What are PET basics?

The basic principle of PET 1. Positron-emitting tracer is injected into the body 2. Emitted positrons (+) travel 1 – 3 mm 3. Positrons collide with electrons (-) causing an “annihilation” 4. Annihilation emits energy in the form of two 511keV energy gamma rays at ~180 degrees 5. Gamma rays are detected by opposing detectors 6. Energy discrimination (an “energy window”) is used to ensure that each gamma is ~511 keV 7. Timing discrimination (a “coincidence time window”) is used to ensure that each gamma ray comes from the same annihilation, hence ensuring accurate localization of the tracer 5 1 6 4 2 3 7

Coincidence

Trues One annihilation Detection within coincidence window E energy window One annihilation Detection within coincidence window Energy within energy window trues = const * activity

Randoms Two annihilations Detection within coincidence window Energy within energy window Randoms = const * activity * activity

Correction of randoms Randoms are related to the single rate of each detector Randoms are related to the length of the coincidence window Randoms can be calculated when the singles for each detector are measured, and the coincidence window for each detector pair is known Randoms can be measured and corrected in real time for each LOR, using a delayed coincidence window with exactly the same length as the “direct” coincidence window

Reduction of randoms Relevant parameters: Coincidence window 12 ns random coincidences 12 ns 6 ns 4.5 ns (pico 3D)

Scatter One annihilation Detection within coincidence window Energy loss due to scatter But energy still within energy window Scatter fraction is object dependent!

PET event energy spectra PET events are distributed across a range of energy, not only in the 511 keV range. An energy window is employed to reject scatter. ENERGY WINDOW 425 – 650 ENERGY WINDOW 511 keV PHOTONS LSO SCATTER BGO Counts 350 – 650 100 200 300 400 500 600 700 Energy (keV)

Correction of scatter Scatter is related to mu map Emission Transmission Scatter Corrected Scatter is related to mu map Scatter is patient dependent Scatter needs to be measured for each patient Scatter can be estimated by phantoms (but a cylindrical phantom may be a good approximation for the brain; everywhere else it is a very poor estimation) Scatter can be precisely modeled for each patient using the mu map: Watson method

Correction of attenuation Patient absorbs some of the 511 keV photons Attenuation is patient dependent mu map has to be measured for each patient mu map can be measured with external sources 137Cs for estimated mu map 68Ge for precise definition of mu map X-ray for high statistics and precise mu map

Noise Equivalent Countrate (NEC) Main sources of statistical error in a PET system are randoms and scatter Comparison to a system that is resistant to randoms and scatter NEC describes the effective number of counts measured by the PET scanner as a function of the activity in the FOV

NEC – clinical performance INJECTED DOSE RANGE 185 – 740 MBq 5 – 20 mCi 1 hour uptake Biograph HI-REZ PICO 90 80 Biograph 70 60 50 Noise Equivalent Count Rate [per sec] 2D 40 30 20 10 2 0.1 0.2 0.3 0.4 0.5 4 6 8 10 12 14 16 18 20 Specific Activity kBq/cc [uCi/cc] *Ring difference and energy window unspecified; for Biograph HI-REZ all measurements are clinical Source: Carney, et Al., “Regionally dependent count rate performance analysis of patient data acquired with a PET/CT scanner,” abstract 364, SNM 2003.

Sensitivity Septa employed No septa Low efficiency High efficiency A measure of the number of coincidence events a scanner is able to detect, assuming no dead time. Four to five times improvement with 3D acquisition techniques. 2D acquisition mode 3D acquisition mode Septa employed Low efficiency Higher dose required Lengthy scan times Fewer counts per dose (low count rate) Low scatter No septa High efficiency Lower dose required Short scan times Higher counts per dose (high count rate) High scatter

PET•CT Protocol The typical protocol begins with a CT topogram to identify the scan range. This is followed by a spiral CT exam of the body part of interest.

PET•CT Protocol The patient is then automatically positioned for the start of the PET exam. The PET exam is a series of bed positions during which the radioactive emissions are collected.

attenuation correction PET•CT scan protocol Spiral CT: seconds CT PET Survey CT Recon Fused PET•CT FUSION scatter correction attenuation correction WB PET: 10-20 min PET CT PET Recon

Block detector components 169 crystal elements per detector block 4 photomultiplier tubes (PMTs)/detector block Detector module PMT Channeled scintillation light Detector block

Attenuation artifacts Conventional CT: 50 cm FOV Note: arms not fully imaged, hardening at edges of field of view Emission only PET Note: arms fully imaged Attenuation correction PET Note: artifacts in liver and possible lesion distortion Reduced image quality Reduced accuracy Increased artifacts Potential diagnostic impact

ACPlus™ Attenuation Correction Conventional attenuation scan ~120 sec scan time 106 counts FULL FOV Conventional CT attenuation scan ~10 sec scan time 1012 counts TRUNCATED FOV Siemens ACPlus ~10 sec scan time 1012 counts FULL FOV (NOT TRUNCATED) Extended 70 cm transverse FOV Super fast attenuation scanning Exceptionally high statistics Unmatched attenuation image quality Highest accuracy attenuation correction

Standard PET: filtered backprojection DETECTOR ELECTRONICS GANTRY CROSS SECTION COINCIDENCE TIMING WINDOW (4.5 nsec)

Standard PET: filtered backprojection DETECTOR ELECTRONICS GANTRY CROSS SECTION COINCIDENCE TIMING WINDOW (4.5 nsec)

Time of flight T, TIME DIFFERENCE OF DETECTION DETECTOR ELECTRONICS CONVENTIONAL TOF COINCIDENCE TIMING WINDOW (4.5 nsec) Source: Conti, et al., IEEE 2004

Complex schematic of a PET•CT