Alliance is currently the largest provider of MRI and other diagnostics to the NHS.
Hanover has been briefed to handle the company's press office, media relations, NHS marketing and public affairs activity.
It will target national and clinical press and policy makers as part of the brief. The account is worth in excess of £100,000.
Hanover head of healthcare Andrew Harrison and account director Alexa Knight will report into Alliance Medical head of business development Charlie Fox.
Alliance Medical was founded in Warwickshire in 1989 and has a turnover in excess of £100 million per annum. It supports more than 400 hospitals and clinics in the UK.
Hanover recently expanded its team with the appointment of National Institute for Health and Clinical Excellence comms director Louise Fish .
Fish joined the agency last month as an associate director after seven years at the Government's health rationing body.