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Speakers: S
k L. N. Rothenberg, Ph.D. – Computed Tomography
G. D. Clarke, Ph.D. – Magnetic Resonance Imaging
J. A. Zagzebski, Ph.D. – Ultrasonic Imaging
August 1, 2012
A
Lawrence N. Rothenberg, Ph.D.
Keith S. Pentlow, M.Sc.
Department of Medical Physics
Memorial Sloan‐Kettering Cancer Center
e‐mail: rothenbl@mskcc.org
1970 ‐ Projection Radiography
2D projection of 3D anatomy
What about conventional tomography?
 With x‐ray tube moving above the table and the image receptor moving in a parallel plane below the table, we can get only coronal or sagittal slices.
 Out of plane anatomy blurred over the image
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Complex Tomography for Coronal Images
Philips Polytome & CRG Stratimatic
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Can we use screen‐film to get axial cuts?
 Toshiba found a way to do that
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Toshiba TAT
TAT‐Transverse Axial Tomography:
Toshiba Unit in Radiation Oncology
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CT Overview
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CT Overview and History
 CT was first introduced in Britain in 1971. The Nobel Prize for Medicine was awarded in 1979 to Godfrey Hounsfield of Britain (CT scanner) and Alan Cormack of the US (image reconstruction).
of the US (image reconstruction)
 It’s advantages over radiography are:
 Conventional radiography suffers from collapsing of 3D structures onto a 2D image.
 Although spatial resolution is lower in CT, low contrast resolution is far superior in CT, allowing for the first time the direct visualization of soft tissue organs and tumors.
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CT development
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•
•
•
•
•
EMI Mk1 head only scanner,
introduced 1971
1st generation, i.e. translate-rotate
geometry, parallel rays, pencil
b m one
beam,
n NaI
N I detector
d t t p
per sli
slice
180 rotation, 5 mins per acquisition
5 mins per reconstruction
2 slices produced
Initially 80 x 80 matrix
Water bag/box for bolus
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At Cornell – NY Hospital Neuroradiology
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EMI – Electric & Musical Industries, Ltd.
CT Acquisition
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CT Reconstruction
It  Ioe t
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G. Hounsfield and His Lab CT
ln( I o / I t )   t
3 Projections, 7 Rays Each
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EMI CT Scanning Motion
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First Patient Scans 1971‐72
 Ambrose at Atkinson Morley’s Hospital in London
 Patient with suspected frontal lobe tumor
 Mayo Clinic, NY Hospital and others in US
Problem for Physicists: How to measure patient dose?
Led to MSAD and CTDI
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EMI CT Head Image
What to call this process?
 Computerized Axial Tomography – CAT
 Computerized Transaxial Tomography – CTT
Gray scale 80 x 80 CRT and Polaroid image and paper printout provided
5 minute scan – 5 minute reconstruction
“It looks just like the cross-sectional anatomy books”
(Your
smart phone has much
more computing power than the EMI computer had!)
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 Computed Tomography ‐ CT
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Research and Development Goals
 Faster in‐slice scanning
 5 min – 20s – 5s – 3 s – 1s – 0.8s – 0.5 s – 0.3 s
 Faster image reconstruction
 5 min – a few seconds
f
d
 Better resolution in all three dimensions
 80 x 80 – 160 x 160 – 320 x 320 – 512 x 512 ‐ 1024 x 1024
 We can now achieve sub mm resolution in 3D
 Faster coverage of the entire body
 Minutes – a few seconds
Even Peanuts got into the
got into the act!
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Two Images
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Projection Radiography vs. CT
Radiography Computed Tomography
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CT Reconstruction
It  Ioe
 t
The Mathematicians
ln( I o / I t )   t
 Johan Radon – Radon Transform ‐ 2D and 3D reconstruction from infinite projections
 Alan Cormack – unaware of Radon’s work, developed reconstruction technique in South Africa –
reconstruction technique in South Africa Nobel Prize with Hounsfield in 1979
 David Kuhl – reconstruction techniques for PET
 Shepp and Logan – developed reconstruction algorithms
3 Projections, 7 Rays Each
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1st Generation: Rotate‐Translate
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2nd Generation: Rotate‐Translate w Small Fan Beam
5 Minute Scan Time
20 Second Scan, 30 detectors, 10 deg fan, required bolus
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Robert Ledley, D.D.S. (d. 7/24/12) and the ACTA Body and Head Scanner
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CT development
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Ohio-Nuclear Delta50 head and
body scanner (covers removed),
introduced 1974
2nd generation, i.e. translate-rotate
geometry, parallel rays, pencil
beam, three NaI detectors per
slice
180 rotation, 1 - 3 mins per
acquisition (2 slices)
256 x 256 matrix
Shaped filters
Images in color, CT number range 0 to 200, no bolus required
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3rd Generation: Rotate‐Rotate w Wide Fan Beam
Early CT System Components
Polaroid Camera – later replaced by laser imager, now images go directly to PACS
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3rd Generation
Scan times as low as 0.33 sec, 800 detectors, 40-50 deg fan, 360 deg rotation
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4th Generation
1000’s of stationary detectors, x-ray tube rotates 360 deg,no ring artifacts
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4th Generation Scanner (AS&E)
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CT Numbers‐Hounsfield Units (HU)
CT# (x, y)  1000
 (x, y) -  water
 water
CT# (water)
CT# (air)
CT# (soft tissue)
CT# (bone, I)
= 0
= -1000
= -300 to +100
= up to +3000
Originally EMI used +/-500, Ledley used 0 - 200
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CT Numbers‐Hounsfield Units (HU)
Survey of Early CT Units
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Hounsfield units = 1000 x ( - water) / water
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David White’s Phantoms
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TLD and Film Dosimetry
Scanners with 180 deg or 360 deg rotation, and possibly with overscan
Dr. White’s tissue AAPM History Symposium 2012
formulations used in many commercial phantoms
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Survey of Early CT Units
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CT in Popular Scientific Press
Note color image displays
Manufacturers
Pfizer/A.S. & E
Elscint
EMI
General Electric
Ohio-Nuclear
Philips
Picker
Searle
Siemens
Varian*
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5th Generation: Electron Beam CT (EBCT)
EBCT
• Imatron, (later marketed by
Picker, then Siemens, then
bought by GE), introduced
1980’s
• Electron Beam CT (sometimes
called 5th generation), stationary
scintillator-photodiode
detectors, scanning electron
beam on stationary target ring,
fan beam, four (eight) slice
• 216 rotation, 50 millisecs per
acquisition (8 slices)
R. Robb at Mayo Clinic
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CT Image Display – Window/Level
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Detectors/Detector Arrays‐Xenon Gas
CT was the first widely used digital imaging system
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Solid State Detectors
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Helical Slip Ring Scanners
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Size reduction & continuous rotation–slip rings 1987
Early Slip Ring Scanning
HV Slip Rings – Closed Tunnel
• Improved high voltage
generator technology
made units smaller
• Slip ring technology
enabled them to be
placed on the rotating
part of gantry
• This permitted
continuous rotation
without interscan
delays
• Set the stage for
spiral scanning
Key:
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1. Tube, 2. Collimator, 3. Tube Controller, 4. HV Gen (-),
5. Detector, 6. DAS, 7. HV Gen (+), H. OB Comp., 9. Stat Comp.
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Size reduction
•
Many components now in gantry or under the desk
Spiral CT (Also Helical) 1989
Pitch 
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table movement (mm) per rotation
collimator width (mm) at isocenter
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Multi-slice or multi-detector row CT
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MDCT
Multi-slice or
multi-detector
row CT
Driven by
– X-ray tube
heat loading
– Faster scans
– More
practical thin
slices
– Improved
spiral
interpolation
Or even 64 or 320 rows
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Reconstruction Example:
Iterative Reconstruction
Filtered Backprojection
Iterative Reconstruction - Slow
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Reconstruction Algorithms- GE Phantom
Soft
Standard
Lung
Detail
Bone
Edge
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Multiplanar Reconstruction
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Volume Rendering
Multiplanar Images – Isotropic Resolution
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Adaptive Statistical Iterative Reconstruction (ASIR)
advanced reconstruction technique that reduces image noise and improves low contrast detectability and image quality. Higher diagnostic performance at lower dose Up to 40% less dose with no loss of image quality Improves low contrast detectability up to 30% Reprojection Techniques
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PET-CT
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non-ASIR 120KV@150mAs
Noise=25.9
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ASIR 100KV@150mAs
Noise= 16.5
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Big Bore CT for Radiation Oncology
Computed Tomography
EMI Mk I - 1972
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Multi-slice CT
Angio-CT
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Note: Hopefully AAPM History Symposium 2012
the caption above refers to the scanner, not to me!
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320 Slice CT (Toshiba)
From Popular Science magazine
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Flat Panel Cone Beam CT
Cone Beam CT in Radiation Oncology
1 – MV Source
Accelerator
2 – kV Flat Panel
Imager
3 – MV Flat Panel
Imager
4 – kV Source
X-ray Tube
2
1
4
3
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From Lovelock
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Dental Cone Beam CT
1
Ionizing Radiation!
2
1 – X-ray Tube
2 – Flat Panel Imager
From Dental Planet
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CT History Information
Further Reading for CT
 W. Kalender, PMB 51, R29‐R43, 2006
RSNA/AAPM Teaching Modules:
CT Image Quality and Protocols
CT Systems
Radiation Dose in CT:
Cardiac CT
Physics of Cardiac Imaging with Multiple‐Row Detector CT – Mahesh and Cody. RadioGraphics 2007
Managing Radiation Use in Medical Imaging: A Multifaceted Challenge. Hricak, Brenner et al Radiology March 2011
 Siemens web site
 Impact Scan.org
 Google
 Wikipedia
 Many other sources
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Thanks for slides to:
Sources of Dose/Risk Information
 J. Anthony Seibert, Ph.D., 2011 President AAPM,  ACR & RSNA – radiologyinfo.org
Professor UC Davis, Radiological and Medical Physics Society of NY (RAMPS) October 5, 2010.
 Maynard High, Ph.D., Medical Physicist, Westchester Medical Center, NY.
 The Essential Physics of Medical Imaging 2nd
Edition. J.T. Bushberg, J.A. Seibert, E.M. Leidholdt and J.M. Boone, Lippincott Williams and Wilkins Publisher, 2002 and 3rd Edition 2012
 D. Michael Lovelock, Ph.D., Associate Attending Physicist, MSKCC
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 Image Gently for Pediatric Exams – imagegently.org
 Image Wisely for Adult Exams – imagewisely.org
 FDA Radiological Health
 IAEA Radition Protection of Patients – rpop.iaea.org They don’t just look for illegal nuclear weapons!
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