A proposal of an open PET geometry

Abstract

The long patient port of a PET scanner tends to put stress on patients, especially patients with claustrophobia. It also prevents doctors and technicians from taking care of patients during scanning. In this paper, we proposed an 'open PET' geometry, which consists of two axially separated detector rings. A long and continuous field-of-view (FOV) including a 360 degrees opened gap between two detector rings can be imaged enabling a fully 3D image reconstruction of all the possible lines-of-response. The open PET will become practical if iterative image reconstruction methods are applied even though image reconstruction of the open PET is analytically an incomplete problem. First we implemented a 'masked' 3D ordered subset expectation maximization (OS-EM) in which the system matrix was obtained from a long 'gapless' scanner by applying a mask to detectors corresponding to the open space. Next, in order to evaluate imaging performance of the proposed open PET geometry, we simulated a dual HR+ scanner (ring diameter of D = 827 mm, axial length of W = 154 mm x 2) separated by a variable gap. The gap W was the maximum limit to have axially continuous FOV of 3W though the maximum diameter of FOV at the central slice was limited to D/2. Artifacts, observed on both sides of the open space when the gap exceeded W, were effectively reduced by inserting detectors partially into unnecessary open spaces. We also tested the open PET geometry using experimental data obtained by the jPET-D4. The jPET-D4 is a prototype brain scanner, which has 5 rings of 24 detector blocks. We simulated the open jPET-D4 with a gap of 66 mm by eliminating 1 block-ring from experimental data. Although some artifacts were seen at both ends of the opened gap, very similar images were obtained with and without the gap. The proposed open PET geometry is expected to lead to realization of in-beam PET, which is a method for an in situ monitoring of charged particle therapy, by letting the beams pass through the gap. The proposed open PET geometry will also allow simultaneous PET/CT measurements of the same PET FOV as the CT FOV, in contrast to the conventional PET/CT where each FOV is separated by several tens of centimeters.

Imaging simulations of an “OpenPET” geometry with shifting detector rings

“…1, the central point of the field of view (FOV), where the highest sensitivity is obtained, is opened. In our previous report [1], we showed the following: 1. A long and continuous FOV including a 360-degree open gap between two detector rings can be imaged, allowing a fully 3D image reconstruction of all the possible lines of response (LORs).…”

“…Our previous report [1] also indicated that there is a limited range for the gap width. Figure 2 illustrates an OpenPET geometry having two detector rings of axial length W, separated by a gap G. When G [ W, the FOV becomes axially discontinuous, as shown in Fig.…”

“…In this study, we applied the masked ordered-subset expectation-maximization (OS-EM) algorithm [1], which is an efficient implementation of the original OS-EM algorithm [4]. In order to avoid calculating all variations of the system matrix and projection data for the variable gaps, we modeled the OpenPET as a long and gap-less scanner to which a mask for eliminating LORs corresponding to the gap was applied.…”

“…Therefore, we have proposed an ''OpenPET'' geometry, which consists of two axially separated detector rings (Fig. 1b) [1]. As shown in Fig.…”

Using compressive sensing to recover images from PET scanners with partial detector rings

“…31,32 These systems, however, have large detector gaps which might not be adequately recovered using our proposed approach. In this regard, an in depth investigation about the value of our proposed approach to such systems is warranted.…”