Collimator for Survey

8/7/2019 Collimator for Survey

http://slidepdf.com/reader/full/collimator-for-survey 1/12

Pinhole collimator design for nuclearsurvey system

Wanno Lee*, Gyuseong ChoDepartment of Nuclear Engineering, Korea Advanced Institute of Science and Technology (KAIST),

373-1 Kusong-dong, Yusong-gu, Taejon, 305-701, South Korea

Received 23 August 2001; received in revised form 15 February 2002; accepted 19 February 2002

Abstract

A conventional knife-edge collimator, which is widely used in gamma camera for medicaldiagnosis, is not suitable for nuclear imaging system because many scattering radiations near the

pinhole aperture happen and blur image. A new pinhole collimator, which shapes a channeledaperture for reducing image degradation induced by the scattering radiations, is introduced andits characteristics are analyzed by Monte Carlo simulation. Resolutions dened as the full-widthat half-maximum (FWHM) of point spread function and efficiencies are calculated about severalpinhole diameters from 4 to 8 mm and channel heights from 2 to 10 mm. For this calculation, weassumed that 137 Cs radiation sources with 662 keV mono-energies enter into our designedcollimator at the 1 m distance from the detector plane. The efficiencies and resolutions of thechanneled collimator are compared with those of the conventional collimator. By comparisonresults, it is veried that the new collimator takes advantage more than the conventional col-limator. The optimum channel height and diameter of the pinhole collimator from simulationresults are also proposed and designed. We nally acquired nuclear image mounting this colli-

mator in the nuclear survey system. # 2002 Elsevier Science Ltd. All rights reserved.

1. Introduction

The collimator plays an important role determining image quality in a medicalgamma camera as well as nuclear survey system. However its researches for nucleareld applications have not been accomplished unlike its enough analyses in a medicalsystem (Smith et al., 1997; Mortimer and Anger, 1954; Johnson et al., 1995; Redus

Annals of Nuclear Energy 29 (2002) 2029–2040

www.elsevier.com/locate/anucene

0306-4549/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved.P I I : S0306 -4549 (02 )00031 -2

* Corresponding author. Tel.: 82-42-869-3881; fax: 82-42-869-3810.E-mail address: [email protected] (W. Lee).



et al., 1992). The collimator design for nuclear survey system should be differentlytaken into consideration from the collimator for medical environments because ithas always been used in the high-energy radiation environments.

Several types of collimator could be used in nuclear survey but a pinhole colli-mator is the most useful for nuclear imaging because large area monitoring is pos-sible and it takes advantage of the same angular resolution irrelative to the distancebetween a source and a system.

We have developed a nuclear survey system with the pinhole collimator that is com-posed of combined charge-coupled devices (CCD) and a gamma imaging system thatproduces a color image of gamma rays through the superimposition of white andblack at surroundings of the source (Lee et al., in press). Fig. 1 shows the developedprototype system.

The purpose of this study is to investigate the optimum pinhole aperture design foraccomplishing the improved image at high-energy radiation eld under the conditionthat the collimator efficiency is greater than the minimum value for being able toobtain the distinguished image from background radiation.

In this paper, characteristics of the knife-edge and the channel type collimator areanalyzed by the MCNP 4B photon simulation code, which can be used for neutron,photon, electron, or coupled neutron/photon/electron transport (Briesmeister,1997). The channel type is selected by these analyses and we also verify these simu-lation results through experimental test of two types of collimators.

After determining the collimator shape, our studies consisted of parameter calcu-

lations and Monte Carlo simulations in order to optimize the channel height anddiameter. Experimental studies are used to test and evaluate the channeled pinholecollimator due to the change of two parameters.

2. Materials and methods

In order to simulate interaction with radiation and detector due to aperture diametersand channel heights, the MCNP 4B transport code is used. MCNP input geometryconsists of three components and has the same structure with the real senor of our

developed system.The rst components are the lead and aluminum parts for shielding environmental

radiation, which also support the pinhole aperture. For shielding about 1.4 MeV-environment radiations of 40 K, the lead thickness of 20 mm is selected and it can shieldthe half value of the 40 K. The aluminum thickness of 5 mm for supporting the leadweight is determined. The focal length dened as the distance from the pinhole apertureto scintillator crystal is 108.8 mm and the eld of view of the pinhole collimator is42 when it is inserted into the system.

The second component is the scintillation crystal part. We assume that the crystaltype is NaI(Tl), which was made by Alpha Spectra Co. The crystal thickness, dia-

meter, and density are respectively 10 mm, 51 mm and 3.67 g/cm 3 . The gamma rayenergy resolution of this crystal was proposed about 11% and the measured resultsshowed about 12% at 662 keV.

2030 W. Lee, G. Cho / Annals of Nuclear Energy 29 (2002) 2029–2040



Fig. 1. The developed prototype system: (a) radiation imaging camera without CCD visual camera iscomposed of the sensor, NIM bin module, and data acquisition board and (b) the acquisition software.

W. Lee, G. Cho / Annals of Nuclear Energy 29 (2002) 2029–2040 2031



The third is the pinhole collimator mounted on the lead head. In order to considerthe experimental facility, the collimator is composed of lead cap and tungsten alloypinhole.

Fig. 2(a) shows MCNP input geometry and source distribution is shown inFig. 2(b). The spatial resolution and efficiency are calculated by Monte Carlomethod due to the collimator types and channel heights. The distance between thesource and crystal plane is 1000 mm and solid angle of the source about pinhole

Fig. 2. (a) Input geometry for MCNP simulation and (b) the source distribution passed the pinhole andscattered photons.




plane is 1.8 . Geometries of the conventional and channeled pinhole are shown inFig. 3(a) and (b). In order to evaluate the spatial resolution, we assume that thecrystal is voxelized by 51 Â51Â10. The dimension of each voxel is 1 Â1Â1 mm.Therefore, the maximum accuracy of resolution in this simulation is 1 mm.

The collimator efficiency is calculated by summing photon numbers accumulatedin the each voxel and if they are less than 5% of maximum photon numbers, theyare rejected for reduction of simulation error.

Collimators having several diameters and channel heights for experimental study

are designed based on calculated and simulated results. For comparison of thescattering radiations created near the hole of the knife-edge and channeled colli-mator, we also designed the 4 mm diameter collimator of two types and measured

Fig. 3. Cross section of (a) knife-edge collimator and (b) channeled collimator.




the characteristics with standard point sources shielded by lead. As the nuclearimaging principle was described in the previous study (Lee et al., 2001), the samemethod was used in this paper. We compared the spatial resolutions and efficienciesof collimators having different channel heights with the line phantom and acquiredthe nuclear image for the channeled pinhole collimator.

Fig. 4. Point spread function of (a) knife –edge collimator with the 4 mm pinhole diameter and (b)channeled collimator of 2 mm height with 4 mm pinhole diameter.




3. Results

Resolutions of the knife-edge and the channeled collimator of channel height (2mm) with 4 mm pinhole diameter are shown in Fig. 4(a) and (b). These guresexplain an effect of the channel height about image resolution. The channeledcollimator can reduce image blurring because of decreasing the scattering radiationnear the hole, however the image of the knife-edge collimator at high-energy radia-tion eld broaden because of the scattered photons increased near the hole andshielding material. Therefore, the knife-edge collimator is not suitable for a nuclearsurvey system. According to these calculations, the spatial resolution of the channelcollimator is better than that of the knife-edge by about 37.5% although theefficiency decreases by about 30.6%. These results are similar to the experimentalvalue.

Fig. 5(a) shows the FWHM of point spread function (PSF) due to the change of the channel height and diameter. As the pinhole diameter is narrow and the channelheight is long, the PSF becomes sharp. Under the same conditions of Fig. 5(a), thecollimator efficiencies are calculated and shown in Fig. 5(b). The efficiency inclinesto decrease due to increase the channel height, as expected.

In this paper the system sensitivity, which is determined by other components of an electronic system and data acquisition board besides the collimator (Guru et al.,1995; Redus et al., 1994; Gal et al., 2000), is dened as the minimum activityrequired for a source to be detected signicantly above the noise level of the camera

(Redus et al., 1996). This is described in terms of the signal-to-noise ratio as

S ¼N source À N bkg

ffiffiffiffiffiffiffiffiffiffi N bkgp ¼

C source À C bkg

ffiffiffiffiffiffiffiffiffiffiffiffiffi C bkg =tp ð1Þ

where S is the sensitivity, N source is the total source count detected by the nuclearsurvey system during the setting time, N bkg is the total background count during thesame time without a source, C source is the counting rate of the source plus background,and C bkg is the counting rate of the background without the source.

It is usually the minimum requirement of nuclear survey systems like our devel-oped camera that the sensitivity should be acquired more than 6 for obtaining themeaningful image from the background noise. We used this value about the 137 Cssource of 100 Ci located in the 1000 mm distance from the detector plane during 60s measurement time.

By using Eq (1) when the background count rate ( C bkg ) is 4 #/s, which wasmeasured by our developed system, the source count rate ( C source ) is calculated as a5.55 #/s.

Intrinsic and absolute efficiencies are dened as (Knoll, 1989)

" int ¼number of pulse recorded

number of radiation quanta incident in detectorð2Þ




" abs ¼number of pulse recorded

number of radiation quanta emitted by source: ð3Þ

By using Eqs. (2) and (3), the collimator efficiency is given by

Fig. 5. (a) Resolutions dened as the FWHM of point spread function and (b) collimator efficiencies dueto the change of channel height and diameter.




" col ¼" abs

" int¼

number of radiation quanta incident in detectornumber of radiation quanta emitted by source

: ð4Þ

The peak efficiency assumes that only those interactions that deposit the fullenergy of the incident radiation are counted. The total and peak count efficiency arerelated by the peak-to-total ratio as

r ¼number of peak counts recorded by the full energy deposition

number of pulses recordedð5Þ

Fig. 6. The real image of line phantom shaping a cylinder that has the 10 mm length and 2 mm diameter:(a) the channel height is 2 mm and (b) the channel height is 6 mm with the 4 mm pinhole diameter.




where r is the peak-to-total ratio (Knoll, 1989).If the peak cont rate with our system is measured about the known source activities,

this value is then expressed by

C source ¼ A Â " abs Â r ¼ A Â " col Â " int Â r

" col ¼C source

A Â r Â " int

ð6Þ

where A is the source activity.The count rate ( C source ) and source activity ( A) are proposed above. In addition,

the peak-to-total ration ( r) is determined as 0.027 through measurements as well assimulation. The limit value level of the collimator efficiency is calculated by the Eq.(6) as 1 :5 Â 10À6 because the intrinsic efficiency ( " int ) is actually less than 1. The limitvalue level based on this calculation is shown in Fig. 5(b). The collimator efficiencymore than this limit value is required at the nuclear survey systems.

Fig. 6(a) and (b) shows line images obtained through experiments at the samediameter (4 mm) when the channel height is changed from 2 to 6 mm. These guresapparently prove our simulation results that the increase of the channel heightachieves higher resolution at the expense of decreased efficiency.

Therefore, both factors should be considered simultaneously for the optimumdesign of the pinhole collimator. Fig. 7 shows the relative value considered the reso-lution and efficiency about the height and diameter having more than the limit level.

The 4 mm pinhole diameter with the channel height (2 mm) is proposed as anoptimum condition for the nuclear survey system. The real image using this designed

Fig. 7. The relative value considered the collimator efficiencies and resolutions: it shows that the optimumvalue is the 4 mm pinhole diameter with the 2 mm channel height.




pinhole collimator is given in Fig. 8 when the 137 Cs point source is emitted at the 1 mdistance as explained above.

4. Conclusion

The channeled pinhole collimator for application at high-energy radiation eld isintroduced and its characteristics are analyzed by Monte Carlo method. When wecompared with resolutions of the knife-edge and the channel collimator at the samediameter, resolution degradation of the knife-edge collimator from the scatteringradiation near the hole was heavier than that of the channeled collimator. Using theminimum requirements of the nuclear survey system that the sensitivity should be

more than 6 for obtaining the meaningful images from background noise, the limitvalue of the collimator efficiency is calculated and the optimum height and diameterof pinhole is proposed. We get the real image with this designed pinhole at the 137 Cspoint source. This proposed method could be applied in the optimum pinhole designfor the similar nuclear survey instruments with our developed system.

In the future, we will plan to obtain radiation images in the industrial elds ornuclear facilities with this pinhole.

References

Briesmeister, J.F., 1997. MCNP—A General Monte Carlo N-Particle Transport Code Version 4B. LosAlamos National Laboratory, New Mexico.

Fig. 8. The real radiation images at the 137 Cs point source: (a) left image is displayed by 128 Â128, (b)right image is displayed by 256 Â256.




Gal, O., Jean, F., Laine, F., Leveque, C., 2000. The CARTOGAM portable gamma imaging system.IEEE Trans. Nuclear Science 47 (3), 952–956.

Guru, S.V., He, Z., Wehe, D.K., Knoll, G.F., 1995. A portable gamma imaging system for radiation

monitoring. IEEE Trans. Nuclear Science 42 (4), 940–944.Johnson, E.L., Jaszczak, R.J., Wang, H., Li, J., Greer, K.L., Coleman, R.E., 1995. Pinhole SPECT forimaging in-111 in the head. IEEE Trans. Nuclear Science 42 (4), 1126–1132.

Knoll, G.F., 1989. Radiation Detection and Measurement. John Willey and Sons, Inc., New York.Lee, W., Cho, G., Kim, H.D. in press. A radiation monitoring system with the capability of gamma

imaging and the estimation of exposure dose rate. IEEE Trans. Nuclear Science.Mortimer, R.K., Anger, H.O., 1954. The Gamma Ray Pinhole Camera with Image Amplier. Conv.

Record I.R.E., Pt 9, 2–5.Redus, R.H., Nagarkar, V., Cirignano, L.J., 1992. A nuclear survey instrument with imaging capability.

IEEE Trans. Nuclear Science 39 (4), 948–951.Redus, R., Squillante, M., Gordon, J., Knoll, G.F., Wehe, D., 1994. A combined video and gamma ray

imaging system for robots nuclear environments. NIM in Physics Research A 353, 324–327.

Redus, R.H., Squillante, M., Gordon, J.S., Bennett, P., Entine, G., 1996. An imaging nuclear surveysystem. IEEE Trans. Nuclear Science 43 (3), 1827–1831.

Smith, F.M., Jaszczak, R.J., Wang, H., 1997. Pinhole aperture design for 131 I tumor imaging. IEEETrans. Nuclear Science 44 (3), 1154–1160.


Documents

Collimator for Survey