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Transmission characteristics of a two dimensional antiscatter grid prototype for CBCT
Author(s) -
Altunbas Cem,
Kavanagh Brian,
Alexeev Timur,
Miften Moyed
Publication year - 2017
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1002/mp.12346
Subject(s) - truebeam , imaging phantom , optics , cone beam computed tomography , detector , dosimetry , contrast to noise ratio , offset (computer science) , signal to noise ratio (imaging) , flat panel detector , transmission (telecommunications) , physics , materials science , nuclear medicine , linear particle accelerator , beam (structure) , image quality , computer science , medicine , computed tomography , artificial intelligence , telecommunications , image (mathematics) , radiology , programming language
Aim High fraction of scattered radiation in cone‐beam CT ( CBCT ) imaging degrades CT number accuracy and visualization of low contrast objects. To suppress scatter in CBCT projections, we developed a focused, two‐dimensional antiscatter grid (2 DASG ) prototype. In this work, we report on the primary and scatter transmission characteristics of the 2 DASG prototype aimed for linac mounted, offset detector geometry CBCT systems in radiation therapy, and compared its performance to a conventional one‐dimensional ASG (1 DASG ). Methods The 2 DASG is an array of through‐holes separated by 0.1 mm septa that was fabricated from tungsten using additive manufacturing techniques. Through‐holes’ focusing geometry was designed for offset detector CBCT in Varian TrueBeam system. Two types of ASG s were evaluated: (a) a conventional 1 DASG with a grid ratio of 10, (b) the 2 DASG prototype with a grid ratio of 8.2. To assess the scatter suppression performance of both ASG s, Scatter‐to‐primary ratio ( SPR ) and scatter transmission fraction (Ts) were measured using the beam stop method. Scatter and primary intensities were modulated by varying the phantom thickness between 10 and 40 cm. Additionally, the effect of air gap and bow tie ( BT ) filter on SPR and Ts were evaluated. Average primary transmission fraction ( T P ) and pixel specific primary transmission were also measured for both ASG s. To assess the effect of transmission characteristics on projection image signal‐to‐noise ratio ( SNR ), SNR improvement factor was calculated. Improvement in contrast to noise ratio ( CNR ) was demonstrated using a low contrast object. Results In comparison to 1 DASG , 2 DASG reduced SPR s by a factor of 3 to 6 across the range of phantom setups investigated. Ts values for 1D and 2 DASG s were in the range of 21 to 29%, and 5 to 14% respectively. 2 DASG continued to provide lower SPR and Ts at increased air gap and with BT filter. Tp of 1D and 2 DASG s were 70.6% and 84.7% respectively. Due to the septal shadow of the 2 DASG , its pixel specific primary transmission values varied between 32.5% and 99.1%. With respect to 1 DASG , 2 DASG provided up to factor of 1.7 more improvement in SNR across the SPR range investigated. Moreover, 2 DASG provided improved visualization of low contrast objects with respect to 1 DASG and NOASG setups. Conclusions When compared to a conventional 1 DASG , 2 DASG prototype provided noticeably lower SPR and Ts values, indicating its superior scatter suppression performance. 2 DASG also provided 19% higher average primary transmission that was attributed to the absence of interseptal spacers and optimized grid geometry. Our results indicate that the combined effect of lower scatter and higher primary transmission provided by 2 DASG may potentially translate into more accurate CT numbers and improved contrast resolution in CBCT images.