Module Rhodes

Transcription

1 Module Rhodes

2 Health Physicist Concerned with providing occupation radiation protection and minimizing radiation dose to the public. Diagnostic Imaging has changed our world Live longer Work to accomplish ALARA

3 Cardinal Principles Time - Minimize Exposure = Exposure Rate X Exposure Time Worker exposed to 420 mr/hour. If exposed for 5 minutes, what is the dose? 420 :: x = 35 mr minutes, what is the dose? 420 :: x = 329 mr 60 47

4 Time 5 minute timers Fluoroscopy utilizing pulsed beam 2.1 Rad/min for every ma of operation at 80 kv= Tabletop Dose If a fluoro case lasted for 4 minutes and the tube operated at 1.8 ma, what would the patient radiation exposure be? 4 X 1.8 X 2.1 = Rads

5 Increase Distance Utilize Inverse Square Law to calculate doses I 1 :: D 2 2 I 2 D 2 1 A radiographic tube has an output of 4.7 mr/mas at 40 SID. What would be the radiation exposure at 50 SID? 4.7 :: 50 2 Distance X 40 2 = 3.0 mr/mas

6 Where to stand for fluoro Protective curtains 1/1000 intensity of primary beam in the form of scatter Distance

7 Use it! Lead Atomic number 82 or Concrete - Photoelectric effect Half Value Layer - HVL- Thickness of absorbing material that reduces the intensity of the beam to half of its original value Tenth Value Layer TVL Shielding 1 TVL = 3.3 HVL

8 Shielding - Construction Walls & Doors protect workers and public Primary Protective Barrier prevent direct or unscattered radiation from reaching a person Straight line paths of the beam 1/16 Pb, 7 feet up from floor, when tube is 5 7 feet from wall Secondary Protective Barrier Protection from a beam that has been scattered/leakage 1/32 Pb or equivalent ½ overlap with primary

9 Protective Barriers Primary Barriers Primary beam Lead Concrete Secondary Barriers Leakage radiation Scatter radiation Steel Glass Gypsum Wood 9

10 HVL TVL Tube Potential Lead (mm) Concrete (cm) Lead (mm) 40 kvp kvp kvp kvp kvp kvp Concrete (cm) 10

11 Must be fixed & feet high Control Booth X-rays must scatter minimum 2 times before reaching Secondary protective barrier Window 1.5 mm Pb equivalent Maximum allowance 100 mr/week Exposure switch assures operator behind wall

12 Factors Affecting Barrier Thickness Distance Occupancy Controlled Area vs. Uncontrolled Area Workload Use Factor kvp 12

13 Shielding - Barriers Calculations Barrier Shielding Requirements W x U x T = Workload, Use, Occupancy factors

14 Use Shielding ( U ) The portion of beam On time that the beam is directed at a primary barrier during the week.. Chest room - if 50% of the time the room is in use, the beam is pointed toward the wall behind the chest board, the wall has a use factor of ½ Use Factor for secondary radiation always = 1

15 Workload ( W ) Radiation ON TIME per week Calculated to mas per week or ma per minute Example room is operated 5 days a week Average 20 patients/day Average number of exposures per patient is 2 Average technique is 110 kv, 400 ma,.01 seconds W = 400 x.01 x 2 x 20 x 5 = 800 ma/week

16 Occupancy Factor (T) Modification in construction shielding to address how often space is utilized If no one is present behind a wall- no extra shielding necessary basement closet- 10 th floor exterior wall Fully occupied = 1 Corridors = 1/5 Public toilets, vending =1/20 Outdoor with transient pedestrians, stairwells =1/40

17 Controlled/Uncontrolled Occupationally exposed individual or general public Controlled Radiographer work areas weekly dose allowed is 100 mrem Uncontrolled hallways or patient waiting areas weekly dose allowed is 2mrem Sheilding requirements

18 Calculate Barrier Requirements Workload X Use X Occupancy Determine primary or secondary status Test or make exposures to confirm if needed Leakage radiation is given consideration For 100 KV U = ½, T = 1, W = 15,000 mas/wk = 250 mamin/week

19 Barrier Design

20 Effective Dose Method of converting a partial body dose to a dose as if the whole body were exposed. Utilizing tissue weighting ED is the equivalent whole body dose. Radiographic images collimated to body area Radiation risk estimates are based on total body exposures Absorbed Dose x Rad weighting factor x tissue weighting factor Bushong Page. 545 Charts

21 Weighting Factors Radiation Weighting Factors X -rays 1 Gamma rays 1 Neutrons 5-20 Protons 2 Alpha particles 20 Tissue Weighting Factors Gonads 0.20 Bone marrow 0.12 Colon 0.12 Lung 0.12 Stomach 0.12 Liver 0.05 Breast 0.05 Thyroid 0.05 Skin 0.01 Effective Dose = abs dose rad wtg factor tissue wtg factor 21

22 Protection Features Tube Housing 100 mr/hr at 1 meter distance Control Panel Beam on visible & audible signals SID Indicator tape measure or more elaborate accurate within 2% of the indicated SID Collimation Alignment must be within 2% for light and exposed field Positive Beam Limitation ( PBL ) accurate within 2% Tube/IR alignment Light/laser

23 Filtration Protection Features Below 50 KV KV > 70 KV.5 mm AL 1.5 mm AL 2.5 mm AL Utilize HVL measures to determine added thickness needed Bushong Page. 550

24 Protection Features Reproducibility (same technique over & over ) not greater than 5% variable Linearity ( same technique with different ma/s variations ) 10% variable Operator Shield Fixed, cord limited Mobile Pb apron switch must have 2 meter cord ( 6 feet)

25 Protection Features - Fluoro SID 38 cm ( 15 ) on stationary 30 cm ( 12 ) on mobile Primary Protective Barrier II tube carriage interlock Filtration for tabletop exposures Collimation unexposed boarder visible on monitors Exposure control- Dead man Bucky slot cover -.25 mm pb Protective curtain -.25 mm pb Cumulative Timer 5 min audible Dose Area Product 2.1 R/min/ma at 80 kv (10 Rad/min) not to exceed 20 R/min for mag modes (high level control)

26 Shielded vs. Unshielded Fluoroscopy 26

27 Patient Dose Descriptions Entrance Skin Exposure ESE frequently used easy to calculate Gonadal Dose Important for genetic responses Bone Marrow Estimated from ESE not measured directly Deep & Shallow Doses are calculated based on the energy of the beam of exposure

28 X-Ray Exams and Patient Dose Exam Technique (kvp/mas) ESE (mgy t ) Mean Marrow Dose (mgy t ) Skull 76/ <1 Chest 110/ <1 C-Spine 70/ <1 Gonad Dose (mgy t ) L-Spine 72/ KUB 74/ Pelvis 70/ Extremity 60/ <1 CT Head 125/ CT Pelvis 124/

29 Entrance Skin Exposure - ESE Called Patient Dose TLD most used Nomogram can use to estimate Nomogram for each unit

30 Mean Marrow Dose Average dose to the entire active bone marrow.

31 Gonadal Dose Varies on where the primary beam is located Men approximately 90% of ESE Women approximately 33% of ESE

32 Genetically Significant Dose - GSD The gonadal dose that if received by every member of the population, would produce the total genetic effect on the population as the sum of the individual doses actually received. Averages dose between those exposed and those not. Looking at effect on genetic pool. US 20 mrad/year - not expected to cause damage

33 Mammography Doses Approx 800 mr/view ESE Center tissue 100 mrad/view About 26 KV Grids 4:1 or 5:1 Glandular Dose Approx 15% of ESE

34 CT Doses CT higher for ESE 10% of all US x-rays are CT, but accounts for 70% of dose CT tight collimation critical to dose reduction Sharp boundaries No overlap between slices Couch move at exact increments Multislice/helical have reduced patient dose over single slice due to reduction in overlap Increase # of slices- decrease dose

35 Reduce Patient Dose Reduce Unnecessary Exams Routine exams chest and spine x-rays for new jobs Public choice CT exams Reduce Repeats About 5% Most due to Tech error Proper Technique Selection Fast IR 400 or greater Correct Positioning Area Shielding

36 Area Shields Contact Shadow Cup

37 First 2 weeks all or nothing First trimester major organogenesis Needs 25 Rad or greater to the fetus for concern Collimate Double Shield Post signs in waiting rooms Have questionnaires 25 Rad Rule fetal dose counsel mother Pregnant Patient therapeutic abortion???

38 Radiation Exposure of Pregnant Patients First two weeks of pregnancy High exposure may cause resorption of the embryo No other response likely No possibility of the induction of congenital abnormalities Second week to tenth week Major organogenesis Major organ systems of the fetus developing High exposure may cause congenital abnormalities 38

39 Fetal Doses

40 Dose Trends

41 Occupational Doses Should not exceed 100 mr/year - most less #1 Area is Fluoroscopy interventional Extremity monitoring required for interventional Rads

42 Occupation Doses Mammography Very low CT Very low Surgery if C-arm could be higher Mobile Wear shields, use distance Wear Badges- do not protect awareness only anyone with a chance of receiving 10% of the dose limits should wear a badge Minimize holding Control Badges

43 Lead Aprons

44 Pregnant Radiographer Must declare pregnancy in writing Meet with RSO training baby badge Baby treated as general public - dose limited to 500 mrem for the pregnancy If not declared organization under no obligation to limit worker dose to lesser value Once declared, may limit work locations if high radiation Can rescind declaration