Approach to CT Head On Call

Michael LoretoPGY-2, Diagnostic Radiology

Outline CT basics

Normal anatomy

Search algorithms

Introduction to common call scenarios

Windowing and Grey Scale Different tissues attenuate x-rays to varying degrees The degree to which a tissue absorbs radiation within each voxel (linear

attenuation coefficient, u) is calculated and assigned a value related to the average attenuation of tissues within it = Hounsfield Unit (HU)

Each HU is assigned a grey scale value on the display monitor and presented as a square picture element (pixel) on the image

Modern CT scanners are able to differentiate in excess of 2000 HU, however, the human eye can only differentiate about 30 shades of grey

Contrast can be enhanced by assigning just a narrow interval of CT numbers to the entire grey scale on the display monitor = window technique

Range of CT numbers displayed on the whole grey scale = window width (W) and average value = window level (L)

Specific window settings can be chosen to optimize the evaluation of specific structures/tissues changes in window width alter contrast, and changes in window level select the structures in the image to be displayed on the gray scale (ie. from black to white)

Narrowing the window compresses the grey scale to enable better differentiation of tissues within the chosen window (allowing for differentiation of more subtle differences in attenuation); for example, if a window width of 80 is selected and the window level is centred at 30HU, then CT numbers above 70 will appear white and those below -10 will appear black. Conversely, if the window is widened to 1500 HU, then each detectable shade of grey would cover 50HU (1500/30) and soft tissue differentiation would be lost; however, bone/soft tissue interfaces would be apparent

Numerous presets exist on the imaging workstation with optimal window settings for evaluating various structures/tissues

Tissue CharacteristicsTissue Hounsfield Units

Metallic foreign body > +1000Bone +400 +1000

Calcification > +150Soft tissue +10 +100

*Acute blood clot + 55 +75**Gray matter ~ +40White matter ~ +30

Water (eg. serous fluid, CSF)

0 +20

Fat -60 -100Air -1000

Tissue Characteristics

*Acute hematoma is more dense than flowing blood, due to clot retraction and loss of water; with time blood appears isodense (subacute) and then hypodense (chronic) to the brain parenchyma, due to clot resorption.

**Grey and white matter differ only slightly in density due to differences in fatty myelin content (higher fatty myelin content in white matter)

Image Artefacts Artefact = visual impression in the image of a feature

that does not actually exist in the tissue being imaged

Important to recognize so as not to be confused with pathology

May occur as a result of: scanner malfunction, patient movement or the presence of extrinsic objects eg. a metallic foreign body

Types of Artefacts1. Motion

Occur with voluntary/involuntary patient movement Streaking pattern

2. Partial volume CT number reflects the average attenuation within the voxel and

thus, if a highly attenuating structure is present within the voxel, it will raise the average attenuation value

Contamination can occur especially with thicker slices and near bony prominences

Can be reduced by using thinner slices (eg. posterior fossa)

Types of Artefacts3. Metallic

Attenuation coefficient of metal is much greater than any structure w/in the body Radiation is completely attenuated by metal and information about adjacent

structures is lost Produces a characteristic star-shaped/scattered streak artefact eg. bullet fragments, aneurysm coils, dental work

4. Beam Hardening Results from an increase in the average energy of the x-ray beam as it passes

through a tissue Low energy radiation in x-ray beam is filtered out by high density structures such

as bone, leaving higher energy radiation which is less absorbed by soft tissues, thus reducing tissue differentiation

Characterized by linear bands of low attenuation connecting two areas of high density (eg. bone, posterior fossa)

Motion Artefact

Metallic Artefact

Normal Anatomy Checklist Midline structures

Falx cerebri, septum pellucidum, third ventricle, pineal gland, fourth ventricle

Ventricular system Lateral, third, fourth ventricles

Basal cisterns Suprasellar, interpeduncular, ambient, quadrigeminal, pre-pontine, CPA,

cisterna magna Sylvian fissure and insular ribbon Basal ganglia and deep white matter

Caudate, internal capsule, lentiform nucleus, external capsule, claustrum, extreme capsule

Cerebrum frontal, temporal, parietal, and occipital lobes Cerebellum Brainstem mid-brain, Pons, medulla

Calcifications Falx cerebri/dura

Choroid plexus

Pineal gland

Basal ganglia

Vascular Anatomy - Arterial Anterior circulation ICA system

ICA MCA M1, M2, M3 segments ACA A1, A2, A3 segments A. comm.

Posterior circulation Vertebro-basilar system Vertebral PICA Basilar AICA, SCA PCA P1, P2, P3 segments P. comm.

Vascular Anatomy - Venous Cavernous sinus

Ophthalmic veins

Dural venous sinuses: Superior sagittal Inferior sagittal Straight Torcula/confluence Transverse Sigmoid Internal jugular veins

Types of CT Studies On Call Unenhanced CT

CT with contrast

CT angiogram

CT venogram

Unenhanced CT – Common Indications

Hemorrhage Ischemic stroke Decreased LOC Seizure Headache

Enhanced CT – Common Indications

Assessment of intracranial mass lesion Primary malignancy vs. mets

Abscess/infection eg. meningitis, toxoplasmosis (HIV+)

CTA – Common Indications

Spontaneous SAH Cerebral artery aneurysm AVM

Ischemic stroke Occlusive thrombus Dissection

CTV – Common Indications

Dural venous sinus thrombosis

Unenhanced CT – Search Algorithm Scout free skull/C-spine radiograph

Gestalt

Soft tissue window W: 350, L: 40 Bone window W: 2000, L: 500 Brain window W: 80, L: 40 Subdural window W: 180, L: 80 Stroke window W: 30, L: 30

Unenhanced CT – Soft Tissue Window Extracranial soft tissues:

Laceration, foreign body, swelling/subgaleal hematoma *NB - can help to localize site of trauma to evaluate for

underlying coup and contra-coup injuries

Orbits: Globe Optic nerve EOMs Superior ophthalmic vein Orbital fat Hematoma

Unenhanced CT – Bone Window Paranasal sinuses

Frontal, ethmoid, maxillary, sphenoid opacification Subcutaneous/orbital emphysema/pneumocephalus

Mastoid air cells Opacification Hemotympanum Subcutaneous emphysema/pneumocephalus

Bones (fractures) Facial nasal bone, bony orbit, bony sinuses, mandible Skull base petrous temporal bone fractures (longitudinal vs. transverse) Calvarium linear vs. depressed Occipital condyles

Unenhanced CT – Brain Window Evaluating for:

Asymmetry/displacement Abnormal density

– Hyperdensity:– acute blood free + within vessels

» Extra-axial EDH, SDH, SAH, IVH» Intra-axial» Dense MCA sign clot w/in MCA (acute CVA)» Triangle/delta sign clot w/in confluence (dural venous sinus thrombosis)

– tumour– calcification– foreign body

– Hypodensity– edema/infarct– air (pneumocephalus)

Unenhanced CT – Brain Window Midline structures assess for midline shift

Falx cerebri, septum pellucidum, third ventricle, pineal gland, fourth ventricle

CSF spaces: Ventricles compression, hydrocephalus, blood Sulci effacement, blood Cisterns effacement, blood

Parenchyma Assess for blood both overlying the cerebral hemispheres (extra-

axial) and within the parenchyma (intra-axial)

Unenhanced CT – Subdural Window

ONE MORE LOOK FOR EXTRA-AXIAL BLOOD!!!

Unenhanced CT – Stroke Window

Gray-white differentiation: Insular ribbon sign Basal ganglia sign

Enhanced CT – Search Algorithm

Mass lesion: Abnormal parenchymal enhancement

Abscess/infection: Abnormal parenchymal/meningeal

enhancement

CTA/CTV – Search Algorithm Search ONE vessel at a time:

Right and left vertebral arteries– PICA– Basilar, AICA, SCA– PCA

Right and left internal carotid arteries– ACA, A. comm.– MCA

Search Algorithm – CTA/CTV Dural venous sinuses Post-contrast head abnormal

parenchymal enhancement

Search Algorithm – CTA/CTV Assess for:

Arteries– patency (stenosis/occlusion), dissection, aneurysm– normal variants

– eg. fetal origin of PCA, hypoplastic/absent arteries

Dural venous sinuses– patency

Skull Fractures Calvarial

Linear Depressed

Basal skull petrous temporal bone fractures (3 types): Longitudinal (70-90%) - # parallel to long axis of petrous apex Transverse - # perpendicular to long axis of petrous apex Mixed/complex

NOTE: increased significance if fracture is open or communicates with an adjacent sinus (increased risk of infection)

Skull Fractures – Radiological Features Look closely at the initial SCOUT image

Secondary signs/clues: Overlying soft tissue swelling Underlying brain abnormality blood, pneumocephalus

Common “fakeouts”: Suture lines + vascular grooves Vascular grooves often branch and both have common

locations (look for asymmetry!!!)

Acute Ischemic Stroke Unenhanced CT has low sensitivity – primarily done to rule out

hemorrhage/other causes of patient’s symptoms

Hyperdense MCA = acute intraluminal thrombus (corresponding loss of contrast opacification on CTA); seen in 25-50% of acute MCA occlusions.

Loss of gray-white differentiation: insular ribbon sign basal ganglia sign

Sulcal effacement (secondary to cytotoxic edema)

Global Cerebral Ischemia/Anoxic Brain Injury

Diffuse brain swelling/edema can result in: global loss of gray-white differentiation global sulcal/cisternal effacement pseudo-subarachnoid hemorrhage dense cerebellum

Intracranial Hemorrhage Intra-axial

Intra-parenchymal hemorrhage Cerebral contusions Diffuse axonal injury

Extra-axial Epidural hematoma Subdural hematoma Subarachnoid hemorrhage Intra-ventricular hemorrhage

Intra-parenchymal Hemorrhage 10-15% of CVAs Common Pathophysiology: Small intracerebral arteries often

damaged by chronic HTN rupture blood leaks directly into the brain parenchyma

Risk factors: HTN, underlying brain pathology (tumour, AVM), bleeding diatheses, anti-coagulation therapy, cocaine abuse

Clinical Presentation: Abrupt onset and rapid deterioration Radiologic features:

Hyperdense hemorrhage Surrounding edema Mass effect Common locations for hypertensive bleeds basal ganglia + PF

Cerebral Contusions Traumatic injury to cortical surface of brain Radiological features:

Location:– Often multiple, bilateral involving superficial cortex– Frontal and temporal lobes > parietal, occipital, post.

Fossa– Coup and contra-coup injuries

Unenhanced CT:– Focal/multiple areas of high density (hemorrhage) with

surrounding low density (edema)

Diffuse Axonal Injury (DAI) Shear injury – secondary to severe rotational acceleration and

deceleration forces on the brain

Unenhanced CT: Often normal (50-80%) Small hypodense foci due to traumatic edema Hyperdense petechial hemorrhages at the corticomedullary junction (20-

50%) 10-20% evolve to focal mass lesion (hemorrhage/edema) New lesions may become apparent on delayed scans

Note: T2 GRE MR sequences are the most sensitive and demonstrate hypointense foci at characteristic locations; microbleeds may only be visible on GRE.

Epidural Hematoma (EDH) Arise within the epidural space = potential space

between dura and inner table of skull

Commonly associated with overlying skull fracture with resultant laceration of the middle meningeal artery/vein

Early recognition/intervention imperative delay may result in expansion and cerebral herniation

EDH – Radiological Features Location:

66% temporoparietal (MMA injury) 29% frontal pole, parieto-occipital region Vertex epidural hematoma disruption of sagittal sinus

Unenhanced CT: Biconvex (lentiform) hyperdense collection with a sharply

demarcated border Hematoma does NOT cross suture lines, but may cross the

midline Associated calvarial fracture and mass effect

Subdural Hematoma (SDH) Arises between the inner layer of the dura mater and the

arachnoid mater

Bleeding results from torn bridging veins that cross the potential space between the cerebral cortex and dural venous sinuses

Rebleeding secondary to osmotic expansion or repeat trauma can lead to an “acute on chronic hemorrhage”

Common demographic elderly, alcholics; contributing factors include: large subdural spaces due to age related involution and/or atrophy, coagulopathy, repeated falls

SDH – Radiological Features Location:

blood seen layering over the cerebral convexity; often extends into the interhemispheric fissure, along the tentorium

crosses suture lines, but does NOT cross the midline

bilateral in 15-25%

SDH – CT Features Acute SDH

high density fluid collection layering along the cerebral convexity crescentic (concave inner margin/convex outer margin) associated mass effect (sulcal effacement, ventricular compression,

midline shift)

Subacute SDH (1-2 weeks)– “isodense” to grey matter

Chronic SDH (> 2 weeks)– “hypodense” to gray matter– “acute-on-chronic” hyperdense acute hemorrhage intermixed or

layering dependently within the chronic collection.

Subarachnoid Hemorrhage (SAH)

Etiology: Spontaneous ruptured aneurysm (72%),

AVM (10%), hypertensive hemorrhage Traumatic

Bleeding within the subarachnoid space may lead to obstruction of ventricular outflow of CSF

SAH – Radiological Features Aneurysms (85% anterior circulation); common locations:

ICA terminus, P.comm. junction, MCA bi/tri-furcation, A.comm, basilar tip

Unenhanced CT: Highly sensitive for acute SAH (Sn~98% w/in 12 hours, 93% w/in 24 hours) Location of SAH correlates directly with the location of the aneurysm

rupture in ~70%– eg. A.comm. aneurysm rupture blood in interhemispheric fissure

Most sensitive areas for identification of SAH:– interpeduncular cistern– posterior aspects of Sylvian fissures– occipital horns of lateral ventricles

Intra-ventricular Hemorrhage (IVH) Etiology:

Rupture of sub-ependymal veins Reflux from SAH Extension of parenchymal blood

Increased risk of hydrocephalus (interferes with CSF absorption at the arachnoid granulations)

Layers dependently in the occipital horns

AVMs Congenital abnormality consisting of abnormally dilated

tortuous arteries and veins, with closely packed abnormal pathological vessels which SHUNT blood b/t the two

Most common intracerebral vascular lesion

80% occur < age 40 (20% < age 20)

Clinical presentation headaches, seizure, acute intracranial hemorrhage (50%), progressive neurological deficits; 10% incidental

AVMs – Radiologic Features Location:

Supratentorial (90%) parietal > frontal > temporal > occipital Infratentorial (10%)

Unenhanced CT: Irregular lesion with large feeding arteries and draining veins Mixed density vessels, hemorrhage, calcification 10% not visualized

Enhanced CT/CTA: Dense serpiginous enhancement (tortuous dilated vessels)

Cerebral Artery Aneurysms Common locations:

Bifurcation points ICA terminus, MCA bi/trifurcation, A.comm, P.comm, basilar tip

Threshold for detection CTA highly sensitive for aneurysms > 2mm Giant cerebral aneurysms > 2.5cm diameter Key descriptors:

Location Shape Projection Dimensions dome to neck ratio (implications for treatment)

MIRROR aneurysms in 10% of cases!!! (beware “satisfaction of search”)

Dural Venous Sinus Thrombosis (DVST) Rare cause of stroke that should NOT be forgotten as a possible

etiology Risk factors:

Septic causes mastoiditis/sinusitis, facial cellulitis, meningitis, encephalitis, abscess/empyema

Aseptic causes– Hypercoagulable states pregnancy, OCP– Low-flow states CCF, shock

NOTE: In 1/3 of patients no etiology is found.

DVST – Radiologic Features Unenhanced CT

– Hyperdense material (thrombosed blood) within a dural venous sinus– Cord sign = hyperdense dural sinus– Triangle/delta sign = hyperdense thrombus at torcula/confluence

– Cerebral infarction NOT characteristic of an arterial territory

Enhanced CT/CTV– Filling defect(s) within the dural venous sinuses eg. empty

triangle/delta sign = filling defect w/in the straight/superior sagittal sinus, representing flow around a central non-enhanced clot

– Gyral enhancement peripheral to an infarct

Look for co-existing signs of infection/inflammation (RFs)

Raised ICP The skull defines a fixed volume increasing

the volume of its contents or brain swelling from any cause rapidly increases ICP (and decreases CPP!)

Causes of raised ICP include: Hemorrhage, abscess, meningoencephalitis,

primary/metastatic tumours, hydrocephalus, cerebral edema

Raised ICP – Radiological Features Sulcal and cisternal effacement Herniation of brain parenchyma types of cerebral herniation:

Subfalcine– supratentorial brain extends under the falx– look for deviation of falx/septum pellucidum from the midline

Transtentorial– downward or upward displacement of brain through tentorium at level of

incisura.– descending transtentorial herniation occurs more often than ascending

herniations and includes the subcategory of uncal herniation– the innermost part of the temporal lobe, the uncus, can herniate through the

tentorium, putting pressure on the brainstem, most notably the midbrain– look for asymmetry of the suprasellar cistern and ambient cistern effacement

Cerebellar tonsillar– cerebellar tonsils herniate downward through the foramen magnum

Hydrocephalus CSF is produced in the choroid plexus and absorbed into

the venous system via the arachnoid granulations

Hydrocephalus results from an excess of CSF, due to an imbalance in CSF production and absorption, resulting in increased intra-ventricular pressure

Classification: Communicating (non-obstructive) blockage of CSF

flow beyond the outlet of the 4th ventricle Non-communicating (obstructive) blockage of CSF

flow within the ventricular system, with dilatation proximal to the obstruction

Communicating Hydrocephalus Blockage of CSF flow over the cerebral convexities/absorption at the

arachnoid granulations secondary to:– SAH, meningeal mets, granulomatous meningitis

Rapid CSF production eg. choroid plexus papilloma

Radiological features: Symmetrical enlargement of the lateral, third and fourth

ventricles Normal/effaced cerebral sulci Dilatation of subarachnoid cisterns Periventricular low attenuation transependymal flow of CSF

Non-communicating Hydrocephalus Location of obstruction/causes:

Lateral ventricles ependymoma, meningioma Foramen of Monro third ventricular colloid cyst Aqueduct of Sylvius congenital aqueductal stenosis, IVH Fourth ventricle/foramen of Luschka and Magendie congenital,

tumour, extrinsic compression

Radiological features: Ventricular dilatation proximal to the level of obstruction Earliest indication may be dilatation of the temporal horns Progressive enlargement of the ventricular system which is

disproportionate to narrowed and effaced cortical sulci Periventricular low attenuation (transependymal CSF flow)

Abscesses Etiology:

Extension from adjacent sinonasal infection, mastoiditis, OM Generalized septicemia Penetrating trauma or surgery

Radiological features: Location supratentorial:infratentorial = 2:1; typically at the

corticomedullary junction in the frontal and temporal lobes NECT low density lesion with associated mass effect; +/- gas CECT “ring-enhancement”, with central necrosis and surrounding

edema (lesions <5mm enhance homogeneously) NB – Complication = ventriculitis (extension to ventricular system)

MAGIC DR – DDx for Ring-Enhancing Lesions

M – metsA – abscessG – GBMI – infarctC – contusion

D – demyelinationR – resolving

hematoma

Meningitis Inflammation of the meninges

Anatomic classification: Pachymeningitis inflammation of the dura Leptomeningitis inflammation of the arachnoid membran and

subarachnoid space (more common)

Meningoencephalitis involvement of meninges and parenchyma

Risk factures concurrent infections eg. sinusitis, mastoiditis, otitis media

Meningitis – Radiologic Features Unenhanced CT often NORMAL

Enhanced CT: Meningeal enhancement Meningeal thickening (TB, sarcoidosis) Sulcal effacement (edema)

Mass Lesions Primary tumours:

eg. astrocytoma, GBM, oligodendroglioma, meningioma

Secondary tumours (mets):– Most commonly supratentorial; located at the gray-white junction

Radiological Features: Variable appearances: hypo iso hyperdense May be seen due to associated edema, asymmetry/mass effect GIVE CONTRAST

Key Points ALWAYS follow your search algorithm

Utilize clinical information to help focus your search, but do NOT let it bias your assessment

Beware of satisfaction of search

THE END