Transcript Pediatric Doses

George David
Associate Professor of Radiology
Medical College of Georgia
Computed Radiography (CR)
• Re-usable metal imaging plates replace
film & cassette
• Uses conventional bucky & x-ray
equipment
CR Exposure & Readout
CR Readout
CR Latitude
• Much greater latitude
than screen/film
• Plate responds to many
decades of input
exposure
 under / overexposures
unlikely
• Computer scales image
to provide viewable
densities
 Unlike film, receptor separate
from viewer
Film Screen vs. CR Latitude
CR Latitude:
.01 – 100 mR
100
Digital Radiography (DR)
• Digital bucky
• Incorporated
into x-ray
equipment
Digital Radiography (DR)
• Potentially lower patient dose
than CR
• High latitude as for CR
• Digital bucky fragile
First DR portables coming
to market
Raw Data Image
• Unprocessed image as read from
receptor
 CR
» Intensity data from PMT’s as a result of scanning
plate with laser
 DR
» Raw Data read directly from TFT array
• Not a readable diagnostic image
• Requires computer postprocessing
 Specific software algorithms applied to
image prior to presenting it as finished
radiograph
*
Enhancing Raw Image
(Image Segmentation)
1. Identify collimated image
border
2. Separate raw radiation
from anatomy
3. Apply appropriate tonescale to anatomy
 Done with look-up table (LUT)
This process is
specific to a
particular body
part and
projection
Look Up Table (LUT)
• Converts a raw
data pixel value to
a processed pixel
value
• “Original” raw
data pixel value
indicates amount
of radiation falling
on pixel
Image Segmentation
• Computer must establish
location of collimated border of
image
• Computer then defines
anatomic region
• Finished image produced by
tone scaling
Requires histogram analysis of
anatomic region
Histogram
• Graph
showing how
much of
image is
exposed at
various levels
Film/Screen Limited Latitude
• Film use
has little
ambiguity
about
proper
radiation
exposure
Should I Worry?
In CR & DR,
image density is
no longer a
reliable indicator
of exposure factor
control.
CR / DR Latitude
DANGER
Will
Robinson!!!
• Almost impossible to
under or overexpose CR /
DR
• Underexposures look
noisy
• Overexposures look
GOOD!!!
CR / DR Latitude
More
DANGER
• If adult technique used on
peds patients, images
look GOOD!
• If grid removed for ped
patient & grid technique
used, image looks GOOD
• NO ONE COMPLAINS
Exposure Creep:
Tendency of radiographs toward higherthen-necessary exposures
• High doses have no detrimental effect on
image quality
• Desire to see less noise on radiographs
• Increased exposure latitude
So how do I judge the exposure if I
can’t tell by looking at the image?
Exposure Index
• Each manufacturer provides exposure
feedback to technologist
• Displayed on CR reader monitor
• Displayed on workstations
Exposure Index
• Measure of radiation received by receptor
below anatomy
Not a direct measure of patient exposure
• If exposure index higher than necessary,
patient overexposed
Displayed Exposure
Index Affected by
•
•
•
•
X-Ray technique selection
Improper centering of image on cassette
Improper selection of study or projection
Placing two or more views on same cassette
 Can cause image to appear dark
Exposure Indication Varies
between Manufacturers
Receptor Kodak Fuji S
Exposure
EI
Number
0.5 1700
400
1 2000
200
2 2300
100
4 2600
50
Kodak
Fuji
 Logarithmic scale
 “S” number goes down
 EI goes up 300 when
as exposure goes up!
 S is half when
exposure doubled
exposure doubled
Exposure Index is Logrithmic
• EI = 2000 +1000 * log(exposure)
Logrithmic Exposure Index
3000
2500
Exposure Index
2000
1500
1000
Doubling
500
0
0
1
2
3
4
Receptor exposure (m R)
5
6
7
Initial Set-up for
Exposure Index
• Kodak recommendation for exposure
index: 1800 – 2200
• Manual technique:
Technologist should strive to keep exposure index
consistent
• Phototiming:
Set-up by service & physics according to
manufacturer’s instructions
Imaging is NOT a
Beauty Contest
• CR/DR exposure should be
selected to provide
“Maximum tolerable noise”
Keeps dose as low as possible
(ALARA)
• Noise tolerance depends on
study & objective.
• Technologist requires noise
feedback from radiologists
Phototimed Phantom Image
• 75 kVp
• 88 mAs
• 2460 EI
Let’s Approximately Double mAs
• 75 kVp
• 88 mAs
• 2460 EI
• 75 kVp
• 160 mAs
• 2680 EI
Let’s Go Crazy
• 75 kVp
• 88 mAs
• 2460 EI
• 75 kVp
• 640 mAs
• 3300 EI
How Low Can You Go? Cut mAs in Half!
• 75 kVp
• 88 mAs
• 2460 EI
• 75 kVp
• 40 mAs
• 2060 EI
Let’s Go Crazy Low
• 75 kVp
• 8 mAs
• 1380 EI
• 75 kVp
• 1 mAs
• 550 EI
Fluoroscopy Doses
• Beam time
• Geometry
• Application of features
Last Image Hold
• Computer displays last fluoro image
before radiation shut off.
• Image noisier than for digital spot
Image made at fluoroscopic technique /
intensity
Less radiation than digital spot
• Allows operator to review static
processes while beam off
ideal for teaching
ideal for orthopedic applications such as hip
pinning
Fluoro Frame
Averaging
• Computer averages current with
user-selectable number of previous
frames
Averages current frame & history
• Conventional fluoro only displays
current frame
Fluoro Frame
Averaging Tradeoff
• Advantage:
Reduces quantum noise
• Disadvantage
Because history frames are averaged with
current frame, any motion can result in lag
Fluoro Frame Rates
• Many systems allow option of
using lower frame rates
15, 7.5, 3.75 fps rather than 30
computer displays last frame until next
one
» reduces flicker
• Most implementations lower
patient & scatter exposure
Exposure proportional to frame rate
• dynamic studies may be jumpy
CT Patient
Dose
• Tube rotates
around patient
during study
• Dose distribution
different than
radiography
Patient
USA Today 6/19/2001
“Each year, about 1.6 million
children in the USA get CT
scans to the head and
abdomen — and about 1,500
of those will die later in life of
radiation-induced cancer,
according to research out
today.”
Biggest Question
about CT Doses?
Appropriateness
of Study
Pediatric Color Coding
• Broselow-Luten Pediatric
System
• Based on child’s size &
weight
• Used in ER’s for
Resuscitation & support apparatus
Medications
IV fluids
Pediatric Color Coding In CT
• Slightly modified Broselow-Luten Pediatric
System
• Color code is used to determine complex
CT protocols
 Contrast options
 Scanner protocols
 Multi-slice has even greater variety of options
» Detector configuration
» Gantry rotation speed
» Table speed
• Color coding significantly reduces protocol
errors
CT Dose depends on
X-Ray Beam
• kVp
• mA
• time
• pitch
• filtration
Image Quality &
Processing Selections
• noise
• detector
efficiency
• matrix resolution
• reconstruction
algorithm
CT Contrast Resolution Depends on Noise
Noise is function of mA
CT Dose Measurement
• Lucite phantom
• 5 holes
One center
Four 90o apart in periphery
• Chamber placed in one
hole
• Lucite plugs placed in
remaining 4 holes
• Slice centered on
phantom
Chamber
Plugs
Measuring CT Dose Index
CTDI
• “Pencil” ion chamber used
• Pencil oriented in “Z” direction
Dose Phantom
Chamber
Hole for
Chamber
Active Chamber Area
(exposed area of chamber)
Z
Slice
Z
CTDI100
• Measurement made with 100 mm
chamber
• Includes scatter tails
Dose Phantom
Chamber
100 mm
Slice
Z
CTDIW
• Weighted average of center
& peripheral measurements
• Represents “average” dose
in scan plane
CTDIW = 1/3 CTDI100-center + 2/3 CTDI100-periphery
CTDIC
CTDiP
Beam Pitch
Table motion in one rotation
Beam Pitch = --------------------------------------Beam thickness
Table motion in
one rotation
Beam Thickness
Beam Pitch
Table motion in one rotation
Beam Pitch = --------------------------------------Beam thickness
Beam Pitch > 1
Beam Pitch = 1
CT Beam Pitch
Beam Pitch: 0.75
Beam Pitch: 1.5
Beam Pitch & CT Dose
table motion during one rotation
Beam Pitch = -------------------------------------------Beam thickness
• Dose inversely proportional to pitch
Smaller pitch
Higher dose
Higher pitch
Lower dose
Definition of CTDI vol
CTDIvol = CTDIw / Pitch
Table motion in one rotation
Beam Pitch = --------------------------------------Beam thickness
Dose Length Product
DLP
• CTDIvol* length of scan (in mGy*cm)
• Reported on many scanners
CT Dose Tradeoff
• More dose required to
improve noise for same
spatial resolution
Noise
Resolution
Noise
Dose
Dose
CT Dose Reduction
• Reduce mAs
Increases image noise
Noise inversely proportional to square
root of dose
• Proper technique is maximum
tolerable noise as determined
by radiologist
CT Noise Reduction:
Increase Slice Width
• more photons detected per voxel
BUT
• More partial volume effect
more different tissue types in each voxel
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 z-axis 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 is High Dose Modality
• CT head dose
fairly uniform
• For body CT,
surface doses
approximately
2X dose at
center
~ 4 rads
~ 2 rads
Effective Dose
• “Manufactured” quantity
• Calculated by multiplying actual
organ doses by “risk weighting
factors”
• Units:
Rem
» = 1 rad for x-rays
Sievert
» = 1 Gray for x-rays
Effective Dose Weighting Factors
Tissue /
Organ
Weighting
Factor
Tissue /
Organ
Weighting
Factor
Gonads
.20
Liver
.05
Bone marrow
.12
Esophagus .05
Colon
.12
Thyroid
.05
Lung
.12
Skin
.01
Stomach
.12
Bladder
.05
Bone
.01
Surface
Remainder .05
Effective Dose
• Represents single dose to entire
body that gives same cancer risk
• Allows comparison of different nonuniform exposures
(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
The End
?