CALIBRATION ERRORS IN DIGITAL SUBTRACTION RADIOGRAPHY

Two methods for the measurements of osseous change detected by digital subtraction radiography have been evaluated. Estimates of errors introduced by soft and hard tissues that overlay the region of interest (ROI) or by soft tissue that replaced the bone are made. All estimates are made assuming that the radiation source is a 40 keV monoenergetic beam and the reference Standard is equivalent to compact bone. These assumptions facilitate a theoretical analysis of calibration errors on a relative scale. The radiographic image method uses a calibration wedge on each of the two films. The mean gray value of the ROI on each film is converted into an equivalent thickness of bone by matching the ROI mean gray value to a gray value along the wedge. These thickness values are then subtracted to obtain a measurement of the amount of change. The subtraction image method makes use of a wedge on only one of the two films. The image of the subtraction of the two radiographs is used for detection of the area of change and the mean gray value of the ROI is matched to a gray value along the wedge. The thickness of the wedge at this point is the measured change. The errors introduced by inclusion of the cheek over the ROI and the replacement of bone by soft tissue are estimated for the radiographic image method. The influence of unequal beam attenuation between the reference ramp and the ROI is estimated for the subtraction image method. Other factors influencing the accuracy of estimates of osseous change which are nonlinear in their effects are also considered. These include the effects of scattered radiation, film response characteristics and x‐ray beam hardening. Copyright © 1990, Wiley Blackwell. All rights reserved