Rochester, Minn.-based Mayo Clinic plans to announce an affiliation Wednesday with NorthShore University HealthSystem, which operates four hospitals in the northern suburbs. The multiyear agreement, scheduled to begin Oct. 1, will give NorthShore physicians fast access to Mayo specialists and research targeting three areas of care: cardiovascular, cancer and neurological cases. NorthShore, a nonprofit that owns hospitals in Evanston, Glenview, Skokie and Highland Park, said the partnership will allow it to better treat existing patients, expand its patient base and enhance its expertise in complex cases.