Clinical Indications Background

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Clinical indications for noninvasive vascular diagnostic modalities have increased in the last ten years due to rapid developments in technology and subsequent improvements of spatial resolution. Improvements in imaging of intracranial vessels have occurred in large part due the possibility to acquire larger volumes. Major advantages of MRA over CTA for the study of intracranial circulation are that it is less invasive, is not entirely dependent on the need for contrast media, and permits better separation of arteries from veins.

Prior to the advent of noninvasive vascular techniques, digital subtraction angiography was the primary imaging modality for the diagnosis of intracranial vessel pathology, even though its indications were largely restricted to the exclusion of vascular malformations in the presence of in-tracranial hemorrhage, to the study of tumour circulation and to the evaluation of venous thrombosis. On the other hand, its applicability for the exclusion of arterial vessel thrombosis was limited, since there was no effective treatment for acute stroke. The demonstrable efficacy of intravenous thrombolysis and, to a lesser extent intrarterial thrombolysis, has increased the need of a more complete evaluation of patient with acute stroke.

Possibly the major advantage of MRA for evaluation of the intracranial circulation is the possibility to intergrate this with conventional MR studies of the brain. In this regard recent sophisticated techniques such as diffusion and perfusion MR imaging, in conjunction with MRA, now permit a considerably more precise diagnosis of vascular lesions.

The recent diffusion of Stroke Unit departments for treatment of acute stroke has increased the need for a more precise and fast diagnosis of cerebral infarction and the evidence that MR can reveal the presence of tissue at risk, the so called

"ischenmic penumbra", has further elevated the importance of MRA as a diagnostic technique in the intracranial vasculature. MRA in this setting permits a rapid demonstration of the occluded vessel thereby facilitating rapid decision-making concerning the most appropriate treatment.

Integration of MR imaging and MRA has been shown to be particularly useful for the evaluation of major sinus venous thrombosis and intracranial carotid dissection, two indications for which conventional angiography is less frequently employed.

Although conventional angiography is still considered the gold standard technique for pre-treatment evaluation of intracranial vascular malformations, MRA has progressively gained ground for both diagnosis and follow-up of cerebral malformations, especially of cerebral aneurysms.

Normal Anatomy

The cerebral blood supply is provided by four arteries: the two internal carotid arteries and the two vertebral arteries. The internal carotid artery (ICA) originates from the bifurcation of the common carotid artery and penetrates the skull base through the jugular foramen. The vertebral artery enters the cranium by way of the occipital foramen and, unlike other arteries which dichotomize, they join to form the larger basilar artery.

Schematically, the intracranial arterial system is divided into an anterior portion which consists of the carotid circulation (Fig. 1), and a posterior portion which consists of the vertebro-basilar circulation (Fig. 2). Both vascular territories anastomose by way of the circle of Willis located at the base of the brain (Fig. 3). Three sets of arteries originate from the circle of Willis: the anterior, middle and posterior cerebral arteries. These arteries provide the blood supply to the cerebral hemispheres, while the brainstem and the cerebel-

Circle Willis Sagittal
Fig. 1. Schematic representation of the internal carotid artery, its bifurcation and the anterior cerebral artery with its cortical branches

I Internal carotid artery (ICA) 1 Medial frontobasal artery

II Middle cerebral artery (MCA) 2 Callosomarginal artery

III Anterior cerebral artery (ACA) 3 Pericallosal artery

Fig. 3. Schematic representation of the complete circle of Willis

I Internal carotid artery (ICA)

II Basilary artery (BA)

III Middle cerebral artery (MCA)

IV Anterior cerebral artery (ACA)

V Posterior cerebral artery (PCA)

a Posterior communicating artery (PCoA) b Anterior communicating artery (ACA) c Superior cerebral artery lum are supplied exclusively by branches of the vertebro-basilar circulation.

Internal Carotid Artery

After emerging from the petrosal portion of the temporal bone, the intracranial tract of the ICA passes anteriorly to the petrous apex and runs along the sides of the sella, within the cavernous si-

Vertebral Artery Branches
Fig. 2. Schematic representation of the vertebro-basilar circulation from the vertebral arteries to the distal branches of the posterior cerebral arteries

I Vertebral artery (VA) a Inferior posterior cerebellar artery

II Basilary artery (BA) b Inferior anterior cerebellar artery

III Posterior cerebral artery c Superior cerebellar artery (PCA) d Temporooccipital arteries

IV Anterior spinal artery e Medial occipital arteries nus. This segment is also called the carotid siphon due to its double curved path within the cavernous sinus. After emerging from the cavernous sinus, the ICA lies within the subarachnoid space. Important branches originating from the ICA at this point include the meningohypophyseal trunk, the inferolat-eral trunk, and the ophthalmic arteries (OA).With-in the supra-cavernous segment, the ICA has a vertical orientation and enters the subarachnoid space adjacent to the anterior clinoid process. The branches which originate at this point include the posterior communicating artery (PCoA) and the anterior choroidal artery (AChA). Only the PCoA and OA are generally well represented on MRA images (Fig. 4a,b) (Table 1) [1-3].

The OA originates from the anterio-medial surface of the ICA while the PCoA and the AChA originate from the posterior surface. The PCoA runs immediately above the III cranial nerve and merges with the posterior cerebral artery (PCA). The AChA is the last vessel which originates from the ICA and is not always seen on MRA (Figs. 5-6) [4-7]. The ICA terminates by dividing into two arteries: the middle cerebral artery (MCA) and the anterior cerebral artery (ACA).

Persistence of fetal carotid-vertebrobasilar anastomoses, such as the trigeminal artery which directly connects the intracavernous ICA with the basilar artery, is the most common form of anatomical variation of the ICA (Fig. 7).

Each ACA is subdivided into two tracts: the pre-

I Internal carotid artery (ICA) 1 Medial frontobasal artery

II Middle cerebral artery (MCA) 2 Callosomarginal artery

III Anterior cerebral artery (ACA) 3 Pericallosal artery

Middle Cerebral Artery Internal Carotid

Fig. 3. Schematic representation of the complete circle of Willis

I Internal carotid artery (ICA)

II Basilary artery (BA)

III Middle cerebral artery (MCA)

IV Anterior cerebral artery (ACA)

V Posterior cerebral artery (PCA)

Table 1. Path and branches which originate from the internal carotid artery

Segment

Path

Collateral branches

MRA visibility

Cervical

Vascular space, parapharyngeal space

None

-

Intrapetrous

Carotid canal

Carotico-tympanic artery, vidian and periostea arteries

No

Cavernous

C5

Petrous apex at the posterior bend

Meningohypophyseal trunk

No

C4

Petrous apex at the anterior bend

Inferolateral trunk

No

C3

Intradural segment

McConnel's capsular artery

No

C3-C2

Mostly intradural

Ophthalmic artery

Fair

Supra-cavernous C2-C1

Within the subarachnoid space medial to the anterior clinoid process and under the optic nerve

Superior hypophyseal artery

No

C1

Vertical segment which divides in a T to form the anterior and middle cerebral

Posterior communicating artery

Good

arteries

Anterior choroidal artery

Rarely

Anterior Choroidal Artery

/ AJ

m m

y is y /

N PCoA

P

3 ^

» \ \ *

Y

b

Fig. 4. (a-b). 3D TOF MRA of a normal internal carotid artery, and the anterior and middle cerebral arteries in anterior (a) and lateral view (b) (see Table 1)

Axial Middle Cerebral Artery

Fig. 6.3D TOF MRA (axial view/). Demonstration of the origin and normal course of the right anterior choroidal artery

Fig. 5. 3D TOF MRA (axial view). Both ophthalmic arteries and posterior communicating arteries are well defined

Fig. 6.3D TOF MRA (axial view/). Demonstration of the origin and normal course of the right anterior choroidal artery

Fig. 7.3D TOF MRA (axial view). Persistence of the trigeminal artery (arroW with direct connection of the right cavernous internal carotid artery to the basilar artery mÊÊ

m i A31

w\Wjw communicating A1 tract from which originate the lenticular-striat arteries and, sometimes, Heubner's artery, and the A2 tract, which bifurcates into the frontal-polar and orbital-frontal arteries (Fig. 8a). The ACA divides into the marginal callosum and the pericallosum arteries at the level of the bend of the corpus callosum (Fig. 8b). The most frequent anatomical variants of the ACA are the "azygos" ACA (Fig. 8c) and the triple pericallosal artery (Fig. 8d). The anterior communicating artery (ACoA) forms an anastomosis between the two ACAs. Fen-estrations and duplications of the ACA are rare anatomical anomalies which may be associated with small aneurysms of the ACoA [8,9].

The MCA is divided into four segments (Fig. 9): the horizontal or sphenoidal segment (M1) from which originate the perforating lenticulostriate arteries which are not visible on MRA, the insular segment (M2), the opercular segment (M3) and the M4 segments which are outside of the sylvian fissure and supply the frontal and parietal convexity. Anatomical variants of the MCA are uncommon. The most frequent are duplications, fenestrations, or accessory arteries (Fig. 10) [10].

Azygos Anterior Cerebral Artery

Fig. 8a-d. 3D TOF MRA of the anterior circulation. Representation of the anterior cerebral artery (a), callosal artery (b), azygos artery (c) and triple pericallosal artery (d)

m i A31

Anterior Cerebral Circulation

Fig. 8a-d. 3D TOF MRA of the anterior circulation. Representation of the anterior cerebral artery (a), callosal artery (b), azygos artery (c) and triple pericallosal artery (d)

Anterior Cerebral Artery

Fig. 9. 3D TOF MRA (axial view). Normal representation of the middle and posterior cerebral arteries with their different segments

Fig. 11. Selective angiogram of the left vertebral artery with opacification of the basilar artery, cerebellar arteries and posterior cerebral arteries

Fig. 9. 3D TOF MRA (axial view). Normal representation of the middle and posterior cerebral arteries with their different segments

Vertebro-Basilar System

The vertebro-basilar circulation comprises the vertebral, basilar, and posterior cerebral (PCA) arteries (Fig. 11). The posterior inferior cerebellar artery (PICA) usually originates before the vertebral arteries merge to form the basilar artery. This artery is usually visible on MRA images (Fig. 12). Conversely, the other branches (meningeal, spinal and bulbar) are seldom visible. Asymmetrical variations of the vertebral arteries are common: most frequently, one of the vertebral arteries is either absent, hypoplastic or ends directly in the PICA, resulting in the basilar artery originating directly from the contralateral vertebral artery.

The basilar artery runs within the pre-pontine cistern and terminates by dividing into two at the PCA. The two anterior-inferior cerebellar arteries (AICA) and the two superior cerebellar arteries (SCA) originate from the caudal tract of the basilar artery. Both AICA and SCA are often visible on MRA images (Fig. 13) [11]. Anatomical variants of the basilar artery are uncommon. The most frequent anomaly is fenestration of the proximal segment (Fig. 14) [12].

The PCA is divided into four segments (Fig. 9): the pre-communicating segment (P1), the per-

Posterior Circulation Mra
Fig. 12.3D TOF MRA (coronal view,). Normal representation of the posterior circulation and of the normal left posterior inferior cerebellar artery (PICA) originating from the left vertebral artery
Aica Pica Variant

Fig. 13.3D TOF MRA (coronal view). Normal representation of the posterior circulation and of both anterior inferior cerebellar arteries (AICA) and the superior cerebellar arteries (SCA)

Anomaly Fenestration
Fig. 14.3D TOF MRA (coronal view). Fenestration (arrow) of the proximal segment of the basilar artery

Fig. 13.3D TOF MRA (coronal view). Normal representation of the posterior circulation and of both anterior inferior cerebellar arteries (AICA) and the superior cerebellar arteries (SCA)

Table 2. Vertebro-basilar system and branches

Segment

Path

Collateral branches

MRA visibility

Vertebral arteries

Medullary cistern

PICA

Good

Basilar artery

Prepontine cistern

AICA, SCA

Good

Perforators

No

PCA

Perimesencephalic cistern

Posterior choroidal

No

Lenticulostriate

No

Parieto-occipital

Good

Calcarine

Good

imesencephalic segment (P2) from which originates the lenticulostriate and posterior choroidal (PChA) arteries which are not detectable on MRA, segment (P3) which runs behind the quadrigemi-nal lamina and which divides into two branches which are visible on MRA images, and the parieto-occipital and calcarine arteries, and the most distal branches (P4) (Table 2).

The Circle of Willis

The circle of Willis is the most important system of anastomosis between the carotid and vertebro-basilar systems; it also connects the circulation of the right and left hemispheres thereby providing a possible mechanism for hemodynamic compensation in cases of severe stenosis or occlusion of the

ICA and/or basilar artery. In its complete or "balanced" form which occurs in 20% of cases, the circle of Willis is composed of the two ICAs, the two A1 tracts of the ACA, the ACoA, the two PCoAs and the two P1 segments of the PCA (Fig. 15).

The circle of Willis presents with a wide range of anatomical variants due either to hypoplasia or agenesis of one or more components (Fig. 16). The most frequent sites of hypoplasia or agenesis are the PCoA (34%) and the A1 tract (25%) with "fetal" origin of the PCA from the ICA. Hypoplasia or absence of the P1 segment is also relatively common (17%) (Fig. 17a-b) [3-6].

Anastomosis between the intracranial and ex-tracranial arterial circulation is provided by the OA and leptomeningeal arteries. In pathological

Circle Willis Circulation

Fig. 15.3D TOF MRA (axial view). Normal complete circle of Willis Fig. 16.3D TOF MRA (axial view). Normal variant of the circle of

Willis. Agenesis of the anterior communicating artery (white arrow) and the left posterior communicating artery (black arrow)

Fig. 15.3D TOF MRA (axial view). Normal complete circle of Willis Fig. 16.3D TOF MRA (axial view). Normal variant of the circle of

Willis. Agenesis of the anterior communicating artery (white arrow) and the left posterior communicating artery (black arrow)

Circle Willis Normal VariantCircle Willis Axial

Fig. 17a-b. 3D TOF MRA (axial view). Normal variant of the circle of Willis. Agenesis of the right Al segment of the anterior cerebral artery (a) (arroW and of the left PI segment of the posterior cerebral artery with "fetal" origin from the internal carotid siphon (b) (arrow)

conditions in which the ICA is obstructed, these anastomoses permit the spontaneous revascular-ization of the cerebral arteries [ 13]. While the lep-tomeningeal arteries are seldom visible on MRA images, revascularization of the intracranial ICA by way of the OA can be observed due to the hypertrophy of this vessel.

Cerebral Venous Anatomy

The venous system is composed of dural sinuses, diploic veins, meningeal veins, and superficial and deep cerebral veins. Schematically, the blood is drained from the brain to the deep venous system (centripetal flow) and to the superficial venous system (centrifugal flow). Both these systems drain into the dural venous sinuses which also col lect blood from the diploic and meningeal veins. This is the most important venous drainage pathway of the brain [14,15], (Fig.18a-b).

The dural sinuses are: the superior sagittal sinus (SSS) that runs along the midline within the superior insertion of the falx cerebri and terminates in the confluence of sinuses indicated as tor-cular herophili (Fig. 19a-b) [16]; the inferior sagittal sinus (ISS) that originates at the rostral margin of the corpus callosum and runs within a dural fold in the inferior margin of the falx cerebri following the superior profile of the corpus callosum; and the transverse sinuses (TS) contained within the tentorium of each side and run along the insertion with the internal surface of the occipital bone (Fig. 20a-b). The two TS systems continue caudally with the sigmoid sinus and then finish in the jugular vein. The right TS is generally larger than the

Cerebral Venous Sinuses

Fig. 18a-b. Schematic representation of the cerebral venous drainage system in the lateral (a) and coronal (b) views a lateral view b coronal view

Fig. 18a-b. Schematic representation of the cerebral venous drainage system in the lateral (a) and coronal (b) views a lateral view

I Superior sagittal sinus (SSS)

II Inferior sagittal sinus (ISS)

III Sagittal sinus (SS)

IV Transverse sinus (TS)

V Sigmoid sinus a Internal cerebral vein (ICV)

b Basal vein (BV)

c Vein of Galen (VG)

d Superior cerebral veins e Sinus confluence (SC)

b coronal view

I Superior sagittal sinus (SSS)

II Inferior sagittal sinus (ISS)

III Sinus confluence (SC)

IV Transverse sinus (TS)

V Sigmoid sinus

I Superior sagittal sinus (SSS)

II Inferior sagittal sinus (ISS)

III Sagittal sinus (SS)

IV Transverse sinus (TS)

V Sigmoid sinus a Internal cerebral vein (ICV)

b Basal vein (BV)

c Vein of Galen (VG)

d Superior cerebral veins e Sinus confluence (SC)

Callosomarginal

I Superior sagittal sinus (SSS)

II Inferior sagittal sinus (ISS)

III Sinus confluence (SC)

IV Transverse sinus (TS)

V Sigmoid sinus

Sinus Sigmoid

Fig. 19a-b. Cerebral angiography (a) in the venous phase and corresponding 3D PC MRA (b) of the principal intracranial dural sinuses: Superior Sagittal Sinus (SSS) (white arrow), Straight Sinus (SS) and Transverse Sinus (TS) (blackarrow left but a wide range of anatomical variants may be encountered [17].

The cavernous sinus is located in the anterior portion of the cranial base on both sides of the sphenoid body, within the dural space which constitutes the walls of the various canals within which the siphon runs, perpendicular to the III, IV, V1,2 and VI cranial nerves. The cavernous sinus receives blood from the orbit by way of the superior ophthalmic vein and from the anterior portion of the sfeno-parietal sinus [2]. The superior petrosal sinus and the inferior petrosal sinus flow into the lateral and medial portions, respectively, of the posterior portion of the cavernous sinus (Fig. 21). The superior petrosal sinus provides a connection between the cavernous sinus and the TS. The inferior venous sinus provides a connection between the inferior petrosal sinus and the ipsilateral sig-moid sinus.

The superficial veins of the brain which flow

Inferior Petrosal Sinus
Fig. 20a-b. Cerebral angiography (a) in the venous phase and corresponding 3D PC MRA (b) of the principal intracranial dural sinuses and of the Sinus Confluence (SC)
Cerebral Angiogram

Fig. 21.3D PC MRA of the deep venous system: Vein of Galen (VG), Basilar vein of Rosenthal (BVR), Internal cerebral vein (ICV), Straight sinus (SS); the sphenoparietal sinus (SpS) and the cavernous sinus (CS) are also shown

Table 3. Affluences of major dural sinuses and veins

Venous sinus

Affluent veins (venous inflow from)

MRA visibility

Superior Sagittal Sinus

Superficial veins of the brain

Good

Vein of Trolard

Fair

Transverse sinus

Vein of Labbe

Good

Superficial middle cerebral vein

Good

Emissary veins

Seldom

Vein of Galen

Internal cerebral vein

Good

Basilar vein of Rosenthal

Good

Straight sinus

Vein of Galen

Good

Inferior sagittal sinus

Fair

Cavernous sinus

Superior ophthalmic vein

Good

Sphenoparietal sinus

Good

Superior petrosal sinus

Fair

Inferior petrosal sinus

Fair

Clival venous plexus

Seldom

Deep Middle Cerebral Vein

Fig. 22a-b. Cerebral angiography (a) in the venous phase and 2D PC MRA (b) of the deep and superficial cerebral venous system

Cerebral Angiogram

Fig. 22a-b. Cerebral angiography (a) in the venous phase and 2D PC MRA (b) of the deep and superficial cerebral venous system into the SSS and the TS constitute the superficial venous system (Table 3). The deep venous system consists of the subependimal, terminal, anterior caudate, and the septal veins which merge into the internal cerebral vein (ICV). These veins run within the superior portion of the III ventricle and after reaching the basilar vein of Rosenthal, flow into the vein of Galen (VoG) (Fig. 22a-b).

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  • kip norman
    Can a transverse sinus occulusion cause interior turbinate hypertrophy?
    8 years ago

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