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The ClearPET/XPAD project :
Development of small animal PET/CT scanner

Introduction

While X-ray computerized tomography (CT) allows imaging the mass density of living tissues using an external X-ray tube, positron emission tomography (PET) is imaging gamma rays directly emitted from the tissues. In this case, the emission of gamma rays results from the decay of radioactive nuclei used to label a radiopharmaceutical injected to the patient. Thus, in the contrary to transmission X-ray tomography, emission tomography images the function rather than the structure of living tissues. The development of combined PET/CT imaging systems led to a rapid expansion of this technique in clinical routine. Similarly, the development of dedicated PET scanners and micro-CT scanners for small animals pleads for joining these two modalities in a common gantry.
Rat brain 18FDG scan
Rat brain 18FDG scan
The ClearPET build in Lausanne
The ClearPET build in Lausanne
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However, like with clinical PET/CT, the juxtaposition of both modalities does not allow for extrapolating the exact position of the animal during the PET scan. Therefore, we intend to combine the ClearPET detector1 with the XPAD3 X-ray hybrid pixel detector2,3 in a common rotating gantry. To achieve this goal, we redesigned the partial ring arrangement of the ClearPET detectors. As a result, both detection systems will be merged together with an X-ray tube in a fully integrated PET/CT device that will make it possible to acquire simultaneous emission and transmission scans for mice.The small animal ClearPET scanners have been developed during the past years within the Crystal Clear Collaboration4. All scanners use the same detector technology based on the combination of crystal matrices coupled with multi-anode photomultiplier tubes (MaPMTs).
Since the sensitive area of the MaPMTs is smaller than their outer dimension, there is always an empty space between two axially adjacent detector blocks. This leads to empty slices in the sinograms, which cause loss of scanner sensitivity and severe artefacts in the images. In its original configuration, the ClearPET demonstrator consists of 18 LSO/LuYAP phoswich detector heads arranged in three adjacent partial rings. Each ring consists of 6 detector modules mounted asymmetrically so that only two modules are facing each other (see left picture). The new configuration consists of 21 heads, it still uses partial rings but adjacent and therfore allows to virtually remove the dead space between modules (see right picture), thus increasing the sensitivity.
Old and new PET detectors configuration
Old (left) and new (right) detectors configuration
View of the bi-modal scanner
View of the bi-modal scanner
The new PET detectors configuration is implemented with three partial rings of 7 detector modules. It provides a PET modality axial FOV of 54 mm and a transverse FOV of 110 mm. The CT components are somewhat similar to those used for the PIXSCAN. It consist of an X-ray tube from RTW with 50 µm focal spot size and the 118×76 mm2 XPAD3 X-ray imager with 130×130 µm2 pixels which are positioned within the PET detector ring. The microCT modality has an axial FOV of 59 mm and a transverse FOV of 38 mm with a magnification by a factor 2. The X-ray cone beam passes through the shared FOV without irradiating the PET detectors directly. Hence, the ClearPET/XPAD system will allow for simultaneous scanning of small animals.
The influence of scattered X-ray on the operation of the LSO/LuYAP phoswich detectors modules is currently being studied using GATE simulations and experimental measurements. It has been shown that with an adapted shielding of the PET detectors, the influence of X-ray is reduced enough to operate the PET detectors.
GATE simulation with Shielding and Gamma or Xrays.
GATE simulation with shielding and Gamma (left) or Xrays (right).
Possible layout of the first level of the ClearPET/Xpad
Draft of the first level of the ClearPET/XPAD
The ClearPET/XPAD dual-modality scanner will be mounted on the former ClearPET rotating gantry with some modifications in order to be able to use both modalities simultaneously.
The mechanics is being re-design and re-build to add the X-Ray source, the X-PAD detector and the shielding on the former ClearPET gantry. Due to the increase of the number of elements to be placed on the gantry, it has been decided to have a second circular plate, on the top of the main plate. The various elements needed for both the PET detectors operation and the X-ray micro scanner will thus be distributed among those two levels.
At the same time, using a table top set-up, the ClearPET DAQ is also partially being re-design to take into acount the new PET detectors configuration. In particular instead of only one embeded PC taking the data, there will by four of them, to cope with the increased number of sectors in the new configuration. This imply a modification of the communication software between the main control PC and the embeded PCs.
Table set-up of the ClearPET in its new configuration
Table top set-up of the ClearPET in its new configuration
View from the entrance of the future application room
View from the entrance of the future application room
In paralell, the CPPM has started the procedure to get a fully equiped application room for using PET-CT in the appropriate safety conditions, as required by the law. A first phase has been the definition of the required specifications for such a room. This led to a feasability study and quotation. Our both funding institutions, Université de la Méditerranée and CNRS, now agreed to fund respectively the equipment needed for such a facility, and the infrastructure realisation. We are now in the proccess of getting the infrastructure done by a specialized company and ordering the equipment.

1 J.-B. Mosset et al. Development of an optimized LSO/LuYAP phoswich detector head for the Lausanne ClearPET demonstrator. IEEE Trans. Nucl. Sci. 53 (2006) 25-29.

2 P. Delpierre et al. XPAD: A photons counting pixel detector for material science and small-animal imaging. Nucl. Instrum. Meth. A 572 (2007) 250-253.

3 P. Pangaud et al. XPAD3: A new photon counting chip for X-ray CT-scanner. Nucl. Instrum. Meth. A 571 (2007) 321-324.

4 K. Ziemons et al. The ClearPETTM project: Development of a 2nd generation high-performance small animal PET scanner. Nucl. Instrum. Meth. A 537 (2005) 307-311.

5 M. Rey et al. Count rate performance study of the Lausanne ClearPET scanner demonstrator. Nucl. Instrum. Meth. A 571 (2007) 207-210.

6 S. Jan et al. GATE : A simulation toolkit for PET and SPECT. Phys. Med. Biol. 49 (2004) 4543-4561.


This work is supported by the ANR Programme « Chaire d'excellence 2005 » under grant No. ANR-05-CEXC-01 and by the Conseil Régional PACA.
It was labelled in 2006 by the pole of competitiveness OPTITEC.
This is one of the CERIMED projects, carried out in collaboration with the IBDML.


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