Affiliations: Department of Radiation Oncology, University of
Oklahoma Health Sciences Center, Oklahoma City, OK, USA | Department of Electrical and Computer Engineering,
University of Oklahoma, Norman, OK, USA | Medical Physics Department, Memorial Sloan-Kettering
Cancer Center, New York, NY, USA | Vassar Brothers Medical Center, Poughkeepsie, NY,
USA
Note: [] Corresponding author: Imad Ali, Ph.D., DABR, Assistant
Professor, Medical Physics, Department of Radiation Oncology, University of
Oklahoma Health Sciences Center, 825 NE 10th Street, OUPB 1430, Oklahoma City,
OK 73104, USA. Tel.: +1 405 271 8290; Fax: +1 405 271 9240; E-mail: [email protected]
Abstract: Purpose: To investigate image artifacts caused by a standard
treatment couch on cone-beam CT (CBCT) images from a kV on-board imager and to
develop an algorithm based on spatial domain filtering to remove image
artifacts in CBCT induced by the treatment couch. Methods: Image artifacts in CBCT induced by the treatment couch were quantified by scanning a
phantom used to quantify CT image performance. This was performed by scanning
the phantom setup on a regular treatment couch and in air with the kV on-board
imager. An algorithm was developed to filter image artifacts from the treatment
couch by processing of cone-beam radiographic projections using two scans: one
scan of the phantom and treatment couch and a second scan of the treatment
couch only. This algorithm is based on a pixel-by-pixel removal of beam
attenuation due to the treatment couch from each projection of the phantom and
couch scan. The net couch-filtered projections were then used to reconstruct
CBCT. Results: We found that the treatment couch causes considerable
image artifacts: CT number uniformity is degraded and varies as much as 15%,
and noise in CBCT scans with phantom plus couch (3.5%) is higher than for
the phantom in air (1.5%). The spatial domain filtering technique reduces
noise by more than 1.5%, improves uniformity by a factor of 2, and removes
ringing and streaking artifacts related to the standard treatment couch in CBCT
reconstructed from couch-filtered projections. This filtering technique was
tested successfully to filter other hardware objects such as a patient
immobilization body-fix frame. Conclusions: The standard treatment couch
causes image artifact in CBCT from kV on-board imaging systems. The spatial
domain filtering technique developed in this work improves image quality of
CBCT by preprocessing the projections prior to CBCT reconstruction. This
technique might be useful to filter other hardware objects from CBCT which may
contribute to the degradation of image quality.
