Quantitative SPECT/CT imaging of lead-212: a phantom study

Lead-212 (²¹²Pb) is a promising radionuclide for targeted therapy, as it decays to α-particle emitter bismuth-212 (²¹²Bi) via β-particle emission. This extends the problematic short half-life of ²¹²Bi. In preparation for upcoming clinical trials with ²¹²Pb, the feasibility of quantitative single photon-emission computed tomography/computed tomography (SPECT/CT) imaging of ²¹²Pb was studied, with the purpose to explore the possibility of individualised patient dosimetric estimation.

Both acquisition parameters (combining two different energy windows and two different collimators) and iterative reconstruction parameters (varying the iterations x subsets between 10 × 1, 15 × 1, 30 × 1, 30 × 2, 30 × 3, 30 × 4, and 30 × 30) were investigated to evaluate visual quality and quantitative uncertainties based on phantom images. Calibration factors were determined using a homogeneous phantom and were stable when the total activity imaged exceeded 1 MBq for all the imaging protocols studied, but they increased sharply as the activity decayed below 1 MBq. Both a 20% window centred on 239 keV and a 40% window on 79 keV, with dual scatter windows of 5% and 20%, respectively, could be used. Visual quality at the lowest activity concentrations was improved with the High Energy collimator and the 79 keV energy window. Fractional uncertainty in the activity quantitation, including uncertainties from calibration factors and small volume effects, in spheres of 2.6 ml in the NEMA phantom was 16-21% for all protocols with the 30 × 4 filtered reconstruction except the High Energy collimator with the 239 keV energy window. Quantitative analysis was possible both with and without filters, but the visual quality of the images improved with a filter.

Only minor differences were observed between the imaging protocols which were all determined suitable for quantitative imaging of ²¹²Pb. As uncertainties generally decreased with increasing iterative updates in the reconstruction and recovery curves did not converge with few iterations, a high number of reconstruction updates are recommended for quantitative imaging.