FIG. 6.

Thyroid imaging using ¹²⁴I-iodide in vivo. (A) Biodistribution of ¹²⁴I-iodide in thyroid of genetically modified mice in which thyroid iodide uptake is suppressed by induction of a transgene; 1 week later suppression is relieved and iodide uptake is normalized. Top panels: representative images of uninduced mice, 1 week on doxycyclin to induce the transgene, followed by 1 week off doxycyclin. Positron emission tomography (PET) imaging was performed using an R4 microPET scanner (Concorde Microsystems) with Na¹²⁴I produced on the MSKCC EBCO TR 19-9 (Advanced Cyclotron Systems Inc.) using 16 MeV protons on a tellurium-124 target. Mice were injected via tail vein with 1.7–2.0 MBq (45–55 μCi) of Na¹²⁴I. Mice were imaged 24, 48, and 72 hours later under inhalational isoflurane anesthesia (Forane; Baxter Healthcare) at 1 L/min. List-mode data were acquired for 5 minutes using an energy window of 250–750 keV and a coincidence timing window of 6 nanoseconds, histogrammed into two-dimensional (2D) projected data by Fourier rebinning, and reconstructed by filter back-projection using a cut-off frequency equal to the Nyquist frequency. The image data were normalized to correct for nonuniformity of response of the PET, dead-time count losses, ¹²⁴I positron branching ratio, and physical decay to the time of injection, but no attenuation, scatter, or partial-volume averaging correction was applied. (B) Quantification of thyroid ¹²⁴I-iodide uptake in mice treated with the indicated conditions. ***p<0.001. An empirically determined system calibration factor (in units of [μCi/mL]/[cps/voxel]) was used to convert reconstructed voxel count rates to activity concentrations. The resulting image data were then normalized to the administered activity to parameterize images in terms of the percentage of the injected dose per gram of tissues (%ID/g). Manually drawn 2D regions of interest (ROIs) or three-dimensional (3D) volumes of interest (VOIs) were used to determined the %ID/g (decay corrected to the time of injection) in various tissues. Image visualization and analysis were performed using ASIPro VM software (Concorde Microsystems). (C) Representative gross appearance of thyroid glands at the indicated times. The boundaries of the thyroid are demarcated by dashed lines. Scale bar: 1 mm. ID/g, injected dose/gram Reproduced with permission from Chakravarty et al. (58).