Neuroradiologia

Neuroradiology
Amit A Roy
Katherine Miszkiel
INVESTIGATIONS
Abstract
Neuroradiology is the radiological subspeciality dealing with the diag-nosis,
characterization and, in some cases, treatment of disease entities
affecting the central or peripheral nervous system. It is a rapidly expand-ing
field and one in which technological advances have been pivotal in
driving further progression. The last few years have seen significant
improvements in access to high-quality imaging; modalities and tech-niques
that were once the remit of academic institutions with significant
research interests are now accessible to the majority with reduced cost,
improved availability and concomitant dissemination of expertise. The
trend towards subspecialization has continued in recent years, with
a specialist’s career choice no longer limited to the pursuit of either
a predominantly interventional or diagnostic role. The emergence of
those with dedicated expertise in head and neck imaging, paediatric
neuroradiology, neuro-ophthalmology, neuro-oncology and stroke is
a development that is likely to continue and parallels that which is occur-ring
in body imaging.
The objectives of this chapter are to introduce the principal neuroradio-logical
imaging modalities relevant to clinical practice, discuss what each
offers and convey their respective limitations. Scenarios in which a given
modality is particularly advantageous over others will be discussed as
well as the circumstances that preclude the use of certain techniques.
The list of modalities discussed is not intended to be exhaustive; the
emphasis will be on those that are currently routinely available but
novel developments and those currently limited to specialist centres func-tioning
mainly as research tools will also be mentioned briefly.
Keywords angiography; computerized tomography; Doppler ultrasound;
magnetic resonance imaging; myelography; perfusion imaging; positron
emission; radionuclide scanning
Introduction
Imaging modalities fall into one of two major categories: those that
utilize ionizing radiation and those that rely instead upon some
other physical characteristic of the tissue being interrogated in
order to generate an image. The former group includes traditional
radiographic techniques, such as plain film radiography, angiog-raphy
and myelography, as well as the more recent developments
of radionuclide scanning and computed tomography (CT). The
latter subset includes magnetic resonance imaging (MRI) and
ultrasound.
MRI and CT are presently the modalities of choice in the
evaluation of CNS pathology, with radiography, myelography
and angiography generally regarded as second-line investiga-tions,
reserved for cases when the former are precluded or as
a prelude to therapeutic intervention. Despite being quick, rela-tively
inexpensive and portable, ultrasound presently has
a limited role in the evaluation of neurological disease on
account of the osseous skull vault, which is relatively impervious
to sound wave transmission. There are, however, a few defined
indications where ultrasound provides invaluable adjunctive
information.
In recent years, increasing scrutiny has been placed on the
judicious use of ionizing radiation in diagnostic studies. Well-publicized
incidents in which patients received radiation doses
from CT perfusion studies far in excess of those expected, with
consequent deleterious outcomes, have made this issue front-page
news in the medical press.1 Awareness and acknowledge-ment
of the ALARA principle, which dictates that diagnostic
studies utilize a radiation dose that is ‘as low as reasonably
achievable’ has always constituted a fundamental component of
radiology training. However, there has been progressive
dissemination of this message to the medical community as
a whole with the increasing expectation that tests be justified,
optimized and dose-limited. As such, there is now ever-greater
reliance on computerized post-processing techniques, which
ensure that image quality is maintained in the face of the need to
reduce radiation dose. The ‘image gently’ campaign,2 launched
in the USA in 2008 by The Alliance for Radiation Safety in
Paediatric Imaging, has sought to actively promote this message
specifically in relation to imaging the child and to date has
received over 12,000 pledges from medical practitioners.
Computed tomography
Since its inception in 1967 by the British engineer, Sir Godfrey
Hounsfield, interest in CT has exploded with progressive
refinements over the last four decades rendering the technique
invaluable in the diagnosis of neurological disease. Even today, it
remains the mainstay of imaging diagnosis in this field, not least
on account of its availability and speed; modern-day multislice
scanners, which can image multiple sites in the body simulta-neously,
are able to achieve exceptionally short scanning times,
facilitating the interrogation of ever smaller structures within
a practicable time period, and negating the effects of motion.
Indeed, the substantial evidence base that has recently been built
around the diagnosis and management of patients with stroke
owes a great deal to CT; its ready availability and rapid delivery
of high-quality diagnostic images has been fundamental to the
restructuring and centralization of stroke services, which has
recently revolutionized the management of this condition.
Technique
Conventional radiographic techniques involve the bombardment
of a subject with X-rays, produced by an X-ray tube. The image
generated is a representation of the extent to which the component
tissues constituting the subject prevent the X-rays from passing
through, a property known as attenuation. The attenuation of
Amit A Roy MBBS (Hons) BSc (Hons) MRCS DOHNS FRCR is a Neuroradiology
Fellow at the National Hospital for Neurology and Neurosurgery, Queen
Square, London, UK. Conflicts of interest: none declared.
Katherine Miszkiel BM (Hons) MRCP FRCR is a Consultant Neuroradiologist
at the National Hospital for Neurology and Neurosurgery, Queen
Square, London, UK. Conflicts of interest: none declared.
MEDICINE 40:8 440 2012 Published by Elsevier Ltd.

INVESTIGATIONS
a material is inextricably linked to its density. In this way, a two-dimensional
(2D) representation of a 3D structure is obtained.
CT is a natural extension of this technique; the same under-pinning
physical principles are coupled with powerful computer-processing
power to culminate in a series of images, or slices,
depicting the subject concerned. Central to the understanding of
CT is the notion of ‘voxels’; these are volume elements analogous
to pixels in two dimensions. Each voxel depicts a small piece of
the patient being scanned and is assigned a unit of measurement,
called the Hounsfield unit, based on its attenuation. The
computer derives the average Hounsfield number of the voxel
under consideration via the fixed points of reference, namely the
values assigned for water (HU ¼ 0) and air (HU ¼ 1000).3 The
image begins taking shape as the numerical values for each pixel
are represented on a two-dimensional matrix by a shade of grey.
Tissues of high inherent density are depicted as white (such as
bone, calcification or intravenous contrast) whilst low-density
materials such as air and fat appear black. Soft tissue is of
intermediate density. It follows that this system allows for more
than 2000 shades of grey to be depicted. However, the human
eye is unable to differentiate between such subtle gradations
potentially rendering a large part of this dataset wasted. The
fundamental principle of ‘windowing’ circumvents this problem
and allows the user to tailor the image by focussing on a narrow
range of densities and disregarding all voxels with attenuation
values outside a pre-defined range. This principle is fundamen-tally
important in the interpretation of stroke imaging, as subtle
differences in the attenuation of ischaemic brain compared with
healthy tissue can be made conspicuous only through appro-priate
manipulation of windowing.
In recent years, the advent of multislice scanners has brought
huge advantages both in terms of improving image quality and
reducing scanning time. Present day scanners can assimilate up
to 256 sequential slices through a patient in a single rotation,
making imaging of the brain possible within a fraction of
a second. The attendant benefits in limiting artefact from motion
are huge. In addition, the speeds achievable are sufficient to
image during the first pass of a contrast bolus, obviating the need
for larger volumes that expose the patient to a potentially higher
risk of nephrotoxicity.
The ability to image faster and obtain thinner slices has also
made the notion of ‘isotropic’ voxels a reality. An isotropic voxel is
essentially a cube, dimensionally identical in all three planes. This
feature facilitates true 3D imaging through the generation of multi-planar
reformatted images that lose nothing in terms of resolution.
This is a major advance from the previous generations of CT
scanners, which could achieve this feat of high-resolution non-axial
imaging only by physically altering their gantry.
Contrast-enhanced CT (CECT)
CT of the brain is a rapid and powerful diagnostic tool with
proven benefit in both the acute and non-acute settings.
However, its efficacy can be improved in a number of scenarios
through the coupling of imaging with the administration of
intravenous contrast media. Contrast agents used in CT imaging
are water-soluble iodine macromolecules, either in ionic or non-ionic
forms. The latter represent a more recent development,
generally being the agents of choice today. Fewer associated
adverse effects are seen than when using ionic agents and this is
thought to result from a reduced propensity to dissociate into
component molecules.
Although modern contrast agents are safe, adverse effects do
occur and include idiosyncratic reactions, anaphylaxis, drug
interactions and contrast-induced nephropathy. The rare but real
potential for anaphylactic reactions dictates that resuscitation
facilities are available wherever contrast-enhanced scanning is
being performed. Contrast-induced nephropathy is another
feared complication, accounting for 12% of cases of hospital-acquired
renal failure4; however, it is extremely unlikely in the
absence of recognized risk factors, such as pre-existing renal
impairment and severe diabetes.5 The choice of contrast agent in
the scenario of a patient deemed at high risk of nephropathy has
been the subject of much debate but current advice is that low-osmolality,
non-ionic media are safest.
The utility of contrast-enhanced imaging is in highlighting
areas where the bloodebrain barrier has lost its normal integrity,
as occurs in a number of infective, inflammatory and neoplastic
conditions. When an area of tissue takes up contrast, its density
(and thus the Hounsfield units ascribed to the voxels that depict
that area) also increases. As a consequence, the area of tissue
concerned appears brighter, a phenomenon termed enhance-ment.
The enhancement patterns of certain lesions are predict-able
and reproducible, thereby aiding differential diagnosis
(Figures 1e3).
As an adjunct to standard post-contrast imaging, a number of
other techniques utilizing intravascular contrast delivery have
evolved, simply by adjusting the timing of scanning relative to the
time of peripheral venous injection. CT angiography (CTA) and
venography (CTV) provide powerful non-invasive means by
which the vessels supplying and draining the central and periph-eral
nervous systems can be interrogated (Figures 4 and 5).
Indeed, CTA is often the primary investigation performed in
establishing the aetiology of subarachnoid haemorrhage, with
catheter angiography relegated to situations in which therapeutic
intervention is likely to ensue. The quality and speed with which
CTA can now be performed has also rendered it invaluable in the
assessment of stroke; it currently forms part of the initial imaging
protocol, such that patients can now undergo comprehensive
imaging of the brain and vasculature within minutes of arriving
through the emergency department’s door. This owes a great deal
to the advent of multislice scanners and powerful software
programs, which effortlessly reconstruct the datasets obtained
into formats that are most conducive to rapid diagnosis.
So-called ‘stealth’ or stereotactic CT scanning is another
relatively recent development. This permits pre-operative diag-nostic
imaging to be loaded into a system located in the operating
theatre, which translates the dataset into precise three-dimensional
images, thereby aiding surgical mapping and facil-itating
the safest and least invasive path to a lesion.
CT myelography combines the potential for high-quality
multi-planar reformatted imaging with the instillation of iodine-based
contrast media into the intrathecal space. The resulting
images enable excellent visualization of the terminal spinal cord
and caudal nerve roots. Conventional myelography, in which
plain radiography follows the instillation of contrast, is now
essentially defunct. CT cisternography employs a similar prin-ciple
and can be employed to visualize the CSF spaces around the
brainstem and thus the anatomy of the lower cranial nerves. MRI

INVESTIGATIONS
Figure 1 This 34-year-old woman had disseminated tuberculosis (TB) and multiple neurological signs and symptoms including headache, weakness and
lower cranial nerve palsies. (a and b) Coronal and axial post-contrast T1-weighted magnetic resonance (MR) images demonstrating obstructive hydro-cephalus,
multiple ring-enhancing tuberculomas (arrows) and prominent basal meningeal enhancement. (c) Sagittal post-contrast T1-weighted image of
the cervical spine demonstrates diffuse meningeal enhancement. Prominent enhancement involving the ventral surfaces of the pons and medulla
oblongata is particularly noteworthy (arrow). (d) Axial T2-weighted MR image through the thorax at the level of the upper mediastinum reveals multifocal
patchy pulmonary changes in keeping with active TB.
Figure 2 This 54-year-old woman presented with gradually progressive bony swelling involving the left side of her face. (a) Axial unenhanced CT on bony
windows demonstrates gross bony expansion, sclerosis and deformity involving the left fronto-temporal region. (b and c) Axial T2-weighted and stealth
protocol post-contrast T1-weighted magnetic resonance images show bony sclerosis and expansion, widening of the diploic space, subjacent dural
thickening and enhancement and normal intra-cranial appearances. The features are those of a predominantly intra-osseous meningioma.

Figure 3 This 74-year-old man presented with rapidly progressive left sided weakness and clumsiness. (a and b) Axial T2-weighted magnetic resonance
images demonstrate thickening and signal abnormality in relation to the splenium of the corpus callosum (arrow) with further multifocal areas of
parenchymal hyperintensity involving the parieto-occipital regions bilaterally. (c and d) Multiple peripherally enhancing lesions on both sides of the
midline with involvement of the corpus callosum. The features are in keeping with multifocal high-grade glioma (glioblastoma multiforme).
would now be more appropriate than either of these modalities in
the first instance, but they are invaluable adjuncts when MRI is
precluded.
CT perfusion is another relatively novel technique made
possible by the advent of faster scanning times. It is of particular
relevance in the field of stroke imaging as it permits the rate of
contrast uptake by defined areas of neuroparenchyma to be
quantified. The process culminates in graphical representations
of cerebral blood flow, blood volume and transit time from which
the distributions of infarcted tissue and potentially salvageable
ischaemic parenchyma can be derived.6 Although undoubtedly
efficacious, the technique remains principally a research tool,
limited to centres with experience and relevant expertise.
Magnetic resonance imaging
MRI is currently the modality of choice in the investigation of
neurological disease. It provides the greatest soft tissue
resolution among the techniques presently available and does
not utilize ionizing radiation, rendering it safe in the vast
majority of scenarios. Since its inception in the 1970s, interest in
the technique has exploded, with progressive refinements and
the addition of novel sequences occurring in parallel with
concomitant advancements in CT. The two modalities are
frequently utilized in a complementary fashion, as there are
many circumstances in which the ready availability and scanning
speed of CT render it the more appropriate option.
Technique and principles7
Nuclear magnetic resonance (NMR), the fundamental principle
upon which MRI is based, was discovered as early as the 1930s.
However, it was not until Bloch and Purcell realized its signifi-cance
that NMR spectroscopy was born, their work culminating
in the award of the Nobel Prize for Physics in 1952. The exten-sion
of NMR to a medical imaging technique did not occur until
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INVESTIGATIONS
1973. Since this time, fervent research and development have
brought about huge advances and refinements to the technique,
elevating MRI to the status of current gold standard in imaging
technology.
NMR is based on the observation that isotopes with an odd
number of protons and neutrons demonstrate an intrinsic
magnetic moment and can thus be induced to resonate when
placed within a powerful magnetic field. The functional unit in
clinical MRI is the hydrogen nucleus, or proton, which is abun-dant
within organic tissue and behaves like a magnetic dipole
when placed within an electromagnetic field. During an MRI
scan, energy at a specific frequency is transmitted into the body
as radio waves, causing the abundant protons to resonate and
align against the magnetic field; when the radio wave then
ceases, the protons realign with the original magnetic field and
return energy in the form of further radio waves that constitute
the MR signal. This signal is progressively amplified and
undergoes numerous computer-processing steps to derive the MR
image. Fundamental to the interpretation of MRI is the appreci-ation
that different body tissues comprise hydrogen atoms in
differing quantities and in varying molecular environments; the
nature of the resulting image thus reflects both the abundance of
hydrogen atoms and their chemical surroundings.
Basic sequences
The characteristics of the image obtained can be altered by
manipulating the magnitude and direction of the applied radio-frequency
pulses with pre-defined protocols termed sequences.
The so-called T1 and T2 relaxation times are the fundamental
parameters measured by all scanners, giving rise to T1- and T2-
weighted images respectively, the basic sequences central to
MRI.
T1-weighted images provide excellent anatomical resolution.
Free water molecules (such as those within circulating CSF)
appear of low signal (dark) whilst proteinaceous fluid and
melanin appear brighter than surrounding brain. Subacute
Figure 4 This 64-year-old woman presented with sudden onset of right-sided weakness. (aeh) Acute stroke protocol CT/CTA on admission and MRI per-formed
24 hours later. (a) Axial unenhanced CT. No discernible hypodensity is seen in the left middle cerebral artery (MCA) territory and the cortical insular
ribbon appears intact. (b) Axial unenhanced CT. There is a short segment of hyperdensity within the Sylvian (M2) branch of the left MCA in keeping with
intraluminal thrombus (arrow). (c) Axial CT angiographic (CTA) image at the level of the pterygoid plates. There is no contrast opacification within the left
internal carotid artery (ICA) just distal to the carotid bifurcation (arrow). (d) Axial T2-weighted MR image. Subtle signal hyperintensity is seen within the left
insular cortex (arrow). (e) Coronal fluid attenuated inversion recovery (FLAIR) MR image. The conspicuity of parenchymal changes in the left insular region is
improved by nullifying the signal from adjacent CSF (arrow). (f ) Diffusion-weighted image (DWI): signal hyperintensity is demonstrated involving both grey
and white matter within the left MCA territory. (g) Corresponding apparent diffusion coefficient (ADC) image: signal hypointensity within the left insular,
temporal and parietal lobes is indicative of restricted diffusion and thus acute infarction. (h) Axial gradient-echo T2* image demonstrates intraluminal
thrombus within the left M2 segment corresponding to that seen on the admission unenhanced CT (arrow). (iek) MR angiography (MRA) and
fat-suppressed imaging through the neck. (i) Maximum intensity projection (MIP) MRA images, antero-posterior (AP) projection. There is abrupt cut-off
involving the left ICA just distal to the origin (black arrow). However, there is reconstitution of the terminal ICA via a persistent trigeminal artery (white
arrow). A persistent trigeminal artery is an example of an arterial communication between the carotid and vertebro-basilar systems, which are present
within the fetal circulation but normally involutes in adulthood. (j) Axial fat-suppressed T2-weighted image at the level of the proximal ICAs demonstrates
loss of the normal flow void on the left with signal hyperintensity in keeping with intraluminal thrombus. (k) Right anterior-oblique (RAO) MIP MRA image.

INVESTIGATIONS
Figure 4 (continued)
haemorrhage also appears bright due to the paramagnetic char-acteristics
of iron within methaemoglobin, giving rise to so-called
‘T1-shortening’. T1 images are also employed to demonstrate
contrast enhancement, which occurs with gadolinium-based
agents whose intrinsic ability to alter the magnetic properties
of blood is responsible for signal augmentation. The indications
for performing contrast-enhanced studies are analogous to those
in CT imaging.
T2-weighted images are superior in delineating abnormal
tissues such as those harbouring infection, inflammation and
neoplastic disease.
T2* images are optimized to assess the effects of molecules
with magnetic properties on surrounding tissues. The iron con-tained
within haemoglobin is the commonest example and
demonstrates paramagnetic effects following haemorrhage, which
alter the local magnetic field within its vicinity (Figures 4 and 6).
Scanning protocols
Typically, a routine brain scan comprises several sequences
including not only the above but also those tailored to the
specific indication. It is conventional to include all three
orthogonal planes (axial, coronal, sagittal), although any plane of
imaging is theoretically possible, unlike CT. Spinal scanning
typically includes sagittal imaging and selected axial slices
through any regions of interest.
There are a number of additional sequences that are particu-larly
advantageous in certain scenarios. For example, those that
suppress CSF-signal can be invaluable in visualizing the peri-ventricular
lesions that characterize multiple sclerosis by
greatly improving their conspicuity. FLAIR (fluid-attenuated
inversion recovery) is such a sequence that has also proved to be
valuable in monitoring tumour follow-up.
Fat-suppressed sequences such as STIR (short-tau inverse
recovery) are images created with T2-weighting but with
suppression of signal generated by fat. This improves conspicuity
of entities such as oedema, where the high signal of fat may
obscure the boundaries of a pathological process. Fat-suppressed
axial imaging through the neck is frequently employed in
vascular dissection protocols, where the perceptibility of intra-mural
haematoma is improved. High spatial resolution tech-niques
such as CISS (constructive interference in steady state)
provide exquisitely detailed images of inner ear anatomy and the
lower cranial nerves, facilitating detection of even small
cerebello-pontine angle lesions, for example, without the use of
intravenous contrast. Establishing evidence of vascular contact
in suspected cases of trigeminal neuralgia or hemi-facial spasm
are further applications.
Diffusion-weighted imaging (DWI) utilizes the principle that
the signals generated by protons in water molecules differ
depending upon whether free diffusion is occurring; when
Brownian motion is not permitted due to pathological processes,
a differential MR signal is generated, which may be ‘mapped’.
In normal tissues or those in which vasogenic oedema occurs,
random Brownian motion of water molecules is not limited and
thus no diffusion restriction is seen. In tissues with a tight degree
of cellular packing or those in which cytotoxic oedema occurs,

INVESTIGATIONS
Figure 5 This 28-year-old woman presented with a history of headaches followed by progressive cerebral obtundation. There was a preceding history of
non-Hodgkin’s lymphoma. (a) Axial CT venogram (CTV) image at the vertex demonstrates irregular filling defects within the superior sagittal sinus
(arrows) with segments of non-opacification of the visualized cortical veins. The features are highly suggestive of sagittal sinus and cortical vein
thrombosis. (b) More inferiorly, a discrete filling defect is visible within the superior sagittal sinus e the ‘empty delta sign’ (black arrow). The superior
sagittal sinus is also expanded and hyperdense, suggestive of acute thrombosis. Scattered foci of para-sagittal parenchymal haemorrhage are also visible
(white arrows). (c) CTV midline sagittal maximum intensity projection image further demonstrates multiple filling defects within the superior sagittal sinus
(arrow). (d) Antero-posterior digital subtraction angiography (DSA) image depicting filling defects within the right transverse sinus and superior sagittal
sinus (arrows). A catheter has been placed within the right transverse sinus during attempted mechanical thrombectomy.
restricted diffusion occurs and manifests as decreased signal on
apparent diffusion coefficient (ADC) mapping.
DWI has revolutionized the diagnosis of stroke, demon-strating
unequivocal changes within minutes of infarction, far
earlier than abnormalities are detectable on CT (Figure 4).
Changes classically persist for up to 3 weeks, which can be useful
in distinguishing acute from chronic phenomena. Restricted
diffusion is also a feature of cerebral abscesses, prion diseases
such as CJD8 and certain neoplastic lesions including lymphomas
and high-grade gliomas (Figure 7).
Magnetic resonance angiography (MRA) differs from the
equivalent CT technique in that it is possible non-invasively
to depict the vasculature without the need for contrast media.
This is based on the principle that protons within flowing
blood return signals distinct from those within static tissue.
Post-processing steps are able to extract these differences to
create so-called ‘time-of-flight’ angiographic images (Figure 4).
Selective depiction of the arterial or venous tree is possible.
However, contrast-enhanced MRA (CEMRA) is increasingly
being performed for the diagnosis and follow-up of aneurysms
and other vascular malformations on account of its improved
resolution.
MR spectroscopy remains mostly the remit of research despite
early promise. Its basis lies in the ability of the MR signal to
provide quantitative information regarding chemical composi-tion.
Although the technique may be advantageous in certain
defined situations, such as the differentiation of recurrent
neoplasm from treatment-related change, and the assessment
and monitoring of the leukodystrophies,9 it has largely failed to
make a significant impact on routine clinical practice.
Novel developments and future directions
The intense research activity focused on MRI over the last four
decades shows no signs of diminishing with numerous advances
and technical refinements steadily adding to what is already
available in the world of clinical practice.

INVESTIGATIONS
Figure 6 This 66-year-old man had a history of previous head trauma and hypertension. (a) Axial unenhanced CT demonstrates a probable mature infarct
in the right occipital pole (white arrow) with likely gliotic change from trauma within the right inferior frontal lobe (black arrow). No obvious focus of
haemorrhage is seen. (bee) Phase, magnitude, maximum intensity projection and susceptibility-weighted imaging (SWI) images from the SWI protocol
demonstrate innumerable peripherally located microhaemorrhagic foci, lobar haemorrhage within the right frontal lobe and superficial haemosiderosis.
These features are seen typically in cerebral amyloid angiopathy. (f ) Gradient-echo T2* image fails to demonstrate a number of the microbleeds seen on
the SWI, highlighting the improved sensitivity of this novel technique.
Presently, the majority of scanners in diagnostic use operate
at field strengths of 1.5 T (tesla). 3 T scanners are now relatively
commonplace but, despite definite advantages in terms of signal-to-
noise ratio, commensurate improvements in resolution are not
always apparent. In the research setting, higher field strength
magnets at up to 11 T are achievable, but legitimate safety
concerns and technical hurdles need addressing before such
equipment is used for clinical purposes.
Diffusion tensor imaging (DTI) is a novel development in
which both the magnitude and direction of diffusion within
cerebral white matter can be inferred and graphically repre-sented,
culminating in the generation of elegant tractographic
colour maps. Though principally a research tool confined to
centres with specific expertise, information such as this may be
fundamentally important to surgical planning in the future.10
Perfusion MRI, analogous in principle to the equivalent CT
method, entails scanning immediately and then sequentially after
injection of intravenous contrast. This technique has been coupled
with DWI in order to cross-reference areas of reduced perfusion
with corresponding restricted diffusion; in this manner, potentially
salvageable ischaemic parenchyma may be identified. Perfusion
MRI may also have a role in the discrimination of tumour recur-rence
from radiation necrosis and the predicting of malignant
transformation of low-grade gliomas through the detection of
increases in cerebral blood volume over time. Other examples of
functional MRI include techniques in which dynamic scanning can
demonstrate areas of parenchyma intimately involved in speech,
through the depiction of increased activity. In the past, such tech-niques
have principally been research-oriented, but they are grad-ually
entering the realm of routine clinical practice as information
such as this may be invaluable to surgical planning.
Susceptibility-weighted imaging (SWI) is a novel MRI tech-nique
that is exquisitely sensitive to haemorrhage.11 Potential
clinical applications include the assessment of trauma, stroke,
malignancy and dementia (Figures 6 and 8).
‘Stealth’ and interventional MRI follow similar principles to
the analogous CT techniques; they not only facilitate surgical
mapping of lesions but also provide the means for real-time
imaging feedback intra-operatively.
Table 1 summarizes the advantages and disadvantages of CT
and MRI in neuroimaging. Box 1 lists some of the more common
contraindications and cautions associated with MRI.
Angiography and interventional neuroradiology
Intra-arterial cerebral angiography is usually achieved via selec-tive
catheterization of the carotid or vertebral arteries under
fluoroscopic guidance following percutaneous femoral or

INVESTIGATIONS
Figure 7 This 33-year-old man presented with rapidly progressive dementia. (a) Axial T2-weighted image at the level of the basal ganglia demonstrates
bilateral subtle signal hyperintensity involving the caudate and lentiform nuclei and thalami. (b) Axial fluid attenuated inversion recovery (FLAIR) image
demonstrates subtle cortically based signal hyperintensity involving the para-sagittal frontal lobes and peri-rolandic regions (arrows). (c and d) Axial
diffusion-weighted image (DWI) and apparent diffusion coefficient (ADC) map respectively: hyperintense change is seen within the basal ganglia and
thalami bilaterally with corresponding hypointensity on ADC; the appearances thus signify diffusion restriction. These are hallmark changes seen in
CreutzfeldteJakob disease (CJD).
brachial artery puncture. Iodinated contrast medium is injected
rapidly through the catheter and sequential radiographic expo-sures
delineate the passage of the bolus through progressive
vascular phases. A digital subtraction technique removes bone
and other obscuring soft tissues, leading to a series of post-injection
images optimized to demonstrate the vascular
anatomy at multiple phases. Since the inception of this technique
some 80 years ago, numerous technological refinements
involving every step have occurred, including the engineering of
the catheters, the safety of the contrast media and the sophisti-cation
of the fluoroscopic and image post-processing elements.
Powerful software applications are now able to transform the
dataset such that exquisite 3D representations of the most
complex vascular anatomy are possible, affording the operator
almost limitless potential to manipulate the images as desired.
Despite meticulous technique there remains the small but
significant risk of stroke through inadvertent arterial damage or
introduction of embolic foci. For this reason, catheter

Figure 8 This 48-year-old woman presented with multiple transient ischaemic attacks (TIAs), dementia and progressive pseudobulbar palsy. (a) Axial
T2-weighted magnetic resonance image depicts bilateral peri-ventricular and external capsular signal abnormality with a subcortical infarct in the left
parietal lobe (black arrow). (b) Coronal fluid attenuated inversion recovery (FLAIR) image demonstrating the same features. (c and d) Susceptibility-weighted
imaging (SWI) reveals multiple punctuate foci of signal hypointensity within the corpora striata, thalami and posterior fossa in keeping with
microhaemorrhages. This patient was suspected of suffering from CADASIL (cerebral autosomal dominant arteriopathy with subcortical infarcts and
leukoencephalopathy).
angiography has largely been superseded by CTA or MRA, unless
the intention is to proceed with therapeutic intervention.
The burgeoning field of interventional neuroradiology has
arguably experienced the greatest advancement in recent times, and
owes a great deal to the huge technological stridesmade within both
the imaging and engineering sciences. Many conditions for which
surgery was the only feasible treatment 10e15 years ago can now
be treated successfully via a minimally invasive interventional
approach with significant improvements in morbidity and
mortality. The range of therapeutic options is constantly evolving
and presently includes the exclusion of intra-cerebral aneurysms
through the delivery of endovascular platinum coils, the emboli-zation
of arteriovenous malformations and the treatment of cere-bral
vasospasm (Figures 5 and 9).
The advent of flow-diverting stents for aneurysms previously
deemed untreatable is another noteworthy advance. However,
advancements in the field of stroke treatment have been partic-ularly
exciting and so far-reaching as to prompt radical
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Advantages and disadvantages of CT and MRI in neuroimaging
Imaging modality Advantages Disadvantages
CT C Quicker scanning times
restructuring of stroke service provision; the potential to deliver
thrombolytic agents directly to the site of blockage and deploy
mechanical thrombectomy devices directly to the site of occlu-sion
represents a huge change in the manner in which this
devastating condition is managed. Needless to say, rapid and
accurate diagnosis is a prerequisite; in this way, concomitant
advances in diagnostic CT and CTA have been equally
contributory.
Plain radiography
Plain radiographic techniques in neuroimaging have largely been
superseded by technically superior cross-sectional modalities.
Other than as part of a general skeletal survey, skull radiography
is now scarcely performed. Although spinal films are commonly
undertaken as part of follow-up after surgery, their role in initial
diagnosis is limited. Plain myelography is now largely defunct,
replaced by lumbo-sacral MRI as the modality of choice in the
investigation of lower spinal pathology. In situations where MRI
is precluded, the myelographic technique is now combined with
CT to produce multi-planar images with superior diagnostic
potential.
Ultrasound
Although the osseous skull vault is relatively impervious to
sound wave transmission, a number of defined indications exist
in which ultrasound is particularly favourable, given the absence
of ionizing radiation and its portability, low cost and real-time
feedback potential.
Cervical Doppler (or duplex) has represented a key modality
in the evaluation of occlusive arterial disease within the neck
since the inception of ultrasound as a medical diagnostic tech-nique.
It represents a fast, portable, non-invasive and safe
alternative to intra-arterial angiography, which is now rarely
performed for this indication. The superimposition of Doppler
colour flow and velocity waveforms onto standard B-mode
sonographic imaging permits not only visualization of the
stenotic plaque and its anatomical composition but also quanti-fication
of velocity and pressure gradients.
Transcranial Doppler (TCD) is a more recent development. It
utilizes the principle that velocity measurements within the
major intra-cranial arteries are achievable via duplex ultrasound
performed through thinner bony landmarks such as the temporal
region or through the orbits. Recognized applications include the
assessment of intra-cranial stenosis, subarachnoid haemorrhage
(and potential associated vasospastic complications) and the
confirmation of brain death. Future developments may include
implantable devices linked to therapeutic drug delivery systems,
which may provide a means not only to detect stroke at the
earliest possible opportunity but also, potentially, to initiate
antithrombotic therapy.
Neonatal transcranial ultrasound is the most frequently per-formed
neuroimaging investigation in this age group, making use
Contraindications and cautions with MRI e metallic
objects or implants
C Pacemaker
C Implantable cardiac defibrillator (ICD)
C Aneurysm clips
C Coronary stents (some types)
C Metallic foreign bodies, particularly in or near the eye
C Metal implant, e.g. orthopaedic prosthesis
C Shrapnel or bullet wounds
C Neurostimulator
C Implanted drug infusion device
C Dentures/teeth with magnetic components
Box 1
C Patient more accessible thus preferential
in critically ill or trauma patients
C Currently more sensitive in the assessment
of intra-cranial calcification, acute
haemorrhage and bony disease
C Improvements in 3D scanning with conse-quent
improvements in CT angiography
C Radiation dose, particularly important in
repeated scanning
C Inferior soft tissue contrast compared with
MRI
C Streak artefact from metallic implants
degrades image quality
MRI C No radiation burden
C Superior sensitivity in detecting CNS
pathology
C Ability to image in any plane without need
for reformatting
C Optimal soft tissue contrast
C Lengthy scanning time
C Requirements for general anaesthetic or
sedation in certain non-compliant groups
C Metallic foreign bodies contraindicated
(including medical devices such as cardiac
pacemakers and neurostimulators)
C First trimester of pregnancy is a relative
contraindication
Table 1
INVESTIGATIONS

INVESTIGATIONS
Figure 9 This 26-year-old woman presented with proptosis of the left globe and associated pulsatile swelling. (a and b) Axial T2-weighted magnetic
resonance imaging (MRI) scans depict gross proptosis of the left globe with a large curvilear flow void within the superior left orbit (arrow). (c) Lateral
digital subtraction angiography image following contrast injection via the left internal carotid artery (ICA) in the arterial phase. A prominent anteriorly
directed vessel (arrow) corresponding to that seen on the MRI opacifies via the cavernous carotid segment; the abnormal vessel is a distended left
superior ophthalmic vein and an underlying arteriovenous fistula is responsible. (d) Angiographic image following direct cannulation of the left superior
ophthalmic vein and instillation of embolic material at the origin of the fistula (arrow). Contrast injection via a catheter in the left ICA results in no
discernible flow through the previous fistulous communication.
of the natural acoustic windows of the fontanelles. Its utility lies
principally in the bedside nature of the study, which makes its
deployment on the intensive care unit ideal. Modern-day scan-ners
may facilitate exquisitely detailed visualization of the
superficial neuroparenchyma. However, the technique is heavily
operator-dependent and its success relies wholly on the patency
and calibre of the fontanelles, which begin closing after 10
months or so. Limited visualization of the posterior fossa struc-tures
is a further limitation. Common indications include the
assessment of germinal matrix haemorrhage with its associated
deleterious sequelae.
Radionuclide imaging
In radionuclide imaging, a radiopharmaceutical (comprising
a tracer molecule coupled to a radioactive isotope) is adminis-tered
to the patient and imaging ensues following an appropriate
time interval, during which redistribution of tracer occurs. It is an
example of functional imaging; biological processes such as
blood flow and metabolic activity can be inferred from the
distribution of tracer on the resulting image. Examples of
commonly used radionuclide techniques include FDG-PET
(fluoro-deoxyglucose positron emission tomography) and
HMPAO SPECT (hexamethypropyleneamine oxime single photon
emission computed tomography), the former enabling assess-ment
of cerebral metabolism and the latter depicting blood flow.
Common indications include the investigation of epilepsy and
dementia. In the former, areas of increased blood flow on the
ictal SPECT have been shown to correlate well with epileptogenic
foci. The evaluation of malignancy formerly constituted an
important indication for radionuclide scanning. The advent of
improved MRI and CT technology has all but obviated the need
to perform such tests in these cases, but FDG-PET may still play a
role in differentiating recurrent malignancy from post-treatment
change, a feat that has proved difficult with even the highest
quality anatomical imaging available.

INVESTIGATIONS
Conclusion
Neuroradiology is a burgeoning field and one in which signifi-cant
recent technical advancements have occurred. It is rapidly
expanding in terms of manpower, expertise and resources such
that prompt access to the highest quality imaging is available to
all. Lengthy hospital admissions for diagnostic tests are no
longer a major factor and the emergence of interventional
neuroradiology is a development which has brought exciting
novel treatments to the fore and promises much for the near
future. A
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