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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
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
2
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
3
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
4
CT PATIENT DOSIMETRY CT PATIENT DOSIMETRY ‐‐WHY SO WHY SO ??IMPORTANT?IMPORTANT?
• 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.
5
6
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
7
CT PATIENT DOSIMETRY CT PATIENT DOSIMETRY ‐‐WHY SO WHY SO ??IMPORTANT?IMPORTANT?
• 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
8
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
x 120
93,700 person Sv438,000 person Sv
CT scans by year in US (millio
70 0
CT procedures by year (millions)
57.6
62.0
60.0
70.0
Annual growth of >10% per yAnnual growth > 10%/yr
U.S. population < 1%/yr
Courtesy of Dr. Fred Mettler
45.4
50.1
53.9
50.0
illi
on
s)
30.6
34.9
39.640.0
ced
ure
s (m
i
18.3 19.521.0
22.625.1 26.3
20.0
30.0
No
. o
f p
roc
10.0
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
25.0%
18.0%
19.0% 19.4%
16.1%
15.0% 15.3%
20.0%
12.4%
10.1% 10.0%
13.7% 14.0%
9.6%10.0%
15.0%
3.7%
7.2%6.3%
4.4%5.0%
1.3%2.2%
0.7%
1.6%
0.0%
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
12
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
14
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
15
CT PATIENT DOSIMETRY CT PATIENT DOSIMETRY ‐‐WHY SO WHY SO ??IMPORTANT?IMPORTANT?
• 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
16
Danger of not understanding CT dose
•3 cases reported•MDCT perfusion +DSA (CT fluoroscopy)•Temporary hair loss (3‐5 Gy)
17
CT PATIENT DOSIMETRY CT PATIENT DOSIMETRY ‐‐WHY SO WHY SO ??IMPORTANT?IMPORTANT?
• 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
18
CAPABILITY OF MODERN CT SCANNERPediatric Phantom 5‐year Oldy
Siemens 64‐slice
19
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
20
EXTREME CASES: Siemens 64‐slice
Chest W/O Care - Extreme CaseChest W/O Care Extreme Case
10.1
9
11.6
6
.66
10.8
2
11.8
7
10.2
7
10.9
9
10.7
3
7.04
6.66
7 .43
10
15e
(cG
y)
0.64
5
3.97 5.
2.72
1.23
0.92
0.29
0.14
0.04 0.26
-5
0
5
Org
an D
os
Slice 12- BM/ BM/ Lungs- Esopha BM/ Pancrea Stomac Intestine Small Ascendi BM/
Chest
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
Colon
BM/ Pelvis Ovaries Bladder
Abdomen W/O Care - Extreme Case
.21
50 5 64 6 83 6.30
3
20.4
5
21.7
1
07 68
20.00
25.00
(cG
y)
15
0.19
0.38
14. 5
1.11 1.58 1.95
8.07
13.8
11.2
3 14. 6
13.8 14
. 1
13.4
3
11.2
5 14. 0
14.6
5.00
10.00
15.00
Org
an D
ose
(
Abdomen
0 0
0.00
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-
skin
BM/ Mandibl
eThyroid Kidney
BM/ Ribs-
sternuthymus Breast
Lungs- Heart
Esophagus- Heart
BM/ Spine-
(T Spleen
Pancreas Liver
Stomach
Intestine ULI
Small Intestin
e
Ascending
Colon
BM/ Pelvis Ovaries Bladder
TOPICSTOPICS
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. Research opportunities
22
THE BIG PICTURETHE BIG PICTURE
23
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)
24
DETECTOR WIDTH INCREASED (>1998)
10 mm 20 mm
GE CT/i GE QX/i 4‐slice
25
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
26
MDCT QXi dose profile (dotted line)
1.0
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)
27
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
28
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
15
20CT/iQX/i
17 17 17
5
106.3
8.5 8.3mG
y
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)
29
Snapshot in2007 HEAD SCAN COMPARISON (Duke Data)( )
Variation in patient dose in various CT scanners
1.60
1.80
2.00
1.52 1.58
1.86
1.62 1.62 1 62 1 62 1 69
Comparison of ED (mSv)
0 60
0.80
1.00
1.20
1.401.62 1.62 1.69
1.22
ED (m
Sv)
0.00
0.20
0.40
0.60
30
Scanner Type
DOSE COMPARISON UNDER SAME IMAGE QUALITY
SDCT
MDCTMDCT
31
Comparison of Dose Profile 4 li QX/i d 1 li CT/i4-slice QX/i and 1-slice CT/i
32
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
800
1000
708090100 QXi Fitted
CTi Fitted
%
400
600
40506070
mra
ds
Differen
200
400
10203040
nce
0 10 20 30 40 50 600 0
mm 33
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
35
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
36
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. Research opportunities
37
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
38
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.
39
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
40
FACTORS AFFECTING DOSEFACTORS AFFECTING DOSE
2 Coronary CTA2. Coronary CTA
– Pitch < 0.3 overlapping beams pp gcontribute higher dose (0.22 GE)GE)
41
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
42
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
43
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
44
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
45
FACTORS AFFECTING DOSEFACTORS AFFECTING DOSE
1 Pitch effect1. Pitch effect
2. Focal spot wobble and tracking
3 Ch i f k d3. Choice of kVp on dose
4. Patient thickness
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)
47
FACTORS AFFECTING DOSEFACTORS AFFECTING DOSE
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
48
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
49
MOSFET on the surface
50
51
FACTORS AFFECTING DOSEFACTORS AFFECTING DOSE
1 Pitch effect1. Pitch effect
2. Focal spot wobble and tracking
3 Ch i f k d3. Choice of kVp on dose
4. Patient thickness
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
53
Modulation – axial and helical
5. TUBE CURRENT MODULATION
mA1mA1 mA2
mA2
y
modulation around the z‐axis modulation around the cross‐section
Z x
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
250
300
55%55%40%40%Z Modulation Z Modulation -- Auto mA Auto mA
XYZ ModulationXYZ Modulation
100
150
200
mA
sm
As
0
50
100
0 0 100 0 200 0 300 0 400 0 500 0
55
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
56
GE Healthcare
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
57
DOSE INDICES – YESTERDAY, TODAY AND TOMORROWTOMORROW
58
3. REVIEW OF VARIOUS CT DOSE INDICES3. REVIEW OF VARIOUS CT DOSE INDICES
A. CTDIFDAB. CTDI100100
C. CTDIw &CTDIvolD DLPD. DLP
E. ED
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))
60
CTDI –Mathematical Definition (FDA)
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.
61
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.
62
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.
63
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
65
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.
66
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)
67
68
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.
B
C
A
C
E
D69
CTDI lCTDIvol
CTDIwvol
CTDICTDI =P
where P=pitch
70
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
71
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
SDLP
mSvED(mSv)=E ( )*DLP(mGy*cm)mGy*cm
DLP
yE =anatomy-specific conversion factor
72
BASIS FROM DLP TO ED
73
74
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
75
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
25
White columns=MOSFET method determinationerror bars = one SD
11
16
11
20.6
13.3
16.4
15
20
Dos
e (m
Sv)
4.6
11
3.9
6.814.51
0
5
10
Effe
ctiv
e D
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.
77
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
78
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
q
Radiation Weighting Factor
Tissue Weighting Factors
Effective Dose
79
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
80
BS 0.01Remainder 0.05
CTDI – How to measure?
81
AAPM REPORT NO.96 2008www.aapm.org
82
CTDI – How to measure?
•AAPM Report 1 (1977)
AAPM R 39 (1993)•AAPM Report 39 (1993)
•AAPM Report 96 (2008)
83
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)
Note:
CTDI h t l th 150–CTDI phantom length – 150 mm
–CT pencil ion chamber – 100 mmp
84
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.
85
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
86
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
89
• 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)
90
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)
91
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
3rd
ti Fluorofor organ dose MeasurementsIn radiology
generation of
MOSFETTechnology
Purchased, 2nd
generation of
generationof MODFETtechnology Mobile
Fluoro dose
monitorsoftware
DualEnergy
gypurchased
of MOSFET
MOSFET
92
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.
94
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
95
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
97
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
98
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
99
Siemens ‐ Dual EnergyTwo x‐ray tubes
100
GE Design‐ Single TubeGE Design Single Tube
101
Siemens ‐ Dual EnergykHow it works
80kV80kVBone670 HU Iodine
296 HU
Bone450 HU Iodine
144 HU
140kV
144 HU
102
Single energy CTWhat do we measure?
compactbone
What do we measure?
800
1000
70
80liver
200
400
600
/ HU 50
60
70spong.bone
kidney
pancreas
blood
waterfat
400
‐200
0
CT‐value
20
30
40kidney
air‐800
‐600
‐40010
0
lungs
Material differentiation works because tissues have characteristicair
‐1000because tissues have characteristic densities
103
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
104
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
105
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
106
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
0
65Tissue
‐100Fat
0
CT value
@ 140 kV60‐90 107
Dual Energy CT Functional InformationFunctional Information
… to obtain an individual material map for each material *)
Fat
TissueTissueTissueFat
Tissue
Fat
Tissue
IodineIodineIodineIodineIodineIodineIodine
*) not available in the DualEnergy application, at least not before VA20…108
Calcified stone Uric acid stone
109
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
4.78
4.26
5.0
3.18
2.513.0
4.0
ED (m
Sv)
ED from DLP
1 0
2.0
E Measured ED
0.0
1.0
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
14.82
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)
113
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
114
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
115
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
116
NEW OPPORTUNITIESNEW OPPORTUNITIES
• Radiochromic film dosimetry in diagnosticRadiochromic film dosimetry in diagnostic radiology
117
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
118
Sections 11 13 16unexposed
Sections 11, 13, 16
After exposure
119
Darker color change after exposure
Slice 11 ResultsPhantom
Slice 11 Results
Lung definitionLung definition
CT Image
120
Vertebral body/spinal cord GafchromicDose distribution
Slice 11 Results‐Line ProfilesSlice 11 Results Line Profiles
121
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
122
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.
123