NCRP-147 Shielding Models 147 Shielding Models - Radiation ...

RSMI 2009 Session III
Diagnostic X-Ray Shielding Design
NCRP-147 Shielding Models
Douglas J. Simpkin, Ph.D.
Aurora St. Luke’s Medical Ctr
Milwaukee, WI
dsimpkin@wi.rr.com
http://www.geocities.com/djsimpkin/

Models for Diagnostic X-Ray
Shielding Calculations
Yes
No
2

The Three Models for Diagnostic
X-ray Shielding In NCRP 147
1. First-principle extensions to NCRP 49
2. Given calculated kerma per patient, scale
by # patients and inverse squared distance,
and then use transmission curves designed
for particular room types
3. NT/(Pd 2 )
3

1 st principle extensions to NCRP 49
• (Underlies the other two methods)
• The kerma in the occupied area may have
contributions from
– primary radiation
– scatter radiation
}
– leakage radiation } Secondary radiation
4

Primary, Scatter, and Leakage
Must protect
from primary
radiation
primary
Must protect from
scatter & leakage
radiation
5

1 st 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
(“workload distribution”)
– new model for image receptor attenuation
– new model for leakage
6

1 st principle extensions to NCRP 49
• These primary, scatter, and leakage
radiations may be from multiple x-ray
sources (or tube positions)
• So, simply add up all the contributions to the
kerma, K, from all these sources in the
occupied area behind a barrier of thickness x,
K ( x)
= ( K ( x)
+ K ( x)
K ( x)
)
∑ ∑ +
tubes kVp
P
S
L
7

1 st principle extensions to NCRP 49
• Then iteratively find a barrier thickness x that
decreases that kerma to P/T, the design goal
modified by the occupancy factor
K )
(
x
) =
∑ ∑ ( K
(
x
)
+
K
(
x
)
+
K
(
x
)
=
= P
S
L
tubes kVp
• See http://www.geocities.com/djsimpkin/ for
shareware program XRAYBARR to do this
P
T
8

1 st principle extensions to NCRP 49
• XRAYBARR was written by me in the mid
1990s to perform shielding calculations with
these new models as we developed NCRP-147.
The shielding data and examples in NCRP-147
are based on the output of XRAYBARR.
• Note: Some of the examples in NCRP-147
aren’t duplicated by XRAYBARR because
NCRP-147 takes shortcuts in the tabulated x pre
values. XRAYBARR is right!
9

Primary Radiation Model
• In primary beam,
know kerma per
workload at 1 m,
K W(kVp) , for 3
phase units (data
of Archer et al.
1994)
Primary Kerma at 1 m per workload
10

Unshielded Primary Beam Kerma
KW ( kVp
) W
( kVp
)
K (0)
=
d
W
• At a given kVp,
P
2
• If only a fraction U of the tube’s workload
is directed at this barrier, then
K
K ( 0) =
W
P
( kVp)
d
2
P
P
U W ( kVp)
• U is the use factor for this barrier
11

Kerma Behind a Primary Barrier
• The kerma behind a primary barrier of
transmission B(x, kVp) is
K
K
kVp
U W
kVp
(
) (
)
( x,
kVp)
W
B
( x,
kVp
)
2
d
P =
• For the whole distribution of workloads, total
kerma is
P
K
P
( x)
=
∑
kVp
K
W
( kVp)
U W
2
d P
( kVp)
B(
x,
kVp)
12

Primary Radiation:
i
The Old NCRP-49 Model
x
Barrier of thickness x decreases raw
primary radiation kerma to P/T
13

Primary Radiation:
The Reality
Grid, cassette,
supporting structures
patient
t
Primary radiation is significantly
attenuated t before reaching barrier
14

Primary Radiation:
A Conservative, Realistic Model
Grid, cassette, maybe
image receptor
supporting structures
Even without the patient, primary radiation is still
significantly ifi attenuated t before reaching barrier
15

Primary Radiation:
NCRP-147 Model
Grid, cassette, maybe
supporting structures
No patient!
t!
x pre
} x tot = x + x pre
x
Assume primary beam attenuation in image receptor is due to a
pseudo-barrier whose equivalent thickness x pre gives same
transmission as that seen for actual image receptors.
16

1E+0 Primary Transmission Through Patient,
8
6 Image Receptor, and Supports
4
Data of Dixon (1994)
Transmiss sion
1E-1
2
8
6
4
2
1E-2
8
6
1E-3
4
2
8
6
4
2
1E-4
No patient & grid & cassette:
B = 4.7E-6 kVp 2.181
No patient & grid & cassette &
cassette support structures &
radiographic table:
B = 9.36E-13 kVp 4.917
ers
Wall-Mou unted Grid +
Cassette + Cassette Hold
Type of Radiographic Table
(data of Dixon 1994)
GE RTE Table
GE Advantx Table
Siemens Multix-T Table
Picker Clinix-T Table
40 50 60 70 80 90 100 125 150
kVp
17

1E+3
Values of x pre
(Grid+cassette+support)
p
Gypsum
1E+2 Plate Glass
Concrete
x pre (mm m)
1E+1
Steel
1E+0
Lead
1E-1
1
20 30 40 50 60 70 80 90 100 110 120 130 140 150
kVp
18

x pre for Radiographic Room
Workload Distributions
• From NCRP-147 Table 4.6:
– Grid + cassette:
• 0.3 mm Pb
• 30 mm concrete
– Gid+ Grid cassette +tbl/h table/chest tbucky supports:
• 0.85 mm Pb
• 72 mm concrete
• (See Dixon & Simpkin Health Phys 74;181-
189;1998 for a more complete list.)
19

Calculation of Primary Kerma
• Same as model in NCRP-49 except
– account for workload distribution in kVp
– May account for image receptor shielding x pre
• Primary kerma in occupied area is then
K ( x )
P
1
d 2
P
+ x pre
=
∑ K W ( kVp)
U W ( kVp)
B(
x + x pre , kVp)
kVp
20

Scatter Radiation
patient
21

Scaled Normalized Scatter Fraction
1 m
1 m
1m
K S
K P
1 cm 2 area
primary beam
at 1 m
θ
K
a
′ =
S
× 10 +6
1
K
P
22

Scaled Normalized Scatter Fraction
'
23

Scatter Radiation
• Same theory as old NCRP-49
K
– scatter fraction data of Kelley & Trout reevaluated
by Simpkin & Dixon (Health Phys 1998)
– pri ib beam area F (cm 2 ) measured at pri idistance d F
conveniently taken as image receptor area @ SID
– explicitly itl show kVp dependence d and sum over
workload distribution to yield shielded scatter
kerma
S
−6
a′
× 10 K ( ) ( )
,
1 W
kVp W kVp F
( x θ ) =
∑
B
( x,
kVp
)
2 2
d
d
kVp
S
F
24

Leakage Radiation
Radiation originating from x-
ray tube focal spot but not
emanating from the tube
portal
patient
25

Leakage radiation
• Intensity can’t exceed L = 100 mR/hr at 1 m
when tube is operated at its leakage
technique factors
– maximum potential for continuous operation
kVp max (typically 135-150150 kVp, or 50 kVp for
mammography)
– I max is the maximum continuous tube current
possible at kVp max . Note that this is usually a
low mA, not typical of clinical radiography.
26

Leakage radiation
• These leakage technique factors specify
how thick the shielding in the tube housing
should be
• NCRP49 suggested leakage technique
factors of 3.3 mA at 150 kVp, 4 mA at 125
kVp, 5 mA at 100 kVp; remain fairly
typical today
27

Leakage radiation
• NCRP-147 calculations (and shielding methods
2 and 3) use
– 3.33 mA at 150 kVp
– worst case leakage rates
– (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)
– However, actual leakage rates are 0-30% of the
maximum leakage so we don’t see a problem
28

New Leakage Model
• For tube operating at techniques (kVp, I) with
transmission through the tube housing B housing ,
assume leakage kerma rate at 1 m through tube
housing is
2
K
L ( h i
( kVp
)
∝
kVp
I
Bhousing (
kVp
)
• Assume worst case scenario: leakage kerma rate =
limit L for tube operation at leakage technique
factors (conservative by factors of 3 to ~infinity)
29

New Leakage Model
• Estimate thickness of tube housing by using primary beam
output at leakage technique factors as model for unhoused
leakage radiation.
1931 mGy/hr 100 mR/hr = 0.873 mGy/hr
1 m
Tube operated at
150 kVp, 3.3 mA
1 m
“unhoused”
tube
Tube housing
= 2.32 mm Pb
thick
1 m 1 m
1931 mGy/hr 1931 mGy/hr
30

New Leakage Model
• Write ratio of leakage kerma rates at any kVp
to L at kVp max
• and knowing that at a given kVp, workload
W(kVp) is the time integral of the tube
current: W ( kVp)
= ∫ I dt
•then unshielded d leakage kerma K L (at 1 m) at
that kVp is
K L
(0, kVp)
=
L kVp
2
(1 −U
) W
2
( kVp)
B
housing
kVp I
B
( housing max )
max max
kVp
( kVp)
31

New Leakage Model
• Applying inverse square to distance d L from
tube to shielded area,
• and putting a barrier with transmission
exp(–ln(2)x/HVL) between tube & area yields
K L
( x,
kVp)
=
L kVp
1
d
2
d L
2
kVp
(1 −
U
)
W
2
max
I
max
(
kVp
)
B
⎛ − ln(2) x
× exp⎜
⎝ HVL(kVp(
)
housing
⎞
⎟
⎠
B
housing
( kVp
max
(
kVp
)
×
)
32

How far off is NCRP-49’s leakage model
1E+0
1E-1
1E-2
NCRP-1 147 leakage e/
NCRP-4 49 leakage
1E-3
1E-4
Leakage dose as function of kVp
transmitted through x-ray tube
1E-5 housing of 2.32 mm Pb compared
to that at 150 kVp
1E-6
1E-7
Leakage technique factors:
150 kVp, 3.3 mA for 100 mR/hr
1E-8
1E-9
50 60 70 80 90 100 110 120 130 140 150
kVp
33

Summary: Shielding Model No. 1
• Rigorous model based on the well-accepted
NCRP-49 methods.
• But you need a computer program
(XRAYBARR, for example) to implement
fully!
• Is there a shielding method that allows
paper and calculator solutions
34

NCRP-147 Shielding Model No. 2
• For each clinical workload distribution, of
total workload dWW norm per patient, for both
primary and secondary barriers, NCRP-147
provides:
– K 1 , the kerma per patient at 1 m distance
• Primary kerma per patient K 1 P is in Table 4.5
• Secondary kerma per patient K sec1 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

NCRP-147 Shielding Model No. 2
Primary Air Kerma at 1 m for Workload
Distributions, ib i K 1
36

NCRP-147 Shielding Model No. 2
Secondary Air Kerma at 1 m for Workload
Distributions, K 1 sec
37

NCRP-147 Shielding Model No. 2
• For single kVp operation cf. Simpkin and
Dixon Health Phys. 74(3), (), 350–365 for
secondary kerma per workload at 1 m at
single kVp operation
• All other data is available in NCRP 147
– But be careful reading scientific notation:
1.234 x 10 1 = 12.34
38

Shielding Model No. 2
• Get the unshielded kerma, K(0), by scaling the kerma
per patient at 1 m, K 1 , by
– N patient procedures (suggested values of N are in Table
4.3) or, equivalently
– total workload W tot (where workload/pat = W norm )
– can tweak W tot by a QE-specified different workload per
patient, W site
• Kerma is then
1
K
U N
K( 0) = =
2
d
K
– (where U is replaced dby 1 for secondary barriers)
d
1
2
U W
W
tt tot
norm
39

Shielding Model No. 2
• Ratio of P/T to K(0) is the required transmission
2 2
P / T P d P d W
B
( x
) = = =
norm
K
(0)
1
1
N T UD W T UD
– (again, U is replaced by 1 for secondary barriers)
• Transmission i B is now a function of
– barrier material and thickness
– workload distribution
ib i
– primary or secondary
tot
40

B=0.0047
x=1.2 mm Pb
42

Now the
difficulty is in
reading the
correct curve!
43

Shielding Model No. 3 for
“Representative Rooms”
• Model No. 2 fails for
complicated
assemblages of x-ray
tubes/ positions/
workload
distributions, such as
in a radiographic or
radiographic/
fluoroscopic room
44

Shielding Model No. 3 for
“Representative Rooms”
• (Using XRAYBARR) NCRP-147 shows
barrier thickness requirements calculated for
representative rooms:
– Assume conservatively small room layout
• assures maximum contribution from all sources
– Presumes that the kinds of exposures made
amongst the various x-ray tubes/positions follow
those observed by the AAPM TG-9 survey
45

Representative Radiographic Room
46

Use Factors from AAPM Survey
Rad Room:
Chest Bucky
(Another primary wall gets
U=2% of the floor/ other
barrier distribution; assume
tube is centered overtable)
Cross-table
Lateral Position
U=9%
Overtable Position
U=89% shooting down
at tfloor
Rad Room: floor/
other barriers applies
to Overtable and
Crosstable positions
47

Representative Radiographic Room
Chest
Bucky
wall
primary
Secondary Barrier
ky
dary
est Buck
l second
Che
wal
Cross-table
Lateral Wall
primary
Secondary Barrier
U=2%
primary
wall
48

“Representative 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 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 room
– 25% of patients imaged by fluoroscopy tube
49

“Representative R&F Room”
Chest Rad
tube
Overtable
Rad tube
Image
Intensifier
Crosstable
Lateral Rad
Tube
Undertable Fluoro Tube
50

“Representative Room”
Barrier Requirements
• From Model 2, transmission requirement is
B ( x)
=
2
P d
N T UK
1
• so the barrier thickness requirement must
scale as:
N T
P d
2
51

• Method:
“Representative Room”
Barrier Requirements
– Given N patients/week, need to shield to P/T, a
distance d from the x-ray source
N T
P d
– Calculate in mGy -1 m -2
2
– Look up the required barrier thickness on the
graph appropriate for that workload
distribution, barrier, and barrier material
52

There are 12 NT/Pd 2 graphs
2
• For Representative Radiographic and R&F
Rooms:
–For Lead and Concrete:
• Primary barriers with preshielding
• Primary barriers without preshielding
• Secondary barriers
53

NT/Pd 2 curves have been fit
•The NT/Pd 2 curves have been fit to a
modified Archer eqn:
• See fitting parameters at
– http://geocities.com/djsimpkin/Shielding/Shield
ing.htm
55

NT/Pd 2 : From where is d
Primary Barriers
measured
Floor
overhead radiographic tube
Chest Bucky wall
chest tube (72" SID)
Crosstable Lateral Wall cross-table tube (40" SID)
2% U wall center of table
Secondary Barriers
Floor
Chest Bucky secondary wall
Secondary Wall
Ceiling
patient on table
chest tube (72" SID)
patient on table
patient on table
57

Equivalency of Shielding Materials for
Model No. 3 Calculations
• For “representative room” calculations only,
conservatively conclude
– Steel thickness requirement =
8 × Pb thickness requirement
– Gypsum wallboard thickness requirement =
3.2 × concrete thickness requirement
– Glass thickness requirement =
1.2 × concrete thickness requirement
58

Conclusions
• NCRP-147 utilizes 3 shielding models
– Model No. 1: Extension of the methods of NCRP-49
• With kVp dependence
• With new models for primary and leakage
• Requires computer program to implement tfully
– Model No. 2: Based on data from model no. 1,
• NCRP-147 shows kerma per patient at 1 m and transmission
curves appropriate for a given workload.
• Calculate unshielded kerma and then transmission needed to
reduce to P/T. Look up barrier thickness.
– Model lNo. 3: Based on data from model no. 1,
• For N patients at distance d (for a particular workload
distribution & barrier), calculate NT/Pd 2
• NCRP-147 shows barrier thickness as function of NT/Pd 2
59