Imaging methods for goitre
Diffuse enlargement of the thyroid gland (Glandula thyreoidea) can lead to severe compressive complications, particularly with the development of a retrosternal goitre. In such cases, besides subclinical hyperthyroidism, patients primarily present with severe dyspnoea.
- Contrast-enhanced computed tomography (CT): Axial slices clearly demonstrate narrowing of the tracheal lumen. Due to mass effect and compression of mediastinal veins, extensive collateral venous circulation in the anterior chest wall frequently develops. Coronal reconstructions are essential for assessing the craniocaudal dimension of the goitre and confirming its connection to the cervical portion of the gland.
- Neck and thoracic inlet ultrasound: Color Doppler mapping reveals enlargement of the lobes (often with heterogeneous echogenicity).
- Radionuclide imaging: The use of the iodine isotope (123I) demonstrates prominent radiopharmaceutical uptake by the goitre tissue.
Thymus (Thymus) and thymoma
The size of a normal thymus is variable. Its hyperplasia is frequently associated with myasthenia gravis. The tumour itself (thymoma) is rare in children, so targeted searches for it at a young age in myasthenia are uncommon.
CT appearance of thymoma: The lesion typically has homogeneous density, enhances uniformly, and occasionally contains cystic areas. Most tumours are encapsulated, but capsule invasion is observed in 30–60% of cases. Invasive thymomas invade the mediastinal adipose tissue, adjacent pleura, and produce characteristic pleural metastases (transpleural spread). Important: CT is limited in reliably diagnosing invasion if the process does not extend beyond the thymus.
MRI appearance of thymoma: The normal thymus in young individuals has an intermediate signal (lower than fat, but higher than muscle) on T1- and T2-weighted images. With age, the gland involutes and is replaced by fat, increasing its signal intensity. Thymoma itself is characterized on MRI by low signal intensity on T1-weighted images (similar to muscle) and high intensity on T2-weighted images. The method outperforms CT when data regarding mediastinal structure involvement are ambiguous. On T2-weighted images, lobular architecture, fibrous septa, cystic areas, and haemorrhages causing signal heterogeneity can be distinguished.
Scintigraphy of parathyroid adenomas
Parathyroid adenomas (glandulae parathyreoideae) are frequently identified in patients with chronic kidney disease against the background of secondary hyperparathyroidism accompanied by hypercalcaemia.
The primary imaging modality is scintigraphy using 99mTc-sestamibi. The examination is performed in two phases:
- Early phase (10 minutes): The radiopharmaceutical accumulates in the parenchyma of both the thyroid and parathyroid glands.
- Delayed phase (2 hours): Characterized by persistent radiopharmaceutical retention solely within the adenoma tissue, whereas the agent washes out from the thyroid gland.
For precise anatomical correlation (e.g., along the posterior surface of the lobe), hybrid SPECT-CT (SPECT-CT) technology is utilized. It is also critically important in cases of recurrent hypercalcaemia following parathyroidectomy. SPECT-CT enables the accurate localization of functionally active ectopic adenomas (e.g., in the anterior mediastinum) that were occult prior to surgery.
Briefly: answers to topical questions
What sonographic features are characteristic of diffuse goitre on ultrasound?
Diffuse toxic goitre (Graves disease) is characterized by enlargement of the thyroid gland with alterations in its echostructure.
- Echogenicity — the parenchyma becomes hypoechoic, and its structure heterogeneous (non-uniform).
- Vascularization — color Doppler mapping (CDM) demonstrates a diffuse, pronounced increase in blood flow, known as the specific "thyroid inferno" sign (thyroid inferno).
Which radiopharmaceuticals are used for thyroid scintigraphy?
Radiopharmaceuticals based on iodine and technetium isotopes are used for thyroid scintigraphy.
- Iodine isotopes — iodine-123 ($^{123}\text{I}$) and iodine-131 ($^{131}\text{I}$) are used.
- Technetium isotopes — technetium-99m ($^{99m}\text{Tc}$) is used, specifically in the form of $^{99m}\text{Tc}$-pertechnetate.
These agents allow for the assessment of structural features, functional activity of gland regions, and the identification of active isotope-accumulating "hot nodules".
Into which structures does an invasive thymoma most frequently grow?
An invasive thymoma can grow beyond the capsule into surrounding mediastinal structures.
- Mediastinal adipose tissue — infiltration and obliteration of adjacent fat pads are possible.
- Pleura — pleural involvement, thickening, nodularity, pleural effusion, and pleural metastases may occur.
- Pericardium — the tumour may involve the pericardium.
Where can ectopic parathyroid adenomas be located?
Ectopic parathyroid adenomas (ectopic parathyroid adenoma) can be located in various atypical anatomical regions of the neck and thorax.
- Mediastinum — nodular lesions are identified in the anterior or superior mediastinum.
- Ligamentous apparatus — adenomas may be situated along the thyrothymic ligament (thyrothymic ligament).
- Neck fascial spaces — location in the retropharyngeal or retrooesophageal position is possible.
What ultrasound features are characteristic of a parathyroid adenoma?
On ultrasound examination, parathyroid adenomas exhibit several characteristic features.
- Echogenicity — the lesions are hypoechoic (dark) compared to the surrounding tissue.
- Size — they are visualized as enlarged structures.
- Localization — typically situated posterior to the thyroid gland (most often posterior or slightly inferior to the mid-lower portion of the lobes).
Multi-phase examinations help differentiate them from lymph nodes, which can mimic adenomas.
MRI signal of thymoma: T1 — dark (isointense to muscle), T2 — requires attention (high signal from cystic areas and tumour tissue).
Class and exam questions
Why do chest wall veins dilate in retrosternal goitre?
This is a consequence of mass effect: a large goitre compresses the major mediastinal veins, triggering the development of extensive collateral venous circulation on the anterior chest wall.
What is the main advantage of MRI over CT in evaluating thymoma?
MRI better visualizes the lobular architecture of the tumour and cystic changes on T2-weighted images. Furthermore, MRI is more informative for detecting invasion into mediastinal structures when CT data are equivocal.
How to distinguish a parathyroid adenoma from thyroid tissue on scintigraphy?
On early images (10 minutes), 99mTc-sestamibi is captured by both glands. On delayed images (after 2 hours), the radiopharmaceutical washes out of the thyroid gland, while persistent uptake is retained in the parathyroid adenoma.
What else to study on this topic
- Features of CDM in ultrasound of nodular goitre
- Classification of ectopic parathyroid glands in the mediastinum
- Transpleural metastasis of invasive thymomas
- Changes in thymic MRI signal during age-related fatty involution
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