Arachnoid Granulations in the Transverse and Sigmoid Sinuses: CT, MR, and MR Angiographic Appearance of a Normal Anatomic Variation

James Roche and Denise Warner

PURPOSE: To investigate the imaging characteristics, prevalence, and clinical significance of arachnoid granulations in the transverse and sigmoid venous sinuses. METHODS: We reviewed the imaging findings, clinical signs and symptoms, final diagnoses, and follow-up studies of 32 patients with 41 probable arachnoid granulations. RESULTS: On CT scans, arachnoid granulations appear as well-defined filling defects, wholly or partly within a venous , with the same density as . MR images show these entities as largely isointense with cerebrospinal fluid in all sequences. Linear variations of signal intensity within the granulations are thought to be fibrous septa or vessels. Calcification was present in 3 granulations and altered both CT density and MR signal intensity. The granulations appear as filling defects at MR angiography and at digital subtraction angiography. In some oblique MR angiographic projections, they appear elliptical and could be mistaken for thrombus. No clinical significance could be given to the existence of any of these arachnoid granulations. They occur in 0.3 to 1 of 100 adults in the population. CONCLU- SION: Arachnoid granulations in the transverse and sigmoid venous sinuses are common findings seen with thin-section imaging and are usually of no significance.

Index terms: Arachnoid, anatomy; Dural sinuses

AJNR Am J Neuroradiol 17:677–683, April 1996

Over the last 5 years we collected imaging imaging studies and review the clinical signs studies obtained with computed tomography and symptoms in 32 patients. (CT), magnetic resonance (MR) imaging, MR angiography, and digital subtraction angiogra- phy in which filling defects were seen within the Materials and Methods transverse or sigmoid venous sinuses. We con- sider these to be arachnoid granulations pro- Over a 5-year period we collected all imaging studies that showed any possible arachnoid granulations in the truding into the sinuses, an observation that is transverse or sigmoid sinuses. Because most of these supported by Tokiguchi et al (1) in a report lesions seem to be clinically innocent, additional or fol- containing autopsy proof and by Mamourian low-up CT, MR imaging, or digital subtraction angiography and Towfighi (2). We suggest that these granu- was not indicated; however, three patients volunteered to lations are common, normal variants. We de- have MR imaging in addition to their original CT examina- scribe the appearance of these structures on tions. Various CT scanners were used; MR images were obtained primarily on a 1.5-T superconducting magnet; and digital subtraction angiography was performed with modern 1024-matrix units. All studies were reviewed to Received April 13, 1994; accepted after revision October 26, 1995. record the site and number of granulations, their apparent From the Department of Diagnostic Radiology, Royal North Shore internal morphology, any adjacent , and their effect Hospital, St Leonards (J.R.), and the Department of Diagnostic Radiology, on adjacent bone. The Children’s Hospital, Camperdown (D.W.), New South Wales, Australia. For all patients, we conducted clinical interviews (with Address reprint requests to James Roche, MB, ChB, FRACR, Depart- follow-up periods ranging from 3 months to 5.5 years) in ment of Diagnostic Radiology, Royal North Shore Hospital, St Leonards, which the signs and symptoms leading to the initial exam- New South Wales 2065, Australia. ination, the final diagnosis, and the current states of health AJNR 17:677–683, April 1996 0195-6108/96/1704–0677 were recorded. These interviews were conducted with the ᭧ American Society of Neuroradiology referring specialist, with the general practitioner, or, occa- 677 678 ROCHE AJNR: 17, April 1996

sionally, with the patients themselves. No biopsy, surgical, or autopsy confirmation is available for these patients.

Results Seventeen men and 15 women, 20 to 75 years old, were found to have 41 arachnoid granulations. One patient had 3 granulations (Fig 1) and 7 patients had 2 granulations. Mul- tiple granulations were usually located at similar sites on opposite sides, but occasionally they were adjacent to one another (Fig 2). The most common location was the middle third of the transverse sinus, with 21 granulations; the next most common location was the lateral trans- verse sinus, with 14 granulations (Fig 3). Two granulations were seen at the confluence of the sinuses, 3 were seen in the superior portion of the , and 1 was seen in the middle portion of the sigmoid sinus. The granulations were oval or round, with a diameter that ranged from 3 to 14 mm. Only 2 granulations were larger than 10 mm. Their average size was 7 mm; 7 were between 3 and 5 mm, 20 were between 6 and 8 mm, and 9 were larger than 8 mm. Measurements were not possible for 5 granulations. Fig 1. Precontrast (A) and postcontrast (B) axial CT scans CT scans were available for 26 patients with show three granulations, one in the lateral third of the transverse 31 granulations. Granulations were best appre- sinus on each side and one at the torcular (arrows). ciated on thin-section scans obtained after in- travenous administration of contrast material, and all were visible on the 25 postcontrast stud- ies available. In 6 of the 17 noncontrast studies available, the granulations were not visible. This was most probably because the section thick- ness and levels were not identical with the post- contrast studies. The density of most granula- tions was nearly equivalent to cerebrospinal

Fig 2. Axial T2-weighted MR image (A) and lateral digital subtraction angio- gram, venous phase (B), show a double granulation (arrows) in the right trans- verse sinus with draining into the anterior granulation. AJNR: 17, April 1996 ARACHNOID GRANULATIONS 679

Fig 4. Postcontrast axial CT scan shows a granulation in the lateral aspect of the left transverse sinus with central calcification and a peripheral halo of CSF density (arrow).

Fig 3. Noncontrast (A) and contrast-enhanced (B) axial CT scans show a granulation in the lateral third of the right transverse sinus. Postcontrast coronal scan (C) confirms the position of the Fig 5. Proton density–weighted (top) and T2-weighted (bot- granulation inside the sinus. tom) axial MR images show a large granulation in the lateral third of the left transverse sinus with two septa. fluid (CSF) as determined visually or by region- of-interest measurements. Three granulations weighted images that these granulations were were calcified and could be seen easily on both most readily identified by MR imaging (Figs 2 precontrast and postcontrast studies. One gran- and 5). The one calcified granulation examined ulation appeared to be entirely calcified but the with MR imaging was almost isointense with other two showed varying amounts of central white matter on T1-weighted images and was calcification with a surrounding halo of CSF not noted prospectively. It appeared hypoin- density (Fig 4). tense relative to CSF on intermediate and T2- MR studies were available in 14 patients with weighted images, and showed a prominent hy- 18 granulations. These studies were not always pointense interface with the flow void of the targeted to the and not all sinus. granulations were shown in every sequence. On MR angiography was available in five patients T1-weighted images, 12 granulations were with six granulations. Five of the granulations isointense with CSF and 5 were isointense with were easily seen on projection images but one gray matter. Identification of granulations isoin- smaller granulation could only be identified on tense with gray matter was difficult in 3 of these partition images. Oblique projection images 5 granulations. Intermediate and T2-weighted made three granulations appear elongated, and images showed uncalcified granulations to be they could have been mistaken for thrombus in isointense with CSF. It was on thin-section T2- a different clinical context (Fig 6A). 680 ROCHE AJNR: 17, April 1996

Fig 7. Postcontrast axial CT scan shows granulation in the superior aspect of the sigmoid sinus (arrows). The adjacent bone is indented, a finding that was confirmed on bone window images.

the inner table of the calvaria and there was definite evidence of bone remodeling around the granulation (Fig 7). Two other granulations showed possible bone remodeling. In 16 gran- ulations there was a close relationship with a vessel, or vessels, that appears to be draining into the sinus immediately adjacent to the gran- ulation. In 4 of these 16, the vein is apparently traced directly into the granulation (Fig 2). This feature was best seen on three-dimensional T1- weighted images after injection of contrast ma- Fig 6. A, Phase-contrast MR angiogram shows a granulation terial. Granulations did not change over time, in the lateral part of the transverse sinus. This indents the sinus but the longest period between studies was only and appears as a long elliptical defect on this oblique projection 2 years. view (arrow). A review of the symptoms of patients showed B and C, Phase-contrast MR angiograms, two projection views of a different patient (same as in Fig 3), show the spherical filling that 13 of the 32 had symptoms that could defect of an in the transverse sinus. conceivably be related to the posterior fossa, such as pulsatile tinnitus, vertigo, ataxia, deaf- ness, Meniere syndrome, or ear pain. The re- Venous phase digital subtraction angiogra- maining 19 had symptoms unrelated to the pos- phy showed five granulations in four patients, terior fossa or venous sinuses. Clinical follow- which generally were best seen on frontal views, up, which ranged from 3 months to 5.5 years, but were also seen in the lateral projection (Fig has shown partial or complete resolution of 2B). These appear as well-defined static filling symptoms in 18 patients, no change in 9 pa- defects within a sinus opacified by either carotid tients, and no relevant development of symp- or vertebral injections. toms in 4 patients whose general condition has The morphology of these granulations varies. otherwise deteriorated from systemic disease. Their density/intensity generally parallels that In 1 patient with end-stage renal failure and of CSF. Internal structures that are linear or advanced lymphoma, severe truncal ataxia de- septate in appearance could be seen in 18 of the veloped. The granulation in this patient was un- 41 granulations (Fig 5). This is probably a changed over 3 weeks and there were no other falsely low figure, as these internal structures posterior fossa abnormalities. Symptoms were are best seen on high-resolution MR images. thought by the referring clinicians to be drug- Three granulations had calcium within them. related. No patient has had a diagnosis that was One granulation was immediately adjacent to considered relevant to the venous sinuses, such AJNR: 17, April 1996 ARACHNOID GRANULATIONS 681 as increased intracranial pressure, dural vascu- CSF circulates through these granulations and lar malformation, or sinus thrombosis. thereby enters the venous system. Arachnoid The frequency of these granulations is not granulations are reputed to increase in size and accurately known. Over a 9-month period, number with age (3–5). Upton and Weller (6) 3100 routine CT scans were prospectively described different histologic appearances of viewed and 9 granulations were discovered. arachnoid granulations with age; more cellular, Since not all of these had adjacent thin-section less complex granulations occur in younger pa- views of the posterior fossa after intravenous tients. Wolpow and Schaumburgh (7) described administration of contrast material (our routine variability of size and morphology of arachnoid is 3-mm-thick sections every 5 mm), this prob- granulations in different sites in the same pa- ably underestimates the true frequency. In 200 tient. Radiologically, they have been said to be MR studies obtained for possible acoustic neu- calcified, to simulate destructive lesions in the roma, three-dimensional imaging was per- calvarium (4, 8), to mimic dermoid cysts in the formed, giving 2-mm adjacent sections through young (9), to be a potential cause of CSF otor- the sinuses, and 2 granulations were found. T1- rhea in the (10), and frequently weighted imaging without contrast administra- to extend into the (5, tion is not the optimal way to discover granula- 10–12). tions by MR imaging, so this also under- Granulations in the transverse or sigmoid si- estimates the frequency. The prevalence of nus are best found on adjacent thin-section CT granulations in the adult population would thus scans after intravenous administration of con- appear to be at least between 0.3 and 1 in 100. trast material. When not calcified, they are iso- dense with CSF and are usually visible on non- contrast scans of the same section thickness Discussion and position. With MR imaging, the granulations Arachnoid granulations in the transverse and are most conspicuous on thin-section T2- sigmoid sinuses appear to be normal anatomic weighted images or on T1-weighted images ob- variants that surely have been seen by many tained after contrast administration, because radiologists. Tokiguchi et al (1) recently pub- they contrast with the signal of the sinus. The lished a description of such a granulation, variable low-contrast difference between the shown by CT and confirmed at autopsy, that granulation and the sinus on noncontrast T1- correlates well with the appearances described weighted images means they may often be in this study. Their image of a microscopic missed on this sequence. Spin-echo T1- specimen of a collapsed granulation gives an weighted studies have shown five granulations indication of the internal structure of these bod- to be isointense with gray matter, possibly be- ies. Mamourian et al (2) described the CT and cause of a partial volume effect but more prob- MR appearance of a lesion they concluded was ably because of the varying amount of stromal a giant arachnoid granulation in the transverse tissue that occurs within these spaces (6, 7). In sinus that matches our observations. They also the other 12 granulations seen on T1-weighted described autopsy features of two giant arach- images, and in all the granulations seen on the noid granulations in the that proton density–weighted and T2-weighted im- they regarded as incidental findings. We believe ages, the signal intensity of uncalcified granu- these granulations are common, have a charac- lations was the same as CSF. Previous reports teristic appearance, and are rarely a cause of of surgically proved arachnoid granulations (4, symptoms. This recognition of normality 10) at other sites have described their signal averted the planned surgical excision of a gran- intensity on MR images as being similar or con- ulation from one of our patients, who is symp- tinuous with CSF. With MR angiography, these tom-free 4 years later. granulations are usually well seen (Fig 6). They Arachnoid granulations (pacchionian bodies) are best assessed on the partition images, as are mostly found clustering in parasinusoidal they may be missed on some of the maximum blood lakes alongside the superior sagittal si- intensity projection views. They appear as nus. Anatomic descriptions of them within the rounded or elliptical areas with no signal. The lateral sinus date back to 1920 (3). They are elliptical defects may simulate thrombus, but a bulbous aggregates of fibroelastic tissue that review of T2-weighted images usually shows are continuous with the subarachnoid space. their true nature. 682 ROCHE AJNR: 17, April 1996

Four filling defects seen in the transverse si- fossa. However, the failure of any firm patho- nuses of 14 healthy volunteers on MR angio- logic diagnosis to be made subsequent to all grams were considered to be arachnoid granu- radiologic studies, and the absence of any re- lations (R. Knutzon, J. Tanabe, P. Baum, and W. lated deleterious outcome to the patients, are Dillion, “MR Venography: Artifacts, Normal An- pertinent. No patient had any significant, unex- atomic Variants, and Their Differentiation from pected illness during the follow-up periods, Thrombus,” Proceedings of the 31st Annual which ranged from 3 months to more than 5 Meeting of the American Society of Neuroradi- years. Moreover, the symptomatology of many ology, May 16–20, 1993, 90 [abstract]). Filling of these patients prompted investigation with defects have been previously described in the thin, closely spaced CT or MR sections, which transverse sinus at angiography (13) and have increased the likelihood of discovering granula- been labeled as inflow artifacts from unopaci- tions. fied . Three granulations that The frequency with which these granulations were shown by MR imaging in our study corre- were found indicates that they are so common lated well with defects in the venous phase of that a firm association with such symptomatol- digital subtraction angiography (Fig 2). ogy must be guarded. However, it is possible The internal structures seen in 18 granula- that a large, unilateral granulation in a dominant tions (Fig 5) are not unexpected from histologic sinus could cause partial obstruction of venous descriptions (6, 7) and probably correspond to flow. Tress et al (personal communication, vessels or to a reticulum of fibrous tissue that 1994) have indicated the possibility of partial may appear as discrete septa. These are best obstruction of the sinus related to granulations assessed on MR images, but can occasionally in patients with idiopathic, raised intracranial be noted on CT scans. In 16 granulations there pressure. Pressure gradients were recorded on was a close relationship with a vessel or vessels, either side of these granulations, but pressure which apparently entered the granulation in 4 gradients were also observed between the mid- cases. The 3 giant arachnoid granulations found dle and distal transverse sinuses in patients with at autopsy by Mamourian and Twofighi (2) were idiopathic intracranial hypertension without all shown to contain a relatively large blood granulations or any other abnormality. None of vessel in the center. Two of these lesions were in our patients had symptoms that warranted in- the transverse sinus. Central enhancement in a vasive investigations, such as pressure mea- transverse sinus giant arachnoid granulation in surements. one of their cases also suggested the presence The rate of occurrence we report of between of a central vascular channel. Browder et al (14) 0.3 and 1 in 100 adults appears to be conser- found that all but 2 of 23 nodules in the trans- vative. Four filling defects were noted in the verse sinus were located at the site where the transverse sinuses on MR angiograms of 3 of 14 vein of Labbe´ joins the sinus. Upton and Weller healthy volunteers by Knutzon et al (presented (6) described a small number of endothelial- at ASNR meeting, 1993). Browder et al (14) lined channels on the surface of arachnoid gran- found 23 polypoid tumors in the transverse si- ulations not seen in arachnoid villi. Grossman nuses of 295 autopsy cases. Our experience and Potts (5), in an angiographic and patho- suggests that in the vast majority of cases the logic study, noted 49% of patients over the age discovery of a granulation in a sinus is entirely of 40 had veins closely related to arachnoid innocuous. The differential diagnosis of a small granulations, and they considered this relation- arachnoid cyst could be considered in cases in ship to be casual. The significance of vessels which the defects only indent the sinus but associated with arachnoid granulations remains where their site, size, and overall appearance unclear. Bone remodeling of the inner calvar- suggest they are all the same anatomic varia- ium was present in one of our cases (Fig 7) and tion. suggested in two others. Bone erosion from granulations at other sites has been well de- scribed (4, 8–10), and such a pressure effect Acknowledgments when they lie in a venous sinus is not surprising. We are grateful to S. Blome, D. Brazier, G. Briggs, J. 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