Performance demonstration of a hybrid Compton camera with an active pinhole for wide-band X-ray and gamma-ray imaging

Abstract

X-ray and gamma-ray imaging are technologies with several applications in nuclear medicine, homeland security, and high-energy astrophysics. However, it is generally difficult to realize simultaneous wide-band imaging ranging from a few tens of keV to MeV because different interactions between photons and the detector material occur, depending on the photon energies. For instance, photoabsorption occurs below 100 keV, whereas Compton scattering dominates above a few hundreds of keV. Moreover, radioactive sources generally emit both X-ray and gamma-ray photons. In this study, we develop a "hybrid" Compton camera that can simultaneously achieve X-ray and gamma-ray imaging by combining features of "Compton" and "pinhole" cameras in a single detector system. Similar to conventional Compton cameras, the detector consists of two layers of scintillator arrays with the forward layer acting as a scatterer for high-energy photons (> 200 keV) and an active pinhole for low-energy photons (< 200 keV). The experimental results on the performance of the hybrid camera were consistent with those from the Geant4 simulation. We simultaneously imaged [Formula: see text]Am (60 keV) and [Formula: see text]Cs (662 keV) in the same field of view, achieving an angular resolution of 10[Formula: see text] (FWHM) for both sources. In addition, imaging of [Formula: see text]At was conducted for the application in future nuclear medicine, particularly radionuclide therapy. The initial demonstrative images of the [Formula: see text]At phantom were reconstructed using the pinhole mode (using 79 keV) and Compton mode (using 570 keV), exhibiting significant similarities in source-position localization. We also verified that a mouse injected with 1 MBq of [Formula: see text]At can be imaged via pinhole-mode measurement in an hour.

Figure 1

Comparison of the simulated energy response of the intrinsic efficiency (left) and angular resolution (right) with the results of actual measurements.

Figure 2

(Upper) The experimental configuration of the simultaneous measurement of ${}^{241}$Am and ${}^{137}$Cs. (Lower) The MLEM reconstructed images of the ${}^{241}$Am (60 keV) source analyzed by the pinhole mode (left) and the ${}^{137}$Cs (662 keV) source analyzed by the Compton mode (right).

Figure 3

The MLEM reconstructed images of “L”-shaped sources. Pinhole reconstruction of the ${}^{241}$Am source (left) and Compton reconstruction of the ${}^{137}$Cs source (right) that were measured separately.

Figure 4

Energy spectrum of ${}^{211}$At obtained by a LaBr${}_3$ scintillator coupled to a PMT.

Figure 5

The pinhole MLEM reconstruction image (left) and the Compton MLEM reconstruction image (right) of a bottle with ${}^{211}$At at the center of the FOV (upper) and ${30}^{\circ }$ to the right (lower).

Figure 6

(Left) Experimental configuration of the measurement of the mouse administered with ${}^{211}$At. (Center) The pinhole MLEM reconstructed image obtained by 1 h of measurement. (Right) The Compton MLEM reconstructed image obtained by 11 h of measurement.

Figure 7

The configuration of the hybrid camera (left). Schematic view of the pinhole event (center) for the lower energy range and the Compton event (right) for the higher energy range that are used for the pinhole/Compton reconstruction in the hybrid camera.

Figure 8

(Left) Energy spectrum of all the events detected in either detector (black), the events accumulated by with the front detector (green), the events obtained only with the rear detector (blue) and the events reacted with both detectors (red) accumulated from placing ${}^{241}$Am and ${}^{137}$Cs sources simultaneously in front of the detector. (Right) 2D energy spectrum of coincidence events from the front detector (scatterer) and the rear detector (absorber). The area painted red corresponds to the energy cut range for 662 keV Compton events. The brightest area ($E_{\mathrm{front}}\sim$ 200 keV) corresponds to back-scattering events.