A previously healthy 11-year-old boy presented to the emergency department with a history of left forearm pain. He described a 3-month history of intermittent mild left forearm pain, which worsened after activity. On physical exam, he presented swelling of the distal third of the left forearm, without signs of inflammation or infection. Figs. 1, 2 and 3.

Fig. 1
figure 1

CR

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Fig. 2
figure 2

MRI. a Sagittal T2-weighted FSE, b axial T1-weighted FSE, c axial T2-weighted fat-sat FSE, and d post-contrast subtraction

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Fig. 3
figure 3

Dynamic MR angiography

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