|MadSci Network: Medicine|
First I'll present some "ELEMENTAL" information taken from "The Visual Elements Periodic Table" and "WebElements" (http://www.chemsoc.org/viselements/pages/pertable_fla.htm) and (http://www.webelements.com/) : Thallium is found in several ores, one of which is pyrites, used in the production of sulphuric acid. The commercial source of thallium is as a by-product of pyrites roasting in sulphuric acid production. It can also be obtained from the smelting of lead and zinc ores. Thallium is also present in manganese nodules found on the ocean floor. Crude thallium is present as a component in flue dust along with arsenic, cadmium, indium, germanium, lead, nickel, selenium, tellurium, and zinc. The use of thallium is limited as it is a toxic element. Thallium sulphate is employed as a rodent killer - it is odorless and tasteless - but household use of this poison has been prohibited in the USA. Thallium oxide is used to produce glasses with a high index of refraction, and also low melting glasses which become fluid at about 125K. Elemental thallium has no known biological role. It is considered toxic and teratogenic. Contact of the metal with the skin is considered dangerous, and there is evidence that the vapor is both teratogenic and carcinogenic. This is the "elemental" information readily available and present as "the" information to use in consideration of the harmful effects of thallium. Now, here's the consideration of GREATEST importance. The "compound" and/or molecular structure USED plays the biggest factor in many of the health and/or adverse reactions of any substance in question. For example, there are many drugs on the market that produce medical wonders. However, if something in the manufacturing process should go wrong, and the "structure" of that compound changed, it could then very well become a fatal compound. This is the nature of all man made substances. Even though the "elemental" aspects of thallium say it's dangerous, it's the chemical structure that truly dictates whether or not, or more specifically, how dangerous a compound is. One needs to be aware that the correct references and perspectives are looked at to ensure the safety of a particular substance. And above all, make no assumptions on your own about unknown substances and compounds. Always seek professional (medical) advice if you have ANY questions. And ALL questions are valid questions if you should have any concerns; so ASK. In addition, the amount of the substance in question needs to be considered. There is always the possibility for an unintended "overdose". Even with good things too much can be harmful; i.e. too many aspirin can create ulcers and too much water is called drowning. The amount of thallium in the diagnostic drug is considered a "trace" quantity and not considered deleterious. The main medical reasons thallium is used in many nuclear diagnostic tests is that the biological characteristics of thallium-201 is known to be similar to those of potassium as a transport substrate in the presence of glucose, insulin or phobolmyristate acetate (PMA). This means it readily flows through the bloodstream and is utilized similarly to potassium. The half life of thallium is relatively short (72 hours) and clears the body easily and quickly. Thallium is superior to Tc-Sestamibi (once "the" diagnostic drug of choice to use) in the evaluation of myocardial viability because of several of it's unique properties. One of which it is redistributed after some time (which also is used in a medical diagnosis) after administration. Since you mentioned you were to receive a myocardial perfusion study, thallium stress test, I'll include some information about the important uses of thallium in these tests. In myocardial perfusion imaging, thallium-201 is highly recognized for its ability to image "regional" myocardial blood flow distribution. Thallium-201 is very efficiently extracted by viable myocardial cells. After intravenous administration, thallium distributes in proportion to regional blood flow. Images of the heart shortly after thallium administration show deficits in regions where blood flow is relatively reduced and in zones of nonviable myocardium(e.g., previous myocardial infarction). These areas of "deficit" show up typically as dark areas in the image. Over time, "redistribution" of isotope generally occurs in previously ischemic zones, that is, defects related to ischemic myocardium normalize or "fill in." Defects related to infracted or scarred myocardium typically do not "redistribute" over time and remain fixed. However, imaging at 24 hours or after reinjection of thallium-201 may show viable but hypoperfused segments not otherwise identified by a standard redistribution study performed at 3 to 4 hours after isotope injection. Myocardial perfusion imaging is most commonly used in conjunction with exercise stress, with thallium-201 administered through an indwelling intravenous line at peak exercise. The patient then exercises for an additional 30 to 60 seconds, and images are generally acquired immediately after and again 3 to 4 hours after thallium administration. In patients with stable angina who are unable to exercise, pharmacologic "stress" (i.e., dobutamine, adenosine, or dipyridamole) has been used to induce myocardial hyperemia, with subsequent regional inhomogeneities in the perfusion pattern related to coronary stenoses. Finally, in patients with unstable angina or acute myocardial infarction, a perfusion study can be performed at rest. As with exercise, serial imaging can be performed after pharmacologic or rest thallium administration and demonstrate redistribution in regions of rest ischemia or underperfused but viable myocardium. In the scheme of things, nuclear medicines strive for best utilization. Will the target compound or element behave in a specific/desired way? Will it go to the areas of interest? Are the radiations emitted sufficiently detectable? How long will the "radioisotope" stay in the body? Is the compound medically safe? And as with all "drugs", there may be an allergic reaction to one or more components of the drug - that's just a fact of life. Fortunately, there have been only rare documented cases of an allergic reaction to the thallium tagged drug. Even though most information out on thallium may say it's a dangerous substance, when chemically combined and utilized correctly, it has become one of the many useful tools in diagnostic medicine. It's all in the perspective and structure. I hope this has helped allay some of your fears. And thanks for asking your question. I've included some websites that might also be helpful. Best wishes with your diagnosis. Possible helpful websites: http://www.vh.org/Patients/IHB/NucMed.html http://www.mamc.amedd.army.mil/williams/NucMed/CV_ToC.HTM (highly medical) http://www.americanheart.org/Scientific/statements/1995/049501.html (medical article on heart studies - guidelines) http://chorus.rad.mcw.edu/doc/00391.html
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