title: Hybrid scatter correction applied to quantitative holmium-166 SPECT.
abstract: Ho-166 is a combined beta-gamma emitter of which the betas can be used therapeutically. From the 81 keV gammas of Ho-166, SPECT images can be obtained, which give opportunities to guide Ho-166 therapy. Accurate reconstruction of Ho-166 images is currently hampered by photopeak-scatter in the patient, down-scatter in the detector, collimator and patient caused by the 1.4 MeV photons and by bremsstrahlung. We developed and validated a method for quantitative SPECT of Ho-166 that involves correction for both types of scatter plus non-uniform attenuation correction using attenuation maps. Photopeak-scatter (S) is compensated for by a rapid 3D Monte Carlo (MC) method that is incorporated in ordered subset (OS) reconstruction of the emission data, together with simultaneous correction for attenuation (A) and detector response (D); this method is referred to as OS-ADS. Additionally, for correction of down-scatter, we use a 14 keV wide energy window centred at 118 keV (OS-ADSS). Due to a limited number of available energy windows, the same 118 keV energy window was used for down-scatter correction of the simultaneously acquired Gd-153 transmission data. Validations were performed using physical phantom experiments carried out on a dual-head SPECT system; Gd-153 transmission line sources were used for acquiring attenuation maps. For quantitative comparison of OS-ADS and OS-ADSS, bottles filled with Ho-166 were placed in both a cylindrical phantom and an anthropomorphic thorax phantom. Both OS-ADS and OS-ADSS were compared with an ordered subset reconstruction without any scatter correction (OS-AD). Underestimations of about 20% in the attenuation map were reduced to a few per cent after down-scatter correction. The average deviation from the true activity contained in the bottles was +72% with OS-AD. Using OS-ADS, this average overestimation was reduced to +28% and with OS-ADSS the deviation was further reduced to 16%. With OS-AD and OS-ADS, these numbers were more sensitive to the choice of volumes of interest than with OS-ADSS. For the reconstructed activity distributions, erroneous background activity found with OS-AD was reduced by a factor of approximately 2 by applying OS-ADS and reduced by a factor of approximately 4 by applying OS-ADSS. The combined attenuation, photopeak-scatter and down-scatter correction framework proposed here greatly enhanced the quantitative accuracy of Ho-166 imaging, which is of the uppermost importance for image-guided therapies. It is expected that the method, with adapted window settings, also can be applied to other isotopes with high energy peaks that contaminate the photopeak data, such as I-131 or In-111.
authors.affiliation: Department of Nuclear Medicine, Image Sciences Institute, University Medical Centre Utrecht, Universiteitsweg 100, STR 5.203, Utrecht, The Netherlands. t.c.dewit@azu.nl
authors.forename: Tim C
authors.initials: TC
authors.lastname: de Wit
authors.forename: Jianbin
authors.initials: J
authors.lastname: Xiao
authors.forename: J Frank W
authors.initials: JF
authors.lastname: Nijsen
authors.forename: Fred D
authors.initials: FD
authors.lastname: van het Schip
authors.forename: Steven G
authors.initials: SG
authors.lastname: Staelens
authors.forename: Peter P
authors.initials: PP
authors.lastname: van Rijk
authors.forename: Freek J
authors.initials: FJ
authors.lastname: Beekman
mesh_terms: D006695:Holmium / Q000494:pharmacology*; D006801:Humans; D007091:Image Processing, Computer-Assisted; D015233:Models, Statistical; D009010:Monte Carlo Method; D019047:Phantoms, Imaging; D011868:Radioisotopes / Q000494:pharmacology*; D019275:Radiopharmaceuticals / Q000737:chemistry; D012542:Scattering, Radiation; D015899:Tomography, Emission-Computed, Single-Photon / Q000379:methods*
chemical_list: D011868:Radioisotopes; D019275:Radiopharmaceuticals; D006695:Holmium