Comparative Evaluation of 24-Hour Thyroid 131I Uptake Between   Camera-Based Method Using Medium-Energy Collimator and Standard Uptake Probe-Based Method

Abstract

The objective of the study was to make a quantitative comparison of 24-h thyroid uptake calculated by γ camera-based and thyroid uptake probe-based methods after administration of a diagnostic 131 I capsule in patients with benign thyroid disorders. Methods: The study group comprised 66 patients, of whom 26 were male (28-67 y old) and 40 female (20-65 y old). These patients had benign thyroid disorders (primarily hyperthyroidism [thyrotoxicosis]), most of whom had been referred for evaluation before radioiodine treatment. 131 I (25 μCi [925 MBq]) was administered, and 24-h thyroid uptake was calculated using a probe-based method and a camera-based method with a medium-energy parallel-hole collimator. The paired t test was used to check the variation in values obtained by these 2 methodologies. Result: Of the 66 patients included in this study, 45 had clinical thyrotoxicosis and 21 had nonthyrotoxic multinodular goiter. In the group with thyrotoxicosis, neck uptake ranged from 40.13% to 97.1% by the probe-based method and 36.89% to 95.9% by the camera-based method. In the group with clinically nonthyrotoxic goiter, neck uptake ranged from 1.4% to 38.4% by the probe-based method and 0.6% to 34.8% by the camera-based method. Paired t testing was performed on both groups of patients, and P values were less than 0.05, showing good agreement within the 2 groups of data. Conclusion: The camera-based method is a good substitute for the probe-based method; though not producing identical results, the former could be used to derive useful quantitative information on thyroid function.

γ-Camera–Based Method for Measuring the γ-Count from 131I Capsules: An Alternative to the Thyroid Uptake Probe

“…Sealed gelatin capsules containing 131 I were kept individually in serially numbered test tubes. In the probe-based counting method, each capsule was placed in an acrylic glass thyroid phantom 30 cm from the face of the probe's detector (iso-response region) and counted for 10 s (5). The background count was also measured for 10 s and then subtracted from the capsule count, and the mean and SD were calculated for the 0.925-MBq capsule sets and the 1.85-MBq capsule sets separately.…”

“…Before administering radioactive iodine, our center confirms that the patient has followed the iodine restriction requirements, is not taking any interfering drugs, and is not pregnant or lactating. After the 131 I dose has been administered, neck counts are taken at 2 and 24 h using a thyroid uptake probe that has a NaI(Tl) crystal fitted with a flat-field collimator and the associated electronics (1,5). The patient is positioned supine with the neck extended, and the probe is placed 30 cm away.…”

“…The ratio of the backgroundcorrected neck count to the background-corrected phantom count is calculated to get the percentage neck uptake, or [(neck count 2 neck background count)/(phantom count 2 phantom background count)] ·100 (6). An alternative, camera-based, method has also been found to be a good option (5).…”

Quantification of radioactivity by planar gamma-camera images, a promoted method of absorbed dose in the thyroid after iodine-131 treatment

“…Iodine-131 ( 131 I) is an essential and widely used radioisotope in thyroid diseases, and planar imaging has been considered the most popular method for 131 I thyroid uptake radioactive activity quantification 5 , 10 – 12 . The sensitivity known as count rate per unit activity or calibration factor must be known in order to convert the acquired count rate to activity 5 .…”

“…However, studies of thyroid radioactive activity quantification did not provide a consistent method for defining the ROI. Most researchers defined the ROI according to visual characteristics and drew a region containing the thyroid 10 , 12 , but the visual character is a fuzzy concept. Additionally, several researchers adopted other methods for defining the ROI of the target tissue, such as choosing an 85% isocount contour of the thyroid gland 13 or considering system spatial resolution (FWHM) 5 .…”