Hybrid Imaging PET/CT & PET/MRI

What type of radioactive decay is primarily used in PET imaging?

Positron emission

What happens after a positron encounters an electron in the body?

They annihilate and produce two 511 keV photons

What is the total minimum energy required for positron annihilation to occur?

1.22 MeV

Why does PET imaging generally produce higher resolution images compared to traditional nuclear medicine?

Because of higher energy emissions and less scatter

How do PET tracers differ from standard nuclear medicine tracers?

They emit positrons instead of just gamma rays

What is the energy of each photon emitted after positron annihilation?

511 keV

What happens if the minimum energy threshold of 1.22 MeV is not met?

No annihilation photons are produced

What is one reason PET imaging experiences less scatter in the body?

Higher energy photons that pass through tissue more easily

What is the energy of each photon emitted during positron annihilation?

511 keV

In PET imaging, how far apart do the photons travel after annihilation?

180°

What is the name of the technique PET uses to localize events without collimators?

Annihilation Coincidence Detection

PET detectors do not use:

Absorptive collimators

What causes the two 511 keV photons in PET imaging?

Positron-electron annihilation

Why does PET offer better spatial resolution than traditional nuclear imaging?

Higher photon energy reduces scatter

What type of detectors are used in PET imaging to localize photon paths?

Coincidence ring detectors

PET emission photons are typically detected within what range from the positron’s origin?

A few tenths to a few millimeters

What technique allows PET scanners to localize positron annihilation events without collimators?

Annihilation Coincidence Detection

The detection of annihilation photons in opposing detectors defines:

The volume where the annihilation occurred

ACD detectors commonly have which of the following cross-section shapes?

Square or rectangular

The volume measured by PET detectors using ACD is shaped like:

A voxel or box

Why does the shape of the detector matter in PET imaging?

It defines the spatial resolution and voxel dimensions

What must occur for ACD to detect a valid positron event?

Two 511 keV photons must be detected at the same time in opposing detectors

How is pixel size in a PET image calculated?

DFOV ÷ matrix size

In the example, converting a 30.2 cm DFOV to mm gives:

302 mm

What is the resulting pixel size with a DFOV of 30.2 cm and a 512 matrix?

0.59 mm

Which of the following is needed to calculate voxel size?

DFOV and detector thickness

Why is the matrix shape important when calculating pixel size?

It defines the shape of each pixel (square vs. rectangle)

Which of the following would give the smallest pixel size?

Small DFOV, large matrix

What is the main purpose of calculating pixel size in imaging?

To assess spatial resolution

A voxel is formed by:

A 2D pixel plus detector thickness

What type of photon interaction is PET coincidence logic based on?

Annihilation of a positron and electron

In coincidence logic, what is the typical initial timestamp window placed on photon events?

1–2 nanoseconds

What is the purpose of the 6–12 nanosecond coincidence timing window?

To account for differences in travel distance

If two photons hit opposing detectors 20 nanoseconds apart, what happens?

It’s marked as a random event

Which of the following best describes coincidence logic?

Matching photon pairs based on time and angle

What problem would occur if the timing window was too wide?

Too many random events are accepted

What angle do true coincidence photons typically travel apart?

180°

Why do detectors need to apply digital timestamps to events?

To align and match photon pairs

What does timing resolution in PET imaging help determine?

The exact timing of photon detection

What is the typical range of timing resolution in PET scanners?

0.5–5 nanoseconds

Which components affect the timing resolution of a PET system?

Scintillator material and photodetector used

What happens if the timing resolution is too wide?

More random events may be included

What angular variation from 180° does the timing window typically allow for valid coincidences?

±5%

Why is timing resolution critical in PET systems?

It distinguishes valid events from unrelated emissions

What is the term for PET’s ability to localize photon events without absorptive collimation?

Electronic collimation

What technology enables PET to avoid the use of traditional collimators?

Annihilation Coincidence Detection (ACD)

Compared to SPECT, PET has:

Higher sensitivity

What benefit do multiple opposing detector arrays offer in PET imaging?

Simultaneous projection angle acquisition

Sensitivity in PET imaging is defined as:

Number of true events detected per unit of radiotracer activity

Why is PET collimation more efficient than SPECT collimation?

It uses detector timing instead of physical barriers

What best describes the detector arrangement in a modern PET scanner?

Stationary full-ring detector array

What is one advantage of a 360° PET detector ring?

Speeds up scan by acquiring all angles simultaneously

Which of the following is a true coincidence event in PET?

Two photons emitted 180° apart from the same annihilation

What does a “random” coincidence refer to in PET imaging?

Two unrelated photons being detected simultaneously

Why does the PET system benefit from a stationary ring over rotating cameras?

It minimizes artifacts from patient motion

Which of the following events adds noise rather than signal to PET data?

Random coincidence

What does the red dot represent in Image A?

Annihilation event origin

In conventional PET (Image B), what is the limitation of event localization?

Equal probability along the LOR

What imaging improvement does TOF PET (Image C) offer over conventional PET?

Improved localization of annihilation events

What statistical model does TOF PET use to represent the event's location?

Gaussian distribution

In TOF PET, what does a smaller FWHM indicate?

Better timing resolution

What does Δd represent in TOF PET imaging?

Distance offset from center based on timing