Transcript NSECT - Research Imaging Institute

Neutron Stimulated Emission
Computed Tomography (NSECT)
Drosoula Giantsoudi
Spring 2007
Presentation for Diagnostic Imaging II class
Outline
 Introduction – What is NSECT
 The physics behind
 Methods of NSECT
 Potential applications
 Neutron radiation dose
 Directions of present and future
research
What is NSECT?
 Spectroscopic measurement of element
concentrations in the body using a neutron beam
 γ emission computed tomography (ECT), γ rays
from stable isotopes stimulated by neutron
collisions
 Image the naturally existing distribution of stable
isotopes along the path of the neutron beam
 Not clinically applied yet
The physics behind NSECT
 Fast neutron interactions:
– Elastic scattering
– Inelastic scattering
 Energy given to the nucleus leaving
it in an excited state.
 Characteristic gamma emission from
the γ-decay of the nucleus
 Gamma energies for all elements:
100keV-6600keV except H and
He(25MeV)
Method description
Three required components:
 Neutron source
2.8 MeV deuteron beam  gas cell  6MeV
neutron beam
 Imaging target – sample
– Energy emitted characteristic of the specific
element
 Gamma spectrometer
– High purity germanium (HPGe) detector
Acquisition setup
Acquisition setup
 Collimated neutron beam (2x2cm2, 1x1cm2 or
narrower)
 Low – dose beam is scanned over the body
 Cylindrical HPGe detectors - fine energy resolution
– Energy spectrum acquisition for each position of the beam
– Reconstruction of nuclei distribution into a tomographic
image – seperate for each element
– Image intensity corresponding to the concentration of that
element
Experimental Setup
http://dailabs.duhs.duke.edu
Disadvantages
 Spatial resolution determined by the
collimated beam
 Neutron beam collimation in a small size
not a trivial task
 Limitations: narrow beams result in
decreased neutron flux
 Longer acquisition time - patient motion
Another approach
 Broad neutron beam geometry
– No translation of the target
or the beam necessary
 Collimated Ge detector –
rotational collimators
 Compton Coincidence Detector
(CCD) 2D projection
 Spatial resolution determined by the detector
 Decreased detection efficiency of the CCD due to
multiple coincidence requirement
Considerations-Challenges
 Background suppression
– ‘sample-in-sample-out’ subtraction
– pulsed neutron beam, acquisition in the time window that
the photons are produced - TOF measurements
 Normalization
– monitoring of the deuteron flux: collecting the charge
deposited onto the gas target cell
– neutron monitoring: thin plastic scintillator
– neutron monitor to beam current ratio
 List-mode data acquisition, allowing for post setting
of parameters (detector bias, TOF windows)
Identification of specific elements
 Spectra from basic elements: C, Ca, Fe, Cu,
water phantom (for O)
 Analysis identifying the peaks (NNDC
databases)
 γ-ray flux~(# target nuclei)×(neutron flux)×
(differential cross section)
 Neutron and γ attenuation have small effect
for biomedical targets
http://www.sns.gov/workshops/ian2006/TU2/IAN2006oct_Kapadia.pdf
http://www.sns.gov/workshops/ian2006/TU2/IAN2006oct_Kapadia.pdf
Reconstructed Images
http://dailabs.duhs.duke.edu
Potential medical applications
 Cancer diagnosis:
– difference in trace element concentrations in
tissues when malignancy begins to occur
– In vivo non destructive spectrometry=non invasive
biopsy long before anatomical changes
 Non-invasive measurement of iron
– Important for thalassanemia, heamochromatosis
mutations, chronic blood transfusions
– No delay or morbidity associated with repeated
biopsy
Potential medical applications
 Small animal spectroscopic imaging.
 Molecular biology research
 Study of metallo-protein transport
 Follow the transport of stable
isotopesnew tool for pharmaceutical
development
Neutron radiation dose
 Monte Carlo simulations – GEANT4 for 2.5 MeV neutron beam:
 Two necessary factors:
– Fraction of # of incident neutrons that deposit energy in the subject (0.6)
– Average fraction of each neutron’s energy deposited (1.4MeV)
 For a gamma peak of 30 counts, 10×106 incident neutrons (liver in
the abdomen)
 1cm2 beam area, 10cm path through the liver, density 1.1g cm-3
 Eabs= (Eav per neutron)×(#of incident neutrons)×(fraction of
neutrons interacting)=1.4MeV×107×0.6=8.4×106 MeV=1.34×10-6J
 Effective Dose = Eabs ×QF ×WF=(1.34×10-6 J/0.011 kg)×10×0.5 =
0.0061 mSv < 0.02 mSv for a chest x-ray
Directions of present and
future research
 High flux neutron source
– shorter scan time
– smaller collimation of the neutron beam
 More efficient, multiple or collimated detectors
– better signal to noise ratio
– better spatial resolution
 Extended dose simulations
Thank you!
 References:
– Carey Floyd, C Howell, A Kapadia et al, Introduction
to neutron stimulated emission computed tomography,
Phys. Med. Biol. 51 (2006) 3375–3390
– http://www.sns.gov/workshops/ian2006/TU2/IAN2006
oct_Kapadia.pdf
– http://deckard.mc.duke.edu/research8.html
– http://www.aapm.org/meetings/04AM/VirtualPressRoo
m/NeutronImaging.pdf