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Physics & Instrumentation in Positron Emission Tomography. Paul Vaska, Ph.D. Center for Translational Neuroscience Brookhaven National Laboratory July 21, 2006. Non-invasive Medical Imaging Techniques. CAT. MRI. X-Ray. Anatomical X-ray CAT MRI Ultrasound Functional

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physics instrumentation in positron emission tomography
Physics & Instrumentation inPositron Emission Tomography

Paul Vaska, Ph.D.

Center for Translational Neuroscience

Brookhaven National Laboratory

July 21, 2006

non invasive medical imaging techniques
Non-invasive Medical Imaging Techniques

CAT

MRI

X-Ray

  • Anatomical
    • X-ray
    • CAT
    • MRI
    • Ultrasound
  • Functional
    • “nuclear medicine” - SPECT, PET
    • Optical fluorescence, …
positron emission tomography
Positron 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
technical challenges in pet imaging
Technical 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
pet imaging overview
PET Imaging Overview
  • Synthesize radiotracer
  • Inject radiotracer
  • Measure gamma-ray emissions from isotope (~20-60 min)
  • Reconstruct images of radiotracer distribution (nCi/cc)
slide6

Nucleus

Neutrons

+

+

+

Protons

Electrons

Positron (+) Decay

18F-FDG

slide7

+

+

+

+

+

+

+

+

+

+

+ Decay

Neutron-deficient isotopes can decay by emitting positrons

anti-neutrino

positron

  • Net effect: one proton replaced by
  • neutron
  • anti-neutrino
  • positron
positron annihilation
Positron 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

e+

e-

511 keV

raw data image reconstruction
Raw Data & Image Reconstruction

0

90

180

“sinogram”

90 projection

image reconstruction

0 projection

slide10

Important Detector Properties

  • Spatial resolution
    • Directly controls spatial resolution in reconstructed image
    • Currently ~ 1 - 5 mm
    • Depth-of-interaction?
      • Reduces “parallax”
slide11

55M Events

1M Events

Important Detector Properties

  • Detection efficiency (aka sensitivity, stopping power)
    • Reduces noise from counting statistics
    • Currently > ~ 30% (singles)
slide12

Random (accidental) coincidence

Important Detector Properties

  • Time resolution
    • Affects acceptance of random coincidences
    • Currently ~ 1 - 10 ns
    • Time-of-flight (TOF)?
      • c = ~ 1 ft/ns
      • Need << 1 ns resolution
slide13

Important Detector Properties

  • 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

511 keV

Scatter and Attenuation

prototypical pet detector
Prototypical PET Detector

Optical reflector

light is converted to an electrical signal & amplified

Gamma Ray

Scintillation

Crystal

PMT

Pre-Amplifier

+ Electronics

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

photons are collected at the end of the crystal

Front-end electronics condition the signal for further processing

slide15

New Developments

  • Detectors
  • Multimodality imaging
  • Specialized applications
slide16

25

175

New Developments: Detectors

  • Scintillators
    • No perfect choice - tradeoffs
    • Also practical qualities
      • Rugged?
      • Hygroscopic?
      • Cost?
slide17

New Developments: Detectors

  • Photosensors
    • Photomultiplier tubes
    • Avalanche photodiodes
      • Arrays, position-sensitive
      • Compact but noisier
    • Silicon photomultipliers
      • Very new
      • Best of both?

PMT

APD array

SiPM

slide18

Sa1

Sa2

Z1

Z2

Sc

New Developments: Detectors

  • Solid-state detectors
    • Direct conversion, no photodetector
    • Great dE/E & spatial resolution
    • Poorer timing & stopping power
    • CZT
slide19

New Developments: Detectors

  • Pb converters & ionization

HIDAC

Pb-walled straws (50 cm long)

slide20

vs.

SHV connector

decoupling capacitor

signal output connector

APD

HV filter capacitor

LSO slab

unused APD slot

Current-limiting resistor

crystal holder

New Developments: Detectors

  • 3D gamma-ray event positioning
    • Depth of interaction
    • Reduces parallax problem
slide21

10 Mcts

1 Mcts

5 Mcts

no TOF

300 ps TOF

New Developments: Detectors

  • Time of flight using LaBr3
slide22

New 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
slide23

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
slide24

RatCAP

ASIC

differential

TSPM TDC

optical

PCI card

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, …
slide25

Architecture

high voltage

data, clock, power

all interconnections

LSO APD

18 mm axial FOV

38 mm FOV

ASICs

TSPM

72 mm OD

optical links to PCI

RatCAP

194 g

slide26

Performance

1st prototype: LLD = 150 keV average, variable

  • 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
slide27

Imaging Conditions

  • Anesthetized 250-350 g rats
  • Limited DAQ livetime >> long scans for statistics
  • Artifacts
slide28

F-18 Fluoride Bone Scan

  • 1.3 mCi fluoride

RatCAP

microPET R4

slide29

C-11 Raclopride

  • 1.8 mCi raclopride

In the RatCAP

slide30

C-11 Methamphetamine

Time-activity curve for striatum

thanks
Thanks!

DOE OBER funding