john c. villforth lecture 43 rd annual national conference on radiation control j thomas payne phd
TRANSCRIPT
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John C. Villforth Lecture43rd Annual National Conference
on Radiation Control
J Thomas Payne PhD
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Outline
Evolution of CT scanners
1971 to the present …………
Early Dose Measurements
CTDI from “soup to nuts”
The next phase “Equivalent Dose”
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EMI - Electric and Musical Industries Ltd formed March 1931 by merger of UK Columbia Graphophone and Gramophone Company
Sir Godfrey Hounsfield (1919-2004)EMI Research DivisionNobel Prize - 1979
Oct. 1, 19711st Clinical scan – Atkinson Morley’s Hospital, London
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EMI Mark I - 1st Commercial CAT scanner 1972 • 80x80 Matrix• Water bath• 10 minute scan time • Two (2) 13mm slices
Early EMI CT Head Scans, 80x80 Matrix
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First CT scanner in U.S. Mayo ClinicRochester, MNJune 1973
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EMI Mark I
Polaroid film hard copy
Water bag diaphragm
Wow !Could get a paper printout !!!
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CT Advantage over plain film
Linear Attenuation map• No superimposition • Improved image contrast
0.5% versus 10%
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ACTA with COLOR - 1974
A REAL TURKEY
Watermelon not a cucumber
The RACE begins…..
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Early CT Physics Paper
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1st Gen. 1972-73
4th Gen. 1978-793rd Gen. 1976-78
2nd Gen. 1974-76
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CT Units 1974-78 over 18 vendors, 29 modelsACTAArtronix – 1110 Neuro CAT
AS&ECGRElscint - Scanex
EMI – Mark I, CT5005, CT1010
GE – CT/N, CT/T 6800,7800
Ohio-Nuclear – Delta 50,25, 110,150,190
Omnimedical – Omni 4001
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CT Units 1974-78 over 18 vendors, 29 models
Pfizer – 200FS, 4000
Philips – Tomoscan
Picker – Synerview 300,600
Searle – Photrax 4000
Siemens – Siretom, Somatom
Syntex – System 60
Technicare - 2060
ToshibaVarian Varian
Pfizer
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CT Units 1974-78
McCullough DFOV units -1.5 McC’s
ACTA
Texas Instruments Computer - hexadecimal
Siemens Neurotom
2” NaI detectors
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CT Units 1974-78
Pfizer 200FS 2nd Gen. Trans/Rot
Siemens Neurotom
Mag Tape, Film, Paper printout
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Original Siemens Somatom
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The GE Continuum………
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Picker “Green” 4th Generation
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HV Transformer
HV Transformer
Detector array
X-ray Tube Housing
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In the early 80’s along comes a new kid on the block Nuclear Magnetic Resonance --- MRI
Many projected demise of CT
But wait - SPIRAL CT may save the day !!!!
Here lies “CAT Scanner”Killed by MRI
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Pitch = Table speed (mm/rot) / Beam Width (mm)Pitch = Table speed (mm/rot) / Beam Width (mm)
Willi Kalender, W. Seissler, E. Klotz, P Vock“Spiral volumetric CT with single-breathhold technique, continuous transport and continuous scanner rotation”
Radiology (1990), 176:181-183
SPIRAL CT introduced 1989
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SPIRAL CT 1996 – 2004 and beyond… SPEED BABY !!!
Slide – complements Willi Kalender
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Multiple detector rows – Multislice 1998Dual tube – super FAST Cardiac..Large Detectors – 256 Slice
Tube A
Tube B Detector B
Detector A
SPIRAL CT and even Dual Tube
Siemens Definition
Spiral CTSlip-ring technologyHigher power TubesInterpolation algorithms
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Radiation Dose - Early measurements
Medical Physicists get thee to measuring !
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Radiation Dose - Early measurements
Surface dose and inside a humanoid phantom
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Patient Dose and Dose Measurement• TLD’s • Film
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In comes CTDI circa 1980 ~ 8 years from 1st CT
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CT Dose “Queen” – Cynthia McCollough PhD
Cynthia McCollough, PhDMayo Clinic
Tom Payne, PhD Abbott Northwestern Hosp.
Michael McNitt-Gray, PhD UCLA School of Medicine
Krista Bush, RT(CT) ACR
Thomas Ruckdeschel, MSAlliance Medical Physics
“CT Dose Index and Patient Dose: They are Not the Same Thing” Cynthia McCollough PhD et.al. , Radiology, Vol. 259:No:2,311-316, May 2011
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AAPM Report No. 96January 2008
AAPM website:http://www.aapm.org/pubs/reports/RPT_96.pdf
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AAPM Report No. 96January 2008
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AAPM Report No. 96January 2008
Weighted ave 1/3, 2/3 W. Leitz IEC
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CTDI concept can be used for Helical scanning
CTDIvol = CTDI w / pitch
CTDIvol
Average weighted dose for a helical scan volumeCurrently displayed for a CT scan technique
Remember that CTDIvol is a weighted Dose for “plastic cylinder” with f-factor for “Air” It is NOT PATIENT DOSE but a Dose descriptor
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CTDI ion chamber (100 mm long) CTDI100
Acrylic CTDI phantoms32 cm diameter (body)16 cm diameter (head)Holes for measurements
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ACR CT Accreditation Dose Limits
OldCTDIw (mGy)
CurrentCTDIvol (mGy) CTDIvol (mGy)
60 75 80
35 25 30
25 20 25
X X X
Examination
Pediatric Body5yr old
Reference LevelsPass/Fail Criteria
Adult Head
Adult Abdomen
Pediatric Head1yr old
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Why can’t there be peace in the valley?
Cone beams baby!
New Scanners - CT beam is wider than detector
Dental 3D “CT”SPECT/CT
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Why don’t they just measure the dose?
What a novel idea – just like the therapy physicists do
But I thought we were measuring dose
Courtesy of Bob Dixon
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http://www.aapm.org/pubs/reports/RPT_111.pdf
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Develop concept of “equilibrium dose”Deq and “equilibrium dose- pitch product”Deq where p1 x Deq = p2 x Deq
^
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AAPM TG200 CT Phantom Courtesy John Boone PhD
30cm Diameter60 cm Length (multi-section)High Density Polyethylene (HDPE)
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Future ChallengeCTDI vol compared to Deq
Phantom material
Phantom size
Chamber size
Acrylic vs High Density Polyethylene
16cm/32cm vs 30cm
100mm Length vs 15mm Length
Different :
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DL(0) and CTDI100 Equivalence
Small ion chamber vs. pencil chamber
Integrate charge during axial or helical scan series at an arbitrary pitch p=b/T for a scan length L
z = 0
CTDI100
Pencil chamber
Integrate charge for a single axial scan
p D100(0) = CTDI100
mm
mm
zdzfT
CTDI
50
50
100 )(1
L
2/
2/
)(1
)0(L
L
L zdzfb
D
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Mission Accomplished… Bob Dixon
Fearless Leader of TG 111
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Automatic Exposure Control (AEC)Iterative Reconstruction Techniques
Dose Reduction Techniques
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AEC systems
Z-axis AEC“Digital scout”
mA ModulationDuring rotation
GE Auto mA Smart mA(LightSpeed Pro – only)
Philips Dose Right Dose Right DOM
Siemens CARE Dose 4D CARE Dose 4D
Toshiba Sure Exposure
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Iterative Reconstruction Techniques
GE ASiR ™ Adaptive Selective Iterative Reconstruction
Noise reduction technique works on projection data in recon space not an image filter in image spaceTypical 40-50% dose reduction
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“”It’s his morning CAT scan “
CAT Scan Humor ……The End