title: Monte Carlo study of the effects of system geometry and antiscatter grids on cone-beam CT scatter distributions.
abstract: OBJECTIVE
  The proliferation of cone-beam CT (CBCT) has created interest in performance optimization, with x-ray scatter identified among the main limitations to image quality. CBCT often contends with elevated scatter, but the wide variety of imaging geometry in different CBCT configurations suggests that not all configurations are affected to the same extent. Graphics processing unit (GPU) accelerated Monte Carlo (MC) simulations are employed over a range of imaging geometries to elucidate the factors governing scatter characteristics, efficacy of antiscatter grids, guide system design, and augment development of scatter correction.
  
  
  METHODS
  A MC x-ray simulator implemented on GPU was accelerated by inclusion of variance reduction techniques (interaction splitting, forced scattering, and forced detection) and extended to include x-ray spectra and analytical models of antiscatter grids and flat-panel detectors. The simulator was applied to small animal (SA), musculoskeletal (MSK) extremity, otolaryngology (Head), breast, interventional C-arm, and on-board (kilovoltage) linear accelerator (Linac) imaging, with an axis-to-detector distance (ADD) of 5, 12, 22, 32, 60, and 50 cm, respectively. Each configuration was modeled with and without an antiscatter grid and with (i) an elliptical cylinder varying 70-280 mm in major axis; and (ii) digital murine and anthropomorphic models. The effects of scatter were evaluated in terms of the angular distribution of scatter incident upon the detector, scatter-to-primary ratio (SPR), artifact magnitude, contrast, contrast-to-noise ratio (CNR), and visual assessment.
  
  
  RESULTS
  Variance reduction yielded improvements in MC simulation efficiency ranging from ∼17-fold (for SA CBCT) to ∼35-fold (for Head and C-arm), with the most significant acceleration due to interaction splitting (∼6 to ∼10-fold increase in efficiency). The benefit of a more extended geometry was evident by virtue of a larger air gap-e.g., for a 16 cm diameter object, the SPR reduced from 1.5 for ADD = 12 cm (MSK geometry) to 1.1 for ADD = 22 cm (Head) and to 0.5 for ADD = 60 cm (C-arm). Grid efficiency was higher for configurations with shorter air gap due to a broader angular distribution of scattered photons-e.g., scatter rejection factor ∼0.8 for MSK geometry versus ∼0.65 for C-arm. Grids reduced cupping for all configurations but had limited improvement on scatter-induced streaks and resulted in a loss of CNR for the SA, Breast, and C-arm. Relative contribution of forward-directed scatter increased with a grid (e.g., Rayleigh scatter fraction increasing from ∼0.15 without a grid to ∼0.25 with a grid for the MSK configuration), resulting in scatter distributions with greater spatial variation (the form of which depended on grid orientation).
  
  
  CONCLUSIONS
  A fast MC simulator combining GPU acceleration with variance reduction provided a systematic examination of a range of CBCT configurations in relation to scatter, highlighting the magnitude and spatial uniformity of individual scatter components, illustrating tradeoffs in CNR and artifacts and identifying the system geometries for which grids are more beneficial (e.g., MSK) from those in which an extended geometry is the better defense (e.g., C-arm head imaging). Compact geometries with an antiscatter grid challenge assumptions of slowly varying scatter distributions due to increased contribution of Rayleigh scatter.
authors: Department of Biomedical Engineering, Johns Hopkins University, Baltimore, Maryland 21205, USA.
authors: A
authors: A
authors: Sisniega
authors: W
authors: W
authors: Zbijewski
authors: A
authors: A
authors: Badal
authors: I S
authors: IS
authors: Kyprianou
authors: J W
authors: JW
authors: Stayman
authors: J J
authors: JJ
authors: Vaquero
authors: J H
authors: JH
authors: Siewerdsen
mesh_terms: D000818:Animals; D003196:Computer Graphics; D054893:Cone-Beam Computed Tomography / Q000379:methods*; D006801:Humans; D009010:Monte Carlo Method; D019047:Phantoms, Imaging; D012542:Scattering, Radiation; D059629:Signal-To-Noise Ratio
references: Int J Radiat Oncol Biol Phys. 2008 Mar 15;70(4):1045-56
references: 18029110
references: Med Phys. 2009 Nov;36(11):4878-80
references: 19994495
references: Phys Med Biol. 2012 Jul 7;57(13):4293-307
references: 22705748
references: Med Phys. 2004 Dec;31(12):3506-20
references: 15651634
references: Med Phys. 1992 Mar-Apr;19(2):475-81
references: 1584148
references: Comput Biol Med. 1976 Oct;6(4):307-323
references: 1000957
references: Phys Med Biol. 2007 Feb 7;52(3):577-87
references: 17228106
references: Phys Med Biol. 1983 Dec;28(12):1429-33
references: 6665036
references: IEEE Trans Med Imaging. 2006 Jul;25(7):817-27
references: 16827483
references: Int J Radiat Oncol Biol Phys. 2002 Aug 1;53(5):1337-49
references: 12128137
references: Med Phys. 2004 Nov;31(11):3057-67
references: 15587659
references: Phys Med Biol. 2010 Aug 7;55(15):4335-59
references: 20647608
references: Phys Med Biol. 2009 Jun 21;54(12):3847-64
references: 19491449
references: Br J Radiol. 1986 Dec;59(708):1209-18
references: 3801800
references: Med Phys. 2005 May;32(5):1397-413
references: 15984691
references: Phys Med Biol. 2009 Nov 21;54(22):6931-42
references: 19887715
references: IEEE Trans Med Imaging. 2006 Dec;25(12):1573-87
references: 17167993
references: Phys Med Biol. 2004 Sep 21;49(18):4321-33
references: 15509068
references: Phys Med Biol. 2002 Jan 7;47(1):163-75
references: 11814224
references: Med Phys. 2006 Nov;33(11):4320-9
references: 17153411
references: Med Phys. 2012 Jan;39(1):153-9
references: 22225284
references: IEEE Trans Med Imaging. 2008 Dec;27(12):1791-810
references: 19033095
references: Med Phys. 2005 Sep;32(9):2967-75
references: 16266111
references: Phys Med Biol. 2006 Sep 21;51(18):4567-86
references: 16953043
references: Proc SPIE Int Soc Opt Eng. 2006 Feb 28;6142:6142341-6142347
references: 18509508
references: Skeletal Radiol. 2013 May;42(5):649-57
references: 22990597
references: Med Phys. 2001 Feb;28(2):220-31
references: 11243347
references: Radiology. 2001 Dec;221(3):657-67
references: 11719660
references: Med Phys. 2006 Jan;33(1):187-97
references: 16485425
references: Otolaryngol Head Neck Surg. 2009 Jun;140(6):841-4
references: 19467400
references: Med Phys. 1982 Jul-Aug;9(4):464-72
references: 7110075
references: Med Phys. 2009 Jan;36(1):22-32
references: 19235370
references: Med Phys. 2006 Dec;33(12):4541-50
references: 17278805
references: Med Phys. 2007 Jan;34(1):275-81
references: 17278513
references: Phys Med Biol. 2010 Nov 21;55(22):6695-720
references: 21030750
references: Med Phys. 2012 Aug;39(8):4932-42
references: 22894419
references: Med Phys. 1982 Mar-Apr;9(2):231-9
references: 7087909
references: Med Phys. 1987 Mar-Apr;14(2):178-85
references: 3587135
references: Med Phys. 2004 May;31(5):1195-202
references: 15191309
references: Phys Med Biol. 2010 Aug 21;55(16):4495-507
references: 20668342
references: Med Phys. 1988 Jul-Aug;15(4):567-75
references: 3211049
references: Med Phys. 2010 Oct;37(10):5456-70
references: 21089782
references: Med Phys. 2000 Aug;27(8):1903-14
references: 10984236
references: Med Phys. 2005 Feb;32(2):613-26
references: 15789608
references: Med Phys. 2011 Aug;38(8):4700-13
references: 21928644
references: Med Phys. 2012 Jun;39(6Part20):3856
references: 28517527
references: AJR Am J Roentgenol. 2010 Feb;194(2):W193-201
references: 20093573
references: Phys Med Biol. 2007 Oct 21;52(20):6275-93
references: 17921585
references: Med Phys. 2009 Jun;36(6):2258-68
references: 19610315
references: Med Phys. 1983 Jan-Feb;10(1):40-50
references: 6843512
references: Med Phys. 1982 Nov-Dec;9(6):860-7
references: 7162472
references: Med Phys. 2001 Apr;28(4):469-74
references: 11339743
references: Med Phys. 1983 Nov-Dec;10(6):786-95
references: 6656692
references: IEEE Trans Med Imaging. 2004 May;23(5):584-90
references: 15147011
references: Eur Radiol. 2007 Nov;17(11):2767-79
references: 17587058
references: Med Phys. 2005 Jan;32(1):241-54
references: 15719975
references: Med Phys. 1997 Nov;24(11):1661-70
references: 9394272
references: Phys Med Biol. 2011 Sep 21;56(18):5845-64
references: 21852725
references: Med Phys. 1985 Mar-Apr;12(2):252-5
references: 4000088
references: Med Phys. 1987 Jul-Aug;14(4):567-76
references: 3626996
references: AJR Am J Roentgenol. 2007 Dec;189(6):1312-5
references: 18029864