Neuroimaging: Definition, Methods & Techniques

Instructor: Jason Lulejian

Jason has taught medicine and has a Medical Degree from Western University of Health Sciences

Did the desire to know what CT, MRI or PET come across your mind during an episode of ''ER'' or ''House''? If so, this lesson will teach you the basics of neuroimaging, from definition to finer points of techniques and methodologies.

The Basics of Neuroimaging

Neuroimaging can be broadly described as any 2- or 3-D representation of a person's central nervous system, which includes everything from the brain, spine and the adjacent structures (such as the cerebral spinal fluid). There are several techniques that medical professionals can use to create images of the central nervous system. Let's look at those now.

Computed Tomography (CT): The Bigger Picture

CT scan of head and neck with out contrast
CT scan of head and neck with out contrast

Let's begin our lesson with the most common neuroimaging technique. Have you ever watched Grey's Anatomy? Well, computed tomography, also known as a CT scan, is what would likely be ordered if someone were to come into the show's ER after having a head injury. It is the means by which the medical team can quickly evaluate a patient to see if he or she is bleeding into the brain or has some other large abnormality.

But how does it work? It generally works through similar principles as X-rays, where high-energy beams of radiation are sent through the brain and the skull and allow us to see what is absorbed and what is not. This gives a very primitive picture of what is going on in the brain and central nervous system. However, the beauty of CT is that it uses multiple X-rays at many different angles (usually about 360 degrees), and the computer can then essentially make a 3-D map of the central nervous system structure (i.e., head CT or spine CT). This map is referred to as the tomographic map.

Generally, this is really good for imaging large structures in the brain, such as bone and huge tissue abnormalities. These include really nasty things such as hemorrhages because you are able to see the difference between blood, tissue, and bone. You can think of a CT scan as looking at the relative density of each object: the most dense material, bone, is completely white, and the least dense material (air, moving fluid) is completely black. Brain tissue looks gray to off-white gray. Health-care providers read CTs as 2-D 'slices' that allow them to look at different levels of the structure in question.

However, what happens when you want to see more of the arterial structures? Moving blood is difficult to see on CT. The solution is what we call contrast media, which is essentially dye. Contrast media is generally made out of heavy metals or sometimes complex molecules. After it has been injected into the patient and then swirled around in the arterial system, the reflection of the beams of X-rays makes a stronger signal from the moving blood. This gives us a more complex and complete view of the central nervous system than one without, but it is more costly and dangerous.

Magnetic Resonance Imaging (MRI): The Finer Details

What happens when you need a more precise image that can visualize texture of the tissues in question? This leads us to our second large type of imaging, namely MRI, or magnetic resonance imaging. MRI can be understood in many different ways. The easiest way is to say that you want to find details of tissue. MRI, in the classical sense, generally looks at protons and their energy. What that means is when a health-care provider wants to look at the texture of the tissue, they can use magnetic resonance imaging.

This technique places the patient in a large field of magnetic energy. These magnets pulse, energizing certain molecules or elements. The basic MRI that you usually will see in a hospital usually energizes water, namely protons (hydrogen). They essentially, after being energized, 'wiggle', releasing energy, and that energy is detected by the scanner. This happens in many different pulses. It's like playing Marco Polo with every molecule of water in your brain. MRIs create a 3-D representation; however, just like CTs, MRIs are read in 2-D 'slices', which allow health-care professionals see different views of the structures.

MRI can see all sorts of tiny details in tissues, especially in the central nervous system. You essentially get the same shades, bone being bright white and tissue being off-gray, only with greater resolution than CT scans. MRI can also have different 'filters' on the camera, and there are generally two types of MRIs, T1 and T2.

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