title: Two-dimensional dynamic fluid bowtie attenuators.
abstract: Fluence field modulated (FFM) CT allows for improvements in image quality and dose reduction. To date, only one-dimensional modulators have been proposed, as the extension to two-dimensional (2-D) modulation is difficult with solid-metal attenuation-based fluence field modulated designs. This work proposes to use liquid and gas to attenuate the x-ray beam, as unlike solids, these materials can be arranged allowing for 2-D fluence modulation. The thickness of liquid and the pressure for a given path length of gas were determined that provided the same attenuation as 30 cm of soft tissue at 80, 100, 120, and 140 kV. Liquid iodine, zinc chloride, cerium chloride, erbium oxide, iron oxide, and gadolinium chloride were studied. Gaseous xenon, uranium hexafluoride, tungsten hexafluoride, and nickel tetracarbonyl were also studied. Additionally, we performed a proof-of-concept experiment using a 96 cell array in which the liquid thickness in each cell was adjusted manually. Liquid thickness varied as a function of kV and chemical composition, with erbium oxide allowing for the smallest thickness. For the gases, tungsten hexaflouride required the smallest pressure to compensate for 30 cm of soft tissue. The 96 cell iodine attenuator allowed for a reduction in both dynamic range to the detector and scatter-to-primary ratio. For both liquids and gases, when k-edges were located within the diagnostic energy range used for imaging, the mean beam energy exhibited the smallest change with compensation amount. The thickness of liquids and the gas pressure seem logistically implementable within the space constraints of C-arm-based cone beam CT (CBCT) and diagnostic CT systems. The gas pressures also seem logistically implementable within the space and tube loading constraints of CBCT and diagnostic CT systems.
authors.affiliation: University of Wisconsin-Madison , Department of Biomedical Engineering, 1415 Engineering Drive, Madison, Wisconsin 53706, United States.
authors.forename: James R
authors.initials: JR
authors.lastname: Hermus
authors.affiliation: University of Wisconsin-Madison, Department of Biomedical Engineering, 1415 Engineering Drive, Madison, Wisconsin 53706, United States; University of Wisconsin-Madison, Department of Medical Physics, 1005 WIMR, 1111 Highland Avenue, Madison, Wisconsin 53705, United States; University of Wisconsin-Madison, Department of Radiology, 1005 WIMR, 1111 Highland Avenue, Madison, Wisconsin 53705, United States.
authors.forename: Timothy P
authors.initials: TP
authors.lastname: Szczykutowicz
keywords: computed tomography; dynamic bowtie filter; fluence field modulated computed tomography
references.citation: Med Phys. 2009 Apr;36(4):1184-92
references.pmid: 19472624
references.citation: Med Phys. 2013 Mar;40(3):031910
references.pmid: 23464325
references.citation: Med Phys. 2007 Jul;34(7):2691-703
references.pmid: 17821977
references.citation: PLoS One. 2014 Jul 22;9(7):e103054
references.pmid: 25051067
references.citation: Med Phys. 2004 Nov;31(11):3057-67
references.pmid: 15587659
references.citation: J Comput Assist Tomogr. 1977 Oct;1(4):419-28
references.pmid: 615220
references.citation: Med Phys. 2011 Jul;38 Suppl 1:S2
references.pmid: 21978114
references.citation: Phys Med Biol. 2014 Mar 7;59(5):1305-26
references.pmid: 24556823
references.citation: Med Phys. 2013 Feb;40(2):021905
references.pmid: 23387753
references.citation: Med Phys. 2013 Feb;40(2):021906
references.pmid: 23387754
references.citation: Med Phys. 2015 Apr;42(4):1779-86
references.pmid: 25832068