Measurements of Characteristic Curves in I. I. /TV Digital Imaging Systems

Abstract

How to measure the "characteristic curve" of image intensifier (I. I)/TV digital imaging systems is illustrated based upon our recent research. The curve represents the relationship between the input in terms of relative x-ray intensity and the output in terms of the pixel value. The method used to determine the characteristic curve involves exposing an aluminum stepwedge with a beam that was narrowly collimated in order to reduce the contribution of veiling glare. The relative x-ray intensities transmitted through each step of the stepwedge were determined by using a 100-mm camera, which was attached to the I. I. output, as well as screen/film systems selected. The gradient curve of the system was was derived from the slope of the characteristic curve. Results obtained with a Siemens Digitron 2DSA system showed that its characteristic and gradient curves depended upon the matrix size used only at an initial stage, and did not change with the I. I. field size, iris size, and x-ray beam quality (different kVs). It was found, for accurate measurement of the curve, that it was necessary to collimate the x-ray beams, to narrow width, approximately 0.6cm×10cm, so that the effect of veiling glare from the thinner to the thicker parts of the stepwedge was eliminated. The validity of the characteristic curve was demonstrated by measurement of iodine attenuation curves obtained with the I. I/TV digital system at different exposure levels. The application of the characteristic curve (or the gradient) to image analysis was discussed. The characteristic curve is useful for quantitative analysi (linearization of the system response) and also for monitoring of the system performance for quality assurance.