Rationale and Objectives This study aimed to preliminarily investigate the feasibility of intravoxel incoherent motion (IVIM) theory in the differential diagnosis of benign and malignant thyroid nodules. Materials and Methods Forty-five patients w…
INTRODUCTION
Thyroid nodules are the most common finding in the thyroid gland, present in approximately 40% of the population; however, only 5%–10% of nodules are malignant. An increasing incidence of thyroid cancer has been reported worldwide ( ). The treatment plan and prognosis of thyroid nodules remain controversial, and a notable challenge in this field is the differential diagnosis between benign and malignant nodules.
Ultrasonography is the most widely used modality for defining thyroid lesions; the pattern of sonographic features involving thyroid nodules can be indicative of a risk of malignancy, but remains insufficient to serve as the sole method to distinguish benign from suspicious malignant lesions. Computed tomography is not a sensitive tool for demonstrating intrathyroid lesions; moreover, it is ineffective to attempt to differentiate benign from malignant nodules by measuring the iodine content and enhancement characteristics of nodules on computed tomography scans ( ). Technetium-99m methoxyisobutylisonitrile (99mTc-MIBI) has frequently been used for the evaluation of thyroid nodule function. 99mTc-MIBI scintigraphy is highly sensitive for thyroid neoplasia and has a high negative predictive value for excluding malignant thyroid nodules ( ). Although a negative 99mTc-MIBI scan virtually excludes malignancy in a hypofunctional thyroid nodule, the low specificity and positive predictive value of the scanning modality are problematic. Fine-needle aspiration biopsy is currently considered the most effective method for preoperative evaluation of thyroid nodules, and is primarily used to evaluate nodules >10 mm, based on sonographic pattern ( ). However, 2%–25% sample of the patients are cytologically indeterminate, based upon the Bethesda System ( ); malignancy percentage in this group ranges between 14% and 70% ( ).
This challenge led to the discovery of newer, noninvasive diagnostic modalities as a supplemental approach for assessment of malignancy risk in thyroid nodules. Previous studies suggested that diffusion-weighted imaging (DWI) can be used for the evaluation of thyroid nodules. More restricted diffusion is observed in solid malignant tumors than in benign or cystic lesions, which are indicated by a reduction in the apparent diffusion coefficient (ADC) ( ).
In vivo, the microscopic motion of water molecules detected by DWI is influenced by diffusion of water molecules and by microcirculation of blood in the capillary network ( ). Therefore, ADC values that are influenced by the perfusion effect may limit the reliability of ADC in characterizing thyroid lesions. To avoid this, a pair of large b values (>200 s/mm 2 ) is generally used to estimate pure diffusion (e.g., 500 s/mm 2 and 1000 s/mm 2 ). Diffusion and perfusion, however, are important biological features of organ pathophysiology and can be indicative of various phenomena. Intravoxel incoherent motion (IVIM), first described by Le Bihan et al. ( ), is a promising imaging theory for the separation of tissue perfusion and diffusivity. IVIM theory could resolve ADC, pure diffusion coefficient ( D ), and perfusion-related incoherent microcirculation ( D *), separately, while also identifying the microvascular volume fraction ( f ). In recent years, IVIM has been widely used in the differential diagnosis of lesions in the brain ( ), prostate ( ), liver ( ), and salivary gland ( ); however, its application for thyroid lesions remains largely unknown.
The purpose of this preliminary study was to explore the utility of IVIM for differentiating between benign and malignant thyroid nodules, as well as to determine the IVIM metric with the greatest accuracy.