NCRP #147 CT Approach With Updated Data

CT Scanners:
Estimate t Unshielded d Dose
• Estimate ambient
dose around scanner
• Manufacturer’s
isoexposure curves
• extrapolate using 1/r
2
from isocenter
• scale by mAs used
clinically i ll vs. for
isoexposure curve
• varies with phantom

D(z)
z

Scattered Air Kerma
Proportional to Energy Deposited in
Phantom

CTDI
1
= f ( z)
dz
T
T b
∞
∫
−∫∞
50mm
∫
− mm
1
CTDI 100 = f ( z)
dz
T
Tb
50
mAs
CTDI
nCTDI 100 = mAs
100

Approximate Integral by n CTDI 100

Scatter coefficient

http://www.impactscan.org/download/ctditablesdownload.htm
http://www.
impactscan.org
/download
/ctditablesdownload.htm

1 ⎧1
2
CTDI =
CENTER
⎨ CTDI + CTDI
vol
100
100
p ⎩
3
3
EDGE
⎫
⎬
⎭
DLP = L[CTDI
]
[
vol

DLP Method

Default DLP values
Examination CTDIvol
(mGy)
Scan Length
(L) (cm)
DLP (mGycm)
Head 60 20 1200
Chest 15 35 525
Abdomen 25 25 625
Pelvis 25 20 500
Body Average (chest, abdomen or pelvis
examination)
550

The facility administrator should
therefore be asked to provide not only
the total number of head and body
procedures, but also the number (or
fraction) of those procedures which are
performed both with- and without-
contrast media (the default DLP values in
table 5.2 being doubled for this subset of
procedures).
In the absence of such information,
it is suggested that the QE assume
that 40% of all procedures are
repeated with contrast, resulting in
an overall average DLP of 1.4 times
the default DLP values shown in
table 5.2.

Example CT Calculation
• Real estate office at 3 m from isocenter
• P = 0.02 mGy/wk, T =1
• Procedures/wk : 150 bodies, 30 head
• Assume 40 % of procedures are done
both with and without contrast
• DLP (body)= 550 mGy.cm
• DLP (head) = 1200 mGy.cm

1
6 −4
−1
K
sec
( body)
= 3x10
cm (550mGy.
cm)(1.4)
= 0.28mGy
/
5
procedure
1
sec
=
−5
−1
K ( head)
= 9x10
cm (1200mGy.
cm)(1.4)
0.15mGy
/
procedure

K
2
⎛ 1 m ⎞
sec
= ⎜ ⎟ 2mGywk
−
⎝ ⎠
⎝ 3 m ⎠
[ ]
1
150(0.28) + 30(0.15 = 5.
B
0.02
mGywk
−1
= 3.9 10
−
3
− 1
= x
5.2mGywk

Trans smission
1E+0
8
6
4
2
1E-1
8
6
4
1E-2
8
2
8
6
4
Transmission of CT Scanner
Secondary Radiation Through Pb
140 kVp
Fitting parameters to Equation B.2
kVp α (mm -1 ) β (mm -1 ) γ
120 kVp 120 2.246 5.73 0.547
140 2.009 3.99 0.342
2
1E-3
8
6
4
2
1E-4
0 05 0.5 1 15 1.5 2 25 2.5 3
Lead Thickness (mm)

Health Physics August 2007 93(2) 165-170
S. L. Larson et. al.
Comparison of CT Scatter Fractions of NCRP
147 to Manufacturer’s Scatter Specific Data

1.3 mR/scan @ 1m for 10 mm slice
40 cm scan length per procedure
180 procedures/wk (40
scans/proc.)x 1.3 mR/scan =9380
mR/wk