Asymmetry of Cerebral Hemispheres

The presence of asymmetry in the human cerebral hemispheres is detectable at both the macroscopic and microscopic levels. An association between these asymmetries is inferred but not yet supported by statistical analysis.
Asymmetries seen in comparative studies provide strong evidence for phylogenetic origins of brain lateralization.

Two components of macroscopic asymmetry of  Cerebral Hemispheres are known.
One is “PETALIA” and another is “VOLUME TORQUE”.

“PETALIA”  term describes asymmetrical protrusion of one part of cerebral hemisphere beyond the other. CT and MRI studies show that these petalias are more prominent in right handers.  Among the most prominent observations of brain asymmetry are the right frontal and left occipital 'petalias', the right hemisphere protrudes anteriorly beyond the left, and the left hemisphere extends posteriorly beyond the right.
“VOLUME TORQUE” is a differential distribution of tissue of a cerebral hemisphere compared to other. The right frontal region is often wider than the left, and the left occipital region is often wider than the right, reflect lateralized volume differences in frontal and occipital regions  and is associated with language function in humans, not found in other apes.
“Yakovlevian anticlockwise torque” encompasses includes petalia plus the frequent extension of the left occipital lobe across the midline over the right occipital lobe, bending the interhemispheric fissure towards the right.

References:
Arthur W. Toga and Paul M. Thompson, Laboratory of Neuro Imaging , Department of Neurology, UCLA School of Medicine, Los Angeles, CA, USA MAPPING BRAIN ASYMMETRY
LeMay, M.  Morphological cerebral asymmetries of modern man, fossil man, and nonhuman primate. Ann N Y Acad
Sci. 280, 349-66 (1976).
LeMay, M. & Kido, D. K. Asymmetries of the cerebral hemispheres on computed tomograms. Journal of Computer
Assisted Tomography. 2, 471-6 (1978).
Kertesz, A., Black, S. E., Polk, M. & Howell, J.  Cerebral asymmetries on magnetic resonance imaging. Cortex. 22(1),
117-27(1986).

Spinal epidural Ewing sarcoma with significant cord compression

Epidural spinal cord compression (ESCC) refers to compression of spinal cord or cauda-equina by any pathology, most commonly by benign etiology like protruded disc, focal hyper trophy - ossification of ligamentum flavum. Neoplastic and in Infective Epidural abscess are the other next common causes. Among neoplastic the commonest malignancies involving the spinal epidural space are Metastatic lymphoma, Nerve sheath tumor, Meningiomas, Hemangiomas and Metastases from systemic malignancy. Rarely round cell tumors which can present with ESCC are primary CNS lymphoma, granulocytic sarcoma and extra-osseous Ewing sarcoma.
The presentation in neoplastic causes is predominantly slowly progressive paraplegia with or without pain and bowel bladder dysfunction.
Here I am presenting a case of Spinal Epidural Round cell tumor / Ewing’s sarcoma presented with acute onset paraplegia.

A 11 yo female presented with abrupt onset of weakness in both lower limbs and low back pain.

On admission MRI Dorsal spine shows:
A focal lentiform shaped extra dural soft tissue on sagittal T2 extending from D3 to D5 (5cm in length and 7mm in thickness),
Hypo intense on T1 and iso to hyper intense on T2. Homogenous enhancement on post contrast T1.
Significant cord compression with abnormal intramedullary signals. No abnormal focal cord enhancement on post contrast T1. No similar lesion elsewhere in rest of the whole spine post contrast T1 screening.
Adjacent D4 spinous process show abnormal enhancement on post contrast T1 with a minimal similar soft tissue in left spinalis compartment. Rest of the bone marrow signals normal.

Imaging wise possible DDs: Lymphoma, Sarcoma, Pseudo tumour.
Epidural Abscess and Hematoma - less / unlikely due to homogenous enhancement.

Histopathology report: Ewing’s sarcoma ( Microscopy :  Sections A and B, both shows circular malignant small round 'Blue' cell tumor composed of syncytial aggregates lobular sheets of fairly monotonous appearing small round cells having round to oval hyper chromatic nuceli, finally stippled chromatin and smooth nuclear membranes. The cells have pale ampophilic to eosinophilic cytoplasm with indistinct margins and occasional cells show nucleoli the minimal interstitial stroma shows couple of congested blood vessels and appears eosinophillic granular fibrillary in section B the tumor is seen within the inter trabercular space of bony tissue. No endothelial lined space invasion seen.)

Spinal Epidural Ewing's

Extra-osseous Ewing sarcoma (EOES) is an undifferentiated small round-to-oval cell tumor of uncertain origin. Primary extraosseous Ewing sarcoma is a rare entity, especially in the spinal epidural site. Only 21 cases have been reported in the literature.
It differs in presentation from skeletal Ewing's sarcoma in several respects. The average age of occurrence is around 17 years (range 4-47 years), in contrast to 10 years for skeletal Ewing's sarcoma. EOES, like skeletal Ewing's sarcoma, occurs more commonly in males than females, with a ratio of 2:1. Presentation includes back pain in almost in all patients, paresis of one or both legs in approximately 80% with or without sensory disturbances, bladder and bowel dysfunction. The neurological deficits supposed to be of insidious onset being neoplastic. Rarely acute presentation as in this case.
Lumbar and dorsal region spine is most commonly affected. Cervical involvement is rare.
On MRI these lesions commonly appear isointense to muscle on T1, iso to hyperintense on T2 and show moderate enhancement on contrast-enhanced T1 without bony involvement.
Microscopically, the lesion is composed of sheets of fairly uniform round-to-oval cells. Usually the cells have relatively clear and indistinct cytoplasm but contain large quantities of PAS-positive, diastase-digestible material, indicating high concentrations of glycogen. The tumor cells show diffuse, intense membrane reactivity for CD99 (MIC 2) on immuno-histochemical staining.
In management wide surgical resection with follow-up local irradiation and chemotherapy offer the best chance for long-term disease-free survival.

References:
Turk J Pediatr. 2008 May-Jun;50(3):282-6 Primary spinal epidural Ewing sarcoma: a case report and review of the literature, Hsieh CT, Chiang YH, Tsai WC, Sheu LF, Liu MY.
Cavaliere R, Schiff D. Epidural spinal cord compression. Curr Treat Options Neurol 2004;6:285-95.
Kawasaki K, Wakabayashi K, Koizumi T, Tanaka R, Takahashi H. Spinal cord involvement of primary central nervous system lymphomas: Histopathological examination of 14 autopsy cases. Neuropathology 2002;22:13-8.
Kalayci M, Sumer M, Yenidunya S, Ozdolap S, Acikgoz B. Spinal granulocytic sarcoma (chloroma) presenting as acute cord compression in a non-leukemic patient. Neurol India 2005;53:221-3.
Shin JH, Lee HK, Rhim SC, Cho KJ, Choi CG, Suh DC. Spinal epidural extraskeletal Ewing sarcoma: MR findings in two cases. Am J Neuroradiol 2001;22:795-8.
Gandhi D, Goyal M, Belanger E, Modha A, Wolffe J, Miller W. Primary epidural Ewing's sarcoma: Case report and review of literature. Can Assoc Radiol J 2003;54:109-13.    
Kadri PA, Mello PM, Olivera JG, Braga FM. Primary lumbar epidural Ewing's sarcoma: Case report. Arq Neuropsiquiatr 2002;60:145-9.    
Allam K, Sze G. MR of primary extraosseous Ewing sarcoma. AJNR Am J Neuroradiol 1994;15:305-7.
Kaspers GJ, Kamphorst W, van de Graaff M, van Alphen HA, Veerman AJ. Primary spinal epidural extraosseous Ewing's sarcoma. Cancer 1991;68:648-54.

Why does the spinal cord enlarge at cervical and lumbar region

Because those are the levels of the spinal cord that deal with the innervation for the arms (cervical) and legs (lumbar). There is more mass of nerve cells, bodies, and tracts in order to accommodate the innervation needs of the limbs.
The normal cervical enlargement extends from about the C3 to D2 vertebra with a maximum circumference of about 38 mm. An analogous region in the lower limbs occurs at the lumbar enlargement termed as conus.

Vertebro Basilar hypoplasia presenting with Episodic Dizziness

A 58 yo male with recurrent episodic dizziness since long time, accompanied with episodes of blurring of vision. MRI study of Brain was normal, no any ischemic lesion in posterior circulation territory.
MR Angiography was advised with clinical diagnosis of vertebro basilar insufficiency.

The Non contrast 3 D TOF MR Angiography of brain and neck shows:

Smaller diameter of intra cranial portion of both the vertebral and basilar. Narrowing is diffuse and smooth ,  not irregular as we see in atherosclerotic  patients.

Right intra cranial vertebral measures ~1.7mm

Left intra cranial vertebral measures ~ 1.6mm

Basilar measures ~ 2.2mm

Right Pcoms is continuing as PCA – fetal PCA on right side. Basilar continuing as left PCA. Rest of the intra cranial vessels and neck vessel show course and calibre.

Imaging diagnosis : Hypoplastic bilateral intra cranial vertebral and Basilar.

The Episodic dizziness of patient can be attributed to vertebro basilar hypoplasia and is consistent with clinical diagnosis of  vertebro basilar insufficiency.

Discussion: 

On TOF MRA Vertebral hypoplasia (VH) is when vertebral artery (VA) is less than 2.0 mm in diameter and Basilar hypoplasia (BH) when basilar is less than its normal average diameter of 3 to 5mm.

Basilar hypoplasia is usually accompanied with unilateral vertebral artery hypoplasia; however, BH with bilateral VH composing vertebrao basilar hypoplasia (VBH) is rare but a significant normal anatomical variation in a patient clinically presenting with vertebro basilar insufficiency where other ENT causes of dizziness are ruled out.

Reference: Intracranial Vertebral-Basilar Artery Hypoplasia Presenting with Episodic Dizziness, Ghana Med J, Jiann Jy Chen and Dem Lion Chen, 2010 September; 44.

Special thanks to Dr Sachin Baldawa MS Mch (Neurosurgery) 

Moyamoya Disease MRI

A 31 yo known case of recurrent episodes of ishemia under evaluation.MRI study brain Axial FLAIR image of brain with non contrast 3 D TOF MR Angiography shows:
Bilateral cortical border zone infarcts.
Severe stenosis involving intra cranial portions of both the ICAs. MCAs not visualised along with its cortical branches. Distal portion of PCAs not visualised along with its cortical branches. Multiple vessels from proximal portion of PCAs are the thalamo striate perforators giving is so called Puff of Smoke appearance contributing to basal cerebral anastomotic or collateral network.

Moyamoya disease 

A bilateral steno occlusive disease of the intra cranial internal carotid artery.
Moyamoya is a Japanese word for a "puff" or "cloud of smoke" , and it has been used to refer to an extensive basal cerebral network of small anastomotic vessels at the base of the brain around and distal to the circle of Willis secondary to segmental stenosis or occlusion of the terminal parts of both internal carotid arteries.  The basal vascular network is contributed by lenticulostriate, chorioidal, thalamoperforating, premammilary and thalamogeniculate arteries, as well by to unnamed branches arising directly from the circle of Willis and meningeal arteries via ECA.
Pseudo aneurysms and micro aneurysm are well reported along these collaterals and circle of Willis. Pathogenesis of MD is not well understood. Various theories of inflammatory and immunologic mechanisms remain unproven. Very high concentration of basic fibroblast growth factor (bFGF) with high angiogenic activity in Csf samples of patients typical imaging findings of Moyamoya disease. There are strong evidences to support hereditary and familial factors especially among the Japanese.

Clinical manifestations of MD includes secondary to ischemia and or bleed. Ischemic symptoms common in children. Infarcts are more common along cortical border zones. Intracranial bleed in the form of subarachnoid bleed,  intra parenchymal or intraventricular bleed common in adults. Causes of intracranial haemorrhage in MD are rupture of dilated fragile collaterals or rupture of aneurysms along the circle of Willis and basal cerebral network of collaterals.


Reference: MOYAMOYA DISEASE: CLINICAL AND ANGIOGRAPHIC FEATURES Dragan Stojanov ,  Petar Bošnjaković,  Zoran Milenković, Nebojša Stojanović CLINICAL FEATURES OF MOYAMOYA DISEASE yong seung Hwang,


Other cases of Moya moya disease:
Moyamoya disease DSA 
Intraventricular-hemorrhage-in-moyamoya

Arachnoiditis MRI

Axial T2w image at L5-S1 level show clumping of nerve roots together and adherent to the thecal sac with empty center of the thecal sac in a case with history of laminectomy in dorsal region. 

Syn: Chronic adhesive arachnoiditis.
A broad term denotes inflammation of the spinal meninges and subarachnoid space.
It develops after an acute form of arachnoiditis beginning with an inflammatory exudative reaction and progressing into fibrin deposition along nerve root sheaths and arachnoid surfaces. Collagenous strands then develop, adhering nerve roots and septating the subarachnoid space.
Varies etiologies categorised into infectious, non infectious inflammation and neoplastic.
Infectious includes bacterial, viral, fungal, and parasitic agents.
Noninfectious inflammatory etiologies include surgery, intrathecal hemorrhage, administration of intra thecal myelographic contrast, anesthetics and steroids.
Neoplastic includes the haematogenous spread or direct seeding from primary CNS tumors.

Imaging wise arachnoiditis is diagnosed when multiple nerve roots are adherent to the margins of the thecal sac (Empty sac sign) or clumped together across multiple segments (Pseudotethering). 
Normally on axial sections nerve roots in thecal sac should fall freely in the dependent portions. The lack of normal "fanning" on sagittal views indicates an abnormal position of nerve roots within the sac due to segmental nerve root adherence. Strands of scar can loculate the CSF, creating intradural cysts or a bizarre appearance to the sac. 
There is may be an associated thickening of nerve roots. The thickened nerve roots of adhesive arachnoiditis do not substantially enhance. If there is significant enhancement, other causes such as CSF metastases, Guillan-Barre syndrome, and active granulomatous infection should be considered.


References: Arachnoiditis, MRI Web Clinic, July 2007 by Larry B. Poe, M.D.

Aneurysmal 3rd CN palsy

Third (Occlulomotor) Cranial Nerve Palsy
Signs and symptoms:
Pt usually present with sudden onset unilateral ptosis. Rarely, bilateral if the damage occurs to the third nerve nucleus, frequently accompanied by peri orbital pain or headache. Pt may or may not complains of double vision because the ptosis obscures the vision in the affected eye. If eye lid is manually elevated, the patient will definitely experience diplopia.
On examination limited elevation, depression and adduction. There is an underaction of the superior, inferior, and medial recti muscles and inferior oblique muscle, which may be total or partial. The pupil may be dilated and minimally reactive to light (pupillary involvement), totally reactive and normal (pupillary non-involvement), or may be sluggishly responsive (partial pupillary involvement).


Pathophysiology: 
Third (Oculomotor) CN palsy results from damage to the nerve anywhere in its course from the third nerve nucleus in the dorsal mesencephalon, its fascicles in the brainstem parenchyma, the nerve root in subarachnoid space, or in the cavernous sinus or posterior orbit.
1. Damage to the third nerve nucleus results in an ipsilateral third nerve palsy with contralateral superior rectus under action and bilateral ptosis.
2. Damage to the third nerve fascicles results in an ipsilateral third nerve palsy with contralateral hemiparesis (Weber's syndrome), contralateral intention tremor (Benedikt's syndrome), or ipsilateral cerebellar ataxia (Nothnagel's syndrome). Vascular infarct, metastatic disease and demyelinization are the common causes of brainstem involvement.
3. Damage to the third nerve within the subarachnoid space produces an isolated third nerve palsy. The main causes are compression of the nerve by an aneurysm of the posterior communicating artery or the basilar artery, and ischemic vasculopathy.
4. Damage to the third nerve in the cavernous sinus, superior orbital fissure, or posterior orbit is unlikely to present as isolated third nerve palsy due to the confluence of other structures in these areas. Cavernous sinus involvement may also include pareses of cranial nerves IV, VI and V-1, and an ipsilateral Horner's syndrome. The common causes of damage in these areas include infective, non infective inflammatory, neoplastic and carotid artery aneurysm.

Clinical pearls: 
• Isolated third nerve palsy due to ischemic vasculopathy will spontaneously resolve and recover over a period of three to six months. If the palsy fails to resolve in this time frame, repeat the MRI to search for the true etiology.
• Myasthenia gravis has the ability to mimic virtually any cranial neuropathy, including isolated third nerve palsies.

Aneurysmal Third Nerve Palsy

Third cranial nerve palsy caused by internal carotid - Posterior communicating Aneurysm is one of the most challenging problems facing the ophthalmologist.
This Pcom aneurysm is responsible for 13 - 30% of acquired oculomotor palsy.
Usually present with three clinical signs, includes 1. Ipsilateral facial, orbital or ocular pain, 2. Extraocular muscle and levator involvement and 3. Pupillary paresis.
Isolated third nerve palsy may be the only presenting sign of this aneurysm. A patient with isolated third nerve palsy with pupillary involvement should be considered as aneurismal third nerve palsy until proven otherwise.

Axial T2 image and 3 D TOF Non contrast MR Angiography of Brain shows a left side Pcom Aneurysm presenting with left side isolated 3rd CN Palsy 

Urgent neuro imaging and intervention necessary because of the possibility of aneurysmal rupture and intracranial hemorrhage carries a significant risk for morbidity and mortality.
Treatment options includes craniotomy and aneurysmal clipping. Recent reports indicated that endovascular management also produce effective neural recovery.

References:
Hand book of occular disease management.
J Med Assoc Thai Vol. 87 No.11  2004

Special Thanks to Dr Pandurang Wattamwar MD DM Neurology. 

HIV Encephalitis Vs PML

* A 50 yo male with altered behavior and sensorium. Known Retro positive.On admission drowsy.

MRI Brain Axial FLAIR shows:
Diffuse cerebral cortical atrophy - significant for age. An associated ex vacuo dilatation of lateral ventricles and mid brain atrophy.
Faint ill defined T2 hyper intensity in fronto parietal peri ventricular white matter, bilateral and symmetrical involvement goes in favor of HIV Encephalitis.

* A 31 yo male with left side weakness.
On admission Tridot test positive.

MRI study of Brain shows:
Bilateral patchy ill defined parietal T2 hyper intensities, confined to white matter, peri ventricular as well as sub cortical white matter.
No mass effect.
High signal on Diffusion attributed to prolonged T2 effect, bilateral asymmetrical involvement goes in favor of PML.

HIV Encephalitis – Encephalopathy

Syn: HIV- 1 Encephalitis / Encephalopathy, HIVE.
A syndrome of cognitive, behavioural and motor abnormalities attributed to direct effect of HIV on brain, in absence of opportunistic infection.
Most freqauent neurological manifestation of HIV infection.
Atrophy with bilateral symmetric confluent peri ventricular white matter disease, hypodense on CT and T2 hyperintense on MRI is a diagnostic clue.
Clinical finding should guide imaging finding and not reverse.
Affect any age group and gender.

Progressive Multifocal Leukoencephalopathy (PML)

A sub acute progressive demyelinating disease.
Bad prognosis, even fatal.
Seen in patients with immune disorders particularly impaired cell mediated immune response. Predominantly occurs in patients with AIDS, in ~ 5% of patients with AIDS. Results from JC virus infection - genus Polyomavirus -  family Papovaviridae. Before AIDS epidemic, PML was rare and was associated with other immune compromised conditions like leukemia, lymphoma, organ transplantation and severe combined immunodeficiency (SCID).
Imaging findings:
MRI is the preferred imaging modality over CT.
Usually bilateral and asymmetrical involvement. Patchy and multifocal. Unilateral involvement uncommon.
Predominantly involve fronto parietal white matter; may involve periventricular white matte or sub cortical white matter or both.
On CT, ill defined patcy areas of low attenuation. No enhancement on post contrast.
On MR, ill defined patchy areas of T2 hyperintensity. May show high signal on diffusion due to prolonged T2 effect. No enhancement on post contrast T1.
An associated atrophy,  increase in confluence of lesions, increase in hypo intensity of lesion on T1,  involvement of corpus callosum are poor prognostic indicators.
Faint enhancement seen on post contrast T1 in follow up MRI of PML pts treated with Anti Retroviral Therapy is associated with increase in their CD4+ count, may indicate favourable prognosis.

HIV Encephalitis Vs PML

Imaging wise HIV Encephalitis - Encephalopathy  is usually characterised by bilateral symmetrical Periventricular T2 white matter hyperintensity, An associated diffuse cortical atrophy and ventricular dilatation which are not predominant findings of PML.
In PML involvement is usually bilateral, patchy and asymmetrical. Unilateral involvement is uncommon. Predominantly involve fronto parietal white matter.
Clinically HIV Enecephaltis pt present with altered cognition where as PML is associated progressive focal motor and sensory deficits.
HIV Encephalitis is attributed to direct effect of HIV on brain parenchyma where as PML is attributed to Demyelinating disease and opportunisting infection occurring in pts with advanced AIDS with severely impaired cell mediated immunity and CD 4 counts.

Similar cases:
Hiv-encephalitis-encephalopathy
PML

Dehiscent jugular bulb

Right side Dehiscent jugular bulb 

A high riding jugular bulb is a normal anatomical variation, present very close to middle ear and the intervening sigmoid plate between a high riding jugular bulb and the middle ear may be thin and intact but when it is absent, allowing the wall of the vein to bulge into the middle ear cavity is termed as Dehiscent jugular bulb is a known causes of pulsatile tinnitus.
Can present as a retrotympanic vascular mass and turn out disasterous if mistaken for a middle ear tumour and biopsied.

Circle of Willis and its clinical significance

The circle of Willis (also called Willis' Circle, Loop of Willis, cerebral arterial circle, and Willis Polygon) is a circle of arteries at the base of brain that supply blood to the brain and surrounding structures. It is named after Thomas Willis, an English physician.

The circle of Willis is formed when the internal carotid artery (ICA) enters the cranial cavity bilaterally and divides into the anterior cerebral artery (ACA) and middle cerebral artery (MCA). The anterior cerebral arteries are then united by an anterior communicating (ACOM) artery. These connections form the anterior half (anterior circulation) of the circle of Willis. Posteriorly, the basilar artery, formed by the left and right vertebral arteries, branches into a left and right posterior cerebral artery (PCA), forming the posterior circulation. The PCAs complete the circle of Willis by joining the internal carotid system anteriorly via the posterior communicating (PCOM) arteries.

The arrangement of the brain's arteries into the Circle of Willis creates collaterals in the cerebral circulation. If one part of the circle becomes blocked or one of the arteries supplying the circle is blocked or narrowed, blood flow from the other blood vessels can preserve the cerebral perfusion.

The anterior communicating artery (AcoA) and posterior communicating arteries (PcoA) of the circle of Willis provide the main route for collateral blood flow in cases of carotid artery obstruction.

Reference: Collateral Configuration of the Circle of Willis A. W. J. Hoksbergen, MD;  B. Fülesdi, MD, PhD; D. A. Legemate, MD, PhD; L. Csiba, MD, PhD.

    

In this case left ICA show severe stenosis / occlusion at its origin, its intra cranial as well as cervical portion not visualised on 3 D TOF non contrast MR Angiography. Left ACA and MCA filled via contra lateral anterior circulation via Acom and posterior circulation via Pcom. 

Parenchymal sequences show a small left parietal cortical chronic infarct along cortical border zone. 

Clinical detail of the patient is that he had history of right sided weakness 3months back secondary to this small left parietal infarct along cortical border zone secondary to severe left ICA stenosis but he is now relatively asympatomatic due to efficient collateral circulation of Circle of Willis. 

Tortuosity of cervical ICA clinical significance

Variations in the course of cervical ICA are reported in the literature as coiling, looping, kinking or tortuousness of the vessel.
The clinical relevance of morphological anomalies of extracranial ICA is a matter of debate because of up to date the natural  history of kinking, coiling and tortuosities of this artery is not well known.
It is well known that the intracranial portions of the ICA are highly tortuous, while its cervical course is almost straight. There are two points of fixation at the cervical portion of the ICA: at the bifurcation of the common carotid artery and at its entry into the temporal bone. If the vessel is longer than the distance between these two points, curvatures and loops develop.
Several authors consider these variations as congenital anomalies of the aortic arch arteries  while others consider it being caused by artheriosclerosis, fibromuscular hyperplasia , degenerative lesions, hypertension or aging .
Tourtuous extracranial ICA needs no treatment as long as the patient does not have a cerebrovascular ischemic sign or a neurological complaint.

Pitfalls of MR Angiography due to slow flow in large Aneurysm

This case illustrates a potential pitfall of Non contrast 3D TOF MRA imaging. Contrast enhanced CT study shows a focal aneurysmal dilatation of basilar, but the non contrast 3 D TOF MR Angiography of Brain of same patient fails the aneurysm is not depicted well on lateral view.

The non visualization of aneurysm may be due to slow flow owing to its large size or thrombus formation in the aneurysm because vascular imaging in Non contrast 3D TOF MRA depends on flow related signal from a rapid blood flow in a vessel. In this study the ICAs and basilar show normal flow related signals but the large aneurysm sac appears nearly isointense with nearby static brain.
CT Angiography or MR Angiography with contrast has considerable value in such cases because it is not dependent on flow velocity to create contrast and signals.

Reference: Practical MR Physics and case file of MR artifacts and pitfalls, Alexander C. Mamourian, MD

Organised Brain Abscess

Chronic Abscess with well organised wall fails show enhancement on post contrast study. 

A known case right frontal lobe abscess, steriotectic biospy and histopathology proven pyogenic abscess.

Serial CT studies of brain at approximately 6months interval shows no significant change in size of lesion. The lesion is round has uniform thickness hyper dense wall with no enhancement on post contrast study, an adjacent perilesional Gliosis.

The typical appearance of abscess is a hypodense necrotic center with a peripheral uniform enhancement along its wall. Rarely in chronic cases a well organized abscess wall fail to generate such ring enhancement as in this case.

Meningomyelocele MRI

Sagittal T2 w image shows:
A dorsal mid line fusion defect in lumbo sacral region.

Herniation of meningeal sac out the defect forming a subcutaneous swelling.

Low lying tethered cord with neural elements noted traversing the meningeal sac. Focal dilatation of central canal of cord.

Meningomyelocele

Syn: MMC, myelomeningocele, open spinal dysraphism, spina bifida aperta, spina bifida cystica.

Location: Lumbosacral (44%) > thoracolumbar (32%) > lumbar (22%) > thoracic (2%).

Size varies depending on extent of neural tube defect.

Imaging Findings:

Xray: Posterior spina bifida with wide eversion of lamina.

MRI: The investigation of choice.

Dorsal mid line fusion defect, wide spinal dysraphism, flared laminae.

Loss of normal posterior epidural fat segmentation at the level of anomaly on sagittal sections. The posterior epidural space is wide with contiguous epidural fat.

Low lying tethered cord.

Nerve roots traversing the dural sac originate from ventral placode surface.

Rest of the whole spine screening and brain needed to rule out associated abnormalities like Kyphoscoliosis, vertebral segmentation anomalies, Diastematomyelia, dermal sinus, Syrinx, Chiari II malformation in ~ 100%, hydrocephalus.