Fluid-Attenuated Inversion-Recovery Fast Spin-Echo MR : A Clinically Useful Tool in the Evaluation of Neurologically Symptomatic HIV-Positive Patients

With the introduction of the fast fluid-attenuated inversion-recovery (FLAIR) magnetic resonance (MR) technique, the routine use of this imaging sequence to enhance the detection of intracranial lesions in the cortical and subcortical regions and deep white matter has become practical, as illustrated so well in Thurnher et al’s article in this issue of AJNR (1). The diagnostic advantages of FLAIR can now be applied to patient treatment on a daily basis because of the time savings allowed by the coupling of the fast spin-echo technique to the FLAIR sequence (2–7). This achievement has been made possible by recent technical advances in imaging hardware (2). Whereas FLAIR performed with the conventional flow-compensated spin-echo sequences was diagnostically valuable (8–19), the long acquisition times made it too cumbersome for daily use (especially in today’s climate of managed health care). However, the decrease in acquisition time offered by the fast spin-echo technique has generated renewed interest in the diagnostic utility of FLAIR. Typically, total acquisition times have decreased from roughly 13 minutes for FLAIR to between 5 to 8 minutes for fast spin-echo FLAIR (3, 4, 6). Even shorter acquisition times of between 2 minutes 8 seconds to 4 minutes 16 seconds have been achieved with a modified fast FLAIR technique (5). The nulling of cerebrospinal fluid (CSF) signal on FLAIR images gives FLAIR its diagnostic advantage. With CSF devoid of signal, lesions exhibiting high signal intensity in the white matter adjacent to the ventricles or cortical sulci can be more easily detected (2–5). The contrast between the signal void of CSF and the adjacent hyperintensity of white matter lesions on this still heavily T2-weighted sequence enables the imager to appreciate abnormalities that might otherwise have gone undetected on routine spin-echo T2-weighted images (3, 4). Those hyperintense lesions in the periventricular white matter that are often obscured on T2-weighted images because of volume averaging with the even brighter CSF are more easily appreciated on FLAIR images (4, 5). Furthermore, some lesions on conventional proton-density spinecho images might also not be as apparent as on FLAIR images because of the similarity in signal between the lesions and adjacent gray matter on this sequence and a lower lesion-tobackground ratio than T2-weighted images (3). Multiple sclerosis plaques, for example, have been reported to be isointense with gray matter on proton density–weighted images (3). As emphasized by Hashemi et al (3) in cases of multiple sclerosis and now by Thurnher and colleagues in patients with human immunodeficiency virus (HIV) encephalitis and progressive multifocal leukoencephalopathy (PML), the detection of demyelinating lesions is particularly aided by this FLAIR technique, especially when combined with fast spin-echo. The value of this sequence has also been acknowledged in the diagnosis of acute cerebrovascular disease, acute carbon monoxide poisoning, mesial temporal sclerosis, and partial epilepsy, herpes encephalitis, tuberous sclerosis, acute subarachnoid hemorrhage, and intracranial tumors (3, 5, 6, 14–19). Potential pitfalls with the FLAIR technique have, of course, also been acknowledged (2–6, 13). These include CSF flow and misregistration artifacts, increased fat signal, decreased sensitivity to the detection of low-signal lesions caused by magnetic susceptibility effects such as those containing hemosiderin and, on FLAIR fast spin-echo, lack of white matter