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How To Read A Head CT Scan
1. How To Read A Head CT
Scan
Ahsan Aziz
Assistant Registrar, NIMH
2. Scheme Of The Topic
• Basic Principles of CT scan
• Normal Neuroanatomy as seen on head CT
scan
• How to describe a CT head
• Some abnormal tomography
4. History
• Sir Godfrey Hounsfield- introduced CT scan in
1972
• Won Nobel prize in 1979
5. Parts
1) Gantry- which houses X ray apparatus
2) X ray tube-akin to that in a X ray machine.
3) Detectors
4) Patient couch
5) Viewing console
6.
7. Principle
• Uses X rays applied in sequence of slices
across the organ
• Images reconstructed from X ray absorption
data
• X ray beam moves around the patient in a
circular path
9. PRINCIPLE…..
• CT scan provides a 3D display of the
intracranial anatomy built up from a vertical
series of transverse axial tomograms.
• Each tomogram represents a horizontal slice
through the patient’s head.
14. BASICS….
• X-RAYS ARE ABSORBED TO DIFFERENT DEGREES BY
DIFFERENT TISSUES (rays pass freely through air but
not through bone)
• 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 ( Bone is the
best example)
• Anything darker (lower density) than brain=
hypodense (CSF and air are classical examples)
15. HOUNSFIELD UNITS
• CT scanners record the attenuation (brightness)
of each pixel in Hounsfield Units (HU)
• Related to composition & nature of tissue
• Represent the density of tissue
• Also called as CT NUMBER
• Marks between -1000 to +1000
• More negative more black, more positive more
white
16. Hounsfield Units of different tissues
Air --- 1000
(Darkest
Black)
Fat ---70
Pure water 0
CSF +8
White matter +30
Gray matter +45
Blood +70
Bone/calcification +1000 (White)
19. AXIAL SECTIONS OF CT HEAD
POSTERIOR FOSSA CUTS
-Above the foramen magnum level
-Level of fourth ventricle
-Above the fourth ventricle level
-Tentorial
SUPRATENTORIAL CUTS
-Third ventricular level
-Lateral ventricular level
-Above the corpus callosum
44. 4. Sulci
Identify
• Sulci
• Sylvian fissure
• Central sulcus
• Post central sulcus
• Precentral sulcus
Sulci will be deeper with age
Blood in sulcus is important
64. 8. Midline shift…
1. Select a slide where both lateral ventricles are
prominent.
2. Draw a line joining falx cerebri anteriorly and
posteriorly.
3. The septum between ventricles should not deviate
more than 5mm from midline.
69. Indication of Head CT scan..(Absolute)
• GCS less than 10 any time following head injury
• Skull fracture
• Blood or CSF coming out of nose, ear, or panda eye following
injury
• Sudden severe headache
• Headache with vomiting (Exclude migraine and tension type
headache)
• Focal neurological sign (paralysis, cranial nerve palsy etc.)
• First Seizure after 20 years of age or not responding to anti-
epileptic drugs
• First episode of psychosis
70. CT or MRI scan?
Depends on suspected pathology
CT is better in
Bone pathology
Acute hemorrhagic conditions
MRI is better in
Posterior fossa pathology (Brainstem and cerebellum)
White matter disease
Better gray and white matter differentiation
Orbital or aural cavity pathology
75. Further reading for those who are
interested
• Window level and width (to fix density of
image)
• Contrast CT scan
76. Reference
• Osborn Diagnostic Imaging (Brain), 1st edition
• Radiology in Medical Practice by Prof. A. B. M.
Abdullah
• Head and Neck Imaging, 4th edition
• Fundamentals of Diagnostic Radiology, 4th edition