Imaging the Brain

Question 1

For what 3 reasons may imaging of the brain be performed?

Answer 1

• to confirm a clinical diagnosis / suspicion
• to rule out important diagnoses / pathologies
• to guide or evaluate management / treatment

Question 2

What is the first line imaging technique for acute imaging of the brain and why?

Answer 2

CT is 1^st line for imaging the brain in the acute setting

• it provides excellent visualisation of bony structures
• it provides reasonable visualisation of the brain - cerebral hemispheres and ventricles
  
  • MRI is superior - but also more expensive, time-consuming, etc.
• it allows for fast image acquisition and can be used as an “excludogram” to exclude masses, large infarcts and acute bleeds

Question 3

What are the benefits of using MRI over CT to image the brain (even though it is slower)?

Answer 3

• it provides superior anatomical detail even without the use of contrast
• there is no radiation exposure
• it provides better images of the posterior fossa
  
  • CT images of the posterior fossa are poor as this region has a lot of bone around it
  • the CT struggles to penetrate the dense bone, producing streaks of brightness
• it provides better visualisation of tumours, CVA and meningeal disease
  
  • the _meninges cannot be seen on CT_
  • if meningitis is suspected, a post-contrast MRI is performed opposed to CT

Question 4

What are the following features of the CT image?

Answer 4

• green = head of caudate nucleus
• purple = lentiform nucleus
• yellow = lateral ventricle
• beige = internal capsule
• red = thalamus

Question 5

What are the following features of this MRI scan?

What type of scan is this and how do you know?

Answer 5

• this is a T2 weighted scan as the CSF appears bright
• there should not be any signal coming from the vessels (e.g. sagittal sinus appears black)
• if there is turbulence in the blood flow or flow is slow, there may be some excited blood products present that result in high signal artefact
  
  • this tends to occur in the base of the skull in the jugular foramen

Question 6

When is CT superior to MRI for imaging the brain and why?

Answer 6

• CT is superior in the acute setting as it allows for fast image acquisition
• it provides excellent visualisation of bleeding, however MRI is superior in detecting acute and chronic infarcts
• it has fewer contraindications than MRI

Question 7

What are the benefits of using MRI over CT?

Answer 7

• it has superior anatomical detail, even without contrast
• it does not use ionsing radiation
• it provides good visualisation of the posterior fossa
• it is superior for detecting and evaluating tumours

Question 8

What are the main contraindications to MRI?

What is a drawback that applies to both CT and MRI?

Answer 8

• contraindications to MRI include some pacemakers, cochlear implantsm ICDs and some orthopaedic metalwork
• both scans require the patient to lie still for extended periods of time
• CPR (defib) cannot be used in MRI so caution must be taken in very unwell patients

Question 9

For what 3 reasons are CT scans of the head commonly requested in the ACUTE setting?

Answer 9

• to confirm the diagnosis where there is clinical suspicion (symptoms +/- signs) of acute intracranial pathology
  
  • e.g. stroke, intracranial bleeding
• to exclude serious / life-threatening intracranial pathology, even if there is minimal clinical suspicion
  
  • e.g. stroke, SAH
• to exclude intracranial injury in patients with head injury - CT may be performed if the patient is unable to give a history and/or comply with examination
  
  • e.g. dementia patients, intoxicated patients

Question 10

Why is it important to use CT to differentiate between haemorrhagic and ischaemic strokes in the acute setting?

Answer 10

• haemorrhagic stroke involves a bleed into the part of the brain that is dying
• ischaemic stroke involves poor perfusion to a part of the brain
• aspirin / thrombolysis is the usual treatment, but you do not want to give this if there is already a bleed in the brain as it will make it worse

Question 11

What type of image is taken in a CT scanner and how can this be reconstructed?

Answer 11

• the CT scanner has an X-ray tube and multiple detectors that rotate around the patient
• X-rays pass through tissues differently depending on the tissue density
• X-rays are taken from numerous angles around the patient
• data is reconstructed into an image that appears as “slices” through the patient

Question 12

What is meant by a “profile” in CT?

How many profiles are taken and how thick is each “slice”?

Answer 12

• one “slice” is acquired per rotation
• slice thickness can be modified, but is typically 1mm
  
  • thinner slices can be used if a higher resolution image is needed
• during each rotation, detectors acquire many snapshots (“profiles”) of the attenuated X-ray beam
• the X-ray beam is attenuated as it passes through tissues of different densities within the patient
• typically, 1,000 profiles are taken per rotation
• each profile is then reconstructed into a 2D image of that slice

Question 13

How is radiation exposure measured?

What is UK background radiation and how does this compare to the dose from a CT head scan?

Answer 13

• the dose delivered to a patient from ionising radiation is described in milliseiverts (mSv)
• UK background radiation is 2.7mSV per year
• dose from a CT head is around 1.4mSV - equivalent to 6 months background radiation

Question 14

How do structures on CT appear?

What unit is used to measure density?

Answer 14

• structures on CT appear as differing shades on a greyscale
• bright structures are described as “hyperdense”
• dark structures are described as “hypodense”
• the Hounsfield unit is used to measure the density of tissues on CT

Question 15

What is the HU for bone, water and air?

Answer 15

• air has a value of -1000 HU
• water has a value of 0 HU
• bone has a value of +1000 HU

Question 16

What pathology is shown here?

Answer 16

subdural haemorrhage

• blood collects between the dura and the arachnoid
• the haematoma conforms to the brain surface and can cross suture lines, producing a typical “crescent” shape
• this is an example of a chronic subdural haematoma as the blood appears dark

Question 17

How can you differentiate an acute from a chronic subdural haemorrhage?

Answer 17

• the blood appears darker in a chronic subdural haemorrhage
• the blood appears bright in an acute subdural haemorrhage (“hyperdense”)

Question 18

Who tends to be affected by a subdural haemorrhage?

Why do these tend to occur?

Answer 18

• they commonly occur in the elderly and chronic alcoholics
• there is cerebral atrophy, meaning that a head injury leads to tearing of the dural bridging veins
• these can be chronic with subtle symptoms and signs

Question 19

What is the treatment for subdural haemorrhage?

What can happen in a large haemorrhage if it is not treated?

Answer 19

• sometimes there is no treatment and sometimes it needs draining
  
  • this depends on the size, acuity and age of the patient
• the bleed can compress the brain, causing herniation as a result of increased intracranial pressure
  
  • the cerebellar tonsils are the first structure to herniate through the foramen magnum
• the basal cisterns in the posterior fossa may become compressed, which results in hydrocephalus due to disruption to CSF drainage

Question 20

How could the mass effect be measured and described in this subdural haemorrhage?

Answer 20

moderate degree of mass effect

• the sulci and gyri are lost on the affected side as they are being compressed by the bleed
• the lateral ventricle is also being compressed
• there is midline shift - a line is drawn along the superior sagittal sinus (measured in mm)

Question 21

What type of injury is shown here?

Answer 21

extradural haemorrhage

• the blood appears bright as this is an acute injury
• blood collects between the skull and the dura and rarely crosses suture lines - produces a convex “lemon” appearance on CT

Question 22

What causes an extradural haemorrhage?

How does this correlate with the appearance on CT and what else should be looked for?

Answer 22

• this is an acute injury that is caused by head trauma - therefore blood appears bright
• skull fracture may be seen on CT
• it is commonly caused by a blow to the temporal region, which tears the middle meningeal artery (+/- vein) and results in rapid arterial bleeding

Question 23

What is the classic history associated with extradural haemorrhage?

Why is rapid surgical decompression needed?

Answer 23

• the classic history involves a head injury, followed by a lucid interval and then rapid neurological deterioration
• there is a rapid rise in ICP
• death results from tentorial herniation unless rapid surgical decompression is performed

Question 24

What pathology does this scan demonstrate?

What is the most common cause of this pathology?

Answer 24

subarachnoid haemorrhage

• it has a typical “starfish” appearance on CT scan
• the bleed is between the arachnoid and pia, so blood can get in between the sulci and gyri
• the most common cause is a ruptured aneurysm

Question 25

What is the classical history for SAH?

What are the 2 different types?

Answer 25

• the typical history is of a sudden onset, severe, worst ever headache - this is typically occipital and described as “thunderclap”
• SAH can occur due to trauma or it can be spontaneous
• it is commonly caused by rupture of a cerebral artery aneurysm or bleeding from an AV malformation that the individual is not aware of

Question 26

What are some of the associated symptoms of SAH?

Answer 26

• severe “thunderclap” headache that is sudden in onset
• blood between the arachnoid and pia irritates the meninges and causes signs of meningism - e.g. neck stiffness
• free blood irritates surrounding blood vessels and can induce vasospasm
• bleeds can be catastrophic and cause immediate LOC that the patient never recovers from

Question 27

What does the CT and MRI of someone with SAH look like?

Does a normal CT scan exclude SAH?

Answer 27

• CT shows bright blood in the fissures +/- ventricles
• there will be a flair signal on MRI
• a normal CT does not exclude SAH as a small volume of subarachnoid blood would not be visible
• a lumbar puncture is required to exclude SAH

Question 28

Why should CT brain scan be performed as soon as possible when SAH is suspected?

What happens if the CT comes back as normal?

Answer 28

• in suspected SAH, CT needs to be performed as soon as possible to maximise the sensitivity
• the blood of the SAH becomes less dense as time goes on and becomes more difficult to see
• a normal CT scan is insufficient to rule out SAH and lumbar puncture is required in patients with normal CT scans
• a diagnosis of primary thunderclap headache can only be made once normal brain imaging and CSF have been demonstrated

Question 29

What pathology is demonstrated here?

Answer 29

intracerebral haemorrhage

• as this is an acute event, the blood appears bright on the initial CT scan (hyperdense)
• blood collects in the brain parenchyma and is often catastrophic

Question 30

What is the most common cause of intracerebral haemorrhage?

Answer 30

• it usually occurs secondary to an acute hypertensive episode, which results in rupture of arteries/veins due to increased pressure
• this leads to bleeding into the brain tissue, which causes stroke
• the thalami and pons are most frequently affected

Question 31

How do you decide whether a patient should have a CT scan after a head injury?

Answer 31

• NICE have clear guidance to help clinicians decide whether a patient should have a CT scan after a head injury and how quickly it should be performed
• if you have concerns about a patient even though they do not meet these criteria, it should be discussed with a senior colleague and/or the duty radiologist

Question 32

What does the NICE criteria state about which patients should have a CT scan within 1 hour of the risk factor being identified?

Answer 32

• GCS < 13 on initial assessment
• GCS < 15 at 2 hours after the injury on assessment in the emergency department
• suspected open or depressed skull fracture
• any sign of a basal skull fracture

1. haemotympanum (blood in tympanic cavity of middle ear)
2. “panda eyes”
3. CSF leakage from the ear or nose
4. Battle’s sign (haematoma over the mastoid process)

• post-traumatic seizure
• focal neurological deficit
• more than 1 episode of vomiting

Question 33

What does the NICE criteria state about who else should receive a CT head and when it should be performed?

Answer 33

• patients who have sustained a head injury with no other indications for CT head but are on anticoagulants
• they should have a CT head within 8 hours of injury

Question 34

In what one situation is MRI scanning NOT superior to CT scanning?

Why is MRI not performed in the acute setting?

Answer 34

• CT is superior at visualising acute haemorrhage
• CT is quicker and still gives excellent visualisation of fractures, acute bleeds and masses
• CT requires less patient cooperation and scan time is quicker

Question 35

When is MRI scanning of the brain performed?

Answer 35

• it is used after suspected stroke when CT is unremarkable / equivocal
• it is superior to CT in demonstrating lesions in multiple sclerosis
• it is superior to CT for imaging the posterior cranial fossa
• it is able to assess meningeal disease (post-contrast) whereas CT cannot

Question 36

What is the purpose of contrast agents?

Answer 36

• they can improve the diagnostic value of CT and MRI scans

Question 37

When is a non-contrast CT of the brain performed?

Answer 37

• to detect acute intracranial bleeding - blood is dense (bright) and so is difficult to distinguish from contrast agent
• in the case of suspected cerebral infarction (ischaemic stroke)

Question 38

What type of contrast is used in CT scans of the brain?

When might these scans be performed?

Answer 38

• uses iodine-based contrast (hyperdense / bright)
• CT without contrast is performed first in cases of suspected intracranial tumours
• if there is a clinical suspicion of a space-occupying lesion / suspicious area on non-contrast CT then a CT with contrast is performed
• most tumours enhance with contrast and are usually surrounded by an area of low density (vasogenic) oedema

Question 39

When is contrast used in an MRI scan and how does it appear?

What is this used to look for?

Answer 39

• MRI is used with a gadolinium contrast agent to image brain tumours
• most tumours disrupt the blood-brain barrier, which allows the contrast to be taken up by the tumour
• gadolinium appears bright on a T1 scan
• usually an image is taken before and after contrast to look for post-contrast enhancement

Question 41

What type of scan is this and how do you know?

Is there an abnormality? What could confirm this?

Answer 41

• this is a T1-weighted MRI scan as the CSF is dark and the vessels / sinuses are not completely black
• there is an area of asymmetry seen around the insula and sylvian fissure
• when contrast is added, the lesion enhances

Question 42

What is shown in this image?

What do you expect to see when contrast is added and what treatment might be given?

Answer 42

• oedema appears black
• this is vasogenic oedema that occurs in aggressive tumours due to leaky blood vessels around the tumour
• it is often the oedema that causes the symptoms of raised intracranial pressure, rather the tumour itself
• steroids are given to relieve these symptoms by reducing the amount of oedema
• when contrast is added, the tumour is ring-enhancing, but the oedema does not enhance (remains black)