RESUMEN
In the last ten to fifteen years there has been much research in using amorphous and polycrystalline semiconductors as x-ray photoconductors in various x-ray image sensor applications, most notably in flat panel x-ray imagers (FPXIs). We first outline the essential requirements for an ideal large area photoconductor for use in a FPXI, and discuss how some of the current amorphous and polycrystalline semiconductors fulfill these requirements. At present, only stabilized amorphous selenium (doped and alloyed a-Se) has been commercialized, and FPXIs based on a-Se are particularly suitable for mammography, operating at the ideal limit of high detective quantum efficiency (DQE). Further, these FPXIs can also be used in real-time, and have already been used in such applications as tomosynthesis. We discuss some of the important attributes of amorphous and polycrystalline x-ray photoconductors such as their large area deposition ability, charge collection efficiency, x-ray sensitivity, DQE, modulation transfer function (MTF) and the importance of the dark current. We show the importance of charge trapping in limiting not only the sensitivity but also the resolution of these detectors. Limitations on the maximum acceptable dark current and the corresponding charge collection efficiency jointly impose a practical constraint that many photoconductors fail to satisfy. We discuss the case of a-Se in which the dark current was brought down by three orders of magnitude by the use of special blocking layers to satisfy the dark current constraint. There are also a number of polycrystalline photoconductors, HgI(2) and PbO being good examples, that show potential for commercialization in the same way that multilayer stabilized a-Se x-ray photoconductors were developed for commercial applications. We highlight the unique nature of avalanche multiplication in a-Se and how it has led to the development of the commercial HARP video-tube. An all solid state version of the HARP has been recently demonstrated with excellent avalanche gains; the latter is expected to lead to a number of novel imaging device applications that would be quantum noise limited. While passive pixel sensors use one TFT (thin film transistor) as a switch at the pixel, active pixel sensors (APSs) have two or more transistors and provide gain at the pixel level. The advantages of APS based x-ray imagers are also discussed with examples.
Asunto(s)
Electrónica/instrumentación , Rayos X , Diseño de Equipo , Pantallas Intensificadoras de Rayos XRESUMEN
OBJECTIVE: The objective of the present study was to compare the effects of a 16-week pharmacotherapy with fenofibrate (200 mg) or pravastatin (initially 20 mg for 8-weeks and, if necessary, increased to 40 mg) on low density lipoprotein (LDL) particle size assessed by gradient gel electrophoresis among patients with type IIa dyslipidemia. METHODS: For that purpose, type IIa dyslipidemic patients (cholesterol, 7.45+/-1.18 (S.D.) mmol/l; LDL cholesterol, 5.57+/-1.16 mmol/l; triglycerides (TGs), 1.66+/-0.43 mmol/l) were randomized to either fenofibrate (n=36) or pravastatin (n=43) therapy for 16 weeks. Fasting plasma lipoprotein levels as well as the LDL peak particle size (using 2-16% polyacrylamide gel electrophoresis) were assessed at baseline and after the 16-week treatment period. RESULTS: Whereas significant improvements in the plasma lipoprotein-lipid variables were observed with both fenofibrate and pravastatin treatments, LDL peak particle size was only significantly increased with fenofibrate therapy (+2.11+/-5.18 A, P<0.05). Among patients under fenofibrate therapy, changes in TG levels were negatively associated with changes in LDL peak particle size (r=-0.54, P<0.0007), whereas no such association was found in pravastatin-treated patients. The prevalence of patients with small, dense LDL particles (as defined by LDL particle diameter <255.5 A) was reduced from 69.4 to 30.6% (P<0.05) among fenofibrate-treated patients as opposed to 81.4 to 72.1% (NS) in patients who received pravastatin. CONCLUSION: As pravastatin treatment had no effect on LDL size, it is suggested that the additional effect of fenofibrate therapy on LDL size may contribute to reduce the risk of coronary heart disease (CHD) beyond what can be expected from the reduction in LDL cholesterol concentration in type IIa dyslipidemic patients.