multi detector ct (mdct) dosimetry - hps detector ct (mdct) dosimetry ... age 0-10 age 11-17 age...

Multi‐Detector CT (MDCT) Dosimetry –Multi Detector CT (MDCT) Dosimetry Current Issues and Challenges

NC HPS SPRING MEETINGMARCH 7, 2008CAROLINA BEACH, NC

Terry Yoshizumi, Ph.D.Associate Professor

Duke University Medical CenterDurham NCDurham, NC

yoshi003@mc.duke.edu1

ACKNOWLEDGEMENTACKNOWLEDGEMENT

•James Colsher, PhD, GE HealthcareJames Colsher, PhD, GE Healthcare

•Osama Saba, PhD, Siemens Medical Solutions

•Fred Mettler Jr MD University of New MexicoFred Mettler Jr. MD, University of New Mexico

•Mahadevappa Mahesh, PhD, Johns Hopkins HospitalHospital

TODAY’S TOPICSTODAY S TOPICS

1. CT patient dosimetry ‐ why so important?

2. Recent changes that affect CT dosimetry

3. Factors affecting dose3. Factors affecting dose

4. Dose indices – yesterday, today and tomorrowtomorrow

5. Review of dose estimation methods

6. Emerging issues and opportunities

CT PATIENT DOSIMETRY CT PATIENT DOSIMETRY ‐‐WHY SO WHY SO ??IMPORTANT?IMPORTANT?

• Controversy generated by David Brenner andControversy generated by David Brenner and Eric Hall’s paper in 2007– Cancer risk from CT– Cancer risk from CT

• CT dose ‐ New findings coming out of NCRP Scientific Committee 6 2 (since 2005)Scientific Committee 6‐2 (since 2005)

• Potential danger of modern CT scanners

• Rapid advancement of CT technology

• Rapid expansion of CT applicationsp p pp

• David Brenner and Eric Hall’s controversial paper in 2007p p

– Computed Tomography – An increasing source of radiation exposure

– New England J of Med 2007: 357; 2277‐2284

– Suggested the radiation dose from CT scans is a cause for concern and may be responsible for a small percentage ofconcern, and may be responsible for a small percentage of cancer deaths in the U.S.

– Strong responses from professional societies such as HPS, AAPM, and ACR.

• CT dose ‐ New findings coming out of NCRP Scientific Committee 6 2 (since 2005)Scientific Committee 6‐2 (since 2005)

• CT dose ‐ New findings coming out of NCRP ScientificCT dose New findings coming out of NCRP Scientific Committee 6‐2 (since 2005) – One of the subcommittees was Medical Patient Exposure

– Previous NCRP report was published in 1987 as NCRP Report 93 (20 yrs ago)

Wh h d i h l 20 ?– What changed in the last 20 yrs?

• Medical imaging technology

• Clinical practice• Clinical practice

• Patient dosimetry

Radiation dose from CT: Then (1980) and Now (2006)

• According to NCRP report 100 (1980)According to NCRP report 100 (1980)

– Collective dose for CT 3,700 person Sv

• According to current estimations (2006)According to current estimations (2006)

– Collective dose for CT 438,000 person Sv

Effective dose per capita ~1 5 mSv– Effective dose per capita 1.5 mSv

93,700 person Sv438,000 person Sv

CT scans by year in US (millio

CT procedures by year (millions)

Annual growth of >10% per yAnnual growth > 10%/yr

U.S. population < 1%/yr

Courtesy of Dr. Fred Mettler

39.640.0

18.3 19.521.0

22.625.1 26.3

0.01993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 200610

CT scans of Abdomen and PelvisExam distribution vs US Population*p

19.0% 19.4%

15.0% 15.3%

10.1% 10.0%

13.7% 14.0%

9.6%10.0%

7.2%6.3%

4.4%5.0%

1.3%2.2%

Age 0-10 Age 11-17 Age 18-24 Age 25-34 Age 35-44 Age 45-54 Age 55-64 Age 65-74 Age 75-84 Age 85 and older

% of CTs % of Population% of CTs % of Population

* LNEP 2003~ 20% of population >55 years, receives >55% of CT scans 11

From AAPM 2007 Presentation: Dr. Mahesh, NCRP Committee

Preliminary estimate of changes in

U. S. medical radiation exposure

All other ?? mSv

U.S. 1980 U.S. 2006

Interventional 0.4 mSvAll other

Natural Radiography 0.3 mSv

Nuclear medicine 0 8 mSv

All other ?? mSv

l 2.4 UNSCEAR

CT scanning 1.5 mSv

Nuclear medicine 0.8 mSv

Medical 0.54 mSv

Natural 2.8 mSv

Medical ~3.0 mSv

Medical 0.54 mSv

Total 3.6 mSv per capitaMedical 3.0 mSv

Total ~ 5.413

Computed Tomography (CT)In Summary

• Annual growth over 1993‐2006:Annual growth over 1993 2006:

– CT Procedures > 10% vs US population < 1%

• Nearly 62 million CT procedures in US in 2006

• Data correlated to nearly 7649 hospitals in US• Data correlated to nearly 7649 hospitals in US

• Pediatric CT ~ 8‐10% of total procedures

• CT dose ‐ new Findings coming out of NCRP Scientific Committee 6 2 (since 2005)Scientific Committee 6‐2 (since 2005)

• Potential danger of modern CT• Potential danger of modern CT scanners

–Danger of not knowing the capability of CT scanner– casecapability of CT scanner– case report

–Capability of modern CT scanner–extreme case studyextreme case study

Danger of not understanding CT dose

•3 cases reported•MDCT perfusion +DSA (CT fluoroscopy)•Temporary hair loss (3‐5 Gy)

• Potential danger of modern CT• Potential danger of modern CT scanners

–Danger of not knowing the capability of CT scanner– casecapability of CT scanner– case report

–Capability of modern CT scanner–extreme case studyextreme case study

CAPABILITY OF MODERN CT SCANNERPediatric Phantom 5‐year Oldy

Siemens 64‐slice

Summary: Siemens 64‐sliceSummary: Siemens 64 slice*Care= automatic tube current modulation

ED (mSv)ED (mSv) +/+/--SD (SD (mSvmSv))

Chest W Care*Chest W Care* 3.053.05 0.140.14C es W Ca eC es W Ca e 3.053.05 0. 40. 4

Chest W/O CareChest W/O Care 3.053.05 0.140.14

Ch ECh E 42 9542 95 0 550 55Chest ExtremeChest Extreme 42.9542.95 0.550.55

Abdomen W CareAbdomen W Care 6.836.83 0.140.14

Abdomen W/O CareAbdomen W/O Care 5.915.91 0.290.29

Abdomen ExtremeAbdomen Extreme 118.9118.9 1.851.85

EXTREME CASES: Siemens 64‐slice

Chest W/O Care - Extreme CaseChest W/O Care Extreme Case

3.97 5.

0.04 0.26

Slice 12- BM/ BM/ Lungs- Esopha BM/ Pancrea Stomac Intestine Small Ascendi BM/

OrgansSeries1 10.19 0.64 11.66 5.66 10.82 11.87 10.27 10.99 10.73 7.04 6.66 3.97 5.43 2.72 1.23 0.92 0.29 0.14 0.04 0.26

Breast- skin

Mandible

Thyroid Kidney Ribs-sternum

thymus BreastLungs Heart gus-

HeartSpine-

(T Spleen

Pancreas Liver

Stomach

Intestine ULI

Small Intestine ng

BM/ Pelvis Ovaries Bladder

Abdomen W/O Care - Extreme Case

50 5 64 6 83 6.30

1.11 1.58 1.95

3 14. 6

13.8 14

5 14. 0

Abdomen

OrgansSeries1 15.21 0.19 0.38 14.50 1.11 1.58 1.95 8.07 13.85 11.23 14.64 13.86 14.83 16.30 13.43 20.45 21.71 11.25 14.07 14.68

Slice 21-Pelvis-

BM/ Mandibl

eThyroid Kidney

BM/ Ribs-

sternuthymus Breast

Lungs- Heart

Esophagus- Heart

BM/ Spine-

(T Spleen

Pancreas Liver

Stomach

Intestine ULI

Small Intestin

Ascending

BM/ Pelvis Ovaries Bladder

TOPICSTOPICS

6. Research opportunities

THE BIG PICTURETHE BIG PICTURE

CHANGES THAT AFFECT CT DOSIMETRYT iti P i d ft 1998Transition Period after 1998

From SDCT to MDCT – transition periodFrom SDCT to MDCT transition period 1998 (confusion period)

Detector width increasedDetector width increased

Shorter focal spot to center of ratation distancedistance

Dose profile due to detector width

Duke data between SDCT and MDCT (SnapDuke data between SDCT and MDCT (Snap shot)

DETECTOR WIDTH INCREASED (>1998)

10 mm 20 mm

GE CT/i GE QX/i 4‐slice

Shorter focal spot‐to‐center of rotation distance

Focal spotFocal spot--toto

SDCTSDCTGE CT/iGE CT/i

Focal spotFocal spot to to isocenter shorter for isocenter shorter for MDCTMDCT

630 mm vs. 540 mm630 mm vs. 540 mm

(630/540)(630/540)22 1 36 11 36 1(630/540)(630/540)2 2 = 1.36:1= 1.36:1

MDCT < SDCT

↓mAs 74% with the mAs 74% with the same noisesame noise

MDCTMDCT(1/1.36 =0.74)(1/1.36 =0.74)

MDCTMDCTGE QX/iGE QX/i

MDCT QXi dose profile (dotted line)

profile (dotted line)

QXi 4 x 5.0 mm 0.6

4i mode body (GE manual)

CTI dose profile

CTi 5 b d (GE

(dotted line)

CTi, 5 mm body (GE manual)

CONFUSION PERIOD IN DOSIMETRY 1998 19991998‐1999

• Many facilities continued to adopt old SDCTMany facilities continued to adopt old SDCT scan protocols in new MDCT environment without considering design changes thatwithout considering design changes that might affect patient dose

• Also many hospitals did not differentiate scan• Also many hospitals did not differentiate scan protocols between adults and pediatric patients; this further complicated thepatients; this further complicated the dosimetry issues

DUKE EXAMPLE: DOSE COMPARISON BETWEEN SDCT AND MDCT SNAP SHOT 2001MDCT ‐‐‐ SNAP SHOT 2001

Yoshizumi TT and Nelson RC, Radiation Issues with Multi-Detector Row HelicalCT Critical Reviews in Computed Tomography 2003; 44: 95-117

Relative Dose Comparisonbetween CT/i and QX/i

CT. Critical Reviews in Computed Tomography 2003; 44: 95 117.

between CT/i and QX/i

20CT/iQX/i

17 17 17

8.5 8.3mG

Chest Abdomen Pelvis0

Duke Body CT Protocol (2001)

(using a 32cm acrylic phantom with(using a 32cm acrylic phantom withTLD chips inside the center hole)

Snapshot in2007 HEAD SCAN COMPARISON (Duke Data)( )

Variation in patient dose in various CT scanners

1.52 1.58

1.62 1.62 1 62 1 62 1 69

Comparison of ED (mSv)

1.401.62 1.62 1.69

Scanner Type

DOSE COMPARISON UNDER SAME IMAGE QUALITY

MDCTMDCT

Comparison of Dose Profile 4 li QX/i d 1 li CT/i4-slice QX/i and 1-slice CT/i

DOSE COMPARISON BETWEEN 20 mm QX/i AND 10 mm CT/I

(Scanned to produce the same image noise)(Scanned to produce the same image noise)

QXi (151 A)QXi (151 mA)CTi (265 mA)% Diff.

Dose Comparison (QXi and CTi)

scanned outside scanned plane

708090100 QXi Fitted

CTi Fitted

40506070

Differen

10203040

0 10 20 30 40 50 600 0

RECENT ADVANCES IN CT TECHNOLOGY

F t t b t ti tiFaster tube rotation times • <0.4 s for a full rotation • shorter exposure time > tube current must• shorter exposure time ‐> tube current must increase to get same photon statistics ‐> higher Heat Unitg

Greater Anode Heat Capacity (HU=mA * kVp * seconds)seconds)

•10 MHU (64-slice MDCT)•MDCT (e.g., GE QX/i 6.3 MHU) •SDCT 2-3 MHU (e.g., GE CT/i)•Axial scanner (old days) 1 MHU 34

Toshiba 256 slice CT scanner AAPM 2007

4 0 cm4.0 cm

12.8cm

Toshiba Aquilion One (2008)Toshiba Aquilion One (2008)

• 12.8 cm detector (2007) to 16 cm detector coverage (320 x 0.5 mm), 0.35 SEC ROTATION

6. Research opportunities

FACTORS AFFECTING DOSEFACTORS AFFECTING DOSE

1. Pitch effect1. Pitch effect

2. Focal spot wobble and tracking

3 Choice of kVp on dose3. Choice of kVp on dose

4. Patient thickness

5 Tube current (auto mA modulation)5. Tube current (auto mAmodulation)

6. Tube rotation time

7 Slice thickness7. Slice thickness

8. Beam filter

Pitch = Table travel per rotation li thi kslice thickness

Pitch = 10 mm/10 mm = 1

Pit h 20 /10 2Pitch = 20 mm/10 mm = 2

Thi d fi iti l li tThis definition only applies to

single-detector helical CT.

FACTORS AFFECTING DOSE

1. Axial CT vs. Helical SDCT F i h 1 d hFor pitch=1, doses are the same (axial=helical)

D f it h 2 i SDCT i ½ th t fDose for pitch 2 in SDCT is approx. ½ that of pitch 1

2 Coronary CTA2. Coronary CTA

– Pitch < 0.3 overlapping beams pp gcontribute higher dose (0.22 GE)GE)

1. Pitch effect1. Pitch effect

3 Choice of kVp on dose3. Choice of kVp on dose

5 Tube current (auto mA modulation)5. Tube current (auto mAmodulation)

6. Tube rotation time

7 Slice thickness7. Slice thickness

8. Beam filter

FOCAL SPOT WOBBLE AND TRACKING

• Purpose: to follow the focal spot so that onePurpose: to follow the focal spot so that one can maintain a uniform x‐ray beam that is as narrow as possible on the detector therebynarrow as possible on the detector, thereby reducing dose and avoiding artifacts

• Toth reported a dose reduction of up to 40% on• Toth reported a dose reduction of up to 40% on GE LightSpeed QX/i

FOCAL SPOT WOBBLE AND TRACKING

• focal spot moves in the direction of z‐axis due to thermal pchanges in the tube and mechanical forces associated with gantry rotation and tilt angle

Basic approach: use some detectors beyond those capturing the clinically useful signal to track the wandering of the penumbra regionsuseful signal to track the wandering of the penumbra regions

1 Pitch effect1. Pitch effect

3 Ch i f k d3. Choice of kVp on dose

5. Tube current (auto mAmodulation)

6. Tube rotation time6. Tube rotation time

7. Slice thickness

8 B fil8. Beam filter46

FACTORS AFFECTING DOSE

3 Choice of kVp on dose3. Choice of kVp on dose– The dose varies approximately to the power of

2; increasing kVp results in an increase in dose2; increasing kVp results in an increase in dose

– Example: 120 kVp to140 kVp results in the 36% dose increase, i.e., (140/120)2=1.36dose increase, i.e., (140/120) 1.36

– Image quality issue

New issue Dual Energy CT (New– New issue‐ Dual Energy CT (New Area)

1. Pitch effect. tc e ect2. Focal spot wobble and tracking3. Choice of kVp on dose3. Choice of kVp on dose

4. Patient thickness5 T b t ( t A d l ti )5. Tube current (auto mAmodulation)6. Tube rotation time

li hi k7. Slice thickness8. Beam filter

ABDOMINAL CT FOR OBESE PATINETS

• Schindera etal. (Duke Univ) Acad. Radiol 2007; 14 486 49414:486‐494.

• ‐studied the effect of a modified MDCT protocol for b ti t i lit d di ti dobese patients on image quality and radiation dose

• ‐showed that using a modified abdominal MDCT protocol with 8 cm or more of subcutaneousprotocol with 8 cm or more of subcutaneous fat, image quality can be substantially improved without a significant increase in radiation dose towithout a significant increase in radiation dose to the abdominal organs

MOSFET on the surface

1 Pitch effect1. Pitch effect

3 Ch i f k d3. Choice of kVp on dose

5. Tube current (auto mAmodulation)

6. Tube rotation time6. Tube rotation time

7. Slice thickness

8 B fil8. Beam filter52

5. TUBE CURRENT MODULATION5. TUBE CURRENT MODULATION

Basic Concept:Basic Concept:

Adjust the tube current to accommodate the patient contour and composition

Technical Approach:Technical Approach:

Modulation along z‐axis

Modulation around the cross‐section

Modulation – axial and helical

5. TUBE CURRENT MODULATION

mA1mA1 mA2

modulation around the z‐axis modulation around the cross‐section

modulation around the z axis modulation around the cross section

mA2mA1

54modulation – axial and helical

How smartma works• Projection data from a scout scan measures the patient and determines how to modulate the mA for improve dose efficiency and consistent IQ

100% 100% Fixed mAFixed mAZ ModulationZ Modulation Auto mAAuto mA

55%55%40%40%Z Modulation Z Modulation -- Auto mA Auto mA

XYZ ModulationXYZ Modulation

0 0 100 0 200 0 300 0 400 0 500 0

0.0 100.0 200.0 300.0 400.0 500.0

Jim Colsher, GE Healthcare

An AutomA Example (Noise Index =24)

sd 25.4

sd 23.7 sd 22.6

GE Healthcare

DOSE INDICES – YESTERDAY, TODAY AND TOMORROWTOMORROW

3. REVIEW OF VARIOUS CT DOSE INDICES3. REVIEW OF VARIOUS CT DOSE INDICES

A. CTDIFDAB. CTDI100100

C. CTDIw &CTDIvolD DLPD. DLP

F. ORGAN DOSES

(Often neglected(Often neglected

In the past) 59

A. CTDIFDA (CT Dose Index)

FDA regulations - 21 CFR Part 1020.33 (August 31, 1984))

CTDI –Mathematical Definition (FDA)

z = position along a line perpendicular to the tomographic planeD(z) = dose at position zD(z) dose at position zT = nominal slice thicknessn = number of tomograms produced in a single scanThi d fi iti th t th d fil i t d dThis definition assumes that the dose profile is centered around z=0.

CTDI – Concept (FDA)

FDAFDA

Key points:1. CTDI is designed to represent the dose received by a

patient per slice of a multi-slice studypatient per slice of a multi-slice study.2. It is the dose to a slice of the patient from the primary

x-ray beam and the scatter from fourteen neighboring li ith idslices, seven on either side.

A. CTDIFDA (CT Dose Index)

Limitations:

O i i ll d fi d f i l COriginally defined for axial CT scanners in 1984.

CTDIFDA is not applicable to i h i l d l ieither single‐detector or multi‐detector helical CT scanners.

Meaning of CTDI•The equivalent of the dose value inside the irradiated slice thatirradiated slice that would result if the absorbed radiation dose profile were entirely concentrated to a rectangular profile of g pwidth equal to thenominal thicknessAll d t ib ti•All dose contributions

from outside the nominal slice width are added to the area inside the slice. 64

CTDI –Mathematical Definition

CTDI is by definition only indicator of the level of y ylocal dose in the irradiated slice.

When the number of slices is increased, the irradiated mass grows by the same amount as the energy absorbed. Therefore the dose doesn’t change.

CTDI100

CTDI100 =

n n mber image slices per scann = number image slices per scanT = slice width per imageDa( ) dose profile in a is (absorbed in air)Da(z) = dose profile in z-axis (absorbed in air)

CTDIWCTDIW

CTDI = (2/3)*CTDI + (1/3)*CTDICTDIW = (2/3)*CTDI100 peripheral + (1/3)*CTDI 100central

1. CTDIW consists of 2/3 of the CTDI100 peripheral dose plus 1/3 of the CTDI100central dose.

CTDI lCTDIvol

CTDIwvol

CTDICTDI =P

where P=pitch

DOSE LENGTH PRODUCT (DLP)DOSE LENGTH PRODUCT (DLP)

volDLP (mGy*cm)=CTDI (mGy)*L(cm)L=scan length in cm

Descriptor of the total amount of radiation absorbed by taking into account also theabsorbed by taking into account also the extent of the body region being irradiated

ESTIMATING ED FROM DLP

• ED from DLPadultadult EEDLPDLP

h d ih d i 0 00230 0023ED from DLP– Adult

– Pediatric

head regionhead region 0.00230.0023chestchest 0.0170.017abdomenabdomen 0 0190 019abdomenabdomen 0.0190.019

mSvED(mSv)=E ( )*DLP(mGy*cm)mGy*cm

yE =anatomy-specific conversion factor

BASIS FROM DLP TO ED

ED paperED paper

• Hurwitz, Yoshizumi etal. Effective dose determination using an , ganthropomorphic phantom and MOSFET technology for clinical adult body multidetector array CT protocols, JCAT 31 544 549 200731, 544‐549, 2007

DUKE EXPERIENCE16‐slice GE scanner

• Comparison of Effective Dose Determined by the Dose Length Product (DLP) Method vs Direct Measurement with Metal Oxide Semiconductor Field Effect Transistor (MOSFET) Technology and an Anthropomorphic Phantom

Black columns= DLP method determinationhi l h d d i i

White columns=MOSFET method determinationerror bars = one SD

6.814.51

0Standard Chest PE Chest Coronary CTA Renal Stone Abdomen-

Pelvis

CT Protocol 76

DUKE EXPERIENCEDUKE EXPERIENCE

• While the DLP method has been clinically used toWhile the DLP method has been clinically used to estimate ED due to easy of calculations, this method may not be accurate in modern 16‐slice and 64‐slice scanners.

• The accuracy of absorbed organ dose and ED determination is important in radiation risk stratification for stochastic effects of radiation such

i d tias cancer induction.

TISSUE WEIGHTING FACTORS AND EFFECTIVE DOSEDOSE

• ED: E = Σ w *HED: E = Σ wT HT

– wT =tissue weighting factorwT tissue weighting factor

– HT = equivalent dose

• ED represents equivalent whole‐body dose• ED represents equivalent whole‐body dose that would produce an equivalent detriment to the health of the individualto the health of the individual

CONCEPT OF EFFECTIVE DOSE EQUIVALENT OR EFFECTIVE DOSE

ICRP Report 26 (1977)

Dose Equivalent

ICRP Report 60 (1990)

Equivalent Dose q

Quality Factor

Weighting Factors

Effective Dose Equivalent

Radiation Weighting Factor

Tissue Weighting Factors

Effective Dose

Effective Dose Equivalent Effective Dose

HISTORY OF TISSUE WEIGHTING FACTOR

1977 1990 2005

ICRP 26ICRP 60G d 0 20

ICRP draftICRP 26Gonads 0.25Breast 0.15RBM 0.12

Gonads 0.20Breast 0.05RBM 0.12Lung 0.12

BM, Colon, Breast , Lung, Stomach 0.12

Bladder Gonads Liver

Lung 0.12Thyroid 0.03BS 0.03Remainder 0 30

Thyroid 0.05Stomach 0.05Bladder 0.05Liver 0 05

Oesoph Thyroid 0.05

Skin, B S, Brain, Kidneys, Salivary glands 0.01

Remainder 0.30 Liver 0.05Colon 0.12Oesoph 0.05Skin 0.01

Remainder 0.10

BS 0.01Remainder 0.05

CTDI – How to measure?

AAPM REPORT NO.96 2008www.aapm.org

CTDI – How to measure?

•AAPM Report 1 (1977)

AAPM R 39 (1993)•AAPM Report 39 (1993)

•AAPM Report 96 (2008)

A. CHANGING TECHNOLOGIES

# SLICES & DETECTOR WIDTH

–64 slices – 40 mm width (05‐06)

256 li 128 idth (2007)–256 slices – 128 mm width (2007)

–320 slices – 160 mm width (2008)

CTDI h t l th 150–CTDI phantom length – 150 mm

–CT pencil ion chamber – 100 mmp

RECENT THEORETICAL EVELOPMENT‐CTDI100

Dixon, R. Med Phys 2003; 30: 1272-1280.

…..Also addressed is the concern that as radiation beam widths for multi-slice scanners get wider the currentwidths for multi-slice scanners get wider, the currentmethodology based on the measurement of the integral of the single slice profile using a 10 cm long ion chamberg p g g(CTDI100) may no longer be adequate. It is shown thatthis measurement would underestimate the equilibrium dose and dose line integral by about 20% in the center of the body phantom, and by about 10% in the center of thehead phantom for a 20 mm nominal beam width in a multihead phantom for a 20 mm nominal beam width in a multi-slice scanner.

Toshiba Aquilion One (2008)Toshiba Aquilion One (2008)

• 12.8 cm detector (2007) to 16 cm detector coverage (320 x 0.5 mm), 0.35 SEC ROTATION

CTDI MEASUREMENT

150 mm

Active chamber length 100 mm

20 mm (4-, 8-, 16-slice) 1998-200240 mm (64-slice) 2006

128 mm (256-slice) 2007

Recall the meaning of CTDI•The equivalent of the dose value inside the irradiated slice thatirradiated slice that would result if the absorbed radiation dose profile were entirely concentrated to a rectangular profile of g pwidth equal to thenominal thicknessAll d t ib ti•All dose contributions

from outside the nominal slice width are added to the area inside the slice. 88

Toshiba 160 mm (320‐slice)

New 40 mm (64-slice)

Toshiba 160 mm (320‐slice)

Detector 20 mm (QX/i)

10 cm active length of ion chamber

100 mm

• Ion chamber no longer covers the entire tail portion of the single profileToshiba detector width 16 cm > 10 cm ion chamber

• Toshiba detector width 16 cm > 10 cm ion chamber

FURTHER COMPLICATIONS

as radiation beam widths for multi‐slice scanners get wider, the currentmethodology based on the measurement of the integral of the single slice profile using a 10 cm long ion chamber (CTDI )profile using a 10 cm long ion chamber (CTDI100) may no longer be adequateUK NRPB Monte Carlo look‐up table no longer p gvalid because CTDI100 values are underestimated64‐slice beam width (4 cm) Conventional CTDI no longer applicable in 256Conventional CTDI no longer applicable in 256‐slice (~13 cm) and 320‐slice (16 cm)

REVIEW OF CT DOSE ESTIMATION METHODSREVIEW OF CT DOSE ESTIMATION METHODS

A. Manual Look‐up tables (Outdated)B. Organ dose from CTDI (limited value)g ( )C. Monte Carlo based dose calculator (limited)D. Effective Dose from DLP (Limited)( )E. Anthropomorphic phantom with TLDs (time

consuming)F. Anthropomorphic phantom with

MOSFET(metal oxide semiconductor field effect transistor) detectors (current value)effect transistor) detectors (current value)

History of dosimetry program at DukeHistory of dosimetry program at Duke

1997 2000 2002 2005 20072003 2008

TLD program for organ dose

First ti

Adult, ped Phantoms

ti Fluorofor organ dose MeasurementsIn radiology

generation of

MOSFETTechnology

Purchased, 2nd

generation of

generationof MODFETtechnology Mobile

Fluoro dose

monitorsoftware

DualEnergy

gypurchased

of MOSFET

MOSFET

Interventional vascular radiology

93 GE CT scan

VALIDATION OF MOSFET TECHNOLOGY IN CT DOSIMETRYDOSIMETRY

• Physics paper• Physics paperYoshizumi T, Goodman P, Frush DP, Nguyen G, Toncheva G , Sarder M Barnes L Validation of CT OrganM, Barnes L. Validation of CT Organ Dose Assessment: MOSFET vs. TLD. Am. J.Roentgenol., May 2007; 188: 1332 ‐ 1336.1332 1336.

MOSFET detectors (Best Medical Canada LtdMOSFET detectors (Best Medical Canada Ltd. Ottawa, Canada)

St d d MOSFET d i tSt d d MOSFET d i tStandard MOSFET dosimeter:Standard MOSFET dosimeter:silicon chip 1mmsilicon chip 1mm22

active area 0 2mm x 0 2mmactive area 0 2mm x 0 2mm

active area 0.2mm x 0.2mmactive area 0.2mm x 0.2mm

WHAT IS MOSFET?( metal oxide semiconductor field effect transistor)

ComponentsComponentsMetal gate electrodeMetal gate electrode

SiO2 layerSiO2 layer

Si b t tSi b t t

Current passes from Current passes from source to drain source to drain if a negative voltage (if a negative voltage (threshold threshold

Si substrateSi substrate

voltagevoltage) exists on gate ( ON position)) exists on gate ( ON position)

When irradiated causes a permanent shift in the threshold voltage of the transistor; this shift is proportional to the absorbed dose.

Vt = Vt1 – Vt0 Dose∝96

CIRS ANTHROPOMORPHIC PHANTOMS AT DUKE

ADULT MALE (70 K )ADULT FEMALE (55 Kg)

10‐YR OLD

ADULT MALE (70 Kg)

NEWBORNNEWBORN

5‐YR OLD

EMERGING ISSUES AND OPPORTUNITIESEMERGING ISSUES AND OPPORTUNITIES

11 Dual Energy CT Scanner:Dual Energy CT Scanner:1.1. Dual Energy CT Scanner: Dual Energy CT Scanner:

Two DesignsTwo Designsgg

–– Siemens’ Definition Siemens’ Definition ––Two xTwo x‐‐ray ray t bt btubestubes

–– GE VCTGE VCT –– One xOne x‐‐ray tuberay tubeGE VCT GE VCT One xOne x ray tuberay tube

2.2. Coronary CTA Coronary CTA DosimetryDosimetry

Siemens ‐ Dual EnergyTwo x‐ray tubes

GE Design‐ Single TubeGE Design Single Tube

Siemens ‐ Dual EnergykHow it works

80kV80kVBone670 HU Iodine

296 HU

Bone450 HU Iodine

144 HU

Single energy CTWhat do we measure?

compactbone

What do we measure?

80liver

/ HU 50

70spong.bone

kidney

pancreas

waterfat

‐200

CT‐value

40kidney

air‐800

‐600

‐40010

Material differentiation works because tissues have characteristicair

‐1000because tissues have characteristic densities

Dual EnergykHow it works

Two dominant absorption processes determine the total attenuation, Compton d h l ffand Photoelectric effectμ σ τ= + Mean Energy:

56 kV 76 kV

Two independent

Tube 1

Two independent measurementsare required to solve this

Tube 2

Dual Energy:How it works

Total attenuation decreasesTotal attenuation decreases with increasing energy

Attenuation is characteristic forAttenuation is characteristic for each material, depends on photon energy and material densityy

X‐ray absorption depends on the inner electron shellsDECT i i iDECT is sensitive to atomic number and densityDECT is not sensitive toDECT is not sensitive to chemical binding

Dual EnergyHow it worksHow it works

Therefore:

The previous formula can be written as a material decomposition

μ=α ×μ +α ×μ

It is not possible to distinguish between more than two materials of

1 1 2 2μ=α ×μ +α ×μ

It is not possible to distinguish between more than two materials of arbitrary density – but we can utilize the fact that body materials occur with specific densities and perform a three material decompositiondecomposition

Dual Energy CT Functional Informationu ct o a o at o

Three material decomposition – the basis for clinical applications

CT value

Iodine

CT value

@ 80 kV

65Tissue

‐100Fat

CT value

@ 140 kV60‐90 107

Dual Energy CT Functional InformationFunctional Information

… to obtain an individual material map for each material *)

TissueTissueTissueFat

Tissue

IodineIodineIodineIodineIodineIodineIodine

*) not available in the DualEnergy application, at least not before VA20…108

Calcified stone Uric acid stone

Dual Source CT Functional Information with Dual Energy

Separation of Iodine and Calcium: Automatic Bone‐Removal

Functional Information with Dual Energy

Courtesy of Klinikum Großhadern, Munich110

Dual Energy Liver Protocol Effective DoseEffective Dose

Siemens 64‐slice Definition6.0

2.513.0

ED from DLP

E Measured ED

Liver DE 140/80 Liver 120

111Duke University 2008

DUAL ENERGY LIVER PROTOCOL (DUKE)GE 64‐SLICE VCT

Liver Scan protocol DLP (mGy*cm) ED (mSv)

Helical auto mA 140 kVp 167 78 2 52Helical auto mA 140 kVp 167.78 2.52

DE 1250.25 18.75(ED=DLP*0.015)

Beam tracking 21% reduction applied

140+80 cine combined 1033.29 15.50

140 cine only (0.8 s) 712.21 10.68 (phantom)

80 cine only (0.8 s) 321.09 4.82 (phantom)y ( ) (p )

112Duke University 2007‐RSNA

CORONARY CTACORONARY CTA

• Current issues• Current issuesHigh ED and organ doseg g

• New development

retrospectively gated helical (RGH) vs. prospectively gated axial (PGA)vs. prospectively gated axial (PGA)

HIGH DOSE IN Coronary CT Angiography (CTA)HIGH DOSE IN Coronary CT Angiography (CTA)

PAPERS:• Hurwitz LM, Reiman RE, Yoshizumi TT, Goodman PC, Toncheva G, Nguyen

G, Lowry C. Radiation dose from contemporary MDCT cardiothoracic imaging protocols using an anthropomorphic female phantom: g g p g p p pimplications for cancer induction. Radiology 2007;245:742‐750.

• GE 64‐slice VCT scanner

‐ retrospectively gated helical (RGH)

– ED 18‐32 mSvED 18 32 mSv

–Breast 5‐9 cGy

RETROSPECTIVELY GATED HELICAL (RGH) VS. PROSPECTIVELY GATED AXIAL (PGA)PROSPECTIVELY GATED AXIAL (PGA)

Tube current on continuously vs. tube current turned only during prescribed cardiac phase

Table moves at constant velocity vs table steps (moves) and shootsTable moves at constant velocity vs. table steps (moves) and shoots

PROSPECTIVELY GATED AXIAL (PGA)PROSPECTIVELY GATED AXIAL (PGA)

Reduced dose achieved (Duke Data):Reduced dose achieved (Duke Data):

RGH vs. PGA: 19 mSv vs. 6 mSv at 65 BPM

G 6 li CGE 64‐slice VCT scanner

Adult vs. pediatric:

• Higher HR in ped (~100 bpm) may prevent toHigher HR in ped ( 100 bpm) may prevent to use PGA; must use RGH

NEW OPPORTUNITIESNEW OPPORTUNITIES

• Radiochromic film dosimetry in diagnosticRadiochromic film dosimetry in diagnostic radiology

FROM 2‐D TO 3‐D DOSE VISUALIZATIONFROM 2 D TO 3 D DOSE VISUALIZATION

INSERT FILM AT EACH SECTION

3‐D VISUALIZATION OF DOSE

Sections 11 13 16unexposed

Sections 11, 13, 16

After exposure

Darker color change after exposure

Slice 11 ResultsPhantom

Slice 11 Results

Lung definitionLung definition

CT Image

Vertebral body/spinal cord GafchromicDose distribution

Slice 11 Results‐Line ProfilesSlice 11 Results Line Profiles

CONCLUSIONSCONCLUSIONS

• Witnessing continued advancement of MDCT technologyWitnessing continued advancement of MDCT technology since the introduction in 1998;

• Collective dose to pts from CT continues to climb

• MDCT radiation dosimetry issues still lag behind the technology due to lack of new CTDI concept

• Many new exciting opportunities exist• Many new exciting opportunities exist:– MOSFET, radiochromic film dosimetry, glass dosimetry, Monte Carlo

code development.

• New CTDI still needs to be developed– CBCT geometry

CT DOSIMETRY REVIEW PAPERS IN PRINT

• Mettler FA Jr., Huda W, Mahesh M, Yoshizumi TT. A Catalog Of Effective Doses In Radiology And Nuclear Medicine. In print Radiology 2007.

• Mettler FA Jr., Thomadsen BR, Bhargavan M, Gilley DB, Gray JE, Lipoti JA, , , g , y , y , p ,John McCrohan J, Yoshizumi TT, Mahesh M. Medical Radiation Exposure in the U.S. 2006: Preliminary Results. In Print Health Physics 2008.

• Fred A. Mettler Jr., Mythreyi Bhargavan, Bruce R.Thomadsen, Debbie B. Gilley, Joel E. GrayJill A. Lipoti, Mahadevappa Mahesh, John McCrohan, Terry T. Yoshizumi. Nuclear Medicine Exposure in the US 2005‐2007: y pPreliminary results. Seminars in Nuclear Medicine, Sept 2008.

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