الفهرس 
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Abstract
There is a great increase in the incidence of thyroid cancer. Thyroid cancer is the most common endocrine tumor. Incidence of thyroid cancer is increasing by 6%. Fine needle aspiration cytology guided by ultrasonography can prevent unnecessary thyroidectomy and precise the extent of surgical resection (Abd el Aziz, et al, 2014). Thyroid ultrasonography can provide information with regard to nodule diameter, structure of the nodule whether cystic or solid, presence of calcification , regular margin and absence of halo sign but still no reliable criteria for distinguishing benign from malignant lesions. In addition, it is difficult to diagnose the malignancy of the nodule when it is large or multinodular (Abd el Aziz, et al, 2014). Diagnostic imaging examinations such as radionuclide scintigraphy and sonography are commonly used to attempt differentiation between benign and malignant thyroid nodules. However, these modalities have many limitations, such as poor accuracy with sonography and exposure to ionizing radiation with nuclear scintigraphy (Shi et al, 2013). Ultrasound-guided fine-needle aspiration biopsy (FNAB) has been widely well-accepted as an accurate diagnostic method for evaluation of thyroid nodules. Cancer incidence in individuals with a thyroid nodule has been determined as 9.2-13% with FNAB. FNAB is an efficient method in differentiating malignant from benign nodules (Mutlu et al, 2012).
Recent developments in MRI may show that some MR protocols are of diagnostic value for these types of lesions. Routine T1- and T2-weighted MR imaging can provide information on the location and size of thyroid lesions. But these protocols still don’t have the specificity for distinguishing benign from malignant nodules or assessing the functional status of these thyroid nodules (Wu et al, 2013). DWI is sensitive to changes in the microstructural organization of tissue that may affect water diffusion. It has been used to evaluate head and neck tumors. The Apparent Diffusion Coefficient (ADC) value is a quantitative parameter for distinguishing malignant tumors from benign thyroid nodules (Abd el Aziz, et al, 2014).
The b-value in the DWI was an important factor for image quality. We obtained diffusion-weighted MR images with differentn b-values (L.M. Abd el Aziz et al.2015). The mean ADC values of thyroid nodules were measured for b values of 500 and 1000 s/mm2. The mean ADC value of benign solitary thyroid nodules ranged from 1.56 to 2.28 x10-3 mm2/s (b = 1000 s/mm2) and from 1.64 to 2.70 x10-3 mm2/s (b = 500 s/mm2). The mean ADC value of malignant thyroid nodules ranged from 0.54 to 1.47 x10-3 mm2/s (b = 1000 s/mm2) and from 0.61 to 1.56 x10-3 mm2/s (b = 500 s/mm2). There is significant difference between benign and malignant solitary thyroid nodules, when ADC value of 1.5 x 10-3 mm2/s was used as a cutoff value for the differentiation of benign from malignant thyroid nodules (H.M. Abdel-Rahman et al.2016).
Conclusion
In conclusion, DWI with the ADC values may be feasible, non-invasive and non-radiative method of distinguishing malignant from benign thyroid nodules that do not require the administration of contrast media. The b value is a very important factor, and different b-values have to be acquired for more accurate quantitative analysis of diffusion-weighted images and consequently reliable ADC maps as well as the ADC
measurement.
ADC values of nodules may provide useful data about the nature of a thyroid nodule with significant difference in the ADC value between benign and malignant thyroid lesions, which ADC values in benign lesions being higher than malignant lesions.