S3: Introduction to Neuroradiology Flashcards
Why do we need to image the brain?
There are many disease entities that imaging may be needed to diagnose: Trauma, tumour, stroke, seizure and neurological dysfunction (memory, motor, psychiatric).
List some neurological symptoms that patients may present in an acute setting
Loss of consciousness, seizure, headache, focal neurological deficit, psychiatric (hallucination, personality change), slow neurological development, cranial nerve palsies.
Describe plain film imaging
- Offers good bone detail and is very good at showing us foreign bodies.
- However, provides little information about soft tissue/brain.
- We don’y use this type of scan much anymore, only for detecting foreign bodies, facial views, for shunts or skeletal survey.
- They are no longer used for head injuries (NICE guidelines say first line of imaging for head scan is CT scan). They are also not useful for inter-cranial issues like haemorrhage.
List some imaging before CT was discovered
Plain film, ventriculography (measuring air sitting in ventricles) and angiography (iodinated contrast injected into carotid).
Describe a CT scanner
A CT machine, in very simple terms consists of three main things. An aperture for the person to lay in, a tube to fire x-rays and a detector on the other side. This produces a tomogram which gets images of the whole body but in slices.
What are the different slices CT scans are there?
- Single slice axial CT was used initially.
- Then we had a single slice helical CT (spiral CT) which made scanning someone quicker as it provides a spiral volume of data.
- Now we have multislice CT scan which means increasing numbers of arrays of detectors. It acquires data faster with more detectors. They can scan in under a minute and resolution is good it fine x ray technology, with good special resolution and spiral scanning.
Describe the physics of MRI
- MRI works on the principle that spinning protons (hydrogen ones used) act like little magnets.
- First a strong magnetic field is applied and this causes the protons to align. The protons are still in a low energy state and a radiofrequency pulse is needed to increase the energy. Some protons and tissues are easier to excite then others e.g. Cortical bone is harder to stimulate.
- Then a radiofrequency pulse is emitted to the aligned protons and this causes them to “wobble” and flip.
- When the radiofrequency pulse is removed, the protons emit a particular radiofrequency signal (depending on how easy to excite so different tissues emit different signals) and return to their aligned state (resting) state.
Advantages of MRI
- Has good soft tissue definition.
- More sensitive than CT.
- Has better anatomical detail.
- Gives us more information, the features of the imaging allow us to distinguish between different disease states.
Limitations of MRI
- Long scan times - 30-90 minutes.
- Movement artefact - have to stay very still inside and MRI is so sensitive that a little movement will ruin the images.
- Problems with claustrophobia as the ball of the scanner is so small and it is limited for obese patients as well.
- Ferromagnetic metal equipment and implants (foreign bodies) = contraindications for MRI. For example, cardiac pacemakers, metal valves and metal foreign bodies are not allowed. Metal not only pulls but heats up and can cause bad burns (indicated in old tattoos containing metal).
Describe diffuser tension imaging (DTI)
- DTI is a magnetic resonance imaging technique that allows the measurement of the diffusion of water in tissue in order to produce neural tract images and structure of the brain.
- The technique works due to the Brownian motion of water molecules and the fact they diffuse down fibres.
- DTI collects multidirectional diffusion information and can be used to map white matter tracts (tractography). This can be useful in tumours or problems such as MS, epilepsy or cognitive issues.
Describe functional MRI (fMRI)
- fMRI image the oxygen levels in the brain and show us the metabolically active areas of the brain. They label the oxygen. This is because increased oxygen levels indicate active areas.
fMRI is used for brain mapping, mainly for research purposes but also in preoperative assessment.
Describe MR spectroscopy
- MR spectroscopy is a non-invasive chemical analysis of the brain. Patterns of change in the chemical spectrum (metabolite levels) will occur with certain diseases e.g. tumour, demyelination and infection.
MR spectroscopy acts as problem solving tool, it is not usually definitive or diagnostic on its own. - It is very important to appreciate that brain imaging is part of the examination of a patient AND examination must always be performed in the context of the patients clinical history. This is because we may misinterpret scans if we don’t know the history.
Describe how head trauma is treated
- According to NICE guidelines, first line of trauma imaging is CT scan. Then MRI.
- As well as look at the secondary problems, like increased intracranial pressure, oedema, ischaemia, hydrocephalus.
What may a truama CT show?
Haemorrhage, Brain injury such as contusion, ischaemia, diffuse axonal injury, odema, Ischaemia that could be systemic, or vascular injury like a dissection. May also see fractures.
Describe how stroke is diagnosed and treated
The objectives of imaging a stroke in the acute setting are:
- To confirm it is indeed a stroke.
- Is it a haemorrhagic stroke or ischaemic stroke?
- What is the extent of the infarct and is the patient suitable for thrombolysis.
- Post-thrombolysis.
