Final Report Summary - ASPID (Application of Silicon Photomultipliers to Imaging Detectors)
Positron emision tomography (PET) is a medical imaging technique widely applied in oncology, neurology and cardiology and in preclinical research with small animals (mice and rats). Preclinical tests are necessary in the development of new drugs and the study of many diseases. Tests in mice are an essential step in these studies. However, their small organs require the use of scanners with excellent resolution and efficiency.
With the contribution of the ASPID project, a prototype of a high resolution small animal PET tomograph has been constructed and tested at the Instituto de Física Corpuscular, in Valencia, in collabouration with the University of Pisa and INFN Pisa. This is the first prototype scanner built with the novel photodetectors known as silicon photomultipliers and continuous scintillator crystals.
PET detectors are generally composed of a pixellated crystal array coupled to position-sensitive photomultiplier tubes (PMTs). Silicon photomultipliers are solid state photodetectors that in the last eight years have experienced enormous advances, and can currently be purchased from several manufacturers. In many fields of physics research, they are under study to replace conventional PMTs, given that in addition to their high gain and fast response, they are compact, rugged and cheap in comparison to PMTs.
In pixellated PET detectors, spatial resolution is given by the size of the crystals. In order to achieve a resolution better than 1 mm, the crystals must be very small, increasing the difficulty and cost of production. In addition, a larger number of crystals implies a larger amount of material separating the crystals and thus an increase of the dead area that results in a reduction of the efficiency.
Continuous crystals avoid the loss in efficiency and allow to achieve a high spatial resolution if coupled to highly segmented photodetectors. However, they impose several challenges that must be investigated to achieve a high benefit. Within the ASPID project, the use of continuous crystals coupled to SiPMs has been investigated, achieving excellent results.
PET detectors composed of a LYSO continuous crystal of size 12 mm x 12 mm x 5 mm and 12 mm x 12 mm x 10 mm coupled to monolithic SiPM matrices have been assembled and characterised. The matrices, fabricated by FBK-irst, are made of 64 (8 x 8) SiPM elements, implemented in a common silicon substrate to minimise the dead area between the detectors. Initial characterisation tests were carried out with crystals painted in three different ways: five faces painted white (reflecting configuration), five faces painted black (absorbing configuration) and lateral faces painted black and back face painted white (mixed configuration). The results showed that a superior energy resolution was achieved with the white crystal while a good determination of the interaction position could be achieved, so they were the selected option for the prototype development [1].
The determination of the interaction position in the crystal is one of the main issues of employing continuous crystals. With the centre-of-gravity or Anger method, in which the position is calculated as the average of the centre positions of the photodetector elements that give a signal, weighted with the signal size, the interaction positions close to the crystal edges are artificially biased towards the centre of the crystal. After testing diferent position determination algorithms, a method has been chosen that is based on an analytic model of the solid angle subtended by the interaction position with the different photodetector elements [2]. This method can determine not only planar coordinates of the interaction position (x, y), but also the depth of interaction (DoI) in the crystal, that helps to avoid artifacts in the images due to the parallax error. A resolution of 0.7 mm FWHM has been achieved in (x, y), and tests are ongoing for a precise determination of the DoI.
The prototype assembled consists of two detector heads attached to a rotating stage. Na-22 radioactive sources are placed in the central region of the scanner, and the detector heads rotate around taking data in different positions in order to cover 180º each head. Images are reconstructed both with conventional MLEM image reconstruction algorithms, after applying a virtual pixellisation of the detectors, and also using the simulated one-pass list mode (SOPL) algorithm [6], in order to take full advantage of the continuous data and avoiding any discretisation step. SOPL is based on multiray sampling where each ray-endpoint is obtained from an estimated position including a Gaussian model of uncertainty with 0.7 mm FWHM, as measured in the position determination tests. System matrix elements are calculated on-the-fly and data for reconstruction are stored in list-mode format. Images of point-like sources have been reconstructed and a resolution of 0.8 mm FWHM has been achieved.
Tests with derenzo-like phantoms are planned to estimate the resolution in PET images. Future plans include the prototype optimisation and to estimate the possibility of employing the scanner in preclinical studies with mice.
The ASPID project has contributed to achieve excellent results in frontier research in medical physics, achieving sub-millimeter resolution in a novel detector concept, and it has settled the basis to continue the work for further improvements. The research carried out will contribute to the development of PET imaging scanners with better resolution and efficiency, with a significant impact in preclinical research. The same concept can be applied to human scanners with high potential benefits in medical diagnosis.
[1] Characterisation of a PET detector head based on continuous lyso crystals and monolithic, 64-pixel silicon photomultiplier matrices. G. Llosá, J. Barrio, C. Lacasta, M. G. Bisogni, A. D. Guerra, S. Marcatili, P. Barrillon, S. Bondil-Blin, C. Taille, C. Piemonte. Phys. Med. Biol., 2010, volume 55, pages 7299 - 7315, number 23.
[2] Position reconstruction in detectors based on continuous crystals coupled to silicon photomultiplier arrays. J. Cabello, J. Barrio, C. Lacasta, M. Rafecas, G. Llosá. Nuclear Science Symposium and Medical Imaging Conference (NSS/MIC), 2011 IEEE, 2011, pages 3911 - 3916.
[3] Simulated one-pass list-mode: A highly flexible method of image reconstruction for PET. J. E. Gillam, P. Solevi, J. J. Oliver, M. Rafecas. Nuclear Science Symposium and Medical Imaging Conference (NSS/MIC), 2011 IEEE, 2011, pages 4206 - 4210.
With the contribution of the ASPID project, a prototype of a high resolution small animal PET tomograph has been constructed and tested at the Instituto de Física Corpuscular, in Valencia, in collabouration with the University of Pisa and INFN Pisa. This is the first prototype scanner built with the novel photodetectors known as silicon photomultipliers and continuous scintillator crystals.
PET detectors are generally composed of a pixellated crystal array coupled to position-sensitive photomultiplier tubes (PMTs). Silicon photomultipliers are solid state photodetectors that in the last eight years have experienced enormous advances, and can currently be purchased from several manufacturers. In many fields of physics research, they are under study to replace conventional PMTs, given that in addition to their high gain and fast response, they are compact, rugged and cheap in comparison to PMTs.
In pixellated PET detectors, spatial resolution is given by the size of the crystals. In order to achieve a resolution better than 1 mm, the crystals must be very small, increasing the difficulty and cost of production. In addition, a larger number of crystals implies a larger amount of material separating the crystals and thus an increase of the dead area that results in a reduction of the efficiency.
Continuous crystals avoid the loss in efficiency and allow to achieve a high spatial resolution if coupled to highly segmented photodetectors. However, they impose several challenges that must be investigated to achieve a high benefit. Within the ASPID project, the use of continuous crystals coupled to SiPMs has been investigated, achieving excellent results.
PET detectors composed of a LYSO continuous crystal of size 12 mm x 12 mm x 5 mm and 12 mm x 12 mm x 10 mm coupled to monolithic SiPM matrices have been assembled and characterised. The matrices, fabricated by FBK-irst, are made of 64 (8 x 8) SiPM elements, implemented in a common silicon substrate to minimise the dead area between the detectors. Initial characterisation tests were carried out with crystals painted in three different ways: five faces painted white (reflecting configuration), five faces painted black (absorbing configuration) and lateral faces painted black and back face painted white (mixed configuration). The results showed that a superior energy resolution was achieved with the white crystal while a good determination of the interaction position could be achieved, so they were the selected option for the prototype development [1].
The determination of the interaction position in the crystal is one of the main issues of employing continuous crystals. With the centre-of-gravity or Anger method, in which the position is calculated as the average of the centre positions of the photodetector elements that give a signal, weighted with the signal size, the interaction positions close to the crystal edges are artificially biased towards the centre of the crystal. After testing diferent position determination algorithms, a method has been chosen that is based on an analytic model of the solid angle subtended by the interaction position with the different photodetector elements [2]. This method can determine not only planar coordinates of the interaction position (x, y), but also the depth of interaction (DoI) in the crystal, that helps to avoid artifacts in the images due to the parallax error. A resolution of 0.7 mm FWHM has been achieved in (x, y), and tests are ongoing for a precise determination of the DoI.
The prototype assembled consists of two detector heads attached to a rotating stage. Na-22 radioactive sources are placed in the central region of the scanner, and the detector heads rotate around taking data in different positions in order to cover 180º each head. Images are reconstructed both with conventional MLEM image reconstruction algorithms, after applying a virtual pixellisation of the detectors, and also using the simulated one-pass list mode (SOPL) algorithm [6], in order to take full advantage of the continuous data and avoiding any discretisation step. SOPL is based on multiray sampling where each ray-endpoint is obtained from an estimated position including a Gaussian model of uncertainty with 0.7 mm FWHM, as measured in the position determination tests. System matrix elements are calculated on-the-fly and data for reconstruction are stored in list-mode format. Images of point-like sources have been reconstructed and a resolution of 0.8 mm FWHM has been achieved.
Tests with derenzo-like phantoms are planned to estimate the resolution in PET images. Future plans include the prototype optimisation and to estimate the possibility of employing the scanner in preclinical studies with mice.
The ASPID project has contributed to achieve excellent results in frontier research in medical physics, achieving sub-millimeter resolution in a novel detector concept, and it has settled the basis to continue the work for further improvements. The research carried out will contribute to the development of PET imaging scanners with better resolution and efficiency, with a significant impact in preclinical research. The same concept can be applied to human scanners with high potential benefits in medical diagnosis.
[1] Characterisation of a PET detector head based on continuous lyso crystals and monolithic, 64-pixel silicon photomultiplier matrices. G. Llosá, J. Barrio, C. Lacasta, M. G. Bisogni, A. D. Guerra, S. Marcatili, P. Barrillon, S. Bondil-Blin, C. Taille, C. Piemonte. Phys. Med. Biol., 2010, volume 55, pages 7299 - 7315, number 23.
[2] Position reconstruction in detectors based on continuous crystals coupled to silicon photomultiplier arrays. J. Cabello, J. Barrio, C. Lacasta, M. Rafecas, G. Llosá. Nuclear Science Symposium and Medical Imaging Conference (NSS/MIC), 2011 IEEE, 2011, pages 3911 - 3916.
[3] Simulated one-pass list-mode: A highly flexible method of image reconstruction for PET. J. E. Gillam, P. Solevi, J. J. Oliver, M. Rafecas. Nuclear Science Symposium and Medical Imaging Conference (NSS/MIC), 2011 IEEE, 2011, pages 4206 - 4210.