Mobile C‐arm cone‐beam CT for guidance of spine surgery: Image quality, radiation dose, and integration with interventional guidance
Authors: Schafer S. , Nithiananthan S. , Mirota D. J. , Uneri A. , Stayman J. W. , Zbijewski W. , Schmidgunst C. , Kleinszig G. , Khanna A. J. , Siewerdsen J. H. 
Published: July 2011

Purpose: 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. Methods: 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.
Results: 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  ×  0.3  ×  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  ×  0.3  ×  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 ×  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 μSv/mGy on average at tableside to ∼0.1 μSv/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 (∼40–60 μSv), compared to 5–11 min for the conventional approach. Conclusions: 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.