Examination of aperture signals in digital radiography

Lead discs have been widely used to measure scattered radiation in radiographic imaging. An alternative approach, which offers advantages in terms of practical simplicity and radiation dose efficiency, is based on measurements obtained using a narrow aperture. The use of such an aperture for scatter measurement is examined from an experimental perspective in this paper. The examination is based on analysing features in images of apertures of 0.5 mm to 10 mm in diameter using a digital fluoroscopy system. Clinical imaging conditions were simulated using a 15 cm thickness of scattering material and a kilovoltage of 80 kVp. It was found that images of broad apertures consist of a transmitted signal, with primary and scatter components, which is surrounded by a skirt resulting from scatter. Furthermore, it was found that the skirt and the scatter component of the transmitted signal become negligible for narrow apertures. It was also found that a 1.5 mm aperture generates a scatter-free primary signal within the limits of accuracy of the detection system employed. This outcome was confirmed on the basis of accurate densitometric measurements and aperture signals which were independent of air gap. It is concluded that the signal measured using a 1.5 mm aperture can be related to the open field signal to determine the scatter signal and that this is an accurate method for such measurement with this experimental arrangement.