Performance evaluation of a second prototype PET-MRI system based on four-layer DOI-PET detectors integrated with a RF coil

I.INTRODUCTIONWe are developing a PET/MRI system based on 4-layered depth-of-interaction (DOI) detectors [1] integrated with a birdcage RF-coil [2][3] as shown in Fig. 1. The PET detectors which consist of a scintillator block, photo sensors and front-end circuits are placed close to the objective. Therefore, both high sensitivity and high spatial resolution even at the edge of the field of view are achieved by the four-layer DOI measurement. The photo sensors and front-end circuits are shielded to minimize RF noises from the MRI and influence of noise from the PET detectors on MRI imaging. If the shielding material is inside the RF coils, the RF pulse is blocked by the shielding material and then complete images cannot be obtained. Therefore, each RF coil element is inserted between the scintillator crystal blocks.In the last PSMR conference, we reported the first prototype of a single detector ring to show a proof-of-concept [4]. However, the axial field-of-view (FOV) of the prototype was only 1 cm, which could not be extended due to the size of the readout circuit boards. Therefore, we developed the second prototype which had an extendable axial FOV with optimized readout circuits. In this paper, we present details of the 2nd prototype system and results of an evaluation of imaging performance on the simultaneous measurements.\nII.METHODS AND MATERIALSFig. 2 shows a photograph of the second prototype system. The second prototype consisted of a birdcage head coil and 24 PET detector units with two PET detectors each (Fig. 3). A carbon fiber shielding box was used instead of the Cu shielding which had been used in our previous prototype. Each PET detector consisted of a 14 x 14 x 4-layer array of lutetium fine silicate (LFS) crystals (1.9 mm x 1.9 mm x 4.0 mm), an 8 x 8 multi-pixel photon counter (MPPC, S12641PA-050) array and a readout circuit board which contained an ASIC functioning as amplifiers and temperature control system. The RF coil was dedicated to a 3 T MRI (MAGNETOM Verio, Siemens). There were eight RF-coil elements and the 24 PET detector units were mounted on gaps between the RF-coil elements.We conducted performance tests of the prototype system using the setup in fig. 4. The PET detectors, the RF-coil, a data acquisition system and cables were in an MRI room. A control PC and power supplies for the MPPCs and preamplifiers were outside the MRI room which connected to all the detectors through a penetration panel with metal cables. The cables were shielded with copper foils and grounded at the penetration panel. The data acquisition system was connected to the control PC through optical fibers. Energy spectrum and 2D position histogram were measured for uniform irradiation of a 22Na point-like source with and without MRI measurement. Spin echo (SE) and echo planer imaging (EPI) technique were used in the evaluation experiment of the simultaneous measurements. III.RESULTS AND DISCUSSIONSFig. 5 shows energy spectra for the 511keV annihilation radiations with and without the MRI measurements. Differences of light yield between the crystals were not corrected for each energy spectrum. Three energy spectra were perfectly overlapped. From these results, no degradation of the energy information was observed in simultaneous measurements. Fig, 6 shows flood histograms of the PET detectors obtained with and without MRI measurements (SE and EPI). The spots corresponding with two 14 × 14 × 4 crystal blocks are shown in one flood histogram. The crystals in all four layers clearly appear in all flood histograms. Comparison of the three flood histograms shows no degradation of crystal identification performance for all MRI sequences.\nFig. 7 shows examples of magnitude images measured for the cylindrical phantom using the second prototype. The left and right figures are obtained without and with PET measurement, respectively. The SNRs of the phantom images with and without PET measurement were 85.8 and 80.6, respectively. The degradation of the SNR was ~5%. In addition, effect of secondary magnetic field due to eddy current was reduced by using the carbon shielding box, compared with the previous prototype.\nIV.CONCLUSIONWe developed a second prototype of a RF-coil integrated with DOI-PET detectors and evaluated its performance in simultaneous measurement. The influence of the simultaneous measurements with MRI on the PET performance is negligible. In addition, the SNR of the phantom image in the magnitude images and eddy current effect were reduced, compared with the previous prototype system.PSMR2016 - 5th Conference on “PET/MR and SPECT/MR