Essentials of CT brain (For Undergraduates)

ESSENTIALS OF CT BRAIN

(For Undergraduates)

Dr. Prithwiraj MaitiMBBS

House Physician, Department of Internal MedicineR.G.Kar Medical College

Admin and Founder, Pgblaster IndiaAuthor of: “A Practical Handbook of Pathology Specimens and Slides” and “An

Ultimate Guide to Community Medicine”; published by Jaypee Brothers, India

Topics to be discussed1. Introduction2. Principle of CT scan3. Mechanism of CT scan4. Concept of Hounsfield unit (HU)5. Different Window levels in head CT6. Advantages and limitations of CT scan7. Anatomical structures seen in a normal brain CT8. CT scan in head trauma9. CT scan in cerebrovascular accident (CVA)10. CT scan in CNS infections11. CT scan in diagnosing brain tumors12. CT scan in sinus pathology13. CT scan in evaluation of headache

Introduction• In 1970, Sir Jeffrey Hounsfield combined a mathematical

reconstruction formula with a rotating apparatus that could both produce and detect X-Rays, producing a prototype for the modern-day CT scanner. For this work he received both a Nobel Prize and a knighthood.• The cranial computed tomography (CT) has assumed a critical role in

the practice of emergency medicine for the evaluation of neurological emergencies, both traumatic and atraumatic.

Principle of CT scan• The most fundamental principle behind radiography of any kind is the

following statement: X-rays are absorbed to different degrees by different tissues. • Dense tissues, such as bone, absorb the most x-rays, and hence allow

the fewest through the body part being studied to the film or detector opposite. • Conversely, tissues with low density (air/fat), absorb almost none of

the x-rays, allowing most to pass through to a film or detector opposite.

Mechanism of CT scan• CT scan machines have an X-Ray source and detector, situated 180◦ across

from each other, move 360◦ around the patient, continuously sending and detecting information on the attenuation of X-Rays as they pass through the body. • Very thin x-ray beams are utilized, which minimizes the degree of scatter or

blurring. • Finally, a computer manipulates and integrates the acquired data and

assigns numerical values based on the subtle differences in X-Ray attenuation. • Based on these values, a grey-scale axial image is generated that can

distinguish between objects with even small differences in density.

Concept of Hounsfield unit (HU)

Different Window levels in head CT• Brain Window: Shows subarachnoid hemorrhage• Subdural window: Shows subdural hemorrhage • Bone window: Shows bullet fragment and fracture.

Advantages and limitations of CT scan (Compared to MRI)

ADVANTAGES LIMITATIONS1. Shorter imaging time2. Lower cost3. Superior in detecting the presence of

calcification and bony abnormalities4. Good definition of extra-axial bone

tumors (Ex.: Acoustic neuroma, Meningioma etc.).

1. Poor definition of the extent of edema (so lower rate of tumor detection)

2. Poor neuroanatomical definition compared to MRI

3. Imaging of posterior fossa lesion is limited due to bony artifacts

4. Risk of exposure to radiation.

Anatomical structures seen in a normal brain CTAt the level of Orbit

A= ORBIT B= SPHENOID SINUS C= TEMPORAL LOBE D= EXTERNAL AUDITORY CANALE= MASTOID AIR CELLS F= CEREBELLAR HEMISPHERES

Anatomical structures seen in a normal brain CTAt the level of Sella Turcica

A= Frontal LobeB= Frontal Bone C= Dorsum Sellae D= Basilar Artery E= Temporal Lobe F= Mastoid Air Cells G= Cerebellar Hemisphere

Anatomical structures seen in a normal brain CTAt the level of Midbrain

A= FRONTAL LOBE B= SYLVIAN FISSURE C= TEMPORAL LOBED= SUPRASELLAR CISTERN E= MIDBRAIN F= FOURTH VENTRICLE G= CEREBELLAR HEMISPHERE

Anatomical structures seen in a normal brain CTAt the level of Cerebellum

A= FALX CEREBRI B= FRONTAL LOBE C= ANTERIOR HORN LAT VENTRICLED= THIRD VENTRICLE E= QUADRIGEMINAL PLATE CISTERN F= CEREBELLUM

Anatomical structures seen in a normal brain CTAt the level of Basal ganglia

A= ANTERIOR HORN LAT VENTRICLE B= CAUDATE NUCLEUS C= ANT. LIMB OF INTERNAL CAPSULED= GLOBUS PALLIDUS AND PUTAMEN E= POST. LIMB OF INTERNAL CAPSULE F= THIRD VENTRICLEG= QUADRIGEMINAL PLATE CISTERN H= CEREBELLAR VERMIS I= OCCIPITAL LOBE

Anatomical structures seen in a normal brain CTAt the level of Thalamus

A= GENU OF CORPUS CALLOSUM B= ANT HORN OF LATERAL VENTRICLE C= INT CAPSULED= THALAMUS E= PINEAL GLAND F= CHOROID PLEXUS G= STARAIGHT SINUS

Anatomical structures seen in a normal brain CTAt the level of Lateral ventricle

A= FALX CEREBRI B= FRONTAL LOBE C= BODY OF LATERAL VENTRICLED= SPLENIUM OF CORPUS CALLOSUM E= PARIETAL LOBE F= OCCIPITAL LOBEG= SUPERIOR SAGITTAL SINUS

Anatomical structures seen in a normal brain CTAt the uppermost level

A= FALX CEREBRI B= SULCUS C= GYRUS D= SUPERIOR SAGGITAL SINUS

CT scan in head trauma• Imaging modality of choice in head trauma is non contrast CT scan.• Important CT findings of head trauma:1. Extradural hemorrhage (EDH)2. Subdural hemorrhage (SDH)3. Subarachnoid hemorrhage (SAH)4. Orbital blow out fractures5. Isolated bone fractures.

Extradural hemorrhage• Location: Between the inner surface of skull and outer layer of dura

(periosteum)• As a result, EDHs are usually limited in their extent by the cranial

sutures [unlike SDH, which are not limited by sutures]• Features in CT: Biconvex in shape, hyperdense, sharply demarcated• Features of mass effect (i.e. midline shift and uncal herniation) may be

present.

Subdural hemorrhage• Location: Between the dura and arachnoid mater• In contrast to EDH, SDH is not limited by sutures but are limited by

dural reflections (falx cerebri, tentorium and falx cerebelli)• Appearance of SDH depends upon the time since its formation:Acute SDH (<3 days): Crescentic, hyperdenseSubacute SDH (3-14 days): Nearly isodense, for detection often CECT

is neededChronic SDH (>14 days): Hypodense with enhancing membrane.

ACUTE SUBACUTE CHRONIC

Subarachnoid hemorrhage (SAH)• Location: Subarachnoid space• The most common cause of subarachnoid hemorrhage is trauma• The most common cause of non traumatic subarachnoid hemorrhage

is a ruptured aneurysm• CT finding includes hyperdense material (instead of hypodense CSF)

within the subarachnoid space.

SAH NORMAL CT

Orbital blow out fractures• Typically occurs when there is a fracture of one of the walls of orbit

but the orbital rim remains intact• Inferior blow-out fractures are the most common.

CT in cerebrovascular accident (CVA)

Ischemic stroke1. Immediate phase (<1 hour): The earliest CT sign visible is a

hyperdense segment of a vessel, representing direct visualisation of the intravascular thrombus / embolus and as such is visible immediately

2. Early phase (1-3 hours): Loss of grey-white matter differentiation and Hypo-attenuation of deep nuclei

3. First week: Hypo-attenuation and swelling become more marked resulting in a significant mass effect

4. 2nd-3rd week: Swelling starts to subside, at this time cortex appears near normal.

Day 0: Long segment of hyperdense right MCA artery in keeping with thromboembolism. The grey white matter differentiation of the insular cortex is lost.

Day 3: Large patchy MCA and ACA territory infarct with marked midline shift.

• There are two types of hemorrhagic strokes: those caused by an intracerebral hemorrhage (ICH) and those caused by a subarachnoid hemorrhage (SAH)• Common locations: Basal ganglia and thalamus, pons and cerebellar

hemisphere• Acute blood is markedly hyperdense compared to brain parenchyma,

and as such usually poses little difficulty in diagnosis.

CT in cerebrovascular accident (CVA)Hemorrhagic stroke

BASAL GANGLIA BLEED

CT scan in CNS infectionsTopics to be discussed:1. Tubercular meningitis2. Herpes simplex encephalitis3. Neurocysticercosis4. Brain abscess

Tubercular meningitis (TBM)It is to be noted that NCCT may be absolutely normal.Following are the hallmarks of TBM:1. Dural thickening and calcification around basal cisterns2. Enhancing basal exudates involving basal cisterns3. Cerebral atrophy4. Infarcts resulting from arteritis5. Non communicating hydrocephalus6. Ring enhancing lesions -> Tuberculoma.

Tuberculoma with hydrocephalus

Herpes simplex encephalitis (HSE)• Predominant involvement of limbic system (temporal lobe, subfrontal

region, cingulate gyri)• CT findings:Very early stage: May be normalInitial stage: Low density lesions in temporal lobeLate stage: Hemorrhage is highly suggestive of HSE.

NeurocysticercosisStage of the disease CT finding

Vesicular stage Non-enhancing cystColloidal stage Cyst wall enhancement in form of ring

Granular nodular stage Isodense cyst with calcified scolexCalcified stage Small calcified nodule

Brain abscess• Ring of iso/hyperdense tissue, typically of uniform thickness• Central low attenuation (fluid/pus)• Surrounding low density (due to vasogenic oedema)• Obstructive hydrocephalus common in case of intraventricular spread.

CT scan in diagnosing brain tumorsAlthough the most effective and common tool for diagnosing a brain tumor is the use of a MRI scan, CT scan serves as an effective screening tool for supra-tentorial abnormalities.

CT scan in sinus pathology• Physicians who are interested in treating patients with sinus disease

must be able to read and interpret sinus CT scans• Here, normal sinus radiology and its infectious changes are shown:

Normal sinus CT:+ - Border of maxillary sinus* - Maxillary sinus ostiumU - Uncinate processE - Ethmoid sinusesIT- Inferior turbinate MT- Middle turbinateS – SeptumC - Concha bullosa

CT scan showing bilateral ethmoid and maxillary disease, with complete obliteration of both osteomeatal complexes.

CT scan in evaluation of headacheOne should consider neuroimaging in the following situations:

1. Recent significant change in the pattern/ frequency/ severity of headache

2. Progressive worsening of headache despite appropriate therapy 3. Focal neurologic signs or symptoms 4. Onset of headache with exertion, cough, or sexual activity5. Orbital bruit6. Onset of headache after 40 years of age.

• A head CT scan (± Contrast) is likely to be sufficient in most patients. • An MRI along with MRA is indicated when posterior fossa or vascular

lesions are suspected.

Thank you.

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