title: Mobile C-arm cone-beam CT for guidance of spine surgery: image quality, radiation dose, and integration with interventional guidance.
abstract: OBJECTIVE
  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 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.
  
  
  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.
authors.affiliation: Department of Biomedical Engineering, Johns Hopkins University, Baltimore, Maryland 21202, USA.
authors.forename: S
authors.initials: S
authors.lastname: Schafer
authors.forename: S
authors.initials: S
authors.lastname: Nithiananthan
authors.forename: D J
authors.initials: DJ
authors.lastname: Mirota
authors.forename: A
authors.initials: A
authors.lastname: Uneri
authors.forename: J W
authors.initials: JW
authors.lastname: Stayman
authors.forename: W
authors.initials: W
authors.lastname: Zbijewski
authors.forename: C
authors.initials: C
authors.lastname: Schmidgunst
authors.forename: G
authors.initials: G
authors.lastname: Kleinszig
authors.forename: A J
authors.initials: AJ
authors.lastname: Khanna
authors.forename: J H
authors.initials: JH
authors.lastname: Siewerdsena
mesh_terms: D002102:Cadaver; D054893:Cone-Beam Computed Tomography / Q000379:methods* / Q000706:statistics & numerical data; D006801:Humans; D021621:Imaging, Three-Dimensional / Q000379:methods* / Q000706:statistics & numerical data; D007432:Intraoperative Period; D008159:Lumbar Vertebrae / Q000000981:diagnostic imaging / Q000601:surgery; D019060:Minimally Invasive Surgical Procedures; D019047:Phantoms, Imaging; D011829:Radiation Dosage; D011857:Radiographic Image Interpretation, Computer-Assisted / Q000379:methods*; D015641:Radiography, Interventional; D011874:Radiometry; D013131:Spine / Q000000981:diagnostic imaging* / Q000601:surgery*; D013904:Thoracic Vertebrae / Q000000981:diagnostic imaging / Q000601:surgery; D054854:Vertebroplasty