Total-body dynamic PET/CT imaging reveals kinetic distribution of [¹³N]NH₃ in normal organs

To systematically investigate kinetic metrics and metabolic trapping of [¹³N]NH₃ in organs.

Eleven participants performed total-body [¹³N]NH₃ dynamic positron emission tomography (PET). Regions of interest were drawn in organs to obtain time-to-activity curves (TACs), which were fitted with an irreversible two-tissue compartment model (2TC) to investigate constant rates K₁, k₂ and k₃, and to calculate Ki. Additionally, one-tissue compartment model using full data (1TC_full) and the first four minutes of data (1TC_4min) were fitted to TAC data. K₁ and k₂ were compared among different models to assess [¹³N]NH₃ trapping in organs.

Kinetic rates of [¹³N]NH₃ varied significantly among organs. The mean K₁ ranged from 0.049 mL/cm³/min in the muscle to 2.936 mL/cm³/min in the kidney. The k₂ and k₃ were lowest in the liver (0.001 min^- 1) and in the pituitary (0.009 min^- 1), while highest in the kidney (0.587 min^- 1) and in the liver (0.800 min^- 1), respectively. The Ki was largest in the myocardium (0.601 ± 0.259 mL/cm³/min) while smallest in the bone marrow (0.028 ± 0.022 mL/cm³/min). Three groups of organs with similar kinetic characteristics were revealed: (1) the thyroid, the lung, the spleen, the pancreas, and the kidney; (2) the liver and the muscle; and (3) the cortex, the white matter, the cerebellum, the pituitary, the parotid, the submandibular gland, the myocardium, the bone, and the bone marrow. Obvious k₃ was identified in multiple organs, and significant changes of K₁ in multiple organs and k₂ in most organs were found between 2TC and 1TC_full, but both K₁ and k₂ were comparable between 2TC and 1TC_4min.

The kinetic rates of [¹³N]NH₃ differed among organs with some have obvious ¹³N-anmmonia trapping. The normal distribution of kinetic metrics of ¹³N-anmmonia in organs can serve as a reference for its potential use in tumor imaging.