THYROID IMAGING USING POSITRON EMISSION TOMOGRAPHY - A COMPARISON WITH ULTRASOUND IMAGING AND CONVENTIONAL SCINTIGRAPHY IN THYROTOXICOSIS

Forty-six comparative studies were performed on 41 patients with hyperthyroidism. Clinically these comprised two groups: those with Graves' disease, and those with multinodular goitre. All patients underwent an ultrasound examination and positron emission tomography (PET) using 124I, then gamma camera pinhole imaging following their 131I therapy administration. Although the 131I pinhole imaging was not performed for diagnostic purposes, there was good correlation (78% agreement) between it and 124I PET in determining relative lobe size. Hence either imaging modality could be used as an indicator of the relative radiation dose delivered to each thyroid lobe at a macroscopic level. In terms of gland morphology the PET images correspond well to the high resolution ultrasound images (78% agreement), unlike the pinhole images which correlated poorly (only 28% agreement). The results showed that PET imaging gives better anatomical and physiological detail than 131I pinhole imaging. In 77% of cases where the pinhole image showed a uniform distribution of radioisotope, the improved spatial resolution of the PET images revealed non-homogeneous distribution indicating a non-uniform distribution of radiation dose. Since all dosimetry calculations are based on the assumption of uniform distribution of radioiodine, this non-uniformity could possibly have important consequences in the outcome of radioiodine therapy in thyrotoxicosis.