A 9-year-old boy is being treated for acute hematogenous osteomyelitis of the distal tibia with appropriate IV antibiotic therapy. After three days of treatment, he fails to show any clinical improvement. Advanced imaging is obtained and reveals a 1.5x1.5cm abscess in the distal tibia. The patient subsequently undergoes formal open surgical debridement, without complications. Following surgery, serial evaluations of which of the following studies is the most expeditious method to determine the early success of treatment?