Digital Radiography:
Exposure Factor Selection and ALARA
Andrew Woodward M.A., R.T.(R)(CT)(QM)
Assistant Professor
The University of North Carolina at Chapel Hill
School of Medicine
Department of Allied Health Sciences: Division of Radiologic Science
DISCLAIMER
• Exposure data listed is based upon
simulations performed within a laboratory
setting using anthropomorphic phantoms.
• Application of the concepts contained within
this presentation should be done under the
guidance of a Radiologist and/or Medical
Physicist.
Why this topic?
• “Dose Creep”
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• Image Gently™
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• Image Wisely™
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What do we know?
Film – Intensifying Screen
• Self regulating system – limited dynamic range
▪ The receptor response
▴ Screen speed
Relative
Speed
▴ Film H&D curve
▴ Processing
▪ Selecting appropriate exposure factors assures optimal OD
▴ Over exposed = dark image
▴ Correct exposure = correct OD
▴ Under exposed = light image
Dynamic Range
Response
E++
E+
Eo
E-
Exposure
The ability of the receptor to respond to change in exposure.
6
Under exposed
-mAs - kVp - RS -Chemistry
+ Grid ratio + SID
Correct exposure
Over exposed
+mAs + kVp + RS +Chemistry
- Grid ratio
7
- SID
Film-Screen and the Radiographer
• The radiographer immediately recognized
when an exposure error occurred.
• Viewbox and Repeat / Reject Analysis were
gate keepers for dose related issues.
Digital Imaging Systems
• The receptor and “processing” have changed.
• The radiographer’s responsibility to the patient
remains unchanged.
▪ Produce diagnostic images with minimal radiation
exposure.
• As Low As Reasonably Achievable
Response - OD
Dynamic Range
Film - Screen vs. Digital Detector
Exposure
10
Digital Systems
•
•
•
•
CR
DR
DDR
??????????
•
The acronyms are essentially
meaningless.
•
Cassette based
•
Cassette-less
•
Photostimulable Storage Phosphors
•
Flat Panel with Thin Film Transistor
•
•
▪
Amorphous Selenium
▪
Amorphous Silicon
▴ No scintillator required
▴ Requires a scintillator
Charged Couple Device
▪
Requires a scintillator
Complimentary Metal Oxide
Semiconductor
▪
Requires a scintillator
Saturation
Response
Dynamic Range
E++
E+
Eo
E-
Exposure
The ability of the receptor to respond to change in exposure.
Digital Systems
• Loss of visual cues
▪ Automatic rescaling
▪ Image processing
• Relationships between radiographic factors and image
appearance are decoupled.
▴ “Controlling” factors don’t have same impact on image.
mAs ≠ Density / brightness
 kVp ≠ Contrast

• Application of imaging physics foundation is more critical
13
How do you ensure ALARA with the digital
receptor you have now?
• Establish protocols with a clearly defined range of exposure indicators for
each exam. The exposure indicator values are then “audited” to ensure
technologist compliance with ALARA and image quality.
• Consider use of higher kVp levels in comparison to what was used with
film-screen.
• Consider use of a SID greater than 40”/101.6cm.
▪ 44”/111.76 cm
▪ 48”/121.92cm
• Replace older grid designs with grids made from materials that attenuate
less of the primary beam.
“Defined” Exposure Indicator
• The radiologist(s) should be asked to
determine when the noise level present in an
image prevents them from providing an
accurate interpretation of the image.
• Upon identifying that noise level, the range of
exposure indicator values may be established.
Effect of Automatic Rescaling
16
Dose and Image Noise are related
34 mR
More noise
67 mR
Less noise
“The American Association of Physicists in Medicine (AAPM), ………………..….common
exposure indices and deviation indices to be implemented across all digital
radiography detector types and across all manufacturers and vendors of such
equipment. The document explains a method for placing standardized exposure
information and content in the DICOM metadata in each image associated with
the imaging study. While the details are left to the interested reader [1, 2], it is the
manufacturer’s responsibility to calibrate the imaging detector according to a
detector-specific procedure, to provide methods to segment pertinent anatomical
information in the relevant image region and to generate an exposure index (EI)
that is linearly proportional to detector exposure.”
The standardized exposure index for digital radiography: an opportunity for optimization of radiation dose to the pediatric population
J. Anthony Seibert 1 and Richard L. Morin2
Pediatr Radiol. 2011 May; 41(5): 573–581.
PMCID: PMC3076558
Published online 2011 April 14. doi:
10.1007/s00247-010-1954-6
Higher kVp = Reduced Entrance Skin Exposure
• kVp values for film-screen imaging were
chosen based upon a “desired” level of
radiographic contrast.
• Image processing associated with digital
allows the creation of the image using higher
kVp values while letting us maintain a desired
image contrast.
kVp
• The following images were obtained using AEC and an
anthropomorphic phantom.
• The kVp was incrementally increased based upon the
generator controls and the AEC controlled the mAs value
used for each exposure.
• There was a 10% difference between the highest and
lowest exposure indicator.
• ESE mR values were obtained using an ionization chamber
device placed at the beam entrance level to the skin.
60 kVp
@ 7.4 mAs
42.5 mR ESE
64.5 kVp @ 5.67 mAs
36.9 mR ESE
18% Reduction in ESE
70 kVp @ 4.29 mAs
32.6 mR ESE
23% Reduction in ESE
81 kVp @ 2.74 mAs
27.3 mR ESE
35% Reduction in ESE
60 kVp
@ 7.4 mAs
42.5 mR ESE
81 kVp @ 2.74 mAs
27.3 mR ESE
35% Reduction in ESE
70 kV @ 15.2 mAs
138 mR ESE
81 kV @ 8.94 mAs
108 mR ESE
22% Reduction in ESE
70 kV @ 15.2 mAs
138 mR ESE
81 kV @ 8.94 mAs
108 mR ESE
22% Reduction in ESE
Table 3.
Mean effective dose data at varying kVp.
Projection
kVp
AP Pelvis
63
66 (standard technique)
70
73
77
81
85
90
96
Mean effective dose in mSv
0.78
0.6
0.51
0.42
0.32
0.28
0.25
0.22
0.27
Grondin, Y., et al.,(2004) DOSE-REDUCING STRATEGIES IN COMBINATION OFFERS SUBSTANTIAL POTENTIAL BENEFITS TO
FEMALES REQUIRING X-RAY EXAMINATION
Radiation Protection Dosimetry (2004), Vol. 108, No. 2, pp. 123---132
DOI: 10.1093/rpd/nch015
Exposure Field Size
• The judicious use of collimation results in
improved image quality. Why?
▪ Reduces the amount of scatter radiation produced
and therefore less scatter striking the image receptor.
• It also reduces the total volume of tissue
irradiated and therefore a reduction in exposure
to the patient.
A distinct lack of
“collimation”
Collimation
14” x 14” = 196 square inches = 18% less
14” x 17” = 238 square inches
11” x 14” = 154 square inches = 36% less
10” x 12” = 120 square inches = 50% less
• Somatic*
▪ Skin
▪ Bone Marrow (Red)
▴
▴
▴
▴
▴
▴
Skull
Sternum
Scapula
Pelvis
Vertebrae
Epiphyseal ends of long bones
SID and ESE
• Increasing SID could be used to reduce the
entrance skin exposure.
• Potential to increase spatial resolution.
• Possible issue with grid cut-off if focal range of
grid is not matched.
81 kVp @ 8.94 mAs
40” SID
108 mR ESE
81 kVp @ 11.0 mAs
48” SID
87.9 mR ESE
28% Reduction in ESE
Table 4.
Mean effective dose data at varying FFDs.
Projection
FFD in cm
AP Pelvis
Mean effective dose in mSv
100 (standard technique)
110
120
130
1.15
1.08
0.85
0.81
Grondin, Y., et al.,(2004) DOSE-REDUCING STRATEGIES IN COMBINATION OFFERS SUBSTANTIAL POTENTIAL BENEFITS TO
FEMALES REQUIRING X-RAY EXAMINATION
Radiation Protection Dosimetry (2004), Vol. 108, No. 2, pp. 123---132
DOI: 10.1093/rpd/nch015
Filtration
• The purpose of adding addition filtration is to
increase the overall energy of the beam with
the result being a decrease in patient entrance
skin exposure.
Copper Filtration
• The literature documents the potential for a
dose reduction of 15 to 35% with the addition
of copper filtration.
Anti - scatter grid
A core of very thin lead alloy foil strips, separated
by a radiolucent inter-space material of aluminum, cellulose
fiber, or carbon fiber. It is encased in a sturdy, but
radiolucent material. Possibly aluminum alloy or carbon fiber.
Protective
wrapper
Lead foil strip
Interspace
Grid Construction
• Newer fiber inter-space materials have the
potential to reduce exposure 10 to 40%.
Thank you