Byrne 2008 Medicine

7/30/2019 Byrne 2008 Medicine

1/11

InvestIgatIons

MeDICIne 36:10 545 2008 elir Ld. all rih rrd.

NuroradioloyJ v Byr

Abstract

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imi Dpplr ulrud; mylrphy; prui imi; pirmii mrphy; rdiuclid ciNeuroradiology uses planar scanning, i.e. computerized tomog-

raphy (CT) and magnetic resonance imaging (MRI), alone or

in combination, to identiy most central nervous system (CNS)

pathologies. Traditional radiographic techniques (e.g. plain

radiography, myelography and angiography) are now second-

line modalities. Skull radiography has a role in the emergency

department, but not in general medical or neurology out-patient

departments. Myelography with intrathecal injection o radio-

graphic contrast media is now reserved or patients who areunable to undergo MRI because o implants (e.g. cardiac pace-

makers) or extreme claustrophobia, and is generally used in

combination with CT. Techniques such as Doppler ultrasound

(carotid, transcranial and intra-operative), radionuclide scanning

and unctional MRI have specic indications and are important

research tools with limited roles in routine practice.

JV Byrne FRCS FRCR is Consultant Neuroradiologist at the John Radcliffe

Hospital and Professor of Neuroradiology at the University of

Oxford, UK. His specialist interests are cerebrovascular disease and

interventional neuroradiology. Competing interests: none declared.

CT

Since its introduction in the mid-1970s, CT has been the main-

stay o neuroradiological diagnosis. Its role has evolved in the

last decade with the introduction o aster multislice scanners,which generate considerably more data than previous machines,

allowing better-quality three-dimensional (3D) image displays.

Tcniqu

The basic principle o CT involves rotating an X-ray source and a

series o detectors around the patient and making multiple mea-

surements o radiation attenuation. The measured attenuation

(amount o radiation absorbed) by small volume-elements (vox-

els) o the subject is calculated by computer and used to build

a two-dimensional (2D) picture composed o a matrix o pixels.

Each pixel is assigned a grey-scale value (in Hounseld units),

which is based on the measured attenuation and thereore refects

the density o the scanned tissue. Hounseld units, named aterthe inventor o CT, are scaled with the attenuation value o water

set at zero and maximum density +1000 and minimum density

1000. High-density structures (e.g. bone or calcication) are

depicted in white and low-density tissues (e.g. air) in black; inter-

mediate densities are depicted in grey. By this means, the inher-

ent contrast resulting rom density dierences between tissues is

displayed on a cross-sectional image (Figure 1).

In order to obtain non-axial views it was previously necessary

either to alter the patients position or angle the scanner gantry.

The extent to which the latter can be achieved was limited by the

engineering o the scanner. The problem has been solved by spi-

ral scanning and the introduction o multiple detector systems.

The original scanners collected data or each 2D slice in a single360 rotation using a single detector. Multiple 2D images could

be re-assembled by computer reconstruction to obtain images in

3D displays but the quality was relatively poor. Current scan-

ners move the X-ray source and patient simultaneously, and scan

in a helical pattern to produce volume data images in seconds.

These speeds are achieved using multiple detectors, which col-

lect data as they move. Current multidetector scanners employ

up to 64 detectors and can scan the entire brain in less than

3060 seconds. These speeds reduce arteacts caused by the

patient moving and are ast enough to scan during the rst pass

o a bolus o intravascular radiographic contrast media. This has

been a major advance or neuroradiology.

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InvestIgatIons


CT wit contrast mdia

A water-soluble iodine-containing agent is injected intrave-

nously and accumulates in areas o bloodbrain barrier break-

down. Contrast agents are used to highlight tumours and areas

o infammation (e.g. meningitis, abscesses). The eect o uptake

by a lesion is to increase its density so that it appears brighter

(relative to the surrounding brain). This is termed enhancement

and refects the vascularity o the tissues, i.e. it is increased in

vascular tumours and reduced in areas o inarction.

Contrast media can also be injected into arteries as well as

veins to show vessels, i.e. angiography, or intrathecally by lum-

bar puncture to show the spinal cord, i.e. myelography. CT is thenused to image the structures outlined by the density dierences

within the injected body compartment. This is the principle o

traditional radiographic myelography which CT has replaced

because it is more sensitive than X-ray lm and CT angiography

(CTA). The images are acquired as digital 3D data, which makes

their interpretation easier (Figure 2).

Fast scanning during injections allows the rate o contrast

uptake by a tissue to be measured and is the basis o perusion

scanning. This technique is used to estimate blood fow and

blood volume within tissues by comparing time/density curves

between dierent parts o the brain. It is used to identiy areas o

under-perusion ater stroke.

MRI

MRI is now the modality o choice or the investigation o most

neurological disease because it does not use ionizing radiation

and is extremely sensitive to small changes in tissue watercontent (Table 1).

Principls of MRI

In MRI, imaging is based on the phenomenon o magnetic res-

onance, in which elements with an odd number o subatomic

particles (protons and electrons) can be induced to resonate

Calcification

Frontal horn

Cyst

Cyst

Axial CT scan without contrast,

showing calcification in an

oligodendroglioma that was

found to be partially cystic at

surgery. The calcification was

not evident on MRI

Fiur 1

A coronal view of the frontal lobes (computed tomography angiography) showing a haematoma and the nidus of a brain arteriovenous

malformation (AVM). The scan is performed during the passage of a bolus of radiographic contrast so that the arteries are enhanced and

reconstructed by computer to produce this display.

Draining veins

Nidus of arteriovenousmalformation (AVM)

Haematoma

Anterior cerebral arteriesdisplaced by haematoma

Middle cerebral artery

Enlarged middle cerebralartery and branchessupplying AVM

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InvestIgatIons


when placed in a strong magnetic eld. Clinical MRI is perormed

at the resonance requency o hydrogen nuclei because they are

abundant in biological tissue. Resonating hydrogen protons are

perturbed by electromagnetic energy, applied as a radiorequency

pulse. This causes the protons to emit a weak signal, which is

detected, amplied and, ater computer processing, used to con-

struct an image. Dierent tissues contain dierent amounts o

hydrogen (largely in water) in various molecular environments;the appearance o the image depends principally on the concen-

tration and molecular environment o the perturbed hydrogen

protons and the applied radiorequency pulse.

The appearance o the image can be changed by manipulat-

ing the radio-requency pulse, using electronic protocols (termed

sequences). The parameters commonly measured are T1, T2

and T2* relaxation times (Figure 3),

Images relying principally on contrast produced by T1

relaxation are termed T1-weighted (T1 W) and show cere-

brospinal fuid (CSF) darker than the brain. These sequences

are designed to show the anatomical detail o the brain andspinal cord. They are used with contrast enhancement or simi-

lar indications to CT. The contrast medium used in MRI is a

paramagnetic substance, such as gadolinium, which alters the

Head of caudate nucleus

a Axial T2W and b T1W MRI showing the basal ganglia.

Lentiform nucleus(comprisingputamen,globus,pallidius)

Thalamus

Trigone region oflateral ventricle

White matterof frontal lobe

Sylvianfissure

Externalcapsule

Internalcapsule

Frontal horn oflateral ventricle

Subcutaneous fat

Sulci of

Sylvian fissure

External

capsule

Posterior partition of

lateral ventricle (trigone)

Head of caudate

nucleus

Lentiform

nucleus

Thalamus

Third

ventricle

Interhemispheric

fissure and falx

a b

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InvestIgatIons


magnetic properties o blood and appears brighter on T1 W

images (Figure 4).

Water (and thereore CSF) appears white on T2-weighted (T2 W)

images. T2 W images are very sensitive to increases in cerebral

water, which occur in most infammatory and neoplastic diseases

o the CNS. Cerebral white and grey matter can be distinguished

because o dierences in their water and at content.

T2 star (T2*) is the term used to describe the eects on the scano molecules with inherent magnetic properties. The best example

is the iron contained in haemoglobin (Hb). Ater acute haemor-

rhage, the magnetic resonance signal rom a blood clot is aected

by the extravasated Hb. The magnetic eect, termed susceptibility,

causes local dierences (inhomogeneities) in the magnetic gradi-

ent and loss o signal. T2*-weighted sequences are sensitive to any

magnetic eld inhomogeneity and are best at showing the eects o

both acute and chronic cerebral haemorrhage (Figure 5).

Hydrogen protons in bone or in areas o calcication return

little signal and thereore appear dark on both T1 W and T2 W

sequences. MRI is not as sensitive to brain calcication as CT.

MRI squncs and tcniqusA typical brain scan comprises several sequences designed to

image the brain in three orthogonal planes using a combination

o sequence types. The spine is imaged in the sagittal plane with

axial scans at levels o interest. Any plane o scan can be desig-

nated on MRI (unlike CT). Contrast medium (e.g. gadolinium)

a

b

a Saggital T1-weighted MRI

after administration of

intravenous gadolinium. A

cystic cerebellar tumour shows

partial enhancement.

b Magnetic resonance

angiography enhanced with

gadolinium and reconstructed

in the axial plane. The tumour

is only moderately vascular.

This was confirmed at surgery;

histological examination

revealed a cerebellar

astrocytoma rather than a

more vascular

haemangioblastoma.

Trigone of

lateral ventricle

Enhanced

lateral sinus

Non-enhancing

cystic element of

cerebellar tumourEnhancing

component of cerebellar tumour

Basilar artery

Jugular bulb

Sigmoidsinus

Lateral sinus

Torcula

Posteriorcommunicating artery

Internal carotidartery

Posteriorcerebralartery

Tumour incerebellar

hemispherewithout evidence ofvessel hypertrophy

Fiur 4

Diffrntial dianosis usin MRI

Multifocal wit mattr (i-sinal) lsions on T2-witd

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Tabl 1


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InvestIgatIons


is administered intravenously to highlight tumour and infam-matory lesions that have interrupted the bloodbrain barrier

(Figures 4 and 6).

CSF signal-suppressing sequences: Since T2 W sequences are

best or demonstrating ocal lesions causing increases in cerebral

water, lesions in the periventricular brain (e.g. multiple sclerosis)

may be obscured by the high signal returned by adjacent CSF. A

fuid low-attenuation inversion recovery (FLAIR) sequence was

developed to show periventricular plaques because it produces

a T2 W image with dark CSF but has proved to have wider uses

including tumour ollow-up (Figure 7). It is usually included in a

standard brain scan protocol.

Fat-suppression sequences: Fat appears bright on standard T1W and T2 W sequences. Sequences that suppress the at signal

are used to investigate suspected at-containing tumours and

lesions in areas containing at (e.g. the orbit, Figure 8).

Diffusion-weighted MRI: Physiological and pathological changes

in the Brownian motion o water in the cellular and extracel-

lular compartments o tissue can be distinguished by diusion-

weighted MRI. These movements are detected on ultra-ast scans

which measure the rate and direction o water diusion. Acute

cell swelling ater ischaemic stroke (due to cytotoxic oedema)

restricts normal diusion. DWI scanning can demonstrate this

change and can demonstrate hyperacute inarction (Figure 9),

a Axial T2 W MRI showing an acute haematoma in the frontal lobe. The centre of the haematoma contains deoxyhaemoglobin which distorts the

magnetic field locally and reduces the signal return. This results in a dark area on the image. b is the same patient scanned 2 weeks later when

the effect has reduced as the clot breaks down and the centre now returns a higher (brighter) signal.

a

b

Vasogenic oedema

surrounding haematoma

Signal loss due to magneticeffect of acute haematoma

Third ventricle containing

cerebrospinal fluid

Internal cerebral veins

Early low signal rim to

haematoma due to

haemosiderin

Vasogenic oedema

Bright centre of

subacute/chronic haematoma

Falx

Anterior cerebral arteries

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InvestIgatIons


beore changes are detectable on CT. It is also used to distin-

guish brain abscesses rom tumours since pus in the ormer is

associated with restricted diusion, which is not seen in areas o

tumour-induced vasogenic cerebral oedema.

Functional MRI: Sequences that allow production o images in

less than 1 second can be used to perorm dynamic scanning and

real-time imaging. Fast-scanning techniques such as echoplanar

imaging can show areas o increased brain activity and are used

in unctional MRI or research and to locate eloquent areas o the

brain in patients beore neurosurgery. Perusion imaging tech-

niques (i.e. scanning immediately ater an intravenous injection

o contrast medium) is used with DWI so that reduced perusion

(i.e. contrast uptake) can be compared with areas o restricted

diusion on DWI to identiy ischaemic brain that is potentially

salvagable brain ater stroke.

a

b

c

Axial T1W a and b and sagittal c MRI showing a tumour which enhances after intravenous gadolinium injection b and c. This is a meningioma

which has grown from the anterior margin of the pituitary fossa. Note how the pituitary gland is bright c because it is outside the bloodbrain

barrier and, therefore, also enhances.

Anterior falx

Tumour

Optic tractMid-brain

Enhancement of tumour

Corpus callosum

Straight sinus (enhanced)

Tumour arising from planum sphenoidale

Mid-brain

Pituitary gland

Pons

Sphenoid air sinus

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Magnetic resonance spectroscopy (MRS): Since the MR sig-nal refects the chemical composition o tissue it can measure

cerebral metabolites. This is done by producing a display show-

ing the requency spectrum o chemical composition o an area

o tissue. MRS is used in various specic clinical situations

(e.g. to distinguish radionecrosis rom recurrent neoplasms) and

to assay cerebral levels o drugs (e.g. lithium). Despite its theo-

retical potential, its clinical role has proved limited.

Magnetic resonance angiography (MRA): Because moving

hydrogen atoms in fowing blood return dierent signals rom

those in static tissue, sequences have been designed to capital-

ize on the eect o blood fow and highlight vessels. There are

two types o sequence in clinical use: time-o-fight and phase-contrast. Both rely on computer post-processing o images to

produce an angiogram (i.e. images showing blood vessels). They

can selectively demonstrate arteries (MR arteriography) or veins

(MR venography), based on the velocities and direction o blood

fow, and can be used to demonstrate the patency o intracranial

vessels. No injected contrast is needed, but MRA is increasingly

perormed ater injection o gadolinium to improve the resolution

o vessels and to obtain the temporal dimension. Fast scanning

allows the use o 3D data collection and is now used extensively

or the diagnosis and ollow-up o treated vascular lesions such

as aneurysms and arteriovenous malormation, and to show

tumour vasculature and vessel occlusions (Figure 4).

CSF in frontalsulci

Lateral ventricle

Demyelination

Demyelination

CSF ininterhemisphericfissure

Lateral ventricleDemyelination

a Sagittal T2-weighted and b axial fluid low-attenuation inversion recovery MRI in a patient with multiple sclerosis. In a, plaques of

demyelination return high signal (bright), as does CSF. The sequence used in b is designed to show plaques as high-signal lesions, but with low

signal returned by CSF. The resulting image clearly shows areas of demyelination, against dark CSF in the adjacent ventricles.

a

b

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Wn to us CT or MRI

During CT the patient is more accessible and overall scanning is

quicker, thus CT is saer ater recent trauma and or imaging in

very ill patients.For almost all other indications brain and spine

scanning uses MRI because o the absence o ionizing radiation,

high sensitivity to CNS pathology and the ability to image in anyplane directly. The last is especially useul when imaging the

spine (Figure 10)

The strong magnetic elds used in MRI can aect cardiac pace-

makers and may disturb metal implants. Patients with pacemak-

ers, some types o aneurysm clips or retained metallic oreign

bodies (particularly in the eye) cannot be scanned and so they are

imaged with CT. Other problems that limit the use o MRI are the

claustrophobic nature o the scanner and the need or patients to

remain quite still during scanning. For these reasons, sedation or

general anaesthesia may be required to scan children and patients

who suer claustrophobia. Monitoring patients under general

anaesthesia is more complicated during MRI than CT.

CT is better at showing intracranial calcication (Figure 1),

bone and recent intracranial haemorrhage (Figure 11), particu-

larly subarachnoid haemorrhage, though it seems likely that the

capabilities o MRI will continue to expand. Recent technical

improvements in 3D scanning with CT have increased its use or

angiography, especially ater spontaneous intracranial haemor-

rhage. CT angiography has replaced emergency catheter angiog-raphy in many departments and its complementary role assures

it a place in neuroradiology or the oreseeable uture.

Radiorapic aniorapy

Tcniqu

Cerebral intra-arterial angiography is usually perormed by selec-

tive catheterization o the carotid or vertebral artery. Following

percutaneous emoral or brachial artery puncture, a catheter is

positioned under fuoroscopic control and radio-opaque con-

trast medium is injected rapidly. Passage o the contrast bolus

through tissues rom artery to vein is recorded by a series o

Coronal MRI of the orbits obtained with a T2-weighted sequence

(hence bright CSF), showing the orbital optic nerves. The external

ocular muscles can also be seen, contrasted against the low signal

returned by fat on this sequence. The right optic nerve (arrow)returns the bright signal characteristic of optic neuritis.

(By courtesy of P Pretorius).

Maxillary sinus

Middleconcha

Inferiorconcha

Anterior hemispheric fissure

Orbitaloptic nervesurroundedby externalocularmuscles

Frontal lobe

Fiur 8

Axial diffusion-

weighted MRI at

the level of lateral

ventricles.

Anatomical detail

is vague

compared with

T1-weighted

images (e.g.

Figure 4b), CSFappears dark, and

grey matter is

lighter than white

matter (note the

crossing fibres of

the anterior

corpus callosum).

This scan shows

restricted water

diffusion caused

by infarction in

the left centrum

semiovale, it was

performed within

hours of the onset

of hemiplegia.

Cortical grey matter

Anterior corpus callosum

White matterof frontal lobe

Frontal hornof lateralventricle

Infarct in

centrumsemiovale

Trigone oflateralventricle

Third ventricle

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InvestIgatIons


radiographic exposures timed to show the maximum opaci-

cation o each component o the vascular tree. Subtraction o

each digitized image rom a baseline image (obtained beore

the arrival o intravascular contrast) is perormed electronically,

i.e. by digital subtraction angiography (DSA). This process

removes details o the baseline image (i.e. cranial bone) rom

subsequent images so they show only the opacied vessels.Because digitized systems are sensitive to lower concentra-

tions o contrast medium, images can be obtained ollowing

both intravenous (IV-DSA) and intra-arterial (IA-DSA) injection.

Improved computer reconstruction techniques now allow 3D

image post-processing in DSA. To acquire enough data or 3D

displays, a technique called rotational angiography is used. This

involves rotating the X-ray gantry in a similar way to CT during

the injection o contrast medium.

Pros and cons

IA-DSA carries a risk o arterial damage caused by the catheter or

inadvertent introduction o emboli. IV-DSA is thereore attractive

because it is less likely to cause stroke, but it lacks the precision

o IA-DSA. It has been replaced by CTA and MRA. MRA can

provide images o adequate diagnostic quality without expos-

ing patients to the risks associated with ionizing radiation and

radiographic contrast agents. However, CT angiography is more

robust, provides better bone detail and is quicker in acutely ill

patients.Along with the general shit to less invasive imaging, CT and

MR angiography are replacing catheter angiography or diagnosis

and IA-DSA is increasingly perormed only during endovascular

embolization procedures or the treatment o brain arteriovenous

malormations and aneurysms (Figures 2 and 12).

Plain radiorapy and mylorapy

The chest radiograph is the most important plain radiograph in

neuroradiology. Spinal radiography has a limited role as a primary

investigation, except ater trauma. Radiographic tomography and

myelography have largely been replaced by planar scanning.

a

b

Sagittal T1W (dark CSF) and

T2W MRI (bright CSF) of the

lumbar canal in a patient with

spina bifida. a A tract between

the spinal canal and skin

surface containing fat

(therefore bright on the image)

is best seen. b The spinal cord

extends low into the lumbarcanal and is tethered in the

region of the bony defect

(L5/S1).

L2 vertebral body

Low spinal cord

Lipoma in lumbar canal

Tethered cord and filum

White cerebrospinalfluid in expandedlumbo-sacral canal

Lipoma in lumbar canal

Fat-containing tract to skin dimple

Dark cerebrospinal fluid

S1 vertebral body

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InvestIgatIons


Radionuclid scannin

Brain and spine scanning or the diagnosis o tumours, using

radionuclides such as technetium-99m, has been generally

replaced by CT and MRI. The principle o radionuclide scanning

is that tissues and neoplasms concentrate a substance labelled

with a radioactive isotope (i.e. a radiotracer) and the patient is

scanned at a suitable interval ater its injection. The technique

is most commonly used to identiy sites o metastatic tumour

but tissue metabolism and blood fow can also be assessed with

positron emission tomography (PET) and single-photon emission

computerized tomography (SPECT).

Tcniqus

Positron emission tomography (PET) involves the detection o

two high-energy photons emitted by the annihilation o a posi-

tron rom a radiotracer. The radionuclides used have short hal-

lives and a cyclotron is required to produce them; this, and the

need or a dedicated detector, has previously limited its avail-

ability, but the recent introduction o networked radionuclide

production and systems that are combined with a CT scanner

(i.e. PET-CT) has seen an expansion in its use in oncology.

Single-photon emission computerized tomography (SPECT)

uses gamma ray-emitting radionuclides and can be perormed

using a standard gamma camera. Technetium-99m-labelled

Axial CT scan showing an acute haematoma in the right frontal lobe a. The clot is hyperdense and so appears whiter than the brain. It has

extended into the adjacent ventricle b.

a

b

High-density haemorrhage

in frontal lobe

Calcification in pineal

Part of left lateral ventricle

Haemorrhage in the frontal lobe

Cerebrospinal fluid in left

lateral ventricle

Haemorrhage in the right

lateral ventricle

Falx

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InvestIgatIons


hexamethylpropylene amine oxime (HMPAO) is commonly usedor scanning the brain. This substance is lipophilic and crosses

the intact bloodbrain barrier to bind to unspecied receptors. Its

distribution refects cerebral blood fow and remains stable or

about 1 hour, during which scanning is perormed.

Uss

Both techniques are used to measure cerebral blood fow, but

PET provides unique inormation about cerebral metabolism o

oxygen and glucose, using oxygen-15 and fuorine-18 respec-

tively. They are used to investigate patients with seizure disor-

ders, dementia, neoplasms or ischaemic disease. PET research

has been benecial in the development o SPECT, and both are

used to complement and validate unctional MRI.

Ultrasonorapy

Tcniqu

Ultrasonography is based on the transmission and detection o

refected sound rom tissue planes within the body. Because

bone is relatively impervious to sound, ultrasonography is useul

in the CNS only when perormed through a bone deect (e.g. an

open ontanelle) or ater laminectomy. Duplex ultrasonography

combines high-resolution real-time imaging with Doppler fow

analysis and is used to evaluate the cervical carotid arteries. TheDoppler principle is used to determine the velocity o blood fow

based on requency changes in a continuous sonic signal.

This technique has been rened by the addition o colour

to the displayed images, and high-power instruments are now

available or transcranial use, which are capable o estimating

blood fow velocity in intracranial arteries.

Uss

Transcranial ultrasonography is used to evaluate hydrocephalus

and neonatal haemorrhage in young children. It is simple to use,

but less simple to interpret. Duplex scanning has replaced angiogra-

phy or screening patients with suspected carotid biurcation steno-

sis. It may be combined with MRA, and most vascular surgeons usethe combination or preoperative imaging, instead o IA-DSA.

FURTheR ReADINg

Dmrl P, d. Rc dc i diic urrdily. Brli:

sprir-vrl, 2001.

grm RI, Yum DM. nurrdily: h rquiii (rquii i

rdily). oxrd: Mby, 2003.

obr a, Blr s, slzm K, l. Diic imi: bri, 1 d.

amiry, 2004.

a b

c

2D a and c and 3D b digital subtractionangiography frontal views showing an

aneurysm of the anterior communicating

artery before a and c and during treatment

by coil embolization. The 3D view is

obtained by rotating the X-ray source

during intra-arterial contrast injection and

is used for pre-operative evaluation of the

aneurysm and anatomy.

Pericallosal artery

Aneurysm

Anteriorcerebral artery

Internal

cerebral artery

Middle

cerebral artery

Coils in aneurysm

Microcatheter at

the internal

cerebral artery

termination

Fiur 12

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