Basics

 Presented by:
       Rachel Marsden - Fifth Year Medical Student, The University of Sheffield Medical School
       Dr Ian Bickle - Radiology SpR, Sheffield Teaching Hospitals NHS Foundation Trust
       Patrick Stirling - Fith Year Medical Student, Queen's University Belfast

What is CT?

Computed Tomography or CT uses digital processing to generate a 3-D image from a large number of 2-D X-rays taken around a single axis of rotation. A picture of a modern CT scanner is seen below. It shows the scanner in the background with the control panel in the foreground.

In CT the patient lies supine on the table and the donut shaped scanner rotates taking a large number of x-rays in a single slice. Patients will then move through the scanner to allow a different number of “slices” to be taken through the anatomical area of interest. These x-rays are then re-formatted by the computer and can be viewed by the radiologist in multiple plains.

Interpreting the CT

Images on CT are seen as various shades of grey based on how much radiation a tissue absorbs. When reporting x-rays it is useful to know something about two key words.

• Attenuation – defined as the process by which a beam of radiation is reduced in intensity when passing through material. If a tissue has low attenuation it would suggest that it is relatively transparent, where as high attenuation is a denser material. In terms of CT low attenuation appears dark (air) where as high attenuation (bone) objects appear brighter.
• Hounsfield Units (HU) – Units of x-ray attenuation used in CT scanning. Each tissue has a volume based on how bright/dark it appears; the brighter the tissue, the higher the HU; Bone +1000, Water 0, Air -1000

Head CT: some basics

It is worth knowing how you would expect tissues on CT head to appear. Keeping attenuation and HU in mind you can work out that:

• CSF in the brain will appear black
• Bone will appear white.
• Grey and white matter within the brain have different attenuations; the densely packed nerve cell bodies of the grey matter have a higher attenuation than the nerve axons of the white matter, meaning that perversely white matter is darker than grey matter on CT (TOP TIP: always look for the grey white matter boundary; loss of this boundary can be an early sign of brain injury.)

• Blood contains protein making it dense and areas of acute bleeding appear high attenuation (bright) on CT.
•  Areas of dead or damaged brain tissue will become less dense, meaning that they will appear darker than the surrounding brain (low attenuation)

Contrast

IV contrast in CT highlights blood vessels or vascular areas of the brain. It is also useful for identifying areas of high cell turn over such as tumours and infection. Aneurysms, tumours and abscesses all become brighter post contrast administration on head CT.

Keeping all this in mind we have ten CT heads for you to look at. Don’t panic, just say what you can see, is it higher or lower attenuation than the surrounding brain? Where in the brain is the abnormality? What shape is it? Think about your brain anatomy and remember to take clues from the history.

Enjoy!

Case (1)

Extra Dural Haematoma

A 24 year old female fell from her horse during a cross country competition, hitting her head on a wooden fence on the way to the ground. She was taken by ambulance to A&E where on examination she had a Glasgow coma scale of 11/15. CT scan is shown below.

 Describe the two radiological abnormalities shown above

 Give a radiological diagnosis

 What other abnormality should always be looked for in these cases?

Learning point

Difference between extradural and subdural

 

	Extradural	Subdural
Source of Blood	Middle meningeal artery	Dural Veins
Size	Usually small as limited by skull vaults	Large
Shape	Convex “EGG”	Concave “BANANA”
Crosses sutures?	Unable to cross sutures	Can cross sutures
Crosses Midline?	May Cross midline	Does not cross midline
Position related to injury	Directly adjacent to injury site	Often distant from injury site – contre coup
Attenuation	High attenuation	High attenuation in acute phase, can also be low (chronic) or mixed attenuation (acute on chronic)

Case (2)

Subdural Haematoma

An 84 year old male nursing home resident presents with increasing drowsiness. The care assistant who attended with him informs you he has become increasingly unsteady and confused over the past two weeks since falling in the bathroom and has deteriorated rapidly over the past 24 hours.

CT scan was performed and is shown below

 

 Report this CT scan, describe what you can see?

 What is your radiological diagnosis?

 What is responsible for the different densities seen on CT?

Bleeding in the brain changes in density with time as shown in the table below

Learning points

Phase	Density	Appearance on CT
Acute	Hyperdense	Bright
7-10 days	Isodense	Difficult to distinguish
21-30 days (Chronic)	Hypodense	Dark

So why does acute bleeding appear more hyperdense on CT?

Blood contains haematocrit and a high concentration of protein. Protein has a high electron density and therefore an evolving bleed in the brain has high attenuation. Slowly over the time the protein is reabsorbed via the CSF and the area of haemorrhage becomes isodense again.

Case (3)

Subarachnoid Haemorrhage

A 46 year old male with known Adult Polycystic Kidney Disease (APKD) presents to his GP with sudden onset headache associated with nausea and vomiting.

 This patient has APKD, what vascular abnormality does this pre-dispose to?

His CT scan is shown below

 What is the diagnosis?

 What does his CT scan show? What complication is he developing?

 In what part of the circle of willis do you think his aneurysm arose from?

Learning Points

By far the most common cause of SAH is trauma, although ruptured aneurysms are the most common non-traumatic cause.

SAH is defined as a haemorrhage into the subarachnoid space so when looking on CT look for high attenuation in the basal cisterns, sylvian fissure, in the ventricles and the interhemispheric fissure (although this is rare).

Case (4)

Hypertensive Haemorrhage

A fifty five year-old smoker develops a sudden onset right sided hemi-paresis associated with nausea during a five a side football competition. He is a known hypertensive and on questioning admits to being “hit and miss” with his medication and is not usually very active.

On arrival at A&E he has developed a headache and is having difficulty with his speech

 Describe the abnormality seen on CT

High attenuation/density mass on the left side of the brain in the region of the basal ganglia – intra-parenchymal haematoma

 What is the diagnosis?

Hypertensive haemorrhage

 What risk factors from the history make this diagnosis most likely?

Poorly controlled hypertension

Smoking

Learning Point

Hypertensive bleeds

These bleeds commonly manifest with symptoms of sudden onset hemiparesis and hemiplegia alongside speech difficulties, headache and nausea. Symptoms often occur during periods of activity.

Long periods of hypertension “wear away” at blood vessel walls. In the brain this can lead to vessels become blocked and blood leaks into the brain parenchyma. Blood collects to form a haematoma which radiologically is most often seen in the region of the basal ganglia and thalamus.

Case (5)

Infarct

75 year old smoker presents to A&E with a 24 hour history of increasing right sided weakness.

 Report his CT scan – what can you see?

 Which blood vessel supplies this part of the brain?

 Name the different types of stroke

Learning point

Remember that stroke is a clinical diagnosis; imaging is required to confirm the diagnosis and the type of stroke. It is important to know whether or not you have a haemorrhagic stroke as a haemorrhagic stroke can not be treated with thrombolysis!!! Haemorrhagic strokes are represented by hyperdense areas within a vascular territory.

Case (6)

Hydrocephalus

19 year female presents to A&E after falling down a flight of stairs after a night out. She describes a sudden onset worse ever headache associated with nausea and vomiting over the past 3 hours.

A non contrast CT head was performed.

 What is your initial working diagnosis?

 Report this CT? Don’t panic, just say what you see?

 Is this communicating or non communicating hydrocephalus?

Learning Point

Hydrocephalus can be classed into two categories, communicating and non communicating.

Non-communicating – obstruction to CSF flow is caused by blockage within the ventricles i.e. from an intra-ventricular mass

Communicating – there is free flow of CSF out of the fourth ventricle and the obstruction is due to a defect in re-absorption of CSF. This is often secondary to disease within the subarachnoid space such as meningitis or in this case subarachnoid haemorrhage.

Case (7)

Skull Fracture

A 20 year old pedestrian is hit by a car travelling approximately 25mph. He is thrown to the ground some distance away by the impact and sustains multiple injuries including a head injury. Upon arrival at A&E his GCS is 13/15.

CT scan is shown below, using the bone window filters

 Report the CT scan, what can you see?

 Although difficult to see on this setting, what else would you expect to see given the type of injury he has sustained?

 Why can it be tricky to see skull fractures in some paediatric patients?

Learning Points

There are two main types of skull fractures

Linear – by far the most common type, straight, single break in the bone, beware though, there could be more than one

Comminuted or Depressed – rarer and often the result of more severe trauma. Bone breaks in multiple fragments and fragments are depressed into the skull vault.

Two rarer types but vital not to miss are

Diastic – fracture causes the bone to separate and widen at the skull sutures. Seen in younger patients

Basilar – breaks in the bones at the base of the skull. These fractures are often the result of severe trauma. Warning signs that should lead to a suspicion of skull base fracture are blood in the sinuses, CSF leaks from nose and ears and “racoon eyes” due to bruising of orbits as blood collects.

Case (8)

Brain Abscess

A 24 year old builder is brought to A&E by his worried girlfriend. He has a one week history of fever associated with fatigue and headache and has deteriorated rapidly over the past 24 hours; developing vomiting and increasing confusion and drowsiness. His girlfriend informs you that he has been suffering from toothache on and off for the past fortnight but has been unable to get an appointment with his dentist.

 What are the significant points from his history?

His post contrast CT is seen below

 Report the abnormality, what can you see?

 What is your diagnosis?

 What is the most likely causative organism in this case?

Learning Point

A brain abscess should always be suspected if a patient has symptoms of infection in combination with symptoms of raised intracranial pressure (headache, vomiting, confusion and coma) and focal neurological damage (seizure, hemiparesis).

Most abscesses are caused by spread from local infection such as ear infection; dental abscess and infection of the paranasal sinuses so always keep a look out for these factors in the history and imaging. Brain abscesses can also occur in immuno-comprimised patients such as those with HIV and cancer patients undergoing chemotherapy and these tend to be atypical organisms.

Case (9)

Primary Brain Tumour

A forty year old female teacher presents to her GP after suffering an “episode” which on description appears to be a generalized seizure. On taking a full history in addition to the seizure she has been suffering from a gradually worsening headache over the past month.

Post contrast CT scan is seen below

 Describe the abnormality seen on the CT scan

 Is the lesion supra or infra tentorial

 What radiological abnormalities in the brain could present with seizures?

 What is your diagnosis?

Learning points

There are over 100 different types of primary brain tumour that can be difficult to distinguish between on CT. You can however gain clues as to the type of tumour by the position in the brain.

Try to determine if the lesion is:

• extra-axial or intra-axial

• infra or supra-tentorial

• Intraventricular

Case (10)

Cerebral Metastases

A 48 year old female with a significant past history of right breast carcinoma presents with a three week history of headache, nausea and vomiting and dizziness. Her CT scan is seen below.

 Describe the radiological abnormalities?

Round increased density lesions, with surrounding low density

 How many are there and give regions in the brain?

Three – one in right frontal lobe, one in right temporal lobe and one in the cerebellar vermis

 What would you expect to happen if contrast was given?

The rim would enhance

 What is the most likely diagnosis?

Cerebral metastases – breast primary

• Learning points

  There is a large differential diagnosis of ring enhancing lesions in the brain. You can help decide what pathology is most likely by looking at the areas of the brain, how many lesions there are and the clinical history.

  • Brain abscess

  • Colloid cysts

  • Target lesion – multiple sclerosis

  • Demylination

  • Primary Brain tumour

  • Haematoma

  • Thrombosed aneurysms

  • Thrombosed vascular malformations

  • Calcification