In suspected septic arthritis, when is MRI most appropriate?

• A. As the first-line imaging in all suspected cases.
• B. To guide aspiration when ultrasound is clearly diagnostic.
• C. Not needed if ultrasound shows a simple effusion.
• D. For detailed assessment if ultrasound findings are inconclusive.

Answer: (D)

In suspected septic arthritis, ultrasound is used first to detect fluid and guide joint aspiration. MRI is most appropriate when ultrasound findings are inconclusive or when there is concern for more extensive disease that ultrasound cannot fully characterize. MRI provides superior soft-tissue contrast and can reveal bone marrow edema, occult osteomyelitis, synovitis, abscesses, and the precise extent of joint and surrounding tissue involvement, which is crucial for planning treatment.

If ultrasound clearly shows a simple effusion with no signs suggesting deeper infection, MRI isn’t needed. MRI isn’t typically the first test in all suspected cases, and it isn’t usually used to guide aspiration—that role belongs to ultrasound.