Profound improvements in perfusion and diffusion tensor imaging over the past few decades are changing the ways in which radiologists understand disease processes, especially those involving small blood vessels in the brain, according to Dr. Jonathan Gillard of Cambridge University Hospital in the U.K.
Advancements such as diffusion MRI are now current practice, both for assessing the extent of acute ischemic disease to speed the treatment of stroke patients and for mapping white-matter fibers to identify for neurosurgeons which parts of the brain are functional and should not be touched during surgery, explained Dr. Denis Le Bihan, from NeuroSpin, CEA Saclay Center in Paris.
Future directions for diffusion MRI are not yet clear, but there are two potential applications, he said. The first relates to problems in communication between various parts of the brain when, for example, the frontal lobe is not talking to the hippocampus or not talking in the right way.
"There are some papers showing that in schizophrenia, depression, or autism, the brain anatomy looks almost normal. The issue is that communication is not well established, and communication goes through the white-matter fibers. So if we have a way to map out white-matter fibers, we may be able to show that something is abnormal; there are not as many fibers as we expect, or the fibers are not well-organized. These are things that could be detected with diffusion tensor imaging," Le Bihan said.
Sagittal and coronal projections of the white matter track connecting brain regions, as obtained with diffusion tensor MRI. (Provided by C. Poupon et al., NeuroSpin)
Diffusion MRI also may be used to detect activation of the brain. During activation, the diffusion of water molecules slows down. The reason is not yet known, but it may be because neurons swell when they are activated. If diffusion MRI proves to be accurate in visualizing changes in the microscopic structure of tissues that occur during activation of the neurons, it could reveal a process that is intrinsically linked to brain activation.
"BOLD (blood oxygenation-dependent) MRI is an indirect way to detect brain activation. It detects an increase in the cerebral blood flow in the regions which have been activated. So we don't see activation; we see something related to it. With diffusion MRI, we could have a way to detect directly the regions that are activated," Le Bihan said.
