Intro to CT of Brain

Question 1

common clinical indications for CT Brain

Answer 1

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

Question 2

what is a focal neurological deficit

Answer 2

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

Question 3

what can indicate the area of the brain or nervous system affected

Answer 3

type, location, severity of the problem

Question 4

what is not specific to a certain area of the brain

Answer 4

a non-focal problem - general loss of consciousness

Question 5

what is hydrocephalus

what are some of the symptoms

Answer 5

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

Question 6

how is hydrocephalus treated

Answer 6

VP shunt

endoscopic third ventriculostomy

Question 7

what is a VP shunt

Answer 7

a thin tube that is surgically implanted in the brain and drains the excess CSF into the peritoneal cavity

Question 8

what is ETV

Answer 8

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

Question 9

Benefits of CT Brain - 5 points

Answer 9

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

Question 10

Risks of CT Brain - 3 points

Answer 10

radiation exposure
CT not recommended for pregnant women unless medically necessary (even though risk is minimal for CT head)
adverse reaction to contrast media

Question 11

what are the limitations to a CT Brain - 3 points

Answer 11

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

Question 12

what is the general preparation for a CT Brain - 10 points

Answer 12

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

Question 13

describe helical CT head

Answer 13

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

Question 14

what may poor positioning lead to

Answer 14

cut off relevant anatomy so the scan would have to be repeated - increased patient dose
increased time spent on post processing data

Question 15

describe axial CT head

Answer 15

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

Question 16

what does angulation reduce

Answer 16

radiation dose to the lens of the eye and the thyroid gland

Question 17

Differences between axial and helical

Answer 17

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

Question 18

what are the 2 main artefacts that appear in a CT brain

Answer 18

partial volume effect

beam hardening artefact

Question 19

what is the partial volume effect

Answer 19

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

Question 20

where in a CT brain is the partial volume effect most prevalent

Answer 20

in the posterior cranial fossa due to the presence of both brain tissue and the dense bone of the petrous ridge

Question 21

how can the partial volume effect be reduced

Answer 21

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

Question 22

why is this significant in axial acquisition

Answer 22

slice thickness can be increased part way through the scan

Question 23

why can this not be altered in helical acquisition

Answer 23

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

Question 24

what is the beam hardening artefact

Answer 24

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

Question 25

how is beam hardening artefact reduced?

Answer 25

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

Question 26

what is CT angiography and perfusion imaging

Answer 26

non invasive assessment of major intracranial vessels and brain parenchyma
useful in identifying aneurysms, vessel occlusion and stenosis and stroke

Question 27

what is CT venography

Answer 27

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