Transcript Radiation Exposures and Safety Considerations

Radiation Doses and Safety
Considerations
Medical College
of Georgia
G. David, M.S., DABR
Associate Professor of Radiology
*
Radiation Safety
Whom are we protecting?
 Patient
 Physicians & Staff
 General Public
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
*
It is possible to inflict damage
during radiology procedures!
16-21 weeks
post
fluoroscopic
procedure
18-21 months
post procedure
Close-up
Courtesy FDA Web Site: http://www.fda.gov/cdrh/rsnaii.html
Joint Commission Sentinel Event
Policy
Prolonged fluoroscopy with
cumulative dose >1500 rads to a
single field
Each accredited health care organization is
encouraged, but not required, to report to The Joint
Commission any sentinel event meeting these
criteria.
*****
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)
*
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
Beam Size (Collimation)
 Reduces volume of tissue irradiated
II
Tube
X-Ray
Tube
II
Tube
X-Ray
Tube
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)
Operator Protection
Considerations
 Time
 Distance
 Shielding
 Collimation
Operator Protection - Time
 Minimize “beam-on” time
 Your exposure is directly proportional to
beam time
Operator Protection – Distance
(“Inverse Square Law”)
 Exposure rate falls off quickly with distance
 If distance doubles, exposure rate drops by 4
Exposure Rate Fall-off with Distance
100
90
80
Exposure rate
70
60
50
40
30
20
10
0
0
1
2
3
4
5
Distance
6
7
8
9
10
Radiation Protection of Operator Shielding
 Sources of radiation for operator
 Primary
 Scatter
 Leakage
Primary X-Ray Beam
 Beam coming from x-ray
tube
 Operator should avoid
primary beam
Primary Beam
(High Intensity)
II
Tube
 keep hands, etc. out of
X
primary beam area
 Source of most patient
exposure
X-Ray
Tube
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
Leakage Radiation
 Some radiation leaks
through x-ray tube housing
 Intensity much lower than
scatter
TV Camera
II
Tube
Patient
Table
 Negligible contribution
X-Ray
Tube
Operator Protection - Shielding
 Shield between patient & operator
significantly reduces exposure to
operator
Operator Protection - Shielding
 Apron
 Gloves
 Lead Drapes
 Face Shield
 Thyroid Shield
 Ceiling-mounted shield
Collimation
 Reducing field size significantly reduces scatter radiation
 Smaller scattering volume
 More shielding from patient
Image
Receptor
X-Ray
Tube
Image
Receptor
X-Ray
Tube
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
Protecting the General Public:
Lead 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
Radiation Risk Categories
 Deterministic (non-stochastic)
 Stochastic
Deterministic (non-stochastic)
Radiation Risks
 Effect has known threshold radiation
dose
 Examples
 Erythema
 Cataract formation
 Clearly addressed by regulations
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
Stochastic Effects
 Published data based primarily on
high doses
 Regulations based on a linear
model
 1/10,000 of the dose produces
1/10,000 the frequency of the effect
 Linear model is controversial!!!
Background Radiation
 Earth
 Air
 Cosmic
 People
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
Threshold for Other Biological
Effects from Radiation
 Cataract induction
 200 rads
 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
Threshold for Other Biological
Effects from Radiation
 Fetal doses below 1 rad result in
negligible congenital abnormalities
 Risk from acute doses below 10 rads
considered “small”
 Abortion not commonly considered
Reference: Huda
Diagnostic Radiology Exposures
 Generally very low compared to previous
values
 Greatest concerns
 Fetal doses
 Angiography / cardiac cath /
interventional studies
 CT
Exposure Measurement Protocols
 Standardized methodology for determining
how much radiation patient receives
 Different protocol for each modality
 Usually provided for “average” or “typical”
patient
Exposure Measurement Protocols
 Radiograpy
 Entrance Skin Exposure (ESE)
 Mammography
 Mean glandular dose
 CT
 CT dose index (CTDI)
 Dose length product (DLP)
Radiography / Fluoroscopy
Entrance Skin Exposure
 Ionization measured
where radiation enters
patient
 Does not address
internal doses which
depend upon
 Beam penetrability
 Absorber
R
“Patient”
Tablet op
Entrance Skin Exposures
 PA Chest
10-20 mR
 Abdomen:
~300 mR
Entrance Skin Exposures
 AP Skull
~ 150 mR
 Hand
~
20 mR
 Elbow:
~20 mR
 Femur
~
200 mR
Comparison of Entrance Skin Exposure
Humerus
Finger/Toe
Ankle
Foot
Elbow
Hand/Wrist
Knee
Femur
Shoulder
Ribs (below diaph.)
Ribs (above diaph.)
Chest
Abdomen (KUB)
Pelvis
L Spine
T Spine
C Spine
Skull
0
50
100
150
200
250
mR
Entrance skin exposures.
Internal doses will be substantially less.
300
350
400
450
Typical Fluoroscopy Exposure Rates @
Tabletop
 “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
Legal maximum table top exposure: 10 R/min (20 R/min
in high dose mode)
Angiography / Interventional /
Cardiology
Caution
 Long fluoroscopic beam times
 Multiple imaging exposures
 Cine (cardiology)
 Subtraction images (Angiography)
Mammography
Mean Glandular
Dose (MGD)
 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
Measuring Mean Glandular Dose (MGD)
 Measure ESE with
chamber
Mammo
Tube
 Compression paddle &
accreditation phantom
in place
 MGD calculated from
ESE
Compression
Device
Breast
Support
R
Phantom
Grid
Image
Receptor
Mammography Mean Glandular
Dose
 Limits
 ACR


100 mrad w/o grid
300 mrad w/ grid
 MQSA

300 mrad CC View FDA approved phantom
 Typical
 ~100 mrad (digital)
CT Patient
Dose
 Because tube rotates around patient, dose distribution
different from radiography
 Skull dose distribution
 Fairly uniform
 Body dose distribution
 Dose to center of body ~ half of skin dose
CT Dose Phantom
 Lucite
 5 holes
 One center
 Four in periphery
 Comes in two flavors
 “Head”
 “Body”
CT Dose Measurement
 Chamber placed in one hole
 Lucite plugs placed in remaining 4
holes
 Slice centered on phantom
Chamber
 Standardize technique
 kVp
 mAs
 scan time
 pitch
 beam thickness
Plugs
Measuring CT Dose
 “Pencil” ion chamber used
 Pencil pointed in “Z” direction
Dose Phantom
Beam
Chamber
Z
Typical CT Doses
 4 rads head
 2 rads body
 Surface doses for body
scans may be 2X the
dose at center
CT Usage
 Annual growth
 U.S. Population: <1%
 CT Procedures: >10%
 ~ 67,000,000 procedures in
2006
 about 10% pediatric CT
Computed Tomography — An Increasing Source of Radiation Exposure
David J. Brenner, Ph.D., D.Sc., and Eric J. Hall, D.Phil., D.Sc.
New England Journal of Medicine, 2007
U.S. Per Capita Exposure
6
1982
2006
5
4
mSv 3
2
1
0
Total
Medical
Exposure Increase 1982-2006
40%
500%
medical
exposure increase
in 24 years
60%
CT
Other
CT Usage
 16% of imaging procedures
 23% of total per capita exposure
 49% of medical exposure
CT Causes Cancer?
“On the basis of …data on CT use from 1991 through
1996, it has been estimated that about 0.4% of all cancers
in the United States may be attributable to the radiation
from CT studies…By adjusting this estimate for current
CT use this estimate might now be in the range of 1.5 to
2.0%.”
Computed Tomography — An Increasing Source of Radiation Exposure
David J. Brenner, Ph.D., D.Sc., and Eric J. Hall, D.Phil., D.Sc.
New England Journal of Medicine, 2007
CT Causes Cancer?
In the United States, of approximately
600,000 abdominal and head CT
examinations annually performed in
children under the age of 15 years, a rough
estimate is that 500 of these individuals
might ultimately die from cancer
attributable to the CT radiation.
Estimated Risks of Radiation-Induced Fatal Cancer from Pediatric CT;
Brenner, Elliston, Hall, & Berdon; AJR-176 Feb. 2001
Other Modalities
 Ultrasound
 No known biological effects as used clinically
 Greatest concerns


Fetus
Temperature elevation
 MRI
 No known biological effects as used clinically