Tc99m Myocardial Perfusion Imaging Agents
- Tc99m Sestamibi (Cardiolite)
- Tc99m Tetrofosmin (Myoview)
- Tc99m Noet
- Tc99m Teboroxime
The ideal radiopharmaceutical for myocardial perfusion imaging should have the following characteristics: myocardial uptake directy proportional to blood flow, high extraction fraction, high target-to-background ratio, and good myocardial retention and photon flux for optimal imaging.
Technetium-labeled agents can be substituted for thallium in the rest and stress evaluation of myocardial perfusion. These agents are very different in behavior compared with Thallous chloride. The isotope chart on the following page gives a detailed comparison between the Tc99m compounds used for SPECT myocardial perfusion imaging. The technetium-labeled agents are lost slowly from the myocardium, with comparable clearance from normal and ischemic tissue. Since Tc99m-labeled tracers are essentially fixed in the cells, separate injections are required for the rest and the stress portions of the examination. Optimally, the rest and stress exams are performed on separate days. Same-day studies use as little as 8-10 mCi for the first dose and up to 45 mCi for the second. The second injection should contain 2.5 to 3 times the resting dose. The doses are adjusted for the weight of the patient. When possible, the injections should be separated by a three hour delay, though time constraints may not allow for this.
SESTAMIBI AND TETROFOSMIN
For a two-day imaging protocol, use 25-35 mCi depending on patient weight. SPECT imaging may be begun as soon as 15 minutes up to 3 hours post injection of tracer. With Sestamibi and Tetrofosmin, subdiaphragmatic radioactivity in the field of view can be problematic. A 16-ounce glass of cold water can weigh down the stomach when radioactive tracer refluxes up from the small bowel, removing it from the field of view. A carbonated beverage or a light snack can help remove stomach, bowel, or liver activity.
The trend in myocardial imaging nowadays is to use as much radiotracer as is necessary to get optimal imaging, especially in patients who are obese. Private offices and some hospitals are using doses in the 40 to 50 mCi range for myocardial SPECT in their morbidly obese patients. The radiation load to the patient for a two-day SPECT imaging protocol using conservative doses (25-30 mCi) of Tc99m Tracers such as Cardiolite or Myoview is in the same range as a CT scan, 10.6 mSV (milliSieverts).
Tc99m Sestamibi (Cardiolite)
Sestamibi passively diffuses into the cell and binds to the mitochondria in the nucleus. Like Thallium its initial uptake is flow-dependent, although at 55%, it has a slightly lower extraction rate. Myocardial uptake is 1.4% of the injected dose. Due to prompt excretion and a short halflife, ten times more Mibi may be used than Thallium to yield a similar radiation dose (0.05 Gy) to the target organ (small intestine). The hexakis alkylisonitrile technetium complexes are monovalent cations with a central technetium core octohedrally surrounded by 6 identical ligands coordinated through the isonitrile carbon.
Tc99m Tetrofosmin (Myoview)
Tetrofosmin: a diphosphine 99mTc complex (1,2-bis[bis(2-ethoxyethyl) phosphino] ethane) developed to replace 201Tl in myocardial perfusion imaging. Its first pass extraction rate is 45%; total myocardial uptake is 1 – 1.2% of the injected dose. Unlike Thallium, Tetrofosmin is not a potassium analog. The mitochndrial membrane potential plays a major role in the myocardial uptake and retention of Tetrofosmin. Myocardial uptake is related to the metabolic status of the myocytes, in particular the mitochondrial membrane and the plasma membrane potentials. Imaging quality is comparable to Sestamibi, though it clears more rapidly from the lungs and liver.
Tc99m Teboroxime is a boronic acid-derived, neutral lipophilic compound that, when labeled with 99mTc, has a half-life if 6 hours and produces gamma-energy rays of 140 keV. Myocardial cell uptake is proportional to coronary blood flow. Being a neutral lipophilic compound, it passively diffuses across the myocardial cell membrane. The first-pass myocardial extraction fraction is high (>90%), and the washout is rapid. There is no redistribution. Imaging must be completed within 10 minutes of administration due to rapid washout.The tracer has been approved by the FDA as a myocardial perfusion imaging agent, yet is not commercially available in the US at this time.
[(99m)Tc-N-ethoxy-N-ethyl dithiocarbamato-nitrito] is a new myocardial perfusion imaging agent that is comparable to 201Tl, allowing initial stress and delayed rest imaging. Clearance of pulmonary is rapid. Image quality is good because of low background interference.
COMPARISON OF PERFUSION AGENTS
More Resources for the Technologist to Download…
99mTc Tetrofosmin Pharmacology
Tetrofosmin Package Insert.
Download this continuing education article that breaks down the radiation exposure with Tc99m based radiotracers.
MPI IMAGING and THE DILEMMA OF 99mTC SHORTAGES
Download this continuing education article that details the challenges of reactor closures and Molybdenum shortage with respect to the manufacture of Tc99m based radiotracers. Detailed imaging protocols are offered.