Computed Tomography SCANCO User Meeting 2005

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Computed TomographySCANCO User Meeting 2005

• Dr. Bruno Koller
• SCANCO Medical AG
• www.scanco.ch

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Overview

• X-Ray Basics
• CT Hardware Components
• Measurement
• Reconstruction
• Artefacts

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Introduction

• 3D distribution of tissue-properties
• Density (absorption of X-rays, speed of sound)
• Chemical composition
• Temperature
• ...
• Imaging of these local tissue properties using
  grayscale or color mapping

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Introduction
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Whole Body CT

• Good S/N
• Good contrast bone/soft tissue
• Slice thickness 2-5 mm

2 cm
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Peripheral CT

• Good Contrast Bone/Soft tissue
• Voxelsize 100 mm
• Limited FOV (130 mm)

1 cm
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Microtomography

• Excellent contrast bone/soft tissue
• Slice thickness and in plane resolution lt10 mm
• More noise in images

1 mm
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3D Microtomography
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CT-Basics

• Based on measurement of attenuation of X-rays
  (Beer-Lambert)
• Measurement of a projection value (Sample)

Source
Detector
m
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Measurement of one projection
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Measurement of one projection
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Measurement of one projection
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Measurement of one projection
Io
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t
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Projection Value Measurement
I
I
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X-rays
Source
Object
Detector
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Source

• X-Ray Tubes (most common)
• Continuous, steady output (high flux)
• Small focal spot (lt 10 mm)
• Variable energy and intensity
• Polychromatic beam

I
E
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Attenuation coefficient m 1/cm

• Attenuation coefficient changes with material
• Attenuation coefficient changes with energy

bonemusclefat
m
E
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Beam Hardening

• Soft X-rays are attenuated more than hard X-rays
• Depending on object, spectrum changes

m(E) d
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Detectors

• Usually detect visible light only
• Counting Systems (Photomultipliers)
• Integrating Systems (CCD, Diode Arrays,
  CMOS-Detectors)
• They all need Scintillators
• Convert X-rays into light
• NaI, CsI, CdTe ...
• The thicker, the more efficient, but the thiner,
  the better the spatial resolution (tradeoff
  between high output or high res)
• Fiber optics (straight or tapered) in between to
  protect from remaining X-rays

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CT-Measurement

• For a CT measurement one needs an certain number
  of single projection measurements at different
  angles (theoretically, an unlimited number is
  required)
• In realized Tomography-Systems one usually finds
  a geometrically ordered detector configuration

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1st generation scanner

• Single Detector System
• Translation-Rotation
• 5 min. per slice

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2nd generation scanner

• multichannel-Systems (4, 6, 8, 16)
• Translation-Rotation
• 20 sec. per slice

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3rd generation scanner

• Fan-Beam-Geometry
• multichannel-system (500 detectors), angle gt
  180o
• Rotation of tube and detektorsystem
• no translation
• 1 10 sec. per slice

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Parallel Beam (Synchrotron)

• Parallelbeam
• Rotation of object only
• No collimators required
• 2D-Detector arrays

? A. Kohlbrenner, ETH Zürich
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Cone Beam

• Tube with focal spot
• Linear, 2-D Detector (e.g. 1024 x 1024 Elements,
  CCD)
• Single rotation
• Artefacts due to improper scanning scheme (would
  require to different movements)

? A. Kohlbrenner, ETH Zürich
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Spiral scanning

• Continuous movement of patient during rotation
• Volumetric measurement
• Slicewise reconstruction with variable slice
  thickness by interpolation
• As scanner can continuously rotate, one can
  achieve much faster scan speeds
• Latest models (clinical scanners) with parallel
  detector rings (Multirow, currently up to 64)
• 40 slices per second (150 rpm)
• No need in current MicroCT systems as the
  rotation speed is low

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Reconstruction

• Iterative reconstruction
• ART (Arithmetic Reconstruction Technique)
• Assume image (base image)
• Calculate projections of this base image
• Modify image after comparing calculated
  projections with measured Projections
• Strategy...

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Reconstruction

• Direct method The measured projections are
  backprojected under the same angle as the
  measurement was taken. All projections are summed
  up

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Reconstruction
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Reconstruction
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Convolution-Backprojection
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Artefacts

• Beam Hardening
• Attenuation coefficients depend on energy
• soft X-rays are much more absorbed than harder
  X-rays
• Distribution changes when beams penetrate object
• Segmentation problems

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Artefacts

• Object outside of FOV
• Inconsistent set of projection data (only
  partially within the beam at some angles,
  completely in the beam at other angles)
• Local Reconstruction only for geometry

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Artefacts

• Motion
• Object moves during scan
• May be eliminated by external gating
  (respiratory, heart beat)
• Total absorption of X-Rays e.g. Caused by
  metallic implants (division by 0 in
  reconstruction)
• Other Artefacts
• Wrong geometry (fan-beam-angle)
• Centers artefact
• Mechanical alignment
• Insufficient no. of projections (sampling)
• ...

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Resources

• Volume of 1024 x 1024 x 1200 requires 2.4 GB
  (short integer)
• Doubling the resolution requiers 8x more time to
  calculate
• Doubling the resolution requiers 8x more disk
  space