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Fast Shading Correction of Cone Beam CT in Radiation Therapy Via Tissue Sparsity

L Shi1*, L Zhu2 , J Wei3 , (1) Stanford University, Stanford, CA, (2) University of Science and Technology of China, Hefei, Anhui, (3) Landauer Medical Physics , Newnan, GA


(Monday, 7/30/2018) 9:30 AM - 10:00 AM

Room: Exhibit Hall | Forum 6

Purpose: The quantitative use of cone beam computed tomography (CBCT) in radiation therapy is limited by severe shading artifacts, even with system embedded correction. We recently proposed an effective shading correction method using spatial information in planning CT (pCT). In this work, we further improve our method by eliminating this dependence to make it more clinical adaptable.

Methods: This method is to estimate shading artifact via tissue classification, which is adversely hindered by the same artifact. Tissue intensities tend to be uniform in a local region as the shading artifact varies slowly over image. The proposed method first divides the CBCT into blocks, each of which must contain all primary tissues. An automatic Fuzzy-C-mean algorithm is used for tissue classification in each block. The resulting probability map for each tissue is assigned with an HU value and then combined. The difference images between CBCT and classified image are considered as shading error, but only sparse shading samples are used for correction to avoid classification errors. A Fourier-Transform based technique, local filtration, is used to efficiently process the sparse data for shading correction. Method performance is evaluated on pelvis phantom and 6 patient datasets.

Results: The proposed method improves the CBCT image quality on both phantom and patient to pCT level. The signal non uniformity (SNU) is reduced from 6.38% to 0.87% on phantom, and an average of 7.42% to 1.46% on patient data. The HU difference to pCT is reduced from 38±22 HU to 15±10 HU in muscle and from 52±40 HU to 25±21 HU in fat. Typical processing time for one patient data is about 45 seconds.

Conclusion: We further improve the shading correction method for clinical use. The method can be applied directly to the output images from an on-board CBCT without modification of hardware or clinical protocol.


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