Modified NEMA NU-2 performance evaluation methods for a brain-dedicated PET system with a hemispherical detector arrangement

Performance evaluation of VRAIN: a brain-dedicated PET with a hemispherical detector arrangement

Objective.For PET imaging systems, a smaller detector ring enables less intrinsic spatial resolution loss due to the photon non-collinearity effect as well as better balance between production cost and sensitivity, and a hemispherical detector arrangement is more appropriate for brain imaging than a conventional cylindrical arrangement. Therefore, we have developed a brain-dedicated PET system with a hemispherical detector arrangement, which has been commercialized in Japan under the product name of VRAIN^TM. In this study, we evaluated imaging performance of VRAIN.Approach.The VRAIN used 54 detectors to form the main hemispherical unit and an additional half-ring behind the neck. Each detector was composed of a 12 × 12 array of lutetium fine silicate crystals (4.1 × 4.1 × 10 mm³) and a 12 × 12 array of silicon photomultipliers (4 × 4 mm²active area) with the one-to-one coupling. We evaluated the physical performance of VRAIN according to the NEMA NU 2-2018 standards. Some measurements were modified so as to fit the hemispherical geometry. In addition, we performed¹⁸F-FDG imaging in a healthy volunteer.Main results.In the phantom study, the VRAIN showed high resolution for separating 2.2 mm rods, 229 ps TOF resolution and 19% scatter fraction. With the TOF gain for a 20 cm diameter object (an assumed head diameter), the peak noise-equivalent count rate was 144 kcps at 9.8 kBq ml^-1and the sensitivity was 25 kcps MBq^-1. Overall, the VRAIN provided excellent image quality in phantom and human studies. In the human FDG images, small brain nuclei and gray matter structures were clearly visualized with high contrast and low noise.Significance.We demonstrated the excellent imaging performance of VRAIN, which supported the advantages of the hemispherical detector arrangement.

Performance evaluation of dedicated brain PET scanner with motion correction system

OBJECTIVE

Various motion correction (MC) algorithms for positron emission tomography (PET) have been proposed to accelerate the diagnostic performance and research in brain activity and neurology. We have incorporated MC system-based optical motion tracking into the brain-dedicated time-of-flight PET scanner. In this study, we evaluate the performance characteristics of the developed PET scanner when performing MC in accordance with the standards and guidelines for the brain PET scanner.

METHODS

We evaluate the spatial resolution, scatter fraction, count rate characteristics, sensitivity, and image quality of PET images. The MC evaluation is measured in terms of the spatial resolution and image quality that affect movement.

RESULTS

In the basic performance evaluation, the average spatial resolution by iterative reconstruction was 2.2 mm at 10 mm offset position. The measured peak noise equivalent count rate was 38.0 kcps at 16.7 kBq/mL. The scatter fraction and system sensitivity were 43.9% and 22.4 cps/(Bq/mL), respectively. The image contrast recovery was between 43.2% (10 mm sphere) and 72.0% (37 mm sphere). In the MC performance evaluation, the average spatial resolution was 2.7 mm at 10 mm offset position, when the phantom stage with the point source translates to ± 15 mm along the y-axis. The image contrast recovery was between 34.2 % (10 mm sphere) and 66.8 % (37 mm sphere).

CONCLUSIONS

The reconstructed images using MC were restored to their nearly identical state as those at rest. Therefore, it is concluded that this scanner can observe more natural brain activity.

A 3-dimensional stationary cascade gamma-ray coincidence imager

Objective.For certain radionuclides that decay through emitting two or more gamma photons consecutively within a short time interval-called cascade gamma-rays, the location where a radiopharmaceutical molecule emits cascade gamma-rays can be identified through coincidence detection of the photons. If each cascade photon is detected through a collimation mechanism, the location of the molecule can be inferred from the intersection of the back-projections of the two photons.Approach.In this work, we report the design and evaluation of a three-dimensional stationary imager based on this concept for imaging distributions of cascade-emitting radionuclides in radiopharmaceutical therapy. The imager was composed of two gamma-ray cameras assembled in an L-shape. Both cameras were NaI(Tl) scintillator based, one with a multi-slit collimator, the other with a multi-pinhole collimator. The field of view (FOV) was 100 mm (∅) × 100 mm (L). Based on the unique characteristics of the cascade coincidence events, we used a direct back-projection algorithm to reconstruct point source images for assessing the imager's intrinsic spatial resolution and the standard maximum likelihood expectation maximization algorithm for reconstructing phantom images.Main results.We evaluated the performance of the imager in both simulated and prototype form with radionuclide¹⁷⁷Lu (cascade photon emitter). On the simulated imager, the coincidence detection efficiency at the center of FOV was 3.85 × 10^-6, the spatial resolution was 7.0 mm. On the prototype imager, the corresponding values were 3.20 × 10^-6and 6.7 mm, respectively. Simulated hot-rod and experimental cardiac phantom studies demonstrate the first three-dimensional cascade gamma coincidence imager is fully functional.

Photon counting detectors and their applications ranging from particle physics experiments to environmental radiation monitoring and medical imaging

Photomultiplier tubes (PMTs) and silicon photomultipliers (SiPMs) have played essential roles in various applications, such as fundamental particle physics experiments, nuclear medicine, and environmental radiation monitoring, for several decades. Understandings their physical properties as well as present applications is indispensable for the development and future applications of these detectors. In this review, we describe the physical principles of PMTs and SiPMs and introduce various applications of these detectors.