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Coronary Artery Disease, Parathyroid Adenoma, and 99mTc-SestaMIBI Essay

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Updated: Feb 26th, 2022

Coronary Artery Disease

Coronary artery disease has the highest mortality rate in the entire universe. The World Health Organization (WHO) reported that the disease resulted in over nine million deaths in the year 2016. A comparison between developed and developing countries shows a big difference in the trends of mortalities caused by CAD. The mortality rates in developed nations as the United Kingdom and the United States of America are decreasing. Regardless of that, according to the American Heart Association organization, in the USA, 16.5 million people who were older than 20 years had developed coronary artery disease and the males accounted for 55%. In developing countries, the situation is worse; mortalities associated with the disease are on an increase. The principal factors in decreasing mortalities are the application of primary and secondary prevention methods against the disease. The former aims at reducing cardiovascular events that pose a high risk for the development of CAD but have no previous history. On the other hand, the latter include treatments directed at preventing further damage to the circulatory system in patients who have been suffering from CAD.

CAD is mostly used to outline stenosed coronary arteries that present with symptoms, often referred to as significant or obstructive CAD. Diagnosis of the disease is difficult for patients experiencing acute signs and symptoms. CAD can be diagnosed in patients with severe chest pains if an acute coronary syndrome is confirmed by the use of electrocardiogram and biomarkers even in mild CAD. The most common approach to a suspected case of CAD is performing myocardial stress testing. This test is usually used to keep watch on invasive coronary angiography.

Generally, among the population in the world and patients travailing from end-stage renal disease (ESRD), the leading cause of mortality in coronary heart disease. In both populations, the principles used in assessing and managing cardiovascular risks are the same. Subclinical conditions can now be detected early thanks to the advancements in the non-invasive imaging of coronary arteries. Medical therapy of the disease aims at altering the natural course of the disease and refining symptoms of angina. In the ESRD population, coronary revascularization poses an additional risk and benefit equation. In stable ESRD patients with multivessel CAD, coronary bypass surgery is preferred despite the risks of myocardial infarction, stroke, and chest wound infection. For ESRD patients suffering from the acute coronary syndrome, the most effective treatment is a percutaneous coronary mediation on the affected blood vessels.

Parathyroid Adenoma

Parathyroid adenoma is one of the proliferative disorders of the parathyroid gland. These conditions include parathyroid carcinoma, parathyroid adenoma, and parathyroid hyperplasia. The typical symptoms presented are elevated parathyroid hormone and calcium levels in serum and evidence of primary hyperthyroidism. It presents three types: primary, secondary, and tertiary hyperthyroidism. Hyperplasia of the parathyroid gland is common in cases of secondary hyperthyroidism that is caused by renal disease. Chronic renal diseases cause hypercalcemia, which initiates tertiary hyperthyroidism (self-regulating secretion of parathyroid hormone). Parathyroid adenomas cause 80-85% of the primary hyperthyroidism, while parathyroid hyperplasia and parathyroid carcinoma account for 15% and 5% respectfully. Clinically, patients with primary hyperthyroidism may present with evidence of high serum calcium levels. These include fatigue, weakness, pain, polyuria, polydipsia, and nephrolithiasis. Gastrointestinal tract symptoms exhibited are vomiting, constipation, nausea, and anorexia. In cases of extreme hypercalcemia, cardiac arrhythmia, coma, and death may occur. Currently, the diagnosis of hypercalcemia is an incidental finding during the normal tests for other diseases. High levels of calcium in serum are noted during laboratory screening. Its evaluation involves radiographic studies followed by surgical interventions.

Usually, parathyroid glands cannot be detected by imaging because of their size (5 *3* 1 mm). In the case of parathyroid disease, the gland is enlarged, hence it can be seen with the use of imaging systems. The primary imaging techniques used to visualize diseased glands are 99mTc-sestamibi scintigraphy and sonography. For adenomas, a long and avid uptake of sestamibi is seen on 2-hour delayed images. The use of single-photon emission computed tomography (SPECT) causes an increase in the efficacy of scintigraphy in pinpointing distended glands: It allows the gland to be viewed in three dimensions. In localizing parathyroid adenomas, scintigraphy offers over 90% sensitivity and easy imaging of glands measuring over 500mg.

99mTc-SestaMIBI in Myocardial Perfusion Imaging

99mTc-SestaMIBI is mainly used in carrying out myocardial perfusion scintigraphy. Sestamibi is a cation, and it is lipophilic hence, it can easily diffuse from blood vessels into the surrounding myocardial cells. Intracellularly, it is held in proximity to the mitochondria of the cells. At first, 60% of the diffused sestamibi is extracted from the coronary blood flow and then myocardial clearance is extensively reduced.

Rapid clearance from blood and a subsequent high myocardial uptake are the key characteristics of the biodistribution of sestamibi. After injection of the agent, it takes one hour for the initial strong hepatic activity to be cleared into the gallbladder. Usually, the best target to non-target ratio occurs between 60 – 90 minutes after the initial injection. Calculations of absorbed radiation doses show that the main target organ is the thyroid gland (230 mRad/mCi at rest). These values may be because 99mTc-pertechnetate can be produced in vivo.

An injection of 99mTc-SestaMIBI at rest results in a homogenized uptake. When under stress, coronary blood flow in the normal branch increased 2 times but to a different extent on the stenosed blood vessel. This creates a heterogeneous blood flow that can be observed with 99mTc-SestaMIBI as an area of relatively low uptake.

99mTc-SestaMIBI in Parathyroid Scan

99mTc sestamibi comprises cationic molecules that have lipophilic properties. Once the molecules are injected intravenously, their distribution in the body depends on blood flow. Through passive diffusion, the molecules then cross from the blood vessels through cell membranes into cells. In the cells, they are highly concentrated in the mitochondria area. Therefore, the detectability of hyperplastic parathyroid glands and parathyroid adenomas has a direct relationship with the presence of oxyphil cells rich in mitochondria.

While in circulation, 99mTc sestamibi is distributed to the thyroid gland submandibular and parotid salivary glands, liver and heart. There is no evidence of uptake of the molecules in the normal parathyroid gland. It is normal for the radiotracer to be found in the arm vein that was used to administer the molecule. However, there are chances of the radiotracer accumulating in the oral cavity at mild to moderate levels. This is because of secondary secretion of the radiotracer from the salivary glands. In the bone marrow, a varying mild uptake of radiotracer is evident. In children and youth, there is a mild to moderate uptake in the thymus. Moreover, there is some uptake in brown fat, especially in the supraclavicular region.

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IvyPanda. (2022) 'Coronary Artery Disease, Parathyroid Adenoma, and 99mTc-SestaMIBI'. 26 February.

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