Radiation Dose

Report
John C. Villforth Lecture
43rd Annual National Conference
on Radiation Control
J Thomas Payne PhD
Outline
 Evolution of CT scanners
1971 to the present …………
 Early Dose Measurements
 CTDI from “soup to nuts”
 The next phase “Equivalent Dose”
Sir Godfrey Hounsfield (1919-2004)
EMI Research Division
Nobel Prize - 1979
Oct. 1, 1971
1st Clinical scan – Atkinson Morley’s
Hospital, London
EMI - Electric and Musical Industries Ltd
formed March 1931 by merger of
UK Columbia Graphophone and
Gramophone Company
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
First CT scanner in U.S.
Mayo Clinic
Rochester, MN
June 1973
EMI Mark I
Polaroid film hard copy
Water bag diaphragm
Wow !
Could get a paper printout !!!
CT Advantage over plain film
Linear Attenuation map
• No superimposition
• Improved image contrast
0.5% versus 10%
The RACE begins…..
ACTA with COLOR - 1974
A REAL TURKEY
Watermelon not a cucumber
Early CT Physics Paper
1st Gen. 1972-73
2nd Gen. 1974-76
3rd Gen. 1976-78
4th Gen. 1978-79
CT Units 1974-78 over 18 vendors, 29 models
ACTA
Artronix – 1110 Neuro CAT
AS&E
CGR
Elscint - Scanex
EMI – Mark I, CT5005, CT1010
GE – CT/N, CT/T 6800,7800
Ohio-Nuclear – Delta 50,25,
110,150,190
Omnimedical – Omni 4001
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
Toshiba
Varian
Varian
Pfizer
CT Units 1974-78
ACTA
2” NaI detectors
Siemens Neurotom
Texas Instruments
Computer - hexadecimal
McCullough DFOV units -1.5 McC’s
CT Units 1974-78
Pfizer 200FS 2nd Gen. Trans/Rot
Siemens Neurotom
Mag Tape,
Film, Paper printout
Original Siemens Somatom
The GE Continuum………
Picker “Green”
4th Generation
3rd Generation - Fastest scan time
Improvements 1980-90 and beyond
512x512 matrix - 1981
Slip-rings for X-ray tube – continuous rotation
Faster data transmission – brush contactors / wireless
Multiple detector rows – Multislice 1998
Dual tube – super FAST Cardiac..
16, 64, 128, 256 Slice scanners
Large Detector size +30cm coverage
Modern CT – Covers Off!
We have come along way Baby !!!
X-ray Tube Housing
HV Transformer
Detector array
HV Transformer
In the early 80’s along comes a new kid on the block
Nuclear Magnetic Resonance --- MRI
Many projected demise of CT
Here lies
“CAT Scanner”
Killed by MRI
But wait - SPIRAL CT may save the day !!!!
SPIRAL CT introduced 1989
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
Pitch = Table speed (mm/rot) / Beam Width (mm)
SPIRAL CT 1996 – 2004 and beyond… SPEED BABY !!!
Slide – complements Willi Kalender
SPIRAL CT and even Dual Tube
Multiple detector rows – Multislice 1998
Dual tube – super FAST Cardiac..
Large Detectors – 256 Slice
Tube A
Siemens Definition
Tube B
Spiral CT
Slip-ring technology
Higher power Tubes
Interpolation algorithms
Detector B
Detector A
Radiation Dose - Early measurements
Medical Physicists get
thee to measuring !
Radiation Dose - Early measurements
Surface dose and
inside a humanoid
phantom
Patient Dose and
Dose Measurement
• TLD’s
• Film
In comes CTDI circa 1980
~ 8 years from 1st CT
CT Dose “Queen” – Cynthia McCollough PhD
Thomas Ruckdeschel, MS
Alliance Medical Physics
Tom Payne, PhD
Abbott Northwestern Hosp.
Krista Bush, RT(CT) ACR
Michael McNitt-Gray, PhD
UCLA School of Medicine
Cynthia McCollough, PhD
Mayo Clinic
“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
AAPM Report No. 96
January 2008
AAPM website:
http://www.aapm.org/pubs/reports/RPT_96.pdf
AAPM Report No. 96
January 2008
AAPM Report No. 96
January 2008
Weighted ave
1/3, 2/3
W. Leitz IEC
Remember that CTDIvol is a weighted Dose
for “plastic cylinder” with f-factor for “Air”
It is NOT PATIENT DOSE but a Dose descriptor
CTDI concept can be used for Helical scanning
CTDIvol = CTDI w / pitch
CTDIvol
Average weighted dose for a helical scan volume
Currently displayed for a CT scan technique
CTDI ion chamber (100 mm long) CTDI100
Acrylic CTDI phantoms
32 cm diameter (body)
16 cm diameter (head)
Holes for measurements
ACR CT Accreditation Dose Limits
Reference Levels
Examination
Adult Head
Old
CTDIw (mGy)
60
Pass/Fail
Criteria
Current
CTDIvol (mGy) CTDIvol (mGy)
75
80
Adult Abdomen
35
25
30
Pediatric Body
25
20
25
X
X
X
5yr old
Pediatric Head
1yr old
Why can’t there be peace in the valley?
New Scanners - CT beam is wider than detector
Cone beams baby!
Dental 3D “CT”
SPECT/CT
Courtesy of Bob Dixon
Why don’t they just
measure the dose?
But I thought we were
measuring dose
What a novel idea – just like
the therapy physicists do
http://www.aapm.org/pubs/reports/RPT_111.pdf
Develop concept of “equilibrium dose”
Deq
and “equilibrium dose- pitch product”
^D
where p1 x Deq = p2 x Deq
eq
AAPM TG200 CT Phantom
30cm Diameter
60 cm Length (multi-section)
High Density Polyethylene (HDPE)
Courtesy John Boone PhD
Future Challenge
CTDI vol compared to
Deq
Different :
Phantom material
Acrylic vs High Density Polyethylene
Phantom size
16cm/32cm vs
Chamber size
30cm
100mm Length vs 15mm Length
DL(0) and CTDI100 Equivalence
L/2
1
ion chamber
vs.


DSmall
L (0) 
f
(
z
)
d
z

b L / 2
pencil chamber
p D100(0) =
1
CTDI 100 
T
50 mm

f ( z)dz
50 mm
CTDI100
CTDI
Pencil
100 chamber
L
Integrate charge during axial or
helical scan series at an arbitrary
pitch p=b/T for a scan length L
z=0
Integrate charge for a
single axial scan
Mission Accomplished…
Bob Dixon
Fearless Leader
of TG 111
Dose Reduction Techniques
Automatic Exposure Control (AEC)
Iterative Reconstruction Techniques
AEC systems
Z-axis AEC
“Digital scout”
mA Modulation
During 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
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 space
Typical 40-50% dose reduction
CAT Scan Humor ……
The End
“”It’s
his morning
CAT scan “

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