Magnetic Resonance Imaging (MRI) uses strong magnets and radio waves to generate detailed images of internal organs and tissues. This imaging technique relies on various “sequences” or types of scans, each crafted to highlight distinct characteristics of different tissues. T2 Fluid-Attenuated Inversion Recovery (FLAIR) is one such specialized MRI sequence.
Decoding MRI Signals
MRI signal characteristics are determined by how quickly excited protons in tissues return to their resting state, a process described by relaxation times known as T1 and T2. On T1-weighted images, fatty tissues, such as subcutaneous fat, appear bright, while water-filled structures like cerebrospinal fluid (CSF) appear dark. This contrast helps visualize anatomical structures.
Conversely, T2-weighted images are sensitive to water content, making fluid-filled areas, including CSF, appear bright. Edema and inflammation, which involve increased water, also show as bright signals on these sequences. Different tissue types exhibit varying signal intensities based on their molecular compositions and relaxation properties.
The Specifics of T2 FLAIR
T2 FLAIR, or Fluid-Attenuated Inversion Recovery, modifies the standard T2-weighted sequence. Its defining feature is suppressing the signal from free-flowing fluids, such as cerebrospinal fluid (CSF) that surrounds the brain and spinal cord. This suppression is achieved through a specific radiofrequency pulse sequence that inverts the magnetization of fluid protons.
After a precise delay, a 90-degree pulse is applied, effectively canceling out the water signal. This causes CSF to appear dark on T2 FLAIR images, contrasting with its bright appearance on conventional T2-weighted scans. By rendering the CSF dark, T2 FLAIR enhances the visibility of abnormalities located near fluid-filled spaces, as these abnormalities appear bright against the dark background, preventing them from being obscured.
What T2 FLAIR Reveals
T2 FLAIR sequences are useful for uncovering various neurological conditions. They are sensitive to subtle changes in tissue water content, detecting edema, inflammation, and demyelination. For instance, in multiple sclerosis (MS), T2 FLAIR images identify demyelinating plaques, which appear as bright lesions within the white matter of the brain, standing out against the darkened CSF.
This sequence also helps detect subtle lesions associated with strokes. T2 FLAIR is beneficial for identifying peritumoral edema, the swelling that often surrounds brain tumors, and for visualizing lesions caused by infections or inflammatory processes. The fluid suppression allows for clearer delineation of these abnormalities, which might otherwise be difficult to distinguish from bright CSF on standard T2 images.
Distinguishing T2 FLAIR from Other MRI Sequences
T2 FLAIR shares similarity with standard T2-weighted imaging in its sensitivity to water content, meaning both can highlight areas of increased fluid. The defining difference lies in T2 FLAIR’s specific technique to suppress the signal from free-flowing fluids, such as cerebrospinal fluid (CSF). While CSF appears bright on conventional T2-weighted images, it is intentionally rendered dark on T2 FLAIR scans.
This fluid suppression is a deliberate choice by radiologists to improve the visualization of lesions located adjacent to CSF spaces, which might be obscured by the bright fluid signal on standard T2 images. For example, a small lesion on the surface of the brain might be difficult to discern if it blends with the bright CSF on a standard T2 image. T2 FLAIR effectively separates the lesion from the surrounding fluid, making it more conspicuous. T1-weighted images, in contrast, provide anatomical detail and show fluid as dark, but they do not specifically suppress fluid signals for lesion detection like T2 FLAIR.