REVIEW OF THE MENINGES (1)
The neuraxis is encased by 3 protective coats collectively called the meninges.
These layers are called dura mater, arachnoid mater,
and pia mater.
1.
Dura mater:
The dura mater (DM) is the outermost fibrous covering of the brain,
rich in neurovascular supply. It is also called pachymeninges (Greek: pachy = thick). It ensheathes the cranial nerves and fuses with the epicranium.
The dura also folds upon itself to partition the cranial cavity (falx cerebri, falx cerebelli and tentorium cerebelli).
2.
Arachnoid mater:
The arachnoid mater (AM) is the middle meningeal layer.
It loosely invests the brain,
bridging all,
but two (longitudinal cerebral fissure and stem of the lateral sulcus),
fissures.
3.
Pia mater:
The pia mater (PM) is a vascular membrane adherent to the neuroparenhymal surface extending into the sulci and fissures. Collectively the AM and PM are referred to as the leptomeninges (Greek: lepto = thin).
![](https://epos.myesr.org/posterimage/esr/ecr2018/142016/media/769610?maxheight=300&maxwidth=300)
Fig. 1: THE MENINGEAL LAYERS
BLOOD-BRAIN BARRIER
To ensure an environment for optimal functioning of neural tissues,
a narrow homeostatic range is desirable. This needs a tight surveillance on the materials transported into the brain,
maintained by a physiologico-physical barrier known as the Blood-Brain Barrier (BBB).
(2)
1.
Morphology:
The BBB is formed by endothelial cells (ECs) with intervening tight junction proteins which surround neural vsaculature. Cellular elements (astroglia) surround the ECs,
form an additional stratum of reinforcement.
(2)
![](https://epos.myesr.org/posterimage/esr/ecr2018/142016/media/769611?maxheight=300&maxwidth=300)
Fig. 2: ANATOMY OF THE BLOOD-BRAIN BARRIER
2.
Physiology:
The BBB allows bidirectional movement of water, and is permeable to lipophilic substances,
those with low ionization at physiologic pH and those lacking plasma binding proteins.
(3-7) Glucose and amino acids uptake are dependent on enzyme-mediated processes and carrier proteins.
(8-10)
![](https://epos.myesr.org/posterimage/esr/ecr2018/142016/media/769612?maxheight=300&maxwidth=300)
Fig. 3: PHYSIOLOGY OF THE BLOOD-BRAIN BARRIER
References: REDRAWN AND MODIFIED ILLUSTRATION. REF: https://www.pharmaceutical-journal.com/news-and-analysis/features/a-barrier-to-progress-getting-drugs-to-the-brain/20202723.article
3.
Location:
The BBB is found along blood vessels that supply neuroparenhcyma. Areas of the brain devoid of the BBB include the choroid plexus,
hypophysis,
tuber cinereum,
area prostrema,
paraphysis,
pineal gland,
and preoptic recess.
(11)
RELEVANCE OF THE BLOOD-BRAIN BARRIER IN MRI
Intra-axial pathologies enhance due to disruption of the BBB,
while extra-axial lesions enhance due to vascular prolification or contrast leak from vessels. (12)
Contrast media used in neuroimaging contain gadolinium,
which shortens relaxation times of tissues they accumulate in. The net result increases signals on T1-weighted images (T1WI) and reduces signals on T2-weighted images (T2WI).
(12,
13)
T1W Imaging is used following contrast administration to detect enhancement.
At our institution,
this is coupled with post-contrast Fluid Attenuation Inversion Recovery (FLAIR) imaging.
At clinical doses,
FLAIR is sensitive to contrast related T1 shortening because of its long inversion time.
(15) This,
along with its property to null CSF signals and lack of vascular enhancement,
makes it the preferred sequence for the detection of leptomeningeal disease.
(12,
13)
![](https://epos.myesr.org/posterimage/esr/ecr2018/142016/media/769613?maxheight=300&maxwidth=300)
Fig. 4: PHYSICS OF T1 RELAXATION AND GADOLINIUM EFFECTS ON T1 RELAXATION
![](https://epos.myesr.org/posterimage/esr/ecr2018/142016/media/769614?maxheight=300&maxwidth=300)
Fig. 5: PHYSICS OF FLAIR IMAGING AND CONTRAST ENHANCEMENT IN FLAIR IMAGING