Alexander Ganin

Quiescent period respiratory gating for PET∕CT

PURPOSEnTo minimize respiratory motion artifacts, this work proposes quiescent period gating (QPG) methods that extract PET data from the end-expiration quiescent period and form a single PET frame with reduced motion and improved signal-to-noise properties.nnnMETHODSnTwo QPG methods are proposed andevaluated. Histogram-based quiescent period gating (H-QPG) extracts a fraction of PET data determined by a window of the respiratory displacement signal histogram. Cycle-based quiescent period gating (C-QPG) extracts data with a respiratory displacement signal below a specified threshold of the maximum amplitude of each individual respiratory cycle. Performances of both QPG methods were compared to ungated and five-bin phase-gated images across 21 FDG-PET/CT patient data sets containing 31 thorax and abdomen lesions as well as with computer simulations driven by 1295 different patient respiratory traces. Image quality was evaluated in terms of the lesion SUV(max) and the fraction of counts included in each gate as a surrogate for image noise.nnnRESULTSnFor all the gating methods, image noise artifactually increases SUV(max) when the fraction of counts included in each gate is less than 50%. While simulation data show that H-QPG is superior to C-QPG, the H-QPG and C-QPG methods lead to similar quantification-noise tradeoffs in patient data. Compared to ungated images, both QPG methods yield significantly higher lesion SUV(max). Compared to five-bin phase gating, the QPG methods yield significantly larger fraction of counts with similar SUV(max) improvement. Both QPG methods result in increased lesion SUV(max) for patients whose lesions have longer quiescent periods.nnnCONCLUSIONSnCompared to ungated and phase-gated images, the QPG methods lead to images with less motion blurring and an improved compromise between SUV(max) and fraction of counts. The QPG methods for respiratory motion compensation could effectively improve tumor quantification with minimal noise increase.

Time-of-Flight PET Detector Based on Multi-Pixel Photon Counter and Its Challenges

Geiger-mode multi-pixel APD is being recognized as the best alternative solid-state photo-sensor to vacuum PMT for various specific applications. Especially, its magnetic field immunity and high gain made it popular in MR/PET detector research. In this paper, we utilized its compactness, high gain and high photon detection efficiency in the design of TOF PET detector. In a typical block detector based on PMT, the full timing capability of both PMT and scintillator could not be achieved due to its light sharing for Anger logic scheme. Since Geiger-mode APD is a solid-state based technology, we can apply one-to-one coupling between a scintillator and the photo-sensor to optimize the signal-to-noise ratio. Also, the high photon detection efficiency of MPPC, Geiger mode APD from Hamamatsu, would help to improve timing resolution. So, we made a block detector based on a 4 × 4 array of 3 × 3 mm2 MPPC coupled to a 4 × 4 array of 3 × 3 × 25 mm3 LYSO crystals to evaluate its performance. We have achieved the average of 9% energy resolution and 314 ps coincidence timing resolution with very good uniformity. This block timing resolution showed no degradation in timing compared to individual single channel timing resolution as expected from one-to-one readout. On top of that, the result proves that the solid-state based photo-sensor can be used for TOF PET detector. During the development and setup of the detector, we recognized that a compact and low power electronics readout scheme is one of the biggest challenges, including its cost, for MPPC or other Geiger-mode APD to be used in products.

Evaluation of a position sensitive avalanche photodiode for PET

A gamma ray detector for PET, consisting of an array of mixed lutetium oxyorthosilicate (MLS) scintillator crystals coupled to a position sensitive avalanche photodiode (PSAPD), was evaluated. The scintillator array was constructed from individual MLS crystals with dimensions of 1.5 mm /spl times/ 1.5 mm /spl times/ 15 mm. The assembled 7 /spl times/ 7 array, including intercrystal reflector material, had a pitch of 1.79 mm. The low noise, high gain PSAPD had dimensions of 14 mm /spl times/ 14 mm. Peaks associated with each of the 49 scintillator crystals were readily identifiable in flood histograms, and most of the crystals demonstrated energy resolution in the range of 15% to 20% at 511 keV. Measurements of the timing of the PSAPD in coincidence with a fast-scintillator/PMT detector indicated a timing resolution of approximately 4 ns. The operating characteristics and design attributes, such as compactness and reduced readout channel requirements, of the PSAPD make it attractive for high resolution PET applications.

Measured count-rate performance of the Discovery STE PET/CT scanner in 2D, 3D and partial collimation acquisition modes

We measured count rates and scatter fraction on the Discovery STE PET/CT scanner in conventional 2D and 3D acquisition modes, and in a partial collimation mode between 2D and 3D. As part of the evaluation of using partial collimation, we estimated global count rates using a scanner model that combined computer simulations with an empirical live-time function. Our measurements followed the NEMA NU2 count rate and scatter-fraction protocol to obtain true, scattered and random coincidence events, from which noise equivalent count (NEC) rates were calculated. The effect of patient size was considered by using 27 cm and 35 cm diameter phantoms, in addition to the standard 20 cm diameter cylindrical count-rate phantom. Using the scanner model, we evaluated two partial collimation cases: removing half of the septa (2.5D) and removing two-thirds of the septa (2.7D). Based on predictions of the model, a 2.7D collimator was constructed. Count rates and scatter fractions were then measured in 2D, 2.7D and 3D. The scanner model predicted relative NEC variation with activity, as confirmed by measurements. The measured 2.7D NEC was equal or greater than 3D NEC for all activity levels in the 27 cm and 35 cm phantoms. In the 20 cm phantom, 3D NEC was somewhat higher ( approximately 15%) than 2.7D NEC at 100 MBq. For all higher activity concentrations, 2.7D NEC was greater and peaked 26% above the 3D peak NEC. The peak NEC in 2.7D mode occurred at approximately 425 MBq, and was 26-50% greater than the peak 3D NEC, depending on object size. NEC in 2D was considerably lower, except at relatively high activity concentrations. Partial collimation shows promise for improved noise equivalent count rates in clinical imaging without altering other detector parameters.

Impact of PSF modelling on the convergence rate and edge behaviour of EM images in PET

EM reconstructions with point-spread-function (PSF) modelling is performed to increase the spatial resolution in PET images. These images exhibit slower initial convergence compared to reconstructions without PSF modelling. Furthermore, they exhibit more pronounced ringing around the edges of sharp features. We investigate the effect of different objects and PSF modelling on the convergence rate and edge behaviour of the EM algorithm in two stages: (i) at the initial iterations where the updates are large and (ii) at the later iterations where the updates are small. For the initial iterations, we compare the sharpness of the EM updates with and without PSF modelling. We show via simulations that the PSF modelling during the backprojection step causes smoother updates and consequently smoother images in the early stages of the EM algorithm. For the later iterations, we approximate the image as the ML image plus a perturbation term and develop an approximate update equation for the perturbation, which depends on the Hessian (H) of the log-likelihood. Based on this equation and the spectral analysis of H, we demonstrate how edges with ringing are preserved in the later stages of the algorithm and eliminated only for the case of noiseless data reconstruction with an unrealistically high number of iterations. In addition, we provide an intuitive explanation for the creation of the edge artefacts in terms of the PSF modelling during the backprojection step.

Time-of-flight PET-MR detector development with silicon photomultiplier

Silicon photomultiplier technology based on Geigermode avalanche in p-n junction has made unforeseen progress for the last 5-6 years. With its high gain and high photon detection efficiency, it has shown that it could replace PMTs in many applications including time-of-flight PET. Also, its magnetic immunity and compactness made it very suitable for PET-MR detector. In this paper, we present time-of-flight PET-MR detector based on silicon photomultiplier from its selection, prototype test with discrete electronics, ASIC and a module design. Also, the system performance data is presented.

Comparison of different methods for data-driven respiratory gating of PET data

Respiratory movement degrades image quality in PET/CT. The first step in correcting for movement is to gate the data into different motion states. In current practice, the gating is based on information from external devices that measure physical parameters such as the chest position. Various groups have proposed methods to extract a gating signal out of the PET data. Here we compare methods using PCA, Laplacian Eigenmaps, Spectral Analysis and sensitivity. We test the methods on clinical PET list mode data for different tracers. We evaluate correlation with the chest position as measured by the Varian RPM system, and stability under increased noise in the PET data, both by reducing counts and reducing total duration. We also compare SUVmax and lesion displacement when gating the PET data based on the signals extracted by the different methods.

Evaluation of position sensitive avalanche photodiodes for PET

A gamma ray detector for PET, consisting of an array of mixed lutetium oxyorthosilicate (MLS) scintillator crystals coupled to a position sensitive avalanche photodiode (PSAPD), was evaluated. The scintillator array was constructed from individual MLS crystals with dimensions of 1.5 mm/spl times/1.5 mm/spl times/15 mm. The assembled 7/spl times/7 array, including inter-crystal reflector material, had a pitch of 1.79 mm. The low noise, high gain PSAPD had dimensions of 14 mm/spl times/14 mm. Peaks associated with each of the 49 scintillator crystals were readily identifiable in flood histograms, and most of the crystals demonstrated energy resolution in the range of 15% to 20% at 511 keV. Preliminary measurements of the timing of the PSAPD in coincidence with a fast-scintillator/PMT detector indicated a timing resolution of approximately 4 ns. The operating characteristics and design attributes, such as compactness and reduced readout channel requirements, of the PSAPD make it attractive for high resolution PET applications.

Count-Rate Performance of the Discovery STE PET Scanner Using Partial Collimation

We investigated the use of partial collimation on a clinical PET scanner by removing septa from conventional 2D collimators. The goal is to improve noise equivalent count-rates (NEC) compared to 2D and 3D scans for clinically relevant activity concentrations. We evaluated two cases: removing half of the septa (2.5D); and removing two-thirds of the septa (2.7D). System performance was first modeled using the SimSET simulation package, and then measured with the NEMA NU2-2001 count-rate cylinder (20 cm dia., 70 cm long), and 27 cm and 35 cm diameter cylinders of the same length. An image quality phantom was also imaged with the 2.7D collimator. SimSET predicted the relative NEC curves very well, as confirmed by measurements, with 2.5D and 2.7D NEC greater than 2D and 3D NEC in the range of ~5-20 mCi in the phantom. We successfully reconstructed images of the image quality phantom from measured 2.7D data using custom 2.7D normalization. Partial collimation shows promise for optimized clinical imaging in a fixed-collimator system.

A compact SPECT detector based on a quad PMT

Traditionally, the technology of general purpose SPECT cameras has been based on a large panel of NaI:Tl scintillator, optically coupled with a number of 2” or 3” PMTs [1]. With its rather good performance at low cost, the SPECT camera design has not changed essentially since Hal Anger invented it in the late 1950s. For the last decade, however, with progress in new scintillators and photosensors, there have been renewed efforts to improve spatial and energy resolutions. A series of new compact detectors have been mostly designed for small animal or organ specific SPECT cameras. In this paper, we present a concept of a compact SPECT detector design using a NaI:Tl crystal array and a quad anode PMT. By using a crystal array and a light guide, we demonstrate that all individual crystals can be identified without any dead area at the edge of the detector. This proves the possibility of a compact SPECT detector having high spatial resolution using an array of regular types of PMTs, for example, a 2×2 or 3×3 array of PMTs. Also, we show that this block structure design enables very small gaps at the edge of the detectors resulting in a more compact geometry when packed in full size SPECT cameras.