CdZnTe Detectors for the Positron Emission
Tomographic Imaging of Small Animals
Arnaud Drezet, Olivier Monnet, Guillaume Montémont, Jacques Rustique, Gérard Sanchez, Loïck Verger*
Abstract—We report the timing performances between two
cadmium zinc telluride (CZT) detectors equipped with a specific
electrode geometry and a dedicated electronic setup for Positron
Emission Tomography (PET) application. The measured
coincidence times reach 2,6ns Full Width at Half Maximum
(FWHM) for a 500V bias voltage and 300keV energy threshold.
Subsequently, a simulation study was carried out to assess the
spatial and efficiency performances of these detectors which allow
the Depth Of Interaction (DOI) measurement. Preliminary results
show that the proposed design could reach a resolution better
(1mm Full Width at Half Maximum instead of 2,5mm in the
middle of the field of view) than what is achieved with the
standard PET scintillation detectors. Experimental confirmation
of this result is in progress. µ
Index Terms—CdZnTe, positron emission tomography, timing
resolution, depth of interaction measurement.
I.
INTRODUCTION
last 10 years, the development of dedicated
Oanimalthescanners
for Positron Emission Tomography
VER
(PET) imaging has been acknowledged as a necessity in many
medical fields : research on new radiopharmaceuticals [1],
validation of kinetic and disease models for clinical PET
[2],[3] or studies of genetically modified rodents [4]. Most of
the time, these dedicated systems are based upon the structure
of clinical tomographs, especially as far as the detectors are
concerned. Though, imaging small animals presents some
specificities compared to scanning human patients. The major
difference lies in the required spatial resolution. Achieving at
least 1-2mm FWHM resolution is compulsory to perform well
in the application fields mentioned above [1]. Among other
consequences, this entails the need for finely segmented
detectors placed as close as possible to the imaged animal.
With standard PET materials, namely inorganic scintillators
like Bismuth Germanate (BGO), Lutetium Orthosilicate (LSO)
or Lutetium Aluminum Perovskite (LuAP), very fine twodimensional square segmentation are now achieved (0,975mm
for microPET II [5] or 0,8mm for the MiCES PET system [6]).
The authors belong to LETI-CE Recherches Technologies, CEA Grenoble,
17 rue des Martyrs, 38054 Grenoble cedex 9, France.
(*corresponding author : loick.verger@cea.fr)
The main limit for this kind of material lies in the lack of
segmentation in the depth of the detector. Indeed, the third
dimension pixellisation is an important requirement for a small
diameter system : if the gamma ray to be absorbed is
perpendicular to the face of a detector, the depth of the latter
does not contribute to the degradation of the spatial resolution.
But if the gamma ray is emitted from an off axis point in the
transaxial plane of the tomogaph, it can penetrate in a detector
from its side, thus creating a severe uncertainty in the original
annihilation position, commonly referred to as the depth of
interaction (DOI) problem.
To overcome this DOI effect in scintillators, a huge variety of
stratagems has been experimented since the initial design
proposed by Wong [7]. Today, the most intensively
investigated approach consists in a segmented version of the
phosphor-sandwich (phoswich) detector concept. Thanks to
this technique, the length of the crystal can be divided into two,
three or four pieces. Yet, the phoswich design suffers from
important drawbacks (complex and time-consuming
fabrication, degradation of the light signal at the crystal
interfaces) and does not allow to attain very fine depth
resolution.
What we propose here is to study the suitability of cadmium
telluride (CdTe:Cl and CdZnTe) detectors to address the
spatial resolution issue associated with small animal PET
systems. The main advantage of semi-conducting detectors
over scintillation detectors lies in the fact that the segmentation
can be obtained very easily thanks to the electrode
pixellisation. Subsequently, the segmentation pitch can be
chosen as small as desired. Furthermore, cadmium telluride
shows high intrinsic stopping power features for 511keV
photons (see Table I).
TABLE I
CADMIUM TELLURIDE STOPPING POWER PROPERTIES AT 511KEV
Properties
Atomic Number
Density (g.cm-3)
Attenuation Coefficient (cm-1)
Photofraction (%)
CdTe:Cl
48-52
6
0.51
18
CdZnTe
48-30-52
6.2
0.57
18
It is commonly acknowledged that one drawback of semiconducting detectors is their alleged insufficient mobility for
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PET [8]. In the past 17 years, several studies were undertaken
to evaluate the timing performances of CdTe and CdZnTe
pertaining to the PET application [9] – [19]. The results
obtained, with a mean coincidence value around 10ns FWHM
for cadmium telluride detectors of small volume, can be
considered to be promising, though not entirely satisfying. To
this connection, it is worth noticing that all these research
works focused on the optimization of the physical and
geometrical characteristics of the detectors, but did not study
the influence of the electronic setup, specifically the crucial
preamplifier stage.
In this work, we present the results of the optimization of the
electronic setup in terms of improvement of the coincidence
timing response between cadmium telluride detectors. We also
report on simulations carried out thanks to the Monte Carlo
package Penelope to assess the efficiency and the spatial
resolution reachable by CdTe detectors.
II. DETECTOR GEOMETRY AND PREAMPLIFIER STAGE
To obtain the fastest response from the CZT detectors without
jeopardizing their detection efficiency, we choose and adapt
the planar transverse field configuration presented in [20] for
the electrode geometry. In our case, both faces have stripped
electrodes as shown in Fig. 1.
Fig. 2. Experimental setup to measure the timing resolution between two
cadmium telluride detectors in coincidence (Spectrometric amplifier : Ortec
Model 570, Time to Amplitude Converter : Ortec Model 467, Multi-Channel
Analyzer : Canberra AccuSpec). The home-made x10 amplifier stages are
needed to provide the Constant Fraction Discriminator (Fast Comtec Model
7029A) with signals of sufficient amplitude.
Several studies were undertaken to assess the role played by
different parameters (energy threshold, inter-electrode
distance…) in the timing resolution finally obtained. The most
striking results are presented in Fig. 3. They deal with the
influence of the applied bias voltage between the detector
electrodes on the coincidence timing resolution. These
measurements are performed with all anode strips connected to
each other, as well as all cathode strips.
At 500V, we measure a timing resolution of 2,6ns FWHM.
This result suits well with preliminary measures carried out
between a CZT detector and a BaF2 crystal (1,9ns FWHM),
and is compatible with the timing window of a PET scanner of
the order of 10ns.
Fig. 1. Design of the cathode and anode faces. Detectors are 20mm long,
16mm wide, and the inter-electrode distance is 0.9mm. The anode and cathode
strips are 0.9mm (pitch 1mm) and 3.9mm (pitch 4mm) wide, respectively. The
cathode segmentation allows the DOI measurement.
The customized preamplifier is composed of a low noise
(10-20C.Hz-1/2 at 20MHz when connected to a 10pF capacity
detector) charge-sensitive amplifier with a fast differentiator
stage delivering output signal rise times as short as 6ns.
III. EXPERIMENTAL RESULTS
The setup consists of two CZT detectors arranged on opposite
sides of a 68Ge source emitting two back-to-back 511keV
photons. We use standard nuclear instrumentation modules as
shown in Fig. 2 [14].
Fig. 3. Coincidence timing resolution between two CZT detectors as a
function of applied bias voltage.
IV. SIMULATION RESULTS
We use the Monte Carlo package Penelope [21] to simulate the
3D detection module, shown in Fig. 4, and to evaluate its
efficiency and spatial resolution performances. It corresponds
approximately to 2 arrays of 16 piled up detectors presented in
Fig. 1.
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We compare the CZT results, with and without depth of
interaction (DOI) measurement, to what is obtained with three
scintillator detector modules, namely those from ATLAS,
microPET Focus and microPET 2 [5], conducting an almost
equivalent simulation procedure (see Fig. 6, 7 and 8). The main
difference is that we use a barycentric algorithm to reconstruct
the interaction location in the case of scintillators instead of
having a direct electronic localization in the voxels.
Fig. 4. Schematic view of the simulated CZT detector. The 16x16 module is
16mm wide, 16mm high, 40mm deep, and is divided into 2560 voxels.
The one-dimensional photon beam created by a punctual
radiation source perpendicularly hits the detector surface on the
central voxel. Compton and photo-interactions occur in the
material, and we monitor the voxels in which an energy
superior to a predetermined threshold is deposited. If one or
two voxels are excited above this threshold, we derive the
transaxial spatial resolution reachable with the detector from
their localization. The simulation procedure is summarized
through the Fig. 5 shown below.
Fig. 6. Geometry of the simulated LGSO/GSO ATLAS detector. Each crystal
is 2mm wide and 2mm high. The 9x9 module is 18mm wide, 18mm high,
comprises 2 layers of LGSO 7mm and GSO 8mm deep.
Fig. 7. Geometry of the simulated LSO microPET Focus detector. Each
crystal is 1,6mm wide and 1,6mm high. The 12x12 module is 19,2mm wide,
19,2mm high, 10mm deep.
Fig. 8. Geometry of the simulated LSO microPET 2 detector. Each crystal is
1mm wide and 1mm high. The 14x14 module is 14mm wide, 14mm high,
12,5mm deep.
Fig. 5. Illustration of the simulation procedure. The studied detector is
illuminated by a 1D gamma ray beam at the surface of a precise voxel. After
interaction in the detector, a spatial profile is constructed on the radial
transaxial plan by taking into account a reference detector in virtual
coincidence with the simulated detector.
As shown in Fig. 9, the DOI measurement in the CZT detector
considerably increases the spatial resolution when moving off
the center of the field of view. Furthermore, this design could
potentially achieve a resolution more than two times better
(1mm against 2,5mm FWHM) than what is obtained with the a
scintillator detector.
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Fig. 11. 3D representation of simulated parallelepiped (left) and trapezoidal
(right) CZT detectors.
Fig. 9. Comparison of CZT and scintillator transaxial spatial resolution.
As for the detection efficiency (see Fig. 8), the 40mm-deep
CZT detector is slightly superior to the 10mm-deep LSO
detector.
Fig. 12. Comparison of trapezoidal CZT and scintillator detector efficiency.
V. EXPERIMENTAL VALIDATIONS
Fig. 10. Comparison of CZT and scintillator detector efficiency.
To confirm the simulation results, a multichannel experimental
bench has been set up. It consists of two detectors, as
described in Fig. 1, put one behind the other to obtain a total
depth of 40mm. The 16 anodes and 10 cathodes are connected
to 26 identical preamplifiers (see Fig. 12). The generated
signals are fed into customized constant fraction discriminators
put on 8 electronic boards (see Fig. 13), each controlled by an
FPGA. The data are processed through a Labview program.
To improve the efficiency of CZT detectors, especially the
homogeneity of detection across the field of view, we choose to
evaluate a trapezoidal geometry alternative to the common
parallelepiped form (see Fig. 10).
With such a configuration, the CZT detectors offer an
efficiency equal or superior to all the simulated scintillator
detectors. The results are shown in Fig. 11. Though, it remains
to be seen whether such a geometry can be experimentally
exploited.
Fig. 13. Experimental bench in progress (Motherboard with 16 preamplifiers).
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VII. REFERENCES
[1]
[2]
[3]
[4]
[5]
Fig. 14. Experimental bench in progress (Board with 4 CFD).
Only preliminary settings have been carried out so far. They
prove the bench to be functional (see Fig. 14).
[6]
[7]
[8]
[9]
[10]
[11]
Fig. 15. Temporal dispersion among the electronic channels without delay
compensation. The values obtained, around 1,9ns FWHM for CZT/BaF2
coincidences, are consistent with the timing results presented previously
(2,6ns FWHM for CZT/CZT coincidences).
VI. CONCLUSIONS
[12]
[13]
[14]
We have jointly developed a specific three-dimensional
CdZnTe detector geometry and a preamplifier stage to achieve
the best coincidence timing performance between 2 CZT
detectors. The 2,6ns FWHM measure obtained at 500V is
encouraging, and has led us to simulate the spatial performance
of a detector with depth of interaction (DOI) capability.
Preliminary simulations indicate that the proposed design could
outperform an LSO-based system, with a 1mm spatial
resolution instead of 2,5mm FWHM. The efficiency obtained
for parallelepiped detectors, 40mm CZT equivalent to 10mm
LSO, could be greatly improved, especially pertaining to the
homogeneity across the field of view, if a trapezoidal
geometry for the CZT detector could be experimentally
implemented. Simulations at the scale of a whole small animal
PET system coupled to an experimental validation of these
results are under way. This new 3D detector geometry may
open up new vistas for innovative system architectures,
particularly regarding the sensitivity improvement.
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