Int J CARS (2015) 10:1371–1381 DOI 10.1007/s11548-015-1247-5 ORIGINAL ARTICLE A multi-vertebrae CT to US registration of the lumbar spine in clinical data Simrin Nagpal 1 · Purang Abolmaesumi 2 · Abtin Rasoulian 2 · Ilker Hacihaliloglu 3 · Tamas Ungi 1 · Jill Osborn 4 · Victoria A. Lessoway 5 · John Rudan 6 · Melanie Jaeger 6 · Robert N. Rohling 2 · Dan P. Borschneck 6 · Parvin Mousavi 1 Received: 13 December 2014 / Accepted: 8 June 2015 / Published online: 15 July 2015 © CARS 2015 Abstract Purpose Spinal needle injections are widely applied to alle- viate back pain and for anesthesia. Current treatment is performed either blindly with palpation or using fluoroscopy or computed tomography (CT). Both fluoroscopy and CT guidance expose patients to ionizing radiation. Ultrasound (US) guidance for spinal needle procedures is becoming more prevalent as an alternative. It is challenging to use US as the sole imaging modality for intraoperative guidance of spine needle injections due to the acoustic shadows created by the bony structures of the vertebra that limit visibility of the tar- get areas for injection. We propose registration of CT and the US images to augment anatomical visualization for the clinician during spinal interventions guided by US. Methods The proposed method involves automatic global and multi-vertebrae registration to find the closest alignment between CT and US data. This is performed by maximizing the similarity between the two modalities using voxel inten- sity information as well as features extracted from the input volumes. In our method, the lumbar spine is first globally aligned between the CT and US data using intensity-based registration followed by point-based registration. To account for possible curvature change of the spine between the CT B Parvin Mousavi pmousavi@cs.queensu.ca 1 School of Computing, Queen’s University, Kingston, ON, Canada 2 The University of British Columbia, Vancouver, BC, Canada 3 Rutgers-The State University of New Jersey, Newark, NJ, USA 4 St. Paul’s Hospital, Vancouver, BC, Canada 5 BC Women’s Hospital, Vancouver, BC, Canada 6 Kingston General Hospital, Kingston, ON, Canada and US volumes, a multi-vertebrae registration step is also performed. Springs are used to constrain the movement of the individually transformed vertebrae to ensure the optimal alignment is a pose of the lumbar spine that is physically possible. Results Evaluation of the algorithm is performed on 10 clin- ical patient datasets. The registration approach was able to align CT and US datasets from initial misalignments of up to 25 mm, with a mean TRE of 1.37 mm. These results sug- gest that the proposed approach has the potential to offer a sufficiently accurate registration between clinical CT and US data. Keywords Registration · Ultrasound · Lumbar spine · Spinal interventions · Multi-vertebrae Introduction Spine needle injections are commonly used for analgesia to relieve pain and anesthesia to remove nerve sensation. An example is an injection into the facet joint, which is used to treat chronic lower back pain. Between 25 and 45 % of chronic lower back is attributed to the facet joint [3]. Injec- tions into this region are particularly challenging due to the deep location, proximity to nerves and the narrow joint space. These challenges make it difficult to provide accurate injec- tion into the joint when the procedure is performed without guidance. The current standard to guide the injection is fluo- roscopy or computed tomography (CT). However, there are several drawbacks to these image modalities, including the risks posed by ionizing radiation to the patient and the physi- cian. Another common percutaneous spinal intervention is epidural injection. It is used before surgery as an alternative 123