Introduction 1 3 2 1 4 15 16 18 19 20 18 18 6 21 23 24 25 7 8 Pathomechanisms 24 26 5 27 28 29 30 31 3 5 32 1 2 1 2 3 4 1 2 25 3 4 Fig. 1 dashed arrows solid arrows a b c d  Fig. 2 1 long white arrow short white arrow a b c d dashed arrow  Fig. 3 orange white a solid black arrow dashed arrow b solid black arrow dashed arrow  Fig. 4 3 solid arrow dashed arrow a b  Imaging manifestations 33 5 8 23 8 34 35 25 Fig. 5 a b c d c  Osseous changes (vertebral body) 2 36 37 6 36 36 38 39 6 Fig. 6 a b c  40 41 29 36 5 42 43 44 The trabecular hypertrophy and cortical thickening caused by the osseous involvement by PD results in a variable degree of low signal on both T1- and T2-weighted images. In the severe cases of vertebral sclerosis (ivory vertebra appearance) the whole vertebra demonstrates a diffuse low signal on both T1- and T2-weighted sequences. More commonly however, the signal characteristics are heterogeneous on both T1- and T2-weighted sequences due to changes in the intervening marrow space. PD of the vertebra is easily missed or misinterpreted on MR images in the early stages. This is especially true when there is an absence of classic changes including vertebral enlargement and cortical thickening. This is mainly because PD is a disorder of bone and the vertebral marrow is only secondarily affected. MRI can show discrete areas of marrow signal alteration involving vertebral bodies with low non-specific signal on T1-weighted images and high signal on T2-weighted sequences. The marrow changes due to vertebral involvement by PD are described in detail later. Osseous changes (posterior elements) Neural arch involvement can be difficult to evaluate on radiographs alone. The best modality for neural arch assessment is CT with reconstructions. 5 8 4 45 7 8 Fig. 7 a arrow b  Fig. 8 solid white arrow a b c d e solid white arrow c e  solid arrows dashed arrow  46 47 27 21 46 48 49 Imaging techniques such as PET and PET-CT, can contribute to the differential diagnosis of PD from other disorders in the spine, especially when non-specific PD changes are incidentally discovered, and to assess disease activity after treatment. 50 51 Extra-osseous changes 26 45 9 26 Fig. 9 a solid white arrow b dashed white arrows  52 Pathological complications 52 Bone marrow 53 8 10 11 54 Fig. 10 a b c black arrow d black arrow  Fig. 11 a b c d e f dashed arrows white arrows  Whenever vertebral bone marrow signal changes are seen on MRI, PD has to be considered in the differential diagnosis because of its non-specific and varied appearance in the bone marrow, and subtle bony alterations suggestive of PD have to be sought. Paget’s disease can, however, also coexist with other disorders. 12 10 11 12 Fig. 12 a b c  Pathological complications 21 Back pain 1 21 1 24 5 55 5 8 24 26 56 24 Table 1 Causes of back pain in Paget’s disease Periosteal stretching Vascular engorgement Microfractures Facet arthritis Intervertebral disc disease Overt fractures of vertebrae, sacrum Spondylolysis/-listhesis Sarcoma—very rare Spinal stenosis 2 5 24 25 8 13 24 5 24 57 5 25 13 58 5 24 8 Table 2 Causes of neural dysfunction in Paget’s disease Posterior expansion of the vertebral body Posterior neural arch expansion Facet joint arthritis/overgrowth Ligament ossification Spondylolisthesis Fracture retropulsion Extra-osseous involvement—PD, haematopoiesis, “pseudosarcoma”, malignancy “Arterial steal” syndrome Fig. 13 Sagittal T2-weighted MR image demonstrates cauda equina compression at the L1 level due to pagetic enlargement of the whole vertebra. Note the stenosis caused by expansion of both the vertebral body and posterior elements. Degenerative spondylolisthesis and stenosis at L4/L5 is noted  14 8 13 8 24 59 Fig. 14 a b  Neural dysfunction 24 60 61 5 57 62 2 20 63 5 15 64 64 66 Fig. 15 8 arrows  Compression fracture 16 59 59 67 Fig. 16 Lateral and anteroposterior radiograph of the lumbar spine demonstrates severe compression of a pagetic L3 vertebra with retropulsion into the spinal canal. Note the increased interpediculate distance as a hallmark of PD  16 8 68 69 Facet joint arthropathy 26 9 25 5 24 25 9 9 5 26 70 25 Spondylolysis and spondylolisthesis 17 Fig. 17 a arrow b  17 Intervertebral disc involvement 26 45 68 26 26 8 18 26 71 19 Fig. 18 Disc involvement. Serial radiographs 2 years apart in the same patient demonstrate progressive involvement of the L4/L5 intervertebral disc in Paget’s disease of the L5  Fig. 19 a b white arrows c d black arrows e dashed arrows  Neoplastic transformation 5 8 24 72 73 5 74 75 3 8 54 19 11 76 77 5 20 Table 3 Causes of paravertebral swelling in Paget’s disease Extra-osseous extension Fracture haematoma Extra-medullary haematopoiesis Paget’s sarcoma/other tumours Paget’s pseudosarcoma Fig. 20 a b white arrows black arrow  Conclusion Recognition of the imaging manifestations of spinal PD and the potential clinical complications enables accurate assessment of patients prior to appropriate management. This knowledge should allow subtle PD to be identified on imaging when this is not suspected. This is especially relevant to MRI, as it has become the imaging modality of choice for investigating the spine. Patients presenting with back pain and spinal stenosis have to be assessed for pagetic complications before attributing the symptomatology to the disease itself.