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The modified BSS-EPID and associated dose model provide an improved system for dosimetry measurements using EPIDs. Several important limitations of the current hardware and software are addressed by this system.

A B S T R A C TThis study examines the language-driven aspects of the formation of a classroom-based community of practice (CoP), placing emphasis on ways in which researchers can verify the status of observed practices. Discourse analysis is reinforced by such an evidence-based understanding of the social milieu of a research site. When determining whether an aggregate of people is functioning as a CoP, however, the nature of the measuring stick is a vital question. When institutional forces have brought a group of participants together, how can an observer verify empirically the dynamic development of mutual engagement, joint enterprise, and shared repertoire? In a sample case study, representative features outlined by Wenger (1998:130-31) are identified, and their emergence traced, via analysis of ethnographic fieldnotes and audio recordings. These features provide evidence of the development of localised practices (i.e. ways of doing grounded in this community) as distinct from more widely recognisable practices. Identifying the difference increases the likelihood that results of discourse analysis can be useful to educators. (Community of practice, discourse analysis, nexus of practices, warranting)* I N T R O D U C T I O NThe aim of this article is twofold. The primary aim is to argue that communities of practice theory can provide fruitful constructs for sociolinguistic research, but in some settings at least, the researcher must plausibly demonstrate that the aggregate of people under the researcher's gaze indeed constitutes a community of practice with a localised repertoire of practices, including language. Thus the second aim is to explore how this demonstration might be achieved. Describing social organisation in any research setting is potentially fraught with problems if the describer (i.e. the writer of research) has not paid close attention to how the people there have come to understand one another within a particular setting-what Wenger (1998:79) refers to as their particular response to the conditions in which they find themselves. Thus an ethnographic approach to research provides invaluable

A key requirement for the development of the field of medical x-ray scatter imaging is accurate characterization of the differential scattering cross sections of tissues and phantom materials. The coherent x-ray scattering form factors of five tissues (fat, muscle, liver, kidney, and bone) obtained from butcher shops, four plastics (polyethylene, polystyrene, lexan (polycarbonate), nylon), and water have been measured using an energy-dispersive technique. The energy-dispersive technique has several improvements over traditional diffractometer measurements. Most notably, the form factor is measured on an absolute scale with no need for scaling factors. Form factors are reported in terms of the quantity x = λ(-1)sin (θ/2) over the range 0.363-9.25 nm(-1). The coherent form factors of muscle, liver, and kidney resemble those of water, while fat has a narrower peak at lower x, and bone is more structured. The linear attenuation coefficients of the ten materials have also been measured over the range 30-110 keV and parameterized using the dual-material approach with the basis functions being the linear attenuation coefficients of polymethylmethacrylate and aluminum.

The long-term stability of three clinical electronic portal imaging devices (EPIDs) was studied to determine if longer times between calibrations can be justified. This would make alternatives to flood-field calibration of EPIDs clinically feasible, allowing for more effective use of EPIDs for dosimetry. Images were acquired monthly for each EPID as part of regular clinical quality assurance over a time period of approximately 3 years. The images were analysed to determine (1) the long-term stability of the EPID positioning system, (2) the dose response of the central pixels and (3) the long term stability of each pixel in the imager. The position of the EPID was found to be very repeatable with variations less than 0.3 pixels (0.27 mm) for all imagers (1 standard deviation). The central axis dose response was found to reliably track ion chamber measurements to better than 0.5%. The mean variation in pixel response (1 standard deviation), averaged over all pixels in the EPID, was found to be at most 0.6% for the three EPIDs studied over the entire period. More than 99% of pixels in each EPID showed less than 1% variation. Since the EPID response was found to be very stable over long periods of time, an annual calibration should be sufficient in most cases. More complex dosimetric calibrations should be clinically feasible.

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