Download CT Dose

Dose from
Computed
Tomography
CT Patient
Dose
 Tube rotates around
patient during study
 Dose distribution different
than radiography
Patient
CT Dose depends on
X-Ray Beam
 kVp
Image Quality &
Processing Selections
 noise
 mA
 detector
 time
 slice thickness
 filtration
 collimation
Direct
efficiency
 matrix resolution
 reconstruction
algorithm
Indirect
CT Patient Dose
 In theory only image plane is
exposed
 In reality adjacent slices get
exposed because
X-ray
Tube
 beam diverges
 interslice scatter
Patient
Detector
CT Dose Phantom
 Comes in 2
sizes
 “Head”

16 cm
diameter
 “Body”
 32 cm
diameter
Measuring CT Dose
 “Pencil” ion chamber used
 Pencil pointed in “Z” direction
Measurement Procedure
 Standardize protocol
 kVp
 mAs
 scan time
 slice thickness
 table movement
 field of view
 Select appropriate phantom
 head
 body
Measuring Dose
 “Pencil” ion chamber used
 Pencil pointed in “Z” direction
 Single thin slice irradiated
 Long chamber captures scatter
Dose Phantom
Chamber
Hole for
Chamber
Active Chamber Area
(exposed area of chamber)
Z
Slice
Z
Field Measurement of CTDI
 Pencil ion chamber length must >> slice thickness
 chamber receives radiation from all parts of dose
distribution including scatter
Dose
Chamber Partial Irradiation Correction
 Only part of chamber is irradiated
 Chamber calculates exposure as:

charge from ionization
----------------------------------mass of air in entire chamber
BUT
 Because only part of chamber irradiated, displays incorrect
exposure
 Meter assumes full chamber irradiated
 Partial chamber irradiation correction required
Dose
CT Dose Conventions
 CTDI (Computed Tomography Dose Index)
 CTDI100
 CTDIW
 CTDIVOL
 DLP (Dose Length Product)
 Effective Dose
CTDI100
 Measurement made with 100 mm chamber
 Includes scatter “tails”
Dose Phantom
Chamber
100 mm
Beam
Z
CTDIW
 Weighted average of center & peripheral
measurements
 Represents “average” dose in scan plane
CTDIW = 1/3 Center Dose + 2/3 Avg Periphery Dose
CTDIC
CTDiP
16 cm phantom:
Measurements do not
vary much
32 cm phantom:
Peripheral measurement
about 2X center
Beam Pitch & Dose!
Table motion in one rotation
Beam Pitch = --------------------------------------Beam width
Beam Pitch > 1
Beam Pitch = 1
Beam Pitch & CT Dose
It’s easy. Higher pitch
means table moves
patient through beam
faster
table motion during one rotation
Beam Pitch = -------------------------------------------Beam width
 Dose inversely proportional to pitch
Smaller pitch
Higher dose
Higher pitch
Lower dose
Definition of CTDI vol
CTDIVOL corrects
dose for beam
pitch
CTDIvol = CTDIw / Pitch
Table motion in one rotation
Beam Pitch = --------------------------------------Beam width
Dose Length Product
DLP
 DLP = CTDIvol* length of
scan
 DLP units
 mGy*cm
 Displayed on report page
Effective Dose
 Represents single dose to
entire body that gives same
cancer risk
 Allows comparison of
different non-uniform
exposures
 Units:
 Rem

= 1 rad for x-rays
 Sievert

= 1 Gray for x-rays
Measure dose to various
organs and multiply by these
weighting factors
Tissue /
Organ
Gonads
Weighting
Factor
.20
Bone marrow
.12
Colon
.12
Lung
.12
Stomach
.12
Bladder
.05
Tissue /
Organ
Liver
Weighting
Factor
.05
Esophagus .05
Thyroid
.05
Skin
.01
Bone
.01
Surface
Remainder .05
Effective Dose Example
(2 mSv X 0.12) + 1 mSv X 0.05) = 0.29 mSv
1 mSv
(thyroid)
=
2 mSv
(lung)
0.29 mSv
(whole body)
Effective Doses for Various Studies
Eff. Dose
PET F-18 FDG
Cardiac scan Tc-99m pertechnetate
Bone scan Tc-99m MDP
Brain scan Tc-99m HMPAO
Mammogram
Coronary angiogram (high)
Coronary angiogram (low)
CT body
CT head
Barium enema (10 images, 137 sec fluoro)
Barium swallow (24 images 106 sec fluoro)
IVP (6 films)
0
200
400
600
800
1000 1200 1400 1600 1800
mrem
What affects CT Dose?
 Multi-slice’s wide beam
diverges more
 Is less parallel
 Multi-slice can expose ends
which are not imaged
 Some machines prevent
this
CT Dose Tradeoff
 More dose required to improve spatial
resolution for same noise
 smaller pixels
 maintaining same dose / pixel
 More dose required to improve noise for
same spatial resolution
Noise
Resolution
Dose
CT Dose Reduction
 Reduce mAs
 Increases image noise
 Noise inversely proportional to square root of dose
 Proper technique:
 maximum tolerable noise as determined by radiologist
CT Noise Reduction
 Increase pixel size
BUT
 Larger pixels obscure visualization of small objects
 poorer spatial resolution
CT Noise Reduction:
Increase Slice Width
 more photons reach detectors
BUT
 More partial volume effect
 more different tissue types in each voxel
CT Noise Reduction
 Increase overall detector efficiency
 More photons detected for same technique
CT Phototiming
 Allow operator to specify image quality
 GE nomenclature: “Noise Index”
 Modulate mA as tube
 rotates around patient
 Patient moves through gantry
 Goal: keep photon flux to detector constant
during study
Rotational Beam
Modulation
 Operator specifies maximum mA
 mA reduced as tube rotates around
patient to provide only as radiation as
needed
Z-axis Beam Modulation
 Scanner determines changes in attenuation along zaxis from scout study
 mA changed as patient moves through gantry
CT Beam Modulation
 Note more variation in mA during tube rotation in thorax
than in abdomen.
CT Dose Considerations
http://library.thinkquest.org/
NCRP Reports
Ionizing Radiation Exposure of the
Population of the United States
 # 93 Published in 1987
 Data from early 1980’s
 Extensive data on medical exposures
 # 160
 Re-write in 2008
National Council on Radiation Protection and Measurements
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
1982
Total Exposure
(mSv)
3.6 (360 mrem)
0.53
2006
6.0
3.0
 ~500%
Medical Exposure
(mSv)
medical exposure increase
 24 year exposure increase
• Total: ~2.5 mSv
• CT: ~ 1.5 mSv
CT Usage
 16% of imaging procedures
 23% of total per capita exposure
 49% of medical exposure
Multi-slice CT Doses
Other considerations
 Other considerations
 Tendency to cover more volume (anatomy)
 Better availability of equipment
Other Reasons for High CT Doses
 Repeat Exams
 No adjustment of technique factors for different size
patients
 No adjustment for different areas of body
CT Usage
“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 Dose Risk
 Natural incidence of cancer: 1 in 5
 Increase in risk from typical CT study << natural
incidence
 This is public health concern because of large large
increase in volume for CT.
FDA Website:
http://www.fda.gov/RadiationEmittingProducts/RadiationEmittingProductsandProcedures/Medi
calImaging/MedicalX-Rays/ucm115329.htm
Dose reduction
mechanisms
 Customize settings to patient size
 mA
 kVp
 Reduce z-axis coverage
 Reduce field of view
 Consider alternate procedures
http://www.acr.org/SecondaryMainMenuCategories/quality_safety/app_criteria.aspx
Big Brother
 California SB 1237 would require
 hospitals & clinics to record CT doses
 Accreditation
 Reporting of “overdosing”
Concerns
 Siemens automatically increases mA when pitch is
increased
 GE protocols can be
 Changed
 Over-written
 CTDI (dose index)
 Radiation measured in phantom
 Useful in comparing units
 Of limited use in calculating accurate dose to specific
patient
Concerns
 CT dose report often not convenient to view
 No dose database on CT scanners
 Review of dose information in DICOM headers is
cumbersome manual process