Fluid-attenuated inversion recovery

Fluid-attenuated inversion recovery (FLAIR) is an magnetic resonance imaging (MRI) sequence that is used to image the brain. It is particularly useful for detecting lesions with high water content, making it invaluable in the diagnosis of various cerebral conditions, including multiple sclerosis, stroke, and encephalitis. FLAIR imaging is a type of inversion recovery sequence that has been specifically designed to null the signals from water, providing an image in which the effects of fluid are suppressed, thereby enhancing the visibility of lesions.

Principle

The FLAIR sequence is based on the inversion recovery technique but with a long inversion time (TI). This long TI is chosen to be the null point for free water, meaning that the signal from water is significantly suppressed in the resulting images. This suppression of the water signal allows for an increased contrast between normal and abnormal brain tissues, particularly those tissues that are edematous or infiltrated by pathology, which would otherwise be obscured by the high signal of cerebrospinal fluid (CSF) in standard T2-weighted images.

Clinical Applications

FLAIR imaging is widely used in clinical practice due to its ability to provide clear images of the brain that highlight abnormalities. Some of the key applications include:

• Detection of Multiple Sclerosis (MS): FLAIR is highly sensitive to the periventricular white matter lesions characteristic of MS.
• Assessment of Stroke: FLAIR can help in identifying the age of a stroke, as it is sensitive to the changes in water content that occur in brain tissue after a stroke.
• Evaluation of Encephalitis: The sequence can detect subtle changes in the brain tissue that are indicative of encephalitis.
• Identification of Brain Tumors: FLAIR can be used to delineate the boundaries of brain tumors and to distinguish between tumor tissue and surrounding edema.

Advantages and Limitations

The primary advantage of FLAIR imaging is its ability to suppress the signal from CSF, which enhances the detection and delineation of lesions near or involving CSF spaces. However, there are some limitations to the FLAIR sequence:

• Sensitivity to Motion: Like other MRI sequences, FLAIR is sensitive to patient motion, which can degrade image quality.
• Long Acquisition Times: The need for a long inversion time results in longer scan times compared to other sequences, which can be uncomfortable for patients.
• Limited Use in the Posterior Fossa: The suppression of CSF signal can sometimes be incomplete in the posterior fossa due to inhomogeneities in the magnetic field.

Technical Considerations

When performing FLAIR imaging, several technical factors must be considered to optimize image quality:

• Inversion Time (TI): The TI must be carefully selected to effectively null the signal from water.
• Repetition Time (TR) and Echo Time (TE): The TR and TE should be optimized based on the specific MRI system and the clinical application to achieve the best contrast between lesions and normal tissue.
• Magnetic Field Strength: Higher field strengths can provide better signal-to-noise ratio (SNR) and resolution, but may also increase susceptibility to artifacts.

Conclusion

FLAIR imaging is a powerful tool in the diagnosis and management of various brain pathologies. Its ability to suppress the signal from water allows for enhanced visualization of lesions, making it a critical sequence in the arsenal of brain imaging techniques.

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