CHICAGO (AP) — A suburban Chicago chiropractor has been sentenced to 20 months in prison for billing an insurance company more than $4 million for treatment that was unnecessary or never provided.

Federal prosecutors said Thursday 46-year-old Steven Paul of Northbrook pleaded guilty to one count of health care fraud. U.S. District Court Judge Rondald Guzman said during a sentencing hearing in Chicago he was imposing the shortest prison term possible because of Paul's "extraordinary cooperation" in the investigation.

Paul and co-defendant Bradley Mattson of Lake Forest jointly owned six chiropractic clinics north of Chicago. Both admitted in plea agreements they required patients to receive x-rays and a pre-set schedule of visits even if they weren't medically necessary.

Mattson was sentenced in 2012 to six and a half years in prison.