In the third part of our series, John Frank discusses the techniques used to image head and neck disorders

Radiologists can image the head and neck in a variety of ways to show different pathologies. Radiologically, the head is made up of two separate parts--the cranium and the face. They grow at different rates, as can easily be seen in babies, who have a relatively large cranium that grows slowly, and a smaller face, which grows more quickly. Imaging the head usually relates to the cranium and contents. Always consult an imaging department for the best advice so that radiologists can tailor the correct investigation to the question you need answering.

When to request a plain x ray

Doctors often request plain x rays of the skull for vague symptoms such as headache. The only real indication for plain x rays of the skull in an adult, however, is trauma.¹ Other intracranial problems do not show up in plain x ray images. Rare conditions, such as chronic raised intracranial pressure, will be diagnosed by computed tomography (CT) long before they cause any changes to the inner table of the skull vault. The only time plain x rays are of use is in imaging facial bones to look for fractures or infections such as sinusitis (figs 1 and 2).

Fig 1 (top) X ray image showing opaque left maxillary antrum
due to sinusitis. Sinuses are usually filled with air, which is black in x ray
images; opacity is caused by the air space being filled with pus and fluid

Fig 2 (bottom) X ray image of a fracture of the floor of the
left orbit with an opaque left maxillary antrum filled with blood

Magnetic resonance imaging or computed tomography?

You should investigate any suspected intracranial lesion with either CT or magnetic resonance imaging (MRI) (figs 3 and 4). The commonest suspected lesions are strokes or tumours. Intracranial lesions such as fresh blood, old strokes, and tumours show up well using either technique. CT easily shows strokes, and the important diagnostic point is that fresh blood is shown as a white area within the brain (fig 5), whereas a block in an intracranial artery is shown as a darker wedge in the brain. The importance lies in the further treatment: you do not treat a bleed with aspirin and anticoagulation, but you will treat a blood clot like this.

Fig 3 Computed tomographs showing sections through a normal brain.
Structures, such as the eyes, sinuses, and ventricles, can be easily identified

Fig 4 Normal magnetic resonance images showing the anatomy
of the brain, such as the ventricles, brain stem, and corpus callosum

Fig 5 CT of the head of a patient presenting with a
stroke. Extensive fresh blood, caused by intracranial bleeding, is
visible (white) around the internal capsule and in the lateral ventricles

CT shows tumours by their mass effect, surrounding oedema, and edge enhancement with contrast. MRI shows changes in the signal from the lesion, often with surrounding oedema. CT and MRI may be interchangeable in that they both show the alteration in brain anatomy, but CT is better where bony injury is suspected, and it also has a role to play in imaging facial fractures, whereas MRI is better at showing intracranial disease. Of course, both techniques are useful in investigating other lesions in the head and neck, mainly tumours (fig 6).

Fig 6 Tumours identified using CT (left) and MRI (right).The
computed tomograph shows the mass effect of a tumour shifting
the lateral ventricles,and the contrast media delineates the edge
of the tumour because the blood-brain barrier is altered.The magnetic
resonance image (of a different patient) shows shift of the ventricles and
an abnormal signal from tumour because of different molecular constitution

Nuclear medicine

Nuclear medicine has a smaller part to play in brain imaging because anatomical information from CT or MRI is usually more helpful than physiological information. But images from nuclear medicine can help in diseases such as suspected Parkinson's disease, where the dopamine transporters are diminished in the basal ganglia. Imaging these gives an indication of the degree of severity of the disease. The test uses a dopamine transporter ligand labelled with iodine-123, which shows where the dopamine transporters are situated. This is an expensive and specialised test, and a neurologist needs to assess patients before requesting the test (fig 7).

Fig 7 Uptake of ioflupane, a cocaine analogue, labelled with ¹²³I,
which binds to sites of dopamine transporters in the basal ganglia.
Uptake occurs in the caudate nuclei but not in the putamen, which is typical
of Parkinson's disease. This test is a dopamine transporter scan (DaTScan)

Positron emission tomography, using fluorodeoxyglucose (a glucose analogue) labelled with fluorine-18 can show areas of the brain with increased activity, which is most likely tumour, when MRI is equivocal (fig 8).

Fig 8 An HIV positive patient's brain showing abnormal uptake of
fluorodeoxyglucose labelled with ¹⁸F in the left parietal region. MRI
showed a lesion at this site, but could not distinguish between toxoplasmosis
and lymphoma, both of which may present in HIV positive patients. Uptake indicates
that the patient has a lymphoma--toxoplasmosis has no uptake of fluorodeoxyglucose

Neck imaging

The main structures in the neck that require imaging are the thyroid, parathyroids, the mouth and oesophagus in swallowing, and the cervical lymph nodes when doctors suspect malignancy.

Thyroid imaging

Radiologists can image the thyroid using either ultrasound or nuclear medicine (fig 9 and 10). There have to be specific indications and you should always discuss these with an imaging department.¹ Examples of conditions requiring investigation are large palpable goitres or suddenly developing lumps, especially if they are painful. Altered thyroid function tests, indicating thyrotoxicosis, are not good indications for imaging the thyroid, especially where there are no palpable lumps in the neck.

Fig 9 Ultrasound scan showing the normal
echo pattern within the left lobe of the thyroid

Fig 10 Nuclear medicine scan showing uptake
of technetium-99m within a normal thyroid. Both
lobes of the thyroid are clear with even uptake throughout

When to use which imaging technique is a vexed question, and many people use ultrasound first, especially where there is a palpable nodule. If solid, then a biopsy can be got at the same time. If the ultrasound shows a multinodular goitre, then nuclear medicine may help in determining the metabolic state of the nodules, especially if the patient is also thyrotoxic. However, it is not possible to diagnose cancer from an ultrasound.

Parathyroid imaging

In hypercalcaemia, where a parathyroid adenoma is suspected because of raised serum calcium concentrations, radiologists can take images of the parathyroids using nuclear medicine. This technique relies on differential uptake between normal and abnormal parathyroids. Tumour seeking sestamibi, labelled with ^99mTc, is widely used. This is taken up by both the thyroid and parathyroids, and is cleared rapidly from normal tissue but retained in tumours of the parathyroids (fig 11).

Swallowing problems

Radiologists can investigate swallowing problems either by endoscopy or by studies with barium. Examples of such problems are dysphagia and reflux. The former is best studied with barium, and the latter by endoscopy. Studying dysphagia allows the radiologist to visualise exactly how the tongue moves and how the bolus of barium is cleared from the mouth. The oesophagus may be studied by either technique, as can the stomach; the advantage of endoscopy is that biopsies can be taken at the same time (fig 12).

Lymph node imaging

Radiologists can image lymph nodes in the neck using ultrasound, but this simply shows their size and shape, and gives no information about their metabolic activity, which is especially important if there is a question of malignancy. To demonstrate malignant nodes--as well as a primary malignancy--the best technique to image the patient is using ¹⁸F labelled). This is a glucose analogue taken up by malignant cells which have a much higher glucose metabolism than normal cells. Fluorodeoxyglucose is not broken down to carbon dioxide, water, and energy, as glucose is, but remains in the cells. This can then be imaged using a special nuclear medicine positron camera (positron emission tomography scanner), becasue ¹⁸F is a positron emitter (fig 13 and 14).

After the injection of the isotope, the patient must lie still and not talk to avoid abnormal uptake muscle. Diabetic patients need to have properly controlled normal blood glucose concentrations for fluorodeoxyglucose positron emission tomography scans. If the concentration of glucose in the blood is too high, not enough labelled fluorodeoxyglucose will enter the cells,giving inaccurate results.

Fig 11 (left) Sequential images showing initial activity
within the thyroid area but retention of activity in a left lower
parathyroid adenoma over four hours. The normal thyroid and parathyroids
do not retain activity with time, whereas the parathyroid tumour, an adenoma, does

Fig 12 (right) Lateral view of a normal oesophagus filled with
barium, and its relationship with the cervical spine posteriorly and the
trachea anteriorly. Motility disorders and filling defects within the barium
caused by tumours are seen easily, as are other oesophageal pathologies

Fig 13 Positron emission tomograph showing
extensive abnormal uptake in a mass of nodes on the right side of
the neck, normal uptake in the brain and heart, and some renal excretion

Fig 14 Positron emission tomographs occasionally show unsuspected
disease.This patient presented with a lump in the right side of the neck,which
showed adenocarcinoma on biopsy. The scan shows uptake in the neck and also on
the right side of the tongue, but in addition there is uptake in the left breast and a left
internal mammary node.The patient had dual pathology--carcinoma of the tongue and the breast

Glossary

Technetium-99m--Metastable isotope of technetium used for most nuclear medicine technetium is polyvalent, and can label many carrier molecules

Iodine-123--Isotope of iodine used for imaging. This pure gamma emitter is easily imaged on a gamma camera

Computer tomography--A tube rotates around a patient and a computer produces an image from the raw data. Modern scanners also move the patient through the ring at the same time as the x rays are produced, allowing the computer to construct three dimensional representations

Ultrasound--High frequency sound waves are emitted from a special probe, and the reflections detected by that probe. A computer then generates the images in real time and also allows measurements to be made.

Magnetic resonance imaging (MRI)--Imaging tool using a high strength magnetic field to align all the hydrogen protons, before subjecting them to tuned radiofrequencies (also known as nuclear magnetic resonance)

Positron emission tomography--Some isotopes, such as ¹⁸F, emit positrons--positively charged electrons. These annihilate immediately with an electron to give off two 511 keV photons at 180°. These photons that are detected by a specialised scanner, and a computer analyses the results and creates the images

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