Marker-less assessment of the geometric error of fused cone-beam computed tomography images of the foot constructed using stitching software

Background: Image noise, object repositioning, initial manual image alignment, and stitching of different volumes, i.e. anatomical regions may all affect the stitching error of fused cone-beam computed tomography (CBCT) images. Purpose: To determine the geometric error of fused CBCT images of the hindfoot, lower leg, and forefoot after using stitching software, based on a marker-less validation method. Material and Methods: CBCT images of the hindfoot, lower leg, and forefoot were acquired multiple times on a Planmed Verity scanner with and without repositioning the leg between acquisitions. Prototype stitching software was used to stitch hindfoot–forefoot volumes and hindfoot–lower leg volumes. Stitching error was determined via registration and by calculating the displacement of the tibia, first metatarsal, or proximal phalanges, with respect to the calcaneus, compared to their position on conventional CT. Results: Overall total translation and rotation errors were 1.22 ± 0.62 mm (range 0.43–3.07 mm) and 1.27° ± 0.53° (range 0.29°–2.88°). Lower leg translation was smaller compared to forefoot translation (P < 0.001). No statistical difference was observed between lower leg and forefoot rotation errors (P = 0.186). Cadaver repositioning between acquisitions resulted in larger rotation errors (P < 0.05). Cadaver repositioning did not affect translation errors (P = 0.768). Conclusion: Geometric error of fused CBCT images can be quantified using a marker-less validation method. Stitching of hindfoot volumes with forefoot and lower leg volumes induces translation and rotation errors of up to approximately 3 mm and 3°. Translation errors are larger in hindfoot–forefoot stitches compared to hindfoot–lower leg stitches.