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ABSTRACT: 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.
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, image noise, and contrast-to-noise ratio (CNR) in quantitative phantoms at constant dose.
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 image noise 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.
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.
Medical Physics 01/2012; 39(1):153-9. · 2.83 Impact Factor
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ABSTRACT: A flat-panel detector based mobile isocentric C-arm for cone-beam CT (CBCT) has been developed to allow intraoperative 3D imaging with sub-millimeter spatial resolution and soft-tissue visibility. Image quality and radiation dose were evaluated in spinal surgery, commonly relying on lower-performance image intensifier based mobile C-arms. Scan protocols were developed for task-specific imaging at minimum dose, in-room exposure was evaluated, and integration of the imaging system with a surgical guidance system was demonstrated in preclinical studies of minimally invasive spine surgery.
Radiation dose was assessed as a function of kilovolt (peak) (80-120 kVp) and milliampere second using thoracic and lumbar spine dosimetry phantoms. In-room radiation exposure was measured throughout the operating room for various CBCT scan protocols. Image quality was assessed using tissue-equivalent inserts in chest and abdomen phantoms to evaluate bone and soft-tissue contrast-to-noise ratio as a function of dose, and task-specific protocols (i.e., visualization of bone or soft-tissues) were defined. Results were applied in preclinical studies using a cadaveric torso simulating minimally invasive, transpedicular surgery.
Task-specific CBCT protocols identified include: thoracic bone visualization (100 kVp; 60 mAs; 1.8 mGy); lumbar bone visualization (100 kVp; 130 mAs; 3.2 mGy); thoracic soft-tissue visualization (100 kVp; 230 mAs; 4.3 mGy); and lumbar soft-tissue visualization (120 kVp; 460 mAs; 10.6 mGy)--each at (0.3 x 0.3 x 0.9 mm3) voxel size. Alternative lower-dose, lower-resolution soft-tissue visualization protocols were identified (100 kVp; 230 mAs; 5.1 mGy) for the lumbar region at (0.3 x 0.3 x 1.5 mm3) voxel size. Half-scan orbit of the C-arm (x-ray tube traversing under the table) was dosimetrically advantageous (prepatient attenuation) with a nonuniform dose distribution (-2 x higher at the entrance side than at isocenter, and -3-4 lower at the exit side). The in-room dose (microsievert) per unit scan dose (milligray) ranged from -21 microSv/mGy on average at tableside to -0.1 microSv/mGy at 2.0 m distance to isocenter. All protocols involve surgical staff stepping behind a shield wall for each CBCT scan, therefore imparting -zero dose to staff. Protocol implementation in preclinical cadaveric studies demonstrate integration of the C-arm with a navigation system for spine surgery guidance-specifically, minimally invasive vertebroplasty in which the system provided accurate guidance and visualization of needle placement and bone cement distribution. Cumulative dose including multiple intraoperative scans was -11.5 mGy for CBCT-guided thoracic vertebroplasty and -23.2 mGy for lumbar vertebroplasty, with dose to staff at tableside reduced to -1 min of fluoroscopy time (-4(0-60 microSv), compared to 5-11 min for the conventional approach.
Intraoperative CBCT using a high-performance mobile C-arm prototype demonstrates image quality suitable to guidance of spine surgery, with task-specific protocols providing an important basis for minimizing radiation dose, while maintaining image quality sufficient for surgical guidance. Images demonstrate a significant advance in spatial resolution and soft-tissue visibility, and CBCT guidance offers the potential to reduce fluoroscopy reliance, reducing cumulative dose to patient and staff. Integration with a surgical guidance system demonstrates precise tracking and visualization in up-to-date images (alleviating reliance on preoperative images only), including detection of errors or suboptimal surgical outcomes in the operating room.
Medical Physics 08/2011; 38(8):4563-74. · 2.83 Impact Factor
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ABSTRACT: Purpose: X‐ray scatter has been identified as a principal factor in cone‐beam CT (CBCT) image quality in applications ranging from dental to IGRT and image‐guided surgery. The variety of system geometries entail disparate scatter magnitudes, which bear heavily on the question of utility of antiscatter grids. This paper revisits this question for low‐dose CBCT on a mobile C‐arm for image‐guided surgery, identifying tradeoffs in dose and image quality and answering the question of when (or whether) grids should be employed. Methods: Studies were performed on a mobile C‐arm prototype equipped with a flat‐panel detector for high‐quality CBCT. Antiscatter grids of grid ratio (GR) 6:1 – 12:1 (103 lpi) were tested under varying scatter conditions in “body” surgery (e.g., spine) using task‐specific protocols for bone and soft tissue visibility in the thorax and abdomen. Studies included grid orientation, CT number accuracy, uniformity, contrast, noise, and contrast‐to‐noise ratio (CNR), each evaluated in quantitative / anthropomorphic phantoms. Results: Grid orientation along the detector z‐ axis introduced susceptibility to artifacts attributed to ramp filter amplification of gridlines under motion nonidealities. Orientation along the xy‐axis resolved this effect. Increasing GR improved CT number accuracy from 40% error (no grid) to 4% (12:1), but imparted an increase in noise by ∼20–60%. CNR for high‐contrast objects was largely unaffected by grids, but a significant reduction (2–44%) in CNR was observed in low‐contrast soft‐tissues. Conclusion: While grids showed substantial improvement in CT number accuracy and uniformity, soft‐tissue CNR was reduced due to grid attenuation in both the thorax and abdomen. The CNR could be restored by increasing the dose by a factor of 1.5. This poses a significant drawback to low‐dose, repeat CBCT and diminishes the extent to which grids should be employed, particularly in light of simple scatter correction techniques that offer comparable restoration without increase in dose. Research supported by the National Institutes of Health R01‐CA‐127444 and collaboration with Siemens Healthcare (Erlangen Germany).
Medical Physics 05/2011; 38(6):3875-3875. · 2.83 Impact Factor
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S Schafer,
Y Otake,
A Uneri,
D Mirota,
S Nithiananthan,
W Zbijewski,
J Stayman, C Schmidgunst,
C Bulitta,
A Khanna,
J Siewerdsen
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ABSTRACT: Purpose: A prototype mobile C‐arm incorporating a flat‐panel detector (FPD) for high‐quality cone‐beam CT (CBCT) is under investigation for spine surgery. This work describes the image quality and dose in cervical, thoracic, and lumbar spine surgery and integration with novel navigation subsystems critical to high‐precision surgical guidance, including: tracking, video augmentation, deformable registration, and multi‐modality visualization. Method and Materials: Image quality, patient dose, and in‐room dose were assessed in phantom and cadaver studies approximating spine surgery. An open‐source software architecture was employed for integration of navigation systems based upon the CISST libraries linked to 3D Slicer. For spine surgery, a novel video‐based tracking and augmentation system was developed to facilitate high‐precision minimally invasive techniques. Target registration error (TRE) and accuracy of overlay were assessed. Deformable registration of preoperative and intraoperative images was based upon a fast implementation of the Demons algorithm. Results: Image quality demonstrated sub‐mm spatial resolution and soft‐tissue visibility; protocols were identified for “standard” and “high‐quality” imaging of the spine ‐ respectively for bone visualization (100 kVp, 2.4 mA, 200 projections, 1.6 mGy) and for soft‐tissue (100 kVp, 5.0 mA, 400 projections, 7.1 mGy). The TRE for video‐based and infrared tracking systems was 0.67 and 0.54 mm, respectively, and the accuracy of video overlay was 1.0 mm for the integrated video‐based tracker, compared to 1.3 mm for separate video and infrared trackers. Deformable registration demonstrated accuracy on the order of the voxel size on timescales consistent with surgical application. Conclusion: The first evaluation of an integrated mobile CBCT and guidance systems for spine surgery was demonstrated and characterized, emphasizing a flexible architecture of application‐specific navigation technologies. Drawing upon a decade of research in C‐arm CBCT, a new clinical prototype C‐arm is realized and undergoing evaluation and translation to spine surgery trials. Research supported by the NIH and Siemens Healthcare.
Medical Physics 05/2010; 37(6):3394-3394. · 2.83 Impact Factor
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ABSTRACT: The continuing research and further development in flat panel detector technology have led to its integration into more and more medical x-ray systems for two-dimensional (2D) and three-dimensional (3D) imaging, such as fixed or mobile C arms. Besides the obvious advantages of flat panel detectors, like the slim design and the resulting optimum accessibility to the patient, their success is primarily a product of the image quality that can be achieved. The benefits in the physical and performance-related features as opposed to conventional image intensifier systems, (e.g., distortion-free reproduction of imaging information or almost linear signal response over a large dynamic range) can be fully exploited, however, only if the raw detector images are correctly calibrated and postprocessed. Previous procedures for processing raw data contain idealizations that, in the real world, lead to artifacts or losses in image quality. Thus, for example, temperature dependencies or changes in beam geometry, as can occur with mobile C arm systems, have not been taken into account up to this time. Additionally, adverse characteristics such as image lag or aging effects have to be compensated to attain the best possible image quality. In this article a procedure is presented that takes into account the important dependencies of the individual pixel sensitivity of flat panel detectors used in 2D or 3D imaging and simultaneously minimizes the work required for an extensive recalibration. It is suitable for conventional detectors with only one gain mode as well as for the detectors specially developed for 3D imaging with dual gain read-out technology.
Medical Physics 10/2007; 34(9):3649-64. · 2.83 Impact Factor
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ISCAS;
