What percentage of patients with TIA symptoms and a negative MRI within 12 hours of symptom onset go on to have a stroke in 90 days?

As per one small study of 120 patients, 4.2%.

14.7% if diffusion-weighted MRI is positive.


Source

Coutts SB, Simon JE, Eliasziw M, et al. Triaging transient ischemic attack and minor stroke patients using acute magnetic resonance imaging. Ann Neurol. 2005; 57:848-854.