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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
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. PRINCIPLEâŚ..
⢠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 patientâs head.
14. BASICSâŚ.
⢠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
ďSulci will be deeper with age
ďBlood in sulcus is important
64. 8. Midline shiftâŚ
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
ď Bone pathology
ď Acute hemorrhagic conditions
MRI is better in
ď Posterior fossa pathology (Brainstem and cerebellum)
ď White matter disease
ď Better gray and white matter differentiation
ď 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