Intraoperative cone‐beam and slot‐beam CT: 3D image quality and dose with a slot collimator on the O‐arm imaging system

Abstract

Purpose To characterize the 3D imaging performance and radiation dose for a prototype slot-beam configuration on an intraoperative O-arm (TM) Surgical Imaging System (Medtronic Inc., Littleton, MA) and identify potential improvements in soft-tissue image quality for surgical interventions. Methods A slot collimator was integrated with the O-arm((TM)) system for slot-beam axial CT. The collimator can be automatically actuated to provide 1.2 degrees slot-beam longitudinal collimation. Cone-beam and slot-beam configurations were investigated with and without an antiscatter grid (12:1 grid ratio, 60 lines/cm). Dose, scatter, image noise, and soft-tissue contrast resolution were evaluated in quantitative phantoms for head and body configurations over a range of exposure levels (beam energy and mAs), with reconstruction performed via filtered-backprojection. Qualitative imaging performance across various anatomical sites and imaging tasks was assessed with anthropomorphic head, abdomen, and pelvis phantoms. Results The dose for a slot-beam scan varied from 0.02-0.06 mGy/mAs for head protocols to 0.01-0.03 mGy/mAs for body protocols, yielding dose reduction by similar to 1/5 to 1/3 compared to cone-beam, owing to beam collimation and reduced x-ray scatter. The slot-beam provided an similar to 6-7x reduction in scatter-to-primary ratio (SPR) compared to the cone-beam, yielding SPR similar to 20-80% for head and body without the grid and similar to 7-30% with the grid. Compared to cone-beam scans at equivalent dose, slot-beam images exhibited an similar to 2.5x increase in soft-tissue contrast-to-noise ratio (CNR) for both grid and gridless configurations. For slot-beam scans, a further similar to 10-30% improvement in CNR was achieved when the grid was removed. Slot-beam imaging could benefit certain interventional scenarios in which improved visualization of soft tissues is required within a fairly narrow longitudinal region of interest (+/- 7 mm in z)--for example, checking the completeness of tumor resection, preservation of adjacent anatomy, or detection of complications (e.g., hemorrhage). While preserving existing capabilities for fluoroscopy and cone-beam CT, slot-beam scanning could enhance the utility of intraoperative imaging and provide a useful mode for safety and validation checks in image-guided surgery. Conclusions The 3D imaging performance and dose of a prototype slot-beam CT configuration on the O-arm((TM)) system was investigated. Substantial improvements in soft-tissue image quality and reduction in radiation dose are evident with the slot-beam configuration due to reduced x-ray scatter.