All patients who underwent neck DECT scanning from August 2017 to October 2019 were enrolled in this study. Inclusion criteria were as follows: patients who had a thyroid hormone test, which was performed 3 days within CT scanning. Exclusion criteria were as follows: (1) uneven thyroid density, with low-density lesions and calcification; (2) a history of thyroid surgery or thyroid artefacts seriously affected by the environment; (3) abnormal thyroid function; (4) recent treatment with thyroid preparations or iodine-containing drugs. A total of 43 patients (20 males and 23 females), aged 22-79 years, with an average age of 55.00±13.67 years, were enrolled in the study. All selected cases were approved by the hospital ethics committee, and informed consent was signed by the patients before scanning.
Computed tomography scanning and post-processing
CT scanning was performed using Siemens Definition Flash（SOMATOM
Definition flash; Siemens Healthcare, Forchheim, Germany). The scanning parameters were as follows: A tube voltage 100 kV, reference current 186 mAs; B tube voltage Sn140 kV, reference current 125 mAs, fusion coefficient 0.5, pitch 0.65, open CARE Dose 4D, Q30 (SAFIRE strength 3); and slice thickness/interval, 1.5/1.5 mm. Scanning ranged from the skull base to the thoracic entrance. The patient lay supine on the examination bed. The mandibular and shoulder positions were required in order to avoid the influence of clavicle artefacts. Instructions regarding breath holding and no swallowing were given to avoid breathing and swallowing artefacts.
Measurement and data analysis
For measurement of the CT values and the iodine concentration of the thyroid glands, CT data were transferred to a standard post-processing workstation (Syngo Via workstation, Siemens Healthcare, Forchheim, Germany). The iodine map was obtained by choosing the “CT Dual Energy” mode. The iodine map image and
the conventional 120 kVp images were generated from the low- and high-voltage CT data sets with a slice thickness of 1.5 mm. The iodine concentrations and the CT values were measured from those images.
The slices for the ROI setting were carefully selected with use of the following criteria: (a) minimal beam hardening artefacts; (b) homogenous area; and (c) no
nodular lesions. We manually marked the ROIs on the right and left lobes of the thyroid gland. The largest possible ROI (round or oval-shaped) was marked taking care not to include the margins of the thyroid tissue. The iodine concentration and CT value were measured three times. The average value of the ROI was set to 20 mm2. The left and right thyroid volumes, including the isthmus of the thyroid, were obtained through the outlined layer by layer, with the VOI Freehand option using the CT Bone Reading program. The mean iodine concentration and volume of thyroid tissue were measured, and the total iodine content (total iodine content = (mean iodine concentration × thyroid volume) was calculated.
Detection of thyroid function
Fasting venous blood samples were collected in the morning, and serum free triiodothyronine (FT3), total triiodothyronine (TT3), free thyroxine (FT4), total thyroxine (TT4) and thyroid hormone (TSH) were detected by chemiluminescence immunoassay and analysed by gamma-ray radioimmunoassay. The instrument obtained the corresponding results.
SPSS 20.0 software was used for statistical analysis. The correlation between mean iodine concentration, volume, total iodine content, age and thyroid function was analysed by Spearman correlation analysis. Statistical significance was defined at p < 0.05.