Cone-beam computed tomography with a flat-panel imager: Magnitude and effects of x-ray scatter

A system for cone-beam computed tomography (CBCT) based on a flat-panel ima ger (FPI) is used to examine the magnitude and effects of x-ray scatter in FPI-CBCT volume reconstructions. The system is being developed for applicat ion in image-guided therapies and has previously demonstrated spatial resol ution and soft-tissue visibility comparable or superior to a conventional C T scanner under conditions of low x-ray scatter. For larger objects consist ent with imaging of human anatomy (e.g., the pelvis) and for increased cone angle (i.e., larger volumetric reconstructions), however, the effects of x -ray scatter become significant. The magnitude of x-ray scatter with which the FPI-CBCT system must contend is quantified in terms of the scatter-to-p rimary energy fluence ratio (SPR) and scatter intensity profiles in the det ector plane, each measured as a function of object size and cone angle. For large objects and cone angles (e.g., a pelvis imaged with a cone angle of 6 degrees), SPR in excess of 100% is observed. Associated with such levels of x-ray scatter are cup and streak artifacts as well as reduced accuracy i n reconstruction values, quantified herein across a range of SPR consistent with the clinical setting. The effect of x-ray scatter on the contrast, no ise, and contrast-to-noise ratio (CNR) in FPI-CBCT reconstructions was meas ured as a function of SPR and compared to predictions of a simple analytica l model. The results quantify the degree to which elevated SPR degrades the CNR. For example, FPI-CBCT images of a breast-equivalent insert in water w ere degraded in CNR by nearly a factor of 2 for SPR ranging from similar to 2% to 120%. The analytical model for CNR provides a quantitative understand ing of the relationship between CNR, dose, and spatial resolution and allow s knowledgeable selection of the acquisition and reconstruction parameters that, for a given SPR, are required to restore the CNR to values achieved u nder conditions of low x-ray scatter. For example, for SPR=100%, the CNR in FPI-CBCT images can be fully restored by: (1) increasing the dose by a fac tor of 4 (at full spatial resolution); (2) increasing dose and slice thickn ess by a factor of 2; or (3) increasing slice thickness by a factor of 4 (w ith no increase in dose). Other reconstruction parameters, such as transaxi al resolution length and reconstruction filter, can be similarly adjusted t o achieve CNR equal to that obtained in the scatter-free case. (C) 2001 Ame rican Association of Physicists in Medicine.