Acknowledgement. Diagnostic X-Ray Shielding. Nomenclature for Radiation Design Criteria. Shielding Design Goal (Air Kerma):

Size: px

Start display at page:

Download "Acknowledgement. Diagnostic X-Ray Shielding. Nomenclature for Radiation Design Criteria. Shielding Design Goal (Air Kerma):"

Garey Cobb
7 years ago
Views:

1 Diagnostic X-Ray Shielding Multi-Slice CT Scanners Using NCRP 47 Methodology Melissa C. Martin, M.S., FAAPM, FACR Therapy Physics Inc., Bellflower, CA AAPM Annual Meeting, Orlando, FL Refresher Course Thursday, August 3, 3, 006 8:30 am Acknowledgement Slides Courtesy of: S. Jeff Shephard,, M.S., DABR M.D. Anderson Cancer Center, Houston, TX Ben Archer, Ph.D,, FACR Baylor College of Medicine, Houston, TX Nomenclature for Radiation Design Criteria Required thickness = NT/Pd where: N = total no. of patients per week T = Occupancy Factor P = design goal (mgy( mgy/wk) d = distance to occupied area (m) Shielding Design Goal (Air Kerma): Uncontrolled Areas Annual: P = mgy per year Weekly: P = 0.0 mgy per week Controlled Areas Annual: P = 5 mgy per year Weekly: P = 0. mgy per week

BR M.D. Anderson Cancer Center, Houston, TX Ben Archer, Ph.

2 Distance (d) The distance in meters from either the primary or secondary radiation source to the occupied area. New recommendations in Report 47 for areas above and below source. Where in the occupied area do you calculate the dose? To the closest sensitive organ! 0.5 m.7 m = 5.5 ft 0.3 m = ft Recommended Occupancy Factors for Uncontrolled Areas: T= Clerical offices, labs, fully occupied work areas, kids play areas, receptionist areas, film reading areas, attended waiting rooms, adjacent x-rayx rooms, nurses stations, x- ray control rooms T=/ Rooms used for patient examinations and treatments T=/5 corridors, patient rooms, employee lounges, staff rest rooms T=/8 corridor doors Recommended Occupancy Factors for Uncontrolled Areas: T=/0 public toilets,, vending areas, storage rooms, outdoor area with seating, unattended waiting rooms, patient holding areas T=/40 minimal occupancy areas; ; transient traffic, attics, unattended parking lots, stairways, janitor s s closets, unattended elevators

3 m = ft Recommended Occupancy Factors for Uncontrolled Areas: T= Clerical offices, labs, fully occupied work areas, kids play areas, receptionist areas, film reading areas, attended waiting rooms,

3 Equivalency of Shielding Materials Table 4.8 Page 67 Steel thickness requirement: 8 Pb thickness requirement Gypsum wallboard thickness requirement: 3. concrete thickness requirement Plate Glass thickness requirement:. concrete thickness requirement Light-weight concrete thickness requirement:.3 std-weight concrete thickness requirement mm /8 inch 0.79 mm /3 inch.00 mm 5/8 inch.9 mm 3/64 inch.58 mm /6 inch.98 mm 5/64 inch.38 mm 3 3/3 inch Nominal Thickness of Lead (mm and inches) and Nominal Weight (lb ft - ) at bottom of each bar Multi-Slice Helical CT Shielding Larger collimator (slice thickness) settings generate more scatter Offsets advantages of multiple slices per rotation Environmental radiation levels typically increase Ceiling and floor deserve close scrutiny Problem Question: Do I really need to put lead in the ceiling of a 6-slice CT scanner room? 3

7 mm /8 inch 0.79 mm /3 inch.00 mm 5/8 inch.9 mm 3/64 inch.58 mm /6 inch.98 mm 5/64 inch.38 mm 3 3/3 inch.0.5 3.0 4.0 5.0 6.0 8.

4 Method Calculate the unshielded weekly exposure rate at 0.5 m beyond the floor above. Find the maximum weekly exposure at m from isocenter and inverse-square square this out to the occupied area beyond the barrier. Apply traditional barrier thickness calculations to arrive at an answer. Occupancy, permissible dose, attenuation of concrete, etc. Weekly Air Kerma at m (K sec ) sec (head) = к head * DLP sec (body) =. * к body * DLP K sec K sec NCRP 47 DLP Method head = 9x0-5 / cm body = 3x0-4 / cm к head к body Use inverse square to find unshielded weekly exposure at barrier from K sec NCRP 47 DLP Method DLP (Dose-Length Product) = CTDI VOL * L CTDI VOL = CTDI W /Pitch CTDI W = /3 Center CTDI 00 + /3 Surface CTDI 00 (mgy) L = Scan length for average series in cm Units of mgy-cm = [ / 3 CTDI 00, Center + / 3 CTDI 00, Surface ] * L/p NCRP 47 DLP Method Procedure Head Body Abdomen Pelvis Body (Chest, Abdomen, or Pelvis) CTDI Vol (mgy) * Double the value shown for w/wo contrast Scan Length (L) (cm) DLP* (mgy- cm)

Apply traditional barrier thickness calculations to arrive at an answer. Occupancy, permissible dose, attenuation of concrete, etc.

5 Example 80 Procedures/week 50 Abdomen & Pelvis 30 Head 40% w&w/o contrast 3 (4. m) ceiling height (finished floor to finished floor) GE LightSpeed 6 Preliminary Information Architectural drawings (Plan view) of exam room, floor above, and floor below Elevation sections through scanner location for floor and ceiling Occupancy factors for floors above and below Two rooms away for possibility that remote areas may be more sensitive than adjacent areas Composition of walls, ceilings and floors Materials and thickness Scanner placement from vendor Distance from scanner to protected areas beyond barriers N N Mechanical N N CT Control CT Scan Tech Corridor R I S Fa x RI S ' P A C S Copy ' CT6 B3.4587a 5

Elevation sections through scanner location for floor and ceiling Occupancy factors for floors above and below Two rooms away for possibility that remote areas may be more

6 Unshielded Weekly Exposure at Barrier Air Kerma/procedure at m (K sec 40% w&w/o contrast K sec K sec sec ) sec (head) = к head * DLP =.4 * 9x0-5 cm - * 00 mgy-cm = 4.9 mgy sec (body) = к body * DLP =.4 *. * 3x0-4 cm - * 550 mgy-cm = 4.6 mgy Unshielded Weekly Exposure at Barrier Weekly Air Kerma (K( sec ) at Ceiling: 30 head procedures/wk 50 body procedures/wk sec = 4. m m m = 3.7 m K sec D sec sec (head) = 30 * 4.9 mgy * (m/3.7m) = 0.36 mgy K sec sec (body) = 50 * 4.6 mgy * (m/3.7m) = 3.04 mgy Unshielded Weekly Exposure at Barrier Required Transmission (B) Weekly Air Kerma (K( sec ) at Ceiling: K sec (Total) = K sec (head) + K sec (body) K sec (Total) = 0.36 mgy mgy B = P K sec * T P = Maximum permissible weekly exposure T = Occupancy Factor K sec (Total) = 3.40 mgy 0.0 mgy = = 3.87x mgy * 6

7 m K sec D sec sec (head) = 30 * 4.9 mgy * (m/3.7m) = 0.36 mgy K sec sec (body) = 50 * 4.6 mgy * (m/3.7m) = 3.

7 Total Shielding Required Use Simpkin curve fit equations or look up on published attenuation diagrams (NCRP 47 Fig. A-) A Transmission Transmission of CT Scanner Secondary Radiation Through Lead (0 kv).00e+00.00e-0.00e x0-3.00E-03.00E mm Lead Thickness (mm) Existing Shielding Measure existing attenuation in walls with Tc- 99m source and Na-I I detector (determine lead- equivalence usually 0. mm Pb-eq eq) Floors and ceilings Find lead equivalence from documentation of concrete thickness. Find thickness by drilling a test hole and measuring. Always assume light weight concrete, unless proven otherwise (30% less dense than standard density, coefficients used in NCRP 47) Transmission.00E+00.00E-0 9x0 -.00E-0.00E-03.00E-04 Transmission of CT Scanner Secondary Radiation Through Concrete (0 kv) mm Concrete (mm) Transmission.00E+00.00E-0 9x0 -.00E-0 3 light concrete =. std concrete = 53 mm std concrete B = 9x0 - = 0.45 mm Pb-eqiv Transmission of CT Scanner Secondary Radiation Through Lead (0 kv) Existing Shielding Subtract existing lead-equivalence equivalence from total required Convert to /3 inch multiples (round up) Total lead to add = (Total required) (Existing) =.54 mm 0.45 mm =. mm Round up to /6 Pb Additional Lead required.00e-03.00e mm-eq Lead Thickness (mm) 7

87x0-3.00E-03.00E-04 0.0 0.5.0.54.5mm.0.5 3.0 3.

$s s CT evaluation center) website has measured axial and peripheral CTDI 00 for most scanners on the market in Excel format. Where: к is the scatter fraction at one meter per cm scanned.$

8 CTDI Method CTDI Method Unshielded weekly exposure calculation: Secondary exposure per procedure at one meter K s L = к x p x mas/rotation x CTDI 00, peripheral /mas x Scan kv CTDI kv ImPACT (the UK s s CT evaluation center) website has measured axial and peripheral CTDI 00 for most scanners on the market in Excel format. Where: к is the scatter fraction at one meter per cm scanned. L is the length of the scanned volume. p is pitch. К (head) К (body) 9x0-5 cm - 3x0-4 cm - Calculate K sec for head and body separately, then combine with weighting factors depending on percentage of total workload. K s (total) = CTDI Method % heads * K s (head) + % body * K s (body) 00% Finally, inverse-square this exposure out to each area to be protected. Isodose Map Method Assume an isotropic exposure distribution based on the maximum exposure rate in the vendor-supplied exposure distribution plots (approx. 45 o to the scanner axis). Overestimates shielding needed in the gantry shadows and the shadows of the patient. 8

L is the length of the scanned volume. p is pitch. www.impactscan.

9 3.3E-4mGy 4 m 5.9E-4 mgy 70 mas 40 kv 3 m.3e-3 mgy. m m m 5.3E-3 mgy 9

10 Acceptable exposures outside of this line 0.5 m aff 8th floor Lead Lead 7th floor Led Lead Lead Drop Ceiling ' 9 6 Feet AFF (below) 7th floor Acceptable exposures outside of this line 6 6th floor Comparison of Methods DLP CTDI 00 Isodose Head Body Head Body Head Body K sec Combined Weekly Exposure at Ceiling 3.4 mgy 0.38 mgy 0 mgy Add Lead /6 /3 3/3 0

floor Acceptable exposures outside of this line 6 6th floor Comparison of Methods DLP CTDI

Shielding References Simpkin,, DJ, Transmission of scatter radiation from computed tomography (CT) scanners determined by a Monte Carlo calculation. Health Physics 58(3):363-367, 367, 990.

11 Shielding References Simpkin,, DJ, Transmission of scatter radiation from computed tomography (CT) scanners determined by a Monte Carlo calculation. Health Physics 58(3): , 367, 990. Dixon, RL and Simpkin,, DJ. New Concepts for Radiation Shielding of Medical Diagnostic X-ray X Facilities. In Proceedings of the 997 AAPM Summer School. NCRP (005), National Council on Radiation Protection and Measurements. Structural Shielding Design for Medical X-Ray Imaging Facilities,, NCRP Report #47 (National Council on Radiation Protection and Measurements, Bethesda, Maryland) Contact Information Melissa C. Martin, M.S., FACR, FAAPM Certified Medical Physicist Therapy Physics Inc. 956 Rose St., Bellflower, CA Office Phone: Office Fax: Cell Phone: MelissaMartin@Compuserve.com

NCRP (005), National Council on Radiation Protection and Measurements.

Elements of required physical infrastructures: space, schielding, and patient flow..

Elements of required physical infrastructures: space, schielding, and patient flow.. IAEA Following the IAEA guidelines, adapded by Anna Benini for workshop on Health Technology IUPESM Task Group, Porto