The document provides a comprehensive overview of interpreting head CT scans, detailing the basic principles, normal neuroanatomy, and descriptions of typical findings. It covers techniques for identifying structures, evaluating densities, and recognizing abnormalities, alongside the historical context of CT technology. Additionally, it discusses the appropriate usage of CT versus MRI in various clinical scenarios.

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

3. BASIC PRINCIPLES OF
CT SCAN

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

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

8. PRINCIPLES OF CT………

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.

10. TECHNIQUE…..
Slice thickness may
vary, but in general, it is
between 5 and 10 mm
for a routine Head CT

11. CT Planes

12. Axial Plane Coronal Plane Sagittal Plane

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)

17. Densities on CT scan…….

18. NORMAL NEUROANATOMY AS
SEEN ON HEAD CT SCANS

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

20. Lateral View of Brain

21. NORMAL ANATOMY…….
A= ORBIT , B= SPHENOID SINUS , C= TEMPORAL LOBE, D=EXTERNAL AUDITORY CANAL
E= MASTOID AIR CELLS F= CEREBELLAR HEMISPHERES

22. NORMAL ANATOMY…….
A=Frontal Lobe, B= Frontal Bone (Superior Surface of Orbital Part), C= Dorsum Sellae,
D=Basilar Artery E= Temporal Lobe F= Mastoid Air Cells G=Cerebellar Hemisphere

23. NORMAL ANATOMY…….
A=FRONTAL LOBE B= SYLVIAN FISSURE C=TEMPORAL LOBE
D=SUPRASELLAR CISTERN E=MIDBRAIN F=FOURTH VENTRICLE
G= CEREBELLAR HEMISPHERE

24. NORMAL ANATOMY……..
A=FALX CEREBRI B=FRONTAL LOBE C=ANTERIOR HORN LAT VENTRICLE
D=THIRD VENTRICLE E=QUADRIGEMINAL PLATE CISTERN F=CEREBELLUM

25. NORMAL ANATOMY……..
A=ANTERIOR HORN LAT VENTRICLE B=CAUDATE NUCLEUS C=ANT LIMB INT CAPSULE
D=GLOBUS PALLIDUS AND PUTAMEN E=POST LIMB INT CAPSULE F=THIRD VENTRICLE
G=QUADRIGEMINAL PLATE CISTERN H=CEREBELLAR VERMIS I=OCCIPITAL LOBE

26. NORMAL ANATOMY……..
A=GENU OF CORPUS CALLOSUM B=ANT HORN OF LATERAL VENTRICLE C=INT
CAPSULE
D=THALAMUS E=PINEAL GLAND F=CHOROID PLEXUS G=STARAIGHT SINUS

27. NORMAL ANATOMY…….
A=FALX CEREBRI B=FRONTAL LOBE C=BODY OF LATERAL VENTRICLE
D=SPLENIUM OF CORPUS CALLOSUM E=PARIETAL LOBE F=OCCIPITAL LOBE
G=SUPERIOR SAGITTAL SINUS

28. NORMAL ANATOMY……..
A=FALX CEREBRI B=SULCUS C=GYRUS D=SUPERIOR SAGGITAL SINUS

29. Normal neuroanatomical Shapes
to Identify distortions

30. Orbits and Ethmoid air cells

31. Base of skull, Midline bony prominence,
Prepontine cistern, Pretrous bone, Frontal sinus

32. Star shape ~ Circle of Willis,
Fourth ventricle, Temporal horn ~ slit

33. Midbrain and interpeduncular Cistern (Heart shaped)

34. Third ventricle (Smiley face)

35. Third ventricle, basal ganglia

36. Body of lateral ventricle (Twin banana)

38. Reading a CT Head
Outside to Inside steps

39. 1. Skin and
Soft Tissue
Examine Normal
Contour of skin
• Hematoma
• Foreign body
• Surgical
conditions
• Skin and soft
tissue
pathology

40. 2. Bone
Examine
• Bony contour (fracture, bone tumor)
• Sutures
• Mastoid air cells

41. 3. Sinuses
Evaluate
• Superior sagittal sinus
• Frontal sinus
• Ethmoid sinus
• Sphenoid sinus
• Maxillary sinus

42. 3. Sinuses …..
• Look for
• Sinusitis
• Mucosal thickening
• Blood in sinus (Traumatic)
• Mucous retention cyst and polyp

43. 3. Sinuses …..

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

45. 4. Sulci…..

46. 5. Brain parenchyma
Identify lobes
Frontal, Parietal, Temporal, Occipital
Identify structures
Midbrain, Pons, Medulla, cerebellum
Identify deep structures
Corpus callosum, Caudate nucleus, Lentiform
nucleus, Thalamus, Internal capsule

47. 5. Brain parenchyma….
Look for
• Symmetry between both sides
• Gray and white matter differentiation
• Change in density

48. Midbrain and Lentiform Nucleus

49. 5. Brain Parenchyma

50. 5. Brain Parenchyma….
Epidural hemorrhage Lens Shaped

51. 5. Brain Parenchyma….
Subdural hemorrhage Crescent shape

52. 5. Brain Parenchyma….
Subarachnoid hemorrhage

53. 5. Brain Parenchyma….
Intraparenchymal hemorrhage

54. 6. Cisterns
Identify cisterns
Supraseller, Ambient, Prepontine, Cisterna
magna
Cisterns can be filled with blood in subarachnoid
hemorrhage

55. Supraseller and Basal Cisterns

56. 7. Ventricles
Identify ventricles
Evaluate for any change in
• Size
• Shape
• Symmetry
• Density
• Mass effect

57. 7. Ventricles
Lateral Ventricles Third ventricle

58. 7. Ventricles….
Cerebral Aqueduct Fourth Ventricle

59. 7. Ventricles ….
Hydrocephalus

60. 7. Ventricles…
Mass effect

61. 8. Midline shift
Identify midline structures
• Falx Cerebri and Fornix
• Pineal Gland
• Great vein of Galen

62. 8. Midline shift….
Falx cerebri Fornix

63. 8. Midline shift…
Pineal Gland
(Calcified)
Great vein of
Galen

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.

65. 8. Midline shift…
Prominent ventricles Draw a midline

66. 8. Midline Shifts…..
Right sided abscess causing
left sided midline shift

67. Recap
1. Skin and Soft tissue
2. Bony contour
3. Sinuses
4. Sulci
5. Brain parenchyma
6. Cisterns
7. Ventricles
8. Midline shift

68. Other concerns (Calcification)

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

71. Prefer CT scan in …
• Acute trauma or bony fracture
• Calcified lesions
• Suspected acute intracerebral hemorrhage
• Suspected subarachnoid hemorrhage

72. Prefer MRI scan in…
• Arteriovenous malformations
• Epilepsy and adult onset seizures
• Hemangiomas
• Hydrocephalus
• Infection / Inflammation
• Infratentorial tumors
• Lacunar infarcts
• Lyme disease

73. Prefer MRI in…
• Meningioma
• Multiple sclerosis
• Orbit and optical nerve disease
• Pituitary dysfunction
• Posterior fossa abnormality
• Space occupying lesion
• Stroke (use Diffusion weighted MRI)

74. Prefer MRI in…
• Syringomyelia
• Vascular abnormalities
• White matter disease

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