PET / PET CT

Question 1

Cyclotron Production

Answer 1

High energy particles (protons, deuterons) are fired at a target e.g. energy of 18 MeV

Often done as negative ions (H-)

Product is passed on for chemical processing

Essential for C11, F18 and other low atomic weight isotope

Question 2

How are FDG tracers used?

Answer 2

Fluoro Deoxy Glucose “a fluorine instead of oxygen (version of) glucose

Cancer cells use lots of glucose - most of the time, even when they have plenty of oxygen - so take up more FDG

Question 3

How is a PET image formed?

Answer 3

Anti-matter (positron) meets matter (electron) they annihilate converting into energy (2 gamma rays)

511 keV photons detected as simultaneous arrival of the 2 * gamma

Co-incidence events recorded registered via Lines of Response, LOR

Question 4

How is PET detection filtered?

Answer 4

Use energy discrimination and ToF to discern trues, random or scattered counts.

Question 5

PET corrections required

Answer 5

a) System correction map
b) Missing photon corr - pulse pile-up, dead times
c) Attenuation corr - x-ray CT / LoR / Im segmentation

Question 6

PET reconstruction methods

Answer 6

1) FBP
2) Iterative reconstruction
3) Using mutual information from anatomical IMs (CT, MR)

Question 7

PET applications

Answer 7

Cancer (18F-FDG) diagnosis
Brain (18F-FDG) dementia/epilepsy
Heart (blood flow, ammonia 13N)
metabolism (18F FDG)

Question 8

How does FDG work?

Answer 8

Taken up preferentially in cancer cells

Converted to FD glucose-6-phosphate and gets trapped in the cells.

Question 9

What is PSMA?

Answer 9

Prostate-specific membrane antigen (PSMA) is expressed in nearly all prostate cancers with increased expression in poorly differentiated, metastatic, and hormone-refractory carcinomas

Can image prostate cancer via 68Ga labelled PSMA inhibitors

Question 10

How does Rb Cardiac perfusion imaging?

Answer 10

Rubidiumisrapidlyextractedfromtheblood andistakenupbythemyocardium inrelationtomyocardialperfusion.

Veryshortphysicalhalf­lifemakes82Rb suitableforrepeatedandsequential perfusionstudies (10min interval)

82Rbshortt1/2 givesrelativelylow radiationexposureforpatients.

sharper images than pet-ct

Question 11

Describe the two stages of Dementia imaging

Answer 11

Two stages:

1) dementia syndrome
2) subtype diagnosis

Question 12

What are the some additional requirements for PET-CT planning?

Answer 12

Positioning lasers
Special (flat pallet) bed
Positioning accessories
Radiation shielding?
[CT contrast injector]
Image transfer to planning computers

Question 13

What is time of flight?

Answer 13

Measure time lag between detection of two gammas Helps to determine position along line of response

Question 14

What is the PET PSF?

Answer 14

Variation in resolution over the detector. Sensitivity is highest at the centre.

LOR can be misplaced if it intercept the detector at an angle. PSF is used to create a correction factor

Question 15

How is depth of interaction encoded?

Answer 15

Can either use a dual layer system with two different scintillation materials or a pin photodiode system with individual LSO crystal elements undernearth.

Can determine response depth based on attenuation chracteristics.

Question 16

What is PET-MR how does it work?

Answer 16

Combines PET and MRI imaging in a single device
Produced combo images using registration
PMTs can be affected by mag field, SiPM works better

Question 17

What are the advantages of PET-MR

Answer 17

● Simultaneous imaging information 
● High spatial resolution of MRI 
● High sensitivity of PET 
● PET and MR in one visit 
● Multi-parametric imaging 
● Lower radiation dose 
● MR for PET motion correction

Question 18

What are the disadvantages of PET-MR?

Answer 18

● Compromise on PET performance? 
● Compromise on MR performance? 
● Attenuation correction 
● Limited choice of MR coils 
● Safety arrangements (radiation monitors?) 
● Cost

Question 19

How is attenuation corrected for in PET (3 types) ?

Answer 19

1) Standard PET - transmission scan - gamma source rot to produce corr map (crude im used with segmentation)
2) With CT - X-ray CT: low dose x-ray corr map
3) With MRI - MR used to simulate CT image
Needs segmentation (intobone,soft­tissue,lungs,air) im intensity doesnt reflect attenuation

Question 20

Examples of PET - MR corrections

Answer 20

1. Dixon based - Fat/water mri
2. Template based approach mri-ct pairs
3. Bone imaging with MRI - ultrashot ech time mRI