Cerebral Venous Sinuses
The most important feature regarding venous sinuses, and veins in general, is to keep in mind that veins are like rivers. The size of any vein or sinus is determined by the sum total of the tributaries it gathers, much like a river is exactly as large as the total volume of streams which feed it. Nearly all variations in venous size can be explained by this simple and powerful analogy.
The main sinuses are well known. The superior sagittal sinus is nearly always present but can be variable in its anterior extent; its hypoplasia may be associated with dominance of the cavernous sinus draining the frontal lobes. Often the SSS divides before reaching the torcula, an anatomical variant of no clinical significance but sometimes leading to unnecessary imaging to “conclusively” rule out an imaginary thrombus. The frequent variations at the torcular region bear witness to its late embyologic formation from the primordial venous plexus, substantially after the anterior and middle sinus have been formed.
The inferior sagittal sinus is smaller and inconstant. It usually collects tributaries from corpus calossum and singulate gyrus regions, and drains into the straight sinus.
The transverse sinuses are often asymmetric, the left being more often hypoplastic than the right (pulsations of the right atrium are thought to be responsible for larger capacity of the right jugular system) The jugular foramen on smaller side is correspondingly small as well, helping distinguish developmental hypoplasia from acquired thrombosis.
The sigmoid sinuses are not always of the same caliber as the transverse ones, especially when a large vein of Labbe empties into the proximal sigmoid sinus to enlarge it substantially as compared with its transverse tributary. Extensive emissary veins (mastoid, occipital, condylar) often drain in part and occasionally in toto the transverse-sigmoid sinus complex, and the corresponding jugulars may be hypoplastic. Isolated findings of this nature are rarely due to consequences of pathologic shunting, and looking on bone windows for emissary channels can confirm this as a non-pathologic anatomical disposition.
The cavernous sinus collects extensive drainage and sports multiple egress routes. It has multiple compartments and may in fact be regarded, at least functionally, as a collection of compartmentally separate entities. Not seeing it on an ICA injection does not mean its not there — only one compartment is hypoplastic. Embryologically, although the cavernous sinus is a very early structure, in its earliest form it does not participate in any way in the drainage of the brain — rather collecting tributaries of the future orbital/opthalmic veins, facial and sphenopalatine systems. Even in the newborn, the connections between sylvian and basal venous systems and cavernous sinus are usually nonexistent — maturation of these connections is one mechanism which allows for stabilization of venous drainage in infants affected by high flow shunts such as some Vein of Galen Malformations. The developmentally late capture of the cavernous sinus by Sylvian and basal veins explains the marked variability in the extent of such capture in adults, while the ophthalmic vein to cavernous sinus connection is essentially constant.
The sphenoparietal sinus runs along the ridge of sphenoid lesser wing and collects tributaries of the Sylvian veins to empty into the cavernous sinus. It is, admittedly, a misnomer, and a very embarrassing one at that. Embryologically, a sinus by that name does exist, collecting the venous drainage of the very thin cortical mantle (future superficial sylvian veins), and directing it from the sphenoid area towards the sigmoid sinus, underneath the temporal lobe. This sinus usually disappears in the adult, except for some very rare instances. The very proximal portion of it appears to persist and connect to the cavernous sinus, however it is not clear whether this is in fact the case and if, instead, another channel does not run along the sphenoid ridge. Nevertheless, the erroneous name stuck, having nothing to do with the parietal bone.
The superior petrosal sinus runs along the petrous ridge from cavernous to the sigmoid sinus. Inferior petrosal sinus runs down the petrous pyramid towards the jugular foramen or, more precisely, towards the very distal extra-cranial jugular vein.
An inconstant, in the adult, and more likely present than absent in the child, occipital sinus drains inferior from the torcula towards the marginal sinus which loosely surrounds the foramen magnum, to exit through it into suboccipital veins, perivertebral venous plexus, or around magnum to the jugular foramen.
Some sinuses are present less commonly than others: the more uncommon ones are labeled in white. The superior and inferior sagittal sinuses are not shown to full extent for clarity. Note how the sphenoparietal sinus runs along the edge of the lesser sphenoid wing, and superior petrosal sinus follows the petrous ridge. Most people have quite asymmetric sinuses.
Superior Sagittal Sinus
The superior sagittal sinus is a more variable structure than is generally assumed from straight-as-an-arrow diagrams. Although traditionally a single channel spanning the midline and terminating at the torcula, in practice you are likely to see additional dural channels, particularly in the posterior portion of the sinus, usually connecting witht the transverse sinus system somewhat off midline. Often the sinus is deviated to the side, again over the occipital region. This is most noticeable on angio or stacked MIP images, as midline deviations when viewed sequentially on cross-sectional imaging are not as impressive. There is no particualr clinical significance to this. It does become important in surgical planning occasionally, and may play a role in evolution of sinus thrombosis cases (there are reports of “recanalization” of falcine sinus and other dural sinuses in patients with sagittal sinus thrombosis. Ultimately, these additional channels underline the evolution of dural sinuses, which are formed by coalescence of multiple channels, some of which can persist. In the patient below, the posterior portion of the SSS is particularly grotesque, with additional left (yellow) and right (pink) dural channels. The “true” SSS is in purple. The torcula is orange. Notice also a vein (green) which may appear like a dural sinus channel. However, it is only visualized from the left injection and is therefore unlikely to be a sinus.
Superior sagittal sinus angulation: It is extremely common to see the SSS angulated with respect to midline. Here, a small midline dural channel (blue) leads to the torcula proper (pink), while the “main” SSS (red) is deviated to the right. Which is the true torcula here is a matter of semantics; the important point is to recognise the arrangement.
Inferior Petrosal Sinus MRI — some cross-sectional imaging to help identify sinus outflow pathways; inferior petrosal sinus extends along the lateral aspect of the dorsum sella towards the jugular foramen.
Cavernous Sinus=blue; inferior petrosal sinus=light blue; sigmoid sinus=purple
The inferior sagittal sinus is highly variable in extent of development and course. This one has an unusual craniocaudal orientation. Notice complete lack of cavernous sinus capture by the sylvian veins, which drain instead towards the sigmoid region.
Labbe=dark blue. Inferior temporal vein=beige. Inferior sagittal sinus=light blue. Straight sinus=black. Basal vein=red. Internal Cerebral Vein=green. Notice dominance of drainage to the sigmoid sinus system with no visualization of the sphenoparietal sinus or cavernous sinus. The basal vein drains exclusively posterior. This patient would be very unlikely to tolerate sigmoid sinus or straight sinus thrombosis.
In this patient the inferior sagittal sinus did not develop. Corpus calossum and adjacent teritorry which may be expected to drain into the inferior sagittal sinus is instead collected by secondarily prominent caudate veins emptying into the thalamostriate and internal cerebral veins. Notice also a well-developed superior petrosal sinus receiving the superficial sylvian system.
Blue Circle=tumor blush. The superficial sylivan vein is prominent within the circle and extends over the temporal lobe towards the superior petrosal sinus (dark blue arrow). Labbe=black. No trolard is seen, various superior cortical veins drain into the SSS. Basal vein=light blue, dominant posterior drainage. Very nice demonstration of the deep venous system tributaries. Anterior Septal vein=bright green, capturing territory of hypoplastic anterior cerebral vein. The inferior sagittal sinus is absent. Thalamostriate vein with large longitudinal caudate vein=yellow. Direct lateral vein=pink. Posterior caudate/splenial veins=brown. A prominent Pericalossal Vein empties into a large inferior sagittal sinus. Also note hypoplasia of the superior sagittal sinus proximal to a large superior frontal convexity tributary.
Occipital Sinus — The sinus connecting torcular to the marginal sinus (sinus which abuts, partially or completely surrounds the foramen magnum) is called the occipital sinus, after th
Dark blue=percalossal vein. Light blue=inferior sagittal sinus. Pink=frontal convexity vein. Orange=anterior septal vein. Yellow=thalamostriate vein. Red=Internal cerebral vein. Occipital sinus (blue arrows on the sagittal and MRI axial projections), draining into the marginal sinus (dark blue arrows on the AP projections.) The occipital sinus is more commonly seen in children.
Dural Sinuses — it is somewhat a misnomer, as all sinuses are dural. Naturally, major dural sinuses have unique names like the superior sagittal, transverse or sigmoid sinus. There is certainly a developmental propensity for these to form in most people — Dorcas Padget felt that sinuses form at intersection of dural covers due to the resistance of the channel to pressure-related collapse. However, not infrequently we observe existence of other dural venous channels, unnamed, draining this or that group of veins. Eventually these sinuses will join a more well-established sinus. The extra sinuses go under a broad term of “dural sinus” — the best thing to do, as always, is to simply describe what exists — for example “left parietal dural sinus draining into the superior sagittal sinus”.
Here is an example of a right parieto-occipital sinus (purple arrows, stereo pair) which collects regional supratentorial and infratentorial veins, emptying into the transverse sinus. Again, this is really only of importance to a surgeon or interventionalist of a procedure in the area is being contemplated.
Cavernous sinus. Cavernous sinus is a metaphysical entity. It is a collection of anatomically and functionally separate venous compartment which, altogether, constitute the single venous space we have come to regard as a distinct anatomical structure. It is critical for the neurointerventionalist to understand this, because his or her treatments will be, of necessity, targeting these varied and complex compartments. This is a distinctly different view than the classical question of whether or not the sinus as a whole is involved in a disease process.
Embryologically, although the cavernous sinus is a very early structure, in its earliest form it does not participate in any way in the drainage of the brain — rather collecting tributaries of the future orbital/opthalmic veins, facial and sphenopalatine systems. Even in the newborn, the connections between sylvian and basal venous systems and cavernous sinus are usually nonexistent — maturation of these connections is one mechanism which allows for stabilization of venous drainage in infants affected by high flow shunts such as some Vein of Galen Malformations. The developmentally late capture of the cavernous sinus by Sylvian and basal veins explains the marked variability in the extent of such capture in adults, while the ophthalmic vein to cavernous sinus connection is essentially constant.
The classical view holds:
1) Basal vein of Rosenthal — typically flows toward CC, but easily reverses flow in cases of fistula, etc.2) Ophthlamic vein — again typically flow is into the sinus, but can easily reverse itself3) Sphenoparietal sinus — also reverses easily. Drains sylvian venous network into the sinus.
1) Superior Petrosal Sinus
2) Inferior Petrosal Sinus
3) Foramen Rotundum, Foramen Ovale, and other skull base foramina to the pterygoid venous plexus
4) Contralateral Cavernous sinus thru transcavernous channels
5) Clival venous plexus down to foramen magnum region, and from there into jugular veins or marginal sinus. A neat way of projecting arterial phase as a mask for venous phase to demonstrate carotid artery relationship to the cavernous sinus. Many tributaries and egress routes of the cavernous sinus are visible.
Cavernous Sinus=orange. Tributaries: Sphenoparietal sinus=brown. Outflow: Superior petrosal sinus=dark blue; Clival venous plexus=purple; Foramen Ovale=green; Foramen Rotundum=red; Pterygoid venous plexus = light blue and yellow. Notice shadow of skull base just below the sphenoparietal sinus. Cavernous Sinus (dark blue). The sylvian network and sphenoparietal sinus (orange) is well seen draining into the cavernous sinus with a well-developed inferior petrosal sinus (light blue). Notice superimpositing of the basal vein (purple) and the anterior choroidal artery (red)
Another demonstration of a small cavernous sinus (purple) receiving sphenoparietal sinus (dark blue) inflow and draining into the superior petrosal sinus (light blue). Notice the relationship of the sphenoparietal sinus to the sphenoid ridge on the unsubtracted views. The basal vein (yellow) is superimposed on the anterior choroidal artery (green)
Ophthalmic Vein Cavernous Sinus anatomy via a carotid cavernous fistulogram
Left ICA injection demonstrating enlarged superior (purple) and inferior (light blue) ophthalmic veins draining a carotid-cavernous sinus (dark blue) fistula. The fistula was approached via an orbitotomy cutdown gaining access into the superior ophthalmic vein (lateral center and AP right images) and closed by coiling. The microcatheter is labeled in red.
Sphenoparietal SinusThe size of this sinus, like any other, is determined by the sum total of the tributaries it gathers, much like a river is exactly as large as the total volume of streams which feed it. The principal feeders of the sphenoparietal sinus, which runs along the sphenoid ridge, as in the above figure, are the superficial Sylvian veins; their relative prominence dictates the size of the sphenoparietal sinus. Classically, as shown above, this sinus empties into the anterolateral aspect of the cavernous sinus. However, this need not always be the case, a fact best appreciated angiographically rather than through destructive anatomical dissections which do not provide information on flow and violate hemodynamically separate venous compartments. For example, in this patient with a sizable CN V shwannoma, consisting of intracavernous (yellow) and prepontine, intradural (white) components artificially separated by the posterior cavernous sinus wall dura, the cavernous sinus compartment is completely filled with tumor. This fact cannot be appreciated on an MRI study, where both tumor and sinus are expected to enhance, however it is well seen angiographically. The sphenoparietal sinus (pink) receives superficial (purple) and deep (orange) Sylvian veins. Notice that the sphenoparietal sinus is situated lateral to the mass, and therefore outside of the lateral wall of the cavernous sinus which contains, among other cranial nerves, the CNV portion involved by the tumor.
Preoperative angiogram of the same patient demonstrates complete occlusion of the cavernous sinus, as shown by lack of contrast opacification in the venous phase of the angiogram, and widening of the cavernous sinus space (blue double arrow). The arterial phase image mask shows the location of the ICA. The sphenoparietal sinus drains independently into the pterygopalatine venous plexus (dark blue arrow) below the skull base.
Stereo pair of the lateral projection, better demonstrating the deep sylvian contribution (orange)
Similar disposition is seen in a different patient, with no cavernous sinus pathology, where at least a portion of the sphenoparietal sinus (pink) receiving prominent Sylvian veins (purple) drains into the pterygopalatine plexus (dark blue) via a channel (light blue) separate from the cavernous sinus proper (green).
Finally, the compartmentalization of the cavernous sinus is shown here by another pathologic entity, the cavernous sinus dural arteriovenous fistula. Injections of the right and left carotid arteries demonstrate a fistula at the posterior aspect of the left cavernous sinus medial compartment (pink), supplied by various branches of the left and right MHT. The venous drainage of this compartment is directed into the engorged superior ophthalmic vein (red). At the same time, the patient’s normal hemispheric venous drainage, via dominant deep sylvian veins, proceeds via the sylvian veins and sphenoparietal sinus (light blue) into an anatomically distinct lateral compartment of the cavernous sinus (purple), which empties via the foramen ovale (yellow) into the pterygopalatine venous plexus.
The same arrangement is shown in the lateral projections of early (left) arteriovenous shunting and brain venous phase (right) lateral compartment drainage. Neither compartment communicates with the other, as evidenced by their mutually separate drainage routes. Understanding this anatomy allows the operator to consider transvenous coiling of the fistulous medial compartment without compromise of the sylvian venous outflow.
Multiple routes of egress from cavernous sinus demonstrated in a case of CC fistula.
Red=ECA (catheter seen on lateral); Orange=foramen rotundum branch; Yellow=MMA; green=cavernous sinus; dark blue=ophthalmic vein; light blue=facial angular vein; purple=basal vein; bright green=straight sinus; black=sigmoid sinus; brown=jugular bulb; pink=superior petrosal sinus; white=inferior petrosal sinus; double yellow=sphenoparietal sinus; double light blue=reflux into brain veins via basal vein (likely pontomesencephalic and lateral mesencephalic veins.
Falcine Sinus — developmental variation of persistence of a falcine channel (which can be anywhere along the falx, but most commonly seen in the parietal region). These venous channels within the falx cerebri are transiently present during embryogenesis before formation of definitive sinuses, and usually regress. The persistence of such sinus attests to the robust potential of the dura to form sinuses pretty much anywhere. For certain hemodynamic reasons the confluence of the dural reflections are where sinuses form normally — such as at the junction of the convexity dura and the tentorial sinus or the falx cerebri. However, sinuses can and do form within the dural reflections in other places on occasion — a dural sinus within the falx cerebri is the so-called falcine sinus.
Persistence of falcine sinus is associated with developmental arterioevnous shunts, best exemplified by the Vein of Galen malformation. In the true VOG malformation, the shunt involves tributaries of what would otherwise become the vein of Galen (internal cerebral, basal vein). When this kind of high-flow shunting involves the primitive venous system, the true vein of Galen and the straight sinus are absent. Instead, the more developmentally primitive falxine sinus persists. This is the the very simple way to recognise a true, congenital Galen malformation — the Galen itself (and straight sinus) are missing! It is therefore true that the name VOG malformation is an unfortunate misnomer. If, on the other hand, you see an AVM or some other shunt in the region (such as quadrigeminal plate AVMs or periatrial ones) but the Galen and straight sinus is present, this means that the lesion was not hemodynamically active in the embryonic phase. The same goes for dural shunts of the trigonal area. Look for the Falxine sinus to tell you whether the lesion was hemodynamically active in utero or not. This has important treatment implications, as one must be especially careful not to occlude the true vein of Galen with Onyx, glue or other embolic (if you think it is the true Galen!), as many other small but vital veins drain there also.
In this child with a left parasagittal posterior splenial/atrial AVM, stereo MIP image of post-contrast MRI following successful treatment (volumetric T1 post is better than TOF MRV for veins) shows the falxine sinus (blue), with tributaries of inferior sagittal sinus (green), internal cerebral vein (purple) coming into the false vein of Galen (pink). Notice absence of the straight sinus, with superior cerebellar veins (yellow) instead draining directly into the torcular. Also seen is a “sheet” of venous blood along the tentorial leaf (white arrows), another vestige of primitive drainage.
The pre-embo vertebral injection images of the AVM nidus (black) draining into the false Galen (pink) and the Falxine sinus (blue). The AVM is supplied via the posterior lateral choroidal arteries (red).
Same AVM from the ICA injection, with a large primary atrial vein (dark blue).
Following embolization, resection, recurrence (as frequently the case with childhood AVMs) and gamma-knife, things look good. Stereo, of course.
In this following case of falxine sinus with no associated shunt, the facine sinus connects the parietal portion of the sagittal sinus iwth the straight sinus. The sagittal sinus distal to the falcine sinus is hypoplastic and in fact is draining “retrograde” towards the falcine sinus which empties into the straight sinus. There is no torcula. A very prominent inferior sagittal sinus is present, which is also somewhat unusual.
Falcine sinus=light blue; Superior sagittal sinus=dark blue; Straight sinus=purple; Inferior sagittal sinus=pink; Internal cerebral vein=yellow. Emissary Veins:These are usually present in the posterior occipital region — being emissary (meaning going from intracranial sinus thru some sort of unnamed hole in the bone into the soft tissues) from the mastoid or occipital regions. They are often seen on MRI and angio and should not, by themselves, promt concern for some kind of fistula unless other evidence of fistula is present. There are however emissary veins present in various other places such as along the superior sagittal sinus. The example below illustrates such a situation — the emissary vein near the vertex runs in the subgaleal space on the left towards the pterygoid plexus. It is usually seen later than even the late venous phase of the brain, as these veins take time to fill.
In most patients, the jugular foramina and various cavernous sinus exit routes are not the only means by which venous blood leaves the skull. Most of us have a number of additional foramina which assist, sometimes quite a lot. Most of us are familiar with occipital emissary veins, which are located in the occipital bone, posterior to the jugular foramina. Below is a typical example of occipital emissary veins (white arrows)
These can sometimes be very prominent, especially when there is associated absence or compression of the jugular vein (this structure can be frequently compressed between the C1 lateral mass and the styloid process). They can become quite efficient.
More interesting, perhaps, are the less frequently appreciated but sometimes quite important emissary veins located elsewhere, for example over the frontoparietal convexities. Dry skull specimens sometimes sport a few seemingly random holes on the top, which are emissary foramina for these veins, typically connected to the superior sagittal sinus. The single emissary vein below (white) traverses the skull, marked by its fainter appearane due to skull density. Once part of the scalp, it splits into the ipsilateral (light blue) and contralateral (purple) channels which ultimately empty into the ipsilateral (dark blue) and contralateral (pink) pterygopalatine fossae venous plexuses. The emissary vein can be appreciated without the stereo capability by its course anterior to the sagittal sinus. Since the sagittal sinus is a midline structure, anything anterior to it on a standard lateral projection has to be either intra-osseous or trans-osseous.
Even more interestingly, and also more commonly, frontoparietal emissary channels are frequently directed into Diploic Veins. Normally, diploic veins drain the diploic space of the skull. Their presence can be recognized on skull radiographs as lucent channels which do not correspond to the known course of middle meningeal artery and vein complexes, as seen on the following radiograph, where these frequently bilateral symmetric and therefore superimposed channels are marked with black (ipsilateral) and white (contralateral) arrows
These are diploic veins. You know they are thinning the skull because of the lucencies they produce (the actual sharp line is due to a bone-tissue interface). The below digital subtraction angiogram (middle image), in late venous phase, nicely shows the corresponding venous channels filling in the diploic venous spaces seen on the skull radiograph (above and left image). Venous phase unsubtracted view (right image) shows contrast in the vein, for those who still remain in doubt. The patient is post clipping of cerebral aneurysm (no relationship to diploic veins…). It is important to extent the acquisition into late venous phase, since these emissary veins are often opacified quite late.
Stereo pairs of the same:
The emissary veins are not without pathologic significance. Usually, they are simply a curiosity. Sometimes, however, surgeons do get in trouble when they “turn a flap”, seemingly in an innocent place, to encounter profound bleeding from the bone. This may be followed by an unexplained (venous) postoperative brain infarct. It is likely that such stories are due to encounter with a particularly large and functionally important emissary vein which, once destroyed, leaves the brain with insufficient alternative drainage.
Here is an example of a patient with what appears to be a congenitally hypoplastic superior sagittal sinus (no evidence of meningioma, history of or active venous thrombosis, etc.) Noncontrast head CT shows multiple diploic venous channels (blue arrows) with corresponding emissary foramina within the innter table (purple arrows) but not outer table
A poorly timed, venous phase CTA shows small caliber of sagital sinus
Left internal carotid artery injection demonstrates hypoplasia of the superior sagittal sinus (black arrows). This results in a number of very interesting adaptations.
1. Large posterior frontal emissary veins (yellow and dark yellow arrows), with corresponding superimposed skull lucencies (white arrows), drain the frontoparietal convexities. The emissary veins drain into the pterygopalatine venous plexus (red)
2. The superficial sylvian veins (purple) are dominant and collect the territory of the operculum, bypassing the sagittal sinus into the superior petrosal sinus (pink).
The internal cerebral vein (dark green) prominently collects several medullary veins (light green)
Nice stereo image of the same:
Right ICA injection demonstrating the same veins, from the other side.
Early (left) and late (right) venous phase lateral images of right ICA injection show a similar adaptive disposition to that seen on the left. A large vein of Labbe (blue) drains the temporal lobe and frontoparietal operculum into sigmoid sinus, without using the hypoplastic SSS (black). Large diploic emissary veins (yellow, dark yellow) drain into the pterygopalantine venous plexus (red).
Vertebral artery injections, other than demonstrating a hypoplastic SSS (black) are unremarkable.
Sagittal Sinus Thrombosis — collaterals. All of the above anatomic knowledge can become very useful in evaluation of venous thrombosis. Numerous collateral pathways develop in this setting attempting to compensate for the loss. The most dramatic cases usually involve the largest channel — the superior sagittal sinus. In this case, a man presented with what initially was thought to be vasculitis-related brain hemorrhage. Subsequent workup led to an angiogram, where sagittal sinus thrombosis with extensive trans-cerebral and trans-osseous emissary vein collateral channels was seen. In retrospect, these findings were present on the patient’s earlier contrast MRI. “Venovibe” or other contrast-enhanced MR venograms can very sensitive, particularly when interpreted with the appropriate index of suspicion. Noncontrast 2-D time of flight MRV I consider to be next to useless as a problem-solving technique. Any thin-slice postcontrast T1 study is vastly superior.
2-D TOF (top left). Sagittal post-contrast MRI (top middle and right). Late (bottom right) and super-late (bottom left) venous phase images of a catheter angiogram. MRI demonstrates a parietal hemorrhage (green, additional sequences confirm hemorrhagic nature). Notice that while the anterior frontal and occipital lobes of the bottom right angiogram are in venous phase, the posterior frontal and parietal lobe drainage is delayed (green arrows). No SSS is visible. Numerous trans-cerebral veins (dark blue) , normally invisible, have enlarged to provide alternative dranage of cortex into the internal cerebral vein (purple). The Trolard (brown), previously draining towards the thrombosed sagittal sinus (because it is larger in caliber near the top) now drains inferiorly into a parietooccipital vein and into the sigmoid sinus. The sylvian network (light blue) is prominent, maximizing its cortical territory. Notice also transosseosus emissary veins (red) draining the brain into scalp veins (orange) on the delayed venous phase bottom left.
Transverse Sinus Asymmetry
The transverse sinus is more often asymmetric than not — usually the right one is bigger, some say because pulsations of the right atrium are propagated cranially in a valveless system to impart a larger capacitance to the ipsilateral jugular system and intracranial sinuses. The above images illustrate an additional layer of complexity — a prominent vein of Labbe (dark blue) empties into the distal right transverse sinus, significantly enlarging its caliber and that of the right sigmoid sinus (pink), whereas the more proximal right transverse sinus is hypoplastic. In this person the left transverse sinus is dominant (yellow). Unless this anatomy is understood, the appearance (particularly on MRI and MRV) may be misconstrued as transverse sinus thrombosis. Notice presence of bilateral emissary veins at the sigmo-jugular junction (white).