Reference-free thyroid uptake measurement.

PURPOSE

An easy-to-implement scintigraphic method that enables the assay of thyroid iodine uptake with improved accuracy and without the use of a reference source was developed with an aim to avoid unnecessary high radiation burden to patients undergoing (131)I treatment for benign thyroid disease.

MATERIALS AND METHODS

Dual-energy (123)I planar imaging involving simultaneous recording of both the primary (159 keV) and scatter (130 keV) frames was followed by generation of subtraction 'scatter-free' images. A uniform cutoff level of 15% was chosen to delineate the functional thyroid tissue with the help of isocontour regions of interest, which in turn enabled quantitation of the scatter-free thyroid counts. The ratio of the total counts being recorded in each of the above frames, the 'primary-to-Compton' ratio, was determined and compared with the uniform absorption-free ratio ascribed to all of the thyroid images. This enabled determination of the upscaling factor needed to restore the scatter-free counts to their unmoderated level. The corrected count content was subsequently converted to the iodine dose accumulated by the thyroid with the help of the camera-specific net detection efficiency. As a result, the true uptake value could be determined without the need to image a reference source or phantom.

RESULTS

Clinical evaluation of the proposed reference-free method was carried out by reviewing the 24-h uptake values obtained for the 32 patients suffering from Graves' disease, multinodular struma, or autonomous nodule. A comparison with concurrently assayed thyroid uptake values derived from the counts of the 24-h urine samples measured by the well-type scintillation counter (direct assay method) revealed very good linear correlation (r > 0.96). Furthermore, a comparison with uptake values determined using the 159 keV images of the patient as well as those of the reference sample in a commonly used so-called 'classic' manner was also carried out. The outcomes of the classic assay were found on average to underestimate the thyroid uptake in absolute terms by ∼ 25% because of unattended scatter and absorption-induced count losses. In contrast, the results of the reference-free method exhibited slight overestimation only, which on average amounted to less than 2%. However, because of the time-related fluctuations of the gamma camera and the dose calibrator as well as uncertainties induced by the scatter and absorption corrections, the average relative error associated with thyroid uptake values as determined by the proposed reference-free method was found to amount to nearly 10%.

CONCLUSION

Strong evidence in support of the validity of the reference-free method designed to measure thyroid uptake with improved accuracy and without relating to a reference phantom image was produced in a clinical setting. As a result, the unnecessary high radiation burden to patients undergoing (131)I therapy because of systematic uptake underestimation could be avoided.