[18F]FLT PET Tracer Production, PET/MR Imaging and ex vivo Biodistribution

Fluorine-18 was produced as [¹⁸F]fluoride at a PETtrace cyclotron using the ¹⁸O(p,n)¹⁸F nuclear reaction, and [¹⁸F]FLT was synthesized as described elsewhere [27]. Approximately 11–13 MBq [¹⁸F]FLT was intravenously (i.v.) injected into the Ye-infected and PBS-treated control animals under anesthesia (1.5% isoflurane and 0.8 L/min 100% oxygen), and the mice remained conscious during the 90 min uptake time. The imaging protocol included sequential PET/MR imaging of the PBS-treated control and Ye-infected mice on three consecutive days. The animals were examined at 1, 2 and 3 days post infection (p.i.). Mice were imaged using a small animal PET scanner (Inveon, Siemens Preclinical Solutions, Knoxville, TN, USA) that yielded a spatial resolution of approximately 1.3 mm in the reconstructed images. A 10 min static PET scan was acquired after the uptake time with no attenuation correction. During the PET and MR imaging, the animals were anesthetized with 1.5% isoflurane mixed with 100% oxygen and anesthesia was monitored via measurement of respiratory frequency, and the body temperature was maintained at 37°C using a heating pad placed underneath the mouse. MR imaging was performed on a 7 T small animal MR tomography system (Clinscan, Bruker Biospin MRI, Ettlingen, Germany) to obtain anatomical information directly after the PET scan. The bed with the anesthetized mouse was carefully passed from the PET scanner into the MR without moving the animal. A T2-weighted 3D-space sequence (TE / TR 202 / 2500 ms, image matrix of 137 x 320, slice thickness 0.27 mm) was used for whole-body imaging. The iterative two-dimensional ordered subset expectation maximization algorithm (OSEM2D) reconstructed PET images were normalized to each other according to the injected activity, were manually fused using anatomical landmarks to the respective MR images and analyzed using Inveon Research Workplace software (IRW, Siemens Preclinical Solutions) by drawing regions of interests (ROIs) following anatomical information retrieved from MR images. The results are expressed as the % of the injected dose per cm³ (%ID/cc) of tissue. After the final PET scan, all animals were sacrificed by cervical dislocation under deep anesthesia and dissected. The organs were removed, and radioactivity in the tissue samples was quantified with an aliquot of the injected radiotracer in a γ-counter (Wallac 1480 WIZARD 3” Gamma Counter; Perkin Elmer, Waltham, MA, USA) using an energy window between 350 and 650 keV. The results are expressed as % of the injected dose per g (%ID/g) of tissue or as the spleen-to-muscle ratio.