A Study of External End-shields for PET

T. Hasegawa Kitasato University, School of Allied Health Sciences, Japan

H . Murayama and T. Nakajima National Institute of Radiological Sciences

H . Matsuura and Y. Wada Siemens-dsahi Medical Technologies Ltd., Nuclear Medicine Product Group

Abstract The effects of external end-shields in screening

radioactivity out-of the field-of-view were studied by phantom experiments with a help of Monte-Carlo simulation for ECAT EXACT HR+. Random coincidence rates were measured for a standard cylindrical phantom at out-of the field-of-view with and without an external end-shield. The results were well reproduced with Monte-Carlo simulation. Various single count rates were calculated as a function of the detector ring number by Monte-Carlo simulation.

I. INTRODUCTION Radioactivity out-of the field-of-view (FOV) has been

a concern in 3D mode of Positron Emission Tomography (PET) [l, 2, 31. It possibly increases accidental (random) coincidence, which increases the statistical noise of images and deteriorates the count rate characteristics. In addition, i t may affect the scatter correction accuracy since most of the standard correction algorithms do not have the potential to correct scatters from out-of-FOV radioactivity at present.

It was studied that an external end-shield was efficient to screen the out-of-FOV radioactivity 14, 51. A body- shield around a subject also was shown to be efficient [SI.

The efficiency of an end-shield depends on its diameter, thickness, substance, position and the distribution of radioactivity and materials. The aim of this study is to understand the efficiency of an external end-shield for a PET scanner by phantom experiments with a help of Monte- Carlo simulation.

11. MATERIALS AND METHODS

A . Phantom experiments The PET scanner was ECAT EXACT HR+

(CTI/SIEMENS, Knoxville, TN, USA.) It was operated in 3D mode with the standard data-taking parameters (span 9 and mrd 22). Each end-shield was made of a circular lead board attached with a steel board. Their sizes are shown in table 1. It was installed at an edge of the patient port as shown in Fig. 1. A standard cylindrical phantom (20 cm in inner-diameter and 18.5 cm in inner-length) filled with water containing "F radioactivity was set at 34 cm apart from the FOV by the

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Figure 1: An external end-shield (tlbd350) installed on the PET scanner temporarily.

B. Monte-Carlo simulation Our Monte-Carlo code was based on GEANT3.21 [7].

The lower energy threshold was set at 350 keV. It was confirmed that simulated sensitivity and scatter fraction agreed with phantom measurements with relative accuracy of better than 5 %.

A single count rate was obtained from the simulation for each detector ring. Random count rates were calculated from the single count rates. The present preliminary study simulated 100 M decays in the out-of-FOV phantom for each condition.

111. PHANTOM EXPERIMENTS Figure 2 shows measured random count rates. For

the lightest end-shield (tlO-d350), the random count rate became about 3 percent of that without an end-shield (no-exsh). The rate was slightly smaller for the thicker end-shield with the same diameter (t20-d350). For the end-shield with the smaller inner-diameter (tlO-d300), the rate decreased largely. These results show that the geometrical area of the end-shield is the most important specification at the first approximation.

IV. MONTE-CARLO CALCULATIONS

Simulated random count rates also were shown in Fig. 2. They were in good agreement with the experimental results.

abbreviations no-exsh tlGd350 t20-d350 t 10-d300

0.01 0.1 1 simulated random rates (/(MBq) 9

lead thickness steel thickness inner diameter outer diameter 0tnU-i 0 mm 10 mm 3.2 mm 350 mm 550 mm 20 mm 6.4 mm 350 mm 550 mm 10 mm 3.2 mm 300 mm 550 mm

Figure 2: Measured random count rates in comparison with Monte-Carlo calculations.

Figure 3 and 4 show projection views of simulated gamma-ray tracks with and without an external end-shield, respectively. These figures support intuitively that the end-shield effects are primarily determined by the geometrical screening. In addition, it can be seen that some tracks were scattered by the external end-shields to be detected.

Various single counts were calculated by Monte-Carlo simulation and shown in Fig. 5. It was clearly shown that the difference of the lightest end-shield and the thicker end- shield came from the single counts with scattering in the end-shields. The single count with end-shield scattering for the end-shield t10-d300 amounted to about one-fifth of the total single count.

Figures 6, 7, 8 and 9 show simulated single counts with and without end-shield scattering as a function of the detector ring number. As expected from the simple geometrical consideration, the counts were significantly larger in the rings far from the out-of-FOV radioactivity than in the rings close to it.

V. DISCUSSION In considering the effects of an external end-shield, the

positions of the end-shield and the radioactivity source are important although they were not changed in this

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Figure 3: Projected tracks for 500 decays without an external end-shield.

study. The random rates are expected to change largely with changing their position as expected from the simple geometrical consideration and as shown in other studies. In addition, the material distribution also should be cared since it increases scattering as well as absorption. In order to know the effects of an additional material closer to the FOV, Monte-Carlo simulation was performed with an additional water cylindrical phantom (2-ph) and the lightest end-shield (tlO-d350) as shown in Fig. 10. Calculated single counts are shown in Fig. 11. The total singles decreased slightly in this case.

A body-shield made of a flexible thiner lead board was experimentally studied to be efficient for reducing the random count rates [6]. Single counts were calculated by

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410'

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singles with exsh scattorlng (110-6350) -

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Figure 6: Simulated single counts as a function of the detector ring numbera for no external end-shield per 100 M decays in the outof-FOV phantom.

Figure 4: end-shield (tlO-d350).

Projected tracks for 500 decays with an external

Figure 7: Simulated single counts as a function of the detector ring numbers for the external end-shield t10-d350 per 100 M decays in the out-of-FOV phantom.

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410' . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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d tolpl angle ccunla per 10M decays

Figure 5: out-of-FOV phantom.

Monte-Carlo simulation for a body-shield with thickness of 3.2 mm, a length of 20 cm and a diameter of 21 cm at the 10 cm close to the FOV from the phantom by the centers, and compared with the results for the lightest end-shield in Fig. 11. The single count with the shield

Simulated single counts per 10 M decays in the rlng-nmber

Figure 8: Simulated single counts as a function of the detector ring numbers for the external end-shield t20-d350 per 100 M decays in the out-of-FOV phantom.

scattering had a larger fraction than for the end-shield. In finding the beat shield, their thickness, shape and

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*IO',.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

total sln$es wllh watering In sMeM total 6ingles with scaneflng in 1st phantom total singles Mlh sc8terlna in 2nd phantom

0 1 lo" total singles with end wiRou( scattering

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ring-number Figure 11: Simulated single counts per 10 M decays in the outof-Fov phantom for a body-shield (body-+h) and

end-ehield (t10-d350), in comparison with the results for one phantom and an end-shield (tlO-d350).

Figure 9: Simulated single counts as a function of the detector ring numbers for 'he external t10-d300 per loo one phantom (l-ph), and for two phantoms (2-ph) and an decays in the outof-FOV phantom.

VI. CONCLUSION It was confirmed by phantom experiment and

Monte-Carlo simulation that the random reduction due to the external end-shields was primarily determined by the simple geometrical screening effect. In addition, it was shown that the scattering by the shields should be paid an attention in particular for a thinner shield with a larger area. The Monte-Carlo simulation was shown to play an indispensable role in understanding the effects of the shields.

VII. ACKNOWLEDGMENTS We are grateful to C. Michel about the Monte-Carlo

code and K. Kawashima. This study was supported in part by grants from the Ministry of Education, Science and Culture of Japan (Grant-in-Aid No. 10770453) and from Kitasato University (Grant-in-Aid No. SAHS-B033- 1997 and SAHS-B065-1998).

VIII. REFERENCES [l] V.Sossi, J.S.Barney, R.Harrieon and T.J.Ruth, "Effect

of scatter from radioactivity outaide of the field of view in 3D PET", IEEE Trans. Nucl. Sci., vol. 42(4), 1995

[Z] N.C.Ferreira, R.Tr4bossen and B.Bendriem, "Assessment of 3D-PET quantitation: influence of out of the field of view radioactive sources and attenuating media" 1 IEEE Z h n s - NuC1. Sc2.1 vol. 45(3), 1998 pp. 1670-1675.

[3] T.Hasegawa, M. Suzuki, H. Murayama, T. Irie, K. Fukushi and Y. Wada, IEEE %ns. Nucl. Sci., (in press), 1999.

[4] S.Yamamoto, S.Miura, Y.Shoji, H.Iida and I.Kanno, "Development of a front shield for a 3D positron emission tomograph", Kakuigotu, vol. 33(6), 1996 pp. 641-646 (in Japanese with English abstract).

pp. 1157-1161.

Figure 10: Projected tracks for 500 decays with two phantoms (2-ph) and an end-shield (tlPd350).

position should be carefully selected for a specific PET study with the knowledge about the radioactivity and material distribution as an important premise.

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[5] T.J Spinks, M.P.Miller, D.L.Bailey, P.M.Bloodeld, L.Livieratos and TJones, “The effect of activity outside the direct field of view in B 3D-ody wholebody positron tomograph”, Phys. Med. Biol., vol. 43, 1998

[6] M.E.Daube-Witherspoon, A.Belakhlef, S.L.Green and I.Zanzi, “Design of Patient Shielding to Reduce the Effects of Out-of-Field Radioactivity in 3D PET”, Conference Record of 1998 IEEE Nuclear Science .Symposium and Medical Imaging Conference, 8-14 November 1998, Tronto, Canada.

[7] European Laboratory for Particle Physics (CERN, Conseil European pour la Recherche Nucleaire), lnformation Technology Division (IT), CH-1211 Geneva 23 (Switzerland).

pp. 895-904.

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