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Nuclear Medicine and Tc-99m
Transcript of Nuclear Medicine and Tc-99m
- Gamma ray energy emitted should match imaging equipment while minimizing tissue absorption and radiation dose; ideally between 80-200 keV
- Beta emission and conversion electrons should be minimal because they increase radiation to the patient but do not improve the scan image
- The half-life should be short so the substance is absent from the patient's system quickly Production of Tc-99m - Commercially produced by fission of highly enriched uranium in nuclear reactors; Mo-99 is one product
- Mo-99 is used as the transportable form of Tc-99m due to the longer half-life, leaving medical institutions to produce Tc-99m from Mo-99 on their own Uses in Medicine Powell Richards first realized Tc-99m's potential as a medical radiotracer in the 1950s
Tc-99m is used in 20 million diagnostic procedures annually and 85% of all diagnostic imaging uses the Tc-99m isotope
Because Tc-99m emits 140.5 keV gamma rays, the wavelength emitted by conventional x-ray machine, it can be readily detected by medical equipment
Bone Scans - Technetium-99m is used predominately in bone scans
- The pertechnetate ion can be used directly in bone scans, because it is taken up by osteoblasts that aggregate around injury or tumor sites Other Scans - For other scans, TcO4- is not suitable, so:
A reducing agent is added to bring the oxidation number of Tc down to +3 or +4
A ligand is added to form a coordination complex, the ligand chosen to have an affinity for the organ to be targeted
For example, for myocardial imaging, Technetium sestamibi is used to show how blood flows through the heart: - Radionucleotides combine with other substances to form chemical compounds that localize to specific organs or receptors; this allows processes of those locations to be imaged
- Scans work by taking advantage of how the body handles substances differently when there is pathology present; for example, a tumor in the brain takes up more glucose than other areas in the brain, which can be visualized using radioactive glucose and radio-imaging
* The radionucleotide is known as a "tracer"
- Radio-imaging differs in that it shows the physiological functions
rather than the physical anatomy that scans like the MRI and CAT
scans show Tc-99m emits gamma rays 88% of the time, minimizing other radiation, and emits easily detectable 140 keV gamma rays Tc-99m decays with minimal electron conversion and no beta emission, resulting in minimal unnecessary radiation to the patient Tc-99m has a half-life of 6 hours, so 93.7% decays in 24 hours, making it ideal for scanning while preventing long-term patient exposure to radiation Decay of Tc-99m Technetium is atomic number 43 and its most stable isotope has an atomic mass of 98 amu and a half life of 4,200,000 years
Technetium-99m is a metastable nuclear isomer, indicated by the "m"
The decay into Tc-99 occurs by gamma emission 88% of the time, with the other 12% occurring by internal conversion
Tc-99m was discovered in 1938 by Emilio Segre and Glenn T. Seaborg after they bombarded Molybdenum producing Mo-99 which decayed into Tc-99m with a half life of 2.75 days The "Cintichem Process" Tc-99m Generators - Molybdenum-99 spontaneously decays by emitting beta particles. Over 87% of decays lead to the 142 keV excited state of Tc-99m 99Mo → 99mTc + β− + νe - First generator was developed in 1958
- Generator using column chromatography:
# Mo-99 is put into its water soluble form, (MoO4) and absorbed onto (Al2O3)
# As Mo-99 decays, TcO4- (pertechnate ion) is formed
# Saline solution can then be pulled through the column to produce a saline solution containing Tc-99m as the dissolved sodium salt of pertechnate Instant Tc-99m This instant method was developed in 1971 using Mo-100, a stable form, as the starting point Isotopically enriched Mo-100 to >99.5% is bombarded by 22 MeV protons in medical cyclotrons to produce Tc-99m Prostate
Cancer Compared to Iodine Radioisotopes - Tc-99m can be used in smaller quantities because there is less tissue absorption, decreasing radiation risk to the patient and a lower cost
- In comparison, Tc-99m has higher accuracy and precision In Conclusion... Technetium-99m is a highly useful radioisotope with attributes that make it ideal for medical diagnostics