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

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

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

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

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

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)

Scanogram or Scout View

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Brain CT

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Scalp Hematoma

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

Courtesy Dr. Jim Potchen

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CTA Coronary

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