Hyperintensity in the Subarachnoid Space on FLAIR MRI

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1 MRI of Subarachnoid Space Neuroradiology Pictorial Essay Stephen L. Stuckey 1 Tony D. Goh 2 Theresa Heffernan 3 David Rowan 4 Stuckey SL, Goh TD, Heffernan T, Rowan D Keywords: brain, CSF, FLIR, MRI, subarachnoid space DOI: /JR Received January 21, 2004; accepted after revision May 13, Department of Radiology, Princess lexandra Hospital, Ipswich Rd., Woolloongabba, Queensland, ustralia ddress correspondence to S. L. Stuckey. 2 Department of Radiology, Christchurch Hospital, Christchurch, New Zealand. 3 Department of Radiology, The Wesley Hospital, uchenflower, Queensland, ustralia. 4 Department of Radiology, The lfred Hospital, Prahran, Victoria, ustralia. JR 2007; 189: X/07/ merican Roentgen Ray Society Hyperintensity in the Subarachnoid Space on FLIR MRI OJECTIVE. The purposes of this essay are to illustrate the causes of FLIR hyperintensity in the subarachnoid space and to outline the mechanisms of the findings. CONCLUSION. FLIR subarachnoid space hyperintensity may be encountered with both pathological conditions and artifacts. Knowledge of these conditions and appearances coupled with any associated findings may suggest the cause of the FLIR subarachnoid space hyperintensity. diffuse distribution and a lack of ancillary findings often remain nonspecific and may require clinical correlation and CSF analysis. t many institutions, the FLIR pulse sequence has become a routine part of MRI studies of the brain. First described by Hajnal et al. [1] in 1992, FLIR MRI techniques consist of an inversion recovery pulse to null the signal from CSF and a long echo time to produce a heavily T2-weighted sequence. The FLIR technique produces images highly sensitive to T2-weighted prolongation in tissue. In addition, factors that affect the T1-weighted relaxation time of CSF may interfere with its suppression and result in CSF hyperintensity. Compared with conventional T2-weighted and proton density weighted imaging, use of the FLIR sequence improves detection of lesions within the subarachnoid space and brain parenchyma, particularly of lesions near the brain CSF interface. When disease occurs within the subarachnoid space, the relaxation time of CSF is altered. This change translates into lesser degrees of CSF signal nulling and resultant hyperintensity of the CSF or subarachnoid space during the FLIR sequence. Such findings have been well described in a wide range of pathologic conditions, such as subarachnoid hemorrhage (SH), meningitis, and leptomeningeal spread of malignant disease. Other, less common causes of subarachnoid FLIR hyperintensity are artifacts. With increased routine use of the FLIR sequence, radiologists need to be familiar with the causes of subarachnoid FLIR hyperintensity. nalysis of the distribution of sulcal FLIR hyperintensity and the associated imaging findings related to the primary pathologic condition (e.g., the presence of an adjacent mass) can help elucidate the cause. In many instances, however, particularly when there are no ancillary findings and the distribution is diffuse, the finding remains nonspecific, and clinical correlation alone or in combination with CSF analysis may be needed. This pictorial essay illustrates the known causes of FLIR hyperintensity in the subarachnoid space and briefly outlines the mechanisms for each of the findings. Pathologic Causes of Hyperintensity Subarachnoid Hemorrhage The appearance of SH on MRI has been a controversial topic [2]. cute SH is notoriously difficult to detect on conventional T1- and T2-weighted sequences, and FLIR has been appreciated and less challenged as being superior for detection of SH in the subacute phase [2, 3]. Results of both in vivo and in vitro studies have suggested that FLIR imaging is as sensitive as or more sensitive than CT in the evaluation of acute SH, but compared with the findings at lumbar puncture, the findings on FLIR imaging are not definitive in excluding acute SH [4 6]. The FLIR sequence is particularly useful in visualization of acute SH in areas where CT may be limited because of beam-hardening artifacts [7] (Fig. 1). The hyperintense appearance of acute SH on FLIR images relates to several factors and effects on both T1- and T2-weighted relaxation times. T1-weighted shortening of bloody CSF due to the higher protein content causes an off- JR:189, October

2 set in the null point of CSF inversion times, resulting in increased signal intensity. T2- weighted prolongation also occurs as a result of the high protein content of blood and inflammatory products in both dilute and dense blood CSF mixtures [7]. Oxyhemoglobin, which is diamagnetic and the initial product of blood degradation, may also contribute to T2- weighted prolongation. SH differs from intraparenchymal hemorrhage in that the mix of blood with high-oxygen-tension CSF delays generation of paramagnetic deoxyhemoglobin, and oxyhemoglobin remains present longer than in intraparenchymal hemorrhage [7]. Variability in the appearance of SH after 48 hours most likely relates to hemoglobin degradation, which adds further complexity to the MRI signal intensity [8]. Meningitis Unenhanced FLIR MRI has been shown to be a sensitive technique for the detection of inflammatory meningitis (Figs. 2 and 2) [9]. Elevations in CSF protein and cellular concentrations that occur in meningitis result in shortening of the T1 relaxation time, alteration of the point at which CSF is nulled, and T2 prolongation of CSF relaxation time [10]. Unenhanced FLIR MRI, although moderately sensitive, is not superior to contrast-enhanced T1-weighted imaging in the detection of meningitis [11]. Contrast-enhanced FLIR images have been shown to be superior to contrast-enhanced T1- weighted images in visualization of inflammatory leptomeningeal disease [12, 13]. Leptomeningeal disease can be more easily visualized on contrast-enhanced FLIR images than on contrast-enhanced T1-weighted images because FLIR imaging allows clearer distinction between enhancing meninges and enhancing cortical veins, cortical veins becoming less clearly enhanced on FLIR images (found in only 9% of cases in one series) [12, 14] (Fig. 2C). The relatively less prominent effect of gadolinium on venous appearance with FLIR compared with conventional spin-echo sequences does not appear to have been previously explained, especially in view of the apparently incongruous greater sensitivity of FLIR to lower concentrations of gadolinium [12]. The relative persistence of flow voids with FLIR may reflect more prominent time-offlight effects secondary to differences in acquisition technique (i.e., several interleaved acquisitions are often used for FLIR). Gadolinium enhancement of subarachnoid pathology should result in shortening of T1-weighted values and FLIR hyperintensity. Meningeal Carcinomatosis FLIR images may depict evidence of leptomeningeal malignancy without the use of IV gadolinium in patients with known or suspected neoplastic disease (Fig. 3) [15, 16]. Some results [9, 16] have suggested that FLIR images are superior to contrast-enhanced T1-weighted images for the diagnosis of subarachnoid space metastases. Other studies, however, have not reproduced these results, although cases in which unenhanced FLIR may be superior to gadolinium-enhanced T1-weighted imaging have been described. Other studies [17, 18] have had mixed results on the value of contrastenhanced FLIR imaging versus contrastenhanced T1-weighted imaging. With use of the contribution of gadolinium enhancement mediated T1-weighted shortening to the FLIR image, contrast-enhanced FLIR images may be of value in the depiction of leptomeningeal carcinomatosis, but the relative value of this technique remains an area of controversy. Figure 4 shows the use of gadolinium-enhanced FLIR in imaging of a 60- year-old man with non-hodgkin s lymphoma. The axial contrast-enhanced FLIR image reveals more intense and conspicuous bilateral enhancement of the internal auditory canals and adjacent seventh cranial nerves within the temporal bone secondary to lymphomatous infiltration than does the corresponding gadolinium-enhanced T1-weighted image. The FLIR hyperintensity of leptomeningeal carcinomatosis in the subarachnoid space results from elevations in CSF cellular and protein concentrations. The situation is similar to that described for inflammatory meningitis. Gadolinium enhancement similarly results in shortening of T1-weighted values and FLIR hyperintensity. Leptomeningeal Melanosis Leptomeningeal melanosis is part of the neurocutaneous melanosis congenital phakomatosis. The leptomeningeal pathologic condition can be melanosis or melanoma. MRI performed with FLIR may show subarachnoid hyperintensity similar to that of meningeal carcinomatosis. It has been speculated that the FLIR hyperintensity may be due not only to T2-weighted prolongation reflecting the elevated protein content but also to the T1-weighted shortening effects of melanin [19, 20]. For these reasons, FLIR hyperintensity can be seen with or without gadolinium contrast enhancement (Fig. 5). Fat-Containing Tumors The FLIR imaging technique entails an inversion pulse to null the signal intensity of CSF as these spins pass through the zero point determined by the tissue-specific (in this case CSF) T1-weighted relaxation time. ny shortening of the CSF T1-weighted relaxation time negates this effect. Fat-containing tumors, such as lipoma, in the subarachnoid space that appear hyperintense on T1-weighted images therefore appear hyperintense on FLIR sequences (Fig. 6). Fat droplets in the subarachnoid space from a ruptured dermoid are similarly hyperintense on FLIR sequences (Fig. 7). y a similar mechanism, droplets of residual oil-based contrast medium (e.g., iophendylate) are a potential cause of FLIR hyperintensity in the subarachnoid space. cute Stroke Vascular hyperintensity in the subarachnoid space on FLIR images may be produced by severe (> 90%) vascular stenosis or occlusion of major cerebral vessels with resulting slow flow [21]. bnormalities in vascular signal intensity on FLIR images as a result of large vessel occlusion or high-grade stenosis is reported to occur as the earliest MRI sign of ischemia in some patients and thus may be seen before detectable abnormalities on diffusionweighted images [21, 22]. rterial hyperintensity on FLIR images in association with acute stroke may reflect retrograde collateral circulation, vascular congestion, slow flow, or T1- weighted shortening and T2-weighted prolongation of thromboembolism or stationary blood. Compared with those obtained with conventional T2-weighted sequences, images obtained with the FLIR sequence clearly depict arterial occlusion as intraarterial areas of high signal intensity against a dark CSF background (Figs. 8 and 8) [23]. The area of intraarterial signal intensity is larger than the area of abnormality on diffusion-weighted images (Fig. 8C). In combination, the two sequences can be useful for predicting whether an area of cerebral tissue is at risk of infarction (the ischemic penumbra). There may be important implications in the management of hyperacute cerebral ischemia [23]. The areas of intravascular signal intensity on FLIR images appear to correlate with the perfusion abnormality [23]. Moyamoya Disease In 1995, Ohta et al. [24] reported diffuse leptomeningeal enhancement on contrast-enhanced T1-weighted images of children with moyamoya disease. Those authors named this 914 JR:189, October 2007

3 MRI of Subarachnoid Space finding the ivy sign because it resembled ivy creeping on stones. In a more recent report [25], high signal intensity in the subarachnoid space on FLIR images in a patient with moyamoya disease also was termed the ivy sign. Retrograde slow flow of engorged pial arteries through leptomeningeal anastomoses has been proposed as the most likely mechanism of this finding [25] (Fig. 9). Sturge- Weber syndrome can have a similar appearance but in a more limited distribution. Elevated lood Pool to CSF Ratio Taoka et al. [26] postulated that an increased blood pool to CSF ratio within a sulcus can be a cause of sulcal hyperintensity on FLIR images. The relatively small volume and oxygenation status of the blood in vascular structures within the sulcus normally has a minimal effect on the local magnetic field and signal intensity of CSF protons. The blood pool effect can theoretically increase when the relative volume ratio of CSF to blood in a voxel is decreased. Taoka et al. suggested that this phenomenon may explain the hyperintensity of the subarachnoid space on FLIR images in instances of reduced CSF space or relative increase in intravascular volume, such as that caused by mass effect due to hydrocephalus or vascular disease such as cortical vein or dural sinus thrombosis. Taoka et al. also suggested that assessment of the FLIR distribution of sulcal hyperintensity may provide a clue to causation, vascular causes being more likely to be associated with a diffuse sulcal FLIR hyperintensity and the mass effect being more likely to be focal sulcal FLIR hyperintensity. In their series, however, Taoka et al. found the distinction not statistically significant. Contrast Media Mamourian et al. [27] found that IV contrast material can cause sulcal FLIR hyperintensity in healthy dogs and that this effect was most marked on triple-dose delayed imaging. In three patients with renal impairment, Lev and Schaefer [28] found diffuse CSF FLIR hyperintensity due to delayed leakage of gadolinium into the subarachnoid space, mimicking the appearance of SH and other pathologic conditions. In a larger series of 33 patients, ozzao et al. [29] concluded that when FLIR images were acquired 2 24 hours after IV administration of gadolinium contrast material to patients with pathologic conditions characterized by an altered blood brain barrier or neovascularization near the subarachnoid space or ventricles (e.g., stroke, neoplasm, and surgery), CSF signal change is likely and should not be confused with hemorrhage. rtifact-related Causes of Hyperintensity Supplemental Oxygen bnormalities in CSF signal intensity on FLIR images have been found in patients undergoing MRI examinations while receiving supplemental oxygen [30, 31]. n approximately 4- to 5.3-fold increase in signal intensity with 100% supplemental oxygen has been found [32]. Other studies have shown these effects with 100% oxygen but no increase in signal intensity with 50% oxygen mixtures [33]. The weakly paramagnetic effect of supplemental oxygen results in reduction of CSF T1-weighted relaxation time and subsequent high signal intensity on FLIR images [32] (Fig. 10). CSF Pulsation CSF flow artifact can produce artifactual FLIR hyperintensity in the subarachnoid space. Intense CSF pulsation results in inflow of CSF and thus protons, which have potentially not undergone the inversion pulse, into the imaging plane. Such artifacts tend to occur in the basal, prepontine, and cerebellopontine angle cisterns (Fig. 11) and in sections containing foramina of the ventricular system. These artifacts are less common and less intense over the convexities of the cerebral hemispheres, where CSF flow is diminished [34]. Mechanisms such as k-space reordering by inversion time at each slice position, tailored radiofrequency pulses, increasing the number of interleaving acquisitions, and adiabatic inversion pulses have been used to reduce or eliminate CSF pulsation artifacts [35 38]. Vascular Pulsation Vascular pulsation, because the motion is potentially periodic as a function of k-space location, produces artifacts on FLIR images that reproduce the size, shape, and alignment of the responsible vessel along the phase-encoding direction of the image (Fig. 12). Such ghosting artifacts produced by periodic motion in which there is synchrony between the phase-encoding steps and the motion are commonly seen on MR images. These artifacts may display components with alternating high and low signal intensities and in rare instances can be mistaken for hyperintensity in the subarachnoid space on FLIR images. Similar phase-encoding ghosting artifacts from eyeball motion can project over the subarachnoid space (Fig. 13). Magnetic Susceptibility rtifact Magnetic susceptibility artifact can result in artifactual increased FLIR signal intensity in the subarachnoid space. t tissue interfaces where the magnitude of local magnetic field inhomogeneities is pronounced, incomplete nulling of CSF by the slice-selection inversion pulse can occur. Susceptibility artifacts and resultant FLIR subarachnoid hyperintensity most commonly occur in the presence of metal (Fig. 14), but more subtle local incomplete nulling of CSF can be caused by air in the paranasal sinuses and the temporal bones. Motion rtifact Marked head motion can simulate the appearance on FLIR images of pathologic changes in the subarachnoid space. Comparison of several slices usually confirms the marked head motion and inconsistency in FLIR sulcal hyperintensity (Fig. 15). This problem occurs because CSF in the imaging plane does not undergo the inversion pulse. References 1. Hajnal JV, ryant DJ, Kasuboski L, et al. Use of fluid attenuated inversion recovery (FLIR) pulse sequences in MRI of the brain. J Comput ssist Tomogr 1992; 16: tlas SW. MR imaging is highly sensitive for acute subarachnoid hemorrhage: not! Radiology 1993; 186: Noguchi K, Ogawa T, Seto H, et al. Subacute and chronic subarachnoid hemorrhage: diagnosis with fluid-attenuated inversion-recovery MR imaging. Radiology 1997; 203: Mohamed M, Heasly DC, Yagmurlu, Yousem DM. Fluid-attenuated inversion recovery MR imaging and subarachnoid hemorrhage: not a panacea. m J Neuroradiol 2004; 25: Noguchi K, Seto H, Kamisaki Y, Tomizawa G, Toyoshima S, Watanabe N. Comparison of fluid-attenuated inversion-recovery MR imaging with CT in a simulated model of acute subarachnoid hemorrhage. m J Neuroradiol 2000; 21: Woodcock RJ Jr, Short J, Do HM, Jensen ME, Kallmes DF. Imaging of acute subarachnoid hemorrhage with a fluid-attenuated inversion recovery sequence in an animal model: comparison with noncontrast-enhanced CT. m J Neuroradiol 2001; 22: Noguchi K, Ogawa T, Inugami, et al. cute subarachnoid hemorrhage: MR imaging with fluid-attenuated inversion recovery pulse sequences. Radi- JR:189, October

4 ology 1995; 196: akshi R, Kamran S, Kinkel PR, et al. Fluid-attenuated inversion-recovery MR imaging in acute and subacute cerebral intraventricular hemorrhage. m J Neuroradiol 1999; 20: Singer M, tlas SW, Drayer P. Subarachnoid space disease: diagnosis with fluid-attenuated inversion-recovery MR imaging and comparison with gadolinium-enhanced spin-echo MR imaging blinded reader study. Radiology 1998; 208: Melhem ER, Jara H, Eustace S. Fluid-attenuated inversion recovery MR imaging: identification of protein concentration thresholds for CSF hyperintensity. m J Neuroradiol 1997; 169: Kamran S, ener, lper D, akshi R. Role of fluid-attenuated inversion recovery in the diagnosis of meningitis: comparison with contrast-enhanced magnetic resonance imaging. J Comput ssist Tomogr 2004; 28: Mathews VP, Caldemeyer KS, Lowe MJ, Greenspan SL, Weber DM, Ulmer JL. rain: gadolinium-enhanced fast fluid-attenuated inversion-recovery MR imaging. Radiology 1999; 211: Splendiani, Puglielli E, De micis R, Necozione S, Masciocchi C, Gallucci M. Contrast-enhanced FLIR in the early diagnosis of infectious meningitis. Neuroradiology 2005; 47: Goo HW, Choi CG. Post-contrast FLIR MR imaging of the brain in children: normal and abnormal intracranial enhancement. Pediatr Radiol 2003; 33: Tsuchiya K, Katase S, Yoshino, Hachiya J. FLIR MR imaging for diagnosing intracranial meningeal carcinomatosis. JR 2001; 176: Singh SK, gris JM, Leeds NE, Ginsberg LE. Intracranial leptomeningeal metastases: comparison of depiction at FLIR and contrast-enhanced MR imaging. Radiology 2000; 217: Singh SK, Leeds NE, Ginsberg LE. MR imaging of leptomeningeal metastases: comparison of three sequences. m J Neuroradiol 2002; 23: Ercan N, Gultekin S, Celik H, Tali TE, Oner Y, Erbas G. Diagnostic value of contrast-enhanced fluidattenuated inversion recovery MR imaging of intracranial metastases. m J Neuroradiol 2004; 25: Pirini MG, Mascalchi M, Salvi F, et al. Primary diffuse meningeal melanomatosis: radiologic pathologic correlation. m J Neuroradiol 2003; 24: Hayashi M, Maeda M, Maji T, Matsubara T, Tsukahara H, Takeda K. Diffuse leptomeningeal hyperintensity on fluid-attenuated inversion recovery MR images in neurocutaneous melanosis. m J Neuroradiol 2004; 25: Kamran S, ates V, akshi R, Wright P, Kinkel W, Miletich R. Significance of hyperintense vessels on FLIR MRI in acute stroke. Neurology 2000; 55: Maeda M, Yamamoto T, Daimon S, Sakuma H, Takeda K. rterial hyperintensity on fast fluid-attenuated inversion recovery images: a subtle finding for hyperacute stroke undetected by diffusionweighted MR imaging. m J Neuroradiol 2001; 22: Toyoda K, Ida M, Fukuda K. Fluid-attenuated inversion recovery intraarterial signal: an early sign of hyperacute cerebral ischemia. m J Neuroradiol 2001; 22: Ohta T, Tanaka H, Kuroiwa T. Diffuse leptomeningeal enhancement, ivy sign, in magnetic resonance images of moyamoya disease in childhood: case report. Neurosurgery 1995; 37: Maeda M, Tsuchida C. Ivy sign on fluid-attenuated inversion-recovery images in childhood moyamoya disease. m J Neuroradiol 1999; 20: Taoka T, Yuh WT, White ML, Quets JP, Maley JE, Ueda T. Sulcal hyperintensity on fluid-attenuated inversion recovery MR images in patients without apparent cerebrospinal fluid abnormality. JR 2001; 176: Mamourian C, Hoopes PJ, Lewis LD. Visualization of intravenously administered contrast material in the CSF on fluid-attenuated inversion-recovery MR images: an in vitro and animal-model investigation. m J Neuroradiol 2000; 21: Lev MH, Schaefer PW. Subarachnoid gadolinium enhancement mimicking subarachnoid hemorrhage on FLIR MR images: fluid-attenuated inversion recovery. JR 1999; 173: ozzao, Floris R, Fasoli F, Fantozzi LM, Colonnese C, Simonetti G. Cerebrospinal fluid changes after intravenous injection of gadolinium chelate: assessment by FLIR MR imaging. Eur Radiol 2003; 13: Frigon C, Shaw DW, Heckbert SR, Weinberger E, Jardine DS. Supplemental oxygen causes increased signal intensity in subarachnoid cerebrospinal fluid on brain FLIR MR images obtained in children during general anesthesia. Radiology 2004; 233: Deliganis V, Fisher DJ, Lam M, Maravilla KR. Cerebrospinal fluid signal intensity increase on FLIR MR images in patients under general anesthesia: the role of supplemental O2. Radiology 2001; 218: nzai Y, Ishikawa M, Shaw DW, rtru, Yarnykh V, Maravilla KR. Paramagnetic effect of supplemental oxygen on CSF hyperintensity on fluid-attenuated inversion recovery MR images. m J Neuroradiol 2004; 25: raga FT, da Rocha J, Hernandez Filho G, rikawa RK, Ribeiro IM, Fonseca R. Relationship between the concentration of supplemental oxygen and signal intensity of CSF depicted by fluidattenuated inversion recovery imaging. m J Neuroradiol 2003; 24: Rydberg JN, Hammond C, Grimm RC, et al. Initial clinical experience in MR imaging of the brain with a fast fluid-attenuated inversion-recovery pulse sequence. Radiology 1994; 193: Wu HM, Yousem DM, Chung HW, Guo WY, Chang CY, Chen CY. Influence of imaging parameters on high-intensity cerebrospinal fluid artifacts in fast- FLIR MR imaging. m J Neuroradiol 2002; 23: Herlihy H, Hajnal JV, Curati WL, et al. Reduction of CSF and blood flow artifacts on FLIR images of the brain with k-space reordered by inversion time at each slice position (KRISP). m J Neuroradiol 2001; 22: Tanaka N, be T, Kojima K, Nishimura H, Hayabuchi N. pplicability and advantages of flow artifactinsensitive fluid-attenuated inversion-recovery MR sequences for imaging the posterior fossa. m J Neuroradiol 2000; 21: Hajnal JV, Oatridge, Herlihy H, ydder GM. Reduction of CSF artifacts on FLIR images by using adiabatic inversion pulses. m J Neuroradiol 2001; 22: JR:189, October 2007

5 MRI of Subarachnoid Space Fig year-old man 3 days after traumatic subarachnoid hemorrhage., xial FLIR MR image shows posttraumatic subarachnoid hemorrhage (arrows) overlying temporal lobes., CT scan corresponding to shows subarachnoid hemorrhage overlying temporal lobe is more difficult to appreciate owing to beam-hardening artifact (arrows) from adjacent calvarium. C Fig year-old man with meningitis., xial T1-weighted contrast-enhanced MR image shows prominent leptomeningeal and vascular enhancement over both cerebral hemispheres., Unenhanced axial FLIR MR image corresponding to shows subtle areas of abnormal hyperintensity (arrows) in subarachnoid space. C, Contrast-enhanced FLIR MR image shows intense diffuse leptomeningeal enhancement. Some vessels enhanced in are not enhanced. JR:189, October

6 Fig year-old man with leptomeningeal metastasis of medulloblastoma., xial contrast-enhanced T1-weighted MR image shows widespread leptomeningeal metastatic lesions (arrows) involving surfaces of cerebellum and cerebral hemispheres., xial FLIR MR image shows mild subarachnoid hyperintensity and nodularity over cerebellar and cerebral surfaces in keeping with presence of widespread leptomeningeal metastatic lesions (arrows). Fig year-old man with non-hodgkin s lymphoma., xial contrast-enhanced MR image obtained with FLIR sequence shows bilateral intense enhancement (arrow) of internal auditory canals and adjacent seventh cranial nerves within temporal bone secondary to lymphomatous infiltration., Contrast-enhanced axial T1-weighted MR image corresponding to shows less conspicuous meningeal enhancement (arrow) of internal auditory canals. Fig year-old woman with neurocutaneous melanosis and melanocytic tumor of leptomeninges. (Courtesy of razier D, Sydney, ustralia), FLIR MR image shows extensive sulcal hyperintensity., Contrast-enhanced T1-weighted MR image shows extensive conspicuous sulcal hyperintensity and enhancement. 918 JR:189, October 2007

7 MRI of Subarachnoid Space Fig year-old man with old right posterior circulation infarct., xial T1-weighted MR image shows small focus of hyperintensity (arrow) due to lipoma at right cerebellopontine angle., xial FLIR MR image corresponding to shows lipoma (arrow) at right cerebellopontine angle. C Fig year-old woman with recurrent headache after resection of ruptured dermoid., Preoperative axial T1-weighted MR image shows left parasellar T1-weighted hyperintense dermoid (arrow)., xial T1-weighted MR image shows high-signal-intensity fat droplets (arrows) in subarachnoid space in keeping with dermoid rupture. C, xial FLIR MR image shows focal subtle hyperintense fat droplets (arrows) in subarachnoid space. JR:189, October

8 C Fig year-old woman with right cerebral infarct., xial FLIR MR image obtained soon after onset of neurologic symptoms shows hyperintensity (arrow) due to thrombotic occlusion or slow flow in right middle cerebral artery., Time-of-flight MR angiogram shows occlusion of M1 segment of right middle cerebral artery. C, xial diffusion-weighted MR image shows acute right lenticulostriate infarct (arrow) in perforator territory but no established infarct in rest of right middle cerebral artery territory. Perfusion imaging was not performed. Further ischemia-related diffusion abnormality is evident in left periventricular white matter. Fig year-old woman with moyamoya disease. xial FLIR MR image shows vascular hyperintensity (arrows) in subarachnoid space most likely due to slow or retrograde flow. Fig year-old intubated man with meningioma undergoing follow-up imaging. xial FLIR MR image shows diffuse hyperintensity (arrows) in subarachnoid space thought to be caused by supplemental oxygen. Fig year-old man with left sensorineural hearing loss. xial FLIR MR image shows focal increased signal intensity (arrow) due to CSF flow artifact in right aspect of prepontine cistern. 920 JR:189, October 2007

9 MRI of Subarachnoid Space Fig year-old man with right sensorineural hearing loss. xial FLIR MR image shows vascular pulsation artifact (arrow) from left transverse sinus that can be mistaken for hyperintensity in subarachnoid space. Phase-encoding direction is anteroposterior, whereas in most FLIR examinations phase encoding is left to right. Fig year-old man after resection of cavernous hemangioma., FLIR MR image shows magnetic susceptibility artifact due to metallic clips (arrow) from craniotomy performed 2 years earlier., FLIR MR image shows subtle artifactual hyperintensity (arrow) in subarachnoid space overlying superior aspect of right temporal lobe. Fig year-old woman with left sensorineural hearing loss. xial FLIR MR image shows hyperintensity (arrow) close to subarachnoid space (in phase-encoding direction) due to ghosting artifact from eye movement. Phase-encoding direction is anteroposterior, whereas in most FLIR examinations phase encoding is left to right. Fig year-old woman with delirium. FLIR MR image shows area of artifactual hyperintensity (arrow) in subarachnoid space due to marked head motion. JR:189, October