Effects of Tube Voltage on Phase-Contrast Imaging for Different Microfocus X-Ray Tubes

doi:10.4236/eng.2013.510B121

Paper Menu >>

Journal Menu >>

Engineering, 2013, 5, 590-594

http://dx.doi.org/10.4236/eng.2013.510B121 Published Online October 2013 (http://www.scirp.org/journal/eng)

Effects of Tube Voltage on Phase-Contrast Imagin g for

Different Microfocus X -Ray Tubes*

Jianbao Gui, Z hanli Hu, Peter Z. Wu, Hairong Zheng

Paul C . Lauterbur Research Center for Biomedical Imaging, Institute of Biomedical and Health Engineering,

Shenzhen Institutes of Advanced Technology, Chinese Academy of Sciences, Shenzhen, China

Email: [email protected], hr.zheng@si at.ac.cn

Received 2013

ABSTRACT

In the past decade, phase-contrast imaging (PCI) has beco me a hot research with an increased improvement of the im-

age contrast with respect to conventional absorption radiography. In this paper, effects of tube volta ge (kVp ) on propa-

gation-based phase-contrast imaging have been investigated with two types of microfocus x-ray tubes, a conventional

sealed x-ray tube with the focal spot size of 13 - 20 μm and an open x-ray tube with minimum focal spot size less than 2

μm. A cooled x-ray CCD detector with the pixel size of 24 μm was used to acquire digital images. Two thin plastic

sheets with different thickness were used as radiography phantoms. Two different phenomena were observed for the

t wo x -r a y t ub e s. For the o pen tub e, p hase -contr a st e ffect has a sli ght dr op with the incr ea si ng of t ub e volt age , ho weve r,

it is opposite for the sealed tube. A further investigation indicates that the variation of focal spot size causes the abnor-

mal result for the sealed tube. It also s hows t hat pha se-contrast effect is more sensitive to focal spot size than tube vol-

tage.

Keywords: Phase-Contrast Imaging; Edge Enhancement ; X-Ray; Tube V oltage

1. Introduction

Conventional x-ray imaging is based on the attenuation

properties of an object, however, for some weak absorp-

tion objects, such as biological tissue, polymers, and fi-

ber composites, the use of conventional x-ray imaging is

limited due to poor image con trast. While the applica tion

of x-ray phase-contrast imaging can provide a way to

generate the images with better image-contrast because

the interaction cross-section of x-ray phase-shift is about

a thousand times larger than that of absorption for soft

tissues [1].

Wilkins and his colleague firstly developed a classical

theory formula (PGW theory) [2] on propagation-based

phase-contrast imaging (PB-PCI) in 1997. This PGW

theory is based on the paraxial Fresnel–Kirchhoff dif-

fraction for spatially coherent x-ray sourc es . I n 2 00 4 , W u

and Liu publicized a more comprehensive theory based

on Wigner distribution function (WDF) [3], which can

deal with partiallycoherent x-ray and the detector’s im-

pact. According to the PGW theory, for a weak absorp-

tion and weak phase-shi ft object,

( )( )

I xrzx

λρ

′′

≈−

, (1)

where re is the classical electron radius and ρe the pro-

jected electron density, λ denotes the wavelength and z

the propagation distance. From the formula, we can

know that a longer wavelength and a larger propagation

distance are helpful to the forming of phase contrast,

which means a lower x-ray energy expected.

We can also consider phase-contrast effect from lateral

coherence length for the PB-PCI d ep end i ng o n a p ar tia ll y

coherence microfocus x-ray source,

λσ λγ

⊥

= =

, (2)

where σ is the source size, γ is the angular width of the

source as viewed from the observation point. The lateral

coherence length is larger, phase-contrast effect is better.

Fro m this wa y, we ca n se e t hat l on ger wavel engt hs (lo w-

er x-ray energies), longer SOD distances and smaller

focus-spot sizes, are beneficial for the phase-contrast

imaging.

Donnelly investigated some influencing factors by ex-

periments based on different PCI systems [4-6]. It is in-

teresting that Donnelly showed that phase enhancement

effect had a slightl y drop with the increa sing of t ube vo l-

tage ( kVp) in t his p ap er [4 ], but the a utho r als o sho wed a

This work was supported in part

by National Science & Technology

Pillar Progr am (2012B AI13B 04), Nation al Natural Sci ence Foundati on

of China (81401410), Special Fund Project

and Basic Research Pr

gram of Shenzhen (

CXZZ20140505091419405,

JCYJ201304011703

06796, JCYJ20140 417113430558

, JCYJ20130401170306880).

J. B. GUI ET AL.

591

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-

J. B. GUI ET AL.

592

hancement, the effect of focal-spot size is greater. From

our results, it can be derived that phase-contrast effect is

insensitive to tube voltage (or equivalently, wavelength

of x -ray photos), but sensitive to focal-spot siz e.

3.2. Open X-Ray Tube

3.2.1. Constant Target Power

Figure 3(a) shows the phantom image obtained at 80

kVp when a constant target power of 3 W was held.

Representative profiles of the pixel intensity values

across the edge images obtained at different tube voltag-

es are shown in Figure 3(b). EEI as a function of kVp

are shown in Fig ure 3(c). These indicate a different re-

sult that edge enhancement drops as kVp increases. In

Figure 4, images of JIMA resolution test-pattern show

that the spatial resolutions are almost same at different

kVp.

3.2.2. Constant Target Current

Whena constant target current was held, representative

profiles of the pixel intensity values across the edge im-

ages obtained at different tube voltages are shown in

Figure 5(a). EEI as a function of tube voltage are shown

in Figure 5(b). These results also show that edge en-

hancement has a drop as the kVp increases. In Figure 6,

images of JIMA resolution test-pattern shows the spatial

resolutions are almost same at different kVp.

(a) (b) (c)

Figure 1 . (a) The 0.5 mm plastic edge phantom image obtained at 50 kVp a n d t he S OD o f 1 8 5 mm, ( b) re pr e se nt ative pr of il es

near to the rig ht edge at dif ferent tube volt ages and (c) EEI as a f untio n of kVp. The t ube p ower was he ld cons tant at 10 W.

This resul t was obtai ned based on the sealed type x-ray tube (UltraBright, Oxford Instruments).

(a) (b) (c) (d)

Figure 2. Representative images of the JIMA resoluti on test- patter n a t different tube voltage, 20 kVp, 30 kVp, 40 kVp and 90

kVp, display window is [0.9, 1.5]. All images were acquired at the tube power of 10 W. The resolution pattern from left to

right in every figure is 15, 10 and 7 μm, respectively. (a ) 20 kV p, 10 W; (b ) 30 kV p , 10 W ; (c) 40 kV p , 10 W ; (d) 90 kV p , 10 W.

(a) (b) (c)

Figure 3. (a) The 2 mm plastic edge phantom image obtained at 80 kVp and the SOD of 40 mm, (b) representative profiles

near to the edge at different tube voltages and (c) EEI as a funtion of kVp. The tube power w as held const ant at 3 W. This

result was obtained based on the open type x-ray tube (FXE 160.51, YXLON).

1200 1250 1300 1350 1400

0.85

0.9

0.95

1.05

Pix e ls

Relati ve intensity

20 kVp

40 kVp

60 kVp

90 kVp

20 40 60 80 100

0.005

0.01

0.015

Tube vol tage (kVp)

Upstroke index

1000 1050 1100 1150

0.75

0.8

0.85

0.9

0.95

1.05

1.1

Pix el

Relative intensit

40 kVp, 75 µA

60 kVp, 50 µA

80 kVp, 37.5 µA

100 kVp, 30 µA

120 kVp, 25 µA

40 60 80100 120

2.4

2.6

2.8

Tube Voltage(kVp)

Edge E nhancement Index

J. B. GUI ET AL.

593

(a) (b) (c)

Figure 4. I mages of JIM A res o lution test-pattern at different tube voltages with constant target power, (a) 40 kVp, (b) 80 kVp,

(a) (b)

Figure 5. (a) Representative profiles near to the edge at different tube voltages and (b) EEI as a funtion of kVp. The target

curr ent was held constant at 30 μA. This result was obtained based on the open type x-ray tube (FXE 160.51, YXLON).

(a) (b) (c)

Figure 6. Ima ges of JIMA resolution test-pattern at different tube voltage with constant target current, (a) 40 kVp, (b) 80

kVp and (c) 120 kVp. All i mages sho w the same spatial resoltion of 3 μm.

Both the study at constant target power and the study

at constant target current demonstrate that edge en-

hancement has a drop wit h the increa s ing of t ube vol t age,

but the drop is slight since the edge enhancement at the

high voltage of 120 kVp is still obvious. This result is

consistent with the PGW theory and lateral coherence

length formula, longer wavelength (lower x-ray energy)

is beneficial for the phase-contrast imaging. The use of

high tube voltages over 100 kVp is very significant in

reducing patient dose if the phase-contrast imaging

technology will be applied in human breast diagnosis in

the future.

4. Conclusion

We investigated the effect of tube voltage (kVp) on the

propagation-based phase-contrast imaging with two types

of microfocus x-ray tubes, a conventional sealed x-ray

tube and an open x-ray tube. Two different phenomena

were observed for the two x-r ay tube s . For t he o p en t ub e ,

phase-contrast effect has a slight drop with the increasing

of tube voltage, however, it is opposite for the sealed

tube. A further investigation indicates that the variation

of focal spot size causes the abnormal result for the

sealed tube. It also shows that phase-contrast effect is

more sensitive to focal spot size than tube voltage.

1000 1050 1100 1150

0.8

0.9

1.1

Pix el

Relative intensit

40 k Vp

60 k Vp

80 k Vp

100 k Vp

120 k Vp

40 60 80 100 120

2.2

2.4

2.6

2.8

Tube Voltage(kVp)

Edge Enhancement Index

J. B. GUI ET AL.

594

REFERENCES

[1] X. Z. Wu and H. Li u, “Clini cal Implementation of X-Ray

Phase-Contrast Imaging: Theoretical Foundations and

Design Considerations,” Medical Physics, Vol. 30, No. 8,

2003, pp. 2169-2179.

http://dx.doi.org/10.1118/1.1593836

[2] A. Pogany, D. Gao and S. W. Wilkins, “Contrast and

Resolution in Imaging with a Microfocus X-Ray Source,”

Review of Scientific Instruments, Vol. 68, No. 7, 19997,

pp. 2774-2782. http://dx.doi.org/10.1063/1.1148194

[3] X. Z. Wu and H. Liu, “Phase-Space Formulation for Phase-

Contrast X-Ray Imaging,” Applied Optics, Vol. 44, No.

28, 2005 , pp. 5847-5854.

http://dx.doi.org/10.1364/AO.44.005847

[4] F. D. Edwin and R. P. Ronald, “Quantification of the

Effect of kVp on Edge-Enhancement Ind ex in Phase-Con-

trast Rad io graph y,” Medica l Ph ysics, Vol. 29, No. 6, 2002,

pp. 999-1002. http://dx.doi.org/10.1118/1.1477416

[5] F. D. Edwin, et al., “Characterization of the Phase-Con-

trast Radiography Edge-Enhancement Effect in a Cabinet

X-Ray System,” Physics in Medicine and Biology, Vol.

51, No. 1, 2006 , p. 21.

http://dx.doi.org/10.1088/0031-9155/51/1/002

[6] F. D. Edwin, R. P. Ronald and R. P. David, “Quantifica-

tion of the Effect of System and Object Parameters on Edge

Enhancement in P hase-Contrast Radiography,” Medical

Physics, Vol. 30, No. 11, 2003, pp. 2888-2896.

http://dx.doi.org/10.1118/1.1617430