CT: Shielding and Radiation Protection

1 AAPM 2012 Summer School on Medical Imaging using ionizing RadiationCT: Shielding andRadiation ProtectionJune 28, 2012 James Kofler, Clinic, Rochester, MNJeff Brunette, CHPR adiation SafetyMayo Clinic, Rochester, MNAAPM 2012 Summer School on Medical Imaging using ionizing RadiationNothing to discloseDisclosuresAAPM 2012 Summer School on Medical Imaging using ionizing RadiationLearning Objectives Understand approaches to specifying theshielding requirements for a CT scan room Review three acceptable methods for specs Review Radiation Protection methods for CTpersonnel Review patient Shielding practicesAAPM 2012 Summer School on Medical Imaging using ionizing RadiationAAPM 2012 Summer School on Medical Imaging using ionizing RadiationAAPM 2012 Summer School on Medical Imaging using ionizing RadiationShielding: NCRP Report 147 THE ReferenceDepending on of these could be your best friend!Woof!AAPM 2012 Summer School on Medical Imaging using ionizing Radiation Shielding : PurposeTo limit Radiation exposure to employeesand members of the general public to anacceptable level.

2 Design Goals (P) (in air kerma)Controlled Area: mGy/week (5 mGy/yr)Uncontrolled Area: mGy/week (1 mGy/yr)AAPM 2012 Summer School on Medical Imaging using ionizing RadiationShielding: DefinitionsControlled AreaLimited access area under supervision by anindividual in charge of Radiation and working conditions arecontrolled for Radiation Protection AreaNot a controlled 2012 Summer School on Medical Imaging using ionizing RadiationShielding: DefinitionsOccupancy FactorAverage fraction of time that the maximallyexposed individual is present while thex-ray beam is anyperson, but the singleperson whospends the most time 2012 Summer School on Medical Imaging using ionizing RadiationSuggested Occupancy Factors (T)LocationOffices, labs, pharmacies, receptionist areas, attendedwaiting rooms, kid s play areas, x-ray rooms, nursestations, reading rooms, control roomsTExam and treatment roomsCorridors, patient rooms, employee lounges & rest rooms Corridor doorsPublic toilets, vending areas, storage rooms, outdoor areasw/ seating, unattended waiting rooms, patient holdingOutdoors, unattended parking lots, attics, stairways,unattended elevators, janitor closets11/21/51/81/201/40 AAPM 2012 Summer School on Medical Imaging using ionizing RadiationShielding.

3 DefinitionsBarrier TransmissionX-raySourceKermaDetectorK(0) xK(x)B(x) = K(x)K(0)AAPM 2012 Summer School on Medical Imaging using ionizing RadiationBarrier Transmission UsageK(x)B(x) = PT x KsecB(x) = K(0)K(0)xK(x)Design Goal (P)CT: Only secondaryOccupancy FactorUse B(x) to lookup barrier thicknessAAPM 2012 Summer School on Medical Imaging using ionizing Radiation1 Shielding : General SchemeObtain tube output (at given distance)for given procedures3 Decide usage of adjacent & nearby spaces(for occupancy factor & design goal)Estimate number of procedures per week2 Calculate barrier transmission factor4 Look-up required Shielding thickness5 AAPM 2012 Summer School on Medical Imaging using ionizing RadiationCT: Step 1. Obtain tube output Three Different Methods- CTDI-100-Dose Length Product (DLP)-Isodose MapAAPM 2012 Summer School on Medical Imaging using ionizing RadiationCTDI-100 MethodBody Phantom (32 cm)Head Phantom (16 cm)10 cmPencilChamberAAPM 2012 Summer School on Medical Imaging using ionizing RadiationCTDI-100 MethodKsec= 1 LpCTDI100 Secondary air kerma per patient at 1 mSecondaryAt 1 mAAPM 2012 Summer School on Medical Imaging using ionizing RadiationCTDI-100 MethodKsec= 1 LpCTDI100 Secondary air kerma per patient at 1 mScatter fraction per cm(peripheral phantom axis)Head = 9 x 10-5cm-1 Kappa (K):Body = 3 x 10-4cm-1 Considers leakage too!

4 AAPM 2012 Summer School on Medical Imaging using ionizing RadiationCTDI-100 MethodKsec= 1 LpCTDI100 Secondary air kerma per patient at 1 mLength of scan (cm)PitchAAPM 2012 Summer School on Medical Imaging using ionizing RadiationCTDI-100 MethodKsec= 1 LpCTDI100 Secondary air kerma per patient at 1 mAt typical techniqueAAPM 2012 Summer School on Medical Imaging using ionizing RadiationCTDI-100 MethodKsec= 1 LpnCTDI100 Secondary air kerma per patient at 1 m*From measured or published valuesmAsNormalizedper unit mAs*AAPM 2012 Summer School on Medical Imaging using ionizing RadiationCTDI-100 MethodKsec= LpnCTDI100 Secondary air kerma per patient at D mmAsD m1 m2 Inverse square lawAAPM 2012 Summer School on Medical Imaging using ionizing RadiationCTDI-100 MethodKsec= LpnCTDI100 Secondary air kerma per week at D mmAsD m1 m2 Number of patients/weekNUnits of mGy/weekAAPM 2012 Summer School on Medical Imaging using ionizing RadiationCT Shielding : EXAMPLEAAPM 2012 Summer School on Medical Imaging using ionizing RadiationCT Shielding .

5 EXAMPLECTC ontrolRoomScanRoomIsocenterReading 2012 Summer School on Medical Imaging using ionizing RadiationCT Shielding : EXAMPLECTC ontrolRoomScanRoomIsocenterReading 2012 Summer School on Medical Imaging using ionizing RadiationCT Shielding : EXAMPLECTC ontrolRoomScanRoomIsocenterReading = 1T = 1T = 1/5T = 1/5T = 1/40 AAPM 2012 Summer School on Medical Imaging using ionizing RadiationCT Shielding : EXAMPLE Workload (Body scanner)175 Patients/work week (35 Patients/day) Scans/patient (pre- & post-contrast)60 Average scan length (cm) Gantry rotation time (s)120 kVp300 Average effective mAs(Implies 300 mAs at pitch of ) Average total scan time (s) (mGy/mAs)AAPM 2012 Summer School on Medical Imaging using ionizing RadiationCT Shielding : EXAMPLECTC ontrolRoomScanRoomIsocenterReading = 1T = 1T = 1/5 What Shielding isrequired here? ( m inside Reading Room wall) m from isocenter mAAPM 2012 Summer School on Medical Imaging using ionizing RadiationCTDI-100 MethodKsec= LpnCTDI100 Secondary air kerma per week at mmAsD m1 mGy/wkAAPM 2012 Summer School on Medical Imaging using ionizing RadiationCTDI-100 Method Design Goal (P) (in air kerma)- Uncontrolled Area: mGy/week (1 mGy/yr)B(x) = PT mGy/wk(1) mGy/wk== x 10-3 Note: Occupancy factor was 1 So Transmission factor is:AAPM 2012 Summer School on Medical Imaging using ionizing RadiationCT Shielding : EXAMPLENCRP Report No.

6 147B(x) = x mm leadLook to Fig. see whatto specifyAAPM 2012 Summer School on Medical Imaging using ionizing RadiationCT Shielding : EXAMPLEF igure NCRP Report No. mmAAPM 2012 Summer School on Medical Imaging using ionizing RadiationCT Shielding : EXAMPLEB(x) = x mm leadCould use:5 lb lead( mm,5/64 inch)But wait!!NCRP Report No. 147 AAPM 2012 Summer School on Medical Imaging using ionizing RadiationCTDI-100 Method: Isotropic AssumptionAAPM 2012 Summer School on Medical Imaging using ionizing RadiationCTDI-100 Method: Measured ScatterAdapted from Wallace, et. al, J. Radiol. Prot. 32 (2012) 39-50 AAPM 2012 Summer School on Medical Imaging using ionizing RadiationCT Shielding : EXAMPLECTC ontrolRoomReading Methodcan significantlyoverestimatethe requiredshielding alongsides of thescanner!AAPM 2012 Summer School on Medical Imaging using ionizing RadiationDose-Length Product (DLP) MethodDLP = CTDI volx Scan LengthCTDIvol=1/3 CTDI100, center + 2/3 CTDI100, periphery PitchPeripheryCenterPhantomDLP (and CTDIvol) are displayed on scanner consoleAAPM 2012 Summer School on Medical Imaging using ionizing RadiationDose-Length Product (DLP) MethodKsec(head) = KheadDLP1 Ksec(body) = KbodyDLP1 Secondary air kerma per procedure at 1 mAs before, multiply by the number of proceduresand inverse-square for the wall 2012 Summer School on Medical Imaging using ionizing RadiationCT Shielding : EXAMPLECTC ontrolRoomScanRoomIsocenterReading = 1T = 1T = 1/5 What Shielding isrequired here?

7 ( m from wallin corridor) m from isocenter mT = 1/5 AAPM 2012 Summer School on Medical Imaging using ionizing RadiationDLP Method: EXAMPLE Workload (Body scanner)175 Patients/work Scans/patient (pre- & post-contrast)= procedures per weekAll are chest, abdomen, pelvis or comboNeed DLP values!AAPM 2012 Summer School on Medical Imaging using ionizing RadiationDLP Method: EXAMPLE Typical DLPs from Table (NCRP 147)Head1,200 Chest525 Abdomen625 Pelvis500 Body Average550(chest, abdomen, or pelvis)ProcedureDLP (mGy cm)AAPM 2012 Summer School on Medical Imaging using ionizing RadiationDLP Method: EXAMPLECAUTIONBe careful about using published typical valuesEstimates should be based onsite-specific data, when possibleAAPM 2012 Summer School on Medical Imaging using ionizing RadiationDLP Method: EXAMPLEKsec(body) = x 3x10-4cm-1x 550 mGy-cm1 Secondary air kerma per procedure at 1 m= mGy/procedureSecondary air kerma per week at m= procsx mGy/procKsec= 2012 Summer School on Medical Imaging using ionizing RadiationDLP Method: EXAMPLE Design Goal (P) (in air kerma)- Uncontrolled Area: mGy/week (1 mGy/yr)B(x) = PT mGy/wk(1/5) mGy/wk== x 10-2 So Transmission factor is.

8 AAPM 2012 Summer School on Medical Imaging using ionizing RadiationCT Shielding : EXAMPLENCRP Report No. 147B(x) = x mm leadCould use:2 lb lead( mm,1/32 inch)But wait!!AAPM 2012 Summer School on Medical Imaging using ionizing RadiationConsistency in lead thickness Must document lead in walls (so always know), Having consistency with all walls- Easier for carpenters- Easier if scanner changes- Easier if room use changes- Doors and windows often re-used in remodelingprojects (these must be same as in walls) Exception may be if one wall needs more leadAAPM 2012 Summer School on Medical Imaging using ionizing RadiationCT Shielding : EXAMPLECTC ontrolRoomScanRoomIsocenterReading = 1T = 1T = 1/5If this were an occupied space(office, etc.)it may dominatethe shieldingrequirements!AAPM 2012 Summer School on Medical Imaging using ionizing RadiationIsodose MethodGESiemensAAPM 2012 Summer School on Medical Imaging using ionizing RadiationScatter Distribution from : Gy/scan140 kV,100 mAs,40 mm to scaleto weeklysite usageAAPM 2012 Summer School on Medical Imaging using ionizing RadiationScatter Distribution from :mGy/week120 kV,300 mAs/proc, procs,40 mm to scaleand overlayon blueprintAAPM 2012 Summer School on Medical Imaging using ionizing RadiationScatter Distribution from ManufacturerInterpolate,extrapolate (r-squared)to get unshielded kermaat wallsCTControlRoomScanRoomIsocenterReadi ng = 1T = 1T = 1 Shielding isneeded here?

9 Call it 2012 Summer School on Medical Imaging using ionizing RadiationIsodose Method: EXAMPLE Design Goal (P) (in air kerma)- Uncontrolled Area: mGy/week (1 mGy/yr)B(x) = PT mGy/wk(1/40) mGy/wk== x 10-1 So Transmission factor is:AAPM 2012 Summer School on Medical Imaging using ionizing RadiationCT Shielding : EXAMPLENCRP Report No. 147B(x) = x mm leadCould use:2 lb lead( mm,1/32 inch)But recallconsistency!AAPM 2012 Summer School on Medical Imaging using ionizing RadiationCT Shielding : Three Methods CTDI100- Neglects scanner Shielding - Over-estimates Shielding on sides- Most variables easy to find Dose Length Product- Neglects scanner Shielding - Over-estimates Shielding on sides- Easy to use Isodose- Can be very cumbersome (errors)- Accounts for scanner shieldingAAPM 2012 Summer School on Medical Imaging using ionizing RadiationCT Shielding : Other Considerations Floors and ceilings- For CTDI100and DLP methods:Calculate requirements same way- For isodose method: Spin plot to vertical orientation- Different types of concrete slabs and decking Penetrations (plumbing, electrical, etc.)

10 - Wrap, overlap, get creative if necessary- No line of sight to isocenterAAPM 2012 Summer School on Medical Imaging using ionizing RadiationFloors and m(1 ft) mFloor belowScan roomFloor m(~7 ft)Not drawn to scaleAAPM 2012 Summer School on Medical Imaging using ionizing RadiationFloors and CeilingsFloor belowScan roomFloor aboveShieldingRoomceilingInterstitial spaceNot drawn to scaleAAPM 2012 Summer School on Medical Imaging using ionizing RadiationAAPM 2012 Summer School on Medical Imaging using ionizing RadiationAAPM 2012 Summer School on Medical Imaging using ionizing RadiationShielding Personnel Distance Time- Minimize time in room while x-ray ON Shielding - Maximize distance from scanner while ON- Personal protective Shielding should be available-Aprons, eyewear, gloves, etc. as appropriate- Moveable shields if practicalAAPM 2012 Summer School on Medical Imaging using ionizing RadiationShielding PersonnelAt Mayo - Relative exposure lines Step back one line to halve exposure AAPM 2012 Summer School on Medical Imaging using ionizing RadiationAAPM 2012 Summer School on Medical Imaging using ionizing RadiationAAPM 2012 Summer School on Medical Imaging using ionizing RadiationBismuth and Other Patient ShieldsAAPM Position Statement on the Use of Bismuth Shielding for the Purpose of Dose Reduction in CT Scanning.