Introduction to CT Brain: The Basic Principles

DR. WAN NAJWA ZAINI WAN MOHAMED
RADIOLOGIST AND HEAD,
JPD, HQE II
30-31 JULY 2015 HQE II BASIC CT BRAIN INTERPRETATION

 HISTORY
 BASIC PRINCIPLES
 INDICATIONS, PREPARATIONS
 ARTIFACTS

 CT BRAIN is an extremely useful diagnostic
tool used routinely in hospital care.
 Because many disease processes are time
dependent and require immediate action, a
quality physician needs to be able to accurately
interpret and act upon certain CT findings
without specialist (e.g., radiologist) assistance.
 It has been shown that even a brief educational
intervention can significantly improve the
physician’s ability to interpret cranial CT scans.

 Founder of X-ray
on 8th November,
1895
 German physicist,
Wilhelm Conrad
Röntgen
 First X-Ray taken
– wife’s hand

 Founder of CT in 1972
 Sir Godfrey Newbold
Hounsfield
 British Engineer in
EMI, LTD.
 “The Beatles’
greatest legacy”

 Sir Allan McLeod
Cormack
 Physics Professor in US
 Worked independently
to develop solutions to
mathematical problems
 Both shared the Nobel
Price in 1979

 Original scanners took approximately 6
minutes to perform a rotation (one slice) and 20
minutes to reconstruct.
 Current generation CT scans can complete a
full brain imaging in seconds.
 Despite many technological advances since
then, the principles remain the same.

generation configuration detector beam Min scan time
first Translate -rotate 1-2 Pencil thin 2.5min
second Translate -rotate 3-52 Narrow fan 10sec
Third
Rotate- rotate 256-1000 Wide fan 0.5sec
fourth Rotate- fixed 600-4800 Wide fan 1sec
fifth Electron beam 1284 Wide fan
electron beam
33ns

 CT - Computed Tomography
 Tomography – a 2-D radiographic image
representing a slice through a body part. All
anatomy not at the target level is blurred.
 CT scan – provides a 3D display of the
intracranial anatomy built up from a vertical
series of transverse axial tomograms.

Think like looking into a loaf of bread by cutting it into
thin slices and then viewing the slices individually.

 Each tomogram represents a horizontal slice
through the patient’s head.
 CT Scan combines X-Rays and detectors
coupled with a computer to create cross
sectional image of any part of the body.

 CT X-Ray beams moves around the patient in
a circular path.
 The transmitted X-rays are received and
absorbed by arrays of detectors across the
patient on the opposite side of the circle from
the X-Ray source.
 Images are then reconstructed from the X-ray
absorption data using mathemathical
processes.

 Gantry
1. X-Ray tube
2. Generators
3. Collimators & Filters
4. Detector arrays & DAS
 Patient couch
 Computer console

1. X-ray tube &
collimator
2. Detector assembly
3. Tube controller
4. High freq. generator
5. Onboard computer
6. Stationary computer

 Set scan parameters – kVp, mA,
scan time, etc
 Set scan mode – Digital
radiograph, axial or volume
 Houses reconstructor
 Review and archive images
 Post-processing

 Cross sectional layer of
the body is represented
as an image matrix.
 Each square of the
image matrix is called
pixel(picture element)
and it represents tiny
block of tissue called
voxel (volume element)

 The average linear
attenuation coefficient
(µ), between tube and
detectors
 Attenuation coefficient
reflects the degree to
which the X-ray intensity
is reduced by a material

Air - 1000
Fat -70
Pure water 0
CSF +8
White matter +30
Gray matter +45
Blood +70
Bone/calcification +1000

 Window width (W) : range of CT numbers
displayed in shades of gray, ranging from black
to white.
 CT numbers > window : white;
 CT number < window : black
 Window level (L) : describes the centre of the
scale.

 Allows the reader to focus on certain tissues
within a set of parameters. Most CT imaging
include windows that are optimized for brain,
blood and bone.
 BRAIN : W 155, L 40
 STROKE : W 30, L 30
 SUBDURAL : W 150, L 5
 BONE : W 3000, L 570

 X-RAYS ARE ABSORBED TO DIFFERENT
DEGREES BY DIFFERENT TISSUES
 Always describe CT findings as densities –
isodense/ hypodense/ hyperdense.
 Higher the density = whiter is the appearance.
 Lower the density = darker the appearance.
 Brain is the reference density.

 Anything of the density as brain = isodense.
 Higher density than brain = hyperdense ( skull
is the best example).
 Anything darker (lower density) than brain =
hypodense (CSF and air are classical
examples).

 Acute head trauma
 Suspected acute intracranial hemorrhage
 Vascular occlusive disease (acute and chronic) or vasculitis
(including use of CT angiography and/or venography)
 Aneurysm evaluation
 Detection or evaluation of calcification
 Immediate postoperative evaluation following surgical
treatment of tumor, intracranial hemorrhage, or hemorrhagic
lesionS
 Treated or untreated vascular lesions
 Mental status change
 Increased intracranial pressure
 Headache

 Acute neurologic deficits
 Suspected intracranial infection
 Suspected hydrocephalus
 Certain congenital skull and brain lesions (such as, but not
limited to, craniosynostosis, macrocephaly, and
microcephaly)
 Evaluating psychiatric disorders
 Brain herniation
 Suspected mass or tumor
 CT guidance and image integration for neurosurgical,
neurointerventional, and other therapeutic procedures
 Certain skull lesions (such as, but not limited to, fibrous
dysplasia, Paget disease, histiocytosis, osteolytic
lesions,and skeletal tumors)

 When MR imaging is unavailable or contraindicated, or if the supervising
physician determines CT to be appropriate
 Diplopia
 Cranial nerve dysfunction
 Seizures
 Apnea
 Syncope
 Ataxia
 Suspicion of neurodegenerative disease
 Developmental delay
 Neuroendocrine dysfunction
 Drug toxicity
 Congenital morphologic brain abnormalities
 Abusive head trauma and postmortem forensic investigations
 Brain death
 Suspected shunt malfunctions or shunt revisions

 Informed consent.
 Remove all metallic accessories, eyeglasses,
jewelleries, dentures, hearing aid.
 Enquire about pregnancy, diabetes, renal
dysfunction, food allergies, asthma, cardiac
and other medical illness.

 Avooid food at least 4 hours prior to a contrast
study.
 Adequate hydration pre and post contrasted
scan.

 Sedation – children, uncoorperative patients.
 Avoid breasfeeding 24 hours after contrasted
study.
 Equipped to deal with anaphylactic reactions.
 ALARA concept.

 Digital projection
– AP, PA, Lat or Oblique projection
– Surview, Scanogram
 Conventional CT
– Axial
– Start/stop
 Volumetric CT
– Helical or spiral CT
– Continuous acquisition

 A scannogram/ topogram of the head is done
using digital projection method.
 For head scans, conventional axial or slice by
slice method is commonly used.
 For CTA studies, volumetric or helical/spiral CT
is used.

 Artifacts are distortions or errors in the image
that are unrelated to the object scanned.
 Most common artifacts in CT are:
 Motion artifacts
 Streak artifacts
 Beam hardening artifacts
 Partial voluming artifacts
 Ring artifacts

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