Fast Fluid Attenuated Inversion Recovery Flair Imaging

FLAIR is an MRI sequence that produces heavily T2-weighted images with cerebrospinal fluid (CSF) signal suppression by employing a specific inversion pulse placed at the CSF null point.2 Suppression of the CSF signal leads to better lesion-to-CSF contrast, allowing better delineation of masses adja

figure 27.1. (A) Oligodendroglioma: nonen-hanced computed tomography (CT) scan showing left frontal hypodense mass with calcifications. (B) Intradiploic epidermoid cyst: nonenhanced CT scan showing bony erosion and expansion.

cent to ventricles and sulci (Figure 27.2); however, that is true only for relatively large lesions, as small tumors may be lost among the periventricular gliosis that appears bright on FLAIR.3 Based on multiple studies, FLAIR was found to be superior to both proton density (PD)- and T2-weighted images in delineating intraparenchymal lesions. In a large prospective, blinded analysis, Maubon et al. evaluated 102 patients with a multitude of neurologic presentations, including brain tumors, using turbo spin echo (TSE), turbo FLAIR, and gradient and SE (GRASE) images. They found that FLAIR was significantly superior to both GRASE and turbo SE for white matter disease (P less than 0.05), superior only to TSE (P less than 0.05) for vascular disease, but not superior to either gradient SE or TSE for tumors.4 Multiple descriptive studies with smaller numbers of patients showed more encouraging results: increased sensitivity of detection and better con-spicuity of lesions using FLAIR sequences compared to T2-weighted images,5 better appreciation of peritumoral edema, and better definition between edema and tumor than T2-weighted and proton density-weighted images.6 In a retro spective analysis including only 18 patients, Bynevelt et al. found FLAIR to be superior for appreciation of the lesion (91% of studies) and for demonstration of its margin (92%) and suggested that FLAIR can replace PD- and T2-weighted spin-echo imaging in radiologic follow-up of low-grade glioma.7 All three studies, however, relied on the subjective evaluation of the quality of images by different readers.

FLAIR imaging has also been used in the differentiation of intracranial epidermoid from other pathologies, related mostly to the incomplete signal suppression due to the presence of keratin and cholesterol crystals in epidermoids (Figure 27.3B). In a series of eight patients with a surgically confirmed diagnosis of epidermoid, Chen et al. compared conventional MR sequences with fast fluid-attenuated inversion recovery (fast-FLAIR) and echo-planar diffusion-weighted (DW) MR imaging. On fast-FLAIR imaging, the mean signal intensity of epidermoid tumors was significantly higher than that of CSF but significantly lower than that of the brain. The authors concluded that fast-FLAIR imaging is superior to conventional MR imaging in depicting intracranial epidermoid

figure 27.2. (A) T2-weighted and (B) fast fluid-attenuated inversion recovery (FLAIR) image of left frontal lobe anaplastic astrocytoma. FLAIR delineates the tumor border more clearly as a result of inherent cerebrospinal fluid (CSF) signal suppression.

figure 27.3. (A) T2-weighted images, (B) FLAIR images, (C) diffusion-weighted image (DWI), and (D) apparent diffusion coefficient (ADC) maps of posterior fossa epidermoid cyst eroding the bone. Note incomplete suppression of signal on FLAIR images and restricted diffusion on DWI and corresponding ADC maps.

figure 27.3. (A) T2-weighted images, (B) FLAIR images, (C) diffusion-weighted image (DWI), and (D) apparent diffusion coefficient (ADC) maps of posterior fossa epidermoid cyst eroding the bone. Note incomplete suppression of signal on FLAIR images and restricted diffusion on DWI and corresponding ADC maps.

cysts.8 Similar results confirming the superiority of FLAIR to other sequences in the evaluation of epidermoid cysts were reached by Ikushima et al.9

A recent application of FLAIR imaging is the evaluation of leptomeningeal spread of tumors, whether primary or metastatic. High signal intensity in the sulci and fissures is suggestive of tumor involvement (Figure 27.4A). In one study evaluating 70 patients with cytologically proven leptomeningeal metastases, FLAIR imaging was found to have a sensitivity of only 34% for disease detection, compared to 66% for gadolinium-enhanced MR10 (Figure 27.4B). So, although FLAIR can help support the diagnosis, it alone cannot be used for the exclusion of leptomeningeal metastases, and contrast-enhanced T1-weighted imaging remains essential for that diagnosis. Contrast-enhanced FLAIR imaging, on the other hand, can improve detection of lep-tomeningeal disease in pediatric patients when compared to routine contrast-enhanced T1-weighted imaging, partly because of suppression of signal intensity from normal vascular structures on the surface of the brain, allowing easier visualization of abnormal leptomeninges.11 That study, however, was limited by the small number of patients with a history of medulloblastoma (n = 6).

The lack of definite proof of the usefulness of enhanced FLAIR images has hampered the routine implementation of this sequence in the clinical evaluation of brain tumor patients.

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