A 28-year-old male presents to the emergency department 6 hours after a crush injury to the right leg sustained in a road traffic accident. He complains of severe pain in the anterior compartment of the leg that is disproportionate to the clinical findings and is not relieved by analgesics. On examination, pain is elicited on passive dorsiflexion of the foot. Compartment syndrome is suspected. Which investigation is the gold standard for confirming the diagnosis?