RSMI 2009 Session III RSMI 2009 Session III Diagnostic XDiagnostic X--Ray Shielding DesignRay Shielding Design

NCRPNCRP--147 Shielding Models147 Shielding Models

Douglas J. Simpkin, Ph.D.A St L k ’ M di l CtAurora St. Luke’s Medical Ctr

Milwaukee, WIdsimpkin@wi.rr.com

http://www.geocities.com/djsimpkin/http://www.geocities.com/djsimpkin/

Models for Diagnostic XModels for Diagnostic X--RayRayModels for Diagnostic XModels for Diagnostic X Ray Ray Shielding CalculationsShielding Calculations

Yes No

2

Yes

The Three Models for DiagnosticThe Three Models for DiagnosticThe Three Models for Diagnostic The Three Models for Diagnostic XX--ray Shielding In NCRP 147ray Shielding In NCRP 147

1. First-principle extensions to NCRP 492. Given calculated kerma per patient, scale

by # patients and inverse squared distance, y p q ,and then use transmission curves designed for particular room typesp yp

3. NT/(Pd2)

3

1st principle extensions to NCRP 49

• (Underlies the other two methods)• The kerma in the occupied area may have p y

contributions from– primary radiationprimary radiation– scatter radiation– leakage radiation Secondary radiation}leakage radiation }

4

Primary, Scatter, and LeakagePrimary, Scatter, and Leakage

Must protect from primary radiationradiation

primary

Must protect from

5

scatter & leakage radiation

1st principle extensions to NCRP 49

• The models for primary, scatter, and leakage in NCRP-147 are extensions to what’s in NCRP-49 (1976):– x-ray tubes operating over ranges of potentials y p g g p

(“workload distribution”)– new model for image receptor attenuation– new model for leakage

6

1st principle extensions to NCRP 49• These primary scatter and leakage• These primary, scatter, and leakage

radiations may be from multiple x-ray sources (or tube positions)sources (or tube positions)

• So, simply add up all the contributions to the k K f ll h i hkerma, K, from all these sources in the occupied area behind a barrier of thickness x,

( )∑ ∑ ++=tubes kVp

LSP xKxKxKxK )()()()(

7

1st principle extensions to NCRP 49• Then iteratively find a barrier thickness x that• Then iteratively find a barrier thickness x that

decreases that kerma to P/T, the design goal modified by the occupancy factormodified by the occupancy factor

( )∑ ∑ =++= LSPPxKxKxKxK )()()()( ( )∑ ∑ =++=

tubes kVpLSP T

xKxKxKxK )()()()(

• See http://www.geocities.com/djsimpkin/ for shareware program XRAYBARR to do this

8

shareware program XRAYBARR to do this

1st principle extensions to NCRP 49• XRAYBARR was written by me in the mid• XRAYBARR was written by me in the mid

1990s to perform shielding calculations with these new models as we developed NCRP-147.these new models as we developed NCRP 147. The shielding data and examples in NCRP-147 are based on the output of XRAYBARR.p

• Note: Some of the examples in NCRP-147 aren’t duplicated by XRAYBARR because p yNCRP-147 takes shortcuts in the tabulated xprevalues. XRAYBARR is right!

9

Primary Radiation ModelPrimary Radiation ModelPrimary Radiation ModelPrimary Radiation ModelPrimary Kerma at 1 m per workload

• In primary beam, know kerma per pworkload at 1 m, KW(kVp) , for 3 W pphase units (data of Archer et al. 1994)

10

Unshielded Primary Beam KermaUnshielded Primary Beam KermaUnshielded Primary Beam KermaUnshielded Primary Beam Kerma

At i kV )()()0( W kVpWkVpKK• At a given kVp, 2

)()()0(P

WP d

ppK =

• If only a fraction U of the tube’s workload is directed at this barrier, then

2

)()()0( WP d

kVpWUkVpKK =

• U is the use factor for this barrierPd

11

Kerma Behind a Primary BarrierKerma Behind a Primary BarrierKerma Behind a Primary BarrierKerma Behind a Primary Barrier• The kerma behind a primary barrier of

transmission B(x, kVp) is

)()()()( kVBkVpWUkVpKkVK W

• For the whole distribution of workloads total

),()()(),( 2 kVpxBd

ppkVpxKP

WP =

• For the whole distribution of workloads, total kerma is

),()()()( 2 kVpxBd

kVpWUkVpKxKP

W

kVpP ∑=

12

dPkVp

i i ii i iPrimary Radiation:Primary Radiation:The Old NCRPThe Old NCRP--49 Model49 ModelThe Old NCRPThe Old NCRP 49 Model49 Model

x

Barrier of thickness x decreases raw i di ti k t P/T

13

primary radiation kerma to P/T

Primary Radiation:Primary Radiation:Primary Radiation:Primary Radiation:The RealityThe Reality

Grid, cassette, supporting structures

ti tpatient

Primary radiation is significantly tt t d b f hi b i

14

attenuated before reaching barrier

Primary Radiation:Primary Radiation:Primary Radiation:Primary Radiation:A Conservative, Realistic ModelA Conservative, Realistic Model

Grid, cassette, maybe image receptor supporting structures

Even without the patient, primary radiation is stilli ifi tl tt t d b f hi b i

15

significantly attenuated before reaching barrier

Primary Radiation:Primary Radiation:Primary Radiation:Primary Radiation:NCRPNCRP--147 Model147 Model

Grid, cassette, maybe supporting structures

N ti t!No patient!

xxpre } xtot = x + xpre

Assume primary beam attenuation in image receptor is due to a pseudo-barrier whose equivalent thickness xpre gives same

16

transmission as that seen for actual image receptors.

68

1E+0 Primary Transmission Through Patient, Image Receptor, and Supports

2

4

1E-1 ersNo patient & grid & cassette:

Data of Dixon (1994)

2

468

1E-1

sion

unte

d G

rid +

Cas

sette

Hol

d

B = 4.7E-6 kVp 2.181

468

1E-2

Tran

smis

s

No patient & grid & cassette &cassette support structures &radiographic table:

Wal

l-Mou

Cas

sette

+ C

68

2

1E-3

g pB = 9.36E-13 kVp4.917 Type of Radiographic Table

(data of Dixon 1994)

GE RTE Table

C

2

46

1E 4

GE Advantx Table

Siemens Multix-T Table

Picker Clinix-T Table

1740 50 60 70 80 90 100 125 150

kVp

1E-4

1E+3 Values of x pre

(Grid+cassette+support)

1E+2 Plate GlassGypsum

p

1E 2

m)

Concrete

1E+1

x pre

(mm

Steel

1E+0Lead

1E-1

18

20 30 40 50 60 70 80 90 100 110 120 130 140 150kVp

1E 1

xx for Radiographic Roomfor Radiographic Roomxxprepre for Radiographic Room for Radiographic Room Workload DistributionsWorkload Distributions

• From NCRP-147 Table 4.6:– Grid + cassette:

• 0.3 mm Pb• 30 mm concrete

G id + tt + t bl / h t b k t– Grid + cassette + table/chest bucky supports:• 0.85 mm Pb• 72 mm concrete72 mm concrete

• (See Dixon & Simpkin Health Phys 74;181-189;1998 for a more complete list.)

19

189;1998 for a more complete list.)

Calculation of Primary KermaCalculation of Primary Kerma

• Same as model in NCRP-49 except– account for workload distribution in kVpaccount for workload distribution in kVp– May account for image receptor shielding xpre

• Primary kerma in occupied area is then• Primary kerma in occupied area is then

=+ preP xxK )(

∑ + preW kVpxxBkVpWUkVpKd

),()()(12

20

kVpPd 2

Scatter RadiationScatter Radiation

ipatient

21

Scaled Normalized Scatter FractionScaled Normalized Scatter FractionScaled Normalized Scatter FractionScaled Normalized Scatter Fraction

1 mKS

1 m1 m1 m

6+′ SK

K

θ

61 10+×=′

P

S

KKa

22

KP 1 cm2 area primary beam at 1 m

Scaled Normalized Scatter FractionScaled Normalized Scatter Fraction'

23

Scatter RadiationScatter RadiationScatter RadiationScatter Radiation• Same theory as old NCRP-49

– scatter fraction data of Kelley & Trout reevaluated by Simpkin & Dixon (Health Phys 1998)

i b ( 2) d i di d– pri beam area F (cm2) measured at pri distance dF conveniently taken as image receptor area @ SID

li itl h kV d d d– explicitly show kVp dependence and sum over workload distribution to yield shielded scatter kermakerma

),()()(10),( 22

61 kVpxB

dF

dkVpWkVpKaxK W

S ∑−×′

=θ

24

)()( 22 pdd FkVp S

S ∑

Leakage RadiationRadiation originating from x-ray tube focal spot but not emanating from the tube portal

i

portal

patient

25

Leakage radiationLeakage radiationLeakage radiationLeakage radiation

• Intensity can’t exceed L = 100 mR/hr at 1 mIntensity can t exceed L 100 mR/hr at 1 m when tube is operated at its leakage technique factorstechnique factors– maximum potential for continuous operation

kVp (typically 135-150 kVp or 50 kVp forkVpmax (typically 135 150 kVp, or 50 kVp for mammography)

– Imax is the maximum continuous tube current max s t e a u co t uous tube cu e tpossible at kVpmax . Note that this is usually a low mA, not typical of clinical radiography.

26

Leakage radiationLeakage radiationLeakage radiationLeakage radiation

• These leakage technique factors specify how thick the shielding in the tube housing should be

• NCRP49 suggested leakage technique gg g qfactors of 3.3 mA at 150 kVp, 4 mA at 125 kVp, 5 mA at 100 kVp; remain fairly p p ytypical today

27

Leakage radiationLeakage radiationLeakage radiationLeakage radiation• NCRP-147 calculations (and shielding methodsNCRP 147 calculations (and shielding methods

2 and 3) use – 3 3 mA at 150 kVp3.3 mA at 150 kVp– worst case leakage rates

(Subsequently we’ve found that assuming 4 mA at– (Subsequently, we ve found that assuming 4 mA at 125 kVp leakage technique factors specifies barriers that are 10-20% thicker than in the report)p )

– However, actual leakage rates are 0-30% of the maximum leakage so we don’t see a problem

28

g p

New Leakage ModelNew Leakage Modelgg

• For tube operating at techniques (kVp, I) with p g q ( p, )transmission through the tube housing Bhousing, assume leakage kerma rate at 1 m through tube housing is

)()( h i2 kVpBIkVpkVpKL ∝

• Assume worst case scenario: leakage kerma rate =

)()( housing kVpBIkVpkVpKL ∝

limit L for tube operation at leakage technique factors (conservative by factors of 3 to ~infinity)

29

New Leakage ModelNew Leakage Modelgg• Estimate thickness of tube housing by using primary beam

output at leakage technique factors as model for unhoused p g qleakage radiation.

1931 mGy/hr 100 mR/hr = 0.873 mGy/hr

1 m 1 mTube operated at 150 kVp, 3.3 mA

“unhoused” tube

Tube housing = 2.32 mm Pb thick

1 m 1 m

30

1931 mGy/hr 1931 mGy/hr

New Leakage ModelNew Leakage Modelgg• Write ratio of leakage kerma rates at any kVp

to L at kVpto L at kVpmax

• and knowing that at a given kVp, workload W(kVp) is the time integral of the tubeW(kVp) is the time integral of the tube current: h hi ld d l k k ( 1 )

∫= dtIkVpW )(• then unshielded leakage kerma KL (at 1 m) at

that kVp is

)()()()1(

),0(maxhousingmax

2max

housing2

kVpBIkVpkVpBkVpWUkVpL

kVpKL

−=

31

)( maxhousingmaxmax kVpkVp

New Leakage ModelNew Leakage Modelgg

• Applying inverse square to distance dL fromApplying inverse square to distance dL from tube to shielded area,

• and putting a barrier with transmission• and putting a barrier with transmission exp(–ln(2)x/HVL) between tube & area yields

)()()1(2 kVBkVWUkVL×

−=

)()()()1(

),(maxhousingmax

2max

housing2

kVpBIkVpkVpBkVpWUkVpL

kVpxKL

⎟⎠

⎞⎜⎜⎝

⎛ −×

)()2ln(exp1

2

g

kVpHVLx

d32

⎠⎝ )(kVpHVLdL

How far off is NCRPHow far off is NCRP--49’s leakage model?49’s leakage model?

1E-1

1E+0

1E-3

1E-2

e/

1E 5

1E-4 Leakage dose as function of kVp transmitted through x-ray tube housing of 2.32 mm Pb compared 14

7 le

akag

e49

leak

age

1E-6

1E-5 ous g o 3 b co pa edto that at 150 kVp

Leakage technique factors:150 kVp, 3.3 mA for 100 mR/hrN

CR

P-1

NC

RP-

4

1E-8

1E-7

3350 60 70 80 90 100 110 120 130 140 150

kVp

1E-9

Summary: Shielding Model No 1Summary: Shielding Model No 1Summary: Shielding Model No. 1Summary: Shielding Model No. 1• Rigorous model based on the well-accepted

NCRP-49 methods.• But you need a computer program y p p g

(XRAYBARR, for example) to implement fully!f y

• Is there a shielding method that allows paper and calculator solutions?paper and calculator solutions?

34

NCRPNCRP--147 Shielding Model No 2147 Shielding Model No 2NCRPNCRP 147 Shielding Model No. 2147 Shielding Model No. 2• For each clinical workload distribution, of

l kl d W i f b htotal workload Wnorm per patient, for both primary and secondary barriers, NCRP-147 provides:provides:– K1 , the kerma per patient at 1 m distance

• Primary kerma per patient K 1 is in Table 4 5• Primary kerma per patient KP is in Table 4.5• Secondary kerma per patient Ksec

1 is in Table 4.7– B, the transmission of the radiation ,

generated by this workload distribution for primary or secondary barriers (cf App B & C)

35

NCRPNCRP--147 Shielding Model No 2147 Shielding Model No 2NCRPNCRP 147 Shielding Model No. 2147 Shielding Model No. 2

Primary Air Kerma at 1 m for Workload Di ib i K1

36

Distributions, K1

NCRPNCRP--147 Shielding Model No 2147 Shielding Model No 2NCRPNCRP 147 Shielding Model No. 2147 Shielding Model No. 2

Secondary Air Kerma at 1 m for Workload

37

S y WDistributions, K1

sec

NCRPNCRP--147 Shielding Model No. 2147 Shielding Model No. 2

•• For single kVp operation For single kVp operation cf. Simpkin and Dixon Health Phys. 74(3), 350–365 for y ( ),secondary kerma per workload at 1 m at single kVp operationg p p

• All other data is available in NCRP 147But be careful reading scientific notation:– But be careful reading scientific notation: 1.234 x 101 = 12.34

38

Shielding Model No. 2Shielding Model No. 2• Get the unshielded kerma K(0) by scaling the kerma• Get the unshielded kerma, K(0), by scaling the kerma

per patient at 1 m, K1, by – N patient procedures (suggested values of N are in Table

4.3) or, equivalently – total workload Wtot (where workload/pat = Wnorm)– can tweak W by a QE-specified different workload per– can tweak Wtot by a QE-specified different workload per

patient, Wsite

t tWUKNUK 11• Kerma is then

( h U i l d b 1 f d b i )norm

tot

WdWUK

dNUKK 22)0( ==

39

– (where U is replaced by 1 for secondary barriers)

Shielding Model No. 2Shielding Model No. 2• Ratio of P/T to K(0) is the required transmission• Ratio of P/T to K(0) is the required transmission

1

2

1

2

)0(/)( WdPdP

KTPxB norm===

– (again, U is replaced by 1 for secondary barriers)T i i B i f i f

11)0()(

UDTWUDTNK tot

• Transmission B is now a function of – barrier material and thickness

kl d di ib i– workload distribution– primary or secondary

40

41

B=0.0047

42x=1.2 mm Pb

Now theNow the difficulty is in reading the

correct curve!

43

Shielding Model No 3 forShielding Model No 3 forShielding Model No. 3 for Shielding Model No. 3 for “Representative Rooms”“Representative Rooms”

• Model No. 2 fails for complicatedcomplicated assemblages of x-ray tubes/ positions/ workload distributions, such as i di hiin a radiographic or radiographic/ fluoroscopic room

44

fluoroscopic room

Shielding Model No 3 forShielding Model No 3 forShielding Model No. 3 for Shielding Model No. 3 for “Representative Rooms”“Representative Rooms”

• (Using XRAYBARR) NCRP-147 shows barrier thickness requirements calculated forbarrier thickness requirements calculated for representative rooms:

Assume conservatively small room layout– Assume conservatively small room layout• assures maximum contribution from all sources

– Presumes that the kinds of exposures madePresumes that the kinds of exposures made amongst the various x-ray tubes/positions follow those observed by the AAPM TG-9 survey

45

y y

Representative Radiographic RoomRepresentative Radiographic RoomRepresentative Radiographic RoomRepresentative Radiographic Room

46

Use Factors from AAPM SurveyUse Factors from AAPM SurveyUse Factors from AAPM SurveyUse Factors from AAPM SurveyCross-table

Rad Room:Chest Bucky

Lateral Position U=9%

Overtable Position U=89% shooting down t fl

Rad Room: floor/ other barriers applies

at floor

(Another primary wall gets other barriers applies to Overtable and Crosstable positions

( p y gU=2% of the floor/ other barrier distribution; assume tube is centered overtable)

47

tube is centered overtable)

Representative Radiographic RoomRepresentative Radiographic Roomp g pp g pCross-table Lateral Wall Secondary Barrier

Chest

primary y

Chest Buckywall U=2%primary

ky dary

U 2% primary wall

est B

uck

l sec

ond

48

Che

wal Secondary Barrier

“Representative R&F Room”“Representative R&F Room”Representative R&F RoomRepresentative R&F Room• Also assume a “Representative R&F room”

– Has same layout as “Standard Radiographic Room except an undertable fluoro x-ray tube and image intensifier are added centered over tableintensifier are added, centered over table

– Does fluoro as well as standard radiographic work, with table and chest buckies and crosstable work

• Assume– 75% of patients imaged as if in radiographic roomp g g p– 25% of patients imaged by fluoroscopy tube

49

“Representative R&F Room”“Representative R&F Room”

Overtable Rad tube

Chest Rad tube

Rad tube

IImageIntensifier

Crosstable Lateral Rad Tube

50

TubeUndertable Fluoro Tube

“Representative Room”“Representative Room”Representative RoomRepresentative RoomBarrier RequirementsBarrier Requirements

• From Model 2, transmission requirement is2

1

2

)(UKTNdPxB =

• so the barrier thickness requirement must scale as:

2dPTN

51

2dP

“Representative Room”“Representative Room”Representative RoomRepresentative RoomBarrier RequirementsBarrier Requirements

• Method:– Given N patients/week, need to shield to P/T, aGiven N patients/week, need to shield to P/T, a

distance d from the x-ray source– Calculate in mGy-1 m-2TN y

– Look up the required barrier thickness on the

2dP

Look up the required barrier thickness on the graph appropriate for that workload distribution, barrier, and barrier material

52

22There are 12 There are 12 NT/PdNT/Pd22 graphsgraphs

• For Representative Radiographic and R&FRooms:– For Lead and Concrete:

• Primary barriers with preshieldingy p g• Primary barriers without preshielding• Secondary barriers

53

54

22NT/PdNT/Pd22 curves have been fitcurves have been fit

• The NT/Pd2 curves have been fit to a modified Archer eqn:

• See fitting parameters atSee fitting parameters at– http://geocities.com/djsimpkin/Shielding/Shield

ing.htm

55

g

56

NT/PdNT/Pd22: From where is : From where is ddmeasured?measured?

Primary BarriersFloor overhead radiographic tubeChest Bucky wall chest tube (72" SID)Chest Bucky wall chest tube (72 SID)Crosstable Lateral Wall cross-table tube (40" SID)2% U wall center of table

Secondary BarriersFloor patient on tableFloor patient on tableChest Bucky secondary wall chest tube (72" SID)Secondary Wall patient on tableCeiling patient on table

57

Ceiling patient on table

Equivalency of Shielding Materials forEquivalency of Shielding Materials forEquivalency of Shielding Materials for Equivalency of Shielding Materials for Model No. 3 CalculationsModel No. 3 Calculations

• For “representative room” calculations only, conservatively concludey– Steel thickness requirement =

8 × Pb thickness requirementq– Gypsum wallboard thickness requirement =

3.2 × concrete thickness requirement– Glass thickness requirement =

1.2 × concrete thickness requirement

58

ConclusionsConclusions• NCRP-147 utilizes 3 shielding models

Model No 1: Extension of the methods of NCRP 49– Model No. 1: Extension of the methods of NCRP-49• With kVp dependence• With new models for primary and leakage

R i t t i l t f ll• Requires computer program to implement fully– Model No. 2: Based on data from model no. 1,

• NCRP-147 shows kerma per patient at 1 m and transmission i f i kl dcurves appropriate for a given workload.

• Calculate unshielded kerma and then transmission needed to reduce to P/T. Look up barrier thickness.

M d l N 3 B d d f d l 1– Model No. 3: Based on data from model no. 1, • For N patients at distance d (for a particular workload

distribution & barrier), calculate NT/Pd2

NCRP 147 h b i hi k f i f NT/Pd2

59

• NCRP-147 shows barrier thickness as function of NT/Pd2