1 Center for Fast Ultrasound Imaging Department of Electrical Engineering Computer Tomography Systems Jørgen Arendt Jensen and Mikael Jensen (Risø DTU) October 5, 2011 Center for Fast Ultrasound Imaging, Build 349 Department of Electrical Engineering Technical University of Denmark Center for Fast Ultrasound Imaging, Department of Electrical Engineering Technical University of Denmark Computer tomography - outline •The idea behind computer tomography •Simple reconstruction •First and second generation systems •Fan beam and helical scan systems •The gantry and detection system •Fast CT systems •Image examples •Advise for the assignments Center for Fast Ultrasound Imaging, Department of Electrical Engineering Technical University of Denmark Why use CT? • Conventional radiography suffers from the collapsing of 3D structures onto a 2D image • Although resolution is lower in CT, it has extremely good low contrast resolution, enabling the detection of very small changes in tissue type • CT gives accurate diagnostic information about structures inside the body From: W. A. Kalender; Computer Tomography, Publicis, 2005 Center for Fast Ultrasound Imaging, Department of Electrical Engineering Technical University of Denmark What are we measuring? • The average linear attenuation coefficient, , between tube and detector: • Attenuation coefficient reflects the degree to which X-ray intensity is reduced by the material 4/x ) exp( x I I o Center for Fast Ultrasound Imaging, Department of Electrical Engineering Technical University of Denmark What do we measure? • Intensity measured by detector: • Conversion to attenuation: • Attenuation values are scaled relative to water: 0 ln 1 I I x 1000 water water tissue HU ) exp( x I I o Center for Fast Ultrasound Imaging, Department of Electrical Engineering Technical University of Denmark Early CT scanner geometry Single slice Single projection Single direction Very long scan time!
Transcript
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Center for Fast Ultrasound ImagingDepartment of Electrical Engineering
Computer Tomography Systems
Jørgen Arendt Jensen and Mikael Jensen (Risø DTU)
October 5, 2011
Center for Fast Ultrasound Imaging, Build 349Department of Electrical EngineeringTechnical University of Denmark
Center for Fast Ultrasound Imaging, Department of Electrical Engineering Technical University of Denmark
Computer tomography - outline
•The idea behind computer tomography•Simple reconstruction•First and second generation systems•Fan beam and helical scan systems•The gantry and detection system•Fast CT systems•Image examples
•Advise for the assignments
Center for Fast Ultrasound Imaging, Department of Electrical Engineering Technical University of Denmark
Why use CT?
• Conventional radiography suffers from the collapsing of 3D structures onto a 2D image
• Although resolution is lower in CT, it has extremely good low contrast resolution, enabling the detection of very small changes in tissue type
• CT gives accurate diagnostic information about structures inside the body
From
: W. A
. Kal
ende
r; C
ompu
ter T
omog
raph
y, P
ublic
is, 2
005
Center for Fast Ultrasound Imaging, Department of Electrical Engineering Technical University of Denmark
What are we measuring?
• The average linear attenuation coefficient, , between tube and detector:
• Attenuation coefficient reflects the degree to which X-ray intensity is reduced by the material
4/x
)exp( xII o
Center for Fast Ultrasound Imaging, Department of Electrical Engineering Technical University of Denmark
What do we measure?
• Intensity measured by detector:
• Conversion to attenuation:
• Attenuation values are scaled relative to water:
0
ln1II
x
1000
water
watertissueHU
)exp( xII o
Center for Fast Ultrasound Imaging, Department of Electrical Engineering Technical University of Denmark
Early CT scanner geometry
Single sliceSingle projectionSingle direction
Very long scan time!
2
Center for Fast Ultrasound Imaging, Department of Electrical Engineering Technical University of Denmark
Measurement of attenuation
From
: W. A
. Kal
ende
r; C
ompu
ter T
omog
raph
y, P
ublic
is, 2
005
Center for Fast Ultrasound Imaging, Department of Electrical Engineering Technical University of Denmark
Algebraic reconstruction
Io Io
Io
Io
From: W. A. Kalender; Computer Tomography, Publicis, 2005
Center for Fast Ultrasound Imaging, Department of Electrical Engineering Technical University of Denmark
Algebraic reconstruction
)exp()exp(
)exp()exp(
)exp()exp(
)exp()exp(
43022
21021
24012
31011
xdxII
xdxII
xdxII
xdxII
unknownsequations
xII
xII
xII
xII
4,4
)ln(
)ln(
)ln(
)ln(
431
22
0
211
21
0
241
12
0
311
11
0
Center for Fast Ultrasound Imaging, Department of Electrical Engineering Technical University of Denmark
Back projection
1 2 3 4 5 3 2 1 2 3 4 5 6 7 1
5 3
7
For all projections, the measured values are added to all contributing pixels
This will be further addressed in the next lecture
Center for Fast Ultrasound Imaging, Department of Electrical Engineering Technical University of Denmark
From
: W. A
. Kal
ende
r; C
ompu
ter T
omog
raph
y, P
ublic
is, 2
005
Center for Fast Ultrasound Imaging, Department of Electrical Engineering Technical University of Denmark
Sir Godfrey N. Hounsfield (Nobel Prize 1979)
• Obituary in The Telegraph: “The invention of the CAT scanner was a remarkable achievement, not least because of the complex algebraic calculations involved in the computer programming. Other research teams with larger resources than EMI had already dismissed such a device as impossible to develop, and one prominent British scientist remarked that Hounsfield's machine used "mathematics I wouldn't pretend to understand now or at any stage of my career".
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Center for Fast Ultrasound Imaging, Department of Electrical Engineering Technical University of Denmark
First and second generation scans
From
Hou
nsfie
ld’s
orig
inal
pat
ent
Center for Fast Ultrasound Imaging, Department of Electrical Engineering Technical University of Denmark
First CT scanner (EMI scanner)
• 80 x 80 pixel matrix• 4 min rotation time• 8 gray levels• Overnight image
reconstruction
http
://fa
rm4.
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ic.fl
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.com
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jpg
Center for Fast Ultrasound Imaging, Department of Electrical Engineering Technical University of Denmark
First clinical CT images from Atkinson Morley's Hospital in 1971
Center for Fast Ultrasound Imaging, Department of Electrical Engineering Technical University of Denmark
Hounsfield units:
•Intensity measured by detector:
•Conversion to attenuation:
•Attenuation values are scaled relative to water: 0
ln1II
x
1000
water
watertissueHU
)exp( xII o
Center for Fast Ultrasound Imaging, Department of Electrical Engineering Technical University of Denmark
Typical H.U. Values
Center for Fast Ultrasound Imaging, Department of Electrical Engineering Technical University of Denmark
Hounsfield units
From
: W. A
. Kal
ende
r; C
ompu
ter T
omog
raph
y, P
ublic
is, 2
005
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Center for Fast Ultrasound Imaging, Department of Electrical Engineering Technical University of Denmark
Windowing
Center for Fast Ultrasound Imaging, Department of Electrical Engineering Technical University of Denmark
From
: W. A
. Kal
ende
r; C
ompu
ter T
omog
raph
y, P
ublic
is, 2
005
Center for Fast Ultrasound Imaging, Department of Electrical Engineering Technical University of Denmark
Modern CT system generations
From
: W. A
. Kal
ende
r; C
ompu
ter T
omog
raph
y, P
ublic
is, 2
005
Center for Fast Ultrasound Imaging, Department of Electrical Engineering Technical University of Denmark
Modern CT scanner
Center for Fast Ultrasound Imaging, Department of Electrical Engineering Technical University of Denmark
The development in image quality
a) Early CT images with 80 x 80 pixels
b) View from several scans
c) Modern CT image with 1024 x 1024 pixels
d) Combined scans for 3D
From
: W. A
. Kal
ende
r; C
ompu
ter T
omog
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y, P
ublic
is, 2
005
Center for Fast Ultrasound Imaging, Department of Electrical Engineering Technical University of Denmark
From
: W. A
. Kal
ende
r; C
ompu
ter T
omog
raph
y, P
ublic
is, 2
005
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Center for Fast Ultrasound Imaging, Department of Electrical Engineering Technical University of Denmark
Center for Fast Ultrasound Imaging, Department of Electrical Engineering Technical University of Denmark
From
: http
://be
lley.
org/
ct/c
t/obj
ectiv
e03/
imag
es/c
t-gan
try-in
side
-fron
t.jpg
Center for Fast Ultrasound Imaging, Department of Electrical Engineering Technical University of Denmark
Tube for X-raygeneration in CT
From
: W. A
. Kal
ende
r; C
ompu
ter T
omog
raph
y, P
ublic
is, 2
005
Center for Fast Ultrasound Imaging, Department of Electrical Engineering Technical University of Denmark
From
: W. A
. Kal
ende
r; C
ompu
ter T
omog
raph
y, P
ublic
is, 2
005
Center for Fast Ultrasound Imaging, Department of Electrical Engineering Technical University of Denmark
Slip rings in the CT gantry
From: http://w
ww
.bmim
ed.com/filebin/JP%
20Pictures/G
E%2064%
20Gantry%
20005.jpg
Center for Fast Ultrasound Imaging, Department of Electrical Engineering Technical University of Denmark
Slip Rings
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Center for Fast Ultrasound Imaging, Department of Electrical Engineering Technical University of Denmark
Detector types in CT
From
: W. A
. Kal
ende
r; C
ompu
ter T
omog
raph
y, P
ublic
is, 2
005
Center for Fast Ultrasound Imaging, Department of Electrical Engineering Technical University of Denmark
Modern CT system generations
From
: W. A
. Kal
ende
r; C
ompu
ter T
omog
raph
y, P
ublic
is, 2
005
Center for Fast Ultrasound Imaging, Department of Electrical Engineering Technical University of Denmark
Center for Fast Ultrasound Imaging, Department of Electrical Engineering Technical University of Denmark
Center for Fast Ultrasound Imaging, Department of Electrical Engineering Technical University of Denmark
Imatron scanner
Center for Fast Ultrasound Imaging, Department of Electrical Engineering Technical University of Denmark
Multi-slice CT system and fast scanning
From
: W. A
. Kal
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r; C
ompu
ter T
omog
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y, P
ublic
is, 2
005
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Center for Fast Ultrasound Imaging, Department of Electrical Engineering Technical University of Denmark
Number of detectors
• Low-end scanners: 400-600 detectors/row
• High-end scanner: 650-900 detectors/row
• Multi-slice scanners:
64, 256, 384 rows today
Number of projections
• Low-end scanners: 600-1000 per rotation/slice
• High-end scanner: 1100-1800 per rotation/slice
From
: W. A
. Kal
ende
r; C
ompu
ter T
omog
raph
y, P
ublic
is, 2
005
Center for Fast Ultrasound Imaging, Department of Electrical Engineering Technical University of Denmark
Examples of CT images
Center for Fast Ultrasound Imaging, Department of Electrical Engineering Technical University of Denmark
Center for Fast Ultrasound Imaging, Department of Electrical Engineering Technical University of Denmark
Center for Fast Ultrasound Imaging, Department of Electrical Engineering Technical University of Denmark
Center for Fast Ultrasound Imaging, Department of Electrical Engineering Technical University of Denmark
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Center for Fast Ultrasound Imaging, Department of Electrical Engineering Technical University of Denmark
Center for Fast Ultrasound Imaging, Department of Electrical Engineering Technical University of Denmark
Examplesof high-quality CT images with 3D reconstruction, segmentation and rendering
Center for Fast Ultrasound Imaging, Department of Electrical Engineering Technical University of Denmark
3D reconstruction, segmentation and shading
Center for Fast Ultrasound Imaging, Department of Electrical Engineering Technical University of Denmark
3D reconstruction and segmentation
Center for Fast Ultrasound Imaging, Department of Electrical Engineering Technical University of Denmark
Segmentation
Center for Fast Ultrasound Imaging, Department of Electrical Engineering Technical University of Denmark
Cardiac CT image – multi-slice and ECG gated
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Center for Fast Ultrasound Imaging, Department of Electrical Engineering Technical University of Denmark
Summary
• CT developed since the 1970’ties• Several generations have been
made• Overview of technology used• Helical, multi-slice scanning
possible today• Excellent image quality obtained
today
• Next lecture: filtered backprojection algorithm and other reconstruction methods
Center for Fast Ultrasound Imaging, Department of Electrical Engineering Technical University of Denmark
Advise for assignments
• Get started• Get started
•Get started …..
• Make a validated Matlab estimator and show its performance• Document it in your report with a description and plots of results• Remember always to put SI units (or some scaled version) on the axis• Validate using exercise 3 data that your estimator works• Show performance with different parameters• Discuss results also if they are wrong (find the error)
• Report should contain all code in appendix• It should be handed in as a pdf document that is searchable
