Peak Noise Equivalent Count Rate Research Articles

Performance Evaluation of the Philips “Gemini” PET/CT System

Methods to test the practical performance of the Philips Gemini PET/CT system during clinical imaging are described and results presented. The test methods used were based on the NEMA standards and those suggested by a task group of the Commission of European Communities (EEC). These were performed using the EEC emission phantom, and phantoms provided by Philips. Scans were acquired using the routine clinical protocols. Images were reconstructed using a 3D row action maximum likelihood algorithm (RAMLA). Attenuation correction (AC) was applied using CT data (CTAC) and Cs-137 source transmission scans (CsAC). Images can be acquired with the PET and CT gantries closed or open. Results of tests on the gantry alignment show that the static offsets varied by a maximum of 4.5 mm when the system was first installed, but as the floor has settled these variations have decreased to a maximum of 2.5 mm. The mean negative nonuniformity was $-4.8 pm 1.1hbox%$ and $-5.3 pm 1.1hbox%$ for CsAC and CTAC respectively and the mean positive nonuniformity was $4.7 pm 1.4hbox%$ and $5.5 pm 1.6hbox%$ . Scatter fraction measurements were made in the EEC body phantom. The reconstructed scatter fractions were 0.18 and 0.16 with and without out-of-field-of-view activity respectively ( $pm$ OFOVA) for both CsAC and CTAC images. The sinogram-derived (NEMA NU 2-1994) scatter fraction was 0.44 and 0.43 $pm$ OFOVA. At the centre of the field of view (FOV) the transverse spatial resolution (SR) was 5.0 mm in air (5.0 mm in water) and the axial SR was 6.0 mm (6.8 mm). 10 cm away from the centre the transverse radial SR was 5.4 mm (5.3 mm), the transverse tangential SR was 5.2 mm (5.4 mm) and the axial SR was 6.6 mm (7.2 mm). The recovery coefficients demonstrate the partial volume effect. The AC accuracy was similar in water (both $0.2 pm 0.1 hbox%$ ) and Teflon [ $10.9 pm 1.9hbox%$ (CTAC) and $10.0 pm 2.0hbox%$ (CsAC)], for both types of AC, but very different in air [ $14.3 pm 2.2hbox%$ (CTAC) and $31.3pm 1.5hbox%$ (CsAC)]. If the transmission maps were not segmented prior to AC these gave much higher and very different residual AC errors. The peak noise equivalent count rate was 45.2 kcps at 184 MBq and 25.2 kcps at 127 MBq $pm$ OFOVA. The system sensitivity to true and scattered counts was 34.1 kcps/(kBq/cm $^3$ ) and to true counts was 19.4 kcps/(kBq/cm $^3$ ).

Impact of high energy resolution detectors on the performance of a PET system dedicated to breast cancer imaging

We are developing a high resolution, high sensitivity PET camera dedicated to breast cancer imaging. We are studying two novel detector technologies for this imaging system: a scintillation detector comprising layers of small lutetium oxyorthosilicate (LSO) crystals coupled to new position sensitive avalanche photodiodes (PSAPDs), and a pure semiconductor detector comprising cadmium zinc telluride (CZT) crystal slabs with thin anode and cathode strips deposited in orthogonal directions on either side of each slab. Both detectors achieve 1 mm spatial resolution with 3–5 mm directly measured photon interaction depth resolution, which promotes uniform reconstructed spatial resolution throughout a compact, breast-size field of view. Both detector types also achieve outstanding energy resolution (<3% and <12%, respectively for LSO-PSAPD and CZT at 511 keV). This paper studies the effects that this excellent energy resolution has on the expected system performance. Results indicate the importance that high energy resolution and narrow energy window settings have in reducing background random as well as scatter coincidences without compromising statistical quality of the dedicated breast PET data. Simulations predict that using either detector type the excellent performance and novel arrangement of these detectors proposed for the system facilitate ∼20% instrument sensitivity at the system center and a peak noise-equivalent count rate of >4 kcps for 200 microCi in a simulated breast phantom.

Optimization and performance evaluation of the microPET II scanner for in vivo small-animal imaging

MicroPET II is a newly developed PET (positron emission tomography) scanner designed for high-resolution imaging of small animals. It consists of 17 640 LSO crystals each measuring 0.975 × 0.975 × 12.5 mm3, which are arranged in 42 contiguous rings, with 420 crystals per ring. The scanner has an axial field of view (FOV) of 4.9 cm and a transaxial FOV of 8.5 cm. The purpose of this study was to carefully evaluate the performance of the system and to optimize settings for in vivo mouse and rat imaging studies. The volumetric image resolution was found to depend strongly on the reconstruction algorithm employed and averaged 1.1 mm (1.4 µl) across the central 3 cm of the transaxial FOV when using a statistical reconstruction algorithm with accurate system modelling. The sensitivity, scatter fraction and noise-equivalent count (NEC) rate for mouse- and rat-sized phantoms were measured for different energy and timing windows. Mouse imaging was optimized with a wide open energy window (150–750 keV) and a 10 ns timing window, leading to a sensitivity of 3.3% at the centre of the FOV and a peak NEC rate of 235 000 cps for a total activity of 80 MBq (2.2 mCi) in the phantom. Rat imaging, due to the higher scatter fraction, and the activity that lies outside of the field of view, achieved a maximum NEC rate of 24 600 cps for a total activity of 80 MBq (2.2 mCi) in the phantom, with an energy window of 250–750 keV and a 6 ns timing window. The sensitivity at the centre of the FOV for these settings is 2.1%. This work demonstrates that different scanner settings are necessary to optimize the NEC count rate for different-sized animals and different injected doses. Finally, phantom and in vivo animal studies are presented to demonstrate the capabilities of microPET II for small-animal imaging studies.

Count-Rate Dependent Event Mispositioning and NEC in PET

Most current PET detector designs suffer from event mispositioning at high count rates, as scintillation light from nearby and nearly simultaneous gamma ray conversions becomes mixed. We have used the NEMA NU 2-2001 70 cm test phantom and a Na-22 point source to quantify this effect as a function of activity on two block-detector tomographs (the Siemens/CTI HR+ and the General Electric Discovery LS), and two Anger-type PET tomographs (the Siemens ECAM DUET and the Philips CPET+). After accounting for event losses due to dead time, we find that the number of counts on LORs passing through a cylinder of diameter and height equal to the point-source full width at tenth-maximum measured at low rate surrounding the point source decreases by between 9% (HR+) and 35% (CPET+) at the activity giving rise to peak noise equivalent count (NEC) rate. Mispositioned events act to reduce signal-to-noise ratio, both by reducing apparent activity at the originating location and by increasing the signal background. We have reformulated the conventional expression for NEC rate to account for this phenomenon. The new formulation of NEC, which we call NEC/sup */, results in a lower peak value which in turn occurs at a lower activity concentration than for the conventional formulation.

NEMA NU 2-2000+ performance measurements on an ADAC MCD camera

The clinically oriented NEMA NU-2 2000+ performance standards have been performed on an ADAC molecular coincidence detection dual-head camera. At 1 cm radius, the transverse resolution full-width at half-maximum was measured to be 5.2 (10.0) mm and the axial was 5.0 (10.6) mm. At 10 cm radius, the transverse radial resolution was 4.7 (7.4) mm and the transverse tangential was 8.5 (20.9) mm. The axial resolution was 14.0 (41.1) mm. The peak true count rate was 4370 counts/s at a concentration of 4.3 kBq/ml. The peak noise equivalent count rate (NECR) was 1620 counts/s at 2.9 kBq/ml. The system scatter fraction was 48%, The maximum relative count rate error observed at the NECR peak was -42%. The sensitivity was 1010 counts/sm/rSq in the center and 870 counts/s/MBq at an offset of 10 cm. The contrast for the hot spheres ranged from 18.5 (11.6)% to 11.6 (7.9)% for a contrast ratio of 8:1 (4:1). For the cold spheres, it ranged from 41% to 25% with an overall background variability of 7%. The residual error in scatter and attenuation correction was approximately 50%. All measurements were performed at the maximum allowed crystal-to-crystal distance of 810 mm.

Design of a high-resolution, high-sensitivity PET camera for human brains and small animals

The authors present design parameters for a 3-D PET camera with high sensitivity (35 cm detector ring diameter, 15 cm axial field of view) and isotropic high resolution provided by detector modules capable of depth of interaction (DOI) measurement. Detector modules are made of LSO crystals (3 mm square by 30 mm deep)-the small module size and short decay time of LSO reduce the detector dead time by a factor of 14 compared to conventional EGO detector modules and narrow the coincidence window width to 4 ns. This yields an expected peak noise equivalent count rate of 800 kcps and noise equivalent sensitivity of 1370 kcps//spl mu/Ci/cc with a 20 cm diameter phantom-three to five times higher than conventional scanners. With 5 mm fwhm DOI resolution, the expected reconstructed spatial resolution is <3.0 mm fwhm throughout the entire field of view. Depth of interaction measurement information is incorporated into the reconstruction algorithm by rebinning onto a regularly spaced grid. Attenuation correction is performed with an orbiting single transmission source.

Use of face masks during a plague epidemic

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). <b>Methods:</b> 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 ²²Na 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 ¹⁸F line source insert. The NEMA image-quality phantom and rodent imaging were also performed to access the overall imaging capabilities of the scanner. <b>Results:</b> 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. <b>Conclusion:</b> 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.

EFFECT OF CLOFIBRATE AND ANDROSTERONE ON HYPERCHOLESTEROLAEMIC XANTHOMATOSIS.

The recent introduction of simultaneous whole-body PET/MR scanners has enabled new research taking advantage of the complementary information obtainable with PET and MRI. One such application is kinetic modeling, which requires high levels of PET quantitative stability. To accomplish the required PET stability levels, the PET subsystem must be sufficiently isolated from the effects of MR activity. Performance measurements have previously been published, demonstrating sufficient PET stability in the presence of MR pulsing for typical clinical use; however, PET stability during radiofrequency (RF)-intensive and gradient-intensive sequences has not previously been evaluated for a clinical whole-body scanner. In this work, PET stability of the GE SIGNA PET/MR was examined during simultaneous scanning of aggressive MR pulse sequences. <b>Methods:</b> PET performance tests were acquired with MR idle and during simultaneous MR pulsing. Recent system improvements mitigating RF interference and gain variation were used. A fast recovery fast spin echo MR sequence was selected for high RF power, and an echo planar imaging sequence was selected for its high heat-inducing gradients. Measurements were performed to determine PET stability under varying MR conditions using the following metrics: sensitivity, scatter fraction, contrast recovery, uniformity, count rate performance, and image quantitation. A final PET quantitative stability assessment for simultaneous PET scanning during functional MRI studies was performed with a spiral in-and-out gradient echo sequence. <b>Results:</b> Quantitation stability of a ⁶⁸Ge flood phantom was demonstrated within 0.34%. Normalized sensitivity was stable during simultaneous scanning within 0.3%. Scatter fraction measured with a ⁶⁸Ge line source in the scatter phantom was stable within the range of 40.4%–40.6%. Contrast recovery and uniformity were comparable for PET images acquired simultaneously with multiple MR conditions. Peak noise equivalent count rate was 224 kcps at an effective activity concentration of 18.6 kBq/mL, and the count rate curves and scatter fraction curve were consistent for the alternating MR pulsing states. A final test demonstrated quantitative stability during a spiral functional MRI sequence. <b>Conclusion:</b> PET stability metrics demonstrated that PET quantitation was not affected during simultaneous aggressive MRI. This stability enables demanding applications such as kinetic modeling.