Integrated Thyroid Imaging: Ultrasound and Scintigraphy

Abstract

Thyroid sonography has made substantial progress over the last decades in terms of spatial resolution and additional parameters including vascularity, perfusion, and elasticity of lesions. The improved depictability of thyroid nodules has led to a more detailed sonographic characterization of malignant thyroid nodules considering features like microcalcification, capsular invasion, and reduced elasticity. Thus, ultrasound (US) has become the most important single tool for risk assessment of thyroid nodules. Predominantly cystic and spongiform nodules as well as many nodules with a mixed composition of solid and cystic components can safely be identified as benign on ultrasound and need no further work-up for risk assessment. Predominantly solid nodules, however, require a structured and consistent sonographic assessment including the evaluation of nodule composition, echogenicity, calcifications, shape, margins, and additional sonographic parameters. One should adhere to one of the TIRADS when reporting ultrasound findings on thyroid nodules categorizing each nodule to a certain risk class. However, the risk of malignancy reported in studies for each class may not be readily transferrable to daily routine, since the prevalence of malignant nodules in tertiary care centers is as high as 20% whereas in primary care units the prevalence may be as low as 1 per mille, thus lowering positive predictive values by one to two orders of magnitude. In addition, some suspicious features found in those studies may not be directly transferable to daily practice: microcalcifications are often difficult to discern from benign colloidal spots, even after having run through learning sessions; a taller-than-wide shape of nodules with contact to the dorsal parts of the thyroid gland does not imply malignancy according to a pole concept which has been recently developed.Thyroid scintigraphy (TS) has long been an indispensable modality for functional characterization of thyroid nodules. It should be applied to rule out malignancy in a substantial proportion of predominantly solid nodules by showing a hyperfunctioning (“hot”) nodule. Such nodules are almost always benign with few exceptions (e.g., “trapping only” nodules). If laboratory findings and scintigraphic appearance of a hot nodule do not match, further diagnostic work-up is warranted including 123I− scintigraphy with late imaging at 24 h and fine-needle aspiration cytology (FNAC). In settings with a rather low prevalence of malignant thyroid nodules such as in primary or secondary care units, a combined use of US and TS may be adopted to rule in suspicious nodules for further work-up by FNAC, including predominantly solid hypofunctioning (“cold”) nodules. MIBI scintigraphy can be used in hypofunctioning nodules with indeterminate cytology and if contraindications preclude patients from FNAC.TS is a valuable tool to detect functional abnormalities not only in nodules but also in the thyroid gland as a whole by assessing the overall radionuclide uptake. As such, it has long been used to confirm the diagnosis of Graves’ disease and thyroid autonomy. For disseminated autonomy, it is the only tool to definitely verify the diagnosis. In destructive thyroiditis, TS reliably shows a reduced overall radionuclide uptake justifying its application in selected cases with doubtful sonographic or laboratory findings. The user of TS is encouraged to calculate a site-specific normal range for the uptake value normalized to TSH in addition to the raw uptake. Thus, even subtle functional disorders of the thyroid can be detected and graded.Functional or metabolic imaging is increasingly combined with morphological imaging and is acquired and displayed as volume data rather than planar images including SPECT/CT, PET/CT, and PET/MRI. These combined modalities increase the restricted morphological field of view from ultrasound enabling to reliably image substernal, ectopic, or dystopic localizations of thyroid tissue. Combined modalities also benefit from the increasing spectrum of functional or metabolic tracers including MIBI, iodine isotopes (e.g., 124I-) and newer PET tracers.

Integrated Diagnostics of Thyroid Nodules

“…In patients with thyroid nodules, low TSH levels may be related to autonomously functioning thyroid nodule(s) and thyroid scintigraphy is indicated. A normal TSH excludes a clinically significant autonomy but, especially in countries with low iodine intake, cannot exclude compensated autonomy: in those regions, thyroid scintigraphy may properly exclude such nodules (frequently suspicious at neck US) from inappropriate FNAC [13,39]. Routine measurement of serum anti-thyroid peroxidase (TPO) antibodies is not necessary for thyroid nodule evaluation [10,40] and routine measurement of serum thyroglobulin (Tg) is strongly discouraged as it may be elevated in different thyroid diseases, including benign ones, and is aspecific and relatively insensitive for thyroid cancer [41].…”

“…Since the thyroid gland is superficially located with its posterior border generally situated less than 4 cm from the skin, highresolution (≥12 MHz) probes provide excellent image quality. High-resolution conventional B-mode (i.e., gray-scale ultrasound) evaluation is now integrated with multiparametric ultrasound (MPUS), including vascularization assessment (spectral Doppler, SD; color Doppler, CD; power Doppler ultrasound, PD; superb microvascular imaging, SMI; contrast-enhanced ultrasound, CEUS) and tissue stiffness assessment (sonoelastography), respectively [13].…”

“…Today, thyroid US allows an accurate evaluation of morphologic features, which have been used to propose a standardized risk stratification for thyroid nodules (i.e., Thyroid Imaging And Data Reporting Systems (TI-RADS)) attempting to reduce the admittedly high inter-operator variability [13]. Among these, the American College of Radiology (ACR)-TI-RADS, European (EU)-TI-RADS, Korean (K)-TI-RADS, British Thyroid Association (BTA), American Thyroid Association (ATA) classification and American Association of Clinical Endocrinologists (AACE), American College of Endocrinology (ACE), and Associazione Medici Endocrinologi (AME) classification systems are commonly used [18,[45][46][47][48][49].…”