The design of megavoltage projection imaging systems: some theoretical aspects

This study investigates factors associated with the imaging of a patient using a high-energy radiotherapy treatment beam. Both single-stage (e.g., solid-state detector) and two-stage (e.g., scintillation screen plus TV) systems are considered. First an expression is derived that relates dose at the buildup depth in the object to the structure of the object, the scatter-to-primary signal-variance ratio and the differential-signal-to-noise ratio in the image. Second the number of bits required to digitize the image is derived. Third the effect of scattered radiation is investigated for photon counting, photopeak, and Compton detector types. Fourth the effect of noise in the detection process is considered. Finally, the relationship between x-ray source size, detector aperture, and image magnification is derived. The optimum magnification for given source size and detector aperture is discussed in terms of the system transfer function. The study indicates that at a primary beam energy of 2 MeV, a dose of 10(-3) cGy is required to detect reliably the presence of a bone section of area 10 x 10 mm and thickness 4 mm in 250 mm of soft tissue. For this example, it is also estimated that a digitization accuracy of 10 bits is required. The calculations indicate that for a Compton detector, the scatter-to-primary signal-variance ratio drops from a value of around 30% at the exit surface of the object to 5% at a distance of 80 cm from the object with a consequent small reduction in the dose required to form the image.