Brain imaging techniques provide detailed views of the brain’s physical structure and its activity. These tools help medical professionals and researchers diagnose conditions, guide treatments, and expand neuroscience.
The Brain’s Major Structures
An image of the brain reveals several large-scale structures that perform distinct functions. The cerebrum, the largest part, sits at the top and is divided into two hemispheres, the left and right, connected by the corpus callosum. Each hemisphere controls the opposite side of the body, with the left associated with language and arithmetic, and the right with creativity and spatial abilities. The cerebrum’s outermost layer, the cerebral cortex, is called “grey matter” and is responsible for higher functions like thought, memory, and perception.
Beneath the cerebral cortex, “white matter” consists of myelinated nerve fibers, acting as communication pathways connecting different brain regions. The cerebrum is further divided into four lobes:
The frontal lobe handles planning, problem-solving, and movement.
The parietal lobe processes sensory information like touch, temperature, and taste.
The temporal lobe assists with hearing, smell, and memory.
The occipital lobe is dedicated to processing visual information.
Positioned beneath the cerebrum at the back of the head, the cerebellum coordinates muscle movements, maintains posture, and helps with balance. The brainstem connects the cerebrum and cerebellum to the spinal cord, controlling automatic functions such as breathing, heart rate, and sleep cycles.
How Brain Images Are Created
Brain images are generated using various technologies, each relying on different physical principles to visualize internal structures. Magnetic Resonance Imaging (MRI) employs powerful magnets and radio waves to create detailed pictures of soft tissues like the brain. The strong magnetic field aligns hydrogen atoms within the body’s water molecules, and then radiofrequency pulses briefly knock them out of alignment. As these atoms return to their original alignment, they release energy signals that the MRI scanner detects and a computer translates into high-resolution images.
Computed Tomography (CT) scans, in contrast, utilize a series of X-ray images taken from multiple angles around the body. A motorized X-ray source rotates around the patient, sending narrow beams through the body, which are then detected on the opposite side. A computer processes these X-ray data to construct cross-sectional “slices” of the brain, which can then be stacked to form a three-dimensional view. This method is particularly effective for visualizing bone structures and detecting fresh bleeding.
Positron Emission Tomography (PET) scans offer a different perspective by visualizing metabolic processes or blood flow within the brain. A small amount of a radioactive tracer, often a glucose analog called fluorodeoxyglucose (FDG), is injected into the bloodstream. As the brain’s cells use glucose for energy, the tracer accumulates in active areas. The PET scanner detects the emitted radiation, allowing a computer to create an image map that highlights regions with higher metabolic activity, appearing “brighter” on the scan.
Differentiating Structural and Functional Imaging
Brain imaging techniques are broadly categorized into structural and functional, each serving a distinct purpose in understanding the brain. Structural imaging focuses on creating a static map of the brain’s physical anatomy, revealing its shape, size, and any abnormalities present. Techniques such as standard MRI and CT scans fall into this category, providing detailed views of brain tissues, bone, and fluid-filled spaces. These images are comparable to a detailed architectural blueprint of a building, showing where different rooms and walls are located.
Functional imaging, conversely, measures brain activity by detecting changes in blood flow, metabolism, or electrical signals as the brain performs tasks or rests. PET scans and functional MRI (fMRI) are examples of functional imaging. fMRI, for instance, measures changes in blood oxygen levels, as active brain regions demand more oxygenated blood. This type of imaging is akin to a real-time traffic report, illustrating which parts of the city are busy at a given moment, rather than just showing the layout of the roads.
Interpreting Brain Scans in Health and Disease
Interpreting brain scans involves recognizing patterns indicative of health and identifying deviations that signal disease or injury. A healthy brain on a scan appears symmetrical, with clear distinctions between the darker grey matter of the cerebral cortex and the lighter white matter beneath. Fluid-filled spaces, like the ventricles, should be appropriately sized and symmetrical, without signs of compression or enlargement. Blood vessels should follow expected pathways, appearing organized and free from blockages or aneurysms.
In cases of disease or injury, these normal patterns can be disrupted. A stroke, caused by interrupted blood flow, may appear on a CT scan as a distinct, localized area of reduced density (hypoattenuation) within hours, indicating tissue damage. On an MRI, especially with diffusion-weighted imaging (DWI), a stroke can be detected within minutes by showing changes in fluid balance in the brain, and older strokes may appear as areas of tissue loss or white spots. Brain tumors appear as abnormal masses or growths on MRI, sometimes displacing normal brain structures or showing irregular contrast enhancement after dye injection, making them appear brighter. For conditions like Alzheimer’s disease, structural MRI scans can reveal widespread brain shrinkage, or atrophy, particularly in regions associated with memory, such as the temporal and parietal lobes and the hippocampus, reflecting the loss of brain cells.