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Does Off-Centering of Treatment Isocenter Impact On CBCT Image Quality for Brain-Met Patients?

N Mail*, F Li, M diMayorca, R Lalonde, M Huq, UPMC Shadyside Hospital, Pittsburgh, PA

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PO-GePV-M-158 (Sunday, 7/10/2022)   [Eastern Time (GMT-4)]

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Purpose: Acquisition of cone beam tomographic images (CBCT) and matching it with planning CT images is a standard practice for setting up of brain metastases patients prior to the delivery of stereotactic radiosurgery (SRS) treatments. Depending on the location of targets, the isocenter location for such treatments can vary. The present study investigates the impact of the location of the isocenter on the quality of CBCT images.

Methods: Catphan-504 and Head rando phantoms were imaged with the full bowtie-filter at four different treatment isocenters and four different FOVz (2-18 cm, SI) to examine the influence of off-centering on CBCT image quality. The image quality parameters investigated include noise, contrast, CNR, spatial resolution, and CT number accuracy; these were quantified using MATLAB 2018. The impact of off-centering on skull bone’s CT number and profiles was thoroughly studied and compared with those obtained from CT-sim images.

Results: Increasing the FOVz from 2 to 18 cm significantly lowers the values of the image quality parameters for all 5 metrices. Specifically, the CNR was lowered by a factor of 1.5 as the FOVz increased from 2-18 cm. The skull bone CT number was lowered by 250 HU for off-centering the isocenter at 18 cm FOV. Geometric integrity in CBCT images was preserved at all extreme positions of the isocenter. A reduction of 17% in MTF was seen for 6 cm off-centering from midline, which is attributed to variation in scatter frequency and magnitude.

Conclusion: Quantitative evaluation in phantoms demonstrates significant artifacts with increasing FOVz. These artifacts were significantly higher in the off-centered scans, which is primarily attributed to scatter variation and frequency as a function of gantry rotation. The framework used in this study can be useful for evaluating scatter correction methods and for comparison between systems.Keywords: FOV, Off-centering, scatter, Image-quality, Catphan-phantom, image-truncation

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