Imaging Of The Central Nervous System
Imaging of the central
nervous system (CNS) is essentially designed to look either at structure
(computed tomography [CT], magnetic resonance imaging [MRI], angiography) or
function (functional MRI [fMRI], positron emission tomography [PET], and single
photon emission CT [SPECT]). In clinical practice it is the former that is the
mainstay of practice, with the latter tests reserved for patients being
investigated for specific problems or as part of a research project.
· In general structural scanning is
undertaken to determine if there is any abnormality in the CNS on imaging.
· If there is an abnormality, then where
is it and does it fit with the history and clinical examination?
· What is the likely nature of that
abnormality pathologically based on its radiological appearance?
This information can be
used for the future investigation and management of patients with neurological
disorders. This chapter outlines the major imaging modalities used in
clinical practice, their indications, value and drawbacks.
Structural imaging
CT imaging
• Basic principle: this technique uses X-rays to scan the brain or spine (typically lumbar)
and then reconstruct an image of that structure; it can be performed with or
without a contrast agent, the latter being used to better define blood vessels
and abnormalities in the blood–brain barrier.
• Use: imaging
of the brain looking for major abnormalities, in particular stroke, head
trauma, hydrocephalus or tumour, especially in the acute medical situation. It
can also be used to look for skull fractures and prolapsed intervertebral discs
in the lumbar spine, and in some cases to look for cerebral aneurysms.
· Advantages: widely available, and often gives
useful and vital information especially in acute situations. It
is well tolerated by nearly all patients, even those who cannot fully
cooperate, and if general anaesthesia is needed, this is more easily performed
with CT than MRI.
· Disadvantage: it has poor contrast resolution
compared with MRI and as such is not so good at identifying lesions in the posterior
fossa and cervicothoracic spine. This is because dental fillings can often
result in several artefacts on scans of the posterior fossa. It also involves
radiation, which can be an issue in some situations – e.g. pregnancy.
MRI
· Basic principle: this technique places the patient
in a strong magnetic field which is then subject to a series of magnetic perturbations
(scan sequence), which alter the orientation of hydrogen ions, such that their
change and subsequent shift back to normal position is detected. Thus, it does
not use X-rays and is very sensitive to subtle changes in water content, which makes
it a highly sensitive scan.
· Use: most patients with neurological problems
should have an MRI scan, given its superior spatial resolution compared with CT
scanning and the fact that any part of the neural axis can be scanned with it.
Thus, it is employed in patients with chronic neurological problems (e.g. multiple
sclerosis) as well as those with evolving acute disorders (e.g. herpes
encephalitis). It can also be used with a contrast agent (gadolinium)
and to image blood vessels both on the arterial side (magnetic resonance
angiography [MRA]) looking for carotid artery disease or intracerebral
aneurysms and on the venous side (magnetic resonance venography [MRV]), especially
to look for major venous sinus thromboses.
· Advantages: high spatial resolution and the
fact that any part of the neural axis can be imaged, along with the major
vessels, without recourse to X-ray exposure or invasive procedures.
· Disadvantage: it is a noisy, claustrophobic
experience and requires the patient to be cooperative to some extent. Some
patients cannot cope with the claustrophobia while agitated patients will move
in the scanner causing major artefacts on the images. It also cannot be used in
patients with metallic magnetic materials such as a cardiac pacemaker.
Angiography
· Basic principle: this is the imaging of blood
vessels and it can be carried out using CT and MRI, but in some cases it
requires the direct visualization of blood vessels using a radiolucent contrast
agent injected into an artery with video fluoroscopy to follow its course.
Thus, the flow of the dye can be followed through the vasculature and X-rays
taken to capture various different phases of the injection; this can identify
problems on the arterial and venous sides of the circulation.
• Use: its
main value is the identification of vascular abnormalities such as
aneurysms, arteriovenous malformations and venous sinus disease. In all cases
angiography is either performed to confirm an equivocal MRA/MRV result or as a
prelude to a more invasive procedure to deal with the underlying abnormality
such as the obliteration of vascular malformations through intravascular
occlusion techniques (gluing or coiling).
• Advantage:
it is the most high-resolution scan for identifying vascular abnormalities and
is essential if intravascular interventional therapies are being considered.
• Disadvantage:
it is an invasive procedure with a small but nevertheless real complication
rate of stroke and local haemorrhage/haematoma at the site at which the
catheter is passed into the artery (typically the femoral artery in the groin).
Functional scanning
This embraces SPECT, PET
and fMRI. Although there are a number of different types of scan, they can be
thought of as looking at either:
• Blood
flow/metabolism, using glucose and oxygen markers to reflect neuronal activity
and pathology, such that a loss of activity reflects an area that contains
dysfunctional or dead neurones. So, for example, in Alzheimer’s disease, there
will be hypoperfusion in the parietotemporal cortices. Such ‘metabolic’ scans
can be under- taken for diagnostic and therapeutic purposes in some patients in
routine clinical practice. Another related approach, which is currently only
used in research, relies on looking at oxygen extraction in areas of the brain
while the patient is being tested on a particular task while being imaged in
the MRI scanner. The resultant scan will show which areas of the brain are
activated by that task. This is called fMRI, and has been used, for example, to
see which brain areas are activated by specific types of cognitive or visual processing
tasks.
• Specific
neurochemical markers which are used to identify and label particular aspects
of a neurotransmitter pathway. In Parkinson’s disease this may involve looking
at the dopamine transporter (e.g. DAT scans) or certain types of dopamine
receptors (e.g. 11C-raclopride labelling of D2 receptors in PET). The former
types of scan are found in many nuclear medicine departments and are widely
available, while PET scanning is still only an experimental tool and found in a
few research centres. However, [18F]2-fluoro- 2-deoxy-D-glucose
(FDG)-PET scanning is being increasingly used to find small tumours in patients
with suspected paraneoplastic syndromes (see Chapter 62). This is because they
can detect small metabolically active tumours that cannot be seen using
traditional imaging modalities.
