PET Quantification

Question 1

Why is quantification theoretically possible in PET and not in SPECT?

Answer 1

The attenuation along a known line of response is independent of the unknown position of the emission along that line.

Question 2

What is the purpose of quantification of PET?

Answer 2

• Lesion characterisation
• Response assessment
• Data reduction in trials & statistical analysis
• Dose optimisation/response assessment
• Testing drug targeting
• Radiotherapy target identification – functional target volumes with
  reduced inter-observer variability

Question 3

What are the types of corrections and calibrations required to calculate the activity concentration?

Answer 3

• Randoms Correction
• Normalisation
• Dead Time Correction
• Scatter Correction
• Attenuation Correction
• PET Scanner Calibration

Question 4

What are ‘Randoms’?

Answer 4

Randoms arise when two uncorrelated single detection events happen with sufficient temporal proximity to fall within the trues detection window. They can be estimated from singles count rates.

Question 5

What is the singles count rate equation?

Answer 5

C_ij = 2τr_ir_j

Question 6

What are the different types of randoms corrections? (3)

Answer 6

1. Estimation from singles rates
2. Delayed coincidence channel
3. Tailfitting

Question 7

What is the purpose of normalisation?

Answer 7

To compensate for variable sensitivities.

An individual correction factor is required for each line of response in the scanner.

Question 8

What are the causes of variable sensitivity?

Answer 8

• Axial data summing and “mashing”
• Detector Efficiency
• Geometric & Solid angle effects
• Time window alignment
• Structural alignment
• Septa

Question 9

What are the reasosn for detector efficiency variation?

Answer 9

• Position within the block; this is systematic and predictable and is called the
  “block profile”.
• Physical differences in the crystal and light guide
• PM tube gains varying

The latter two should be familiar from sensitivity map corrections of gamma cameras.

Question 10

What are the geometric and solid angle effects?

Answer 10

These effects occur as lines of response are narrower closer to the edge of the field of view. The position and angle of incidence of the incoming photon affects the depth of mateiral along its path and hence the probability of interaction causing varying detection efficiencies. Reducing acceptance angles which causes a reduction in sensitivity.

Question 11

How do you mitigate geometric and solid angle effects?

Answer 11

The effects are calculated analytically and incorporated into the recontruction algorithms, termed the ‘arc correction’.

Question 12

What is the purpose of time window alignment?

Answer 12

Important for coincidence timing to be accurately synchronised.
Asychronicity offsets the coincidence window which reduces sensitivity to true coincidences, but not the
number or random events detected

Question 14

What is the effect of minor structural misalignment of the module sin the ring system?

Answer 14

They affect the sensitivity of the lines of response.

Question 15

Describe the method of direct normalisation?

Answer 15

• Illuminating all possible lines of response using a positron source
• An analytical correction is made fro non-unifrom radial illumination
• Normalisation coefficients combining all the effects are taken to be in inversely proportional to the counts acquired in each LOR

Positron source typically used is 68Ge

Question 16

What are the problem with drect normalisation? (3)

Answer 16

1. Long aquisitions
2. Dependant on excellent source uniformity
3. Scatter isnt comparable to patient scatter which can cause artefacts and bias