PET Shielding Is Complex
PET/CT
Shielding Evaluation
• More lead is used than for typical
radiographic installations
• The safety factor is much less than for
most diagnostic Installations
• Exposure usually occurs from multiple
sources
G. Donald Frey, Ph.D.
Department of Radiology
Medical University of South Carolina
Charleston, SC
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1
Shielding Evaluation
My Priorities
• To Insure
• Check for proper construction
• Determine the adequacy of the
shielding
• NCRP 147
Radiation Doses are below the levels
required by regulation
Are consistent with the shielding design
That the shielding is properly installed
7
Instrumentation
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Instrument of Choice
• We have used three types of
instruments
• All three devices gave approximately
equal readings
• All could produce accurate
measurements
• The portable NaI(Tl) survey meter was
somewhat more sensitive and
convenient to use
Large volume ionization chamber
• Radcal 9010 w/ 10X5-1800
Portable pressurized ionization chamber
• Innovision 451P
Portable NaI(Tl) Survey Meter
• Exploranium GR-135
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Evaluating the Annual
Exposure at a Location
Proper Construction
• Use source in each location
• Use meter to scan for gaps and voids
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2
Scaled Source Method
• Find scaled source strengths that give
the same radiation exposure (air kerma)
as the total activity that is used in the
room
• Place sources in all patient locations
• Measure dose at appropriate
Annual Dose = Dose Rate x
Number of Patients x
Time Each Patient is in the Room
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Source
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Activity =
Average Patient Dose x
Number of Patients x
Time Each Patient is in the Room
Source
Source
=
Annual Dose = Dose Rate x
1 hour
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Average Patient Activity
Uptake Room
Or
ANτ
R A = D Hx, y, zL = D Nτ
• Factors
A
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Administered activity
Time patient spends in room
Number of patients in the room
Patient self attenuation factor
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3
Average Patient Activity
Uptake Room
Au = ζ A0 ‡
τu
H 1 ê 2 L t ê T 1 ê2
t
A u = ζ H A 0 ê ln H2 LL HT 1 ê2 ê τ u L H1 − H1 ê 2 L
0
Annual Dose
Uptake Room
H τ u ê T 1 ê2 L
Au
D ui Hx, y, z L
A u N ui τ u
D ui Hx, y, z L N ui τ u
L
D ui Hx, y, z L
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Average Patient Activity
Scan Room
Average Patient Activity
Scan Room
• Factors
A s = ζ A 0 H 1 ê 2 L H τ u ê T 1 ê2 L κ ‡
Administered activity
Time patient spends in uptake room
Fraction of activity voided by patient
Time patient spends in scan room
Number of patients in the room
Patient self attenuation factor
τs
A s = ζ H A 0 ê ln H2 LL H1 ê 2 L
x H 1 − H 1 ê 2 L H τ s ê T 1 ê2 L L
0
H 1 ê 2 L t ê T 1 ê2
H τ u ê T 1 ê2 L
κ HT 1 ê2 ê τ s L
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Annual Dose
Scan Room
As
D si Hx, y, z L
A s N si τ u
D si Hx, y, z L N si τ s
t
Dose from All Rooms
D si Hx, y, z L
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‚ As Nsi τu + ‚ Au Nui τu
i
D Hx, y, zL
i
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4
So from all rooms
Scaling Factor
So we need to Scale the Activity
500
mCi
Effective Scaled
Activity Activity
20536
41
5134
10
9001
18
Room
Uptake Room 1
Uptake Room 2
Scan Room
i
y
j
zìσ
j
j‚ As Nsi τu + ‚ Au Nui τuz
z
i
ki
{
D Hx, y, zL ê σ
αui = Au Nui τu ê σ
αsi = As Nsi τs ê σ
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Compare Results to
Design Values
So from all rooms
Scaling Factor
Room
Uptake Room 1
Uptake Room 2
Scan Room
p ê TD Hx, y, zL b 1
500
mCi
Effective Scaled
Activity Activity
20536
41
5134
10
9001
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• Correct for Occupancy Factors
• Correct for decay of sources during the
measurement
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Inside
Source Size Calculation
Facility
Date
Administered Activity
Half Life of Radionuclide
Self Absorption Factor
Medical University of South Carolina
PET/CT Facility
Location
Control Room Operator
Control Room Secondary Operator
Door to Corridor
Entrance Corridor
Corridor Door to Uptake Room
Corridor Door to Uptake Room
Outside Porch
1/9/2006
555 MBq
110 min
Inside
15 mCi
0.72
Room
Time between injection and beginning of room use
Decay Factor
Time patient remains in room
Decay Dose Rate Factor
Voiding Factor (Retention when patient enter room)
Patients per week
Effective Activity
Uptake Room 1
0 min
1
90
0.76
1
32
759826 MBq
20536 mCi
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Distance
Design Limit
Annual Limit
Design
Value (P)
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5
5
5
5
50
5
Me ter
Time
Annual
Time
Rea ding Corrected Estimate
8:43 AM
2.30
1.42
0.71
8:45
5.30
3.30
1.65
8:45
4.60
2.87
1.43
8:47
8.90
5.62
2.81
8:48
30.00
19.06
9.53
8:48
30.00
19.06
9.53
8:49
14.10
9.01
4.51
Loca tion
Control Room Operator
Control Room Secondary Operator
Door to Corridor
Entrance Corridor
Corridor Door to Uptake Room
Corridor Door to Uptake Room
Outside Porch
Time
Annual
Corrected Estimate
1.42
0.71
3.30
1.65
2.87
1.43
5.62
2.81
19.06
9.53
19.06
9.53
9.01
4.51
Percent
P-Value
14%
33%
29%
56%
191%
19%
90%
Status
Pass
Pass
Pass
Pass
Fail
Pass
Pass
Distance
Design Limit
Annual Limit
Percent
P-Value
14%
33%
29%
56%
191%
19%
90%
Status
Pass
Pass
Pass
Pass
Fail
Pass
Pass
Occupa ncy Annual
Factor
Estimate
1.00
0.71
1.00
1.65
1.00
1.43
1.00
2.81
1.00
9.53
1.00
9.53
1.00
4.51
Design
Value (P)
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5
5
5
50
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Occupancy Annual
Factor
Estimate
1.00
0.71
1.00
1.65
1.00
1.43
1.00
2.81
1.00
9.53
1.00
9.53
1.00
4.51
Percent
P-Value
14%
33%
29%
56%
191%
19%
90%
Status
Pass
Pass
Pass
Pass
Fail
Pass
Pass
Meter
Time
Reading
8:43 AM
2.30
8:45
5.30
8:45
4.60
8:47
8.90
8:48
30.00
8:48
30.00
8:49
14.10
Percent
P-Value
14%
33%
29%
56%
191%
19%
90%
Status
Pass
Pass
Pass
Pass
Fail
Pass
Pass
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5
Evaluating Lead in Wall
•
•
•
•
•
Use a source in room
Measure air kerma at point outside wall
Determine B
Evaluate using chart from TG 108
Calculate Using Archer Equation
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Direct Calculation
How Much Activity Do You Need?
100000
• The thickness can also be calculated
using the model of Archer et al
10000
Required MBq
x = H1 ê αγ L ln 8@B −γ + Hβ ê α LD ê @1 + Hβ ê α LD<
D c = A0 G ê d2
B = Dm d2 ê A0 G
x = H1 ê αγ L ln
−γ
8@HD m d 2 ê A 0 Γ L + Hβ ê α LD ê @1 + Hβ ê α LD<
1000
100
0
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Desired distance
Desire Air KERMA
US
Inches
0
1/16
1/8
1/4
3/8
1/2
3/4
1
Metric
mm
0
1.6
3.2
6.4
9.5
12.7
19.1
25.4
5
10
15
Lead (mm)
20
25
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3 mete rs
10 uSv/hr
Attenuation
1.000
0.829
0.674
0.430
0.267
0.164
0.062
0.023
Activity Unshielded
MBq
uSv/hr
629
10
759
12
933
15
1465
23
2357
37
3826
61
10168
162
27078
430
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6
Lead T hickness Evaluation
F-18 Source
Calibration Time
Gamma Constant
Alpha
Beta
Gamma
Barrier
1742.7
3:35 PM
0.143
1.5430
-0.4408
2.1360
None
Uptake 1 (Rear)
Uptake 1 (Rear)
uSv-m2/MBq-h
per cm
per cm
Unshielded
Measured Measured
NET
Calculated
Activity Distance Air Kerma
Bkg
Air Kerma Air Kerma
MBq
m
uGy/hr
nGy/hr
uGy/hr
uGy/hr
3:50 PM
1586
3.0
25.5
110
25.39
24
3:58 PM
1508
3.0
5.3
110
5.19
23
4:04 PM
1452
1.5
19
110
18.89
89
Time
Symbols
MBq
B
1.04
0.22
0.21
Calculated Specified
Lead
Lead Deviation
mm
mm
-0.4
0
10.7
10
7%
11.1
10
11%
A0 = Administered Activity
Au = Average activity per patient in the uptake room
As = Average activity per patient in the scan room
T1ê2 = Physical Half − life for F − 18
τu = time patient waits after injection
τs = time it takes to complete the PET ê CT exam
κ = fraction of the administered activity the patient voids
ζ = self absorption factor
N = total patients per year
Nui = number of patients per year in uptake room i
Nsi = number of patients per year in scan room i
N = ‚ Nsi = ‚ Nui
i
i
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Dui Hx, y, zL = The average dose rate at any location Hx, y, zL from
a patient in uptake room 1
Dsi Hx, y, zL = The average dose rate at any location Hx, y, zL from
a patient in scan room 1
Dui Hx, y, zL = the annual dose to any location Hx, y, zL from all
of the patients in uptake room i
Dsi Hx, y, zL = the annual dose to any location Hx, y, zL from all
of the patients in scan room i
D Hx, y, zL = the annual dose to any location Hx, y, zL from all the
patients in the facility
σ = the dose scaling parameter
αui = the scalled activity for uptake room i
αsi = the scalled activity for scan room i
p = design value Hp = 1 mSv ê yr for non − controlled areas,
5 mSv for controlled areas L
T = occupancy factor
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