The typical brain scan shows a muted gray rendering of the brain, easily distinguished by a series of convoluted folds. But according to Van Wedeen, a neuroscientist at Massachusetts General Hospital, in Boston, that image is just a shadow of the real brain. The actual structure–a precisely organized tangle of nerve cells and the long projections that connect them–has remained hidden until relatively recently.
Traditional magnetic resonance imaging, or MRI, can detect the major anatomical features of the brain and is often used to diagnose strokes and brain tumors. But advances in computing power and novel processing algorithms have allowed scientists to analyze the information captured during an MRI in completely new ways.
Diffusion spectrum imaging (DSI) is one of these twists. It uses magnetic resonance signals to track the movement of water molecules in the brain: water diffuses along the length of neural wires, called axons. Scientists can use these diffusion measurements to map the wires, creating a detailed blueprint of the brain’s connectivity.
On the medical side, radiologists are beginning to use the technology to map the brain prior to surgery, for example, to avoid important fiber tracts when removing a brain tumor. Wedeen and others are now using diffusion imaging to better understand the structures that underlie our ability to see, to speak, and to remember. Scientists also hope that the techniques will grant new insight into diseases linked to abnormal wiring, such as schizophrenia and autism.
On the next page is an animation of the wiring of a marmoset monkey.
The marmoset brain, shown above, is about the size of a plum. By scanning a dissected brain for 24 hours, scientists were able to generate a map with a spatial resolution of 400 microns. “The image quality and resolution are much higher than we can obtain in a living subject,” says Wedeen.
As the brain rotates, you can see that all the neural fibers are visualized in half of the brain: the spiky fibers that look like pins in a pincushion are part of the cerebral cortex. The sparser half of the image displays only the fibers originating in the opposite side.
It’s easy to see that this brain lacks the folding that is characteristic of the human brain. “The human brain would look 25 times as complicated,” says Wedeen. “Every gyrus [fold] has its own story to tell.”
Credit: George Day, Van Wedeen, Ruopeng Wang at MGH, and John Kaas at Vanderbilt