Summer 2007

DTI Tractography Provides White Matter 'Roadmap'

UCSF scientists are using innovative signal processing algorithms to map white matter tracts, providing neuroscientists and neurosurgeons with valuable information about the brain.

"This is a very new technique, and a lot of what we are doing is basic science," says UCSF neuroradiologist and researcher Pratik Mukherjee, M.D., Ph.D. "But neurosurgeons are already using our images to plan surgeries."

Modern imaging methods such as computed tomography (CT) and magnetic resonance imaging (MRI) can distinguish between gray matter and white matter in the brain, and functional MRI (fMRI) can be used to create maps of brain function in the cortex. Such images are used to guide surgeons' preoperative planning as they try to decide how to resect a tumor, arteriovenous malformation or epileptogenic area without disrupting vital cognitive centers.

White matter, on the other hand, is not well defined by these imaging technologies, and its important role in cognition has necessarily been neglected. "fMRI tends to show the brain as a collection of distinct processing areas, but the truth is that the brain is a network and it's important to understand the connections," Mukherjee says. Cutting the connection between important processing areas in the cortex, or between the cortex and subcortical centers such as the spinal cord, can be just as damaging as disrupting those areas themselves.

Diffusion tensor imaging (DTI) data allow Mukherjee to perform tractography to map those connections by following the bundles of axonal tracts that make up white matter. DTI is done by means of the same large magnets that produce MRIs, but the mathematical algorithms that Mukherjee uses look at the movement of water in the brain.

DTI tractography relies on the fact that it is easier for water to diffuse along the axon of a neuron than it is to diffuse across the axonal membrane. By defining the greatest movement of water molecules in those neural cells, Mukherjee can define the direction that axons are running in any given part of the brain. Linking up that information from many points across the whole brain provides direct axonal pathways from one part of the brain to others.

Neurological surgeons like UCSF's Mitch Berger, M.D., continue to electrically map brain function around a lesion once the surgery begins, but DTI provides vital information that other imaging technology can't. Preoperative imaging still includes CT scans, MRI, magnetoencephalography (MEG) and EEGs, but only DTI tractography will show white matter connections between cortical and subcortical processing areas.

UCSF: at the Forefront of MRI Technology

In addition to the standard 1.5-tesla MRI scanners, UCSF is home to three 3-tesla machines and a very rare 7-tesla machine. One of the 3T scanners, located at the UC Imaging Center on the Parnassus campus, is available for outpatient imaging. The other two 3T systems and the 7T system are used for clinical research. The 7T MRI scanner, housed in a new building at UCSF's Mission Bay campus, is the first of its kind on the West Coast and one of only about 10 such machines in the world.

These high-powered scanners increase the signal-to-noise ratio of the scans, providing clearer images in less time than standard 1.5T MRI machines. An image that might take 10 minutes to acquire in a 1.5T machine is created in three minutes or less in the 7T machine. The more powerful magnetic fields also give the 3T and 7T scanners special capabilities that would be less feasible on a standard 1.5T platform.

Although both the 3T and 7T machines are provided for clinical research, they are available to most neurological surgery patients at UCSF. Two additional 3T MRI machines are scheduled to be installed in Moffitt Hospital during the first quarter of 2009 to become part of the regular clinical service for use by inpatients and outpatients.

Dr. Pratik Mukherjee can be reached at (415) 353-1668.

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