On whole-body MRI for staging of multiple myeloma, diffuse homogeneous low T1 signal in the marrow (below paraspinal muscle signal) replacing normal fat signal is seen. What does this indicate compared to a normal pattern?

• A Normal age-related red marrow reconversion
• B Iron overload causing diffuse T1 shortening
• C Post-treatment marrow fibrosis
• D Diffuse infiltrative pattern of myeloma — high disease burden ✓

Explanation

On MRI, normal bone marrow contains abundant fat with high T1 signal. In multiple myeloma, diffuse marrow infiltration by plasma cells replaces fat, producing uniform low T1 signal below the signal of paraspinal muscles. This diffuse infiltrative pattern (as opposed to focal lytic lesions) indicates high tumour burden and is associated with adverse prognosis. Whole-body MRI is now preferred over skeletal survey as it detects ≥30% plasma cell infiltration at the lesion site. Iron overload causes low T1 and low T2 signal from paramagnetic effect. Red marrow reconversion is reversible and shows patchy, symmetric distribution.

Reference: Grainger & Allison's Diagnostic Radiology, 7th ed.

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