Effect of region of interest on ADC and interobserver variability in thyroid nodules

To determine the effect of region of interest (ROI) on tumor's apparent diffusion coefficient (ADC) and interobserver variability in thyroid nodules.

Thirty-three individuals with 45 pathologically-confirmed thyroid nodules were assessed by preoperative diffusion-weighted imaging (DWI) with b values of 0 and 400 s/mm², respectively. Two readers evaluated the ADC values of lesions based on three ROI techniques: whole-volume, single-slice and small solid-sample groups. Interobserver variability was analyzed for all ROI techniques, and the mean ADCs of benign and cancerous thyroid nodules were compared.

For the mean ADCs of non-cancerous thyroid nodules, average differences and limits of agreement (LOAs) between readers were 0.00 [- 0.17-0.17] × 10^- 3 mm²/s for whole-volume ROI (ICC = 0.967), 0.00 [- 0.26-0.26] × 10^- 3 mm²/s for single-slice ROI (ICC = 0.932) and - 0.02 [- 0.38-0.41] × 10^- 3 mm²/s for small solid-sample ROI (ICC = 0.823). For the mean ADCs of cancerous thyroid nodules, average differences and LOAs between readers were - 0.05 [- 0.23-0.13] × 10^- 3 mm²/s (ICC = 0.885), 0.01 [- 0.23-0.25] × 10^- 3 mm²/s (ICC = 0.839) and - 0.07 [- 0.52-0.39] × 10^- 3 mm²/s (ICC = 0.579) for the three ROI methods, respectively. The mean ADC values were more scattered in the small solid-sample ROI group in comparison with the whole-volume and single-slice groups, in noncancerous and cancerous specimens. Of all three ROI techniques, whole-volume ROI-determined ADC had the highest combined sensitivity (80.0%), specificity (88.3%) and Youden index (0.683), with a cut-off of 1.84 × 10^- 3 mm²/s.

The ROI method overtly affects ADC measurements in benign and cancerous thyroid nodules. Small solid-sample ROI yielded the worst interobserver variability of average ADC measurements.