carey floyd cef at deckard.duhs.duke.edu
Fri Oct 15 13:10:48 CEST 2004

This is an introduction as well as a request for advice (even if the 
advice is to look somewhere else). I am developing a form of gamma 
emission computed tomography for imaging and identifying stable 
isotopes in which the stable isotopes are stimulated by an external 
neutron beam to emit characteristic gamma photons. This technique which 
I call NSECT for Neutron Stimulated Emission Computed Tomography, 
places unusual requirements on the detection system as the gamma 
energies of interest lie between 500 and 6200 keV. The detection system 
would ideally report both the spatial location as well as the energy of 
the detected photons. No physical collimation of which I am currently 
aware can effectively collimate photons of these energies. I had 
initially written my own Monte Carlo codes to simulate the NSECT 
acquisition using a 1st generation CT approach (which is how I have 
reduced the concept to practice and have produced experimental 
tomographic reconstructions of multi-element phantoms. While these 
codes were successful, as I began to design specialized detectors, the 
complexity of the geometries and  physical interactions lead me to 
consider MCNP and GEANT as more effective for my needs. Having written 
a neutron transport MC code that read ENDF files and computed libraries 
back in 1982, I was not eager to repeat this effort. Starting this 
project in June, I decided to use GEANT as I was impressed with the 
flexibility of the code design and with the online support that was 

I have been intrigued by your efforts with GATE and seek advice as to 
whether GATE in its current and evolving form would be a good match to 
my needs. I understand that GATE is built upon GEANT (although it seems 
to lag a few releases behind which I can fully understand), My 
question: do you feel that it would be easier to incorporate neutrons 
into GATE, or tomography into GEANT (which I have already done to some 
extent)? The immediate task facing me is to evaluate Compton Scatter 
Cameras for tomography of high energy (500-6200 keV) gammas emitted 
from the body. I will be evaluating a number of different detector 
materials including solid state detectors (such as Germanium and CZT) 
as well as scintilators (such as BGO). While the primary neutrons are 
around 3-10MeV, there seem to always be a cloud of thermalized neutrons 
hanging about and we learned the hard way that NaI is not a good gamma 
spectrometer with Na capturing the thermal neutrons and emitting its 
own capture gammas internally! (Yes, we should have known better but we 

Any thoughts and help are welcome. I need to be able to compute patient 
dose as well. While we have been successful thus far with GEANT, our 
C++ skills are limited and GEANT is written with such elegance that we 
are sometimes left in the dark due to our own lack of such elegance.

Carey E. Floyd Jr. Ph.D.
Professor of Radiology and Biomedical Engineering
Director Digital Imaging Research Division
Duke University
919-684-4138 ph
919-684-3934 fax
919-970-3644 page
919-684-3856 staff assistant Melissa Allen

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