Cathy Liu wore her trademark smile last month as she watched her former colleagues celebrate."I was proud of them," she says of the young doctors in training who graduated to the next level of medical residency at the University of California, Davis."I wanted to be there. I felt happy."But deep inside, Liu admits, she also felt a twinge of sadness.A year ago, she was a newly minted physician and had just started her clinical training at UC Davis.Then the doctor became a patient.Two days after she watched Independence Day fireworks last year, Liu was hit by a car during a morning jog. She ended up in the intensive care unit of her own hospital with a severe brain injury.Her medical career seemed shattered. Her very life was in jeopardy.But since then, as she has seen other UC Davis residents move forward in their careers, Liu has achieved some milestones of her own.She has tossed away most of her prescription medications. She has discarded the thick plastic brace that once supported her weak right leg, and she walks nearly a mile to physical therapy. She can cut her food with her once-useless right arm. Once unable to speak more than a word or two at a time, she expresses herself in full sentences. Unable to comprehend complex ideas in the first months after her accident, she can read magazine articles and novels. She lives mostly independently, and has flown by herself to visit friends across the country.But Dr. Liu is far from finished.At age 26, her goal remains the same as it has been since she was 8 years old. "I want to practice medicine," she says.First, though, she must master the art of daily living.After the accident, her injured left brain refused to communicate the correct signals to the right side of her body, making walking, talking and speaking difficult. But day by day, all of those things are getting easier."I'm still working on it," says Liu.In recent months Liu has traveled across the country twice, once with her boyfriend to visit her sister in New York; and once by herself, to a friend's wedding in Atlanta."At first I was not too sure about it," Pam Liu, Cathy's mother, says of the solo trip. "But she was pretty adamant about it. And I knew that if she got into any trouble or got confused, she would ask someone for help."As it turned out, even that was unnecessary.Traumatic brain injuries are complex and unpredictable, her caretakers say. When Cathy arrived in the UC Davis emergency room a year ago as an unidentified "Jane Doe," she was comatose and bleeding inside her skull. She endured three major brain surgeries. Her doctors could not promise she would survive, much less regain her physical and intellectual abilities."She has definitely far exceeded expectations," says Kia Shahlaie, a young neurosurgeon who operated on Liu and became emotionally attached to her after learning that she was one of the hospital's own.Tests that will help doctors understand the effects of various procedures on Liu's "amazing" recovery are not yet complete, he says. But her case already has had a significant impact.Based on their success with Liu, neurosurgeons at UC Davis have changed the way they treat traumatic brain injuries. Shahlaie has presented Liu's case at national and international conferences, and uses it to train junior neurosurgery residents. It is likely to be published in at least one major medical journal."True to her profession and oath, Cathy has, albeit indirectly, helped save lives at UC Davis Medical Center and abroad," says Shahlaie.If she is to regain the ability to study and practice medicine again, however, Liu will have to overcome a condition that affects her ability to communicate. Called aphasia, it typically occurs when the brain's left temporal lobe is deprived of oxygen as a result of a stroke, tumor or trauma.Because of her aphasia, Liu sometimes knows what she wants to say but has trouble voicing her thoughts."Her speech production is more impaired than her understanding," says Christine Davis, chief of speech language pathology at UC Davis.Yet Cathy has made incredible strides and continues to improve, Davis says."She is very bright and very motivated, and both of those things bode well for her," she says. "We are seeing successes in her that you never expect to see with this kind of brain injury."Further progress is possible, even years into the future, Davis says."I have not seen a plateau with Cathy. I assume there will be one at some point, but I just don't know. She's still improving. She has all the right components to be successful."E-mail Cynthia Hubert at chubert(at)sacbee.com (Distributed by Scripps Howard News Service, www.scrippsnews.com.)