Fluid Attenuated Inversion Recovery

Fluid-attenuated inversion recovery (FLAIR) is an MR sequence that suppresses CSF by applying an initial inverting 180° pulse, which negatively magnetizes the CSF and then waiting approximately for the T1 of CSF so that its magnetization (and hence signal) crosses a zero point just as the imaging data begins to be acquired (19). Thus, water signal is suppressed. It can be used in conjunction with long time-to-echo (T2-weighted) or short time-to-echo (T1-weighted) fast spin echo sequences. FLAIR suppresses signal from CSF and any other fluid with a comparable T1, thus it is nonspecific.

FLAIR T2-weighted images improve the conspicuity of lesions, especially those located near CSF spaces, such as near the cerebral ventricles. Moreover, because fluid

Figure 2 Value of blood volume maps in assessing brain tumors. Glioblastoma multiforme on T1-weighted (upper left), T1-weighted postcontrast (upper middle), and FLAIR (upper right) demonstrate, respectively, no abnormality, subtle enhancement, and edema in the region of the tumor indicative of a low grade tumor. The T2* perfusion blood volume map (lower left), however, demonstrates increased blood volume suggesting a higher-grade tumor.

Figure 2 Value of blood volume maps in assessing brain tumors. Glioblastoma multiforme on T1-weighted (upper left), T1-weighted postcontrast (upper middle), and FLAIR (upper right) demonstrate, respectively, no abnormality, subtle enhancement, and edema in the region of the tumor indicative of a low grade tumor. The T2* perfusion blood volume map (lower left), however, demonstrates increased blood volume suggesting a higher-grade tumor.

is present along the cerebral convexities, FLAIR greatly improves the detectability of lesions located near the dura. Thus, FLAIR T2-weighting has become a new standard in imaging the brain (20,21).

Contrast enhancement is a critical component of brain MRI, because it identifies sites of breakdown in the blood brain barrier which is often associated with

Figure 3 Comparison of MRI, PET, and blood volume maps. Left to right: FLAIR image, FDG PET, blood volume, and postcontrast T1-weighted image. Respectively these images depict: a lesion in the mass intermedia demonstrating high signal on FLAIR, relatively low metabolic activity on PET but increased blood volume despite absence of enhancement in the thalamus and insula suggesting a large, high-grade glioma.

Figure 3 Comparison of MRI, PET, and blood volume maps. Left to right: FLAIR image, FDG PET, blood volume, and postcontrast T1-weighted image. Respectively these images depict: a lesion in the mass intermedia demonstrating high signal on FLAIR, relatively low metabolic activity on PET but increased blood volume despite absence of enhancement in the thalamus and insula suggesting a large, high-grade glioma.

Figure 4 Advantage of FLAIR for detecting leptomeningeal metastases. Left to right: T1-weighted MRI before contrast, the T1-weighted MRI after intravenous contrast, and the FLAIR after contrast. Note that the enhancement of the subarachnoid space indicative of leptomeningeal involvement of the occipital lobe is seen only on the FLAIR.

Figure 4 Advantage of FLAIR for detecting leptomeningeal metastases. Left to right: T1-weighted MRI before contrast, the T1-weighted MRI after intravenous contrast, and the FLAIR after contrast. Note that the enhancement of the subarachnoid space indicative of leptomeningeal involvement of the occipital lobe is seen only on the FLAIR.

tumor. Conventional T1-weighted imaging depicts enhancing lesions as foci of increased signal. For the brain parenchyma, T1-weighting alone is usually sufficient, however, it can be difficult to detect leptomeningeal invasion on conventional T1-weighted scans (Fig. 4). By using FLAIR sequences after contrast administration, it is easier to detect enhancement in the meninges or parenchyma since the CSF is suppressed in this sequence (Fig. 5) (19).

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