Feasibility of dual-resolution cone beam breast CT: A simulation study

The feasibility of using the dual-resolution cone beam breast CT technique to obtain high-resolution images inside a selected volume-of-interest (VOI). The spatial resolution improvement, dose saving and scatter reduction with this technique are studied and demonstrated with simulations. With the dual-resolution cone beam CT technique, the breast is first scanned with a low resolution detector at a lower exposure level. A selected volume-of-interest (VOI) in the breast is then scanned with a small field, high-resolution detector at a higher exposure level. The two image sets are then combined together to reconstruct high-resolution 3-D images for the VOI. The spatial resolution that can be achieved was estimated by obtaining 3-D images of point objects and use them to compute the MTFs for evaluation as a function of the geometric magnification, detector blurring function and focal spot size. Monte Carlo simulation based on the Geant4 package was used to estimate the degree of dose saving and scatter reduction for a cylinder shaped breast phantom. The VOI images generated with the dual-resolution cone beam CT technique demonstrated the same visibility of micro-calcifications as those generated with the full-breast, high resolution image acquisition. It has been shown that the spatial resolution can be increased by factor of 1.2 with smaller focal spot size and larger magnification. With the exposure level outside the VOI reduced by a factor of 15, scatter components can be reduced by a factor of 5.5 or greater in and outside the VOI. Dose can be reduced by a factor of 5.5 inside the VOI and up to 20 outside the VOI. We have demonstrated that high spatial resolution inside the VOI may be achieved with a high-resolution detector (e.g. CCD/CsI), reduced focal spot size (0.3 mm), and optimized geometrical magnification (e.g. 1.63). Exposure reduction outside the VOI has been shown to reduce the scatter components in the high-resolution projection image data for the VOI. This has also led to significantly lowered doses inside and outside the VOI.