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A liquid xenon PET camera for neuro-science
J. Collot, S. Jan, E. Tournefier
To cite this version:
IXInt. Conf.onCalorimetryinPart. Phys.-Annecy,Oct.9-14,2000
ALIQUID XENON PETCAMERA FOR NEURO-SCIENCE
J.Collot,S. Jan,E. Tourne er
InstitutdesSciencesNucleaires
IN2P3/CNRS -JosephFourierUniversity
53 av. desMartyrs
38026 Grenoble cedex ,France
ABSTRACT
Acamerawhichmakesuseofliquidxenoninthescintillationmodeisstudied
forPositronEmissionTomographyofhumanbrain.
Asimulationwhichtakesintoaccountthebasicphysicalprocessesonly,shows
thattheintrinsicspaceresolutiononecanreachisr
FWHM
min
=2:1mm.
ResultsontheperformanceofaHamamatsupositionsensitivePMToperating
intheUVrange(180nm) arepresented.
1 Introduction
Theaim of the projectis to developa PositronEmission Tomograph (PET)
basedontheuseofliquidxenon(LXe)asanactivemedium. ThisPETwillbe
Itsapplicationforanonline ( C)PETcamerain thecontextof C
hadron-therapyisalsoconsidered.
ThisdevelopmentisproposedbythreelaboratoriesoftheJosephFourier
Uni-versityof Grenoble (ISN, IRM/CHU, Nuclear Medicine department/CHU)
andoneindustrialpartner,DTA/AirLiquide,forthecryogenicequipments.
Inthisproject,weonlywanttoexploitthescintillationdetectionmodeofliquid
xenon. Thescintillation decaytimeofLXe (3ns)couldresultin asigni cant
progresson the time resolution, the sensitivity and theselectivity of the
de-tector. Bene ting from this, webelievea high imageresolution (' 3mm on
image)andanincreasebyafactor5ofthecountingratecapabilitycompared
tothepresenttomographscouldbeobtained.
The execution of this project is organized along two phases : rst, the
de-velopmentof a full PET simulation (GEANT 4- ROOT - IDL) and a R&D
investigation which includes the constructionof asmall prototypeto con rm
theprojectfeasibility, followedby thedevelopmentand the constructionofa
fulldevicewhich couldbeamicro-PETcamera.
Wewillpresenttheoperationprinciplesofthisdevice,itspreliminarysimulated
performanceandthe rstresultsobtainedduringtheongoingR&Dphase.
2 Liquid xenonas compared to crystals
Liquid xenon is a known gamma detection medium which features gamma
interactionpropertiescomparabletoNaI.However,itsscintillationeÆciencyis
twiceashighasthatofNaI,whichisthemosteÆcientinorganiccrystal,and
itsscintillation decaytimeismorethantentimesshorterthanthebestvalue
ofallcrystalsconsideredin PETdevelopment(40nsforLSO).
Animportantaspectofthisprojectisthefactthatweonlywanttousethe
LXescintillationproperties
2)
andnotitschargecollectionmode
3)
,because
thescintillationyieldwhencomparedtothechargecollectioneÆciencyofLXe
ismuch lesssensitiveto apollutionoftheliquid: itmaytolerateuptoafew
ppmof impurities( O
2
...) versusafew ppbin charge mode. Ontop ofthis,
thedriftvelocityoffreeelectronsismuchtooslow.
Bycomparisontocrystal-basedcameras,theuseofaliquidactivemedium
may enable us to design novel detector geometries which could result in a
(fast) (ns) 3 (98%) 40 (slow) (ns) 25 (2%) Photons/MeV 7.8 10 4 3.2 10 4 Wave length (nm) 178 420
Table1: ComparisonbetweenLSOandliquidxenonasanactivemedium.
respectto thetomographsavailable today
4)
, are: afactor 1.5for the axial
andtransaxialspace resolutions ('3mm onreconstructed images with
18 F
) , a factor 5 for the counting rate , a time coincidence window 5 ns (
reductionofrandom coincidences). Agoodenergyresolutionto discriminate
thescatteredphotonsandto lterthe"Comptonnoise"couldalsobeobtained
ifonepreservesahighcollectioneÆciencyofthelightemitted.
3 Monte-Carlo simulation
OursimulationprogramisbasedontheGEANT4toolkit
5)
. Ittakescareof
thecompletegeometricaldescriptionoftheapparatus,thegenerationandthe
interaction of positons in awater standardphantom , theirannihilation into
photonpairsandtheinteraction trackingofthesubsequent511keVphotons.
ROOT is used to analyzethe simulationresults and constructsinograms
6)
which arethen fed into IDLin order to buildthe images( usingtheFiltered
Back-projectionmethod).
The liquid xenonis containedin a5cm thickring of 30 cm of internal
radiuswhichcovers20cmintheaxialFieldOfView(FOV).Thetotalvolume
ofLXeis20.5l.
A standard phantom for simulating the conditions prevailing during a
brainstudyisahollowcylinder,20cminlengthanddiameter,madeofathin
Plexiglasvessel lledwith water. Thephantomis placedatthe center ofthe
FOVinthescanner. The
18
F
+
energyspectrumissampledwiththeclassical
VonNeumannalgorithm(Fig. 1).
Oneimportantphysicale ect whichlimitsthePETspatial resolutionis
the
+
rangebeforeitannihilates,andmoreprecisely,the
+
distanceof ight
MeV
0
0.1
0.2
0.3
0.4
0.5
0.6
Counts
0
20
40
60
80
100
120
140
160
180
200
Mean = 0.274
Positron emission energie
for F18 radionuclide in water
Mean = 0.274
Figure1:Simulatedkineticenergydistributionofthe
+
fromthe
18
Fspectrum
seethat themeandistanceof ight, inthecaseof
18
F,before annihilationis
0.5mm(Fig. 2). Thesecondphysicsprocesswhichhasanimportantimpact
onthespatialresolution,istheacolinearityofthetwoannihilation 's. Thisis
essentiallydueto theorbital motionoftheatomicelectronswhichparticipate
to the positon annihilations
7)
. The acolinearity angle can be modeled by
aGaussiandistributionwith a0.25
Æ
standarddeviation. Thecontribution of
thise ect,at FWHM,is1.6mmforourgeometry.
3.1 Firstresults
Theenergydepositedbythetwo intheliquidxenonwassimulated. 200000
+
eventsweregenerated in thewaterphantom. Thedeposited energy
spec-trum for real ( photo-electric + Compton interactions ) coincidences with a
thresholdat300keVforeach is shownin gure3. Thefraction ofselected
events(enteringinthis gure)givestheuppersensitivity(S
max
)limitofthe
tomograph: S
max
=1:8%
Only thecontribution of thephysicale ects to thespatial resolution of
the detector was simulated. Figure 4shows the space resolution obtained (
for100000 eventssimulated) : 2.1mm at FWHM, which iswhat weexpect
fromthequadraticconvolutionofthepositondistanceof ightandthephoton
mm
0
0.5
1
1.5
2
2.5
Counts
0
500
1000
1500
2000
2500
3000
3500
Mean = 0.546
Distance of flight
for F18 radionuclide in water
Mean = 0.546
Figure2: Simulateddistributionofthe
+
distanceof ightinwaterfromthe
18 Fspectrum
MeV
0.6 0.65 0.7 0.75 0.8 0.85 0.9 0.95
1
1.05
0
200
400
600
800
1000
1200
1400
Nent = 3678
Nent = 3678
Figure 3: Simulated deposited energy spectrum for real coincidences with a
mm
-10
-8
-6
-4
-2
0
2
4
6
8
10
0
2
4
6
8
10
12
14
16
18
Chi2 / ndf = 3.049 / 7
Constant = 17.05 +- 2.409
Mean = 0.07941 +- 0.1001
Sigma = 0.8865 +- 0.07811
Chi2 / ndf = 3.049 / 7
Constant = 17.05 +- 2.409
Mean = 0.07941 +- 0.1001
Sigma = 0.8865 +- 0.07811
Figure4: Space resolutionfor
18
Feventssimulatedwithawaterphantom
4 R&D phase
4.1 Testof aHamamatsuPMT
Apositionsensitivephoto-multipliertube,equippedwithaquartzwindowand
aRbCs photo-cathode , (HAMAMATSU R5900-00-C12)wastested with the
set-upshownin gure5. Itsanodeiscomposedoftwoplanesofcrossedplates
which enable us to detect the x and y barycenters of the light pulses with a
verygoodresolution. Theprimaryaimofthesetestsistomeasuretheintrinsic
spatialresolutionofthisPMTat=180nm.
The outputsignalsfrom the crossed-plateanodes areampli edand
un-dergoAnalog-to-DigitalConversion(Fig. 5). Then thesesignalsareread out
byacomputerfordigitalprocessingtolocatethecenter ofgravity.
Theresultspresentedin this paperwereobtainedatroom temperature.
ADC
COMPUTER
POSITION SENSITIVE PMT
X-Y STAGE
FILTER AT 180 nm
UV
LAMP
X1
X2
Y1
HT
PREAMP
Y2
QUARTZ OPTICAL FIBER
Figure 5: Schematic blockdiagramforpositiondetectionset-up.
div.
1000
1020
1040
1060
1080
1100
1120
Counts
0
10
20
30
40
50
Chi2 / ndf = 26.56 / 32
Constant = 42.77 +- 2.23
Mean = 1038 +- 0.320
Sigma = 5.901 +- 0.208
X bary.
sigma = 5.9 div.
0.018 mm/div.
Chi2 / ndf = 26.56 / 32
Constant = 42.77 +- 2.23
Mean = 1038 +- 0.320
Sigma = 5.901 +- 0.208
A liquid xenon cryogenic system is being built by the DTA branch of Air
Liquide. Itwill beready inafewweeks. Weexpect the rstresultswith this
systembefore theend of the year. It will allow us to liquefyand monitorin
temperatureupto5lofLXe. Thissystemcanbeusedforprototypingaswell
asfortheconstructionofasmalldevice(micro-PETcamera).
5 Conclusions and perspectives
Thepreliminaryresultsofafullsimulationallowedustodeterminetheintrinsic
performance of this camera. For apoint-like sourcelocated at the center of
thetomograph,wehaveshownthattheintrinsicachievablespaceresolutionis
2:1mm.
Thenextstepinthesimulationwillbetostudyandoptimizetheinstrumental
response of the camera so as to limit the degradation of its intrinsic space
resolutionasmuchaspossible. Itincludesthesimulationofthelightcollection
inopticalguides(Altubes,quartztubes...).
As a rst result of our R&D activities, we may conclude that the position
sensitive PMT Hamamatsu R5900 is a good candidate for the detection of
light at =180 nm. This has to be con rmed at the temperature of liquid
xenon(165 K). In parallel,wealso envisageto test Si photodiodes equipped
withquartzwindows.
Asthetestcryostatandtheliquidxenonstationwillbeshortlyoperationalat
thelaboratory, weforeseeto build and test asmall prototypecell (2 x 2x3
cm 3
)at thebeginningof2001, soastocon rmtheinstrumentalperformance
obtainedbysimulation.
6 Acknowledgements
Theauthorswould liketo express theirgratitudeto their medicalcolleagues,
J.F.Le Bas and D. Fagret , who by their solid support have contributed to
make this projectstart. We address special thanks to P. de Saintignonwho
contributedtothedesignofthecryogenicequipments. Wealsowish tothank
thetechnicalsta ofISNandin particular: J.Ballon,J.Berger,C.Barnoux,
theJoseph FourierUniversityofGrenoble.
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