Performance evaluation of a whole-body prototype PET scanner with four-layer DOI detectors

Abstract

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.

Evaluation of image quality at the detector’s edge of dedicated breast positron emission tomography

“…dbPET achieves higher sensitivity and resolution than whole-body PET/CT by i) reduction of respiratory movement of the breast by acquisition in the prone position, and ii) bringing the detector close to the breast. The 4-layer depth-of-interaction detector used in dbPET can maintain sensitivity and resolution at the edges of the transverse eld of view [24,25]. However, if the background mammary gland shows physiological FDG uptake, the contrast of the lesion and background is higher than that of PET/CT.…”

Evaluation of image quality at the detector’s edge of dedicated breast positron emission tomography

“…dbPET achieves higher sensitivity and resolution than whole-body PET/CT by i) reduction of respiratory movement of the breast by acquisition in the prone position, and ii) bringing the detector close to the breast. The 4-layer depth-of-interaction detector used in dbPET can maintain sensitivity and resolution at the edges of the coronal eld of view [28,29]. However, if the background mammary gland shows physiological FDG uptake, the measured dbPET contrast was higher than the measured PET/CT contrast in the same lesion.…”

Evaluation of image quality at the detector’s edge of dedicated breast positron emission tomography

“…dbPET achieves higher geometric sensitivity and spatial resolution than whole-body PET/CT by i) DOI detector and smaller voxel size in the former than in the latter, ii) reduction of respiratory movement of the breast by acquisition in the prone position, and iii) bringing the detector close to the breast. Especially, the 4-layer DOI detector used in dbPET can maintain geometric sensitivity and spatial resolution at the edges of the coronal eld of view [28,29]. The 4-layer DOI detector can detect gamma-ray interaction positions accurately by detecting Compton scattering events, and DOI information can dramatically reduce the voxel size.…”