1. a liquid xenon Compton camera
3 g medical imaging with
a liquid xenon Compton camera
and Scandium-44 radionuclide.
J.P. Cussonneau on behalf of XEMIS project
Subatech (IMT-Atlantique, In2p3/CNRS, Nantes University)

2. Outline XEMIS project 3γ Imaging XEMIS1: performances
XEMIS2: small animal imaging
Image - Simulation
Conclusion

3. XEMIS (Xenon Medical Imaging System)
(β+, γ) emitter for functional imaging, e.g. 44Sc
• Liquid xenon Compton camera
Time projection chamber (TPC)
- Direct 3D location of the radioactive source
- Administered dose reduction &/or shorter acquisition time
XEMIS1
R&D
XEMIS2
XEMIS3
Human body
imaging
Small animal imaging
3γ : 20 kBq
100+ times less of injected activity
Large field-of-view
High sensitivity
30 kg LXe
200 kg LXe

4. 3 imaging with Compton telescope
(E1, x1, y1, z1)
(E2, x2, y2, z2)
LOR θ Δ
γ 511keV
Compton Telescope
 E0
Cone
Emitter (β+,γ)
Reconstructed  direction
Spatial resolution cone axis (∆)
Energy resolution opening angle ()
- Direct 3D location of the radioactive source: res. along LOR ~ 1 cm (FWHM)
- Reduction of injected activity: 100 times less
Both new radiopharmaceutical and new camera technology !

5. 3g imaging using 44Sc based pharmaceuticals
44Sc is the best candidate
β % γ e+
Emission β+
Emax = MeV
T1/2 = 4 h
Range in H2O ~ 2.8mm
B.R. ~ 100% MeV 
Good for Compton imaging
Fast emission
Precise time coincidence
Production: ARRONAX cyclotron
Radiopharmaceutical : CRCINA/INSERM
Ready for tests with
[44Sc]-DOTA

6. XEMIS1: Compton camera XEMIS1 is a single phase liquid xenon TPC
Field rings
XEMIS1 is a single phase liquid xenon TPC
PMT
30 kg ultra pure -110°C
Active volume 2.5 x 2.5 x 12 (or 6) cm3
1” square UV sensitive PMT
Segmented anode (2.5 x 2.5 cm2 active) in 64 pixels
Frisch grid. Gap 0.5 (or 1) mm
Field shaping rings for homogeneous drift field up to 2.5 kV/cm
12 cm
Frisch grid
Anode

7. Why liquid xenon? Neon Argon Krypton Xenon LXe provides: Atomic number10 18 36 54
Density (g/cm3)
1.2
1.4
2.4 3
Boiling Point (K)
27.1
87.3
119.8
165.0
Light yield (UV/MeV) ( )
30000
40000
25000
42000
Ionization yield (/MeV) ( )
46000
49000
64000
Decay Time (ns)
10, 15400
6.3, 1500
7.0, 85
2.2 , 27 , 45
Wavelength (nm) 85
128
150
178
J.A. Nikkel et al JINST
LXe provides:
Simpler cryogenics
Simultaneous production of a scintillation and an ionization signal
Fast decay, high scintillation light yield and high ionization yield

8. XEMIS1: Time Projection Chamber
Particle interaction in the active volume produces prompt scintillation light and ionization electrons
Scintillation light (PMT) t0
LXe γ
Field rings
Cathode
Anode
PMT
LXe
PMT
Fast scintillation
Decay times: 2.2, 27, 45 ns +
Cathode
Ionization
(Frisch grid, Anode, FEE)
Energy + (x, y) + t1
LXe
λ = 178 nm
λ = 178 nm
Field rings UV z
Vdrift is known ~ 2 mm/ms
Z = vdrift.(t1 - t0)
Frisch Grid
Anode γ
Energy + 3D Positions
of each interaction z x y

9. XEMIS1: FEE - Ionization read-out
Noise (RMS): 85 +/- 5 e C )
IDEF-X Asics Developed for IrFU (Gevin et al. 2006)
Adapted by Subatech for LXe
511 keV kV/cm)  e-
LXe
Anode 64 pixels
3.1 x 3.1 mm2

10. Ionization : signal @ 511 keV for Photoelectrics
1 ch. = 80 ns c

11. Ionization : signal @ 511 keV for Compton
1 ch. = 80 ns c

12. XEMIS1: Ionization results @ 511 keV
Energy res.(s)= 4.92 ± kV/cm
Primary electron
E  102 keV
thermalization
Secondary electrons
Tertiary electrons E2 E1 E0
Thomas et al Phys. Rev. A
Recombination in LXe:
E0  5 eV
E1  5 keV
E2  20 keV
Thomas Imel Model is not so good!
MC simulations required… NEST/Geant4
M Szydagis et al Jinst. , Oct 2011

13. XEMIS1: Drift time – spatial resolution
Photoelectric 511 keV and 1 kV/cm
Beginning γ
End
Drift time resolution: ∼ 50 ns
Z resolution: ∼ 100 μm
Drift velocity ~ 2 mm/ms

14. XEMIS1: Angular resolution
Acceptance & resolution limited by active area of XEMIS1
Improvement expected at higher electric field
XEMIS2 is the key
Equivalent to ΔL = 8.2 mm (FWHM) for a 5 cm distant source α
0.75 kV/cm
Source: 22Na (E0 = MeV)
BaF2
BaF2
LXe TPC
Gallego et al. NIMA 2015

15. XEMIS2: Small animals imaging
Purification
ReStoX
XEMIS2 Camera
Purification
ReStoX
XEMIS2
XEMIS2 at Nantes Hospital:
1st image: 1st semester 2018
preclinical researches: until 2020

16. Install a liquid xenon camera in a hospital ?
XEMIS2: ReStoX
ReStoX:
Recovery and Storage system of Xenon
Scientific collaboration:
Install a liquid xenon camera in a hospital ?
XEMIS2
ReStoX
Xenon cryogenics of XEMIS2 :
- compact (210 kg capacity)
safe (from -110°C to RT)
High cooling power LN2(10 kW)
ultra clean (~ppb impurities)

17. XEMIS2 : ReStoX commissioning
~9.5 days
In case of LN2 failure, RT should be reached in ~ 1 year

18. Commissioning: camera filling (XEMIS2 is pre-cooled @ LXe temp.)
125 kg filled in ~ 1 hour at 30 nl/mn thanks to the pump. GXe flows through getters

19. Commissioning : assisted gravity recovering
Bypass of the circulation circuit, direct connection of gaseous phases
LN2 valve closed on ReStoX
End of LXe Recovering
125 kg LXe recovered in less than 9 mn ! (w/o cooling power)

20. From XEMIS1 to XEMIS2
XEMIS2: Full liquid xenon cylindrical camera dedicated to small animal imaging
Scintillation
What we have learnt from XEMIS1:
380 x 1’’ PMTs in LXe
LXe: 200 kg
cathode
- Drift time resolution: < 50 ns
- 3D spatial resolution: < 1 mm
- Energy resolution (s): ~ 511 keV
- Very low elec. noise: 85 e- (~ 1.5 keV)
- Ionization, attenuation length: > 1 m
Field Rings
LXe
Air
120mm
120mm
- Frisch grid / gap properties affect:
- Charge collection
- Signal shape
- Charge and time measurement
Vacuum
LXe TPC
Ionization
Active volume
‐ axial : 2 x 12 cm
‐ radius : 7 -> 19 cm
- Optimal time and charge measurement
→ CFD method: XTRACT front-end
pixels: 3.1 x 3.1 mm

21. XEMIS2 : DAQ for Ionization
IDeF-X HD_LXe
Imaging Detector Front-end
XTRACT: Xemis TPC Readout for Acquisition of Charge and Time
∼20000 electronic channels
Challenge: continuous read-out with negligible dead-time
XTRACT v2 for summer 2017

22. XEMIS2 : ionization DAQ scheme
Vacuum
LXe
Cooled directly by LXe
Air PU
X 8
IDEF-X
XTRAC
X 32
XTRACT
pixels
6 x FPGA
Board
20480
80
LVDS
> 2m
8 x 32A
2A+1D
640x Idef-X
(32W) 50mW/chip
640x XTRACT
(32W) 50mW/Chip
80 x PU
(40W) 500mW/PU
TIdef-X = -100°C
TXtract = -80°C
TPU = -30°C
Challenge : Continuous read-out with negligible dead-time
Goal : record on disc ~104 charge and time signals/pixel/s

23. XEMIS2: full Gate/Geant4 simulation 3g detection efficiency
MOUSE PHANTOM
Cylinder:
radius = 2.6 cm
length = 12 cm
Sphere:
radius = 5 mm
SOURCE
44Sc
Total: 20.0 kBq
Hot Sphere: kBq
Contrast  = 15
XEMIS2 CAMERA
TPC active area
Radius: 7 -> 19cm
Axial length: 24 cm
Liquid Xenon
Acquisition time :
tA = 20 min
PMTs
Mouse
Phantom
Cathode
Anodes
3g detection efficiency
At least 4 Hits in LXe :
1 for each 511 keV photons
2 for 1157 keV photon
Phantom length
Phantom radius
Quite homogeneous !

24. XEMIS2 image reconstruction 20 kBq, 20 mn
Image reconstruction -> Iterative ML-EM assuming Poisson Distribution
Reconstructed 23 iterations. Central transverse slices (x,y)
Simulated true distribution
It’s completely new and it works !
Expected resolution : [2-3 mm] all over the field of view
A lot of works for the future …

25. Conclusion
3g imaging can be a new medical modality thanks to LXe technology.
Big challenges and innovations for the design of the camera.
XEMIS2 is under construction and will demonstrates the potential of LXe for medical imaging.
Expected image qualities are very promising:
- very low injected activity,
- good spatial resolution all over the FOV,
- reduced acquisition time on a large FOV.
LXe technology is scalable: design of large whole body camera could be investigated.