nuclear medicine

what forces act inside the nucelus

1. coulomb force
   - protons repel each other (due to same charge)

2. strong nuclear force
   - holds protons and neutrons together

what causes nucelus to be unstable

too many neutrons or protons

Why can’t hospitals just ship Tc-99m directly?

Because it has a very short half-life (6 hours) → decays too fast

What is a radionuclide generator?

A system that produces radioactive material on-site

What is the “parent” isotope?

Molybdenum-99 (Mo-99)
Half-life = 66 hours

What is the “daughter” isotope?

Technetium-99m (Tc-99m)

Produced by beta decay of Mo-99

What is “milking”?

Extracting Tc-99m from the generator using a solvent

Why is Tc-99m used so much?

• Half-life = 6 hours (perfect balance)

• Emits 140 keV gamma rays

• Safe enough but detectable

What does Tc-99m decay into?

Tc-99 (very long half-life → basically harmless)

What does a gamma camera do?

Creates a 2D image of where radiation is in the body

Shows function, not structure

What is a collimator?

A lead grid that only allows straight gamma rays in

• Improves image accuracy

• Blocks angled/scattered rays

What is the NaI crystal?

A sodium iodide crystal that converts gamma rays → light

• scintillation

What do PMTs (photomultiplier tubes) do?

• Convert light → electrical signal

What do X and Y circuits do?

Determine location of the signal on the image

What does the Z circuit do?

Measures energy of the photon

Why Z circuit matters (what is compton scattering, why is this a problem, how does the camera fix this?)

A photon hits something → changes direction + loses energy

This could place the signal in the wrong direction

So, the Z-circuit checks the energy and if the energy is too low the photon is rejected

What is SPECT?

A rotating gamma camera → creates 3D images

How does SPECT work

Takes many 2D images → computer builds a 3D model

what is SPECT’s weakness and how is this fixed

• poor resolution leading to blurry images

• combined with MRI or CT scans, putting function and structure tgt

What type of decay does PET use?

• uses Positron (β⁺) decay

What happens to the positron in PET and what is produced?

• positron travels, hits an electrons, then annihilated

• produces two gamma photons each 511 keV that travel in opposite directions

how does PET detect where decay happened?

a ring of detectors

coincidence counting?

If two detectors opposite each other detect photons at the same time,event happened somewhere beteween them

why isnt PET perfectly accurate

Because of positron travel distance

Before annihilation, positron travels 2-3mm randomly, scan shows where it ended not where it started

how does scatter and resolution affect PET

• limit - 2-3mm’

• if photon changes direction, system draws wrong line