Endocrine System PDF

# The Endocrine System ## Thyroid Imaging - The thyroid gland is located in the anterior neck. - It consists of two lobes connected by an isthmus. - It's highly vascular and receives blood from the superior and inferior thyroid arteries. - Thyroid imaging is based on the process of thyroid hormone...

# The Endocrine System ## Thyroid Imaging - The thyroid gland is located in the anterior neck. - It consists of two lobes connected by an isthmus. - It's highly vascular and receives blood from the superior and inferior thyroid arteries. - Thyroid imaging is based on the process of thyroid hormone production. - Thyroid hormones are manufactured from iodine absorbed from the digestive tract. - Iodine is transported and trapped by the thyroid follicular cells. - Iodine is oxidized to form MIT or DIT. - T3 is formed from the coupling of one MIT and one DIT molecule. - T4 is formed from the coupling of two DIT molecules. - Thyroid hormones are stored in the thyroid gland before being released into the body. - Thyroid hormones are important for a wide range of metabolic processes, including growth and development, body temperature regulation, and metabolism of proteins, lipids, carbohydrates, vitamins, and minerals. - Thyroid hormone production and secretion are controlled by a negative feedback mechanism. - TSH, secreted by the anterior pituitary gland, regulates thyroidal iodide uptake and release. - Low levels of circulating hormone cause more TSH to be produced which stimulates the thyroid to produce more T3 and T4. - Increased levels of circulating hormones signal the anterior pituitary to suppress TSH secretion. - This may affect the hypothalamus, which releases TRF, stimulating the anterior pituitary gland to produce TSH. ## Radiopharmaceuticals - 99mTc-pertechnetate, 123I-sodium iodide, and 131I-sodium iodide are used for thyroid imaging. - 131I-sodium iodide is not recommended for routine imaging due to its higher radiation dose. ## Clinical Indications - Thyroid imaging relates gland structure to function, including gland size, nodules, and ectopic thyroid tissue. ## Common Drugs and Chemical Substances Influencing Thyroid Uptake of Iodine | Substance | Duration of effect | |---|---| | Iodine-containing products | 2-4 weeks | | Lugol's solution, SSKI | 2 weeks or longer| | Topical iodine products | 2-4 weeks | | Kelp | 2-4 weeks | | Certain vitamin/mineral supplements | 2-4 weeks | | Some cough medicines | 2-4 weeks | | Radiographic contrast media | 2-4 weeks | | Water-soluble intravenous contrast | 2-4 weeks | | Other oral and fat-soluble contrast media | 2-4 weeks | | Thyroid medications | 4-6 weeks | | Thyroxine |2-3 weeks | | Triiodothyronine | 2-3 weeks | | Anti-thyroid medications | 2-8 days | | Propylthiouracil | 2-8 days | | Methimazole | Unknown | | Salicylates | Unknown | | Adrenocorticotropic hormone, adrenal steroids| 8 days | | Competing anions | 1 week | | Perchlorate | 1 week | | Pertechnetate | 1 week | ## Clinical Procedure - Before administering a dose, the technologist should question the patient about previous thyroid problems, neck surgery, medications, and recent radiographic procedures. - A low-iodine diet may be recommended for 3-10 days before radioiodine administration. - Certain thyroid medications may need to be discontinued. - Female patients of child-bearing age should be questioned to rule out pregnancy or breastfeeding. - The radiopharmaceutical is administered orally if radioiodine is used and intravenously if 99mTc-pertechnetate is used. - Imaging is performed 15-30 min after 99mTc-pertechnetate administration, 3-4 or 16-24 hr after 123I-sodium iodide, and 6-24 hr after 131I-sodium iodide. - The patient is placed in the supine position with the neck hyperextended. - Using a scintillation camera, they are positioned at a distance which is centered in the field of view, and produces an image the size of the thyroid gland. - Images are acquired in anterior and oblique projections, and images with markers placed on the suprasternal notch or over palpable modules may be useful. - Magnified views are obtained by moving the collimator closer to the surface of the neck. - An anterior view of the mediastinum is indicated if ectopic thyroid tissue is suspected. - A thyroid uptake is usually performed before or after imaging when radioiodine is used. ## Image Findings - The normal thyroid gland appears as a butterfly-shaped structure with a uniform, symmetric distribution of activity. - The right lobe is slightly larger than the left. - The isthmus may not be visualized. - A pyramidal lobe (third lobe) is sometimes visualized. - Abnormal findings include gland enlargement and visualization of functioning or nonfunctioning thyroid nodules. - Thyroid imaging determines whether a nodule concentrates tracer. ## Technical Considerations - Medications, food supplements, and diagnostic tests can interfere with tracer uptake. - Movement can introduce a motion artifact into the diagnostic image. - The patient should avoid moving the head, swallowing, coughing, or talking. ## Thyroid Uptake - Thyroid uptake is a common measure of the amount of radioactive iodide taken up and retained in the thyroid gland. - The value depends on how long after tracer administration the uptake is measured. - 2-6 hr after tracer administration reflects iodide trapping and organification within the gland. - 24-48 hr after administration reflects the rate at which iodine is lost. ## Radiopharmaceuticals - Radioiodine (123I or 131I) is preferred, although it is possible to determine thyroid uptake with 99mTc-pertechnetate. - Radioiodine capsules are easier to handle than radioiodine liquid and minimize the potential for accidental spills and contamination. ## Clinical Procedure - The patient should be instructed not to eat after midnight on the night before the test and to continue fasting for 2 hr after capsule administration. - Data is collected with a thyroid uptake probe. - The detector should provide a field of at least 10 cm in diameter at the surface of the patient's neck. - If a patient dose capsule is used as the standard, the capsule is placed in a neck phantom. - The thyroid probe is positioned perpendicular to the capsule at a distance of 25-30 cm from the face of the collimator. ## Technical Considerations - A correction factor must be applied to correct for physical decay. - Some facilities use a duplicate capsule as a standard. - Non-thyroid background measurements are also collected. ## Calculations - Thyroid uptake is calculated using the following equation: $$ % uptake = \frac{N-T}{S \times D} - B \times 100 $$ - where: - N = patient neck counts per minute. - T = patient background counts per minute. - S = standard counts per minute. - D = decay factor. - B = room background counts per minute. - A decay factor is not needed if a duplicate capsule is used as the standard. - Normal values are 6-18% for a 4 hr uptake and 10-35% for a 24 hr uptake. ## Technical Considerations - Medications containing iodine and radiographic procedures using iodinated contrast media will suppress radioiodine uptake. - Thyroid medications containing T4 and antithyroid drugs will suppress thyroid uptake. - 123I has a low-energy photon of 159 keV, so correction for attenuation by soft tissue may be considered. - Patients with extremely large goiters or heavy necks may attenuate the photons, causing erroneous results because of variable thyroid depths. - Each time an uptake measurement is performed, the patient's prior positioning must be reproduced to ensure constant counting geometry. - Standardized distances between the detector and the patient's neck or the neck phantom must be maintained. - To account for residual activity, patients who have received previous radioactive tracers must have a count taken over their thyroid gland to correct for this activity before the tracer is administered. ## Radioiodine Whole-Body Imaging - Whole-body imaging with radioiodine may be performed after total thyroidectomy for differentiated thyroid carcinoma. - The purpose of this type of imaging is to identify residual functioning thyroid tissue and/or areas of metastases. - 131I-sodium iodide is the tracer most commonly used to perform this type of imaging. - The optimum dosage of 131I-sodium iodide is controversial. - 123I-sodium iodide is now being considered as an alternative to 131I for whole-body imaging. - Patient preparation is similar to that for radioiodine thyroid imaging. - Sensitivity of whole-body radioiodine imaging can be enhanced by ensuring that the patient has complied with a low-iodine diet and by determining that the serum TSH level is greater than 30-50 milliunits (mU)/L. - Pregnancy and lactation should be ruled out in female patients. - Anterior and posterior images of the body, including the head to mid-femur, are acquired at 24-48 hr after 131I sodium iodide administration (at 24 hr if 123I is used). - Anatomical landmarks should be placed on the images to assist in precise identification of areas of iodine uptake. - Activity may be seen in the salivary glands, thyroid tissue remnants, stomach, esophagus, and thymus, as well as in distant functioning metastases. ## Parathyroid Imaging - The four parathyroid glands are located on the posterior aspects of the poles of the thyroid gland. - Their location can be extremely variable. - They produce and secrete parathyroid hormone (PTH), the hormone responsible for regulating the level and distribution of calcium and phosphorus. - Radionuclide imaging of the parathyroid is useful when primary hyperparathyroidism is suspected. This condition results from a tumor in one of the parathyroid glands or from hyperplasia of all four glands, both of which lead to excess secretion of PTH. - Excess PTH stimulates removal of large amounts of calcium from the bones, causing weakening and increased fracture risk. - The excess calcium level in the blood affects the nervous system function and muscle contraction. - The excess calcium may also be deposited in various tissues as calcium phosphate. - Death may result in extreme cases. - Treatment for primary hyperparathyroidism is surgical removal of the hyperplastic glands or the tumor. - Radionuclide imaging is a means of identifying the location of the parathyroid tissue before surgical intervention. -Definitive localization of ectopic tissue or tumor is particularly useful. - Parathyroid imaging may be accomplished with either the dual-phase technique or a dual-tracer technique. ## Clinical Procedure - No special patient preparation is required. - The patient is placed in the supine position with the neck hyperextended. - Both the neck and the upper mediastinum should be imaged. - The patient must remain in the same position for an extended period. - Patient comfort should be optimized. - Sandbags or restraining devices to immobilize the head and neck are recommended. - An intravenous line may be placed into an arm vein for easy administration of the tracers. - Imaging may be performed as a static image acquisition or as single-photon emission computed tomography imaging acquisition. ## Dual-phase Technique - 99mTc-sestamibi (5-25 mCi [185-925 MBq]) is administered and localizes in both thyroid and parathyroid tissue. - The tracer washes out of normal thyroid tissue more rapidly than it does from abnormal parathyroid tissue. - Early imaging (10 minutes after tracer administration) and delayed imaging (1.5-2.5 hr after tracer administration) may demonstrate retention of the tracer in abnormal parathyroid tissue that becomes more obvious on the delayed images. ## Dual-tracer Technique - This technique uses two tracers: - one that delineates normal thyroid tissue (99mTc-pertechnetate or 123I-sodium iodide) - one that localizes in both thyroid and abnormal parathyroid tissue (99mTc-sestamibi or thallium-201 [201Tl]-thallous chloride). - There are both advantages and disadvantages for which is administered first. - Downscatter from the higher-energy radionuclide into the lower-energy window, length of time the patient must remain still, administered activity of each tracer, and time required for 123I-sodium iodide localization are considerations when choosing the protocol to be used. - In all protocols, both images are normalized so the counts per pixel in the thyroid are the same in both images. - Then the 99mTc-pertechnetate or 123I-sodium iodide image is subtracted from the 201Tl-thallous chloride or 99mTc-sestamibi image to reveal the area of abnormal parathyroid tissue. ## Technical Considerations - Thyroid disease is common in patients with hyperparathyroidism. - Such disease may cause non-uniform uptake of both tracers in the thyroid gland and lead to false-positive results when the images are processed with computer subtraction. - The diagnostic quality of the images depends on patient cooperation. - Movement during data acquisition can have adverse effects. - Image registration, if applicable, also must be precise. - When using the dual-tracer technique, 99mTc-pertechnetate may be administered first to minimize the length of time the patient must remain in one position. - After 99mTc administration, the patient does not have to be positioned until the tracer has concentrated in the thyroid and imaging is ready to begin. - This method necessitates correction of the thallium image for 99mTc downscatter. - There are several advantages to the dual-phase technique. - Only tracer, one intravenous injection and one tracer are required. - Computer subtraction is not required. - The radiation dose to the patient may be decreased with the use of a single 99mTc-labeled agent. ## Adrenal Imaging - The adrenal glands are located at the superior poles of the kidneys. - They are small glands, weighing only 6-7 grams. - They consist of an outer cortex and inner medulla. - The cortex produces steroid hormones (aldosterone, cortisol, etc.). - Cholesterol is a precursor or building block of these hormones. - Cholesterol can be radiolabeled with 131I and has been used to image adrenal adenomas and certain other adrenal pathology. - Iodocholesterol is an investigational drug and not available commercially. - The adrenal medulla manufactures catecholamines (epinephrine and norepinephrine)—hormones that control the body's response to stress. - Tumors of the adrenal medulla are called pheochromocytomas. - They secrete excessive amounts of catecholamines and may be benign or malignant. - Symptoms of pheochromocytoma include increased catecholamine levels in the blood and urine, as well as hypertension. - Nuclear medicine imaging can be used to identify sites of excessive catecholamine secretion within the adrenal bed or in metastatic sites outside of this area. - The radiopharmaceutical used to image the adrenal medulla is 123I or 131I-methyliodobenzylguanidine (MIBG; also known as iobenguane sulfate). - This compound is structurally similar to norepinephrine but does not exert any pharmacologic effect. ## Clinical Procedure - In preparation for imaging, the patient should receive Lugol's solution (a concentrated solution of potassium iodide) at least 1 day before tracer administration and for 6-7 days thereafter. - The solution saturates the thyroid gland with "cold" iodine, preventing the uptake of any "free" radioiodine (radioiodine not attached to MIBG) that may be present in the tracer, thereby minimizing unnecessary radiation exposure to the thyroid gland. - Approximately 0.5 mCi (18.5 MBq) of 131I-MIBG and approximately 10 mCi of 123I-MIBG is administered intravenously. - Anterior and posterior imaging from the top of the skull to the pelvis is performed at 1, 3, and 7 days after tracer administration for 131I-MIBG and at 24 hr but not later than 48 hr post-injection for 123I-MIBG. - The patient should be asked to void immediately before imaging, because free iodine is excreted through the urine. ## Image Findings - 131I-MIBG and 123I-MIBG uptake is visualized normally in the liver, spleen, and heart. - The salivary glands and bladder may also be visualized as a result of uptake of free iodine in the tracer. - Areas of abnormal uptake persist over time. - Pheochromocytomas may occur in the adrenal bed or in other places in the thorax and abdomen. - Metastases from a malignant pheochromocytoma may be visualized in the liver, bone, lymph nodes, heart, lungs, or other sites. - 131I-MIBG and 123I-MIBG are used in the detection, localization, staging, and follow-up of neuroendocrine tumors and their metastases, in particular for: \- pheochromocytomas \- neuroblastomas \- ganglioneuroblastomas