A 35-year-old woman is admitted with acute compartment syndrome of the forearm following a supracondylar fracture of the humerus in her 8-year-old child (she sustained the injury while protecting the child). Clinical examination shows severe pain, swelling, and loss of finger sensation. Compartment pressure measurement is being arranged. Which of the following investigations would be most appropriate to assess the extent of muscle necrosis and guide the need for amputation if fasciotomy is delayed?