1. CT Scanning and Patient Dose Past Present and Future
John C. Villforth Lecture
43rd Annual National Conference
on Radiation Control
J Thomas Payne PhD

2. Outline Evolution of CT scanners 1971 to the present …………
Early Dose Measurements
CTDI from “soup to nuts”
The next phase “Equivalent Dose”

3. CT – The beginning Sir Godfrey Hounsfield (1919-2004)
EMI Research Division
Nobel Prize
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

4. 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

5. First CT scanner in U.S.
Mayo Clinic
Rochester, MN
June 1973

6. EMI Mark I Polaroid film hard copy Water bag diaphragm Wow !
Could get a paper printout !!!

7. CT Advantage over plain film
Linear Attenuation map
No superimposition
Improved image contrast
0.5% versus 10%

8. The RACE begins….. ACTA with COLOR - 1974 A REAL TURKEY
Watermelon not a cucumber

9. Early CT Physics Paper

10. Oh those GENERATIONS !!!! 1st Gen. 1972-73 2nd Gen. 1974-76
3rd Gen
4th Gen

11. 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

12. 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
Toshiba
Varian
Pfizer
Varian

13. Computer - hexadecimal
CT Units
ACTA
2” NaI detectors
Texas Instruments
Computer - hexadecimal
Siemens Neurotom
McCullough DFOV units -1.5 McC’s

14. CT Units 1974-78 Siemens Neurotom Mag Tape, Film, Paper printout
Pfizer 200FS 2nd Gen. Trans/Rot
Siemens Neurotom
Mag Tape, Film, Paper printout

15. Original Siemens Somatom

16. The GE Continuum………

17. Picker “Green” 4th Generation

18. And the Winner is……. 3rd Generation - Fastest scan time
Improvements and beyond
512x512 matrix
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

19. Modern CT – Covers Off! We have come along way Baby !!!
X-ray Tube Housing
HV Transformer
Detector array
HV Transformer

20. 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 !!!!

21. 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:
Pitch = Table speed (mm/rot) / Beam Width (mm)

22. SPIRAL CT 1996 – 2004 and beyond… SPEED BABY !!!
Slide – complements Willi Kalender

23. 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
Detector B
Spiral CT
Slip-ring technology
Higher power Tubes
Interpolation algorithms
Detector A

24. Medical Physicists get
Radiation Dose - Early measurements
Medical Physicists get
thee to measuring !

25. Radiation Dose - Early measurements
Surface dose and inside a humanoid phantom

26. Patient Dose and
Dose Measurement
TLD’s
Film

27. In comes CTDI circa 1980 ~ 8 years from 1st CT

28. CT Dose “Queen” – Cynthia McCollough PhD
Thomas Ruckdeschel, MS
Alliance Medical Physics
Tom Payne, PhD
Abbott Northwestern Hosp.
Krista Bush, RT(CT) ACR
Cynthia McCollough, PhD
Mayo Clinic
Michael McNitt-Gray, PhD
UCLA School of Medicine
“CT Dose Index and Patient Dose: They are Not the Same Thing”
Cynthia McCollough PhD et.al. , Radiology, Vol. 259:No:2, , May 2011

29. AAPM Report No. 96 January 2008 AAPM website:

30. AAPM Report No. 96
January 2008

31. AAPM Report No. 96
January 2008
Weighted ave
1/3, 2/3
W. Leitz IEC

32. CTDI concept can be used for Helical scanning CTDIvol = CTDI w / pitch
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

33. CTDI ion chamber (100 mm long) CTDI100
Acrylic CTDI phantoms
32 cm diameter (body)
16 cm diameter (head)
Holes for measurements

34. ACR CT Accreditation Dose Limits
Pass/Fail Criteria
Reference Levels
Old
CTDIw (mGy)
Current
CTDIvol (mGy) 60 75 80 35 25 30 20 X
Examination
Adult Head
Adult Abdomen
Pediatric Body
5yr old
Pediatric Head
1yr old

35. 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

36. Why don’t they just measure the dose?
Courtesy of Bob Dixon
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

38. Develop concept of “equilibrium dose”
Deq
and “equilibrium dose- pitch product”
Deq where p1 x Deq = p2 x Deq ^

39. AAPM TG200 CT Phantom Courtesy John Boone PhD
30cm Diameter
60 cm Length (multi-section)
High Density Polyethylene (HDPE)

40. Future Challenge CTDI vol compared to Deq Different : Phantom material
Phantom size
Chamber size
Acrylic vs High Density Polyethylene
16cm/32cm vs 30cm
100mm Length vs 15mm Length

41. DL(0) and CTDI100 Equivalence
Small ion chamber vs. pencil chamber
z = 0
CTDI100
Pencil chamber
Integrate charge for a single axial scan
p D100(0) = CTDI100 L
Integrate charge during axial or helical scan series at an arbitrary pitch p=b/T for a scan length L

42. Mission Accomplished…
Bob Dixon
Fearless Leader of TG 111

43. Dose Reduction Techniques
Automatic Exposure Control (AEC)
Iterative Reconstruction Techniques

44. (LightSpeed Pro – only)
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
Toshiba
Sure Exposure

45. 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

46. CAT Scan Humor ……
The End
“”It’s his morning
CAT scan “