Speaker
Dr
Dennis R. Schaart
(Delft University of Technology)
Description
In recent years, small-animal positron emission tomography (PET) has gained much
interest. Most current designs use small scintillation crystals coupled to position-
sensitive photomultiplier tubes. It would seem relatively straightforward to increase
the resolution in such designs by decreasing the pixel size. However, this results in
loss of sensitivity because of the increased dead space between the pixels. Also,
resolution improvement may be impeded by inter-crystal scatter and parallax errors.
Monolithic scintillation detectors, consisting of a few cm3 of scintillating material
coupled to one or more position-sensitive light sensors, in particular avalanche
photodiode (APD) arrays, can avoid these problems. In such detectors, the entry point
of an incoming annihilation photon can be estimated from the distribution of the
scintillation light on the APD arrays.
Using the Monte Carlo code GATE, a small-animal PET scanner based on monolithic
scintillation detectors is simulated in order to investigate the resolution of the
reconstructed images and the count rate performance. The scanner consists of four
rings of 38 detector modules each, with an inner diameter of ~128 mm. The modules
consist of 20x10x20 mm3 LSO crystals coupled to two Hamamatsu S8550 APD arrays.
Crystals with a trapezoidal rather than a rectangular shape are also investigated. By
increasing the width of the crystal on the outside of the scanner, the dead space
between the detector modules is minimized allowing for a further increase of the
detection efficiency. These monolithic designs are compared to a scanner with the
same dimensions using pixellated crystals.
We have previously shown that detectors based on monolithic scintillators have an
intrinsic spatial resolution similar to that of scintillation detectors using
pixellated crystals (< 2 mm FWHM). Reconstructions showing the high resolution
achieved in actual PET images can be simulated in a realistic matter using
experimentally determined error distributions. These error distribution are obtained
as a function of the photon entry point and the incidence angle by determining the
difference between the true entry point and the measured one for a large number of
events. Images and detailed spatial resolution results will be shown at the
conference.
The monolithic scintillation detector appears to offer a point-source sensitivity
that is roughly a factor of two larger than a pixellated one. The same appears to be
true for the noise equivalent count rate (NECR) for activities in the range of
interest for small-animal PET (less than ~50 MBq), even at a relatively long dead
time of 1000 ns. NECR curves calculated for a cylindrical water-filled phantom for
different dead times, coincidence windows and energy windows will be presented at the
conference.
Authors
Mr
D.J. (Jan) van der Laan
(Delft University of Technology)
Dr
Dennis R. Schaart
(Delft University of Technology)
Dr
Hugo W.A.M. de Jong
(VU Amsterdam Medical Centre)
Co-authors
Prof.
Carel W.E. van Eijk
(Delft University of Technology)
Mr
Cedric Lemaître
(Vrije Universiteit Brussel)
Mr
Marnix C. Maas
(Delft University of Technology)
Dr
Peter Bruyndonckx
(Vrije Universiteit Brussel)
