shielding for diagnostic x-rays: uk guidance - itn for...shielding for diagnostic x-rays: uk...

Shielding for Diagnostic X-rays:UK Guidance

Jerry WilliamsRoyal Infirmary of EdinburghRoyal Infirmary of Edinburgh

BIR/ IPEM Working Party ReportBIR/ IPEM Working Party Report

British Institute of Radiology

Institute of Physics and Engineering in Medicine

Working Party:

David Sutton Jerry Williams Colin Martin Don McIntosh Tony

Institute of Physics and Engineering in Medicine

David Sutton, Jerry Williams, Colin Martin, Don McIntosh, Tony Cotterill, Graham Hart, David Gallacher

Publication date: 2000

BIR/ IPEM Working Party Report

Content:

BIR/ IPEM Working Party Report

• Design criteria and dose constraints

• Primary & Secondary radiations• Primary & Secondary radiations

• Building materials

• X-ray transmission factorsay t a s ss o acto s

• Assessment of shielding

• Worked examples

Working Party:

David Sutton Jerry Williams Colin Martin Don McIntosh Tony David Sutton, Jerry Williams, Colin Martin, Don McIntosh, Tony Cotterill, Graham Hart, David Gallacher

Publication date: 2000

dRadiation Sources

• Radiography (film/ screen)• Radiography + Fluoroscopy• Radiography + Fluoroscopy• Angiography

CT ( i l li )• CT (single slice)• Mammography• Dental• DEXA• DEXA

h lkThis talk

• Design criteria• Transmission factors• Transmission factors• Shielding materials

P i d S d R di ti• Primary and Secondary Radiations

Design Criteria: Design Criteria: Dosimetric Quantities

• Dose limits–Effective dose, E mSvEffective dose, E mSv

• Dose monitoringOperational quantity–Operational quantity

–Personal dose equivalent, Hp(d) mSv

Shi ldi l l ti• Shielding calculations–Air kerma, K mGy

Design Criteria

• Dose constraint (people)– Based on public dose limit (1 mSv/ year)

A li d ( ff bli )– Applied to everyone (staff + public)– 0.3 mGy/ year (6 µGy/ week)

• Occupancy factor • Occupancy factor – ≥5%– Dose to be < 6 mGy/ year– (Controlled area)

• Dose constraint (film or CR)/ d ( / k)– 0.4 µGy/ day (2 µGy/ week)

fOccupancy factors

• Based on individual occupancy• Examples:p

Office 100%Reception area 100%Reception area 100%X-ray control room 100%Patient examination room 50%Corridor 20%Corridor 20%Toilets/ bathrooms 10%Outdoor area with seating 10%Store rooms 5%Store rooms 5%Unattended waiting rooms 5%

fX-ray Transmission factors

• Adopted Archer’s empirical model1−

⎤⎡ ⎞⎛ γγα

αβ

αβ1 ⋅⋅

⎥⎦⎤

⎢⎣⎡ −⎟

⎠⎞⎜

⎝⎛ += teB

• Used Simpkin’s α, β, γ values• Added data for brick and barium Added data for brick and barium

plaster

Archer et al (1983) Health Physics, 44, 507.

Simpkin (1995) Health Physics, 68, 704

h ld lShielding material

UK Standard lead thicknesses

Code t mm3 1.324 1.805 2.246 2.65

Primary and Secondary Primary and Secondary Radiations

• Leakage radiationg• Primary/ transmitted beams• ScatterScatter

X-ray Tube LeakageX ray Tube Leakage

Assumptions:•Tube leakage factor: 150 kV/ 3.3 mA

•Field size: 1000 cm2

•Distance: 1 m from tube/ patient

WP Recommendation:WP Recommendation:

•Leakage contribution may be ignored

Primary & Transmitted Primary & Transmitted Radiation

• Fluoroscopy– Beam restricted to

image intensifier/ flat X-raybeam

image intensifier/ flat panel detector

– Detector has > 2mm lead equivalent

Patient

lead equivalent

• Mammography

Imagingdevice

Radiography: Film dose method

• Dose to film/ imaging plate– 400 speed system– Dose ≤10 µGy

Dose ≤10 µGy

• Lead equivalence (100 kV)– Cassette

• 0 19 mm• 0.19 mm– Cassette + grid

• 0.26 mm– Cassette/grid/tableCassette – Cassette/grid/table

• 0.8 mm– Cassette/grid/chest Bucky

• 0 7 mm

Bucky system

• 0.7 mm

Lead equivalence data derived from Dixon (1994) Med Phys, 21, 1785

Radiography: Film dose method

• Dose to film/ imaging plate

– 400 speed system– Dose ≤10 µGyDose ≤10 µGy

• Lead equivalence (100 kV)– Cassette

• 0 19 mm• 0.19 mm– Cassette + grid

• 0.26 mm– Cassette/grid/table– Cassette/grid/table

• 0.8 mm– Cassette/grid/chest Bucky

• 0 7 mm• 0.7 mm

Attenuated primary: limiting HVLAttenuated primary: limiting HVL

Limiting HVL: = ln(2) / α1

Limiting HVL: ln(2) / α

0.1 to eDD ⋅α−=

0 2

0.3

VL m

m

0.01

Tran

smis

sion

0.1

0.2

Lim

iting

H

0.001

T

040 60 80 100 120

kV0.0001

0 0.5 1 1.5 2 2.5 3

Lead thickness mm

Radiography: ESD methodRadiography: ESD method

• Unattenuated primary beam

p y– Beam outside patient

• Entrance Surface Dose (ESD)• Situations include

– Beam not collimated to cassette– Beam not directed to Bucky

• Examples– Chest radiography– Cross table radiography– Out of Bucky radiography (e.g.

extremities)

extremities)• Area of wall irradiated

– Not easy to predict– Variable– Variable

lExamples

Chest Radiography (film dose)Chest Radiography (film dose)

• ParametersParameters– 100 films/ week– 90 kV– ESD = 0.15 mGy– Film dose = 10 µGy– FFD = 1.8 m– FSD = 1.4 m

Fil ll 0 7 – Film-wall = 0.7 m

Best Radiographic Practice

Calculation (Film-dose method)

• Attenuated kerma at wall per week:Film dose x Workload x ISL correction

mGy52.07.08.1

8.110010102

3 =⎥⎦⎤

⎢⎣⎡

+××× -

⎦⎣

• No of HVLs required (6 µGy/ week) = 6.5 • Limiting HVL of lead @ 90 kV = 0.23 mmLimiting HVL of lead @ 90 kV 0.23 mm• Total equivalent lead thickness = 1.5 mm• Additional lead shielding =

0 8 0.8 mm

Chest Radiography (ESD)Chest Radiography (ESD)

• ParametersParameters– 100 films/ week– 90 kV– ESD = 0.15 mGy– Film dose = 10 µGy– FFD = 1.8 m– FSD = 1.4 m

Fil ll 0 7 – Film-wall = 0.7 m

Calculation (ESD method)( )• Worst case assumption:

• Beam size greater than patient and Bucky system

• Unattenuated kerma at wall per week:ESD x Workload x ISL correction

mGy7.47.08.1

4.110015.02

=⎥⎦⎤

⎢⎣⎡

+××

• Maximum transmission permitted= 6x10-3/ 4 7= 6x10 3/ 4.7= 0.0013

• Lead shielding @ 90 kV1.4 mm

Summary

• Best practice –Beam collimated to patient/ image Beam collimated to patient/ image

plate/ Bucky–0.8 mm lead

• Poor practice–Beam outside patient and BuckyBeam outside patient and Bucky–1.4 mm lead

Scatter

Scatter ModelNCRP 49 method

400FKaK us ⋅⋅=

Ks – scatter kerma; Ku – primary kermaF– Field size (cm2); a – scatter factorTrout and Kelly (1972), Radiology, 104, 161

Ku . F ⇒ Area-air kerma product (AKP) orArea-air kerma product (AKP) orDose area product (DAP)

DAPSK DAPSKs ⋅=

hWhy DAP?

• No assumptions on field size• Availability of DAP data• Availability of DAP data

–Requirement in UK legislation to record patient doserecord patient dose

–DAP preferred patient dose metric (fluoroscopy and radiography)(fluoroscopy and radiography)

–National surveys of patient dose

Scatter factor normalised to DAP (S)Scatter factor normalised to DAP (S)12

m2 )-1

125 kVp

8

10µG

y.(G

y.cm 100 kVp

85 kVp 70 kVp 50 kVp

4

6

r fac

tor,

S µ

0

2

0 30 60 90 120 150 180

Scat

ter

0 30 60 90 120 150 180

Angle of scatter

( ) ( )[ ]1f85kVdbS 234 θθθθ

JR Williams (1996) Br J Radiol, 69, 1032

( ) ( )[ ]1f85kVedcbaS 234 +−×+θ+θ+θ+θ=

Scatter factor normalised to DAP (S)

12

-1

Scatter factor normalised to DAP (S)

8

10

Gy.

(Gy.

cm2 )-

125 kVp100 kVp 85 kVp 70 kVp 50 kVp

4

6

fact

or, S

µG

a -1.04E-07b 3 27E 05

0

2

0 30 60 90 120 150 180

Scat

ter f b 3.27E-05

c -2.75E-03d 8.37E-02e 1 58E+000 30 60 90 120 150 180

Angle of scatter

( ) ( )[ ]1f85kVdbS 234 θθθθ

e 1.58E+00f 5.99E-03

( ) ( )[ ]1f85kVedcbaS 234 +−×+θ+θ+θ+θ=

JR Williams (1996) Br J Radiol, 69, 1032

Calculation vs measurement

Cli i l li tiMeasured/ Calc doseNo of DAP/ week Ave Calc Mean Min Max

Interventional Radiology (liver disease) 8 520 0.71 0.22 0.02 0.42Abdominal + lower limb angiograpy 8 1120 2.15 0.17 0.02 0.43

Clinical application positions Gy cm2 Dose mGy

Cardiac angiography 7 780 0.81 0.99 0.71 1.26Cerebral angiography 7 460 1.48 0.97 0.67 1.23Ba contrast studies (o/c tube) 9 430 0.66 0.74 0.50 1.13B t t t di ( / t b ) 10 960 1 62 0 28 0 05 0 86Ba contrast studies (u/c tube) 10 960 1.62 0.28 0.05 0.86General Radiography 10 460 0.96 0.64 0.30 1.06

Validation of scatter data by Validation of scatter data by Monte-Carlo

Scatter fraction (S) @ 85 kV12

Measured

6

8

10

Gy

cm2

MeasuredMC

2

4

6

S µG

y/ G

00 30 60 90 120 150 180

Angle

Comparison with NCRP 147

Scatter fraction (S) @ 85 kV12

UK

6

8

10

Gy

cm2 )-1

UK

NCRP 147

MonteCarlo

2

4

6

S µG

y (G

00 30 60 90 120 150 180

Angle

Differences in scatter factors?

• Phantom• X-ray spectrum• X ray spectrum• Measurement methods

Method in practicep12

m2 )-1

125 kVp

8

10µG

y.(G

y.cm 100 kVp

85 kVp 70 kVp 50 kVp

4

6

r fac

tor,

S µ

0

2

0 30 60 90 120 150 180

Scat

ter

( )

0 30 60 90 120 150 180

Angle of scatter

( ) ( )[ ]1f85kVedcbaS 234 +−×++++= θθθθ

h d ( )Method in practice (1)

0 6

0.8

1.0

doseθ d

0 2

0.4

0.6

Rel

ativ

e

r

0.0

0.2

0 30 60 90 120 150 180

S tt i l

( )[ ] ( ) 12max cmGyµGy5.2kV031.0S

−+×=

Scattering angle

( )d = 1 m; θ = 117°

θ d = 1 m

∫∫ ⋅= θ

θθd

d)(SS15030

ave ∫ θd

h d ( )Method in practice (2)

• Save over C-arm rotation – (30 to 150°)(30 to 150 )

• Smax for parallel beam

kV Smax Save

50 4.05 3.73

70 4.67 4.30

85 5.14 4.72

100 5 60 5 15100 5.60 5.15

125 6.38 5.86

b dTo be continued…..

• Application examples–FluoroscopyFluoroscopy–Radiography

• Comparisons with NCRP 147• Comparisons with NCRP 147