Development of a full-ring 'add-on PET' prototype: A head coil with DOI-PET detectors for integrated PET/MRI

摘要

We developed a full-ring "add-on PET' prototype which is brain-dedicated and consists of a RF-head coil with four-layer depth-of-interaction (DOI) PET detectors for integrated PET/MRI in order to evaluate performance of our previously proposed add-on PET system and to investigate the mutual influences between the individual PET and MRI modalities when they are integrated in simultaneous measurements. In this add-on PET prototype, the DOI detectors are mounted on the head coil and close to the patient head. As a result, higher sensitivity and higher spatial resolution can be achieved for the integrated PET/MRI, compared with conventional whole body PET/MRI systems. In addition, implementation cost can be reduced, tuning of the RF-coil can be optimized and PET and MRI images can be obtained simultaneously in exactly the same positions. Specifically, the full-ring prototype consists of eight DOI-PET detectors and a birdcage type head coil of a 3T MRI. The radius of the PET ring is 123.9 mm. The distance from the center to the RF-coil elements is 130.5 mm. The scintillator blocks consist of lutetium-yttrium oxyorthosilicate scintillators arranged in 19 × 6 × 4 layers with reflectors inserted between them. The size of each crystal element is 2.0 mm × 2.0 mm × 5.0 mm. We evaluated performance of the full-ring prototype in simultaneous measurements of the integrated PET/MRI. We obtained spatial resolutions of 2.3 mm at the center of the field-of-view (FOV) and lower than 3.5 mm in the whole FOV. The energy resolution of 19.4% was obtained for 511 keV gamma-rays. In addition, we observed no degradation of PET performance caused by the MRI measurement The signal-to-noise ratio (SNR) of the MRI image was 209.4 in simultaneous measurements with the PET. The maximum ∆B_0 and maximum difference of the secondary magnetic field due to the eddy current effect were smaller than 0.8 ppm and ± 5.0 μT, respectively. We concluded that sufficient spatial resolution and detector performance of the PET were obtained for the add-on PET prototype even in the simultaneous measurements with the MRI. The influence of the PET detectors on the static magnetic field and the SNR of the MRI images were not problems, although the eddy current effect must be suppressed.