title: (90)Y -PET imaging: Exploring limitations and accuracy under conditions of low counts and high random fraction.
abstract: OBJECTIVE
  (90)Y -positron emission tomography (PET) imaging is becoming a recognized modality for postinfusion quantitative assessment following radioembolization therapy. However, the extremely low counts and high random fraction associated with (90)Y -PET may significantly impair both qualitative and quantitative results. The aim of this work was to study image quality and noise level in relation to the quantification and bias performance of two types of Siemens PET scanners when imaging (90)Y and to compare experimental results with clinical data from two types of commercially available (90)Y microspheres.
  
  
  METHODS
  Data were acquired on both Siemens Biograph TruePoint [non-time-of-flight (TOF)] and Biograph microcomputed tomography (mCT) (TOF) PET/CT scanners. The study was conducted in three phases. The first aimed to assess quantification and bias for different reconstruction methods according to random fraction and number of true counts in the scan. The NEMA 1994 PET phantom was filled with water with one cylindrical insert left empty (air) and the other filled with a solution of (90)Y . The phantom was scanned for 60 min in the PET/CT scanner every one or two days. The second phase used the NEMA 2001 PET phantom to derive noise and image quality metrics. The spheres and the background were filled with a (90)Y solution in an 8:1 contrast ratio and four 30 min acquisitions were performed over a one week period. Finally, 32 patient data (8 treated with Therasphere(®) and 24 with SIR-Spheres(®)) were retrospectively reconstructed and activity in the whole field of view and the liver was compared to theoretical injected activity.
  
  
  RESULTS
  The contribution of both bremsstrahlung and LSO trues was found to be negligible, allowing data to be decay corrected to obtain correct quantification. In general, the recovered activity for all reconstruction methods was stable over the range studied, with a small bias appearing at extremely high random fraction and low counts for iterative algorithms. Point spread function (PSF) correction and TOF reconstruction in general reduce background variability and noise and increase recovered concentration. Results for patient data indicated a good correlation between the expected and PET reconstructed activities. A linear relationship between the expected and the measured activities in the organ of interest was observed for all reconstruction method used: a linearity coefficient of 0.89 ± 0.05 for the Biograph mCT and 0.81 ± 0.05 for the Biograph TruePoint.
  
  
  CONCLUSIONS
  Due to the low counts and high random fraction, accurate image quantification of (90)Y during selective internal radionuclide therapy is affected by random coincidence estimation, scatter correction, and any positivity constraint of the algorithm. Nevertheless, phantom and patient studies showed that the impact of number of true and random coincidences on quantitative results was found to be limited as long as ordinary Poisson ordered subsets expectation maximization reconstruction algorithms with random smoothing are used. Adding PSF correction and TOF information to the reconstruction greatly improves the image quality in terms of bias, variability, noise reduction, and detectability. On the patient studies, the total activity in the field of view is in general accurately measured by Biograph mCT and slightly overestimated by the Biograph TruePoint.
authors.affiliation: Department of Nuclear Medicine, University Hospital of Nantes, Place Alexis Ricordeau, Nantes 44093, France and CRCNA-UMR 892 INSERM 6299 CNRS, 8 quai Moncousu BP 70721, Nantes 44007, France.
authors.forename: Thomas
authors.initials: T
authors.lastname: Carlier
authors.affiliation: Institute of Medical Physics, University of Sydney, Camperdown, New South Wales 2006, Australia.
authors.forename: Kathy P
authors.initials: KP
authors.lastname: Willowson
authors.affiliation: Department of Radiation Oncology, Allegheny General Hospital, Pittsburgh, Pennsylvania 15212.
authors.forename: Eugene
authors.initials: E
authors.lastname: Fourkal
authors.affiliation: Faculty of Health Sciences, University of Sydney, Lidcombe 2141, Australia and Department of Nuclear Medicine, Royal North Shore Hospital, St Leonards, New South Wales 2065, Australia.
authors.forename: Dale L
authors.initials: DL
authors.lastname: Bailey
authors.affiliation: Department of Diagnostic Imaging, Fox Chase Cancer Center, Philadelphia, Pennsylvania 19111.
authors.forename: Mohan
authors.initials: M
authors.lastname: Doss
authors.affiliation: Siemens Healthcare Molecular Imaging, 810 Innovation Drive, Knoxville, Tennessee 37932.
authors.forename: Maurizio
authors.initials: M
authors.lastname: Conti
mesh_terms: D000465:Algorithms; D006801:Humans; D064847:Multimodal Imaging / Q000295:instrumentation; D019047:Phantoms, Imaging; D017785:Photons; D049268:Positron-Emission Tomography / Q000295:instrumentation / Q000379:methods*; D019275:Radiopharmaceuticals; D014867:Water; D055114:X-Ray Microtomography / Q000295:instrumentation; D015021:Yttrium Radioisotopes
chemical_list: D019275:Radiopharmaceuticals; D015021:Yttrium Radioisotopes; D014867:Water