TEP 5.5.01 -01

ranchocucamongabrrrAI and Robotics

Nov 6, 2013 (3 years and 7 months ago)

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P2550100




PHYWE Systeme

GmbH & Co. KG © All rights reserved

1

TEP

5.5.01

-
01


Principles of Digital X
-
ray
Imaging

www.phywe.com

Related topics

exposure time, detector saturation, full well capacity, detector calibration,
beam


intensity
, detec
tor offset,
pixel specific gain,

attenuation and transmis
sion, image
processing.



Principle

With digital X
-
ray imaging, X
-
ray photons that interact with the detector are converted to a digital signal.
This permits to record digital radiographies. With this experiment, the principles of digital detectors for X
-
ray imaging a
re laid out.


Equipment

1

XRE 4.0 X
-
ray expert set

09110
-
88

1

XRCT 4.0 X
-
ray Computed Tomography upgrade set

09180
-
88

1

XR 4.0 Accessories for CT

09057
-
44





Additional equipment



PC, Windows® 7 or higher







Fig. 1:

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PHYWE Systeme GmbH & Co. KG © All rights reserved P2
550100

Principles of Digital X
-
ray Imaging


TEP

5.5.01

-
01


Tasks

1.

Define a
good exposure time.

2.

Calibrate the detector

3.

Take some radiographies of an object
and process the images

with the image viewer


Set
-
up

Attach the XRIS to its stage.

Place the Digital X
-
ray detector XRIS on the rail at position 25 cm. The back side of the
XRIS stage co
r-
responds to its position on the rail. This position is called the 'source to detector distance' SDD (mm).










Fig.

2
: Set
-
up of the
XRIS


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PHYWE Systeme

GmbH & Co. KG © All rights reserved

3

TEP

5.5.01

-
01


Principles of Digital X
-
ray
Imaging

www.phywe.com

Note

Details concerning the operation of the X
-
ray unit
and
Detector

as well as information on how to ha
n-
dle the
detector

can be found in the respective o
p-
erating
instr
u
ctions
.

Procedure

-

Connect the X
-
ray unit via USB cable to the
USB port of your computer (the correct port of
the X
-
ray unit is marked in Fig.
3
).

-

Connect the usb cable of the detector to the
computer


-

Start the “measure
CT
” program. A virtual X
-
ray
unit
,

rotation stage and Detector
will be di
s-
played on the screen.

The green

indication

LED
on the left of each components indicates that its
presence has been detected

(Fig. 4)

-

Y
ou can change the
High V
oltage and current
of the X
-
ray tube

in the corresponding
input
windows

or manually on the unit
.

(Fig.4)

-

When clicking on the unit
pictogram
additional
information concerning the unit can be r
e-
trieved
( Fig.4)

-

The status pictogram indicate the status of the
unit and can also be used to control the unit
such as swi
tching on and off the l
ight or

the X
-
rays

(Fig4.)

-

The position of the digital detector can be a
d-
justed to its real position either by moving the
XRIS pi
c
togram or by filling in the correct value
in the input window
. (Fig.4)


-

The settings of the XRIS can be adjusted using
the input windows. The exposure time controls
the time between two frames are retrieved from
the detector, the number of frames defines how
many frames are averaged and with the binning
mode the char
ge of neighbouring pixels is ave
r-
aged to reduce the total amount of pixels in one
frame.












Fig.
4
: Part of the user interface of the software


Fig. 3
: Connection of the
computer




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PHYWE Systeme GmbH & Co. KG © All rights reserved P2
550100

Principles of Digital X
-
ray Imaging


TEP

5.5.01

-
01



Experiment execution


1. Exposure time and saturation


-

Adjust the XRIS settings and X
-
ray

unit settings according to fig.5
or load the configuration from the
predefined
CTO file 'Experiment 1' (see Fig
.

5
).


-

Start a new experiment, give it a unique name a
nd fill in your details (fig.6
). Alternatively it is also
possible to load this experiment

with pre
-
recorded images and open this manual.

The correct co
n-
figuration will
be loaded automatically as well but the functionalities of the software will be limited to
avoid overwriting the existing data.




Fig.
5
:

The settings for this experiment (left panel) and the method load and adjust the settings (right panel)



Fig.
6
:

How to create a new or open an
existing experiment

Overview of the settings of the XRIS and X
-
ray unit
:

-

35kV, 1.00mA

-

exposure time 0.5 sec

-

Number of

frames: 1

-

Binning mode 500x500

-

SDD= 250



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PHYWE Systeme

GmbH & Co. KG © All rights reserved

5

TEP

5.5.01

-
01


Principles of Digital X
-
ray
Imaging

www.phywe.com


-

Switch on the X
-
rays and activate the 'Live view' (see fig
.7)
. When the Live view is activated, every
new image that is retrieved from the X
-
ray detector is
displayed. The Detec
tor exposure load bar
(see f
ig
.7
) indicates the average degree of fill for each pixel.
I
t is very important to remain below
the maximal fill degree of the detector
.
O
therwise the detector will be saturated and
won't work
properly.
I
f the saturation level is reached, the load bar will tur
n red. (see theory for more explan
a-
tion)
.
















Increase and decrease the exposure time of the detector between 0.23 sec and 2 sec
while watching the load bar. Note: after th
e adjustment of the exposure time, leave suff
i-
cient time for the detector to physically adjust itself to the new settings.


The fill degree of the detector is linearly dependant on the exposure time

but is also influenced by

the i
n-
tensity of the beam. This

beam

intensity can be modified in three ways:

1.

Change of maximal kV

2.

Change of current

3.

Variation in SDD


-

Change the maximal kV between 10 and 35 kV for a given current and exposure time while watc
h-
ing the Load bar
.

-

Change the current between 0.1 and 1.00 mA

for a given kV and exposure time while watching the
Load bar

-


-

The increase in fill degree is not linearly proportional to the adjustment of the kV range, see exper
i-
ment 3 for more explanation.

-


-

The fill degree of the detector is
also influenced by the di
stance of the detector to the source.

The

fill
degree is proportional to the intensity of the beam that falls on the detector which itself is proportio
n-
al to its opening angle.

The intensity is governed by th
e inverse square law (see experiment 3
).

-

Vary th
e SDD by manually placing the detector closer or further from the source and watch the load
bar:




Open the Unit

Fig.
7: Activation of the Live View and the detector exposure fill degree



6





PHYWE Systeme GmbH & Co. KG © All rights reserved P2
550100

Principles of Digital X
-
ray Imaging


TEP

5.5.01

-
01





Change the SDD




Switch on the X
-
rays and wait 10 sec




Look at the load bar


Note:
For all the further experiments it is important to remain
under the saturation level,
watch the load bar closely after each modification

of one of these three parameters.


2
.
Detector calibration


After having defined the desir
ed and adequate configuration for

the
experiment

(fig
.5
)
, the detector
needs to be
calibrated. Each digital detector has a different and variable offset and pixel
-
specific gain

(see theory)
. During the calibration these variations will be measured and stored. Such corrections
are
performed automatically by
the soft
ware:




Automatic correc
tion
:

-

Make sure there is no object between the source and the detector, otherwise the calibr
a-
tion will be incorrect.

-

deactivate the 'Live view'

-

click on calibrate:




-

After successful calibration, the red LED in the

calibrate button will turn green. From now
on the detector is calibrated and the displayed images will be corrected for offset and pi
x-
el gain

-

The Load bar will not be visible anymore while the intensity bar is made visible. With the
intensity bar the co
ntrast

in the displayed frame can be adjusted.



Note: if the configuration is altered, the calibration will be incorrect, this will result in the deactivat
ion of
the calibration

status
.

Calibration will have to be performed again.


Fig.
8: Calibrate the detector


Fig.
9: After calibration, the fill degree bar disappears and the contrast/intensity bar is shown


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GmbH & Co. KG © All rights reserved

7

TEP

5.5.01

-
01


Principles of Digital X
-
ray
Imaging

www.phywe.com

3
.
Applications


Once the detector is calibrated, goo
d quality images can be displayed, recorded and saved.


-

Place one of the demo objects between the source and the detector,
switch on

the X
-
rays and
activate the liv
e view. To place the object in front of the detector, a stand can be used.

Adjust
the contrast if desired.








-

The colours of the displayed image can also be altered w
ith three levels (Low, Middle and
High). The 'High' colour corresponds with the pixels having a value higher than the upper co
n-
trast/intensity handle. The 'Low' colour corresponds with the pixels having a value lower than
the lower contrast/intensity handl
e. The 'Middle' colour corresponds with the pixels having a va
l-
ue between than the upper and lower contrast/intensity handle.









-

To save the resulting image, stop the live view and click on save. Four formats of
images can
be saved (tif, png, jpg and bmp). For the three later image

formats, the contrast adjustments are
retained during saving while this is not the case for the

tif format. F
or
tif, the images are saved in
a raw format.
If the images will be used for

further processing or
measuring in the
measureCT sof
t
w
are it is importa
nt to save them as
tif
.










Fig.
10: Adjusting the contrast/intensity

Fig.
11: Adjusting the colours



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PHYWE Systeme GmbH & Co. KG © All rights reserved P2
550100

Principles of Digital X
-
ray Imaging


TEP

5.5.01

-
01



-

To save
a
sequence of image, stop the live view and click on record sequence. Four formats of
images can be saved (tif, png, jpg and bmp). For the

three later image formats, the contrast a
d-
justments are retained during saving while this is not the case for the tif format. For tif, the i
m-
ages are saved in a raw format.
If the images will be used for further processing or
measu
r-
ing in the measureCT so
f
t
w
are it is importa
nt to save them as
tif
.













To further process an image you need to open the image viewer. When the viewer is opened, the
image that is present in the frame display will be transferred automatically to the image viewer.
The image viewer can be opened in two ways, either from the
task
bar or using the shortcut bu
t-
ton
.

When you are finished with the viewer, close it, otherwise the next time you open it the r
e-
maining image will still be present.









To process images that were saved as tif, open the

destination directory

(fig
15
)
. All the tif ima
g-
es that are present in the folder will be displayed in the list.
The viewer will typically open while
listing the current operational folder and the current operational image for ease of use.









Fig.
13: Recording


a series of images

Fig.
14: Open the Image Viewer

Fig.
15:

Open a directory with tif
-

images


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GmbH & Co. KG © All rights reserved

9

TEP

5.5.01

-
01


Principles of Digital X
-
ray
Imaging

www.phywe.com

T
he image viewer has several functions available.


The tif files that are present in the current operational directory are listed (fi
g.1
6
.1) and
the current operational image is displayed in the image viewer display
(fig
.1
6
.
2). In this
display, the line profile rulers are present (fig
.1
6
.
3). Additional functionalities can be a
c-
cessed through the menu (fig
.1
6
.
4). The histogram of the image and the contrast se
t-
tings are listed on the right (fig
.1
6
.
6) as well as the line

profile data (fig
.1
6
.
7) and the r
e-
gion parameters (fig
.1
6
.
8). Saving and exporting options are available through several
buttons (fig
.1
6
.
9) and the colour of the image levels van be adjusted ((fig.16.10).

















Using the image buttons (fig
.1
6
.
.5) it is possible:

-

To

select the ruler function (fig.1
7
.
1) which allows to change the line profile rulers from po
s
i-
tion (fig.1
6
.3).

-

To selec
t several zoom functions (fig.1
7
.2 and fig
.1
7
.4) to zoom in

on a region of the image

-

To select the move function (fig
.1
7
.3) to change the position of the image once it is zoomed in
on.










Fig.
16: The Image Viewer
functionalities

Fig.
17: The image buttons options



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PHYWE Systeme GmbH & Co. KG © All rights reserved P2
550100

Principles of Digital X
-
ray Imaging


TEP

5.5.01

-
01



Histogram


The histogram of the current image is
displayed in the histogram
display (fig
18
.1). Such a histogram refers to the number of pixels (Y
-
scale) with a certain grey
-
value (X
-
scale) that are present in the i
m-
age. By adjusting the lower and/or upper level

handle

(fig
18
.2), the
image contrast ca
n be

altered and with the autos
c
a
le bu
t-
ton(fig
18
.3), the lower and upper contrast level are reset to optimal
conditions. The histogram can be saved as an exc
e
l file (fig
18
.4).



Line profile


The line profile function calculates the gray
-
value of each pixel a
long
a line from the centre of one ruler to the centre of the other ruler (fig
16
.3). The result is displayed in the line profile display (fig
19
.1). The
line profile can be saved as an excel file (fig
19
.2).


Regions
parameters


The rulers of the line profile also define a
square region
. By clicking on the calculate bu
t-
ton from the region parameters (fig
20
), the a
v-
erage grey value and the spread of that region
are calculated.


Play, Save, AVI

and Convert


With the play

button

(fig
21.1
)
, the images that are listed in the d
i-
r
ectory

are subsequently displayed, starting from the first selected
image.

It is possible to save one single image under a different
fo
r
ma
ts

or with other
colours

(fig21
.2)
. A series of images that are
se
l
ected can be saved as an avi
-
movie using the AVI
-
button
(fig
21
.3)
and a series of images can be converted to a different file
fo
r
mat using the convert button
(fig
21
.4)
.




Fig.
18: Histogram options

Fig.
19: Line profile options

Fig.
20: Region parameters

Fig.
21: Play, save, AVI functions


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11

TEP

5.5.01

-
01


Principles of Digital X
-
ray
Imaging

www.phywe.com



Image viewer
menu


Apart from opening a new directory, the image viewer menu
(figxxx.4) has also several other functionalities.


-

set

play

speed
: changes the speed for playing the images





































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PHYWE Systeme GmbH & Co. KG © All rights reserved P2
550100

Principles of Digital X
-
ray Imaging


TEP

5.5.01

-
01


T
heory


Detector saturation


With digital
X
-
ray imaging, X
-
ray photons that interact with the detector are converted to a digital signal.
Such a digital detector is composed of a raster of pixels (picture elements) and each pixel can be co
n-
sidered as bucket. For each interaction of an X
-
ray photon

with the detector, a series of electrons are
produced in the pixel corresponding with the location of the photon interaction. These electrons are
stored in the pixel, gradually filling up the bucket. After a set time interval, "
exposure time
", the electro
n
content of the pixel is measured by emptying it. For the same intensity of X
-
ray's, a longer exposure time
will results in a larger number of pixels in the bucket.

Each digital detector has a limited bucket size which is called the
'full well capacity
'
of the detector.
When this level off fill is reached, additional electrons are thrown away because the detector is satura
t-
ed. A saturated detector will
cause inconsistent measurements and has thus to be avoided.



Detector calibration


Each digital detec
tor has a different and variable offset and pixel
-
specific
output
. During the calibration
these variations will be measured and
used in the subsequent imaging.

Even without the X
-
rays on, the detector will generate a read
-
out value that is different from 0
, called
'dark image'. This has several reasons from which the main reasons are an electronic offset and read
-
out noise. When determining the beam intensity I
0
, it is important to subtract this offset (I
D
) from the
measured read
-
out (I
0,M
).















Another issue that exists, is that
the sensitivity of every detector pixel is slightly different, resulting in a
wide variation of I
0

values for every pixel. During calibration, these variations are recorded.


After calibration, the

Transmission value

for every pixel (I) is recalculated based on the beam intensity of
that pixel at that time (I
M
), the beam intensity during calibration(I
0,M
) and the dark current intensity (I
D
)
du
r
ing calibration.



















If the calibration was successfu
lly performed, after calibration the images have grey
-
values between 0
and 1.











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13

TEP

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-
01


Principles of Digital X
-
ray
Imaging

www.phywe.com

Transmission and Attenuation


In X
-
ray imaging, the transmission or attenuation of X
-
rays through a certain object are measured. D
e-
pending on the settings of the source,

a beam with a certain intensity I
0

is measured by the detector
when no object is placed between the source and the detector. When an object is placed in the path of
the beam, this object will attenuate the beam so that the detector measures a smaller inte
nsity I instead
of I
0
. The remaining intensity I compared to the original I
0

is called transmission (T), which is the opposite
of the attenuation (A) of the object.

For a calibrated detector, the beam intensity is rescaled to a value between 1 and 0. With

T= 1 for the
beam without an object in front of the detector (I
0
).