Absolute quantification in multi-pinhole micro-SPECT for different isotopes

In preclinical Single Photon Emission Tomography (SPECT), absolute quantification is interesting, expressed in percentage of injected radioactive dose per gram of tissue. This allows for accurate evaluation of disease progression and precise follow-up studies without the need for sacrificing animals. Accurate modeling of image degrading effects is currently under development for isotopes different from ^99mTc. The aim of this work is to develop absolute micro-SPECT quantification for three different isotopes: ^99mTc, ¹¹¹In and ¹²⁵I. This selection of isotopes covers a wide range of energies, is pre-clinically relevant and allows us to optimally validate the algorithms used for image reconstruction. Furthermore, we will mix these isotopes with additional iodine-based CT contrast agent, to mimic contrast-enhanced SPECT/CT protocols. For each isotope, both a calibration phantom and three 1-ml vials were scanned on the CZT-based FLEX Triumph-I scanner (GM-I). The calibration phantom allows the conversion of reconstructed voxel counts to MBq/ml. The 3-vial phantom consists of 3 different concentrations of radioactivity. Two vials contain iodine-based CT contrast agent to significantly increase the attenuation. All datasets were reconstructed using a GPU-based reconstruction algorithm, which includes resolution recovery, pinhole penetration, geometrical sensitivity correction, scatter correction and attenuation correction. We show good quantification for ^99mTc and ¹¹¹In. The absolute quantification of I is suboptimal, due to insufficient scatter correction. No influence can be seen when iodine-based CT contrast agent is used together with ¹²⁵I.