© 1998 IEEE. Personal use of this material is permitted. Permission from IEEE must be
obtained for all other uses, in any current or future media, including reprinting/republishing
this material for advertising or promotional purposes, creating new collective works, for
resale or redistribution to servers or lists, or reuse of any copyrighted component of this
work in other works.
Detector Module with Improved Spatial Sampling
Nuclear Medicine Department
National Institutes of Health, Bethesda MD 20892
small ring diameter PET scanners
must possess depth-of-interaction (DOI) encoding, increased
spatial sampling, high sensitivity and the ability to handle high
photon input rates without excessive pulse pileup or random
coincidences. We created such a module by optically
coupling an entrance array of individual LGSO crystals to an
exit array of individual
(and other) crystals that was, in
turn, optically and directly coupled to a miniature PSPMT.
DO1 was determined for each event by delayed charge
integration (DCI), a technique that exploits differences
decay time between
3D was increased by introducing a half
crystal pitch spatial offset between the entrance and exit arrays
directions. Position detection accuracy in
both the LGSO and
layers, and the accuracy of DO1
assignment of events to either layer was high.
suggest that this combination of scintillators and
acquisition/processing methods may be particularly useful in
the design of high performance, small ring diameter PET
scanners for small animal imaging.
As the diameter of a PET ring scanner shrinks, the need for
depth-of-interaction (DOI) information, finer spatial sampling
and improved count rate performance increases. Numerous
methods have, in fact, been advanced that address one or
more of these issues. We propose here a module that, in
principle, meets all or most of these conditions
simultaneously, while possessing good position detection
module is comprised of two layers of optically coupled
scintillators that differ from each other in light decay time (for
DO1 determination). Second, all of the crystals
are geometrically identical and possess small entrance cross-
sections (potentially good position detection accuracy). Third,
the entrance layer is spatially offset with respect
layer by half a crystal pitch
both spatial directions
(increased spatial sampling in 3D). Fourth, both scintillators
have short light decay times, high densities, good light outputs
and an extended combined length (potentially high count rates
with low pulse pileup, good stopping power, good position
detection accuracy, good sensitivity).
The module combines five different features.
Finally, these layers are directly coupled to a thin window,
miniature position-sensitive photomultiplier tube with
favorable performance characteristics (good timing properties
Current address: Concorde Microsystems, 10427 Cogdill
Knoxville, Tennessee 37932
for DO1 discrimination, efficient conversion of scintillation
light to photoelectrons for good position detection accuracy).
In the work described below, we evaluated a test module
with these features for the purpose of determining whether this
module might be suitable as the detection element of a high
resolution, small diameter rodent PET scanner.
Geometrical layout of the crystal arrays. The entrance array
is centered on,
coupled to, the exit array
All crystal surfaces were finished by chemical etching and
all crystals were wrapped on their long sides with a double
layer of Teflon tape. The entrance end of each crystal in the
entrance layer was also covered with Teflon tape.
The entrance layers consisted only of LGSO crystals while
the exit layer consisted of GSO and other scintillator types.
(These other crystal types were included to evaluate different
scintillator combinations and DO1 methods, but here we shall
focus only on the properties of the LGSOIGSO portion
Note that the entrance layer is offset by half a crystal pitch
1.1 mm) with respect to the exit array
directions. The entrance layer was optically coupled
to the exit layer with optical grease and the exit layer coupled
with optical grease to a Hamamatsu R5900-C8 position-
sensitive photomultiplier tube (PSPMT). The entire assembly
a light-tight box.
For each scintillation event within its field-of-view, the
signals, one for each of the four X-anode
plates, one for each of the four Y-anode plates and a ninth
signal from the last dynode for triggering and DO1
The dynode signal is first used to
ns-wide trigger signal for the charge-
integrating ADCs (LeCroy FERA). It is further split into two
halves that are also integrated.
All anode signals and one of the dynode signals are
delayed for 100 ns to ensure that they arrive at the ADCs
immediately after the trigger signal (Full Charge Integration,
The other dynode signal is not delayed
ADC can only capture the charge that is still present after 100
ns (Delayed Charge Integration, Figure
If the full charge
integral is plotted against the delayed charge integral (Figure
4A), two radial “spokes” appear in the resulting diagram, one
for each scintillator.
the spoke with the greater slope
correspond to events
the fast scintillators LGSOLSO, while
the spoke with the smaller slope corresponds to events in the
scintillator. If ROIs are defined that encompass
the LGSO and
regions individually (Figure 4B),
assignment of an event to one or the other of these regions
also identifies the layer of interaction.
Thus, when an event occurs, depth is determined for that
event by delayed charge integration (DCI) and the crystal
that layer determined by centroid calculation
using the anode signals. (Subsequent refinement of the DCI
technique has improved the separation between these two
scintillators substantially as shown in Figure 4C).
Experiments to Determine DOI and Event
The ability of these methods to place a scintillation event
in the correct crystal were evaluated qualitatively by flood-
field illumination of the module from the front. Because other
scintillators were included
the exit array, the calculated
pattern of events assigned to each layer can be compared to
the known pattern of crystals
Delayed Charge Integration
the Delayed Charge Integration method.
DO1 accuracy was determined by scanning a 1.1 mm wide
F-18 slit source along the depth direction of the scintillator
arrays. At each point, the number of events occurring
scintillation layers was determined.
Similarly, position detection accuracy was determined for
each layer by scanning a highly collimated beam of F-18
mm steps down the LGSO column
entrance array that overlay the two columns
in the exit array. At each spatial position, the counts occumng
in each crystal in both arrays were determined.
The results of field flood illumination of the module are
If the DO1 selection conditions are not
applied, an image of the field-of-view appears as
When the DO1 conditions are applied, however, this image
separates into two images, one corresponding to the fast
scintillators LGSOLSO (Figure 5B) and the other to the slow
(Figure 5C). Note that the pattern of
Figure 5B and the combined pattern
the entrance and exit arrays shown in these figures correspond
to the known pattern of crystals in the actual arrays (Figure
DO1 and position detection accuracy are illustrated in
In each of these graphs the fraction of events
assigned to each crystal is plotted against the position of the
illuminating source. No additional event rejection conditions
were applied in the creation of the DO1 graph. However,
spatial rejection conditions were applied before creating the
position detection accuracy graph in Figure
These results were obtained by placing small spatial
around each crystal center and rejecting all events that
occurred outside these regions. Under these conditions,
position detection accuracy is high
in both layers) but
of all events
accepted). When large
ROIs are used, position detection accuracy decreases
but sensitivity is increased
of all events
created by plotting the full
charge integral (vertical axis) against the delayed charge integral
(horizontal axis). Definition of
Separation is improved in
by reducing the
dynode time constant.
phoswich/PSPMT detector module comprised of two
fast, high stopping power scintillators with offset entrance and
exit arrays can accurately locate the endpoints of annihilation
events in space, albeit coarsely in the depth dimension. The
spatial offset between entrance and exit arrays, moreover, does
not appear to unduly degrade the accuracy of event positioning
across the field-of-view. In addition, since both
have short light decay times, pulse pileup can be
minimized by utilizing short pulse integration times
The ability to acquire accurate DO1 and position
information at high rates and with relatively fine
sampling suggests that this module may be useful in
applications where these advantages are particularly
significant, i.e. in small ring diameter PET scanners intended
for small animal imaging.
Field flood illumination of the detector module ignoring
This image separates into an
image (C) when the Delayed Charge Integration
method is applied.
Phoswich layer accuracy
each phoswich layer when
Vaquero is supported, in part,
a grant from CICYT
Siegel was supported
from the National Research Council.
Eriksson, M.E. Casey, M.S. Andreaco, C.
Wienhard, G. Flagge, R. Nutt. “Performance
Results of a New DO1 Detector Block for a High
Resolution PET-LSO Research Tomograph HRRT”.
IEEE Nuclear Science Symposium and Medical Imaging
Siegel, W.R. Candler, M.V.
Green. “Performance Characteristics of a Compact