Simpkin - Radiation Shielding for Medical Instalations

RSMI 2009
Session VI – Workshops & Tutorials
Diagnostic Shielding Design
Douglas J. Simpkin, Ph.D.
Aurora St. Luke’s Medical Ctr
Milwaukee, WI
dsimpkin@wi.rr.com
http://www.geocities.com/djsimpkin/

Shielding a Low-Use Portable C-arm
Fluoro Unit
• In the US, a “Pain Clinic” uses a portable C
arm fluoro unit for short times for a number
of patients per day. Do we need shielding
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Shielding a Low-Use Portable C-arm
• Guess at workload:
Fluoro Unit
– Not more than 1 min fluoro/patient
– 5 patients/day
– 5days/week
– Guess at conservative technique: 4 mA over 70-
90 kVp
• Total workload: 4 mA × 1 min × 5 patients/
day × 5day/wk= 100 mA·min/wk
min/wk
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Shielding a Low-Use Portable C-arm
Fluoro Unit
• d = 3 m to fully occupied, uncontrolled area
• P/T = 002mGy/wk
0.02 • Load into XRAYBARR
– 730 cm 2 image receptor size at t1 m SID
–90° scatter
– Secondary radiation i only (image intensifier ifi is a
primary beam-stop)
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Comments
• Shielding design need not be an academic pursuit!
• Indeed for new installations
– Put 1/16” or 2 mm Pb in walls
• In USA because of distribution scheme, 1/16” Pb-lined drywall
costs the same as is 1/32” Pb and much more readily available
• BUT, the detailed calculations
– Support your design
– Are needed for low use situations: “Do you need Pb”
– Are absolutely needed for ceilings/floors in high use
CT/interventional labs
– Should be done when changing gx-ray systems
• Especially going from single slice to multi-slice CT
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Information o Missing from NCRP 147….
• We have no info on Cu-filtered x-ray beams!
– Commonly seen for Cath Labs/EP Labs/ Interventional Labs
and now some under-table R&F Fluoro tubes
– For the same dose rates to the image receptor, Cu filtered
workloads (mA·min) min) are an order of magnitude greater than
for Al filter
– No workload data presently available
• Complicated since filter automatically adjusts to patient attenuation,
with higher Cu filtration used with thinner patients
• Clinical observation: 70-80 kVp used to maintain Iodine image contrast
• DAP (measured before patient) will decrease with Cu filtration
– May explain discrepancy in predicted d unshielded d doses between recent
UK/Irish DAP-based data & NCRP-147
– No output data, scatter fractions, or transmission data for Cu
filtered beams
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Primary Beam incident on patient
80 kV X-Ray Beams, Variable Filters
Num mber of X Rays
0 mm
Al filtered
copper
beam filter
0.2 mm
copper
filter
0.3 mm
copper
filter & Siemens has a 0.6
mm Cu filter option!
0 10 20 30 40 50 60 70 80
X Ray Energy (keV)
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80 kV X-Ray Beams, Variable Filters
Num mber of X Rays
0 mm
Al filtered
copper
beam filter
Fat patient
0.2 mm
copper
filter
0.3 mm
copper
filter
Thin patient
0 10 20 30 40 50 60 70 80
X Ray Energy (keV)
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Information Missing from NCRP 147….
• Gut feeling: Amount of fluoro/cine per patient
has not changed “that much” since workload
survey of the early 1990s.
• Cu filtration is like taking shielding off the wall
and sticking it into the primary beam filter!
– So use the NCRP-147 Al-filtered beam and workload
data for Al-filtered beams, sticking with N
patients/wk
– This should adequately shield heavy-use rooms (and
over-shield lightly-used l rooms)
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