Friday, August 3, 2007, 1:56 AM

Lane Garrison, who pleaded guilty to vehicular manslaughter and drunken driving in a crash that killed a teenage passenger, was ordered Thursday to undergo a 90-day evaluation to help a judge decide his sentence.

The former "Prison Break" actor will undergo a "diagnostic" by parole officers and psychologists in a prison before returning for an Oct. 31 appearance before Superior Court Judge Elden S. Fox.

Garrison, 27, was driving a 2001 Land Rover on Dec. 2 when he lost control and rammed a tree. The crash killed Vahagn Setian, a Beverly Hills High School student who would have been 18 on Wednesday. Two other passengers, both 15-year-old girls, survived.

Garrison had a blood-alcohol content of 0.20 percent, more than twice the legal limit for driving, and was under the influence of cocaine, according to police.

About a dozen of the victim's friends and relatives were in court Thursday, all wearing T-shirts with Setian's picture on the front and the James Dean saying "Dream as if you'll live forever, live as if you'll die today" on the back.

Garrison pleaded guilty in May to one count of vehicular manslaughter without gross negligence, one count of driving under the influence with a blood-alcohol level of 0.15 percent or higher, and a misdemeanor of providing alcohol to a minor.

He apologized to Setian's family during the court proceeding.

"I relive that night every day and I think about the bad decision I made that day. All I can say to you is, `I'm so sorry,'" he said.

Garrison's character on Fox's "Prison Break" was killed in the show's Oct. 2 episode. The Dallas native's other credits include the film "Crazy."

He was recently featured in a public service announcement that encourages viewers not to drink and drive. The PSA re-enacts his crash and he appears to be holding back tears as he talks about what a stupid and costly mistake he made.

With hope, -.Later.-