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The Effect of Off-Focus Radiation in Scatter Correction for Cone Beam CT

L Shi1 , L Zhu2*, (1) Stanford University, Stanford, CA, (2) University of Science and Technology of China, Hefei, Anhui


(Tuesday, 7/31/2018) 7:30 AM - 9:30 AM

Room: Room 205

Purpose: We aim to identify the source of the significant difference between the scatter distributions estimated by two methods proposed in our group for dedicated cone beam breast CT (CBBCT) and to investigate its effects on CBBCT image quality.

Methods: We recently proposed two novel methods for scatter correction in CBBCT, a library based (LB) technique and a forward projection (FP) model. These two methods obtain very different scatter distributions. We hypothesize that the off-focus radiation (OFR) is the contributor which is ignored in the LB method. A thin wire experiment is used to study the effect of OFR on CBBCT spatial resolution by measuring the point spread function (PSF) and the modulation transfer function (MTF). A narrow collimator is used to suppress the OFR-induced signals. In addition, Pseudo "PSFs" and "MTFs" are measured on clinical CBBCT images obtained via FP and LB methods using small calcifications as point sources. The improvement of spatial resolution achieved by suppressing OFR in both studies is quantified by the improvement ratios of PSFs and spatial frequencies at different MTF values. Our hypothesis is verified if these ratios obtained from experimental and clinical data are consistent.

Results: In the wire experiment, the suppression of OFR increases the maximum signal of the PSF by about 14% and reduces the full-width-at-half-maximum (FWHM) by about 12.0%. Similar improvement on spatial resolution is achieved by the FP to LB methods in patient study. The improvement ratios of spatial frequencies at different MTF values match very well in both studies at a level of around 16%.

Conclusion: The results indicate that the main difference between the LB and FP methods is the OFR-induced signals. As OFR significantly affect the image spatial resolution, this finding is therefore important in further improvement of diagnostic performance of CBBCT, especially for micro-calcification detection.


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