Download CT Scanning

CT Scanning
Gerald R. Aben, MD, FACR
Department of Radiology
College of Osteopathic Medicine
6/12/2012
DEPARTMENT OF RADIOLOGY
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Computed Tomography (CT)
• Donut Shaped machine
• Uses x-ray energy and computer generation of
images
• Advantages
• Sensitive to slight density difference
• Cross sectional anatomy
• Attenuation: reductions in intensity of x-ray beam as
it traverses matter either by absorption or deflection
• Special terms used on CT reports
• High attenuation, Low attenuation
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CT Terms
• High attenuation
• Absorption of x-ray photon
• Presented as white on image
• Low attenuation
• Free passage of photon
• Presented as black on image
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CT Scanner
Tilting
Gantry
Examination
Bed or Couch
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64+ Slice CT
• Faster scan times
• Reduced patient motion
• Increased resolution 0.35mm isotropic
resolution
• 3-D reconstructions
• Improved diagnostic accuracy
• Reduced need for ‘high risk’ somewhat
more invasive examinations
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CT Scanner
Patient couch or
bed
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DEPARTMENT OF RADIOLOGY
Gantry with
Rotating
Tube and
Detector
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Basic Principles of CT
• CT imaging system
moves around the
body part at a fixed
location
• Attenuation
information
obtained in
multiple planes
• Reconstruct of this
attenuation
information into a
simple grid
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DEPARTMENT OF RADIOLOGY
Basic Principles of CT
• Each body section
divided into 3
dimensional boxes –
voxel
• 2 dimension grid of
pixels
• Calculate attenuation
in each direction
• Add up all attenuations
in each pixel
• Normalize to a common
scale
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Basic Principles of CT
• Density of each
pixel varies
resulting in a
pictorial
representation of
the density of
structures within
that section
• Repeat for each
subsequent slice
• The smaller the
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pixel,
the higher
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Spiral (Helical) CT:
• Table moves at constant speed
• X-ray tube and detectors continuously rotating
• Multiple views are acquired which are not in-plane
(helical data set-volumetric data)
• Computer reconstructs views to form a slice (similar
principle to that presented earlier)
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Spiral (Helical) CT:
• Faster image acquisition than conventional CT (less
motion artifact)
• Allows high resolution 2-D and 3-D reformations
• Isotropic Voxels
• Can also obtain conventional axial image at a single
location (i.e. head CT, high resolution lung CT)
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Spiral (Helical) CT Scanning
Continuous
Tube
Rotation
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2000# Spinning
Instrument Package <1rps
DEPARTMENT OF RADIOLOGY
Continuous
Table
Motion
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Hounsfield Units
Hounsfield units (HU) = CT Numbers =
Arbitrary scale based on attenuation with water assigned a
CT number of 0
One CT number (HU) =
1/1000 of water attenuation
value
= 0.1% change in attenuation
relative to water
-1,000
Air
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0
Water
DEPARTMENT OF RADIOLOGY
+1,000
Dense bone
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Typical CT Numbers (HU)
+1000 ------Dense Bone
100
Fresh Blood(+45 to +90 HU)
Liver, spleen, muscle,
Aorta, gray matter, white
matter,(+25 to +75 HU)
0
CSF, cystic lesions, water
50
0
-100
-700
Fat
Lung
-1000
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Scanogram or Scout View
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Brain CT
Scalp
Hematoma
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CT Brain Contrast
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CT Brain Axial MIP
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CT Brain Coronal MIP
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CT Brain Sagital MIP
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CT Abdomen
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3-D CT Cervical Spine
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CT Colonography
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Courtesy Dr. Jim Potchen
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Courtesy Dr. Jim Potchen
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CTA Coronary
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