Detectors for the future of X-ray imaging.

In recent decades, developments in detectors for X-ray imaging have improved dose efficiency. This has been accomplished with for example, structured scintillators such as columnar CsI, or with direct detectors where the X rays are converted to electric charge carriers in a semiconductor. Scattered radiation remains a major noise source, and fairly inefficient anti-scatter grids are still a gold standard. Hence, any future development should include improved scatter rejection. In recent years, photon-counting detectors have generated significant interest by several companies as well as academic research groups. This method eliminates electronic noise, which is an advantage in low-dose applications. Moreover, energy-sensitive photon-counting detectors allow for further improvements by optimising the signal-to-quantum-noise ratio, anatomical background subtraction or quantitative analysis of object constituents. This paper reviews state-of-the-art photon-counting detectors, scatter control and their application in diagnostic X-ray medical imaging. In particular, spectral imaging with photon-counting detectors, pitfalls such as charge sharing and high rates and various proposals for mitigation are discussed.

Diffraction in air-clad fibres.

The angular transmissivity of high numerical aperture air-clad fibres is measured as a function of skewness of the launched light. Within the experimental limits the measured transmissivity of skew rays is significantly lower compared to theoretical predictions for air-clad fibres with uniform cladding surfaces. The discrepancy is attributed to diffractive losses of skew rays from the periodic corrugations at the pump core-cladding air interface.

Assessment of chloride secretion in human nasal epithelial cells by X-ray microanalysis.

The genetic disease cystic fibrosis (CF) is due to defective epithelial chloride transport. Different treatments have been proposed that could restore chloride transport in CF patients. A new method is proposed for measuring the chloride secretion in easily accessible epithelial cells. Fresh nasal epithelial cells were obtained by nasal brushing and made to attach to titanium grids for electron microscopy. Chloride efflux through the cystic fibrosis transmembrane regulator channel was stimulated by 20 microM forskolin and 100 microM isobutyl-methylxanthine (IBMX), in standard Ringer's solution (SR). Chloride efflux through the calcium-regulated channel was stimulated by 200 microM adenosine triphosphate (ATP) in SR. The cells were rinsed after the exposure, in order to remove the experimental medium, frozen and freeze-dried. The elemental composition of the cells was determined by X-ray microanalysis. Rinsing with distilled water or ammonium acetate appeared to cause damage to the cells, whereas rinsing with isotonic mannitol preserved the ionic composition. Stimulation of cells from healthy controls with forskolin and IBMX in a chloride-containing medium caused a significant (28 +/- 6%) decrease in chloride concentration, which is indicative of net chloride efflux. In similar conditions, stimulation with ATP induced a 29 +/- 5% decrease in the chloride concentration. Stimulation of cells from CF patients with forskolin and IBMX in a chloride-containing medium caused no significant change in the intracellular chloride concentration, whereas ATP stimulation induced a response similar to that obtained in cells from healthy controls. It is concluded that X-ray microanalysis of nasal epithelial cells may be used to determine chloride secretion in CF patients in an easily accessible cell type.

Annealing properties of gratings written into UV-presensitized hydrogen-outdiffused optical fiber.

Accelerated aging of gratings written in UV presensitized hydrogen outdiffused optical fibers show that these gratings are more stable than standard gratings written in hydrogen-loaded fibers. They are observed to grow initially by as much as 2%. The predicted decay after 25 years at 80 degrees C is ~0.1% . The results suggest that the index modulation decay is so slow that postfabrication annealing can be avoided.

Ultraviolet-induced absorption losses in hydrogen-loaded optical fibers and in presensitized optical fibers.

Reduced hydroxyl formation in presensitized fibers exposed to cw 244-nm light after hydrogen outdiffusion is reported. The OH band in the presensitized fiber shifts toward 1390 nm. In the fully hydrogen-loaded fiber the OH band is centered at 1397 nm and does not shift with fluence.

Birefringence control in plasma-enhanced chemical vapor deposition planar waveguides by ultraviolet irradiation.

Complete birefringence compensation is demonstrated in plasma-enhanced chemical vapor deposition waveguides by 193-nm postexposure. A single relaxation process dominates the decay in stress anisotropy, indicating that compressive stress from the substrate leads to an elastic stress anisotropy at the core.

Correlation of ultraviolet-induced stress changes and negative index growth in type IIa germanosilicate waveguide gratings.

The stress anisotropy in planar germanosilicate waveguides is used to follow the changes in stress, measured as a birefringence, with UV irradiation. Compressive stress increases with the growth of the negative index grating, saturating when the grating modulation saturates.

Locking in photosensitivity within optical fiber and planar waveguides by ultraviolet preexposure.

Strong photosensitivity is locked in permanently after hydrogen outdiffusion of hydrogen-loaded waveguides presensitized with UV light by either pulsed 193-nm or cw 244-nm laser output.

High-temperature stable gratings in germanosilicate planar waveguides.

Negative index gratings in planar germanosilicate waveguides were found to be stable up to 500 degrees C. The annealing properties are similar to those of negative index fiber gratings.