Medical Imaging (english) - b

Report
SCANCOMEDICAL
Computed Tomography
SCANCO User Meeting 2005
Dr. Bruno Koller
SCANCO Medical AG
www.scanco.ch
SCANCOMEDICAL
Overview
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X-Ray Basics
CT Hardware Components
Measurement
Reconstruction
Artefacts
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SCANCOMEDICAL
Introduction
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3D distribution of tissueproperties
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Density (absorption of Xrays, speed of sound…)
Chemical composition
Temperature
...
Imaging of these local
tissue properties using
grayscale or color mapping
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SCANCOMEDICAL
Introduction
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SCANCOMEDICAL
Whole Body CT
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Good S/N
Good contrast bone/soft tissue
Slice thickness 2-5 mm
2 cm
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SCANCOMEDICAL
Peripheral CT
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Good Contrast Bone/Soft tissue
Voxelsize 100 mm
Limited FOV (130 mm)
1 cm
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SCANCOMEDICAL
Microtomography
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Excellent contrast bone/soft
tissue
Slice thickness and in plane
resolution <10 mm
More noise in images
1 mm
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SCANCOMEDICAL
3D Microtomography
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SCANCOMEDICAL
CT-Basics
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Based on measurement of attenuation of X-rays (BeerLambert):
Source
m
Io

d
Detector
I
I  Io  e
 md
Measurement of a projection value (Sample):
 I 
P (t )  ln o    mdl
 I t   L
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SCANCOMEDICAL
Measurement of one projection
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SCANCOMEDICAL
Measurement of one projection
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SCANCOMEDICAL
Measurement of one projection
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SCANCOMEDICAL
Measurement of one projection
Io
t
I
t
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SCANCOMEDICAL
Projection Value Measurement
I
I
I0
m
X-rays
Source
Object
Detector
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SCANCOMEDICAL
Source
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X-Ray Tubes (most common)
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Continuous, steady output (high flux)
Small focal spot (< 10 mm)
Variable energy and intensity
Polychromatic beam
I
E
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SCANCOMEDICAL
Attenuation coefficient m [1/cm]

Attenuation coefficient changes with material:
 Io 
   m x, y dl
P (t )  ln
 I t   L

Attenuation coefficient changes with energy:
m
bone
muscle
fat
E
BK/ 16
SCANCOMEDICAL
Beam Hardening
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Soft X-rays are attenuated more than hard X-rays
Depending on object, spectrum changes
I
m(E)
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I
d
E
E
BK/ 17
SCANCOMEDICAL
Detectors
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Usually detect visible light only
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They all need Scintillators
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Counting Systems (Photomultipliers)
Integrating Systems (CCD, Diode Arrays, CMOS-Detectors)
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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SCANCOMEDICAL
CT-Measurement
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For a CT measurement one needs an certain number of
single projection measurements at different angles
(theoretically, an unlimited number is required)
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In realized Tomography-Systems one usually finds a
geometrically ordered detector configuration
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SCANCOMEDICAL
1st generation scanner
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Single Detector System
Translation-Rotation
5 min. per slice
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SCANCOMEDICAL
2nd generation scanner
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multichannel-Systems (4, 6, 8,
16)
Translation-Rotation
20 sec. per slice
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SCANCOMEDICAL
3rd generation scanner
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Fan-Beam-Geometry
multichannel-system (500+
detectors), angle > 180o
Rotation of tube and
detektorsystem
no translation
1 – 10 sec. per slice
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SCANCOMEDICAL
Parallel Beam (Synchrotron)
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Parallelbeam
Rotation of object only
No collimators required
2D-Detector arrays
 A. Kohlbrenner, ETH Zürich
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SCANCOMEDICAL
Cone Beam
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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
BK/ 24
SCANCOMEDICAL
Spiral scanning
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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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SCANCOMEDICAL
Reconstruction
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Iterative reconstruction
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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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SCANCOMEDICAL
Reconstruction
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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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SCANCOMEDICAL
Reconstruction
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SCANCOMEDICAL
Reconstruction
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SCANCOMEDICAL
Convolution-Backprojection
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SCANCOMEDICAL
Artefacts
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Beam Hardening
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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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SCANCOMEDICAL
Artefacts
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Object outside of FOV
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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
BK/ 32
SCANCOMEDICAL
Artefacts
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Motion
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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
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Wrong geometry (fan-beam-angle)
Centers artefact
Mechanical alignment
Insufficient no. of projections (sampling)
...
BK/ 33
SCANCOMEDICAL
Resources
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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
BK/ 34

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