[en] X-ray diagnostics gives the largest contribution to the population dose from man-made radiation sources. Strategies for reduction of patient doses without loss of diagnostic accuracy are therefore of great interest to society and have been focussed in general terms by the ICRP (ICRP 1996) through the introduction of the concept of diagnostic reference levels. The European Union has stimulated research in the field, and, based on patient dose measurements and radiologists' appreciation of acceptable image quality, good radiographic techniques have been identified and recommended (EUR 1996a, b) for conventional screen-film imaging. These efforts have resulted in notable dose reductions in clinical practices (Hart et al 1996). In spite of 100 years of use of x-rays for diagnostics, the choice of technique parameters still relies to a great extent on experience. Scientific efforts to optimize the choice in terms of finding the parameter settings which yield sufficient image quality at the lowest possible cost in dose are still rare. True optimization requires (1) estimation of the image quality needed to make a correct diagnosis and (2) methods to investigate all possible means of achieving this image quality in order to be able to decide which of them gives the lowest dose. The paper by Tapiovaara, Sandborg and Dance published in this issue of Physics in Medicine and Biology (pages 537-559) addresses the optimization of paediatric fluoroscopy, a timely and important topic. Fluoroscopy procedures, used to guide x-ray examinations or interventional procedures, are little standardized and may result in high dose levels; radiation exposure in childhood is likely to result in a higher lifetime risk than the same exposure later in life. The authors represent an interesting mix of expertise within various scientific fields: the theory of medical imaging and assessment of image quality, the physics of diagnostic radiology and radiation dosimetry. They provide good insights into the technical performance and limitations of clinical x-ray equipment and the functioning of x-ray image intensifiers (XRIIs), and discuss the problem in an educational manner. The authors have succeeded in producing a paper that combines high scientific merit and valuable practical guidelines towards the optimization of paediatric fluoroscopy and radiography. This information, if properly utilized by practitioners, will contribute to significant (50%) reduction in radiation doses without sacrificing image quality and diagnostic accuracy. Fluoroscopy using XRIIs is one of the x-ray procedures that allows the possibility of bringing patient doses to an absolute minimum: quantum-noise due to the inevitable stochastic nature of the interactions of x-rays with the image receptor will ultimately limit the possibility for further dose reduction. In a well designed (quantum-noise limited) system, patient dose ( D) increases proportionally with the square of signal-to-noise ratio (SNR). The SNR of the ideal observer (ICRU 1996) may be used as image quality descriptor. To ascertain minimum patient dose, the just detectable SNR level of critical image details in a given diagnostic procedure should be determined. The dose-to-information conversion factor SNR²/D is independent of patient dose, thus providing a useful figure-of-merit for optimization of the technique parameters of an imaging task. In the cited study, the strategy for optimization is to maximize the SNR²/D ratio, leaving the absolute requirement on SNR (and patient dose) to be determined by the user. Technique factors which strongly influence the SNR²/D ratio are the energy spectrum (tube potential and total filtration) and the choice of anti-scatter technique. These can be adjusted by the user of clinical x-ray equipment and are focussed in the paper. A powerful tool used in executing the study is a carefully developed and validated computational model of the imaging chain (Tapiovaara and Sandborg 1995) allowing simultaneous calculation of SNR and D, and flexible variation of the parameters. The results indicate that optimal parameter settings may differ significantly from those recommended on the basis of previous experience: low tube potential techniques are suggested to improve dose efficiency as well as use of appropriately designed anti-scatter (fibre) grids. The detection task, or the type of important detail, also influences the choice of preferred technique. The current study has important implications for other x-ray imaging procedures which, like XRII fluoroscopy, are not restricted by the requirement of optical density as in conventional screen-film imaging. It suggests the importance of optimizing the imaging techniques for digital radiographic systems so that their advantages over conventional systems can be fully utilized. Another natural extension of the current work is high dose fluoroscopy in interventional procedures which has been a major concern of radiation risks to patients (Shope 1996). A search for improved techniques to minimize exposure in general fluoroscopy is a continued challenge to imaging physicists. An even greater challenge is posed to the fluoroscopic x-ray equipment manufacturers. Some systems will need to be redesigned to allow users greater flexibility of selecting a desired strategy for the automatic brightness compensation control. It will also be essential to provide higher output for fluoroscopic systems to accommodate low kV and heavy filtration operations. In summary, the paper by Tapiovaara et al (1999) is an important contribution to development of dose efficient techniques which will influence future practices and manufacturers of x-ray equipment. Scientific investigations such as this one will serve as a driving force for practical implementation of these dose efficient techniques. (author)