A 62-year-old male underwent posterior spinal instrumented fusion for degenerative lumbar spondylolithesis one year ago. He presents to office complaining of persistent lower back pain. The pain initially improved but over the last 6 months he has had recurring pain at the site of the surgery primarily with activity. He denies back pain at rest or night pain. Physical examination reveals a well healed wound and no physical abnormalities. He has no tenderness to palpation to the thoracic or lumbar spine. He has no neurological deficits. His laboratory results show an erythrocyte sedimentation rate (ESR) = 8 mm/h and C-reactive protein (CRP) = 3 mg/L at the last visit which are both within normal limits. Figure A shows a series of radiographs from his pre-operative, 3 month post-operative and 1 year post-operative clinic visits, respectively. Which of the following investigations would best confirm the suspected underlying diagnosis?