Estimating regional flood discharge during Palaeocene-Eocene global warming.

Among the most urgent challenges in future climate change scenarios is accurately predicting the magnitude to which precipitation extremes will intensify. Analogous changes have been reported for an episode of millennial-scale 5 °C warming, termed the Palaeocene-Eocene Thermal Maximum (PETM; 56 Ma), providing independent constraints on hydrological response to global warming. However, quantifying hydrologic extremes during geologic global warming analogs has proven difficult. Here we show that water discharge increased by at least 1.35 and potentially up to 14 times during the early phase of the PETM in northern Spain. We base these estimates on analyses of channel dimensions, sediment grain size, and palaeochannel gradients across the early PETM, which is regionally marked by an abrupt transition from overbank palaeosol deposits to conglomeratic fluvial sequences. We infer that extreme floods and channel mobility quickly denuded surrounding soil-mantled landscapes, plausibly enhanced by regional vegetation decline, and exported enormous quantities of terrigenous material towards the ocean. These results support hypotheses that extreme rainfall events and associated risks of flooding increase with global warming at similar, but potentially at much higher, magnitudes than currently predicted.

Autogenic geomorphic processes determine the resolution and fidelity of terrestrial paleoclimate records.

Terrestrial paleoclimate records rely on proxies hosted in alluvial strata whose beds are deposited by unsteady and nonlinear geomorphic processes. It is broadly assumed that this renders the resultant time series of terrestrial paleoclimatic variability noisy and incomplete. We evaluate this assumption using a model of oscillating climate and the precise topographic evolution of an experimental alluvial system. We find that geomorphic stochasticity can create aliasing in the time series and spurious climate signals, but these issues are eliminated when the period of climate oscillation is longer than a key time scale of internal dynamics in the geomorphic system. This emergent autogenic geomorphic behavior imparts regularity to deposition and represents a natural discretization interval of the continuous climate signal. We propose that this time scale in nature could be in excess of 104 years but would still allow assessments of the rates of climate change at resolutions finer than the existing age model techniques in isolation.

Fluvial response to abrupt global warming at the Palaeocene/Eocene boundary.

Climate strongly affects the production of sediment from mountain catchments as well as its transport and deposition within adjacent sedimentary basins. However, identifying climatic influences on basin stratigraphy is complicated by nonlinearities, feedback loops, lag times, buffering and convergence among processes within the sediment routeing system. The Palaeocene/Eocene thermal maximum (PETM) arguably represents the most abrupt and dramatic instance of global warming in the Cenozoic era and has been proposed to be a geologic analogue for anthropogenic climate change. Here we evaluate the fluvial response in western Colorado to the PETM. Concomitant with the carbon isotope excursion marking the PETM we document a basin-wide shift to thick, multistoried, sheets of sandstone characterized by variable channel dimensions, dominance of upper flow regime sedimentary structures, and prevalent crevasse splay deposits. This progradation of coarse-grained lithofacies matches model predictions for rapid increases in sediment flux and discharge, instigated by regional vegetation overturn and enhanced monsoon precipitation. Yet the change in fluvial deposition persisted long after the approximately 200,000-year-long PETM with its increased carbon dioxide levels in the atmosphere, emphasizing the strong role the protracted transmission of catchment responses to distant depositional systems has in constructing large-scale basin stratigraphy. Our results, combined with evidence for increased dissolved loads and terrestrial clay export to world oceans, indicate that the transient hyper-greenhouse climate of the PETM may represent a major geomorphic 'system-clearing event', involving a global mobilization of dissolved and solid sediment loads on Earth's surface.

Assessment of paediatric CT dose indicators for the purpose of optimisation.

OBJECTIVES: To establish local diagnostic reference levels (LDRLs) at the Royal Children's Hospital (RCH) Melbourne, Parkville, Australia, for typical paediatric CT examinations and compare these with international diagnostic reference levels (DRLs) to benchmark local practice. In addition, the aim was to develop a method of analysing local scan parameters to enable identification of areas for optimisation. METHODS: A retrospective audit of patient records for paediatric CT brain, chest and abdomen/pelvis examinations was undertaken. Demographic information, examination parameters and dose indicators--volumetric CT dose index (CTDI(vol)) and dose-length product (DLP)--were collected for 220 patients. LDRLs were derived from mean survey values and the effective dose was estimated from DLP values. The normalised CTDI(vol) values, mAs values and scan length were analysed to better identify parameters that could be optimised. RESULTS: The LDRLs across all age categories were 18-45 mGy (CTDI(vol)) and 250-700 mGy cm (DLP) for brain examinations; 3-23 mGy (CTDI(vol)) and 100-800 mGy cm (DLP) for chest examinations; and 4-15 mGy (CTDI(vol)) and 150-750 mGy cm (DLP) for abdomen/pelvis examinations. Effective dose estimates were 1.0-1.6 mSv, 1.8-13.0 mSv and 2.5-10.0 mSv for brain, chest and abdomen/pelvis examinations, respectively. CONCLUSION: The RCH mean CTDI(vol) and DLP values are similar to or lower than international DRLs. Use of low-kilovoltage protocols for body imaging in younger patients reduced the dose considerably. There exists potential for optimisation in reducing body scan lengths and justifying the selection of reference mAs values. The assessment method used here proved useful for identifying specific parameters for optimisation. Advances in knowledge Assessment of individual CT parameters in addition to comparison with DRLs enables identification of specific areas for CT optimisation.

Comparison of organ dosimetry methods and effective dose calculation methods for paediatric CT.

Computed tomography (CT) is the single biggest ionising radiation risk from anthropogenic exposure. Reducing unnecessary carcinogenic risks from this source requires the determination of organ and tissue absorbed doses to estimate detrimental stochastic effects. In addition, effective dose can be used to assess comparative risk between exposure situations and facilitate dose reduction through optimisation. Children are at the highest risk from radiation induced carcinogenesis and therefore dosimetry for paediatric CT recipients is essential in addressing the ionising radiation health risks of CT scanning. However, there is no well-defined method in the clinical environment for routinely and reliably performing paediatric CT organ dosimetry and there are numerous methods utilised for estimating paediatric CT effective dose. Therefore, in this study, eleven computational methods for organ dosimetry and/or effective dose calculation were investigated and compared with absorbed doses measured using thermoluminescent dosemeters placed in a physical anthropomorphic phantom representing a 10 year old child. Three common clinical paediatric CT protocols including brain, chest and abdomen/pelvis examinations were evaluated. Overall, computed absorbed doses to organs and tissues fully and directly irradiated demonstrated better agreement (within approximately 50 %) with the measured absorbed doses than absorbed doses to distributed organs or to those located on the periphery of the scan volume, which showed up to a 15-fold dose variation. The disparities predominantly arose from differences in the phantoms used. While the ability to estimate CT dose is essential for risk assessment and radiation protection, identifying a simple, practical dosimetry method remains challenging.

Radiation exposure and the justification of computed tomography scanning in an Australian hospital emergency department.

In an emergency department (ED), computed tomography (CT) is particularly beneficial in the investigation of high-speed trauma patients. With the advent of multidetector CT (MDCT) scanners, it is becoming faster and easier to conduct scans. In recent years, this has become evident with an increasing number of CT requests. Patients who have multiple CT scans during their hospital stay can receive radiation doses that have an increased theoretical risk of induction of cancer. It is essential that the clinical justification for each CT scan be considered on an individual basis and that due consideration is given to the radiation risk and possible diagnostic benefit. The current lack of a central State or Commonwealth data repository for medical images is a contributing factor to excessive radiation dosage to the population. The principles of justification and radiation risks are discussed in this study.

The clinical application of PET/CT: a contemporary review.

The combination of positron emission tomography (PET) scanners and x-ray computed tomography (CT) scanners into a single PET/CT scanner has resulted in significant improvements in the diagnosis and staging of disease, particularly in the field of oncology. A decade on from the publication of the details of the first PET/CT scanner, we review the technology and applications of the modality. We examine the design aspects of combining two different imaging types into a single scanner, and the artefacts produced such as attenuation correction, motion and CT truncation artefacts. The article also provides a discussion and literature review of the applications of PET/CT to date, covering detection of tumours, radiotherapy treatment planning, patient management, and applications external to the field of oncology.