Low Contrast Sensitivity of Radiologic, CT, Nuclear Medicine, and Ultrasound Medical Imaging Systems

Abstract

The physical sensitivity of a medical imaging system is defined as the square of the output signal-to-noise ratio per unit of radiation to the patient, or the information/radiation ratio. This sensitivity is analyzed at two stages: the radiation detection stage, and the image display stage. The signal-to-noise ratio (SNR) of the detection stage is a physical measure of the statistical quality of the raw detected data in the light of the imaging task to be performed. As such it is independent of any software or image processing algorithms which belong properly to the display stage. The fundamental SNR approach is applied to a wide variety of medical imaging applications and measured SNR values for signal detection at a given radiation exposure level are compared to the optimal values allowed by nature. It is found that the engineering falls short of the natural limitations by an inefficiency of about a factor two for most of the individual radiologic system components, allowing for great savings in the exposure required for a given imaging performance when the entire system is optimized. The display of the detected information is evaluated from the point of view of observer efficiency, the fraction of the displayed information that a human observer actually extracts. It has been found that the human observer is able to extract more than 50 percent of the displayed information in simple images when they are presented such that the noise is easily visible; otherwise, the internal noise sources of the observer degrade observer efficiency for lesion detection to much lower values. Methods for optimizing both the detection and the display of medical image information are presented in terms of the information/radiation ratio together with brief descriptions of the measurement methodology required to assess the images. Copyright © 1984 by The Institute of Electrical and Electronics Engineers, Inc.