Medical imaging plays a key role in the detection, diagnosis, and treatment monitoring of brain disorders. It provides a rich source of anatomical, functional and molecular information.
Neuroimaging techniques can reveal both form and function, revealing how different parts of the brain connect and communicate and how that creates our thoughts, emotions and actions. The most common modalities are:
What is neuroimaging?
Neuroimaging is the use of a variety of cutting-edge techniques to produce images of the brain or other parts of the central nervous system. These images allow scientists to study the brain’s structure and function without having to surgically remove tissue or subject participants to invasive procedures.
Neuroimaging techniques can measure structural changes in the brain, such as vascular malformations and tumors, or its functional properties, including neural activity, oxygen consumption, receptor sites and metabolism. These imaging modalities can also be used in conjunction with other clinical measures such as blood flow, electrical activity and neurotransmitters.
Different imaging methods have distinct strengths and weaknesses; for example, MEG records the magnetic or electrical fluctuations of individual neurons, but has a poor time resolution (milliseconds), while fMRI can better localize neural activity with high spatial resolution, but has a lower temporal resolution (seconds). Machine learning is an important component of neuroscience, as it enables us to analyze neuroimaging data to perform tasks such as segmentation, classification and prediction.
The EEG records the electrical activity of neurons in your brain. It produces a graph that displays voltages on the vertical axis and time on the horizontal axis. A neurologist or clinical neurophysiologist interprets the results by visual inspection.
Its main advantage is its millisecond-range temporal resolution and a low initial cost, compared to fMRI, PET or MRS. However, the meninges and skull smear the signal and limit its spatial resolution.
EEG is used in intensive care units to monitor neural function during surgery (for example, assessing depth of anesthesia or the effects of amobarbital) and to predict seizures in patients with epilepsy. It’s also used in the more experimental field of neurofeedback and as a component of brain computer interfaces. The test requires you to relax in a comfortable position while the technician places dozens of electrodes around your head using gels, saline solutions and pastes to maintain conductivity.
Computed tomography (CT)
The computerized tomography (CT) test, also known as the CAT scan, is a noninvasive diagnostic imaging procedure that uses X-rays to produce horizontal, or axial, images, called slices, of an area of the body. It allows physicians to view bones, muscles, fat, organs and blood vessels with greater detail than is possible with standard X-rays.
CT can be used alone or in conjunction with other diagnostic procedures, such as MRI, to better evaluate some types of brain tumors. Sometimes a contrast dye, often made of iodine, is administered before CT in order to enhance the picture.
Patients should be aware that total, or whole-body, CT exposes them to a significant amount of radiation. Women who are pregnant or breastfeeding should not undergo this test.
Magnetic resonance imaging (MRI)
MRI uses magnetic fields and radio waves to create images without the use of ionizing radiation. It is a very versatile technique that can produce high-resolution cross-sectional images of the brain and other body structures, with exceptional soft tissue contrast. It is based on nuclear magnetic resonance (NMR), which describes the magnetization of certain atomic nuclei, particularly hydrogen, and their evolving spin polarization in response to an applied external magnetic field.
The NMR signals are detected by hardware called receiver coils. During an MRI scan, you will hear loud clicks that are part of the scanner’s operation. You should wear earplugs or headphones to help block the noise. You should also remove any jewelry or metal items that could interfere with the scan. You may be given a contrast dye before some MRI scans to improve the clarity of the images.
Positron emission tomography (PET)
Positron emission tomography is a nuclear medicine scan used to assess the function of tissues and organs, such as the brain, heart, and cancer. It involves injecting a safe amount of a radioactive substance that binds to cells and is absorbed by them.
The positrons that are emitted when the substance decays are detected by detectors, which produce images of tissue metabolic activity. PET is a functional imaging technique, unlike CT or MRI, which are more suited to measuring anatomy and morphologic changes. Fluorine-18-fluoro-2-deoxyglucose (FDG) is an example of a commonly used PET radiotracer. It is used to assess glucose metabolism in the brain and to visualize tumors. PET is also useful in evaluating the response to treatment in oncology. The tracer kinetics provide quantitative information about derangements at molecular level, which is helpful in selecting appropriate mode of treatment.