My 4 Year Old Son Has Ankle Contraction/weakness And Is

My 4 year old son has ankle contraction/weakness and is a toe walker. He has lost the ability to put his feet flat. We are also being followed by an ortho. He had an odd reaction to his first serial casting and lost the ability the walk 4 days after the casts were removed. He was able to walk after 3 days of resting. We were referred to a neurologist who has done a series of tests. He is negative for muscular dystrophy. They did an MRI on his brain and lower spine. The brain was normal but I recieved the Report on the spine and it has the following assessment.

The lumbar vertebral body heights are adequately maintained as are the disc spaces. There is no disc protrusion. The conus medullaris is normal in morphology and terminates at L2. There is no evidence of a tethered cord. There is a prominent central canal within the visualized portions of the lumbar spine;at its widest on the study today the central canal measures 2mm at t10 and the central canal is slightly dilated in all the thoracic and lumbar cord that was imaged. There is no visualized intraspinous or paraspinous mass. There is no occult spinal dsyraphism. Visualized portions of the retro peritoneum are within normal limits.
Impression: prominent central canal dilation in the visualized thoracic and lumbar cord. Because of this, a total spine is recommended with contrast for further evaluation. No other abnormalities are identified.

What are they looking for with the full mri? I am just trying to get an understanding of he possible implications of this reading. Any help would be great.

Thank you!