History: 23-year-old with 4 months history of progressively worsening weakness and difficulty extending her thumb and fingers. (Figs 1, 2, 3, 4, 5, 6, and 7)

Fig. 1
figure 1

T1 Axial forearm

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Fig. 2
figure 2

T2 SPAIR axial forearm

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Fig. 3
figure 3

T1 W axial slice just proximal to the supinator muscle origin

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Fig. 4
figure 4

T2 W SPAIR axial slice just proximal to the supinator muscle origin

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Fig. 5
figure 5

US long axis at the antecubital fossa

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Fig. 6
figure 6

US Transverse view of lateral intermuscular septum within the antecubital fossa

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Fig. 7
figure 7

US Transverse view just proximal to the supinator origin

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