The border of a lesion is a good indicator of its biological activity. Benign lesions like haemangioma, enostosis, osteoid osteoma, osteoblastoma and ABC have a clear demarcation, known as geographic appearance (Fig. 1.8). In contrast, aggressive benign lesions like aggressive haemangioma and malignant lesions such as osteosarcoma, Ewing sarcoma and metastases have a large transition zone. Cortical destruction is also a sign of aggressiveness.

The structure or matrix of a tumour can help the radiologist in diagnosing tumours arising from the bone or cartilage.

Osteoblastic tumours contain amorphous ossifications on plain radiography or CT. The matrix lacks an organized trabecular pattern and is usually less dense than normal bone. Dense osteoblastic lesions display a low T1–T2 signal intensity pattern on MR imaging. Enostosis is characterized by solid, dense, cortical bone within the spongy bone of the vertebral body. A typical feature of osteoid osteoma is calcification within a lucent nidus with surrounding reactive sclerosis (Fig. 1.6). Osteoblastoma is histologically similar to osteoid osteoma, with areas of calcification in the matrix, but is larger and more expansive (Fig. 1.5).

Cartilage-forming tumours typically show punctate, arc or ring calcifications at radiography and CT. These calcifications appear as low-signal-intensity foci at MR imaging. Chondroid tissues with ring- and arc-type calcifications are typical for osteochondroma and chondrosarcoma. This pattern may, however, also occur in osteoblastoma and chondroid types of chordoma (Fig. 1.5).

ABC, osteoblastoma and to a lesser extent aggressive haemangioma can have an expansive nature (Figs. 1.5 and 1.8). Malignant lesions such as Ewing sarcoma and chondrosarcoma demonstrate expansion in combination with soft tissue extension, a feature that may, sometimes, be found in benign lesions like osteoblastoma and GCT as well.

A soft tissue component can be seen in malignant lesions like Ewing sarcoma but also in benign lesions like osteoblastoma. Aggressive haemangiomas can also be accompanied by a large soft tissue component. These lesions are frequently symptomatic.

Distinction between lesions can be made based on their osteoblastic or osteolytic nature (Table 1.1). The amount and degree of matrix mineralization in osteoblastic lesions is widely variable; thus the appearance on plain radiography or CT may range from densely blastic to nearly completely lytic. The differential diagnosis of osteoblastic tumours includes osteoblastic metastasis, bone island, lymphoma and osteosarcoma. Osteoid osteoma and osteoblastoma are essentially no bone-forming tumours but display a high density due to adjacent reactive bone sclerosis.

As in osteoblastic lesions, a gradient in osteolytic lesions exists. A very lytic appearance is usually associated with a more aggressive behaviour. Less lytic lesions tend to be less aggressive. The differential diagnosis of a lytic vertebral lesion, ranked from moderately lytic to extremely lytic, includes ABC, chordoma, GCT, lymphoma, metastasis, plasmacytoma/multiple myeloma and sarcoma.

A SPECT study is very sensitive to osteoblastic activity. Osteoblastic and less aggressive osteolytic lesions with partial osteoblastic activity will be detected using this technique. Purely osteolytic tumours might escape detection (Table 1.2).

In some cases the density of a lesion on CT facilitates further characterization. A low density (−50 to −100 HU) can be found in fatty lesions like lipomas and haemangiomas. By measuring the density, fat-containing lesions can be differentiated from air (−800 to −1000 HU), occasionally seen in degenerative disc disease. A very dense tumour in the spongy vertebral bone with a density similar to cortical bone will most likely be a bone island, also known as enostosis (Fig. 1.9).

The behaviour of a lesion on T1-weighted and T2-weighted MRI sequences is another important parameter. The majority of pathological lesions have low signal intensity on T1-WI and high signal intensity on T2-WI (Fig. 1.10). However, a few exceptions exist (Table 1.3). Haemangioma and eosinophilic granuloma contain fat and will be bright on T1-WI (Figs. 1.11 and 1.12). Another exception is the dark appearance of a GCT on T2-WI due to the high cellularity and collagen and haemosiderin content [8]. Bone islands are dark on both T1-WI and T2-WI since they are histologically identical to cortical bone.

The enhancement pattern after the administration of a gadolinium chelate can be very suggestive in some cases, like a ring and arc pattern in chondroid tumours (Fig. 1.13).

Some morphological patterns can offer, if present, a very specific diagnosis or differential diagnosis. It is, however, important to interpret these findings with caution and correlate them with all other features. Vertebra plana is characteristic for EG but can also be seen in rare cases of other tumours like GCT. A polka-dot pattern on axial CT images is very typical for a haemangioma (Fig. 1.14b). Osteoid osteoma displays a specific morphology that can be detected on radiography, CT and MRI: prominent reactive sclerosis surrounding a lucent central nidus (with variable central calcification) (Fig. 1.6). Paget’s disease and lymphoma may result in an expanded vertebra with marked sclerosis, known as an ivory vertebra. Although typically linked to ABC fluid–fluid levels, a result of sedimentation of blood degradation products can sometimes be found in osteoblastoma, chondroblastoma, telangiectatic osteosarcoma and rarely in GCT and fibrous dysplasia (Fig. 1.15) [9]. On axial MR images, a plasmacytoma may resemble the morphology of the brain, called mini-brain appearance [10]. Chordomas may be shaped as a dumbbell or mushroom, preserving the disc space (Fig. 1.13). The spider pattern can be observed in plasmacytoma but also in haemangioma (Fig. 1.16).