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Physics & Instrumentation inPositron Emission TomographyPhysics & Instrumentation in

Positron Emission Tomography

Paul Vaska, Ph.D.

Center for Translational NeuroscienceBrookhaven National Laboratory

July 21, 2006

P matrix = 0.6mm, = 0.1

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Non-invasive Medical Imaging Techniques

Non-invasive Medical Imaging Techniques

Anatomical• X-ray• CAT• MRI• Ultrasound

Functional• “nuclear medicine” - SPECT, PET• Optical fluorescence, …

CAT

X-Ray

MRI

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Positron Emission TomographyPositron Emission Tomography

Recent mainstream acceptance- relatively expensive

- cyclotron for tracer production

- detectors must stop high-energy gamma-rays

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

- BUT unique functional capabilities

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

Radiochemistry - better tracers Imaging Physics - better images by

• Detector design– Spatial resolution– Sensitivity

• Image processing– Corrections for physical effects– Image reconstruction algorithms

Data Analysis & Biological Modeling - better interpretation of images

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PET Imaging OverviewPET Imaging Overview

- Synthesize radiotracer- Inject radiotracer- Measure gamma-ray

emissions from isotope (~20-60 min)

- Reconstruct images of radiotracer distribution (nCi/cc)

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+

+ +

NucleusNucleus

NeutronsNeutrons

ProtonsProtons

ElectronsElectrons

Positron (Positron (++) Decay) Decay

18F-FDG

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++ Decay Decay

+

+

+

+

+

+

+

+

+

Neutron-deficient isotopes can decay by emitting Neutron-deficient isotopes can decay by emitting positronspositrons

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anti-neutrinoanti-neutrino

positronpositron Net effect: one Net effect: one proton proton replaced by replaced by

• neutronneutron• anti-neutrinoanti-neutrino• positronpositron

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Positron annihilationPositron annihilation

Annihilation gives• 2x 511 keV gamma rays• 180 degrees apart• Line of response

Positron range & gamma noncollinearity

Scanner is just a photon counter!• Counts gamma-ray pairs vs.

single gammas• Time window ~ 1 ns

511 keV

511 keV

e+

e-

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Raw Data & Image ReconstructionRaw Data & Image Reconstruction

0 projection

0

180

90

90 p

roje

ctio

n

image reconstruction

“sinogram”

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Important Detector PropertiesImportant Detector Properties

- Spatial resolution- Directly controls spatial resolution

in reconstructed 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 noise from counting statistics- Currently > ~ 30% (singles)

55M Events1M Events

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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 << 1 ns resolution

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Important Detector PropertiesImportant Detector PropertiesImportant Detector PropertiesImportant Detector Properties

Scatter and Attenuation

511 keV

- Energy resolution- Scattered gammas change

direction AND lose energy- Affects acceptance of scattered

coincidences- Currently ~ 20%

- Deadtime- Handle MHz count rates!

511 keV 400 keV

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ScintillationCrystal

PMTPre-Amplifier+ Electronics

Gamma photon converts to optical photons (proportional to gamma energy, typ. 1000’s)

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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New DevelopmentsNew Developments

• Detectors

• Multimodality imaging

• Specialized applications

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Scintillator NaI(Tl) BGO GSO LSO LuAP LPS LaBr (ns) 230 300 60 40 18 30 35

(cm-1) 0.35 0.95 0.70 0.86 0.95 0.70 0.47E/E (%) 6.6 10.2 8.5 10.0 ~15 ~10 2.9

Rel. light output (%) 100 15 25 70 30 73 150175

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New Developments: DetectorsNew Developments: Detectors

• Scintillators• No perfect choice - tradeoffs

• Also practical qualities• Rugged?• Hygroscopic?• Cost?

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New Developments: DetectorsNew Developments: Detectors

• 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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New Developments: DetectorsNew Developments: Detectors

• Solid-state detectors• Direct conversion,

no photodetector • Great dE/E &

spatial resolution• Poorer timing &

stopping power• CZT Z2

Z1

Sa2

Sa1

Sc

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New Developments: DetectorsNew Developments: Detectors

• Pb converters & ionization

HIDAC Pb-walled straws (50 cm long)

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New Developments: DetectorsNew Developments: Detectors

• 3D gamma-ray event positioning• Depth of interaction• Reduces parallax problem

LSOslab

APDslab

s

vs.

LSO slab

crystal holder

APD

decoupling capacitor

HV filter capacitor

Current-limiting resistor

signal output connector

SHV connector

unused APD slot

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New Developments: DetectorsNew Developments: Detectors

• Time of flight using LaBr3n

o T

OF

300 p

s T

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

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RatCAP: Rat Conscious Animal PET

Eliminate anesthesia in preclinical neuroscience using PET in order to:

• Remove confounding effects of anesthetic on neurochemistry

• Enable stimulation in animal PET

• Enable correlations of behavior and neuro-PET

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Architecture

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

Timestamp & Signal Processing Module• Programmable real-time logic (FPGA)• 1 ns bins (debugging, now 10 ns)

Data acquisition• PCI card in standard PC• Up to 70 MB/s = ~10 Mcps singles• Offline software for coincidences, corrections, recon, …

TSPM TDC

PCI card

ASIC

opticaldifferential

RatCAP

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Architecture

RatCAP

TSPM

LSO APD

ASICs

all interconnections

38 mm FOV

72 mm OD optical links to PCI

high voltage

194 g

data, clock, power

18 m

m a

xial F

OV

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Performance

Spatial resolution (FWHM @ CFOV)• FBP: 2.1 mm• MLEM: <1.5 mm

Energy resolution: 23% FWHM

Time resolution: 14 ns FWHM• window = 30 ns

Sensitivity (point @ CFOV): 0.7%

Peak Noise Equivalent Count rate: 14 kcps @ 5 Ci/cc

1st prototype: LLD = 150 keV average, variable

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Imaging Conditions

Anesthetized 250-350 g rats

Limited DAQ livetime >> long scans for statistics

Artifacts

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F-18 Fluoride Bone Scan

1.3 mCi fluoride

RatCAP microPET R4

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C-11 Raclopride

1.8 mCi raclopride

In the RatCAP

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C-11 Methamphetamine

Time-activity curve for striatum

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Thanks!Thanks!

DOE OBER funding