A silicon photomultiplier (SPM) is a large area detector consisting of a parallel array of photon counting microcells. Each microcell consists of a Geiger Mode photodiode with an integrated quenching element. Each microcell is then connected to a common output. The microcells have a uniform gain of up to 10 and provide an identical charge output signal for each photon detected. Under illumination the summed output of the detector is proportional to the number of Geiger pulses and hence proportional to the incident photon flux. This combination gives extremely high performance comparable to that of a conventional photomultiplier tube (PMT). We report on the characterization of two different 1 mm 2 SPM detector designs with 620 and 920 microcells at room temperature (20 deg) and down to . We assess detection efficiency, breakdown voltage, gain, dark rate, crosstalk, timing jitter and dynamic range. The SPM detector operates over the visible region of the spectrum, characterized here from 400 to 800 nm. The peak photon detection efficiency of 15% occurs at 500 nm with a cooled () dark rate of 600 at a bias voltage of 31 V. In a test for positron emission tomography (PET), an energy resolution of 25% was recorded for the detection of 511 keV gamma radiation using 1 mm1 mm15 mm LYSO scintillator crystal. The SPM has many applications such as medical imaging, microscopy, high-energy physics, and homeland security.

We have grown single YAlO3 crystal fibers using the so-called μ-pulling down technique for scintillation applications. Good quality un-doped and Ce-doped crystal fibers with diameters in the range 0.3–2.5mm and length up to 150mm have been grown. A first structural, spectroscopic and scintillation characterization will be presented.

Small-animal PET systems are now striving for sub-millimetre resolution. Current systems based upon PSPMTs and finely pixellated scintillators can be pushed to higher resolution, but at the expense of other performance parameters and a rapidly escalating cost. Moreover, depth of interaction (DOI) information is usually difficult to assess in such systems, even though this information is highly desirable to reduce the parallax error, which is often the dominant error for such high-resolution systems. In this study we propose a high-resolution detector head for a small-animal PET imaging system with intrinsic DOI information. Instead of a pixellated scintillator, our design is based upon the classic Anger camera principle, i.e. the head is constructed of modular layers each consisting of a continuous slab of scintillator, viewed by a new type of compact silicon photodetector. The photodetector is the recently developed silicon photomultiplier (SiPM) that as well as being very compact has many other attractive properties: high gain at low bias voltage, excellent single-photoelectron resolution and fast timing. A detector head of about 4 × 4 cm2 in area is proposed, constructed from three modular layers of the type described above. We perform a simulation study, using the Monte Carlo simulation package Geant4. The simulation results are used to optimize the geometry of the detector head and characterize its performance. Additionally, hit estimation algorithms are studied to determine the interaction position of annihilation photons correctly over the whole detector surface. The resulting detector has a nearly uniform efficiency for 511 keV photons of ∼70% and an intrinsic spatial resolution of less than ∼0.4 mm full width at half maximum (fwhm).

Positron Emission Tomography (PET) for small animal studies requires high-resolution gamma cameras with high sensitivity. Traditionally, inorganic scintillators are used and, in recent times, coupled to position sensitive PMTs to achieve a higher resolution. Such PSPMTs are costly, operated at high voltage and have a relatively low packing fraction. However, their advantage, compared to current solid state photodetectors, is their high signal-to-noise ratio. The Silicon Photomultiplier (SiPM) is a silicon diode detector that shows great promise as a photodetector for scintillators and hence application in nuclear medicine imaging applications. The microcell MRS (Metal–Resistor–Semiconductor) structure of the SiPM leads to a self-quenching, Geiger-mode avalanche photodiode (GAPD), that produces a large gain (5×105) at low bias voltage (50V) and proportional output for moderate photon flux. Such a compact silicon detector, with a performance similar to a PMT, is obviously well disposed to being developed into a close-packed array in order to have a position-sensitive detection surface. We propose a miniature, high-resolution camera for a small-animal PET imaging system that is based on such an array of SiPM. The design is based upon the classic Anger camera principle; each detector module consists of a continuous slab of scintillator, viewed by a matrix of SiPM. A detector head of 4×4cm2 in area is proposed, constructed from three such modules of the continuous camera described above. The stacked layers would give the system intrinsic depth of interaction (DOI) information. A summary of measured SiPM performance and results of a simulation of the proposed camera, using the Monte Carlo package GEANT4, are presented. It is shown that using three layers of 5mm thick LSO, gives an efficiency of 68% with maximum count rates in the front layers. Intrinsic spatial resolution of <0.4mm FWHM was found although this is degraded at the edges. Although the inclusion of DOI information increases the overall spatial resolution, the parallax error was still found to be the limiting factor in a small animal system.

The operation and performance of multi-pixel, Geiger-mode APD structures referred to as Silicon Photomultiplier (SPM) are reported. The SPM is a solid state device that has emerged over the last decade as a promising alternative to vacuum PMTs. This is due to their comparable performance in addition to their lower bias operation and power consumption, insensitivity to magnetic fields and ambient light, smaller size and ruggedness. Applications for these detectors are numerous and include life sciences, nuclear medicine, particle physics, microscopy and general instrumentation. With SPM devices, many geometrical and device parameters can be adjusted to optimize their performance for a particular application. In this paper, Monte Carlo simulations and experimental results for 1mm(2) SPM structures are reported. In addition, trade-offs involved in optimizing the SPM in terms of the number and size of pixels for a given light intensity, and its affect on the dynamic range are discussed.

A prototype for positron emission mammography is under development at the Department of Physics of Pisa University. The device will be composed of two opposing detectors (parallel plane geometry). The active part of each detector head consists of a matrix of 900 YAP: Ce pixel scintillators, with a 2x2 mm(2) pitch and a 30 mm thickness. The read out is performed by an array of nine metal channel dynode PSPMTS (mod. R8520-00-C12) from Hamamatsu. In the previous version of the head, the PSPMTS were independently read out. For the clinical implementation of the prototype we have designed a simplified circuitry for the readout of the nine tubes based on a multiplexed resistive divider, reducing the number of channels from 36 to 4. A simulation study for an optimised amplifier has been carried out. The housing for each of the two yap-pem detectors has been fully engineered and is in the assembly stage.

The Silicon PhotoMultiplier (SiPM) APD represents an interesting advance in photodetection and could soon be a rival to traditional PMTs in many applications. The SiPM is effectively a densely packed 2D array of Geiger-mode APD microcells, each having individual resistive quenching and multiplexed outputs. In this way the SiPM acts as a linear, high-gain photodetector for moderate photon flux (N-photon < N-cells). The Metal-Resistor- Silicon (MRS) structure SiPM, produced by CPTA Russia, has been characterised and tested for scintillator light detection in medical applications such as PET. We present a summary of measurements of the device's primary operating characteristics and results of the application to scintillator readout. (c) 2006 Elsevier B.V. All rights reserved.

Functional imaging of small animals,such as mice and rats,using high-performance positron emission tomography (PET)and single-photon emission tomography (SPECT), is becoming a valuable tool for studying animal models of human disease. The possibility to use either PET or SPECT allows the scientist to exploit the advantages of both techniques, e.g., the wide range of easily accessible single-photon radiotracers for SPECT and the exquisite performance of PET. The combination of PET and SPECT techniques for small-animal studies could offer the unique possibility of developing new and interesting protocols for the investigation of many biological phenomena more effectively than with PET or SPECT modality alone.

A new type of silicon device has been realised that has many properties comparable to or better than a conventional PMT (photomultiplier tube). This paper presents the first results of using these devices in place of PMT for the readout of scintillators for eventual application in PET (positron emission tomography). This device, the silicon photomultiplier (SiPM) is effectively an avalanche photodiode operated in Geiger mode. In Geiger-mode detectors, a very large current signal is produced regardless of the size of the input, giving just logical rather than proportional information. However, the SiPM is subdivided into a large number (1500) of microcells that act as independent and virtually identical Geiger-mode photodiodes. The outputs of all these individual microcells are connected so that the total output signal is the sum of the signals from all of the microcells that were fired. In this way proportional information can be obtained. As a consequence of their design, these detectors have potentially very fast timing (<100 ps), high gain (105-106) at low bias voltage (~50 V), a high quantum efficiency (35% at 500 nm), excellent single photoelectron resolution and are cheap to manufacture. Here their characterisation and initial results when used with pulsed LED and scintillator pixels are presented

In this study a miniature, high-resolution detector head for a small-animal PET imaging system that has intrinsic depth of interaction (DOI) information is proposed. The design is hased upon the classic Anger camera principle, i.e. one detector module layer consists of a continuous slab of scintillator, viewed by a new type of compact silicon photodetector. The photodetector is the recently developed Silicon Photomultiplier (SiPM) that as well as being very compact has many other attractive properties: high gain at low bias voltage, excellent single-photoelectron resolution and fast timing. A detector head of 4/spl times/4 cm/sup 2/ in area is proposed, constructed from three module layers of the continuous detector described above. Here, a simulation study is carried out, using the Monte Carlo simulation package GEANT4. The simulation results are used to determine the performance of a single detector head and to optimize the geometry of the detector, resulting in a spatial resolution of up to /spl sim/0.6 mm full-width at half maximum (FWHM).

Commercially constructed crystal matrices are characterised for use with PSPMT detectors for PET system developments and other nuclear medicine applications. The matrices of different scintillation materials were specified with pixel dimensions of 1.5×1.5mm2 in cross-section and a length corresponding to one gamma ray interaction length at 511keV. The materials used in this study were BGO, LSO, LYSO, YSO and CsI(Na). Each matrix was constructed using a white TiO loaded epoxy that forms a 0.2mm septa between each pixel. The white epoxy is not the optimum choice in terms of the reflective properties, but represents a good compromise between cost and the need for optical isolation between pixels. We also tested a YAP matrix that consisted of pixels of the same size specification but was manufactured by a different company, who instead of white epoxy, used a thin aluminium reflective layer for optical isolation that resulted in a septal thickness of just 0.01mm, resulting in a much higher packing fraction. The characteristics of the scintillation materials, such as the light output and energy resolution, were first studied in the form of individual crystal elements by using a single pixel HPD. A comparison of individual pixels with and without the epoxy/dielectric coatings was also performed. Then the matrices themselves were coupled to a PSPMT in order to study the imaging performance. In particular, the system pixel resolution and the peak to valley ratio were measured at 511 and 122keV.

In this paper we present some recent results we have obtained in the development of detectors for small animal PET and for PEM, based on the use of Position Sensitive PMTs or Hybrid Photo Diodes (HPDs) coupled to crystal matrices. New ideas and future developments are discussed.

Arrays of small scintillation crystals are being used increasingly for high-resolution imaging applications in nuclear medicine. Although the degree of pixellation now available is high for some scintillation materials, this spatial-resolution is often achieved at the expense of degraded energy-resolution due to the lower, and more variable, light-collection efficiency. The energy-resolution of a detector is, however, especially important in nuclear medicine where events that have been scattered in the body need to be rejected efficiently. The light-output from a range of CsI(Tl) arrays was measured in terms of the number of photoelectrons detected, using a hybrid photodiode. This data, used in conjunction with the measured energy-resolution and an estimate of the intrinsic energy-resolution of CsI(Tl), was used to assess the magnitudes of the various contributions to the overall energy-resolution of these detectors. This information suggested that there is an opportunity to improve their energy-resolution by carefully choosing the geometry and reflector material to minimise the variance in the light collection. If the nature of this variance is understood sufficiently well, there maybe an opportunity to apply a similar post-processing technique to that which has dramatically improved the performance of other standard scintillation detectors. This possibility depends on the use of either the uniform quantum efficiency of the M-HPD photocathodes or a monolithic array of PIN diodes. This would ensure that no additional or indeterminate variance in the light-collection is introduced. Measurements made using fine BGO arrays for higher-energy applications and columnar grown CsI(Tl) for X-ray imaging will also be presented.

In this paper, we present the design and the results of a feasibility study of a novel, ultra-fast and high-resolution PET detector. It is based on the use of an LSO crystal array read-out by a Multi-pixel Hybrid PhotoDiode (M-HPD) using an encoded fiber light-guide. The fiber encoding method enables one to readout more than 400 discrete crystal elements using a single M-HPD tube having 61 pixels. One of the key features of the detector is that the encoded fiber light-guide, used in conjunction with a digital readout system, eliminates the need for the use of ADCs in finding the address of the crystal in which interaction occurred. The readout time of a detector comprising around four hundred 2x2x10mm discrete crystals, defined as the time between identifying a coincidence and providing the address of the crystal hit, can be reduced to less than 0.1mus. As a consequence, one could significantly improve the noise-effective-count-rate (NECR) performance of a PET system based on this detector design. Another advantage of this detector design is that the parallel readout scheme used, greatly simplifies the readout electronics by eliminating the use of ADC in the readout system. The feasibility of this detector design has been confirmed by measurements using a prototype detector module based on a 5x5 array of 2x2x10mm LSO crystals. A signal level of around 200 photoelectrons has been measured for 511keV energy deposited in the detector. An energy-resolution of similar to30% and a timing resolution of less than 4 ns were achieved in this study.

The basis for a design of a high-resolution PET imaging system, utilising a wavelength-shifting (WLS) fibre readout technique is presented here. Scintillation light is transported from the individual LSO pixels in the detector array through the WLS fibres to a multi-pixel hybrid photodiode (M-HPD). Each detector array consists of a number of 2/spl times/2/spl times/10 mm LSO crystals viewed by two orthogonal ribbons of WLS fibres. An array of as many as 30/spl times/31 crystals can thus be read-out by a single, small 61-pixel M-HPD tube. The design is both compact, inexpensive and suitable for application not only in the field of small-animal PET or in a dedicated PEM system, but could also operate in a combined PET/MRI system. Initial tests have shown that the signal level generated at the detector by a single 511 keV energy-deposit, is 20-25 photoelectrons. Separate tests have indicated that, at this signal-level, a resolving time of /spl sim/25 ns could be achievable.

This paper describes the design of a fast imaging system aimed at providing the accurate location of the centroid of a sentinel lymph node. Through the proposed use of a new 80mm diameter multi-pixel hybrid photo-diode, equipped with a fibre-optic window, a CsI(T1) scintillation crystal and a specially optimised collimator, we expect to achieve a precision of +/- 3mm within 20 s for the location of a lymph node. It has been assumed that the affected gland has a 10mm diameter and had accumulated 3 mu Ci of Tc-99 through drainage of the region of the breast. The paper discusses the likely imaging quality of the larger tube based on measurements made with a smaller prototype detector. The design of two collimators is presented; one to provide a rapid location of the centroid of the SLN and the other to provide the surgeon with it measure of the depth of the potentially infected organ. A method for relating the image information to the patient's anatomy to guide the surgeon is also presented.