RADIATION DOSE AND IMAGE INFORMATION IN COMPUTED RADIOGRAPHY. A phantom study of angle measurements in the weight-bearing knee

Purpose: To establish a correlation between radiation dose and diagnostic accuracy when employing a new digital method for angle determinations. The specific intention was to determine how far the radiation dose can be reduced without losing measuring accuracy and to compare this radiation dose with that employed with our conventional method. Material and Methods: An image succession of an anthropomorphic phantom was generated with a computed radiography (CR) system, by reducing the exposure stepwise. The images were archived and transferred to a workstation for evaluation. The intraobserver variation of two angle determinations was used as an indicator of the evaluation accuracy. Patient radiation doses were measured with thermoluminescent dosimeters. The energy imparted, indicating the relative risk associated with exposure to ionising radiation, and the effective dose, which determines the absolute risk, were calculated. Results and Conclusion: No significant correlation was found between patient dose and measuring accuracy within the evaluated exposure interval. At the lowest exposure of the CR system, the energy imparted to the patient was 30 μJ. Compared with our conventional analogue method this is a reduction by 98%. The effective dose was as low as 1.5 μSv. The CR technique creates possibilities to adapt exposure parameters, and thus the radiation dose to the patient, according to the purpose of the investigation.