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Brain IMAGING TECHNIQUE  

Imaging techniques 

In most neurological disorders, plain films are either normal or the abnormalities are too non-specific 
for the diagnosis to be made. Skull radiographs are rarely performed except as part of skeletal surveys 
in suspected non-accidental injury or myeloma . Computed tomography (CT) and magnetic resonance 
imaging (MRI) give vastly more information and one or the other investigation is indicated in 
practically all patients with intracranial disease. 
 

Computed tomography 

A routine CT examination of the brain involves making 20

–30 axial sections. The axial plane is also 

the routine viewing projection but, if sufficiently thin, reconstructions can be made from the axial 
sections, which then provide images in any other plane .However, to enable good differentiation of 
grey and white matter, a slice thickness of 3

–5 mm is needed. The window settings are selected for 

brain tissue or bone, depending on the structure being assessed 

  

Contrast enhancement for computed tomography

 

The brain parenchyma does not normally enhance following an intravenous injection of contrast 
medium due to the blood

–brain barrier (BBB) – the endothelial lining of cerebral vessels preventing 

passage of solutes. Contrast enhancement of a brain lesion is therefore a consequence of breakdown of 
the BBB such as with ischaemia, inflammation and neoplasms. Intracranial lesions (such as 
meningiomas) supplied by the external carotid artery, which lacks a BBB, will often enhance avidly. 
There is also no BBB in the pituitary, pineal and choroid plexuses, which will normally enhance. 
Contrast is therefore only used routinely to evaluate vessels and extra-axial lesions or to increase the 
conspicuity of brain lesions. 

Intracranial enhancement on computed tomography and magnetic resonance imaging 
Physiological 
• Choroid 
• Anterior pituitary gland 
• Arteries 
• Dural venous sinuses 
Pathological 
• Metastases 
• Some primary gliomas 
• Meningiomas 
• Abscess 
• Acute demyelination

 


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Computed tomography angiography

 

If thin slices are acquired following contrast administration

 

detailed reconstructions of the vessels can 

be created in

 

multiple planes and with surface-shaded three-dimensional

 

images. CT angiography has 

replaced conventional

 

angiography for the initial diagnosis of arterial

 

occlusions, aneurysms and 

arteriovenous malformations

 

(AVMs). The venous phase of the angiogram can give information

 

on the 

venous sinuses of the brain such as when

 

looking for thrombosis. Perfusion CT is a new technique

 

that 

can quantify the passage of contrast through the brain

 

to evaluate the presence and extent of infarction 

and ischaemia

 

in stroke patients who may be candidates for thrombolysis

 

treatment.

 

 

Normal head computed tomography 

The

 

cerebrospinal fluid (CSF) is seen as water density within the ventricular system and subarachnoid 

space surrounding

 

the brain. It is possible to distinguish the white and grey

 

matter of the brain due to 

the higher fat content within myelinated white matter, which is therefore of lower attenuation.

 

The larger arteries at the base of the brain can usually be identified within the CSF-containing basal 
cisterns. 
Calcification is normally seen in the pineal gland and choroid plexus particularly in the lateral 
ventricles. 
Pathological calcification can be seen in abnormal vessels such as an AVM or aneurysm and some 
types of brain tumour. 
The supratentorial regions are usually well shown, but details of the posterior fossa may be obscured by 
artefact from the surrounding bone. 


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Abnormal head computed tomography 

When an abnormality is seen, it is important to decide whether it has an intra-axial or extra-axial 
location as the pathologies and therefore the differential diagnosis are very different. Intra-axial lesions 
can involve the white and grey matter structures of the brain parenchyma, while extra-axial lesions may 
involve the meninges, extracerebral spaces and skull vault. Specific diagnoses are suggested by 
combining the clinical features with information about multiplicity, size, position and density of the 
lesion. 
The key signs of an abnormality on a CT scan are: 
• abnormal tissue density 
• mass effect 
• enlargement of the ventricles. 

Abnormal tissue density 

Abnormal tissue may be of higher or lower density than the normal surrounding brain. High density is 
seen withacute haemorrhage ,calcification and areas of contrast enhancement . Low density can be due 
to cytotoxic oedema associated with infarcts, or to vasogenic oedema, which commonly surrounds 
neoplasms, abcesses and other areas of inflammation.  
Cytotoxic oedema will involve both the white and grey matter structures ,whereas vasogenic oedema is 
limited to the white matter and characteristically shows finger-like projections into the subcortical 
white matter in the gyri.  

 
 


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Mass effect 

The normally symmetrical lateral ventricles should be examined to see if they are displaced or 
compressed. Shift of midline structures, such as the septum pellucidum, the third ventricle or the pineal 
gland away from a lesion indicates a significant mass effect. Ventricular dilatation will occur if the 
mass obstructs the flow of CSF. A mass effect may also show itself by effacing the basal cisterns, such 
as when the suprasellar cistern is obscured by downward movement of the medial temporal lobes over 
the tentorium cerebellum (uncal herniation )or the cerebellar tonsils through the foramen magnum. 
 

Enlargement of the ventricles

 

There are two basic mechanisms that cause the cerebral ventricles to enlarge: 
• Obstruction to the CSF pathway, either within the ventricular system (obstructive hydrocephalus) or 
over the surface of the brain  

 

 
• Secondary to atrophy of the surrounding brain tissue 

 

 
 
 
 
 
 
 


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Magnetic resonance imaging 

The advantage of brain MRI is its superior spatial and contrast resolution without the artifact problems 
associated with local bone structures on CT. Therefore, the anatomy of the brain can be exquisitely 
displayed and enables much better visualization of brain pathologies, particularly in areas such as 
around the skull base for the pituitary gland and the posterior fossa which are poorly seen by CT. The 
routine sequences used for MRI vary but usually include T1- and T2-weighted sequences in orthogonal 
planes 
A FLAIR (fluid attenuated inversion recovery) sequence is useful as the pulse sequence nulls the bright 
signal from CSF, making brain pathology (usually T2 bright) more visible. Magnetic resonance can 
also recognize flowing blood within larger arteries and veins, which can be examined without the need 
for contrast medium to create intracranial angiograms . 
The diffusion-weighted imaging (DWI) sequence looks at the normally random movement of water 
molecules in brain tissue and is extremely useful, particularly when the patient is suspected of having 
an acute stroke. Within areas of cytotoxic oedema such as an infarct, the movement 
of water molecules in the extracellular space is limited by the swollen, dying cells and this produces 
bright signal, whereas the movement of water within vasogenic oedema is not limited (free) and does 
not produce any signal change. The DWI scan becomes positive within minutes of an acute stroke 
whereas CT abnormalities can take hours and will miss very small infarcts. Restricted diffusion within 
a cystic mass is also relatively specific for pus within a pyogenic abscess. 
The water molecules in normal axons making up the white matter are unlikely to traverse the myelin 
sheath and therefore flow more rapidly in the direction of the axon bundle. An advance in diffusion 
imaging uses this feature to demonstrate the location and orientation of the tracts connecting various 
parts of the brain: an investigation known as diffusion tensor imaging or tractography, which may be 
helpful for studying white matter pathways in disease and for surgical planning. 
Functional MRI utilizes different techniques to measure the blood flow to parts of the brain which 
changes depending on the neuronal activity in that location at the time of scanning. The patient is 
usually scanned whilst performing various tasks such as memory recall and the scan produces a map of 
brain activity superimposed on the anatomical location. The technique may be useful in understanding 
brain function, particularly in psychiatric disorders The disadvantages of MRI compared with CT 
include the limited visualization of calcification and lack of bone detail. Each sequence has to be 
acquired separately so the overall scan time is much longer, during which the patient has to lie still, and 
monitoring seriously ill patients within the scanner can be difficult. If intubated, the life support and 
monitoring equipment must be MRI compatible. Any intracranial ferrous metal present such as 
aneurysm clips or cochlear implants are absolute contraindications

 


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Contrast enhancement for magnetic resonance imaging

 

the gadolinium compounds used for MRI enhancement are excluded from the normal brain substance 
by the BBB. 
Breakdown of the BBB, such as by tumours or abscesses, means that contrast will accumulate within 
these pathological processes and show high signal intensity (i.e. they appear white) on T1-weighted 
images   

 

 


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Abnormal magnetic resonance imaging of the brain 

The range of normal anatomy and abnormalities that can be shown by MRI is very great. Fat, 
haemorrhage, oedema, CSF and flowing blood all have characteristic signal intensities. 
Thus, it is more often possible to make a specific diagnosis of an intracranial disorder with MRI than 
with CT. MRI is the preferred investigation in intracranial sepsis, tumours, inflammatory diseases, 
epilepsy and congenital malformations. Haemorrhage can be seen on MRI and the blood can be aged as 
haematomas develop a specific signal pattern owing to the breakdown products of haemoglobin, such 
as methaemaglobin or haemosiderin. These have different paramagnetic effects that profoundly alter 
the MR signal in a way that can be recognized on T1- and T2-weighted scans 


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Specific brain disorders

  

 

Glioma

 

Glioma is a non-specific term used to describe a group of tumours that arise from glial cells, which 
normally support the brain neurons, such as astrocytes (astrocytoma is the commonest tumour type). 
They range from low grade (e.g. childhood pilocytic astrocytoma) to high grade (e.g. glioblastoma 


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multiforme). At CT, a glioma typically appears as a solitary, irregular, low attenuation lesion .Local 
mass effect can usually be demonstrated although this can be minimal as the glioma is replacing rather 
than expanding brain tissue. Gliomas may calcify 

– particularly oligodendrogliomas. For accurate 

assessment of gliomas, both pre and post contrast scans should be performed as enhancement can be 
associated with a higher grade of malignancy. 
The MRI features are similar to those of a mass, often with adjacent signal change. However, it is 
important to realize that tumour cells will be present beyond the margin of any signal change seen 
around a glioma. The mass may show a variety of signal intensities but, in general, the tumour is lower 
in signal intensity than the normal brain on the T1-weighted images and higher in signal intensity 
on the T2-weighted images. Calcification, though sometimes recognizable as absence of signal, is less 
evident than it is with CT. 

 

 

 

 

 

 

 


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Brain metastases

 

Metastases in the brain are often low density on CT unless they are haemorrhagic. On CT and MRI, 
they usually show contrast enhancement and are often surrounded by substantial oedema. Metastases 
are typically multiple but a solitary metastasis can be indistinguishable from a primary intracerebral 
brain tumour with either technique. A parenchymal lesion in the posterior fossa of an adult should be 
considered to be a metastasis until proven otherwise. 

 

 

 

 

 

 

 


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Meningioma

 

Meningiomas are the commonest non-glial intracranial tumour and arise from the meninges of the 
vault, falx or tentorium. The commonest sites are the parasagittal region, over the cerebral convexities 
and the sphenoid ridges. On an unenhanced CT scan, a meningioma is often denser than the brain 
because of calcium within the lesion . Following intravenous contrast, the tumour shows marked 
homogeneous enhancement (. Reactive sclerosis and blistering of the adjacent bone with thickening 
and enhancement of the local dura may also be seen. The multiplanar imaging capability of MRI makes 
it possible to predict the site of origin of the tumour with greater confidence than is usually possible 
with CT. Once it can be ascertained that an enhancing tumour is extra-axial, by compressing the brain 
from outside, the diagnosis of meningioma becomes highly likely. 

 

 

Acoustic neuroma 

The term 

‘acoustic neuroma’ is a misnomer as they are schwannomas that arise from the vestibular 

branch of the vestibulocochlear nerve. Vestibular schwannomas typically arise on the nerve within the 
internal auditory canal and may extend out medially into the cerebellopontine angle . When large, they 
can be recognized at CT or MRI. When small, they may only be identifiable with high resolution MRI 
As schwann cells are not glial in origin, they enhance avidly.  

 

 


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Stroke 

Stroke is defined as a sudden, focal neurological deterioration due to a disturbance in the blood supply 
to the brain. It is a common cause of hospital admission and has a high morbidity. The important 
causes of stroke are: 
• Cerebral infarction, which may be due to in situ thrombus or embolus from the proximal artery or 
heart. 
• Intracerebral haemorrhage. 
• Subarachnoid haemorrhage. 
Acute cerebral infarction and haemorrhage are often clinically similar, but it is important to distinguish 
between these two conditions as subsequent investigation and treatment differ greatly. The acute 
management of thromboembolic infarct is aimed at destroying the clot with thrombolysis, but this is 
contraindicated in the presence of haemorrhage 

– therefore CT is the best first test.  

Cerebral infarction 

There are four main outcomes of CT performed for an acute stroke: 
 

• The presence of haemorrhage precludes thrombolysis treatment and is described below. 

• Stroke mimics are conditions that present like stroke, such as a subdural haematoma or brain tumour, 
for which different treatments are required. 
• A normal scan either means the patient is not having a stroke (stroke mimic not identifiable by 
imaging, such as hemiplegic migraine) or is at the very early stages of a stroke before the CT becomes 
abnormal and therefore is an ideal candidate for thrombolysis. 
• Then there are the early signs of a stroke seen on CT. The dense artery sign is high density clot 
visualized within a major intracranial artery. Later, infarcted brain becomes lower attenuation which is 
initially best seen in areas such as the lentiform nucleus and later may involve the whole vascular 
territory. The infarct will gradually resolve, leaving an atrophic area of low attenuation gliosis.  
Diffusion-weighted imaging is the most sensitive method for the early detection of an infarct and will 
show changes within minutes of the onset . However, its use acutely is limited by scanning time, 
availability and problems obtaining safety clearance. 

 

 
 


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Intracerebral haemorrhage 

Acute haemorrhage is seen on CT as high attenuation, frequently causing a mass effect .The initial high 
density lessens over time, leaving a low density area indistinguishable from an infarct. MRI is useful in 
the follow-up of intracerebral haemorrhages to exclude underlying vascular malformation or occult 
metastasis, which may be obscured by the presence of blood. If no cause is identified, formal cerebral 
angiography may be required to exclude a subtle vascular anomaly.

 

 

 

 


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Subarachnoid haemorrhage 

Spontaneous subarachnoid haemorrhage is usually due to a ruptured intracranial aneurysm or vascular 
malformation. CT is the best initial investigation to diagnose a subarachnoid haemorrhage and to 
demonstrate the site of bleeding. A subarachnoid haemorrhage is recognized by high density blood 
outside the brain in the sulci, Sylvian fissures and basal cisterns. Subarachnoid haemorrhage on CT will 
obviate the need for lumbar puncture and CSF examination, but sensitivity of CT decreases with time 
after a haemorrhage and a normal examination does not exclude the diagnosis. CT angiography can be 
used to demonstrate the aneurysm and to plan treatment by neurosurgical clipping or by interventional 
radiological microcatheter techniques which occlude the aneurysm with metal coils. Ateriovenous 
malformations may be coiled or embolized to reduce the size and risk of haemorrhage 

 


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Infection 

In acute meningitis CT and MRI are usually normal and antibiotics should start immediately and not 
await the result of a scan. A lumbar puncture is frequently performed to obtain CSF to confirm the 
diagnosis. A CT scan prior to lumbar puncture is only essential if there is evidence of raised 
intracranial pressure, focal neurological signs or change in conscious level. 
Encephalitis is caused by infection, usually viral. CT and MRI show unilateral or asymmetrical 
bilateral, focal abnormal areas, often in a characteristic distribution appearing as low attenuation on CT 
and high signal on a T2-weighted MRI scan. The commonest cause of viral encephalitis 
is herpes simplex, which typically produces abnormalities in the medial temporal lobe, insular cortex 
and inferior frontal lobes. These areas may contain areas of haemorrhage and enhancement. 


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An abscess can be caused by bacterial, tuberculous, fungal or parasitic organisms. Necrosis and pus 
formation occur in the centre of the abscess, which appears as low density on CT or fluid on MRI. The 
wall of the abscess enhances with intravenous contrast and may be surrounded by oedema, giving an 
appearance known as 

‘ring enhancement’ . The pus within the centre of a pyogenic abscess will 

typically demonstrate restricted diffusion. 

 

 

 

 


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Ageing 

Various changes can be seen on CT and MRI in elderly patients that often bear little correlation with 
the clinical state of the patient. Atrophy of the brain occurs, resulting in dilatation of the ventricles and 
widening of the cortical sulci. Small vessel atherosclerotic ischaemia can produce low attenuation areas 
in the deep white matter on CT, normally seen in the periventricular regions, which are 
T2=hyperintense on MRI. 

 

 

Extracerebral haematoma 

Extracerebral haematomas comprise extradural and subdural haematomas, depending on the location 
of the blood in relation to the dura mater layer of the meninges. An extradural haematoma is seen as a 
lens-shaped, high density area situated over the surface of the cerebral hemisphere that does not cross 
sutures as it lies below the periosteal layer of the skull . It is normally an arterial bleed from a 
meningeal artery which was damaged by a skull fracture 

– a common associated finding – and 

therefore occurs at the site of head impact (coup injury). As it is an arterial bleed, an initial period of 
lucidity is followed by rapid loss of consciousness as the intracranial pressure increases, requiring 
emergency surgical evacuation.  
A subdural haematoma is seen as a crescenteric collection of blood that conforms to the shape of the 
underlying brain and occurs most commonly over the convexity of the brain where it is not limited by 
any of the skull sutures, but can also extend along the falx and tentorium . It is normally a venous bleed 
from the bridging veins which cross the subdural space and therefore is more commonly seen in 
patients who have cerebral atrophy, making the veins more prone to injury. They often occur on the 
side opposite to the head impact (contracoup injury) or may be bilateral following a shaking injury (as 
seen in nonaccidental injury). Acutely, the blood is high density for 1

–2 weeks following injury; 


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however, as the feeding system is low pressure, the haematoma may be clinically occult and become 
chronic and consequently low (CSF) density after several weeks . In the intervening period, 
haematomas pass through a phase of being isodense with the brain and are, therefore, less obvious on 
CT scans. They should be suspected if there is any midline displacement or ventricular compression. 
The displacement may not be obvious if the haematomas are bilateral, when effacement of the sulci 
may be the only clue to their presence  

 

 

Fracture 

Fractures of the skull base or vault should be looked for on bone window settings . Fractures of the 
skull vault should not be confused with normal sutures or vascular markings. Assessment should be 
made of any significant depression of the fracture as these may require surgical elevation and are more 
likely to be associated with underlying brain injury. If there is a penetrating skull injury or a fracture 
involves the normally pneumatized paranasal sinuses, middle ears or mastoids, air may enter the 
cranium and be seen on CT as locules of very low density gas . CT can also demonstrate fluid (blood) 
in the sinuses and mastoid air cells or air in the orbits, suggesting a facial or skull base fracture. 

 

 

                                                                                                                                                                                      Noor Rahman 




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