Atsushi Ohtani

Performance evaluation of a high-sensitivity large-aperture small-animal PET scanner: ClairvivoPET.

ObjectiveIn this study, we evaluated the performance of a newly commercialized small-animal positron emission tomography (PET) scanner, ClairvivoPET, which provides significant advantages in spatial resolution, sensitivity, and quantitative accuracy.MethodsThis scanner consists of depth of interaction detector modules with a large axial extent of 151 mm and an external 137Cs source for attenuation correction. Physical performances, resolution, sensitivity, scatter fraction (SF), counting rate including noise equivalent count (NEC) rate, quantitative accuracy versus activity strength, and transmission accuracy, were measured and evaluated. Animal studies were also performed.ResultsTransaxial spatial resolution, measured with a capillary tube, was 1.54 mm at the center and 2.93 mm at a radial offset of 40 mm. The absolute sensitivity was 8.2% at the center, and SFs for mouse-and rat-sized phantoms were 10.7% and 24.2%, respectively. Peak NEC rates for mouse-and rat-sized uniform cylindrical phantoms were 328 kcps at 173 kBq/ml and 119 kcps at 49 kBq/ml, respectively. The quantitative stability of emission counts against activity strength was within 2% over 5 half-lives, ranging from 0.6 MBq to 30 MBq. Transmission measurement based on segmented attenuation correction allowed 6-min and 10-min scans for mouse-and rat-sized cylindrical phantoms, respectively. Rat imaging injected with 18F-NaF resulted in visibility of fine bone structures, and mouse imaging injected with 18F-D-fluoromethyl tyrosine demonstrated the feasibility of using this system to obtain simultaneous time activity curves from separate regions, such as for the heart and tumors.ConclusionsClairvivoPET is well suited to quantitative imaging even with short scan times, and will provide a number of advantages in new drug development and for kinetic measurement in molecular imaging.

Development of a high resolution whole-body DOI PET system

High spatial resolution Positron Emission Tomography (PET) imaging provides functional data about a patients body and provides us with a range of new possibilities, including medicine development, new diagnostic methods, and others. The use of Depth of Interaction (DOI) detectors is one effective technique for the improvement of whole-body PET imaging, as is evident in the fact that most small animal PET images are taken using a DOI system. In this study, we developed a prototype whole-body DOI- equipped PET system and evaluated its performance. This scanners DOI detectors consist of dual-layer (front and rear) 9 x 10 array of GSO/GSO crystals, a light guide and two rectangular double-anode photo multiplier tubes (PMTs). The DOI layers are discriminated according to decay time differences, which are controlled by Ce concentrations. The DOI detectors are arranged in a circular detector ring, with a diameter of 664 mm. In order to avoid parallax errors, coincidental events, including rear layer detection, were addressed to the nearest neighbor LOR front-layer coincidence pair. To evaluate the spatial resolution of this PET system at various radial positions, 22Na point sources at 1, 10, 20, and 25 cm offset from the center were reconstructed, both with and without DOI information. Results showed that spatial resolution degradation at 25 cm improved from 195 % to 154 % as a result of using DOI data. During a visual evaluation using a Derenzo phantom, image reconstruction using DOI data clarified the image and corrected hot rod shapes. The results presented here show that DOI detectors were useful in creating an effective whole-body PET system.

A scatter-compensated crystal interference factor in component-based normalization for high-resolution whole-body PET

On a positron emission tomography (PET) scanner consisting of block detectors, coincidence responses to scattered radiation may differ from those to true depending on the crystal pair position within a coincidence block pair. Furthermore, these differences are considered to vary according to the radial position of the coincidence block pair. These conditions create ringing artifacts in the reconstructed image due to the lack of scatter compensation in detector normalization. In component-based normalization, a scatter-compensated crystal interference factor is therefore required in addition to the scatter-compensated block profile and intrinsic crystal efficiencies. In this study, we propose a scatter-compensated component-based normalization scheme using an annulus phantom, which provides true and scattered radiations over a large transaxial field of view, and evaluates the quality of three different-sized phantom images with whole-body PET. The results showed that the proposed normalization method significantly reduces the ringing artifacts in reconstructed images with different scattered/true fractions. The proposed algorithm, which introduced the scatter-compensated crystal interference factor, worked well under different scattered/true ratio conditions and was considered to be a robust, practical normalization method in high-resolution whole-body PET.

Optimization of enhanced energy window on a whole-body DOI PET system

The use of depth of interaction (DOI) detectors is one effective technique to improve the spatial resolution of whole-body PET imaging. We have developed a whole-body PET system with DOI detectors that achieves less than 3 mm (FWHM) uniform spatial resolution, independent of the radial position in the transaxial field of view. On the other hand, improvements in sensitivity and noise equivalent count rate (NECR) were expected by implementing the DOI-dependent extended energy window (DEEW) method, employing a different energy window for each layer. Furthermore, the energy-based selective coincidence (ESC) method reduces multiple coincidences. In this study, we investigated sensitivity and NECR improvements using the DEEW method and the optimized enhanced energy window setting. The DOI detector consists of dual-layer GSO crystals, and the DOI detectors are arranged into a circular detector ring with a diameter of 664 mm. We evaluated the sensitivity and NECR performance based on the NEMA NU2-2001 standard. ESC and DEEW (1st layer: 412-624 keV, 2nd layer: 200-300 keV + 400-624 keV) resulted in better system performance than the conventional method.

Scatter-compensated geometrical components for detector normalization in whole-body PET

On a PET scanner consisting of block detectors, coincidence responses to scattered radiation may differ from those to trues depending on the crystal pair position within a coincidence block pair. Furthermore, these differences are considered to vary according to the radial position of the coincidence block pair. In component-based normalization, a scatter-compensated geometrical component is therefore required in addition to the scatter-compensated block-profile and intrinsic crystal efficiencies. In this study, we propose a scatter-compensated component-based normalization scheme using an annulus phantom which provides true and scattered radiations over a large transaxial field-of-view. We evaluate the normalization accuracy using three different-sized phantoms with whole-body PET. Results showed that the proposed normalization method significantly reduces the ringing artifacts in reconstructed images with different scattered/true fractions. The proposed algorithm worked well under different scattered/true radiation conditions, and was considered to be a practical normalization method in whole-body PET.

Development of a high-spatial-resolution whole-body DOI PET system using LGSO/GSO crystals

Use of Depth of Interaction (DOI) detectors is one effective technique for improving whole-body PET imaging.