Imaging description

The FLAIR pulse sequence completely suppresses the bright signal from cerebrospinal fluid (CSF) due to a 180-degree inversion pulse followed by an excitation pulse applied at the null point for CSF. Dark CSF signal on a T2-weighted image increases the detection sensitivity of hyperintense lesions near the CSF surfaces, making FLAIR an indispensible sequence for brain imaging.

Non-suppression of CSF signal on FLAIR is generally attributed to increased protein concentration in the CSF, resulting in decreased T1 relaxation time, and this finding can be seen in a variety of conditions including acute and subacute subarachnoid hemorrhage (SAH), meningitis, leptomeningeal metastases and inflammation, vascular congestion secondary to venous occlusion or arterial collateral flow, and artifact from field inhomogeneity created by metal or other foreign material. Perhaps the most common reason for sulcal hyperintensity on FLAIR images in a hospital setting, however, is high concentrations of oxygen inhalation during MRI, which is a routine procedure for patients receiving general anesthesia for MRI scanning (Fig. 51.1). It has been shown that administration of a high fraction of inspired oxygen during general anesthesia correlates with increased CSF signal intensity on brain FLAIR MR images, and that patients receiving 100% oxygen almost invariably show sulcal hyperintensity, whereas under 50% oxygen this phenomena is unlikely to occur [1,2]. While the exact mechanism is not clear, increased oxygen concentration in CSF, coupled with the mild paramagnetic feature of oxygen, is believed to be responsible from T1 shortening of CSF.