A 15-year-old male presents with swelling around the distal femur. X-ray shows a lytic lesion in the metaphysis with Codman's triangle and a 'sunburst' periosteal reaction. MRI shows the lesion breaching the cortex with a soft tissue mass showing heterogeneous T2 signal. The most characteristic MRI feature that indicates tumour matrix mineralisation in osteosarcoma is:
- A Areas of low T1 and low T2 signal within the mass (tumour osteoid) ✓
- B Diffuse T2 hyperintensity with perilesional oedema
- C Fluid-fluid levels within the lesion
- D Skip metastases in the same bone appearing as separate T2 bright foci
Correct answer: A. Areas of low T1 and low T2 signal within the mass (tumour osteoid)
Explanation
Tumour osteoid (mineralised matrix) in osteosarcoma appears as areas of low signal on both T1 and T2 MRI sequences due to its calcium content and lack of mobile protons — this distinguishes it from soft tissue or haemorrhage. Diffuse T2 hyperintensity represents oedema and non-mineralised tumour. Fluid-fluid levels suggest aneurysmal bone cyst component. Skip metastases are a separate feature and do not indicate matrix mineralisation.
Reference: Grainger & Allison's Diagnostic Radiology, 7th ed.
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Written and medically reviewed by the StethoPrep medical team.