Innovation is in our genes. 1 Siemens Medical Solutions Molecular Imaging What are PET basics?

Innovation is in our genes.

1 Siemens Medical Solutions Molecular Imaging

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

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Coincidence



Trues

E energy 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

12*2*1 tsnglssnglsrandoms



Reduction of randoms

Relevant parameters:

Coincidence window

coincidence window

ran

do

m c

oin

cid

ence

s

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

350 – 650

ENERGYWINDOW

425 – 650

Co

un

ts

Energy (keV)

0 100 200 300 400 700

600500

SCATTER

511 keV PHOTONS

BGO

LSO



Correction of scatter

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

Emission Transmission Scatter Corrected



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

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dxdyyxlyx

lp

)sincos(),(

)sincos(),(exp

),(



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

rst

tNEC

)2(

2



NEC – clinical performance

*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.

INJECTED DOSE RANGE185 – 740 MBq5 – 20 mCi1 hour uptake

0.1 0.2 0.3 0.4 0.52 4 6 8 10 12 14 16 18 20

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20

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60

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80

90

Specific Activity kBq/cc [uCi/cc]

No

ise

Eq

uiv

alen

t C

ou

nt

Rat

e [

per

sec

]

2D

Biograph HI-REZ PICO

Biograph



Sensitivity

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.

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

2D acquisition mode 3D acquisition mode



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.



scatter correction attenuation correction

PET Recon

Spiral CT: seconds CT

CT PET

Survey

WB PET: 10-20 min

PET

CT PET

CT Recon

Fused PET•CT

FUSION

PET•CT scan protocol



Block detector components

169 crystal elements per detector block

4 photomultiplier tubes (PMTs)/detector block

Detector block

PMT

Detector module

Channeled scintillation light



Attenuation artifacts

Conventional CT: 50 cm FOV

Note: arms not fully imaged, hardeningat 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

Extended 70 cm transverse FOV Super fast attenuation scanning Exceptionally high statistics Unmatched attenuation image quality Highest accuracy attenuation correction

Conventional attenuation scan

~120 sec scan time106 counts

FULL FOV

Conventional CT attenuation scan

~10 sec scan time1012 counts

TRUNCATED FOV

Siemens ACPlus~10 sec scan time1012 counts

FULL FOV (NOT TRUNCATED)



Standard PET: filtered backprojection

COINCIDENCE TIMING WINDOW (4.5 nsec)

DETECTOR ELECTRONICSGANTRY CROSS SECTION



Standard PET: filtered backprojection

COINCIDENCE TIMING WINDOW (4.5 nsec)

GANTRY CROSS SECTIONDETECTOR ELECTRONICS



Time of flight

DETECTOR ELECTRONICS

COINCIDENCE TIMING WINDOW (4.5 nsec) T, TIME DIFFERENCE OF DETECTION

CONVENTIONAL TOF

Source: Conti, et al., IEEE 2004



Complex schematic of a PET•CT

Innovation is in our genes. 1 Siemens Medical Solutions Molecular Imaging What are PET basics?

Documents

energy window randoms

energy window scatter

coincidence window randoms

e energy window

energy window unspecified

coincidence window energy

patient scatter

coincidence time window