Positron Emission Tomography (PET)

What is the general principle of PET? (3 things)

• inject biologically-active molecule into the body, labelled with a positron emitter

• positrons travel a short distance in tissue and annihilate with an electron, giving a pair of photons

• they travel at 180 deg. to each other and are detected.

What are the isotopes like used for PET? (2 things)

• have fewer neutrons

• have short half lives

If the isotopes have short half-lives, is that good or bad?

Need a cyclotron on site. So despite the hazardous nature of a cyclotron, need to deliver ASAP for use in the patient.

Sketch graphs of counts per second as a function of time for PET and SPECT. Label where a scan time may take place, and compare their SNRs based on this.

What is coincidence detection? (2 things)

• photons that reach the detector at the same time are recorded

• photons travel at the speed of light so there is drastic difference when measured/observed against noise on the same timescale.

What happens to the SNR during coincidence detection?

SNR is increased by having two photons recorded at the same time (± 5ns)

How does image reconstruction work?

• if 2 coincident photons are detected, we know the source must lie on the path between 2 detectors.

• same maths as CT

• use similar back projection reconstruction technique like CT scanning for making functional images.

Note:

• for SPECT, use collimators for getting straight lines

What is the spatial resolution affected by in PET?

• the non-linearity of gamma rays (not totally opposite each other)

• gamma rays at 180 deg ± 0.25 deg to each other

State disadvantages of PET

• results in blurring of final images

• positron has to find an electron to annihilate it

• the distance travelled in the tissue by a positron limits resolution

  • 2mm

Describe some applications of PET

• cancer is most common

• FDG is used (analog of glucose)

  • A marker of tissue uptake of glucose (linked to metabolism)

  • get high uptake in areas that require high energy e.g. cancers