I have a magnetic implant (linx) that doesn't allow mris above 0.7t - if i needed an mri in the future for something such as multiple sclerosis, are 0.7t/0.3t open mris reliable enough for ms? what about spinal tap? is my only option implant removal?

The higher Tesla MRIs provide higher resolution and are capable of finding more lesions. The lower-rated and especially open MRIs may miss smaller lesions. Spinal fluid can evaluate oligoclonal bands and neurofilament light levels and could add to diagnostic confirmation. But all of this is a moot point if you lack symptoms consistent with MS or other neurological disorders.