Transcript 89

IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology
RADIATION PROTECTION IN
DIAGNOSTIC AND
INTERVENTIONAL RADIOLOGY
Part 12.1 : Shielding and X-ray room design
Practical exercise
IAEA
International Atomic Energy Agency
Overview / Objectives
• Subject matter : design and shielding
calculation of a diagnostic radiology
department
• Step by step procedure to be followed
• Interpretation of results
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12.1 : Shielding and X-ray room design
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IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology
Part 12.1 : Shielding and X-ray room
design
Design and shielding calculation of a diagnostic
radiology department
Practical exercise
IAEA
International Atomic Energy Agency
Radiation Shielding - Calculation
• 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/mAmin @ 1 meter
• 125 kVp, dose/unit workload = 7.17 mGy/mAmin @ 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
(FSD, usually about 80 cm)
• 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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Shielding Calculation - Lead Required
• From tables or graphs of lead shielding,
we can find that the necessary amount
of lead is 2.5 mm
• 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
105
50
75 kV
100
150
200 kV
250
104
300 kV
103
102
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Lead Required
1
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12.1 : Shielding and X-ray room design
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Radiation Shielding Parameters
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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
• Workload
• Protective barriers
• Protective clothing
2.5 Gy/1000 mAs-scan
Typical scatter dose distribution around
a CT scanner
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Protective barriers
• Workload (W): The weekly workload is usually expressed in
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=
6 . 40 . 40 . 200
60
= 32000 mAmin/week
• 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
KuX =
Scattered radiation
Typical maximum scatter radiation around a CT :
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
assuming scattered photons with the same
penetrating capability of those of useful beam
dsec
P (dsec )2
WSctT
Secondary barrier
Example: 120 kV; P = 0.04 mSv/week,
dsec= 3 m, W= 32000 mAmin/week, T= 1
KuX =
0.04 (3.0) 2
(32000) (0.0025) (1)
= 0.0045
Requires 1.2 mm of lead or 130 mm of concrete
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Where to Get More Information
• National Council on Radiation Protection
and Measurements “Structural Shielding
Design for Medical X Rays Imaging
Facilities” 2004 (NCRP 147)
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