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```									 IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology

DIAGNOSTIC AND

Part 12.1 : Shielding and X-ray room design
Practical exercise

IAEA
International Atomic Energy Agency
Overview / Objectives

• Subject matter : design and shielding
department
• Step by step procedure to be followed
• Interpretation of results

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Part 12.1 : Shielding and X-ray room
design

Design and shielding calculation of a diagnostic
Practical exercise

IAEA
International Atomic Energy Agency

• Based on NCRP 147
• Assumptions used are very pessimistic, so
overshielding is the result
• Various computer programs are available,
giving shielding in thickness of various
materials

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Shielding Calculation - Principle

• We need, at each calculation point, the dose
per week per mA-min, modified for U and T,
and corrected for distance
• The required attenuation is simply the ratio
of the design dose to the actual dose
• Tables or calculations can be used to
estimate the shielding required

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Shielding Calculation - Detail

Dose per week - primary
• Data being used for NCRP 147 suggests
that for :
• 100 kVp, dose/unit workload = 4.72 mGy/mA-
min @ 1 meter

• 125 kVp, dose/unit workload = 7.17 mGy/mA-
min @ 1 meter

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Shielding Calculation - Detail

• Thus if the workload were 500 mA-min/week
@ 100 kVp, the primary dose would be :

500 x 4.72 mGy/week @ 1 meter = 2360 mGy/
week

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Sample Shielding Calculation

• Using a typical x-ray room, we will calculate
the total dose per week at one point

Office

Calculation Point

2.5 m

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Shielding Calculation - Primary

If U = 0.25, and T = 1 (an office) and the
distance from the x-ray tube is 2.5 m,
then the actual primary dose per week
is :

(2360 x 0.25 x 1)/2.52 = 94.4
mGy/week

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Shielding Calculation - Scatter

• Scatter can be assumed to be a certain
fraction of the primary dose at the patient
• We can use the primary dose from the
previous calculation, but must modify it to the
shorter distance from the tube to the patient
• The “scatter fraction” depends on scattering
angle and kVp, but is a maximum of about
0.0025 (125 kVp @ 135 degrees)

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Shielding Calculation - Scatter

• Scatter also depends on the field size is
simply related to a “standard” field size of 400
cm2 - we will use 1000 cm2 for our field
• Thus the worst case scatter dose (modified
only for distance and T) is :
(2360 x 1 x 0.0025 x 1000)
--------------------------------          = 3.7 mGy
(400 x 2.52 x 0.82)

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Shielding Calculation - Leakage

• Leakage can be assumed to be at the
maximum allowable (1 mGy.hr-1 @ 1 meter)
• We need to know how many hours per week
the tube is used
• This can be taken from the workload W, and
the maximum continuous tube current
• Leakage is also modified for T and distance

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Shielding Calculation - Leakage

• For example: if W = 300 mA-min per week and the
maximum continuous current is 2 mA, the “tube on”
time for leakage calculation
= 300/(2 x 60) hours
= 2.5 hours

• Thus the leakage = 2.5 x 1 x 0.25 / 2.52 mGy
= 0.10 mGy

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Shielding Calculation - Total Dose

• Therefore the total dose at our calculation
point:
= (94.4 + 3.7 + 0.1) = 99.2 mGy / week

• If the design dose = 0.01 mGy / week
then the required attenuation
= 0.01/99.2
= 0.0001
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• From tables or graphs of lead shielding,
we can find that the necessary amount
• There are tables or calculation formula
for lead, concrete and steel at least
• The process must now be repeated for
every other calculation point and barrier

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Shielding Calculation

Reduction factor
50       75 kV       100 150           200 kV
105
250
104
300 kV
103

102

1     2     3    4      5           6           7             8 mm
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Room Shielding - Multiple X-Ray
Tubes

• Some rooms will be fitted with more than
one x-ray tube (maybe a ceiling-mounted
tube, and a floor-mounted tube)

• Shielding calculations MUST consider the
TOTAL radiation dose from all tubes

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CT room design

• General criteria:
• Large room with enough space for:
• CT scanner
• Auxiliary devices (contrast media injector, emergency bed and
equipment, disposable material containers, etc)
• 2 dressing-rooms
• Other spaces required:
• Console room with large window large enough to see the patient
all the time
•   Patient preparation room
•   Patient waiting area
•   Report room (with secondary imaging workstation)
•   Film printer or laser film printer area

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Room shielding

• Protective barriers
• Protective clothing

2.5 Gy/1000 mAs-scan

Typical scatter dose distribution around
a CT scanner

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Protective barriers

milliampere minutes.
• The workload for a CT is usually very high
• Example:
6 working day/week, 40 patients/day, 40 slices/patient,
200 mAs/slice, 120 kV

W=                        = 32000 mAmin/week
6 . 40 . 40 . 200
60

• Primary beam is fully intercepted by the detector assembly.
Barriers are interested only by scattered radiation

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Computation of secondary protective barriers

P(dsec )2
Typical maximum scatter radiation around a CT :
KuX =         WSct T
Sct= 2.5 Gy/mAmin-Scan @ 1 meter and 120 kV.
This quantity may be adopted for the calculation of
protective barriers

The thickness S is otained from the attenuation
curve for the appropriate attenuation material
Secondary barrier
assuming scattered photons with the same                       d sec
penetrating capability of those of useful beam

Example: 120 kV; P = 0.04 mSv/week,
dsec= 3 m, W= 32000 mAmin/week, T= 1
0.04 (3.0) 2
KuX =     (32000) (0.0025) (1)   = 0.0045

Requires 1.2 mm of lead or 130 mm of concrete

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