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Arachnoid Granula- Anatomy and Imaging

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Arachnoid Granula- Anatomy and Imaging REVIE\N ARTICLE Imaging of arach- Arachnoid granula- noid granulations • The reported prevalence of arach- fions anatomy noid granulations in anatomic investi- gations varies considerably from 8.4% and imaging to 66%.11,12In these two series most were described as being present with- in the transverse sinuses. Grossman one-way valve between the relatively and Potts' found impressions in the Ian C Duncan high pressure subarachnoid space skull due to arachnoid granulations in FFRad(D) SA and the lower pressure veins and 46% of 400 randomly selected skull Unitas Interventional Unit sinuses. The actual transfer of fluid These increased in prevalence PO Box 14031 takes place partly through small Lyttelton and depth with increasing age (Figs 0410 intracellular channels in a passive laIb). Most were seen in the anteri- manner, but mainly through active or parietal region adjacent to the mid- Arachnoid granulations were orig- transport via a vacuole-mediated line (average distance: 14 mm). They inally described by Pacchioni in 1705 trans cellular mechanism." eSF is also reviewed 100 cerebral angio- and hence are also called Pacchionian also absorbed to a much lesser grams and found veins related to granulations. They are specialised degree through the leptomeninges in araclmoid granulations in 38%. In focal herniations of the arachnoid the spinal subarachnoid space. one further case there was a discrete membrane, surrounding the brain, Arachnoid villi differ only from filling defect in the superior sagittal extending into the dural venous sinus- granulations in size, being much sinus. No arachnoid granulations es. They may also be found in associa- smaller. They are often only seen related to the transverse sinuses were tion with the bridging veins that cross with the aid of magnification." noted. The reported incidence of the subarachnoid space from the cere- Arachnoid villi are present as early as arachnoid granulations with sectional bral cortex to drain into the venous the 26th week of pregnancy. They imaging and angiography varies con- sinuses. Their function is thought to may be seen from the 39th week and siderably from 0.3% to 90% (Table I). be the resorption of eSF into the cir- are macroscopically evident (granu- This considerable variability is related culation." The arachnoid granula- lations) by 18 months of age.' to various technical issues such as tions extend into the sinus or adja- Arachnoid granulations may there- thicker scan slicethickness and lack of cent venous lacunae through tight fore be derived from arachnoid contrast administration during Tl defects in the meningeal dura. The villi.2.8They enlarge considerably by weighted image acquisition in the ear- space in the centre of the granulation the age of 3 - 4 years" and continue to liest series." The availability of higher is in direct contact with the sub- enlarge throughout adult life. They imaging resolution together with con- arachnoid space. The endothelium rarely grow larger than 1 cm in size trast -enhanced imaging has led to of the sinus is reflected on the herni- and never larger than 2 cm. The larg- greater imaging sensitivity and thus ated granulation and may cover this er ones can produce an erosion of detection of arachnoid granulations structure entirely, or be deficient the inner table of the adjacent cal- (Fig. 1C).14.15 leaving a number of the arachnoid varium. The depth of these 'depres- Descriptions of the anatomic loca- cells of the wall of the granulation in sions' may increase with increasing tions of arachnoid granulations have direct contact with the sinus lumen. age." Occasionally the 'depressions' also shown considerable variability, as These exposed cells are called arach- may be deep enough to affect the shown in Table II. From these data, noid cap cells." The granulations are outer table of the skull as well, and however, we can see that arachnoid responsible for the drainage of eSF rarely 'giant' granulations may be granulations are found in relation to from the subarachnoid space into seen." Arachnoid granulations can all major dural sinuses and many are the venous circulation. This forms a calcify in later life. related to bridging cortical veins or 5 SA JOURNAL OF RADIOLOGY. November 2003 REVIEVV ARTICLE 21% of cases examined by Gailloud et lations may appear isointense with ai" (Figs 3a, 3b). In the reports by parenchyma on Tl-weighted images Leach et al" and Liang et al;" granu- and hypointese relative to CSF on T2- lations were also described in the weighted images." The size of granu- straight sinus, typically at the junction lations can vary between 1and 14mm of the sinus and the Vein of Galen (average 2 - 7 mm), with the largest (Figs 4a-g). The average number of being found in relation to the trans- arachnoid impressions seen on skull verse sinuses.IO,12,13,15 'Ectopic' granula- radiographs was shown by Grossman tions have been described presenting Fig. ta. Left extemal carotid digital subtraction and Potts' to be 2.7 in patients over 19 in the convexity of the skull some dis- angiogram. Slight mlsregistration of the mask allows visualisation of an indentation of the inner years of age, with the number and tance from the sagittal sinus," table of the skull at the site of an arachnoid gran- depth of the impressions also increas- although Grossman and Potts' ulation (arrow). ing with increasing age. Calcification showed the presence of arachnoid of granulations was reported by impressions in randomly selected Roche and Warner13 in 3 of 31 cases individuals up to 28 mm from the seen at CT and 1 of 18 cases seen at midline. MR! (8%). No calcified granulations The most important differential were seen in the cases reviewed by diagnosis for these granulations by Leach et al" An interesting discrep- virtue of the appearances as intra- .. ancy between angiographic studies and sectional imaging is that the for- mer commonly describe the presence of arachnoid granulations in or close to the superior sagittal sinus which tend not to present typically as intra- Fig. tb. Selective left intemal carotid arteriogram, luminal filling defects within the late venous phase, showing slight indentation of sinus, whereas those in the transverse the superior sagittal sinus at the site of the same granulation. (arrow) and other sinuses do protrude into the sinuses seen as discrete intrasinus fill- ing defects on sectional imaging. The Fig. 2a. Coronal gadolinium-enhanced Tt MR scan showing the passage of the 'bridging'seg- CT density of granulations varies ment of the cortical veins traversing the arachnoid from being isointense with CSF to spaces exaggerated by the presence of bilateral chronic subdural fluid collections (arrow). being isointense with brain. They do not enhance after contrast adminis- tration, being seen as discrete hypointense filling defects surround- ed by enhanced blood within the 12 14 sinuS. ' With MR imaging, granula- Fig. te. Parasagittal gadolinium-enhanced Tt MR tions are typically hypo intense relative scan showing the CSF-isointense granulation to brain and isointense to CSF on Tl- (white arrow) with associated enhancing conical veins (black arrows). weighted and on fluid-attenuated inversion recovery (FLAIR) sequences cortical venous entry points (Figs (Figs le, 4a-c). They are again non- enhancing, being seen as discrete Fig. 2b. Sagittal image, same case, again show- 2a-e). One relatively common site of ing the bridging veins traversing the arachnoid occurrence of an arachnoid granula- hypointense filling defects within the spaces (arrows). No arachnoid granulations are macroscopically evident in this patient who is 2 tion is at the confluence of the Vein of sinuses on contrast enhanced Tl- years old. Labbe and the transverse sinus, seen in weighted images.":" Calcified granu- 6 SAJOURNAL OF RADIOLOGY • November 2003 o REVIEVV ARTICLE Table I. Studies showing the prevalence of arachnoid granulations with imaging Estimated Study Imaging methods Results prevalence (%) Roche & Warner" CT, MR, MRA, DSA 9/3100 on CT 0.3 - 1% 2/2000nMR Leach et al." Contrast-enhanced CT, 138/573 on CECT (24%) 24 - 66 MR, cadaveric specimens 13/100 on CEMRI (13%) 19/29 cadavers (66%) Caseyet al." CT venography 11/20 on CT venography 55 Gailloud et al." Cerebral angiography 12/57 patients 21.1 Liang et al." MRI (contrast-enhanced 3D 90/100 patients 90 MPRAGE) Fig. 2c. Selective left internal carotid digital sub- Fig. 2d. Same arteriogram, PA projection. The Fig. 2e. Selective left internal carotid arteriogram, traction arteriogram, left anterior oblique projec- granulation is visible parasagittal/y (arrow) . late venous phase, in another patient showing tion, showing a wel/-circumscribed fil/ing defect multiple fil/ing defects protruding into the superior indenting the bridging segment of a cortical vein sagittal sinus lumen itself (arrow). These are also (arrow). This appearance is typical of an arach- compatible with granulations. noid granulation. Table II.Anatomic location of arachnoid granulations Study Method Anatomical location Cortical vein association Grossman & Potts' Cerebral All parasagittal (Av:14 mm) 38/100 (38%) angiography Mainly anterior parietal Roche &Warner" CECT,MRI, Transverse sinus: 85% 16/41 (39%) MRA,DSA Torcula: 5% Sigmoid sinus: 10% Leach et al." CECT,MRI Right transverse sinus: 39% 95/154 on CT (62%) Left transverse sinus: 52% Sigmoid sinus: 2% 11/13 on MRI (85%) Torcula: 5% Distal superior sagittal sinus: 0.6% Straight sinus: 1% Casey et al." CT venography Mostly transverse sinus Not mentioned Gailloud et al." Cerebral Transverse sinus at the 12/12 (100%) angiography confluence of the vein of Labbé Liang et al." 3D Contrast-enhanced MPRAGE MRI Superior sagittal sinus: 53.8% 414/433 (96%) Transverse sinus: 28.1 % (Right tra! erse sinus: II %) (Lett transverse sinus: 17.1%) Straight sinus: 17.6% Vein uf Galen: 0.5% 7 SAJOURNAL OF RADIOLOGY • November 2003 REVIEVV ARTICLE Fig.
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