In this work, we investigated the effects of scattered radiation and beam quality on the low contrast performance
relevant to cone beam breast CT imaging. For experiments, we used our benchtop conebeam CT system and constructed
a phantom consisting of simulated fat and soft tissues. We varied the field of view (FOV) along the z direction to observe
its effect on scattered radiation. The beam quality was altered by varying the tube voltage from 50 to 100 kV. We
computed the contrast-to-noise ratio (CNR) from reconstructed images and normalized it to the square root of dose
measured at the center of the phantom. The results were used as the figure of merit (FOM). The effect of the beam
quality on the scatter to primary ratio (SPR) had minimal impact and the SPR was primarily dominated by the FOV. In
the central section of the phantom, increasing the FOV from 4 to 16 cm resulted in drop of CNR in the order of 15-20%
at any given kVp setting. For a given FOV, the beam quality had insignificant effect on the FOM in the central section of
the phantom. In the peripheral section, a 10 % drop in FOM was observed when the kVp setting was increased from 50
to 100. At lower kVp values, the primary x-ray transmission through the thicker parts of the phantom was severely
reduced. Under such circumstances, ring artifacts were observed due to imperfect flat field correction at very low signal
intensities. Higher kVp settings and higher SPRs helped to increase the signal intensity in highly attenuating regions and
suppressed the ring artifacts.
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