2004 as a single map at 1:1,000,000 scale along with extensive descriptive and interpretive text. A major issue addressed by the mapping was the relative ages of the extensive plains of Acidalia Planitia and the knobs and mesas of Cydonia Mensae. The mapping results clearly favor a younger age for the plains. Topical studies included a preliminary analysis of the very abundant small domes and cones to assess the possibility that their origins could be determined by detailed mapping and remote-sensing analysis. We also tested the validity of putative shorelines by using GIs to co-register full-resolution MOLA altimetry data and Viking images with these shorelines plotted on them. Of the 3 proposed shorelines in this area, one is probably valid, one is definitely not valid, and the third is apparently 2 shorelines closely spaced in elevation. Publications supported entirely or in part by this grant are included.
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- 2004
17. Mars Data Analysis Program
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McGill, George E
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Lunar And Planetary Science And Exploration - Abstract
Grant NAGS12158 addressed a major NASA objective concerning the possibility of a palm ocean or large lake in the northern lowlands of Mars. Our overall approach for this study was an analysis of the graben-bounded giant polygons of Utopia Planitia, but specifically those grabens that define circles rather than open polygons. These circular grabens overlie buried impact craters, and the grabens form because of differential compaction of the overlying material over crater rims and floors. Several years ago, I predicted that the graben circles would bound depressions, and that the depths of these depressions would scale with the diameters of the graben circles. These predictions have been verified by earlier analysis. During this one-year grant (with one-year no-cost extension) we greatly increased the sample size and validated the earlier research robustly. What remained unexplained was why most of the graben circles in Utopia Planitia were double. A new model, involving volumetric compaction rather than simply 2-D compaction, satisfactorily explains the double rings and also provides a measure of relative thickness of the cover material burying the craters as a function of radial distance from the center of the Utopia Basin. Only two materials are likely candidates for the compacting cover material: volcanic ash, or wet sediment. The water in the wet sediment is largely responsible for the volumetric compaction; dry ash will compact vertically but experiences very limited lateral shrinkage. Thus the depressions within the circular grabens and the model explaining the double rings strongly favor wet sediment and thus provide evidence in favor of a past body of standing water in the northern lowlands. Publications supported entirely or in part by this grant are listed below.
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- 2004
18. Age of the Mars Global Northerly Slope: Evidence From Utopia Planitia
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McGill, George E
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Lunar And Planetary Science And Exploration - Abstract
Recent results from the Mars Orbiter Laser Altimeter (MOLA) experiment on Mars Global Surveyor (MGS) indicate that most of Mars is characterized by a very gentle, roughly northerly slope. Detailed mapping in north-central Arabia Terra combined with superposition relations and crater counts indicate that, in that region at least, this northerly slope must have been formed no later than Late Hesperian, with the most likely time of formation being Late Hesperian. Current research in Utopia Planitia intended as a test of extant models for the formation of giant polygons has turned up good evidence for a Late Hesperian age for the northerly tilt in this region as well, as will be discussed.
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- 2002
19. Mola Topography Supports Drape-Folding Models for Polygonal Terrain of Utopia Planitia, Mars
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McGill, George E and Buczkowski, D. L
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Lunar And Planetary Science And Exploration - Abstract
One of the most important questions we ask about Mars is whether or not there have ever been large bodies of standing water on the surface. The polygonal terrains of Utopia and Acidalia Planitiae are located in the lowest parts of the northern lowlands, the most logical places for water to pond and sediments to accumulate. Showing that polygonal terrain is sedimentary in origin would represent strong evidence in favor of a northern ocean. A number of hypotheses for the origin of the giant martian polygons have been proposed, from the cooling of lava to frost wedging to the desiccation of wet sediments, but Pechman showed that none of these familiar processes could be scaled up to martian dimensions. Two models for polygon origin attempt to explain the scale of the martian polygons by postulating drape folding of a cover material, either sedimentary or volcanic, over an uneven, buried surface. The drape folding would produce bending stresses in the surface layers that increase the probability of Fracturing over drape anticlines and suppress the probability of fracturing over drape synclines. However, both models require an additional source of extensional strain to produce the total strain needed to produce the observed troughs.
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- 2002
20. Investigation into attitudes towards older people with dementia in acute hospital using the Approaches to Dementia Questionnaire
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de Vries, Kay, primary, Drury-Ruddlesden, Jenny, additional, and McGill, George, additional
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- 2019
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21. A training needs analysis of admiral nurses to facilitate advance care planning in dementia
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Harrison Dening, Karen, primary, Scates, Caroline, additional, McGill, George, additional, and De-Vries, Kay, additional
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- 2019
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22. Investigation into attitudes towards older people with dementia in acute hospital using the Approaches to Dementia Questionnaire.
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de Vries, Kay, Drury-Ruddlesden, Jenny, and McGill, George
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ATTITUDES toward mental illness ,DEMENTIA ,QUESTIONNAIRES - Abstract
Background: Health care professionals have poor recognition of cognitive impairment among older patients and often have difficulties in providing care for people with dementia in acute hospitals. There are also reported high levels of stigma and negative staff attitudes towards people with dementia. Methodology: A one-time survey of staff working on acute medical and orthopaedic wards of five District Health Boards in New Zealand using the 'Approaches to Dementia Questionnaire'; a 19-item Questionnaire that aims to capture two attitudinal domains (Hope and Person-Centredness), towards people with dementia. A total of 563 questionnaires were distributed. Data were analysed using R. Results: Three hundred and four (53.99%) respondents completed the survey. Ninety-four of the respondents (31.1%) were aged 51 years or older, and nearly all (88.4%) were women. One hundred and sixty-four of the respondents (53.9%) were registered nurses, 10 were student nurses (3.3%), 24 (7.9%) were healthcare assistants, 70 (23.0%) were allied health professionals, 27 (8.9%) were doctors and 9 (3%) were pharmacists. The mean total score of 72.7 reflected positive attitudes overall on the part of the respondents. Allied health professionals, student nurses and pharmacists had high scores on both scales whilst healthcare assistants scored lowest on both scales. Conclusion: Overall, the Approaches to Dementia Questionnaire is a useful measure, but it can be difficult to differentiate between genuine attitudinal differences and confounding influences such as level of knowledge and education amongst such a diverse group of participants. Differences between staff may be considered as a reflection of the high level of direct care that is expected of some staff and not of others. Further, the Approaches to Dementia Questionnaire may not be the most appropriate measure to accurately understand attitudes to dementia care in acute environments; and that it more usefully reveals knowledge about dementia rather than attitudes related to real-time practice. [ABSTRACT FROM AUTHOR]
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- 2020
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23. Hotspot evolution and Venusian tectonic style
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Mcgill, George E
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Lunar And Planetary Exploration - Abstract
Because hotspots represent an important manifestation of heat loss on Venus, their geological evolution is of fundamental importance for any attempt to understand Venusian tectonics. Eistla Regio is an approximately 7500-km-long, moderately elevated region inferred to overlie one or more large mantle upwellings or hotspots. It also contains many shield volcanoes and coronae believed due to the rise of thermal plumes in the mantle. Central Eistla Regio includes two large volcanoes, Sappho and Anala, and several coronae in close proximity. Detailed mapping in this region results in two conclusions of tectonic significance: (1) Sappho and Anala occur near the intersection of two major extensional deformation zones, and (2) the coronae are older than the large volcanoes. Several of the coronae occur as a chain along Guor Linea, one of the major extensional deformation zones. Stratigraphic relationships indicate that the coronae began forming very soon after the emplacement of the widespread regional plains materials. Thus Central Eistla Regio was the site of a swarm of plumes that first formed coronae and then later formed shield volcanoes. The expected result of such a swarm would be thermal thinning of the elastic lithosphere with time. However, model results, geological observations, and gravity data suggest that the change from coronae to shield volcanoes was accompanied by a thickening of the lithosphere with time. This thickening is interpreted to be the result of global cooling of the lithosphere following the most recent episode of near-global resurfacing. The global cooling must have occurred faster than local heating of the lithosphere due to the impingement of thermal plumes.
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- 1994
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24. Wrinkle ridges, stress domains, and kinematics of venusian plains
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Mcgill, George E
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Lunar And Planetary Exploration - Abstract
Wrinkle ridges are nearly ubiquitous landforms on the plains of Venus. By analogy with similar structures on other planets, venusian wrinkle ridges are inferred to trend normal to the direction of maximum principal compression in the crust, an inference that is verified by geometrical relationships with positive and negative relief features on Venus. Because plains are the dominant terrain on Venus, wrinkle ridges provide an excellent opportunity to determine the orientations of shallow crustal principal stress trajectories over most of the planet. In most places there are two or more sets of wrinkle ridges, and commonly one of these persists over a large area, defining a regional stress domain. Intersection relationships indicate that these domains differ in age.
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- 1993
25. Plains tectonism on Venus: The deformation belts of Lavinia Planitia
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Squyres, Steven W, Jankowski, David G, Simons, Mark, Solomon, Sean C, Hager, Bradford H, and Mcgill, George E
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Lunar And Planetary Exploration - Abstract
High-resolution radar images from the Magellan spacecraft have revealed the first details of the morphology of the Lavinia Planitia region of Venus. A number of geologic units can be distinguished, including volcanic plains units with a range of ages. Transecting these plains over much of the Lavinia region are two types of generally orthogonal features that we interpret to be compressional wrinkle ridges and extensional grooves. The dominant tectonic features of Lavinia are broad elevated belts of intense deformation that transect the plains with complex geometry. They are many tens to a few hundred kilometers wide, as much as 1000 km long, and elevated hundreds of meters above the surrounding plains. Two classes of deformation belts are seen in the Lavinia region. 'Ridge belts' are composed of parallel ridges, each a few hundred meters in elevation, that we interpret to be folds. Typical fold spacings are 5-10 km. 'Fracture belts' are dominated instead by intense faulting, with faults in some instances paired to form narrow grabens. There is also some evidence for modest amounts of horizontal shear distributed across both ridge and fracture belts. Crosscutting relationships among the belts show there to be a range in belt ages. In western Lavinia, in particular, many ridge and fracture belts appear to bear a relationship to the much smaller wrinkle ridges and grooves on the surrounding plains: ridge morphology tends to dominate belts that lie more nearly parallel to local plains wrinkle ridges, and fracture morphology tends to dominate belts that lie more nearly parallel to local plains grooves. We use simple models to explore the formation of ridge and fracture belts. We show that convective motions in the mantle can couple to the crust to cause horizontal stresses of a magnitude sufficient to induce the formation of deformation belts like those observed in Lavinia. We also use the small-scale wavelengths of deformation observed within individual ridge belts to place an approximate lower limit on the venusian thermal gradient in the Lavinia region at the time of deformation.
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- 1993
26. Resolving topographic detail on Venus by modeling complex Magellan altimetry echoes
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Lovell, Amy J, Schloerb, F. Peter, and Mcgill, George E
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Lunar And Planetary Exploration - Abstract
Magellan's altimeter is providing some of the finest resolution topography of Venus achieved to date. Nevertheless, efforts continue to improve the topographic resolution whenever possible. One effort to this end is stereoscopic imaging, which provides topography at scales similar to that of the synthetic aperture radar (SAR). However, this technique requires two SAR images of the same site to be obtained and limits the utility of this method. In this paper, we present another method to resolve topographic features at scales smaller than that of an altimeter footprint, which is more globally applicable than the stereoscopic approach. Each pulse which is transmitted by Magellan's altimeter scatters from the planet and echoes to the receiver, delayed based on the distance between the spacecraft and each surface element. As resolved in time, each element of an altimetry echo represents the sum of all points on the surface which are equidistant from the spacecraft. Thus, individual returns, as a function of time, create an echo profile which may be used to derive properties of the surface, such as the scattering law or, in this case, the topography within the footprint. The Magellan project has derived some of this information by fitting model templates to radar echo profiles. The templates are calculated based on Hagfor's Law, which assumes a smooth, gently undulating surface. In most regions these templates provide a reasonable fit to the observed echo profile; however, in some cases the surface departs from these simple assumptions and more complex profiles are observed. Specifically, we note that sub-footprint topographic relief apparently has a strong effect on the shape of the echo profile. To demonstrate the effects of sub-resolution relief on echo profiles, we have calculated the echo shapes from a wide range of simple topographic models. At this point, our topographic models have emphasized surfaces where only two dominant elevations are contained within a footprint, such as graben, ridges, crater rims, and central features in impact craters.
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- 1993
27. Venus tectonics - An overview of Magellan observations
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Solomon, Sean C, Smrekar, Suzanne E, Bindschadler, Duane L, Grimm, Robert E, Kaula, William M, Mcgill, George E, Phillips, Roger J, Saunders, R. S, Schubert, Gerald, and Squyres, Steven W
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Lunar And Planetary Exploration - Abstract
Magellan observations of the tectonic characteristics of highland regions on Venus are discussed with reference to competing theories for highland formation and evolution. Complex rigid terrain, or tessera, and the extent to which these elevated blocks of intensely deformed crust may be genetically related to highlands are then considered. Further, the tectonics of plains and lowland regions are examined, including deformation belts and coronae, and possible relations between such features and mantle dynamics. Implications of these observations for the global tectonics of Venus are discussed.
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- 1992
28. Plains tectonism on Venus - The deformation belts of Lavinia Planitia
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Squyres, Steven W, Jankowski, David G, Simons, Mark, Solomon, Sean C, Hager, Bradford H, and Mcgill, George E
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Lunar And Planetary Exploration - Abstract
The belts of the Lavinia Planitia region of Venus are discussed in detail with reference to high-resolution radar images from the Magellan spacecraft. Two types of deformation belts are identified: ridge belts and fracture belts. Ridge belts are composed of parallel ridges, each a few hundred meters in elevation, that are interpreted as folds. Typical fold spacings are 5-10 km. Fracture belts are dominated by intense faulting, with faults in some instances paired to form narrow grabens. The formation of ridge and fracture belts is explored using simple models.
- Published
- 1992
29. Origin of giant Martian polygons
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Mcgill, George E and Hills, L. S
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Lunar And Planetary Exploration - Abstract
Extensive areas of the Martian northern plains in Utopia and Acidalia planitiae are characterized by 'polygonal terrane'. Polygonal terrane consists of material cut by complex troughs defining a pattern resembling mudcracks, columnar joints, or frost-wedge polygons on earth. However, the Martian polygons are orders of magnitude larger than these potential earth analogues, leading to severe mechanical difficulties for genetic models based on simple analogy arguments. Plate-bending and finite element models indicate that shrinkage of desiccating sediment or cooling volcanics accompanied by differential compaction over buried topography can account for the stresses responsible for polygon troughs as well as the large size of the polygons. Although trough widths and depths relate primarily to shrinkage, the large scale of the polygonl pattern relates to the spacing between topographic elevations on the surface buried beneath polygonal terrane material. Geological relationships favor a sedimentary origin for polygonal terrane material, but our model is not dependent on the specific genesis. Our analysis also suggests that the polygons must have formed at a geologically rapid rate.
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- 1992
30. Derivation of surface properties from Magellan altimetry data
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Lovell, Amy J, Schloerb, F. Peter, and Mcgill, George E
- Subjects
Lunar And Planetary Exploration - Abstract
The fit of the Hagfors model to the Magellan altimetry data provides a means to characterize the surface properties of Venus. However, the derived surface properties are only meaningful if the model provides a good representation of the data. The Hagfors model provides a good representation of the data. The Hagfors model is generally a realistic fit to surface scattering properties of a nadir-directed antenna such as the Magellan altimeter; however, some regions of the surface of Venus are poorly described by the existing model, according to the goodness of fit parameter provided on the ARCDR CD-ROMs. Poorly characterized regions need to be identified and fit to new models in order to derive more accurate surface properties for use in inferring the geological processes that affect the surface in those regions. We have compared the goodness of fit of the Hagfors model to the distribution of features across the planet, and preliminary results show a correlation between steep topographic slopes and poor fits to the standard model, as has been noticed by others. In this paper, we investigate possible relations between many classes of features and the ability of the Hagfors model to fit the observed echo profiles. In the regions that are not well characterized by existing models, we calculate new models that compensate for topographic relief in order to derive improved estimates of surface properties. Areas investigated to date span from longitude 315 through 45, at all latitudes covered by Magellan. A survey of those areas yields preliminary results that suggest that topographically high regions are well suited to the current implementation of the Hagfors model. Striking examples of such large-scale good fits are Alpha Regio, the northern edges of Lada Terra, and the southern edge of Ishtar Terra. Other features that are typically well fit are the rims of coronae such as Heng-O and the peaks of volcanos such as Gula Mons. Surprisingly, topographically low regions, such as the ubiquitous plains areas, are modeled poorly in comparison. However, this generalization has has exceptions: Lakshmi Planum is an elevated region that is not well fit compared to the rest of neighboring Ishtar, while the southern parts of topographically low Guinevere Planitia are characterized quite well by the Hagfors model. Features that are candidates for improved models are impact craters, coronae, ridges of significant scale, complex ridged terrains, moderate-sized mountains, and sharp terrain boundaries. These features are chosen because the goodness of fit is likely to be most affected either by departures from normal incidence angles or by sharp changes in terrain type within a single footprint. Most large features that are elevated with respect to their surroundings will suffer from steep slope effects, and smaller coronae and impact craters will probably suffer due to rapid changes in their appearance within a single footprint (10-20 km).
- Published
- 1992
31. Structure and kinematics of a major tectonic contact, Michipicoten greenstone belt, Ontario
- Author
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Mcgill, George E
- Subjects
Geophysics - Abstract
The Michipicoten greenstone belt, Ontario, experienced a complex history of folding, faulting, and fabric development. Near Wawa, a major east-west contact, here named the Steep Hill Falls (SHF) contact, extends entirely across the belt. The SHF contact is both an angular unconformity and a fault and is interpreted to be a regionally significant tectonic contact separating distinct northern and southern terranes, both of which include volcanic rocks of probable island-arc origin. The amount of horizontal transport involved in bringing the two terranes together along the SHF contact is not known. Mapping and structural analysis suggest that regionally significant horizontal displacements took place, with movement vectors that changed with time. Early faults, folds, and fabrics imply north-south to northeast-southwest (with respect to present directions) convergence, with a vergence reversal occurring during this complex event. The most likely models infer early south vergence and later north vergence. Transecting the earliest structures are younger (but still Archean) northeast-striking steep cleavages with associated upright folds that may relate to northwest-southeast assembly of the Superior Province craton. The craton assembly event thus involved a transport direction at a high angle to that inferred for the earlier assembly of the Michipicoten greenstone belt.
- Published
- 1992
32. Wrinkle ridges on Venusian plains: Indicators of shallow crustal stress orientations at local and regional scales
- Author
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Mcgill, George E
- Subjects
Lunar And Planetary Exploration - Abstract
The plains regions of Venus exhibit a complex array of structural features, including deformation belts of various types, wrinkle ridges, grabens, and enigmatic radar-bright linears. Probably the most pervasive of these structures are the wrinkle ridges, which appear to be morphologically identical to their counterparts on the Moon and Mars. Almost all workers agree that wrinkle ridges result from horizontal compressive stresses in the crust; they either are explained as flexural fold structures, or alternatively as scarps or folds related to reverse faults. Wrinkle ridges generally are narrow, have small amplitudes, and commonly are closely spaced as well, characteristics that imply a shallow crustal origin. If wrinkle ridges are due to horizontally directed compressive stresses in the shallow crust, as generally has been inferred, then the trends of these features provide a means to map both local and regional orientations of principal stresses in the uppermost part of the venusian crust: maximum compressive stress is normal to the ridges, minimum compressive stress is normal to the topographic surface, and thus the wrinkle ridge trends trace the orientation of the intermediate principal stress. Because there are few plains areas on Venus totally devoid of wrinkle ridges, it should be possible to establish a number of interesting relationships on a near-global scale by mapping the trends of wrinkle ridges wherever they occur. The present study is addressing three questions: (1) Do the trends of wrinkle ridges define domains that are large relative to the sizes of individual plains regions? If so, can these domains be related to large-scale topographic or geologic features? (2) Are regional trends of wrinkle ridges affected by local features such as coronae? If so, is it possible to determine the relative ages of the far-field and local stresses from detailed study of trend inheritance or superposition relationships? (3) What is the relationship between wrinkle ridges and the larger ridges that make up ridge belts?
- Published
- 1992
33. Stratigraphy of the Perrine and Nun Sulci quadrangles (Jg-2 and Jg-5), Ganymede
- Author
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Mcgill, George E and Squyres, Steven W
- Subjects
Lunar And Planetary Exploration - Abstract
Dark and light terrain materials in the Perrine and Nun Sulci quadrangles are divided into nine map units, four dark, and five light. These are placed in time-stratigraphic sequence primarily by means of embayment and cross-cutting relationships. Dark terrain is generally more heavily cratered and thus older that light terrain but, at least in these quadrangles, crater densities are not reliable indicators of relative ages among the four dark material units. The four mapped material units within dark terrain are: cratered dark materials (dc), grooved dark materials (dg), transitional dark materials (di), and dark materials, undivided (d). The five mapped units within light terrain are: intermediate light materials (li), grooved light materials (lg), irregularly grooved light materials (lgl), smooth light materials (ls), and light materials, undivided.
- Published
- 1991
34. Structural geology and geological history of the Perrine and Nun Sulci Quadrangles (Jg-2 and Jg-5), Ganymede
- Author
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Mcgill, George E and Squyres, Steven W
- Subjects
Lunar And Planetary Exploration - Abstract
Grooves are the dominant structural features on Ganymede. While single grooves are found in many areas, it is somewhat more common to find them grouped together in groove sets (groupings of grooves with common structural trends). It is generally believed that the grooves are extensional features. Their underlying geologic nature cannot be determined from Voyager images, however. It appears likely that they are grabens, but the images are insufficient to rule out the possibility that they are modified extension fractures or some kind of ductile necking features. The oldest materials within the Nun Sulci and Perrine quadrangles occur within the cratered dark materials unit, as is generally the case for all of Ganymede. This is the most widespread of the dark units mapped in these two quadrangles. Crater densities within cratered dark materials are consistent with an age on the order of several billion years, and thus cratered dark terrain probably represents crust that has survived from the end of the primordial intense bombardment phase of solar system history. The brief geologic history is greatly oversimplified as a result of the poor resolution and unfavorable viewing geometry of the images covering the area.
- Published
- 1991
35. Tectonic evolution of Lavinia Planitia, Venus
- Author
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Squyres, Steven W, Frank, Sharon L, Mcgill, George E, and Solomon, Sean C
- Subjects
Lunar And Planetary Exploration - Abstract
High resolution radar images from the Magellan spacecraft have revealed the first details of the morphology of the Lavinia Planitia region of Venus. Lavinia is a broad lowland over 2000 km across, centered at about 45 deg S latitude, 345 deg E longitude. Herein, the tectonic evolution of Lavinia is discussed, and its possible relationship to processes operating in the planet's interior. The discussion is restricted to the region from 37.3 to 52.6 deg S latitude and from about 340 to 0 deg E longitude. One of the most interesting characteristics of Lavinia is that the entire region possesses a regional tectonic framework of striking regularity. Lavinia is also transected by a complex pattern of belts of intense tectonic deformation known as ridge belts. Despite the gross topographic similarity of all of the ridge belts in Lavinia, they exhibit two rather distinct styles of near surface deformation. One is composed of sets of broad, arch-like ridges rising above the surrounding plains. In the other type, obvious fold-like ridges are rare to absent in the radar images. Both type show evidence for small amounts of shear distributed across the belts.
- Published
- 1991
36. Origin of the Martian global dichotomy by crustal thinning in the late Noachian or early Hesperian
- Author
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Mcgill, George E and Dimitriou, Andrew M
- Subjects
Lunar And Planetary Exploration - Abstract
The marked dichotomy in topography, surface age, and crustal thickness between the northern lowland (NL) and southern upland of Mars has been explained as due to an initially inhomogeneous crust, a single megaimpact event, several overlapping large basin impacts, and first-order convective overtum of the Martian mantle. All of these hypotheses propose that the dichotomy was formed before the end of the primordial heavy bombardment. Geological data indicate episodes of fracturing and faulting in the late Noachian and the early Hesperian, within the NL and along the lowland/highland boundary. Igneous activity also peaked in the late Noachian and early Hesperian. These data suggest a tectonic event near the Noachian/Hesperian boundary characterized by enhanced heat loss and extensive fracturing, including formation of the faults that define much of the highland/lowland boundary. It is argued that the major result of this tectonic event was formation of the dichotomy by thinning of the crust above a large convection cell or plume.
- Published
- 1990
37. The management of lupus in young people
- Author
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McGill, George, primary and Ambrose, Nicola, additional
- Published
- 2018
- Full Text
- View/download PDF
38. HERE IT IS AGAIN!—EFFORT TO ABRIDGE POWER OF FEDERAL JUDGES IN JURY TRIALS
- Author
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Ashurst, Henry F., King, William H., Neely, M. M., Van Nuys, Frederick, McCarran, Patrick, Logan, M. M., Dieterich, William H., McGill, George, Hughes, James H., Steiwer, Frederick, Hatch, Carl A., Burke, Edward R., Pittman, Key, Connally, Tom, O'Mahoney, Joseph C., Borah, William E., Norris, George W., and Austin, Warren R.
- Published
- 1937
39. Identification of brown adipose tissue in an adult human using parametric data reconstructed from a 2-point Dixon magnetic resonance imaging sequence acquired simultaneously with FDG PET
- Author
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Law, W. Phillip, primary and McGill, George, additional
- Published
- 2016
- Full Text
- View/download PDF
40. The readout thickness versus the measured thickness for a range of screen film mammography and full-field digital mammography units
- Author
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Hauge, Ingrid Helen Ryste, Hogg, Peter, Szczepura, Katy, Connolly, Paul, McGill, George, and Mercer, Claire
- Subjects
Diagnostic radiography ,Medisinske Fag: 700::Klinisk medisinske fag: 750::Radiologi og bildediagnostikk: 763 [VDP] ,Breast compression ,Biological tissues ,VDP::Medisinske Fag: 700::Klinisk medisinske fag: 750::Radiologi og bildediagnostikk: 763 ,Mammography - Abstract
Purpose: To establish a simple method to determine breast readout accuracy on mammography units. Methods: A thickness measuring device (TMD) was used in conjunction with a breast phantom. This phantom had compression characteristics similar to human female breast tissue. The phantom was compressed, and the thickness was measured using TMD and mammography unit readout. Measurements were performed on a range of screen film mammography (SFM) and full-field digital mammography (FFDM) units (8 units in total; 6 different models/manufacturers) for two different sized paddles and two different compression forces (60 and 100 N). Results: The difference between machine readout and TMD for the breast area, when applying 100 N compression force, for nonflexible paddles was largest for GE Senographe DMR+ (24 cm × 30 cm paddle: +14.3%). For flexible paddles the largest difference occurred for Hologic Lorad Selenia (18 cm × 24 cm paddle: +26.0%). Conclusions: None of the units assessed were found to have perfect correlation between measured and readout thickness. TMD measures and thickness readouts were different for the duplicate units from two different models/manufacturers.
- Published
- 2011
41. Tectonic and Geologic Evolution of the Española Basin, Rio Grande Rift: Structure, Rate of Extension, and Relation to the State of Stress in the Western United States
- Author
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GOLOMBEK, MATTHEW P., primary, McGILL, GEORGE E., additional, and BROWN, LAURIE, additional
- Published
- 1983
- Full Text
- View/download PDF
42. 291. The Disease Spectrum of Jia in Patients Attending a Dedicated Adolescent Rheumatology Service
- Author
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Ambrose, Nicola, primary, Radziszewska, Ania, additional, Suffield, Linda, additional, McGill, George, additional, Murphy, Grainne, additional, Fischer, Corinne, additional, Sen, Debajit, additional, and Ioannou, Yainnis, additional
- Published
- 2014
- Full Text
- View/download PDF
43. Venus: preliminary topographic and surface imaging results from the Pioneer orbiter
- Author
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Pettengill, Gordon H., Ford, Peter G., Brown, Walter E., Kaula, William M., Masursky, Harold, Eliason, Eric, and McGill, George E.
- Subjects
Pioneer (Space probe) -- Observations ,Venus (Planet) -- Observations ,Space probes -- Observations ,Science and technology - Published
- 1979
44. The readout thickness versus the measured thickness for a range of screen film mammography and full-field digital mammography units
- Author
-
Hauge, Ingrid H. R., primary, Hogg, Peter, additional, Szczepura, Katy, additional, Connolly, Paul, additional, McGill, George, additional, and Mercer, Claire, additional
- Published
- 2011
- Full Text
- View/download PDF
45. Basement controls on the scale of giant polygons in Utopia Planitia, Mars
- Author
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Cooke, Michele, primary, Islam, Fariha, additional, and McGill, George, additional
- Published
- 2011
- Full Text
- View/download PDF
46. Rallying: Gronholm's date with a ditch could end title challenge
- Author
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McGill, George
- Subjects
PSA Peugeot Citroen ,Transportation equipment industry ,General interest ,News, opinion and commentary - Abstract
Byline: George McGill in Perth SEBASTIEN LOEB led the Rally of Australia after 10 stages, while the world champion, Marcus Gronholm, feared his title hopes were over after plunging his [...]
- Published
- 2003
47. Buried topography of Utopia, Mars - Persistence of a giant impact depression
- Author
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Mcgill, George E
- Subjects
Lunar And Planetary Exploration - Abstract
Knobs, partially buried craters, ring fractures, and some mesas permit a qualitative determination of the topography buried beneath younger northern plains materials. These features are widely distributed in the Utopia area but are absent in a large, roughly circular region centered at about 48 deg N, 240 deg W. This implies the existence of a circular depression about 3300 km in diameter buried beneath Utopia Planitia that is interpreted to represent the central part of a very large impact basin. The presence of buried curved massifs around part of this depression, and a roughly coincident mascon, lend further support. Present topography, areal geology, and paleotopography of buried surfaces all point to the persistence of this major depression for almost the entire history of Mars.
- Published
- 1989
48. Ages of fracturing and resurfacing in the Amenthes region, Mars
- Author
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Maxwell, Ted A and Mcgill, George E
- Subjects
Lunar And Planetary Exploration - Abstract
An attempt is made to determine whether there is any tectonic evidence in the relatively recent history of the boundary zone that will place contraints on the origin of the Martian dichotomy. It is found that the timing of resurfacing events and structural modification of outlier plateaus and mesas in the Martian eastern hemisphere provides a contraint on the history of tectonic events along the cratered terrain-northern plains boundary. The circumferential grabens surrounding the Isidis basin ceased forming before the final emplacement of ridged plains on the adjacent northern lowlands. The cratered plateau east of the Isidis basin includes two crater populations; stripping of the rims of craters was complete before downfalling of the transition zone between the cratered terrain and the northern plains, and a young population of craters on the plateau records the same age as the ridged plains units north of the boundary.
- Published
- 1988
49. Structure of Southwestern Michipicoten Greenstone Belt, Ontario: Evidence for Archean accretion
- Author
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Mcgill, George E and Shrady, Catherine H
- Subjects
Geophysics - Abstract
The results of detailed mapping of a well exposed (by a large fume kill) Archean Michipicoten greenstone belt of Ontario is discussed. Numerous structural features are described including soft sediment deformation, thrust faults, isoclinal folds, and sill intrusion, which apparently formed prior to the earliest flattening cleavage. It was inferred that all of these events may have taken place before the rocks were completely lithified, and the question was raised as to whether early soft sediment and tectonic deformation were coeval. If so, the overall picture would permit (but not compel) recognition of a convergent oceanic environment such as an accretionary wedge or forearc basin.
- Published
- 1988
50. Age of fracturing and mesa development in the Elysium Area, northern Martian plains
- Author
-
Mcgill, George E
- Subjects
Lunar And Planetary Exploration - Abstract
One of the fundamental questions of Martian crustal history is the origin of the crustal dichotomy between northern plains and southern highlands. Hypotheses for the origin of the dichotomy may be constrained by global scale, geophysical considerations, or by detailed geological studies of the genesis and relative ages of materials and landforms in the northern plains of Mars and along the boundary between the plains and the highlands. This abstract summarizes progress on one aspect of a long-range geological study intended to constrain hypotheses for the dichotomy by tracing the history of the northern plains from the most recent events backward -- essentially the same approach used to understand old events in Earth history. Both the giant impact and the mantle convection models for the crustal dichotomy imply a major coeval fracturing event. As has been known for some time, the present dichotomy boundary lies well south of its original position, at least in some places. Nevertheless, there is severe fracturing of this present dichotomy boundary in many places. The specific objectives of this study are to determine the ages of fracturing along the present dichotomy boundary.
- Published
- 1987
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