Table 1

MRI lesion definitions

Normal bone marrow signal	The bone marrow signal in the centre of the vertebral body, if normal, constitutes the reference for designation of normal signal. If the usual reference point is not normal, then the closest vertebra with normal marrow signal in the centre of the vertebral body is used as reference.
Central vs lateral slices	Definitions of central and lateralslices: Central sagittal slices (referred to as central slices) are slices that include the spinal canal. The pedicle may be partially seen but is not continuous between the vertebral body and the posterior elements. Lateral sagittal slices (referred to as lateral slices) are slices that do not include the spinal canal and where the pedicle is continuous between the vertebral body and the posterior elements, or the slice is lateral to the pedicle. Vertebral body inflammatory lesions and fat lesions are scored on central and lateral slices. Vertebral body bone erosion and new bone formation are scored on central slices only. Reference point for slice position: The slice position is defined by the discovertebral unit (DVU) and not by the vertebral body. Therefore, the slice position will always be the same for both endplates of a DVU. The reference structure is the pedicle related to the lower endplate of the DVU. For example, the slice position of both endplates at L1/L2 is defined by the presence/absence of the L2 pedicle on that image. Reference sequence: Whether T1-weighted (T1W) or short tau inversion recovery (STIR) is used to designate slice position will depend on the lesion domain that is being evaluated, that is, for fat lesions, bone erosion and new bone formation this would be the T1W sequence, and for inflammatory lesions this would be the STIR sequence.
Slices in the cervical spine	In the cervical spine, facet joints are located laterally to the vertebral body and are not seen on the same sagittal image. Therefore, in the cervical spine, all images that include the vertebral body are defined as central slices. For C7/T1, C7 is scored as a cervical vertebra while T1 is scored as a thoracic vertebra.
Inflammatory lesion	Inflammatory lesion in vertebral bodies: Increased signal in bone marrow on STIR/T2FS in a vertebral body. Corner inflammatory lesion(CIL), subdivided into anterior (aCIL) and posterior (pCIL): Inflammatory lesion at the vertebral corner on at least one central slice. Type A CIL: The inflammatory lesion itself reaches the corner. Type B CIL: The inflammatory lesion does not extend to the corner but does extend to both the vertebral endplate and the anterior/posterior vertebral cortex (usually because of the presence of fat or erosion in the corner). If a type B CIL is present, a type A CIL cannot be scored. Non-corner inflammatory lesion(NIL): Inflammatory lesion adjacent to the endplate on a central slice but not involving the anterior or posterior vertebral corner. Lateral vertebral body inflammatory lesion (LIL), subdivided into anterolateral (aLIL) and posterolateral (pLIL): Inflammatory lesion adjacent to the endplate on a lateral slice without distinction between corner and non-corner lesions. Inflammatory lesions involving the posterior elements of the spine, not the vertebral bodies: Facet joint inflammatory lesion: Increased signal in bone marrow on STIR/T2FS in at least one facet of a facet joint. Transverse process inflammatory lesion: Increased signal in bone marrow on STIR/T2FS in the transverse process. Rib inflammatory lesion: Increased signal in bone marrow on STIR/T2FS in the rib. Spinous process inflammatory lesion: Increased signal in bone marrow on STIR/T2FS in the spinous process. Soft tissue inflammatory lesion: Increased signal in soft tissue at entheseal attachments on STIR/T2FS.
Fat lesion	Focally increased signal in bone marrow on T1W. Corner fat lesion (CFAT), subdivided into anterior (aCFAT) and posterior corner fat lesion (pCFAT): Fat lesion at the vertebral corner on at least one central slice. Non-corner fat lesion (NFAT): Fat lesion adjacent to the endplate on any central slice but not involving the anterior or posterior vertebral corner. Lateral vertebral body fat lesion(LFAT), subdivided into anterolateral (aLFAT) andposterolateral vertebral body fat lesion(pLFAT): Fat lesion adjacent to the endplate on a lateral slice without distinction between corner and non-corner lesions. Facet joint fat lesion: Increased signal in bone marrow on T1W in at least one facet of a facet joint. Fat lesions in the upper cervical spine: To be considered a true fat lesion in the upper part of the cervical spine, a potential lesion has to be very homogeneous and to have a clearly increased signal on T1W that is clearly demarcated from adjacent bone marrow. This applies especially for C2 and C3 lower anterior corners, where a diffuse signal increase on T1W is common in healthy subjects. It may be necessary to use different windowing of the image for optimal visualisation of the upper vs the lower cervical spine.
Bone erosion	Full-thickness loss of dark appearance of cortical bone at its anticipated location and loss of normal bright appearance of adjacent bone marrow on T1W. Vertebral bone erosions are scored on central slices only. Corner bone erosion, subdivided into anteriorand posterior corner erosion: Bone erosion involving the vertebral corner on a central slice. Facet joint bone erosion: Bone erosion involving at least one facet of a facet joint.
New bone formation	Because of the current limitations of MRI, the following definitions are restricted to only those bone spurs within which bone marrow signal is detectable. In the future, MRI may be able to detect mineralisation within the bone spur at an earlier stage. Bone spur: Bright signal on T1W extending from the vertebral endplate towards the adjacent vertebra. Corner bone spur (syndesmophyte) (COS), subdivided into anterior (aCOS) andposterior corner bone spur(pCOS): Bone spur at the vertebral corner on a central slice. Non-corner bone spur: Bone spur involving the endplate, which involves neither the anterior nor the posterior vertebral corner on a central slice. Ankylosis: Bright signal on T1W extending from a vertebra and being continuous with the adjacent vertebra. Corner ankylosis, subdivided into anteriorand posterior corner ankylosis: Ankylosis at the anterior or posterior corner on a central slice. Non-corner ankylosis: Ankylosis which involves the endplate, but neither the anterior nor the posterior vertebral corner on a central slice. Facet joint ankylosis: Ankylosis at the facet joint.
Assessment of size of inflammatory lesion/fat lesion/bone erosion	Size is only assessed for lesions on central slices in the thoracic and lumbar spine; the slice where the lesion is largest is used. Size of corner lesions is assessed vertically perpendicular to the vertebral endplate and horizontally parallel to the vertebral endplate. Corner lesions are large if they involve 25% or more of the anterior-posterior diameter of the vertebral endplate and/or the height of the vertebral body. Non-corner lesions are large if they involve more than 25% of the vertebral body height. If either new bone formation or bone erosion is distorting the configuration of the corner, size is measured from the point at which the original cortex and endplate would have met. Inflammatory lesions, fat lesions and bone erosions are assessed for size.
Combined corner and non-corner inflammatory lesion/fat lesion	If a corner inflammatory lesion/fat lesion on any central slice involves more than 50% of the anteroposterior diameter of the vertebra (ie, it extends beyond the midpoint), it is scored as both a corner inflammatory lesion/fat lesion and a non-corner inflammatory lesion/fat lesion. Size of the corner component can only be assessed by vertical extent at the anterior/posterior vertebral cortex (horizontal size cannot be assessed). Size of the non-corner component is assessed by height at the midpoint of the anteroposterior diameter of the vertebra.
Combined central slice and lateral slice inflammatory lesion/fat lesion	An inflammatory lesion/fat lesion may be observed extending across both central and lateral slices. It is then scored as being both a central (aCIL/pCIL/NIL) and lateral (aLIL/pLIL) inflammatory lesion, or a central (aCFAT/pCFAT/NFAT) and lateral (aLFAT/pLFAT) fat lesion.

T2FS, T2-weigted image with fat saturation.