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Acquisition Settings for PET of ¹²⁴I Administered Simultaneously with Therapeutic Amounts of ¹³¹I

¹ Department of Nuclear Medicine and PET Research, VU University Medical Centre, Amsterdam, The Netherlands; ² Department of Otolaryngology and Head and Neck Surgery, VU University Medical Centre, Amsterdam, The Netherlands; and ³ Department of Nuclear Medicine, Maastricht University Hospital, Maastricht, The Netherlands

Correspondence: For correspondence or reprints contact: Mark Lubberink, PhD, Department of Nuclear Medicine and PET Research, VU University Medical Centre, P.O. Box 7057, 1007 MB Amsterdam, The Netherlands. E-mail: mark.lubberink{at}vumc.nl

Radiation dosimetry of thyroid cancer therapy with ¹³¹I can be performed by coadministration of ¹²⁴I followed by longitudinal PET scans over several days. The photons emitted by ¹³¹I may affect PET image quality. The aim of this study was to assess the influence of large amounts of ¹³¹I on PET image quality and accuracy with various acquisition settings. Methods: Noise equivalent count (NEC) rates of ¹²⁴I only were measured with a standard clinical PET scanner. Apart from the standard 350- to 650-keV energy window, 425- to 650-keV and 460- to 562-keV windows were used and data were acquired both with (2-dimensional) and without (3-dimensional [3D]) septa. A phantom containing 6 hot spheres, filled with a combination of ¹³¹I and ¹²⁴I and with a sphere-to-background ratio of 18:1, was scanned repeatedly with energy window settings as indicated and emission and transmission scan durations of 7 and 3 min, respectively. NEC rates were calculated and compared with those measured with the phantom filled with only ¹²⁴I. Sphere-to-background ratios in the reconstructed images were determined. One patient with known metastatic thyroid cancer was scanned using energy window settings and scan times as indicated 3 and 6 d after administration of 5.5 GBq of ¹³¹I and 75 MBq of ¹²⁴I. Results: The highest ¹²⁴I-only NEC rates were obtained using a 425- to 650-keV energy window in 3D mode. In the presence of ¹³¹I, the settings giving the highest NEC rate and contrast were 425–650 keV and 460–562 keV in 3D mode, with the clinical scans giving the highest quality images with the same settings. Conclusion: Acquisition in 3D mode with a 425- to 650-keV or 460- to 562-keV window leads to the highest image quality and contrast when imaging ¹²⁴I in the presence of large amounts of ¹³¹I using a standard clinical PET scanner.

Key Words: PET • ¹²⁴I • dosimetry • data acquisition