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An attenuation method for reducing count rate losses in preclinical PET during intratherapeutic imaging

Mellhammar, Emma LU ; Dahlbom, Magnus; Evans-Axelsson, Susan LU ; Axelsson, Johan; Tran, Thuy LU and Strand, Sven-Erik LU (2017) 2016 IEEE Nuclear Science Symposium, Medical Imaging Conference and Room-Temperature Semiconductor Detector Workshop, NSS/MIC/RTSD 2016 2017-January.
Abstract

In pre-clinical imaging, tumor response to radionuclide therapy can be monitored with PET imaging. Radionuclides used for therapy such as 177Lu emit a significant amount of low energy photons. These photons may have an energy high enough to penetrate the imaged object and are the prone to be detected. Although these phtons are likely to be rejected electronically, they add dead-time to the system since they need to be processed by the electronics. This is a problem in high-sensitivity pre-clinical PET system with a low number of readout channels, such as the Genisys G8 investigated in this work. The low energy gammas may also affect image quality due to increased probability of pulse pile-up. The use of high-attenuating... (More)

In pre-clinical imaging, tumor response to radionuclide therapy can be monitored with PET imaging. Radionuclides used for therapy such as 177Lu emit a significant amount of low energy photons. These photons may have an energy high enough to penetrate the imaged object and are the prone to be detected. Although these phtons are likely to be rejected electronically, they add dead-time to the system since they need to be processed by the electronics. This is a problem in high-sensitivity pre-clinical PET system with a low number of readout channels, such as the Genisys G8 investigated in this work. The low energy gammas may also affect image quality due to increased probability of pulse pile-up. The use of high-attenuating shields designed to absorb most of the low energy photons emitted from the therapeutic radionuclide were investigated. Cylindrical led shields were constructed with thicknesses between 1 and 3 mm. A 3mm thick cylindrical shield was also constructed out of Rose metal (50% Bi, 28% Pb, and 22% Sn). The diameter of the shield was wide enough to accommodate a NEMA IQ phantom and a mouse. The attenuation of the shields for annihilation radiation was measured with a 22Na point source placed at the center of the FOV. Measurements of the coincidence rate were performed with the lead shields in place. At a of thicknesses of 1 and 3mm, the coincidence rate was reduced by a factor or 0.70 and 0.40, respectively. To study the effect of the presence of a background of low energy gammas on the coincidence count rate and the efficacy of the lead shields, 177Lu was added to a 1 cm diameter hollow sphere. In the presence of 100 MBq of 177Lu, the coincidence count rate was reduced by a factor 0.20 due to the detector dead-time. Although the count rate was reduced by a factor of 0.40 with the 3mm shield around the source, the dead-time effects due to the 177Lu background were less than 7%. 18F imaging of a NEMA phantom and a tumor bearing mouse showed dramatic image distortions in the presence of the 177Lu background. When imaging of with the 3mm shield in place, the image distortions were eliminated and were comparable in to the images acquired without the background activity.

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author
organization
publishing date
type
Chapter in Book/Report/Conference proceeding
publication status
published
subject
host publication
2016 IEEE Nuclear Science Symposium, Medical Imaging Conference and Room-Temperature Semiconductor Detector Workshop, NSS/MIC/RTSD 2016
volume
2017-January
publisher
Institute of Electrical and Electronics Engineers Inc.
conference name
2016 IEEE Nuclear Science Symposium, Medical Imaging Conference and Room-Temperature Semiconductor Detector Workshop, NSS/MIC/RTSD 2016
conference location
Strasbourg, France
conference dates
2016-10-29 - 2016-11-06
external identifiers
  • scopus:85041818922
ISBN
9781509016426
DOI
10.1109/NSSMIC.2016.8069462
language
English
LU publication?
yes
id
55f834a5-c583-4ea6-9025-918040452f0a
date added to LUP
2018-02-23 11:56:20
date last changed
2019-02-20 11:08:27
@inproceedings{55f834a5-c583-4ea6-9025-918040452f0a,
  abstract     = {<p>In pre-clinical imaging, tumor response to radionuclide therapy can be monitored with PET imaging. Radionuclides used for therapy such as <sup>177</sup>Lu emit a significant amount of low energy photons. These photons may have an energy high enough to penetrate the imaged object and are the prone to be detected. Although these phtons are likely to be rejected electronically, they add dead-time to the system since they need to be processed by the electronics. This is a problem in high-sensitivity pre-clinical PET system with a low number of readout channels, such as the Genisys G8 investigated in this work. The low energy gammas may also affect image quality due to increased probability of pulse pile-up. The use of high-attenuating shields designed to absorb most of the low energy photons emitted from the therapeutic radionuclide were investigated. Cylindrical led shields were constructed with thicknesses between 1 and 3 mm. A 3mm thick cylindrical shield was also constructed out of Rose metal (50% Bi, 28% Pb, and 22% Sn). The diameter of the shield was wide enough to accommodate a NEMA IQ phantom and a mouse. The attenuation of the shields for annihilation radiation was measured with a 22Na point source placed at the center of the FOV. Measurements of the coincidence rate were performed with the lead shields in place. At a of thicknesses of 1 and 3mm, the coincidence rate was reduced by a factor or 0.70 and 0.40, respectively. To study the effect of the presence of a background of low energy gammas on the coincidence count rate and the efficacy of the lead shields, <sup>177</sup>Lu was added to a 1 cm diameter hollow sphere. In the presence of 100 MBq of <sup>177</sup>Lu, the coincidence count rate was reduced by a factor 0.20 due to the detector dead-time. Although the count rate was reduced by a factor of 0.40 with the 3mm shield around the source, the dead-time effects due to the <sup>177</sup>Lu background were less than 7%. 18F imaging of a NEMA phantom and a tumor bearing mouse showed dramatic image distortions in the presence of the <sup>177</sup>Lu background. When imaging of with the 3mm shield in place, the image distortions were eliminated and were comparable in to the images acquired without the background activity.</p>},
  author       = {Mellhammar, Emma and Dahlbom, Magnus and Evans-Axelsson, Susan and Axelsson, Johan and Tran, Thuy and Strand, Sven-Erik},
  isbn         = {9781509016426},
  language     = {eng},
  location     = {Strasbourg, France},
  month        = {10},
  publisher    = {Institute of Electrical and Electronics Engineers Inc.},
  title        = {An attenuation method for reducing count rate losses in preclinical PET during intratherapeutic imaging},
  url          = {http://dx.doi.org/10.1109/NSSMIC.2016.8069462},
  volume       = {2017-January},
  year         = {2017},
}