Signal-to-Noise Ratio Analysis for the Voltage-Mode Read-Out of Quartz Tuning Forks in QEPAS Applications.

Quartz tuning forks (QTFs) are employed as sensitive elements for gas sensing applications implementing quartz-enhanced photoacoustic spectroscopy. Therefore, proper design of the QTF read-out electronics is required to optimize the signal-to-noise ratio (SNR), and in turn, the minimum detection limit of the gas concentration. In this work, we present a theoretical study of the SNR trend in a voltage-mode read-out of QTFs, mainly focusing on the effects of (i) the noise contributions of both the QTF-equivalent resistor and the input bias resistor RL of the preamplifier, (ii) the operating frequency, and (iii) the bandwidth (BW) of the lock-in amplifier low-pass filter. A MATLAB model for the main noise contributions was retrieved and then validated by means of SPICE simulations. When the bandwidth of the lock-in filter is sufficiently narrow (BW = 0.5 Hz), the SNR values do not strongly depend on both the operating frequency and RL values. On the other hand, when a wider low-pass filter bandwidth is employed (BW = 5 Hz), a sharp SNR peak close to the QTF parallel-resonant frequency is found for large values of RL (RL > 2 MΩ), whereas for small values of RL (RL < 2 MΩ), the SNR exhibits a peak around the QTF series-resonant frequency.

A Redo Percutaneous Emergency Intervention of Left Ventricular Assist Device Graft Occlusion.

In patients with advanced heart failure (HF), left ventricular assist devices (LVADs) have demonstrated to be effective in improving the quality of life and reducing further hospitalizations. Although uncommon, LVAD outflow graft obstruction (OGO) is a potentially life-threatening complication and percutaneous treatment has been proposed as a standard intervention strategy in such cases. We report the case of a 69 year old man admitted due to LVAD failure causing unstable HF. Past medical history included percutaneous intervention on the outflow graft with stent implantation one year before. The patient was under chronic treatment with vitamin K antagonists (VKA). Emergent percutaneous angiography was performed, showing recurrent OGO due to thrombosis located at a kinking site, distally to the previously treated segment. Using distal anchoring technique, a balloon-expandable 10 × 79 mm endoprosthesis (GORE® Viabahn® VBX) was effectively positioned and post-dilated. Final angiography confirmed the patency of the stent implanted one-year before. Despite the procedure succeeding in restoring LVAD function, the patient died due to septic shock ten days after. Our case suggests that recurrent OGO can be effectively treated with percutaneous redo and that long-term stent patency can be achieved with a standard antithrombotic treatment, despite further thrombotic events in other segments of the graft are still possible (especially at the kinking site). Moreover, other noncardiac conditions as infective complications, can dramatically impact the clinical course and lead to unfavorable outcomes.

Analytical Study of Front-End Circuits Coupled to Silicon Photomultipliers for Timing Performance Estimation under the Influence of Parasitic Components.

Full exploitation of the intrinsic fast timing capabilities of analog silicon photomultipliers (SiPMs) requires suitable front-end electronics. Even a parasitic inductance of a few nH, associated to the interconnections between the SiPM and the preamplifier, can significantly degrade the steepness of the detector response, thus compromising the timing accuracy. In this work, we propose a simple analytic expression for the single-photon response of a SiPM coupled to the front-end electronics, as a function of the main parameters of the detector and the preamplifier, taking into account the parasitic inductance. The model is useful to evaluate the influence of each parameter of the system on the slope of its response and to guide the designer in the definition of the architecture and the specifications for the front-end electronics. The results provided by the model have been successfully compared with experimental measurements from a front-end circuit with variable configuration based on a bipolar junction transistor (BJT), coupled to a 3 × 3 mm2 SiPM stimulated by a fast-pulsed laser source.