Evaluation of the effects of sagging shifts on isocenter accuracy and image quality of cone-beam CT from kV on-board imagers.
To investigate the effects of sagging shifts of three on-board kV imaging systems (OBI) on the isocenter positioning accuracy and image quality of cone-beam CT (CBCT). A cubical phantom having a metal marker in the center that can be aligned with the radiation isocenter was used to measure sagging shifts and their variation with gantry angle on three Varian linacs with kV on-board imaging systems. A marker-tracking algorithm was applied to detect the shadow of the metal marker and localize its center in the two-dimensional cone-beam radiographic projections. This tracking algorithm is based on finding the position of maximum cross-correlation between a region-of-interest from a template image (including the metal marker) and the projections containing the shadow of the metal marker. Sagging shifts were corrected by mapping the center of the metal marker to a reference position for all projections acquired over a full gantry rotation (0-360 degrees). The sag-corrected radiographic projections were then used to reconstruct CBCT using Feldkamp back-projection. A standard quality assurance phantom was used to evaluate the image quality of CBCT before and after sagging correction. Sagging affects both the positioning accuracy of the OBI isocenter and the CBCT image quality. For example, on one linac, the position of the marker on the cone-beam radiographic projections depends on the angular view and has maximal shifts of about 2 mm along the imager x-direction (patient's cross-plane). Sagging produces systematic shifts of the OBI isocenter as large as 1 mm posterior and 1 mm left in patient coordinates relative to the radiation isocenter. Further, it causes spatial distortion and blurring in CBCT image reconstructed from radiographic projections that are not corrected for OBI sagging. CBCT numbers vary by about 1% in full-fan scans and up to 3.5% in half-fan scans because of sagging. In order to achieve better localization accuracy in image-guided radiation therapy, sagging shifts of the kV OBI need to be corrected. In addition, correction of sagging improves image and provides better visualization of internal structures. Frequent quality assurance is required to monitor and maintain standards of variations in the mechanical accuracy of isocenter and image quality of CBCT because of sagging shifts.
Ali
Imad
I
Department of Radiation Oncology, Oklahoma University Health Sciences Center, Oklahoma City, OK, 73104, USA.
Ahmad
Salahuddin
S
Department of Radiation Oncology, Oklahoma University Health Sciences Center, Oklahoma City, OK, 73104, USA.
D054893:Cone-Beam Computed Tomography / Q000295:instrumentation / Q000379:methods; D006801:Humans; D019047:Phantoms, Imaging; D011785:Quality Assurance, Health Care
Med Phys. 2003 Aug;30(8):2089-115
12945975
Phys Med Biol. 2000 Feb;45(2):329-47
10701507
Med Phys. 2000 Jan;27(1):39-46
10659736
Med Phys. 2005 Mar;32(3):819-29
15839355
J Appl Clin Med Phys. 2004 Spring;5(2):18-28
15738910
Med Phys. 2001 Feb;28(2):220-31
11243347
Int J Radiat Oncol Biol Phys. 1988 Feb;14(2):373-81
3276655
Med Phys. 1994 Jul;21(7):1163-78
7968850
Int J Radiat Oncol Biol Phys. 2008 Aug 1;71(5):1572-80
18539405
Med Phys. 2006 Nov;33(11):4431-47
17153422
Med Phys. 2005 Aug;32(8):2606-14
16193791
Med Phys. 2007 Dec;34(12):4563-6
18196781
Med Phys. 2004 Dec;31(12):3506-20
15651634
Med Phys. 2003 Oct;30(10):2762-92
14596315
Med Phys. 2006 Apr;33(4):1165-79
16696494
Med Phys. 2000 Jun;27(6):1311-23
10902561
Int J Radiat Oncol Biol Phys. 2000 Jan 15;46(2):329-35
10661339
Int J Radiat Oncol Biol Phys. 2008 May 1;71(1):297-304
18406894
Med Phys. 2006 Sep;33(9):3489-97
17022245
Int J Radiat Oncol Biol Phys. 2002 Aug 1;53(5):1337-49
12128137
Int J Radiat Oncol Biol Phys. 2010 Jul 15;77(4):1092-7
19879059