Topic 18: Instrumentation

Question 1

Probability of detection of a photon depends on?

Answer 1

1. Place and direction of incidence of photon on the detector block
2. Energy
3. Scintillator
4. PMT/APD/SiPM efficiency
5. Electronics
6. Incidence rate (-> singles dead-time)

Question 2

Probability of detection of a pair of photons depends in addition on

Answer 2

• Timing circuit
• Downstream processing (-> coincidence dead-time)

Question 3

What factors influence PET image quality?

Answer 3

Accidental coincidences

Detection efficiency

Count Rate

2D vs 3D PET

Time of flight

Question 4

What are examples of determining the detector efficiencies?

Answer 4

The Fan-sum method: suppose you put a symmetric uniform phantom cylinder in pet scanner. measure counts of responses. that go through one detector, then next detector. If they are the same response from all crystals thats good! the sum of coincidence counts seen by 1 detector is proportional to its efficiency.

Question 5

Types of PET coincidences

Answer 5

1. True (unscattered)
2. Scattered coincidence
3. Accidental : two emissions that happen at the same time (four photons in total and you detect only two of them.) And the detector cannot tell whether they were from two annihilations or one annihilations.

Question 6

Randoms from singles (RFS) ?

Answer 6

provides nearly noiseless estimate of the mean background.

(an estimate for the randoms)

Question 7

SPECT resolution is mostly determined by what?

Answer 7

Resolution mostly determined by the collimator.

Question 8

how do you calculate the distance from emission origin to centre of LOR?

Answer 8

d = (t2-t1)c/2

Question 9

What does a higher TOF timing resolution mean?

Answer 9

Reduced uncertainty in localisation

Reduced noise

Question 10

TOF benefits?

Answer 10

TOF converges faster and achieves better contrast for a given noise

Question 11

Scintillator needs to be what?

Answer 11

Dense (efficient) and have high light output (energy resolution)

Question 12

PET detectors

Answer 12

• Scintillator - > needs to be dense and high light output
• Optical photon detector - > converts to electrical signal, SiPM, PMT, APD
• Other types of detectors in research

Question 13

Explain the features of an avalanche photodiode ?

Answer 13

Solid state readout (replaces PMT)

• Converts light to electrical signal
• Suitable to amplify low light levels
• Compact and robust
• Compatible with magnetic field
• Too slow for time-of-flight (PET)

Question 14

Explain the features of a silicon photo-multiplier.

Answer 14

SiPM = array of APDs operated in Geiger mode

• Large gain similar to PM tube
• Insensitive to magnetic field
• Can be used for photon counting
• Faster than APD and PMT so suitable for gathering time-of-flight information
• Relatively low quantum efficiency
• May be suitable for SPECT

Question 15

What is carried out in quality assurance

Answer 15

check procedure

check acquisition

check instrument performance

Question 16

What is adaptive acquisition?

Answer 16

Image reconstruction of this data could proceed as follows:

1. Use the first scan to obtain a low resolution image
2. Use this as an initial image for reconstruction from the second scan data, where only the region of the myocardium is reconstructed (while the rest of the image is kept fixed).

Question 17

What are the benefits of adaptive acquisition?

Answer 17

Improved sensitivity

• Use of wide angle collimators with high geometric efficiency
• Unique ability to concentration acquisition on region-of-interest

Improved spatial resolution

Improved energy resolution

Question 18

Things included on a Spect QC?

Answer 18

Visible damage, or noise

High count uniformity

Centre of rotation check

Compare sensitivity of dual heads

Question 19

What artifact can be seen if there is a uniformity error?

Answer 19

A ring artifact

Question 20

RP principles?