Advancing optical methods to study the living brain
Imaging the Living Brain
Imaging the brain after death is easy because it is no longer being used. Imaging the living brain at a single point in time through MRI is also easily done. However, when you want to observe changes in the brain in real time and over time, it requires careful preparation and advanced technology.
That’s the domain of Elizabeth M.C. Hillman, PhD, Associate Professor of Biomedical Engineering and Radiology, and the members of her Laboratory for Functional Optical Imaging at Columbia University. According to Dr. Hillman, “Our research focuses on using advanced optical techniques–many of which we have developed ourselves–to capture information about the form and function of the living brain. A major theme of our lab is in vivo neuroimaging, in particular to understand the relationship between blood flow changes in the brain and underlying neuronal activity.”
Blood flow in the brain is modulated or changed locally in response to neural activity. So when neurons fire in a particular area of the brain, there’s an accompanying increase in blood flow in that area. This process is called neurovascular coupling. Blood flow, of course, is critical for the brain to maintain normal function and health, but several aspects of blood flow control in the brain are curious. For example, blood flow changes are much slower than neuronal responses, so sometimes higher blood flow rates don’t reach the responding area until after the neuronal activity has already finished. We also don’t know what mechanisms are needed for a firing neuron to signal the need for blood flow to change. This is important, both to be able to interpret the cause of blood flow changes in the brain, and to develop treatments for brain conditions in which neurovascular coupling is broken.
“We want to use our imaging systems to better understand neurovascular coupling,” says Dr. Hillman. “But there are two challenges: one is that we need to observe a living brain with neuronal interconnectivity and intact vascular networks.”
The second, according to Dr. Hillman, has to do with planning the experiment and preparing the test subjects.
“Many of our studies observe the anesthetized brain, but we have to be careful to understand the influence of anesthetics on both neuronal activity and blood flow,” Dr. Hillman explains. “In some cases, we want to observe the brain when it is not under the effects of anesthesia. This requires careful training of lab animals and designing methods that enable us to image a normally operating brain in various states.”
Highly advanced tools and techniques
As far as the imaging technology itself, Dr. Hillman’s lab uses a variety of tools, including in vivo two-photon microscopy, multispectral optical intrinsic signal imaging (MS-OISI) and laminar optical tomography (LOT). The former uses near-infrared light to excite molecules within tissues with fluorescent contrast. MS-OISI uses strobed light emitting diodes (LEDs) and a high-speed camera to image the surface of the brain. LOT uses measurements of scattered light to add a depth dimension to MS-OISI.
Our high-speed imaging systems collect enormous amounts of data. According to Dr. Hillman, data sets typically consume 200 gigabytes. Her lab has “a closet full of massive computers,” where close to 100 terabytes of data are stored.
One of the things she is looking at with these amazing machines is oxygenation dynamics. “Through high-speed, high-resolution imaging,” she says, “we can see changes in oxygenation of blood in brain tissues when neurons are firing. By combining this with real-time measurements of vessel dilations, blood flow, calcium dynamics in neurons and fluorescent metabolites, we can learn a great deal about how the brain maintains its energy supply.”
Neuro vs. vascular
Dr. Hillman’s experiments also provide valuable opportunities to look at neurovascular causes of disease, since abnormal blood flow and oxygenation to different parts of the brain may be at the root of neurological diseases, such as Alzheimer’s and multiple sclerosis. "There is evidence that blood vessels in Alzheimer’s patients experience structural changes that inhibit the ability of neurons to make proper connections," she says, "we need to understand these changes before we can learn how to fix or prevent them."
In addition, Dr. Hillman is studying how the brain behaves in its "resting state," when blood flow in the brain is changing in what appears to be a random way. Researchers have found that in the normal resting brain, there are often parts of the brain in which two or more regions exhibit the same random fluctuations in blood flow, suggesting that they may be functionally connected in some way. A wealth of results in recent years have shown that these connections appear different in people with pathologies such as schizophrenia and ADHD. Dr. Hillman believes that some of these conditions could, therefore, have underlying impairments in neurovascular coupling.
“We have just received the first grant awarded by the National Institutes of Health to study this phenomenon,” she says. “If we can understand whether certain pathologies are driven more by neural or by vascular abnormalities, we can better focus our research on how to repair them.”
How close are we to doing that? According to Dr. Hillman, “When we started, a lot of our work was purely observational, just watch and see, because we were not yet sure what we were looking for. Having established an understanding of neurovascular coupling and being able to mine all that data, we’re able to be much more hypothesis-driven now. We believe we’ve found a critical component of the mechanism, so the next step is to see whether and how it is affected in different diseases and conditions.”