Antiscatter grids in mobile C-arm cone-beam CT: Effect on image quality and dose
(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.
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).
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.
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.
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°).
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.
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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