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Basics of CT scan
by
Chakradhar
Neurology resident
HISTORY
 Computed tomography (CT) scan machines uses X-rays, a powerful
form of electromagnetic energy.
 Sir Godfrey hounsfield-1972
 Nobel prize in 1979 with cormack
 six generation of scanners
 Latest 128 multidetector ct
G.N.HOUNSFIELD ALLAN M. CORMACK
PRINCIPLE
 Internal structure of an object can be reconstructed from multiple
projections of the object.
 Uses x rays applied in sequence of slices across the organ
 Images reconstructed from x-ray absorption data
 Xray beam moves around the patient in a circular path
Beam of light projected in two
direction's, detecting two different
shadows
Terminology
 Hounsfield Unit (HU)- mean attenuation of x-rays by
different tissues.
PIXEL & VOXEL
Each square of the image matrix is called pixel
Thickness of block of tissue called voxel
Linear attenuation coefficient
 The linear attenuation coefficient ( ) of each pixel is
determined by :
1. Composition of the voxel
2. Thickness of the voxel
3. Quality of the radiation beam
Hounsfield units represent logarithmic scale of CT
density.
Pure water has an HU value of ‘0’.
Current CT scans measure from – 1204 to + 3407.
DESCRIPTION Approx. HU DENSITY
Calcium > 1000 Hyperdense
Acute blood 60-80 Hyperdense
Grey matter 38 (32-42) Hyperdense
White matter 30 (22-32) Hyperdense
CSF 0-10 ISODENSE
Fat -30 to - 100 Hypodense
Air - 1000 Hypodense
PARTS
1)xray tube-akin to that in a x ray machine.
2)detectors
3)gantry- which houses xray apparatus
4)patient couch
5)viewing console
1.X-ray tube & collimator
2.Detector assembly
3.Tube controller
4.High freq. generator
5.Onboard computer
6.Stationary computer
X ray tube Internal structure of gantry
FILTERS
Compensation filter is being used
 To absorb low energy x rays
 To reduce patient dose
 To provide a more uniform beam
COLLIMATORS
 To improve image quality
 Collimator width
determines the slice
thickness
FIRST GENERATION
 Narrow pencil beam
 Single detector per slice
 Translate –Rotate movements
of Tube- detector
combination
 Scan time-5min
 Designed only for evaluation
of brain
FIR ST G EN ER A TIO N
SECOND GENERATION
 Narrow fan beam
(30-100)
 Linear detector array(30)
 Translate-Rotate movements of
Tube-Detector combination
 Fewer linear movements are
needed as there are more
detectors to gather the data.
 Between linear movements, the
gantry rotated 30o
 Only 6 times the linear
movements got repeated
 Scan time~20secs
THIRD GENERATION
 Rotate(tube)-
Rotate(detectors) Translatory
motion is completely
eliminated
 Pulsed wide fan beam(500-
550)
 Arc of detectors(600-900)
 Detectors are perfectly
aligned with the X-Ray tube
 Both Xenon and scintillation
crystal detectors can be used
 Scan time< 5secs
FOURTH GENERATION
 Continuous wide fan beam(500-550)
 Ring of detectors(> 2000)
 Rotate(tube)-Fixed(detector)
 X-ray tube rotates in a circle inside
the detector ring
 When the tube is at predescribed
angles, the exposed detectors are
read.
 Scan time< 2 secs
TYPES
 Spiral ct-
uses principle of volumetric acquisiton.
no respiratory misregistration
 HRCT
 CT cisternography and myelography
OTHER SCAN
CONFIGURATIONS
Interest in faster scan times evolves from a
desire to image moving structures such as the
wall of the heart and contrast material in blood
vessel and heart chambers and to overcome
motion artifacts due to cardiac rhythm and
patient breathing .
 Dynamic Spatial Reconstructor(DSR)
 Electron beam computed tomography
DYNAMIC SPATIAL RECONSTRUCTOR
 28 X-ray tubes
 X-ray tubes are aligned with
28 light amplifiers and TV
cameras that are placed
behind a single curved
fluorescent screen
 The gantry rotates about the
patient at a rate of 50 RPM
 Data for an image acquired
in about 16 ms.
 Reconstruct 250 C.S. images
from each scan data
DSR
 The Dynamic Spatial Reconstructor (DSR) is a high-temporal
resolution,
 studies of cardiovascular structure and function. 3-D dynamic
images can be obtained after reconstruction.
 DSR is currently used involves studying selected pediatric patients
with complex congenital heart disease
advantages
Disadvantages
 High Cost
 Mechanical motion is not eliminated
 This instrument represents a novel concept in the use of x-ray to obtain fast
tomographic scanning. In contrast to the DSR and conventional CT, EBCT
has no mechanical parts moving around the patients, resulting in lower
heat production and enabling fast scanning.
 An electron beam, originating from an electron gun located behind the
patient is magnetically deflected sequentially onto four tungsten target
rings, producing eight fan beams (two from each target ring) of x-ray
radiation that pass through the patient.
 Eight almost simultaneous renal tomographic sections can thereby be
obtained,
Electron Beam CT
Electron Beam Computed
Tomography
 Electron gun
 Large Arcs of tungsten targets
 Detector ring
 17 slices per second
EBCT
Why Is It Done?
 This test is used to identify calcium buildup in heart arteries, which
can be a risk factor for coronary artery disease (CAD). It may be
used as a screening tool to detect hardening of the arteries in
people who are at high risk of developing atherosclerosis.
 only CT method which can scan the beating
heart.
 EBCT in measuring RBF
 slices are thicker (8 mm) than those produced by
the DSR.
 temporal resolution is lower than that offered by
the DSR (50 or 100 msec/image),
Disadvantages
Radiation dose from EBT scans
compared to other sources of
radiation
X ray chest-0.1 mSV
Ct brain-2 mSV
EBCT-0.5 to 0.7 mSV
Environmental radiation per year-0.02 mSV
Image Quality in CT
Image quality is the visibility of diagnostically
important structures in the CT image.
The factors that affect CT image quality are
 Quantum mottle (noise)
 Resolution : Spatial and contrast
 Patient exposure.
The factors are all interrelated
CT ARTIFACTS
Artifacts are distortions or errors in the
image that are unrelated to the object
scanned .
Most common artifacts in CT are
 Motion artifacts
 Streak artifacts
 Beam hardening artifacts
 Partial volume averaging artifacts
 Ring artifacts
STREAK ARTIFACTS
Cause: Presence and movements
of objects of very high
density(contrast media, metallic
implants,surgical clips)
Appearance: Streaks
REMEDY:-
•Remove the offending object
if possible. Use a smoothing
algorithm. e.g. Standard
algorithm.
DENTURES
PRODUCING STREAK
ARTIFACT
SURGICAL
CLIP IN HEART
PRODUCING
STREAK ARTIFACT
RING ARTIFACTS
 CAUSE : Detector failure or miscalibration of
a detector
 APPEARANCE:-
Ring
 Rectification : regular quality assurance checks
RING APPEARANCE
Ct brain presentation
CECT
 To detect abnormal disrution caused by tumor,abscess ,infarct etc
 Uses ionic or non ionic contrast(6 fold reduction in allergic reactioin
0.04%)
 In normal CNS vessels,pituitary choroid and dura enhance
Indications for non ionic contrast
 Prior adverse reaction
 BA
 Allergy or atopy hx
 <2yr
 RF(Cr>2)
 Cardiac
 DM
 Severe debilitation
CT
 Advantages –
 Easy availabilty
 Fast
 Better for bone and acute blood,lesions of skull base and calvarium
 Calcification
 Less limited by patient factors
 Disadvantages-
 high radiation
 poor visualisation of posterior fossa lesions
Ct brain presentation
Ct brain presentation
INTERPRETATION OF CT BRAIN
 1-GENERAL INFORMATION
 2-EXTRACRANIAL TISSUE
 3-CRANIAL BONE
 4-BLOOD
 5-CSF FLOW
 A-VENTRICULAR SYSTEM
 B-CISTERNS
 6-BRAIN TISSUE
 A-MASS LESIONS
 B-SULCI & GYRI
 C-GRY & WHITE DIFFERENTIATION
Low density High density
Csf Bone
Fluid Calcification
Air Blood
Fat Contrast
Ct brain presentation
Ct brain presentation
Ct brain presentation
Ct brain presentation
Ct brain presentation
Ct brain presentation
Ct brain presentation
Physiologic calcifications
 Chorid plexus-rare before 10yrs
 Basal ganglia-rare before 40ys
 Pineal gland-common after 30 yr rare before 10yr
 Falx
 Dentate nuclei
INDICATIONS
 To diagnose neuro infections and their complications
 Stroke to distinguish infarct from hemorrhage
 Ct angio before thrombolysis
 Ct venogram for cerebral venous thrombosis(cvt)
 Acute changes in mental status
 Focal neurologic findings
 Trauma
 Suspected SAH
 Cns tumors
Peidural hematoma
Convex shape
Subdural hematoma
Cresent shape
Skull fracture
Infarcts
Anterior cerebral artery infarct
Middle cerebral artery infarct
Posterior cerebral artery infarct
Hyper dense MCA sign
Internal cerebral artery infarct
ACA+MCA
hemorrhage
Intra parenchymal hemorrhage
in putamen Sub arachnoid hemorrhage
hyperdensities in sylvian fissure,basal cysterns
Neuro infections
bacterial meningitis
radiological signs
meningeal enhancement
cerebral edema
complications
abscess stages
sub dural abscess,epidural abscess
Bacterial meningitis
Indicatations for ct brain
before lumbar puncture-
to look for obstructive hydrocephalus-to prevent herniation
to conform meningeal involvement—by meningeal
enhancement
meningitis complications
suggested by seizures, altered sensorium, focal deficits
encephalitis- cerebral edema is seen
others
cerebral abscess
epidural/sub dural empyema
arteritis leading to infarct
hydrocephalus
seen well effaced
Gyri and sulci
Normal parenchyma
cerebral edema
hydrocephalus
 Sub dural effusion
 arteritis leading to infarct
 cerebral abscess
stages
Early cerebritis
early capsule, thin rim
Late capsular, thick rim
Multi loculated
Late cerebritis
d/d for multiple ring enhancing
lesions Tuberculoma
Neurocysticerosis
cns crptococcosis
Metastasis
Abscess (also cerebritis)
Glioblastoma, Granuloma
Infarct (esp. Basal ganglia)
Contusion (rare)
AIDS (Toxoplasmosis, etc.)
Lymphoma (common in AIDS )
neurosarcoidosis
Demyelination (active)
Resolving hematoma, Radiation change (necrosis)
TUBERCULOMA
 Non contrast ct normal or may show complications
 On contrast basal enhancing exudates,meningeal anhancement,
tubeculomas with ring enhancement,ependimitis
Basal exudate enhancement
Tuberculomas with perilesional edema
Coalising tuberculomas
NEUROCYSTICERCOSIS
 Stages
vesicular stage- live stage
only hypo dense lesion with out perilesional edema/ring enhancement
colloidal stage- perilesional edema with ring enhancement
granular stage- scolex gets calcified resulting in central hyper density
nodular stage- entire lesion gets calcified nodular stage-
vesicular stage-
colloidal stage- granular stage- nodular stage-
 Tuberculous granuloma neurocysticercosis
>20 mm size <20mm
large perilesional edema usually small area
irregular margine regular margin
Coalising lesions noncoalising
These findings are not path gnomic,above signs can be seen viceversa
neurocysticercosis
TUBERCULOMAs
central dot sign Stary sky
TOXOPLASMOSIS
CT - (70-80 % cases ) multiple B/L hypo dense contrast enhancing focal lesions with
predisposition to the basal ganglia and subcortical region.
A double dose contrast with increased delay scan time may increase the
sensitivity.
Ct brain presentation
Why not MRI them all???
- MRI is generally preferable to CT for evaluating intracranial
neoplasms
- CT is preferred for visualizing tumor calcification or intratumor
hemorrhage.
Cns tumors
Commonly Calcified and
Hemorrhagic Lesions
Calcified Hemorrhagic
Oligodendroglioma Glioblastoma multiforme
Choroid Plexus tumor Oligodendroglioma
Ependymoma Metastatic:
Central neurocytoma Melanoma
Craniopharyngioma Breast
Teratoma Lung
Chordoma
meningioma
Pilocytic Cerebellar Astrocytoma
Cystic mass with nodular enhancement in the wall
Ependymoma
Enhancing lesion with in 4th ventricle
Glioblastoma Multiforme
Cystic,solid and partially calcified
Image finding
irregularly enhancing with necrotic centre
medulloblastoma
 Premitive neuro ectodermal tumor
 Usually arises from roof of 4th ventricle
 It is partially enhancing on ct
meningioma
 Extra axial tumour
 imaging- homogenously hyper enhancing
Ct brain presentation
Ct brain presentation
Ct brain presentation
Ct brain presentation

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Ct brain presentation

  • 1. Basics of CT scan by Chakradhar Neurology resident
  • 2. HISTORY  Computed tomography (CT) scan machines uses X-rays, a powerful form of electromagnetic energy.  Sir Godfrey hounsfield-1972  Nobel prize in 1979 with cormack  six generation of scanners  Latest 128 multidetector ct G.N.HOUNSFIELD ALLAN M. CORMACK
  • 3. PRINCIPLE  Internal structure of an object can be reconstructed from multiple projections of the object.  Uses x rays applied in sequence of slices across the organ  Images reconstructed from x-ray absorption data  Xray beam moves around the patient in a circular path Beam of light projected in two direction's, detecting two different shadows
  • 4. Terminology  Hounsfield Unit (HU)- mean attenuation of x-rays by different tissues. PIXEL & VOXEL Each square of the image matrix is called pixel Thickness of block of tissue called voxel Linear attenuation coefficient  The linear attenuation coefficient ( ) of each pixel is determined by : 1. Composition of the voxel 2. Thickness of the voxel 3. Quality of the radiation beam
  • 5. Hounsfield units represent logarithmic scale of CT density. Pure water has an HU value of ‘0’. Current CT scans measure from – 1204 to + 3407. DESCRIPTION Approx. HU DENSITY Calcium > 1000 Hyperdense Acute blood 60-80 Hyperdense Grey matter 38 (32-42) Hyperdense White matter 30 (22-32) Hyperdense CSF 0-10 ISODENSE Fat -30 to - 100 Hypodense Air - 1000 Hypodense
  • 6. PARTS 1)xray tube-akin to that in a x ray machine. 2)detectors 3)gantry- which houses xray apparatus 4)patient couch 5)viewing console 1.X-ray tube & collimator 2.Detector assembly 3.Tube controller 4.High freq. generator 5.Onboard computer 6.Stationary computer X ray tube Internal structure of gantry
  • 7. FILTERS Compensation filter is being used  To absorb low energy x rays  To reduce patient dose  To provide a more uniform beam
  • 8. COLLIMATORS  To improve image quality  Collimator width determines the slice thickness
  • 9. FIRST GENERATION  Narrow pencil beam  Single detector per slice  Translate –Rotate movements of Tube- detector combination  Scan time-5min  Designed only for evaluation of brain FIR ST G EN ER A TIO N
  • 10. SECOND GENERATION  Narrow fan beam (30-100)  Linear detector array(30)  Translate-Rotate movements of Tube-Detector combination  Fewer linear movements are needed as there are more detectors to gather the data.  Between linear movements, the gantry rotated 30o  Only 6 times the linear movements got repeated  Scan time~20secs
  • 11. THIRD GENERATION  Rotate(tube)- Rotate(detectors) Translatory motion is completely eliminated  Pulsed wide fan beam(500- 550)  Arc of detectors(600-900)  Detectors are perfectly aligned with the X-Ray tube  Both Xenon and scintillation crystal detectors can be used  Scan time< 5secs
  • 12. FOURTH GENERATION  Continuous wide fan beam(500-550)  Ring of detectors(> 2000)  Rotate(tube)-Fixed(detector)  X-ray tube rotates in a circle inside the detector ring  When the tube is at predescribed angles, the exposed detectors are read.  Scan time< 2 secs
  • 13. TYPES  Spiral ct- uses principle of volumetric acquisiton. no respiratory misregistration  HRCT  CT cisternography and myelography
  • 14. OTHER SCAN CONFIGURATIONS Interest in faster scan times evolves from a desire to image moving structures such as the wall of the heart and contrast material in blood vessel and heart chambers and to overcome motion artifacts due to cardiac rhythm and patient breathing .  Dynamic Spatial Reconstructor(DSR)  Electron beam computed tomography
  • 15. DYNAMIC SPATIAL RECONSTRUCTOR  28 X-ray tubes  X-ray tubes are aligned with 28 light amplifiers and TV cameras that are placed behind a single curved fluorescent screen  The gantry rotates about the patient at a rate of 50 RPM  Data for an image acquired in about 16 ms.  Reconstruct 250 C.S. images from each scan data
  • 16. DSR  The Dynamic Spatial Reconstructor (DSR) is a high-temporal resolution,  studies of cardiovascular structure and function. 3-D dynamic images can be obtained after reconstruction.  DSR is currently used involves studying selected pediatric patients with complex congenital heart disease advantages Disadvantages  High Cost  Mechanical motion is not eliminated
  • 17.  This instrument represents a novel concept in the use of x-ray to obtain fast tomographic scanning. In contrast to the DSR and conventional CT, EBCT has no mechanical parts moving around the patients, resulting in lower heat production and enabling fast scanning.  An electron beam, originating from an electron gun located behind the patient is magnetically deflected sequentially onto four tungsten target rings, producing eight fan beams (two from each target ring) of x-ray radiation that pass through the patient.  Eight almost simultaneous renal tomographic sections can thereby be obtained, Electron Beam CT
  • 18. Electron Beam Computed Tomography  Electron gun  Large Arcs of tungsten targets  Detector ring  17 slices per second
  • 19. EBCT Why Is It Done?  This test is used to identify calcium buildup in heart arteries, which can be a risk factor for coronary artery disease (CAD). It may be used as a screening tool to detect hardening of the arteries in people who are at high risk of developing atherosclerosis.  only CT method which can scan the beating heart.  EBCT in measuring RBF  slices are thicker (8 mm) than those produced by the DSR.  temporal resolution is lower than that offered by the DSR (50 or 100 msec/image), Disadvantages
  • 20. Radiation dose from EBT scans compared to other sources of radiation X ray chest-0.1 mSV Ct brain-2 mSV EBCT-0.5 to 0.7 mSV Environmental radiation per year-0.02 mSV
  • 21. Image Quality in CT Image quality is the visibility of diagnostically important structures in the CT image. The factors that affect CT image quality are  Quantum mottle (noise)  Resolution : Spatial and contrast  Patient exposure. The factors are all interrelated
  • 22. CT ARTIFACTS Artifacts are distortions or errors in the image that are unrelated to the object scanned . Most common artifacts in CT are  Motion artifacts  Streak artifacts  Beam hardening artifacts  Partial volume averaging artifacts  Ring artifacts
  • 23. STREAK ARTIFACTS Cause: Presence and movements of objects of very high density(contrast media, metallic implants,surgical clips) Appearance: Streaks REMEDY:- •Remove the offending object if possible. Use a smoothing algorithm. e.g. Standard algorithm.
  • 24. DENTURES PRODUCING STREAK ARTIFACT SURGICAL CLIP IN HEART PRODUCING STREAK ARTIFACT
  • 25. RING ARTIFACTS  CAUSE : Detector failure or miscalibration of a detector  APPEARANCE:- Ring  Rectification : regular quality assurance checks
  • 28. CECT  To detect abnormal disrution caused by tumor,abscess ,infarct etc  Uses ionic or non ionic contrast(6 fold reduction in allergic reactioin 0.04%)  In normal CNS vessels,pituitary choroid and dura enhance
  • 29. Indications for non ionic contrast  Prior adverse reaction  BA  Allergy or atopy hx  <2yr  RF(Cr>2)  Cardiac  DM  Severe debilitation
  • 30. CT  Advantages –  Easy availabilty  Fast  Better for bone and acute blood,lesions of skull base and calvarium  Calcification  Less limited by patient factors  Disadvantages-  high radiation  poor visualisation of posterior fossa lesions
  • 33. INTERPRETATION OF CT BRAIN  1-GENERAL INFORMATION  2-EXTRACRANIAL TISSUE  3-CRANIAL BONE  4-BLOOD  5-CSF FLOW  A-VENTRICULAR SYSTEM  B-CISTERNS  6-BRAIN TISSUE  A-MASS LESIONS  B-SULCI & GYRI  C-GRY & WHITE DIFFERENTIATION
  • 34. Low density High density Csf Bone Fluid Calcification Air Blood Fat Contrast
  • 42. Physiologic calcifications  Chorid plexus-rare before 10yrs  Basal ganglia-rare before 40ys  Pineal gland-common after 30 yr rare before 10yr  Falx  Dentate nuclei
  • 43. INDICATIONS  To diagnose neuro infections and their complications  Stroke to distinguish infarct from hemorrhage  Ct angio before thrombolysis  Ct venogram for cerebral venous thrombosis(cvt)  Acute changes in mental status  Focal neurologic findings  Trauma  Suspected SAH  Cns tumors
  • 44. Peidural hematoma Convex shape Subdural hematoma Cresent shape Skull fracture
  • 45. Infarcts Anterior cerebral artery infarct Middle cerebral artery infarct Posterior cerebral artery infarct Hyper dense MCA sign Internal cerebral artery infarct ACA+MCA
  • 46. hemorrhage Intra parenchymal hemorrhage in putamen Sub arachnoid hemorrhage hyperdensities in sylvian fissure,basal cysterns
  • 47. Neuro infections bacterial meningitis radiological signs meningeal enhancement cerebral edema complications abscess stages sub dural abscess,epidural abscess
  • 48. Bacterial meningitis Indicatations for ct brain before lumbar puncture- to look for obstructive hydrocephalus-to prevent herniation to conform meningeal involvement—by meningeal enhancement
  • 49. meningitis complications suggested by seizures, altered sensorium, focal deficits encephalitis- cerebral edema is seen others cerebral abscess epidural/sub dural empyema arteritis leading to infarct hydrocephalus seen well effaced Gyri and sulci Normal parenchyma cerebral edema
  • 51.  Sub dural effusion
  • 52.  arteritis leading to infarct
  • 53.  cerebral abscess stages Early cerebritis early capsule, thin rim Late capsular, thick rim Multi loculated Late cerebritis
  • 54. d/d for multiple ring enhancing lesions Tuberculoma Neurocysticerosis cns crptococcosis Metastasis Abscess (also cerebritis) Glioblastoma, Granuloma Infarct (esp. Basal ganglia) Contusion (rare) AIDS (Toxoplasmosis, etc.) Lymphoma (common in AIDS ) neurosarcoidosis Demyelination (active) Resolving hematoma, Radiation change (necrosis)
  • 56.  Non contrast ct normal or may show complications  On contrast basal enhancing exudates,meningeal anhancement, tubeculomas with ring enhancement,ependimitis Basal exudate enhancement Tuberculomas with perilesional edema Coalising tuberculomas
  • 58.  Stages vesicular stage- live stage only hypo dense lesion with out perilesional edema/ring enhancement colloidal stage- perilesional edema with ring enhancement granular stage- scolex gets calcified resulting in central hyper density nodular stage- entire lesion gets calcified nodular stage- vesicular stage- colloidal stage- granular stage- nodular stage-
  • 59.  Tuberculous granuloma neurocysticercosis >20 mm size <20mm large perilesional edema usually small area irregular margine regular margin Coalising lesions noncoalising These findings are not path gnomic,above signs can be seen viceversa neurocysticercosis TUBERCULOMAs central dot sign Stary sky
  • 60. TOXOPLASMOSIS CT - (70-80 % cases ) multiple B/L hypo dense contrast enhancing focal lesions with predisposition to the basal ganglia and subcortical region. A double dose contrast with increased delay scan time may increase the sensitivity.
  • 62. Why not MRI them all??? - MRI is generally preferable to CT for evaluating intracranial neoplasms - CT is preferred for visualizing tumor calcification or intratumor hemorrhage. Cns tumors
  • 63. Commonly Calcified and Hemorrhagic Lesions Calcified Hemorrhagic Oligodendroglioma Glioblastoma multiforme Choroid Plexus tumor Oligodendroglioma Ependymoma Metastatic: Central neurocytoma Melanoma Craniopharyngioma Breast Teratoma Lung Chordoma meningioma
  • 64. Pilocytic Cerebellar Astrocytoma Cystic mass with nodular enhancement in the wall
  • 66. Glioblastoma Multiforme Cystic,solid and partially calcified Image finding irregularly enhancing with necrotic centre
  • 67. medulloblastoma  Premitive neuro ectodermal tumor  Usually arises from roof of 4th ventricle  It is partially enhancing on ct
  • 68. meningioma  Extra axial tumour  imaging- homogenously hyper enhancing

Editor's Notes

  1. Prepaired in
  2. Environmental radiation per year-0.02mSV
  3. .