PET CT

Question 1

What does PET stand for?

Answer 1

Positron Emission Tomography

Question 2

What is PET CT?

Answer 2

Tomographic imaging technique also based on injecting a radiotracer which emits positrons, which have a short half-life.

Question 3

Why is CT combined with PET?

Answer 3

to improve spatial resolution and fuse morphological and functional information

Question 4

What is a disadvantage of the short half life of the radiotracer used in PET?

Answer 4

Cyclotron needs to be on site, this is expensive to run

Question 5

What is a positron?

Answer 5

an antielectron

Question 6

What does the distance traveled by the positron depend on?

Answer 6

The distance the positron travels in tissue depends on its initial energy and the number of
interactions with the atoms in the tissue

Question 7

What are the principles of PET?

Answer 7

Emitted positrons annihilate with electrons in the body resulting in the formation of two gamma rays, with an energy of 511 keV each travelling in opposite directions.

The two gamma rays emitted from the patient have trajectories that are 180 degrees apart & strike solid-state detectors which are positioned in a series of complete rings around the patient.

Question 8

What do the detectors commonly consist of?

Answer 8

1000’s of small crystals of bismuth germanate (BGO) and are coupled to PMT’s

Question 9

Other scintillation crystals used for PET more recently include..

Answer 9

lutetium oxyorthosilicate (LSO) or gadolinium orthosilicate (GSO)

Question 10

Why isn’t NaI used as the scintillator like in SPECT?

Answer 10

NaI scintillator (SPECT) is not used as its detection efficiency for the high energy gamma rays is too low

Question 11

What happens when the gamma rays hit the detector?

Answer 11

When gamma rays hit the crystals, photons are produced, which are then converted into an amplified electrical signal, by PMTs

Question 12

What are the most commonly used radionuclides in PET?

Answer 12

Fluorine-18 (F18)

It is combined with deoxyglucose as fluorodeoxyglucose (FDG). This is a tracer for glucose
metabolism.

Question 13

Why is FDG used?

Answer 13

FDG is taken up by metabolically active cells.
It is not able to take part in glycolytic pathway because it cannot be phosphorylated
Accumulates in the metabolically active cells. The more active a cell is, themore will accumulate.

Question 14

Does PET use a collimator?

Answer 14

PET does not require a collimator and, therefore, eliminates the weakest link in the SPECT image
formation process.

Question 15

What is coincidence detection used for?

Answer 15

Coincidence detection is used to distinguish photons arising from positron
annihilation, based on temporal discrimination.

Question 16

How is PET more advantageous than SPECT?

Answer 16

in terms of spatial resolution, statistical
quality and quantitative accuracy

Question 17

What are the ideal qualities of the scintillator?

Answer 17

High linear attenuation coefficient for the 511 keV photons

High ratio of Photoelectric to Compton interactions

Large number of light photons produced per gamma-ray interaction

Short scintillation light decay time

Question 18

Why are photomultiplier tubes (PMT) used?

Answer 18

PMTs have been the preferred photodetector because their high gain
results in an electrical output with a good signal to noise ratio.

Question 19

What is coincidence counting, & why is it important?

Answer 19

If two opposite detector elements detect a gamma ray photon simultaneously it is considered a coincidence count.

As well as this, A line of response (LOR) joining the two detectors is assumed to intersect the unknown location of the
annihilation event

Question 20

What is Time of Flight?

Answer 20

It has long been appreciated that the difference in the detection times of the two annihilation photons provides a mechanism for precisely localizing the site of individual positron-electron annihilations.

Recent addition which has improved image quality & spatial resolution

Question 21

the benefit of time of flight in PET imaging:

Answer 21

in large patients where high attenuation and scatter mean that image quality is usually the poorest.

Question 22

Why is scatter correction needed in PET?

Answer 22

because of the limited energy resolution of PET systems

Question 23

What can be used to correct scatter in PET?

Answer 23

Pulse height analyser, as only photons within the required energy window are counted as true events. Generally, the scattered photons will have lower energy than the photons traveling directly to
the detector

Question 24

What is the primary role of PET CT?

Answer 24

Primary tumour assessment and staging, Radiotherapy planning, treatment response assessment

Question 25

What is the main scintillator crystal used in PET detectors?

Answer 25

Bismuth Germanate

Question 26

Are Photomultiplier tubes used in PET CT?

Answer 26

Yes

Question 27

What is the function of a photomultiplier tube in PET CT?

Answer 27

as part of the detection system

When the gamma rays interact with the scintillation crystals, the crystals emit flashes of light (scintillations).
The PMTs convert these scintillations into electrical signals. They amplify the light signal and measure its intensity, which correlates with the energy of the detected gamma ray.

Question 28

What does the Line of Response (LOR) represent?

Answer 28

represents the path between two detectors that have simultaneously detected a pair of gamma rays produced by positron annihilation.