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Cone‐beam computed tomography with a flat‐panel imager: Magnitude and effects of x‐ray scatter
Author(s) -
Siewerdsen Jeffrey H.,
Jaffray David A.
Publication year - 2001
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.1339879
Subject(s) - cone beam computed tomography , optics , physics , streak , scanner , coherent backscattering , tomography , image resolution , scattering , computed tomography , medicine , radiology
A system for cone‐beam computed tomography (CBCT) based on a flat‐panel imager (FPI) is used to examine the magnitude and effects of x‐ray scatter in FPI‐CBCT volume reconstructions. The system is being developed for application in image‐guided therapies and has previously demonstrated spatial resolution and soft‐tissue visibility comparable or superior to a conventional CT scanner under conditions of low x‐ray scatter. For larger objects consistent 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‐primary energy fluence ratio (SPR) and scatter intensity profiles in the detector 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°), 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 in reconstruction values, quantified herein across a range of SPR consistent with the clinical setting. The effect of x‐ray scatter on the contrast, noise, and contrast‐to‐noise ratio (CNR) in FPI‐CBCT reconstructions was measured as a function of SPR and compared to predictions of a simple analytical 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 were degraded in CNR by nearly a factor of 2 for SPR ranging from ∼2% to 120%. The analytical model for CNR provides a quantitative understanding of the relationship between CNR, dose, and spatial resolution and allows knowledgeable selection of the acquisition and reconstruction parameters that, for a given SPR, are required to restore the CNR to values achieved under 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 factor of 4 (at full spatial resolution); (2) increasing dose and slice thickness by a factor of 2; or (3) increasing slice thickness by a factor of 4 (with no increase in dose). Other reconstruction parameters, such as transaxial resolution length and reconstruction filter, can be similarly adjusted to achieve CNR equal to that obtained in the scatter‐free case.