EEssttiimmaattiinngg PPaattiieenntt RRaaddiiaattiioonn DDoossee ffrroomm CCoommppuutteedd TToommooggrraapphhyy
CC.. CCaaggnnoonn,, JJ.. DDeeMMaarrccoo,, EE.. AAnnggeell,, MM.. MMccNNiitttt --GGrraayy
UCLA David Geffen School of Medicine
1 Patient Dose from CT Cagnon - ACMP 2008 AAddvvaanncceess iinn TTeecchhnnoollooggyy ......
HHeelliiccaall CCTT:: HHiigghh oouuttppuutt XX --rraayy ttuubbeess,, ccoonnttiinnuuoouuss ggaannttrryy rroottaattiioonn//ttaabbllee mmoottiioonn MMDDCCTT:: OOvveerr ppaasstt 1155 yyeeaarrss ttoottaall ddeetteeccttoorr rroowwss aanndd bbeeaamm wwiiddtthh hhaavvee iinnccrreeaasseedd − 2, 4, 8, 10, 16, 20, 32, 40, 64, 128, 256, and now 320 slice scanners BBeeaamm wwiiddtthh uupp ttoo 1166ccmm SSppeeeedd –– ggaannttrryy rroottaattiioonn ttiimmee ddoowwnn ttoo 11//33 sseecc →→ ......
2 Patient Dose from CT Cagnon - ACMP 2008 IInnccrreeaassiinngg ssccaann ssppeeeedd && aannaattoommiiccaall ccoovveerraaggee IImmpprroovveedd tteemmppoorraall rreessoolluuttiioonn ““ffrreeeezzeess ”” pphhyyssiioollooggiicc mmoottiioonn − Scan entire chest in a breath hold (10 sec) − Scan entire heart in a heart beat (1 rotation) GGrroowwiinngg nnuummbbeerr ooff cclliinniiccaall aapppplliiccaattiioonnss − Emergency room trauma scanning − Cardiac applications − Perfusion studies (repeated scans in one location) − Oncology treatment planning ((…… aanndd bbiilllliinngg ooppppoorrttuunniittiieess)) →→ ...... 3 Patient Dose from CT Cagnon - ACMP 2008 MMoorree ppeeddiiaattrriicc aapppplliiccaattiioonnss
Fast scanners allow for pediatric CT imaging that previously may have required anesthesia, etc. 200% increase in pediatric CT over last few years D.Frush Concerns − ↑ radio -sensitivity − ↑ organ and effective doses, particularly when technical factors are not adjusted 600,000 annual CT scans on children under 15 might result in 500 additional cancer deaths from IR D.Brenner - AJR
4 Patient Dose from CT Cagnon - ACMP 2008 CCTT ddoossee:: AA ccoonncceerrnn??
AAss ooff 22000066,, 6622 mmiilllliioonn CCTT pprroocceedduurreess ppeerrffoorrmmeedd aannnnuuaallllyy AAPM96/IMV Report in CT
IInn UU..SS..,, CCTT ccoommpprriisseess oonnllyy 1155%% ooff aallll eexxaammss bbuutt ggeenneerraatteess 7700%% ooff ddeelliivveerreedd ddoossee ddoossee Mettler 2003
5 Patient Dose from CT Cagnon - ACMP 2008 DDiiaaggnnoossttiicc XX --rraayy ccoonnttiinnuueess ttoo iinnccrreeaassee iinn pprrooppoorrttiioonn ooff tthhee ppooppuullaattiioonn ’’ss ttoottaall eexxppoossuurree ttoo II..RR.. − 11% of total burden in 1980 to 17% in 2000 − Up to 60% of manmade exposure PPrrooppoorrttiioonn ooff tthhaatt bbuurrddeenn dduuee ttoo CCTT hhaass ddrraammaattiiccaallllyy iinnccrreeaasseedd iinn llaasstt ttwwoo ddeeccaaddeess − Growing at rate of 10 -15% annually
NEXT – CT Shrimpton & Edyvean Mettler
6 Patient Dose from CT Cagnon - ACMP 2008 CCTT aass aa SSccrreeeenniinngg TTooooll??
RRiisskk vvss.. bbeenneeffiitt −− wwhheenn ssccaannnniinngg aa ssyymmppttoommaattiicc ppaattiieenntt ttoo rreennddeerr aa ddiiaaggnnoossiiss
vvss..
−− WWhheenn uusseedd aass aa ssccrreeeenniinngg ttooooll oonn aann aassyymmppttoommaattiicc ppooppuullaattiioonn
7 Patient Dose from CT Cagnon - ACMP 2008 FFeeddeerraall SSttaannccee ((2005)
XX--rraayyss ooffffiicciiaallllyy iinndduucctteedd ttoo FFDDAA lliisstt ooff ccaarrcciinnooggeennss SSaannccttiioonn ooff tthhee nnoo --tthhrreesshhoolldd mmooddeell NNoo ssaaffee ddoossee ooff rraaddiiaattiioonn AAnnyy iinnccrreeaassee iinn ddoossee iinnccrreeaasseess rriisskk
8 Patient Dose from CT Cagnon - ACMP 2008 CCTT DDoossee ddiissttrriibbuuttiioonn ddiiffffeerrss ffrroomm pprroojjeeccttiioonnaall
Unlike conventional projectional radiography where beam exit energy is a fraction of entrance, in CT rotating source encircles the body so that PA and AP entrance dose are nearly identical leading to a more uniform dose distribution and ge nera lly high Absorbed energy in tissue (60 keV) er o rgan 1 d ose
0.9
0.8 gn i n 0.7 rme i a
n 0.6 o i rt c a
f 0.5
0.4
0.3 0 10 20 30 40 depth (cm) 10 Patient Dose from CT Cagnon - ACMP 2008 CCTT ddoossee ppaarraaddiiggmm
TThhee llooccaall ttiissssuuee ddoossee ffrroomm aa ssiin sslliiccee iiss nno nggllee ott tthhee ssaammee aass tthhee ddoossee iinn tthhee vveerryy ssaammee ttiissssuuee wwhheenn aaddddiittiioonnaall aaddjjaacceenntt sslliicceess aarree mmaaddee...... bbeeccaauussee eeaacchh aaddddiittiioonnaall sslliiccee ssccaatttteerrss rraaddiiaattiioonn iinnttoo aaddjjaacceenntt sslliicceess..
11 Patient Dose from CT Cagnon - ACMP 2008 DD((zz)) == ddoossee pprrooffiillee aalloonngg zz --aaxxiiss ffrroomm::
Single slice acquisition Multiple slice acquisition
D(z) D(z)
z z 12 Patient Dose from CT Cagnon - ACMP 2008 DDoossee ffrroomm mmuullttiippllee sslliicceess
Even if non -overlapping slices (and ignoring beam penumbra) scatter tails of multiple contiguous scans overlap and contribute to an increased integral dose profile 1.5 Function of: −Single Scan Profile Width (T)
−Number of scans (N) 1 −Spacing (I) I
0.5 e s o d e v i t a l e r T Cas → e fo r I = 0. 0 5 T 0.0 50.0 10z0 a.x0 is location 150.0
13 Patient Dose from CT Cagnon - ACMP 2008 CCoommppuutteedd TToommooggrraapphhyy DDoossee IInnddeexx ((CCTTDDII)) –– ddeeffiinneedd Total area of D(z ) under width T • of central scan of multiple scan CTDI = (1/T ) ∫ D(z)dz profile = total area of single scan - • dose profile (including scatter tails)
D(z) D(z)
z z T 14 Patient Dose from CT Cagnon - ACMP 2008 MMeeaassuurriinngg CCTTDDII
Single axial scan (in phantom to emulate patient scatter) of nominal beam thickness NT where:
− N is number of slices of thickness T
We measure the total integral area D(z ) of the single scan with pencil ion 1° & scatter chamber long enough to collect scatter scatter tails
electrometer NT CCTTDDII –– FFDDAA
FFDDAA ddeeffiinneedd CCTTDDII oovveerr 1144 sslliicceess ((nn iiss tthhee nnuummbbeerr ooff sslliicceess//aaccqquuiissiittiioonn))
7T CTDI = (1/nT) ∫∫ -7T D(z) dz
TThhiiss aassssuummeedd tthhaatt yyoouu eeiitthheerr hhaadd:: − TLDs or film to measure D(z) profile ... OR − A 100 mm chamber covering 14 - 7mm slices − Can overestimate dose for thin slices
16 Patient Dose from CT Cagnon - ACMP 2008 CCTTDDII 110000 Standardizes integration limits across 100 mm chamber (rather than 14 slices of varying size)
1 5cm CTDI 100 = D(z)dz → = ( ELCf )/(NT) NT ∫- 5cm w here • E = measured value of integrated exposure • L = active length of pencil ion chamber, typically 100 mm • f = conversion factor from exposure to dose • C = electrometer calibration factor - typically close to1.0
• N = actual number of data channels used during one axial scan • T = nominal slice width of one axial image (scan collimation)
17 Patient Dose from CT Cagnon - ACMP 2008
be am col lima tion CCTTDDII 110000
CTDI 100 Measurements are done: − In Both Head and Body Phantoms − Using ONLY AXIAL scan techniques (CTDI = A rea under the single scan dose profile) − At isocenter and at least one peripheral position in each phanto m
20 mGy 20 mGy
16 cm 20 10 20 20 10 20 32 cm 20
20
Head Body
18 Patient Dose from CT Cagnon - ACMP 2008 QQuueessttiioonn:: DDoossee ttoo wwhhaatt??
When determining CTDI 100 , one is calculating dose to ...?
11.. aaiirr 33% 22.. ttiissssuuee 20% 33.. aaccrryylliicc 40%
0% 44.. nnoonnee ooff tthhee aabboovvee
7% 55.. CCTTDDII 100 iiss nnoott aa ffuunnccttiioonn ooff mmaatteerriiaall WWhhaatt ff ffaaccttoorr ttoo uussee??
WWhheenn ddeetteerrmmiinniinngg CCTTDDII 100 ,, oonnee iiss ccaallccuullaattiinngg ddoossee ttoo →→ aaiirr
AAPM Report 96: Measurement, Air - f factor of 0.87 rad /R Reporting, & Management of → Radiation Dose in CT (Jan-08) Tissue - f factor of 0.94 rad /R
Acrylic - f factor of 0.78 rad /R
CTDI FDA and often what vendors report
20 Patient Dose from CT Cagnon - ACMP 2008 CCTTDDII ww 20 mGy Due to attenuation CTDI is not 20 10 20 homogeneous across the FOV – particularly in 32 cm body phantom where 20 gradient between periphery and center is on order of 2:1 To arrive at a single descriptive value we use a weighted average of center and
peripheral CTDI 100
C , c + , p CTTD 10 , ent + (( 10 , eriip DIIw 0 er 22//3 0 her w == 3)) CC rall ((11// TTDDI 33)) CC I TTDDII 21 Patient Dose from CT Cagnon - ACMP 2008 CCTTDDII ww LLiimmiittaattiioonnss
CTDIw does describe dose within a ssiin nggllee xx,,yy measured for a single aa scan sscca xxiiaall ann pp llaanne does reflect scatter contributions from adjacent sceans assumes contiguous, non-overlapping scans
CTDIw ddoo ... eess n noott describe dose for helical scans (t thou gh cllos e fo do well for non-adjacent slices frequently ser epinit in tch helical scanning where X-ray beam may overlap =1) (common with MDCT) or where extended pitch leaves gaps between rotations.
22 Patient Dose from CT Cagnon - ACMP 2008 VVoolluummee CCTTDDII ((CCTTDDII vvooll )) CCaallccuullaatteedd ffrroomm CCTTDDIIww RReepprreesseennttss tthhee aavveerraaggee ddoossee iinn tthhee cceennttrraall rreeggiioonn ooff aa mmuullttiippllee ssccaann eexxaamm AAvveerraaggeess oovveerr xx,, yy aanndd zz AAccccoouunnttss ffoorr hheelliiccaall ppiittcchh
CTDI vol = 1 • CTDI w pitch
PITCH = table index per rotation (I) / to nominal scan width (NT) tal
23 Patient Dose from CT Cagnon - ACMP 2008 QQuueessttiioonn : Which of the following CT scan protocols - all made on different scanners with similar measured CTDI 100 values - would you expect to result in the highest CTDI vol ? (All are done at 120 kVp and identical beam collimations)
11.. mmAA == 440000,, rroottaattiioonn ttiimmee == 00..55 sseecc,, ppiittcchh == 11 0% 22.. mmAA == 110000,, rroottaattiioonn ttiimmee == 11 sseecc,, ppiittcchh == 00..55 17% 33.. EEffffeeccttiivvee mmAAss == 220000,, ppiittcchh == 00..55 17% 4. Effective mAs = 200, pitch = 1 17% 4. Effective mAs = 200, pitch = 1
50% 55.. TThheerree iiss nnoo ddiiffffeerreennccee iinn CCTTDDII vol ffoorr tthheessee pprroottooccoollss Not all scanners provide same information to user: Siemens & Phi lips use concept of “effective mAs ” which already takes pitch into account
Effective mAs = ( mA * sec)/pitch
1. mA = 400, rot. time = 0.5 sec, pitch = 1: e ff. m mAs = ( 2. mA = 100, rot. time = 1 sec, pitch = 0.5: eff 400 f.mA *0. As = 5)/1 (1 1 = 3. Effective mAs = 200, pitch = 0.5: eff 00* 200 .mA 1)/0 s = 0.5 200 = 2 4. Effective mAs = 200, pitch = 1 eff. 0 00 mAs s = 200 5. There is no difference in CTDIvol for these protocols – allll hav e th e sa ame eff ecti ive m mAs 25 Patient Dose from CT Cagnon - ACMP 2008 CCTTDDII LLiimmiittaattiioonnss
SSaayyss nnootthhiinngg aabboouutt lleennggtthh ooff ssccaann ......
26 Patient Dose from CT Cagnon - ACMP 2008 CTDI vol = 20 mGy
CTDI vol still = 20 mGy
How do we represent the greater biologic risk?
27 Patient Dose from CT Cagnon - ACMP 2008 DDoossee LLeennggtthh PPrroodduucctt ((DDLLPP))
RReepprreesseennttss iinntteeggrraatteedd ddoossee iinn tteerrmmss ooff ttoottaall ssccaann lleennggtthh ((## sslliicceess •• sslliiccee wwiiddtthh))
DDLLPP == CCTTDDII vvooll ((mmGGyy)) •• ssccaann lleennggtthh ((ccmm)) DDLLPP rreefflleeccttss ttoottaall eenneerrggyy aabbssoorrbbeedd
28 Patient Dose from CT Cagnon - ACMP 2008 DLP = 200 mGy •cm CTDI vol = 20 mGy ten 1-cm slices
CTDI still = 20 mGy DLP = 400 mGy •cm vol twenty 1 -cm slices
29 Patient Dose from CT Cagnon - ACMP 2008 DDLLPP ffrroomm CCTTDDII vvooll
AAPM Report 96
30 Patient Dose from CT Cagnon - ACMP 2008 PPrroobblleemmss wwiitthh CCTTDDII mmeetthhooddoollooggyy
AAssssuummeess eennttiirree ssccaatttteerr ttaaiillss aarree ccaappttuurreedd iinn tthheeiirr eennttiirreettyy bbyy tthhee cchhaammbbeerr
IInn bbooddyy pphhaannttoomm,, CCTTDDII 100 uunnddeerreessttiimmaatteess MMSSAADD ((ffoorr ppiittcchh 11)) bbyy aapppprrooxxiimmaatteellyy 3300%% (Boone) TThhee wwiiddeerr tthhee ccoolllliimmaattiioonn tthhee wwoorrssee tthhee pprroobblleemm ((MMDDCCTT 2255 && 3300 mmmm bbeeaamm wwiiddtthhss nnooww ccoommmmoonn,, 116600 mmmm nnooww bbeeiinngg iinnttrroodduucceedd))
31 Patient Dose from CT Cagnon - ACMP 2008 PPrroobblleemmss wwiitthh CCTTDDII mmeetthhooddoollooggyy
Chamber is 100 mm & phantom is 150 mm length Chamber under -reports: can capture primary, but scatter tails are lost 1° & scatter How to measure wider beams? scatter − Longer phantoms & pencil chambers Expensive & impractical
− Multiple scans with small chamber May be sensitive to dose inhomogeneities or “dose striping ” nT from diverging beam (especially at periphery) and tube position
32 Patient Dose from CT Cagnon - ACMP 2008 CCTTDDII LLiimmiittaattiioonnss:: IInnccrreeaassiinngg bbeeaamm wwiiddtthh ……
1 x 5 4 x 1 16 x 0.5 64 x 0.5 320 x 0.5 1998 2001 2004 2007 Courtesy K. Geleijns
(Le idin Univ 33 Patient Dose from CT Ceargsnion - ACMP 2008 ty, T he N ethe rlan ds) Courtesy K. Geleijns
(Le idin Univ ersi ty, T he N ethe CTDI100 rlan ds)
CTDI300
34 Patient Dose from CT Cagnon - ACMP 2008 Courtesy K. Geleijns
Correction factors … (Le Correction factors … idin Univ ersi ty, T he N ethe rlan ds)
35 Patient Dose from CT Cagnon - ACMP 2008 Small volume measurement devices can be sensitive to non-uniform CT radiation pattern – particularly at surface
Film profile @ surface
As well as p has in helical scans e (po siti a on o De t s f t Mar tar ub 32 cm CTDI phantom co ‘ t o e 05 f sc con axials an) tigu ous
36 Patient Dose from CT Cagnon - ACMP 2008 MMDDCCTT ddoossee eeffffiicciieennccyy:: OOvveerr --bbeeaammiinngg
For each detector row in MDCT to see same radiation intensity, all must be located in the umbra region of beam, the penumbra (which could be integrated into the signal of a single detector) is not utilized and adds to patient dose
Percentage loss greatest with relatively few detector rows
Cody, et al. Report of CT dose for MDCT scanners used in NLST - suggests improved dose efficiency in more complex scanners. 4 x 1 1m 6 x m 1.25 mm
37 Patient Dose from CT Cagnon - ACMP 2008 PPhhaannttoommss vvss.. PPaattiieennttss
Phantoms easy to work with - symmetric, homogeneous, and standardized. Patients exhibit none of these features Not a good estimate for objects that vary in size and shape from reference cylinders − CTDI tends to overestimate dose for large patients and underestimate for small/pediatric patients − Without dose modulation, entrance dose increased in the lateral relative to the AP projection (1/r 2 source –skin distance)
38 Patient Dose from CT Cagnon - ACMP 2008 WWhhaatt iiss tthhee qquueessttiioonn bbeeiinngg aasskkeedd??
CCTTDDII mmaayy pprroovviiddee aa uusseeffuull bbeenncchhmmaarrkk ffoorr ccoommppaarriinngg ssccaannnneerrss aanndd pprroottooccoollss BByy iittsseellff CCTTDDII iiss nnoott aa ggoooodd eessttiimmaattee ooff oorrggaann ddoossee oorr rraaddiiaattiioonn rriisskk TToo eessttiimmaattee rriisskk wwee nneeeedd ttoo iinnvveessttiiggaattee ddoossee ttoo oorrggaannss aanndd EEffffeeccttiivvee ddoossee
39 Patient Dose from CT Cagnon - ACMP 2008 EEffffeeccttiivvee DDoossee ((HH EE))
Effort to equate the partial body ICRP60 wT values exposure of diagnostic X -ray to a whole body equivalent stochastic risk gonads 0.2 stomach 0.12 colon 0.12 lung 0.12
HHE((SSvv )) == SS wwT ¥¥ HHT ((SSvv )) bone marrow 0.12 breast 0.05
esophagus 0.05 bladder 0.05
liver 0.05 thyroid 0.05 wT values are tissue weighting factors that assign stochastic risk bone 0.01 skin 0.01 relative to radiating whole body surfaces remainder 0.05
40 Patient Dose from CT Cagnon - ACMP 2008 DDeetteerrmmiinniinngg TTiissssuuee//OOrrggaann DDoossee??
TTLLDDss aanndd MMOOSSFFEETTss oonn aanntthhrrooppoommoorrpphhiicc pphhaannttoommss aann iimmpprroovveemmeenntt oovveerr hhoommooggeenneeoouuss ppllaassttiicc ccyylliinnddeerrss,, bbuutt ...... − Absolute dose measurements require reference calibration of solid state detectors and appropriate energy corrections − “Point ” measurements must be extrapolated to organs, whole body. − Measurements specific to a given phantom size, habitus , and composition not necessarily generalizable to actual patients
41 Patient Dose from CT Cagnon - ACMP 2008 OOrrggaann ddoossee wwiitthh MMoonnttee CCaarrlloo
CCoommppuuttaattiioonn iinntteennssiivvee mmeetthhoodd tthhaatt ttrraacckkss llaarrggee nnuummbbeerrss ooff iinnddiivviidduuaall pphhoottoonnss tthhrroouugghh mmaatthheemmaattiiccaall mmooddeellss ooff pphhaannttoommss aanndd ppaattiieennttss aanndd ccaallccuullaatteess ddoossee ddeeppoossiittiioonn ffrroomm iinnddiivviidduuaall pphhoottoonn//ttiissssuuee iinntteerraaccttiioonn pprroocceesssseess
42 Patient Dose from CT Cagnon - ACMP 2008 MMoonnttee CCaarrlloo ddoossee eessttiimmaatteess ooff ssttaannddaarrdd pphhaannttoomm mmooddeell
NRPB: Monte Carlo transport of CT spectra through MIRD phantom Based on mathematically described organ models of a standard, hermaphroditic, adult of given tissue composition. Dose estimates based on original, contiguous axial scan data from earlier scanners but can accept input for helical, modern MDCT protocols with scanner “matching ” factors. Input CT model, scan protocol, and scan extent and database generates table of organ doses and an effective dose. Multiple methods have evolved or are derived from this method (Huda, Atherton, ImPACT , LeHeron , Kalendar )
43 Patient Dose from CT Cagnon - ACMP 2008 NNoorrmmaalliizzeedd eeffffeeccttiivvee ddoossee (( kk)) ccooeeffffiicciieennttss
EEuurrooppeeaann WWoorrkkiinngg GGrroouupp -- CCTT QQuuaalliittyy gguuiiddeelliinneess SSeett ooff ffaaccttoorrss rreellaattiinngg MMoonnttee CCaarrlloo bbaasseedd oorrggaann ddoossee ttoo DDLLPP vvaalluueess SSttaannddaarrddiizzeess CCTT ddoossee rreeppoorrttiinngg ffoorr ttyyppiiccaall ssccaannss TThhuuss eeffffeeccttiivvee ddoossee ccaann bbee eessttiimmaatteedd ((wwiitthhiinn 1100%%)) ddiirreeccttllyy ffrroomm DDLLPP vvaalluueess ((wwhhiicchh ccaann bbee mmeeaassuurreedd iinn pphhaannttoomm aanndd//oorr oobbttaaiinneedd ffrroomm ssccaannnneerr ccoonnssoollee))
44 Patient Dose from CT Cagnon - ACMP 2008 EEffffeeccttiivvee ddoossee ªª ((kk)) ** DDLLPP
AAPM Report 96
45 Patient Dose from CT Cagnon - ACMP 2008 EExxaammppllee:: ““LLooww ddoossee ”” CChheesstt pprroottooccooll
Protocol: 120 kVp , 80 mA , 0.5 sec. rot., 4 x 2.5 beam width
100mm pencil chamber at center position of 32cm phantom reading: 26.7 mR
Center CTDI 100 = ( ELCf )/(NT) = 2.3 mGy
(26 .8mR )( 100 mm )( 1.0)( 0.87 rad / R) 1R 10 mGy • ( • )= 2.3mGy (4)( 2.5mm ) 1000 mR rad
Similarly, CTDI 100 at 12:00 = 4.7 mGy
CTDI w = (2/3 center + 1/3 periphery) = 3.9 mGy
46 Patient Dose from CT Cagnon - ACMP 2008 EExxaammppllee:: LLooww ddoossee CChheesstt pprroottooccooll
Applying the measured axial CTDI values to a helical protocol wh ere the table incrementation is 15mm/rotation
CTDI vol = CTDI w/pitch = CTDI w x (NT)/I = CTDI w (4 x 2.5/15) = (3.9 mGy ) (0.667) = 2.6 mGy Assuming a 35 cm long chest: DLP = (2.6 mGy)(35) = 91 mGy -cm
And using the k factor for an adult chest of 0.014: Effective dose = (91)(0.014) = 1.3 mSv
47 Patient Dose from CT Cagnon - ACMP 2008 LLiimmiittaattiioonnss ooff ccuurrrreenntt ddoossee ddeetteerrmmiinnaattiioonn mmeetthhooddss
No specific modeling of MDCT − Schmidt & Kalendar recently modeled Siemens Vol. Zoom MDCT have broader beam widths, different shape & composition of bow -tie filters, shorter focal to isocenter distances Helical scanners have variable pitch (0.5 -2) and introduce non -contiguous slices Recent advent of tube current modulation - both in plane and longitudinally along gantry axis - requires specific modeling techniques to simulate
48 Patient Dose from CT Cagnon - ACMP 2008 LLiimmiittaattiioonnss EExxttrraappoollaattiinngg ccaallccuullaatteedd oorrggaann ddoossee eessttiimmaatteess ffoorr mmaatthheemmaattiiccaall mmooddeellss ooff aa ssttaannddaarrdd mmaann ttoo aaccttuuaall ppaattiieennttss iiss pprroobblleemmaattiicc AAccttuuaall ppaattiieenntt ssiizzee && mmoorrpphhoollooggyy ccaann hhaavvee ssiiggnniiffiiccaanntt iimmppaacctt oonn ddoossee (Huda, Cody) aass ccaann aaggee,, ggeennddeerr,, aanndd eetthhnniicciittyy
49 Patient Dose from CT Cagnon - ACMP 2008 NNeexxtt sstteepp →→
SSppeecciiffiicc ssccaannnneerr,, ssccaann pprroottooccooll,, aanndd ppaattiieenntt mmooddeelliinngg
MMoonnttee CCaarrlloo ssiimmuullaattiioonnss tthhaatt mmooddeell ccuurrrreenntt MMDDCCTT ssccaannnneerrss iinn hheelliiccaall ((oorr aaxxiiaall)) mmooddee ttrraannssppoorrttiinngg pphhoottoonnss tthhrroouugghh vvooxxeelliizzeedd mmooddeellss ooff rreeaall ppaattiieennttss
50 Patient Dose from CT Cagnon - ACMP 2008 MMooddeellss tthhaatt eexxpplliicciittllyy ssppeecciiffyy ssccaannnneerr && ssccaann ggeeoommeettrryy iinncclluuddiinngg::
−− SSoouurrccee ttoo iissoocceenntteerr −− BBeeaamm ccoolllliimmaattiioonn −− BBeeaamm ppaatthh ((hheelliiccaall,, nnoonn ccoonnttiigguuoouuss,, eettcc.. −− BBeeaamm ssppeeccttrruumm ((vveennddoorr ssuupppplliieedd)) −− BBooww --ttiiee eeqquuaalliizzaattiioonn ffiilltteerr ((mmooddeelleedd oonn pprroopprriieettaarryy vveennddoorr iinnffoorrmmaattiioonn ooff sshhaappee aanndd ccoommppoossiittiioonn))
51 Patient Dose from CT Cagnon - ACMP 2008 Collimation & scanner geometry
j CT X-ray spectra q direction vectors (u,v,w) f ( w, q, & j )
w
sampled source position: x, y, z
CT source: U S ser can def ner ined & P , roto col Spe Bow-tie filter cific
52 Patient Dose from CT Cagnon - ACMP 2008 MMoonnttee CCaarrlloo SSiimmuullaattiioonn AApppplliiccaattiioonn:: EEvvoolluuttiioonn ooff PPhhaannttoomm DDoossiimmeettrryy
Calculate irradiation patterns, organ Model and benchmark against dose, whole-body effective dose in “conventional” dosimetry phantoms patient-specific voxelized phantoms PPaattiieenntt BBaasseedd PPhhaannttoommss Voxelized Patient Models − From GSF (Petoussi-Henss, Zankl et al, PMB, 2002) − Visible Human based upon the CT data from the Visible Human Project of the National Library of Medicine
Body -Mass GSF Model Age (yr) Gender Weight (kg) Height (cm) Index (kg/m 2) Baby 8 wks Female 4.2 57 12.9 Child 7 Female 22 115 16.4 Irene 32 Female 51 163 19.2 Golem 38 Male 69 176 22.2 Donna 40 Female 79 170 27.3 Frank 48 Male 65* 97* *** Helga 26 Female 81 170 28.0 Visible Human 38 Male 103 180 30.9 PPaattiieenntt BBaasseedd PPhhaannttoommss
The GSF data is se basgemd upon or enta gan tionof the original CT scan sets.
ICRU 44 elemental composition and mass density
GGSSFF -- GGoolleemm
55 Patient Dose from CT Cagnon - ACMP 2008 PPaattiieenntt bbaasseedd:: SSiimmuullaattiioonn SSccaann PPrroottooccoollss
For each size patient model, simulate whole -body scan (MDCT scanner - GE Lightspeed 16) , mapping HUs of tissues into Monte Carlo materials Evaluate: − organ dose − whole -body effective dose Scan Protocol: − Top of head to mid -thigh − Helical scan, pitch=1, 120 kVp − Body bowtie for Adults, head bowtie for Baby and Child − 16 x 1.25 mm nominal beam collimation − On a per 100 mAs basis
56 Patient Dose from CT Cagnon - ACMP 2008 QQuueessttiioonn :: OOnn aa nnoorrmmaalliizzeedd -- ppeerr mmAAss bbaassiiss ((ee..gg.. mmSSvv//mmAAss )) -- hhooww wwoouulldd yyoouu eexxppeecctt eeffffeeccttiivvee ddoossee ttoo vvaarryy wwiitthh iinnccrreeaassiinngg ppaattiieenntt ssiizzee//wweeiigghhtt?? 11.. IInnccrreeaassee
22.. DDeeccrreeaassee
20% 33.. RReemmaaiinn tthhee ssaammee 20% 20% 44.. CCaannnnoott bbee ddeetteerrmmiinneedd 20% 55.. EEffffeeccttiivvee ddoossee iiss iinnddeeppeennddeenntt ooff ppaattiieenntt 20% ssiizzee 10 PPaattiieenntt bbaasseedd –– WWhhoollee BBooddyy SSccaann 25.0 )
20.0 D eMa rco, et al, 2 15.0 007 s A m 0 0 1 / v S m (
10.0 e v i t c e f
f E e s 5.0
0.0 Increasing y a b d n ga m nk Body Size a il e n el u a ird B h n o H sH Fr o D y d o B e l o h W i M C re em D V I ol G Whole-Body Scan, Whole-Body Effective Dose (mSv per 100 mAs) RReessuullttss –– WWhhoollee BBooddyy SSccaann ) 30.0 D Lung
mA s e Mar co, 25.0 et a Liver 100 l, 2 007 Stomach 20.0 Colon mGy/
e( 15.0
10.0 anD os
gOr 5.0
0.0 vear ge
Increasing A y d e a k a b il n m n n g m D a h le n a l u R re o r e H I Body Size B C I o F H is M G D V
Whole-Body Scan, Average Organ Dose
59 Patient Dose from CT Cagnon - ACMP 2008 RReessuullttss –– WWhhoollee BBooddyy SSccaann
22.0
Variation of Whole-Body 20.0 ie v y = 0.0278x 2 - 1.7433x + 38.151
Dose with W t c e eig f f 18.0 R2 = 0.9683 E ht … ) s A 16.0 22.0 y = -0.1001x + 19.936 mmS0 /0 v 1
2 ( 14.0 e 20.0 R = 0.9482
e oh l s o do y e D B 12.0 W ) 18.0 10.0 16.0 10.0 15.0 20.0 25.0 30.0 35.0 i ot y e s v c e D Ef f Body-Mass Index (kg/m2)
mS/m0 v 0 s 1 A 14.0 ( odB
hole 12.0 W … and B ody 10.0 Ma 0.0 20.0 40.0 60.0 80.0 100.0 120.0 ss Ind Body Weight (kg) ex
60 Patient Dose from CT Cagnon - ACMP 2008 LLuunngg SSccrreeeenniinngg PPrroottooccooll FFoorr eeaacchh mmooddeell,, ssiimmuullaattee llooww ddoossee lluunngg ccaanncceerr ssccrreeeenniinngg pprroottooccooll EEvvaalluuaattee −− wwhhoollee --bbooddyy eeffffeeccttiivvee ddoossee −− oorrggaann ddoossee ttoo lluunngg aanndd tthhyyrrooiidd SSccaann PPrroottooccooll − Thoracic inlet to base of lungs (into liver) − Helical scan, pitch=1.375, 120 kVp − 16 x 1.25 mm nominal beam collimation − 80 mAs (0.5 sec rotation time at 160 mA )
61 Patient Dose from CT Cagnon - ACMP 2008 RReessuullttss –– LLooww DDoossee TThhoorraacciicc SSccaann
3.0
2.0 DvL P s . o ar l
1.0 ton CM oef o
ati 0.0 R
y d e a a k d b il n m n g m n ir a h e le n l u a B C Ir o o e H r M G D H is F V
DLP = CTDI VOL ×(scan length )
Effective Dose mSv = DLP ×k
European Guidelines, Jessen , 1999 FFeettaall DDoossee wwiitthh MMDDCCTT
Existing methods: − Idealized geometric models Not pregnant − Single dose estimate Limited allowance for patient size Single gestational age (<8 weeks) Homogenous fetus − No standard of truth for comparison How well do these represent a range of patient anatomies & gestational age?
63 Patient Dose from CT Cagnon - ACMP 2008 MMoonnttee CCaarrlloo AApppprrooaacchh Model CT scanner characteristics − GE LightSpeed 16, pitch 1 Model 27 pregnant patients of gestational age of < 5 weeks to 37 weeks − Voxelized models created from actual patient image sets − including early and late term pregnancies − Mother ’s size − Fetal size − Gestational age − Fetal composition (bone, tissue)
64 Patient Dose from CT Cagnon - ACMP 2008 << 55 wweeeekkss ((ggeessttaattiioonn ssaacc nnoott vviissiibbllee))
Uterus
Uterus
65 Patient Dose from CT Cagnon - ACMP 2008 77 wweeeekkss ((eemmbbrryyoo nnoott vviissiibbllee))
Gest. Sac Uterus
Gest. Sac Uterus
66 Patient Dose from CT Cagnon - ACMP 2008 Average Maturity: 24 weeks
Fetus
Gest. Sac Uterus Most Mature Fetus: 36 weeks
68 Patient Dose from CT Cagnon - ACMP 2008 VVooxxeelliizzeedd PPaattiieenntt MMooddeellss
RRaaddiioollooggiisstt ccoonnttoouurreedd oorrggaannss:: − Fetus (if visible) − Gestational sac (if visible) − Uterus − Within the fetus, voxels assigned to bone or soft tissue Outside of uterus voxels assigned to one of 6 tissue types (based on HU):
− Lung, fat, water, muscle, bone, air 70 Patient Dose from CT Cagnon - ACMP 2008 An gel, et FFeettaall rreessuullttss al. 2 008
Fetal dose (mGy/100 mAs )
METHOD Average Range
Patient Specific 10.8 7.3 -14.3 (n=27) ImPACT (n=1) 12 *** (MIRD based uterine dose) Felmlee (n=1) 11.3 9-13.6 (size allowance +/ - 20%)
Comparison models (that don’t account for gestational age) tend to overestimate fetal dose
71 Patient Dose from CT Cagnon - ACMP 2008 Normalized radiation dose d appear to correlate oes with gneosttational age …
but d correlate with oes mother’s perimeter
An gel, et al. 2 008 WWhhaatt aabboouutt DDoossee//TTuubbee CCuurrrreenntt MMoodduullaattiioonn??
UUssee ssccaannnneerr//ppaattiieenntt ssppeecciiffiicc mmooddeelliinngg ttoo ddeetteerrmmiinnee::
−− HHooww mmuucchh iiss oovveerraallll ddoossee rreedduucceedd??
−− WWhhaatt hhaappppeennss ttoo iinnddiivviidduuaall oorrggaann ddoossee??
73 Patient Dose from CT Cagnon - ACMP 2008 VVaarriiaabbllee mmAA MMoonnttee CCaarrlloo mmooddeell
FFoorr vvaarriiaabbllee ttuubbee ccuurrrreenntt,, mmooddiiffyy ssccaannnneerr XX -- rraayy ssoouurrccee mmooddeell ttoo vvaarryy oouuttppuutt aass aa ffuunnccttiioonn ooff ggaannttrryy aannggllee ((iinn ppllaannee mmoodduullaattiioonn)) aanndd zz aaxxiiss ppoossiittiioonn aalloonngg ssoouurrccee ppaatthh UUssee ddaattaa ffrroomm aaccttuuaall ppaattiieenntt ssccaann tthhaatt pprroovviiddeess:: − tube current vs. tube angle vs. table position − (mA vs. θ vs. z)
74 Patient Dose from CT Cagnon - ACMP 2008 CCoonnvveennttiioonnaall LLoonngg AAxxiiss MMoodduullaattiioonn
Shoulder Breast Lung Region Tissue Region Abdomen 600
500
400 180 degrees
Am ) ( t (LAT ) 300
200 TuCbene ur r 90 degrees 100 (AP)
0 0 50 100 150 200 250 300 Table Position (mm) Siemens Sensation 16 Effective Dose Table (w/ weight factors) applied to GSF “Donna” (ex trap ola variable mA constant mA Ratio ted) Lung 0.750 1.473 1.96 Gonad 0.004 0.007 1.81 Skin 0.016 0.030 1.85 Liver 0.258 0.482 1.87 Bone Marrow 0.114 0.187 1.64 Bone Surface 0.040 0.065 1.64 Breast 0.284 0.581 2.04 Stomach 0.737 1.354 1.84 Thyroid 0.813 1.044 1.28 Esophagus 0.350 0.595 1.70 Bladder 0.000 0.001 2.37 Colon 0.073 0.166 2.28 Remainder Organs 0.082 0.146 1.77 Total 3.522 6.131 1.74
Ratio of maximum mA to average mA = 1.77 2.5 ) A m e l 2.0 b a i r a V
1.5
1.0 s v A m t n a t s n o C ( e s
o 0.5 D e v i
t c e 0.0
f f E f t l ng w e s s er s ta u ad ro c a ch u d n n o L n in r fa re a id ag d lo a T o k r a ur B o h la o rg G S e M S om r p B C O o o i t a iv t y o L ne e S h s er R o n T E B o nd B ai em R
Reference mA ??
77 Patient Dose from CT Cagnon - ACMP 2008 DDoossee ttoo iinnddiivviidduuaall oorrggaannss ffrroomm ttuubbee ccuurrrreenntt mmoodduullaattiioonn??
DDoossee rreedduuccttiioonn iiss rreellaattiivvee –– wwhhaatt iiss ccoommppaarriissoonn mmAA ?? CCuurrrreenntt wwoorrkk bbeeiinngg ddoonnee oonn eeffffeecctt ooff ddoossee mmoodduullaattiioonn sscchheemmeess oonn bbrreeaasstt ddoossee
78 Patient Dose from CT Cagnon - ACMP 2008 VVooxxeelliizzeedd PPaattiieenntt MMooddeell Voxelized models from actual patient images Radiologist contoured breast tissue − (glandular +adipose) Glandular tissue automatically segmented Lung tissue semi -automatically segmented − 5 lung density categories, depending on HU Voxels outside the breast and lung regions automatically assigned to material types: − fat, water, muscle, bone, or air − further sub -divided into 17 density categories, depending on its HU value
79 Patient Dose from CT Cagnon - ACMP 2008 *(as defined by ICRU 44) Original Image Radiologist ’s Contour
Segmented Image Voxelized Model glandular breast
lung PPaattiieenntt ssppeecciiffiicc ccaallccuullaattiioonnss iinn tthhee cclliinniicc??
Validated Monte Carlo modeling can be used to ask specific questions and could serve as a “gold ” reference standard Not generally practical on an individual patient basis Patient and scanner specific Monte Carlo modeling probably better suited for characterizing dose - and identifying factors that effect dose - across a range of patient, scanners, and scan protocols Create data tables of scaling factors to allow estimation of patient and organ dose calculated from standardized measurements made on a CT scanner
81 Patient Dose from CT Cagnon - ACMP 2008 RReeccoommmmeennddeedd rreeaaddiinngg
AAAAPPMM RReeppoorrtt 9966:: TThhee MMeeaassuurreemmeenntt,, RReeppoorrttiinngg,, aanndd MMaannaaggeemmeenntt ooff RRaaddiiaattiioonn DDoossee iinn CCTT ((JJaann --0088))
82 Patient Dose from CT Cagnon - ACMP 2008 AAcckknnoowwlleeddggeemmeennttss
MMoonnttee CCaarrlloo eeffffoorrtt ffuunnddeedd bbyy NNaattiioonnaall IInnssttiittuuttee ooff BBiioommeeddiiccaall IImmaaggiinngg aanndd BBiiooeennggiinneeeerriinngg GGrraanntt RR0011 EEBB000044889988
83 Patient Dose from CT Cagnon - ACMP 2008