Indian Journal of Nuclear Medicine
Home | About IJNM | Search | Current Issue | Past Issues | Instructions | Ahead of Print | Online submissionLogin 
Indian Journal of Nuclear Medicine
  Editorial Board | Subscribe | Advertise | Contact
Users Online: 852 Print this page  Email this page Small font size Default font size Increase font size

 Table of Contents     
Year : 2019  |  Volume : 34  |  Issue : 1  |  Page : 24-26  

Evaluation of acceptance angle in iodine-131 single photon emission computed tomography imaging with monte carlo simulation

1 Department of Physics, LPHE, Modeling and Simulations, Faculty of Science, Mohammed V University, Rabat, Morocco
2 Department of Medical Physics, Academic Hospital of Udine, Udine, Italy

Date of Web Publication17-Jan-2019

Correspondence Address:
Dr. Youssef Bouzekraoui
Department of Physics, LPHE, Modeling & Simulations, Faculty of Science, Mohammed V University, Rabat
Login to access the Email id

Source of Support: None, Conflict of Interest: None

DOI: 10.4103/ijnm.IJNM_109_18

Rights and Permissions

Introduction: In iodine-131 (I-131) imaging, the image quality is degraded by scatter and penetration in a collimator. In this work, we assessed the penetrated and the scattered photon fractions in the photopeak energy window using Monte Carlo Simulation code. Materials and Methods: The Siemens Medical System equipped with high-energy collimator was simulated. We evaluated the acceptance angle values on geometric, penetration, and scatter components in a separate file. Binary images in a data file are obtained and each one of them was imported in software ImageJ. Full-width at half-maximum (FWHM) and sensitivity were calculated and compared. Results: The simulation data show that for the acceptance angle value equal to 4.845°, the geometric, scatter, and penetration components were 93.20%, 4.13%, and 2.67%, respectively. Moreover, the resolution is improved (FWHM = 7.21 mm, full width at tenth maximum = 12.36 mm) for a point source at 12 cm from the detector. Conclusion: The small acceptance angle has a major impact on the image quality in I-131 single photon emission computed tomography.

Keywords: Acceptance angle, geometrical, I-131, penetration, scatter, simulation Monte Carlo imaging nuclear detector

How to cite this article:
Asmi H, Bentayeb F, Bouzekraoui Y, Bonutti F. Evaluation of acceptance angle in iodine-131 single photon emission computed tomography imaging with monte carlo simulation. Indian J Nucl Med 2019;34:24-6

How to cite this URL:
Asmi H, Bentayeb F, Bouzekraoui Y, Bonutti F. Evaluation of acceptance angle in iodine-131 single photon emission computed tomography imaging with monte carlo simulation. Indian J Nucl Med [serial online] 2019 [cited 2022 Oct 4];34:24-6. Available from:

   Introduction Top

The accurate quantification of iodine-131 (I-131) activity, which is estimated from scintigraphic images, has great importance because of the recent success of using it in thyroid cancer therapy, as well as in liver cancer therapy.[1] However, quantitative imaging is limited mainly by the phenomena of scattering and penetration into the septa of a collimator which leads to an error in the determination of activity. The highest intensity emissions of I-131 are 284 (6.1%), 364 (82%), 637 (7.2%), and 723 keV (1.8%).[2] The photons of 637 and 723 keV undergo only a slight attenuation in the phantom, and they have a higher probability of penetrating the collimators. They are counted in the window of the photopeak 364 keV.[3],[4],[5] The I-131 imaging was evaluated using its 364-keV photons.[6] Several methods have been proposed to correct scattering and septal penetration in I-131 imaging.[7],[8],[9] However, no method has yet been universally successful. The solution is to use a Monte Carlo simulation code such as simulation Monte Carlo imaging nuclear detector (SIMIND),[10] for scatter and penetration evaluation to develop a method for correcting these events. The aim of this work was to evaluate the imaging parameters of I-131, especially the collimator acceptance angle to obtain the optimum conditions allowing as much as possible the precise quantification of the activity.

   Materials and Methods Top

We simulated the Siemens Medical Systems equipped with high-energy collimator using Monte Carlo simulation SIMIND code version 5.0. We used the following imaging parameters: 0.95 cm NaI (Tl) crystal thickness, intrinsic spatial resolution of 1.2 cm, and energy resolution of 9.80% at 140 keV. The dimension of the crystal was 50 cm × 40 cm. The energy window setting was 20% at 364 keV. The collimator data used during the simulation are given in [Table 1].
Table 1: Design parameters of high energy collimator

Click here to view

A cylindrical water phantom of dimension 22 cm × 30 cm × 22 cm was placed at 20 cm from the detector surface. We used the SIMIND Monte Carlo simulation to acquire the data from I-131 point source of 0.005-cm diameter located at the center of the cylinder phantom.

The images have 0.3 cm pixel size and 128 × 128 matrix size. We imported binary images created by SIMIND in ImageJ software, National Institutes of Health and the Laboratory for Optical and Computational Instrumentation (LOCI, University of Wisconsin.[11]

In this study, we used the acceptance angles to evaluate the image quality.

Photons are isotropically emitted from a point source O, but the photons which are only propagating within the angle α can be detected [Figure 1]. The angle α can be defined as the collimator acceptance angle, and it is determined by the ratio of the hole size and length of the collimator.[12]
Figure 1: Schematic of acceptance angle

Click here to view

Practically, the acceptance angle α is small to eliminate most of the tilted rays.

   Results and Discussion Top

The contribution of geometric, penetration, and scatter photons was calculated for a point source at 20 cm from the detector surface as shown in [Table 2]. When the acceptance angle is 45°, the geometric component is very weak and does not carry even half of the detected photons. The scattering and penetration components are increased by the contribution of the 637-keV and 723-keV photons in the photopeak energy window, and thus, the geometric photons are decreased.[3],[4] When 4.845° is used, we notice that the geometrical component goes up to (82% at 20 cm) that explains why the contribution of the photons of the upper peaks becomes weak. This is due to small acceptance angle.
Table 2: Contribution of geometric, septal penetration, and scattered photons

Click here to view

[Figure 2] diagram shows that blue and green spectra are obtained when the acceptance angles are 45° and 4.845°, respectively. The counts of blue spectrum are more than the one in the green spectrum. That can be explained by the existence of other peaks of high energies such as 637 keV and 723 keV which can be detected by the detector after the scattering in the collimator. Hence, the photopeak energy window in the case of 45° contains an important part of the scattered photons in comparison to the case of 4.845°.
Figure 2: Energy spectra for iodine-131 source for two acceptance angles values: Blue spectrum (45°), green spectrum (4.845°)

Click here to view

A star like appears in the image of [Figure 3]a resulted from septal penetration, while this star is not as clear as the image for [Figure 3]b.
Figure 3: Images of iodine-131 point source obtained with two acceptances angles: (a) 45° and (b) 4.845°

Click here to view

According to [Table 3], the best resolution is obtained when the distance between the source and the detector is 12 cm full-width at half-maximum (FWHM = 7.21 mm, full-width at tenth maximum [FWTM] =12.36 mm), but the sensitivity slightly decreases at the same distance.
Table 3: The results of the simulations at three different distances from the detector

Click here to view

[Figure 4] shows that the sensitivity becomes large when using the 3-cm thickness value.
Figure 4: Variation of sensitivity with NaI (Tl) crystal thickness

Click here to view

   Conclusion Top

In this study, we have evaluated the imaging parameters for I-131 using Monte Carlo simulation. The obtained results show that the image quality very depends on the acceptance angle. When its value equal to 4.85°, the good results were obtained for the geometric component (81.98%) and the resolution (FWHM = 7.21 mm, FWTM = 12.36 mm).

Financial support and sponsorship


Conflicts of interest

There are no conflicts of interest.

   References Top

van der Have F, Ivashchenko O, Goorden MC, Ramakers RM, Beekman FJ. High-resolution clustered pinhole (131) Iodine SPECT imaging in mice. Nucl Med Biol 2016;43:506-11.  Back to cited text no. 1
Kocher DC. Radioactive Decay Data Tables: A Handbook of Decay Data for Application to Radiation Dosimetry and Radiological Assessment. DOE/TIC-11026. Springfield, VA: National Technical Information Service, U.S. Department of Energy; 1981. p. 133-4.  Back to cited text no. 2
Autret D, Bitar A, Ferrer L, Lisbona A, Bardiès M. Monte Carlo modeling of gamma cameras for I-131 imaging in targeted radiotherapy. Cancer Biother Radiopharm 2005;20:77-84.  Back to cited text no. 3
Dewaraja YK, Ljungberg M, Koral KF. Characterization of scatter and penetration using Monte Carlo simulation in 131I imaging. J Nucl Med 2000;41:123-30.  Back to cited text no. 4
Rault E, Vandenberghe S, Van Holen R, De Beenhouwer J, Staelens S, Lemahieu I, et al. Comparison of image quality of different iodine isotopes (I-123, I-124, and I-131). Cancer Biother Radiopharm 2007;22:423-30.  Back to cited text no. 5
Kojima A, Gotoh K, Shimamoto M, Hasegawa K, Okada S. Iodine-131 imaging using 284 keV photons with a small animal CZT-SPECT system dedicated to low-medium-energy photon detection. Ann Nucl Med 2016;30:169-75.  Back to cited text no. 6
Lee YS, Kim JS, Kim KM, Lim SM, Hee-Joung K. Determination of energy windows for the triple energy window scatter correction method in I-131 on a Siemens SYMBIA gamma camera: A GATE simulation study. Sissa Medialab 2015;10:1-8.  Back to cited text no. 7
Dewaraja Y, Li J, Koral K. Quantitative 131I SPECT with triple energy window Compton scatter correction. IEEE Trans Nucl Sci 1998;45:3109.  Back to cited text no. 8
Bong JK, Son HK, Lee JD, Kim HJ. Improved scatter correction for SPECT images: A Monte Carlo study. IEEE Trans Nucl Sci 2005;52:1263.  Back to cited text no. 9
Ljungberg M. The SIMIND Monte Carlo program home page. Available from: [Last accessed on 2018 Aug].  Back to cited text no. 10
Ferreira T, Rasband W. Image J Program. Available from: [Last accessed on 2018 Aug].  Back to cited text no. 11
Cao Z, Qian L. A triple-head SPECT system with parallel-hole collimators of different acceptance angles. Int J Med Inform 1997;46:145-57.  Back to cited text no. 12


  [Figure 1], [Figure 2], [Figure 3], [Figure 4]

  [Table 1], [Table 2], [Table 3]


    Similar in PUBMED
   Search Pubmed for
   Search in Google Scholar for
 Related articles
    Access Statistics
    Email Alert *
    Add to My List *
* Registration required (free)  

  In this article
    Materials and Me...
    Results and Disc...
    Article Figures
    Article Tables

 Article Access Statistics
    PDF Downloaded167    
    Comments [Add]    

Recommend this journal