PerkinElmer white paper evaluating XRpad flat panel X-ray detectors for security applications

1. Authors:
Patrick Talla
Siegfried Arnold
PerkinElmer Technologies GmbH & Co KG
Walluf, Germany
WHITE
PAPER
Evaluation of PerkinElmer XRpad Flat Panel
Detector for Security Applications
Abstract
The first generation of XRpad Flat Panel Detectors (FPDs), introduced by
PerkinElmer in 2013 have primarily been used in diagnostics applications.
Leveraging their 100 µm pixel resolution, and high level of image quality,
they are also well suited for the field of security inspection. The detectors
are lightweight, battery powered, and have wireless communication. This
enables a fast setup to perform remote scans in the case of real threats
such as inspection of unattended luggage at various transportation hubs.
In this paper, we study the usability of the XRpad FPDs for security
applications. We investigate image quality at low radiation dose, the
level of radiation hardness, and performance at operational temperatures
ranging from -20 °C to 70 °C. The results indicate that the XRpad detector
family, due to its outstanding image quality, fast image acquisition
capabilities, mobility, and ease of integration is well suitable for the security
application, and can provide additional safety to security professionals.
Introduction
Over the past 15 years, ensuring security has become an important task,
primarily due to the threat of terrorism. Safeguarding people and their
belongings is a major challenge for governments everywhere. As a result,
digital X-ray radiography has emerged as a technique which is used by
security experts to examine suspicious packages at various locations.
Fig. 1 shows a metal pipe with electronic components inside. This X-ray
image benefits from being acquired by a detector with high spatial and
contrast resolution, which allows for a clear identification of the content.
The first generation of XRpad flat panel detectors (FPDs), introduced by
PerkinElmer in 2013, has been extensively used in medical and veterinary
applications. Benefitting from a 100 µm pixel pitch, the XRpad detectors
have excellent resolution and overall outstanding image
quality, which makes them good candidates for security
applications. This study investigates these detectors’
imaging performance, their level of radiation hardness,
and extended operating temperature range, to further
explore their suitability for security applications.
XRpad Overview
This section presents the main features and operating
modes of the XRpad detector. Parameters related
to image quality, such as resolution and detective
quantum efficiency, are also addressed.
Features and Operating Modes
The PerkinElmer XRpad detectors are wireless and wired,
lightweight cassette sized FPDs for digital radiography.
Featuring a best-in-class 100 μm pixel pitch, and direct
deposition CsI or an optimized Gadox scintillator, the
XRpad detector series provides exceptional image quality.
The XRpad detectors are available in three imaging sizes,
featuring an imaging area of 43 × 43 cm2
, 43 × 35 cm2
or 30 × 25 cm2
. Figure 2 shows a photograph of the
XRpad detector in imaging size of 43 × 35 cm2
.
Figure 1. X-ray image of pipe with electronics.
Figure 2. XRpad 4336 detector with an imaging size of 43 × 35 cm2
.

2. Different synchronization modes simplify the integration
with various X-ray sources. Both a triggered Data Delivery on
Demand (DDD) mode and an Automatic Exposure Detection
(AED) mode are available for use with various X-ray systems,
including battery operated and continuous or pulsed sources.
The XRpad offers selectable gain settings which can be used to
optimize image quality at different X-ray exposure levels. It offers
both rapid imaging to provide a quick view from single exposures,
as well as the capability to integrate a number of X-ray pulses, or
several seconds of continuous X-ray. This wide range of application
modes provides significant benefits for security inspection.
The XRpad detector can operate in a wireless Infrastructure
Network where it connects through a WiFi Access Point (router)
to a host computer, or it can operate as a Wireless AP itself
connecting directly to the host. The Infrastructure Network
offers advantages in expanding the data transfer distance,
while the direct AP Network is easier to set up in a portable
application with a direct connection to a notebook or tablet
computer. The data tranfer can also be performed through a
wired Ethernet connection while the XRpad is battery powered.
Resolution of the XRpad Detector
A line-pair phantom (Nuclear Associates Model 07-501 was
imaged to demonstrate the resolution capability of the XRpad
detector.1
The image was acquired with the following X–ray
tube settings and geometry: 120 kVp, 150 mA, 10 ms, a 180
cm source-to-image distance, and a 0.6 mm tube focal spot with
a 21 mm Al filter. Offset and gain corrections were applied. The
profile plots are along the two red lines drawn in the image as
shown on Fig. 3 (left). Line pairs at 4.86 lp/mm can be clearly
resolved using the XRpad detector (Figure 3) (right)).
Detective Quantum Efficiency (DQE)
The Detective Quantum Efficiency (DQE) is a measure of the
efficiency with which a detector passes signal to noise ratio from
its input to its output. DQE is the principal objective measurement
used to determine imaging performance.
Figure 4 compares the DQE of the XRpad to three other
commercially available FPDs.2
The DQE of the XRpad detector
is the highest at all spatial frequencies. In particular at the
2
higher spatial frequency of 3 cycles/mm, the DQE of the XRpad is
approximately 80 % higher than the others. This implies that in
comparison the dose to the XRpad can be reduced by up to 45%
without compromising image quality. The DQE data indicate that
the XRpad even at low dose levels, may be used in single rapidly
acquired overview images, offers good imaging performance,
and provides an additional safety factor for the security inspector.
Further, in this comparison, it is the only detector with a limiting
resolution of 5 cycles/mm. As a result, very fine structures can be
imaged and identified better using the XRpad, supporting the
primary aim of mobile security inspection.
Radiation Hardness of the XRpad Detector
As the XRpad is a lightweight portable FPD, it does not have
internal shielding of its electronics. Hence, it is particularly
important to determine the level of radiation hardness of the
detector to have an estimate of the lifetime of the device when
it is used for mobile security.
For this study, an accelerated radiation test is performed by applying a
dose of approximately 1 kGy to the XRpad detector. This is achieved
by using a standard industrial continuous X-ray tube at a source-to-
imager distance of 110 cm, at 225 kVp, 5.3 mA for about 24 hours.
Figure 5 (left) shows a picture of the detector used in the experiment.
Two 23.3 cm × 8.5 cm, 6 mm thick tungsten copper shielding
plates are placed on top of the detector to provide a reference
non-irradiated region. The plates are positioned in a region of the
detector that does not contain internal circuit board electronics.
Figure 3. Resolution of the XRpad detector: (Left) line-pair phantom, (Right) Normalized
profiles plots showing that a resolution of 5 lp/mm can be achieved with the XRpad.
Figure 4. DQE curves for different commercially available FPDs.
Figure 5. (Left) Picture of the detector used for the degradation test. (Right) X-ray image
acquired with the XRpad after exposure at 225 kV and 5.3 mA to indicate ROIs defined in
the shielded and unshielded region of the detector.

3. 3
Temperature Study
In security applications, the FPD may be used under various
ambient conditions, and we therefore study the impact of
temperature on detector performance. In this test, the XRpad is
placed in a thermal chamber, while being powered continuously
by an external power supply. The temperature within the
thermal chamber is varied from -20 °C to +70 °C. A temperature
cycle with plateaus at -20°C, -10 °C, +20 °C, +30 °C, +40 °C,
+50 °C, +60 °C, and +70 °C was started and the detector
remained for several hours at each of the levels. The total
duration of the test was about 148 hours. The impact on
reliability due to high temperature was not part of this study,
but no failures occurred.
Impact of Temperature on Image Quality
No functional issues were observed while operating the detector
over this expanded temperature range. Images were acquired
using internal triggering with an integration time of 400 ms.
During the test, a dark image was stored every 10 minutes, but
only images captured at the temperature plateaus were used to
analyze the influence of temperature on the dark signal.
Figure 8 shows the median dark signal versus temperature. The
error bars represent one standard deviation. For temperatures
below 50 °C the median dark signal varies by less than 5%. At
60 °C and 70 °C, the dark signal default gain setting increases
from 2000 counts to 2500 and 3500, respectively, which is still
negligible when compared to the 16 bit, 65535 maximum level.
Figure 7. (Left) Offset and gain corrected test image. (Right) Profile of pixels counts after
multiple rows averaging.
The applied 1 kGy dose corresponds to an estimated cumulative
use duration of more than seven years, when portable battery
powered X-ray sources such as the GemX-160 and GemX-200
from X-RIS3
or XR200 and XR-3 from Golden Engineering4
are
used, assuming that 80 radiographs with an average exposure
time of five seconds are acquired per day.
Impact of Radiation on the Dark Signal of the Detector
Figure 5 (right) shows an X-ray image obtained after exposure
with the previously described X-ray conditions. For data analysis,
two ROIs are defined: one in the unshielded region of the
detector, and another one in the shielded region.
In order to study the change in dark current due to radiation,
the dark image level (retrieved from the associated ROI) for the
shielded and unshielded regions of the detector are plotted.
Figure 6 shows data points representing the median dark image
value with error bars of one standard deviation as a function
of exposure level. The dark signal value versus dose follows the
same trend for both regions. The maximum deviation observed
is approximately 1.5% when comparing signal levels for both
regions at the same dose. The overall variation in dark signal is
attributed to temperature variations in the X-ray chamber, which
was not thermally controlled.
Impact of Radiation on the Detector Uniformity
Following the 1 kGy exposure, a test measurement (RQA5
spectrum) is performed to evaluate the uniformity of the detector.
Figure 7 (left) shows a region of interest (ROI) of an offset and
gain corrected test image (Exposure time of 400 ms) at half
gain intensity, acquired post radiation and after removing the
copper plates (Figure 5, left). The red vertical arrow in the image
illustrates the limit between the region (previously shielded
by the copper plates) and the unshielded region. A ROI (red
band) encompassing multiple rows of pixels is drawn from the
shielded to the unshielded region. Figure 5 (right) displays the
distribution of the pixel counts after averaging multiple rows of
pixels. The plot (Figure 7, right) shows no trend indicating a drop
or degradation of uniformity due to radiation. The variation of
median values in the shielded and unshielded region is about
0.2 %. This indicates that the XRpad has no significant
degradation of the imaging sensor following the 1kGy exposure.
Figure 6. Dark signal as a function of dose for shielded and unshielded regions.
Figure 8. Median dark signal as a function of temperature.

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Conclusion
This two-part study leads to the following main conclusions:
1. The XRpad detector meets a seven year radiation hardness level with
no detrimental impact on image quality, assuming 80 daily, five second
exposures with a portable battery powered x-ray source .3, 4
2. The XRpad detector has a stable response over a wide
temperature range from -20 °C to +60 °C, and maintains a
dynamic range in excess of 80 dB.
Results from this study demonstrate the suitability of PerkinElmer’s
XRpad X-ray detectors for security applications. The XRpad
provides reliability and robustness at a wide temperature range.
The outstanding image quality and high resolution, fast image
acquisition, mobility and ease of integration enable security X-ray
systems manufacturers to optimize their products, and to enhance
the safety of the task force in the field.
Acknowledgment
The authors would like to thank M. Stockerl, J. Thumerer, L. Lars
and F. Huang for their support.
References
1. Nuclear Associates, Carle Place, NY
2. Development and Evaluation of 2D and 3D Image Quality
Metrics S.N. Murphy, Duke U. (2011)
3. XRIS: http://www.xris.eu
4. Golden Engineering: http://www.goldenengineering.com
Dynamic range of the detector is computed from the acquired
dark images as the ratio of the difference between the maximum
and measured median dark signal level and the median standard
deviation of the dark signal of 10 image frames. Figure 9 is a plot
of the dynamic range of the XRpad as a function of temperature.
For temperatures below 30 °C, the dynamic range exceeds 82 dB.
Between 30 °C and 60 °C, the dynamic range is reduced by less
than 2 dB, and at 70 °C it is reduced by an addition 1 dB.
In order to reduce the variability in dark signal and maintain
high dynamic range, it is advised to use the detector at ambient
temperatures below 60 °C, and to restrict prolonged high
temperature conditions. Further, in order to get the best image
performance, a matching dark image should be obtained with
each X-ray acquisition. For temperatures below 0 °C one must be
ensured that condensation within the detector does not occur.
Figure 9. Dynamic range of the XRpad as a function of temperature.
Unless otherwise specified, PerkinElmer Flat Panel X-ray Detectors are components intended to be integrated into products by X-ray system manufacturers.
System manufacturers are responsible for qualifying and validating their products for their intended uses and meeting all applicable regulatory requirements.
Contents in this document are subject to change without notice.
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