valuation of a Prototype of New P0siti0n—SensitiVe
etector for a Small Animal PET System
Liu Huafeng Omura Tomohide Watanabe Mitsuo Yamashita Takaji
(State Key Laboratory of Modern Optical Instrumentation，Zhejiang University，Hangzhou 310027，China)
(Central Research Laboratory，H amamatsu Photonics K ．K．，Hamakita city 434——8601，Japan)
Abstract PET has become a powerful tool for scientific studies．This paper describes some performance of a
new position—·s~nsitive detector using double layer LSO array and a newly developed compact PS—-PMT (Hama——
matsu R7600一C12)．The results suggest that this detector is a useful device for a small animal PET．Finally，the
initial design of a micro PET scanner employing the new detector is introduced．
Key words Positron emission tomography(PET) Position——sensitive PMT (PS—-PMT) LSO

sensitive detector which consisting of double—layer
l Introduction LS0 array coupled with a new compact position—
Positron emission tomography (PET)relies on
the idea of injecting chemical compound labeled with
positron—emitting isotopes into a body tO measure the
location and concentration of these compounds externally．
It has long been recognized that small laborato—
ry animal PET system is a powerful tool not only for
basic study on the bio—-functions of experimental ani—-
mals but also for pre—clinical research of human be—
ings．One of the major concerns in animal PET sys—
tem is the design and optimum of the detectors．M uch
effort has been dedicated tO develop various animal
PET detectors by a number of researchers ～ ．
W e have proposed a high——resolution position——
sensitive photomultiplier tube(PS-PMT)．Up tO pre—
sent．most of the commercially available PET systems
use BG0 block detectors．The reason of our using the
new proposed inorganic scintillator LSO is，its large
detection efficiency (effective atomic number一66)，
high light yield(50％一75 of the NaI)and short de—
cay time (< 40ns) The preliminary study in the
performance of the compact detector and the initial
design of an animal PET scanner were presented in
this paper．
2 Detector Design
2．1 The Compact PS—PMT
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第2 期 Evaluation of a Prototype of New Position—Sensitive Detector for a Small Animal PET System 1 1 9
Up to present，a variety of PS—PM Ts have been
developed to apply in the PET detectors． PS—PMTs
employ the principle，which the emitted photoelec—
trons from the photocathode are multiplied by the
channel dynodes while retaining the position informa—
tion，and then the multiplied electrons are read out by
crossed multi—plate anodes． Recently， Hamamatsu
has developed a new compact PS—PMT (R7600一C12)，
which has a 25．7 mm square x 20 mm high and the ef—
fective area is 22x22mm。．The R7600一C12 has a Bial—
kali photocathode，1 1-stage of metal channel dynodes
and 6 crossed plate anodes in both X and Y directions．
Tests of position linearity and application to a radia—
tion imaging devices were carried out and reported in
reference E
． The initial evaluation resuIts indicate that
R7600一C12 is a promising PS—PM T for nuclear imag—
ing instrumentation．
2．2 B1oek Detector Design
lo x 1o LSo
Atray
l】x l】
LSO Atray
Fig．1 Configuration of a detector for PET
W e designed and constructed the block detector
as shown in Fig．1，where the upper 10×10 LSO array
is placed on the lower 1 lxl 1 LSO array with a shift of
half the segment pitch in the both X and Y directions．
Each crystal in the LSO array was optically isolated
by a 0．2 mm Teflon tape．The block detector is 22
mm wide x 22 mm long x 20 mm deep，of which the
cross section area of the lower LSO array is just the
same dimension as the Dhotosensitive area of the PS—
PM T．The principle of event positioning is that the
centroid of light distribution from each crystal locates
at different position in PS—PM T。which can be calcu—
lated by the centroid calculation methodE ．
3 Block Detector Performance
3·1 Position Response
The experimental setup for positioning map mea—
surement is shown in Fig．2．The cross—wired anodes
of PS—PMT are connected to two resistor chains in
each X and Y direction，and four output signals from
the resistor chains are amplified and integrated．Each
output signal is then converted to 1 2 bit digital code
and acquired into PC through GPIB interface in list
mode．The dynode signal is also amplified and then
sent to the CFD，which offering an output signal as
the start trigger of the CAM AC ADC．
Fig．2 Experimental setup for positioning histogram mea—-
surem ents
Fig．3 shows the positioning map obtained by ex．．
posing the modules to a point。。Na gamma ray source
and accumulating about millions events． Each seg—
ment on the center of upper and lower array has a
well localized distribution and is clearly separated in
the center． However， the image of the peripheral
crystals especially for the corner crystals，becomes
blurred due to the useful area limitation of the PS—
PM T．Count profiles along one row of upper and low．．
er array are also shown in Fig．3．Each peak corre—
sponds to a different crystal in the block detector．
3．2 Spatial Linearity
In order to evaluate spatial positioning linearity，
we calculated the crystal separation along the main di—
agonal in the flood image，comparing to the physical
crystal separation for upper and lower array，respec—
tively．The results are summarized in Fig．4 and indi—
cate that peak separation is mainly depended on the
position of LSO crystals relative to the PS—PM T．
Strong non—-linearity in the peripheral regions is ob—-
served．This effect is due to the capability of metal
维普资讯 http://www.cqvip.com
120 仪器仪表学报 第2 2卷
channel dynodes to produce a charge distribution com
parable with light distribution．
C
(a)
(b) (c)
Fig．3 (a)Positioning histogram of a 10 x 10+ 1ix 11
LSO detector(256 x 256一pixel image)．(b)Image pro—
files for 10 upper segments corresponding to the line indi—
cated by the arrow B．(c)Image profiles for 1 1 lower seg—
ments corresponding to the line indicated by the arrow
C．
营
董：3
笤
冒
曼3
尝2
叠l
舀0
Crystal number
(a)
3 5 7 9
Crystal number
(b)
Fig．4 (a)Spatial positioning linearity along the main di—
agonal of upper array (b)Spatial positioning linearity a—
long the main diagonal of lower array．
3．3 Energy Resolution
The energy resolution is extracted from the posi—
tion response data of the block detector．The extract—
ed method can be described as follows：the X and Y
value of a position signal event are first computed，
then a crystalidentification method is to define the re—
gions that correspond to each crystal on the two——di——
mensional flood field histogram．After generating the
position look—up—table(LUT)map。the energy spec—
trum for each crystal is obtained． />翌
C
8
。
旦
Channel numbe r
璺
C
8
Channel numbe r
Fig．5 Typical energy spectra (a)for the single crystal in
the lower array (b)for the single crystal in the upper ar—
ray·
Fig．5(a)and (b)show the typical energy spec—
tra of a single crystal in the upper and lower array，
respectively． The average energy resolution of the
lower array was about 20％ and that for the upper ar—
ray was about 24．4％．
3．4 Gain Uniformity
基。
Z
(a) (b)
Fig．6 (a)Normalized gain module Vs．crystal position in
the upper array．(b)Normalized gain module Vs．crystal
position in the lower array．
The gain non——uniformity of a PS——PM T across the
surface area of entrance window is known to con—
tribute the non—·uniform energy resolution of scintilla．．
tion detectors．The R7600一C12 PS—PMT has reduced
this problem by introducing the thinner glass window
and changing the dynode structure． The effects of
these gain variations are excepted to correct especially
for small gamma ray detectors．The gain uniformity
of the detector module was evaluated．The gain of the
elements。which was defined as channel number of the
photopeak maximum of the pulse height distribution，
was calculated．Then the gain variation distribution of
the upper and lower array was plotted in the Fig．6．
3．5 Timing Resolution
The coincidence timing resolution was measured
with between a BaF2 detector and the detector，whose
LSO array was constructed a simple structure of one
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第2 期 Evaluation of a Prototype of New Position—Sensitive Detector for a Small Animal PET System 121
LSO crystal placed on the lower 2×2 array，in order to
remove degradation factors depending on nonunifor—
mity of the PS—PM T，for 511 keV gamma rays of fl
Na source．Fig．7 shows the timing spectrum and ex—
hibits 0．80 ns in FWHM and 1．53 ns in FW TM for
the 1ower side，and that for the upper side is 0．84 ns
in FW HM and】．67 ns in FW TM ．
0 200 400 6oo 800 1000
Channel Number (0．4ns／ch)
Fig．7 Timing spectra for the
side
0 200 400 6o0 8o0 lo00
Channel Number(0．4ns／ch)
(a)Jower side (b)upper
4 Initial design of a M icro PET Scanner
Those preliminary experimenta1 results of the
new detector as mentioned above indicate that fl high
resolution PET scanner can be built．
A micro PET scanner．which is for the studies of
small animals，such as rat etc，is under developing．
The 16 detectors with fl tota1 of 3536 crystals wil1 be
arranged in fl ring of 1 1 2 mm diameter．Fig．8 shows
the arrangement of the block detectors in the gantry．
The transaxia1 field of view (F0V)is 73mm and the
axial F0V is 22mm．The new PET system will be op—
crated in the 3D mode．in which the detection sensi—
tivity is drastically increased．
Fig．8 Detector ring configuration for an animal PET
5 Conclusions
A prototype of new compact position—sensitive
detector has been evaluated。and an anima1 PET sys—
tern using the detectors is under development．Since
the detector employs the PS—PM T，the improvement
of the gain uniformity particularly at peripheral re—
gions in the photosensitive area will improve the per—
formance of the detector．The energy resolution of the
upper array is worse than that of the lower array due
to the relative poor light output．The timing resolu—
tion of the detector is 0．84 ns with the upper array
and 0．80 ns with the lower array in reference to fl
BaF2 detector．Additions1 work wil1 be needed to eval—
uate and optimize the detector，such as count rate ca—
pability，improving the performance of PS-PM T etc．
Preliminary studies on the block detector suggest that
this device will allow us to realize fl high performance
PET system．
2
3
4
7
Reference
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