Multi-Pixel Photon Counter (MPPC) is a Geigermode APD developed by Hamamatsu Corp. We proposed that it could be a suitable photo-sensor for next-generation time-offlight PET detectors due to mainly its high photon detection efficiency. Therefore, we concentrated on the measurement of coincidence timing performance of various MPPCs in conjunction with LYSO crystal scintillators. With 3mm x 3mm devices of 50Jlm sub-pixels coupled to 3mm x 3mm x lOmm crystals, we have demonstrated a strong dependence of timing performance on over-voltage and temperature, and analyzed how changes in photon detection efficiency and dark counts would explain the measurements. The best coincidence timing resolution we have achieved between two identical LYSO/MPPC detectors was 240ps in FWHM. This was worse than the timing resolution of 220ps obtained with Hamamatsu H6533 fast PMT, and contradicted the expected improvement from higher photon detection efficiency. The contradiction could be explained by slow rise-time of MPPC pulse shape, transit time spread, dark counts and electronics noise from large capacitance of MPPC. In particular, the slow rise-time of MPPC pulse suggested that the need for a very low trigger threshold on the timing circuit. Since it in turn makes the detector system more sensitive to noise, this poses additional challenges for ganging multiple devices together into a commercially viable time-of-flight PET block detector. We will discuss it in detail including other challenge imposed by MPPC characteristics.

The Multi-Pixel Photon Counter (MPPC) is a Geiger-mode avalanche photo-diode (APD) developed by Hamamatsu Corp. We propose that it could be a suitable photo-sensor for next-generation time-of-flight PET detectors due to its high photon detection efficiency. We concentrate on the measurement of coincidence timing performance of various MPPCs in conjunction with LYSO crystal scintillators. With 3 mm 3 mm devices of 50 m sub-pixels coupled to 3 mm 3 mm 10 mm LYSO crystals, we have demonstrated a strong dependence of timing performance on over-voltage and temperature, and analyzed how changes in photon detection efficiency and dark counts would explain the measurements. The best coincidence timing resolution we have achieved between two identical LYSO/MPPC detectors was 240 ps in FWHM. This was worse than the timing resolution of 220 ps obtained with a Hamamatsu H6533 fast PMT, and contradicted the expected improvement from higher photon detection efficiency. The contradiction could be explained by the slow rise-time of MPPC pulse shape, transit time spread, dark counts and electronic noise from the large capacitance of the MPPC. In particular, the slow rise-time of the MPPC pulse suggests the need for a very low trigger threshold on the timing circuit. Since this makes the detector system more sensitive to noise, this poses additional challenges for ganging multiple devices together into a commercially viable time-of-flight PET block detector. We will discuss this work in detail including other challenge imposed by MPPC characteristics.

It is known that arteries in their natural position are always subject to a longitudinal stress. However, the effect of this strong longitudinal tension has seldom been addressed. In this paper, we point out that the traditional pulse wave velocity formulae considering only the circumferential elasticity fail to include all the important energies. We present a vigorous derivation of a pressure wave equation, the pressure wave equation with total energy, which considers all the important energies of the whole arterial system by treating the arterial wall and the blood as one system. Our model proposes that the energy transport in the main arterial system is primarily via the transverse vibration motion of the elastic wall. The final equation indicates that the longitudinal stress is essential and the high frequency phase velocity is related to the longitudinal tension along the arterial wall and its Young's shearing modulus. By applying this equation, we suggest that longitudinal elastic property is an important factor in hemodynamics and in the treatment of cardiovascular diseases.

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