A 32-year-old man presented with a 8-month history of vision blurring when bending forward. He also reported experiencing worsening headaches in the morning for the past 6 months. Apart from that, he had no other health issues. Upon examination, he had a body mass index of 30 and no other abnormalities were found. However, bilateral papilloedema was observed during neurological examination. An MR venogram was conducted, but the results were normal. What would be the most suitable next step in investigation?