BALTIMORE – Kimberly Spletter's knee would pop backward involuntarily, or her leg would shake uncontrollably.

"I had to live with it," said Spletters, 50, who has Parkinson's disease.

That changed after Spletter underwent a groundbreaking procedure at the University of Maryland Medical Center that guided ultrasound waves through her skull to kill the brain cells interfering with her motor skills. She's participating in a clinical study looking at a cutting-edge way — known as focused ultrasound — to possibly treat certain Parkinson's symptoms noninvasively.

As many as 1 million people in the United States live with Parkinson's, a nervous system disorder that affects movement and becomes progressively worse over time. The disease most often afflicts older people, but about 4 percent of those suffering from it are younger than 50. Symptoms include tremors, stiff limbs, trouble balancing and a general slowdown in movement.

The procedure performed on Spletter is one of many in the growing medical field of focused ultrasound. When the Focused Ultrasound Foundation was founded nine years ago, the therapy was being looked at to treat three conditions. Today, there are more than 58 ways the treatment is being used — from treating fibroid tumors to brain tumors to obsessive-compulsive disorder.

Spletter was the first at the University of Maryland Medical Center to participate in the Parkinson's study, which also is being conducted in Korea, Canada and at the University of Virginia. "I feel like I got a new lease on life," she said. "My quality of life has improved so much."

The question is for how long. Scientists already know that targeting specific cells in the brain can minimize Parkinson's symptoms. The more common method is to cut a hole into a patient's head and put electrodes on a specific part of the brain.

There were no cuts made to Spletter's skull. Ultrasonic waves were directed through Spletter's skull, much like a magnifying glass aims sunlight at a certain spot. The waves provide energy that is low enough that they don't damage the brain as they pass through, but, where they meet at single point, the energy adds up, creating enough heat to kill the cells.

"The effects are immediate," said Dr. Paul Fishman, a professor of neurology at the University of Maryland School of Medicine. "We saw improvement in her Parkinson's while doing her procedure."

Spletter could not believe what was happening.

"It was a miracle," she said.