Spatial calibration of large volume photogrammetry based metrology systems

Photogrammetry systems are used extensively as volumetric measurement tools in a diverse range of applications including gait analysis, robotics and computer generated animation. For precision applications the spatial inaccuracies of these systems are of interest. In this paper, an experimental characterisation of a six camera Vicon T160 photogrammetry system using a high accuracy laser tracker is presented. The study was motivated by empirical observations of the accuracy of the photogrammetry system varying as a function of location within a measurement volume of approximately 100 m3. Error quantification was implemented through simultaneously tracking a target scanned through a sub-volume (27 m3) using both systems. The position of the target was measured at each point of a grid in four planes at different heights. In addition, the effect of the use of passive and active calibration artefacts upon system accuracy was investigated. A convex surface was obtained when considering error as a function of position for a fixed height setting confirming the empirical observations when using either calibration artefact. Average errors of 1.48 mm and 3.95 mm were obtained for the active and passive calibration artefacts respectively. However, it was found that through estimating and applying an unknown scale factor relating measurements, the overall accuracy could be improved with average errors reducing to 0.51 mm and 0.59 mm for the active and passive datasets respectively. The precision in the measurements was found to be less than 10 μm for each axis

Delayed Carotid Surgery: What Are the Causes in the North West of England?

AbstractIntroductionCarotid endarterectomy (CEA) should be performed within two weeks of symptoms for patients with carotid stenosis >50%. Whether these standards are being achieved and causes of delay between symptoms and CEA were investigated.DesignAn analysis of prospectively collected multi-centre data.MaterialsConsecutive data for patients undergoing CEA between January-2006 and September-2010 were collected. Asymptomatic patients and those with no details on the timing of cerebral symptoms were excluded.Methods‘Delay’ from symptom to CEA was defined as more than two weeks and ‘prolonged-delay’ more than eight weeks. Univariable and multivariable analyses were used to identify factors associated with these delays.ResultsOf 2147 patients with symptoms of cerebral ischaemia, 1522(70.9%) experienced ‘delay’ and 920(42.9%) experienced ‘prolonged delay’. Patients with ischaemic heart disease were more likely to experience ‘delay’ (OR = 1.56; 95% CI 1.11–2.19, p = 0.011), whereas patients with stroke (OR = 0.77; 95%CI 0.63–0.94, p = 0.011) and those treated at hospitals with a stroke-prevention clinic (OR = 0.57; 95%CI 0.46–0.71, p < 0.001) were less likely to experience ‘delay’. Patients treated after the publication of National Institute for Health and Clinical Excellence (NICE) guidelines were less likely to experience ‘prolonged delay’ (OR = 0.77; 95%CI 0.65–0.91, p = 0.003) but not ‘delay’.ConclusionFew patients achieved CEA within two weeks of symptoms. Introducing stroke-prevention clinics with one-stop carotid imaging appears important