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Core
Ultrasound uses high frequency sound waves (typically around 2 to 15 megahertz, MHz, which means millions of sound-wave cycles per second) to produce a 2D image. The ultrasound transducer (the hand-held probe) produces the ultrasound waves, which travel through the tissues and are reflected back to varying degrees, depending on the density of the tissue encountered. These ‘echoes’ are interpreted in terms of space and contrast in order to produce a 2D greyscale image in real-time.
Advantages of ultrasound scans (USS):
- It is cheap and widely available.
- It is very good for soft tissue imaging.
- It produces a dynamic image.
- Doppler imaging (which measures the change in frequency of echoes from moving blood) can be used to assess blood flow.
- Does not involve ionising radiation (radiation that can damage DNA), unlike x-ray/CT.
Disadvantages of USS:
- The quality of the scan depends heavily on the skill of the person doing it.
- Not very useful for assessing deep structures or dense tissues (e.g. bone).
Uses of USS in the head and neck:
- Imaging of the thyroid - e.g. goitre (an enlarged thyroid gland) or a thyroid mass (lump)
- Evaluation of other neck lumps
- Evaluation of superficial infections - e.g. neck abscess
- Carotid Doppler - used to assess stenosis (narrowing) of the carotid artery due to an atheroma (a fatty plaque in the artery wall)
Diagram - Doppler ultrasound of the carotid artery
Creative commons source by National Heart Lung and Blood Institute (NIH) [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
A radiograph uses X-rays to produce a static 2D image. A beam of X-rays is produced and transmitted through the body. As the x-rays pass through the body, they are absorbed to varying degrees by the body’s tissues. Denser tissues (e.g. bone) absorb more x-rays while softer tissues (e.g. muscle) absorb fewer x-rays. The x-rays that remain unabsorbed reach the detector plate and turn the image dark. Therefore, dense structures appear white while less dense structures appear dark. The pattern of absorption is interpreted by a computer which produces the image.
Advantages of radiographs:
- They are cheap, quick and widely available.
- Good for assessing bone and joint pathology.
- Involves a much smaller dose of radiation compared to CT.
Disadvantages of radiographs:
- Poor soft tissue information.
- Involves a small dose of ionising radiation, which carries a very small increased risk of cancer (carcinogenesis).
Uses in the head and neck:
- Head trauma (rarely used). NICE says not to use skull X-rays to diagnose important brain injury before a discussion with a neuroscience unit, which is a specialist brain and nerve surgery team. CT is the test for this. Children with suspected non-accidental injury, meaning injury caused on purpose by another person, may need a skeletal survey, which is a set of X-rays of the whole skeleton.
- Cervical spine trauma (in adults CT is the first scan; plain X-rays have a smaller role, for example in some children)
- Maxillofacial trauma (injury to the face and jaws)
- Foreign body within the neck (plain films miss many swallowed objects, so a normal film does not rule one out)
X-rays of the skull are becoming increasingly uncommon due to the increasing availability and superior sensitivity of CT imaging.
The cranial vault (the dome of the skull) is viewed using PA (posterior-anterior) and lateral films. In normal anatomy the suture lines (the joints between the skull bones) and blood vessels will be visible, which to the untrained eye may be mistaken for a fracture.
Image - Normal lateral radiograph of the skull. Note the visibility of the coronal suture and blood vessels
Creative commons source by Hellerhoff [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
The facial skeleton is assessed with occipito-mental views. These are X-rays taken from the back of the head towards the chin, with the head tilted at different angles (usually 15 and 30 degrees). The Water’s view is one of these occipito-mental views, and the angle quoted for it varies between sources (30, 37 or 45 degrees). A Water’s view can also be useful in assessing the paranasal sinuses (the air-filled spaces in the bones around the nose).
Image - Occipito-mental projection (Water’s view) of the skull
Creative commons source by Ptrump16 [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
The mandible and temporomandibular joint (TMJ, the jaw joint) are assessed with an orthopantomogram (OPG, a panoramic X-ray of the jaws and teeth) and mandible views. The mandible is a ring-like bone, so it often fractures in more than one place, and a fracture may be accompanied by dislocation of the TMJ.
Image - Orthopantomogram showing a fracture through the body of the mandible
Creative commons source by Coronation Dental Specialty Group [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
In adults, NICE recommends CT rather than plain X-rays as the first scan when imaging is needed for a suspected cervical spine (C-spine) injury. Plain X-rays have a smaller role, for example in some children. When plain films of the C-spine are used, AP (anterior-posterior), lateral and ‘open mouth’ views are taken. A Jefferson fracture refers to a fracture of the atlas (C1, the first neck vertebra). A Hangman’s fracture refers to a fracture through the pars interarticularis (the bridge of bone between the upper and lower joint surfaces of the vertebra) of C2. A fracture of the odontoid peg (the peg of C2 that projects up into C1) is best seen with an ‘open mouth’ view. For more information, check out our article on Conditions of the Back and Neck.
Image - Lateral radiograph of the C-spine showing a Jefferson’s fracture (C1)
Creative commons source by James Heilman, MD [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
CT scans project multiple x-rays from different angles in order to produce 2D cross-sectional images which can potentially be reconstructed into a 3D image.
An important concept in CT imaging is windowing. Consider the CT scan as a greyscale image. Depending on their density, tissues show up as different shades of grey, with most dense structures appearing white and the least dense structures appearing black. Due to such a wide range of densities, it can be difficult to contrast tissues of similar densities as they will appear as very similar shades of grey. A CT window allows us to focus in on a certain range of densities, measured in Hounsfield Units (HU, a scale of how strongly a tissue absorbs X-rays). This enhances the contrast between the structures whose density falls within this window. In essence, it allows us to better distinguish between similar shades of grey. Common CT windows include bone windows (to show fractures), brain windows (to show blood and brain tissue) and lung windows (to show the lungs and airways).
Advantages of CT:
- CT shows bone, soft tissue and the brain in much more detail than plain radiographs.
- It is much quicker than MRI and is therefore better for acute presentations.
Disadvantages of CT:
- Involves a significant dose of ionising radiation, therefore increased risk of carcinogenesis.
- Soft tissue detail is not as good as MRI.
Uses in the head and neck:
- Head trauma - particularly to check for intracranial (inside the skull) haemorrhage
- Maxillofacial trauma
- Cervical spine trauma
- Acute focal neurology (sudden new symptoms from one part of the brain, such as weakness on one side) - particularly to rule out haemorrhagic stroke (a stroke caused by bleeding)
- Infection - e.g. meningitis, encephalitis (inflammation of the brain itself). For suspected bacterial meningitis, NICE advises against routine imaging before a lumbar puncture (a needle test to sample the cerebrospinal fluid). Imaging is done first only if there are risk factors for a space-occupying lesion (a mass such as an abscess or tumour) or signs of raised intracranial pressure (pressure inside the skull), such as new focal neurological features (signs from one part of the brain), abnormal pupil reactions, a Glasgow Coma Scale score of 9 or less (a low level of consciousness), or a progressive or rapid fall in level of consciousness. NICE says to take bloods, give antibiotics and stabilise the person before any imaging. For suspected encephalitis, CT is often the first scan, and MRI is preferred where it is available.
- Angiography (imaging of blood vessels after injecting contrast dye) - to evaluate the Circle of Willis (the ring of arteries at the base of the brain) and the venous sinuses (channels that drain blood from the brain)
Image - 3D CT reconstruction showing a fracture through the body of the mandible
Creative commons source by Coronation Dental Specialty Group [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
CT plays an important role in the diagnosis of intracranial haemorrhage (ICH, bleeding inside the skull). ICH can occur within the brain tissue itself (intracerebral haemorrhage) or outside it, in the spaces around the brain. There are 3 main types of ICH outside the brain tissue:
- Extradural (outside the dura, the tough outer membrane around the brain) - an arterial bleed (usually from the middle meningeal artery) caused by head trauma. Blood collects between the periosteal dura (the layer of dura attached to the skull) and the inner table (inner layer) of the skull bone. It appears as a biconvex (lens-shaped) area of hyperdensity (an area that looks brighter than the brain). The blood is limited by the suture lines (the joints between skull bones).
- Subdural (below the dura) - bleeding from bridging veins (small veins that cross from the brain surface to the dural sinuses) due to trauma. The bleed can be acute or chronic. Blood collects within the subdural space. The collection of blood is crescentic (crescent-shaped). Acute bleeds appear bright (hyperdense) on CT whereas chronic bleeds appear darker (hypodense).
- Subarachnoid (below the arachnoid, the middle layer of the meninges) - bleeding from one of the cerebral arteries. It can be spontaneous (ruptured berry aneurysm, a small balloon-like swelling of a brain artery) or traumatic. The classical CT appearance is a central star-shaped area of hyperdensity. Blood enters the subarachnoid space and collects within the basal cisterns (the cerebrospinal fluid-filled spaces at the base of the brain). NICE notes that CT is most accurate within 6 hours of the start of symptoms. If a CT done within 6 hours is reported by a radiologist (a doctor who specialises in reading scans) as normal, NICE advises against routinely offering a lumbar puncture (a needle test to sample the cerebrospinal fluid). If a CT done more than 6 hours after the start of symptoms is normal, NICE advises considering a lumbar puncture, and allowing at least 12 hours after the start of symptoms before it is done. NICE says to diagnose a subarachnoid haemorrhage if the sample shows xanthochromia (raised bilirubin, a yellow pigment made when red blood cells break down).
Image - Axial CT head showing a spontaneous subarachnoid haemorrhage
Creative commons source by James Heilman, MD [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
Magnetic Resonance Imaging (MRI)
MRI uses a strong magnetic field and radio waves to produce very detailed cross-sectional images, which can potentially be reconstructed into a 3D image.
Advantages of MRI:
- Provides very high soft tissue resolution (fine detail), allowing accurate assessment of the edges of a tumour or other lesion.
- Does not involve any ionising radiation.
Disadvantages of MRI:
- It is an expensive imaging modality (type of scan).
- It usually takes longer than CT, so it may be less suitable for a very unwell patient who needs a scan urgently.
- The MRI scanner is noisy and is an enclosed space, which can be distressing to some patients (for example, those with claustrophobia, a fear of enclosed spaces).
- Because of the strong magnetic field, MRI is contraindicated (should not be done) if the patient has metal implants or metal fragments that are not MRI-compatible, for example some pacemakers (devices that regulate the heartbeat). Some implants are ‘MR conditional’, which means they have been shown to be safe in the scanner only within defined conditions, so everyone is screened for implants and metal before going into the scan room.
Uses in the head and neck:
- Imaging of the brain, e.g. epilepsy and tumour evaluation, where fine soft tissue detail matters
- Spinal cord pathology (disease of the spinal cord), where the same fine detail is needed
Image - Sagittal MRI of the cervical spine showing herniation (bulging out of place) of the C6/C7 intervertebral disc
Creative commons source by user:Anthonp [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
Reviewed by: Dr. Thomas Burnell and Dr. Marcus Judge
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