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. 2018 Jul 5;8(1):10167.
doi: 10.1038/s41598-018-28571-y.

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

Yuhao Li  1 , Huawei Cai  1 , Guohua Shen  1 , Fuwen Pang  1 , Ping Dong  1 , Lin Li  2
Affiliations

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

Yuhao Li et al. Sci Rep. .

Abstract

Iodine-131 (131I) is an essential and widely used radioisotope in thyroid diseases and animal experiments. Planar imaging has been considered the most popular method for 131I thyroid uptake radioactive activity quantification. The ROI defining section is essential and can affect the accuracy of quantitative results. However, a consistent method has not been proposed. In this study, a UC-ROI defining method based on ULWL setting and colour display grade was applied. Three steps were performed: image acquisition of five standard activity models and obtaining the exact value that the counts per radioactive activity contributes to the ROI; image acquisition of 20 rat thyroids and obtaining the counts of the ROI (thyroid); and calculating the rat thyroid radioactive activity and comparing these values with the true values. The accuracy of quantification activity of 131I in rat thyroid reached 2.62% ± 0.41%. The mean quantification within 5% could be achieved in 19 of 20 rat thyroids. No significant difference existed between calculated thyroid 131I activity and true values with a paired matched-test (t = -0.384, P = 0.706 > 0.05). The results indicated that with the UC-ROI defining method, more accurate thyroid uptake 131I radioactive activity quantification by SPECT planar imaging can be achieved in vivo rat study.

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Conflict of interest statement

The authors declare no competing interests.

Figures

Figure 1
Figure 1
Relationship between colour brightness and windows level. The change in colour brightness maintains a linear relationship with the counts per pixel within the range from LLWL to ULWL. For the pixels with larger counts than ULWL, the colour display grade remains the brightest.
Figure 2
Figure 2
Four kinds of colour display patterns. The colour display patterns for examples, left to right-GE colour, French, Hot Iron, and XT1 Rainbow. The shade of colour represents the counts per pixel, and the counter lines of the colour display grade can be observed in different patterns. It is the counter lines that are used for defining ROI borders visually. The colour display pattern of GE colour was used in this research.
Figure 3
Figure 3
Planar images with different μ values of the same radioactive source (a rat). We tested μ with different values (from 1 to 80) and obtained various results for the images (the μ for default ULWL is 366). A suitable value of μ must satisfy the requirement that the colour display border of the target tissue should be recognized. In this research, a value of 50 for μ was adopted (other values such as 40, 60, and 70 will also be reliable; the key point is that the value of μ should stay the same for all data processing and ROIs defining).
Figure 4
Figure 4
Method of ROI selection. Three background ROIs were drawn according to the traditional method in (a). The average counts per pixel for ROI-1, ROI-2, and ROI-3 are 18.143, 21.714 and 30.500, respectively. Distinct differences could be noticed and these differences would result in inaccuracy for radioactive activity quantification. In (b) the ROI-1 in blue colour represents the target tissue (rat thyroid) defined according to the UC-ROI drawing method. The ROI-2 in yellow colour, which is drawn in a narrow circle shape next to ROI-1, represents the background region.
Figure 5
Figure 5
Examples for ROI selection of standard model and rat thyroid. (a) Shows an example of processing of the standard model. The default ULWL is 838. According to equations (1) and (2), we adjusted the value of ULWL to 454 (μ = 50 was adopted). Then, the ROI-1 for the target tissue and ROI-2 for the background were selected according to the UC-ROI defining method and the ring-like background defining method. (b) Is a planar image of a rat thyroid. The processing method was maintained with (a) and we adjusted the ULWL from the default value (3701) to 416 (μ = 50).
Figure 6
Figure 6
The relationship between averages for deviations and µ values. With different µ values, though subtle differences existing, the accuracy of radioactivity quantification is stable generally. And a better quantitative ability can be observed when µ equals to 30, 40 or 50.
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