Quantitative Imaging of Scandium Isotopes with a Long Axial Field-of-View PET/CT
Objective. $^{43}$Sc and $^{44}$Sc are positron-emitting isotopes of scandium that have attracted growing interest for theranostic applications in conjunction with the therapeutic isotope $^{47}$Sc. Both isotopes are non-pure positron emitters, and the associated prompt-gamma emission together with the high positron energy introduce complexities in achieving precise quantitative imaging. Approach. We evaluated the quantitative imaging performance of $^{44}$Sc and a $^{43/44}$Sc mixture on a long axial field-of-view (LAFOV) PET/CT using a NEMA IEC body phantom. We assessed contrast recovery coefficients, image noise and bias for different numbers of iterations and for absolute versus relative scatter correction, and implemented an isotope-specific point-spread-function (PSF) correction based on FLUKA-simulated positron ranges. Main results. The quantitative imaging performance of both isotopes is comparable to $^{18}$F, and the vendor's prompt-gamma correction effectively suppresses the additional random coincidences and lung error caused by prompt-gamma emission. The isotope-specific PSF correction further improves contrast recovery. While the bias obtained for $^{44}$Sc meets the EARL acceptance criterion for non-standard radionuclides, the bias for the $^{43/44}$Sc mixture exceeds it, which we attribute to the mixture-specific cross-calibration of the dose calibrators rather than to the PET imaging. Significance. Our results support the use of LAFOV PET/CT for quantitative imaging of scandium isotopes and provide practical reconstruction recommendations for clinical protocols.