Improvement in Signal-to-Noise-Ratio at variable Random Fraction in TOF PET

Time of Flight (TOF) Positron Emission Tomography (PET) produces an improvement in signal-to-noise ratio (SNR) over conventional non-TOF reconstruction. This improvement increases with the size of the patient and with better coincidence time resolution of the system. As a first approximation in TOF PET reconstruction, this gain in SNR is independent of the scatter and random components of the total signal. On the other hand, recently a model has been proposed in which the SNR gain of TOF PET vs. conventional PET is a growing function of the random events fraction. This would provide an additional advantage of TOF PET reconstruction over conventional PET, particularly in the presence of a large random fraction. In this work we show measurements on a NEMA image quality phantom on a Siemens Biograph mCT TOF PET/CT scanner, which confirms the prediction of the model for filtered back projection reconstruction algorithms, in the case of a non uniform source distribution. The implication is that not only is TOF reconstruction more advantageous for large patients, but also the improvement is higher for PET scans performed at higher count rates, compared to a conventional non-TOF reconstruction

Signal-to-Noise gain at variable random fraction in TOF PET

Time of Flight (TOF) Positron Emission Tomography (PET) produces an improvement in signal-to-noise ratio (SNR) over conventional non-TOF reconstruction. This improvement increases with the size of the patient and with better coincidence time resolution of the system. As a first approximation in TOF PET reconstruction, this gain in SNR is independent of the scatter and randoms components of the total signal. On the other hand, recently a model has been proposed in which the SNR gain of TOF PET vs. conventional PET is a growing function of the randoms ratio (number of random coincidences divided by number of trues plus scattered coincidences). This would provide an additional advantage of TOF PET reconstruction over conventional PET, particularly in the presence of a large randoms ratio. In this work we show measurements with a NEMA image quality phantom on a Siemens Biograph mCT TOF PET/CT scanner, which confirm the prediction of the model for filtered back projection reconstruction as well as for iterative reconstruction algorithms. The implication is that not only is Time of Flight (TOF) reconstruction more advantageous for large patients, but also that the improvement is higher for PET scans performed at higher count rates, compared to a conventional non-TOF reconstruction

Time-based position estimation in monolithic scintillator detectors

Gamma-ray detectors based on bright monolithic scintillation crystals coupled to pixelated photodetectors are currently being considered for several applications in the medical imaging field. In a typical monolithic detector, both the light intensity and the time of arrival of the earliest scintillation photons can be recorded by each of the photosensor pixels every time a gamma interaction occurs. Generally, the time stamps are used to determine the gamma interaction time while the light intensities are used to estimate the 3D position of the interaction point. In this work we show that the spatio-temporal distribution of the time stamps also carries information on the location of the gamma interaction point and thus the time stamps can be used as explanatory variables for position estimation. We present a model for the spatial resolution obtainable when the interaction position is estimated using exclusively the time stamp of the first photon detected on each of the photosensor pixels. The model is shown to be in agreement with experimental measurements on a 16 mm × 16 mm × 10 mm LSO : Ce,0.2%Ca crystal coupled to a digital photon counter (DPC) array where a spatial resolution of 3 mm (root mean squared error) is obtained. Finally we discuss the effects of the main parameters such as scintillator rise and decay time, light output and photosensor single photon time resolution and pixel size.Radiation, Science and TechnologyApplied Science

Towards monolithic scintillator based TOF-PET systems: Practical methods for detector calibration and operation

Gamma-ray detectors based on thick monolithic scintillator crystals can achieve spatial resolutions <2 mm full-width-at-half-maximum (FWHM) and coincidence resolving times (CRTs) better than 200 ps FWHM. Moreover, they provide high sensitivity and depth-of-interaction (DOI) information. While these are excellent characteristics for clinical time-of-flight (TOF) positron emission tomography (PET), the application of monolithic scintillators has so far been hampered by the lengthy and complex procedures needed for position- and time-of-interaction estimation. Here, the algorithms previously developed in our group are revised to make the calibration and operation of a large number of monolithic scintillator detectors in a TOF-PET system practical. In particular, the k-nearest neighbor (k-NN) classification method for x,y-position estimation is accelerated with an algorithm that quickly preselects only the most useful reference events, reducing the computation time for position estimation by a factor of ∼200 compared to the previously published k-NN 1D method. Also, the procedures for estimating the DOI and time of interaction are revised to enable full detector calibration by means of fan-beam or flood irradiations only. Moreover, a new technique is presented to allow the use of events in which some of the photosensor pixel values and/or timestamps are missing (e.g. due to dead time), so as to further increase system sensitivity. The accelerated methods were tested on a monolithic scintillator detector specifically developed for clinical PET applications, consisting of a 32 mm × 32 mm × 22 mm LYSO : Ce crystal coupled to a digital photon counter (DPC) array. This resulted in a spatial resolution of 1.7 mm FWHM, an average DOI resolution of 3.7 mm FWHM, and a CRT of 214 ps. Moreover, the possibility of using events missing the information of up to 16 out of 64 photosensor pixels is shown. This results in only a small deterioration of the detector performance.RST/Radiation, Science and TechnologyRST/Applied Radiation & Isotope

Sub-3 mm, near-200 ps TOF/DOI-PET imaging with monolithic scintillator detectors in a 70 cm diameter tomographic setup

Recently, a monolithic scintillator detector for time-of-flight (TOF)/depth-of-interaction (DOI) positron emission tomography (PET) was developed. It has a detector spatial resolution of ∼1.7 mm full-width-at-half-maximum (FWHM), a coincidence resolving time (CRT) of ∼215 ps FWHM, and ∼4.7 mm FWHM DOI resolution. Here, we demonstrate, for the first time, the imaging performance of this detector in a 70 cm diameter PET geometry. We built a tomographic setup representative of a whole-body clinical scanner, comprising two coaxially rotating arms, each carrying a detector module, and a central, rotating phantom table. The fully automated setup sequentially acquires all possible lines of response (LORs) of a complete detector ring, using a step-and-shoot acquisition approach. The modules contained 2 × 2 detectors, each detector consisting of a 32 mm × 32 mm × 22 mm LYSO crystal and a digital silicon photomultiplier (dSiPM) array. The system spatial resolution was assessed using a Na-22 point source at different radial distances in the field-of-view (FOV). Using 2D filtered back projection (2D FBP, non-TOF), tangential and radial spatial resolutions of ∼2.9 mm FWHM were obtained at the center of the FOV. The use of DOI information resulted in almost uniform spatial resolution throughout the FOV up to a radial distance of 25 cm, where the radial and tangential resolution are ∼3.3 mm FWHM and ∼4.7 mm FWHM, respectively, whereas without DOI the resolution deteriorates to ∼9 mm FWHM. Additional measurements were performed with a Na-22 filled Derenzo-like phantom at different locations within the FOV. Images reconstructed with a TOF maximum-likelihood expectation-maximization (TOF ML-EM) algorithm show that the system is able to clearly resolve 3 mm diameter hot rods up to 25 cm radial distance. The excellent and uniform spatial resolution, combined with an energy resolution of 10.2% FWHM and a CRT of ∼212 ps FWHM, indicates a great potential for monolithic scintillators as practical high-performance detectors in TOF/DOI-PET systems.RST/Medical Physics & TechnologyEMSD AS-south Project technician

Experimental Validation of an Efficient Fan-Beam Calibration Procedure for k -Nearest Neighbor Position Estimation in Monolithic Scintillator Detectors

Monolithic scintillator detectors can achieve excellent spatial resolution and coincidence resolving time. However, their practical use for positron emission tomography (PET) and other applications in the medical imaging field is still limited due to drawbacks of the different methods used to estimate the position of interaction. Common statistical methods for example require the collection of an extensive dataset of reference events with a narrow pencil beam aimed at a fine grid of reference positions. Such procedures are time consuming and not straightforwardly implemented in systems composed of many detectors. Here, we experimentally demonstrate for the first time a new calibration procedure for k-nearest neighbor ( k-NN) position estimation that utilizes reference data acquired with a fan beam. The procedure is tested on two detectors consisting of 16 mm ×16 mm ×10 mm and 16 mm ×16 mm ×20 mm monolithic, Ca-codoped LSO:Ce crystals and digital photon counter (DPC) arrays. For both detectors, the spatial resolution and the bias obtained with the new method are found to be practically the same as those obtained with the previously used method based on pencil-beam irradiation, while the calibration time is reduced by a factor of ~ 20. Specifically, a FWHM of ~ 1.1 mm and a FWTM of ~ 2.7 mm were obtained using the fan-beam method with the 10 mm crystal, whereas a FWHM of ~ 1.5 mm and a FWTM of ~ 6 mm were achieved with the 20 mm crystal. Using a fan beam made with a ~ 4.5 MBq 22Na point-source and a tungsten slit collimator with 0.5 mm aperture, the total measurement time needed to acquire the reference dataset was ~ 3 hours for the thinner crystal and ~ 2 hours for the thicker one.RST/Radiation, Science and TechnologyApplied Science

A 32 mm × 32 mm × 22 mm monolithic LYSO: Ce detector with dual-sided digital photon counter readout for ultrahigh-performance TOF-PET and TOF-PET/MRI

New applications for positron emission tomography (PET) and combined PET/magnetic resonance imaging (MRI) are currently emerging, for example in the fields of neurological, breast, and pediatric imaging. Such applications require improved image quality, reduced dose, shorter scanning times, and more precise quantification. This can be achieved by means of dedicated scanners based on ultrahigh-performance detectors, which should provide excellent spatial resolution, precise depth-of-interaction (DOI) estimation, outstanding time-of-flight (TOF) capability, and high detection efficiency. Here, we introduce such an ultrahigh-performance TOF/DOI PET detector, based on a 32 mm × 32 mm × 22 mm monolithic LYSO:Ce crystal. The 32 mm × 32 mm front and back faces of the crystal are coupled to a digital photon counter (DPC) array, in so-called dual-sided readout (DSR) configuration. The fully digital detector offers a spatial resolution of ∼1.1 mm full width at half maximum (FWHM)/∼1.2 mm mean absolute error, together with a DOI resolution of ∼2.4 mm FWHM, an energy resolution of 10.2% FWHM, and a coincidence resolving time of 147 ps FWHM. The time resolution closely approaches the best results (135 ps FWHM) obtained to date with small crystals made from the same material coupled to the same DPC arrays, illustrating the excellent correction for optical and electronic transit time spreads that can be achieved in monolithic scintillators using maximum-likelihood techniques for estimating the time of interaction. The performance barely degrades for events with missing data (up to 6 out of 32 DPC dies missing), permitting the use of almost all events registered under realistic acquisition conditions. Moreover, the calibration procedures and computational methods used for position and time estimation follow recently made improvements that make them fast and practical, opening up realistic perspectives for using DSR monolithic scintillator detectors in TOF-PET and TOF-PET/MRI systems.RST/Applied Radiation & IsotopesImPhys/Algemee

Improved image quality using monolithic scintillator detectors with dual-sided readout in a whole-body TOF-PET ring: A simulation study

We have recently built and characterized the performance of a monolithic scintillator detector based on a 32 mm × 32 mm × 22 mm LYSO:Ce crystal read out by digital silicon photomultiplier (dSiPM) arrays coupled to the crystal front and back surfaces in a dual-sided readout (DSR) configuration. The detector spatial resolution appeared to be markedly better than that of a detector consisting of the same crystal with conventional back-sided readout (BSR). Here, we aim to evaluate the influence of this difference in the detector spatial response on the quality of reconstructed images, so as to quantify the potential benefit of the DSR approach for high-resolution, whole-body time-of-flight (TOF) positron emission tomography (PET) applications. We perform Monte Carlo simulations of clinical PET systems based on BSR and DSR detectors, using the results of our detector characterization experiments to model the detector spatial responses. We subsequently quantify the improvement in image quality obtained with DSR compared to BSR, using clinically relevant metrics such as the contrast recovery coefficient (CRC) and the area under the localized receiver operating characteristic curve (ALROC). Finally, we compare the results with simulated rings of pixelated detectors with DOI capability. Our results show that the DSR detector produces significantly higher CRC and increased ALROC values than the BSR detector. The comparison with pixelated systems indicates that one would need to choose a crystal size of 3.2 mm with three DOI layers to match the performance of the BSR detector, while a pixel size of 1.3 mm with three DOI layers would be required to get on par with the DSR detector.RST/Applied Radiation & Isotope