General Nuclear Medicine

Radiotracer exams

  • In studies which use multiple radio-tracers (i.e. V-Q scan) you will need to use the radiotracer which has the lower keV first. This is because if we use a radiotracer with a keV of 100 for example, there will be a significant amount of particles which do not hit the target at 100 keV but may be at 80 due to down-scatter. So if you were to start with the higher tracer which has a keV of 200 and you get scatter causing some of the particles to hit the target at 100 keV then you would not know if the “hot” areas seen were from scatter from the high keV or from the lower tracer which has a keV of 100.

Exam Tips

  • When looking at images make note of the timing of the study and time between studies as this will clue you in to what tracer is used. This is because if you know the half lives of the tracers (which you should!), if you see that timing indicates days between a study then the use of tracers with a half liver of minutes or hours would not be possible.

Spills

  • What constitutes a major and minor spill varies depending of the isotope

  • Tl-201 & Tc-99m

    • > 100 mCi = major

    • < 100 mCi = minor

  • I-131

    • >1 mCi = major

    • < 1 mCi = minor

  • Ga-67 & In-111 & I-123

    • > 10 mCi = major

    • < 10 mCi = minor

  • Appendix N of NUREG-1556 defines specific doses of radioisotopes in nuclear medicine departments used for imaging, nonimaging, and therapy to determine whether a major spill/contamination procedure or a minor spill/contamination procedure will be implemented.

Collimators

  • These are lead filters that only allow specific photons to reach the detector

  • Pinhole collimator

    • Will magnify very small structures

    • High spatial resolution & low sensitivity

    • When to use:

      • Thyroid scan

      • Parathyroid scan

      • Pediatric joints

  • Low energy collimators (< 150 keV)

    • Used with low energy radiotracers —> Tc-99m

    • When to use:

      • Bone scans

      • Hepatobiliary studies

      • Brain perfusion

  • Medium energy collimators (150-300 keV)

    • Used with the following tracers

      • In-111

      • Ga-67

      • I-123

  • High energy collimators (> 300 keV)

    • Used with I-131

Effective Half Life

  • Effective Half life = 1/Tp + 1/Tb

    • Tp = physical half life time

    • Tb = biological half life time

References: