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Oct 2007 Adapted from Tom Lewellen, PhD

Robert MiyaokaDivision of Nuclear MedicineUniversity of Washington Medical CenterSeattle, Washington USA

An introduction to PET physics

Oct 2007 Adapted from Tom Lewellen, PhD

PET - the basic steps to make an image

Decoding Detector corrections

PrimaryDetection

Coin.processing

Databinning

Datacorrections

Imagereconstruction

Oct 2007 Adapted from Tom Lewellen, PhD

NN

N P

PP P

P N NN

NN

N P

PP P

P P NN

ν

e-e+

Proton decays toneutron in nucleus -

Positron combines withelectron and annihilates

γ

γ

Two anti-parallel 511 keVphotons produced

Unstable parentnucleus

positron and anti-neutrinoemitted

PET basics - positron annihilation

Oct 2007 Adapted from Tom Lewellen, PhD

Spatial Resolution(positron range)

18F 11C 15Omax eng (MeV) 0.633 0.959 1.734most probable eng (MeV) 0.203 0.326 0.696

combined effects of tomoresolution and positron range

*Palmer and Brownell, IEEE Trans Med Img vol.11(3), 1992

Oct 2007 Adapted from Tom Lewellen, PhD

Spatial Resolution(photon non-collinearity)

Photon non-collinearity:Range of angle is + 0.25degrees, centered at 180degrees

R180ο = 0.0022 X D

~2.2 mm for 1 m diameterPET detector system

Oct 2007 Adapted from Tom Lewellen, PhD

What do we use for detectors?

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Oct 2007 Adapted from Tom Lewellen, PhD

Some details about scintillators

0

0.1

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NaI:Tl BGO LSO GSO:Ce

Scintillator

To

tal lin

. attn

. (cm

-1

)

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20

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100

120

NaI:Tl BGO LSO GSO:Ce

Scintillator

Relative Lig

ht Y

ield

Ideally - want dense,fast, bright, cheap

So far, we can notget all at once

Detection efficiency Energy resolution, etc.

Oct 2007 Adapted from Tom Lewellen, PhD

• 3 cm thick bismuth germanateoxide - BGO

µ = 0.91 cm -1

Interaction probability = 0.93Coincidence probability = 0.87

• 2.5 cm thick Sodium Iodide -NaI(Tl)

µ = 0.34 cm -1

Interaction probability = 0.57Coincidence probability = 0.33

No energy discrimination

BGO versus NaI(Tl) - a simplistic comparison

Dedicated BGORing system

Dual headCoincidenceGamma camera

Oct 2007 Adapted from Tom Lewellen, PhD

Two basic detector design approaches

• Driven by scintillator characteristics

• Continuous detectors - no longer a significant player in dedicated PET

• Pixilated detectors - discrete crystals orsaw cut “slabs”

Oct 2007 Adapted from Tom Lewellen, PhD

Large pixilated detectors - evolutionfrom large NaI(Tl) based PET systems.

1 2 3 4trigger

Current system (Philips) uses 4x6x20 mm GSOcrystals mounted to a light guide and the arrayof PMTs - decoding is done essentially thesame way as was done for the NaI version

Oct 2007 Adapted from Tom Lewellen, PhD

An example of a BGO Detector Block

Reflective material

Discrete BGO crystals4 mm transverse8 mm axial30 mm radial

Plastic light shield

2 dual photocathode PMTs

Alternative to saw cuts - discrete crystals with surface treatments/ coupling compound difference to produceequivalent light sharing.

Oct 2007 Adapted from Tom Lewellen, PhD

Tran

sver

se Axial

Block Crystal Decoding

A

B

C

D

A,B,C,D = PMTs

Example for a 6x6array of crystals

E = A + B + C + D

Axial = Z = C + D( ) E

Transverse = X = B + D( ) E

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Oct 2007 Adapted from Tom Lewellen, PhD

Example Crystal Map

Oct 2007 Adapted from Tom Lewellen, PhD

Block Energy

Peaks fordifferent crystals

at differentpositions

Window centerand width

adjusted for eachcrystal

Oct 2007 Adapted from Tom Lewellen, PhD

Spatial Resolution(discrete crystal detector system)

intrinsic(~0.5 w)

w

photon penetration(DOI)

scatter withindetector

statisticaluncertainty(positioning)

dense, good photoelectric cross-section, high light output

Oct 2007 Adapted from Tom Lewellen, PhD

A few things to note about positron decay

1. Range of positron ( < 1 mm for F-18)

2. Non-collinearity - the gamma rays are not exactly at 180 degrees, the FWHM of the angular spread is ~ 0.3 degrees

3. Resolution effects, like gamma cameras, generally add in quadrature:

FWHM = FWHMdet

2+ FWHMcolin

2+ FWHMrange

2

Oct 2007 Adapted from Tom Lewellen, PhD

Channel 1

Channel 2

SummedChannel

γ

γ

coincidence events

“Electronic” collimation => high sensitivity

PET basics - coincidence detection

Oct 2007 Adapted from Tom Lewellen, PhD

Data is binned into a sinogram

Data Acquisition

Image ReconstructionActivity

Distribution(single slice) Projection Data “Sinogram”

“distance”, d

θ

Y

Xθ

d

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Oct 2007 Adapted from Tom Lewellen, PhD

Data is binned into a sinogram

neg pos

Distance from center (bin)-125 0 125

0 90 179

Angle

-125 0 125

-125 0 125-125 0 125

-125 0 125-125 0 125

negpos

negpos

negpos

negpos

Oct 2007 Adapted from Tom Lewellen, PhD

True

Scatter

Randoms

Types of coincidence events

Oct 2007 Adapted from Tom Lewellen, PhD

Coincidence detection

# events

time between detector response

Randoms and count rate

Prompt window

Delayed window

Oct 2007 Adapted from Tom Lewellen, PhD

Randoms and count rate - II

• We can also measure the detector singles rates

– R = 2tS1S2 Where t = resolving time, S is the detector singles rates

– Need to some other corrections - particularly accounting for energywindowing and proper dead time corrections.

– Pro= less noise in the correction.

– Con = is getting the corrections right and accounting for any rapidchanges in counting rate during acquisition (works best in list mode ifthe activity is rapidly changing during the scan)

Oct 2007 Adapted from Tom Lewellen, PhD

Scintillator speed and randoms

Slow Fast

Randoms ~ constant versus time

In this example, #trues = constant

“fast” has ~ 1/2 the randomsOct 2007 Adapted from Tom Lewellen, PhD

Count Rate 3D (LLD = 375keV)

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600

0 0.5 1 1.5

Activity Conc. (uCi/cc)

Count

rate

(k

cps)

NECR (2R)

T

R

S

Randoms go as activity squared!

For full ring systems using BGO, LSO, or GSOrandoms limit the maximum usable count rate.

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Oct 2007 Adapted from Tom Lewellen, PhD

So, we can correct for randomsand we have a handle on countrate limitations.

Another major effect is that ofphoton attenuation in the patient

How do we correct for that?

Oct 2007 Adapted from Tom Lewellen, PhD

Direct coincidence measurement with rotatingsource

Singles measurements

Combined PET/CT scanner

Options in collecting and processingtransmission scan data

Oct 2007 Adapted from Tom Lewellen, PhD

Transmission rod sourcesrotate around the patient

Acquisition is a high energy CT scan

Data allows accurate attenuation correction

Transmission scanning in PET

Oct 2007 Adapted from Tom Lewellen, PhD

Effects of Attenuation in PET

P1= e

! µ x( )dx0

"x

#

P2= e

! µ x( )dx"x

a

#

PC= P

1P2= e

! µ x( )dx0

a

"

Oct 2007 Adapted from Tom Lewellen, PhD

Coincidence + gating rejects mostscatter

Problem - count rate limited bydetector closest to rod source

Count rate limited by thefar detector

Problems - scatter; hardwaremodifications to tomograph

Gated coincidence transmission Singles transmission

Solution - use image segmentation on attenuation data setsOct 2007 Adapted from Tom Lewellen, PhD

T+E - Transmission + emission(transmission after dose is injected)

Essential for clinical scanning with conventionalPET scanner

standard technique (without CT scanner)

Collect emission data

Collect transmission data

Use emission data to subtract emission contamination from transmission data

Either interleave T and E data or takeall E data first and then T data

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Oct 2007 Adapted from Tom Lewellen, PhD

Segmented Attenuation Correction(SAC)

Smooth andreconstruct

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Series1

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Series2

Histogramµ values

Extractclusters

Classifyimage Forward

project

Oct 2007 Adapted from Tom Lewellen, PhD

Images courtesy of CTI and GEMS web sites.

PET/CT

Oct 2007 Adapted from Tom Lewellen, PhD

1. Scout scan(5-20 sec)

CT PET

4. Whole-body PET(6-40 min)

CT PET

PET/CT scan protocol

3. Helical CT (1-2min)

CT PET

2. Selectionof scanregion (1-2min)

Scout scan image

Oct 2007 Adapted from Tom Lewellen, PhD

CT-based Attenuation Correction• The mass-attenuation coefficient (µ/ρ) is remarkably similar for all non-bone

materials since Compton scatter dominates for these materials. Bone has ahigher photoelectric absorption cross-section due to presence of calcium

Oct 2007 Adapted from Tom Lewellen, PhD

CT-based Attenuation CorrectionBi-linear scaling methods apply different scale factors forbone and non-bone materials

air-watermixture

water-bonemixture

air soft tissue dense bone

Oct 2007 Adapted from Tom Lewellen, PhD

Effect of Contrast Agent on CT to PETScaling

The presence of Iodine confounds the scaling process as Iodinecannot be differentiated from bone by CT number alone.

Curve thatshould be usedfor contrastagent

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Oct 2007 Adapted from Tom Lewellen, PhD

What we have so far:

Basics of positron decay

The kinds of coincidence events

An overview of randoms and count rate issues

Several options for attenuation correction

What are the options in basic tomograph designs?

Oct 2007 Adapted from Tom Lewellen, PhD

Some test questions

Oct 2007 Adapted from Tom Lewellen, PhD

D68. Positron cameras detect:

A. Positrons of the same energy in coincidence.B. Positrons and electrons in coincidence.C. Photons of different energies in coincidence.D. Annihilation photons in coincidence.E. Annihilation photons in anticoincidence.

Oct 2007 Adapted from Tom Lewellen, PhD

D69. Spatial resolution of PET systems is determined by:

A. Detector size.B. The ring diameter of the system.C. The detector material.D. Energy of the positron emitter in use.E. All of the above.

Oct 2007 Adapted from Tom Lewellen, PhD

D77. The major limitation on the resolution of an FDG scan on a modernwhole body PET scanner is:

A. Range of the positron.B. Image matrix size.C. The physical size of the individual detectors.D. The non-collinearity between the annihilation photons.E. Attenuation correction.

Why? Camera res ~ 5 mm, positron range depends onisotope but for 18F is < 1 mmNon-collinearity (100 cm diameter) ~3.0 mm.Physical size of detector (~4 mm ) -> ~2.0 mm

Reconstruction filter -> ~10 mm

Oct 2007 Adapted from Tom Lewellen, PhD

D85. All of the following are true statements about PET scanning,except:

A. Radioisotopes are cyclotron produced.B. Positrons are not detected directly.C. Coincident detection at 180° is required.D. Images are generally axial tomograms.E. The detector photopeak is centered at 1.02 MeV.