Photon Factory Activity Report 2016 #34 (2017)

BL-14C/2013G196

An attempt to measure the linearity of diagnostic flat-panel detectors
with monochromatic X-rays
Chisato Kimura 1,*, Toshiaki Yamanouchi 2, Youko Saigou 3, Kazuyuki Hyodo 4
1
Teikyo University, Kaga, Itabashi-ku, Tokyo 173-8605, Japan
2
Kanagawa Institute of Technology, Atsugi, Knagawa 243-0292, Japan
3
Teikyo University Hospital, Kaga, Itabashi-ku, Tokyo 173-8606, Japan
4
KEK-PF, Tsukuba, Ibaraki 305-0801,Japan

1 Introduction
At present, diagnostic flat-panel detectors (FPDs) areclinically used on the premise that exposure doses andpixel values form a linear relationship with conventional
diagnostic X-ray photography. Radiographs are therefore
taken in conditions where X-ray input/output
characteristics of FPDs are stable.
In the generation of diagnostic images with FPDs, images
are often processed to suit a specific diagnostic purpose.
Image processing often involves gradation processing, inwhich the tone varies, and it is questionable whether thelinear input/output relationship between conventional
diagnostic X-rays and FPDs is preserved.
To address this question, in this experiment, we attempted
to measure the linearity of FPDs to X-rays via
monochromatic X-rays using a diagnostic range and analuminum step wedge.
2 Major equipment used
Experiment station : BL-14C
Monochromatic X-ray energy : 33 keV
FPD：CXDI-80CWireless (Canon)
Pixel size : 125μm×125μm
Aluminum step wedge : 10 stair-steps (Fig.1)
Shutter (Fig.2)
Aluminum plate : 10-mm thick plate

3 Methods
Ⅰ. Preparation of images for measurementⅰExperimental alignment (Fig.3)
・An aluminum plate and a shutter were placedon the radiation port side.
・ Distance between the radiation port and theshutter : 38cm
・Distance between the shutter and the diffraction
crystal : 77cm
・Distance between the diffraction crystal and theFPD : 100cm
・Radiation field : 40mm×40mm
ⅱIrradiation method
・Considering the time for afterglow in the FPD
to become extinct, 60 s was chosen as theminimum irradiation interval.
・Irradiation time : 1 s・Application of irradiation : 3 times
・Three images were prepared (Fig. 4).

Photon Factory Activity Report 2016 #34 (2017)

Ⅱ. Relationship between the aluminum step wedge andpixel values (Fig.6, Fig.7)
・The correlation between the stair-step numbers of
the aluminum step wedges and pixel values isshown in all three images.
・ A correlation was found between the stair-stepnumbers of the aluminum step wedges and themean pixel values of the three images (correlation
coefficient ：0.997)

Ⅱ. Measurement of pixel values (Fig.5)
・Using ImageJ, regions of interest (ROIs) were set
on each stair-step of the aluminum step wedge
and in an area outside the aluminum step wedge(referred to as the zeroth stair-step of thealuminum step wedge), and the mean pixel valueof the three images was determined for each ROI.
・ ROIs on the first and tenth stair-steps of thealuminum step wedge had an area half as large as
the ROIs on the other stair-steps as images of theformer two stair-steps were narrower than thoseof the other stair-steps.
・The image of the zeroth stair-step of thealuminum step wedge was wide; thus, the ROI
on this step had an area twice as large as ROIs on
the second to ninth stair-steps.
・Number of ROIs set
Aluminum step wedge : 10
Outside the aluminum step wedge : 1
・Area of ROI (number of pixels)
First and tenth stair-steps of the aluminum
step wedge : 148000 pixels
Second to ninth stair-steps of the aluminum
step wedge : 296000 pixels
Outside the aluminum step wedge (zerothstair-step) : 592000 pixels

4 Result
Ⅰ. The pixel values (Table.1)
・Pixel values of the three images ranged fromapproximately 1500 to 3500.

Photon Factory Activity Report 2016 #34 (2017)

5 Discussion
・A correlation was found between the stair-stepnumbers of the aluminum step wedges and pixel
values from each of the three images or mean pixel
values of the three images. Therefore, it can beconcluded that the linearity of FPDs to X-rays ispreserved.
・Based on these results, we propose that X-rays need
to be monochromatic to generate diagnostic X-ray
images. Alternatively, it is desirable to make X-ray
spectra as close to monochromatic as possible.
・Although X-ray scattering from aluminum step
wedges was not considered in the present experiment,
scattered X-rays may need to be measured toimprove the measurement accuracy.
6 Conclusion
In this experiment, we attempted to clarify therelationship between incident X-ray doses (exposuredoses), which represent the basis of input/output
characteristics of FPDs, and pixel values using aluminum
step wedges.
Our results showed that the linearity of FPDs wasmaintained, indicating that FPDs are suitable for thegeneration of digital images.
7 Future tasks Problem
We propose that (1) measurement of X-ray scatteringfrom aluminum step wedges, which was not considered in
the present experiment, and (2) measurement of linearity
using diagnostic X-rays (continuous spectrum X-rays)
should be performed in studies for obtaining clinicalimages.
* kchisa@med.teikyo-u.ac.jp