CT scans use X-rays to create cross-sectional images of the body. The document discusses the history and principles of CT scanning, describing how images are reconstructed from X-ray absorption data. It outlines the components of a CT scanner including the X-ray tube and detectors. It discusses different generations of CT scanners and how they have improved over time, allowing for faster scan times. The document also covers CT imaging techniques, common artifacts, and applications of CT for evaluating various brain conditions.

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

26. RING APPEARANCE

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

50. hydrocephalus

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)

55. TUBERCULOMA

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

57. NEUROCYSTICERCOSIS

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

65. Ependymoma
Enhancing lesion with in 4th ventricle

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