Optimization of x-ray imaging geometry (with specific application to flat-panel cone-beam computed tomography)

A theoretical method is presented that allows identification of optimal x-r ay imaging geometry, considering the effects of x-ray source distribution, imaging task, x-ray. scatter, and imager detective quantum efficiency (DQE) . Each of these factors is incorporated into the ICRU-recommended figure of merit for image quality, the detectability index, which is maximized to de termine the optimal system configuration. Cascaded systems analysis of flat -panel imagers (FPIs) is extended to incorporate the effects of x-ray scatt er directly in the DQE, showing that x-ray scatter degrades DQE as an addit ive noise source. Optimal magnification is computed for FPI configurations appropriate to (but not limited to) cone-beam computed tomography (CBCT). T he sensitivity of the results is examined as a function of focal spot size, imaging task (e.g., ideal observer detection or discrimination tasks), x-r ay scatter fraction, detector resolution, and additive noise. Nominal condi tions for FPI-CBCT result in optimal magnification of similar to 1.4-1.6, d epending primarily on the magnitude of the x-ray scatter fraction. The meth odology is sufficiently general that examination of optimal geometry for ot her FPI applications (e.g., chest radiography, fluoroscopy, and mammography ) is possible. The degree to which increased exposure can be used to compen sate for x-ray scatter degradation is quantified. (C) 2000 American Associa tion of Physicists in Medicine. [S0094-2405(00)00708-2].