Room: Exhibit Hall | Forum 5
Purpose: Bowtie filter used in cone beam CT (CBCT) image-guided radiotherapy (IGRT) provides image quality improvement and dose reduction. Certain models of CBCT systems require manually placing the appropriate bowtie filter according to the relevant imaging protocol. Inadvertently using a wrong bowtie filter or no bowtie filter could cause degraded image quality, unexpected artifacts and increased patient dose. In this work, we investigated all possible scenarios of bowtie filter misplacement for a Varian Trilogy machine.
Methods: A Catphan phantom and a Body Norm phantom were used to evaluate image quality. CT dose index (CTDI) was measured for each scenario of correct or wrong/missing bowtie filter to quantify imaging doses. Three protocols were evaluated, i.e. Standard Head (100 kVp, 145 mAs, full bowtie), Pelvis (125 kVp, 680 mAs, half bowtie) and Low-Dose Thorax (110 kVp, 262 mAs, half bowtie). For Standard Head, half bowtie and no bowtie filter were studied for comparison with the correct full bowtie acquisition. For Pelvis and Low-Dose Thorax, three scenarios, i.e. full bowtie, reversed direction half bowtie (half-R), and no bowtie were studied for comparison with the correct half bowtie acquisition.
Results: For Standard Head, CTDI-half/full ratio is 1.35 and CTDI-no/full is 1.94. For pelvis and low thorax, CTDI-full/half are 0.77 and 0.76, CTDI-half-R/half are 1.56 and 1.59, CTDI-no/half are 1.99 and 2.12, respectively. No substantial differences of spatial resolution were observed. Errors of CT numbers, crescent artifacts were found in wrong/missing bowtie filter. Decreased contrast-to-noise ratio and uniformity were found in half and no bowtie for Standard Head, and in full bowtie for Pelvis and Low-Dose Thorax.
Conclusion: One should verify the correct bowtie filter during CBCT procedures especially for IGRT system that requires manual placement of a bowtie filter. The direction of half bowtie filter should be verified at the time of installation/acceptance test.