(Radiology. 2000;214:671-677.)
© RSNA, 2000
MR Imaging and Histologic Features of Subinsular Bright Spots on T2-weighted MR Images: Virchow-Robin Spaces of the Extreme Capsule and Insular Cortex1
Chang June Song, MD 2,
Jung H. Kim, MD,
E. Leon Kier, MD and
Richard A. Bronen, MD
1 From the Departments of Diagnostic Radiology, Section of Neuroradiology (C.J.S., E.L.K., R.A.B.), and Pathology, Section of Neuropathology (J.H.K.), Yale University School of Medicine, PO Box 208042, New Haven, CT 06520-8042. Received November 19, 1998; revision requested January 14, 1999; revision received June 17; accepted July 30. Address reprint requests to R.A.B. (e-mail: richard.bronen@yale.edu).
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Abstract
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PURPOSE: To determine the cause and frequency of high-signal-intensity foci detected in the insular cortex and extreme capsule on thin-section, high-spatial-resolution, coronal, T2-weighted magnetic resonance (MR) images.
MATERIALS AND METHODS: The authors assessed high-signal-intensity areas in the insular cortex and extreme capsule on coronal MR images obtained in 56 patients with seizure and five control subjects. Images were obtained with thin-section, high-spatial-resolution, T2-weighted, fast spin-echo; three-dimensional, spoiled gradient-recalled-echo; and fluid-attenuated inversion-recovery sequences. In two formalin-fixed brain specimens, MR imaging findings were correlated with gross anatomic and histologic findings.
RESULTS: Subinsular bright spots were found in 53 of the 56 (95%) patients (96 of 112 [86%] hemispheres) and all five control subjects. The spots were elliptical in 30 patients, round in 14 patients, linear in 22 patients, and dotlike in seven patients and often had a featherlike configuration. The spots were isointense to cerebrospinal fluid on T2-weighted, fast SE images and were located in the anterior extreme capsule white matter and insular cortex. MR imaging of brain specimens revealed bilateral elliptical areas of high signal intensity that corresponded to small multiple cavities at gross anatomic inspection. At microscopic examination, these cavities were perivascular spaces of mostly arteriolar origin.
CONCLUSION: High-signal-intensity subinsular foci at MR imaging are due to enlarged perivascular spaces. In most cases, these foci can be visualized on thin-section, high-spatial-resolution, coronal T2-weighted images; they should not be mistaken for pathologic conditions when they occur unilaterally.
Index terms: Brain, anatomy, 13.92 • Brain, MR, 13.121411, 13.121412, 13.121413
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Introduction
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Recently, we began obtaining high-spatial-resolution, 3-mm-thick, coronal, T2-weighted magnetic resonance (MR) images to evaluate patients with seizure and encountered a unilateral, small, high-signal-intensity change in the region of the insular cortex and subcortical white matter (extreme capsule). Although clinical symptoms were inconsistent with the lesion as the source of the seizures, it was thought that this abnormality may be related to postictal MR changes. Although we did not believe the abnormality represented a neoplasm, we were still concerned about this possibility because small cortical neoplasms are known to cause epilepsy. It was soon discovered, however, that areas of extreme high signal intensity in the capsule were frequently visualized on thin-section, coronal, T2-weighted images. The configuration, signal intensity, and constant location in the anteroinferior subinsular area, as well as the lack of correlation with clinical symptoms, led us to suspect that these were Virchow-Robin or perivascular spaces that were not previously recognized on thicker-section MR images.
Because of the possibility of confusing this signal intensity change with postictal MR changes or changes due to neoplasm, we investigated the nature of the high signal intensity in the insula and extreme capsule. Herein, we use the term "subinsular bright spots" to refer to areas of high signal intensity found both within the insular cortex and in the extreme capsule white matter.
The aim of this study was to (a) report the prevalence and MR imaging characteristics of the subinsular high-signal-intensity foci and (b) determine the cause of these abnormalities by correlating MR imaging findings of brain specimens with those from gross anatomic inspection and histologic examination.
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MATERIALS AND METHODS
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We retrospectively assessed MR images obtained in 56 consecutive patients with medically intractable epilepsy during a 7-month period. Patients younger than 2 years had been excluded because of the difficulty of distinguishing high-signal-intensity, unmyelinated white matter from subinsular bright spots on T2-weighted images. In addition, we prospectively evaluated five control subjects with no evidence of a neurologic disorder. Consent was obtained from the control subjects in accordance with institutional review board requirements.
All patients underwent coronal, T2-weighted, fast spin-echo (SE); three-dimensional (3D), fast spoiled gradient-recalled-echo (SPGR); and fluid-attenuated inversion-recovery (FLAIR) imaging by using a 1.5-T magnet. Imaging parameters for the T2-weighted, fast SE sequence were as follows: repetition time (TR) of 2,400–4,900 msec, echo time of 104–120 msec (2,400–4,900/104–120), three or four signals acquired, 256 x 256 matrix, 18-cm field of view, 16-kHz bandwidth, and 3-mm-thick sections with a 0.3-mm gap.
Fast, 3D, SPGR imaging was performed with the following parameters: 17/3 (minimum TR/minimum echo time), one signal acquired, 256 x 192 matrix, 22-cm field of view, 7.8-kHz bandwidth, and 1.5-mm-thick sections without a gap.
FLAIR imaging was performed with the following parameters: 1,002/148/2,200 (TR msec/echo time msec/inversion time msec), one signal acquired, 256 x 192 matrix, 22-cm field of view, 32-kHz bandwidth, and 4–5-mm-thick sections with a 1.5-mm gap.
The five control subjects underwent imaging by using a separate 1.5-T magnet without the capability of performing fast 3D SPGR or FLAIR sequences. The control subjects underwent T2-weighted, fast SE imaging with the same parameters as those used for the patients.
The insular cortex and extreme capsule were assessed by means of consensus interpretation by two experienced neuroradiologists (C.J.S. and R.A.B. for patients and R.A.B. and E.L.K. for control subjects) to evaluate the frequency, bilaterality, signal intensity, shape, size, and location of the subinsular bright spots on T2-weighted, fast SE; 3D fast SPGR; and FLAIR images. To help distinguish subinsular bright spots from partial volume averaging of sylvian fissure sulci, the high signal intensity on long-TR images had to involve a portion of the extreme capsule and could not be contiguous with adjacent sulci. The shape of the subinsular bright spot was categorized as elliptical, round, linear, or dotlike (Fig 1). The signal intensity of the subinsular bright spot was compared on T2-weighted, fast SE; fast, 3D, SPGR; and FLAIR images and was contrasted with that of Virchow-Robin spaces in the (a) white matter of cerebral convexities (superolateral surfaces of cerebrum) and (b) anterior perforated substance.
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Figure 1a. Thin-section, T2-weighted, fast SE, coronal MR images (4,000/112, three signals acquired) obtained in four patients illustrate the four configurations of subinsular bright spots 0.3-1.3 cm anterior to the anterior commissure. S = sylvian fissure. (a) Elliptical configuration in a 53-year-old woman. Image shows an elliptical right subinsular bright spot (arrow) involving the deep insular cortex and extreme capsule. The feather configuration is suggestive of vascular structures coursing through this area and helps distinguish this entity from a pathologic condition. (b) Round configuration in a 60-year-old woman. Image shows a round left subinsular bright spot (straight arrow). This unilateral abnormality could be difficult to distinguish from a pathologic condition. The linear area of high signal intensity (curved arrow) extending through the bright spot, however, helps establish the linear and round subinsular bright spots as perivascular spaces. (c) Linear configuration in a 35-year-old woman. Image shows two high-signal-intensity linear foci (arrow) in the left subinsular area. (d) Dotlike configuration in a 17-year-old boy. Image shows two dotlike bright foci (arrow) in the left subinsular area.
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Figure 1b. Thin-section, T2-weighted, fast SE, coronal MR images (4,000/112, three signals acquired) obtained in four patients illustrate the four configurations of subinsular bright spots 0.3-1.3 cm anterior to the anterior commissure. S = sylvian fissure. (a) Elliptical configuration in a 53-year-old woman. Image shows an elliptical right subinsular bright spot (arrow) involving the deep insular cortex and extreme capsule. The feather configuration is suggestive of vascular structures coursing through this area and helps distinguish this entity from a pathologic condition. (b) Round configuration in a 60-year-old woman. Image shows a round left subinsular bright spot (straight arrow). This unilateral abnormality could be difficult to distinguish from a pathologic condition. The linear area of high signal intensity (curved arrow) extending through the bright spot, however, helps establish the linear and round subinsular bright spots as perivascular spaces. (c) Linear configuration in a 35-year-old woman. Image shows two high-signal-intensity linear foci (arrow) in the left subinsular area. (d) Dotlike configuration in a 17-year-old boy. Image shows two dotlike bright foci (arrow) in the left subinsular area.
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Figure 1c. Thin-section, T2-weighted, fast SE, coronal MR images (4,000/112, three signals acquired) obtained in four patients illustrate the four configurations of subinsular bright spots 0.3-1.3 cm anterior to the anterior commissure. S = sylvian fissure. (a) Elliptical configuration in a 53-year-old woman. Image shows an elliptical right subinsular bright spot (arrow) involving the deep insular cortex and extreme capsule. The feather configuration is suggestive of vascular structures coursing through this area and helps distinguish this entity from a pathologic condition. (b) Round configuration in a 60-year-old woman. Image shows a round left subinsular bright spot (straight arrow). This unilateral abnormality could be difficult to distinguish from a pathologic condition. The linear area of high signal intensity (curved arrow) extending through the bright spot, however, helps establish the linear and round subinsular bright spots as perivascular spaces. (c) Linear configuration in a 35-year-old woman. Image shows two high-signal-intensity linear foci (arrow) in the left subinsular area. (d) Dotlike configuration in a 17-year-old boy. Image shows two dotlike bright foci (arrow) in the left subinsular area.
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Figure 1d. Thin-section, T2-weighted, fast SE, coronal MR images (4,000/112, three signals acquired) obtained in four patients illustrate the four configurations of subinsular bright spots 0.3-1.3 cm anterior to the anterior commissure. S = sylvian fissure. (a) Elliptical configuration in a 53-year-old woman. Image shows an elliptical right subinsular bright spot (arrow) involving the deep insular cortex and extreme capsule. The feather configuration is suggestive of vascular structures coursing through this area and helps distinguish this entity from a pathologic condition. (b) Round configuration in a 60-year-old woman. Image shows a round left subinsular bright spot (straight arrow). This unilateral abnormality could be difficult to distinguish from a pathologic condition. The linear area of high signal intensity (curved arrow) extending through the bright spot, however, helps establish the linear and round subinsular bright spots as perivascular spaces. (c) Linear configuration in a 35-year-old woman. Image shows two high-signal-intensity linear foci (arrow) in the left subinsular area. (d) Dotlike configuration in a 17-year-old boy. Image shows two dotlike bright foci (arrow) in the left subinsular area.
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To determine the rostral-caudal location of the subinsular bright foci, we used the anterior commissure as a landmark, estimating the distance on the 3-mm-thick, T2-weighted, fast SE coronal sections obtained with a 0.3-mm gap.
To determine the cause and exact location of the subinsular bright spots, MR imaging–histologic correlation was performed in two autopsy specimens. Coronal sections of brains from an 86-year-old man and a 35-year-old man, both of whom died of nonneurologic causes, were examined after 2–3-week fixation in 10% formalin. The specimen sections containing or adjacent to the anterior commissure were examined. The sections, which were approximately 1 cm thick, were placed in a plastic container filled with water. Care was taken to remove any air bubbles.
The brain specimens were imaged with T2-weighted, fast SE and fast, 3D, SPGR sequences in a quadrature head coil with a 1.5-T magnet by using protocols similar to those described earlier. T2-weighted, fast SE parameters were as follows: 4,000/104–120, six signals acquired, 256 x 256 matrix, 16-cm field of view, 16-kHz bandwidth, and 3-mm-thick contiguous sections. The following parameters were used for the fast, 3D, SPGR sequence: 23/5, one signal acquired, 256 x 192 matrix, 18-cm field of view, 16-kHz bandwidth, and 1.5-mm-thick sections without a gap.
The insular cortex and extreme capsule were assessed (by C.J.S. and R.A.B.) to evaluate the absence or presence of altered signal intensity on T2-weighted, fast SE and fast, 3D, SPGR images and the configuration of these abnormalities.
Specimens were evaluated by means of gross and microscopic examination (by J.H.K.). Microscopic analysis was performed from the region of the insula and extreme capsule on 5-µm-thick embedded paraffin tissue stained with hematoxylin-eosin. Luxol fast blue stain was used to stain myelin. van Gieson stain was used to stain for the internal elastic lamina of arterioles (to distinguish arterioles from venules).
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RESULTS
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Demographics
Patients ranged in age from 2 to 70 years (mean age ± SD, 20.9 years ± 16.3). There were 35 patients aged 2–20 years, 14 patients aged 21–40 years, six patients aged 41–60 years, and one patient aged 70 years. There were 30 female and 26 male patients. The control subjects ranged in age from 16 to 28 years (mean age ± SD, 21.0 years ± 5.1). Two control subjects were 16 years old, and three were 22–28 years of age. There were three male and two female control subjects.
Anatomy and Histology
The two brain specimens showed bilateral elliptical areas of high-signal-intensity foci in anterior aspects of the extreme capsule and insular cortex on T2-weighted, fast SE MR images (Fig 2). These foci were isointense to brain tissue at SPGR imaging and could not be visualized. Coronal gross brain specimens showed collections of numerous small holes that were linear or dotlike in the anteroinferior aspect of the deep (medial) cortex of the insula and subinsular extreme capsule.
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Figure 2a. Coronal (a) gross, (b, c) MR imaging, and (d-f) histologic sections of a formalin-fixed brain from an 86-year-old man who died of nonneurologic causes. (a) Magnified photograph of the right hemisphere. The gross specimen shows numerous linear clefts or dotlike small cavities (arrow) within the anterior aspect of the deep cortex of insula (adjacent to white matter) and subinsular white matter. (b) T2-weighted, fast SE (4,000/108, six signals acquired) and (c) 3D, SPGR (23/5, one signal acquired) images show bilateral subinsular bright spots (arrows) with featherlike features. The spots are hyperintense to brain in b and iso- to hypointense to brain in c. The featherlike features are due to partial volume averaging of the large collections of perivascular spaces demonstrated in a. (d) Photomicrograph of a coronal histologic slice through the anterior insula seen in a shows numerous linear vascular structures surrounded by enlarged perivascular spaces (arrowheads and straight arrows). Luxol fast blue stain demonstrates the gray matter-white matter junction (curved arrows) by staining white matter myelin and thus helps confirm that the enlarged perivascular spaces are situated in both the deep insular cortex (arrowheads) and the subinsular white matter (straight arrows). G = gray matter, W = white matter. (Luxol fast blue stain; original magnification, x18; section obtained from region indicated by arrow in a through the middle of the slice section in a.) (e) Photomicrograph with higher magnification demonstrates the size disparity between the small vessels (arrowhead) and the markedly enlarged perivascular spaces (arrow) that surround them. (Luxol fast blue stain; original magnification, x44.) (f) Photomicrograph shows a large perivascular space (curved arrow) surrounding several small vascular structures. The internal elastic lamina (arrow) is stained with elastic van Gieson stain, which helps confirm that this is an arteriole. (Elastic van Gieson stain; original magnification, x220.)
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Figure 2b. Coronal (a) gross, (b, c) MR imaging, and (d-f) histologic sections of a formalin-fixed brain from an 86-year-old man who died of nonneurologic causes. (a) Magnified photograph of the right hemisphere. The gross specimen shows numerous linear clefts or dotlike small cavities (arrow) within the anterior aspect of the deep cortex of insula (adjacent to white matter) and subinsular white matter. (b) T2-weighted, fast SE (4,000/108, six signals acquired) and (c) 3D, SPGR (23/5, one signal acquired) images show bilateral subinsular bright spots (arrows) with featherlike features. The spots are hyperintense to brain in b and iso- to hypointense to brain in c. The featherlike features are due to partial volume averaging of the large collections of perivascular spaces demonstrated in a. (d) Photomicrograph of a coronal histologic slice through the anterior insula seen in a shows numerous linear vascular structures surrounded by enlarged perivascular spaces (arrowheads and straight arrows). Luxol fast blue stain demonstrates the gray matter-white matter junction (curved arrows) by staining white matter myelin and thus helps confirm that the enlarged perivascular spaces are situated in both the deep insular cortex (arrowheads) and the subinsular white matter (straight arrows). G = gray matter, W = white matter. (Luxol fast blue stain; original magnification, x18; section obtained from region indicated by arrow in a through the middle of the slice section in a.) (e) Photomicrograph with higher magnification demonstrates the size disparity between the small vessels (arrowhead) and the markedly enlarged perivascular spaces (arrow) that surround them. (Luxol fast blue stain; original magnification, x44.) (f) Photomicrograph shows a large perivascular space (curved arrow) surrounding several small vascular structures. The internal elastic lamina (arrow) is stained with elastic van Gieson stain, which helps confirm that this is an arteriole. (Elastic van Gieson stain; original magnification, x220.)
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Figure 2c. Coronal (a) gross, (b, c) MR imaging, and (d-f) histologic sections of a formalin-fixed brain from an 86-year-old man who died of nonneurologic causes. (a) Magnified photograph of the right hemisphere. The gross specimen shows numerous linear clefts or dotlike small cavities (arrow) within the anterior aspect of the deep cortex of insula (adjacent to white matter) and subinsular white matter. (b) T2-weighted, fast SE (4,000/108, six signals acquired) and (c) 3D, SPGR (23/5, one signal acquired) images show bilateral subinsular bright spots (arrows) with featherlike features. The spots are hyperintense to brain in b and iso- to hypointense to brain in c. The featherlike features are due to partial volume averaging of the large collections of perivascular spaces demonstrated in a. (d) Photomicrograph of a coronal histologic slice through the anterior insula seen in a shows numerous linear vascular structures surrounded by enlarged perivascular spaces (arrowheads and straight arrows). Luxol fast blue stain demonstrates the gray matter-white matter junction (curved arrows) by staining white matter myelin and thus helps confirm that the enlarged perivascular spaces are situated in both the deep insular cortex (arrowheads) and the subinsular white matter (straight arrows). G = gray matter, W = white matter. (Luxol fast blue stain; original magnification, x18; section obtained from region indicated by arrow in a through the middle of the slice section in a.) (e) Photomicrograph with higher magnification demonstrates the size disparity between the small vessels (arrowhead) and the markedly enlarged perivascular spaces (arrow) that surround them. (Luxol fast blue stain; original magnification, x44.) (f) Photomicrograph shows a large perivascular space (curved arrow) surrounding several small vascular structures. The internal elastic lamina (arrow) is stained with elastic van Gieson stain, which helps confirm that this is an arteriole. (Elastic van Gieson stain; original magnification, x220.)
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Figure 2d. Coronal (a) gross, (b, c) MR imaging, and (d-f) histologic sections of a formalin-fixed brain from an 86-year-old man who died of nonneurologic causes. (a) Magnified photograph of the right hemisphere. The gross specimen shows numerous linear clefts or dotlike small cavities (arrow) within the anterior aspect of the deep cortex of insula (adjacent to white matter) and subinsular white matter. (b) T2-weighted, fast SE (4,000/108, six signals acquired) and (c) 3D, SPGR (23/5, one signal acquired) images show bilateral subinsular bright spots (arrows) with featherlike features. The spots are hyperintense to brain in b and iso- to hypointense to brain in c. The featherlike features are due to partial volume averaging of the large collections of perivascular spaces demonstrated in a. (d) Photomicrograph of a coronal histologic slice through the anterior insula seen in a shows numerous linear vascular structures surrounded by enlarged perivascular spaces (arrowheads and straight arrows). Luxol fast blue stain demonstrates the gray matter-white matter junction (curved arrows) by staining white matter myelin and thus helps confirm that the enlarged perivascular spaces are situated in both the deep insular cortex (arrowheads) and the subinsular white matter (straight arrows). G = gray matter, W = white matter. (Luxol fast blue stain; original magnification, x18; section obtained from region indicated by arrow in a through the middle of the slice section in a.) (e) Photomicrograph with higher magnification demonstrates the size disparity between the small vessels (arrowhead) and the markedly enlarged perivascular spaces (arrow) that surround them. (Luxol fast blue stain; original magnification, x44.) (f) Photomicrograph shows a large perivascular space (curved arrow) surrounding several small vascular structures. The internal elastic lamina (arrow) is stained with elastic van Gieson stain, which helps confirm that this is an arteriole. (Elastic van Gieson stain; original magnification, x220.)
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Figure 2e. Coronal (a) gross, (b, c) MR imaging, and (d-f) histologic sections of a formalin-fixed brain from an 86-year-old man who died of nonneurologic causes. (a) Magnified photograph of the right hemisphere. The gross specimen shows numerous linear clefts or dotlike small cavities (arrow) within the anterior aspect of the deep cortex of insula (adjacent to white matter) and subinsular white matter. (b) T2-weighted, fast SE (4,000/108, six signals acquired) and (c) 3D, SPGR (23/5, one signal acquired) images show bilateral subinsular bright spots (arrows) with featherlike features. The spots are hyperintense to brain in b and iso- to hypointense to brain in c. The featherlike features are due to partial volume averaging of the large collections of perivascular spaces demonstrated in a. (d) Photomicrograph of a coronal histologic slice through the anterior insula seen in a shows numerous linear vascular structures surrounded by enlarged perivascular spaces (arrowheads and straight arrows). Luxol fast blue stain demonstrates the gray matter-white matter junction (curved arrows) by staining white matter myelin and thus helps confirm that the enlarged perivascular spaces are situated in both the deep insular cortex (arrowheads) and the subinsular white matter (straight arrows). G = gray matter, W = white matter. (Luxol fast blue stain; original magnification, x18; section obtained from region indicated by arrow in a through the middle of the slice section in a.) (e) Photomicrograph with higher magnification demonstrates the size disparity between the small vessels (arrowhead) and the markedly enlarged perivascular spaces (arrow) that surround them. (Luxol fast blue stain; original magnification, x44.) (f) Photomicrograph shows a large perivascular space (curved arrow) surrounding several small vascular structures. The internal elastic lamina (arrow) is stained with elastic van Gieson stain, which helps confirm that this is an arteriole. (Elastic van Gieson stain; original magnification, x220.)
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Figure 2f. Coronal (a) gross, (b, c) MR imaging, and (d-f) histologic sections of a formalin-fixed brain from an 86-year-old man who died of nonneurologic causes. (a) Magnified photograph of the right hemisphere. The gross specimen shows numerous linear clefts or dotlike small cavities (arrow) within the anterior aspect of the deep cortex of insula (adjacent to white matter) and subinsular white matter. (b) T2-weighted, fast SE (4,000/108, six signals acquired) and (c) 3D, SPGR (23/5, one signal acquired) images show bilateral subinsular bright spots (arrows) with featherlike features. The spots are hyperintense to brain in b and iso- to hypointense to brain in c. The featherlike features are due to partial volume averaging of the large collections of perivascular spaces demonstrated in a. (d) Photomicrograph of a coronal histologic slice through the anterior insula seen in a shows numerous linear vascular structures surrounded by enlarged perivascular spaces (arrowheads and straight arrows). Luxol fast blue stain demonstrates the gray matter-white matter junction (curved arrows) by staining white matter myelin and thus helps confirm that the enlarged perivascular spaces are situated in both the deep insular cortex (arrowheads) and the subinsular white matter (straight arrows). G = gray matter, W = white matter. (Luxol fast blue stain; original magnification, x18; section obtained from region indicated by arrow in a through the middle of the slice section in a.) (e) Photomicrograph with higher magnification demonstrates the size disparity between the small vessels (arrowhead) and the markedly enlarged perivascular spaces (arrow) that surround them. (Luxol fast blue stain; original magnification, x44.) (f) Photomicrograph shows a large perivascular space (curved arrow) surrounding several small vascular structures. The internal elastic lamina (arrow) is stained with elastic van Gieson stain, which helps confirm that this is an arteriole. (Elastic van Gieson stain; original magnification, x220.)
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The vascular anatomy of the middle cerebral artery was also examined. Branches of the M2 division of the middle cerebral artery were seen penetrating into the insula, and these penetrations were more numerous in the anterior aspect of the insula than in the posterior aspect. Results of microscopic analysis helped confirm that these collections of holes were perivascular spaces located within the subinsular white matter of the extreme capsule and deep cortex of insula (Fig 2). An abundance of vascular structures (and their surrounding perivascular spaces) traversed the anterior aspect of the extreme capsule; the more posterior regions of the extreme capsule, on the other hand, had a relatively decreased amount of vascularity. Most of these vessels in the anterior regions of the deep insular cortex and extreme capsule were small arterioles rather than small veins, as determined with elastic van Gieson stain.
MR Imaging Characteristics
The subinsular bright spots occurred in 53 of 56 (95%) patients (96 of 112 [86%] hemispheres) and were bilateral in 43 (77%) (Table). The subinsular bright spots were often multiple, with 84 occurring in the right hemisphere and 77 in the left hemisphere, for a total of 161 in the 53 patients. The prevalence of high-signal-intensity foci did not differ substantially according to age and occurred in 59 of 70 (84%) hemispheres of patients aged 20 years or younger, 25 of 28 (89%) hemispheres in patients aged 21–40 years, 10 of 12 (83%) hemispheres in patients aged 41–60 years, and two of two (100%) hemispheres in the patient older than 60 years. The subinsular bright spots were found in eight of 10 hemispheres in all five control subjects (ie, bilateral involvement occurred in three subjects, and unilateral involvement occurred in two subjects).
We compared the prevalence of perivascular spaces in the insula and extreme capsule region with the prevalence of Virchow-Robin spaces in other locations. All patients and control subjects had numerous high-signal-intensity foci due to Virchow-Robin spaces on thin-section, T2-weighted, coronal images in cerebral convexity white matter and anterior perforated substance or basal ganglia regions.
The subinsular bright spot was categorized as one of four shapes: elliptical, round, linear, or dotlike. The most common shape was linear, occurring in 76 (47%) of 161 subinsular bright spots in patients. When this configuration was present, multiple linear shapes frequently occurred together in the same patient; thus, this configuration was found in only a minority (39%) of patients (but frequently in that small group). Most patients (54%) had elliptical abnormalities (Table).
In terms of distinguishing the subinsular bright spot from pathologic conditions on T2-weighted, fast SE images, the most troubling shapes were the elliptical and round configurations (occurring in 66% of patients), especially when these particular shapes were unilateral in manifestation. In 20% (11 of 56 patients) of patients, unilateral elliptical or round areas of high signal intensity were present on T2-weighted, fast SE images. Because linear areas of high signal intensity could often be visualized emanating from these round to elliptical high-signal-intensity abnormalities, however, the correct diagnosis of perivascular spaces was usually apparent because of the featherlike appearance (Fig 1).
In the five control subjects, the configurations were elliptical in two, round in one, linear in one, and dotlike in one (in terms of hemispheres, the configurations were elliptical in three of 10, round in two of 10, linear in one of 10, and dotlike in two of 10).
For the patients, the mean craniocaudal, anteroposterior, and right-left dimensions of elliptical abnormalities were 0.99 cm (range, 0.3–1.5 cm), 0.48 cm (range, 0.1–1.5 cm), and 0.28 cm (range, 0.1–0.5 cm), respectively. The mean diameter of round abnormalities was 0.47 cm (range, 0.2–0.9 cm). The mean length of linear abnormalities was 0.68 cm (range, 0.2–1.5 cm).
For control subjects, the mean craniocaudal, anteroposterior, and right-left dimensions of the elliptical abnormalities were 0.8 cm (range, 0.3–1.1 cm), 0.3 cm (range, 0.1–0.6 cm), and 0.4 cm (range, 0.3–0.5 cm), respectively. The mean diameter of the round abnormalities (for both hemispheres) was 0.3 cm. The mean length of the linear abnormality was 0.4 cm.
The signal intensity of the subinsular bright spot was evaluated on thin-section, long-TR, T2-weighted, fast SE images; 3D, fast, SPGR images; and FLAIR images. Most bright spots (80% [45 of 56]) on T2-weighted images were isointense to the cerebral convexity Virchow-Robin spaces. The remaining 20% (11 of 56) were slightly hypointense to convexity Virchow-Robin spaces. The subinsular bright spots were isointense to slightly hyperintense to Virchow-Robin spaces in the anterior perforated substance (and the anterior perforated substance spaces were slightly hypointense to the convexity Virchow-Robin spaces but markedly hyperintense to brain tissue).
On FLAIR images, these bright spots were not detectable because they were isointense to the insular cortex or underlying white matter. They were also difficult to visualize on SPGR images because they were slightly hypointense to isointense to brain tissue (Fig 3).
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Figure 3a. Comparison of (a) T2-weighted, fast SE; (b) 3D, fast, SPGR; and (c) FLAIR coronal images. (a) T2-weighted, fast SE image (4,000/112, three signals acquired) in a 6-year-old girl shows an elliptical area of high signal intensity (arrow) in the right subinsular extreme capsule. (b) Fast, 3D, SPGR image (17/3, one signal acquired) reveals a low-signal-intensity change (arrow) in the same location as the area in a. (c) FLAIR image (10,002/148/2,200, one signal acquired). The perivascular space is not clearly visualized because it is isointense to brain parenchyma. In a-c, S = sylvian fissure.
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Figure 3b. Comparison of (a) T2-weighted, fast SE; (b) 3D, fast, SPGR; and (c) FLAIR coronal images. (a) T2-weighted, fast SE image (4,000/112, three signals acquired) in a 6-year-old girl shows an elliptical area of high signal intensity (arrow) in the right subinsular extreme capsule. (b) Fast, 3D, SPGR image (17/3, one signal acquired) reveals a low-signal-intensity change (arrow) in the same location as the area in a. (c) FLAIR image (10,002/148/2,200, one signal acquired). The perivascular space is not clearly visualized because it is isointense to brain parenchyma. In a-c, S = sylvian fissure.
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Figure 3c. Comparison of (a) T2-weighted, fast SE; (b) 3D, fast, SPGR; and (c) FLAIR coronal images. (a) T2-weighted, fast SE image (4,000/112, three signals acquired) in a 6-year-old girl shows an elliptical area of high signal intensity (arrow) in the right subinsular extreme capsule. (b) Fast, 3D, SPGR image (17/3, one signal acquired) reveals a low-signal-intensity change (arrow) in the same location as the area in a. (c) FLAIR image (10,002/148/2,200, one signal acquired). The perivascular space is not clearly visualized because it is isointense to brain parenchyma. In a-c, S = sylvian fissure.
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The configuration seen on T2-weighted, fast SE images did not always match that seen on the 3D, fast, SPGR images. In some cases, the elliptical and round abnormalities seen on T2-weighted, fast SE images appeared to be composed of multiple linear or dotlike hypointense structures on SPGR images, resulting in different configurations with these two sequences.
Twelve foci that were difficult to evaluate on 3D SPGR images because of higher bandwidth and motion artifacts were excluded from the study.
Location
The subinsular bright spots were located within the deep (medial) insular cortex and the subcortical extreme capsule on T2-weighted, fast SE images, with the predominant findings in the extreme capsule. They were not distributed evenly throughout the extreme capsule. Most spots were found in the anteroinferior extreme capsule area, usually occurring on coronal sections containing or adjacent to the anterior commissure.
In the patients, the spots were distributed a mean distance of 0.55 cm anterior to the anterior commissure (with a range of 1.6 cm anterior to 1.3 cm posterior to the anterior commissure). The location of the bright spots differed slightly from the left to right hemispheres. The mean distance was 0.50 cm anterior to the anterior commissure (range, 1.3 cm anterior to 0.3 cm posterior to the commissure) in the left hemisphere. In the right hemisphere, the spots had a mean distance of 0.6 cm anterior to the commissure (range, 1.3 cm posterior to 1.6 cm anterior to the commissure).
In the control subjects, the subinsular bright spots were a mean distance of 0.48 cm anterior to the anterior commissure (range, 0–1.0 cm).
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DISCUSSION
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Virchow-Robin spaces are cerebrospinal fluid spaces covered by pia that accompany arteries, arterioles, veins, and venules as they perforate the brain. In the past, the perivascular space was studied only by anatomists and pathologists because no imaging technique before the introduction of MR imaging had the resolution required to depict such anatomic detail (1,2). The development of MR imaging permitted the detection of these small cerebrospinal fluid–filled perivascular spaces that are routinely visualized in regions of the anterior perforated substance, basal ganglia, cerebral convexity, and midbrain (3–6).
The pattern of cerebral vasculature helps determine the location of the perivascular spaces. The insula, extreme capsule, and claustrum receive their blood supply from 30–40 small arteries that arise from the insular segments of the middle cerebral artery (Fig 4) (7–9). This pattern of vascular supply is determined during embryologic development (10). There was a greater abundance of arterioles observed in our study perforating the insular cortex anteriorly compared to posteriorly, which is consistent with our observations that the subinsular bright spots occurred in the anterior aspects of the insula and extreme capsule. The anterior commissure is a useful landmark for determining the anteroposterior position of these high-signal-intensity perivascular spaces on long-TR images.
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Figure 4. Photograph of a coronal slice of a human brain specimen through the anterior insular cortex. Note how the region of the subinsular bright spot (as seen in Figs 1-3) corresponds to the penetration of arterioles from the M2 segment of the middle cerebral arteries (arrows) through the insular cortex and into the subcortical white matter of the extreme capsule. Opacification of the arterial tree was obtained with injection of a 10% solution of barium sulfate (Micropaque; Guerbet, Aulnay-sous-Bois, France) into the arterial tree followed by administration of barium sulfate and gelatin for blockage of large vessels. (Reprinted, with permission, from reference 9.)
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In our series, subinsular bright spots were invariably located within the subinsular white matter and deep (medial) portion of the insular cortex. In a study of Virchow-Robin spaces that occurred in the anterior perforated substance and basal ganglia, Adachi et al (11) noted that perivascular spaces, rather than being uniform throughout the length of the accompanying arteriole, are prominent in subcortical white matter. They postulated that the Virchow-Robin space begins to dilate at the end of the pial membrane accompanying intracerebral vessels. These findings may explain why most of the high-signal-intensity foci were found in the extreme capsule rather than the insular cortex.
The prevalence of Virchow-Robin spaces and their relationship to hypertension, dementia, parenchymal white matter punctate foci, and age has been a subject of dispute (4,12–14). In our study, the prevalence of subinsular perivascular spaces was higher than that reported for Virchow-Robin spaces in other regions. The high prevalence of perivascular spaces in our study may be related to the imaging parameters that were used. We used high-spatial-resolution, 3-mm-thick, T2-weighted coronal images, whereas most previous investigators used 5-mm-thick, T2-weighted, transverse images. Heier et al (4) found the prevalence of perivascular spaces to be 38% in 816 MR images reviewed; however, they performed that study a decade ago and used long-TR images with a 5-mm section thickness and a 2.5-mm gap.
The high-spatial-resolution, long-TR, fast SE sequence has enabled us to detect small perivascular spaces. It is presumed that these subinsular bright spots were not visualized previously at our institution because prior protocols used thicker sections for long-TR sequences. Detection of these bright spots in all five control subjects helps support the notion that these small Virchow-Robin spaces will be depicted routinely on high-spatial-resolution, long-TR images and that these perivascular spaces represent normal anatomy. Identification of these bright spots in the control subjects as well as in the patients with seizure supports the hypothesis that they are not related to the seizure condition but rather to the common thin-section, high-spatial-resolution imaging parameters used for both populations. Although other investigators found changes with age (4), we found no substantial difference in the frequency of perivascular spaces in the different age groups.
We believe that the signal intensity of the subinsular bright spots is due to partial volume averaging of collections of numerous small perivascular spaces with intervening brain parenchyma rather than due to solitary perivascular spaces. This hypothesis is supported by the results of gross and microscopic examination of the brain specimens and helps explain why we failed to detect areas of low signal intensity on SPGR images in cases that showed bright spots on T2-weighted images (Figs 2, 3). It also helps explain why the nodular shapes seen on T2-weighted images were sometimes depicted as being composed of multiple linear or small dotlike low-signal-intensity shapes on 1.5-mm-thick SPGR images. The different configuration patterns may be partially explained by how the imaging plane intersects the long axis of the perivascular space. If the perivascular space is conceptualized in a simple model as a cylinder, a dotlike, elliptical, or linear configuration occurs in the cross-sectional image depending on the angle of the plane of section through that cylinder.
It is important to recognize that the subinsular bright spot represents a Virchow-Robin space and not a pathologic condition. Imaging for patients with seizure requires high-spatial-resolution imaging to detect subtle epileptogenic abnormalities in or adjacent to the cortex (15). High-spatial-resolution sequences will result in the visualization of the insular perivascular spaces.
Because 20% (11 of 56) patients may have a unilateral 3–15-mm-diameter elliptical or round high-signal-intensity structure on long-TR, fast SE images, there is a potential for mistaking these perivascular spaces as pathologic changes. On long-TR images, the perivascular signal intensity is similar to that seen with such brain abnormalities as small vessel ischemic disease, neoplasm, demyelinating disease, or postictal changes.
The subinsular bright spot, however, has a number of MR features suggestive of Virchow-Robin spaces, enabling one to distinguish them from pathologic conditions. Most bright spots are bilateral, and all are found in the anterior insula and extreme capsule. Often, there are multiple signal intensity changes when images are closely scrutinized. The elliptical and round shapes often have a featherlike configuration. The spots do not have high signal intensity on FLAIR images (and probably would not have high signal intensity on other long-TR, short–echo time images).
In summary, subinsular bright spots are due to enlarged perivascular spaces and could be mistaken for pathologic conditions when they occur unilaterally. However, the subinsular location, featherlike configuration, signal intensity, and bilaterality can lead one to the correct interpretation.
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Acknowledgments
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We thank Hedwig Sarofin, RTR, MR, and Terry Hickey, RTR, N, MR, for their invaluable assistance in imaging the brain specimens and control subjects.
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Footnotes
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2 Current address: Department of Radiology, Chung-Nam National School of Medicine, Taejon, South Korea. 
Abbreviations: FLAIR = fluid-attenuated inversion recovery
SE = spin echo
SPGR = spoiled gradient-recalled echo
TR = repetition time
3D = three-dimensional
Author contributions: Guarantor of integrity of entire study, R.A.B.; study concepts, R.A.B.; study design, C.J.S., R.A.B.; definition of intellectual content, C.J.S., R.A.B.; literature research, C.J.S.; clinical studies, C.J.S., R.A.B.; experimental studies, C.J.S., J.H.K., E.L.K., R.A.B.; data acquisition, C.J.S., R.A.B., J.H.K., E.L.K.; data analysis, C.J.S., R.A.B.; manuscript preparation, C.J.S.; manuscript editing and review, R.A.B., E.L.K., J.H.K.
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TOP
Abstract
Introduction
