NUV-Sensitive Silicon Photomultiplier Technologies Developed at Fondazione Bruno Kessler

Different applications require different customizations of silicon photomultiplier (SiPM) technology. We present a review on the latest SiPM technologies developed at Fondazione Bruno Kessler (FBK, Trento), characterized by a peak detection efficiency in the near-UV and customized according to the needs of different applications. Original near-UV sensitive, high-density SiPMs (NUV-HD), optimized for Positron Emission Tomography (PET) application, feature peak photon detection efficiency (PDE) of 63% at 420 nm with a 35 um cell size and a dark count rate (DCR) of 100 kHz/mm². Correlated noise probability is around 25% at a PDE of 50% at 420 nm. It provides a coincidence resolving time (CRT) of 100 ps FWHM (full width at half maximum) in the detection of 511 keV photons, when used for the readout of LYSO(Ce) scintillator (Cerium-doped lutetium-yttrium oxyorthosilicate) and down to 75 ps FWHM with LSO(Ce:Ca) scintillator (Cerium and Calcium-doped lutetium oxyorthosilicate). Starting from this technology, we developed three variants, optimized according to different sets of specifications. NUV-HD–LowCT features a 60% reduction of direct crosstalk probability, for applications such as Cherenkov telescope array (CTA). NUV-HD–Cryo was optimized for cryogenic operation and for large photosensitive areas. The reference application, in this case, is the readout of liquid, noble-gases scintillators, such as liquid Argon. Measurements at 77 K showed a remarkably low value of the DCR of a few mHz/mm². Finally, vacuum-UV (VUV)-HD features an increased sensitivity to VUV light, aiming at direct detection of photons below 200 nm. PDE in excess of 20% at 175 nm was measured in liquid Xenon. In the paper, we discuss the specifications on the SiPM related to different types of applications, the SiPM design challenges and process optimizations, and the results from the experimental characterization of the different, NUV-sensitive technologies developed at FBK.

Performance of a high-resolution depth-encoding PET detector module using linearly-graded SiPM arrays

The goal of this study was to exploit the excellent spatial resolution characteristics of a position-sensitive silicon photomultiplier (SiPM) and develop a high-resolution depth-of-interaction (DOI) encoding positron emission tomography (PET) detector module. The detector consists of a 30  ×  30 array of 0.445  ×  0.445  ×  20 mm3 polished LYSO crystals coupled to two 15.5  ×  15.5 mm2 linearly-graded SiPM (LG-SiPM) arrays at both ends. The flood histograms show that all the crystals in the LYSO array can be resolved. The energy resolution, the coincidence timing resolution and the DOI resolution were 21.8  ±  5.8%, 1.23  ±  0.10 ns and 3.8  ±  1.2 mm, respectively, at a temperature of -10 °C and a bias voltage of 35.0 V. The performance did not degrade significantly for event rates of up to 130 000 counts s-1. This detector represents an attractive option for small-bore PET scanner designs that simultaneously emphasize high spatial resolution and high detection efficiency, important, for example, in preclinical imaging of the rodent brain with neuroreceptor ligands.

Indicators of Complex Care During the Consultation-Liaison Psychiatry Activity at the Transplant Center of the Policlinico Hospital, Modena

OBJECTIVE

The objective of this study was to identify possible biopsychosocial predictors of organizational complexity in patients referred to the consultant psychiatrist for assessment before liver transplantation.

METHODS

This was a case-control study. All psychiatric consultations performed before and after liver transplantation from January 1, 2008 to December 31, 2013 were included. Complexity was operationalized as "undergoing two or more psychiatric consultations". Controls were defined as patients who were assessed only once by the consultant. Cases were represented by patients who underwent two or more consultations. Statistical analysis was performed with STATA 13.1, using logistic regressions.

RESULTS

In this study, 515 consultations were requested for 309 patients potentially eligible for liver transplantation. Controls were 209 (67.6%); cases were 100 (32.4%). Positive psychiatric history (odds ratio [OR] = 2.44; 95% confidence interval [CI], 1.43-4.16), viral or toxic (alcohol- or drug-related) liver disease (OR = 1.93; 95% CI, 1.09-3.42), use of psychotropic medications at the baseline (OR = 2.15; 95% CI, 1.14-4.07), and female gender (OR = 1.77; 95% CI, 1.01-3.11) were significantly associated with an increased probability of being cases.

CONCLUSIONS

Positive psychiatric history, viral or toxic liver disease, use of psychotropic medications at the index referral, and female gender are possible biopsychosocial predictors of complexity in patients eligible for liver transplantation.

The Screening for Depression and Neurocognitive Disorders in Subjects Newly Diagnosed with HIV

Background

Inflammatory mediators may be relevant to explain the frequent comorbidity between depression, neurocognitive disorders and HIV. HIV induces activation of inflammatory mediators, mainly cytokines, that have been involved in the onset of depression and response to antidepressant treatment.

Aim

To identify recurring profiles of inflammatory biomarkers subtending depression, effectiveness of antidepressants and neurocognitive disorders among HIV-infected individuals.

Methods

All adult newly HIV-diagnosed out-patients attending HIV clinics in three towns of Northern Italy were screened, assessed for depression and studied immunologically and for neurocognitive disorders.

Results

Twenty-five patients have been enrolled so far: of these, 35% were positive to PHQ-9 screening, of which 6 were positive to the diagnostic assessment for depression. No neurocognitive disorders were found among the patients. As the project will develop, it is expected that frequency of depression, neurocognitive disorders and effective antidepressant treatment will be found to correlate to the profile of immune biomarkers. These findings might help to understand the etiology of depression in HIV, and specifically the role of inflammation and immunological changes.

Disclosure of interest

The authors have not supplied their declaration of competing interest.

Development of clinical simultaneous SPECT/MRI

There is increasing clinical use of combined positron emission tomography and MRI, but to date there has been no clinical system developed capable of simultaneous single-photon emission computed tomography (SPECT) and MRI. There has been development of preclinical systems, but there are several challenges faced by researchers who are developing a clinical prototype including the need for the system to be compact and stationary with MRI-compatible components. The limited work in this area is described with specific reference to the Integrated SPECT/MRI for Enhanced stratification in Radio-chemo Therapy (INSERT) project, which is at an advanced stage of developing a clinical prototype. Issues of SPECT/MRI compatibility are outlined and the clinical appeal of such a system is discussed, especially in the management of brain tumour treatment.

Reaching 200-ps timing resolution in a time-of-flight and depth-of-interaction positron emission tomography detector using phosphor-coated crystals and high-density silicon photomultipliers

Current research in the field of positron emission tomography (PET) focuses on improving the sensitivity of the scanner with thicker detectors, extended axial field-of-view, and time-of-flight (TOF) capability. These create the need for depth-of-interaction (DOI) encoding to correct parallax errors. We have proposed a method to encode DOI using phosphor-coated crystals. Our initial work using photomultiplier tubes (PMTs) demonstrated the possibilities of the proposed method, however, a major limitation of PMTs for this application is poor quantum efficiency in yellow light, corresponding to the wavelengths of the converted light by the phosphor coating. In contrast, the red-green-blue-high-density (RGB-HD) silicon photomultipliers (SiPMs) have a high photon detection efficiency across the visible spectrum. Excellent coincidence resolving time (CRT; [Formula: see text]) was obtained by coupling RGB-HD SiPMs and [Formula: see text] lutetium fine silicate crystals coated on a third of one of their lateral sides. Events were classified in three DOI bins ([Formula: see text] width) with an average sensitivity of 83.1%. A CRT of [Formula: see text] combined with robust DOI encoding is a marked improvement in the phosphor-coated approach that we pioneered. For the first time, we read out these crystals with SiPMs and clearly demonstrated the potential of the RGB-HD SiPMs for this TOF-DOI PET detector.

Bismuth germanate coupled to near ultraviolet silicon photomultipliers for time-of-flight PET

Bismuth germanate (BGO) was a very attractive scintillator in early-generation positron emission tomography (PET) scanners. However, the major disadvantages of BGO are lower light yield and longer rise and decay time compared to currently popular scintillators such as LSO and LYSO. This results in poorer coincidence timing resolution and it has generally been assumed that BGO is not a suitable scintillator for time-of-flight (TOF) PET applications. However, when a 511 keV photon interacts in a scintillator, a number of Cerenkov photons are produced promptly by energetic electrons released by photoelectric or Compton interactions. If these prompt photons can be captured, they could provide a better timing trigger for PET. Since BGO has a high refractive index (increasing the Cerenkov light yield) and excellent optical transparency down to 320 nm (Cerenkov light yield is higher at shorter wavelengths), we hypothesized that the coincidence timing resolution of BGO can be significantly improved by efficient detection of the Cerenkov photons. However, since the number of Cerenkov photons is far less than the number of scintillation photons, and they are more abundant in the UV and blue part of the spectrum, photosensors need to have high UV/blue sensitivity, fast temporal response, and very low noise in order to trigger on the faint Cerenkov signal. In this respect, NUV-HD silicon photomultipliers (SiPMs) (FBK, Trento, Italy) are an excellent fit for our approach. In this study, coincidence events were measured using BGO crystals coupled with NUV-HD SiPMs. The existence and influence of Cerenkov photons on the timing measurements were studied using different configurations to exploit the directionality of the Cerenkov emissions. Coincidence resolving time values (FWHM) of ~270 ps from 2  ×  3  ×  2 mm3 BGO crystals and ~560 ps from 3  ×  3  ×  20 mm3 BGO crystals were obtained. To our knowledge, these are the best coincidence resolving time values reported for BGO to date. With these values, BGO can be considered as a relevant scintillator for TOF PET scanners, especially if photodetectors with even better near UV/blue response can be developed to further improve the efficiency of Cerenkov light detection.

SPAD aptasensor for the detection of circulating protein biomarkers

The need for decentralized clinical tests together with the concept of time and cost saving are pushing the development of portable, miniaturized, compact biosensors with diagnostic and prognostic purpose. Here, we propose an innovative detection system based on a Single Photon Avalanche Diode (SPAD) with high sensitivity and low noise, crucial features for an efficient chemiluminescence biosensor. The SPAD detector, having 60 µm diameter, has a Photon Detection Efficiency higher than 55% at 425 nm and a Dark Count Rate lower than 100 Hz at room temperature. Our design allows a good optical coupling efficiency between sample and detector. A specific biofunctional surface was implemented taking advantage of aptamers, short DNA sequences having high selectivity and affinity toward their targets. We successfully detected physiological levels of Vascular Endothelial Growth Factor (VEGF), a circulating protein biomarker highly correlated with cancer. The SPAD aptasensor showed a Limit of Detection (LoD) in the pM range, stability (up to 42 days) and re-usability (up to seven cycles). This compact biosensor is therefore a promising step toward the actual use of portable microdevices in diagnostics.

Timing and Energy Resolution of new near-UV SiPMs coupled to LaBr3:Ce for TOF-PET

The high light output and fast decay time of LaBr₃:Ce scintillation detectors leads to excellent timing performance. To realize the potential of timing resolution with LaBr₃:Ce we have investigated the performance with SiPMs, which enable 1-to-1 coupling to individual crystals, and which have been optimized for the near-ultraviolet (NUV) scintillation light emission of LaBr₃:Ce. Coincidence timing resolution (CTR) of 100 ps was measured for a 4×4×5 mm³ LaBr₃:30%Ce crystal directly coupled to a large-area 4×4 mm² NUV-SiPM. Results show very little dependence on temperature, in the range of -20° to 20°C, and bias voltage, from 2 V to 5 V over breakdown. Optimal performance was achieved at an over-voltage (OV) range of 3 V - 5 V, at which high gain and high photon detection efficiency are achieved. Though saturation was evident at 511 keV, an energy resolution of 6.8% was measured after correcting for non-linearity. We also measured a CTR of 110 ps for a 4×4×5mm³ LaBr₃:5%Ce crystal and 245 ps for a 4×4×30 mm³ LaBr₃:5%Ce crystal using the NUV-SIPM. The poorer timing measurement for the 30-mm long crystal is due mainly to a systematic shift in the time pick-off as a function of the depth-of-interaction. The excellent temperature stability, fast rise time, high gain, and low noise of the NUV-SiPM make it a practical and highly appealing photodetector for the readout of a LaBr₃:Ce TOF-PET detector.

Sensitivity encoded silicon photomultiplier--a new sensor for high-resolution PET-MRI

Detectors for simultaneous positron emission tomography and magnetic resonance imaging in particular with sub-mm spatial resolution are commonly composed of scintillator crystal arrays, readout via arrays of solid state sensors, such as avalanche photo diodes (APDs) or silicon photomultipliers (SiPMs). Usually a light guide between the crystals and the sensor is used to enable the identification of crystals which are smaller than the sensor elements. However, this complicates crystal identification at the gaps and edges of the sensor arrays. A solution is to use as many sensors as crystals with a direct coupling, which unfortunately increases the complexity and power consumption of the readout electronics. Since 1997, position-sensitive APDs have been successfully used to identify sub-mm crystals. Unfortunately, these devices show a limitation in their time resolution and a degradation of spatial resolution when placed in higher magnetic fields. To overcome these limitations, this paper presents a new sensor concept that extends conventional SiPMs by adding position information via the spatial encoding of the channel sensitivity. The concept allows a direct coupling of high-resolution crystal arrays to the sensor with a reduced amount of readout channels. The theory of sensitivity encoding is detailed and linked to compressed sensing to compute unique sparse solutions. Two devices have been designed using one- and two-dimensional linear sensitivity encoding with eight and four readout channels, respectively. Flood histograms of both devices show the capability to precisely identify all 4 × 4 LYSO crystals with dimensions of 0.93 × 0.93 × 10 mm(3). For these crystals, the energy and time resolution (MV ± SD) of the devices with one (two)-dimensional encoding have been measured to be 12.3 · (1 ± 0.047)% (13.7 · (1 ± 0.047)%) around 511 keV with a paired coincidence time resolution (full width at half maximum) of 462 · (1 ± 0.054) ps (452 · (1 ± 0.078) ps).

Intermed self-assessment: developing an instrument to evaluate biopsychosocial complexity

Introduction

INTERMED is a method to assess biopsychosocial case complexity and a screening instrument to identify patients with multiple care needs. It is based on an interview, brief and easy to use, that can be conducted also by a nurse. Various studies in the last 10 years have confirmed its face-validity and reliability as a clinimetric tool. Its self-assessment version (IM-SA) was developed for further implementation in clinical settings.

Aim

To evaluate language, structure and fluency of the IM-SA questionnaire.

Methods

IM-SA, Italian version 1.0, was administered to 25 patients admitted to an internal medicine ward and to other 25 in the waiting room of a Primary Care outpatient clinic. At the end of the questionnaire, nine extra-questions were specifically added to evaluate comprehension and difficulties encountered by patients while filling in the questionnaire.

Results

Patients admitted to the hospital scored higher and were found to be “more complex” (p < .01). 90% of respondents found the questionnaire easy to answer and 96% answered that the structure of the sentences was not too complex. The majority of patients found IM-SA useful and/or interesting. Contradictions in results also were found, suggesting need for improvement of structural and linguistic properties of the tool.

Conclusion

IM-SA seems to be a feasible and reliable self-assessment method to evaluate biopsychosocial complexity. Further similar studies in different languages are being organized to reach a final version of IM-SA, which will be afterwards compared to the IM professional interview.