Anterior cerebral artery

On either side ICA divides into ACA which travel anteromedially and MCA which travel laterally.
ACA includes three segments A1, A2 and A3.
A1. From ICA to anterior communicating artery.
A2. From anterior communicating artery to the origins of the pericallosal and supramarginal arteries.
A3. Distal ACA or cortical branches.


A1 segment and Acom complex
The A1 segment also known as precommunicating segment, extends from the ICA bifurcation to its junction with the anterior communicating Artery. It travels superior to the optic chiasm or optic nerves and inferior to the anterior perforated substance.

In most cases, the A1- Acom complex assumes one of the four configurations.
A) Single or duplicated Acom forms a bridge between the ACAs.
B) A single large branch arising from the A com
C) Absent A-com and the two ACAs join together directly.
D) Azygos ACA.

Single or duplicated Acom forms a bridge between the ACAs. 

A single large branch arising from the A com 

Absent A-com and the two ACAs join together directly 

Azygos ACA.

A1 Branches: 
Perforating branches of A1 divided into superior and inferior branches.
i. Approximately 2–15 superior branches are medial lenticulostriate arteries that travel superiorly and posteriorly into the anterior perforated substance and supply the anterior hypothalamus, septum pellucidum, anterior commissure, fornix, and the anterior striatum.
ii. Inferior branches supply the optic chiasm and optic nerves.
Acom branches
Perforating branches of the Acom divided into subcallosal, hypothalamic and chiasmatic branches, according to their vascular territories.  The subcallosal branch is usually single and the largest branch of the A com, supplies the septum pellucidum, columns of the fornix, corpus callosum and lamina terminalis.
Recurrent artery of Heubner, most often an A2 branch, may arise from the A1 segment in upto 17% of cases and from the ACA-A-comm junction in 35% of cases.

Variations 
A1 variants
Asymmetry:The left and right A1 segments are asymmetric in size in upto 80% of cases. About 10% of the A1 vessels are hypoplastic.
Absence:Absence of one A1 segment is seen in 1–2% of cases.
Infraoptic ACA:  The A1 segment may travel inferior to or through the optic nerve, rare.
Fenestration of A1 segment.
Accessory ACA: An atypical branch of the ICA courses under the optic nerve and ACA to give rise to the orbitofrontal and frontopolar arteries.
Anomalous origin of A1 from the cavernous ICA or from the contralateral ICA.
Acom variants
A normal Acom, is a single vessel which forms a link between two ACAs, is present in only about 40% of cases.
Anomalous Acom anatomy is present in the remaining 60% of cases. Some 227 A-comm artery complex variations or pattern have been described. These patterns included plexiform (i.e., multiple complex vascular channels, 33%), dimple (i.e., incomplete fenestration, 33%), fenestration (21%), duplication (18%), string (18%), fusion (12%), median artery of the corpus callosum (6%), and azygos ACA (3%). The Acom is absent in some 5% of cases.

A2
The A2 segment travels in a vertical direction, adjacent to the genu of the corpus callosum, extend from the Acom to its division into the pericallosal and callosomarginal arteries. Averages 43 mm in length.
The left and right A2 segments usually travel together in the interhemispheric fissure. Right A2 is more often (~72% of cases) anterior to the left A2 in the sagittal plane.

A2 Branches
Perforating branches penetrate the gyrus rectus and olfactory sulcus.
Recurrent artery of Heubner, also known as the medial striate artery or long central artery.  Thelargest medial lenticulostriate perforator branch from ACA. Arises from the A2 segment in most (57–78%) cases, may arise from A1 segment in up to 17% of cases and from the ACA-A-comm junction in 35% of cases. Called ‘Recurrent’ because it takes U turn after origin, goes ‘back’ laterally over A1, parallel to A1, in opposite direction of A1, towards the terminal ICA, so it often looks like a smaller artery running alongside and above the relatively bigger A1.
Orbitofrontal artery, this artery runs close to the midline in an anterior direction to the gyrus rectus, olfactory bulb and medial aspect of the inferior frontal lobe.
Frontopolar artery, this artery travels anteriorly and superiorly towards the frontal pole.

Variations 
Azygos ACA, a single unpaired A2 segment that arises from the junction of the A1s. It is present in < 1% of the general population. Commonly associated with terminal aneurysm in ~41% of cases.  Associated with holoprosencephaly.
Duplicated A2.
Superior anterior communicating artery, an anomalous communicating vessel between the ACAs near the corpus callosum.

A3
‘A3’ include all the ACA cortical branches distal to the origin of the pericallosal and callosomarginal arteries. Some authors have further subdivided the distal ACA into A4 and A5 segments.
The distal ACA branches have extensive anastomoses with distal branches of the MCA and PCA with a watershed zones inbetween which is most vulnerable to ischemia during hemodynamic failure.

A3 Branches 
Pericallosal artery, the pericallosal artery comprises the main trunk of the ACA as it passes posteriorly over the corpus callosum, gives off multiple small branches 'short callosal arteries'  that travel laterally along the corpus callosum and anastomoses with the splenial artery the 'posterior pericallosal branch', a branch of the PCA.
Callosomarginal artery, is the second largest distal branch of the ACA, after the pericallosal artery. It travels superiorly over the cingulate gyrus to run in a posterior direction within the cingulate sulcus.
Frontal branches are Anterior frontal, middle frontal and posterior frontal. These branches arise from the pericallosal or the callosomarginal artery, are identified according to which part of the superior frontal gyrus they supply.
Paracentral artery, supply the paracentral lobule.
Parietal arteries, the final and most distal branches of the ACA, anastomose with the parietooccipital branch of the PCA. They can be divided into Superior parietal and inferior parietal artery.

Cortical ACA Branches:
Orbitofrontal Artery.
Frontopolar Artery.
Internal Frontal Branches (Anterior, Middle, and Posterior).
Paracentral Artery.
Parietal Arteries (Superior and Inferior).

Reference: Handbook of Cerebrovascular Disease and Neurointerventional Technique.  Mark R. Harrigan, John P. Deveikis and Agnieszka Anna Ardelt. 

Circle of Willis Anatomy

The circle of Willis is the ring of interconnecting vessels that encircles the pituitary infundibulum and provides important collateral circulation between the carotid territories and the vertebro basilar system.

It is actually a heptagon, a seven sided structure, not a circle.
Although it bears the name of Thomas Willis, named in honor of Willis by his student Lower who actually described this structure for first time.

Vessel contributing in formation of Circle of Willis are ICA from either side and PCAs from Basilar. Right and left ACA A1 segments anteriorly from ICAs on either side and Acom in between.
Pcoms from ICA on either side.
Right and left PCA P1 segments posteriorly from PCAs on either side.

A complete well-developed and symmetric circle is found in <50% of cases.
In some 60% of cases, at least one component of the circle is relatively hypoplastic and diminished in its capacity to provide collateral flow.
Sources of asymmetry in the circle of Willis.
Vessel: Variant and incidences
A1 segment: Hypoplastic in 10%; Absent in 1–2%1
Acom: Absent in 5%
Pcom: Hyperplastic (Fetal) in 18–22%; Hypoplastic in 34%; Absent in 0.6%
ICA: Hypoplastic in 0.079%; Absent in 0.01%
P1 segment: Hypoplastic in 15–22% ; Absence is Rare.

Asymmetry of the circle of Willis results in significant asymmetry of flow and is an important factor in the development of intracranial aneurysms and atypical ischemic stroke.

Reference: Handbook of Cerebrovascular Disease and Neurointerventional Technique.  Mark R. Harrigan, John P. Deveikis and Agnieszka Anna Ardelt. 

Carotid siphon

The carotid siphon is an S-shaped part to the ICA; it begins at the posterior bend of the cavernous ICA and ends at the ICA bifurcation.
Cavernous and supra clinoid portions of the ICA forms carotid siphon. Cavernous portion contributes greater part of the carotid siphon, the cavernous portion consist of sub segments as (a) Posterior vertical, (b) Posterior bend, (c) Horizontal, (d) Anterior bend, and (e) Anterior vertical.

The siphon can have an open or a closed configuration.
A closed siphon anatomy can be attributed to exaggerated tortuosity of the ICA, can be seen in patients with advanced age or fibromuscular dysplasia. Clinical significance is during the endovascular navigation where it becomes difficult to negotiate catheter in such close configuration turn.

Haughton's view
During DSA 'Haughton view' is used to open up the carotid siphon and to prevent overlapping of MCA branches within the Sylvian fissure. This view is also helpful for imaging of ICA and MCA aneurysms, PCom origin and anterior choroidal artery.
The lateral arc is positioned as if the patient’s head is tilted away from the side of the injection and away from the xray tube. In simple words the X-ray tube should touch the shoulder on the side of interest.

Reference: 
Handbook of Cerebrovascular Disease and Neurointerventional Technique.  Mark R. Harrigan, John P. Deveikis and Agnieszka Anna Ardelt.

Carotid - Vertebrobasilar Anastomoses

These are transient connections which appear during embryonic development between the carotid and vertebro basilar circulations. These embryonic anastomotic connections usually disappear as the posterior communicating arteries develop.
In rare cases these vessels persist into adulthood.
From superior to inferior, these persistent fetal vessels are:
1. Fetal PCA, the most common of all with prevalence of ~20% of genernal population.
2. Trigeminal, named after the cranial nerve it parallels.
3. Otic,
4. Hypoglossal, named after the cranial nerve it parallels.
5. Pro atlantal intersegmental artery.

1. Persistent Fetal PCA
Normally the posterior communicating artery arises from the ICA just proximal to the ICA bifurcation joins PCA at the junction of p1 and p2 segments. Fetal PCA or  Persistent fetal origin of PCA defined as a prominent P com that gives rise to or continues as PCA p2 segment and onwards with same diameter.  Ipsilateral PCA p1 segment is usually hypoplastic or may be absent.




2. Persistent Trigeminal Artery
The next most common.
Extends from the cavernous ICA to the upper part of the basilar artery and often perforates the dorsum sella.
The vertebrobasilar system proximal to the upper basilar artery may be hypoplastic, with the primitive trigeminal artery supplying most of the flow to the PCAs and the SCAs.
Two main variants.
Saltzman Type I. The persistent trigeminal artery supplies the PCA and SCA territories. The posterior communicating arteries and the basilar artery proximal to the anastomosis are hypoplastic.
Salzman Type II. The PCAs are supplied by the posterior communicating arteries, and the persistent trigeminal artery joins the basilar artery at the level of the SCAs.
Clinical significance is association with intracranial aneurysms. May have an intrasellar component and should not be mistaken for a pituitary mass.

3. Persistent Otic aArtery
Rarest of all carotid-basilar anastomosis.
Extends from the petrous ICA to the basilar system via the internal auditory canal.


4. Persistent Hypoglossal Artery
Common carotid-basilar anastomosis next to trigeminal.
Extends from the cervical ICA to the basilar artery via the hypoglossal canal.
The ipsilateral vertebral artery is usually hypoplastic.
May be associated with an aneurysm.

5. Proatlantal Intersegmental Artery
Extends from the cervical ICA or ECA to the vertebrobasilar system via the foramen magnum.
Relatively rare.
Associated with aplasia or hypoplasia of the vertebral arteries in 50% of cases.

Reference: Handbook of Cerebrovascular Disease and Neurointerventional Technique.  Mark R. Harrigan, John P. Deveikis and Agnieszka Anna Ardelt.

Internal carotid artery Anatomy

Among various classification available for describing the portions of ICA the most simple and widely used is based on the description by Gibo and colleagues.
ICA on either side divided in to four segments or portions.
1. Cervical
2. Petrous
3. Cavernous
4. Supraclinoid

1. Cervical portion 
This portion begins at the carotid bifurcation (usually at the level of C3) and ends at the skull base and usually has no branches. The ICA receives approximately 80% of flow from the CCA.
Further divided into two divisions.
a. Carotid bulb, a focal dilation of the ICA at the origin, ~ 7.4 mm in diameter, compared to 7.0 mm diameter for the CCA.
b. Ascending cervical segment ~4.7mm diameter which remains relatively constant throughout its course.

2. Petrous portion 
The petrous portion extends from the opening of the carotid canal in the skull base to the posterior edge of the foramen lacerum. The proximal portion is vertical followed by horizontal portion with a genu portion which is a bend in the vessel of ninety degree.

3. Cavernous portion
The cavernous segment is S-shaped, extends through the cavernous sinus, is surrounded by areolar tissue, fat, postganglionic sympathetic fibers and the interconnecting venous chambers of the cavernous sinus. The ICA rests directly against the lateral surface of the body of the sphenoid bone in a groove called the carotid suclus. Further divided into five sub segments.
a. Posterior vertical,
b. Posterior bend,
c. Horizontal,
d. Anterior bend,
e. Anterior vertical.

4. Supraclinoid portion 
The supra clinoid portion further divided into three sub segments.
a. Clinoidal segment,
b. Ophthalmic segment and
c. Communicating segment.
The clinoidal segment comprises a tiny wedge-shaped part of the ICA between the proximal and distal dural rings. The anterior clinoid process lies superior and lateral to the clinoidal ICA, over the part of widest separation between the dural rings.
The Ophthalmic segment is the most proximal intradural part of the ICA and extends from the distal dural ring to the origin of the posterior communicating artery gives the ophthalmic artery, which arises from the anterior aspect of the ICA medial to the anterior clinoid process.
The communicating segment begins just proximal to the origin of the posterior
communicating artery and ends with the bifurcation of the ICA into the ACA and the MCA. Gives off posterior communicating artery.

Reference: 
Handbook of Cerebrovascular Disease and Neurointerventional Technique.  Mark R. Harrigan, John P. Deveikis and Agnieszka Anna Ardelt

Salt and Pepper sign

The term is used to describe the speckled appearance of the tissue.
Used in many instances in radiology as well as even some pathology textbooks and journals mentions this term while describing the tissue but most commonly used while described the tissue during MRI interpretation.

* Vascular tumours

Highly vascular tumours such as a paragangliomas which contain flow void and areas of haemorrhage they are Glomus tympanicum , Glomus jugulare , pheochromocytoma and , carotid body tumour.
‘Salt' represents the hyperintensity of mass due to areas of hemorrhages in these hypervascular tumors.
‘Pepper' represents multiple black dots due to signal void of vessels.
The appearance is seen on T1 w images, may be seen on T2w images. Such lesions show ‘blooming’ low signal intensity hemosderin staining on T2*GRE, intense enhancement on post contrast due to the hyper vascularity of the mass.
* Vertebral haemangioma
A less common usage for the term is for vertebral haemangiomas which have a courser black and white dotted appearance especially on axial T2 and T1 images (salt = fat, pepper = coarsened trabeculae).
* Sjogren syndrome
The parotid gland in Sjogren's syndrome has also been described with this appearance, due to a combination of punctate regions of calcification (pepper) and fatty replacement (salt)

Glomus jugulare 
Rare, slow-growing, hypervascular, benign but locally invasive tumor.
Seen at Cp angle near jugular foramen of the temporal bone.
Represent small collections of paraganglionic tissue, derived from embryonic neuroepithelium admixed with autonomic nervous system, found in the region of the jugular bulb.
Globus jugulare is a part of group Paraganglioma, also referred to as chemodectomas or nonchromaffin paragangliomas. Paragangliomas are found at other sites also, including the middle ear (glomus tympanicum), the carotid body (carotid body tumor), and the vagus nerve in proximity to the inferior (nodosum) vagal ganglion (glomus vagale tumor, glomus intravagale tumor).
Association of glomus jugulare is reported with Pheochromocytoma, parathyroid adenoma, and thyroid carcinoma.
Histologically Glomus jugulare described as dense matrix of connective tissue among nerve fascicles.
Expand within the temporal bone via the pathways of least resistance such as air cells, vascular lumens, skull base foramina and the eustachian tube. Spares the ossicular chain.
Often noted as an incidental finding. May go unnoticed due  to non specific and insidious onset symptoms. Predominantly occur in female, common during their fifth and sixth decades of life, more common on the left side.

A 40 yo female presented with hydrocephalus due to mass effect of a Hemangioblastoma (H) at the floor of posterior fossa. The Glomus jugulare tumor (G) with typical salt and pepper appearance on T1w images noted as an incidental finding.

Multicentric tumors are known in ~ 3-10% of sporadic cases and in 25-50% of familial cases.
Metastases from glomus tumors occur in approximately 4% of cases, includes lung, lymph nodes, liver, vertebrae, ribs, and spleen.
CE MRI is investigation of choice, characteristic salt and pepper appearance on T1 and T2-weighted images.

Thornwaldt's cyst MRI

Syn: Pharyngeal bursa, Thornwaldt bursa, Tornwaldt cyst.
A cystic density / signal intensity lesion at the roof of nasopharynx.
A developmental cyst, represent potential space developing in the nasopharynx at the point where the notochord retains its union with the pharyngeal ectoderm.

MRI is investigation of choice, signal intensity on MRI varies on both T1 and T2 weighted images depending up on its protein content and if any associated haemorrhage in the lesion.

The lesion is a mucosal cyst situated in the mid line between the longus capitus muscles, without associated inflammatory changes or edema in the surrounding soft tissues or without any adjacent bone involvement.

DDs:
Usually nil due to its typical location and imaging appearance. May include normal / prominent adenoidal tissue with cystic degeneration, mucous retention cyst.

Age group is second or third decades, no sex predilection.
Clinical symptoms:
Most often asymptomatic. Noted as an incidental findings on CT / MRI.
Symptoms may include postnasal drip, halitosis, headaches, eustachian tube dysfunction resulting in earache.

References:
1. Weissman JL. Thornwaldt cysts. Am J Otolaryngol 1992; 13: 381–5.
2. Miyahara H, Matsunaga T. Tornwaldt's disease. Acta Otolaryngol Suppl (Stockh) 1994; 517: 36–9.
3. Ikushima I, Korogi Y, Makita O, Komohara Y, Kawano H et al. MR imaging of Tornwaldt's cysts.AJR 1999; 172: 1663–5.
4. Goodwin RW. Tornwaldt's disease. Characteristics headaches syndrome and etiology. Laryngoscope 1944; 54: 66–75.
5.Chong VF, Fan YF. Radiology of the nasopharynx: pictorial essay. Australas Radiol 2000; 44: 5–13.
6. Battino RA, Khangure MS. Is that another Thornwaldt's cyst on MRI? Australas Radiol 1990; 34: 19–23.

Vein of Trolard MR Venogram of Brain

Syn:  Superior Anastamotic Vein.
The vein of Trolard, is a part of the superficial venous system of the brain, often located in post central sulcus, connects superficial middle cerebral vein of Sylvius to superior sagittal sinus.

Also important know here about vein of Labbé, the inferior anastomotic vein crosses the temporal lobe, connects the superficial middle cerebral vein of Sylvius to the ipsilateral lateral sinus.
The dominance of these anastomotic veins is dictated by the relative size of the superficial middle cerebral vein and the other anastomotic vein. There is inverse relationship between the size of the superficial middle cerebral vein, the anastomotic vein of Trolard and the vein of Labbé, as all three shares a same drainage area.

Clinical significance:
Surgery : Important to know about these vein , to preserve the vein during lobectomy or Decompressive craniecotmy.
Thrombosis : isolated thrombosis of this vein is known where MR Venogram may show controversial findings like normal superior sagittal sinus with an adjacent hemorrhagic venous infarct which may be otherwise mistaken for a hemorrhagic contusion.

Vein of Labbe MR Venogram Brain

Named after 17th century French surgeon Charles Labbé who described it in his 3rd year of medical school.

The vein is a part of the superficial venous system of the brain, crosses the temporal lobe from sylvian fissure to end in ipsilateral lateral sinus mostly at the junction of transverse and sigmoid sinus. Exact location in temporal region is variable may be anterior temporal, middle temporal or posterior temporal of which middle temporal is most common. The structural anatomy of the vein itself is also variable, with a dominant single channel, multiple branching channels and even venous lakes having been described.

Vein of Labbé is also known as Inferior anastomotic vein, connects the superficial middle cerebral vein of Sylvius from sylvinan fissure to the lateral sinus.

Also important know here about Vein of Trolard, also known as superior anastomotic vein, often located in post central sulcus, connects the superficial middle cerebral vein of Sylvias from sylvian fissure to the superior sagittal sinus.

The dominance of these anastomotic veins is dictated by the relative size of the superficial middle cerebral vein and the other anastomotic vein. There is inverse relationship between the size of the superficial middle cerebral vein, the anastomotic vein of Trolard and the vein of Labbé, as all three shares a common drainage area. 

Clinical significance:

Surgery : Important to know about the vein and to preserve the vein during Temporal lobectomy for temporal lobe epilepsy or Decompressive craniecotmy.

Thrombosis : isolated thrombosis of this vein is known where MR Venogram may show controversial findings like normal or congenital absent ipsilateral lateral sinus with a hemorrhagic venous infarct which may be otherwise mistaken for hemorrhagic contusion. 

Intradiscal Vacuum Phenomenon

Sagittal T1 and T2 w images show intradiscal vacuum phenomenon at L4-5. 

Gas in the disc space first analysed by Ford in 1977, is an ~ 90% nitrogen combined with oxygen, carbon dioxide and other traces of gases.
VP is described in every segment of the spine including the disc space, Schmorl nodes, vertebral  body, the epidural and intradural spaces and facetal joints.
Intradiscal VP is commonest, observed in  up to 20% elderly individuals.
Most of the articles related to intra discal VP are based on plain radiographs and CT findings, although MRI being the preferred diagnostic method for spine, suffers a reduced sensitivity in detecting gas.
Intradiscal VP can be marginal or central. An anterior marginal VP means a crack in the peripheral fibres of the annulus fibrosus caused by a traumatic or degenerative process. A central VP can be found in many conditions characterized by the development of horizontal intra discal clefts. Disc degeneration is the commonest cause of a centrally positioned VP.
On MRI intra discal vaccum phenomenon is seen as a streak of signal void (‘z’ black) on T1 as well as T2 w images. Most commonly seen in lumbar region. Associated with degenerative changes in the disc like disc desiccation, reduced disc height with endplate sclerosis, modic changes, marginal osteophytosis and adjacent segment degeneration.

Sagittal reformatted images of spiral CT study at L5-S1 level shows Grade I anterior subluxation of L5 off S1, break in L5 pars on either side. Height of disc space reduced. Degenerative intra discal vacuum phenomenon noted as a 'Z'black air density in the disc space.  

Reference: The spectrum of vacuum phenomenon and gas in spine, B. Coulier