Magnetic resonance imaging (MRI) is the preferred modality for evaluating the perimesencephalic cisterns due to its superior spatial resolution, ability to distinguish cerebrospinal fluid (CSF) from adjacent brain structures, and excellent soft tissue contrast. While computed tomography (CT) has a role in specific contexts, MRI’s ability to detect both normal and pathological conditions is unparalleled. It is particularly crucial in the evaluation of acute subarachnoid haemorrhage (SAH), where MRI can detect blood within the cisterns and associated mass effects, such as ventricular compression. MRI is also effective in identifying neoplastic conditions, including gliomas, metastases, and other tumours, which can cause effacement or displacement of these cisterns. Contrast-enhanced MRI further aids in delineating tumours and their relationships with surrounding structures.

In addition to neoplastic conditions, the perimesencephalic cisterns are involved in vascular pathologies such as aneurysms, arteriovenous malformations (AVMs), and other vascular anomalies. Advanced imaging techniques, including MR angiography and CT angiography, are invaluable for assessing these conditions. MRI is also an excellent tool for evaluating cranial nerve pathologies and detecting infectious lesions, tumours, or inflammatory processes affecting the cisterns and adjacent brain parenchyma. The variety of MRI sequences enables precise imaging and detailed assessment of both the cisterns and related abnormalities, making it an essential diagnostic tool in these complex cases.

Applied anatomy

Fig 1: Axial section at the level of midbrain highlighting the perimesencephalic cisterns.

Fig 2: Illustration depicting the sagittal section of brain with emphasis on the brainstem, surrounding structures and cisternal spaces.

Fig 3: Illustration of the axial section at the level of midbrain showing the perimesencephalic cisterns and surrounding clinico-radiologically significant structures.

Categorization of Pathologies

1. Neoplastic Lesions

• Primary CNS Tumors: Pinealoblastoma, Meningioma, Astrocytoma
• Skull Base Tumors: Chordoma
• Metastatic Lesions: Leptomeningeal carcinomatosis

2. Non-Neoplastic Mass Lesions

• Infectious Lesions: Tuberculoma
• Congenital Lesions: Lipoma, Epidermoid Cyst, Dermoid Cyst, Ruptured dermoid cyst.
• Miscellaneous Lesions: Mammillary Body Lesions

3. Vascular Lesions

• Aneurysms: P3 Segment of Posterior Cerebral Artery Aneurysm, Basilar Tip Aneurysm
• Malformations: Dolichoectasia
• Hemorrhage: Traumatic subarachnoid hemorrhage, Non-Traumatic, Non-Aneurysmal Subarachnoid Hemorrhage

4. Cranial Nerve-Related Lesions

• Neoplastic: Optic Chiasma Glioma, Lymphoma with cranial nerve enhancement
• Inflammatory/Infiltrative: Leukemic Meningitis, Seronegative Neuromyelitis Optica, IgG4 disease

5. Miscellaneous Lesions

• Herniation Syndromes: Ascending Transtentorial, Uncal Herniation
• Inflammatory Disorders: Tubercular Meningitis, Neurosarcoidosis
• CSF Dynamics and Structural Abnormalities: Intracranial Hypotension
• Neurodegenerative Conditions: Progressive Supranuclear Palsy (PSP)

IMAGING FEATURES OF VARIOUS PATHOLOGICAL ENTITIES INVOLVING THE PERIMESENCEPHALIC CISTERNS:

1. Dermoid cyst

Dermoid cysts are congenital inclusion cysts of ectodermal origin. Intracranial dermoid cysts mostly occur at midline, close to embryonic tissue fusion planes. Their characteristic imaging appearance is a result of epithelial desquamation, sebaceous secretions, and calcifications ⁽²⁾.

Fig 4: Figures A and B: Axial DWI and Sagittal T1-weighted images demonstrate a T1 hyperintense lesion in the interpeduncular cistern without evidence of diffusion restriction, consistent with the fat content of a dermoid cyst. Figure C: Axial T2/FLAIR image shows a T2 hyperintense lesion in the interpeduncular cistern, reflecting its lipid-rich nature. Figure D: Axial T1-weighted post-contrast image reveals no significant enhancement, a typical feature of a dermoid cyst.

2. Meningioma

Meningiomas are the most common dural tumour of the brain. Typical imaging features include an extra-axial enhancing mass with dural tail sign and underlying parenchymal compression ⁽³⁾.

Fig 5: Figures A, B & C: Axial T1, Sagittal T1, and Axial T2-weighted images demonstrate an extra-axial lesion in the right cerebellopontine angle. The lesion is hypointense on T1 and hyperintense on T2, causing effacement of the quadrigeminal and right ambient cisterns with resultant displacement of the midbrain. Figure D: Axial T1-weighted post-contrast image shows avid homogeneous enhancement of the lesion, a hallmark feature of meningioma.

3. Epidermoid cyst

These are congenital ectodermal and extra-axial benign lesions. They characteristically restrict diffusion owing to the abundance of cholesterol and keratin within them ⁽⁴⁾.

Fig 6: Figures A, B & C: Axial DWI, Axial T1, and Axial T2-weighted images demonstrate an extra-axial lesion in the right medial temporal lobe. The lesion is diffusion-restricting, hypointense on T1 and hyperintense on T2, causing effacement of the right crural cistern with resultant compression of right cerebral peduncle. Figure D: Axial T1-weighted post-contrast image shows no significant post contrast enhancement.

4. Thrombosed posterior cerebral artery (PCA) aneurysm

The PCA courses along the ambient and crural cisterns.

Fig 7: Figures A, B, & C: Axial T1, Axial T2/FLAIR and Axial SWI weighted images demonstrate a focal out-pouching from the P3 segment of right posterior cerebral artery, appearing T1 iso-intense and T2/FLAIR hypo-intense with blooming - findings suggestive of a thrombosed aneurysm of P3 segment of right posterior cerebral artery in the right ambient cistern.

5. Ascending transtentorial herniation.

It is the upward displacement of the cerebellum through the tentorial incisura, usually caused by a mass in the posterior fossa ⁽⁵⁾.

Fig 8: Figures A, B & C: Axial T1-weighted, Coronal T2-weighted, and Axial T2/FLAIR-weighted images demonstrate a lobulated extra axial T1 hypointense and T2/FLAIR hyperintense lesion in the left cerebellar hemisphere. Figure D: Sagittal T1 weighted image demonstrates ascending trans tentorial herniation with effacement of quadrigeminal cistern and displacement of the brainstem anteriorly. Figure E : Axial T2/FLAIR weighted image showing effacement of peri mesencephalic cisterns with displacement of the mid brain to the right.

6. Basilar tip thrombosed aneurysm

The tip of the basilar artery lies in the interpeduncular cistern.

Fig 9: Figures A, B, C & D: Axial DWI, Sagittal T1-weighted, Axial T2/FLAIR-weighted, and Coronal T2-weighted images demonstrate a focal dilation at the tip of the basilar artery, appearing T1 hypointense and T2/FLAIR hyperintense, without diffusion restriction—findings suggestive of a thrombosed basilar tip aneurysm. The aneurysm partially effaces the interpeduncular cistern.

7. Clival chordoma

Chordomas are rare malignant tumours of the axial skeleton that originate from the embryonic remnants of the primitive notochord. They are usually expansile and cause widespread bony destruction ⁽⁶⁾.

Fig 10: Figures A&B: Sagittal T1 and T2 weighted images reveal a lobulated T1 hypointense and T2 hyperintense retro-clival prepontine mass infiltrating the posterior aspect of the clivus and compressing the brain stem and displacing it posteriorly. Figures C&D: Axial T1 and T2 weighted images demonstrate displacement of the midbrain posteriorly with effacement of the cisterns secondary to the mass effect.

8.  Diffuse astrocytoma with uncal herniation

Astrocytomas are low-grade intra-axial brain tumours arising from astrocytes. They are usually CNS WHO grade 2 ⁽⁷⁾.

Fig 11: Figures A, B & C: Axial DWI, T1-weighted, and T2/FLAIR images show a non-diffusion-restricting, T1-hypointense, and T2/FLAIR-hyperintense lesion in the right medial temporal lobe, causing effacement of the adjacent cisterns and right uncal herniation. Figure D: Axial T1 post-contrast image shows no significant enhancement. Figure E: Arterial spin labeling (ASL) demonstrates a focal area of increased perfusion in the right temporal lobe. Postoperative histopathology confirmed a diagnosis of astrocytoma.

9. Intracranial hypotension

It is an uncommon, benign entity caused by low cerebrospinal fluid (CSF) pressures, usually due to a CSF leak. It can be spontaneous or secondary to an iatrogenic cause or trauma ⁽⁸⁾.

Fig 12: Figure A: Coronal T2-weighted image demonstrates bilateral frontal subdural collections. Figure B: Sagittal T1-weighted image shows brainstem sagging with descent of cerebellar tonsils Figure C: Sagittal T1-weighted image demonstrates pituitary gland enlargement. Figure D: MR venogram reveals engorgement of the dural venous sinuses. Findings are suggestive of intracranial hypotension.

Fig 13: Figure A: Sagittal T1-weighted image demonstrates the mamillopontine distance, measured from the inferior aspect of the mamillary bodies to the superior aspect of the pons. The normal range is 6.5–7.5 mm. Figure B: Axial T2-weighted image demonstrates the interpeduncular angle, formed by the posterior half of the cerebral peduncles on axial images. The normal value is ≥ 40.5°. Figure C: Sagittal T1-weighted image demonstrates the pontomesencephalic angle, defined as the angle between a line drawn along the anterior margin of the midbrain and the anterior superior margin of the pons. The normal value is 65° ± 10°.

10. Neurosarcoidosis

Sarcoidosis is a granulomatous, multisystem disease of unknown aetiology. It most often disease affects lungs and mediastinal lymph nodes. Cerebral manifestations include meningeal thickening, sarcoid nodules, cranial neuropathies and thickening and enhancement of the hypothalamus and hypophysis ⁽⁹⁾.

Fig 14: Figures A, B & C: Axial DWI, axial FLAIR, and sagittal FLAIR-weighted images demonstrate a non-diffusion-restricting T2/FLAIR hyperintense focus involving the hypothalamus and infundibulum of the pituitary stalk. Additional T2/FLAIR hyperintensities are seen in the midbrain. Figures D & E: Axial and sagittal post-contrast T1-weighted images demonstrate avid contrast enhancement of the lesion focal areas of pachymeningeal thickening.

11. Optic chiasmal glioma

Optic pathway primary tumours are uncommon entities which usually present with visual loss. Gliomas most commonly affect the chiasm ⁽¹⁰⁾.

Fig 15: Figures A, B & C: Axial DWI, axial T1-weighted, and axial T2/FLAIR-weighted images demonstrate a non-diffusion-restricting lesion that appears hypointense on T1-weighted imaging and hyperintense on T2/FLAIR, involving the right optic nerve and optic chiasm. Figures D & E: Sagittal post-contrast T1-weighted and sagittal post-contrast T2-weighted images show avid contrast enhancement of the lesion, with resultant posterior displacement of the midbrain and obliteration of the perimesencephalic cisterns.

12. Tubercular meningitis and tuberculomas

TB meningitis is caused by the extrapulmonary haematogenous seeding of tubercular bacilli into the central nervous system. It is more common in the developing world, with an estimated prevalence of 30-50% of all cases of bacterial meningitis. They typically manifest as meningeal thickening, basal exudates and ring-enhancing tuberculomas  ⁽¹¹⁾.

Fig 16: Figure A: Axial DWI demonstrates an acute infarct in the left caudate nucleus. Figures B&C: Axial T2/FLAIR, Sagittal T2/FLAIR demonstrate hyperintensity in the basal cisterns, consistent with exudates. Figures D&E: Axial post-contrast T1-weighted images demonstrate diffuse leptomeningeal enhancement with hydrocephalus.

Fig 17: Figure A: Axial DWI demonstrates an acute infarct in the right lentiform nucleus. Hydrocephalus noted. Figures B&C: Axial T1, Axial T2/FLAIR demonstrate multiple T1 hypointense and T2 hyperintense lesions around the basal cisterns. Figures D: Axial post-contrast T1-weighted images demonstrate ring enhancement of these lesions, Suggestive of tuberculomas.

13. Ruptured dermoid cyst

Rupture of an intracranial dermoid produces a distinct MR appearance of T1 hyperintensities in cisternal and sulcal spaces, which represent extruded fat droplets ⁽¹²⁾.

Fig 18: Figures A,B&C: Sagittal T1, Axial T1 and Axial T2 weighted images demonstrate a mass in the left para-sellar region appearing heterogeneous hyperintense signal on T1 and T2 weighted sequences with mass effects on the mid brain. Figure D: Axial T1 weighted imaging demonstrates droplets of T1 hyperintense material throughout the subarachnoid sulcal spaces. Case courtesy: Antonio Rodrigues de Aguiar Neto, Radiopedia (2019).

14. Pineal germinoma

These are the most common tumours of the pineal region ⁽¹³⁾

Fig 19: Figures A,B,C&D: Axial DWI, Axial T1, sagittal T1 and axial T2 weighted images demonstrate ill-defined lobulated solid cystic mass lesion noted in the anatomical region of pineal gland and tectum region with diffusion restriction in the solid component. This lesion compresses the midbrain with effacement of quadrigeminal cistern

15. Leukemic meningitis with oculomotor nerve involvement

Fig 20: Figures A,B,&C: Axial FLAIR, Axial T1,and Axial T1+C weighted images demonstrates focal area of pachymeningeal and leptomeningeal thickening and enhancement in the right anterior frontal region with a well defined extra-axial dural based enhancing T1/T2 iso-intense lesion in the right anterior frontal region – suggestive of leukemic meningitis in a known case of B-ALL Figure D : Sagittal T1+C demonstrate bulky infundibulum and pituitary gland with homogenous enhancement. Figure E: Axial T1+C demonstrates thickening of bilateral oculomotor nerves with enhancement.

16. Leptomeningeal carcinomatosis

It is the malignant seeding of the meninges that can occur secondary to solid tumours, brain tumours and haematological cancers ⁽¹⁴⁾.

Fig 21: Figure A: CECT Chest demonstrates relative asymmetry in the right breast with an enhancing ill defined soft tissue attenuating lesion in the retro areolar region causing mild retraction of the right nipple – suggestive of breast carcinoma Figure B: Axial T1+C demonstrates diffuse leptomeningeal and pachymeningeal thickening with plaque like thickening in the left frontal region. Figures C&D: Sagittal T1+C and Axial T1+C demonstrates abnormal meningeal enhancement in posterior aspect of tectum.

17. Progressive supranuclear palsy (PSP)

It is a neurodegenerative taupathy, which is similar to Parkinson’s disease ⁽¹⁵⁾.

Fig 22: Figure A: Axial T2 weighted image demonstrates loss of lateral convex margin of tegmentum of mid brain (Morning glory sign) Figure B: Sagittal T1 weighted imaging demonstrates flattening of the superior aspect of mid brain (Hummingbird sign) Figure C: Sagittal T1 demonstrates reduced mid brain to pons ratio(<0.52).

18. Subarachnoid haemorrhage (SAH)

Fig 23: Figure A: Axial non-contrast CT demonstrates diffuse subarachnoid haemorrhage involving the cisternal spaces and bilateral Sylvian fissures in a patient with Road traffic accident (RTA). Figure B: Axial CTA and reconstructed images demonstrate an incidental outpouching arising from the anterior communicating artery (ACOM) – suggestive of aneurysm. Figure C: Intracranial Digital Subtraction Angiography (DSA) demonstrates a focal outpouching with no active contrast extravasation, suggesting an unruptured aneurysm.

19. Non-traumatic, non-aneurysmal subarachnoid haemorrhage

Fig 24: Figure A: Axial non-contrast CT scan showing subarachnoid haemorrhage in the right crural and ambient cisterns, with no history of trauma. Figures B & C: Axial SWI images demonstrating blooming artifacts in the interpeduncular and cerebellopontine cisterns, suggestive of haemorrhage. Intracranial DSA was performed and showed no abnormalities, supporting a diagnosis of non-traumatic, non-aneurysmal subarachnoid haemorrhage.

20. Wolfram syndrome

It is a rare, neurodegenerative disorder characterised by a spectrum of findings – diabetes insipidus, diabetes mellitus, optic atrophy and deafness; acronymised as ‘DIDMOAD’ ⁽¹⁶⁾.

Fig 25: Figure A: Sagittal T1 weighted imaging demonstrates absent posterior pituitary bright spot. Figure B: Sagittal T1 weighted imaging demonstrates atrophy of brain stem. Figure C: Axial T1 weighted imaging demonstrates atrophy of optic chiasma.Features suggestive of Wolfram (DIDMOAD) syndrome.

21. Oculomotor metastasis

Fig 26: Figure A: Axial T1+C at the level of midbrain showing thickening and enhancement of both oculomotor nerves. Figure B: Sagittal T1+C revealing an avidly enhancing mass in the pineal region Figure C: Axial T1+C demonstrating left frontal leptomeningeal enhancement and a left frontal subcortical parenchymal lesion Biopsy revealed a high grade small round blue cell tumour of the pineal gland (WHO grade 4) with oculomotor, leptomeningeal and parenchymal metastasis.