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Antiscatter grids in mobile C-arm cone-beam CT: Effect on image quality and dose
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/content/aapm/journal/medphys/39/1/10.1118/1.3666947
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http://aip.metastore.ingenta.com/content/aapm/journal/medphys/39/1/10.1118/1.3666947
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Figures

Image of FIG. 1.

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FIG. 1.

(a) Experimental setup for antiscatter grids applied to mobile C-arm CBCT. (1) Antiscatter grid mounted on the FPD. (2) Phantom setup (a head-equivalent extension was removed for better visualization). (3) X-ray tube shown at the start-scan position. (b) Thorax phantom; (c) reconstruction of tissue-equivalent inserts in the thorax phantom.

Image of FIG. 2.

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FIG. 2.

Zoomed and magnified projection image showing (a) an example of gridline aliasing stemming from incorrect angle-dependent gain calibration, and (b) the removal of such shadows/aliasing with correctly applied angle-dependent gain calibration. Grid line artifacts in projection images cause high frequency artifacts in the reconstructions (c), an effect that does not occur in correctly processed images (d).

Image of FIG. 3.

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FIG. 3.

Hounsfield unit inaccuracy, Δ (left axis), and voxel noise (right axis) as a function of grid ratio. Grids are seen to improve HU accuracy but impart an increase in voxel noise unless accompanied by an increase in exposure technique.

Image of FIG. 4.

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FIG. 4.

CNR as a function of grid ratio for (a) thorax phantom bone protocol, (b) thorax phantom soft-tissue protocol, (c) abdomen phantom bone protocol, and (d) abdomen phantom soft-tissue protocol. At fixed patient dose (see Table I) grids impart a significant reduction in soft-tissue CNR.

Image of FIG. 5.

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FIG. 5.

Tissue-equivalent inserts in the thoracic and abdomen phantoms for the bone and soft-tissue protocols at various grid ratios. The images correspond to the CNR measurements of Fig. 4. A reduction in scatter artifacts extending vertically from the cortical bone insert toward the spine insert is visible. The horizontal high- and low-intensity streak artifacts around the cortical bone insert arise from the incomplete orbit of the C-arm (∼178°).

Image of FIG. 6.

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FIG. 6.

Axial and sagittal views of the anthropomorphic body phantom with implanted spine hardware (resolution 0.3 × 0.3 × 0.9 mm3). Images with a 10:1 grid in place (c, d) exhibit little or no improvement in overall image quality compared to the gridless images (a, b) and show a slight increase in image noise unless the dose to the patient is increased to counter the loss in primary fluence.

Tables

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TABLE I.

Technique settings and dosimetry for task-specific protocols in the thorax and abdomen. “Bone protocol” refers to a lower dose technique sufficient for high-contrast bone visualization. “Soft-tissue protocol” refers to a higher dose technique sufficient for soft-tissue visualization.

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/content/aapm/journal/medphys/39/1/10.1118/1.3666947
2011-12-14
2014-04-23

Abstract

Purpose: X-ray scatter is a major detriment to image quality in cone-beam CT(CBCT). Existing geometries exhibit strong differences in scatter susceptibility with more compact geometries, e.g., dental or musculoskeletal, benefiting from antiscatter grids, whereas in more extended geometries, e.g., IGRT, grid use carries tradeoffs in image quality per unit dose. This work assesses the tradeoffs in dose and image quality for grids applied in the context of low-dose CBCT on a mobile C-arm for image-guided surgery.Methods: Studies were performed on a mobile C-arm equipped with a flat-panel detector for high-quality CBCT. Antiscatter grids of grid ratio (GR) 6:1–12:1, 40 lp/cm, were tested in “body” surgery, i.e., spine, using protocols for bone and soft-tissue visibility in the thoracic and abdominal spine. Studies focused on grid orientation, CT number accuracy, imagenoise, and contrast-to-noise ratio(CNR) in quantitative phantoms at constant dose.Results: There was no effect of grid orientation on possible gridline artifacts, given accurate angle-dependent gain calibration. Incorrect calibration was found to result in gridline shadows in the projection data that imparted high-frequency artifacts in 3D reconstructions. Increasing GR reduced errors in CT number from 31%, thorax, and 37%, abdomen, for gridless operation to 2% and 10%, respectively, with a 12:1 grid, while imagenoise increased by up to 70%. The CNR of high-contrast objects was largely unaffected by grids, but low-contrast soft-tissues suffered reduction in CNR, 2%–65%, across the investigated GR at constant dose.Conclusions: While grids improved CT number accuracy, soft-tissue CNR was reduced due to attenuation of primary radiation.CNR could be restored by increasing dose by factors of ∼1.6–2.5 depending on GR, e.g., increase from 4.6 mGy for the thorax and 12.5 mGy for the abdomen without antiscatter grids to approximately 12 mGy and 30 mGy, respectively, with a high-GR grid. However, increasing the dose poses a significant impediment to repeat intraoperative CBCT and can cause the cumulative intraoperative dose to exceed that of a single diagnostic CT scan. This places the mobile C-arm in the category of extended CBCT geometries with sufficient air gap for which the tradeoffs between CNR and dose typically do not favor incorporation of an antiscatter grid.

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Scitation: Antiscatter grids in mobile C-arm cone-beam CT: Effect on image quality and dose
http://aip.metastore.ingenta.com/content/aapm/journal/medphys/39/1/10.1118/1.3666947
10.1118/1.3666947
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