John C. Villforth Lecture43rd 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”

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

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 ClinicRochester, MNJune 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%

ACTA with COLOR - 1974

A REAL TURKEY

Watermelon not a cucumber

The RACE begins…..

Early CT Physics Paper

1st Gen. 1972-73

4th Gen. 1978-793rd Gen. 1976-78

2nd Gen. 1974-76

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

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

CT Units 1974-78

McCullough DFOV units -1.5 McC’s

ACTA

Texas Instruments Computer - hexadecimal

Siemens Neurotom

2” NaI detectors

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

HV Transformer

HV Transformer

Detector array

X-ray Tube Housing

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

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

SPIRAL CT 1996 – 2004 and beyond… SPEED BABY !!!

Slide – complements Willi Kalender

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

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

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

AAPM Report No. 96January 2008

AAPM website:http://www.aapm.org/pubs/reports/RPT_96.pdf

AAPM Report No. 96January 2008

AAPM Report No. 96January 2008

Weighted ave 1/3, 2/3 W. Leitz IEC

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

CTDI ion chamber (100 mm long) CTDI100

Acrylic CTDI phantoms32 cm diameter (body)16 cm diameter (head)Holes for measurements

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

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

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

http://www.aapm.org/pubs/reports/RPT_111.pdf

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

^

AAPM TG200 CT Phantom Courtesy John Boone PhD

30cm Diameter60 cm Length (multi-section)High Density Polyethylene (HDPE)

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 :

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

Mission Accomplished… Bob Dixon

Fearless Leader of TG 111

Automatic Exposure Control (AEC)Iterative Reconstruction Techniques

Dose Reduction Techniques

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

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

“”It’s his morning CAT scan “

CAT Scan Humor ……The End