Microstructure and its effect on field electron emission of grain-size-controlled nanocrystalline diamond films Ultrananocrystalline diamond ͑UNCD͒ films 0.1-2.4 m thick were conformally deposited on sharp single Si microtip emitters, using microwave CH 4 -Ar plasma-enhanced chemical vapor deposition in combination with a dielectrophoretic seeding process. Field-emission studies exhibited stable, extremely high ͑60-100 A/tip͒ emission current, with little variation in threshold fields as a function of film thickness or Si tip radius. The electron emission properties of high aspect ratio Si microtips, coated with diamond using the hot filament chemical vapor deposition ͑HFCVD͒ process were found to be very different from those of the UNCD-coated tips. For the HFCVD process, there is a strong dependence of the emission threshold on both the diamond coating thickness and Si tip radius. Quantum photoyield measurements of the UNCD films revealed that these films have an enhanced density of states within the bulk diamond band gap that is correlated with a reduction in the threshold field for electron emission. In addition, scanning tunneling microscopy studies indicate that the emission sites from UNCD films are related to minima or inflection points in the surface topography, and not to surface asperities. These data, in conjunction with tight binding pseudopotential calculations, indicate that grain boundaries play a critical role in the electron emission properties of UNCD films, such that these boundaries: ͑a͒ provide a conducting path from the substrate to the diamond-vacuum interface, ͑b͒ produce a geometric enhancement in the local electric field via internal structures, rather than surface topography, and ͑c͒ produce an enhancement in the local density of states within the bulk diamond band gap.

The three CT components with the greatest impact on image quality are the X-ray source, detection system and reconstruction algorithms. In this paper, we focus on the first two. We describe the state-of-the-art of CT detection systems, their calibrations, software corrections and common performance metrics. The components of CT detection systems, such as scintillator materials, photodiodes, data acquisition electronics and anti-scatter grids, are discussed. Their impact on CT image quality, their most important characteristics, as well as emerging future technology trends for each, are reviewed. The use of detection for multi-energy CT imaging is described. An overview of current CT X-ray sources, their evolution to support major trends in CT imaging and future trends is provided.

Computed tomography is a widely used medical imaging technique that has high spatial and temporal resolution. Its weakness is its low sensitivity towards contrast media. Iterative reconstruction techniques (ITER) have recently become available, which provide reduced image noise compared with traditional filtered back-projection methods (FBP), which may allow the sensitivity of CT to be improved, however this effect has not been studied in detail. We scanned phantoms containing either an iodine contrast agent or gold nanoparticles. We used a range of tube voltages and currents. We performed reconstruction with FBP, ITER and a novel, iterative, modal-based reconstruction (IMR) algorithm. We found that noise decreased in an algorithm dependent manner (FBP > ITER > IMR) for every scan and that no differences were observed in attenuation rates of the agents. The contrast to noise ratio (CNR) of iodine was highest at 80 kV, whilst the CNR for gold was highest at 140 kV. The CNR of IMR images was almost tenfold higher than that of FBP images. Similar trends were found in dual energy images formed using these algorithms. In conclusion, IMR-based reconstruction techniques will allow contrast agents to be detected with greater sensitivity, and may allow lower contrast agent doses to be used.

The transport of low-energy (<3 eV) photoelectrons in CsI and CsBr films was investigated by direct photoionization in the film and by photoinjection of electrons from underlying K–Cs–Sb, Cs3Sb, and CsI photocathodes. Photoelectron energy distributions and the photoyield dependence on film thickness were studied, assisted by in situ x-ray photoelectron spectroscopy surface characterizations. A model describing electron transport through the coating film was used, which correlated well with experimental results from the various material combinations, coating thickness, and photon energies. The model provides valuable information on the interface potential barrier of theses systems, as well as quantum-yield attenuation length and photoelectron energy distributions.