Imaging the Brain Flashcards
1
Q
For what 3 reasons may imaging of the brain be performed?
A
- to confirm a clinical diagnosis / suspicion
- to rule out important diagnoses / pathologies
- to guide or evaluate management / treatment
2
Q
What is the first line imaging technique for acute imaging of the brain and why?
A
CT is 1st 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
3
Q
What are the benefits of using MRI over CT to image the brain (even though it is slower)?
A
- 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
4
Q
What are the following features of the CT image?
A
- green = head of caudate nucleus
- purple = lentiform nucleus
- yellow = lateral ventricle
- beige = internal capsule
- red = thalamus
5
Q
What are the following features of this MRI scan?
What type of scan is this and how do you know?
A
- 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
6
Q
When is CT superior to MRI for imaging the brain and why?
A
- 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
7
Q
What are the benefits of using MRI over CT?
A
- 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
8
Q
What are the main contraindications to MRI?
What is a drawback that applies to both CT and MRI?
A
- 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
9
Q
For what 3 reasons are CT scans of the head commonly requested in the ACUTE setting?
A
- 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
10
Q
Why is it important to use CT to differentiate between haemorrhagic and ischaemic strokes in the acute setting?
A
- 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
11
Q
What type of image is taken in a CT scanner and how can this be reconstructed?
A
- 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
12
Q
What is meant by a “profile” in CT?
How many profiles are taken and how thick is each “slice”?
A
- 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
13
Q
How is radiation exposure measured?
What is UK background radiation and how does this compare to the dose from a CT head scan?
A
- 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
14
Q
How do structures on CT appear?
What unit is used to measure density?
A
- 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
15
Q
What is the HU for bone, water and air?
A
- air has a value of -1000 HU
- water has a value of 0 HU
- bone has a value of +1000 HU
16
Q
What pathology is shown here?
A
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
17
Q
How can you differentiate an acute from a chronic subdural haemorrhage?
A
- the blood appears darker in a chronic subdural haemorrhage
- the blood appears bright in an acute subdural haemorrhage (“hyperdense”)
18
Q
Who tends to be affected by a subdural haemorrhage?
Why do these tend to occur?
A
- 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
19
Q
What is the treatment for subdural haemorrhage?
What can happen in a large haemorrhage if it is not treated?
A
- 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
20
Q
How could the mass effect be measured and described in this subdural haemorrhage?
A
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)
21
Q
What type of injury is shown here?
A
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
22
Q
What causes an extradural haemorrhage?
How does this correlate with the appearance on CT and what else should be looked for?
A
- 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
23
Q
What is the classic history associated with extradural haemorrhage?
Why is rapid surgical decompression needed?
A
- 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
24
Q
What pathology does this scan demonstrate?
What is the most common cause of this pathology?
A
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
25
What is the classical history for SAH?
What are the 2 different types?
* 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
26
What are some of the associated symptoms of SAH?
* **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
27
What does the CT and MRI of someone with SAH look like?
Does a normal CT scan exclude SAH?
* 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
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?
* 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
29
What pathology is demonstrated here?
**_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**
30
What is the most common cause of intracerebral haemorrhage?
* 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
31
How do you decide whether a patient should have a CT scan after a head injury?
* **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
32
What does the NICE criteria state about which patients should have a CT scan within 1 hour of the risk factor being identified?
* **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**
33
What does the NICE criteria state about who else should receive a CT head and when it should be performed?
* 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
34
In what one situation is MRI scanning NOT superior to CT scanning?
Why is MRI not performed in the acute setting?
* 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
35
When is MRI scanning of the brain performed?
* 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
36
What is the purpose of contrast agents?
* they can **improve the diagnostic value** of CT and MRI scans
37
When is a non-contrast CT of the brain performed?
* 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)_**
38
What type of contrast is used in CT scans of the brain?
When might these scans be performed?
* 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**
39
When is contrast used in an MRI scan and how does it appear?
What is this used to look for?
* 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**
40
41
What type of scan is this and how do you know?
Is there an abnormality? What could confirm this?
* 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
42
What is shown in this image?
What do you expect to see when contrast is added and what treatment might be given?
* 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)
