title: Quantitative accuracy of (177)Lu SPECT reconstruction using different compensation methods: phantom and patient studies.
abstract: BACKGROUND
  In targeted radionuclide therapy (TRT), accurate quantification using SPECT/CT images is important for optimizing radiation dose delivered to both the tumour and healthy tissue. Quantitative SPECT images are regularly reconstructed using the ordered subset expectation maximization (OSEM) algorithm with various compensation methods such as attenuation (A), scatter (S) and detector and collimator response (R). In this study, different combinations of the compensation methods are applied during OSEM reconstruction and the effect on the (177)Lu quantification accuracy is studied in an anthropomorphic torso phantom. In addition, the phantom results are reflected to (177)Lu-DOTA-Tyr3-octreotate ((177)Lu-DOTATATE)-treated patient data and kidney absorbed dose estimates.
  
  
  METHODS
  The torso phantom was imaged with nine various sized (0.4-104.4 cm(3)) spherical inserts, filled with known (177)Lu activity ranging from 0.5 to 105.5 MBq. Images were reconstructed using OSEM algorithm using A, AR and ARS compensation method combinations. The compensation method combinations were compared by calculating the concentration recovery coefficient (cRC) for each insert. In addition, ten (177)Lu-DOTATATE-treated patient's post-therapy dosimetry acquisitions were reconstructed, and the absorbed dose to kidneys was estimated.
  
  
  RESULTS
  cRC values depend on the insert size for all compensation methods. AR and ARS produced significantly higher cRC values than attenuation correction alone. There were no cRC value differences between the methods for the smallest 1-cm-diameter insert, cRC being 0.18. However, the collimator and detector response compensation method (R) made the 1.3-cm-diameter insert clearly visible and improved cRC estimate from 0.19 to 0.43. ARS produced slightly higher cRC values for small- and medium-sized inserts than AR. On the patient data, a similar trend could be seen. AR and ARS produced higher kidney activities than using attenuation correction alone; the total absorbed doses to the right and left kidneys were on average 15 and 20 % higher for AR and 19 and 25 % higher for ARS, respectively. The effective half-life decay estimated from time-activity curves however showed no notable difference between the compensation methods.
  
  
  CONCLUSIONS
  The highest cRC values were achieved by applying ARS compensation during reconstruction. The results were notably higher than those using attenuation correction alone. Similarly, higher activity estimates and thus higher absorbed dose estimates were found in patient data when all compensation methods were applied. ARS improved cRC especially in small-sized sources, and it thus might aid tumour dosimetry for (177)Lu PRRT treatments.
authors.affiliation: HUS Medical Imaging Center, Helsinki University Central Hospital, POB 340, FI-00029 HUS, Helsinki, Finland. eero.hippelainen@helsinki.fi.|Department of Physics, University of Helsinki, P.O. Box 64, FI-00014, Helsinki, Finland. eero.hippelainen@helsinki.fi.
authors.forename: Eero
authors.identifier: http://orcid.org/0000-0002-4021-5845
authors.initials: E
authors.lastname: Hippeläinen
authors.affiliation: Department of Oncology, Cancer Center, Helsinki University Hospital, POB 180, FI-00029 HUS, Helsinki, Finland.
authors.forename: Mikko
authors.initials: M
authors.lastname: Tenhunen
authors.affiliation: Department of Oncology, Cancer Center, Helsinki University Hospital, POB 180, FI-00029 HUS, Helsinki, Finland.
authors.forename: Hanna
authors.initials: H
authors.lastname: Mäenpää
authors.affiliation: Department of Nuclear Medicine, Joint Authority for Päijät-Häme Social and Health Care, Keskussairaalankatu 7, FI-15850, Lahti, Finland.|HERMES Medical Solutions, Stockholm, Sweden.
authors.forename: Antti
authors.initials: A
authors.lastname: Sohlberg
keywords: 177Lu; Image reconstruction; OSEM; PRRT; Quantitation; Time-activity curve