obstruction of the cystic duct leading to the gallbladder. The gallbladder is not visualized because the radiotracer cannot enter it. Some other clinical conditions that can be diagnosed by biliary images are common bile duct obstruction, biliary leak following surgery, biliary atresia, and a chole-dochal cyst.

The product is prepared by adding sodium pertechnetate (99mTc) to the shielded vial containing disofenin and stannous chloride. The preparation is allowed to incubate for 4 minutes. The product should be used within 6 hours of preparation. The structure of the technetium complex is shown.22,23 Hepatobiliary agents are commonly referred to as HIDA agents, or hepatobiliary aminodiacetic acids.

The fasting patient receives an intravenous injection of 5 mCi (185 MBq), which is taken up by the hepatocytes in the liver by active anionic transport. Patients with bilirubin levels greater than 5 mg/dL require an 8 mCi (296 MBq) dose. Then the radiopharmaceutical is excreted in bile, via the biliary canaliculus, into the bile ducts, with accumulation in the gallbladder and finally excretion via the common bile duct into the small bowel. The normal patient exhibits early accumulation of the radiopharmaceutical in the liver and the gallbladder.

Technetium ("mTc) Exametazime (Ceretec).

Technetium (99mTc) exametazime5 is a mixture of unstable lipophilic enantiomers that rapidly cross the blood-brain barrier and is trapped in the tissues. The proposed trapping mechanism for localization includes reduction by glutathione. A similar diffusion and trapping process occurs with autologous lymphocytes in vitro.

Exametazime is also known as hexamethylpropyle-neamine oxime or HMPAO. The radiolabeled complex is in dicated for cerebral perfusion in stroke, but is most commonly used for the radiolabeling of autologous leukocytes as an adjunct in the localization of intra-abdominal infection and inflammatory bowel disease.

Each kit includes several components: (a) reaction vials containing a mixture of exametazime, stannous chloride, and sodium chloride; (b) vials of 1% methylene blue; (c) vials of phosphate buffer in 0.9% NaCl; and (d) 0.45-^m syringe filters. Product preparation depends on the intended use.

For use as a cerebral perfusion agent, the kit can be prepared with or without stabilizer. Shelf life is 4 hours with stabilizer, but only 30 minutes without stabilizer. Stabilizer is prepared by injecting 0.5 mL of 1% methylene blue in the phosphate buffer vial. Fresh sodium pertechnetate (99mTc) elution (<2 hours old) is added to the shielded vial containing exametazime. The vial is gently agitated for 10 seconds and used immediately because this unstabilized complex has a shelf life of approximately 30 minutes. For stabilization, 2 mL of the methylene blue/phosphate buffer mixture is added to the reconstituted exametazime vial within 2 minutes of the technetium (99mTc) exametazime vial reconstitution. Since the stabilized product is dark blue, a syringe filter is utilized to prevent inadvertent administration of particulate matter. Unstabilized product should be visually inspected for a clear, colorless appearance prior to administration. The patient receives an intravenous dose of 10 to 20 mCi (370-740 MBq). Dynamic imaging begins immediately following injection for up to 10 minutes. Static imaging may be performed from 15 minutes to 6 hours after injection.

For the radiolabeling of autologous leukocytes, approximately 50 mL of a patient's blood is drawn. The blood will be processed to separate the leukocytes from the

Technetium (99mTc) Exametazime plasma and other cellular components. Fresh sodium pertechnetate (99mTc) elution (<2 hours old) is added to the shielded vial containing exametazime. The vial is gently agitated for 10 seconds and used immediately. The leukocytes will be incubated with technetium (99mTc) exametazime for 15 minutes, washed with saline, resus-pended in the patient's own plasma, and reinfused to the patient as soon as possible. The goal in leukocyte preparation is to achieve 7 to 25 mCi of activity in the tagged cells. Images are acquired 2 to 4 hours after reinjection of the radiolabeled cells, allowing the cells to localize at the suspected site of infection.

Technetium ("mTc) Macroaggregated Albumin. Technetium (99mTc) macroaggregated albumin (99mTc-MAA) is a sterile white suspension of human albumin aggregates formed by denaturing human albumin at 80°C for 30 minutes at pH 5 (isoelectric point of albumin).4 The particle size and number of particles can be estimated with a hemo-cytometer grid. The particle composition within the suspension should be between 10 and 100 ¡¿m, with no particles greater than 150 ¡¿m. Commercial manufacturing practices provide a consistent range for the number of particles per vial as well as a consistent range of particle size. This primary use of this agent is for the imaging of the pulmonary microcirculation to assess potential pulmonary emboli. Another much less common indication is to evaluate peritoneovenous (LeVeen) shunt patency.

In general, the kit is prepared by adding sodium pertechnetate (99mTc) to the shielded vial containing albumin, aggregated albumin, and stannous chloride, gently inverting the reconstituted suspension (to prevent foaming), and allowing the mixture to incubate at room temperature for 15 minutes. Gentle inversion of the vial is recommended to provide a uniform suspension prior to withdrawing a patient dose into a syringe. If the syringe is not used immediately, it should be gently inverted to resuspend the particles prior to injection. The tagged product is to be used within 12 hours of preparation. The precise structure of the radiolabeled complex is unknown at this time.

The patient receives an intravenous injection of 2 to 5 mCi (74-185 MBq) of the Tc-99m albumin aggregates, which lodge in some of the small pulmonary arterioles and capillaries. The number of aggregates recommended for good image quality and patient safety is 100,000 to 500,000 particles; thus, only a small fraction of the 280 billion capillaries are occluded. Multiple images of the lung are obtained to assess lung perfusion. The distribution of the particles in the lung is a function of regional blood flow; consequently, in the normal lung, the particles are distributed uniformly throughout the lung. When blood flow is occluded because of emboli, multiple segmental "cold" (decreased radioactivity) defects are seen. This procedure is almost always combined with a xenon-133 gas or technetium-99m DTPA aerosol lung ventilation scan (should be normal in pulmonary embolism) and same-day chest radiograph x-ray (should be normal).

Technetium ("mTc) Mebrofenin (Choletec).

Technetium (99mTc) mebrofenin22,23 is a 2:1 complex of ligand:metal that is rapidly eliminated via the hepatobil-iary system. The presence of bromine within the structure of mebrofenin makes the drug more lipophilic, and thus it has higher hepatic extraction than other hepatobiliary iminodiacetic acids (HIDA). Clearance from the blood is also affected by bilirubin levels, but generally to a much lesser extent because of the higher hepatic extraction. The clinical indication for this product is hepatobiliary imaging.

The product is prepared by adding sodium pertechne-tate (99mTc) to the shielded vial containing mebrofenin, stannous fluoride and preservatives, and allowing the mixture to incubate for 15 minutes. The presence of preservatives allows this kit to be used for up to 18 hours after preparation.

The fasting patient receives an intravenous injection of 5 mCi (185 MBq), which is taken up by the hepatocytes in the liver by active anionic transport. Patients with biliru-bin levels greater than 1.5 mg/dL may require a 10 mCi (370 MBq) dose.

Technetium 9mTc) Medronate. Technetium (99mTc) Medronate (99mTc-MDP)24 is a diphosphonate complex with technetium-99m indicated for bone imaging to identify areas of altered osteogenesis. The clinical use of this agent is for investigation of skeletal problems such as metastatic disease to the bones, osteomyelitis, Paget disease, fractures, primary bone tumors, avascular necrosis, metabolic bone disease, and loose or infected hip prostheses. Stress fractures can be diagnosed by bone imaging when x-rays are completely normal. Bone radioscintigraphy is one of the most commonly performed nuclear medicine diagnostic procedures because the whole-body survey allows evaluation of the entire skeleton, which cannot be done as cost-effectively by any other imaging modality.

The relatively weak complex is prepared by adding sodium pertechnetate (99mTc) to the shielded vial containing medronic acid, a stannous salt (varies among manufacturers) with or without an antioxidant stabilizer. Radiolabeling

Technetium (99mTc) Mebrofenin

Technetium (99mTc) Mebrofenin requires an incubation of 1 to 2 minutes, and the recommended shelf life is 6 hours.

The patient receives an intravenous injection of 10 to 20 mCi (370-740 MBq) of Tc-99m medronate, which localizes in bone according to the degree of metabolic activity. Tc-99m medronate is absorbed onto hydroxyapatite crystals at sites of new bone formation with about 50% to 60% of the injected dose distributed throughout the skeleton within 3 hours; the rest is excreted by the kidneys. The patient should void immediately prior to imaging to minimize interference from the bladder. Images are obtained 1 to 4 hours postinjection.

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