Medical Radiology - Lazar A.P. 2008

Radiation examination of endocrine glands
Radiation diagnostics of thyroid and parathyroid gland diseases

The Thyroid Gland is one of the largest Endocrine glands, weighing 25–30 g. It is the only superficial endocrine organ that is readily palpable in the anterior neck. The transverse diameter of the thyroid is 4–5 cm, and the vertical diameter is 5–6 cm. The thickness of each lobe is 18–20 mm, and that of the isthmus is 6–8 mm. The gland reaches its maximum size in individuals aged 25–30 years. After the age of 50, its size decreases, and adipose tissue accumulates beneath the capsule. After 60 years, some thyroid follicles become empty, and Connective Tissue proliferation increases. The gland is generally larger in women than in men and may temporarily enlarge during menstruation.

The thyroid gland is located at the level of the C5–C7 vertebrae, beneath the Skin, Infrahyoid Muscles, and deep Cervical fascia. The posterior concave surface of the gland lies anteriorly and laterally against the thyroid Cartilage (from which the gland derives its name), the cricoid cartilage, and the upper 5–6 tracheal rings.

The thyroid gland consists of right and left lobes connected inferiorly by an isthmus. The thyroid isthmus is located at the level of the 1st–3rd tracheal rings. Posteriorly, the thyroid lobes are adjacent to the Esophagus, Pharynx, and the neurovascular bundle of the neck. In about one-third of people, an additional lobe—the pyramidal lobe—extends upward from the isthmus.

Externally, the thyroid gland is enclosed in a connective tissue fibrous capsule. Septa extend inward from the capsule, forming the framework of the organ and dividing its parenchyma into lobules. These lobules contain follicles lined internally by cuboidal epithelial follicular Cells. These cells produce a dense colloid containing the THYROID Hormones thyroxine and triiodothyronine. Because these hormones incorporate iodinated Amino Acids, follicular cells actively take up iodine ions, resulting in a tissue iodine concentration that is 300 times higher than that in Blood Plasma.

Thyroxine (tetraiodothyronine, T4) and triiodothyronine (T3) regulate all METABOLISM/4.html">Types of Metabolism by enhancing the Oxidation of Proteins, fats, and CARBOHYDRATES. They promote the Excretion of Water, calcium, and potassium, and stimulate the Central Nervous system, Adrenal Glands, and Gonads. Hypoplasia of the thyroid gland and the resulting decrease in hormone production lead to conditions such as Myxedema and cretinism. Conversely, thyroid hypersecretion causes Graves' disease. An enlargement of the thyroid gland is known as a goiter. In certain regions of Ukraine, endemic goiter occurs due to a deficiency of dietary iodine, leading to compensatory proliferation of glandular tissue.

The Parathyroid glands are typically four small, oval bodies located on the posterior surface of the right and left thyroid lobes, comprising paired superior and inferior parathyroid glands. The dimensions of each parathyroid gland are 4–7 mm in length, 3–4 mm in width, and 1–2 mm in thickness.

Each parathyroid gland is enclosed in its own fibrous capsule, with connective tissue septa extending inward. The parenchyma of the parathyroid glands produces parathyroid hormone (PTH), which regulates Calcium and phosphorus metabolism in The Human Body. Complete surgical removal of the parathyroid glands in animals leads to death from tetany (severe Muscle spasms).

Diagnostic imaging Methods for evaluating the thyroid and parathyroid glands include radionuclide scintigraphy, ultrasound (US), computed tomography (CT), and Magnetic Resonance Imaging (MRI).

Radionuclide imaging is the primary method for evaluating the anatomy and function of the thyroid and parathyroid glands. Thyroid scintigraphy is performed on an empty Stomach, following the temporary discontinuation of medications and foods containing iodine and bromine. Thyroid scintigraphy is performed after intravenous administration of 80–100 MBq of ^99mTc-pertechnetate. Parathyroid scintigraphy is performed by computer subtraction of the image obtained after ²⁰¹Tl-chloride administration from that obtained after ^99mTc-pertechnetate administration. The radionuclide image of the thyroid gland resembles a butterfly.

To assess iodide uptake function and thyroid hormone synthesis, thyroid radioiodine uptake measurement is performed after the patient, on an empty stomach, ingests a sodium iodide solution with an activity of 0.5 MBq of ¹²³I. The radiation detector is positioned 30 cm from the anterior neck surface, and measurements are taken at 2, 4, and 24 hours after radiopharmaceutical administration.

In vitro radioimmunoassay is used to assess the release of thyroid hormones into the blood, total and free hormone concentrations, their binding to thyroxine-binding globulin (TBG), anti-thyroglobulin antibody levels, and parathyroid hormone levels.

Ultrasound examination of the thyroid and parathyroid glands is performed using a specialized transducer. For superficial structures located up to 5 cm deep, a high-frequency ultrasound of 7.5–10 MHz is used, whereas deeper structures are evaluated with 5–7.5 MHz. Under normal conditions, the thyroid appears as a homogeneous, fine-grained Structure up to 0.8 cm in thickness. The parathyroid glands may not be visualized; when visible, they appear as rounded structures with smooth contours and a homogeneous structure up to 0.5 cm in diameter.

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Fig. 228. DIFFUSE TOXIC GOITER (radionuclide scan).

The scan (A) shows an enlarged, rounded thyroid gland in its typical anatomical Location, with a homogeneous distribution of the radiopharmaceutical. Radiograms from detectors positioned over the thyroid (B) and The Heart (C) demonstrate increased functional activity of the gland.

CT and MRI are performed when mass lesions, structural or dimensional changes, or spatial relationships with adjacent structures are suspected in the thyroid and parathyroid glands. On tomograms, the glandular structure is homogeneous with well-defined contours; the densitometric density is approximately +80 to +110 HU, and the MR signal intensity is high.

Endemic goiter is characterized by enlargement of the thyroid gland. Its clinical forms include diffuse, nodular, and mixed goiter.

In the diffuse form, imaging modalities reveal an enlarged thyroid gland, the inferior margin of which may extend behind the manubrium sterni. The internal structure, echogenicity, densitometric density, and signal intensity of the gland remain unchanged, with smooth and clear contours. Scintigraphy demonstrates diffuse thyroid enlargement with uniform radiopharmaceutical uptake, unlike thyroiditis, where glandular enlargement is accompanied by heterogeneous radiopharmaceutical accumulation (Fig. 228).

In the nodular form, foci of hyperplasia with altered echogenicity, signal intensity, and decreased densitometric density are detected within the gland. On scintigrams, nodules appear as "cold" areas—defects in radiopharmaceutical uptake.

In myxedema, The production of thyroid hormones decreases, which clinically manifests as fatigue, drowsiness, and muscle weakness. Radionuclide imaging reveals a reduction in the functioning thyroid parenchyma (Fig. 229).

On sonography, a thyroid cyst appears as a rounded, hypoechoic lesion with well-defined margins.

Fig. 229. Myxedema (radionuclide scan).

The scan (A) reveals a small thyroid gland with an uneven distribution of the radiopharmaceutical. Radiograms from detectors positioned over the thyroid (B) and the heart (C) indicate reduced functional activity of the gland. Blood iodine concentration decreases slowly, reaching 25% of its peak level by the end of the second day. On CT and MRI scans, the cyst exhibits a homogeneous structure surrounded by a thin capsule, with a densitometric density of +4 to +30 HU; the MR signal intensity depends on its iodine and protein content. Intravenous contrast administration does not enhance the cystic lesion.

Toxic Thyroid Adenoma appears on sonography as a solitary echonegative lesion with sharp borders. Scintigraphy reveals a "hot" area of increased radiopharmaceutical uptake. Radiotracer uptake in the remaining glandular tissue is reduced, but normalizes following thyrotropin administration (stimulation test).

Carcinoma accounts for approximately 90% of all malignant thyroid tumors. On ultrasound, a cancerous nodule typically appears as a hypoechoic lesion with irregular yet well-defined margins. Nodules with increased and heterogeneous echogenicity are less common. Microcalcifications are frequently observed along the tumor periphery. CT and MRI reveal asymmetric enlargement of the thyroid gland (Fig. 230), while large tumors cause tracheal compression and blurring of its contours. The Internal Structure of the nodule is heterogeneous, the margins are lobulated, the densitometric density is reduced, and the MRI signal intensity varies. Radionuclide scanning reveals a "cold" focal lesion corresponding to the malignant tumor in the gland.

Fig. 230. Thyroid Cancer on a computed tomography scan.

1 - area of decreased densitometric density in the enlarged right lobe of the gland;

2 - enlarged Lymph node with decreased densitometric density.

Fig. 231. BONES OF THE forearm in hyperparathyroidism.

1 - pseudotumor cyst; 2, 5 - areas of radiolucency;

3 - subperiosteal resorption; 4 - Bone Marrow resorption.

Parathyroid adenoma typically causes hyperparathyroidism—an elevated blood concentration of parathyroid hormone, which is detected by radioimmunoassay. The adenoma can be visualized using ultrasound, CT, MRI, and scintigraphy. It is typically round in shape, with well-defined margins, measuring up to 1.5 cm, hypoechoic, and may exhibit a heterogeneous structure containing calcifications. Hyperparathyroidism leads to hyperparathyroid osteodystrophy (Recklinghausen's disease), which radiographically manifests as generalized Osteoporosis and The formation of cysts throughout the Skeletal System (Fig. 231); Nephrolithiasis (Kidney stones) is also frequently detected.