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Vascular stuctures
ARTERIAL VARIANTS AND PITFALLS
Knowledge of the existence and clinical relevance of normal variants such as
fenestrations, duplications and persistent fetal arteries is important for a correct
diagnosis and management of many cases of stroke and subarachnoid hemorrhage and
may also aid in surgical planning. The most common arterial variants are detailed below.
However, the high number of variants and asymptomatic anomalies described in the
literature exceed by far this presentation. For further information, we strongly
recommend Dimminck et al. review published in 2009 (see bibliography).
CT angiography.
Several pitfalls of CTA have been described: including difficulty in visualization of small
arteries, differentiating the infundibular dilatation at the origin of an artery from an
aneurysm, venous structures that can simulate or hide aneurysms, and an inability to
identify thrombosis and calcification on 3D images.
ARTERIAL VARIANTS AND PITFALLS
Hypoplasia of some of the component vessels of the
circle of Willis are frequent anatomical variants.
Hypodynamic posterior communicating arteries (A) and
absence of these arteries on right or left side is the
most common variant in most of the studies. Hypoplastic
anterior communicating artery is also frequent. In
absence of stenosis and other vascular anomalies, its has
no clinical significance.
A
Duplicity of the anterior communicating artery: Duplication is defined as two distinct
arteries with separate origins and no distal arterial convergence. Fenestration is defined
as a division of the arterial lumen with a more plexiform appaerance. Duplication of the
anterior communicating artery is seen in 18% of cases, whereas fenestration of the
anterior communicating artery is present in 12% to 21% of people.
ARTERIAL VARIANTS AND PITFALLS
Duplicate middle cerebral arteries have no direct clinical significance; however, there
are reports of aneurysms occurring at the origin of a duplicate middle cerebral artery.
Accessory middle cerebral artery: an accessory middle cerebral artery is an artery
that arises from the anterior cerebral artery and courses parallel to the M1 segment of
the middle cerebral artery. It may be difficult to differentiate an accessory middle
cerebral artery from a duplicated middle cerebral artery. A smaller middle cerebral
artery branch arising from the anterior cerebral artery is designated as an accessory
middle cerebral artery, whereas a smaller middle cerebral artery branch arising from
the distal carotid artery is called a duplicated middle cerebral artery.
ARTERIAL VARIANTS AND PITFALLS
Azygos Anterior Cerebral Artery: An azygos anterior cerebral
artery represents the persistence of the embryonic median artery of
the corpus callosum. This anomaly could be clinically relevant in the
event of anterior cerebral artery occlusion, since the resulting
ischemia could affect both hemispheres.
Anterior Cerebral Artery Trifurcation: Trifurcation of the
anterior cerebral artery is defined as the occurrence of three A2
segments. This normal variant most likely represents a persistent
median callosal artery.
ARTERIAL VARIANTS AND PITFALLS
A1 Segment Absence or Hypoplasia: Hypoplasia of an anterior cerebral artery A1
segment is present in 10% of autopsies. The contralateral anterior cerebral artery may
supply part or all of the territory of the normal anterior cerebral artery via a large
anterior communicating artery.
Case A
Case B
ARTERIAL VARIANTS AND PITFALLS
Accessory Middle Cerebral Artery: An accessory middle cerebral
artery is an artery that arises from the anterior cerebral artery and
courses parallel to the M1 segment of the middle cerebral artery,
supplying the anterior and inferior regions of the frontal lobe.
Persistent Trigeminal Artery: This is a vessel that originates from
the internal carotid artery immediately after its exit from the
cavernous sinus and anastomoses to the midbasilar artery. This
normal variant may be associated with hypoplasia of the
vertebrobasilar system below the anastomosis.
Fetal Origin of the Posterior Cerebral Artery: Fetal origin of the posterior cerebral
artery occurs when the embryonic posterior cerebral artery fails to regress. In the
presence of this anomaly, the caliper 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 artery.
Posterior Communicating Artery Infundibulum: A region of dilatation less than 2 mm at
the origin of the posterior communicating artery from the internal carotid artery must be
distinguished from an aneurysm.
ARTERIAL VARIANTS AND PITFALLS
Aberrant Internal Carotid Arteries: Two different types of aberrant internal carotid
arteries have been described: Intratympanic internal carotid artery and lateral
pharyngeal internal carotid artery.
The intratympanic internal carotid artery may be secondary to disturbed
differentiation of the third branchial artery. It is characterized by an enlarged inferior
tympanic artery that anastomoses with the horizontal petrous part of the internal carotid
artery. On CT images, a mass in the hypotympanum and absence of the vertical segment of
the carotid canal can be seen.
The lateral pharyngeal internal carotid artery is an anomalous vessel that extends to
or near the midline of the posterior pharyngeal wall.
VENOUS VARIANTS AND PITFALLS
Dominant transverse sinus (left or right) It is a common variant without pathological
significance. Maximun intensity projection (MIP) of images obtained during the venous
phase of an enhanced MRA demonstrates enlarged left transverse sinus and jugular bulb.
VENOUS VARIANTS AND PITFALLS
High position of the jugular bulb: A “high riding” jugular bulb is one that extends above
the floor of the internal auditory canal into the middle ear cavity.
Jugular foramen asymmetry is a common finding as a result of a dominant right or left
jugular vein, with enlargement of the ipsilateral jugular foramen (see figures below).
Enlarged superior ophthalmic vein: Asymmetric enlargement of the right superior
ophthalmic vein has no clinical significance.
PROCESSING TECHNIQUES MISTAKES
Maximum intensity projection (MIP): The MIP technique involves selection of the
brightest pixels to make the image while discarding the rest of them. Sometimes MIP
shows the calcification and stenosis better than volume rendered images. However, unlike
volume rendering, MIP does not provide a good sense of depth of the original data and the
structures may look overlapped or pruned. The width of the MIP slab can be changed to
match the vessel diameter and avoid the overlapping of structures.
MIP
VR
PROCESSING TECHNIQUES MISTAKES
Maximum intensity projection (MIP): Too thick MIP reconstructions may underestimate
vascular stenosis. Source images should always be observed to obtain a right stenosis
measurement. In the example below, a several right internal carotid stenosis (arrow)
visible on 0,7 mm original thickness (A) could be underestimated on 5 mm MIP
reconstruction (B).
A
B
PROCESSING TECHNIQUES MISTAKES
Maximum intensity projection (MIP) and volume rendered images (VR) of the circle of
Willis. The right vertebral artery seems to be occluded. To find out the cause, which is a
thrombosed aneurysm (arrow), 2D images must be reviewed.
MIP
VR
PROCESSING TECHNIQUES MISTAKES
Calcific plaques within the wall may give the appearance of “bubbles” on the volumerendered images and sometimes resemble an aneurysm (A). The source images and an
adequate reconstruction algorism which is capable to differentiate calcium from enhanced
vessels are essential (B). Strategically located enhancing lesions like meningiomas or
schwannomas can mimic an aneurysm too. Reviewing the source images may be helpful to
differentiate this pitfall.
A
B
Sometimes it may be difficult to differentiate an infundibulum from a small aneurysm,
such as in cases where the branch arising from the tip is beyond the resolution of the
CTA. In some cases, review of the source images may be of help to differentiate an
aneurysm from the adjacent normal artery.
PROCESSING TECHNIQUES MISTAKES
Pitfalls at window settings: In the volume-rendering technique, inappropriate window
settings can create pseudostenosis. A very clear background may be one indication that
windowing is too wide. On the other side, a very “dirty” background because of a too wide
window may hide some vascular anomalies.
Images show how important is appropriate windowing. 3D volume-rendered image on right
with window width and center at 240/270 H shows artifactual lack of vasculature. Image
in the middle with width and center of 360/259 shows a normal circle of Willis. Image on
the left (window settings 270/200) shows many more peripheral branches.
PROCESSING TECHNIQUES MISTAKES
Stair step artefacts appear around the edges of structures in multiplanar and 3D
reconstructions when wide collimations and non-overlapping reconstruction intervals are
used.
Many vendors provide automated or semiautomated curved multiplanar reconstruction
capability in which the vessel is traced based on the Hounsfield units. However, the
automated tracing may not be reliable along the closely lying contrast-filled veins and
high-density bone.