PET Scanner Instrumentation Study Notes

PET Scanner Instrumentation

Objectives

• Diagram a PET Scanner

• Discuss the major components of a PET scanner and their function

• Describe how PET scanners acquire and store data

• Explain the fundamental operation of dedicated PET scanners and their design

• Describe transmission imaging and its need and use in attenuation-correcting PET imaging

Essential PET

• Detection of annihilation photons from a positron emitting radionuclide

  • When a radionuclide atom decays, it emits a positron.

  • The positron travels a short distance, losing kinetic energy through ionization and excitation interactions.

  • After losing energy, it annihilates with an electron, producing two annihilation photons emitted at a 180° angle, each with 511 keV of energy.

  • If both photons are detected by two detectors arranged at 180° within a short coincidence timing window (CTW), the event is considered a “good” event, generating a count in the line connecting the detectors.

  • In reality, the positron travels only a few millimeters or less before annihilating.

Physics of Positron Emitters and Annihilation Photons

• Positron-emitting radionuclides include:

  • C-11

  • O-15

  • N-13

  • F-18 (all are biological materials)

  • Cyclotron production yields short half-lives, limiting availability.

  • Cu-64 is also produced in cyclotrons.

  • Rb-82 is generator-produced and used for myocardial perfusion imaging.

  • Ga-68 is also generator-produced.

• Annihilation photons

  • Emit at 511 keV and travel in opposite directions (180° apart).

  • The location of annihilation may not precisely correspond to the location of positron emission due to scattering effects.

• Coincidence imaging

  • Detects the two photons using two detectors within a short CTW.

  • Eliminates the need for collimators through electronic collimation.

Coincidence Imaging Process

• Light is generated (scintillation photon) during the interaction between gamma photons and a crystal.

• This light is subsequently converted into an electronic pulse by a photo-multiplier tube (PMT) or silicon photomultiplier (SiPM).

  • SiPM represents a digital approach in PET imaging.

• The electronic pulse then reaches the electronic circuits for processing, employing a coincidence timing window to discern between true, random, and scattered events.

  • True counts are logged in a sinogram based on the line of response (LOR) of the event detected.

Scintillation Crystals for Annihilation Photon Detection

• Types of scintillation crystals:

  • NaI(Tl)

  • Low detection efficiency for 511-keV photons

  • Now largely replaced

  • BGO (bismuth germanate)

  • Highest detection efficiency but long decay time and low scintillation light yield.

  • Newer scintillators:

  • LSO (lutetium oxyorthosilicate), GSO (gadolinium oxysilicate), LYSO (lutetium yttrium orthosilicate)

  • Offer good detection efficiency, high light yield, and short decay times

### Summary of Scintillation Properties

• The decay time affects scanner deadtime, random coincidence rates, and TOF PET imaging capability.

Time-of-Flight PET (TOF-PET)

• Utilizes scintillators with fast decay times to localize the annihilation reaction along the LOR from the two detecting crystals.

• Timing difference of photon arrival allows precise determination along the LOR.

Basic System Design

• Utilizes multiple small detectors (0.5 x 0.5 x 30 mm), arranged in rings within a gantry.

• Each event is detected by two opposite detectors, defining its location along the LOR.

• Data storage occurs in a sinogram, where each pixel represents an individual LOR.

Types of Events in PET

• True Event: Two annihilation photons from a single annihilation detected within the CTW.

• Single Event: Only one photon detected.

• Random Event: Singles detected in two detectors from separate annihilations.

• Scatter Event: True coincidence where one photon is scattered prior to detection.

• Impact of Activity Concentration on Counts: Increased concentration alters count rates of trues, randoms, and singles.

Correction for Attenuation

• Attenuation is critical in PET due to the dependency on both annihilation photons being detected.

• Correction: The location of the radiopharmaceutical does not affect attenuation, enabling exact measurements through external photon sources or CT.

• Correction Diagram:

  • Visualizes relation between event LOR and external rod source photons before correction.

Time-of-Flight PET Equations

• The TOF-PET technique standardizes the reconstruction focus using the LOR as a potential location for the annihilation event:
  
  egin{align} ext{Let } riangle x & = ext{location from the center of LOR,} \ riangle t & = ext{difference in arrival times,} \ c & = ext{speed of light.} \ riangle x = c imes riangle t ext{This relates the annihilation event's depth to the LOR.} \end{align}
  

Quantitative Abilities

• PET’s qualitative results measure physiologic quantities in vivo, crucial for clinical applications like drug development.

• Successful quantitative measurements necessitate the adjustment for attenuation, scattering, and the calibration factor correlating count density to activity concentration.

Overview of PET Tomograph Composition

• Scintillation crystals: Generally rectangular prisms sized 4-6 mm square x 20-30 mm long, oriented radially.

• Rings of detectors formed in 18-32 configurations housed in a gantry with a bore dimension of 50-60 cm.

• Field of View (FOV):

  • Axial FOV (gantry bore) is 15-18 cm;

  • Transaxial FOV determined by coincident detectors.

Event Detection

• A combination of detectors forms blocks to capture annihilation photons detected in coincidence.

• Coincidences in a CTW of 5-12 nsec long need to be registered between detector blocks.

Overview of Acquisition and Image Display

• Multiple acquisitions performed at each bed position to cover the entire area of interest; images can be reconstructed from all recorded data.

Common acquisition approaches include:

• Histogram mode (similar to single-photon imaging frame mode)

• List mode acquisition

• Prompt sinogram: Captures events detected in the CTW, providing data for trues, randoms, and scatters.

• Delay sinogram: Offers insight on random coincidences providing comparative data analysis.

• Transmission sinogram: Assists in better correction processes (in non-CT PET tomographs).

Performance Measures in PET

• Spatial resolution dependent on several factors:

  • Radionuclide choice;

  • Scintillator size;

  • Detector location in the gantry;

• Sensitivity is affected by scintillator type, detector geometry, acquisition modes, correction accuracies, 3D acquisition modes resulting in greater sensitivity.

• Scatter fraction reflects the ratio of scattered coincidences against total counts post-random event subtraction.

• Count rate denoted with NECR (noise-equivalent count rate) accounts for random and scatter events in PET acquisition and adjusts operation at peak NECR levels.

Standardized Uptake Value (SUV)

• SUV provides semi-quantitative measures indicating tissue uptake.

• SUV Formula:
  $<br>SUV = rac{{ ext{MBq in area of interest (ml)}}}{{ ext{injected dose (MBq)}} imes ext{body weight (g)}}<br>$

• SUV value interpretations and variabilities heavily influenced by factors like blood sugar at injection, time lapse to imaging, and concurrent uptake by other tissues (e.g., myocardium).

Summary

• The interplay of diverse factors determines PET imaging appearances, including tomographic design and choice of radiopharmaceuticals used.

• SUVs are emerging as essential markers in research, emphasizing the necessity of standardization across institutions.