Radiation Doses and Safety Considerations

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Radiation Doses and Safety Considerations

• Medical Collegeof Georgia

G. David, M.S., DABR Associate Professor of
Radiology
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Radiation SafetyWhom are we protecting?

• Patient
• Physicians Staff
• General Public

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Patient Dose Factors / Considerations

• Fluoroscopic exposure time or of radiographic
  exposures
• Beam parameters
• Intensity
• Penetration
• Distance from x-ray tube
• Beam size
• Sensitivity of exposed organs
• damage threshold

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It is possible to inflict damage during radiology
procedures!

16-21 weeks post fluoroscopic procedure
Close-up
18-21 months post procedure
Courtesy FDA Web Site http//www.fda.gov/cdrh/rsn
aii.html
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Patient Dose Depends Upon

• patient
• thickness
• body part in beam
• Operator-controlled factors
• Technique settings
• magnification mode
• operational mode
• normal / high dose
• Collimation (beam size)

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Patient Dose -Exposure Time / exposures

• Fluoroscopy
• patient exposure proportional to beam-on time
• Radiography
• studies ordered
• of films / study
• Cine / angio
• Long fluoro times
• Many images recorded

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Beam Size (Collimation)

• Reduces volume of tissue irradiated

II Tube
II Tube
X-Ray Tube
X-Ray Tube
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Minimizing Patient Exposure

• Consistent with clinical goals minimize
• fluoroscopic beam-on time
• of exposures
• cine / angio fluoro times images
• Beam size (as small as clinically feasible)

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Operator Protection Considerations

• Time
• Distance
• Shielding
• Collimation

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Operator Protection - Time

• Minimize beam-on time
• Your exposure is directly proportional to beam
  time

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Operator Protection Distance(the inverse
square law)

• Exposure rate falls off quickly with distance
• If distance doubles, exposure rate drops by 4

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Radiation Protection of Operator - Shielding

• Sources of radiation for operator
• Primary
• Scatter
• Leakage

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Primary X-Ray Beam

• Beam coming from x-ray tube
• Operator should avoid primary beam
• keep hands, etc. out of primary beam area
• Source of most patient exposure

II Tube
X
X-Ray Tube
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Portable C-Arm
X-Ray Tube
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Scatter (Indirect) Radiation

• Arises mostly from patient
• Emitted in all directions
• intensity varies
• Much lower intensity than primary
• Source of virtually all operator exposure

TV Camera
II Tube
Patient
Table
X-Ray Tube
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Leakage Radiation
TV Camera

• Some radiation leaks through x-ray tube housing
• Intensity much lower than scatter
• Negligible contribution

II Tube
Patient
Table
X-Ray Tube
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Operator Protection - Shielding

• Shield between patient operator significantly
  reduces exposure to operator

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Operator Protection - Shielding

• Apron
• Gloves
• Lead Drapes
• Face Shield
• Thyroid Shield
• Ceiling-mounted shield

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Collimation

• Reducing field size causes significant reduction
  in scatter radiation

II Tube
II Tube
X-Ray Tube
X-Ray Tube
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Minimizing Operator Exposure

• Consistent with clinical goals minimize time
• fluoroscopic exposure times
• cine run lengths frame rates
• Use available lead protective apparel whenever
  possible.
• Collimate as tightly as feasible
• Education

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Protecting the General PublicLead Shielding for
x-ray Rooms

• Physicist calculates shielding for each wall or
  barrier
• Shielding requirement depends on
• Workload
• Distances
• Exam Types
• Use of adjacent space

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Radiation Risk Categories

• Deterministic (non-stochastic)
• Stochastic

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Deterministic (non-stochastic) Radiation Risks

• Effect has known threshold radiation dose
• Examples
• Erythema
• Cataract formation
• Clearly addressed by regulations

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Stochastic Radiation Risks

• Radiation affects probability of condition which
  also occurs naturally
• Cause of condition cannot be determined
• Severity of condition independent of dose
• Examples
• Genetic effects
• Fetal abnormalities
• Cancer

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Stochastic Effects

• All published data based on high doses
• Regulations based on a linear model
• 1/10,000 of the dose produces 1/10,000 the
  frequency of the effect
• No scientific basis or proof for linear model

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Linear Model

• If a 1,000 pound lion can kill 100 Romans in an
  hour

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Linear Model

• A 10 pound puddy tat can kill 1 Roman in an hour.

Oh no!
Suffering succotash I mean ROAR
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Background Radiation

• Earth
• Air
• Cosmic
• People

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Radiation Units

• Rad
• measures energy radiation deposits in a
  biological system
• mrad (milli-rad)
• 1/1000th rad
• Average natural background
• 360 mrad / year

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Threshold for Skin Effects from Radiation

• 300 rad
• temporary epilation
• 600 rad
• main erythema
• 1500-2000 rad
• moist desquamation
• dermal necrosis
• secondary ulceration

Reference Triumf Safety Group
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Threshold for Other Biological Effects from
Radiation

• 200 rads
• Cataract induction
• Acute radiation syndrome
• 100-200 rads whole body irradiation
• Permanent Sterility
• 300-400 rads to gonads
• females
• 500-600 rads to gonads
• males

Reference Huda
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Threshold for Other Biological Effects from
Radiation

• Fetal doses below 1 rad result in negligible
  congenital abnormalities
• Acute doses below 10 rads considered small
  compared to normal pregnancy risks
• Abortion not commonly considered
• Fetal risk depends on gestation period

Reference Huda
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Diagnostic Radiology Exposures

• Generally very low compared to previous values
• Greatest concerns
• Fetal doses
• Angiography / cardiac cath / interventional
  studies
• CT

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Exposure Measurement Protocols

• Standardized methodology for determining how much
  radiation patient receives
• Different protocol for each modality
• Usually provided for average or typical
  patient

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Exposure Measurement Protocols

• Radiograpy
• Entrance Skin Exposure (ESE)
• Mammography
• Mean glandular dose
• CT
• CT dose index (CTCI)

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Typical Plain X-Ray Study Exposures
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Entrance Skin Exposure

• Ionization measured at point where radiation
  enters patient
• Does not address internal doses which depend upon
• Beam penetrability
• Absorber

Patient
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Entrance Skin Exposures

• Abdomen 300 mR

• PA Chest 10-20 mR

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Entrance Skin Exposures

• AP Skull 150 mR

• Hand 20 mR

• Femur 200 mR

• Elbow 20 mR

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Typical Fluoroscopy Exposure Rates

• Cruise control varies exposure rate
  automatically
• Varies greatly with
• Patient
• Imaged anatomy
• Typical Skin Exposure for Average patients

2 - 5 R / minute
Beam on time
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Angiography / Interventional / Cardiology
Caution

• Long fluoroscopic beam times
• Multiple imaging exposures
• Cine (cardiology)
• Subtraction images (Angiography)

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Mean Glandular Dose

• Calculated from entrance skin exposure
• Typical breast assumptions
• 4.2 cm thick (accreditation phantom)
• Breast firmly compressed
• Breast composed of 50 adipose / 50 glandular
  tissue
• average breast closer to 70 adipose / 30
  glandular tissue

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Measuring Mean Glandular Dose (MGD)

• Measure ESE with ion chamber
• Compression paddle scattering phantom in place
• Accreditation Phantom used
• MGD calculated from ESE using conversion factor
• Conversion factor a function of
• Target / filter
• kVp
• Half value layer of beam

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Mammography Mean Glandular Dose

• Limits
• ACR
• 100 mrad w/o grid
• 300 mrad w/ grid
• MQSA
• 300 mrad CC View FDA approved phantom
• Typical
• 150-200 mrad

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CT Patient Dose

• Because tube rotates around patient, dose
  distribution is different than for radiography
• Skull dose distribution
• Fairly uniform
• Body dose distribution
• Dose to center of body half of dose near skin

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CT Patient Dose

• In theory only image plane is exposed
• In reality adjacent slices get some exposure
  because
• beam diverges
• interslice scatter

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CT Dose Phantom

• Made of lucite
• Five holes provided
• One center
• Four 90o apart in periphery
• Comes in two common sizes
• Head
• Body
• larger diameter

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CT Dose Measurement

• Chamber placed in one hole
• Lucite plugs placed in remaining 4 holes
• Slice centered on active area of chamber
• Standardize technique
• kVp
• mAs
• scan time
• slice thickness
• table movement

Chamber
Plugs
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Measuring CT Dose

• Pencil ion chamber used
• Pencil pointed in Z direction

Dose Phantom
Chamber
Hole for Chamber
Active Chamber Area
Z
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Typical CT Doses

• 4 rads head
• 2 rads body
• Surface doses for body scans may be up to twice
  the dose at the center

Note Only plane of interest is exposed in CT
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Other Modalities

• Ultrasound
• No known biological effects as used clinically
• Greatest concerns
• Fetus
• Temperature elevation
• MRI
• No known biological effects as used clinically

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