Journal Article IAEA Safety Series Report Series: No. 60: 'Radiation Protection in Newer Medical Imaging Techniques: Cardiac CT'IAEA Safety Series Report Series: No. 61: 'Radiation protection in Newer Medical Imaging Techniques: CT Colonography' Get access IAEA SAFETY SERIES REPORT SERIES: NO. 60: 'RADIATION PROTECTION IN NEWER MEDICAL IMAGING TECHNIQUES: CARDIAC CT'Published by: International Atomic Energy Agency, Vienna, AustriaISBN-978-92-0-112808-8, 19 pp. ( 2008). Euro 28 (Soft cover)IAEA SAFETY SERIES REPORT SERIES: NO. 61: 'RADIATION PROTECTION IN NEWER MEDICAL IMAGING TECHNIQUES: CT COLONOGRAPHY'Published by: International Atomic Energy Agency, Vienna, AustriaISBN-978-92-0-1113805-8, 27 pp. ( 2008). Euro 28 (Soft cover) Dawn Banghart, CHP, Dawn Banghart, CHP Stanford University Search for other works by this author on: Oxford Academic PubMed Google Scholar Ralph H. Thomas Ralph H. Thomas University of California (Retired)Moraga, California Search for other works by this author on: Oxford Academic PubMed Google Scholar Radiation Protection Dosimetry, Volume 138, Issue 1, January 2010, Pages 92–96, https://doi.org/10.1093/rpd/ncp248 Published: 03 November 2009

Since sea level rise over the past century can be only partially explained by ocean warming and melting of non‐polar ice, we can safely assume that the volume of the Greenland and Antarctic ice sheets probably is decreasing. However, many decades of glaciological observations have yet to yield even a good estimate of the collective mass balance of these two ice sheets. Until recently this uncertainty applied equally to both ice sheets. Then NASA's Program for Arctic Regional Climate Assessment (PARCA) researched the mass balance of the Greenland ice sheet by measuring changes in icesheet volume through different approaches, by analyzing satellite and aircraft data to investigate various characteristics of the ice sheet, and by conducting in situ measurements aimed at understanding observed changes.

The spatial pattern of elevation change estimates derived from satellite radar altimeter data for the period from 1978 to 1988 for the southern Greenland ice sheet is examined. As reported previously, the results from 12 ice cores widely distributed in the study area indicate that much of the spatial variability in the elevation change estimates can be explained by temporal variations in accumulation. Most notably, the areas of largest thickening and thinning, east and west of the ice divide around 66°N, recently experienced substantial decadal fluctuations in accumulation sufficient to explain the observed elevation change rates that span the range of ±24 cm yr−1. Elevation change estimates in the NW of the study area around the 2000‐m elevation contour were found to be inconsistent with both short‐ and long‐term changes in accumulation fluctuations. Examination of stratigraphy in several shallow ice cores in the NW indicates that significant melting took place in 1977 and 1978, and this was followed by many years of very little melt activity. This likely resulted in a significant downward bias in the 1978–1988 elevation change estimates in the NW. Comparison of measured surface velocities with those calculated assuming steady state balance show very good agreement overall, but there are many instances in the southeast where the steady state velocities are significantly smaller than measured surface velocities. We cannot rule out the possibility that increased ice flow in lower‐elevation outlet glaciers has migrated farther upstream thereby causing negative long‐term mass imbalance in southeast Greenland.

During September 1991, April 1992 and June/July 1993, a NASA P-3 aircraft, equipped with a scanning laser altimeter, flew numerous transects of the Greenland ice sheet. The aeroplane location was measured precisely using differential Global Positioning System (GPS) surveying techniques, allowing all altimetry data to be converted into measurements of ice-surface elevation relative to the Earth ellipsoid. Results from flight data indicate that ice-surface elevations can be reliably measured to an accuracy of similar to 20 cm (and possibly to similar to 10 cm) over baselines of more than seven hundred kilometres.

This paper presents a mixing study using gas injection at low transverse angles into M = 3 and M = 6 crossflows. Effects of injectant asymmetries on the mixing processes were also tested by including low injector yaw angles. Ambient temperature helium was injected at matched pressure conditions as well as at 5 x matched pressure into a Mach 6 freestream, with Reynolds number of 5.4 x 10(7)/m. Complementary tests at Mach 3, Re = 5.0 x 10(7)/m, were performed. The primary data are concentration measurements and meanflow measurements with nanosecond exposure shadowgraphs and surface now visualization. At Mach 6, the matched pressure injection case, as compared to the underexpanded case, was found to show a greater injectant core penetration growth rate and a greater concentration decay rate. When the Mach 6 results were compared to similar experiments with a Mach 3 freestream, only a small decrease in the downstream core penetration and mixing rates was observed. However, due to much less initial mixing near the injector at the higher Mach number, the Mach 6 results showed a longer distance for the injectant core to decay to a concentration level corresponding to the H-2-air stoichiometric ratio. Other results showed that the plume remained supersonic and that moderate total pressure losses were found. Also observed was that, for both Mach 3 and Mach 6 freestreams, injector yaw did not increase the rate of decay of the maximum concentration, but that yaw did cause an increase in the overall injectant plume cross section, thus increasing the size of the mixing region.

A critical review of the available high speed mixing experimental database is presented. Experiments of concern involve measurements of species concentration downstream of sonic or supersonic injection of a light gas into a supersonic air stream. Several classes of injection are considered including transverse jets, wall slots, and some hybrid cases. The review is made difficult by the sparseness of the current database. Mixing data is summarized primarily in the form of the downstream decay of the maximum concentration. Most of the data in the far field can be fitted to a. power law curve with the exponent giving an indication of the rate of mixing. The resulting plot of the experiments presented provides a basis for preliminary comparison of high speed injection concepts. Spacing of jets below a critical value can reduce the rate of mixing by a factor ot two. Initial mixing for transverse jets is strongly dependent on the dynamic pressure ratio. The farfield mixing rate for wall slot injectors is found to be comparable to that for arrays of transverse jets, not slow as has been believed. As a crude estimate, it is shown that all the data for all the configurations falls into a band with a decay rate of about.T0,8. The need for more high quality experiments for high speed mixing in support of such difficult problems as supersonic combustion is apparent. The need for careful sampling probe design is emphasized. Finally, the state of analyses for these flows is discussed. A new analysis for arrays of transverse jets is presented.

The three-dimensional Reynolds-averaged Navier-Stokes equations are solved by a finite element method for three slender body conditions. The propeller is modeled by an actuator disk. A zonal turbulence model is used: an integrated turbulent, kinetic energy model in the core of the flow, a mixing-length formulation in the outer region, and a Prandtl wake model downstream of the strut. The turbulence model is incorporated into a modified version of the finite element code FIDAP. Predictions compare very favorably with wind tunnel experiments for the three conditions: self-propelled, 100% overthrust, and propulsion from an ideal-rotor. The swirl was better predicted than previous attempts at this problem. The multilayer turbulence model was essential to proper prediction of the main features of the flow.

A combination of tangential and normal air injection into a Mach 3 airflow was experimentally studied. A rearward facing slot producing tangential injection at a nominal Mach number 1.7 was operated at several different total pressures. An array of transverse tubes of height equal to the slot height and placed just downstream of the slot was operated at two dynamic pressure ratios at both Mach 1 and 2.2. Mean flow measurements of static and total pressures were taken up to 20 slot heights downstream from which Mach number, density and velocity profiles, and entrainment rates were calculated. Various dimensions and spreading angles of the mixing regions were measured directly from nanoshadowgraphs and spark schlieren photographs. Large eddy structures were produced in several cases, leading to increased entrainment of the freestream. For some cases, heated air was injected through the normal tubes, and the jet total temperature decay was measured downstream. It can be seen from the data that the mixing rate can be significantly increased by the combined tangential-normal injection design over tangential slot injection alone, with up to 92% more entrained mass.

Detailed measurements of surface topography, ice motion, snow accumulation, and ice thickness were made in January 1974 and again in December 1984, along an 8 km stake network extending from the ice sheet, across the grounding line, and on to floating ice shelf in the mouth of slow-moving Ice Stream C, which flows into the eastern side of Ross Ice Shelf, Antarctica. During the 11 years between surveys, the grounding line retreated by approximately 300 m. This was caused by net thinning of the ice shelf, which we believe to be a response to the comparatively recent, major decrease in ice discharge from Ice Stream C. Farther inland, snow accumulation is not balanced by ice discharge, and the ice stream is growing progressively thicker. There is evidence that the adjacent Ice Stream B has slowed significantly over the last decade, and this may be an early indication that this fast-moving ice stream is about to enter a period of stagnation similar to that of Ice Stream C. Indeed, these large ice streams flowing from West Antarctica into Ross Ice Shelf may oscillate between periods of relative stagnation and major activity. During active periods, large areas of ice shelf thicken and run aground on seabed to form extensive “ice plains” in the mouth of the ice stream. Ultimately, these become too large to be pushed seaward by the ice stream, which then slows down and enters a period of stagnation. During this period, the grounding line of the ice plain retreats, as we observe today in the mouth of Ice Stream C, because nearby ice shelf, no longer compressed by ice-stream motion, progressively thins. At the same time, water within the deformable till beneath the ice starts to freeze on to the base of the ice stream, and snow accumulation progressively increases the ice thickness. A new phase of activity would be initiated when the increasing gravity potential of the ice stream exceeds the total resistance of the shrinking ice plain and the thinning layer of deformable till at the bed. This could occur rapidly if the effects of the shrinking ice plain outweigh those of the thinning (and therefore stiffening) till. Otherwise, the till layer would finally become completely frozen, and the ice stream would have to thicken sufficiently to initiate significant heating by internal deformation, followed by basal melting and finally saturation of an adequate thickness of till; this could take some thousands of years.

As part of a systematic analysis of Seasat radar altimetry data to obtain Antarctic ice fronts and ice-shelf elevations north of lat. 72° S., Fimbulisen (between long. 12°W. and 08°E.) and the Amery Ice Shelf (around long. 72°E.) are mapped. Interactive computer analysis is used to examine and correct the altimetry range measurements and derive the ice-front positions. Surface elevations and ice-front positions from radar altimetry are compared with ice fronts, ice rises, crevasse zones, and grounding lines identified in Landsat imagery. By comparison of the visible features in imagery and the computer-contoured elevations from radar altimetry, the radar-elevation mapping on some ice rises is confirmed, but some spurious contours are also identified. During the interval between the 1974 Landsat imagery and the 1978 radar altimetry, the central part of the Amery Ice Shelf front advanced 1.5 ± 0.6 km/a, which is in agreement with the ice-velocity measurements of 1.1 ± 0.1 km/a (Budd and others 1982), suggesting negligible calving in the central part of the ice shelf. The undulating surface and small mean slope from the grounding line to about lat. 70°S. suggest a zone of partial grounding similar to Rutford Ice Stream, On Fimbulisen, some previously unmapped ice rises are identified. The ridge of the Jutul-straumen ice tongue is shown to be about 20 m above the surrounding ice and laterally expanding as it flows northward to the ice front. Icebergs within the sea ice and a zone of shore-fast ice are also identified with the same technique used to map the ice-shelf front.

In a recent paper, Thomas et.al. (1984) showed how the coast of Antarctica could be mapped using satellite altimetry data. As the satellite approached the continent from the ocean, the Seasat altimeter obtained strong reflections from sea ice, even for a short time after passing over the ice front. Measured ranges are actually oblique distances to the nearest portion of sea ice, yielding a false drop in surface elevation. From the sequence of oblique ranges during a single orbit crossing of the ice cliff, the horizontal position of a segment of the ice cliff is mapped. Currently, the entire Seasat data set is being analyzed to map most of the Antarctic coastline north of 72 °S to an accuracy of ± 0.1 to 1 km, which is a major improvement over existing surveys. The altimeter waveforms corresponding to each range measurement are computer analyzed (“retracked”), using procedures that account for the specular reflections from sea ice and the diffuse reflections from firn. Each waveform analysis, along with the corrected range for data obtained in the vicinity of an ice cliff crossing, is verified or recomputed on an interactive computer, which also computes and maps the position of the ice front. The locations of several tabular icebergs have also been mapped with the same procedures, which can ultimately be used to obtain an estimate of iceberg population density in polar waters, initial results include the mapping of the Larsen ice shelf on the eastern side of the Antarctic Peninsula, showing, for example, the protrusion at approximately 68.5 °S. Estimates of errors in the derived horizontal position are obtained from the analysis of data from repeating orbit tracks. Comparison of these results with results from future altimetry missions will reveal changes in the position of coastal ice cliffs, due to ice movement and/or iceberg calving. Systematic measurements over several years would probably distinguish the effects of iceberg calving, which is intermittent, from those of ice movement, which is continuous.

Colonies of Microcystis in Abbots Pool, Avon, UK, were found to regulate their buoyancy according to light (photon flux density). The autumnal decline of the population was associated with an increase in the proportion of colonies that were non-buoyant, and with declining temperatures in the pond. Non-buoyant colonies taken from the pond regained buoyancy in the dark rapidly at 20°C but only slowly at 12°C and below. A laboratory strain of Microcystis behaved in a similar manner. Comparisons of the behaviour of this organism placed at 8°C and 20°C were made; in high photon flux density buoyancy was lost at both temperatures due to accumulation of dense carbohydrate. When transferred to the dark cells at 20°C became buoyant again as carbohydrate was utilized and more gas vesicles were made; at 8°C much less carbohydrate was used and no increase in gas-vacuolation occurred. The failure to regain buoyancy in the dark at low temperatures accounts for the loss of buoyancy and sedimentation of the Microcystis in Abbots Pool.

Liquid and slurry jets were injected through a circular orifice transverse to a M = 3 airflow. Mass samples of both jets were taken across the plume 30 injector diameters downstream. Pitot and static pressure surveys were taken across the liquid jet. These data allowed the calculation across the liquid jet plume of the Mach number, air mass flow, liquid-to-air ratio, and momentum flux. In the center third of the plume area, there is a core region of subsonic airflow that carries two-thirds of the mass collected in the plume. In the core, the liquid mass flow is nearly constant from side to side at a given height and the average velocity of the liquid is only 30-60% of the local air velocity. A supersonic mixing region surrounds the core region. Comparison with these direct sampling results indicate that correlations developed from photography are inadequate in determining the penetration and width of either jet. The slurry jet showed substantial phase separation. When a 30% mass-loaded slurry of 1-5 p,m silicon dioxide particles mixed with water was injected, the local loading varied from a low of 13% at the bottom of the plume to 100% outside the plume.

Colonies of Microcystis aeruginosa have dominated the phytoplankton in Lund tube C, a limnetic enclosure in Blelham Tarn, English Lake District, during the summer and autumn in recent years. Following holomixis in autumn the previously buoyant colonies sedimented from the water column onto the bottom mud. In all samples gas vesicles, which provided the colonies with buoyancy, were present in sufficient volume to negate the combined ballast provided by protein, carbohydrate, lipid and phosphate, the major cell components. The gas vesicles, which accounted for about 10% of the cell protein, were too strong to be collapsed or regulated by cell turgor pressure. Consequently, the loss of buoyancy could not be explained by an increase in cell ballast or by disappearance of gas vesicles. Colonies collected in sediment traps were found to be buoyant after they had been agitated and diluted with lake water, which removed a colloidal precipitate from them. Similarly, 66% of the sinking fraction of a net tow sample was found to be buoyant after it had been treated in the same way. Previously buoyant colonies could be made to sink on mixing with the colloidal precipitate. This demonstrated the ability of the precipitate to trap colonies and to cause their sedimentation. The colloid comprised approximately equal amounts of organic and inorganic matter and was rich in iron. Colloids of this type form when the dissolved iron in the anoxic water of the hypolimnion becomes oxidized on mixing with the aerated water of the epilimnion.

A strain of the gas-vacuolate cyanobacterium Microcystis was found to float in cultures grown at low light intensities and to sink in those grown at high intensities. The loss of buoyancy that occurred within 1 to 5 h on increasing the photon flux density from 10 to l00 μmol m–2 s–1 was investigated by centrifuging the cell suspensions in a horizontally placed capillary with a rectangular cross-section, and then separately counting the floating cells under the upper tube surface and sinking cells on the lower surface. Buoyancy loss was not accompanied by loss of gas vesicles, as occurs in some other planktonic cyanobacteria, but was caused by a relative increase in dry matter, principally carbohydrate, without a corresponding increase in gas vesicles. The increase in light intensity gave an increase in cell turgor pressure but this was insufficient to collapse the strong gas vesicles present in this strain, which had a median critical pressure of 0·75 MPa (7·5 bar).

(1984). “Tamper Control Packaging with Plastics”. Drug Development and Industrial Pharmacy: Vol. 10, No. 2, pp. 203-210.

Ranges obtained by radar altimetry from a satellite to a sloping surface on the earth are not measurements of the surface elevation at the subsatellite point. Instead, the reflecting point is displaced upslope from the subsatellite point causing a 'slope‐induced error' between the true range to the subsatellite point and the indicated range. For the SEASAT 1 altimeter, this range error is nearly 80 m for a 0.8 degree slope. Ice sheet surface slopes are frequently large enough to cause errors of 10 m or more. In addition, undulations in the surface about a mean slope cause pronounced variations in these errors. Altimeter measurements of ranges to modeled irregular surfaces are simulated and two correction schemes are used to reconstruct the modeled surfaces from the simulated data. The results illustrate the fundamental limitations inherent in single‐beam radar altimetry for mapping irregular surfaces. In the simpler two‐dimensional case, for which cross‐track slopes are neglected, a relocation scheme, which constructs a surface consistent with the altimeter ranges, removes 85% of the rms error. The mean error along profiles of about 75 km or longer is usually reduced more than the rms error. An alternate slope correction scheme, which uses the local slope to calculate the expected error, is less effective in the two‐dimensional case. However, over a simulated three‐dimensional surface, where groundtracks are widely spaced and cross‐track slopes are significant, the slope‐correction method must be used in at least the cross‐track direction. The effectiveness of the three‐dimensional correction depends on the relative size of the errors caused by the local slope on surface undulations in comparison to the regional slope.

Surface elevations of the ice sheets are contoured at 50‐m intervals for the region of Greenland covered by SEASAT radar altimetry south of 72°N and at 100‐m intervals for a region of East Antarctica north of 72°S. The surface elevations were obtained from computer retracking of the radar altimeter waveforms, which were recorded at 0.1‐s intervals corresponding to 662‐m spacings on the surface. The precision of the elevation measurements before adjustment for radial orbit errors is 1.9 m as shown by analysis of elevation differences at orbital crossover points. This precision is partly determined by radial errors of approximately 1.0 m in orbit determination and partly by noise due to ice surface irregularities. Adjustment of the radial components of the orbits to minimize the differences in elevations at crossovers over a small, relatively flat region reduces the rms difference to 0.25 m, which is indicative of the optimum precision obtainable over the ice sheets. However, the precision degrades as the slope of the surface or amplitude of the undulations increases, yielding an overall precision of ±1.6 m. The preliminary contour maps are not corrected for slope‐induced displacements. A 2‐m contour map in a region of highest data density illustrates the three‐dimensional characteristics of some surface undulations.

Results of the Ross Ice Shelf Geophysical and Glaciologlcal Survey (RIGGS) provide the most complete data set available for any large portion of the polar ice sheets. In this paper, we use RIGGS data to calculate some ice-shelf characteristics. These include steady-state particle trajectories through the ice shelf and the depth of isochronous surfaces, which are of particular importance in choosing a drilling site where ice from the grounded West Antarctic ice sheet is likely to be near the surface. Our estimates for depth to ice originating from the 500 m elevation contour show good agreement with depths to a glaciochemical transition in four ice cores that is believed to be associated with this elevation. This suggests that, for much of the ice shelf, there have been no dramatic and sustained departures from steady state during the past 1.5 to 2.5 ka. With the RIGGS data and an assumed bottom melting rate distribution we calculate steady-state temperature profiles at each of the measurement stations. Then, adopting an ice-flow law deduced from laboratory experiments and ice-shelf measurements, we obtain an effective flow-law parameter for each of these sites. Using these values, the measured strain rate field is transformed to an equivalent stress field over the ice shelf. The stresses are determined by the ice-shelf freeboard and by the force field exerted on the ice shelf by its sides and by ice rises, and our analysis yields estimates of these restraining forces F for the Ross Ice Shelf. An apparent increase in F very close to the ice front suggests that the ice shelf possesses a narrow seaward fringe of anomalously stiff ice. We suspect that this represents the effects of increased bottom-melting rates (and therefore colder and stiffer ice) very close to the ice front. In order to illustrate the role of the restraining forces in controlling ice-shelf behavior, we calculate the strain-rate field for an unrestricted Ross Ice Shelf, i.e. one that is detached from its sides and contains no ice rises. Currently the creepthinning rate for most of the ice shelf is 0.5 to 1m a−1 for an unrestricted ice shelf, but it would increase to 1 to 10 m a −1, with values up to 60 ma− 1 up-stream of the ice rises This paper has been accepted for publication in the Journal of Glaciology.