We characterize a compact MR-compatible PET insert for simultaneous preclinical PET/MRI. Although specifically designed with the strict size constraint to fit inside the 114-mm inner diameter of the BGA-12S gradient coil used in the BioSpec 70/20 and 94/20 series of small-animal MRI systems, the insert can easily be installed in any appropriate MRI scanner or used as a stand-alone PET system. Methods: The insert consists of a ring of 16 detector-blocks each made from depth-of-interaction–capable dual-layer-offset arrays of cerium-doped lutetium-yttrium oxyorthosilicate crystals read out by silicon photomultiplier arrays. Scintillator crystal arrays are made from 22 × 10 and 21 × 9 crystals in the bottom and top layers, respectively, with respective layer thicknesses of 6 and 4 mm, arranged with a 1.27-mm pitch, resulting in a useable field of view 28 mm long and about 55 mm wide. Results: Spatial resolution ranged from 1.17 to 1.86 mm full width at half maximum in the radial direction from a radial offset of 0–15 mm. With a 300- to 800-keV energy window, peak sensitivity was 2.2% and noise-equivalent count rate from a mouse-sized phantom at 3.7 MBq was 11.1 kcps and peaked at 20.8 kcps at 14.5 MBq. Phantom imaging showed that features as small as 0.7 mm could be resolved. 18F-FDG PET/MR images of mouse and rat brains showed no signs of intermodality interference and could excellently resolve substructures within the brain. Conclusion: Because of excellent spatial resolvability and lack of intermodality interference, this PET insert will serve as a useful tool for preclinical PET/MR.

A full-ring PET insert consisting of 16 PET detector modules was designed and constructed to fit within the 114 mm diameter gradient bore of a Bruker 7 T MRI. The individual detector modules contain two silicon photomultiplier (SiPM) arrays, dual-layer offset LYSO crystal arrays, and high-definition multimedia interface (HDMI) cables for both signal and power transmission. Several different RF shielding configurations were assessed prior to construction of a fully assembled PET insert using a combination of carbon fibre and copper foil for RF shielding. MR-compatibility measurements included field mapping of the static magnetic field (B0) and the time-varying excitation field (B1) as well as acquisitions with multiple pulse sequences: spin echo (SE), rapid imaging with refocused echoes (RARE), fast low angle shot (FLASH) gradient echo, and echo planar imaging (EPI). B0 field maps revealed a small degradation in the mean homogeneity (+0.1 ppm) when the PET insert was installed and operating. No significant change was observed in the B1 field maps or the image homogeneity of various MR images, with a 9% decrease in the signal-to-noise ratio (SNR) observed only in EPI images acquired with the PET insert installed and operating. PET detector flood histograms, photopeak amplitudes, and energy resolutions were unchanged in individual PET detector modules when acquired during MRI operation. There was a small baseline shift on the PET detector signals due to the switching amplifiers used to power MRI gradient pulses. This baseline shift was observable when measured with an oscilloscope and varied as a function of the gradient duty cycle, but had no noticeable effect on the performance of the PET detector modules. Compact front-end electronics and effective RF shielding led to minimal cross-interference between the PET and MRI systems. Both PET detector and MRI performance was excellent, whether operating as a standalone system or a hybrid PET/MRI.

A Small Animal PET detector block made with a Dual Layer Offset crystal array with 1.27 mm wide LYSO crystals on a Philips PDC3200-22-44 Digital Photon Counter (DPC) array was characterized while operating near room temperature. Crystal map peak to valley ratio, energy resolution, and timing resolution were characterized as a function of various device settings of the DPC and temperature. In addition, rates of count loss due to the phenomena of incomplete neighbor logic and dark-readout deadtime were measured. Device settings of interest were: Trigger scheme-defining the threshold of when a DPC will generate a timestamp and enter a readout cycle, inhibit fraction-the fraction of noisy cells which are disabled, and RTL-refresh-a setting which reduces the probability of the DPC being triggered from dark noise. At 15°C, peak to valley ratios were measured to be around 11, and energy resolution around 11.5% regardless of device settings. Timing resolution ranged from near 300 ps to 1.5 ns. Count loss from dark readout deadtime was insignificant compared to incomplete neighbor logic, which ranged from as high as 95% to 5% of coincidences. It was found that trade-offs had to be made between timing resolution and count loss. With the most optimal device settings for small animal PET, a timing resolution of 1.4 ns and coincidence losses of 5% were achieved. At these settings, the detector block had little sensitivity to a 5°C temperature fluctuation.

BACKGROUND:Significant numbers of patients with obsessive compulsive disorder (OCD) respond minimally to currently available treatments. Furthermore, the application of both high- and low-frequency repetitive transcranial magnetic stimulation (rTMS) to dorsolateral prefrontal cortex has shown to be ineffective in the reduction of OCD symptoms. In this study, we instead targeted the medial prefrontal cortex (mPFC) and applied low-frequency rTMS to patients with OCD and assessed their clinical response.METHODS:In an open-label design, 10 OCD patients with no other current axis I psychiatric disorders were recruited. Twelve hundred pulses of 1-Hz frequency were applied over the mPFC (Brodmann areas 24 and 32) using a double-cone coil for 10 days. Regions of interest were located on participants' MRIs using neuronavigation software. Patients' symptoms were rated using the Yale-Brown Obsessive Compulsive Scale (Y-BOCS).RESULTS:All patients demonstrated improvement in their OCD symptoms after 10 sessions of rTMS as shown by a mean improvement in Y-BOCS score of 39% (SD = 15%; P < .001, F = 62.95). This improvement persisted 1 month following the last session of rTMS.CONCLUSIONS:Our results suggest the use of low frequency deep rTMS as a promising and robust intervention in OCD symptom reduction. However, this study is limited by its open-label nature and its lack of a control group, so further randomized clinical trials are needed to confirm these results.

Digital Photon Counters (DPCs) are a new technology capable of detecting light similar to a Silicon Photomultiplier. In previous work we characterized the Philips DPC-3200-22_C DPC array for use in small-animal PET. Among other performance aspects studied was a reduction in detector efficiency from Neighbor Logic (NL) malfunction. NL forces neighboring DPC elements into a readout cycle after the first DPC is triggered, and was found to be necessary to readout a finely pixelated array of scintillator crystals. In previous work a detected event was discarded from the data set when NL malfunctioned for the Anger Logic coordinate could not be properly calculated. When the missing DPC is sufficiently far from the crystal of interaction, the Anger Logic coordinate is altered by enough that it cannot be identified using the crystal identification map (CIM; segmented histogram of Anger Logic coordinates) created from events in which all DPCs reported. However, the event may still hold enough information to accurately identify the crystal of interaction through using a CIM generated solely using events where that particular DPC malfunctioned. Here we propose that separate CIMs be used for each permutation of missing DPC elements when NL malfunction occurs. We found that detection efficiency is improved for all combinations of device settings tested. For easier to pass trigger schemes (TSs, which set a statistical minimum threshold of cell breakdowns to trigger the DPC) where NL count loss was significant, there was a very large improvement in efficiency (i.e. 87 to 52% loss for the easiest scheme). This method of event recovery leads to no degradation in timing resolution and a minimal degradation in energy resolution. Use of this method could allow one to use lower TSs which have excellent timing resolution compared to TS4, which was previously found to be necessary to preserve efficiency.

We have developed an MR compatible PET insert prototype to enable simultaneous PET-MR imaging of small animals inside a 7T animal MR imaging system. The 16 detectors in this system are based on the SensL SPMArray4B silicon photomultipliers (SiPMs) and dual layer offset LYSO crystal blocks with crystal size of 1.2 × 1.2 × 4 mm3 in the top layer and 1.2 × 1.2 × 6 mm3 in the bottom layer, with 409 total crystals per detector. Each detector was evaluated by generating flood histogram images and measuring per-crystal photopeak amplitude and energy resolution. The system was tested for MR compatibility and no visible artefacts were seen in the MR images or in the PET detector flood images. The temperature of the detector modules was monitored and found to stabilize after approximately two hours of operation between 32°C and 34°C. Photopeak amplitude, energy resolution and count rates recorded per crystal were studied at a baseline temperature of 30°C and at temperatures ranging from 20°C to 40°C in increments of 2°C. When bias voltage to the detectors was fixed at the value corresponding to an overvoltage of 2.5V at 30°C, the photopeak amplitude changed by as much as 19% and 28% at 20°C and 40°C respectively. Energy resolution and count rates demonstrated similar degrees of change. Maintaining a fixed overvoltage at each temperature point reduced but did not completely eliminate the change in these output characteristics. A lookup table based on empirical measurements was employed for the adjustment of bias voltage at different temperatures to maximize the stability of detector modules. Measurements obtained by using this lookup table method demonstrate better stability when compared to results obtained by using a fixed overvoltage.

PIXELATED Geiger-mode avalanche photodiodes (PPDs), often called silicon photomultipliers (SiPMs) are emerging as an excellent replacement for traditional photomultiplier tubes (PMTs) in a variety of detectors, especially those for subatomic physics experiments, which requires extensive test and operation procedures in order to achieve uniform responses from all the devices. In this paper, we show for two PPD brands, Hamamatsu MPPC and SensL SPM, that at room temperature, the dark noise rate, breakdown voltage and rate of correlated avalanches can be inferred from the sole measure of dark current as a function of operating voltage, hence greatly simplifying the characterization procedure. We introduce a custom electronics system that allows measurement for many devices concurrently, hence allowing rapid testing and monitoring of many devices at low cost. Finally, we show that the dark current of Hamamastu Multi-Pixel Photon Counter (MPPC) is rather independent of temperature at constant operating voltage, hence the current measure cannot be used to probe temperature variations. On the other hand, the MPPC current can be used to monitor light source conditions in DC mode without requiring strong temperature stability, as long as the integrated source brightness is comparable to the dark noise rate.

Silicon photomultipliers (SiPMs) have high gain, excellent timing performance, and are well suited to PET/MRI applications due, in part, to their MR-compatibility and small form factor. Within the constraints of a resistor-based multiplexing circuit, it is useful to evaluate the four generations of SiPM arrays manufactured by SensL: the SPMArray4, ArraySL-4, ArraySM-4, and ArraySB-4. Breakdown voltage and dark current were measured as a function of temperature in two each of the four generations of SensL SiPM arrays. Flood histograms were created with a 68Ge-irradiated 9×9 LYSO crystal array at temperatures of 5 °C to 45 °C in 5 °C increments and overvoltages of 2 to 4 V in 0.5 V increments. Measurements of dark current vs. bias voltage increased as temperature increased, with a corresponding increase in the breakdown voltage, Vb. The temperature dependence of Vb is similar between all four generations of SiPM arrays with slopes ranging from 17.0 to 23.8 mV/°C. Notably, the ArraySB-4 has lower values for the breakdown voltage, with Vb = 24 V at 0 °C. Mean energy resolution for individual LYSO crystals showed improvements in each successive generation. The average energy resolution of the ArraySB-4 was 11.9% after correcting for non-linearity in the SiPM pixels. The linearity of the SensL SiPM arrays as a function of temperature and breakdown voltage makes them a suitable choice for a high-resolution, small animal PET/MRI system. Based on its improved resolvability and energy resolution, lower sensitivity to temperature and higher PDE, the ArraySB-4 will be used in our PET system.

Construction of a full PET system requires scaling up from a few detectors on the bench top to dozens or even hundreds of detectors all operating simultaneously and connected to high channel count electronics. Our collaboration is building a MRI compatible PET insert system that will contain 16 detector modules in the prototype phase and 64 detector modules in the final phase. This number of detectors makes it difficult, if not impossible, to manually configure and monitor each detector in the system. In order to make possible the scaling of the system for up to 64 detectors, we require a scalable slow control system to manage the low level functions of the PET system, such as controlling detector bias and monitoring temperature and power consumption, that can be interfaced to and controlled by a host computer. We have implemented this slow control system in conjunction with a detector interface board (DIB) that is an intermediary between the detectors and the OpenPET digitizer system. Each DIB is connected to four detector modules and is controlled by a Raspberry Pi ® computer directly attached to it. The Raspberry Pi ® computers report to a host PC software program developed in National Instruments LabWindows™/CVI to provide the capability of central monitoring and control.

Aging is associated with decreasing memory (Luo, 2008). At any age, however, it is possible to sharpen and strengthen memory (Kesler et al., 2011). The objective of this study is to investigate the effect of brain exercises on associative memory in elderly individuals with no apparent sign of impaired cognition. We hypothesize that frequent and regular exercise of associative memory improves mental state. To test our hypothesis, two associative memory exercises targeting the connection between the left and right brain hemispheres were designed and applied on 9 elderly between 70 to 90 years of age, during 8 consecutive weeks, with 3 exercise sessions/week. All participants’ memory and mental condition were assessed using the Wechsler Memory Scale (WMS III) questionnaire and the Montreal Cognitive Assessment (MOCA). These two assessments were conducted at the beginning and the end of the exercise regime, and again one month afterwards. The results show that, on average, exercising participants’ memory and mental states improved significantly over the eight weeks of trials. The results are encouraging; they suggest that the designed memory exercises could be used as a tool to improve the cognitive state of the older population. Hence, engaging in the proposed memory exercises regularly may delay the onset of dementia or Mild Cognitive Impairment.

Obstructive Sleep Apnea (OSA) is a respiratory disorder with serious consequences that is characterized by repetitive cessation of breathing for more than 10s often associated with a drop of more than 4% in the blood's Oxygen saturation level. The gold standard for OSA diagnosis is full-night Polysomnography (PSG), which is a time-consuming, inconvenient, and costly assessment. On the other hand, our team has showed that the analysis of tracheal respiratory sounds recorded during wakefulness holds promises to be used as a simple and effective tool for screening moderate and severe OSA. In this paper, we examine the nonlinear characteristics of tracheal breath sounds and the possibility to extract features from Higher Order Spectra (HOS) for OSA screening. The data used in this study were recorded during wakefulness in two body positions, supine and upright, and during mouth and nose breathing. We estimated the bispectrum of the sounds in each respiratory cycle, calculated the median bifrequencies and the energy of the bispectrum, and investigated any statistically significant differences between the extracted features in two groups of non-OSA and severe OSA data. The differences in the features between body positions and nose/mouth breathing were also looked at. One-way ANOVA revealed significant differences in the features between non-OSA individuals and those with severe OSA. The results encourage the use of these features in future studies for OSA screening.