Thyroid Ultrasound Anatomy

Thyroid ultrasound is a crucial non-invasive tool for visualizing and evaluating thyroid gland anatomy. It plays a significant role in diagnosing thyroid disorders, including goiters, nodules, and malignancies. 

Understanding normal thyroid ultrasound anatomy is essential for radiologists and clinicians interpreting these images. This article will provide a comprehensive review of the thyroid gland’s ultrasound appearance, key surrounding anatomical structures, and how this impacts clinical decision-making. 

Healthcare providers can ensure accurate diagnosis and optimal patient management by understanding normal vs abnormal thyroid sonography anatomy.

Preparing for a Thyroid Ultrasound: Clinician Considerations

While no specific patient preparation is necessary for a thyroid ultrasound like dietary restrictions and medication adjustment, clinicians should keep the following in mind to optimize the procedure:

• Patient Comfort:
• Explain the procedure briefly to the patient to reduce any anxiety.
• Provide a pillow or rolled towel under the neck, tilting the head back for better gland exposure.
• Image Quality:
• Select an appropriate frequency transducer (usually a linear array) for optimal thyroid visualization.
• Apply a generous amount of ultrasound gel to the neck for clear transmission of sound waves.
• Documentation:
• Ensure accurate patient information for labeling images and subsequent reports.
• Record any relevant medical history, especially prior thyroid surgeries or radiation exposure.

Additional Notes

• Informed Consent: In some settings, get informed consent from the patient before initiating the ultrasound.
• Contrast Agents: In rare cases, contrast agents might be used for enhanced imaging. Discuss any allergies or contraindications with the patient beforehand.
  
  

Performing a Thyroid Ultrasound: Technical Considerations

Transducer Selection and Gel Application:

• Choose a high-frequency linear array transducer. The frequency range will depend on the specific ultrasound machine, but is between 5-15 MHz.
• Apply a generous amount of ultrasound gel to the patient’s neck area.

Systematic Examination:

• Begin with a transverse view, visualizing the isthmus and both thyroid lobes. Assess for overall size, shape, and echogenicity of the thyroid parenchyma.
• Proceed with longitudinal views for a complete evaluation of each thyroid lobe. Take care to identify, measure, and characterize any nodules or lesions.
• Examine the surrounding anatomy, including major blood vessels (carotid arteries, jugular veins), muscles, and lymph nodes.

Doppler Imaging:

• Employ color doppler or power doppler to evaluate vascular patterns within the thyroid gland and surrounding tissues. This helps assess nodule vascularity or thyroid inflammation (e.g., thyroiditis).

Documentation

• Measurements of the thyroid lobes and isthmus.
• Number, size, and characteristics of any nodules (solid/cystic, presence of calcifications).
• Vascular patterns.
• Any abnormalities of surrounding structures.

• Thoroughly document all findings, including:
• Capture and save relevant images for your report and future reference.

Additional Considerations

• Contrast Agents: In specific cases, contrast-enhanced ultrasound may be indicated for a more detailed examination.
• Fine-Needle Aspiration: Ultrasound may guide a fine-needle aspiration biopsy if required.
  
  

Thyroid Ultrasound Anatomy

• Normal parenchyma: The glandular tissue of the thyroid appears homogenous with medium-level echogenicity, slightly hyper-echoic relative to the adjacent strap muscles. Assess for symmetry between the lobes and any focal areas of altered echogenicity that might suggest pathology.
• Lobes and Isthmus: Document precise measurements of each lobe in three dimensions (length, width, thickness). Also, measure the isthmus thickness. Remember that normal measurements can vary based on the patient's age, sex, and body mass index.
• Nodules: When nodules are present, meticulous characterization is crucial. Note:
• Location: Within a specific lobe or the isthmus.
• Size: Precise measurements in all three dimensions.
• Composition: Solid, mostly solid, mostly cystic, or cystic.
• Margins: Well-defined, ill-defined, or infiltrative.
• Echogenicity: Hypoechoic, hyper-echoic, isoechoic, or mixed.
• Calcifications: microcalcification (often associated with malignancy), macrocalcifications, or rim calcifications.
• Vascularity: Internal vascularity patterns using doppler, classified as absent, peripheral, or marked.
• Anatomical variations: While the butterfly shape is typical, be aware of anatomical variations. Some individuals may have a pyramidal lobe extending from the isthmus, or an accessory thyroid tissue along the embryological descent path of the thyroid gland.
  
  

Surrounding Structures

• Vasculature: Identify the common carotid arteries on either side of the thyroid, noting any displacement or stenosis. Assess the jugular veins for potency and flow direction.
• Musculature: The strap muscles lie anterior to the thyroid gland, while the thicker sternocleidomastoid muscles are lateral. The Longus coli muscles are visible posteriorly.
• Lymph Nodes: Carefully examine the central and lateral neck compartments for enlarged or abnormally echogenic lymph nodes, which could signify metastasis or inflammatory processes.
• Other: The trachea appears as an anechoic structure with posterior acoustic enhancement, while the esophagus may transiently appear to the left of the trachea during swallowing. The hyoid bone serves as a useful anatomical landmark.
  
  

Interpreting Thyroid Ultrasound Results

Interpreting thyroid ultrasounds based on thyroid ultrasound anatomy requires careful consideration of sonographic findings, the patient’s clinical context, and relevant diagnostic guidelines. Focus on:

• Diffuse parenchymal changes: Evaluate for:
• Hypoechogenicity, heterogeneity, and increased vascularity are suggestive of thyroiditis (consider specific patterns for Hashimoto’s versus Graves’ disease).
• Diffuse enlargement is consistent with a goiter (multinodular or non-nodular).
• Nodules: Assess each nodule, noting:
• Size (document in three dimensions).
• Composition (solid, cystic, mixed).
• Margins (well-defined, irregular, infiltrative).
• Echogenicity (hypoechoic, hyperechoic, isoechoic, mixed).
• Calcifications (microcalcifications, macrocalcifications, rim).
• Vascularity patterns on Doppler (absent, peripheral, marked).
• Lymph nodes: Examine central and lateral neck compartments. Abnormalities may include:
• Enlargement (>1cm short axis).
• Rounded shape.
• Loss of fatty hilum.
• Abnormal echotexture (heterogeneous or cystic).
• Other Anatomical Considerations:
• Major vessel compression or displacement.
• Tracheal deviation.
• Prevertebral soft tissue abnormalities.
  
  

Diagnostic Integration

• Clinical Picture: Correlate ultrasound findings with the patient’s history, physical examination, and thyroid function tests (TSH, T4, T3). Additional laboratory testing (e.g., thyroid antibodies) might show.
• Nodule Risk Stratification: Employ standardized classification systems like TIRADS, ACR TI-RADS, or ATA Guidelines to assess a nodule’s malignancy risk and guide management recommendations (e.g., surveillance vs. biopsy).
• Radiology Report: Provide a clear, concise report including:
• Precise measurements of the thyroid gland and any nodules.
• Descriptive characteristics of nodules using standardized terminology.
• Lymph node status and other relevant findings.
• Differential diagnosis and/or management recommendations based on guideline criteria.