Alexander V. Stolin

A large area, silicon photomultiplier-based PET detector module

The introduction of silicon photomultipliers (SiPM) has facilitated construction of compact, efficient and magnetic field-hardened positron emission tomography (PET) scanners. To take full advantage of these devices, methods for using them to produce large field-of-view PET scanners are needed. In this investigation, we explored techniques to combine two SiPM arrays to form the building block for a small animal PET scanner. The module consists of a 26 × 58 array of 1.5 × 1.5mm2 LYSO elements (spanning 41 × 91mm2) coupled to two SensL SiPM arrays. The SiPMs were read out with new multiplexing electronics developed for this project. To facilitate calculation of event position with multiple SiPM arrays it was necessary to spread scintillation light amongst a number of elements with a small light guide. This method was successful in permitting identification of all detector elements, even at the seam between two SiPM arrays. Since the performance of SiPMs is enhanced by cooling, the detector module was fitted with a cooling jacket, which allowed the temperature of the device and electronics to be controlled. Testing demonstrated that the peak-to-valley contrast ratio of the light detected from the scintillation array was increased by ∼45% when the temperature was reduced from 28 °C to 16 °C. Energy resolution for 511 keV photons improved slightly from 18.8% at 28 °C to 17.8% at 16 °C. Finally, the coincidence timing resolution of the module was found to be insufficient for time-of-flight applications (∼2100 ps at 14 °C). The first use of these new modules will be in the construction of a small animal PET scanner to be integrated with a 3T clinical magnetic resonance imaging scanner.

Evaluation of imaging modules based on SensL array SB-8 for nuclear medicine applications

Recent advances in the development of silicon photomultipliers (SiPM) offer new opportunities for medical imaging applications. Specifically, novel imaging devices for positron emission (PET) and single photo-emission computed tomography (SPECT) are becoming feasible. In this investigation, we tested a monolithic array of new generation SiPMs, an SB-8 array from SensL. 8×8 array of 6-mm square SiPMs was studied with two different multiplexing readouts, 4- and 16-channels. SB-8 detector was coupled to various scintillation arrays and resulting prototypes were evaluated. Testing demonstrated that the new device is capable of resolving 1 mm LYSO and 1.5 mm NaI crystals with 16-channel readout and 1.57 mm LYSO and 3 mm NaI crystals with 4-channel readout. Energy resolution of approximately 15% at 511 keV and 19% at 122 keV were obtained with LYSO and NaI crystals, respectively. A timing resolution of 1.52 ns was measured with 1.57 mm LYSO array and the 16-channel readout. It is concluded that new B-series SiPMs from SensL are suitable for use in high spatial resolution nuclear medicine particle detectors.

Towards 1mm PET resolution using DOI modules based on dual-sided SiPM readout

Parallax error in PET modules can be reduced by measuring the annihilation photon depth of interaction (DOI) in the scintillation crystal on an event-by-event basis. Following implementations described in prior literature, we selected a dual-sided readout PET module design in which SiPMs are placed at both ends of a scintillation array and the ratio of the signal from one photodetector (A) divided by the signal sum of both detectors (A+B), as well as the plot of Signal A vs. Signal B are used to measure DOI. Our experimental apparatus consisted of a 12×12 scintillation array with 1×1×10mm3 pixels [from Proteus] with 50μ Toray Lumirror septa for DOI optimization. Both polished ends of the scintillation array were optically coupled to a 12×12×2mm3 anti-reflection coated UV fused silica light spreader window [from Edmund Optics]. The reverse side of each window was optically coupled to a low profile (∼1mm thick) 4×4 element SiPM with 3 × 3mm2 active area pixels [SPMArray2 from sensL]. Flexible Printed Circuit (FPC) cables were used to interface the DOI PET module to custom 16 channel differential pre-amplifiers connected to evaluation/power supply boards [SPMArray2-A0 and SPMArray2-A1, respectively, from sensL]. Average DOI resolution of 1.5±0.1mm FWHM and energy resolution of 20% FWHM was obtained. We conclude that for the limited instances of parallax error expected in realistic PET systems combined with ∼1mm spatial resolution in the scintillation plane, the obtained result for DOI spatial resolution indicates that ∼1mm spatial reconstruction resolution PET imaging is possible with the selected technical approach. Applications for such a compact high DOI resolution PET module include a prostate PET probe working in conjunction with a standard clinical PET imager or a dedicated prostate PET imager aiding in prostate cancer diagnosis and biopsy guidance, as well as a carotid artery PET probe imaging vulnerable plaque or a surgical breast imaging probe.

Molecular Imaging of Conscious, Unrestrained Mice with AwakeSPECT

We have developed a SPECT imaging system, AwakeSPECT, to enable molecular brain imaging of untrained mice that are conscious, unanesthetized, and unrestrained. We accomplished this with head tracking and motion correction techniques. Methods: The capability of the system for motion-corrected imaging was demonstrated with a 99mTc-pertechnetate phantom, 99mTc-methylene diphosphonate bone imaging, and measurement of the binding potential of the dopamine transporter radioligand 123I-ioflupane in mouse brain in the awake and anesthetized (isoflurane) states. Stress induced by imaging in the awake state was assessed through measurement of plasma corticosterone levels. Results: AwakeSPECT provided high-resolution bone images reminiscent of those obtained from CT. The binding potential of 123I-ioflupane in the awake state was on the order of 50% of that obtained with the animal under anesthesia, consistent with previous studies in nonhuman primates. Levels of stress induced were on the order of those seen in other behavioral tasks and imaging studies of awake animals. Conclusion: These results demonstrate the feasibility of SPECT molecular brain imaging of mice in the conscious, unrestrained state and demonstrate the effects of isoflurane anesthesia on radiotracer uptake.

Performance Study of a Wide-Area SiPM Array, ASICs Controlled

In this paper, the capabilities of a wide-area gamma ray photosensor based on a SiPM array are investigated. For this purpose, we have mounted an array of 144 (12×12) SiPMs with individual active area of 3 ×3 mm2 and a pitch of 4.2 mm, thus covering an active area of 50.2 ×50.2 mm2. The measurements were performed by coupling the SiPM array to LYSO crystal arrays of different pixel size ( 2×2 mm2, 1.5 ×1.5 mm2, and 1 ×1 mm2) and 10-12 mm thicknesses. The SiPM array was controlled by means of three ASICs, and the SiPM signals were multiplexed in order to determine the gamma ray impact position by means of implementing the Anger logic algorithm in the ASIC. The optimum bias voltage and temperature dependence of the gamma ray sensor were determined. An energy resolution as good as 8%, for individual crystal pixels, were reached at 5 V overvoltage. The ASICs design allows one to “activate” different photosensor array areas. This feature has been used to evaluate the detector performance as a function of the crystal pixel size and the photosensor dark noise contribution. In this work we also show the system capability to provide depth-of-interaction (DOI) information by means of implementing a two-layer staggered approach. We have found that accurate DOI information is obtained when the ASICs enabled an SiPM active area as high as 32×32 mm2( 8 ×8 SiPMs).

A novel brain PET insert for the MINDView project

The Multimodal Imaging of Neurological Disorders (MINDview) project aims to develop a high resolution and sensitivity dedicated brain Positron Emission Tomography (PET) system capable of visualizing neurotransmitter pathways and their disruptions for mental disorders for diagnosis and treatment follow-up. Moreover, this compact PET system should be fully compatible with a Magnetic Resonance Imaging (MRI) system in order to allow its operation as a brain insert in a hybrid imaging setup with most MRI scanners. The proposed design will enable current installed MRI base to be easily upgraded to PET/MRI systems. The current design for the PET insert consists of a 3 rings configuration, 20 modules per ring, with an axial field of view of ~15 cm and a geometrical aperture of ~33 cm in diameter. When coupled to the new head Radio Frequency (RF) coil, the inner diameter of the complete PET-RF coil insert is reduced to 26 cm. Main features of the PET detector insert for the MINDView project in terms of its overall design, electronic readout, and MRI compatibility will be presented. In addition, the main parameters of the PET detector insert, such as expected spatial and energy resolution, depth of interaction (DOI) capabilities and sensitivity will be discussed in terms of the different approaches considered so far for the construction of the first MINDView prototype. Laboratory tests results associated with the current MINDView PET module concept in terms of key parameters optimisation such as scintillator crystal, photosensor configuration and signal readout will be also presented.

Development of a “resistive” readout for SiPM arrays

We are developing the charge division (“resistive”) readout for several arrangements of Silicon Photomultiplier (SiPM) arrays, based on devices from Hamamatsu and SensL. The challenge with the SiPM arrays, as opposed to position sensitive photomultipliers (PSPMTs), is that the noise level is known to be high, and signal to noise ratio (S/N) is lower than in PMTs. In addition, the S/N decreases quickly with the increasing size of the module and with increasing temperatures. Key parameters to optimize are: size and coverage of the SiPM arrays, operational temperature (potential necessity of introducing system cooling), and bias voltage. All these parameters have impact on the S/N, and in consequence on the spatial resolution and the energy resolution of the detector modules. Our somewhat arbitrary but practical goal is to achieve operation similar to the one offered by H8500/H9500 flat panel PSPMTs when using LYSO scintillation arrays in applications to small PET imaging modules. Ultimately we would like to use the reduced channel number readout in the depth-of interaction (DOI) modules. Our first application is to construct ∼5cm×5cm compact PET modules for the HelmetPET brain imager prototype under construction at WVU.

HelmetPET: A silicon photomultiplier based wearable brain imager

We are developing the HelmetPET, a wearable human PET brain imager which has the potential application of evaluating brain function utilizing PET based radiopharmaceuticals in standing, balancing or moving patients. The HelmetPET is composed of two rings of radiation detectors together providing a cylindrical reconstructed volume with an axial length of 5 cm. Each ring is composed of twenty 2.5 cm2 silicon photomultiplier (SiPM) based detector modules. Each detector module is composed of a 5×5 array of twenty-five Hamamatsu S10362-33-050P Multi Pixel Photon Counters (MPPCs). The 3 mm2 MPPCs are arranged on a 5mm step. Coupled to each of the MPPC modules is a LYSO scintillator crystal array coupled to the MPPC array using to two different LYSO pixel arrays: 1.0×1.0×10 mm3 and 1.5×1.5×10 mm3. The current phase of the project is to equip the forty 2.5 cm2 detector modules with resistive readout and assemble them in a helmet type head support and suspend from a flexible mechanical mount.

Concept of an upright wearable positron emission tomography imager in humans

Positron Emission Tomography (PET) is traditionally used to image patients in restrictive positions, with few devices allowing for upright, brain‐dedicated imaging. Our team has explored the concept of wearable PET imagers which could provide functional brain imaging of freely moving subjects. To test feasibility and determine future considerations for development, we built a rudimentary proof‐of‐concept prototype (Helmet_PET) and conducted tests in phantoms and four human volunteers.

Analysis of Image Combination Methods for Conjugate Breast Scintigraphy

The main objective of the present study was to determine if combining the two images from a conjugate counting system might improve the contrast and signal-to-noise ratio (SNR) of small lesions in all regions of the breast compared to images from a single camera. Several methods for combining the opposing pixels of the two camera images were compared: multiplication, geometric mean, and summation. The image quality metrics measured were spatial resolution, lesion contrast and lesion SNR. These quantities were evaluated both theoretically and experimentally. A capillary phantom was used to measure the spatial resolution as a function of lesion depth and to assess the translation and angular offsets between the two cameras. An acrylic box phantom, with spherical lesions suspended inside, was used to evaluate contrast and SNR as a function of lesion position. Both theoretically and experimentally the spatial resolution in the product images was superior to that in the single images, geometric mean or summation images. Relative to the single camera images, the geometric mean or the summed images, the lesion contrast and SNR of the product images were superior, irrespective of lesion depth, and were more constant with changing lesion depth compared to the single camera images. These findings suggest that improved lesion detectability is possible by imaging simultaneously from both sides of the breast, and forming a combined image using pixel-by-pixel multiplication. This may be especially important if the location of the lesion within the breast is not known a priori.