Tc-99m sestamibi (also known as sesta-methoxyisobutylisonitrile) is one of the technetium radiopharmaceuticals.
- photon energy: 140 KeV
- physical half-life: 6 hours
- normal distribution: thyroid, parathyroid, heart (myocardium)
- excretion: hepatobiliary and renal
- target organ: colon, kidneys, bladder, gallbladder
Tc99m-sestamibi is a radiopharmaceutical that is taken up by cells with a high concentration of mitochondria i.e. highly cellular/metabolically active tissues such as the myocardium of the heart, but also abnormal tissues such as oxyphil cell parathyroid adenomas, myeloma and breast cancer (the basis of so-called scintimammography). This latter technique has a high specificity for cancer but uses significant radiation dose.
Uses, doses and timings
- adult dose: 740 MBq (20 mCi) IV
- timing: early scan at 15 min; delayed scan at 2 hours
When the radiopharmaceutical is used for parathyroid imaging, immediate and delayed imaging of the neck and mediastinum is performed; parathyroid adenomas are best demonstrated on delayed imaging (1-2 hours). A thyroid scan using Tc-99m pertechnetate follows. Parathyroid adenomas do not demonstrate uptake on thyroid scans, whereas normal thyroid tissue does.
Myocardial perfusion scintigraphy
- adult dose: 400MBq rest phase, 1.1GBq stress phase
- timing: image 45-60 minutes following injection allowing for clearance of tracer from the lungs and liver
- indications: investigation of chest pain, suspicion of coronary artery disease, pre-surgical workup including prior to renal transplantation
Tc-99m sestamibi is the most common agent for performing myocardial perfusion scintigraphy. Sestamibi is a lipophillic cation, which diffuses from the blood into the myocardial cells. The tracer is withheld intracellularly adjacent the mitochondria which facilitates imaging. 60% of sestamibi is extracted first pass from the coronary blood flow and there is minimal myocardial clearance.
Myocardial perfusion scintigraphy is performed with a rest and stress phase, looking for areas of reversible ischemia (exercise stress) or impaired myocardial flow reserve (pharmacologic stress). Rest imaging is usually performed first, with a stress phase conducted on the same day with a higher dose injection or a subsequent day.
An exercise stress test requires the patient to exercise on a treadmill (or other equipment in some centers) to achieve 85% of their maximum predicted heart rate e.g. following the Bruce or Modified Bruce protocol. Tc-99m sestamibi is injected at 85% predicted maximum heart rate and the patient continues to exercise for an additional 60 seconds while the tracer is extracted by the myocardium. The patient is imaged 30-60 minutes later. Beta-blockers should be avoided in the days prior to the exercise stress test as they may prevent the patient from reaching higher heart rates.
A pharmacological stress test involves the infusion of a coronary vasodilator such as adenosine, persantin or regadenoson. Each drug has a unique infusion protocol. Caffeine is contraindicated for 24 hours prior to persantin studies. Pharmacological stress testing examines coronary flow reserve, with diseased coronary vessels having impaired physiological response to the infused medication. Other territories of the heart with normal flow will have a relatively higher amount of Tc-99m tracer than those supplied by the diseased vessel.
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