Abstract
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Introduction: Dual-modality small animal PET/MRI imaging is a very powerful tool in biomedical research. A MRI compatible small animal PET scanner called SIAT aPET was developed by using dual-ended readout depth encoding detectors to simultaneously achieve a uniform high spatial resolution and high sensitivity at Shenzhen Institutes of Advanced Technology (SIAT). In this work, the mutual interference is measured and simultaneous PET/MRI imaging is performed by inserting the SIAT aPET into the uMR790 3T MRI scanner of United Imaging. Methods: SIAT aPET consists of 4 detector rings with 12 detector modules per ring. The detector module consists of one 23×23 LYSO array with crystals of 1.0×1.0×20.0 mm3, read out by two 8×8 SiPM arrays from both ends. The SiPM array is the Hamamatsu S13361-3050NE-08 array with a pixel size of 3.0×3.0 mm2 and a pitch of 3.2 mm. The detector ring diameter of the scanner is 111 mm and the axial field of view is 106 mm. It achieves a spatial resolution of <1 mm and a sensitivity of 16.0% at center of the scanner. The SIAT aPET scanner contains three parts, the insert, electronics cabinet, and accessories. The insert part accommodates detectors and front-end electronics that are shielded by an inner and an outer carbon fiber tubes. The electronics cabinet that houses the signal process electronics is placed at the corner of the MRI room about 5 meters away from the MRI FOV. The accessories including air cooler, power supplies and data acquisition computer are placed outside the MRI room. A dedicate birdcage RF coil with an inner diameter of 6 cm and a length of 15 cm was developed. The SIAT aPET was inserted into the bore of the uMR790 3T MRI scanner. A uniform water cylinder phantom was imaged by using a few common MRI sequences, GRE with a short TR (250 us) and a long TR (850 us), T1 weighted SE and T2 weighted SE under the conditions of PET power-off and PET power-on. The signal-to-noise ratios (SNRs) of the transverse slices of the MRI images were compared by calculating the noise from areas of both the background and phantom. The flood histogram and energy resolution of SIAT aPET detectors as well as the sensitivity of the PET scanner were also measured under the condition of MRI imaging sequence turn-off and running. A Derenzo phantom filled with 18F-FDG was scanned for the PET scanner outside the MRI bore and inside the MRI bore with the MRI sequence running. Last, simultaneous PET/MRI imaging of a 19g mouse was performed. The mouse was injected with 280 uCi 18F-FDG. After 80 minutes waiting time, the mouse was scanned for 25 minutes on SIAT aPET and at the same time a MRI scan was performed with the GRE sequence. Results: Compared to that of PET power-off, the average SNR of the MRI images when PET power-on was reduced for about 30% and 10% if the noise is calculated from areas of the background and phantom, respectively for the four commonly used MRI sequences. The MRI imaging had negligible effect on the PET performance. The average energy resolution of the detector was changed from 18.0 to 18.3% and the sensitivity of the scanner was only degraded by 0.04%. The PET images of the Derenzo phantom obtained with PET scanner outside and inside MRI were similar. Simultaneous PET/MRI images of a mouse were successfully acquired.
Conclusions: SIAT aPET, a high spatial resolution, high sensitivity and MRI compatible small animal PET insert, was developed by using dual-ended readout depth encoding detectors. The mutual interference of the SIAT aPET and the uMR790 3T MRI scanner were measured. The results showed that the MRI imaging had negligible effect on the PET performance and the PET scanner had accepted effect on the MRI imaging. The SIAT aPET can be successfully inserted into MRI scanners for simultaneously PET/MRI studies of small animals such as mice, rats and marmosets. Acknowledgments: This work is supported by the Scientific Instrument Innovation Team of CAS (GJJSTD20180002), the Hundred-Talent Program of the CAS (Yongfeng Yang) and the Key Laboratory for Magnetic Resonance and Multimodality Imaging of Guangdong Province.