Therapeutic Radioiodine Interference With Subsequent Diagnostic Imaging

The patient was a 42-year-old man who had been treated with 9 mCl of l-131 for Gravesʼ hyperthyroidism. Two weeks after treatment, an attempt was made to acquire a multigated cardiac blood-pool study after the intravenous administration of 30 mCl of Tc-99m pertechnetate using an ultra-tag kit; image acquisition proceeded using a LEAP collimator. This image demonstrated an inordinate amount of background activity at the anterior mediastinal area and a poor target-to-nontarget ratio due to septal penetration. At this time, it was realized that the patient had received the therapeutic dose 2 weeks earlier. A medium-energy collimator was then used for acquisition, which produced acceptable images of the heart without excess background activity.