DBS has proven to be highly effective in the treatment of movement disorders when standard drug therapies either do not work or have lost their effectiveness. However, the fact that it is an extremely long, difficult and expensive operation, which involves implanting electrodes deep in the brain, has limited its availability.
Since the procedure's approval in 1998, the number of DBS operations performed has grown gradually to about 3,000 annually, although more than 100,000 people a year could stand to benefit from it as a way of treating the tremor, rigidity, stiffness and slowed movement they experience as a result of neurological disorders ranging from dystonia to multiple sclerosis, to Parkinson's disease, to obsessive-compulsive disease.
To improve the procedure further, a team of electrical engineers and neuroscientists at Vanderbilt University has developed a pilot guidance system that automates the most difficult part of the operation: identifying the proper location to insert the electrodes. To work, the electrodes must pass through small nuclei deep in the brain that are about the size of a pea and are not visible in brain scans or to the naked eye. The researchers – writing in a special issue of the journal IEEE Transactions on Medical Imaging published this month – report that the new system can do a better job of identifying the initial location to insert the electrodes than an experienced neurosurgeon.
"The biggest problem with the procedure is that the surgeons cannot see the structure where they have to put the electrode and, as a result, they must spend a considerable amount of time searching for it," says Benoit Dawant, professor of electrical engineering, computer engineering and radiological sciences at Vanderbilt University, who is developing the guidance system in collaboration with Peter Konrad, associate professor of neurological surgery and biomedical engineering.
The only way that the target region can be identified is by its electrical characteristics. So the surgeons must first insert a recording electrode and monitor the electrical activity of the neurons that it touches. Sometimes they have to remove and reinsert the electrode two or more times. Sometimes they have to insert three or four electrodes at the same time in order to find the elusive spot.
"I tell patients that it is something like playing a big game of Battleship," says Konrad, who helped pioneer the procedure. "Like the game, you don't know where the target is until you've made a hit."
Each time the surgeons are forced to reinsert the electrode, it increases the risk of damage to the brain and the length of the operation. When surgeons decide that they have hit the right spot, they implant a stimulating electrode and test it to determine if it reduces the patient's symptoms. Because muscle disorders typically occur only while a person is awake, the patient must remain conscious through the entire procedure.
The operation can take as long as eight to 12 hours to properly place one electrode. (Most patients require two, one in each hemisphere.) "This is extremely rough on patients, who have to be awake through the surgery and have to be locked to the bed," says Konrad. "Anybody who performs this surgery quickly appreciates the need to trim the procedure down to a shorter process."
The computer-aided guidance system compensates for variations in the three-dimensional brain structure of each patient, something that it is very difficult for surgeons to do on their own. This reduces operating times by increasing the odds that the surgeons begin searching closer to the target. The system consists of a three-dimensional brain atlas that was built up by combining the brain scans of 21 post operative DBS patients into one another using sophisticated computer-mapping methods. To predict the location of the target area in a new patient, the researchers map the reference atlas onto the patient's brain scan. When the neurosurgeons have used the system's predictions, they have hit the target area on the first insertion two out of three times, compared with one out of five times when working without it.
"Now, with the use of the atlas, what we are basically doing is plugging the patient's MRI brain scan into the computer, and about three to four hours later it spits back a target that we can use to plan the following week's surgery," says Konrad. This innovation, along with other improvements such as the use of individually made insertion platforms, has substantially reduced the length of the operation: "We have reduced a two-day procedure down to five hours," he says.
Not only does the guidance system save the patient from the risk of a prolonged procedure or undergoing two procedures, it also should cut hospital costs significantly, Konrad adds.
The researchers plan a number of improvements to the guidance system. They have begun to collect data on the effectiveness of the operations and will use that to refine their predictions. They have also set up a system that will collect electrophysiological data from the patient's brains that is collected during the procedure so they can add it to the brain atlas as well. And finally they intend to begin creating individual atlases for different conditions – Parkinson's, essential tremor, dystonia, etc. – in case the precise location of the neurological damage may differ.
The research was funded by Vanderbilt University and FNRS, the Belgian Science Foundation.
Journal
IEEE Transactions on Medical Imaging