Long-length tomosynthesis and 3D-2D registration for intraoperative assessment of spine instrumentation

Abstract

Purpose. A system for long-length intraoperative imaging is reported based on longitudinal motion of an O-arm gantry featuring a multi-slot collimator. We assess the utility of long-length tomosynthesis and the geometric accuracy of 3D image registration for surgical guidance and evaluation of long spinal constructs. Methods. A multi-slot collimator with tilted apertures was integrated into an O-arm system for long-length imaging. The multi-slot projective geometry leads to slight view disparity in both long-length projection images (referred to as ‘line scans’) and tomosynthesis ‘slot reconstructions’ produced using a weighted-backprojection method. The radiation dose for long-length imaging was measured, and the utility of long-length, intraoperative tomosynthesis was evaluated in phantom and cadaver studies. Leveraging the depth resolution provided by parallax views, an algorithm for 3D-2D registration of the patient and surgical devices was adapted for registration with line scans and slot reconstructions. Registration performance using single-plane or dual-plane long-length images was evaluated and compared to registration accuracy achieved using standard dual-plane radiographs. Results. Longitudinal coverage of ∼50–64 cm was achieved with a single long-length slot scan, providing a field-of-view (FOV) up to (40 × 64) cm2, depending on patient positioning. The dose-area product (reference point air kerma × x-ray field area) for a slot scan ranged from ∼702–1757 mGy·cm2, equivalent to ∼2.5 s of fluoroscopy and comparable to other long-length imaging systems. Long-length scanning produced high-resolution tomosynthesis reconstructions, covering ∼12–16 vertebral levels. 3D image registration using dual-plane slot reconstructions achieved median target registration error (TRE) of 1.2 mm and 0.6° in cadaver studies, outperforming registration to dual-plane line scans (TRE = 2.8 mm and 2.2°) and radiographs (TRE = 2.5 mm and 1.1°). 3D registration using single-plane slot reconstructions leveraged the ∼7–14° angular separation between slots to achieve median TRE ∼2 mm and <2° from a single scan. Conclusion. The multi-slot configuration provided intraoperative visualization of long spine segments, facilitating target localization, assessment of global spinal alignment, and evaluation of long surgical constructs. 3D-2D registration to long-length tomosynthesis reconstructions yielded a promising means of guidance and verification with accuracy exceeding that of 3D-2D registration to conventional radiographs.

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

“…The collimator could potentially be implemented in combination with the motorized motion of the gantry along the ‐axis as a basis for “long‐length” intraoperative imaging. Such is the basis for the 2D “Long Film” capability reported by Zhang et al 36 . and Ladd et al 42 .…”

“…The experimental setup is illustrated in Figure 1. As detailed in previous studies, 37,38 The system was modified to include a slot collimator described previously for long-length 2D imaging 36 -used in this work as a basis for slot-beam 3D imaging to investigate potential benefits to x-ray scatter and 3D image quality. The collimator gives a slot-beam spanning 1.2 • F I G U R E 1 (a) A prototype O-arm™ system featuring a slot collimator that reduces the extent of the x-ray beam in the longitudinal (z) direction and is actuated within a motorized collimator assembly to enable slot-beam 3D imaging.…”

“…35,36 Motivated by the potential benefits associated with slot-beam scanning demonstrated in previous and ongoing research, the work reported below implemented and evaluated a prototype slot-beam configuration on the O-arm ™ system (Medtronic, Littleton, MA) for 3D intraoperative imaging, leveraging a slot collimator recently introduced for long-length, slot-scanning tomosynthesis. 36 The slot collimator reduces the beam width in the longitudinal direction, opening the possibility of slot-beam CT over a limited FOV z with reduced x-ray scatter, reduced patient dose, and improved softtissue image quality. There is also the potential for future implementations in which the slot-beam configuration is used in a helical acquisition mode.…”

“…Prepatient beam collimation (in particular, limiting the longitudinal field of view, FOV z ) reduces scatter in a dose‐efficient manner. The benefit of minimizing FOV z in a manner that still covers structures of interest is well recognized in CBCT, 28,29 and the benefit of a “slot” beam is evident in various embodiments found in digital mammography, 32,33 chest radiography, 34 and long‐length imaging 35,36 …”

“…The benefit of minimizing FOV z in a manner that still covers structures of interest is well recognized in CBCT, 28,29 and the benefit of a "slot" beam is evident in various embodiments found in digital mammography, 32,33 chest radiography, 34 and long-length imaging. 35,36 Motivated by the potential benefits associated with slot-beam scanning demonstrated in previous and ongoing research, the work reported below implemented and evaluated a prototype slot-beam configuration on the O-arm ™ system (Medtronic, Littleton, MA) for 3D intraoperative imaging, leveraging a slot collimator recently introduced for long-length, slot-scanning tomosynthesis. 36 The slot collimator reduces the beam width in the longitudinal direction, opening the possibility of slot-beam CT over a limited FOV z with reduced x-ray scatter, reduced patient dose, and improved softtissue image quality.…”