Download detector exposure indicator in ge x-ray systems

DEI – Detector Exposure Indicator
DETECTOR EXPOSURE INDICATOR
IN GE X-RAY SYSTEMS
Prepared by Sung Jeon, GE Healthcare
Abstract:
1.1.Introduction
The direct connection between the level of detector
exposure and optical density is well established
in film-screen radiography. However, that is not
the case in processed digital images because of
dynamic detector range and automatic image
processing that optimizes image display (Figure 1.)
Over- or under-exposure is not easily detectable in
digital imaging. Lack of visual cue for over-exposure
is more problematic: the radiographer tends to
attempt to achieve noiseless images in order to
satisfy the radiologist, and this tendency could lead
to unnecessary increases in patient exposure. This
is well known as “Dose Creep.” To help address such
behaviors, system manufacturers have provided
ways for the users to read and understand detector
exposure information.
Underexposure
Optimal
Overexposure
Digital
1.Background
Film
Detector exposure indicator (DEI) is a feature that
provides visual information for detector exposure in
digital X-ray images. The lack of visual/quantitative
information in digital X-ray images led several X-ray
system manufacturers to develop DEI feature on their
systems over the last few years. However, different
DEI calculation methods and reports by different
vendors often caused confusion to the users. In
recent years, efforts for DEI standardization have
been driven by the medical physics and radiology
communities. In response to standardization efforts,
the IEC developed and published the standard IEC
62494-1. In this whitepaper, we discuss the DEI
feature in GE X-ray systems using the terminologies
defined in IEC 62494-1. This whitepaper is intended to
provide essential information of the DEI feature in GE
X-ray systems for typical users as well as DEI design
details for advanced users/physicists.
Figure 1: Exposure level is easy to detect in Film Radiography (top row) while it is
difficult to detect in Digital Radiography (bottom row.)
1.2. DEI standardization
Over the past few years, different manufacturers,
including GE Healthcare, have come up with their
own methods for calculating and displaying detector
exposure indicator/index (DEI). This has been a
source of confusion among the end users. With
facilities having multi-vendor equipment, it became
a challenge to interpret among the DEI values of
different vendors and to have effective Quality
Control (QC) or Quality Assurance (QA) in place.
To address such diversities, the radiology community
acted on harmonizing detector exposure indicator in
the past few years: The International Electrotechnical
Commission (IEC) published the standard in 2008 [1.]
The American Association of Physicist in Medicine
(AAPM) published the report based on the efforts
driven by Task Group 116 in 2009 [2.] ImageGently
stakeholders agreed to adopt the IEC standard in
2010 [3.]
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DEI – Detector Exposure Indicator
In response to standardization efforts, GE Healthcare
introduced a new design for DEI starting from the
products released in year 2011 (e.g. Precision* 500D/
Proteus* WDR1 Upgrade, Optima* XR220amx, and
Discovery* XR656). The new design is intended to
provide detector entrance exposure information as
defined in IEC 62494-1.
This whitepaper describes the DEI feature in GE
Healthcare x-ray systems. Section 2 contains basic
information about the DEI feature, including the user
interface, available operation options and guidelines for
the users. Section 3 includes details about DEI design
and algorithm, which may be of interest to advanced
users or physicists.
Note that all references to DEI in the current paper
refer to the new DEI design observed in GE Healthcare
x-ray systems introduced since 2011, unless stated
otherwise.
DI is a relative indicator showing how far EI deviates
from EIT with the base 10 logarithmic scale. A scaling
factor of 10 is applied to calculate DI number. When EI
is close to EIT, DI becomes close to 0. When EI is 25%
higher than EIT, DI becomes 1. In other words, DI = 1
means that detector entrance exposure for the image
is 25% larger than the expected exposure index for the
selected anatomy and view. DI becomes -1 when EI is
20% lower than EIT. Similarly, DI becomes 3 when EI is
100% higher than EIT, and it becomes -3 when EI is 50%
lower than EIT.
2.3. Displaying DI values on GE Healthcare X-Ray Systems
The DI result for each exposure is displayed with a
graphical bar indicator as shown in Figure 2, and both
EI and DI are displayed in the annotation as well. By
default, DI ranges and limits for GE X-ray systems are
set as following:
• If -3 <= DI <=2: Display ‘Optimal Range’ text
message with green color in the bar indicator.
2. DEI – Typical Usage
2.1. Basic components
DEI design in GE X-ray systems provides detector
entrance exposure values in terminologies as defined
in IEC 62494-1 [1]. The following definitions of main DEI
components are excerpted from IEC 62494-1.
• Exposure Index (EI) – Measure of the detector
response to radiation in the relevant image region
of an image acquired with a digital X-ray imaging
system.
• Target Exposure Index (EIT) – Expected value of the
exposure index when exposing the X-ray image
receptor properly.
• Deviation Index (DI) – Number quantifying the
deviation of the actual exposure index from a target
exposure index.
2.2. Interpretation of DEI values
EI allows the operator to judge if an image was
taken at a detector exposure level suitable for the
intended level of image quality. EI is proportional to the
detector entrance exposure: If the entrance exposure
is increased, EI value is increased. The value of EI
divided by 100 is comparable to detector entrance
dose measurement in the unit of µGy at calibration
conditions.
Figure 2: Color Coded DI calculation result, DI limits, and text messages are displayed for
each acquired image and reprocessed image. The user has an option for not displaying
DI information at all or displaying numerical DI result only.
• If -3 > DI >=-5 or 2 < DI<= 4: Display ‘Acceptable
Range’ text message with yellow color in the bar
indicator.
• If DI <-5 or DI> 4: Display ‘Out of desired range’ text
message with orange color in the bar indicator.
The same DI ranges and limits are applied to all digital
images. IEC 62494-1 does not specify requirements
for displaying DI range and limit. AAPM Task Group
116 recommends 5 ranges with 4 limits in their report,
which is similar to the values used in GE X-ray systems,
but their initial recommendation was tighter than GE’s
default range and limit setting [2.]
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DEI – Detector Exposure Indicator
2.5. Adjusting EIT
DEI feature on GE X-ray systems is designed to allow
the user to set EIT values independently for different
combinations of anatomy, view, and patient size
(Adult/Pediatric). For each anatomy, view, and patient
size, there are two EIT values: Factory EIT and Custom
EIT as shown in Figure 4. The Custom EIT is the used for
DI calculation and is the adjusted value based upon
Factory EIT and EIT Adjustment factor. At installation,
Custom EIT value equals the Factory EIT value. Changing
the EIT Adjustment by 1.0 adjusts the Custom EIT by 1
Renard step. For a fixed exposure, mAs increase of 1
Renard step increases the mAs by approximately 25%.
Figure 3: The limits and text messages are configurable in the user preference
(Preference 6 DI 6 DI Control Limits.)
GE X-ray systems also provide an option to change
the limits and the corresponding messages in the user
preference (Figure 3.)
2.4. DEI export/DICOM
The DI export option, which can be found in Preferences
6 DI 6 DI Export Logs, allows the user with the ability
to retrieve DEI information onto a CD or USB flash drive
in the form of a CSV file. The exported CSV file lists DEI
information with acquisition time and date, relevant
X-ray techniques, selected anatomy/view/patient size
for past exposures. It can be useful to analyze trends of
X-ray techniques and the corresponding DEI values for
different patient anatomy/view/size. The information in
this file also may be useful to adjust EIT values.
For each acquired image and processed image, the
different DEI values are also stored in the DICOM tags.
The table below lists the details.
DICOM Tags
Description
0011,1036
Median (Anatomy) Count
0011,1037
DEI limit values
0018,1411
Exposure Index
0018,1412
(Compensated) Target Exposure Index
0018,1413
Deviation Index
0011, 106D
Uncompensated Detector Sensitivity
0011,1034
Compensated Detector Exposure
0018,6000
Compensated Detector Sensitivity
Figure 4: EIT values are listed in Preferences 6 DI 6 DI-Preferences. For each anatomy/
view/patient size, ‘Factory Target EI’ is a reference, and ‘Custom Target EI,’ which is equal
to ‘Factory Target EI’ by default, can be modified by changing the ‘Target EI Adjustment’
factor. In the example above, indicated by the square box, the EIT Adjustment was set to
(-2) for Abdomen/antero posterior/pediatric, making Custom EIT approximately 2 Renard
steps lower than the Factory EIT for Abdomen/antero posterior/pediatric exposures.
2.6. Steps for customizing DEI
Users should not rely on the DI result alone in order
to determine appropriate X-ray techniques. It is
also suggested not to modify Custom EIT as a first
mitigation step when DI value deviates from the
desired range based on one or two exams. The
tradeoff of IQ and dose should be the dominant
factor to determine appropriate EIT and proper X-ray
technique based on each site’s preference. Advanced
users and physicists who understand DEI and the IQ/
Dose tradeoff should be involved in making Custom
EIT modification decisions.
Below are general guidelines for adjusting X-ray
techniques and/or EIT when necessary.
Table 1: Relevant DICOM tags for the DEI feature.
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DEI – Detector Exposure Indicator
Step 1: Confirm that detector calibrations were recently
completed by the local field engineer. Poor or out of
date calibration results can adversely impact EI and
DI accuracy.
Step 2: Make sure that imaging conditions are
appropriate and do not deviate from the nominal. For
example, check following factors when EI/DI values
are unusual.
• Collimation and shuttering.
• Unusual body habitus.
• Presence of prosthetics.
• Shielding material in the imaging region of interest.
2.7. Impact of re-shuttering on EI/DI
EI/DI are calculated immediately after image
acquisition and after every instance of image
reprocessing. After a reshuttering operation, EI/DI
recalculation is essential to account for any change
in the relevant image region (Figure 5). If re-shuttering
the image changes the extent of anatomical content
in shuttered region of image, then the value of
interest used in the DEI algorithm could also change,
thus affecting the EI/DI values. Users should note
that the new EI/DI values are generated only after
the image reprocessing operation is fully complete,
which happens only when the user selects the image
reprocess button after adjusting the shutter position.
• AEC functionality and patient positioning relative to
enabled ion chamber cells.
Step 3: If image quality and/or patient entrance dose
are within user’s expectation, but DI is beyond desired
range for the selected exposure techniques, consider
the following:
• When DI is higher than 0, increase EIT Adjustment in
order to set the Custom EIT higher than the Factory EIT.
• When DI is lower than 0, decrease EIT Adjustment in
order to set the Custom EIT lower than Factory EIT.
• It’s not advisable to make a change based on
only a few results. When DI consistently deviates
from default EIT, the users may consider modifying
Custom EIT.
Step 4: If image quality and/or patient entrance dose
do not meet the expectation:
• When assessing image quality, always ensure
that Window Width/Window Level (WW/WL) is
set correctly.
• Adjust kVp, mAs, and/or speed in AEC mode if
necessary to achieve desired image quality and
patient entrance dose.
Figure 5: (1) Processed image after data acquisition and (2) Re-processed image after
manual shuttering. Both of them share identical raw data. EI and DI values were changed
because manual shuttering changed the median anatomy count.
Note: the date and time and institution name on these two images are blocked.
2.8. Benefits of the DEI feature
Some benefits and practical usages of DEI information
are listed below.
• If DI is still out of range although image quality is
acceptable, customize EIT as described in Step 3.
• DEI features provides appropriate detector
exposure level in clinical imaging, which is not easily
detectable in digital radiographic images.
Depending on the system type, patient entrance dose
or DAP is available from direct DAP meter reading or
Dose/DAP calculation based on the x-ray techniques.
• In Fixed mode, EI/DI values can be monitored to
minimize the tendency to increase exposure over
time (prevent ‘dose creep’.)
• In AEC mode, variation in EI/DI values can be
monitored as indicators of patient and ion chamber
positioning or collimation problems.
• DEI feature enables technologists to monitor
patient-specific detector exposure levels and to
maintain consistent IQ with minimum dose.
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DEI – Detector Exposure Indicator
Recent studies and investigations performed by
different research groups [4-8,] have strengthened
the case that DEI can be a useful tool for QA/QC in
X-ray digital images.
3.DEI design details – for advanced
users/physicists
2.9.Summary
Following factors impacts on DEI calculation:
DEI is designed to provide detector entrance
exposure information in digital radiography where
under- or over-exposure is not obvious from the
digital image due to the wide dynamic range of the
digital detector and automatic image processing.
Harmonized DEI terminologies found in recent GE
X-ray systems would help reduce the potential
confusion for utilizing DEI information.
• X-ray techniques including filter/grid status
DEI is different from and cannot replace patient
entrance dose, which is provided by the DAP meter
or Dose/DAP calculation in GE Healthcare systems.
The relationship between patient entrance dose and
detector entrance dose depends on many variables
including patient thickness, kV, collimation, etc. X-ray
beam quality is different before and after passing
through the patient and thereby DEI doesn’t directly
convey dose to the patient.
EI is proportional to detector entrance exposure, and
EI divided by 100 reflects detector entrance dose
in the unit of µGy at calibration conditions. DI is to
calculate the deviation of EI from EIT. DI is a metric for
understanding appropriateness of the exposure for
each anatomy/view/patient size. It’s important
to understand that EI or DI is not the indicator for
image quality.
Appropriateness of the exposure based on DI number
comes with the assumption that EIT is properly set for
each anatomy/view/patient size although the default
EIT may not be optimized. Each site may consider
adjusting EIT if DI is consistently not in desired ranges
despite acceptable image quality. Recent studies by
different research groups have also discussed the
challenges and the importance of EIT optimization [3-5.]
3.1. Algorithm overview
• Collimation and Electronic Shuttering
• Raw data (image)
• Detector calibration results
• Anatomy segmentation performance
• Pre-defined scaling factors addressing X-ray
technique variation compared to the calibration
(near-RQA5) condition
The figure below illustrates DEI calculation workflow,
which is executed for each exposure.
Figure 6: DEI calculation workflow diagram. Values in bold are recorded in the DICOM
tags as listed in Table 1.
3.2. Raw Image/Raw data in GE X-ray systems
DEI calculation is always done using the original data,
not the processed data. Per IEC 62494-1, data type
definitions are as following [1]:
• Raw data – pixel values read directly after the
analogue-digital-conversion from the digital X-ray
imaging device without any software corrections.
• Original data – Raw data to which the allowed
corrections are applied. Allowed corrections include
bad pixel corrections, flat-field correction (gain
and offset corrections), and geometrical distortion
correction.
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DEI – Detector Exposure Indicator
In GE documents (e.g. service manual, operator
manual), the terminology ‘raw data’ is in fact referring
to the original data defined in IEC 62494-1. In this
document, raw and original data are not differentiated,
and both of them refer to original data defined in IEC
62494-1.
3.3. Details for Exposure Index (EI)
(1)
According to IEC 62494-1, EI is related to the value of
interest V:
(2)
Figure 7: (1) Anthropomorphic phantom image and (2) Output of anatomy segmentation
algorithm. The black area in the subsampled image is considered as the relevant
anatomic image region for DEI calculation.
EI=C0∙g(V)
where C0=100µGy-1 and g(V) is an equipment-specific
inverse calibration function. The value of interest is
defined as the central tendency of the original data in
the relevant image region [1.]
The inverse calibration function in GE Healthcare
systems is proportional to the median image count
in the raw data, the value of interest V, and inversely
proportional to the uncompensated detector
sensitivity (counts/µGy.) Hence, the above equation is
implemented as following on the GE Healthcare system.
EI=100 [µGy-1]∙
Median Count [counts]
Detector sensitivity
counts
µGy
The original data comes from the image read from
the digital detector after bad pixel, offset, and gain
correction and is available to the user. The relevant
image region in GE Healthcare systems is defined as
the anatomic region under all clinical conditions. The
anatomic region is identified from image processing by
removing the collimator blades and the areas of raw
radiation. An example of anatomic region identification
is shown in Figure 7. The detector calibration procedure
determines the value of conversion factor (CF,
electrons/X-ray photon,) which is used to calculate the
detector sensitivity value. Hence, proper detector CF
calibration is vital in order to achieve accurate
EI values.
As the count in the raw data is linear to the amount of
exposure, EI shows direct linear relationship with Air
Kerma measurement. At calibration conditions, the
relationship between EI/100 and measured dose in the
unit of µGy is one-to-one. Figure 8 demonstrates the
relationship between Air Kerma measurement and EI. In
the test, EI accuracy was below the specified accuracy
limit (20%) in IEC 62494-1 at calibration conditions.
Figure 8: Calculated EI vs Direct Detector Air Kerma measurement. Detector Air Kerma
was measured at the center of the detector position using a dosimeter (RadCal 9010
with 6 cc probe.) (1) Linear relationship is apparent between EI and Detector Air Kerma
measurement. (2) The error is calculated using the formula 100*(Air Kerma –EI/100)/
Air Kerma, and it is done for different mAs. The errors are below the accuracy limit
(20%) specified in IEC 62494-1. The measurement was done in Discovery XR656 Digital
Cassette mode with 20 mmAl phantom, SID = 100 cm, No Copper filter, No Grid, 80 kVp,
41cmx41cm collimator opening, and DI technical mode.
Note that the result in Figure 8 was obtained with
‘DI Technical Mode’ with 20 mmAl flat field phantom
in the X-ray beam and fully open collimation FOV. EI
in clinical conditions does not necessarily equal the
detector entrance dose due to differences in beam
hardening and scattering. It is important to set the
detector Air Kerma measurement condition equal
to the detector EI measurement condition when
evaluating EI performance – e.g. (1) Position of the
dosimeter probe should be at the center of the image
plane, (2) The dosimeter probe should be located at
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DEI – Detector Exposure Indicator
the same source-to-image distance (SID) compared to
the detector: otherwise, the distance ratio adjustment
(R-squared correction) may also be considered if the
probe cannot be placed at the detector position, (3)
Attenuation, scattering and backscattering condition
for dose measurement should be set equivalent to
those for the detector, and (4) The acceptance angle of
the dosimeter probe should be similar to the one for
the detector.
Proper collimation is an essential step to help minimize
- dose to patient, and it also helps identifying the
relevant image region properly. Incorrect collimation
may lead to improper anatomy segmentation, and it
may result in biased EI and DI.
3.4. Details for Target Exposure Index
At system installation, Custom EIT is equal to Factory EIT.
These default EIT values were determined empirically
based on statistical data analysis of median count
values for different anatomy and view combinations
in a fixed radiographic system. During the design
phase, the data used for setting EIT in most cases
were acquired with in AEC mode. Each anatomy has
different EIT values, because expected median count
values and contrast are different for each anatomy.
For example, EIT for chest AP/PA is different from that
for abdomen because the radiographic contrast in the
anatomical area of the chest image is different from
that of abdomen. The contrast is important because
the location of the ion chambers used for AEC is in the
lung area, but a large fraction of the image has lower
signal (Spine, heart, diaphragm.)
The Factory EIT should not be considered as the gold
standard target value in all circumstance for following
reasons: (1) Factory EIT does not address variability in
system usage or patient size/position because limited
data were used for the analysis. (2) Although the system
lets the user set different EIT depending on patient size,
Factory EIT values for pediatric mode are currently
equal to those for adult mode. Since the exposure level
for pediatric is generally lower than the one for the
adult, EIT for pediatric should ideally be lower than EIT
for adult. (3) Factory EIT values for the mobile system
(Optima XR220amx) were derived from those for the
fixed system while the factory X-ray technique setting
for each anatomy and view are not same between the
mobile and the fixed system.
GE Healthcare continues to collect clinical feedback
and perform ongoing experimental analysis to monitor
and to evaluate the performance of the DEI feature.
This ongoing effort will be continually used to provide
improved Factory EIT for the users.
3.5. Details for Deviation Index
DI in GE X-ray systems is defined as the following:
where EIT’ (Compensated EIT) is based on Custom EIT
shown in Figure 4 and accounts for kVp, Cu filter, grid,
and receptor difference between actual exposure and
the condition when setting the default EIT.
EIT’ and Custom EIT can be different depending on X-ray
techniques being used. EIT’ is saved in DICOM header
as described in Table 1 and the DI Export log file. The
relationship between EIT and EIT’ can be expressed as
the following.
Film equivalent speeds other than 400 will impact
EIT’. Details for the speed factor are discussed in the
next two sections. kVp, Grid, Cu filter, and receptor
changes compared to the condition for default EIT also
impact EIT’. Their contributions were predetermined
and stored in look-up tables based on laboratory
tests with different thickness of a PMMA (Poly Methyl
MethAcrylate) phantom, also known as Plexiglas, at
different kVp.
3.6. Speed setting for X-ray systems
‘Speed’ should be used and understood differently
between Film and CR/DR. This section provides
clarification on the use of ‘Speed’ on digital radiography
systems.
‘Speed’ for Film
• Speed is a characteristic of the film/screen system
and is inversely proportional to the exposure to
the film/screen required to reach a specific optical
density.
• In AEC mode, exposure to the film/screen is
controlled by the system based on clinical
protocols.
• In non-AEC mode, a clinically relevant exposure for
each application (based on anatomy/view) requires
the correct film/screen combination to be selected.
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‘Speed’ for Digital Systems
In “Fixed” exposure mode:
• On a digital system, equivalent film speed is NOT
related to specific image brightness. Post image
processing algorithms determine brightness and
contrast values for display.
• When the speed is increased, EI doesn’t change, but
EIT’ is scaled down inversely proportional to speed.
As a result, the calculated DI increases for the same
fixed exposure.
• Speed in digital X-Ray is not determined by the
sensitivity of the digital detector.
• When the speed is decreased, EI doesn’t change,
but EIT’ is scaled up inversely proportional to speed.
As a result, the calculated DI decreases for the
same fixed exposure.
• In AEC mode, exposure to the detector is controlled
by the system based on clinical protocols.
–– Different clinical protocols are required to meet
different imaging needs. Speed is considered a
metric for the exposure and is included in each
clinical protocol.
–– High Speed: pediatric application requiring
low dose.
–– Low Speed: higher signal-to-noise ratio, higher
dose imaging applications.
• In non-AEC mode, a clinically relevant exposure
for each application (based on anatomy and view)
is determined by the specific X-ray techniques in
protocols not by the speed.
3.7. Impact of speed change on DI calculation
The amount of exposure is controlled by the equivalent
film speed setting in AEC mode while it is manually
controlled by mA and exposure time in Fixed exposure
technique. In both AEC and Fixed mode, equivalent
film speed is used as a factor adjusting the expected
exposure level. The DI calculation result will be different
between AEC and Fixed mode depending on the speed
setting because speed change does not contribute to
exposure change in Fixed mode.
In AEC mode:
• When the speed is increased, EI becomes lower
as the amount of exposure decreases, and EIT’ is
scaled down inversely proportional to speed. As a
result, the calculated DI is constant.
• When the speed is decreased, EI becomes higher as
the amount of exposure increases, and EIT’ is scaled
up inversely proportional to speed. As a result, the
calculated DI is constant.
3.8. DEI (DI) Technical Mode
DEI (DI) Technical Mode option is provided in DI
preferences (a checkbox shown in Figure 4.) Once it is
enabled, the anatomic segmentation step is disabled,
and the region of interest (ROI) used is the central
rectangular region (512x512 pixels. Median count
is obtained from the fixed ROI. A central ROI is also
used for the CF calibration. CF calibration impacts the
detector sensitivity and thereby EI/DI calculation result.
The technical mode can be useful for evaluating DEI
performance since it will help eliminate the potential
variability introduced by the anatomy segmentation,
but it shouldn’t be used for regular clinical exposures
for the same reason.
3.9. Compensated Detector Exposure (CDExp)
CDExp is not defined in IEC 62494-1. It has been used to
display detector exposure information in the previous
DEI feature and is carried forward into the new DEI
feature. To calculate CDExp, the median count in the
identified anatomy is converted to exposure in μGy
based on the compensated detector sensitivity, which
is the adjusted value of uncompensated detector
sensitivity based on receptor type, kVp, Grid, and Cu
filtration changes.
Unlike EI, CDExp reports detector entrance exposure
in simulated clinical conditions. In order to simulate
patient attenuation, a set of Plexiglas has been used
as attenuation material resulting in a different X-ray
spectral attenuation than RQA5 beam condition.
However, a set of Plexiglas still cannot account for
expected variations in actual patient thicknesses and
body compositions. Depending on patient size and
type, CDExp may over- or underestimate detector
entrance exposure. CDExp, Although the units of CDExp
are Gray, it should not be considered as an absolute
detector entrance exposure but as another indicator for
the detector exposure.
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To demonstrate the difference between EI and CDExp, test results are presented in Table 2 EI represents Detector
Air Kerma at the calibration condition whereas CDExp closely represents detector Air Kerma at the ‘simulated’
clinical imaging condition. In the table below, actual detector Air Kerma is close to reported EI/100 with 20 mm
Al plate (GE flat-field phantom.) Air Kerma is close to CDExp with Plexiglas. Depending on the systems’ calibration
status and measurement setup, the test results will vary. It’s important to understand that EI relates to Air Kerma
at a hard beam (near RQA5) condition realized with a 20 mm Al plate, and CDExp relates to Air Kerma at a soft
beam condition realized with a set of Plexiglas.
X-ray technique
20 mm Al Plate
4.75 inches Plexiglas
kV
mAs
EI
CDExp [µGy]
Air Kerma
[µGy]
EI
CDExp [µGy]
Air Kerma
[µGy]
80
1
377
7.7
3.64
466
9.52
9.44
2
769
15.69
7.4
948
19.36
19.15
Table 2: Comparison among Air Kerma, EI, and CDExp. The test was done in Digital Cassette mode without grid.
Note: As more manufacturers follow the IEC 62494-1
standard for expressing detector entrance exposure
and as more customers interpret detector entrance
exposure using EI/DI from the IEC 62494-1 standard,
CDExp won’t play a significant role for delivering
detector exposure information. In some instances, the
customers are confused for understanding detector
exposure information in the context of CDExp along
with EI/DI. Therefore, to minimize such confusion and to
simplify DEI information, DEI feature on future systems
may not include CDExp on the image display. CDExp
value will continue to be available in DICOM headers
and the DI log export file.
3.10.Difference with previous DEI design – for the
users who are familiar with previous DEI design
Prior to introducing new DEI design, GEHC X-ray
systems released earlier than year 2010 (e.g. Definium
5000, Definium 6000, Definium 8000, Discovery XR650,
Optima XR640, Definium AMX 700) displayed three DEI
values characterizing detector entrance exposure.
• Uncompensated Detector Exposure (UDExp):
Median count value in the anatomy converted to
exposure in μGy based on the detector sensitivity
in counts/μGy at 80 kVp, no Cu filtration. And no
grid. It includes a factor to correct to a Soft Beam
condition.
• Detector Exposure Index (DEI): The raw count value
in the segmented anatomy divided by the expected
count value for the given acquisition parameters
(Speed setting, kVp, Cu, Grid). The expected count
value is specific for certain thickness of Plexiglas
(e.g. 5 inches for 80 kVp.) Ideal DEI (index) ranges
vary depending on anatomy/view.
• Compensated Detector Exposure (CDExp): Same
definition with new DEI design.
UDExp in prior DEI design is replaced by EI in new DEI
design. The difference is that UDExp characterizes
detector entrance exposure in soft beam condition (5
inches of Plexiglas) whereas EI characterizes detector
entrance exposure in hard beam condition (calibration
condition.) In addition, the unit for UDExp is μGy while EI
is unitless and equal to 100 times μGy.
Detector exposure index (displayed as ‘DEI’ on the
monitor) in previous DEI design is replaced by DI in the
new DEI design. Detector exposure index is the ratio
of measured count divided by the expected median
count. Expected median count is derived at specific
conditions (different thickness of Plexiglas for different
kVp) while raw median count is calculated for each
clinical exposure. Therefore, optimal detector exposure
index for each anatomy is different while the optimal DI
is fixed. To understand the optimal detector exposure
in the context of detector exposure index, the user
must know the reference ranges for detector exposure
index, which were predetermined based on empirical
data and can be configured by users. For example, the
factory default detector exposure index range for chest
PA is set to 0.2-0.6 and the one for Abdomen is set to
0.56-1.68. The chart below summarizes the difference
between Detector exposure index in the previous
design and DI in new design.
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Data subject to change without notice
9
DEI – Detector Exposure Indicator
Property
Detector exposure index in old design
DI in new design
Expected Count or EIT
(denominator in index or DI
calculation) per Anatomy/View
Fixed
Varies and configurable
Optimal outcome (score) per
Anatomy/View
Dependent
Independent
Limit (range) per Anatomy/View
Varies and configurable
Constant
Number of ranges
3
5
Linearity with dose
Linear
Log with the base 10
Table 3: Comparison between DEI (Index) in previous design and DI in the new design.
GE Healthcare Proprietary – Do not redistribute the document
Data subject to change without notice
10
4.References:
1.
‘Medical electrical equipment – Exposure Index of digital X-ray
imaging systems – Part 1: Definitions and requirements for
general radiography,’ IEC 62494-1 Ed 1.0 2008-08
2.
5.
S. Jeff Shepard et al, ‘An Exposure Indicator for digital
radiography: AAPM Task Group 116 (Executive Summary),’ Med.
Phys. 36(7), July 2009, p2898-2914
J. Anthony Seibert et al, ‘The standardized exposure index for
digital radiography: an opportunity for optimization of radiation
dose to the pediatric population,’ Pediatr Radiol, 841, 2011, p573581
6.
3.
Quentin Moore et al, ‘Image Gently: using Exposure Indicators To
Improve Pediatric Digital Radiography,’ Radio Tech, 84(1), Sep/
Oct 2012, p93-99
Steven Don et al, ‘New Exposure Indicators for Digital
Radiography Simplified for Radiologists and Technologists,’ AJR,
199, Dec 2012, p1337-1341
7.
4.
Menglong Zhang et al, ‘Dose optimization for different medical
imaging tasks from exposure index, exposure control factor, and
mAs in Digital Radiography,’ Health Physics, 103(3), Sep 2012,
p235-240
H Warren-Forward et al, ‘An assessment of exposure indices in
computed radiography for the posterior-anterior chest and the
lateral lumbar spine,’ The British Journal of Radiology, 80, Jan
2007, p26-31
8.
M.L. Butler et al, ‘Are exposure index values consistent in
clinical practice? A multi-manufacturer investigation,’ Radiation
Protection Dosimetry, 2010, p1-4
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