Performance of electronic portal imaging devices (EPIDs) used in radiotherapy: image quality and dose measurements.

The aim of our study was to compare the image and dosimetric quality of two different imaging systems. The first one is a fluoroscopic electronic portal imaging device (first generation), while the second is based on an amorphous silicon flat-panel array (second generation). The parameters describing image quality include spatial resolution [modulation transfer function (MTF)], noise [noise power spectrum (NPS)], and signal-to-noise transfer [detective quantum efficiency (DQE)]. The dosimetric measurements were compared with ionization chamber as well as with film measurements. The response of the flat-panel imager and the fluoroscopic-optical device was determined performing a two-step Monte Carlo simulation. All measurements were performed in a 6 MV linear accelerator photon beam. The resolution (MTF) of the fluoroscopic device (f 1/2 = 0.3 mm(-1)) is larger than of the amorphous silicon based system (f 1/2 = 0.21 mm(-1)), which is due to the missing backscattered photons and the smaller pixel size. The noise measurements (NPS) show the correlation of neighboring pixels of the amorphous silicon electronic portal imaging device, whereas the NPS of the fluoroscopic system is frequency independent. At zero spatial frequency the DQE of the flat-panel imager has a value of 0.008 (0.8%). Due to the minor frequency dependency this device may be almost x-ray quantum limited. Monte Carlo simulations verified these characteristics. For the fluoroscopic imaging system the DQE at low frequencies is about 0.0008 (0.08%) and degrades with higher frequencies. Dose measurements with the flat-panel imager revealed that images can only be directly converted to portal dose images, if scatter can be neglected. Thus objects distant to the detector (e.g., inhomogeneous dose distribution generated by a modificator) can be verified dosimetrically, while objects close to a detector (e.g., a patient) cannot be verified directly and must be scatter corrected prior to verification. This is justified by the response of the flat-panel imaging device revealing a strong dependency at low energies.

Current and future benefits from the use of GM technology in food production.

For the current generation of genetically modified (GM) crops the improvement of agronomic traits (e.g. herbicide tolerance, insect resistance) has been a major objective. The lack of obvious and direct benefits for the consumer has been a main point of criticism. Future trends will increasingly encompass the modification of quality traits, such as the improvement of sensory and especially nutritional properties. Some of the ongoing developments try to meet the desire of consumers for 'healthy' or 'high-tech' foods in developed countries. Others are intended to assist in adjusting the nutritional status of foods to the needs of consumers in developing countries. Considering the increasing world population and the limited amount of arable land, GM technology may also become a valuable tool to ensure food security. The major prerequisite for the applicability of the technique is the safety of the resulting products. The increasing complexity of modifications intended might require adjustments and improvements of the strategies applied to the safety assessment of GM foods. Present research activities try to meet these new challenges.