Neuroradiology

Computerized axial tomography

CT scan is the most commonly available form of neuroimaging in the UK (Fig. 2a). The patient lies with the head in a ring which contains both X-ray emission and detection apparatus. Images are formed in slices, as the head is moved through the ring. The dose of X-rays is relatively large and CT is contraindicated in pregnancy except in emergencies. CT scan remains the method of choice, however, for the demonstration of acute intracranial haemorrhage and intracranial calcification. CT is relatively insensitive at detecting pathology of the spinal cord but can detect most herniated lumbar intervertebral discs and is useful in delineating bony abnormalities. Modern CT can be used to examine arterial and venous structures and provides a non-invasive high-quality alternative to conventional angiography.

Fig. 2 Normal axial images of the brain. (a) CT scan; (b) MRI, T1 axial; (c) MRI, T2 axial.

Magnetic resonance imaging

X-rays are intuitively easy to understand because the denser the tissue, the less penetration by X-ray. MRI is a more complicated imaging technique and the following is a simple (perhaps simplistic) account of some features. The MRI scanner uses the interaction of a strong magnetic field and a pulsed electric field to alter the energy state of protons in the patient’s tissues. This energy is released again after each pulse and is used to form the image. The rate of energy release depends on how tightly the protons are bound, hence on the chemical composition of the tissue. Since by far the greatest number of protons is in water, this forms the major contrast of the image. The scans can be reconstructed in axial, sagittal or coronal planes.

There are two commonly performed types of scan:

T1 – water is dark and anatomical resolution is optimized (Fig. 2b).

T2 – water is bright and pathological tissue is often highlighted, but with some loss of anatomical resolution (Fig. 2c).

Cerebrospinal fluid (CSF) is chemically close to water, so to tell whether an image is T1 or T2, see if the ventricles are bright or dark.

MRI parameters may be set to detect blood flow and MRI may be used as another non-invasive form of angiography (MRA; Fig. 3) of both arterial and venous circulations. MRI can be tailored so that tissues alter appearance according to scan parameters (e.g. suppressing signal from fat), and it is the combination of appearances on different types of scans that leads to the diagnosis.

Fig. 3 Magnetic resonance angiogram showing the vessels viewed from above.

Special MRI sequences may be used for particular purposes. Diffusion-weighted imaging (DWI) is very useful in identifying distinguishing acute infarction from other pathologies (Fig. 2, p. 69) and gradient-echo imaging highlights haemosiderin deposition from previous haemorrhage.

MRI does not involve high doses of ionizing radiation. However, despite this, MRI is relatively contraindicated in the first trimester of pregnancy because of uncertainty about toxicity. Patients may find the procedure claustrophobic and noisy.

MRI is contraindicated in patients with pacemakers, metallic foreign bodies in their eyes or intracranial implants of certain metals, because of the effect of the magnetic field.

Myelography

Myelography is still used for imaging the spinal cord and roots in centres where MRI is not available or if it is contraindicated. A lumbar puncture is performed and water-soluble contrast medium is introduced into the CSF. This is run up and down the spinal column by tipping the patient. Compression is detected by indentation of the column of fluid or by blockage of flow. The contrast medium also outlines the nerve roots, and compression here (e.g. by a disc) can be seen as failure of the nerve root sheaths to fill with contrast medium.

Myelography may cause severe headache. Rare complications include introduction of infection (meningitis) and subdural haematoma. Older contrast agents that were never completely cleared from the subarachnoid space were associated with a chronic, progressive inflammation of the meninges; ‘chemical arachnoiditis’.

Angiography

This is still the ‘gold standard’ test for imaging blood vessels intracranially and in the neck (Fig. 4). Advances in MRA or CT angiography may replace this technique for diagnostic purposes in the future. A cannula is inserted into the femoral artery under local anaesthetic, manoeuvred into the aortic arch and into the carotid or vertebral arteries. Contrast is injected and X-rays are taken. Delayed X-rays allow visualization of the venous system. The examination is relatively safe, major complications such as stroke occurring in <0.5%. Transient neurological deficits due to vasospasm are seen occasionally. Bruising may occur at the site of arterial puncture and bleeding is occasionally severe. Most units monitor pulse and blood pressure very closely for several hours after angiography.

Fig. 4 Normal digital subtraction carotid angiogram.

Recently, interventional angiographic treatment techniques have been developed, for example insertion of glue or coils into aneurysms and other vascular malformations, and balloon dilatation of carotid and vertebral artery stenosis. These developments can be used in conjunction with or as an alternative to surgery.

Functional neuroimaging

A variety of techniques are becoming available to explore regional cerebral function. These are generally research techniques. The most widely available is single photon emission computed tomography (SPECT) (Fig. 5). This utilizes radioactive isotopes whose distribution mirrors cerebral perfusion. Regional uptake of these isotopes is determined by CT detection of radioactive decay emissions. This has very limited temporal and spatial resolution.

Fig. 5 Normal SPECT scan.

Positron emission tomography (PET) uses rapidly decaying substances that are attached to metabolic molecules (such as glucose) to detect metabolically active regions, or attached to water which maps blood flow. These substances emit positrons which are detected by CT using a specialized detector. The substances decay so rapidly that their activity disappears within minutes and they have to be produced by an on-site linear accelerator, restricting the technique’s use. It is more sensitive than SPECT and can be used to detect neurotransmitter activity, with appropriate isotopes, as well as local metabolism.

Echo-planar MRI is a variation of MRI technique and can be used to detect variations in oxyhaemoglobin and deoxyhaemoglobin, which relate to tissue oxygen uptake and, therefore, to metabolic rate.

Neuroradiology

Plain radiography gives little useful information regarding the CNS and should be restricted to assessing bony structures.

The investigation of choice for acute intracranial haemorrhage, e.g. trauma, is CT.

The investigation of choice for most other brain and spinal imaging is MRI.

MRI can be tailored to different purposes.

Myelography has an occasional role.

Angiography remains the best way to visualize the intracranial circulation.