LTCC Based Highly Integrated SiPM Module With Integrated Liquid Cooling Channels for High Resolution Molecular Imaging
Authors: Dohle Rainer , Sacco Ilaria , Rittweg Thomas , Friedrich Thomas , Henning Gerold , Goßler Jörg , Fischer Peter 
Published: November 2017

Abstract
We present a very compact hybrid detection module based on an advanced liquid-cooled LTCC substrate. The double sided hybrid combines 144 photo detectors and 4 specialized flip-chip readout ASICs used for the readout of scintillation crystals with application in time-of-flight (TOF) Positron Emission Tomography (PET) combined with Magnetic Resonance Imaging (MRI). Positron Emission Tomography is mostly known for its use in oncology applications, cardiovascular disease, and imaging of brain functions. If MRI images and PET images are combined, completely new medical diagnostic and treatment prospects are feasible since MRI delivers precise anatomical information. One of the biggest challenges is the development of miniaturized detector modules that are highly functional and MRI compatible.
Our SiPM module has an area of 32.8 by 32.0 mm2 and contains 12 × 12 SiPMs in a pitch of 2.5 mm2. The SiPM readout of the 144 channels is performed by four PETA6 ASICs. The LTCC substrate with 2.1 mm thickness has been manufactured using the most advanced technologies developed at MSE. In order to guarantee the manufacturability in serial or mass production, DP951 P2 green tape has been used. For the cooling channels, special technology has been developed by MSE. The liquid cooling channels inside the LTCC substrate provide excellent cooling for the ASICs, the SiPMs, and thermal insulation between ASICs and SiPMs and allow a very compact design of the detector modules, reducing their height by 50% compared with other technical solutions. This makes both smaller scanners and larger detector rings possible, the latter being needed for heavier patients. We can insert a ring of our modules in an existing MR scanner. Operating the SiPMs at low temperature improves their performance, reducing the effects of dark count rate and improving image quality. There is no heat sink, heat pipe, or other cooling element attached to the back side of the ASICs. In order to avoid interference between the PET and MRI system, short signal length is required for minimizing pickup loops and eddy currents. The 12 SiPM arrays with 2×6 geometry are wire bonded only at the edges of the SiPMs to the LTCC, enabling the use of nearly the whole detector area for photon detection, which is of paramount importance for excellent image quality. At the opposite side of the substrate, four ASICs with 272 μm bump pitch are flip-chip solder assembled to the LTCC substrate including underfilling, and a few SMD components are mounted. A scintillator crystal array on top of the SiPMs converts gamma-rays (511 keV photons produced from positron-electron annihilation) into light. LTCC substrates and all components are fully MRI compatible, which allows integration of PET with magnetic resonance imaging without mutual interference. The performance of a PET detector is characterized by its time resolution, energy resolution, detection efficiency, and spatial resolution. Each of these factors has a huge influence on the quality of the final PET image. All of these factors are improved with our novel technical solution. With measurements on prototypes, state-of-the-art coincidence time resolution (CTR) for pairs of identical detectors in combination with high spatial resolution have been obtained. The paper elucidates the impact of the employed technology on the performance of advanced PET/MRI detector modules. Novel features of the detector modules will help to enhance the “molecular sensitivity” of PET/MRI scanners.