Although digital mammography is currently being used all over the world, most of the mammographic units are still based on screen-film systems. In these systems, the choice of the best combination between the screen and the film is important to assure image quality. This work presents a computer simulation method to help choosing the proper screen-film system to each application, showing sensitometric parameters for each combination, like speed, latitude and contrast determined by sensitometric curves film and screen-film combination, measured experimentally, using commercial calibrated sensitometer and densitometer, allowing further comparison between them according to the application required. Panthom images are presented showing the results that will be obtained in clinical use. The influence of each screen is also determined. Phantom images were obtained using a known screen-film combination. These images were digitized in a laser scanner. The exposures information is used to predict the final image by using the screen-film sensitometric curve, chosen by the user. The computer simulation was used for evaluating several mammographic films combined with different screens, currently used for mammography. Simulated results were in good agreement with values obtained experimentally. The results obtained with the proposed algorithm confirm the possibility of using this method for evaluating the performance of any screen-film combination considering the sensitometric curve. It can be an important tool for quick evaluation of a screen-film system as it predicts the image characteristics with no unnecessary X-ray exposition.
This work presents a computer algorithm to evaluate film digitizers in terms of its spatial resolution by using image processing techniques. A testing pattern containing slits of different widths, not-equally spaced, was developed. When digitizing this pattern, the algorithm automatically analyses the digital image and evaluate spatial resolution capabilities of the digitizer. These analyses were made by calculating computationally digital image slit width and the distance between the slits. Sampling distances in both directions (parallel and perpendicular to scan direction) are determined by comparing calculated values with previous measurements made by using a calibrated microscope. Evaluation also includes the determination of the presampling modulation transfer function (MTF). The algorithm was used for the measurement of effective pixel size and presampling MTF of a Lumiscan 50 laser digitizer and an Umax Powerlook II optical scanner in both directions. Results showed that the laser digitizer presents significative difference between parallel and perpendicular MTF and the optical scanner presents better MTF considering high frequencies components. Results obtained with the proposed algorithm confirm the possibility of evaluating spatial resolution limitations of any film digitizer using an automatic and simple method.
This work presents a computational model for practical application of the transfer function method for radiographic units evaluation, in order to reduce some experimental constraints involved to its determination. With the proposed algorithm, the complete Optical Transfer Function (Modulation Transfer Function and Phase Transfer Function) can be easily determined as well as the effective focal spot sizes at any field location without using a microdensitometer. All measurements are done from a digitized slit image obtained experimentally at the field center position. The effective focal spot sizes can be calculated by using the Line Spread Function Root Mean Square (RMS) value or by the Modulation Transfer Function (MTF) first minimum. Besides, considering the variation of the effective focal spot size given by the field characteristic equations, all these parameters can also be determined at any location on the radiation field. The computer scheme was used for evaluating slit images obtained from nine different x-ray equipments. Results confirmed the possibility of using the transfer function method for quality evaluation of any radiological system in a simple and automatic way. This computer scheme replaces some of the expensive and specific devices necessary to the experimental MTF evaluation by quite more accessible and low cost equipments.