Magnetic Resonance Imaging (MRI) uses strong magnetic fields and radio waves to generate detailed images of the body’s interior by harnessing the magnetic properties of hydrogen atoms, which are abundant in water molecules. The query of whether MRI is structural or functional is answered by the fact that it is capable of both, depending entirely on the specific imaging sequence chosen. Modern MRI scanners are versatile instruments, able to shift their focus from static anatomical mapping to dynamic physiological tracking.
Structural MRI: Mapping Anatomy
Structural MRI (sMRI) creates high-resolution, static images that map the physical anatomy and composition of tissues. The technique relies on subtle differences in how hydrogen protons in various tissues return to their equilibrium state after being excited by a radiofrequency pulse. This process involves two main properties: T1 (longitudinal relaxation) and T2 (transverse relaxation). By manipulating the timing of the radiofrequency pulses and signal reception—the repetition time (TR) and echo time (TE)—technicians emphasize one property over the other to produce distinct image contrasts.
T1-weighted imaging uses short TR and TE times, making fat and tissues with short T1 relaxation times appear bright, which provides clarity for outlining anatomical structures like the cerebral cortex. T2-weighted imaging employs longer TR and TE times, causing water and fluid-filled areas, such as cerebrospinal fluid (CSF) or areas of edema, to appear bright. T2 weighting is effective for detecting pathology, as many diseases involve increased water content. This structural approach is instrumental for identifying physical abnormalities, measuring organ size, or locating lesions and tumors.
Functional MRI (fMRI): Capturing Activity
Functional MRI (fMRI) captures the dynamic physiological changes associated with brain activity. This technique does not image the neurons firing directly but instead measures the resulting change in blood flow, known as the hemodynamic response. When a brain region becomes active, it rapidly consumes oxygen, triggering a localized surge of oxygenated blood that exceeds the tissue’s immediate need.
The core mechanism of fMRI relies on the Blood-Oxygen-Level-Dependent (BOLD) contrast, which detects the magnetic difference between oxygenated and deoxygenated hemoglobin. Deoxygenated hemoglobin is paramagnetic. The influx of oxygen-rich blood during activity flushes out this paramagnetic substance, leading to a local increase in the MRI signal, which is then mapped as an area of activation. While fMRI provides lower spatial resolution than a structural scan, it offers high temporal resolution, allowing researchers and clinicians to observe when and where the brain is engaged in a specific task, such as moving a finger or speaking.
Specialized Techniques: Mapping Connectivity
Advanced MRI techniques focus on the organization necessary for activity. Diffusion Tensor Imaging (DTI) is a prominent example, which measures the movement of water molecules within the brain. Water diffusion is not uniform in all directions; within the tightly bundled axons of white matter tracts, water moves more freely parallel to the fibers than perpendicular to them, a property called anisotropic diffusion.
DTI captures this directional movement of water to reconstruct the pathways that form the brain’s complex wiring. It provides an assessment of the microstructural integrity of the connections themselves. The resulting images, often visualized using a process called tractography, provide a map of structural connectivity, revealing the highways of information transfer that link distant brain regions.
Clinical Applications: When and Why Both Are Needed
In clinical practice, the greatest diagnostic power comes from combining structural and functional data, as neither provides the full picture alone. In neurosurgical planning, a structural MRI precisely locates a brain tumor or lesion and provides the anatomical boundaries for resection. This structural map is then overlaid with functional data from an fMRI, which identifies the eloquent cortex—regions responsible for motor control or language—to ensure they are avoided during surgery.
This integrated approach provides a comprehensive view of disease progression. Structural scans can quantify brain atrophy, such as the shrinking of the hippocampus seen in Alzheimer’s disease. This structural information is then complemented by DTI, which can show the corresponding loss of white matter integrity and connectivity, or fMRI, which may reveal altered patterns of functional communication between brain regions. The MRI platform is a versatile technology offering a suite of sequences to reveal the architecture, activity, or connectivity of the body’s tissues.