RADIOLOGICAL ANATOMY OF NORMAL CT BRAIN

DR. PIYUSH OJHADM RESIDENT

DEPARTMENT OF NEUROLOGYGOVT MEDICAL COLLEGE, KOTA

LOBES OF BRAIN

MEDIAL SURFACE OF THE BRAIN

CT History

SIR GODFREY N. HOUNSFIELD

• 1979 Nobel Laureate in Medicine

COMPUTED TOMOGRAPHY (CT)

• Also known as Computer Assisted Tomography (CAT)

• The term Tomography refers to a process for generating 2D image slices of an examined organ of three dimensions (3D).

• Based on differential absorption of X- ray by various tissues.

Structure of CT scan collimator

TECHNIQUE

• Patient is placed on the CT table in a supine position and the tube rotates around the patient in the gantry.

• To prevent unnecessary irradiation of the orbits, Head CTs are performed at an angle parallel to the base of the skull.

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

• Intravenous contrast is not routinely used, but may be useful for evaluation of tumors, cerebral infections.

• The degree of x-ray absorption by various tissues is expressed and displayed as shades of gray in the CT image.

• Darker shades correspond to lesser attenuation.

• The shades of gray correspond to a number on an arbitrary linear scale, expressed as Hounsefield Units(HU). (-1000 to +3000)

• WINDOWING- changing the settings to preferentially display tissues of interest in image. Eg. Bone window in cases of craniofacial trauma.

Pure water has an HU value of ‘0’.

DESCRIPTION Approx. HU DENSITY

Calcium > 1000 Hyperdense

Acute blood 60-80 Hyperdense

Grey matter 38 (32-42) Hyperdense(light grey)

White matter 30 (22-32) Hyperdense(dark grey)

CSF 0-10 ISODENSE

Fat -50 to - 80 Hypodense

Air - 1000 Hypodense

Low density High density

CSF Bone

Fluid (Edema) Calcification

Air Blood

Fat Contrast

Metallic Foreign Bodies

CT ARTIFACTS

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 volume averaging artifacts

• Ring artifacts

MOTION ARTIFACT

STREAK ARTIFACTSCause: Presence and movements of objects of very high density(contrast media, metallic implants,surgicalclips)

Appearance: Streaks

REMEDY:-

•Remove the offending object if possible.

RING ARTIFACT DUE TO DEFECT IN DETECTOR SYSTEM OF THE MACHINE

CT Scan

• Advantages –– Easy availabilty– Fast– Better for bone and acute blood,lesions of skull

base and calvarium– Calcification– Less limited by patient factors

• Disadvantages-– high radiation– poor visualisation of posterior fossa lesions

Normal CT

Systemic Approach to Head CT & MRI

Interpretation

• Symmetry – Compare left and right side of the cranium

• Midline – Look for midline shift

• Cross-sectional anatomy – Review anatomical landmark for each section.

– Brain tissue : gray matter, white matter , intracerebrallesions

– CSF space : ventricle (dilated or not)

– Skull and soft tissue : scalp swelling, fractures, sinuses, orbit

• Subdural windows : Look for blood collection adjacent to the skull

• Bone windows : Skull, orbit and sinuses, intracranial air

Normal CT of Brain

• Ventricles are normal sized,

the grey versus white

distinction is clear.

• Midline is straight.

• Sulci are symmetrical on

bothsides.

• Skull is intact with no

scalp edema.

1 day 1 year 2 years

AXIAL SECTIONS OF CT HEAD

POSTERIOR FOSSA CUTS• Above the Foramen Magnum Level• level of the Fourth Ventricle• Above the Fourth Ventricular Level• Tentorial Cuts

Supratentorial Sections :• third ventricular level• lateral ventricular level• above the level of Lateral Ventricle

A

B

CD

E

F

G

ABOVE THE LEVEL OF FORAMEN MAGNUM

A

B

CD

E

F

G

AT THE LEVEL OF FOURTH VENTRICLE

A

B

C

D

E

F

ABOVE THE LEVEL OF FOURTH VENTRICLE

A

BC

D

E

F

G

H

I

AT THE THIRD VENTRICULAR LEVEL

AB

C

D

E

F

G

AT THE LATERAL VENTRICULAR LEVEL

B

A

C

D

E

F

G

ABOVE THE VENTRICULAR LEVEL

A

B

C

D

ABOVE THE VENTRICULAR LEVEL

Physiologic Calcification