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Neuroimaging

This document provides an overview of neuroimaging techniques including CT scans and MRI. It discusses the basics of CT scans, including how they work and what different tissue densities look like. It also covers the basics of MRI, describing different sequences like T1-weighted, T2-weighted, FLAIR, DWI, and susceptibility weighted images. It explains how these sequences are used to identify different pathological processes and provides examples of what various conditions look like on neuroimaging. The document is intended to help readers differentiate brain structures, identify pathologies, and understand the appropriate use of different neuroimaging techniques.

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CROSS-SECTIONAL BASICS OF NEUROIMAGING GUIDE DR. A. PAURANIK DR. SANGIT CHAUDHARI CANDIDATE DR. RAHUL SAHU 1
• 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
1. IMAGING MODALITIES: a) CT SCAN b) MRI 2. NORMAL ANATOMY 3. PATHOLOGIC PROCESSES 3
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
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
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
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
Low density High density CSF Bone Fluid (Edema) Calcification Air Blood Fat Contrast Metallic Foreign Bodies 8
9
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
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
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
CT SCAN (contd.) 13
14
15
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
• 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
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
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
20
1. T1WI (T1 Weighted images) 2. T2WI (T2 Weighted images) 3. FLAIR (Fluid Attenuated Inversion Recovery) 4. DWI (Diffusion Weighted images) 5. TI-Weighted images with Gd (Gadolinium contrast) 6. ADC (Apparent Diffusion Coefficient) 7. T2* (T2 Star / SWI/ Susceptibility Weighted Images) MRI SEQUENCES 21
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
Structures bright on T1: • Fat • Sub acute hemorrhage • Melanin • Protein rich fluid • Slowly flowing blood • Paramagnetic substances (gadolinium, copper, manganese). T1 Characteristics 23
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
T2 Characteristics Structures dark on T2: • Fibrous tissue • Deoxyhemoglobin, Methemoglobin (intracellular), Iron, hemosiderin • Melanin 25
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
• 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
T2-WI FLAIR 28
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
• 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
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
T2-WI DWI 32
OTHER CAUSES OF POSITIVE DWI • Bacterial absecess • Epidermoid tumour • Tumours undergoing central necrosis • Acute Encephalitis 33
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
• 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
65 year male- acute Rt ACA Infarct 36
• 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
GREFLAIR Hemorrhage in right parietal lobe 38
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
SUSCEPTIBILITY WEIGHTED IMAGES (SWI) GRADIENT ECHO T2 SWI 40
SUSCEPTIBILITY WEIGHTED IMAGES (SWI) 41
• 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
• 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
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
• 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
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
ABSOLUTE CONTRAINDICATIONS OF MRI 1. Cardiac pacemakers 2. Cochlear implants 3. Metallic foreign body in the eyes 47
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
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
• 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
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 head1 200 (2.0) Lumbar spine series6 180 (1.8) Thoracic spine series6 140 (1.4) Cervical spine series6 27 (0.27) Radiation Dose Considerations 51
NORMAL ANATOMY OF BRAIN 52
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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
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
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
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
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
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
CROSS-SECTIONAL VIEW AT THE LEVEL OF BASAL GANGLIA 60
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
External capsule insular cistern Quadrigeminal and ambient cisterns Insula 62
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
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
PATHOLOGIES 65
66
TRAUMA 67
68
MENINGES 69
White DarkGray 70
EXTRADURAL HEMORRHAGE • Convex • Might be associated with a fracture • Does not cross sutures • Less mass effect 71
EXTRADURAL HEMORRHAGE 72
EXTRADURAL HEMORRHAGE 73
SUBDURAL HEMORRHAGE • Concave/Sickle/Crescent shaped • Can cross sutures • Not associated with fracture • More mass effect 74
SUBDURAL HEMORRHAGE 75
SUBDURAL HEMORRHAGE 76
77
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
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
SUB-ARACHNOID HEMORRHAGE (SAH) 80
SUB-ARACHNOID HEMORRHAGE (SAH) 81
SUB-ARACHNOID HEMORRHAGE (SAH) 82
Stroke Hemorrhagic Ischemic 83
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Blood 85
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Hyperdense MCA sign 89
Lost G-W matter differentiation 90
ISCHEMIC STROKE ON MRI BRAIN ACUTE RIGHT MCA TERRITORY INFARCT T1-WI T2-WI 91
ISCHEMIC STROKE ON MRI BRAIN ACUTE RIGHT MCA TERRITORY INFARCT FLAIR DWI ADC 92
PONTINE INFARCT 93
PONTINE INFARCT 94
ISCHEMIC STROKE WITH HEMORRHAGIC TRANSFORMATION 95
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
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
MENINGITIS- UNENHANCED T1WI 98
MENINGITIS- T1W Gd ENHANCED MRI 99
MENINGITIS 100
TUBERCULOUS MENINGITIS 101
102
TUBERCULOUS MENINGITIS-BASAL EXUDATES AND TUBERCULOMAS 103
104
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
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
ENCEPHALITIS • MRI CHARACTERSTICS: • T1WI: Gyral effacement • T2WI: high signal of temporal lobe and cingulate gyrus • Contrast administration may show gyral enhancement. 107
HSV ENCEPHALITIS 108
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
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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Neuroimaging

  • 1.
    CROSS-SECTIONAL BASICS OF NEUROIMAGING GUIDE DR.A. PAURANIK DR. SANGIT CHAUDHARI CANDIDATE DR. RAHUL SAHU 1
  • 2.
    • By theend 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.
    1. IMAGING MODALITIES: a)CT SCAN b) MRI 2. NORMAL ANATOMY 3. PATHOLOGIC PROCESSES 3
  • 4.
    CT SCAN • Alsoknown 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 hasan 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 Highdensity CSF Bone Fluid (Edema) Calcification Air Blood Fat Contrast Metallic Foreign Bodies 8
  • 9.
  • 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 isplaced 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.
  • 14.
  • 15.
  • 16.
    MRI: AT AGLANCE • 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 isbased 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 IONSARE 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 anexternal 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
  • 20.
  • 21.
    1. T1WI (T1Weighted images) 2. T2WI (T2 Weighted images) 3. FLAIR (Fluid Attenuated Inversion Recovery) 4. DWI (Diffusion Weighted images) 5. TI-Weighted images with Gd (Gadolinium contrast) 6. ADC (Apparent Diffusion Coefficient) 7. 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 • Hemorrhage (hyper acute, chronic) • Low proton density 22
  • 23.
    Structures bright onT1: • 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 darkon 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 imagingsuppresses 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.
  • 29.
    Which scan bestdefines 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 normalmotion 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-weightedMRI 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.
  • 33.
    OTHER CAUSES OFPOSITIVE 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 ADCmay 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 featureof 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.
  • 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.
  • 42.
    • Contrast withGadolinium • 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 areasappears 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 OFINTRACRANIAL 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 OFMRI 1. Cardiac pacemakers 2. Cochlear implants 3. Metallic foreign body in the eyes 47
  • 48.
    IS CT ORMRI 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 orMRI 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 doesnot 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 SingleRadiographs 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 head1 200 (2.0) Lumbar spine series6 180 (1.8) Thoracic spine series6 140 (1.4) Cervical spine series6 27 (0.27) Radiation Dose Considerations 51
  • 52.
  • 53.
  • 54.
    Post Contrast AxialMR 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 AxialMR 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 AxialMR 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 AxialMR 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 AxialMR 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 AxialMR 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 ATTHE LEVEL OF BASAL GANGLIA 60
  • 61.
    Lateral ventricle (anteriorhorn) 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 Quadrigeminaland ambient cisterns Insula 62
  • 63.
    Post Contrast AxialMR 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 AxialMR 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.
  • 66.
  • 67.
  • 68.
  • 69.
  • 70.
  • 71.
    EXTRADURAL HEMORRHAGE • Convex •Might be associated with a fracture • Does not cross sutures • Less mass effect 71
  • 72.
  • 73.
  • 74.
    SUBDURAL HEMORRHAGE • Concave/Sickle/Crescent shaped •Can cross sutures • Not associated with fracture • More mass effect 74
  • 75.
  • 76.
  • 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 • Sensitivity of CT scan 95% within 12 hours, 90-95% at24 hours decreasing further to 30% at 2 weeks 79
  • 80.
  • 81.
  • 82.
  • 83.
  • 84.
  • 85.
  • 86.
  • 87.
  • 88.
  • 89.
  • 90.
    Lost G-W matterdifferentiation 90
  • 91.
    ISCHEMIC STROKE ONMRI BRAIN ACUTE RIGHT MCA TERRITORY INFARCT T1-WI T2-WI 91
  • 92.
    ISCHEMIC STROKE ONMRI BRAIN ACUTE RIGHT MCA TERRITORY INFARCT FLAIR DWI ADC 92
  • 93.
  • 94.
  • 95.
    ISCHEMIC STROKE WITHHEMORRHAGIC TRANSFORMATION 95
  • 96.
    MENINGITIS • Inflammation ofleptomeninges (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: • T1WImay 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.
  • 99.
    MENINGITIS- T1W GdENHANCED MRI 99
  • 100.
  • 101.
  • 102.
  • 103.
  • 104.
  • 105.
    ENCEPHALITIS • Encephalitis refersto 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 • HSVtype 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.
  • 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 ResonanceImaging of the Brain by Paul M. Parizel • Hagga textbook of Radiology • Various references from internet: – www.radiopedia.com – www.radiologyassistant.com 110
  • 111.

Editor's Notes

  • #52 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.
  • #83 Blood filling the suprasellar cistern