In this module, we will be discussing some different approaches to neuroimaging methods. Neuroimaging uses various techniques to directly, or indirectly image the Central Nervous System. The approach can either focus on Structural Imaging, creating an image of the anatomy and the pathology or injury or focus on Functional Imaging, where one tries to create an image of the metabolic or pharmacologic response of the brain or cognitive functioning. There are many techniques that can be employed to obtain either structural or functional images of the nervous system. EEG is a very commonly used method for functional registration of brain function, CT or Computed Axial Tomography, Single-photon Emission Computed Tomography or SPECT, Positron Emission Tomography or PET, Magnetic Resonance Imaging, Functional Magnetic Resonance Imaging, Diffusion Tensor Imaging or Spectroscopy Imaging. Now, I will very briefly step through some of these techniques, and some of these techniques will be explained in greater detail in future modules. For EEG, it measures the electrical activity in the brain, through electrodes that are placed on top of the scalp, as you can see in the top image here. There are standard methods, or standard places for these electrodes to be placed on the scalp so that studies can make comparisons between one another. It measures the activity at rest, or in response to the presentation of a stimulus, depending on the research question or clinical question, and it assesses localized brain responses or brain network responses, through temporal and spatial correlations of activation. On the middle-right image, you can see an example of the location of the various electrodes that are placed on the scalp, and at the bottom, you can see an example readout of what that EEG or electrical activity of the brain looks like. Each line represents one of the electrode placements on the brain. This method is very commonly used in epilepsy research when clinicians or researchers are trying to figure out where the locus of an epileptic seizure is. For Computed Axial Tomography, the system, as you can see on the top-right, essentially uses a series of x-rays. The x-rays are sent through the volume or through the person, in this particular case, and the amount of x-ray absorption in the volume is measured. When you do this using a series of cross-sections, you can reconstruct a 3D volumetric image, following the methods that you see in the middle. An x-ray is sent through the volume, from many, many different angles, and over time, when you combine these images, you can see densities or lesser densities in the image, that can be used to create a 3D volume of the item, or the brain, or the nervous system, that you are trying to image. It is very fast, it is widely available, but it does involve a moderate dose of radiation, as these do contain x-rays that are being used to create these images. For Single Photon Emission Computed Tomography, a gamma-emitting tracer is injected intravenously. And therefore, it is considered a nuclear-imaging technique. It is injected either as a soluble ion, or it is attached to a ligand. A ligand is essentially a vehicle, that the gamma-emitting tracer is attached to, and the system that measures the uptake or the spread of that ligand, throughout the central nervous system, most commonly the brain. The SPECT scanner measures gamma rays in a series of 2D images, from multiple angles, to determine where in the brain the gamma emissions come from, which would represent the amount of uptake of those gamma-emitting tracers in the brain. In the middle-top, you can see a schematic representation of what that looks like. A gamma camera is essentially creating a series of 2D images of that uptake. At the bottom, you can see an example of a SPECT image, which represents increased activity or uptake in warmer colors, the red and the yellows, and decreased activity or less amount of uptake, in the blue or green colors. Now, note that the spatial resolution of these images is not fantastic. It is on the scale of about a couple of centimeters to about a centimeter. Positron Emission Tomography is essentially a refinement of the SPECT technique. It is also a nuclear-imaging technique, that uses a positron-emitting radionuclide, attached to a biologically-active molecule, just like SPECT. However, in this case, it's a positron which reacts with an electron, causing annihilation. The annihilation then sends out two gamma rays in opposite directions. The PET scanner is essentially a series of gamma cameras all the way around the subject, as you can see in the middle-top. It tries to detect the pairing of gamma photons, and the time delay between the measurement, and the measurement on the opposite side can give you an indication as to where in the volume, these gamma rays came from. As such, it gives you a slightly better resolution, or actually, a reasonably better resolution, compared to SPECT, because you're now measuring the difference between two gamma rays, rather than just one. It generates an image of active molecule-binding and depending on the modality that you are interested in, you can use different molecules and different binding ligands, to create an image of what you are interested in. For example, FDG-PET is a marker used for the uptake of glucose, very commonly consumed by the brain, obviously. This is very relevant for tumors, for Alzheimer's disease, where you can see significant differences in glucose uptake, which would be seen as differences in activation or brain activity, which obviously, requires glucose uptake. On the top-right you see an example of such a difference, where on the left-hand side you see normal glucose uptake in a normal aging person, and on the right-hand side you can see a different image, where there is less glucose uptake in the posterior aspect of the brain, indicative of early Alzheimer's disease. Similar, 18F-AV-45 PET images, use a marker that is binding to beta amyloid, a misfolded protein, that occurs in the brain of patients with Alzheimer's disease. By using F18-AV-45, you can create very similar images to what you can see on the bottom right, of a person who shows a beta amyloid positive brain scans or PET scans, with the red and yellow colors, whereas on the bottom hand, you see a healthy control subject, where there is not such binding to beta amyloid, and you can see the colder colors, there's the absence of the red and yellow colors. Finally, 11C-PBB3 is a tau-marker, which can be used for tau accumulation in the brain. Tau is a different misfolded protein, that occurs in patients with Alzheimer's disease, and this marker or this ligant can be used to create a three-dimensional image of the binding and the accumulation of that tau protein, in the brain of Alzheimer's disease, as well as healthy controls. Both of these are predominantly used for research purposes at this time, although the hope obviously, is that they become clinically-relevant with more research. Turning to Magnetic Resonance Imaging, which is also a nuclear-imaging technique, it uses both static and variable magnetic fields to perturb hydrogen atoms. The perturbation of these hydrogen atoms causes a magnetic resonance, that is then measured by radio-frequency coils. These radio-frequency receivers will convert this information into a three-dimensional image, and different pulse-sequences can be used to generate different contrasts, depending on the tissue type. On the top right, you see an example of an MRI scanner and a schematic representation of what that looks like on the inside. There's both a static magnetic field in the blue boxes, there are gradient coils, which are the variable magnetic fields that can be applied in different directions, and then the radio frequency coil is used to read out the magnetic perturbance that you're applying to the system, which can then be converted into an image. You see two examples of such images below. On the left-hand side, you can see that the CSF in the brain is shown as black, whereas in the right-hand image the CSF is predominately shown as white or lighter colors. This is a function of the type of image sequences that are being used or pulse sequences that are being used to visualize the brain. The magnetic field for an MRI scanner is very, very strong, and it's about 60,000 times the strength of the Earth's magnetic field. It's very dangerous to have metallic objects in the vicinity obviously. When participants participate in such studies, they have to complete an extensive screening form to make sure that it's safe for them to go in such an environment. For example, patients with pacemakers should never undergo an MRI, as the magnetic field will interfere significantly with the pacemaker and potentially could cause life-threatening damage. There are many other things that need to be considered. Again, there is a significant checklist that a person will have to go through, like shunt placements or aneurysm clips, that could be placed in the brain, that could interact with the scanner and could potentially cause harm. To give you an example of what could go wrong if you don't obey the rules, here's an image where a hospital bed is pulled into the MRI scanner because the person did not follow the safety guidelines. This image also gives you a very good idea of the strength of the magnetic field. That is an entire hospital bed that is suspended in the air, due to the magnetic force that is being exerted on it. So the safety precautions are very important in these types of environments. But, following those safety regulations, there is no radiation associated with Magnetic Resonance Imaging, and there are no known side effects, even with repeated exposure to the magnetic field. So it's a very safe procedure for people who can safely undergo MRI. It has a very good spatial resolution, and it has a reasonable temporal resolution, providing us with good information about the brain. Let's look a little bit more at the makeup of an MRI scanner. As you can see in the middle, there's a cut-through of the MRI scanner. You see the magnet indicated in yellow, which is always on. It's not something that is being turned on once the patient enters the scanner, it's always on. On top of that are gradient coils. These are these variable magnetic fields that can be applied in different directions, and at the top, you see highlighted, the radio frequency coils that are being used to read out the signal from the brain or from the central nervous system. On the bottom, you see a segment through the scanner. Again, to show you a little bit about how that magnetic field is created, the magnetic field or the magnet within the scanner, essentially consists of a wire that is looped around, very much the same as an electro-motor. In some cases, it can extend up to 20 kilometers' worth of wire that is looped around the scanner bore, and this is submerged in liquid helium, to cool down the metal to minus 270 Degrees Celsius. This makes the metal super-conducting, and allows for the generation of this magnetic field. Obviously, the helium has to be carefully handled and carefully managed, but it is contained within the scanner system and very safe. Overall it creates this magnetic field through the center of the magnet, that serves as the static magnetic field, which we will discuss in much greater detail in our next module. When it concerns brain research, often a head coil is used, as you can see in this image here, which has additional antennae that are used to read out the signal from the brain. On top of that, you see a mirror, which is often used to allow the participant to look outside of the scanner, to make them feel a little bit less claustrophobic. Functional Magnetic Resonance Imaging is essentially a refinement of that same technique. In this particular case, the magnetic resonance from oxygenated versus deoxygenated blood is used to create an image of the functional activity of the brain. Blood and oxygenated blood, particularly, is necessary to support brain function. So, by measuring the amount of oxygenated versus deoxygenated blood, we can get an idea of areas of activation in the brain. This is used to assess localized brain function, as you can see an image of on the bottom left-hand side. On the top in the middle, you see an example of where that mirror that I just spoke about on top of the head coil is now used to present images so that a person can respond to stimuli. Very often, the person will have buttons in their hand, with which they can make choices, response choices, depending on what type of cognitive paradigm is employed. Diffusion Tensor Imaging is also a refined application of Magnetic Resonance Imaging, and uses all the same equipment, but it measures the directionality and diffusion of water, to generate a particular image. This is particularly used to assess fiber projections and the integrity of white matter. If you recall from our previous modules, white matter forms essentially the axon bundles that facilitates communication between neurons, and they're highly directional. By measuring the directionality of the water molecules within those fiber bundles, we can create an image, as you can see in the bottom right, giving a representation of the structural integrity of white matter, and the projections from one area to the next. Finally, we have Spectroscopy Imaging, which is again a refined application, and uses the same hardware as a Magnetic Resonance Imaging. It measures magnetic signatures of various metabolites in the brain, and it creates a frequency distribution of metabolites for defined locations. As you can see on the top-hand side on the right, a white box is outlined, from which this measurement is taken, and you can see the spectrograph, showing the peaks of certain metabolites that can be detected in that area. This is very commonly used for stroke patients, or in some cases cancer, but it can also be used for research, to figure out which metabolites play an important role, in association with certain brain areas or with certain brain functions. The bottom table shows you a short list of the types of metabolites that can be measured with Spectroscopy Imaging. In the next module, we will go deeper into the basis of the MRI signal, and how the MRI technology is used to create structural as well as functional images of the human nervous system.
