multidetector computed tomographic (ct) angiography

Multidetector computed tomographic (CT) angiography : FREQUENTLY

ANATOMICAL VARIATIONS OF THE CIRCLE WILLIS 1

Multidetector computed tomographic (CT) angiography :

FREQUENTLY ANATOMICAL VARIATIONS OF THE CIRCLE

WILLIS

ICONOGRAPHIC REVIEW

Dra. Ximena González Larramendi Dr. Fernando Landó Baison

ABSTRACT:

Objetives: To describe the variations of cerebral arterial blood flow related to the circle of Willis, excluding those related to the persistence of basilar – carotid and to the skull base anastomosis. Clinical relevance, characteristics and incidence from the data provided by the literature will be evaluated.

Methods: A retrospective observavational descriptive analysis of all circle of Willis angiotomographys performed in the hospital of Tacuarembó, during a period from december 1st 2009 until august 31st 2013 was made.

In cases in wich relevant normal variationes were identified, 3D and MIP reconstruccions were performed, allowing a correct anatomical description of them.

The data relating to the prevalence and association with other anomalies and aneurysms were taken from the literature.

Results: The most common variants found in our study are linked to the PCA and to the A1 segment of ACA, wich is consistent with the literature.

Conclusions: The circle of Willis presents variations in its anatomy in 60% of the cases. Some of them have clinical relevance in relation to an increase in the incidence of aneurysms and malformations, in relation to oclusive events and surgical planing, so it is essencial to recognize them.

ABREVIATIONS ATC: angioTc CI: internal carotid artery ACA: anterior cerebral artery ACM middle cerebral artery ACoA: anterior comunicating artery ACP: posterior cerebral artery ACoP: posterior comunicating artery ACS: superior cerebellar artery TB: basilar artery

2 ICONOGRAPHIC REVIEW

INTRODUCTION:

The circle of Willis, described by Thomas Willis in 1664, exhibits a known and systematized anatomy.

It is a vascular circuit interconnecting the supraclinoid portion of both internal carotid, the horizontal segment (A1) of the anterior cerebral artery and the horizontal segment (P1) of the posterior cerebral artery through the anterior communicating artery and the posterior communicating arteries, respectively .

Fewer than 40% of cases is presented complete and well developed so they are very frequent variations in its anatomy.

Some of them are associated with an increased incidence of intracranial aneurysms or encephalic malformations . They may have implications with arterial occlusive event to determine ways of substitutions and extent of the ischemic area.

Sometimes represent information relevant to surgical planning; such as the identification of the azygos ACA, the

trifurcation of the ACA and the accessory MCA in surgeries of anterior aneurysms.

With the emergence of multislice scanners ATC has become a frequent examination incorporated into the work routine due to its high spatial resolution and high sensitivity and specificity in the detection of vascular abnormalities.

We will a iconographic review of the most common variants of the circle of Willis through retrospective image analysis ATC, obtained in the service of Tacuarembo tomography Hospital, in the period from December 1, 2009 to August 31, 2013; exclude those related to persistent carotid-‐basilar anastomosis and skull base.

RESULTS:

DUPLICATION

It is the presence of two separate vessels of similar origin, route and destination. The most common is to ACoA duplication, with a prevalence of 18%.

FENESTRATION

Is defined as a division of the arterial lumen into distinctly separate channels, each with its own endothelial and muscularis layers. Intracranial arterial fenestration is more common in the vertebrobasilar arteries than in the arteries of the anterior circulation. Very rare in the posterior cerebral and communicating arteries, have a lower prevalence of 4% in the ACA (A1 segment), and 5% in the ACoA. An association has been observed between fenestration and aneurysm formation.

Fig. 1: ACoA DUPLICATION: each vessel originating separately from an anterior cerebral artery. formed two separate channels. (arrowheads).



ABSENCES AND HYPOPLASIAS

Sometimes some segment of the polygon can be absent or present a diminished caliber. In these cases, usually the contralateral artery supplies part or all the vascular territory, which implies an additional risk in case of ischemia.

Fig. 2: FENESTRATION OF THE ACA: dual

channels with a common origin from the

anterior cerebral artery (arrows).

Fig 3: HYPOPLASIA A1 SEGMENT OF THE LEFT ACA (black arrows) : posterior comunicating carotid aneurysm (white arrow) and another variant, a fetal origin of the ipsilateral PCA (arrowhead) is also noted

Fig. 4: AGENESIS A1 SEGMENT OF THE ACA : right-‐ A2 segments (arrows) originating from the left A1 segment and absent right A1 segment (arrowhead)

Fig 5: AGENESIS OF ACoA: (arrow)

Fig 6: ABSENCE OF LEFT ACoA : arrowhead


AZYGOS ANTERIOR CEREBRAL ARTERY

The prevalence of azygos anterior cerebral arteries is 0.2%–4.0%

Represents persistence of the embryonic median artery of the corpus callosum .

It is characterized by the convergence of the two A1 segments in a unique A2, disposition medial segment which provides both hemispheres vascularization. It may be associated with intracranial aneurysms, holoprosencephaly and abnormalities in neuronal migration.

TRIFURCATION OF THE ANTERIOR CEREBRAL ARTERY :

Is defined as the occurrence of three A2 segments

This normal variant most likely represents persistence of the median callosal artery.

BIHEMISPHERIC ANTERIOR CEREBRAL ARTERY

It is recognized by the presence of a dominant A2 segment that is the main blood supply to both hemispheres with hypoplasia of the contralateral A2 segment. The clinical relevance is similar to the azygous ACA.

Fig 8: TRIFURCATION OF ACA: A and B: three A2 segments (thin arrows) are observed C: trifurcation of ACA (thin arrows) associated ACoA aneurysm (thick arrow)

Fig 7. AZYGOS ACA: volumetric reconstructions (A) and MIP (B) of the anterior sector of the polygon. Two segments A1 (arrowheads) originating a single A2 segment are observed

Fig. 9: BIHEMISPHERIC ACA: volumetric and MIP reconstructions , A and B: hypoplastic segment A2 (arrow), bi-‐hemispheric dominant contralateral A2 segment (arrowhead).



MIDDLE CEREBRAL ARTERY ACCESSORY

Arises from the ACA and runs parallel to the M1 segment of the MCA, supplying the anterior inferior frontal lobe region. Its importance is that it represents a collateral blood supply to the distal territory of the middle cerebral artery. Can associated an aneurysm at source.

POSTERIOR COMMUNICATING ARTERY INFUNDIBULUM

Is a simetric dilatation at the origin of the posterior communicating artery from the internal carotid artery.. Has a diameter of less than 2 mm and thins distally. Must be distinguished from aneurysms of the posterior communicating artery.

FETAL ORIGIN OF THE POSTERIOR CEREBRAL ARTERY

It may occur on the right side (10% of the general population), the left side (10% of the general population), or bilaterally (8% of the general population)

It is the persistence of the embryonic posterior cerebral artery.

In the presence of this anomaly, the caliber of the posterior communicating artery may be the same as or greater than that of the ipsilateral P1 segment, and the dominant blood supply to the occipital lobes comes from the internal carotid arter.

The P1 segment may be absent or hipoplasic.

Fig. 10 ACM ACCESORIA: A-‐ volumetric reconstructions (black arrow) B-‐ MIP reconstruction

Fig. 11: POSTERIOR COMMUNICATING ARTERY INFUNDIBULUM: A-‐ Volumetric Reconstruccion; B-‐MIP Recostruction: Right infundibulum (arrow). Left carotid aneurysm (thick arrow) were also observed.


COMMON POSTERIOR CEREBRAL AND SUPERIOR CEREBELLAR TRUNK

It has a prevalence of between 2-‐22%.

This anomaly does not have any reported clinical significance .

CONCLUSIONS:

The circle of Willis has variations in their normal anatomy as much as 60% of cases. Some of them have clinical relevance in relation to increase in incidence of malformations and aneurysms, occlusive events before and surgical planning, which is fundamental for recognizing them.

REFERENCES

-‐ 1-‐DimmickSimmon, Faulder Kenneth: Normal variants of the cerebral circulation al multidetector CT angiography. RadioGraphics 2009; 29:1027-‐1043.

-‐2-‐OsbornAnne. Angiografia Cerebral 2ª edicion 2000. Marban.

-‐3-‐Martinez F, Spagnuolo E, Calvo A, Sgarbi N, Soria V. Variaciones del sector anterior del poligono de Willis, correlación anatomo-‐angiografica y su implicancia en la cirugía de aneurismas intracraneanos. Neurocirugía 2004; 15 : 578-‐589

-‐4-‐Grossman –YousemNeurorradiologia 2da edición 2007 Marban.

Fig. 12: Fig. 12: FETAL ORIGIN OF THE LEFT ACP (black arrows) A-‐ Associated with hypoplastic P1 segment of the left PCA (arrowhead). B-‐ Associated agenesis of P1 segment of the left PCA.

Fig 13. : TRUNK COMMON OF ACP AND ACS (arrow).

multidetector computed tomographic (ct) angiography

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