Development of the helmet-chin PET prototype

There is a strong potential demand for high-sensitivity and low-cost brain positron emission tomography (PET) imaging that is applicable to early diagnosis of Alzheimer's disease. Therefore, we have proposed a high-sensitivity dedicated brain PET geometry composed of a helmet detector having a hemisphere shape and a chin detector, which we call helmet-chin PET. Because the shape of a human head is a sphere, the hemispherical arrangement of the detectors allows closer positioning of detectors and better sensitivity than the conventional cylindrical arrangement. In addition, adding detectors around the chin position significantly improves the sensitivity at the center where the cerebellum is located. For a proof-of-concept of the helmet-chin PET, we developed the first prototype of the helmet-chin PET using 4-layer depth-of-interaction (DOI) detectors. The helmet detector for the prototype system was realized by multiple rings having different numbers of detectors and a cross-shaped part covering the top. We used in total 54 DOI detectors, each of which consisted of 1,024 Zr-doped GSO crystals with dimensions of 2.8 × 2.8 × 7.5 mm3 and a high-sensitivity, 64-ch flat-panel photomultiplier tube. In performance evaluations, we determined there were uniform spatial resolutions of 3.0 mm by an analytical method and 1.4 mm by an iterative method. Peak sensitivity was measured as 10 % at a region near the top of the head, which was almost equivalent to the central sensitivity of the cylindrical PET composed of 120 DOI detectors. In addition, we performed an initial imaging test with a brain phantom and we reconstructed the images with and without the chin detector. We found the slice near the bottom of the helmet detector had strong noise without the chin detector, while the slice had good imaging performance with it, and the overall image quality was improved. Therefore, we concluded that the helmet-chin PET had high potential for realizing high-sensitivity, low-cost, and accurate brain imaging.