The T1 time of the FLAIR pulse sequence is adjusted to the relaxation time of the component that should be suppressed. For fluid suppression the inversion time (long T1) is set to the zero crossing point of fluid, resulting in the signal being ‘erased’.
This type of sequence is particularly useful in the detection of subtle changes at the periphery of the hemispheres and in the periventricular region close to CSF.
Axial fluid-attenuated inversion recovery MRI image demonstrating tumor-related infiltration involving lenticular nucleus.
The usefulness of FLAIR sequences has been evaluated in diseases of the central nervous system such as:
head injuries, and others
Increase in FLAIR signal of the fluid within the resection cavity is described as a highly specific and early sign for tumor recurrence in gliomas.
An increase in FLAIR signal of the fluid within the resection cavity might be a highly specific and early sign of local tumor recurrence/tumor progression also for brain metastases 1).
Single dose gadolinium (Gd) enhanced fluid-attenuated inversion recovery (FLAIR) is helpful for visualizing superficial parenchymal metastases.
Cerebral MRI may be proposed if the patient’s clinical condition allows it. FLAIR imaging is more sensitive than CT to demonstrate a subarachnoid hemorrhage and offers greater degrees of sensitivity for the diagnosis of restricted subarachnoid hemorrhage in cortical sulcus 2).
Low grade glioma
Fluid-attenuated inversion-recovery (FLAIR) imaging has established its utility in neuroimaging.
Bynevelt et al.propose this imaging sequence as a replacement for proton density image (PD) and T2-weighted spin-echo sequences in the follow-up of low-grade glioma.
Magnetic resonance imaging (MRI) with T2-weighted and Fluid Attenuated Inversion Recovery (FLAIR) images best delineates the extent of insular glioma infiltration, which can be limited to the insular lobe (Yasargil type 3a) or reach the perisylvian opercula (type 3b) and other paralimbic areas, namely the orbitofrontal and temporopolar regions (type 5), with or without involvement of core limbic structures 3).
MRI examinations of 18 patients with such tumours were reviewed by three neuroradiologists and a neurosurgeon. FLAIR was found to be superior for appreciation of the lesion (91% of studies) and for demonstration of its margin (92%). FLAIR was also better at showing different tumour components, particularly in regions difficult to demonstrate in some planes, such as the vertex in axial imaging. The sequence also defines the postoperative cavity, shows the least amount of susceptibility effect associated with surgical clips, and demonstrates local spread (to white matter tracts, subependymal and capsular) more distinctly.
FLAIR can replace PD and T2-weighted spin-echo imaging in radiological follow-up of low-grade glioma 4).
Lee et al. that measures of spatial diversity from point pattern analysis of intensity habitats from T1 post-contrast and T2 fluid-attenuated inversion recovery images are associated with both tumor subtype status and 12-month survival status and may therefore be useful indicators of patient prognosis, in addition to providing potential guidance for molecularly-targeted therapies in Glioblastoma multiforme 5).
A, B,C: Preoperative FLAIR MR images of a low grade glioma infiltrating the left operculo-insular region and the fronto-orbital, including the perforated substance (white arrow), temporopolar and hyppocampal regions, type 5 B of Yasargil classification (Yasargil et al, 1992). D: Postoperative T1 gadolinium-weighted and E,F: Postoperative FLAIR MR images, showing the subtotal removal of the lesion. The boundaries of the resection are set based on anatomical (perforated substance, white arrow) as well as neurofunctional (subcallosal fasciculus, yellow arrow; inferior occipitofrontal fasciculus, green arrow; arcuate fasciculus, blue arrow) criteria.
A study supports the established association between extent of resection (EOR) and survival and presents additional data that pushing the boundary of a conventional 100% resection by the additional removal of a significant portion of the FLAIR abnormality region, when safely feasible, may result in the prolongation of survival without significant increases in overall or neurological postoperative morbidity. Additional supportive evidence is warranted 6).