Akram Mohammadi

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 Alzheimers 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.

Feasibility of secondary 15O beam production for in-beam PET

For the purpose of in situ visualization of an irradiation field in particle therapy, it is ideal to use radioactive beams for in-beam positron emission tomography (PET) imaging. In the Heavy Ion Medical Accelerator in Chiba (HIMAC), there is a beam course to produce 11C and 10C as secondary beams, but generation of 15O has never been investigated. This study was devoted to analysis of the feasibility of secondary 15O production for in-beam PET by 16O primary beam in different absorbing materials (targets) with various thicknesses in order to find out the optimum target material and its thickness. The study was performed using analytical method, LISE++ code, and the Monte Carlo method, PHITS code. The calculation results showed that the highest intensity of the secondary 15O beam observed for primary 16O beam decelerating in liquid hydrogen but practically almost impossible to apply. The final optimum target was considered to be polyethylene with thickness of 11 cm. The 15O beam was produced in the HIMAC secondary beam line using the optimum target and the in-beam PET imaging was presented for a PMMA phantom irradiated with the produced beam.

Development of a Whole-Body Dual Ring OpenPET for in-Beam PET

For in-beam positron emission tomography (PET), which is an in situ monitoring method for charged particle therapy, we developed the world’s first open-type PET system “OpenPET.” Following our previous studies for small prototypes, in this paper, we finally developed a whole-body dual ring OpenPET (WBDROP). This scanner has two separated rings. Each ring has two detector rings of 40 detector blocks, each of which has the capability of 4-layer depth-of-interaction identification. The ring diameter is 66 cm, and the adjustable open gap is fixed as 9 cm. We found that the system sensitivity was 4.4%. The average spatial resolution, which was measured through filtered back projection, was about 3.5 mm. For in-beam PET, imaging performance was confirmed through phantom experiments. Phantom study results with 11C beam and 10C beam irradiations of about 3 Gy showed that the beam stopping position in the target could be measured with a precision of better than 2 mm. The developed WBDROP promises high performance for in-beam PET imaging.

Radiation damage of the multi-pixel photon counter to be used for in-beam PET in carbon therapy

We are developing the OpenPET for in-beam imaging in carbon ion therapy. We have succeeded in showing a proof-of-concept by developing small OpenPET prototypes that we used in the HIMAC. One of the major issues to realize the OpenPET is radiation damage to detectors because a number of fragmented particles are incident on the detectors located downstream from the target. While we are currently using photo-multiplier tubes in the OpenPET, some groups reported feasibility of use of a semiconductor photo-detector in hadron therapy. In this paper, therefore, we tested radiation hardness of the multi-pixel photon counter (MPPC) to show feasibility of its use in the OpenPET for carbon ion therapy. We used a single pixel MPPC (S10931-050) to evaluate radiation hardness. The experiment was performed in the PH2 course of the HIMAC. The energy of the 12C beam was 290MeV/u and beam intensity was 1.6 × 109 particles per second (pps) which was ten times higher than the typical clinical beam intensity. The carbon ions entered and stopped in a water phantom (10 × 10 × 20 mm3). The MPPC was 30 cm from the back end of the water phantom at an angle of 30 degrees. We measured the energy spectrum for 22Na point source and the dark count spectrum before and after 10, 20, 30, 40, 50, 70 min irradiations. We found pulse height and energy resolution of the energy spectra were degraded as the irradiation time was increased.