ct scanning - educationeducation.rad.msu.edu/courses/radcontent/unit1/ct/computed...spiral (helical)...
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CT Scanning Gerald R. Aben, MD, FACR Department of Radiology College of Osteopathic Medicine
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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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Tilting
Gantry
Examination Bed or Couch
CT Scanner
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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
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Gantry with Rotating Tube and Detector
Patient couch or bed
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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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Basic Principles of CT
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• 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
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• 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 pixel, the higher th l ti
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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
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Continuous Table Motion
Continuous Tube Rotation
2000# Spinning Instrument Package - <1rps
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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
Hounsfield Units
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-1,000 0 +1,000
Air Water Dense bone
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Liver, spleen, muscle, Aorta, gray matter, white matter,(+25 to +75 HU)
Typical CT Numbers (HU)
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+1000 ------Dense Bone
-1000
-700
0
-100
CSF, cystic lesions, water
Fat
Lung
0
50
100 Fresh Blood(+45 to +90 HU)
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Scanogram or Scout View
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Brain CT
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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 6/12/2012 DEPARTMENT OF RADIOLOGY 25
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Courtesy Dr. Jim Potchen
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CTA Coronary
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