Transcript
Page 1: NEUTRON COMPUTED TOMOGRAPHY - Digilib-BATANdigilib.batan.go.id/atom-indonesia/fulltex/v21-n1-1-95/Mardiyanto.pdf · NEUTRON COMPUTED TOMOGRAPHY ... NEUTRON COMPUTED TOMOGRAPHY. Computed

NEUTRON COMPUTED TOMOGRAPHY

Mardiyanto* and Mohtar*

ABSTRACT

NEUTRON COMPUTED TOMOGRAPHY. Computed tomography is anon-destructive testing method which can visualize cross-section of materials based on theirnuclear characteristics. In the previous work, X-ray was used as its radiation media. Theaim of this experiment was to improve the computed tomography technique using neutronbeam. For reconstructing the cross-section image of materials, a filtered back projectionwas used. Results indicated that a minimum hole shown was 3 mm in diameter using ablack and white presentation. While using eight colour levels, a hole of 2 mm in diametercould be seen clearly. It is expected that neutron computed tomography can improve theresults of non-destructive testing.

ABSTRAK

"COMPUTED TOMOGRAPHY" DENGAN NEUTRON. Computed tomographyadalah metode uji tidak merusak yang bisa menampakkan tam pang lintang suatu bahanyang berdasarkan pada sifat-sifat nuklirnya. Pada penelitian sebelumnya, sinar-X digunakansebagai medium radiasi. Tujuan daTi eksperimen ini adalah untuk menambah kemampuanteknik computed tomography dengan menggunakan berkas neutron. Untuk merekonstruksicitra tam pang Jintang suatu bahan. digunakan proyeksi balik yang di filter ("filtered backprojection"). Dari hasil penelitian ditunjukkan bahwa lubang minimum yang tampak adalahyang berdiameter 3 mm bila dipresentasikan dengan warna hitam putih. Apabila digunakan8 warna, lubang dengan diameter 2 mm dapat dilihat dengan jelas. Dengan penelitian inidiharapkan bahwa computed tomography dengan neutron dapat melengkapi informasihasil-hasil dari uji tidak merusak.

INTRODUCTIONThe inspection of critical industrial components is similar as routine

medical check-ups. Two methods can be used to examine the existingdefects within an object, these are destructive and non-destructive testing(NOT). In destructive testing, the sample object needs to be destructed andfollowed by visual exmination, while in the latter destruction is notnecessary.

Computed tomography (CT) is one of NOT methods which can visualizecross-section images of materials based on nuclear characteristics. CT wasoriginated in medical radiology and is becoming increasingly prevalent inindustrial NOT. Several CT algorithms have been presented in order toobtain the accurate reconstruction as well as the minimum reconstructiontime.

CHO [1] presented general thoughts on the physical aspects with

• Materials Science Research Centre, SATAN

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reference to X-rays below 100 KeV, specifically, the interaction of theX-rays with materials and a brief review of a number of computer

algorithms emphasizing the linear superposition technique with com­

pensation [I]. SHEPP and LOGAN [2] used a simulated phantom to

compare the Fourier and the search algorithm. CHO and CHAN [3] madea comparative study of 3-dimensional image reconstruction algorithms with

reference to a number of projections and noise filtering. MARTZ et al

[4] proposed computed tomography systems and their industrial application

using X-rays, while FUJINE et al [5] introduced the utilization of online

video image processing system in the neutron radiography facility.Although a lot of work has been done in this field, more studies on

CT using neutron beam need to be carried out in order to improve themethod.

The objective of this research is to study the application of CT usingneutron beam for industrial applications. Because of the limited number

of projections, it is hoped that this method is able to represent at least

macroscopic defects in the order of 10-1 mm.

MATERIAL AND METHOD

In this experiment, the Kyoto University Research Reactor neutron

radiography facility equiped with on-line video image processing systemwas used. This system consisted of a neutron TV system, video image

processing and personal computer. For CT purposes a stepping motor for

turning the object sample, a stepper motor control and driving unit which

could be operated manually as well as by using a PC were added to the

neutron radiography facility. The PC was a NEC PC-980 1 type, and wasused for arithmetic calculation in CT application and receiving image data

through an RS-232C interface from the video image processing system.

The whole system is illustrated in Figure 1.The sample object was a cylindrical aluminum. In contrast with X-rays,

neutron can penetrate aluminum easily since aluminum has a very lowneutron attenuation coefficient. Artificial defects were made in the form

of holes with different diameter and filled with paraffin in order to increase

their neutron attenuation (Figure 2). The diameters of the holes were 1.5;2.0; 3.0; 3.5; 4.0; 4.5; 4.9; 5.5; and 6.0 mm. This sample was mounted

on the sample table which was rotated with a stepping motor.This research was carried out using the neutron radiography facility at

the Kyoto University Research Reactor E-2 beam tube as thermal neutronsource, with thermal neutron flux was 1.2 x 106 n.Cm -2 .sec-1. The cadmium

ratio measured with a gold foil was 400 and the neutron-gamma ratio was106n.Cm-2.R-I. An aluminium plug was used to prevent the release of

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(1)

radioactive Ar-4l. As a gamma filter, a bismuth single crystal I cm thickwas placed in the aluminium plug. The neutron shutler was made fromboron carbide (B4C) and was situated at the middle of the collimator.

In this experiment four steps were performed, i.e. computer codepreparation, system arrangement, data collection and image reconstruction.

A computer code for collecting, refining data and reconstructing animage was developed and written in BASIC language.

Mathematically, the reconstruction can be described as an estimationof neutron attenuation, g(x,y), of its real value g(x,y) in one fixed plane.

Suppose an integral :

Pg(L) = ~ g ds

along a certain line L in a plane is a linear attenuation coefficient alongthe line which is measured through neutron transmission along L. Theattenuation Pg(L) can be represented as

Pg(L(t,e» = I gdsL (t,e)

where L(t,e) is the line whose normal axis passes through the origin asshown in Figure 3.Based on the figure, t can be written as :

Pg(t,e) = (2)

t = x cas e + y sin e (3)

Let p(w,e) be the Fourier transform of p(t,e) that is equal to theFourier transform of g in polar-coordinates, so that :

P (w,e) = g (w,e)

where

g(w,e) = f_~f~ g(x,y)e-iw(x cos 8+y sin 0) dxdy

(4)

(5)

In this case if p(t,e) is taken for all lines L, then :

g(x,y) = 1/(4n2) i7tde f_~p(w,e)eiW(X cos O+y sin 0) Iwl dw (6)

is the inverse Fourier transform of g(w,e), where the Iwl comes from theJacobian of the transformation into polar coordinates. The projection datap(t,e) is taken from measurements so it has discrete values of p(tk,ej).Where: tk= ka, k= 0, ±I, ±2, ±3, .... ; 8j=jn/n; j= 0, I, 2, .... n-I; a is

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the f:lY sp:lcing Md n is the number of projections. The approximatereconstruction in the discrete domain of Equation (6) can be representedas :

n-I 00

g~(x,y) = a/(2n) L: L: P(tk,8j) /l (x cos 8j+y sin 8j-tk) (7)j=O k=-oo

where /l(w) is Fourier transform of /let) and is a filter function. The sumon k in (7) is finite, namely k ~ lIa and if m rays cover the unit in eachprojection, then a=2/m.

The code for reconstructing the image was made based on the abovealgorithm. The reconstruction can be represensted by a simple way as :

(Reconstructed image )=(Projection data)* {W(Ka)}Where : W(Ka) = the weighting function,

k = 0; ± 1; ±2; anda = the ray spacing.In order to save computing time of the image reconstruction SHEPP

and LOGAN proposed a linear discrete weighting function :')

W (0) - 4/ (4a-) k=O') 2

W (ka) = -4 {na- (4k -I)} k=±1; ±2;The system as shown in Figure I was arranged and, the recording of

the object image was carried out using a video tape recorder. The steppingmotor and the object sample were located in front of the outlet apertureof the collimator. The object sample was arranged at the axis of thecollimator, so that, the whole object was irradiated. The neutron beampassed through the sample was attenuated by the atoms in the sample andthe degree of attenuation depended on the length of the neutron path andthe attenuation coefficient of the atoms. The transmitted neutrons which

carried the information of the object sample were converted into visiblelight. A NE-426 neutron scintillator, a LiF+ZnS(Ag) type, was used asthe converter. The light was, then, detected using a TV camera, displayedin a TV monitor, and the image was recorded by a video tape recorder.The sample was turned in 21 steps of 9° each, and recorded for two minutes.For this purpose it was considered enough to turn the sample only onehalf cycle.

To reduce the collection time, the data was stored digitally in a floppy

diskette. The analog data was converted into digital using analog to digitalconverter before being stored in the floppy diskette. The data was obtainedfrom the line 330 of each image frame and the image was the result of1000 frames integration.

Finally, the collected data was refined by subtracting its background

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noise before the image reconstruction. For refinement the data obtainedfrom the image was subtracted by the data obtained from the backgroundimage, and the image reconstruction was carried out using filter backprojection algorithm.

RESULTS AND DISCUSSIONOne of the digital data is illustrated in Figure 4. The datum was the

result of integrating 1000 image frames. The curve shows the variation

of ~ray levels of the integrated 1000 frames along line 330 from the50t to 550th column. The image was represented in 256 gray levels. Behindthe holes which were filled with paraffin, the areas were darker than theother due to the higher neutron attenuation cross-section of the parrafin.Almost all of the neutrons which passed through the paraffin were absorbed.

The results of reconstructions from line 330 are illustrated in

Figure 5 and 6. In Figure 5, the image was reconstructed in two colorlevels. These were black and white, the white representing the holes. Themaximum digital output data of the filtered back projection was around80. To determine the minimum value of the white ]evel for imagereconstruction, values between 40 and 60 were tested and the best imagewas found when the minimum white level was 50. The figure shows thatthe holes with diameters between 1.5 and 2.0 mm can not be seen.

Figure 6 shows output data from the filtered back projection in eightcolour levels. Only the smallest hole which can not be seen from this figure.The digital output of the filtered back projection calculation was dividedinto eight groups and each group represented one colour.

From the first technique, the datum was only represented into blackand white colour. Consequently, the image information detail was lost,hence, it has low resolution. For the second method, the digital data canbe represented in more detail. As a result, the resolution was improved,however, there was a slight decrease in the sharpness.

These results do not agree with the prediction. The above discussionis only based on the computer representation. Mathematically, the smallesthole which can be imaged has a diameter of 1 mm, because the samplingof spatial frequency is approximately 0.5 line/mm. There are twopossibilities which influence the imperfectness of the results, the lack ofdata for reconstruction, so that the output data oscillates, and the holeswere empty due to the difficulty in filling them with paraffin. If thathappens, it seems that there is no defect because aluminum has low neutronattenuation coefficient.

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CONCLUSION

From the experiments it can be summarized as follows :

1. NCT is one of non-destructive testing techniques which can be usedas a complementary technique to neutron radiography.

2. The results do not agree with the prediction. They are possiblyinfluenced by the representation of the image, sampling of spatialfrequency, and the imperfectness of the sample preparation.

3. The reconstructed image is represented into two ways, namely inblack-white and eight colour levels. The smallest hole which can beimaged using the first technique is 3.0 mm while by the second is2.0 mm.

ACKNOWLEDGEMENTSThe authors would like to express their thanks to DR. S. Fujine and

Mr. K. Yoneda from Kyoto University for their guidance and fruitfuldiscussion.

REFERENCES

1. Z.H. CHO, "General View on 3-D Image Reconstruction andComputerized Traverse Axial Tomography". IEEE Transactions onNuclear Science, Vol. NS-21, June 1974

2. L.A. SHEPP and B.F. LOGAN, "The Fourier Reconstruction of a HeadSection", IEEE Transactions on Nuclear Science, Vol. NS-2l, June1974

3. Z.H. CHO and J.K. CHAN, "A Comparative Study of 3-D ImageReconstruction Algorithms with Reference to Number of Projectionsand Noise Filtering", IEEE Transactions on Nuclear Science,Vol. NS-22, February 1975

4. H.E MARTZ, S.G. AZEVEDO, J.M. BRASE, K.E. WAL TJEN andOJ. SCHNEBERK, "Computed Tomography Systems and TheirIndustrial Applications", Appl. Radiat. Isot., 41(10/11) (1990) 943-961

5. S. FUJINE, K. YONEDA and K. KANDA, "An On-line Video Image

Processing System for Real-Time Neutron Radiography", NuclearInstruments and Methods, 215 (1983) 277-289

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NEUTRONS

IlJl!!1

SCINTILLATOR

CRT MONITORS

DIRECT B&W COLOR

Dog.J

COLOR

NEUTRON TV SYSTEM IMAGE PROCESSING SYSTEM PERSONAL COMPUTER SYSTEM

Figure 1. Neutron TV system block diagram (quoted from Ref. 5).

,-'1\ /

-"_I-.

I

r~ I \,

I,,: ''-- ; \

I

- ~ I \

• nil I

"\ ~.

,~ G6" \ )

.

\\. 7U \~\

1 ~. \

.~ ~I

Figure 2. Sample object.

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y

Figure 3. Line L on X-V coordinates

x

50 100 200

-------.--

300

(DOT DATA)

400 500

-+-'------.,.-./-

550 (LEVEL)

255:WHITE

O:BLAC~-' --_.~.

30

Figure 4. Digital data

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40

106

79

125

J

...J..

84 0

NeT

MARDIYANTO

KURRI

SEPT. 1990

Y

125

Figure 5. Image reconstruction in black and white colour

Figure 6. Image reconstruction in 8 colour levels

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