Photomultiplier tube (PMT)-based scintillation cameras are predominant in molecular imaging but have the drawback that position estimation is severely degraded near the edges (dead edge effect). This leads to sensitivity losses and can cause severe problems in applications like molecular breast imaging and in certain SPECT devices. Using smaller light sensors or semiconductor detectors can solve this issue but leads to increased costs. Here we present a gamma detector based on standard PMTs with a novel light-guide-PMT geometry that strongly reduces dead edges. In our design, a monolithic NaI(Tl) scintillator is read out by square PMTs placed in a staggered arrangement. At the edge of the scintillator we inserted additional light-guides to emulate half-size PMTs. Detector performance was assessed for 99m Tc imaging; an average spatial resolution of 3.6 mm was measured over the whole detector, degrading to 4.0 mm within 30 mm to the critical edge. The dead edge of the scintillator is <; 3 mm. Since a 12-mm seal was used, the overall dead edge is <; 15 mm, which is a significant improvement over conventional Anger cameras (~40-mm dead edge). Therefore, the presented geometry can be useful in creating economical gamma detectors with reduced dead edges.@en

In recent years, breast imaging using radiolabelled molecules has attracted significant interest. Our group has proposed a multi-pinhole molecular breast tomosynthesis (MP-MBT) scanner to obtain 3D functional molecular breast images at high resolutions. After conducting extensive optimisation studies using simulations, we here present a first prototype of MP-MBT and evaluate its performance using physical phantoms. The MP-MBT design is based on two opposing gamma cameras that can image a lightly compressed pendant breast. Each gamma camera consists of a 250 × 150 mm² detector equipped with a collimator with multiple pinholes focusing on a line. The NaI(Tl) gamma detector is a customised design with 3.5 mm intrinsic spatial resolution and high spatial linearity near the edges due to a novel light-guide geometry and the use of square PMTs. A volume-of-interest is scanned by translating the collimator and gamma detector together in a sequence that optimises count yield from the scan region. Derenzo phantom images showed that the system can reach 3.5 mm resolution for a clinically realistic ^99mTc activity concentration in an 11-minute scan, while in breast phantoms the smallest spheres visible were 6 mm in diameter for the same scan time. To conclude, the experimental results of the novel MP-MBT scanner showed that the setup had sub-centimetre breast tumour detection capability which might facilitate 3D molecular breast cancer imaging in the future.

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Much attention is currently being paid to imaging prompt gamma (PG) rays for in vivo proton range monitoring in proton therapy. PG imaging using a collimator is affected by neutron-related background. We study the effectiveness of background reduction experimentally, using a scanning parallel-slit PG collimator as a simplified model of a multislat PG camera. The analysis is focused on the falloff region of the PG intensity profile near the Bragg peak, which is the typical region of interest for proton range estimation. Background reduction was studied for different energy windows, with and without a shifting time-of-flight window that takes into account the proton velocity within the phantom. Practical methods are put forward that apply to cyclotron-based pencil beams. The parallel-slit collimator was placed in front of arrays of cerium-doped lutetium yttrium silicate-coupled digital silicon photomultipliers, used to measure energy and time spectra together with intensity profiles of prompt events emitted from a polymethylmethacrylate phantom irradiated with a 160-MeV proton pencil beam. The best signal-to-background ratio of ~1.6 was similar to that obtained previously with a knife-edge-slit collimator. However, the slope-over-noise ratio in the PG-profile falloff region, was ~1.2 higher for the present collimator, given its better resolution.@en

Silicon photomultipliers (SiPMs) offer high gain and fast response to light, making them interesting for fast timing applications such as time-of-flight (TOF) PET. To fully exploit the potential of these photosensors, dedicated preamplifiers that do not deteriorate the rise time and signal-to-noise ratio are crucial. Challenges include the high sensor capacitance, typically >300 pF for a 3 mm×3 mm SiPM sensor, as well as oscillation issues. Here we present a preamplifier concept based on low noise, high speed transistors, designed for optimum timing performance. The input stage consists of a transimpedance common-base amplifier with a very low input impedance even at high frequencies, which assures a good linearity and avoids that the high detector capacitance affects the amplifier bandwidth. The amplifier has a fast timing output as well as a ‘slow’ energy output optimized for determining the total charge content of the pulse. The rise time of the amplifier is about 300 ps. The measured coincidence resolving time (CRT) for 511 keV photon pairs using the amplifiers in combination with 3 mm×3 mm SiPMs (Hamamatsu MPPC-S10362-33-050C) coupled to 3 mm×3 mm×5 mm LaBr3:Ce and LYSO:Ce crystals equals 95 ps FWHM and 138 ps FWHM, respectively.@en