Effect of grid geometry on the transmission properties of 2D grids for flat detectors in CBCT

Altunbas, Cem; Alexeev, Timur; Miften, Moyed; Kavanagh, Brian D.

doi:10.1088/1361-6560/ab4af4

Cited by 13 publications

(55 citation statements)

References 17 publications

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“…To evaluate the accuracy of scatter intensity measurement, two 2D grid prototypes with grid ratios of 12 and 16 were used. Both grids were fabricated from tungsten, had a grid pitch of 2 mm, and septal thickness of 0.1 mm 25 . The grids were mounted on the flat panel detector, and the gap between the back surface of the grids and the scintillator was set to 2 cm.…”

Section: Methodsmentioning

confidence: 99%

“…All image data were acquired at 125 kVp, without a bow tie filter, and at 0.388 × 0.388 mm 2 detector pixel size in TrueBeam’s offset detector geometry. The experimental setup is described in detail in prior publications by Altunbas et al 23,25 and Alexeev et al 26 …”

Section: Methodsmentioning

confidence: 99%

“…To understand the spatial distribution of primary and scatter intensity on a pixel scale, Monte Carlo simulations were also performed using the Geant4 toolkit 35,36 . An imaging geometry similar to that of the linac‐mounted CBCT system used in the experiments 23,25 was created. A point x‐ray source was used and photon energies were obtained by sampling a 125 kVp bremsstrahlung spectrum 37 .…”

Section: Methodsmentioning

confidence: 99%

“…Recently developed 2D antiscatter grids for CBCT offer an alternative solution to the scatter problem 22,23 . When compared to current high‐performance radiographic grids, 24 2D grids provide substantially improved scatter suppression 25 . Since a 2D grid is a scatter rejection device, it has the potential to provide more robust scatter mitigation than existing scatter correction methods, while simultaneously improving contrast‐to‐noise ratio (CNR) 23,26 .…”

Section: Introductionmentioning

confidence: 99%

“…Since a 2D grid is a scatter rejection device, it has the potential to provide more robust scatter mitigation than existing scatter correction methods, while simultaneously improving contrast‐to‐noise ratio (CNR) 23,26 . However, 2D grids do not fully suppress scatter incident on the detector; a small fraction of scatter fluence (2–8%) is still transmitted through the 2D grid 25 . Such residual scatter in projections reduces CT number accuracy, which in turn challenges implementation of quantitative imaging methods 27–29 .…”

Section: Introductionmentioning

confidence: 99%

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A unified scatter rejection and correction method for cone beam computed tomography

Altunbas

Park

et al. 2021

Medical Physics

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Purpose Scattered radiation is a major cause of image quality degradation in flat panel detector‐based cone beam CT (CBCT). While recently introduced 2D antiscatter grids reject the majority of scatter fluence, the small percentage of scatter fluence still transmitted to the detector remains a major challenge for implementation of quantitative imaging techniques such as dual energy imaging in CBCT. Additionally, this residual scatter is also a major source of grid‐induced artifacts, which impedes implementation of 2D grids in CBCT. We therefore present a new method to achieve both robust scatter rejection and residual scatter correction using a 2D antiscatter grid; in doing so, we expand the role of 2D grids from mere scatter rejection devices to scatter measurement devices. Method In our method, the radiopaque septa of the 2D grid emulate a micro array of beam‐stops placed on the detector which introduce spatially periodic septal shadows. By selecting sufficiently thin grid septa, the primary intensity can be reduced while preserving the uniformity of scatter intensity. This enables us to correlate the modulated pixel signal intensity in septal shadows with local scatter intensity. Our method then exploits this correlation to measure and remove residual scatter intensity from projections. No assumptions are made about the object being imaged. We refer to this as Grid‐based Scatter Sampling (GSS). In this work, we evaluate the principle of signal modulation with grid septa, the accuracy of scatter estimates, and the effect of the GSS method on image quality using simulations and measurements. We also implement the GSS method experimentally using a 2D grid prototype. Results Our results demonstrate that the GSS method increased CT number accuracy and reduced image artifacts associated with scatter. With 2D grid and residual scatter correction, HU nonuniformity was reduced from 65 HU to 30 HU in pelvis sized phantoms, and HU variations due to change in phantom size were reduced from 59 HU to 20 HU, when compared to use of only a 2D grid. With residual scatter correction via GSS method, grid‐induced ring artifacts were suppressed, leading to a 41% reduction in noise. The shape of the modulation transfer function (MTF) was preserved before and after suppression of ring artifacts. Conclusions Our grid‐based scatter sampling method enables utilization of a 2D grid as a scatter measurement and correction device. This method significantly improves quantitative accuracy in CBCT, further reducing the image quality gap between CBCT and multi‐detector CT. By correcting residual scatter with the proposed method, grid‐induced line artifacts in projections and associated ring artifacts in CBCT images were also suppressed with no compromise of spatial resolution.

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Section: Methodsmentioning

confidence: 99%

Section: Methodsmentioning

confidence: 99%

Section: Methodsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 3 more Smart Citations

A unified scatter rejection and correction method for cone beam computed tomography

Altunbas

Park

et al. 2021

Medical Physics

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Evaluation of scatter rejection and correction performance of 2D antiscatter grids in cone beam computed tomography

et al. 2021

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We have been investigating two-dimensional (2D) antiscatter grids (2D ASGs) to reduce scatter fluence and improve image quality in cone beam computed tomography (CBCT). In this work, two different aspects of 2D ASGs, their scatter rejection and correction capability, were investigated in CBCT experiments. To correct residual scatter transmitted through the 2D ASG, it was used as a scatter measurement device with a novel method: grid-based scatter sampling. Methods: Three focused 2D ASG prototypes with grid ratios of 8, 12, and 16 were developed for linac-mounted offset detector CBCT geometry. In the first phase, 2D ASGs were used as a scatter rejection device, and the effect of grid ratio on CT number accuracy and contrast-to-noise ratio (CNR) evaluated in CBCT images. In the second phase, a grid-based scatter sampling method which exploits the signal modulation characteristics of the 2D ASG's septal shadows to measure and correct residual scatter transmitted through the grid was implemented. To evaluate CT number accuracy, the percent change in CT numbers was measured by changing the phantom from head to pelvis size and configuration. Results: When 2D ASG was used as a scatter rejection device, CT number accuracy increased and the CT number variation due to change in phantom dimensions was reduced from 23% to 2-6%. A grid ratio of 16 yielded the lowest CT number variation. All three 2D ASGs yielded improvement in CNR, up to a factor of two in pelvis-sized phantoms. When 2D ASG prototypes were used for both scatter rejection and correction, CT number variations were reduced further, to 1.3-2.6%. In comparisons with a clinical CBCT system and a high-performance radiographic ASG, 2D ASG provided higher CT number accuracy under the same imaging conditions. Conclusions: When 2D ASG is used solely as a scatter rejection device, substantial improvement in CT number accuracy can be achieved by increasing the grid ratio. Two-dimensional ASGs also provided significant CNR improvement even at lower grid ratios. Two-dimensional ASGs used in conjunction with the grid-based scatter sampling method provided further improvement in CT number accuracy, irrespective of the grid ratio, while preserving 2D ASGs' capacity to improve CNR. The combined effect of scatter rejection and residual scatter correction by 2D ASG may accelerate implementation of new techniques in CBCT that require high quantitative accuracy, such as radiotherapy dose calculation and dual energy CBCT.

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3D‐printed large‐area focused grid for scatter reduction in cone‐beam CT

et al. 2022

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Background: Cone-beam computed tomography (CBCT) systems acquire volumetric data more efficiently than fan-beam or multislice CT, particularly when the anatomy of interest resides within the axial field-of -view of the detector and data can be acquired in one rotation. For such systems, scattered radiation remains a source of image quality degradation leading to increased noise, image artifacts, and CT number inaccuracies. Purpose: Recent advances in metal additive manufacturing allow the production of highly focused antiscatter grids (2D-ASGs) that can be used to reduce scatter intensity, while preserving primary radiation transmission. We present the first implementation of a large-area, 2D-ASG for flat-panel CBCT, including grid-line artifact removal and related improvements in image quality. Methods: A 245 × 194 × 10 mm 2D-ASG was manufactured from chromecobalt alloy using laser powder-bed fusion (LPBF) (AM-400; Renishaw plc, New Mills Wotton-under-Edge, UK). The 2D-ASG had a square profile with a pitch of 9.09 lines/cm and 10:1 grid-ratio. The nominal 0.1 mm grid septa were focused to a 732 mm x-ray source to optimize primary x-ray transmission and reduce grid-line shadowing at the detector. Powder-bed fusion ensured the structural stability of the ASG with no need for additional interseptal support. The 2D-ASG was coupled to a 0.139-mm element pitch flat-panel detector (DRX 3543, Carestream Health) and proper alignment was confirmed by consistent grid-line shadow thickness across the whole detector array. A 154-mm diameter CBCT image-quality-assurance phantom was imaged using a rotary stage and a ceiling-mounted, x-ray unit (Proteus XR/a, GE Medical Systems, 80kVp, 0.5mAs). Grid-line artifacts were removed using a combination of exposuredependent gain correction and spatial-frequency, Fourier filtering. Projections were reconstructed using a Parker-weighted, FDK algorithm and voxels were spatially averaged to 357 × 357 × 595 µm to improve the signal-to-noise characteristics of the CBCT reconstruction. Finally, in order to compare image quality with and without scatter, the phantom was scanned again under the same CBCT conditions but with no 2D-ASG. No additional antiscatter (i.e., air-gap, bowtie filtration) strategies were used to evaluate the effects in image quality caused by the 2D-ASG alone. Results: The large-area, 2D-ASG prototype was successfully designed and manufactured using LPBF. CBCT image-quality improvements using the 2D-ASG included: an overall 14.5% CNR increase across the volume; up to 48.8% CNR increase for low-contrast inserts inside the contrast plate of the QA phantom; and a 65% reduction of cupping artifact in axial profiles of water-filled cross sections of the phantom. Advanced image processing strategies to remove grid 240

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Effect of grid geometry on the transmission properties of 2D grids for flat detectors in CBCT

Cited by 13 publications

References 17 publications

A unified scatter rejection and correction method for cone beam computed tomography

A unified scatter rejection and correction method for cone beam computed tomography

Evaluation of scatter rejection and correction performance of 2D antiscatter grids in cone beam computed tomography

3D‐printed large‐area focused grid for scatter reduction in cone‐beam CT

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