Detection of Numeric and Morphological Variation at Lumbosacral Junction: Role of Whole Spine Magnetic Resonance Imaging

Introduction: Numeric variation in presacral vertebral segments and lumbosacral transitions has been reported in different studies with wide range of prevalence (4–36%). There is no standard method for numbering of lumbar vertebrae. In typical regional sequences for lumbar spine magnetic resonance imaging (MRI), vertebral and disc morphology, intervertebral angle, and various local anatomical structures are used as landmarks for numbering. Aim: (a) To document the prevalence in variation of presacral mobile vertebral count and lumbosacral vertebral transition using whole spine MRI. (b) To evaluate the accuracy in the location of the proximal right renal artery (RRA), aortic bifurcation, and conus termination as a landmark for vertebral numbering. Materials and Methods: This prospective observational study includes 317 patients, referred for MRI of the lumbosacral spine. Vertebrae were counted manually using sagittal whole spine localizer images (Mobi View). Short tau inversion recovery (STIR) coronal images were included for classification of lumbosacral transitional vertebrae (LSTV). Prevalence and types of LSTV and level of proximal RRA, aortic bifurcation, and conus termination were documented. Results: About 25.5% of patients showed LSTV and 7.8% of patients showed variation in presacral vertebral count without LSTV. Castellvi Type IIIb LSTV was most prevalent followed by Type IIb. There was significant variation in the level of aortic bifurcation, RRA, and conus termination in patients with normal count and with LSTV. Conclusions: Spinal and paraspinal structures, such as aortic bifurcation, RRA, and conus termination, cannot be considered as landmark for numbering of vertebrae. Counting of vertebrae manually from C2 is recommended for confident documentation of numerical variation in vertebrae. The inclusion of STIR coronal image helps in better identification and characterization of the LSTV.