J. B. GUI ET AL.
Copyright © 2013 SciRes. ENG
different result that phase enhancement effect essentially
wasn’t affected as the tube voltage increased in other two
papers [5,6].
In this paper, we had a detail experimental investiga-
tion on the effect of tube voltage on phase-contrast im-
aging. This stud y was ba sed on two t ypes o f x-r ay t ube s,
a sealed microfocus x-ray tube and an openmicrofocus
x-ray tube, and the later has some better properties suita-
ble for PCI study, for example, smaller and adjustable
focal spot sizes, higher kVps and higher powers. The
study is significant for optimizing system design and ex-
ploring common rules of the PB-PCI technology.
2. Materials and Methods
Two investigations were performed based on a conven-
tional sealed x-ray source (UltraBright, Oxford Instru-
ments) and an open x-ray source (FXE 160.51, YXLON),
respectively. The sealed x-ray source has a focal spot
size of 13 - 20 μm, tube voltage and tube power can been
adjusted in the range of 20 - 90 kVp and 10 - 19 W, re-
spectively. The open x-ray source has a minimum focal
spot size less than 2 μm and the focal spot size can been
adjusted by changing the focusing current, which is the
current flowing through the focusing lenses. The tube
voltage (kVp) can be varied from 20 to 160 kVp, maxi-
mum tube power and target power is 64 W and 10 W,
respectively. A cooled x-ray CCD imaging detector
(Quad-RO 4320, Princeton Instruments) was used to ac-
quire the images. The CCD imaging detector has a high
spatial resolution, large active imaging area and low
noise (24 μm pixel size, 2084 × 2084 array and 50 × 50
mm2 active area, ultra low noise electronics and state-of-
the-art cooling technology), these characteristics are also
helpful for phase-contrast i maging investigation. The source-
to-detector distance (SDD) was held at 710 mm and the
SOD could be adjusted from 10 to 510 mm.
In order to quantify the edge enhancement effect of
phase-contrast imaging, we adopted the same edge en-
hancement index (EEI) as Donnelly [4]. The EEI is de-
fined as follows:
() ()
() ( )
PT PT
EEI HL HL
−+
=−+
, (3)
where P and T are t he peak and tro ugh intensity values at
the ed ge, and H and L are average values of multi-pixels
on the high- and low-intensity regions next to the edge.
Two other indexes [4] were also evaluated: the up-
stroke index (U I), defined as fo llows:
UI = (P − H)/(P + H), (4)
and the downstr oke ind ex (DI), d efined as follo ws:
DI = (L − H)/(L + H). (5)
For the sealed x-ray tube, we have performed the im-
aging at different x-ray tube voltages with constant tube
power of 10 W. In order to focus on the edge-enhanced
phenomenon, a thin rectangular plastic sheet of 0.5 mm
thickness was used as a radiography phantom. The SOD
was kept at 185 mm and exposure time was 8 s.
For the open x-ray tube, to evaluate the effect of the
tube voltage, two sets of experiments were performed. In
the first one, the target power was held constant at 3 W
and the tube voltage was adjusted. In the second one,
target current was held constant at 30 μA and the tube
voltage was adjusted. The SOD was held at 40 mm and
exposure time was adjusted to maintain an approximate
constant exposure intensity. A plastic sheet of 2 mm
thickness was used as a radiography phantom.
The image at each kVp was obtained by averaging 5
frames acquired continuously and profiles of pixel inten-
sity values were obtained by averaging 50 continuous
rows to reduce noise. Quantitative indexes UI, DI and
EEI were computed for all images.
3. Results and Discussions
3.1. Sealed X-Ray Tube
Depending on the sealed type x-ray tube (UltraBright,
Oxford Instruments), the phase-contrast image of the 0.5
mm thickness plastic phantom was acquired at a repre-
sentative tube voltage of 50 kVp, as shown in Fig ure
1(a). From the image we can see obvious edge-enhance-
ment (high- and l o w-intensity ver tical lines) at t wo ed ges
of the phanto m. The pr ofiles along the middle hor izontal
line of the images obtained at different tube voltages are
shown in Figure 1(b). It is interesting to note that this
effect is more pronounced at a higher kVp value. The
quantitative upstroke index shows the same result that
edge enhancement becomes more pro nounc ed a s the kV p
increases.
It is contrary to the common belief that better phase-
contrast occurs at the lower energy since the longer wa-
velength o f x-ray photos at a lo wer tube voltage can im-
prove the phase-contrast. Fortunately, the variation of
focal-spot size at d ifferent ope ration conditions, which is
observed in the other experiments, inspires us to think
about the effect focal-spot size on the phase-contrast im-
aging.
In order to quantify the size of the focal spot in our
system, we imaged a JIMA (Japan Inspection Instru-
ments Manufacturers’ Association) resolution test-pattern
with the micro-focus x-ray system. By using the resolu-
tion test-pattern, the acquired images at different tube
voltages are shown in Figure 2, from which it is seen
that the spatial resolution of the images becomes better
with the increase of kVp value. It means that the varia-
tion of focal-spot size causes the abnormal phenomenon,
though a higher tube voltage can decrease the edge en-
