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
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