Cm Axial Field Of View Research Articles

Performance evaluation of the PennPET explorer with expanded axial coverage

Objective. This work evaluated the updated PennPET Explorer total-body (TB) PET scanner, which was extended to 6 rings with updated readout firmware to achieve a 142 cm axial field of view (AFOV) without 7.6 cm inter-ring axial gaps. Approach. National Electrical Manufacturers Association (NEMA) NU 2-2018 measurements were performed with modifications including longer phantoms for sensitivity and count-rate measurements and additional positions for spatial resolution and image quality. A long uniform phantom and the clinical trials network (CTN) phantom were also used. Main results. The total sensitivity increased to 140 kcps MBq−1 for a 70 cm line, a gain of 1.8x compared to the same system with axial gaps; an additional 47% increase in total counts was observed with a 142 cm line at the same activity per cm. The noise equivalent count rate (NECR) increased by 1.8x without axial gaps. The peak NECR is 1550 kcps at 25 kBq cc−1 for a 140 cm phantom; due to increased randoms, the NECR is lower than with a 70 cm phantom, for which NECR is 2156 kcps cc−1 at 25 kBq cc−1 and continues increasing. The time-of-flight resolution is 250 ps, increasing by <10 ps at the highest activity. The axial spatial resolution degrades by 0.6 mm near the center of the AFOV, compared to 4 mm resolution near the end. The NEMA image quality phantom showed consistent contrast recovery throughout the AFOV. A long uniform phantom demonstrated axial uniformity of uptake and noise, and the CTN phantom demonstrated quantitative accuracy for both 18F and 89Zr. Significance. The performance evaluation of the updated PennPET Explorer demonstrates significant gains compared to conventional scanners and shows where the current NEMA standard needs to be updated for TB-PET systems. The comparisons of systems with and without inter-ring gaps demonstrate the performance trade-offs of a more cost-effective TB-PET system with incomplete detector coverage.

Studies and optimization of scintillation light measurements for the development of the 3-gamma medical imaging XEMIS2 liquid xenon Compton camera

We report the studies and optimization of scintillation light measurements in an updated version of the XEMIS1 prototype for the development of the XEMIS2 camera. A novel monolithic liquid xenon Compton camera, named XEMIS2 (XEnon Medical Imaging System), attempts to achieve low-activity small-animal imaging using the 3-gamma imaging technique. This emerging detector relies on the time projection chamber technique: it will be able to perform a simultaneous detection of the three γ -rays emitted by a specific radionuclide, such as scandium-44, and to produce a good quality image with a remarkable diminution of radiopharmaceutical activity at the same time. Vacuum Ultraviolet (VUV) scintillation light and ionization charge carriers generated from the recoiling particles within the detector are detected and used to reconstruct the interaction position and deposited energy. A cost-effective self-triggering scintillation signal read-out and data acquisition (DAQ) system has been developed to achieve a continuous data read-out with negligible electronics dead time. The DAQ prototype has been installed and qualified in an updated version of the XEMIS1 detector. It reaches the performance specifications in scintillation light measurements. Moreover, scintillation signals can also be used for the virtual segmentation of the monolithic detection volume through the matching algorithm of the scintillation and ionization signals based on the light collection map (LCM). This spatial pre-localization of the physical events, called the virtual fiducialization of the active volume, is used to lower the detector occupancy rate when the administered activity is increased to lessen the examination time. The XEMIS1 experimental LCMs indicate that each PMT owns an individual field of view so as to segment the active volume virtually. The preparation work for the XEMIS2 camera operation has been completed in the updated XEMIS1 detector while the XEMIS2 scintillation light measurement system is under commissioning in Nantes Centre Hospitalier Universitaire. • The XEMIS2 camera oriented to the whole-body small animal 3-gamma medical imaging is presented. • The XEMIS2 system is a monolithic liquid xenon Compton camera with a 24 cm axial field of view. • A dedicated cost-effective 16-channel self-triggering scintillation signal front-end read-out electronics called XSRETOT is described. • The experimental light collection maps of an updated version of the XEMIS1 detector have been built, which can be used for the virtual segmentation of the monolithic detection volume to reduce the detector occupancy rate in the context of increasing the administered activity.

Design study of fully wearable high-performance brain PETs for neuroimaging in free movement

A practical wearable brain PET scanner capable of dynamic neuroimaging during free bodily movement will enable potential breakthrough basic neuroscience studies and help develop imaging-based neurological diagnoses and treatments. Weight, brain coverage, and sensitivity are three fundamental technical obstacles in the development of Fully Wearable High-Performance (FWHP) brain PET scanners. The purpose of this study is to investigate the feasibility of building a FWHP brain PET using a limited volume of lutetium–yttrium oxyorthosilicate (LYSO) scintillator crystals. Six scanners, consisted of the same volume (2.66 kg) of LYSO scintillators with combinations of 2 different crystal pitches (3 mm and 1.5 mm) and 3 different crystal lengths (20 mm, 10 mm, and 5 mm), were simulated. The performances of the six scanners were assessed and compared with Siemen’s HRRT brain PET and mCT whole-body PET, in terms of aperture, axial field of views (AFOV), sensitivity, spatial resolution, count rates, and image noise property. The time-of-flight (TOF) information was included in the image reconstruction to improve the effective sensitivity. The effects of the TOF was assessed by scanning a Jaszczak phantom and reconstructing images with the maximum likelihood expectation maximization (MLEM) algorithm with different timing settings (non-TOF, 500 ps, 200 ps, 100 ps and 50 ps Coincidence Time Resolution, CTR). The signal-noise ratio (SNR) of the images were assessed and compared with those of the HRRT scanner and mCT scanner. The results show that it is possible to construct a FWHP brain PET with better spatial resolution than the dedicated HRRT brain PET, comparable effective sensitivity (with 50 ∼ 100 ps CTR), and whole-brain coverage (23.7 cm inner diameter and 13.4 cm axial field of view) using 2.66 kg of LYSO.

PET systems: the value of added length

The past decade has brought unbelievable advances in CT technology. These have been reflected in a progressive widening of the radiation beam in the axial direction (from “fan beam” to “cone beam” geometry) and in a corresponding increase in the number of detector rings. From the four detectors and 2 cm axial field of view (AFOV), acquired in half a second, that characterised the systems of a decade ago, we now have, with the present technology, 256/312 multi-row detectors that allow the acquisition of an 8/16 cm axial field in less than a third of a second. In addition, dual-source CT is offering a wealth of new possibilities. These new systems, whose realisation demanded the solution of complex mechanics, electronics and information technology problems, have opened up a completely new field of medical investigation: the four-dimensional study of the heart and the vascular system. The past few years have also seen evolutions in the field of PET/CT. Indeed, this combined modality has benefitted not only from improved CT technology, but also from gradual advances in the PET sphere—advances prompted by the introduction of new crystal detectors (such as LSO, GSO, LYSO), new iterative reconstruction algorithms, faster electronics, the “gating” technique (synchronisation of data acquisition with respiratory motion) and, especially, “time of flight” PET technology (TOF) [1–4]. Yet despite these significant improvements, the length of the detector cylinder, or length of the system’s sensitive volume (“field of view”), has remained essentially the same (15–18 cm). With this acquisition geometry, only a very small fraction of the radiation emitted at any given instant is acquired (geometric efficiency ≤0.2). Therefore, to obtain, for example, a “whole-body” study of about 80 cm in length (from the temples to the bladder of the patient), a series of six to eight static scans (“views”) need to be acquired. The adjacent single views, obtained with the movement of the bed, must, of course, also provide an appropriate region of overlap. Obviously, the acquisition time increases in proportion to the number of views. A “standard study” with the injection of 10 mCi of F-FDG takes about 3 min per view and, therefore, approximately 25 min per entire study. To this about a minute should be added for the CT scan. However, when used for wholebody studies in diagnostic oncology—this use accounts for approximately 90% of all PET scans—this procedure presents major limitations. The limited length of the sensitive volume not only imposes the need for multiple views, and thus prolongs the overall duration of the examination, it also (and this is particularly important) limits the maximum “line of response” (LOR) angle (with G. Borasi (*) : F. Fioroni Medical Physics Department, Santa Maria Nuova Hospital, V. le Risorgimento 80, 42100 Reggio Emilia, Italy e-mail: giovanni_borasi@libero.it

System Architecture of the LabPET Small Animal PET Scanner

To address modern molecular imaging requirements, a digital positron emission tomography (PET) scanner for small animals has been developed at Universite de Sherbrooke. Based on individual readout of avalanche photodiodes (APD) coupled to LYSO/LGSO phoswich detectors, the scanner supports up to 4608 channels in a 16.2 cm diameter, 11.25 cm axial field of view with an isotropic ~ 1.2 mm FWHM intrinsic spatial resolution at the center of the field of view. Custom data acquisition boards preprocess and sample APD signals at 45 MHz and compute in real time crystal identification, energy and timing information of detected events at an average sustained rate of up to 1250 raw counts per second per mm2 (10 000 cps/channel). Real time digital signal analysis also filters out events outside the pre-selected energy window with crystal granularity to eliminate Compton events and electronic noise. Retained events are then merged into a single stream through a real-time sorting tree, at which end prompt and delayed coincidences are extracted. A single Firewire link handles both control and data transfers with a host computer. The LabPET features four data recording modes, giving the user the choice to retain data for research or to minimize file size for high coincidence count rate and imaging purposes. The electronic system also supports time synchronized data insertion for flags such as vital signs used in gated image reconstruction. Aside from data acquisition, hardware can generate live energy and discrimination spectra suitable for fast, automatic channel calibration.

SU‐FF‐I‐104: Performance of a PET‐CT Scanner for Hot Lesion at High Activity Concentration

Objective: To evaluate PET‐CT scanner for hot lesion at high activity concentration with and without radioactive background. Method: Four hollow spheres (2.96, 2.00, 1.45 and 0.91 cm) were installed to a PET‐SPECT phantom with three inserts: hot lesions, uniformity linearity, cold lesions and cold spheres. 666 MBq of F‐18 was in cylindrical tank with all inserts and the spheres. Two large spheres contained 26 MBq leading to a lesion to background ratio of 13.2, and two small ones were empty as cold lesions. Three bed sets of 15 cm axial field of view (slice thickness is 3.75 mm) each were scanned for a total of 9 minutes, and CT (120 kV/80 mA) scan as well. Images were at 30 min interval for two hours and then 60 min for last three, and reconstructed with OS‐EM. RoIs and VoIs were defined manually. Same RoI for the largest sphere was used to obtain count as background to both RoI and VoI respectively. Result: Artifacts appeared on PET‐AC and PET‐CT images of the hot spheres and the inserts for first four data sets (0, 30, 60 and 90 min respectively) when the total activity was from 666 to 370 MBq. When it dropped from 370 to 147 MBq, images started to appear normal. However, VoI counts of the hot spheres decreased linearly with decay of the activity over the seven scans. This suggested that the PET‐AC images can still yield accurate VoI counts under the high random and scattering counts from high activity in the cylinder‐background and the CT scan will identify the regions even the images appear distorted. Conclusion: PET‐AC images degraded due to increased random events and scatters at high activity. Regional counts can be measured with AC and RoI from CT data (independent of activity from F‐18) in fused images.

Initial studies using the RatCAP conscious animal PET tomograph

The RatCAP is a small, head-mounted PET tomograph designed to image the brain of a conscious rat without the use of anesthesia. The detector is a complete, high-performance 3D tomograph consisting of a 3.8 cm inside-diameter ring containing 12 block detectors, each of which is comprised of a 4×8 array of 2.2×2.2×5 mm 3 LSO crystals readout with a matching APD array and custom ASIC, and has a 1.8 cm axial field of view. Construction of the first working prototype detector has been completed and its performance characteristics have been measured. The results show an intrinsic spatial resolution of 2.1 mm, a time resolution of ∼14 ns FWHM, and a sensitivity of 0.7% at an energy threshold of 150 keV. First preliminary images have been obtained using 18F-FDG and 11C-methamphetamine, which show comparable image quality to those obtained from a commercial MicroPET R4 scanner. Initial studies have also been carried out to study stress levels in rats wearing the RatCAP.

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.

Strategies to improve 3D whole-body PET image reconstruction

An algorithm is described for three-dimensional whole-body (3DWB) image reconstruction in positron emission tomography. For whole-body applications, improvements to the popular fixed-axial-acceptance-angle technique are achieved by combining axially adjacent projection data available with a long-axis data set. Time-consuming reprojection of unmeasured oblique lines of response is reduced or eliminated by axial overlap of bed positions, while pixel variance and reconstructed axial resolution are made more uniform by the overlap. Improvements in noise and resolution uniformity are accompanied by gains in reconstruction efficiency, and may be optimized against increased acquisition time due to overlapping acquisition segments and reduced axial coverage. An 11-detector-ring overlap improves axial uniformity in coronal images of a long, uniform cylinder from 23% to 8% with uniform axial resolution. Associated with a 37% improvement in reconstruction time is a 34% reduction in axial coverage for four bed positions. A smaller degree of overlap is found to provide the best trade-off between image uniformity, total scan duration, and reconstruction time because of a proportionally greater reduction in reprojected lines of response. Using a sample optimization scheme, we find a three-ring overlap is best for a 60 cm axial field of view and a five-ring overlap for an 80 cm axial field of view.