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AAPM 2012 Summer School on Medical Imaging using Ionizing RadiationCT: Shielding andRadiation ProtectionJames Kofler, Ph.DRadiologyMayo Clinic, Rochester, MNJeff Brunette, CHPRadiation SafetyMayo Clinic, Rochester, MNJune 28, 2012

AAPM 2012 Summer School on Medical Imaging using Ionizing RadiationDisclosuresNothing to disclose

AAPM 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 practices

AAPM 2012 Summer School on Medical Imaging using Ionizing Radiation

AAPM 2012 Summer School on Medical Imaging using Ionizing RadiationShielding: NCRP Report 147THE ReferenceDepending on thecircumstances Woof! both of these could be your best friend!

AAPM 2012 Summer School on Medical Imaging using Ionizing RadiationShielding: Purpose“To limit radiation exposure to employeesand members of the general public to anacceptable level.” Design Goals (P) (in air kerma)Controlled Area: 0.1 mGy/week (5 mGy/yr)Uncontrolled Area: 0.02 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 protection.Occupancy and working conditions arecontrolled for radiation protection purposes.Uncontrolled AreaNot a controlled area.

AAPM 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 on.Not any person, but the single person whospends the most time there.

AAPM 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 roomsT1Exam and treatment rooms1/2Corridors, patient rooms, employee lounges & rest rooms1/5Corridor doors1/8Public toilets, vending areas, storage rooms, outdoor areasw/ seating, unattended waiting rooms, patient holding1/20Outdoors, unattended parking lots, attics, stairways,unattended elevators, janitor closets1/40

AAPM 2012 Summer School on Medical Imaging using Ionizing RadiationShielding: DefinitionsBarrier TransmissionK(0)X-raySourceKermaDetectorK(x)B(x) K(0)K(x)x

AAPM 2012 Summer School on Medical Imaging using Ionizing RadiationBarrier Transmission UsageK(0)K(x)xDesign Goal (P)B(x) K(x)K(0)CT: Only secondaryB(x) PT x KsecOccupancy FactorUse B(x) to lookup barrier thickness

AAPM 2012 Summer School on Medical Imaging using Ionizing RadiationShielding: General SchemeObtain tube output (at given distance)1 for given procedures2 Estimate number of procedures per weekDecide usage of adjacent & nearby spaces3 (for occupancy factor & design goal)4 Calculate barrier transmission factor5 Look-up required shielding thickness

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 Map

AAPM 2012 Summer School on Medical Imaging using Ionizing RadiationCTDI-100 Method10 cmPencilChamberBody Phantom (32 cm)Head Phantom (16 cm)

AAPM 2012 Summer School on Medical Imaging using Ionizing RadiationCTDI-100 MethodSecondary air kerma per patient at 1 mAt 1 m1KsecL CTDI100pSecondary

AAPM 2012 Summer School on Medical Imaging using Ionizing RadiationCTDI-100 MethodSecondary air kerma per patient at 1 mScatter fraction per cm(peripheral phantom axis)1KsecL CTDI100pHead 9 x 10-5 cm-1Kappa (K):Body 3 x 10-4 cm-1Considers leakage too!

AAPM 2012 Summer School on Medical Imaging using Ionizing RadiationCTDI-100 MethodSecondary air kerma per patient at 1 mLength of scan (cm)1KsecL CTDI100pPitch

AAPM 2012 Summer School on Medical Imaging using Ionizing RadiationCTDI-100 MethodSecondary air kerma per patient at 1 m1KsecL CTDI100pAt “typical”technique

AAPM 2012 Summer School on Medical Imaging using Ionizing RadiationCTDI-100 MethodSecondary air kerma per patient at 1 m1KsecL mAs nCTDI100pNormalizedper unit mAs**From measured or published values

AAPM 2012 Summer School on Medical Imaging using Ionizing RadiationCTDI-100 MethodSecondary air kerma per patient at “D” mInverse square law1m2LKsec mAs nCTDI100 pDm

AAPM 2012 Summer School on Medical Imaging using Ionizing RadiationCTDI-100 MethodSecondary air kerma per week at “D” mNumber of patients/week1m2LKsec NmAs nCTDI100 pDmUnits of mGy/week

AAPM 2012 Summer School on Medical Imaging using Ionizing RadiationCT Shielding: EXAMPLE

AAPM 2012 Summer School on Medical Imaging using Ionizing RadiationCT Shielding: omDressingRoomReading RoomElec.CorridorCorridorCorridor

AAPM 2012 Summer School on Medical Imaging using Ionizing RadiationCT Shielding: omDressingRoomReading RoomElec.CorridorCorridorCorridor

AAPM 2012 Summer School on Medical Imaging using Ionizing RadiationCT Shielding: EXAMPLEElec.CorridorT oomT 1/5Reading RoomT 1CorridorElec.CorridorT 1/5T 1/40

AAPM 2012 Summer School on Medical Imaging using Ionizing RadiationCT Shielding: EXAMPLE Workload (Body scanner)1751.25600.5120300Patients/work week (35 Patients/day)Scans/patient (pre- & post-contrast)Average scan length (cm)Gantry rotation time (s)kVpAverage effective mAs(Implies 300 mAs at pitch of 1.0)7.8 Average total scan time (s)0.08 nCTDI100 (mGy/mAs)

AAPM 2012 Summer School on Medical Imaging using Ionizing RadiationCT Shielding: EXAMPLEElec.CorridorIsocenterT 13.5 g RoomT 1CorridorElec.CorridorT 1/5What shielding isrequiredhere?3.5 m from isocenter(0.3 m insideReading Room wall)

AAPM 2012 Summer School on Medical Imaging using Ionizing RadiationCTDI-100 MethodSecondary air kerma per week at 3.5 m1m2LKsec NmAs nCTDI100 pDm2Ksec 218.751m3.5 m3x10-4 cm-160 cm300 mAs 0.08 mGy/mAs1Scans/weekKsec 7.71 mGy/wk

AAPM 2012 Summer School on Medical Imaging using Ionizing RadiationCTDI-100 Method Design Goal (P) (in air kerma)- Uncontrolled Area: 0.02 mGy/week (1 mGy/yr) So Transmission factor is:PB(x) T Ksec0.02 mGy/wk (1) 7.71 mGy/wk 2.6 x 10-3Note: Occupancy factor was “1”

AAPM 2012 Summer School on Medical Imaging using Ionizing RadiationCT Shielding: EXAMPLEB(x) 2.6 x 10-3Look to Fig. 2.3to see whatto specify1.70 mm leadNCRP Report No. 147

AAPM 2012 Summer School on Medical Imaging using Ionizing RadiationCT Shielding: EXAMPLE1.70 mmFigure 2.3 NCRP Report No. 147

AAPM 2012 Summer School on Medical Imaging using Ionizing RadiationCT Shielding: EXAMPLEB(x) 2.6 x 10-3Could use:5 lb lead(1.98 mm,5/64 inch)But wait!!1.70 mm leadNCRP Report No. 147

AAPM 2012 Summer School on Medical Imaging using Ionizing RadiationCTDI-100 Method: Isotropic Assumption

AAPM 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: EXAMPLEElec.CorridorMechanicalCTControlRoomReading RoomElec.CorridorDressingRoomCorridorCTDI-100 Methodcan significantlyoverestimatethe requiredshielding alongsides of thescanner!

AAPM 2012 Summer School on Medical Imaging using Ionizing RadiationDose-Length Product (DLP) MethodDLP CTDIvol x Scan LengthCTDIvol 1/3 CTDI100, center 2/3 CTDI100, peripheryPitchCenterPeripheryPhantomDLP (and CTDIvol) are displayed on scanner console

AAPM 2012 Summer School on Medical Imaging using Ionizing RadiationDose-Length Product (DLP) MethodSecondary air kerma per procedure at 1 m1Ksec1Ksec(head) Khead DLP(body) 1.2 Kbody DLPAs before, multiply by the number of proceduresand inverse-square for the wall distance.

AAPM 2012 Summer School on Medical Imaging using Ionizing RadiationCT Shielding: EXAMPLEElec.DressingRoom5.0 mIsocenterT 1ScanRoomMechanicalCTControlRoomCorridorReading RoomT 1CorridorElec.CorridorT 1/5What shielding isrequiredhere?5.0 m from isocenter(0.3 m from wallin corridor)T 1/5

AAPM 2012 Summer School on Medical Imaging using Ionizing RadiationDLP Method: EXAMPLE Workload (Body scanner)175 Patients/work week1.25 Scans/patient (pre- & post-contrast) 218.75 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 5.2 (NCRP 147)ProcedureDLP (mGy cm)Head1,200Chest525Abdomen625Pelvis500Body Average550(chest, abdomen, or pelvis)

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 possible

AAPM 2012 Summer School on Medical Imaging using Ionizing RadiationDLP Method: EXAMPLESecondary air kerma per procedure at 1 m1Ksec(body) 1.2 x 3x10-4 cm-1 x 550 mGy-cm 0.198 mGy/procedureSecondary air kerma per week at 5.0 m21 m 218.75 procs x 0.198 mGy/procKsec 5.0 m 1.73 mGy/week

AAPM 2012 Summer School on Medical Imaging using Ionizing RadiationDLP Method: EXAMPLE Design Goal (P) (in air kerma)- Uncontrolled Area: 0.02 mGy/week (1 mGy/yr) So Transmission factor is:PB(x) T Ksec0.02 mGy/wk (1/5) 1.73 mGy/wk 5.7 x 10-2

AAPM 2012 Summer School on Medical Imaging using Ionizing RadiationCT Shielding: EXAMPLEB(x) 5.7 x 10-2Could use:2 lb lead(0.79 mm,1/32 inch)But wait!!0.6 mm leadNCRP Report No. 147

AAPM 2012 Summer School on Medical Imaging using Ionizing RadiationConsistency in lead thickness Must document lead in walls (so always know), but 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 lead

AAPM 2012 Summer School on Medical Imaging using Ionizing RadiationCT Shielding: EXAMPLEElec.T 1/5CorridorIsocenterT 1ScanRoomMechanicalCTControlRoomReading RoomT 1CorridorElec.CorridorDressingRoomIf this were anoccupied space(office, etc.)it may dominatethe shieldingrequirements!

AAPM 2012 Summer School on Medical Imaging using Ionizing RadiationIsodose MethodGESiemens

AAPM 2012 Summer School on Medical Imaging using Ionizing RadiationScatter Distribution from ManufacturerTechnique: Gy/scan140 kV,100 mAs,40 mm coll.1.32.6 6.2 10.410.4 6.2 2.61.3Need to scaleto weeklysite usage

AAPM 2012 Summer School on Medical Imaging using Ionizing RadiationScatter Distribution from ManufacturerTechnique:mGy/week0.81.7 4.1 6.86.84.1 1.70.8120 kV,300 mAs/proc,218.75 procs,40 mm coll.Need to scaleand overlayon blueprint

AAPM 2012 Summer School on Medical Imaging using Ionizing RadiationScatter Distribution from hanicalDressingRoomWhat shielding isneeded here?Isocenter 4.1 1.7T 1ScanRoomCall it 4.1 mGy/wk0.8Reading RoomT 1CorridorElec.CorridorT 1/5Interpolate,extrapolate (r-squared)to get unshielded kermaat walls

AAPM 2012 Summer School on Medical Imaging using Ionizing RadiationIsodose Method: EXAMPLE Design Goal (P) (in air kerma)- Uncontrolled Area: 0.02 mGy/week (1 mGy/yr) So Transmission factor is:PB(x) T Ksec0.02 mGy/wk (1/40) 4.1 mGy/wk 1.9 x 10-1

AAPM 2012 Summer School on Medical Imaging using Ionizing RadiationCT Shielding: EXAMPLEB(x) 1.9 x 10-1Could use:2 lb lead(0.79 mm,1/32 inch)But recallconsistency!0.3 mm leadNCRP Report No. 147

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 shielding