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CH2C(CH3)2OCH3 Technetium (99mTc) Sestamibi higher energy y-ray (140 keV) also provides better quality images with contemporary gamma cameras.

Myocardial perfusion imaging is, by far, the most common nuclear medicine procedure. Various myocardial perfusion protocols utilize 99mTc sestamibi for one or both procedures: 1-day rest first, stress second, 1-day stress first, rest second, 1-day stress first, rest if ischemia is present, 2-day protocols, and dual-isotope studies. The 1-day rest first, stress second protocol utilizes 8 to 10 mCi (296-370 MBq) while the patient is at rest (normal blood flow). Imaging occurs 30 to 60 minutes later. Sufficient time is needed for the radiotracer to clear the liver before imaging. One to four hours after the rest study, 25 to 30 mCi (925-1,110 MBq) is injected at peak stress (treadmill or pharmacologic stress with increased blood flow demand), and imaging occurs 15 to 60 minutes later. Two-day protocols typically utilize 20 to 25 mCi (740-925 MBq) doses for both the rest and stress images. Dual-isotope protocols typically perform a thallium rest study first, followed shortly thereafter with a 99mTc stress study.15

The recommended dose for 99mTc sestamibi when used for breast imaging is 20 to 30 mCi (740-1,110 MBq). When used for parathyroid imaging, the recommended dose is 25 to 30 mCi (925-1,110 MBq).

Technetium ("mTc) Succimer Injection. 2,3-di-mercaptosuccinic acid (DMSA, succimer) forms a hy-drophilic complex when labeled with technetium-99m that is useful in evaluating renal function, especially in children.

The pH-dependent preparation of 99mTc-DMSA begins with the addition of sodium pertechnetate (99mTc) to shielded reaction vial containing 2,3-dimercaptosuccinic acid, stannous chloride, ascorbic acid, and inositol at a pH range of 2 to 3. Various metal-DMSA complexes are initially formed, but over the 10-minute incubation period at this acidic pH, the hexacoordinate complex28 of three car-boxylic acids and three sulfhydryls produces a tracer with the highest renal uptake.5 Localization of such a complex prepared at a certain pH is not altered if the environmental pH is subsequently changed.29 This complex has a shelf life of 4 hours.

The patient is injected with 2 to 6 mCi (74-222 MBq) of 99mTc-DMSA and images are taken 1 to 2 hours later.

OO Technetium (99mTc) Succimer

Technetium (99mTc) Sulfur Colloid. Technetium (99mTc) sulfur colloid (99mTc-SC) is a dispersion of particles larger than those found in solution, but small enough to remain suspended in the delivery matrix for a very long time. It is indicated for use by injection to assess functional capacity of reticuloendothelial cells of the liver, spleen, and bone marrow, to evaluate peritoneovenous (LeVeen) shunt patency. It is indicated orally to assess gastric emptying. Another use involves the intralymphatic or peritumoral administration for radioguided sentinel lymph node biopsy.

Kits for the preparation of 99mTc-SC include the following three vials: (a) a reaction vial containing sodium thiosul-fate, edentate disodium, and gelatin; (b) "vial A," which is an aqueous solution of dilute hydrochloric acid; and (c) "vial B," which is an aqueous solution of sodium biphosphate and sodium hydroxide. Sodium pertechnetate (99mTc) solution is added to the shielded reaction vial followed by the addition of the dilute hydrochloric acid. Upon heating in a water bath, the acid reacts with the thiosulfate and 99mTcO4~ to form the colloidal metal sulfide, 99mTc2S7. Short heating periods provide a higher population of smaller colloid particles in the final preparation,30 whereas extended heating periods produce larger particles with increasing amounts of colloidal sulfur.18 After the heating period has ended, the reaction vial is cooled for a brief period, and then the chemical reaction is quenched by the addition of the aqueous solution of sodium biphosphate and sodium hydroxide to the reaction vial. The inclusion of gelatin as an ingredient prevents colloid particles from clumping together. Particle sizes range from 0.1 to 1 ¬°um, with a mean size of 0.3 /m.4

After intravenous injection of 1 to 12 mCi (37-444 MBq) of 99mTc-SC, the tracer is rapidly cleared from the blood by the reticuloendothelial cells of the liver, spleen, and bone marrow. Uptake depends on the relative blood perfusion rate and the functional capacity of the cells. In the normal patient, 85% of the radiocolloid is phagocytized by Kupffer cells in the liver, 7.5% by the spleen, and the remainder by the bone marrow, lungs, and kidneys. Liver-spleen images can be acquired within 20 minutes of administration. Bone marrow imaging studies are performed 1 hour after injection of 10 mCi (370 MBq) of 99mTc-SC. Normal bone marrow will take up the radiocolloid, but diseased bone marrow appears as "cold" defects in patients with tumor deposits in the marrow.

Technetium (99mTc) sulfur colloid use for GI studies includes gastroesophageal reflux (GER) and gastric emptying of solid food. Gastroesophageal reflux imaging is performed after having the patient swallow acidified orange juice mixed with 1 mCi (37 MBq) of 99mTc-SC. Normal patients do not demonstrate reflux. Gastric emptying imaging is performed after the patient swallows solid food (i.e., scrambled eggs or pancakes) radiolabeled with 1 mCi (37 MBq) of 99mTc-SC. In general, the normal gastric emptying half-time is less than 90 minutes for solid food.15 Imaging begins shortly after administration and monitors the migration of the colloid particles.

Identification of sentinel lymph nodes for subsequent biopsy in breast cancer31 and melanoma32 patients is accomplished by the administration of several injections totaling 0.5 to 1.5 mCi (19-56 MBq) of filtered (0.22-^m filter) or unfiltered 99mTc-SC. Smaller colloid particles may translocate more readily into the lymphatic system, whereas larger particles remain at the injection site.15,30 Imaging begins shortly after administration and monitors the migration of the colloid particles.

Technetium ("mTc) Tetrofosmin (Myoview). Techne-tium (99mTc) tetrofosmin is a lipophilic cation complex33 clinically indicated as a myocardial perfusion agent for detecting regions of reversible myocardial ischemia in the presence or absence of infracted myocardium, identifying changes in perfusion induced by pharmacologic stress in patients with coronary artery disease, and assessing left ventricular dysfunction.

Preparation of technetium (99mTc) tetrofosmin begins with the addition of sodium pertechnetate (99mTc) to shielded reaction vial containing tetrofosmin [6,9-bis(2-ethoxyethyl)-3,12-dioxa-6,9-diphosphatetradecane], stannous chloride, disodium sulphosalicylate, sodium D-gluconate, and sodium hydrogen carbonate. Initially, the reduced technetium-99m forms a weak complex with gluconate which undergoes a ligand exchange with tetrofosmin during a 15-minute incubation period at room temperature. The recommended shelf life is 8 hours.

Technetium (99mTc) tetrofosmin accumulates in the myocardium by facilitated diffusion from the Na+/H+ an-tiporter.27 There is essentially no "redistribution," or movement of the tracer out of the myocardium back into the bloodstream. Heart:liver ratios are somewhat higher for tech-netium (99mTc) tetrofosmin as compared with technetium (99mTc) sestamibi and allows for a shorter interval between injection and imaging for technetium (99mTc) tetrofosmin.34

Various myocardial perfusion protocols utilize technetium (99mTc) tetrofosmin for 1-day, 2-day, and dual iso tope studies. One day protocols should utilize 5 to 12 mCi (185-444 MBq) for the first dose (rest or stress). Imaging can commence as early as 15 minutes later. One to four hours after the first study, 15 to 33 mCi (555-1,221 MBq) is injected for the second study. Two-day protocols typically utilize 30 mCi (1,110 MBq) doses for both the rest and stress images. Dual-isotope protocols typically perform a thallium rest study first, followed shortly thereafter with a technetium-99m stress study.15

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