1. Approach to CT Head On Call
Michael Loreto
PGY-2, Diagnostic Radiology

2. Outline CT basics Normal anatomy Search algorithms
Introduction to common call scenarios

3. 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)

4. 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

5. Tissue Characteristics
Hounsfield Units
Metallic foreign body
> +1000
Bone
+400  +1000
Calcification
> +150
Soft tissue
+10  +100
*Acute blood clot
+ 55  +75
**Gray matter
~ +40
White matter
~ +30
Water (eg. serous fluid, CSF)
0  +20
Fat
-60 -100
Air
-1000

6. 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)

7. 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

8. Types of Artefacts Motion
Occur with voluntary/involuntary patient movement
Streaking pattern
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)

9. Types of Artefacts
3. 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)

10. Motion Artefact

11. Metallic Artefact

12. 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

16. Calcifications Falx cerebri/dura Choroid plexus Pineal gland
Basal ganglia

21. 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.

23. Vascular Anatomy - Venous
Cavernous sinus
Ophthalmic veins
Dural venous sinuses:
Superior sagittal
Inferior sagittal
Straight
Torcula/confluence
Transverse
Sigmoid
Internal jugular veins

25. Types of CT Studies On Call
Unenhanced CT
CT with contrast
CT angiogram
CT venogram

26. Unenhanced CT – Common Indications
Hemorrhage
Ischemic stroke
Decreased LOC
Seizure
Headache

27. Enhanced CT – Common Indications
Assessment of intracranial mass lesion
Primary malignancy vs. mets
Abscess/infection
eg. meningitis, toxoplasmosis (HIV+)

28. CTA – Common Indications
Spontaneous SAH
Cerebral artery aneurysm
AVM
Ischemic stroke
Occlusive thrombus
Dissection

29. CTV – Common Indications
Dural venous sinus thrombosis

30. 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

32. 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

34. 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

36. 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)

37. 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)

39. Unenhanced CT – Subdural Window
ONE MORE LOOK FOR EXTRA-AXIAL BLOOD!!!

40. Unenhanced CT – Stroke Window
Gray-white differentiation:
Insular ribbon sign
Basal ganglia sign

42. Enhanced CT – Search Algorithm
Mass lesion:
Abnormal parenchymal enhancement
Abscess/infection:
Abnormal parenchymal/meningeal enhancement

43. 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

44. Search Algorithm – CTA/CTV
Dural venous sinuses
Post-contrast head  abnormal parenchymal enhancement

45. 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

46. 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)

47. 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!!!)

53. 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)

58. 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

60. Intracranial Hemorrhage
Intra-axial
Intra-parenchymal hemorrhage
Cerebral contusions
Diffuse axonal injury
Extra-axial
Epidural hematoma
Subdural hematoma
Subarachnoid hemorrhage
Intra-ventricular hemorrhage

61. 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

64. 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)

66. 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.

68. 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

69. 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

71. 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

72. 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%

73. SDH – CT Features Acute SDH Subacute SDH (1-2 weeks)
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.

76. 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

77. 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

78. 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

80. 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

81. 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)

82. 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”)

89. 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.

90. 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)

93. 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

94. 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

96. 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

97. 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

98. 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)

100. Abscesses Etiology: Radiological features:
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)

103. MAGIC DR – DDx for Ring-Enhancing Lesions
M – mets A – abscess G – GBM I – infarct C – contusion D – demyelination R – resolving hematoma

104. 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

105. Meningitis – Radiologic Features
Unenhanced CT  often NORMAL
Enhanced CT:
Meningeal enhancement
Meningeal thickening (TB, sarcoidosis)
Sulcal effacement (edema)

106. Mass Lesions Primary tumours: Secondary tumours (mets):
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

112. 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

113. THE END