MR/CT image fusion of the spine after spondylodesis: a feasibility study

Abstract

The objective of this study is to evaluate feasibility, accuracy and time requirements of MR/CT image fusion of the lumbar spine after spondylodesis. Sagittal MR and CT images derived from standard imaging protocols (sagittal T2-weighted MR/sagittal reformatted multi-planar-reformation of the CT) of the lumbar spine with correct (n = 5) and incorrect (n = 5) implant position were fused by two readers (R1, R2) using OsiriX in two sessions placing one (session 1) or two (session 2) reference point(s) on the dorsal tip(s) of the cranial and caudal endplates from the second lumbar to the first sacral vertebra. R1 was an experienced musculoskeletal radiologist; R2 a spine surgeon, both had received a short training on the software tool. Fusion times and fusion accuracy, defined as the largest deviation between MR and CT in the median sagittal plane on the ventral tip of the cranial end plate of the most cranial vertebra visible on the CT, were measured in both sessions. Correct or incorrect implant position was evaluated upon the fused images for all patients by an experienced senior staff musculoskeletal radiologist. Mean fusion time (session 1/session 2; in seconds) was 100.4/95 (R1) and 104.2/119.8 (R2). Mean fusion deviation (session 1/session 2; in mm) was 1.24/2.20 (R1) and 0.79/1.62 (R2). The correct/incorrect implant position was identified correctly in all cases. In conclusion, MR/CT image fusion of the spine with metallic implants is feasible, fast, accurate and easy to implement in daily routine work.

Fig. 1

An image fusion example between sagittal reformatted CT and sagittal T2-weighted MR images. The intervertebral cage L4/5 and a small piece of metal are well depicted on the CT (a) and fused image (c), whereas MR rules out disc degeneration of the L5/S1 segment (b). Information from both modalities are combined on the fused image (c)

Fig. 2

An image fusion example between sagittal reformatted CT and sagittal T2-weighted MR images. The two screws in L2 and L3 are positioned close to the cranial endplates, which are intact. The calcifications in the intervertebral disc L2/3 are clearly depicted by the CT (a) and the fused images (c), whereas the intervertebral foramen including the nerve root is shown by the MR (b) and fused images (c). Information from both modalities is combined on one image (c)

Fig. 3

An image fusion example between sagittal reformatted CT and sagittal T2-weighted MR images. The two screws in L5 and S1 are correctly placed, which is demonstrated by the CT (a) and fused images (c). Although anterolisthesis is present between L5 and S1, the intervertebral nerve root is neither compressed nor irritated by metal or osseous structures, as shown by the MR (b) and fused images (c). Important information from both modalities is combined on one image (c)