The objective of this study is to characterize the performance of the preclinical avalanche photodiode (APD)-based TM subsystem of the fully integrated trimodality PET/SPECT/CT Triumph TM scanner using the National Electrical Manufacturers Association (NEMA) NU 04-2008 protocol. The characterized performance parameters include the spatial resolution, sensitivity, scatter fraction, counts rate performance and image-quality characteristics. The PET system is fully digital using APD-based detector modules with highly integrated electronics. The detector assembly consists of phoswich pairs of Lu 1.9 Y 0.1 SiO 5 (LYSO) and Lu 0.4 Gd 1.6 SiO 5 (LGSO) crystals with dimensions of 2 × 2 × 14 mm 3 having 7.5 cm axial and 10 cm transverse field of view (FOV). The spatial resolution and sensitivity were measured using a small 22 Na point source at different positions in the scanner's FOV. The scatter fraction and count rate characteristics were measured using mouse-and rat-sized phantoms fitted with an 18 F line source. The overall imaging capabilities of the scanner were assessed using the NEMA image-quality phantom and laboratory animal studies. The NEMA-based radial and tangential spatial resolution ranged from 1.7 mm at the center of the FOV to 2.59 mm at a radial offset of 2.5 cm and from 1.85 mm at the center of the FOV to 1.76 mm at a radial offset of 2.5 cm, respectively. Iterative reconstruction improved the spatial resolution to 0.84 mm at the center of the FOV. The total absolute system sensitivity is 12.74% for an energy window of 250-650 keV. For the mouse-sized phantom, the peak noise equivalent count rate (NECR) is 183 kcps at 2.07 MBq cc -1 , whereas the peak true count rate is 320 kcps at 2.5 MBq cc -1 with a scatter fraction of 19%. The rat-sized phantom had a scatter fraction of 31%, with a peak NECR of 67 kcps at 0.23 MBq cc -1 and a peak true count rate of 186 kcps at 0.27 MBq cc -1 . The average activity concentration and percentage standard 4 Author to whom any correspondence should be addressed. TM system is able to produce high-quality and highly contrasted images in a reasonable time, and as such it is well suited for preclinical molecular imaging-based research.

The purpose of this work was to evaluate the performance characteristics of the preclinical X-PET subsystem of the FLEX Triumph PET/CT scanner based on the NU 4-2008 standards of the National Electrical Manufacturers Association (NEMA). Methods: The performance parameters evaluated include the spatial resolution, scatter fraction, count losses and random coincidences, sensitivity, and image-quality characteristics. The PET detector array consisted of 11,520 individual bismuth germanate crystals arranged in 48 rings and 180 blocks, with an axial field of view (FOV) of 11.6 cm and a inner ring diameter of 16.5 cm. The spatial resolution was measured with a small 22 Na point source (diameter, 0.25 mm) at different radial offsets from the center. Sensitivity was calculated using the same source by stepping the source axially through the axial FOV of the scanner. Scatter fraction and counting-rate performances were determined using a mouse-and rat-sized phantom with an 18 F line source insert. The NEMA image-quality phantom and rodent imaging were also performed to access the overall imaging capabilities of the scanner. Results: Tangential spatial resolution in terms of full width at half maximum varied between 2.2 mm at the center of the FOV and 2.3 mm at a radial offset of 2.5 cm. The radial spatial resolution varied between 2.0 at the center and 4.4 mm at a radial offset of 2.3 cm. The peak system absolute sensitivity was 5.9% at the center of the FOV. The absolute system sensitivity was 0.67 counts/s/Bq, and the relative total system sensitivity was 73.9%. The scatter fraction for the mouse-sized phantom was 7.9%, with a peak true counting rate of 168 kilocounts per second (kcps) at 0.3 MBq/mL and a peak noise-equivalent counting rate of 106 kcps at 0.17 MBq/ mL. The rat-sized phantom had a scatter fraction of 21%, with a peak true counting rate of 93 kcps at 0.034 MBq/mL and a peak noise-equivalent counting rate of 49 kcps at 0.02 MBq/mL. Recovery coefficients for the image-quality phantom ranged from 0.13 to 0.88. Conclusion: The performance of the X-PET scanner based on the NEMA NU 4-2008 standards was fully characterized. The overall performance demonstrates that the X-PET system is suitable for preclinical research.

The authors demonstrate that the TEW method leads to overestimation in scatter and crosstalk for the CZT-based imaging system while the proposed scatter and crosstalk correction method can provide more accurate self-scatter and down-scatter estimations for quantitative single-radionuclide and dual-radionuclide imaging.

Multimodality molecular imaging using high resolution positron emission tomography (PET) combined with other modalities is now playing a pivotal role in basic and clinical research. The introduction of combined PET/CT systems in clinical setting has revolutionized the practice of diagnostic imaging. The complementarity between the intrinsically aligned anatomic (CT) and functional or metabolic (PET) information provided in a “one-stop shop” and the possibility to use CT images for attenuation correction of the PET data has been the driving force behind the success of this technology. On the other hand, combining PET with Magnetic Resonance Imaging (MRI) in a single gantry is technically more challenging owing to the strong magnetic fields. Nevertheless, significant progress has been made resulting in the design of few preclinical PET systems and one human prototype dedicated for simultaneous PET/MR brain imaging. This paper discusses recent advances in PET instrumentation and the advantages and challenges of multimodality imaging systems. Future opportunities and the challenges facing the adoption of multimodality imaging instrumentation will also be addressed.

Conventional 2-dimensional planar imaging of 123 I-metaiodobenzylguanidine ( 123 I-mIBG) is not fully quantitative. To develop a more accurate quantitative imaging approach, we investigated dynamic SPECT imaging with kinetic modeling in healthy humans to obtain the myocardial volume of distribution (V T ) for 123 I-mIBG. Methods: Twelve healthy humans underwent 5 serial 15-min SPECT scans at 0, 15, 90, 120, and 180 min after bolus injection of 123 I-mIBG on a hybrid cadmium zinc telluride SPECT/CT system. Serial venous blood samples were obtained for radioactivity measurement and radiometabolite analysis. List-mode data of all the scans were binned into frames and reconstructed with attenuation and scatter corrections. Myocardial and blood-pool volumes of interest were drawn on the reconstructed images to derive the myocardial timeactivity curve and input function. A population-based blood-to-plasma ratio (BPR) curve was generated. Both the population-based metabolite correction (PBMC) and the individual metabolite correction (IMC) curves were generated for comparison. V T values were obtained from different compartment models, using different input functions with and without metabolite and BPR corrections. Results: The BPR curve reached the peak value of 2.1 at 13 min after injection. Parent fraction was approximately 58% ± 13% at 15 min and stabilized at approximately 40% ± 5% by 180 min after injection. Two radiometabolite species were observed. When the reversible 2-tissue-compartment fit was used, the mean V T value was 29.0 ± 12.4 mL/cm 3 with BPR correction and PBMC, a 188% ± 32% increase compared with that without corrections. There was significant difference in V T with BPR correction (P 5 2.3e-04) as well as with PBMC (P 5 1.6e-05). The mean difference in V T between PBMC and IMC was −3% ± 8%, which was insignificant (P 5 0.39). The intersubject coefficients of variation after PBMC (43%) and IMC (42%) were similar. Conclusion: The myocardial V T of 123 I-mIBG was established in healthy humans for the first time. Accurate kinetic modeling of 123 I-mIBG requires both BPR and metabolite corrections. Population-based BPR correction and metabolite correction curves were developed, allowing more convenient absolute quantification of dynamic 123 I-mIBG SPECT images. Met aiodobenzylguanidine labeled with 123 I ( 123 I-mIBG) is a norepinephrine analog that has a reuptake mechanism similar to that of norepinephrine without being catabolized in the myocardial sympathetic nerve endings and has been the most widely used sympathetic innervation imaging agent in research and clinical studies for risk stratification of patients with congestive heart failure (HF) (1-5). For conventional 123 I-mIBG imaging, early (15-30 min) and delayed (3-5 h) 2-dimensional (2D) planar images are typically acquired and used for calculating heart-to-mediastinum ratio (HMR) and washout rate (WOR). However, 2D planar imaging is not fully quantitative, because of the superposition of background structures with the 2D heart region of intere...