Intro to CT of Brain Flashcards
common clinical indications for CT Brain
Traumatic Head Injury - to exclude haemorrhage/ skull
Suspected Stoke - distinguish ischaemia from haemorrhage, suspected SAH
Acute Change in Mental Status and Focal Neurological Deficits - to exclude brain tumour, infection (abscess), hydrocephalus
what is a focal neurological deficit
problem with never or spinal cord or brain function
affects a specific location (left side of face, right arm, tongue etc.)
speech, vision and hearing problems
what can indicate the area of the brain or nervous system affected
type, location, severity of the problem
what is not specific to a certain area of the brain
a non-focal problem - general loss of consciousness
what is hydrocephalus
what are some of the symptoms
build of cerebrospinal fluid within the ventricles - puts pressure on the brain which can damage it and cause a wide range of symptoms
headache, vomiting, blurred vision, difficulty concentrating
how is hydrocephalus treated
VP shunt
endoscopic third ventriculostomy
what is a VP shunt
a thin tube that is surgically implanted in the brain and drains the excess CSF into the peritoneal cavity
what is ETV
involves making a hole in the third ventricle allowing the trapped CSF to escape to the surface of the brain where it can be absorbed
Benefits of CT Brain - 5 points
painless, non-invasive, accurate
ability to demonstrate soft tissue and blood vessels
fast and simple - ideal for acute scenario
CT less sensitive to patient movement in comparison to MRI
CT can be performed even with presence of implanted medical device, unlike MRI
Risks of CT Brain - 3 points
radiation exposure
CT not recommended for pregnant women unless medically necessary (even though risk is minimal for CT head)
adverse reaction to contrast media
what are the limitations to a CT Brain - 3 points
not sensitive in detecting meningitis (inflammation of the meninges) - MRI = 1st choice
cannot rule out presence of SAH - Lumbar puncture definitive test but CT rules out other causes to allow LP to go ahead
not sensitive in early diagnosis of infarct - important for thrombylisis
what is the general preparation for a CT Brain - 10 points
all equipment checked for cleanliness
head holder should be attached to the end of the table
if patient on bed/trolley and cannot self transfer - table to appropriate height and PAT slide
PPE should be readily available
ID check
pregnancy status ascertained
justification of request
artefacts removed
if contrast used - allergy check ascertained
full explanation of procedure to obtain optimal images
describe helical CT head
patient positioning not critical because images cane reconstructed retrospectively to eliminate unwanted rotation
however accurate positioning is need as mA modulation is used so the patient must be positioned in the isocentre of the gantry bore for optimal mA modulation - reduce patient dose
what may poor positioning lead to
cut off relevant anatomy so the scan would have to be repeated - increased patient dose
increased time spent on post processing data
describe axial CT head
good technique for positioning is essential - no rotation or tilt of head essential for demonstrating any bilateral asymmetry due to pathological processes
post processing of images to correct poor patient positioning is impossible
acquired parallel to the floor of the anterior cranial fossa - angulation of the gantry
what does angulation reduce
radiation dose to the lens of the eye and the thyroid gland
Differences between axial and helical
ACQUISITION TIME - A = slightly slower, H = sight quicker
ARTEFACTS - A= less in some cases < 16 slice scanners, H = increased artefact
PATIENT DOSE - A = ability to angle gantry and use of mA modulation, H = ability to angle gantry and use of mA modulation
what are the 2 main artefacts that appear in a CT brain
partial volume effect
beam hardening artefact
what is the partial volume effect
a HU is assigned to each voxel and represents the average attenuation at a particular point within a patient
this occurs when a voxel contains two tissues of very different densities
the voxel will represent an average of densities of the two tissues but represent neither accurately
where in a CT brain is the partial volume effect most prevalent
in the posterior cranial fossa due to the presence of both brain tissue and the dense bone of the petrous ridge
how can the partial volume effect be reduced
smaller slice thickness so only one particular anatomy lies within a voxel so no volume averaging is taking place
a narrower beam produces a smaller number of detected photons therefore mAs must increase as slice thickness decreases otherwise image noise will increase
h/e also increases patient dose
why is this significant in axial acquisition
slice thickness can be increased part way through the scan
why can this not be altered in helical acquisition
data is acquired as a volume so the slice thickness cannot be altered part way through so a thicker slice thickness has to be selected for all images
what is the beam hardening artefact
x-ray beam contains varying photon energies, as beam passes through dense bones of petrous ridges the lower energy photons are absorbed resulting in high photon energy - ‘hard’ beam therefore black streaks appear across the image between two dense objects
appear because the portion of the beam that passes through one of the objects is hardened less than the beam that passes through both objects at other tube positions
how is beam hardening artefact reduced?
by beam filtration
Teflon or Aluminium filters reduce the range of photon energies present in the beam before it passes through the patient by removing the lower energy photons and creating a more uniform beam
reduces patient dose
beam hardening artefact software is also available
what is CT angiography and perfusion imaging
non invasive assessment of major intracranial vessels and brain parenchyma
useful in identifying aneurysms, vessel occlusion and stenosis and stroke
what is CT venography
used to assess the dural sinuses within the brain
used in the diagnosis of venous sinus thrombosis - occurs when blood clot forms in the venous sinus’ which prevents blood from draining out the brain - v serious condition that can lead to infarct or haemorrhage and even death
