Vasco PET Review

8/9/2019 Vasco PET Review

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Physics & Instrumentation inPositron Emission TomographyPhysics & Instrumentation in

Positron Emission Tomography

P matrix = 0.6mm, = 0 .1


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QuickTime and adecompressor

are needed to see this picture.


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Positron Emission TomographyPositron Emission Tomography

Recent mainstream acceptance

- relatively expensive

- cyclotron for tracerproduction

- detectors must stop high-energy gamma-rays

- low resolution (>2 mm),limited counting statistics

- BUT unique functionalcapabilities

Applications

- Diagnosis of disease

- cancer (WB), cardiac,

- Research

- brain function

- animal studies


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Technical Challenges in PET ImagingTechnical Challenges in PET Imaging

s Radiochemistry - better tracerss Imaging physics - better images by

Detector design

Spatial resolution Sensitivity

Image processing Corrections for physical effects

Image reconstruction algorithmss Data analysis & biological modeling -

better interpretation of images


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PET Imaging OverviewPET Imaging Overview

- Synthesize radiotracer

- Inject radiotracer

- Measure gamma-rayemissions from isotope

(~20-60 min)- Reconstruct images of

radiotracer distribution(nCi/cc)


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+

++

NucleusNucleus

NeutronsNeutrons

ProtonsProtons

ElectronElectrons

Positron (Positron ( ++) Decay) Decay

18 F-FDG


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Positron Emission Tomography (PET)Positron Emission Tomography (PET) operates by usingoperates by using

at least two opposite to each other positioned rotateable detector.at least two opposite to each other positioned rotateable detector.

PET is based on the principle of detecting annihilationPET is based on the principle of detecting annihilation

radiation with coincidence techniques.radiation with coincidence techniques.

The injected radionuclide must be a positron (The injected radionuclide must be a positron ( ++) emitter. The) emitter. The

positron annihilates after about 1mm path length (depending on density ofpositron annihilates after about 1mm path length (depending on density oftissue material and on the energy of the positron) and emits two 511 keVtissue material and on the energy of the positron) and emits two 511 keV

photons in opposite directions.photons in opposite directions.


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Positron annihilationPositron annihilation

s Annihilation gives 2x 511 keV gamma rays

180 degrees apart

Line of response

s Positron range & gammanoncollinearity

s Scanner is just a photon

counter! Counts gamma-ray pairsvs. single gammas

Time window ~ 1 ns

511 keV

511 keV

e+

e-


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Scintillation

Crystal PMT

Pre-Amplifier

+ Electronics

Gamma photon

converts to optical

photons

(proportional togamma energy, typ.

1000s)

photons are

collected at the end

of the crystal

light is converted to an electrical signal

& amplified

Front-end electronics

condition the signal for

further processing

Prototypical PET DetectorPrototypical PET Detector

Gamma Ray

Optical reflector


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Detection of both photons in coincidence defines aDetection of both photons in coincidence defines a

line along which the annihilation event has taken place.line along which the annihilation event has taken place.

The position of the radionuclide is withinThe position of the radionuclide is within 1mm distance.1mm distance.

This distance as well as a slight deviation from the 180This distance as well as a slight deviation from the 18000emission of the two photons limits the spatial resolution toemission of the two photons limits the spatial resolution to

about 1mm 2mm.about 1mm 2mm.


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The use of annihilation radiation coincidence techniqueThe use of annihilation radiation coincidence technique

in PET improves the quality of image formation considerablyin PET improves the quality of image formation considerably

compared to collimator techniques used in SPECT.compared to collimator techniques used in SPECT.

In SPECT the intensity and the resolution of theIn SPECT the intensity and the resolution of the signalsignal

degrades with increasing depth, due to attenuation through bodydegrades with increasing depth, due to attenuation through body

tissue of increasing thicknesstissue of increasing thickness dd,,

and due to the degradation of collimator resolutionand due to the degradation of collimator resolution cc

withwith

increasing source collimator distanceincreasing source collimator distance zz::

with constant hole diameterwith constant hole diameterddand hole lengthand hole length LL for the collimatorfor the collimator

system.system.


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In annihilation radiation coincidence measurementsIn annihilation radiation coincidence measurements

the resolution is constant with depth due tothe resolution is constant with depth due to

uniformity of the geometric responseuniformity of the geometric response

(defined by the straight line between the two photons).(defined by the straight line between the two photons).


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The intensity of the coincidence signal is defined by theThe intensity of the coincidence signal is defined by the

attenuation in body material from the point of annihilation at depthattenuation in body material from the point of annihilation at depth ddininboth directions,both directions,

with T being the thickness of the body along the line andwith T being the thickness of the body along the line and (x)dx(x)dx ..Therefore the intensity for the annihilation signal along theTherefore the intensity for the annihilation signal along the

line isline is independent of the depth.independent of the depth.


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The absolute count rate for coincidence events is determinedThe absolute count rate for coincidence events is determined

by the count rate for true coincidencesby the count rate for true coincidences IItruetrue

and for randomand for random

coincidencescoincidences IIrandomrandom

which occur when accidentally each detectorwhich occur when accidentally each detector

records an uncorrelated signal within a time windowrecords an uncorrelated signal within a time window

The count rate for true coincidences fromThe count rate for true coincidences from II00

annihilation events is determined by the efficiency e andannihilation events is determined by the efficiency e and

solid anglesolid angle of each detector:of each detector:

For present PETs the total efficiency for coincidence measurement,For present PETs the total efficiency for coincidence measurement,


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The random coincidence count rateThe random coincidence count rate

in the two detectors and by the coincidence time windowin the two detectors and by the coincidence time window : :

Ratio of true to random coincidences:Ratio of true to random coincidences:

which is independent of efficiencies and solid angle and only dependswhich is independent of efficiencies and solid angle and only dependson the intensity of the emitted annihilation radiation, and theon the intensity of the emitted annihilation radiation, and the

coincidence windowcoincidence window (~1(~1 s).s).The intensity of the radiation source inside the body must be at leastThe intensity of the radiation source inside the body must be at least II

00 101066

events/s to obtain a true to random ratio of unity.events/s to obtain a true to random ratio of unity.

This would require a sourceThis would require a source

strength of at least 1 MBq inside tstrength of at least 1 MBq inside t

he body. This gives random rate ofhe body. This gives random rate of


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Raw Data & Image ReconstructionRaw Data & Image Reconstruction

0 projection

0

180

90

90

pro

jec

tion

image

reconstruction

sinogram


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Important Detector PropertiesImportant Detector Properties

-

Spatial resolution- Directly controls spatial

resolution inreconstructed image

-Currently ~ 1 - 5 mm

- Depth-of-interaction?

- Reduces parallax


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Important Detector PropertiesImportant Detector Properties

-

Detection efficiency (aka sensitivity,stopping power)

- Reduces bkg from counting statistics

- Currently > ~ 30% (singles)

55M Events1M Events


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Important Detector IssuesImportant Detector IssuesImportant Detector IssuesImportant Detector Issues

Scatter and attenuation

511 keV

- Energy resolution

- Scattered gammaschange direction andlose energy

- Affects acceptance ofscattered coincidences

- Currently ~ 20%

- Deadtime

- need to handle MHzcount rates!

511 keV 400 keV


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Important Detector PropertiesImportant Detector PropertiesImportant Detector PropertiesImportant Detector Properties

Random (accidental)

coincidence

- Time resolution- Affects acceptance of

random coincidences

- Currently ~ 1 - 10 ns

- Time-of-flight (TOF)?

- c = ~ 1 ft/ns

- Need


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Sc intil lator Na I(Tl) BG O GSO LSO LuAP LP S L aBr

( ) 230 300 60 40 18 30 35 ( 1) 0. 35 0. 95 0. 70 0. 86 0. 95 0. 70 0. 47

/(%) 6. 6 10 .2 8. 5 10 .0 15 10 2. 9

. (% ) 100 15 25 70 30 73 150175

25

New Developments: DetectorsNew Developments: Detectors

Scintillators No perfect choice - tradeoffs

Also practical qualities Rugged? Hygroscopic? Cost?


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Photosensors Photomultiplier tubes Avalanche photodiodes

Arrays, position-sensitive Compact but noisier

Silicon photomultipliers

Very new Best of both?

APD array

PMT

SiPM


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Solid-statedetectors Direct conversion,

no photodetector Great dE/E &

spatial resolution Poorer timing &

stopping power CZT Z2

Z1

Sa2

Sa1

Sc


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Time of flight using LaBr3

noT

OF

300

psT

OF

1 Mcts 5 Mcts 10 Mcts


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New DevelopmentsNew Developments

Multimodality imaging PET/CT

PET/MRI

Specialized applications Brain, breast, prostate

Small animal - microPET

Arterial input function Humans - wrist scanner

Animals - microprobe

Awake rat brain - RatCAP

A hit t


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Architecture

s

Detector blocks x12 LSO 2.2 x 2.2 x 5 mm in 4 x 8 array 1:1 coupling to APD

ASIC - single all digital output

s Timestamp & Signal Processing Module Programmable real-time logic (FPGA)

1 ns bins (debugging, now 10 ns)

s

Data acquisition PCI card in standard PC

Up to 70 MB/s = ~10 Mcps singles

Offline software for coincidences, corrections, recon,

TSPMTDC

PCIcard

ASIC

optical

differential

RatCAP

A hit t


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Architecture

RatCAP

TSPM

LSOAPD

ASICs

allinterconnectio

ns

38 mm

FOV

72 mmOD optical links

to PCI

high voltage

194 g

data, clock,

power

1

8mm

axia

l

FOV

P f


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Performance

s Spatial resolution (FWHM @ CFOV)

FBP: 2.1 mm

MLEM:


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F 18 Fl id B S


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F-18 Fluoride Bone Scan

s

1.3 mCi fluoride

RatCAP microPET R4

C 11 Raclopride


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C-11 Raclopride

s

1.8 mCi raclopride

In the RatCAP

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