The use of CT and MRI in the characterization of intracranial mass

A detailed description of the imaging appearances of all mass lesions is beyond the scope of this review, but we hope to provide the reader with a rational approach to the complex task o

discriminating factors.

N Intravenous administration of contrast material is indicated when an intracranial

mass lesion is detected on CT or MRI

N MRI is the preferred imaging modality for the evaluation of intracranial mass

lesions, but CT is often performed at the initial presentation for practical reasons

DOI: 10.1259/imaging/ 64168868

’2007 The British Institute of Radiology

Abstract Intracranial mass lesions are an important cause

of neurological morbidity and a common indication for cranial

imaging Given the wide range of pathological processes that

can present as intracranial mass lesions, the radiologist has an

important role in limiting the differential diagnosis in an

individual case in order to inform the clinical decision-making

process This review illustrates the use of cranial CT and MRI,

including diffusion weighted imaging (DWI), in the

characterization of intracranial mass lesions A detailed

description of the imaging appearances of all mass lesions is

beyond the scope of this review, but we hope to provide the

reader with a rational approach to the complex task of

producing a short differential diagnosis

The management of intracranial mass lesions was

revolutionized by the development of CT and MRI The

pivotal role of the radiologist in the diagnostic workup of

these patients is summarized in Box 1

The prognosis and management of different

intracra-nial mass lesions vary widely and the radiologist should

therefore always strive to provide the referring clinician

with as short a differential diagnosis as possible Being

able to exclude certain possibilities with confidence from

the differential diagnosis is also very helpful

Some radiologists remain reluctant to offer a short

differential diagnosis, reasoning that the pathologist will

have the final say in any case This is an out-dated and

unnecessarily defeatist view for three reasons (1) Several

brain masses have pathognomonic or virtually

pathogno-monic appearances (e.g cerebral aneurysms [1, 2], lipoma

of the corpus callosum [3], dermoid and epidermoid

tumours [4–7], Lhermitte-Duclos disease (Figure 1) [8],

most vestibular schwannomas [9–11] and most

meningio-mas [12, 13]), rendering biopsy obsolete or even

contra-indicated unless surgery is contra-indicated for clinical reasons

(2) To minimize surgical morbidity, most brain biopsies

are performed through a small burr-hole using a biopsy

needle yielding a tiny fragment of tissue This approach is

obviously prone to sampling errors In a large glioma, for

instance, the biopsy may only yield tissue corresponding

to a World Health Organization (WHO) grade II lesion while the imaging may show contrast enhancement and necrosis indicating a grade IV lesion (3) Certain histolo-gical features are ambiguous and should be interpreted in the light of the imaging findings, e.g Rosenthal fibres are typically found in pilocytic astrocytomas, but can also be seen in reactive gliosis [14]

CT and MRI can be used to obtain information about several features of a mass lesion, including the location,

Figure 1 Using location and specific imaging characteristics

of a lesion to obtain an accurate diagnosis Axial T 2 weighted MRI of a 35-year-old female with a 6-week history of occipital headache shows the characteristic ‘‘tiger-striped’’

or laminated appearance of Lhermitte-Duclos disease, as demonstrated by hyperintense right-sided hemispheric expansion with parallel linear striations on the surface.

size and extent, tissue composition and state of the

blood–brain barrier Chemical and pathophysiological

information obtained from advanced imaging

techni-ques, such as 1H MR spectroscopy and perfusion

imaging, also has a role to play in certain cases but,

since these techniques are not currently widely available,

we have decided to concentrate on modalities and

techniques available in most district general hospitals

What is a mass lesion and what is mass effect?

For the purposes of this review, we include any discrete

intracranial lesion (or lesions) causing mass effect Mass

effect refers to anatomical distortion of tissues

surround-ing the lesion or anatomical distortion and/or

enlarge-ment of the structure in which the lesion arises Judging

the amount of mass effect a lesion exerts is a very

subjective exercise since measurable phenomena, such as

midline shift, are absent in most cases The amount of

mass effect in an individual case depends on the size and

nature of the lesion Not surprisingly, small lesions

generally exert less mass effect than larger lesions, and

in certain parts of the intracranial compartment, such as

the deep cerebral white matter, small mass lesions may

have no appreciable mass effect on imaging studies The

effect of the nature of the lesion on the amount of mass

effect is more complex In brain metastases, for example,

the mass effect is caused not only by proliferating

neoplastic cells pushing away surrounding normal brain

tissue but also by an increase in interstitial water

(vasogenic oedema) in and around the tumour Acute

infarcts, on the other hand, cause mass effect by virtue of

an increase in intracellular water (cytotoxic oedema) [15]

It should come as no surprise that brain tumours

(particularly metastases) are generally associated with

more mass effect than a similar sized acute infarct

Our visual appreciation of mass effect is further

influenced by the exact location of the lesion For

example, a lesion in the brain stem would appear to

have more mass effect than a similar sized lesion in the

deep cerebral white matter, since changes in the relatively

small size and predictable contours of the brainstem are

more noticeable on imaging studies

How do you generate a short differential

diagnosis?

To come up with a differential diagnosis, the

radi-ologist integrates information obtained from imaging

studies — CT and MRI for the purposes of this review —

with demographic and clinical information about the

patient

With experience, this process of assigning different

levels of importance to various pieces of information

becomes intuitive, but it remains one of the most

complex cognitive tasks we perform as radiologists In

this process, it is important to make use of all the

relevant information available, not only on the images

but also on the request card

Certain bits of information have greater discriminating

value than others For example, the age of the patient

strongly influences the differential diagnosis in many

cases while the patient’s sex is usually irrelevant except

in the very specific instances mentioned below Radiology trainees in particular may find our proposed checklist of eight questions (Box 2) useful when trying to come up with a short differential diagnosis of an intracranial mass The fact that four of the eight questions refer to clinical and demographic information reflects the importance of this information The reader should also keep in mind that the relevance of the answer to one question may depend on the answer to another question For example, the anatomical location of

a mass has different implications in different age groups

We shall now briefly discuss the relevance of each of the eight questions using examples This discussion is far from comprehensive, but hopefully serves to illustrate the usefulness of this approach

How old is the patient?

The age of the patient is of particular importance in distinguishing between different types of intra-axial cerebral neoplasms Although most neoplasms have a wide age range, their distribution is often very skewed within that range For example, the most common primary intra-axial brain tumour, glioblastoma multiforme (GBM) has an age range extending from infancy to the ninth decade (and probably beyond), but the overwhelming majority of GBMs occur in adults, with 70% occurring in patients between the ages of 45 years and 70 years [16] A solitary intra-axial ring-enhancing supratentorial mass lesion in a middle-aged or elderly person is therefore most likely to be either a metastasis or a GBM while the same appearance in a child is more likely to represent a primitive neuroectodermal tumour [17] or an infectious condition, such as a bacterial abscess or a tuberculoma On the other hand, an enhancing cerebellar mass in a 3-year-old child is very likely to represent a primary tumour (astrocytoma, medulloblastoma or ependymoma) [17] while a similar appearance in a 60-year-old person is much more likely to represent a metastasis (Figure 2) Extra-axial neoplasms are uncommon in children but the imaging appearances of the two most common types, namely meningiomas and vestibulocochlear schwanno-mas (acoustic neuroschwanno-mas), are so characteristic that it rarely causes confusion when encountered in a child The presence of either of these tumours in a child should raise a suspicion of neurofibromatosis type 2 [11]

How did the patient present?

The clinical presentation can occasionally be very helpful in distinguishing between different intracranial mass lesions As a general rule, dramatic imaging appearances in an eloquent brain area without dramatic neurological deficits, is more in favour of tumour than either infarction or demyelination

For example, a WHO grade II or III astrocytoma can have a very similar appearance to an infarct on CT and MRI but the symptomatology is distinctly different, with tumours usually presenting with seizures and/or head-aches and/or gradual onset of relatively mild neurolo-gical deficits (Figures 3 and 4) [18], while infarcts

typically present with a sudden onset of dramatic motor

and/or sensory deficits Infarcts and demyelination only

rarely present with seizures [19]

Does the patient have a known disease, syndrome

or malignancy?

Only a small minority of patients presenting with an intracranial mass lesion have a predisposing condition, but knowledge of the condition can be extremely helpful

A previous diagnosis of a malignancy with a tendency

to metastasize to the central nervous system (CNS), such

as bronchogenic carcinoma, breast carcinoma, melanoma

or bowel carcinoma, obviously increases the likelihood of enhancing intracranial lesions being metastases [20]

Figure 2 The relevance of patient age in tumours in a particular location Axial gadolinium enhanced T1weighted images through the posterior fossa in (a) a 6-year-old boy and (b) a 62-year-old man, both presenting with recent onset of cerebellar symptoms and signs Based on the ages of the patients, a primary tumour was considered most likely in the child, whereas a metastasis was suspected in the older patient Histology confirmed a medulloblastoma in the child (a) and a non small-cell lung cancer metastasis in the man (b).

Figure 3 Combining clinical and imaging information to

reach the correct diagnosis This 35-year-old man has a

12-year history of temporal lobe epilepsy The coronal

gadoli-nium enhanced T 1 weighted image shows an intra-axial,

cortically based, partially cystic mass with eccentric rim

enhancement The long history of seizures attributable to

this lesion points towards an indolent neoplasm rather than

a more aggressive rim enhancing lesion, such as a metastasis,

glioblastoma multiforme (GBM) or abscess In this location,

ganglioglioma and dysembryoplastic neuroepithelial tumour

(DNET) are the most likely candidates The enhancement

characteristics strongly favour a ganglioglioma and this was

confirmed histologically.

Figure 4 Combining clinical and imaging information to reach the correct diagnosis Coronal T 2 weighted image of a 7-year-old child with precocious puberty and a long history

of gelastic seizures shows a small, well defined hyperintense mass arising from the inferior aspect of the hypothalamus The location and clinical features enable the diagnosis of hypothalamic hamartoma to be made, obviating the need for histological diagnosis.

Even a solitary intra-axial enhancing mass in such a

patient should be considered a metastasis until proven

otherwise On the other hand, similar appearances in a

patient with AIDS would raise the suspicion of primary

CNS lymphoma or toxoplasmosis [21]

Neurocutaneous syndromes (particularly von

Hippel-Lindau (VHL) [22], neurofibromatosis types 1 and 2 [11,

23] and tuberous sclerosis [24–26]) predispose to

parti-cular tumours Knowledge about these conditions allows

the radiologist to make a confident diagnosis of a specific

histological tumour type in many cases (Figures 5 and 6)

At the time of the patient’s first presentation with an

intracranial tumour, the diagnosis of the neurocutaneous

syndrome may not yet have been made but additional

imaging features often give clues to the underlying

genetic condition

Examples of conditions predisposing to non-neoplastic

mass lesions include cyanotic heart lesions and

pulmon-ary arteriovenous malformations, which predispose to

brain abscesses [27, 28], and adult polycystic kidney

disease and coarctation of the aorta, which predispose to

cerebral aneurysms [29, 30]

Is the patient male or female?

While there are slight sex differences in the incidence of

many intracranial mass lesions, the differences are

gen-erally too small to use sex as a meaningful discriminator

There are two rare exceptions to this rule and both involve lesions of midline structures First, lymphocytic hypophysitis is a rare non-neoplastic cause of an enlarged enhancing pituitary gland and or pituitary stalk and is at least eight times more common in women than men [31] We have to stress that the other causes of this appearances, such as pituitary adenoma, Langerhan’s cell histiocytosis, sarcoidosis and germi-noma, are more common in both sexes Second, germinomas in boys and men tend to occur in the pineal gland while the same tumours in women and girls are more common in the suprasellar region [16] It goes without saying that the great (more than one hundred-fold) gender difference in the incidence of breast carcinoma has an influence on how we search for the primary tumour in cases of cerebral metastases from an unknown primary

Where is the lesion/s located?

The location of a mass is a highly discriminating imaging feature and the radiologist should always strive

to get as much information as possible about the exact location of the tumour from the imaging studies The following anatomical descriptors are particularly useful and should be employed when describing the location of

a mass

Intra-axial vs extra-axial The term ‘‘intra-axial’’ implies that a mass arises within the neuraxis, i.e within the substance of the CNS (brain or spinal cord) Extra-axial lesions can arise within the skull, meninges, cranial nerves or blood vessels Extra-axial lesions are easier to classify accurately than intra-axial lesions and often have a pathognomonic appearance [9–13] (Figure 5) The ability to distinguish intra-axial from extra-axial masses is arguably the single most discriminating imaging feature that can be elicited

It can also be a surprisingly difficult distinction to make

in some cases Intra-axial masses are completely sur-rounded by brain tissue except for rare instances of tumours with an exophytic component Lesions within the cerebral or cerebellar white-matter or the deep grey-matter structures can easily be classified as intra-axial However, when a lesion involves the cortex it should be carefully evaluated to decide whether it is protruding into the cortex from outside (extra-axial) or arising within the cortex or subcortical white-matter (intra-axial) Imaging features strongly suggestive of an extra-axial origin include ‘‘trapped’’ cerebrospinal fluid (CSF) and or pial blood vessels between the lesion and the cortex, and a buckled or concertina appearance of the underlying cortex (Figure 7) The presence of oedema is not particularly useful in distinguishing between intra-axial and extra-intra-axial lesions as it can be present or absent

in either [13, 32, 33]

Supra-tentorial vs infra-tentorial This is an easy distinction to make and, in combination with the patient’s age, it is particularly useful in distinguishing between different types of intra-axial tumours For example, GBMs and metastases together

Figure 5 Diagnosing specific histological tumour types as

features of a predisposing genetic syndrome Axial

gadoli-nium enhanced T 1 weighted image demonstrates bilateral,

homogeneously enhancing, extra-axial, cerebellopontine

angle masses with extension into the IAMs This

character-istic appearance of bilateral vestibular schwannomas is

diagnostic of neurofibromatosis type 2 (NF2) Further

enhancing lesions seen bilaterally within Meckel’s cave are

therefore likely to represent bilateral meningiomas or

trigeminal schwannomas.

account for more than half of all intrinsic neoplasms in

adults While both these tumour types occur more

frequently in the supratentorial compartment, only

approximately 1% of GBMs are infratentorial vs

approxi-mately 15% of brain metastases An enhancing cerebellar

mass lesion in an adult is therefore much more likely to

represent a metastasis than a GBM [16]

A cystic lesion with an enhancing mural nodule in the

cerebellum of a child suggests a pilocytic astrocytoma

while the same appearance in an adult or a patient with

VHL syndrome suggests a haemangioblastoma [16, 17,

22, 34] In the supratentorial compartment, particularly

in the temporal lobe of a child or adult with epilepsy the

same appearance is suggestive of a ganglioglioma

(Figure 3) [17, 35]

Specific sites

Certain lesions are specific to — or at least more

common in — certain intracranial locations (Figures 1–6,

8 and 9); for example, pituitary adenomas are limited to

the anterior pituitary gland Similarly, certain sites give

rise to a limited number of lesions; for example, a mass

lesion in the jugular foramen is likely to represent a

glomus tumour, schwannoma, meningioma or

metasta-sis [36] Short differential diagnoses for mass lesions in

the following locations can be found in standard

textbooks of neuroradiology: pituitary/suprasellar

region, cerebello-pontine angle, brain stem, pineal,

intraventricular, jugular foramen, cavernous sinus

Are the lesions solitary or multiple?

Multiple enhancing intra-axial lesions suggest

haema-togenous dissemination of a malignant or infectious

process or a multifocal inflammatory process, such as

Figure 6 Diagnosing specific histological tumour types as features of a predisposing genetic syndrome (a) Axial T 2 weighted and (b) gadolinium-enhanced T 1 weighted MR images in an 8-year-old boy with learning difficulties, epilepsy and a 2-month history of headaches There is a well circumscribed, avidly enhancing mass in a subependymal location associated with the frontal horn of the right lateral ventricle In addition, there are bilateral small subependymal nodules in the trigones of the lateral ventricles as well as a cortical/subcortical area of high signal on T 2 in the left frontal lobe These features, particularly given the clinical history, are pathognomonic of tuberous sclerosis (TS) The enhancing mass therefore represents a subependymal giant cell astrocytoma.

Figure 7 Intra-axial vs extra-axial Axial T 2 weighted MR image demonstrating a right frontal pole meningioma (extra-axial) and a right posterior temporal lobe glioblastoma (intra-axial) Note that there is a cleft of cerebrospinal fluid trapped between the meningioma and the cortex (arrow), while the glioblastoma

is deep to the cortex The presence of oedema is not particularly useful in distinguishing between intra-axial and extra-axial lesions since it can be present or absent in either.

multiple sclerosis or acute disseminated

encephalomye-litis (ADEM)

Primary intra-axial brain tumours are usually solitary

although ‘‘satellite nodules’’ are occasionally seen distant

from the main tumour bulk in high-grade lesions such as

GBMs [37]

Metastatic spread of primary CNS neoplasms is very uncommon with the exception of metastatic seeding of the CSF spaces For example, an intra-axial tumour, such

as a cerebellar medulloblastoma, could give rise to one or more metastatic deposits in the intracranial or spinal subarachnoid space or within the ventricular system

Figure 8 Axial T2 weighted MR images in two different patients demonstrating the importance of location vs signal characteristics (a) Colloid cyst of the third ventricle The shape (well defined and round) and location anteriorly in the third ventricle establishes the diagnosis in this case Colloid cysts are typically non-enhancing and can be hypointense, isointense or hyperintense on T 2 weighted images; therefore the signal characteristics are less helpful than the location in making the diagnosis (b) Cavernoma of the mid-brain An intra-axial lesion anywhere in the central nervous system with these signal characteristics is most likely to represent a cavernoma The peripheral low signal is due to haemosiderin staining while the central high signal denotes the presence of extra-cellular methaemoglobin, indicating a more recent episode of haemorrhage.

In this case, the signal characteristics are more useful than the location of the lesion, as cavernomata occur throughout the neuraxis and look the same in any location.

Figure 9 Using age and location to achieve the correct diagnosis: a 4-year-old mute child with cranial nerve palsies (a) Sagittal midline fluid attenuated inversion recovery (FLAIR) image shows a uniform high signal mass expanding the pons, characteristic

of a pontine astrocytoma Pontine astrocytomas are usually of the diffuse fibrillary type and are at least World Health Organization (WHO) grade II, whereas astrocytomas in the midbrain and medulla are usually pilocytic astrocytomas (WHO grade I) (b) Axial post-gadolinium T 1 weighted image demonstrates patchy contrast enhancement which suggests a WHO grade III lesion.

Other tumours associated with this pattern of

dissemina-tion include ependymomas, germinomas, pilocytic

astro-cytomas and glioblastomas [38, 39]

What are the imaging characteristics on

un-enhanced CT or MRI?

The density of a mass on CT gives some useful

information about its constituents Fluid within a cystic

or necrotic lesion can easily be distinguished from

adipose tissue in a lipoma or lipid material in a dermoid

cyst (Figure 10) However, it is important to keep in

mind that fat can be misinterpreted as gas since the

narrow window levels employed when interpreting

brain CT examinations render fat much darker (and

therefore closer in appearances to gas) than on the wider

window settings employed when viewing CT images of

other body parts It is therefore prudent to widen the

window levels whenever a ‘‘black’’ lesion is seen on a

brain CT scan to distinguish between fat and air

The density of the solid component of an intra-axial

tumour on unenhanced CT scans reflects the cellularity

and extracellular water content of the tumour WHO

grade I and II astrocytomas are generally quite low

density compared with normal white-matter [34, 40]

while hypercellular tumours, such as lymphoma,

germi-noma and medulloblastoma, are usually denser than

normal white matter [41, 42]

Calcification and acute haemorrhage within a mass is

more easily and more reliably detected on CT than MRI

[43, 44]

Diffusion weighted imaging (DWI) has a well

estab-lished role in acute stroke imaging, but it also gives very

useful information about certain intracranial mass

lesions Two circumstances where DWI is particularly useful are in the evaluation of ring enhancing intra-axial mass lesions and CSF signal extra-axial mass lesions The differential diagnosis of ring enhancing intracra-nial mass lesions include metastases, GBM, acute inflammatory demyelination, bacterial abscess and a number of other infectious conditions, such as toxoplas-mosis, tuberculoma and cysticercosis In clinical practice,

it is often important to distinguish between a bacterial brain abscess on the one hand and a tumour on the other hand since the former requires urgent surgical aspiration and intravenous antibiotics while the latter can often be managed with oral corticosteroids and elective surgical biopsy (Figure 11) DWI can be used to help distinguish between these two entities since pus in bacterial abscesses demonstrates markedly restricted diffusion (bright on b51000 s mm–2images and dark on apparent diffusion coefficient (ADC) map) while necrotic tumour material demonstrates facilitated diffusion (dark on b51000 s mm–2images and bright on ADC map), except

in rare instances where haemorrhage has occurred into the necrotic tumour centre [45–47]

An extra-axial mass lesion returning signal isointense

or nearly isointense to CSF on T1 and T2 weighted imaging is likely to be either an arachnoid cyst or an epidermoid tumour Diffusion is facilitated in arachnoid cysts while epidermoid tumours demonstrate restricted diffusion (Figure 12) [48, 49]

What are the enhancement characteristics?

Contrast uptake within brain tissue implies disruption

of the blood–brain barrier (e.g in acute infarcts and demyelinating lesions) or, in the case of neoplasms, it

Figure 10 Utilizing specific imaging characteristics to reach the correct diagnosis This 30-year-old man has a history of periodic headaches and now presents with acute meningeal signs (a) Sagittal midline unenhanced T1weighted image showing a large hyperintense and heterogeneous mass with several, similarly strongly hyperintense foci scattered within the sub-arachnoid space (b) Axial CT image confirms multiple, midline low attenuation foci associated with the falx, confirming these to be fat droplets of a ruptured dermoid cyst Intracranial dermoids usually contain a varying combination of lipid, liquid cholesterol, whorls of hair, calcifications and decomposed epithelial cells producing typical appearances, as illustrated in this case.

implies angioneogenesis with increased endothelial

permeability of the abnormal tumour capillaries [50]

Different morphological patterns of contrast

enhance-ment are recognized Descriptive terms, such as

non-enhancement, uniform non-enhancement, patchy

enhance-ment, gyriform enhancement and ring enhancement

(also called rim or peripheral enhancement) are in

common use and give useful information about the

nature of the lesion

As a general rule, most intracranial tumours —

whether primary or metastatic — show some form of

contrast enhancement The notable exceptions are WHO

grade II astrocytomas and oligodendrogliomas

(Figures 9 and 13) [16, 51, 52], and gliomatosis cerebri

— a rare, diffusely infiltrating glial tumour that is

usually grade III

Relatively uniform enhancement is seen in most solid tumours including meningiomas, schwannomas, pituitary adenomas and pineal tumours, such as germinomas CNS lymphomas typically show uniform enhancement in immunocompetent patients while ring-enhancing lesions have been described in immunodeficient patients [42] Ring enhancement also occurs in aggressive tumours such as GBMs and metastases (around areas of necrosis)

as well as in abscesses and some inflammatory demye-linating lesions (Figure 11) Other infectious lesions, such

as toxoplasmosis, cysticercosis and tuberculomas, can also demonstrate ring enhancement

Inflammatory demyelinating lesions only enhance during the active phase of demyelination [53–55] In multiple sclerosis, this phase usually lasts less than 3 months for individual lesions All the patterns of

Figure 11 The use of diffusion-weighted imagine (DWI) in intra-axial ring enhancing lesions (a) A contrast-enhanced CT scan

of the brain in an elderly woman with headache and a right-sided visual field defect shows a ring enhancing lesion with surrounding vasogenic oedema (b) A DWI (b51000) image of the same patient shows restricted diffusion (high signal) within the fluid content of the lesion Surgical aspiration confirmed an abscess (c) An axial contrast enhanced T 1 weighted MR image in

an elderly lady with a left sided visual field defect demonstrates an intra-axial ring enhancing lesion (d) The DWI (b51000) image shows free diffusion (low signal) of the fluid content Histology confirmed a GBM with a necrotic centre.

enhancement (including no enhancement) listed in Box 2

have been described in demyelinating lesions

Incomplete ring enhancement (also referred to as a

‘‘broken ring’’) is an unusual pattern of enhancement

and is strongly associated with inflammatory

demyeli-nation (Figure 14) including multiple sclerosis although

it has been described in infectious and neoplastic

conditions [56, 57]

Gyriform enhancement within the cerebral cortex

occurs in infarcts and certain encephalitic conditions

that affect the cortex [58, 59]

Conclusions

One of the most important roles of the radiologist in the diagnostic pathway of a patient with an intracranial mass

is to provide the clinicians with a short differential diagnosis The radiologist should integrate all the relevant information available on the images as well as the request card to achieve this Certain bits of information have a higher discriminating value than others and the radiolo-gist should give greater weight to such information when formulating the differential diagnosis

Figure 12 The use of diffusion-weighted imaging (DWI) in cerebrospinal fluid (CSF) signal extra-axial masses (a) Axial T2 weighted MR image demonstrates a CSF signal mass lesion in the cisterna magna displacing the medulla posteriorly (b) The lesion is bright on a b51000 DWI image indicating restricted diffusion The lesion therefore represents an epidermoid rather than an arachnoid cyst.

Figure 13 Contrast enhancement gives information about tumour grade (a) Axial T 1 weighted image in a patient with a longstanding grade II astrocytoma shows an intra-axial mass with relatively uniform hypointense signal compared with white-matter (b) Following gadolinium administration, there is a focal area of enhancement anteromedially (arrow) within the mass indicating anaplastic transformation to a grade III astrocytoma.

Figure 14 Tumefactive demyelination This 61-year-old man with no history of multiple sclerosis presented with progressive right sided hemiparesis and left sided facial numbness The left-sided pontine mass lesion returns high signal on (a) T 2 and low signal on (b) T 1 weighted imaging There is incomplete ring enhancement seen on (c) the axial and (d) sagittal gadolinium enhanced T 1 weighted images Given the patient’s age, a tumour was suspected despite the enhancement pattern Stereotactic biopsy demonstrated demyelination with no evidence of a tumour.