Parallax error caused by the detector crystal thickness degrades spatial resolution at the peripheral regions of the field-of-view (FOV) of a scanner. To resolve this issue, depth-of-interaction (DOI) measurement is a promising solution to improve the spatial resolution and its uniformity over the entire FOV. Even though DOI detectors have been used in dedicated systems with a small ring diameter such as for the human brain, breast and small animals, the use of DOI detectors for a large bore whole-body PET system has not been demonstrated yet. We have developed a four-layered DOI detector, and its potential for a brain dedicated system has been proven in our previous development. In the present work, we investigated the use of the four-layer DOI detector for a large bore PET system by developing the world’s first whole-body prototype. We evaluated its performance characteristics in accordance with the NEMA NU 2 standard. Furthermore, the impact of incorporating DOI information was evaluated with the NEMA NU 4 image quality phantom. Point source images were reconstructed with a filtered back projection (FBP), and an average spatial resolution of 5.2  ±  0.7 mm was obtained. For the FBP image, the four-layer DOI information improved the radial spatial resolution by 48% at the 20 cm offset position. The peak noise-equivalent count rate (NECR) was 22.9 kcps at 7.4 kBq ml−1 and the scatter fraction was 44%. The system sensitivity was 5.9 kcps MBq−1. For the NEMA NU 2 image quality phantom, the 10 mm sphere was clearly visualized without any artifacts. For the NEMA NU 4 image quality phantom, we measured the phantom at 0, 10 and 20 cm offset positions. As a result, we found the image with four-layer DOI could visualize the 2 mm-diameter hot cylinder although it could not be recognized on the image without DOI. The average improvements in the recovery coefficients for the five hot rods (1–5 mm) were 0.3%, 4.4% and 26.3% at the 0, 10 and 20 cm offset positions, respectively (except for the 1 mm-diameter rod at the 20 cm offset position). Although several practical issues (such as adding end-shields) remain to be addressed before the scanner is ready for clinical use, we showed that the four-layer DOI technology provided higher and more uniform spatial resolution over the FOV and improved contrast for small uptake regions located at the peripheral FOV, which could improve detectability of small and distal lesions such as nodal metastases, especially in obese patients.

Comparative study of alternative Geant4 hadronic ion inelastic physics models Comparative study of alternative Geant4 hadronic ion inelastic physics models for prediction of positron-emitting radionuclide production in carbon and oxygen for prediction of positron-emitting radionuclide production in carbon and oxygen ion therapy ion therapy

This work presents an iterative method for the estimation of the absolute dose 24 distribution in patients undergoing carbon ion therapy, via analysis of the distribution 25 of positron annihilations resulting from the decay of positron-emitting fragments 26 created in the target volume. The proposed method relies on the decomposition of the total positron-annihilation distributions into profiles of the three principal 28 positron-emitting fragment species -11 C, 10 C and 15 O. A library of basis functions 29 is constructed by simulating a range of monoenergetic 12 C ion irradiations of a 30 homogeneous polymethyl methacrylate phantom and measuring the resulting one-31 dimensional positron-emitting fragment profiles and dose distributions. To estimate 32 the dose delivered during an arbitrary polyenergetic irradiation, a linear combination 33 of factors from the fragment profile library is iteratively fitted to the decomposed 34 positron annihilation profile acquired during the irradiation, and the resulting weights 35 combined with the corresponding monoenergetic dose profiles to estimate the total 36 dose distribution. A total variation regularisation term is incorporated into the fitting 37 process to suppress high-frequency noise. The method was evaluated with fourteen 38 different polyenergetic 12 C dose profiles in a polymethyl methacrylate target: one 39 which produces a flat biological dose, ten with randomised energy weighting factors, 40 and three with distinct dose maxima or minima within the spread-out Bragg peak 41 region. The proposed method is able to calculate the dose profile with mean relative 42 errors of 0.8%, 1.0% and 1.6% from the 11 C, 10 C, 15 O fragment profiles, respectively, 43 and estimate the position of the distal edge of the SOBP to within an average of 44 0.7 mm, 1.9 mm and 1.2 mm of its true location. 45 1. Introduction 46 Carbon ion therapy is a form of radiotherapy in which accelerated carbon ions are used 47 to deliver a therapeutic dose to the target volume [1, 2, 3]. This treatment modality 48 offers several advantages over photon therapy, such as a well-defined energy-dependent 49 depth of maximum dose shortly before the particles come to rest (known as the Bragg 50 peak), and a high relative biological effectiveness (RBE), particularly at the distal end 51 of the particle range [2, 4, 5]. The Bragg peak can be extended to deliver a uniform 52 dose over a depth range by superimposing monoenergetic beams with different energies 53 and fluences to form a polyenergetic beam, also known as a spread-out Bragg peak 54 (SOBP) [1]. However, anatomical changes, errors in patient positioning and errors in 55 the estimation of ion range may cause significant dose to be delivered outside the target 56 region due to the steep dose gradients between the target region and surrounding healthy 57 tissue [6]. 58 During carbon ion therapy, a variety of target and projectile fragments are produced 59 through nuclear inelastic collisions between ions in the beam and nuclei in the target 60 volum...