Scanner Development & Characterization
Performance evaluation of the MOLECUBES β-CUBE– a high spatial resolution and high sensitivity small animal PET scanner utilizing monolithic LYSO scintillation detectors.
Krishnamoorthy S, Blankemeyer E, Mollet P, Surti S, Holen RV, Karp JS.
The MOLECUBES β-CUBE scanner is the newest amongst commercially available preclinical PET scanners for dedicated small animal imaging. The scanner is compact, lightweight and utilizes a small footprint to facilitate bench-top imaging. It can be used individually, or in combination with the X-CUBE CT scanner, which provides the ability to perform all necessary PET data corrections and provide fully quantitative PET images. The PET detector comprises of an 8 mm thick monolithic LYSO scintillator read-out by an array of 3 mm × 3 mm Hamamatsu silicon photomultipliers. The monolithic scintillator provides the ability to measure depth-of-interaction which aids in the development of such a compact scanner. With a scanner diameter of 7.6 cm and axial length of 13 cm it is suitable for imaging both whole-body mice and rats. This paper presents the design and imaging performance of the β-CUBE scanner. NEMA NU4-2008 characterization and a variety of phantom and animal imaging studies to demonstrate the quantitative imaging performance of the PET scanner are presented. Spatial resolution of 1 mm is measured with a filtered-back projection reconstruction algorithm at the center of the scanner and DOI measurement helps maintain the excellent spatial resolution over the entire imaging FOV. An absolute peak sensitivity of 12.4% is measured with a 255–765 keV energy window. The scanner demonstrates good count-rate performance, with a peak NEC of 300 kcps and 160 kcps measured with ~900 µCi in the NEMA mouse and rat phantoms, respectively. Imaging data with the NEMA image quality phantom and Micro Derenzo phantoms demonstrate the ability to achieve good image quality and accurate quantitative data. Image uniformity of 7.4% and spill-over ratio of 8% were measured. The superior spatial resolution, excellent energy resolution and sensitivity also provide superior contrast recovery, with ~70% recovery for the 2 mm rods. While current commercial preclinical PET scanners have spatial resolution in the 1–2 mm range, the 1 mm3 volumetric resolution presents significant improvement over current commercially available preclinical PET scanners. In combination with the X-CUBE scanner it provides the ability to perform fully quantitative imaging with spatially co-registered high-resolution 3D PET-CT images.
Design of a dual-resolution collimator for preclinical cardiac SPECT with a stationary triple-detector system.
Moore SC, Park M-A, Liu Z, Lyon MC, Johnson LC, Lushear VH, Westberg JG, Metzler SD.
Purpose: One approach to preclinical single‐photon emission computed tomography (SPECT) imaging that provides both high resolution and high sensitivity is based on imaging a mouse inside a collimating tube; many magnified pinhole projection images from a small target region, e.g., the heart, can be recorded simultaneously on multiple detectors with little multiplexing since each pinhole aperture's opening angle is restricted to view mostly the target organ. However, to obtain complete data for reconstruction, it may be necessary to scan the mouse through the target region of the tube. The authors are developing a different approach based on acquisition and reconstruction of both low‐resolution and high‐resolution projection data acquired sequentially through many pinholes embedded in two tungsten tube sections of different diameters, a “scout” section and a high‐resolution section, placed end‐to‐end along the axis of a triple‐head clinical SPECT scanner. This paper describes the design procedures used to determine the geometric parameters of two new collimator‐tube sections, as well as one approach for joint reconstruction of data acquired from both sections.
Methods: The high‐resolution section was designed by projecting as many pinhole views of a simulated mouse heart as possible over each detector's camera, with no overlapping of heart projections and minimal overlapping between adjacent “hot” organ and cardiac projections. The authors then jointly optimized the geometric design of the scout section for a triple‐detector camera system, as well as the number of maximum‐likelihood expectation maximization (MLEM) iterations required to provide minimum mean‐squared error of reconstructed voxel counts throughout a 7‐cm axial range, with the constraints of fixed, 2.4‐mm scout system resolution at the tube center for all apertures, limited multiplexing, and no detector motion. Simulated mouse projection data from both tube sections were then reconstructed to illustrate a simple approach for using high‐resolution data to improve the whole‐body scout images within a cylindrical region surrounding the heart.
Results: The 2‐cm‐inner‐radius high‐resolution tube section accommodated 87 platinum–iridium pinhole inserts, each with a 0.3‐mm square aperture; their radial distances from the centerline of the system ranged from 2.2 to 3.0 cm. The optimal radial distance to the closest scout pinhole and optimal number of MLEM iterations were 4.4 cm and 35 iterations, respectively, and the radial distances of the 39 scout pinholes ranged from 4.4 to 4.8 cm; aperture sizes ranged from 1.1 to 1.7 mm transaxially and 0.9–1.5 mm axially. After including data from the high‐resolution section viewing the heart region into whole‐body mouse reconstructions from scout data, the authors obtained high‐resolution images of the heart, embedded within lower resolution images of the body, with minimal artifacts.
Conclusions: The authors have optimized a dual‐resolution collimator tube that provides both whole‐body projections of a mouse and more targeted projections centered on the heart that can be jointly reconstructed to obtain high‐resolution images of the heart embedded within lower‐resolution whole‐body images.
Introduction of a novel ultrahigh sensitivity collimator for brain SPECT imaging.
Park M-A, Kijewski MF, Keijzers R, Keijzers M, Lyon MC, Horky L, Moore SC.
Purpose: Noise levels of brain SPECT images are highest in central regions, due to preferential attenuation of photons emitted from deep structures. To address this problem, the authors have designed a novel collimator for brain SPECT imaging that yields greatly increased sensitivity near the center of the brain without loss of resolution. This hybrid collimator consisted of ultrashort cone‐beam holes in the central regions and slant‐holes in the periphery (USCB). We evaluated this collimator for quantitative brain imaging tasks.
Methods: Owing to the uniqueness of the USCB collimation, the hole pattern required substantial variations in collimator parameters. To utilize the lead‐casting technique, the authors designed two supporting plates to position about 37 000 hexagonal, slightly tapered pins. The holes in the supporting plates were modeled to yield the desired focal length, hole length, and septal thickness. To determine the properties of the manufactured collimator and to compute the system matrix, the authors prepared an array of point sources that covered the entire detector area. Each point source contained 32 μCi of Tc‐99m at the first scan time. The array was imaged for 5 min at each of the 64 shifted locations to yield a 2‐mm sampling distance, and hole parameters were calculated. The sensitivity was also measured using a point source placed along the central ray at several distances from the collimator face. High‐count projection data from a five‐compartment brain phantom were acquired with the three collimators on a dual‐head SPECT/CT system. The authors calculated Cramer‐Rao bounds on the precision of estimates of striatal and background activity concentration. In order to assess the new collimation system to detect changes in striatal activity, the authors evaluated the precision of measuring a 5% decrease in right putamen activity. The authors also reconstructed images of projection data obtained by summing data from the individual phantom compartments.
Results: The sensitivity of the novel cone‐beam collimator varied with distance from the detector face; it was higher than that of the fan‐beam collimator by factors ranging from 2.7 to 162. Examination of the projections of the point sources revealed that only a few holes were distorted or partially blocked, indicating that the intensive manual fabrication process was very successful. Better reconstructed phantom images were obtained from the USCB+FAN collimator pair than from either LEHR or FAN collimation. For the left caudate, located near the center of the brain, the detected counts were 9.8 (8.3) times higher for UCSB compared with LEHR (FAN), averaged over 60 views. The task‐specific SNR for detecting a 5% decrease in putamen uptake was 7.4 for USCB and 3.2 for LEHR.
Conclusions: The authors have designed and manufactured a novel collimator for brain SPECT imaging. The sensitivity is much higher than that of a fan‐beam collimator. Because of differences between the manufactured collimator and its design, reconstruction of the data requires a measured system matrix. The authors have demonstrated the potential of USCB collimation for improved precision in estimating striatal uptake. The novel collimator may be useful for early detection of Parkinson's disease, and for monitoring therapy response and disease progression.
A dedicated breast positron emission tomography scanner: Proof of concept.
Srinivas SM, Greene LR, Currie GM, Freifelder RH, Reddin JR, Alavi A, Schnall MD, Karp JS.
Purpose: This study developed and tested a novel scanner constructed for dedicated positron emission tomography (PET) of the breast. The breast PET (B-PET) scanner is designed with two opposing detectors using curve plate NaI(Tl) detectors to achieve a combination of high spatial resolution and energy resolution.
Methods: Phantom and clinical studies (n = 20) with 18F-fluorodeoxyglucose were carried out on the whole-body Philips Allegro scanner and the B-PET scanner. Images were subjectively assessed by an expert panel.
Results: Phantom studies indicated improved contrast for B-PET over conventional PET. Of the 20 clinical studies with breast cancer demonstrated on whole-body fluorodeoxyglucose PET, 10 B-PET scans showed agreement. Of the remaining 10 studies, three had breasts that were too small to be imaged, four had lesions that were too deep to be captured in the field of view, and three were excluded due to technical errors.
Conclusions: Compared with conventional PET, B-PET images provided greater detail in breast lesions suggesting that the low-cost and relatively simple design of B-PET may potentially be an important adjunct to traditional mammography in helping determine the nature of a lesion.
Design optimization of a TOF breast PET scanner.
Lee E, Werner ME, Karp JS, Surti S.
A dedicated breast positron emission tomography (PET) scanner with limited angle geometry can provide flexibility in detector placement around the patient as well as the ability to combine it with other imaging modalities. A primary challenge of a stationary limited angle scanner is the reduced image quality due to artifacts present in the reconstructed image leading to a loss in quantitative information. Previously, it has been shown that using time-of-flight (TOF) information in image reconstruction can help reduce these image artifacts arising due to missing angular projections. Our goal in this work is to optimize the TOF, breast scanner design by performing studies for estimating image uniformity and lesion activity uptake as a function of system timing resolution, scanner angular coverage and shape. Our results show that (i) 1.5 × 1.5 × 15 mm 3 lutetium oxy-orthosilicate (LSO) crystals provide a high spatial resolution and system sensitivity relative to clinical scanners, (ii) 2/3 angular coverage scanner design with TOF timing resolution less than 600 ps is appropriate for providing a tomographic image with fewer artifacts and good lesion uptake estimation relative to other partial ring designs studied in this work, (iii) a flat scanner design with 2/3 angular coverage is affected more by larger parallax error than a curved scanner geometry with the same angular coverage, but provides more uniform lesion contrast estimate over the imaging field-of-view (FOV), (iv) 2/3 angular coverage, flat, 300 ps TOF scanner design (for short, practical scan times of ≤ 5 min per breast) provides similar precision of contrast recovery coefficient (CRC) values to a full curved, non-TOF scanner, and (v) employing depth-of-interaction (DOI) measuring detector and/or implementing resolution modeling (RM) in image reconstruction lead to improved and more uniform spatial resolution and lesion contrast over the whole FOV.
Study of PET scanner designs using clinical metrics to optimize the scanner axial FOV and crystal thickness.
Surti S, Werner ME, Karp JS.
The aim of this study is to understand the trade-off between crystal thickness and scanner axial field-of-view FOV (AFOV) for clinical PET imaging. Clinical scanner design has evolved towards 20–25 mm thick crystals and 16–22 cm long scanner AFOV, as well as time-of-flight (TOF) imaging. While Monte Carlo studies demonstrate that longer AFOV and thicker crystals will lead to higher scanner sensitivity, cost has prohibited the building of commercial scanners with >22 cm AFOV. In this study, we performed a series of system simulations to optimize the use of a given amount of crystal material by evaluating the impact on system sensitivity and noise equivalent counts (NEC), as well as image quality in terms of lesion detectability. We evaluated two crystal types (LSO and LaBr3) and fixed the total crystal volume used for each type (8.2 L of LSO and 17.1 L of LaBr3) while varying the crystal thickness and scanner AFOV. In addition, all imaging times were normalized so that the total scan time needed to scan a 100 cm long object with multiple bed positions was kept constant. Our results show that the highest NEC cm–1 in a 35 cm diameter ×70 cm long line source cylinder is achieved for an LSO scanner with 10 mm long crystals and AFOV of 36 cm, while for LaBr3 scanners, the highest NEC cm–1 is obtained with 20 mm long crystals and an AFOV of 38 cm. Lesion phantom simulations show that the best lesion detection performance is achieved in scanners with long AFOV (≥36 cm) and using thin crystals (≤10 mm of LSO and ≤20 mm of LaBr3). This is due to a combination of improved NEC, as well as improved lesion contrast estimation due to better spatial resolution in thinner crystals. Alternatively, for lesion detection performance similar to that achieved in standard clinical scanner designs, the long AFOV scanners can be used to reduce the total scan time without increasing the amount of crystal used in the scanner. In addition, for LaBr3 based scanners, the reduced lesion contrast relative to LSO based scanners requires improved timing resolution and longer scan times in order to achieve lesion detectability similar to that achieved in an LSO scanner with similar NEC cm–1.
Design study of a whole-body PET scanner with improved spatial and timing resolution.
Surti S, Shore AR, Karp JS.
Current state-of-art whole-body PET scanners achieve a system spatial resolution of 4-5 mm with limited sensitivity. Since the reconstructed spatial resolution and image quality are limited by the count statistics, there has not been a significant push for developing higher resolution whole-body PET scanners. Our goal in this study is to investigate the impact of improved spatial resolution together with time-of-flight (TOF) capability on lesion uptake estimation and lesion detectability, two important tasks in whole-body oncologic studies. The broader goal of this project is the development of a new state-of-art TOF PET scanner operating within an MRI while pushing the technology in PET system design. We performed Monte Carlo simulations to test the effects of crystal size (4 mm and 2.6 mm wide crystals), TOF timing resolution (300 ps and 600 ps), and 2-level depth-of-interaction (DOI) capability. Spatial resolution was calculated by simulating point sources in air at multiple positions. Results show that smaller crystals produced improved resolution, while degradation of resolution due to parallax error could be reduced with a 2-level DOI detector. Lesion phantoms were simulated to measure the contrast recovery coefficient (CRC) and area under the LROC curve (ALROC) for 0.5 cm diameter lesions with 6:1 activity uptake relative to the background. Smaller crystals produce higher CRC, leading to increased ALROC values or a reduction in scan time. Improved timing resolution provides faster CRC convergence and once again leads to an increase in ALROC value or reduced scan time. Based on our choice of timing resolution and crystal size, improved timing resolution (300 ps) with larger crystals (4 mm wide) has similar ALROC as smaller crystals (2.6 mm wide) with 600 ps timing resolution. A 2-level DOI measurement provides some CRC and ALROC improvement for lesions further away from the center, leading to a more uniform performance within the imaging field-of-view (FOV). Given a choice between having either an improved spatial resolution, improved timing resolution, or DOI capability, improved spatial or timing resolution provide an overall higher ALROC relative to a 2-level DOI detector.
Design of a PET scanner for in situ dose verification in proton beam therapy.
Surti S, Zou J, Daube-Witherspoon ME, Avery S, McDonough J, Karp JS.
Proton beam therapy can deliver a high radiation dose to a tumor without significant damage to surrounding healthy tissue or organs. One way of verifying the delivered dose distribution is to image the short-lived positron emitters produced by the proton beam as it travels through the patient. A potential solution to the limitations of PET imaging in proton beam therapy is the development of a high sensitivity, in situ PET scanner that starts PET imaging almost immediately after patient irradiation while the patient is still lying on the treatment bed. A partial ring PET design is needed for this application in order to avoid interference between the PET detectors and the proton beam, as well as restrictions on patient positioning on the couch. A partial ring also allows us to optimize the detector separation (and hence the sensitivity) for different patient sizes. Our goal in this investigation is to evaluate an in situ PET scanner design for use in proton therapy that provides tomographic imaging in a partial ring scanner design using time-of-flight (TOF) information and an iterative reconstruction algorithm. GEANT4 simulation of an incident proton beam was used to produce a positron emitter distribution, which was parameterized and then used as the source distribution inside a water-filled cylinder for EGS4 simulations of a PET system. Design optimization studies were performed as a function of crystal type and size, system timing resolution, scanner angular coverage and number of positron emitter decays. Data analysis was performed to measure the accuracy of the reconstructed positron emitter distribution as well as the range of the positron emitter distribution. We simulated scanners with varying crystal sizes (2–4 mm) and type (LYSO and LaBr3) and our results indicate that 4 mm wide LYSO or LaBr3 crystals (resulting in 4–5 mm spatial resolution) are adequate; for a full-ring, non-TOF scanner we predict a low bias (<0.6 mm) and a good precision (<1 mm) in the estimated range relative to the simulated positron distribution. We then varied the angular acceptance of the scanner ranging from 1/2 to 2/3 of 2π; a partial ring TOF imaging with good timing resolution (≤600 ps) is necessary to produce accurate tomographic images. A two-third ring scanner with 300 ps timing resolution leads to a bias of 1.0 mm and a precision of 1.4 mm in the range estimate. With a timing resolution of 600 ps, the bias increases to 2.0 mm while the precision in the range estimate is similar. For a half-ring scanner design, more distortions are present in the image, which is characterized by the increased error in the profile difference estimate. We varied the number of positron decays imaged by the PET scanner by an order of magnitude and we observe some decrease in the precision of the range estimate for lower number of decays, but all partial ring scanner designs studied have a precision ≤1.5 mm. The largest number tested, 150 M total positron decays, is considered realistic for a clinical fraction of delivered dose, while the range of positron decays investigated in this work covers a variable number of situations corresponding to delays in scan start time and the total scan time. Thus, we conclude that for partial ring systems, an angular acceptance of at least 1/2 (of 2π) together with timing resolution of 300 ps is needed to achieve accurate and precise range estimates. With 600 ps timing resolution an angular acceptance of 2/3 (of 2π) is required to achieve satisfactory range estimates. These results indicate that it would be feasible to develop a partial-ring dedicated PET scanner based on either LaBr3 or LYSO to accurately characterize the proton dose for therapy planning.
Design considerations for a limited angle, dedicated breast, TOF PET scanner.
Surti S, Karp JS.
Development of partial ring, dedicated breast positron emission tomography (PET) scanners is an active area of research. Due to the limited angular coverage, generation of distortion and artifact-free, fully 3D tomographic images is not possible without rotation of the detectors. With time-of-flight (TOF) information, it is possible to achieve the 3D tomographic images with limited angular coverage and without detector rotation. We performed simulations for a breast scanner design with a ring diameter and an axial length of 15 cm and comprising a full (180° in-plane angular coverage), 2/3 (120° in-plane angular coverage) or 1/2 (90° in-plane angular coverage) ring detector. Our results show that as the angular coverage decreases, improved timing resolution is needed to achieve distortion-free and artifact-free images with TOF. The contrast recovery coefficient (CRC) value for small hot lesions in a partial ring scanner is similar to a full ring non-TOF scanner. Our results indicate that a timing resolution of 600 ps is needed for a 2/3 ring scanner, while a timing resolution of 300 ps is needed for a 1/2 ring scanner. We also analyzed the ratio of lesion CRC to the background pixel noise (SNR) and concluded that TOF improves the SNR values of the partial ring scanner, and helps to compensate for the loss in sensitivity due to reduced geometric sensitivity in a limited angle coverage PET scanner. In particular, it is possible to maintain similar SNR characteristic in a 2/3 ring scanner with a timing resolution of 300 ps as in a full ring non-TOF scanner.
Imaging performance of A-PET: A small animal PET camera.
Surti S, Karp JS, Perkins AE, Cardi CA, Daube-Witherspoon ME, Kuhn A, Muehllehner G.
The evolution of positron emission tomography (PET) imaging for small animals has led to the development of dedicated PET scanner designs with high resolution and sensitivity. The animal PET scanner achieves these goals for imaging small animals such as mice and rats. The scanner uses a pixelated Anger-logic detector for discriminating 2 /spl times/ 2 /spl times/ 10 mm/sup 3/ crystals with 19-mm-diameter photomultiplier tubes. With a 19.7-cm ring diameter, the scanner has an axial length of 11.9 cm and operates exclusively in three-dimensional imaging mode, leading to very high sensitivity. Measurements show that the scanner design achieves a spatial resolution of 1.9 mm at the center of the field-of-view. Initially designed with gadolinium orthosilicate but changed to lutetium-yttrium orthosilicate, the scanner now achieves a sensitivity of 3.6% for a point source at the center of the field-of-view with an energy window of 250-665 keV. Iterative image reconstruction, together with accurate data corrections for scatter, random, and attenuation, are incorporated to achieve high-quality images and quantitative data. These results are demonstrated through our contrast recovery measurements as well as sample animal studies.
Imaging characteristics of a 3-dimensional GSO whole-body PET camera.
Surti S, Karp JS.
A whole-body 3-dimensional PET scanner using gadolinium oxyorthosilicate (GSO) crystals has been designed to achieve high sensitivity and reduced patient scanning time. This scanner has a diameter of 82.0 cm and an axial field of view of 18 cm without interplane septa. The detector comprises of 4 × 6 × 20 mm3 GSO crystals coupled via an optically continuous light guide to an array of 420 photomultiplier tubes (39-mm diameter) in a hexagonal arrangement. The patient port diameter is 56 cm, and 2.86-cm (1.125 in.) thick lead shielding is used to fill in the region up to the detector ring. Methods: Performance measurements on the scanner were made using the National Electrical Manufactures Association (NEMA) NU 2-2001 procedures. Additional counting rate measurements with a large phantom were performed to evaluate imaging characteristics for heavier patients. The image-quality torso phantom with hot or cold spheres was also measured as a function of counting rate to evaluate different techniques for randoms and scatter subtraction as well as to determine an optimum imaging time. Results: The transverse and axial resolutions near the center are 5.5 and 5.6 mm, respectively. The absolute sensitivity of this scanner measured with a 70-cm-long line source is 4.36 cps/kBq, whereas the scatter fraction is 40% with a 20 × 70 cm line source cylinder. For the same cylinder, the peak noise equivalent count (NEC) rate of 30 kcps at an activity concentration of 9.25 kBq/mL (0.25 μCi/mL) leads to a 7% increase in the peak NEC value. A significant reduction in the peak NEC is observed with a larger 35 × 70 cm line source cylinder. Image-quality measurements show that the small 10-mm sphere in the NEMA NU 2-2001 image-quality phantom is clearly visible in a scan time of 3 min, and there is no noticeable degradation in image contrast at high activity levels. Conclusion: This whole-body scanner represents a new generation of 3D, high-sensitivity, and high-performance PET cameras capable of producing high-quality images in <30 min for a full patient scan. The use of a pixelated GSO Anger-logic detector leads to a high-sensitivity scanner design with good counting rate capability due to the reduced light spread in the detector and fast decay time of GSO. The light collection over the detector is fairly uniform, leading to a good energy resolution and, thus, reduced scatter in the collected data due to a tight energy gate.
Image quality assessment of LaBr3-based whole-body 3D PET scanners: A Monte Carlo evaluation.
Surti S, Karp JS, Muehllehner G.
The main thrust for this work is the investigation and design of a whole-body PET scanner based on new lanthanum bromide scintillators. We use Monte Carlo simulations to generate data for a 3D PET scanner based on LaBr3 detectors, and to assess the count-rate capability and the reconstructed image quality of phantoms with hot and cold spheres using contrast and noise parameters. Previously we have shown that LaBr3 has very high light output, excellent energy resolution and fast timing properties which can lead to the design of a time-of-flight (TOF) whole-body PET camera. The data presented here illustrate the performance of LaBr3 without the additional benefit of TOF information, although our intention is to develop a scanner with TOF measurement capability. The only drawbacks of LaBr3 are the lower stopping power and photo-fraction which affect both sensitivity and spatial resolution. However, in 3D PET imaging where energy resolution is very important for reducing scattered coincidences in the reconstructed image, the image quality attained in a non-TOF LaBr3 scanner can potentially equal or surpass that achieved with other high sensitivity scanners. Our results show that there is a gain in NEC arising from the reduced scatter and random fractions in a LaBr3 scanner. The reconstructed image resolution is slightly worse than a high-Zscintillator, but at increased count-rates, reduced pulse pileup leads to an image resolution similar to that of LSO. Image quality simulations predict reduced contrast for small hot spheres compared to an LSO scanner, but improved noise characteristics at similar clinical activity levels.
Performance of a brain PET camera based on Anger-logic gadolinium oxyorthosilicate detectors.
Karp JS, Surti S, Daube-Witherspoon ME, Freifelder R, Cardi CA, Adam L-E, Bilger K, Muehllehner G.
A high-sensitivity, high-resolution brain PET scanner (“G-PET”) has been developed. This scanner is similar in geometry to a previous brain scanner developed at the University of Pennsylvania, the HEAD Penn-PET, but the detector technology and electronics have been improved to achieve enhanced performance. Methods: This scanner has a detector ring diameter of 42.0 cm with a patient aperture of 30.0 cm and an axial field of view of 25.6 cm. It comprises a continuous light-guide that couples 18,560 (320 × 58 array) 4 × 4 × 10 mm3 gadolinium oxyorthosilicate (GSO) crystals to 288 (36 × 8 array) 39-mm photomultiplier tubes in a hexagonal arrangement. The scanner operates only in 3-dimensional (3D) mode because there are no interplane septa. Performance measurements on the G-PET scanner were made following National Electrical Manufacturers Association NU 2–2001 procedures for most measurements, although NU 2–1994 procedures were used when these were considered more appropriate for a brain scanner (e.g., scatter fraction and counting-rate performance measurements). Results: The transverse and axial resolutions near the center are 4.0 and 5.0 mm, respectively. At a radial offset of 10 cm, these numbers deteriorate by approximately 0.5 mm. The absolute sensitivity of this scanner measured with a 70-cm long line source is 4.79 counts per second (cps)/kBq. The scatter fraction measured with a line source in a 20-cm-diameter × 19-cm-long cylinder is 39% (for a lower energy threshold of 410 keV). For the same cylinder, the peak noise equivalent counting rate is 60 kcps at an activity concentration of 7.4 kBq/mL (0.20 μCi/mL), whereas the peak true coincidence rate is 132 kcps at an activity concentration of 14 kBq/mL (0.38 μCi/mL). Images from the Hoffman brain phantom as well as 18F-FDG patient scans illustrate the high quality of images acquired on the G-PET scanner. Conclusion: The G-PET scanner attains the goal of high performance for brain imaging through the use of an Anger-logic GSO detector design with continuous optical coupling. This detector design leads to good energy resolution, which is needed in 3D imaging to minimize scatter and random coincidences.
Performance measurements of a pixelated NaI(Tl) PET scanner.
Perkins AE, Muehllehner G, Surti S, Karp JS.
A prototype PET scanner for whole body imaging with 4 mm /spl times/ 4 mm /spl times/ 30 mm NaI(Tl) crystals and Anger-logic readout has been built and tested. The scanner is composed of 36,540 NaI(Tl) pixels which are coupled to an optically continuous lightguide and a hexagonal closed packed array of 39 mm photomultiplier tubes (PMTs). The scanner is designed with a crystal-to-crystal diameter of 89 cm and an axial field of view (AFOV) of 25 cm. The main goals of this study are (1) to overcome the count-rate limitation of the continuous NaI(Tl) scanner (CPET), (2) to improve the spatial resolution and image contrast by using small pixels, and (3) to eliminate the relatively large data gaps between the detectors in the continuous NaI(Tl) scanner.
Performance of a whole-body PET scanner using curve-plate NaI(Tl) detectors.
Adam L-E, Karp JS, Daube-Witherspoon ME, Smith RJ.
A whole-body PET scanner, without interplane septa, has been designed to achieve high performance in clinical applications. The C-PET scanner, an advancement of the PENN PET scanners, is unique in the use of 6 curved NaI(Tl) detectors (2.54 cm thick). The scanner has a ring diameter of 90 cm, a patient port diameter of 56 cm, and an axial field of view of 25.6 cm. A 137Cs point source is used for transmission scans. Methods: Following the protocols of the International Electrotechnical Commission ([IEC] 61675-1) and the National Electrical Manufacturers Association ([NEMA] NU-2-1994 and an updated version, NU2-2001), point and line sources, as well as uniform cylinders, were used to determine the performance characteristics of the C-PET scanner. An image-quality phantom and patient data were used to evaluate image quality under clinical scanning conditions. Data were rebinned with Fourier rebinning into 2-dimensional (slice-oriented) datasets and reconstructed with an iterative reconstruction algorithm. Results: The spatial resolution for a point source in the transaxial direction was 4.6 mm (full width at half maximum) at the center, and the axial resolution was 5.7 mm. For the NU2-1994 analysis, the sensitivity was 12.7 cps/Bq/mL (444 kcps/μCi/mL), the scatter fraction was 25%, and the peak noise equivalent count rate (NEC) for a uniform cylinder (diameter = 20 cm, length = 19 cm) was 49 kcps at an activity concentration of 11.2 kBq/mL. For the IEC protocol, the peak NEC was 41 kcps at 12.3 kBq/mL, and for the NU2-2001 protocol, the peak NEC was 14 kcps at 3.8 kBq/mL. The NU2-2001 NEC value differed significantly because of differences in the data analysis and the use of a 70-cm-long phantom. Conclusion: Compared with previous PENN PET scanners, the C-PET, with its curved detectors and improvements in pulse shaping, integration dead time, and triggering, has an improved count-rate capability and spatial resolution. With the refinements in the singles transmission technique and iterative reconstruction, image quality is improved and scan time is shortened. With single-event transmission scans interleaved between sequential emission scans, a whole-body study can be completed in <1 h. Overall, C-PET is a cost-effective PET scanner that performs well in a broad variety of clinical applications.
Dedicated PET scanners for breast imaging.
Freifelder R, Karp JS.
We have used computer simulations to compare two designs for a PET scanner dedicated to breast imaging with a whole-body PET scanner. The new designs combine high spatial resolution, high sensitivity, and good energy resolution to detect small, low-contrast masses. The detectors are position sensitive NaI(Tl) scintillators. The first design is a ring scanner surrounding the breast and the second consists of two planar detectors placed on opposite sides of the breast. We have employed standard performance measures to compare the different designs: contrast, percentage standard deviation of the background, and signal-to-noise ratios of reconstructed images. The results of the simulations show that both of the proposed designs have better lesion detectability than a whole-body scanner. The results also show that contrast is higher in the ring breast system but that the noise is lower in the planar breast system. Overall, the ring system yields images with the best signal-to-noise ratios, although the planar system offers practical advantages for imaging the breast and axilla.
3D imaging characteristics of the HEAD PENN-PET scanner.
Karp JS, Freifelder R, Geagan MJ, Muehllehner G, Kinahan PE, Lewitt RM, Shao L.
A volume-imaging PET scanner, without interplane septa, for brain imaging has been designed and built to achieve high performance, specifically in spatial resolution and sensitivity. The scanner is unique in its use of a single annular crystal of NaI(Tl), which allows a field of view (FOV) of 25.6 cm in both the transverse and axial directions. Data are reconstructed into an image matrix of 1283 with (2 mm)3 voxels, using three-dimensional image reconstruction algorithms. Methods: Point-source measurements are performed to determine spatial resolution over the scanner FOV, and cylindrical phantom distributions are used to determine the sensitivity, scatter fraction and counting rate performance of the system. A three-dimensional brain phantom and 18F-FDG patient studies are used to evaluate image quality with three-dimensional reconstruction algorithms. Results: The system spatial resolution is measured to be 3.5 mm in both the transverse and axial directions, in the center of the FOV. The true sensitivity, using the standard NEMA phantom (6 liter), is 660 kcps/microCi/ml, after subtracting a scatter fraction of 34%. Due to deadtime effects, we measure a peak true counting rate, after scatter and randoms subtraction, of 100 kcps at 0.7 mCi for a smaller brain-sized (1.1 liter) phantom, and 70 kcps for a head-sized (2.5 liter) phantom at the same activity. A typical 18F-FDG clinical brain study requires only 2 mCi to achieve high statistics (100 million true events) with a scan time of 30 min. Conclusion: The HEAD PENN-PET scanner is based on a cost-effective design using Nal(TI) and has been shown to achieve high performance for brain studies and pediatric whole-body studies. As a full-time three-dimensional imaging scanner with a very large axial acceptance angle, high sensitivity is achieved. The system becomes counting-rate limited as the activity is increased, but we achieve high image quality with a small injected dose. This is a significant advantage for clinical imaging, particularly for pediatric patients.
Performance characteristics of a dual head SPECT scanner with PET capability.
Nellemann P, Hines H, Braymer W, Muehllehner G, Geagan M.
Using a dual detector scanner in coincidence avoids many of the factors leading to poor performance in a SPECT system collimated for 511 keV. On the other hand, it would be unrealistic to expect the same good performance achieved with a dedicated PET scanner. It was therefore the goal of the present investigation to develop an ECT scanner which provides the same performance achieved with present SPECT scanners when operated as a SPECT system for imaging single photon emitters and which provides clearly superior performance to a 511 keV collimated SPECT scanner when operated in the coincidence mode.
Effect of increased axial field of view on the performance of a volume PET scanner.
Karp JS, Kinahan PE, Muehllehner G, Countryman P.
The performance of the PENN-PET 240H scanner from UGM Medical Systems is tested and compared to the prototype PENN-PET scanner built at the University of Pennsylvania. The UGM PENN-PET scanner consists of six continuous position-sensitive NaI(Tl) detectors, which results in a 50 cm transverse field-of-view and a 12.8 cm axial field-of-view. The fine spatial sampling in the axial direction allows the data to be sorted into as many as 64 transverse planes, each 2 mm thick. A large axial acceptance angle, without interplane septa, results in a high-sensitivity and low-randoms fraction, with a low-scatter fraction due to the use of a narrow photopeak energy window. This work emphasizes those performance measurements that illustrate the special characteristics of a volume imaging scanner and how they change as the axial length is increased.
Positron emission tomography with a large axial acceptance angle: Signal-to-noise considerations.
Karp JS, Kinahan PE, Mankoff DA.
A volume imaging positron emission tomography (PET) scanner with a large acceptance angle, such as the PENN-PET, offers fine spatial sampling and resolution in three dimensions, and a high sensitivity because of the inclusion of all cross-plane rays. The signal-to-noise ratio (SNR) is used to evaluate image quality for different scanning conditions of the PENN-PET using an activated cylindrical phantom with cold spheres of various sizes. Raising the energy threshold to 400 keV improves the SNR by lowering the scatter fraction, though it also reduces the sensitivity. Increasing the axial acceptance angle from +or-1.3 degrees to +or-6.5 degrees improves the SNR by increasing the sensitivity, even with a two-dimensional reconstruction algorithm, which compromises spatial resolution in the axial direction for points at the edge of the radial field of view. Initial results show that a three-dimensional reconstruction offers an improved SNR over a two-dimensional reconstruction that does not use all cross-plane rays.
Continuous-slice PENN-PET: a positron tomograph with volume imaging capability.
Karp JS, Muehllehner G, Mankoff DA, Ordonez CE, Ollinger JM, Daube-Witherspoon ME, Haigh AT, Beerbohm DJ.
The PENN-PET scanner consists of six hexagonally arranged position-sensitive NaI(TI) detectors. This design offers high spatial resolution in all three dimensions, high sampling density along all three axes without scanner motion, a large axial acceptance angle, good energy resolution, and good timing resolution. This results in threedimensional imaging capability with high sensitivity and low scatter and random backgrounds. The spatial resolution is 5.5 mm (FWHM) in all directions near the center. The true sensitivity, for a brain-sized object, is a maximum of 85 kcps/microCi/ml and the scatter fraction is a minimum of 10%, both depending on the lower level energy threshold. The scanner can handle up to 5 mCi in the field of view, at which point the randoms equal the true coincidences and the detectors reach their count rate limit. We have so far acquired [18F)FDG brain studies and cardiac studies, which show the applicability of our scanner for both brain and whole-body imaging. With the results to date, we feel that this design results in a simple yet high performance scanner which is app
A positron camera using position-sensitive detectors: PENN-PET.
Muehllehner G, Karp JS.
A single-slice positron camera has been develOped with good spatial resolution and high count rate capability. The camera uses a hexagonal arrangement of SIX position-sensitive NaJ(TI)detectorS.Thecountratecapabilityof Nal(Tl)was extendedto 800k cps throughthe use of pulse shortening. In order to keep the detectors stationary, an iterative reconstruction algorithm was modifiedwhich ignores the missing data in the gaps between the six detectorsandgivesartifact-freeimages.Thespatialresolution,as determinedfrom the imageof pointsourcesin air, is 6.5 mm full widthat half maximum.We havealso imageda brainphantomanddoghearts.
Use of position sensitive detectors in positron imaging.
Muehllehner G, Colsher JG.
The use of Anger-type bar cameras for positron imaging results in a totally stationary system. In addition, the design is cost effective since the Anger principle will provide many resolution elements using only a few crystals and photomultipliers. Pulse shortening techniques are used to solve countrate problems associated with Anger-type detectors. When compared to an array of BGO detectors, a bar camera with equal sensitivity has superior spatial resolution.
Performance parameters of a positron imaging camera.
Muehllehner G, Buchin MP, Dudek JH.
Two Anger scintillation cameras have been combined into a positron imaging system. Count rate capability has been extended by using absorbers to reduce scattered radiation from the object, by using 1" thick crystals to improve detection efficiency for annihilation radiation, and by implementing pulse shortening techniques and low dead time D. C. coupled electronics. The main characteristics of the device in its present form are high sensitivity (200 cts/sec/μCi), high resolution (system resolution less than 10 mm FWHM) and useful clinical count rates up to 8000 cts/sec.
A diverging collimator for gamma-ray imaging cameras.
With the advent of stationary scintillation cam eras a new powerful tool became available which offers significant improvement over scanning gamma ray detectors. Cameras image radioisotope distributions in a much shorter time than scanners and also make dynamic studies possible. One disadvantage, however, is that their field of view is presently limited to a diameter of approximately 10 in. while the scanner can view an area appreciably bigger than that. Thus it is often necessary to take two exposures when imaging large objects such as the lungs. This lengthens the total exposure time and introduces additional problems such as how to determine accrately the relative position of the two exposures. These problems can be solved by constructing a larger scintillation detector for the camera. How ever, the technical problems involved increase quickly with increasing diameter. A second solution, described and evaluated in this paper, consists of making a special collimator that can be added to a camera with a useful detector diameter of 10 in. to increase its field of view so that large objects can be imaged.