Assessing the feasibility of quantitative SPECT imaging for low 212Pb activity concentrations using anthropomorphic phantoms

Abstract Background Lead-212 ( 212 Pb) is being investigated for alpha therapies, but it can be challenging to image. To investigate the quantitative accuracy of 212 Pb SPECT images for patient geometries and low activity concentrations, we imaged an anthropomorphic phantom with 212 Pb, and studied the deviations of SPECT derived activity concentrations. Methods Fillable phantom compartment shells of the kidneys, liver and five vertebrae (T11-L3) were 3D-printed based on a patient’s CT-images. The same patient’s 18 FF-PSMA-1007 PET image was used as a basis for the relative distribution of 212 Pb activity within the phantom compartments. The phantom was imaged with a Siemens Symbia Intevo Bold SPECT/CT, with a total of 3.4–3.8 MBq 212 Pb for three acquisitions and 1.0–1.1 MBq 212 Pb for three acquisitions, while recording energy windows centred at 79 keV and 239 keV. The SPECT images were reconstructed with Siemens’ Flash-3D with a variety of iterations, subsets, and matrix sizes. Activity concentrations for each phantom compartment were measured from the images using a calibration factor measured in a uniform cross calibration phantom and compared to the activity concentrations in 1 ml samples extracted from each compartment, which were analysed using a gamma counter. Results Quantification was relatively stable across energy windows and matrix sizes, but best results were achieved using 30–120 reconstruction updates. Low activity concentration volumes representing background and vertebral bodies (0.03–0.05 kBq/ml) were not quantifiable with deviations over 400% for all investigated reconstructions. The activity concentrations in the liver and kidneys were underestimated by 10–50% compared to the gamma counter measurements. Precision between SPECT acquisitions was higher for the larger image matrix, with standard deviations of liver and kidney measurements less than 6% for the higher activity images, and less than 8% for the lower activity images. Conclusion We found that larger volumes, such as liver and kidneys with at least 210 Bq/ml, may be quantifiable with an accuracy of approx. 30–40%. While very low activity concentrations below 54 Bq/ml were not quantifiable, this still indicate carefully used imaging results to be of value in dosimetric calculations, also when characterising latter parts of time activity curves.