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

Lead-212 (²¹²Pb) is being investigated for alpha therapies, but it can be challenging to image. To investigate the quantitative accuracy of ²¹²Pb SPECT images for patient geometries and low activity concentrations, we imaged an anthropomorphic phantom with ²¹²Pb, and studied the deviations of SPECT derived activity concentrations.

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 [¹⁸F]F-PSMA-1007 PET image was used as a basis for the relative distribution of ²¹²Pb 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 ²¹²Pb for three acquisitions and 1.0-1.1 MBq ²¹²Pb 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.

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.

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.