A California Highway Patrol cruiser spun out of control and struck a pickup truck, critically injuring the driver, on Highway 12 just east of the Highway 101 interchange in Santa Rosa Saturday night, the CHP said.

Officer Blair Hardcastle took evasive action when a motorcycle merged onto the highway from the Maple Street onramp about 6:30 p.m., driving partly onto the dirt center strip, Sgt. Kevin Mensior said.

The motorcycle crossed over to the fast lane in front of Hardcastle, who was responding to a call with lights and siren activated, Mensior said.

The cruiser spun across both lanes, struck the guardrail and hit the pickup, a 1991 Chevrolet Silverado driven by Cynthia Mae Dempsey, 44, of Santa Rosa. The pickup rolled over onto its roof, the CHP said.

Dempsey, who sustained a forehead cut and an injury to the right side of her head, was transported to Santa Rosa Memorial Hospital, the CHP said.

Dempsey was in surgery late Saturday and was listed in critical condition, a hospital spokeswoman said.

Officers said they had not determined if Dempsey was wearing a seatbelt.

The motorcyclist, Justin Lee Oliver, 36, of Santa Rosa was not injured and his bike, a 2000 Kawasaki ZR-Z, was not struck.

Mensior said no citations or arrests had been made, and that the accident was still under investigation. A special accident investigation team was en route to the scene at 9 p.m.

Alcohol was not involved, Mensior said.

Westbound traffic on Highway 12 was detoured off the highway onto side streets from Farmers Lane to Santa Rosa Avenue. Officials said the detour might last until midnight.

Hardcastle sustained scrapes to the forearm, likely from his airbag deploying, the CHP said. His cruiser was totaled.