Characterization or Monolithic Scintillation Detectors Etched with Laser Induced Optical Barriers.

Several groups are actively investigating the performance of monolithic (continuous) scintillation detectors using a variety of crystal thicknesses, photo-sensor configurations, and surface treatments. This work explores the performance of thick LYSO crystals that would be applicable to a whole-body PET system. The crystals were etched with laser induced optical barriers (LIOBs) to alter the behavior of the light spread within the crystal in order to improve the performance of the detector. We studied the behavior of the LIOBs in response to optical light using small cubes of LYSO with a variety of laser etching parameters to characterize the impact of the optical barriers. We demonstrated that the opacity of the etchings can be altered by varying the parameters of the laser etching, which influences the depth-dependent light response and spatial resolution in the thick crystal. We successfully etched several crystals, as large as 50×50×25-mm3 thick, with a fine grid of LIOBs, and achieved an average spatial resolution close to 3 mm (FWHM) with 511-keV gammas.

Impact of event positioning algorithm on performance of a whole-body PET scanner using one-to-one coupled detectors.

The advent of silicon photomultipliers (SiPMs) has introduced the possibility of increased detector performance in commercial whole-body PET scanners. The primary advantage of these photodetectors is the ability to couple a single SiPM channel directly to a single pixel of PET scintillator that is typically 4 mm wide (one-to-one coupled detector design). We performed simulation studies to evaluate the impact of three different event positioning algorithms in such detectors: (i) a weighted energy centroid positioning (Anger logic), (ii) identifying the crystal with maximum energy deposition (1st max crystal), and (iii) identifying the crystal with the second highest energy deposition (2nd max crystal). Detector simulations performed with LSO crystals indicate reduced positioning errors when using the 2nd max crystal positioning algorithm. These studies are performed over a range of crystal cross-sections varying from 1 × 1 mm2 to 4 × 4 mm2 as well as crystal thickness of 1 cm to 3 cm. System simulations were performed for a whole-body PET scanner (85 cm ring diameter) with a long axial FOV (70 cm long) and show an improvement in reconstructed spatial resolution for a point source when using the 2nd max crystal positioning algorithm. Finally, we observe a 30-40% gain in contrast recovery coefficient values for 1 and 0.5 cm diameter spheres when using the 2nd max crystal positioning algorithm compared to the 1st max crystal positioning algorithm. These results show that there is an advantage to implementing the 2nd max crystal positioning algorithm in a new generation of PET scanners using one-to-one coupled detector design with lutetium based crystals, including LSO, LYSO or scintillators that have similar density and effective atomic number as LSO.

Impact of detector design on imaging performance of a long axial field-of-view, whole-body PET scanner.

Current generation of commercial time-of-flight (TOF) PET scanners utilize 20-25 mm thick LSO or LYSO crystals and have an axial FOV (AFOV) in the range of 16-22 mm. Longer AFOV scanners would provide increased intrinsic sensitivity and require fewer bed positions for whole-body imaging. Recent simulation work has investigated the sensitivity gains that can be achieved with these long AFOV scanners, and has motivated new areas of investigation such as imaging with a very low dose of injected activity as well as providing whole-body dynamic imaging capability in one bed position. In this simulation work we model a 72 cm long scanner and prioritize the detector design choices in terms of timing resolution, crystal size (spatial resolution), crystal thickness (detector sensitivity), and depth-of-interaction (DOI) measurement capability. The generated list data are reconstructed with a list-mode OSEM algorithm using a Gaussian TOF kernel that depends on the timing resolution and blob basis functions for regularization. We use lesion phantoms and clinically relevant metrics for lesion detectability and contrast measurement. The scan time was fixed at 10 min for imaging a 100 cm long object assuming a 50% overlap between adjacent bed positions. Results show that a 72 cm long scanner can provide a factor of ten reduction in injected activity compared to an identical 18 cm long scanner to get equivalent lesion detectability. While improved timing resolution leads to further gains, using 3 mm (as opposed to 4 mm) wide crystals does not show any significant benefits for lesion detectability. A detector providing 2-level DOI information with equal crystal thickness also does not show significant gains. Finally, a 15 mm thick crystal leads to lower lesion detectability than a 20 mm thick crystal when keeping all other detector parameters (crystal width, timing resolution, and DOI capability) the same. However, improved timing performance with 15 mm thick crystals can provide similar or better performance than that achieved by a detector using 20 mm thick crystals.

Combining Surface Treatments with Shallow Slots to Improve the Spatial Resolution Performance of Continuous, Thick LYSO Detectors for PET.

Positron emission tomography (PET) detectors based on continuous scintillation crystals can achieve very good performance and have a number of practical advantages compared to detectors based on a pixelated array of crystals. Our goal is to develop a thick continuous detector with high energy and spatial resolution, along with high γ-photon capture efficiency. We examine the performance of two crystal blocks: a 46 × 46 × 14 mm3 and a 48 × 48 × 25 mm3 block of LYSO (Lutetium Yttrium Orthosilicate). Using Maximum Likelihood (ML) positioning based upon the light response function (LRF) in the 14 mm thick crystal, we measure a spatial resolution of 3 mm in the central region of the crystal with degradation near the edges due to reflections off the crystal sides. We also show that we can match the spatial resolution achieved using a 14 mm thick crystal by using a 25 mm thick crystal with slots cut into the gamma entrance surface to narrow the LRF. We also find that we can improve the spatial resolution performance near the detector edges by reducing the reflectivity of the crystal sides, albeit with some loss in energy resolution.

Study of PET scanner designs using clinical metrics to optimize the scanner axial FOV and crystal thickness.

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).

DOI Determination by Rise Time Discrimination in Single-Ended Readout for TOF PET Imaging.

Clinical TOF PET systems achieve detection efficiency using thick crystals, typically of thickness 2-3cm. The resulting dispersion in interaction depths degrades spatial resolution for increasing radial positions due to parallax error. Furthermore, interaction depth dispersion results in time pickoff dispersion and thus in degraded timing resolution, and is therefore of added concern in TOF scanners. Using fast signal digitization, we characterize the timing performance, pulse shape and light output of LaBr3:Ce, CeBr3 and LYSO. Coincidence timing resolution is shown to degrade by ~50ps/cm for scintillator pixels of constant cross section and increasing length. By controlling irradiation depth in a scintillator pixel, we show that DOI-dependence of time pickoff is a significant factor in the loss of timing performance in thick detectors. Using the correlated DOI-dependence of time pickoff and charge collection, we apply a charge-based correction to the time pickoff, obtaining improved coincidence timing resolution of <200ps for a uniform 4×4×30mm3 LaBr3 pixel. In order to obtain both DOI identification and improved timing resolution, we design a two layer LaBr3[5%Ce]/LaBr3[30%Ce] detector of total size 4×4×30mm3, exploiting the dependence of scintillator rise time on [Ce] in LaBr3:Ce. Using signal rise time to determine interaction layer, excellent interaction layer discrimination is achieved, while maintaining coincidence timing resolution of <250ps and energy resolution <7% using a R4998 PMT. Excellent layer separation and timing performance is measured with several other commercially-available TOF photodetectors, demonstrating the practicality of this design. These results indicate the feasibility of rise time discrimination as a technique for measuring event DOI while maintaining sensitivity, timing and energy performance, in a well-known detector architecture.

Design Study of a Whole-Body PET Scanner with Improved Spatial and Timing Resolution.

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 (300ps and 600ps), 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 (300ps) with larger crystals (4 mm wide) has similar ALROC as smaller crystals (2.6 mm wide) with 600ps 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 study of an in situ PET scanner for use in proton beam therapy.

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 LaBr(3)) and our results indicate that 4 mm wide LYSO or LaBr(3) 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 LaBr(3) or LYSO to accurately characterize the proton dose for therapy planning.

The imaging performance of a LaBr3-based PET scanner.

A prototype time-of-flight (TOF) PET scanner based on cerium-doped lanthanum bromide [LaBr(3) (5% Ce)] has been developed. LaBr(3) has a high light output, excellent energy resolution and fast timing properties that have been predicted to lead to good image quality. Intrinsic performance measurements of spatial resolution, sensitivity and scatter fraction demonstrate good conventional PET performance; the results agree with previous simulation studies. Phantom measurements show the excellent image quality achievable with the prototype system. Phantom measurements and corresponding simulations show a faster and more uniform convergence rate, as well as more uniform quantification, for TOF reconstruction of the data, which have 375 ps intrinsic timing resolution, compared to non-TOF images. Measurements and simulations of a hot and cold sphere phantom show that the 7% energy resolution helps to mitigate residual errors in the scatter estimate because a high energy threshold (>480 keV) can be used to restrict the amount of scatter accepted without a loss of true events. Preliminary results with incorporation of a model of detector blurring in the iterative reconstruction algorithm not only show improved contrast recovery but also point out the importance of an accurate resolution model of the tails of LaBr(3)'s point spread function. The LaBr(3) TOF-PET scanner demonstrated the impact of superior timing and energy resolutions on image quality.

Experimental evaluation of a simple lesion detection task with time-of-flight PET.

A new generation of high-performance, time-of-flight (TOF) PET scanners have recently been developed. In earlier works, the gain with TOF information was derived as a reduction of noise in the reconstructed image, or essentially a gain in scanner sensitivity. These derivations were applicable to analytical reconstruction techniques and 2D PET imaging. In this work, we evaluate the gain measured in the clinically relevant task of lesion detection with TOF information in fully 3D PET scanners using iterative reconstruction algorithms. We performed measurements in a fully 3D TOF PET scanner using spherical lesions in uniform, cylindrical phantom. Lesion detectability was estimated for 10 mm diameter lesions using a non-prewhitening matched filter signal-to-noise-ratio (NPW SNR) as the metric. Our results show that the use of TOF information leads to increased lesion detectability, which is achieved with less number of iterations of the reconstruction algorithm. These phantom results indicate that clinically, TOF PET will allow reduced scan times and improved lesion detectability, especially in large patients.

Design considerations for a limited angle, dedicated breast, TOF PET scanner.

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 degrees in-plane angular coverage), 2/3 (120 degrees in-plane angular coverage) or 1/2 (90 degrees 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.

A count-rate model for PET scanners using pixelated Anger-logic detectors with different scintillators.

A high count-rate simulation (HCRSim) model has been developed so that all results are derived from fundamental physics principles. Originally developed to study the behaviour of continuous sodium iodide (NaI(Tl)) detectors, this model is now applied to PET scanners based on pixelated Anger-logic detectors using lanthanum bromide (LaBr(3)), gadolinium orthosilicate (GSO) and lutetium orthosilicate (LSO) scintillators. This simulation has been used to study the effect on scanner deadtime and pulse pileup at high activity levels due to the scintillator stopping power (mu), decay time (tau) and energy resolution. Simulations were performed for a uniform 20 cm diameter x 70 cm long cylinder (NEMA NU2-2001 standard) in a whole-body scanner with an 85 cm ring diameter and a 25 cm axial field-of-view. Our results for these whole-body scanners demonstrate the potential of a pixelated Anger-logic detector and the relationship of its performance with the scanner NEC rate. Faster signal decay and short coincidence timing window lead to a reduction in deadtime and randoms fraction in the LaBr(3) and LSO scanners compared to GSO. The excellent energy resolution of LaBr(3) leads to the lowest scatter fraction for all scanners and helps compensate for reduced sensitivity compared to the GSO and LSO scanners, leading to the highest NEC values at high activity concentrations. The LSO scanner has the highest sensitivity of all the scanner designs investigated here, therefore leading to the highest peak NEC value but at a lower activity concentration than that of LaBr(3).

Image quality assessment of LaBr3-based whole-body 3D PET scanners: a Monte Carlo evaluation.

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-Z scintillator, 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.

Design considerations for PET scanners.

In 2-D PET scanners employing septa, scattered radiation is reduced by the septa, placing less importance on good energy resolution. Additionally, the reduced sensitivity in 2-D limits the maximum countrates encountered in clinical FDG studies. In contrast, 3-D PET scanners rely on good energy resolution to reduce the scattered radiation and also must deal with countrates, which are typically 5 times higher than in 2-D mode. To achieve good energy resolution, 3 factors must be considered: 1) choice of a scintillator with good intrinsic energy resolution, 2) choice of a crystal dimension which transmits a uniform amount of light to the PMT in order to avoid light loss along the length of the crystal and 3) choice of a crystal-to-PMT coupling which collects a uniform amount of light from all crystals. As PET scanners are being designed using new, faster scintillators for 3-D imaging, the appropriate trade-off between energy resolution and countrate capability must be found to give the best overall system performance. An example of a fully 3-D PET scanner is the Allegro (ADAC Laboratories), which uses GSO as the detector material. Given the right choice of material and design parameters, good quality, high contrast images can be obtained in 3-D in a relatively short time.