Study of the shielding adequacy of the radiation barriers ... 2/Auditorium/3 CT/5 M… · NCRP Report 147, 2004. STEP 1: Assessment of the lead equivalent thickness of the barriers

Study of the shielding adequacy of the radiation barriers of a room after the substitution of an axial CT scanner by a 16 slice CT scanner

Maria Carmen de SousaMedical Physics Department

Portuguese Institute of Oncology of Coimbra FG, EPE

Context & Aims

� At the beginning of 2011: substitution of an old

axial CT unit by a new 16 slices CT unit

� Unknown barriers radiation shielding of the CT

room

� STEP 1: Assessment of the lead equivalent thickness of the barriers shielding of the room

� STEP 2: Calculation of the necessary lead thickness for an adequate barriers radiation shielding of the room

STEP 1: Assessment of the lead equivalent thickness of

the barriers shielding of the room - METHOD

� “Radiation Shielding for Diagnostic X-Rays” BIR/IPEM Report, 2000

� NCRP Report 147, 2004


the barriers shielding of the room - METHOD

� For each barrier:

� Attenuation factor

� Lead equivalent thickness

ISLfK

KnAttenuatio

decay eradioactivw/barrier,dair,

barriernoair,1m,

ICsource××

=−

&

&

( )nAttenuatiolnµ

1dX

TcPb,

Pbeq99m

×=

calculated from the HVL value given in DL180/2002)( 1

TcPb,cm22,7µ 99m

−=

Barrier Description Barrier thickness

(mm)

Air kerma rate measured behind the

barrier (nGy/s)

(1)

Barrier attenuation

Equivalent lead thickness

dXeq Pb (mm)

A Wall between the MSCT room and the control room

270 - 280 3 nGy/s 182 2,3

B Lead glass of the viewing window

∼ 10 10 nGy/s 905 3,0

C Door between the MSCT room and the control room

32 100 nGy/s 73 1,9

D Wall between the MSCT room and the control room

270 - 280 3,5 nGy/s 176 2,3

D2 The same as D barrier but for a height of 2,1 m

270 - 280 3,5 nGy/s 174 2,3

E Main door of the MSCT room

32 5,8 nGy/s 1266 3,1

F Wall between the MSCT room and the changing room

230 - 240 1,5 nGy/s 488 2,7

G Wall between the CT rooms

∼ 310 0,2 nGy/s 2086 3,4


the barriers shielding of the room - RESULTS



� Estimated value of the

equivalent lead

thickness at the main

entrance door of the CT

room = 3,1 mm

� An inner lead layer of 3

mm of thickness is

visible on the wood in

the side walls of the

door

Left side Right side

STEP 2: Calculation of the necessary lead thickness for an

adequate barriers radiation shielding of the room - METHOD

WORKLOAD – Audit of the local clinical practice

Conditions of local use of the CT TOSHIBA in clinical practice

Exposure parameters Dose indication from the display

Type of examination Mean number of exams per

month

TOSHIBA protocol name used in clinical practice

kV mA x s n x Tn (mm)

(1)

pitch DFOV (2)

L

(cm) (3)

CTDIvol (mGy)

DLP (mGy.cm)

CT head 41 CRÂNEO 120 375 4 x 2 1 S-240 20 93,8 1864

CT maxillofacial 10 PESCOÇO 3 mm 120 7,5* 16 x 0,5 0,938 M-320 30 1 31,3

CT neck (soft parts) 134 PESCOÇO 3 mm 120 7,5* 16 x 0,5 0,938 M-320 30 1 31,3

CT orbits 1 ÓRBITAS 2 mm 120 250 16 x 0,5 0,938 S-240 10,2 74,9 890,9

CT paranasal sinuses 13 SPN 120 150 16 x 0,5 0,938 S-240 10,2 44,9 539,1

CT ears 3 OUVIDOS HELICAL 120 187,5 16 x 0,5 0,938 S-240 8 56,2 539,1

CERVICAL 120 187,5 16 x 0,5 0,688 M-250 18 33,6 648,2

DORSAL 3 mm 120 300 16 x 1 0,938 M-320 30 33,1 1100

CT spine - cervical, thoracic, lumbar, sacrum-coccyx (each segment)

90

LUMBAR SPINE 135 214* 16 x 1 0,938 M-320 30 28,6 953,4

CT chest 372 TORAX 5 mm 120 187,5* 16 x 1 0,938 L-350 40 26,6 1160

CT upper abdomen 291 ABDOMEN HCT 5 mm 120 150* 16 x 1 0,938 L-400 42 21,3 969

CT Kidneys 5 ABDOMEN HCT 5 mm 120 150* 16 x 1 0,938 L-400 42 21,3 969

CT pancreas 1 ABDOMEN HCT 5 mm 120 150* 16 x 1 0,938 L-400 42 21,3 969

CT pelvis 223 ABDOMEN HCT 5 mm 120 150* 16 x 1 0,938 L-400 42 21,3 969

CT limbs (each anatomic segment)

5 OMBRO 2 mm 120 202,5 16 x 1 0,938 M-320 10 22,3 294,8

CT joints 4 OMBRO 2 mm 120 202,5 16 x 1 0,938 M-320 10 22,3 294,8

CT angiography (without iodine contrast)

5 ANGIO AORTA 120 150 16 x 1 0,938 L-400 40,5 21,3 937

1198 Total mean number of examinations per month

17% Percentage of the total number of examinations corresponding to the region of HEAD

83% Percentage of the total number of examinations corresponding to the region of BODY

� Extrapolation of the CT examinations statistics data performed in the Medical Imaging Department between Nov. 2011 and Apr. 2012� Nº of scans per year

� Percentage of examinations of HEAD versus BODY region� An average of 17% of the total nº of the CT exams respect to the

anatomic region of the head and 83% to the region of the body

� Almost all the CT examinations are performed with a tube potential of 120 kV

7.500 patients per year

15.000 exams per year

30.000 scans per year

Avg of 2 exams/patient

Avg of 2 scans/exam (incl.

Exams with contrast media)



OCCUPANCY FACTOR

Barrier Description Occupancy of the adjacent area

T

DL180/2002 NCRP BIR/IPEM

A, D Wall between the MSCT room and the control room

Working area 1 1 1

B Lead glass of the viewing window

Working area 1 1 1

C Door between the MSCT room and the control room

Working area 1 1 1

E Door of the main entrance of the MSCT room

Large corridor 1/4 1/5 1/8 a 1/5

F Wall between the MSCT room and the changing

room

Changing room 1 1/20 1/20

G Wall between the CT rooms Working area 1 1/2 1





� Necessary lead thickness, dXPb

+

+×

×=

−

α

β1

α

βB

lnγα

1dX

γ

Pb α,β,γ values from NCRP 147 for 120 kVp

( ) ( )[ ] ISLTKNKNKbodyhead secbodysecheadsec ×××+×=

Secondary air-kerma for a typical head/body scan

secK

PB =

P: weekly shielding design goal

= 0,4 mSv/week for controlled areas

=0,02 mSv/week for uncontrolled areas(DL180/2002)



head/body100,head/body

head/body

head/body

head/bodysec nCTDImAspitch

LκK

head/body×××=CTDI method

DLP method headheadsec DLPκKhead

×= bodybodysec DLPκ1,2Kbody

××=

Isodose map method head/body

head/body

head/body

head/bodysec mAspitch

LradiationScatterK

head/body××=

Secondary air-kerma for a typical head/body scan

� L, pitch, mAs, DLP]head/body: typical values taken from CT head/chest examination

� nCTDI100, head/body: values from QC corrected for the collimation (nTn & DFOV)

used in CT head/chest exam

Scattered air kerma produced in the CT room during one rotation of the x-ray tube for

exposure parameters typically employed for head/body examinations



A

B

C

D

E

F

G

Main assumptions:

�Isotropic scattered radiation

distribution

�Scattered air kerma corrected

for the collimation (nTn & DFOV)

used in CT head/chest exam


adequate barriers radiation shielding of the room - RESULTS

Barrier Description Barrier thickness

(cm)

Occupancy factor

Weekly shielding

design goal (mSv/wk)

Isocentre-person to

protect distance

d (m)

Weekly secondary radiation air kerma at distance d (mGy/wk))

Necessary lead thickness (mm) Existing equivalent

lead thickness

(mm)CTDI method

DLP method

Isodose map

method

CTDI method

DLP method

Isodose map

method

A Control room wall

27 1 0,4 4,61 9,66 10,59 11,63 0,7 0,7 0,7 2,3

B Viewing window

1 1 0,4 3,74 14,68 16,09 17,67 0,8 0,9 0,9 3,0

C Control room door

3,2 1 0,4 3,57 16,11 17,66 19,39 0,9 0,9 0,9 1,9

D Control room wall

27 1 0,4 3,81 14,14 15,51 17,03 0,8 0,8 0,9 2,3

E Corridor door

3,2 1/4 0,02 5,38 1,77 1,94 2,13 1,1 1,2 1,2 3,1

F Changing room wall

23 1 0,02 3,95 13,16 14,43 15,84 1,9 2,0 2,0 2,7

G G.E. CT room wall

31 1 0,4 2,49 33,11 36,30 53,88 1,1 1,1 1,3 3,4

Margin of increase in the total number of examinations (keeping constant all the other parameters):

factor 5

Conclusions

� The facility provides adequate shielding for the people staying in nearby rooms

� The 3 methods of shielding calculation described in NCRP 147 Report (CTDI, DLP and isodose map) lead to similar results

� The use of a technetium source and the Radcal radiation protection chamber is an appropriate method for the on-site survey of the shielding adequacy based on the measurement of the barriers transmission factor

THANKS FOR YOUR ATTENTION



Comparação dos factores de atenuação pelo chumbo para a radiação gama

de 140 keV emitida pelo 99mTC e para a radiação secundária tipicamente

produzida numa unidade de TC com uma tensão aplicada na ampola de

raios X de 140 kV

1

10

100

1000

10000

100000

0 0,5 1 1,5 2 2,5 3 3,5

Espessura de chumbo (mm)

Ate

nu

açã

o

RadiaçãosecundáriaTC 140 kV

140 keV(99mTc)

Ver se mantidaSe sim, acrescentar-NCRP147- factor quase igual a 9 para 3 mm de Pb

Documents

Study of the shielding adequacy of the radiation barriers ... 2/Auditorium/3 CT/5 M… · NCRP Report 147, 2004. STEP 1: Assessment of the lead equivalent thickness of the barriers