MRI

Investigations

MRI, sagittal t1 weighted mri of the knee
Sagittal T1 weighted MRI of the knee. Image by Ptrump16, Wikimedia Commons, CC BY-SA 4.0.
  • MRI is the investigation of choice for local staging
  • Radiographs are the best first test to characterise a bone lesion
  • CT is superior for matrix, cortex and fine periosteal reaction
  • Perform MRI before biopsy, as haemorrhage and oedema blur margins
  • T1 best shows marrow replacement and true extent for resection length
  • Fat suppressed T2 or STIR overestimate extent by including oedema
  • Contrast T1 fat suppressed separates viable tumour from necrosis and targets biopsy
  • Gradient echo shows haemosiderin blooming, as in tenosynovial giant cell tumour
  • Diffusion and dynamic contrast imaging assess chemotherapy response and recurrence
  • Image the whole bone with a long coronal T1 to detect skip metastases
  • Skip lesions occur in osteosarcoma and change the resection extent
  • Image the whole compartment for soft tissue tumours
  • Calcification and matrix are poorly shown
  • Metal artefact complicates surveillance, helped by metal reduction sequences

Anatomy

  • Longitudinal marrow extent and distance from the joint surface
  • Physeal and epiphyseal involvement decides joint sparing resection in children
  • Cortical breach and soft tissue mass size
  • Intra-articular extension through cartilage, capsule or cruciate attachments
  • Preserved fat plane suggests a free vessel
  • Encasement beyond about 180 degrees suggests vessel involvement
  • Compartmental status for Enneking staging

Differential Diagnosis

  • Most tumours are low T1 and high T2
  • High T1 in lipoma, well differentiated liposarcoma, haemangioma, subacute haemorrhage and melanin
  • Low T2 in desmoid, haemosiderin and dense mineralisation
  • Fluid-fluid levels suggest ABC but also occur in telangiectatic osteosarcoma
  • Cartilage tumours are lobulated, bright on T2, with ring and arc enhancement
  • Peritumoural oedema can mimic tumour extension
  • Signal characteristics are rarely diagnostic alone

Management

  • MRI assesses neoadjuvant response
  • MRI surveys the tumour bed for local recurrence after resection

Reviewed by Professor Phong Tran, Head of Orthopaedic Surgery, Western Health. Last updated 10 October 2026.