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A digital x-ray imaging system for quantitative arterial imaging and blood flow characterization has been developed in our laboratory and evaluated with a phantom study. The system consists of a low noise photo-diode array detector optically coupled to an x-ray image intensifier. The diode array can be considered as an "add-on" to an existing conventional XRII system to produce high quality images of small regions of interest selected from a larger field of view in fluoroscopy mode. X-ray collimation is used to reduce scatter and veiling glare to increase contrast, reduce a non-linear effect in logarithmically subtracted images, and minimize noise.

The speeds of screen -film combinations available in Canada have been measured at 75 kVpwith appropriately filtered beams. The influence on speeds of kVp, attenuation of thefront of the cassette, age and use of screens, reciprocity failure and film variations frombatch to batch is surveyed. The significance of the observed variations in speed (inselecting screen -film combinations) is considered. A set of radiographs of head and bodyphantoms has been used to examine the criteria which radiologists use in defining a 'good'radiograph and from this a qualitative assessment has been made of the priorities inselection of screen -film combinations.IntroductionThe selection of screen -film combinations involves a choice between several conflictingfactors, such as eye appeal of the radiograph, accuracy of diagnosis, dose to the patient,and efficiency and economy of operation of a radiology department. In helping radiologistsand technologists to make a selection we have attempted to answer their questions about thephysical properties of screens and films. Examples of our measurements and results arereported here and their significance to the radiologist is considered. In return radiol-ogists have shown us from sets of test radiographs of phantoms what they consider to be'good' radiographs and have tried to define the parameters they consider important in imagequality. From this we have obtained a qualitative assessment of the relative prioritiesgiven to these parameters in selecting a screen -film combination for general radiography.MethodsSpeeds of screen -film combinations (defined here as the reciprocal of the exposure inmR needed to produce a density of 1.0) were measured using a calibrated three -phase 12pulse generator. Water, lucite and TEMEX water -equivalent rubber phantoms (J. Girdler andSons Ltd., London, U.K.) were used to simulate clinical conditions and provide a suitablyfiltered beam. A simple jig was used to ensure consistent collimation of the X -ray beam.The X -ray exposure of the cassette was measured directly above the front face of thecassette using a pancake -type calibrated ionisation chamber dosimeter. The speeds measuredusing this arrangement were the same as those obtained when the cassette was used with amoving grid in a Bucky tray.Films were processed in DuPont XMD chemistry in a Kodak M6AN processor at 92` F.

The use of narrow x -ray spectra to produce the radiographic image allows a reduction in patient exposurewithout any appreciable degradation in image quality. When this method is combined with the use of rare -earthintensifying screens, the performance of the screens is improved primarily in terms of patient exposure. A number of screens have been tested in a case of bone imaging with aluminum filtered beams and with ytterbium filtered beams. A summary of the results in terms of radiation doses and image quality is presented. Introduction The idea of using a narrow interval of x -ray energies to produce the radiographic image has been discussedby several authors, and it has been in the literature for about twenty years (5). However, it was only veryrecently that applications of this idea in clinical work have been reported (10). At present the most con-venient method to obtain narrow x -ray spectra is by filtering the output of a tungsten anode tube, withelements of relatively high atomic number. Narrow x -ray spectra obtained by this method have been used inclinical experiments in which the use of holmium filter for intravenous pyelograms reduced patient skin ex-posure by approximately 65%, and the use of an ytterbium filter in lumbar spine examinations resulted in areduction of skin exposure by an average of 35% (10). These results have been obtained using only calciumt_ungstate screens, and the reductions in exposure have been achieved without any detectable image degradation.The x -ray spectrum used for the intravenous pyelograms was mostly contained within the range between 33 keVand 56 keV, and the interval of energies used for lumbar spine studies was between approximately h0 keV and

A new x-ray imaging system is being developed for angiographic imaging and blood flow measurments. The system makes use of an x-ray image intensifier optically coupled to a multi-element solid-state photo-detector (Reticon diode array RL1024S). We describe the design of the system and its use in two modes of operation. The first mode is used to produce low noise images by imaging only small regions of interest at a time, thereby reducing noise from the detection of scattered radiation, and by making use of the very large dynamic range (8000:1) of the solid-state detector. The second mode of operation is used to obtain information on the flow of an injected bolus of contrast material through a region of interest. This is accomplished by determining the mean concentration of dye in each 0.33 mm section along the length of an arterial phantom as a function of time over a period of several seconds while the bolus passes through. This data is used to determine areas of flow separation and turbulence due to the presence of simulated stenoses ranging in size from 10% to 84% cross-sectional area reduction. Information on the flow conditions through an artery may be useful for chararterising stenoses or for investigating the conditions associated with the development of atherosclerotic disease.