Radioisotopes and chemicals labeled to isotopes called ‘radiopharmaceuticals’ are used as tracers. They emit gamma rays which are detected by scintillation detectors and therefore of imaging of uptake is possible with a scintillation camera or gamma camera. When positron emitting tracers are used imaging is carried out with a PET camera.
Thallium (201Tl)
201Tl is a cation, biologically analogous to potassium which emits low energy photons. It has a very high myocardial affinity, extraction fraction by myocytes being 85 per cent. Peak uptake in heart muscle occurs in 5 minutes after IV injection, but there is a progressive washout or ‘redistribution’ beginning by 15 minutes post injection.
99mTechnetium (Tc) Labeled Tracers
99mTc-Sestamibi (2-methoxyisobutyl-nitrate) and 99mTc-Tetrofosmin 99mTc agents have the following advantages when compared to thallium:
1) Energy of gamma ray, 140keV, is optimal for gamma camera imaging and can produce higher quality images than Thallium which has lower energy photons [60-83keV].
2) Shorter half-life, (6 hours as opposed to 73 hours of 201Tl) and hence 10-15 times higher dose than thallium can be injected, which results in a higher count rate and shorter imaging time.
3) Production: Technetium is easily available via a generator 24 hours a day. Thallium is cyclotron generated and requires off-site delivery.
99mTc-Sestamibi
Sestamibi is a lipophilic cation (an isonitrile compound), which is extracted by viable myocardium. The extraction fraction for 99mTc-Sestamibi is approximately 65 per cet. The uptake of Sestamibi in the myocardium is proportional to blood flow and hence it is primarily a perfusion agent. The tracer is retained in normal myocardium for several hours. The primary route of excretion is hepatobiliary (33 per cent).
99mTc-Tetrofosmin [1, 2-bis [bis (2-ethoxyethyl) Phosphino] Ethane]
Tetrofosmin is a lipophilic cation which is rapidly taken up by the heart, liver, and kidneys in proportion to blood flow. The agent does not redistribute to any significant degree. Similar to 99mTc-Sestamibi, lesion conspicuity for mild to moderate stenoses may be less apparent than for 201Tl.
Positron Emitters
PET tracers in use include 2-[fluorine 18]-fluoro-2-deoxy-D-glucose (FDG), carbon-11 palmitate,and rubidium 82. Iodinated fatty acids based on iodine-123 include iodohexadecanoic acid and iodophenylpentadecanoic acid.
Thallium (201Tl)
201Tl is a cation, biologically analogous to potassium which emits low energy photons. It has a very high myocardial affinity, extraction fraction by myocytes being 85 per cent. Peak uptake in heart muscle occurs in 5 minutes after IV injection, but there is a progressive washout or ‘redistribution’ beginning by 15 minutes post injection.
99mTechnetium (Tc) Labeled Tracers
99mTc-Sestamibi (2-methoxyisobutyl-nitrate) and 99mTc-Tetrofosmin 99mTc agents have the following advantages when compared to thallium:
1) Energy of gamma ray, 140keV, is optimal for gamma camera imaging and can produce higher quality images than Thallium which has lower energy photons [60-83keV].
2) Shorter half-life, (6 hours as opposed to 73 hours of 201Tl) and hence 10-15 times higher dose than thallium can be injected, which results in a higher count rate and shorter imaging time.
3) Production: Technetium is easily available via a generator 24 hours a day. Thallium is cyclotron generated and requires off-site delivery.
99mTc-Sestamibi
Sestamibi is a lipophilic cation (an isonitrile compound), which is extracted by viable myocardium. The extraction fraction for 99mTc-Sestamibi is approximately 65 per cet. The uptake of Sestamibi in the myocardium is proportional to blood flow and hence it is primarily a perfusion agent. The tracer is retained in normal myocardium for several hours. The primary route of excretion is hepatobiliary (33 per cent).
99mTc-Tetrofosmin [1, 2-bis [bis (2-ethoxyethyl) Phosphino] Ethane]
Tetrofosmin is a lipophilic cation which is rapidly taken up by the heart, liver, and kidneys in proportion to blood flow. The agent does not redistribute to any significant degree. Similar to 99mTc-Sestamibi, lesion conspicuity for mild to moderate stenoses may be less apparent than for 201Tl.
Positron Emitters
PET tracers in use include 2-[fluorine 18]-fluoro-2-deoxy-D-glucose (FDG), carbon-11 palmitate,and rubidium 82. Iodinated fatty acids based on iodine-123 include iodohexadecanoic acid and iodophenylpentadecanoic acid.
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