Neuroimaging

NEUROIMAGING

1. CROSS-SECTIONAL BASICS OF NEUROIMAGING GUIDE DR. A. PAURANIK DR. SANGIT CHAUDHARI CANDIDATE DR. RAHUL SAHU 1

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

3. IMAGING MODALITIES: CT SCAN MRI NORMAL ANATOMY PATHOLOGIC PROCESSES 3

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

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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

13. CT SCAN (contd.) 13

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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 Atoms with an odd number of protons or neutrons have spin A moving electric charge, either positive or negative, produces a magnetic field Body has many such atoms that can act as good MR nuclei ( 1 H, 13 C, 19 F, 23 Na) 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

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21. T1WI (T1 Weighted images) T2WI ( T2 Weighted images ) FLAIR (Fluid Attenuated Inversion Recovery) DWI (Diffusion Weighted images) TI-Weighted images with Gd (Gadolinium contrast) ADC (Apparent Diffusion Coefficient ) T2* (T2 Star / SWI/ Susceptibility Weighted Images) MRI SEQUENCES 21

22. T1 Characteristics In T1WI, White matter appears brighter than Gray matter. Structures Dark on T1 : CSF Edema as in tumor, infection, inflammation H emorrhage (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 , M ethemoglobin (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

28. T2-WI FLAIR 28

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

32. T2-WI DWI 32

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

36. 65 year male- acute Rt ACA Infarct 36

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

38. GRE FLAIR Hemorrhage in right parietal lobe 38

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

40. SUSCEPTIBILITY WEIGHTED IMAGES (SWI) GRADIENT ECHO T2 SWI 40

41. SUSCEPTIBILITY WEIGHTED IMAGES (SWI) 41

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 VENTRICLE S, 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 Cardiac pacemakers Cochlear implants 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 c laustrophobic 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

51. Radiation Dose Considerations Single Radiographs Effective Dose, mrem ( mSv ) Skull (PA or AP) 1 3 (0.03) Skull (lateral) 1 1 (0.01) Chest (PA) 1 2 (0.02) Chest (lateral) 1 4 (0.04) Chest (PA and lateral) 5 6 (0.06) Thoracic spine (AP) 1 40 (0.4) Thoracic spine (lateral) 1 30 (0.3) Lumbar spine (AP) 1 70 (0.7) Lumbar spine (lateral) 1 30 (0.3) CT study Effective Dose, mrem ( mSv ) CT head 1 200 (2.0) Lumbar spine series 6 180 (1.8) Thoracic spine series 6 140 (1.4) Cervical spine series 6 27 (0.27) Radiation Dose Considerations 51

52. NORMAL ANATOMY OF BRAIN 52

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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 th e 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

60. CROSS-SECTIONAL VIEW AT THE LEVEL OF BASAL GANGLIA 60

61. Lateral ventricle (anterior horn) Foramen of Monro Third ventricle Pineal gland Lateral ventricle ( trigone with choroid plexus) Corpus callosum (genu) Caudate nucleus (head) Lentiform nucleus Putamen (outer side), Globus Pallidus (Inner side) Internal capsule Thalamus 61

62. External capsule insular cistern Quadrigeminal and ambient cisterns Insula 62

63. Post Contrast Axial MR Image of the brain 39 40 41 Post Contrast sagittal T1 Wtd M.R.I. Section at th e 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

65. PATHOLOGIES 65

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67. TRAUMA 67

68. TRAUMA Extra-axial Hemorrhage: Extradural Subdural Subarachnoid 68

69. MENINGES 69

70. White Dark Gray 70

71. Trauma EXTRADURAL HEMORRHAGE Convex Might be associated with a fracture Does not cross sutures Less mass effect 71

72. Trauma EXTRADURAL HEMORRHAGE 72

73. Trauma EXTRADURAL HEMORRHAGE 73

74. Trauma SUBDURAL HEMORRHAGE Concave/Sickle/Crescent shaped Can cross sutures Not associated with fracture More mass effect 74

75. Trauma SUBDURAL HEMORRHAGE 75

76. Trauma SUBDURAL HEMORRHAGE 76

77. Trauma 77

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 S ensitivity of CT scan 95% within 12 hours, 90-95% at24 hours decreasing further to 30% at 2 weeks 79

80. SUB-ARACHNOID HEMORRHAGE (SAH) 80

81. SUB-ARACHNOID HEMORRHAGE (SAH) 81

82. SUB-ARACHNOID HEMORRHAGE (SAH) 82

83. Blood Edema 83

84. Hemorrhagic Stroke 84

85. HEMORRHAGIC STROKE Blood 85

86. HEMORRHAGIC STROKE 86

87. HEMORRHAGIC STROKE 87

88. ISCHEMIC STROKE 88

89. Ischemic Stroke Hyperdense MCA sign 89

90. Lost G-W matter differentiation Ischemic Stroke 90

91. ISCHEMIC STROKE ON MRI BRAIN ACUTE RIGHT MCA TERRITORY INFARCT T1-WI T2-WI 91

92. ISCHEMIC STROKE ON MRI BRAIN ACUTE RIGHT MCA TERRITORY INFARCT FLAIR DWI ADC 92

93. PONTINE INFARCT 93

94. PONTINE INFARCT 94

95. ISCHEMIC STROKE WITH HEMORRHAGIC TRANSFORMATION 95

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

98. MENINGITIS- UNENHANCED T1WI 98

99. MENINGITIS- T1W Gd ENHANCED MRI 99

100. MENINGITIS 100

101. TUBERCULOUS MENINGITIS 101

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103. TUBERCULOUS MENINGITIS-BASAL EXUDATES AND TUBERCULOMAS 103

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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

108. HSV ENCEPHALITIS 108

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

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