2. • By the end of this seminar, you will be able to:
– Differentiate between various types of scans
– Localise the important structures of brain on CT
scan and MRI
– Diagnose the pathologies like Stroke, Intra-axial
and Extra-axial hemorhhages.
2
4. CT SCAN
• 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.
• High density tissues such as bone absorb most X-rays
• Low density tissues (e.g. air and fat) absorb almost
none.
4
5. CT SCAN (contd.)
• A pixel (tissue contained within each image unit) within the CT
image absorb a certain proportion of X-Rays passing through
it, and this ability to block X-Rays is called as ATTENUATION.
• For Every body tissue, the amount of attenuation is relatively
constant and is k/as that TISSUE’S ATTENUATION COEFFICIENT
• Unit of measurement of attenuation coefficient is
HOUNSFIELD UNIT [HU] (-1000 HU=>AIR,
+300-500HU=> BONE)
• Higher attenuation =more density = more positive HU = more
bright/white tissue.
5
6. CT SCAN (contd.)
• The brighter the pixel the greater the ability of the
tissue to attenuate X-rays.
• Contrast within the image varies from white (high
attenuation) to black (low attenuation) with the type
of tissue within the voxel.
BLACK →→→→→→→→→→→→→→→→→→→→WHITE
-1000 HU →→→→→→→→→→→→→→→→→→ +1000 HU
AIR (-1000 HU)→→FAT →→CSF→→WHITE MATTER →→ GRAY MATTER
→→ ACUTE HEMORRHAGE →→BONE (+300-500 HU) →→ METAL (+1000
HU)
6
7. Pure water has an HU value of ‘0’.
DESCRIPTION Approx. HU DENSITY
Metal 1000 Hyperdense
Calcium 300-500 Hyperdense
Acute blood 60-80 Hyperdense
Grey matter 38 (32-42) Isodense
(light grey)
White matter 30 (22-32) Isodense
(dark grey)
CSF 0-10 Hypodense
Fat -50 to - 80 Hypodense
Air - 1000 Hypodense
7
8. Low density High density
CSF Bone
Fluid (Edema) Calcification
Air Blood
Fat Contrast
Metallic Foreign
Bodies
8
10. CT SCAN (contd.)
• Pathological processes: alterations in anatomy and
attenuation.
• Pathological processes TYPICALLY increase the
water content in tissues.
• Consequently, pathological processes decrease the
attenuation/brightness of soft tissues.
• Similarly, pathological processes increase
attenuation/brightness of fat.
10
11. CT SCAN (contd.)
• Blood in acute hemorrhage has higher
attenuation/brightness than surrounding soft tissue.
Its attenuation first increases as a clot forms and
then gradually declines over following days.
• Intravenous contrast dye has higher attenuation than
soft tissue: Normally only brightens blood vessels
and tissues without a blood brain barrier like the
choroid plexus.
• Pathological processes typically disturb the blood
brain barrier allowing contrast to enter and
consequent brightening after contrast administration
11
12. 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.
12
16. MRI: AT A GLANCE
• The Patient Is Placed In A Magnetic Field.
• A Radio Frequency Wave Is Sent In.
• The Radio Frequency Wave Is Turned Off.
• The Patient Emits A Signal.
• Which Is Received And Used For Reconstruction Of The
Picture.
16
17. • MRI is based on the principle of nuclear magnetic
resonance (NMR)
• Two basic principles of NMR
1. Atoms with an odd number of protons or neutrons
have spin
2. A moving electric charge, either positive or negative,
produces a magnetic field
• Body has many such atoms that can act as good MR
nuclei (1H, 13C, 19F, 23Na)
17
18. WHY HYDROGEN IONS ARE USED IN
MRI?
• Hydrogen nucleus has an unpaired proton which is
positively charged
• Every hydrogen nucleus is a tiny magnet which
produces small but noticeable magnetic field
• Hydrogen is abundant in the body in the form of
water and fat
• Essentially all MRI is hydrogen (proton) imaging
18
19. Body in an external magnetic field
• In our natural state Hydrogen ions in body are
spinning in a haphazard fashion, and cancel all the
magnetism.
• When an external magnetic field is applied protons in
the body align in one direction.
19
22. T1 Characteristics
In T1WI, White matter appears
brighter than Gray matter.
Structures Dark on T1:
• CSF
• Edema as in tumor,
infection, inflammation
• Hemorrhage (hyper acute,
chronic)
• Low proton density
22
23. Structures bright on T1:
• Fat
• Sub acute hemorrhage
• Melanin
• Protein rich fluid
• Slowly flowing blood
• Paramagnetic substances
(gadolinium, copper,
manganese).
T1 Characteristics
23
24. T2 Characteristics
In T2WI, Gray matter brighter
than white matter.
Structures Bright on T2:
• CSF
• Edema, tumor, infection,
inflammation.
• Methemoglobin in late sub
acute hemorrhage (7-30
days)
24
25. T2 Characteristics
Structures dark on T2:
• Fibrous tissue
• Deoxyhemoglobin,
Methemoglobin
(intracellular), Iron,
hemosiderin
• Melanin
25
26. FLUID-ATTENUATED INVERSION RECOVERY
(FLAIR)
• T2-weighted imaging is well suited for lesion detection in the
brain because most lesions appear hyperintense with this
sequence.
• However CSF also appears hyperintense on T2-weighted spin-
echo (SE) images.
• Therefore, lesions at CSF interfaces, such as cortical sulci and
ventricles, may be mistaken for extensions of CSF or partial
volume effects.
26
27. • FLAIR imaging suppresses signal from free water in
CSF and maintains hyperintense lesion contrast.
• FLAIR sequences are particularly useful in evaluation
of Multiple Sclerosis, infarcts, Sub-Arachnoid
hemorrhage (SAH)
FLUID-ATTENUATED INVERSION RECOVERY
(FLAIR) Contd.
27
29. Which scan best defines the abnormality
T1 W Images: ANATOMY
Subacute Hemorrhage
Fat-containing structures
T2 W Images: PATHOLOGY
Edema
Demyelination
Infarction
Chronic Hemorrhage
FLAIR Images:
Edema
Demyelination (MS)
Infarction especially in Periventricular location
Subarachnoid hemorrhage
29
30. • The normal motion of water molecules within living tissues is
random (brownian motion).
• In acute stroke, there is an alteration of homeostasis
• Acute stroke causes excess intracellular water accumulation,
or cytotoxic edema, with an overall decreased rate of water
molecular diffusion within the affected tissue.
• Therefore, areas of cytotoxic edema, in which the motion of
water molecules is restricted, appear brighter on diffusion-
weighted images because of lesser signal losses
DIFFUSION-WEIGHTED MRI (Contd.)
30
31. DIFFUSION-WEIGHTED MRI
• Diffusion-weighted MRI is a example of endogenous contrast, using
the motion of protons to produce signal changes.
• DWI is obtained by applying pairs of opposing and balanced
magnetic field gradients (but of differing durations and amplitudes)
• The primary application of DW MR imaging has been in brain
imaging, mainly because of its exquisite sensitivity to early
detection of ischemic stroke
31
33. OTHER CAUSES OF POSITIVE DWI
• Bacterial absecess
• Epidermoid tumour
• Tumours undergoing central necrosis
• Acute Encephalitis
33
34. Apparent Diffusion Coefficient (ADC)
• It is a measure of diffusion
• Calculated by acquiring two or more images with a
different gradient duration and amplitude
• The lower ADC measurements seen with early ischemia
34
35. • The ADC may be useful for estimating the lesion age
and distinguishing acute from subacute DWI lesions.
• Acute ischemic lesions can be divided into
hyperacute lesions (low ADC and DWI-positive) and
subacute lesions (normalized ADC).
• Chronic lesions can be differentiated from acute
lesions by normalization of ADC and DWI.
Apparent Diffusion Coefficient (ADC) (Contd.)
35
37. • This feature of GRE sequences is exploited- in
detection of hemorrhage, as the iron in Hb becomes
magnetized locally (produces its own local magnetic
field) and thus dephases the spinning nuclei.
• The main clinical application of GRE sequence is
detection of hemorrhage, micro bleeds, iron
deposition and calcification.
GRADIENT ECHO (GRE)
37
39. SUSCEPTIBILITY WEIGHTED IMAGES (SWI)
• SWI is an MRI sequence which is particularly
sensitive to compounds which distort the local
magnetic fields and as such make it useful in
detecting blood products, calcium etc.
• Most common use of SWI is for identification of
small amounts of hemorrhage/blood products or
calcium, both of which may be inapperent on other
MRI sequences
39
42. • Contrast with Gadolinium
• Gadolinium slows down relaxation phase (shorten T1) &
increases signal on T1 weighted images- relatively more
contrast goes to vascular structures, producing increase in T1
weighted signal intensity
• Useful for visualisation of :
– Normal vessels
– Disruption of BBB
T1WI WITH CONTRAST
42
43. • Pathological areas appears brighter on T1 contrast.
• Like non-contrast T1 but with bright Arteries and
Veins.
• Both contrast enhanced and un-enhanced images
should be compared.
• Contrast contraindicated in ESRD requiring renal
replacement (not recommended with GFR < 30)
T1WI WITH CONTRAST (Contd.)
43
44. APPROACHING MRI FILM
• IMAGE DELINEATION
– For normal anatomy –preferred scan is T1W
– For any pathology Preferred scan is T2W → T1W
– Usual Order: Axial> Sagittal>Coronal.
• SKULL
– Soft tissue
– Diploic Spaces
• VENTRICLES, CISTERNS & SULCI
– Size: Hydrocephalus
– Shape: Mass Effect
– Symmetry.
44
45. • SYMMETRY OF INTRACRANIAL CONTENTS
– Normal grey-white differentiation
– Deep nuclei
– Brainstem & cerebellum
– Sinus and blood vessels
• FOCAL ABNORMALITIES
– Space Occupying Lesion
– Signal Intensity Changes
APPROACHING MRI FILM
45
46. Magnetic Resonance Imaging (MRI)
Advantages: No ionizing radiation
Safer in pregnancy
Better soft tissue contrast
Disadvantages: More expensive
Less available
Unsuitable in unstable or claustrophobic
Unsuitable with foreign objects (aneurysm clips,
pacers, cochlear implant, cardiac stents)
Not optimal for bone
46
47. ABSOLUTE CONTRAINDICATIONS OF MRI
1. Cardiac pacemakers
2. Cochlear implants
3. Metallic foreign body in the eyes
47
48. IS CT OR MRI BETTER FOR BRAIN IMAGING?
• The answer to which imaging modality is better for imaging
the brain is dependent on the purpose of the examination.
• CT and MRI are complementary techniques, each with its own
strengths and weaknesses.
• The choice of which examination is appropriate depends upon
how quickly it is necessary to obtain the scan, what part of
the head is being examined, and the age of the patient,
among other considerations.
48
49. Is CT or MRI Better for Brain Imaging? (contd.)
• CT is much faster than MRI, study of choice in cases of trauma and other
acute neurological emergencies.
• CT considerably less cost than MRI.
• Sufficient to exclude many neurological disorders.
• CT is less sensitive to patient motion during the examination. because the
imaging can be performed much more rapidly
• CT may be easier to perform in claustrophobic or very heavy patients
• CT provides detailed evaluation of cortical bone
• CT allows accurate detection of calcification and metal foreign bodies
• CT can be performed at no risk to the patient with implantable medical
devices, such as cardiac pacemakers, ferromagnetic vascular clips, and
cochlear implants.
ADVANTAGES OF HEAD CT
49
50. • MRI does not use ionizing radiation, and is thus preferred over
CT in children and patients requiring multiple imaging
examinations.
• MRI has a much greater range of available soft tissue contrast,
depicts anatomy in greater detail, and is more sensitive and
specific for abnormalities within the brain itself.
• MRI scanning can be performed in any imaging plane without
having to physically move the patient.
• MRI contrast agents have a considerably smaller risk of
causing potentially lethal allergic reaction.
• MRI allows the evaluation of structures that may be obscured
by artifacts from bone in CT images.
Is CT or MRI Better for Brain Imaging? (contd.)ADVANTAGES OF HEAD MRI
50
54. Post Contrast Axial MR Image of the brain
1
2
3
Post Contrast sagittal T1 Weighted
M.R.I.
Section at the level of Foramen
Magnum
1. Cisterna Magna
2. Cervical Cord
5. Maxillary Sinus
54
55. Post Contrast Axial MR Image of the brain
7
6
Post Contrast sagittal T1W M.R.I.
Section at the level of medulla
6. Medulla
7. Sigmoid Sinus
55
56. Post Contrast Axial MR Image of the brain
8
9
10
11
12
13
14
15
Post Contrast sagittal T1 Wtd
M.R.I.
Section at the level of Pons
8. Cerebellar
Hemisphere
9. Vermis
10. IV Ventricle
11. Pons
12. Basilar Artery
13. Internal Carotid
Artery
14. Internal Auditory
Canal
15. Temporal Lobe
56
57. Post Contrast Axial MR Image of the brain
18
19
20
21
22
Post Contrast sagittal T1 Wtd
M.R.I.
Section at the level of Mid Brain
18. Aqueduct of Sylvius
19. Midbrain
20. Orbits
21. Posterior Cerebral Artery
22. Middle Cerebral Artery
57
58. Post Contrast Axial MR Image of the brain
23
24
25
26
27
Post Contrast sagittal T1W M.R.I.
Section at the level of the
III Ventricle
23. Occipital Lobe
24. III Ventricle
25. Frontal Lobe
26. Temporal Lobe
27. Sylvian Fissure
58
59. Post Contrast Axial MR Image of the brain
28
29
30
31
32
33
34
36
35
37
Post Contrast sagittal T1 Wtd
M.R.I.
Section at the level of Thalamus
28. Superior Sagittal
Sinus
29. Occipital Lobe
30. Choroid Plexus
within the
occipital horn
31 Frontal Horn
32. Frontal Lobe
33. Thalamus
34. Temporal Lobe
35. Internal Capsule
36. Putamen
37. Caudate Nucleus
59
63. Post Contrast Axial MR Image of the brain
39
40
41
Post Contrast sagittal T1 Wtd
M.R.I.
Section at the level of Corpus
Callosum
39. Splenium of corpus callosum
40. Choroid plexus within the
body of lateral ventricle
41. Genu of corpus callosum
63
64. Post Contrast Axial MR Image of the brain
42
43
44
Post Contrast sagittal T1 Wtd
M.R.I.
Section at the level of Body of
Corpus Callosum
42. Parietal Lobe
43. Body of the Corpus Callosum
44. Frontal Lobe
64
78. SUB-ARACHNOID HEMORRHAGE (SAH)
• CT preferred method to
detect SAH
• FLAIR is equivalent to CT
in diagnosing SAH
• Appears as hyperdensity in
one of the cisternal spaces
• Identification of SAH
may help in localising
underlying aneurysm.
78
79. SUB-ARACHNOID HEMORRHAGE (SAH)
• Most common cause- Aneurysm (75-80%)
• Other causes- tumour, trauma, AV malformation
• May result in Hydrocephalus
• Sensitivity of CT scan 95% within 12 hours, 90-95% at24 hours
decreasing further to 30% at 2 weeks
79
96. MENINGITIS
• Inflammation of leptomeninges (arachnoid
membrane and piamater)
CT SCAN
• Non-enhanced CT scans frequently show obliteration
of basal cisterns.
• Contrast enhanced CT scans may show enhancement
in basal cisterns and sylvian fissure.
96
97. MENINGITIS
MRI BRAIN:
• T1WI may show obliteration of basal cisterns
• T2WI: may show abnormal cortical hyperintensity
• FLAIR sequence may show hyperintensity of CSF with in
the subarachnoid space in contrast to hypointense CSF in
the ventricles.
• Contrast enhanced MRI: may show basal cisternal and
sylvian enhancement as well as enhancement deep
within the cortical sulci
97
105. ENCEPHALITIS
• Encephalitis refers to a diffuse, nonfocal inflammatory process of
the brain usually of viral origin.
• Areas of involvement are characterized by mass effect, edema,
hyperintensity, n T2WI and less frequently, small infarctions or
petechial hemorrhages.
105
106. HSV ENCEPHALITIS
• HSV type I is most common cause of sporadic viral encephalitis.
• Prediclection for Subfrontal and medial temporal lobes.
• Insular cortex and cingulate gyrus are also affected
• Lesions Initially unilateral, gradually become bilateral.
• Bilateral temporal lobe involvement nearly pathognomic of HSV
encephalitis
106
107. ENCEPHALITIS
• MRI CHARACTERSTICS:
• T1WI: Gyral effacement
• T2WI: high signal of temporal lobe and cingulate gyrus
• Contrast administration may show gyral enhancement.
107
109. TAKE HOME MESSAGE
• Choose carefully the imaging modality which you
want to confirm your suspected diagnosis
• Always ask for contrast images in suspected
infectious, inflammatory and demyelinating
disorders.
• Always review clinical history while interpreting CT
Scan and MRI.
109
110. REFERENCES
• Magnetic Resonance Imaging of the Brain by Paul M.
Parizel
• Hagga textbook of Radiology
• Various references from internet:
– www.radiopedia.com
– www.radiologyassistant.com
110
1) Wall BF, Hart D. Revised radiation doses for typical x-ray examinations. The British Journal of Radiology 70:437-439; 1997. (5,000 patient dose measurements from 375 hospitals)
5) National Council on Radiation Protection and Measurements. Sources and magnitude of occupational and public exposures from nuclear medicine procedures. Bethesda, MD: National Council on Radiation Protection and Measurements; NCRP Report 124; 1996.
6) United Nations Scientific Committee on the Effects of Atomic Radiation. Sources and effects of ionizing radiation, Vol. 1: Sources. New York, NY: United Nations Publishing; 2000.