WB DW-MRI and ¹⁸ F-FDG PET/TC are valuable tools for the diagnosis and follow-up of MM. ¹⁸ F-FDG PET/CT is considered the gold standard for assessing extramedullary disease, while WB-DWI offers greater sensitivity in detecting diffuse disease, especially in cases of non-nonsecretory myeloma (figure 1).

Fig 1: Comparison between both techniques in the assessment of MM

A standardized protocol has been proposed for WB DW-MRI (figure 2).

Fig 2: WB DW-MRI protocol

On MRI, signs of the disease include (figures 3 y 4 ): 

• Focal or diffuse signal intensity (SI) abnormalities with a diameter > 5 mm.
• Paramedullary or extramedullary soft-tissue lesions.
• Pathological vertebral fractures.
• Restricted Diffusion with Apparent Diffusion Coefficient (ADC) values between 600–1000 µm2 /sec.

Fig 3: Signs of disease in MM depicted on imaging

Fig 4: WB DW-MRI: Example of an active lesion

On PET/TC with¹⁸ F-FDG, signs of the disease include (figure 5 ):

• Hypodense, lytic, or punch-out osseous lesions with a diameter  > 5 mm.
• Diffuse disease: homogeneous standardized uptake values (SUVmax) of the bone marrow greater than liver, or heterogeneous bone marrow uptake. 
• Focal lesions: osseous hyperactive metabolic foci greater than physiological bone marrow uptake.
• Paramedullary or Extramedullary soft-tissue lesions. 

Fig 5: 18F-FDG PET/CT: Example of an active lesion

Multiple myeloma (MM) disease patterns include (figures 6, 7, 8 y 9 ):

• Diffuse pattern: generalized involvement of the bone marrow, which can be homogeneous or heterogeneous.
• Focal on diffuse / combined: independent, well-defined lesions or generalized involvement with some well-defined lesions.
• Micronodular / Variegated / Salt and Pepper" pattern: extensive involvement with nodular lesions <5 mm.
• Paramedullary pattern: bone marrow lesion.
• Extramedullary pattern: soft tissue injury that is not in continuity with spinal disease. Bone lesion.

Fig 6: WB DW-MRI: Patterns of bone marrow infiltration

Fig 7: WB DW-MRI: Patterns of bone marrow infiltration

Fig 8: WB DW-MRI: Patterns of bone marrow infiltration

Fig 9: WB DW-MRI: Example of diffuse and focal disease