Rutao Yao

Initial results from the Sherbrooke avalanche photodiode positron tomograph

The design features and engineering constraints of a PET system based on avalanche photodiode (APD) detectors have been described in a previous report. Here, the authors present the initial results obtained with the Sherbrooke APD-PET scanner, a very high spatial resolution device designed for dynamic imaging of small and medium-sized laboratory animals such as rats, cats, rabbits and small monkeys. Its physical performance has been evaluated in terms of resolution, sensitivity, count rate, random and scatter fractions, contrast and relative activity recovery as a function of object size. The capabilities of the scanner for biomedical research applications have been demonstrated using phantom and animal studies.

Performance characteristics of the 3-D OSEM algorithm in the reconstruction of small animal PET images

Rat brain images acquired with a small animal positron emission tomography (PET) camera and reconstructed with the three-dimensional (3-D) ordered-subsets expectation-maximization (OSEM) algorithm with resolution recovery have better quality when the brain is imaged by itself than when inside the head with surrounding background activity. The purpose of this study was to characterize the dependence of this effect on the level of background activity, attenuation, and scatter. Monte Carlo simulations of the imaging system were performed. The coefficient of variation from replicate images, full-width at half-maximum (FWHM) from point sources and image profile fitting, and image contrast and uniformity were used to evaluate algorithm performance. A rat head with the typical levels of five and ten times the brain activity in the surrounding background requires additional iterations to achieve the same resolution as the brain-only case at a cost of 24% and 64% additional noise, respectively. For the same phantoms, object scatter reduced contrast by 3%-5%. However, attenuation degraded resolution by 0.2 mm and was responsible for up to 12% nonuniformity in the brain images suggesting that attenuation correction is useful. Given the effects of emission and attenuation distribution on both resolution and noise, simulations or phantom studies should be used for each imaging situation to select the appropriate number of OSEM iterations to achieve the desired resolution-noise levels.

Small-Animal PET: What Is It, and Why Do We Need It?

Small-animal PET refers to imaging of animals such as rats and mice using dedicated PET scanners. Small-animal PET has been used extensively in modern biomedical research. It provides a quantitative measure of the 3-dimensional distribution of a radiopharmaceutical administered to a live subject noninvasively. In this article, we will discuss the operational and technical aspects of small-animal PET; make some comparisons between small-animal PET and human PET systems; identify the challenges of, opportunities for, and ultimate limitations in applying small-animal PET; and discuss some representative small-animal PET applications. Education objectives: After reading this article, the technologist will be able to explain the requirements and benefits of small-animal PET in biomedical research, describe the design and general characteristics of a small-animal PET system, list and describe some of the challenges of imaging small animals, and discuss several small-animal PET applications.

Lutetium oxyorthosilicate (LSO) intrinsic activity correction and minimal detectable target activity study for SPECT imaging with a LSO-based animal PET scanner

This work is part of a feasibility study to develop SPECT imaging capability on a lutetium oxyorthosilicate (LSO) based animal PET system. The SPECT acquisition was enabled by inserting a collimator assembly inside the detector ring and acquiring data in singles mode. The same LSO detectors were used for both PET and SPECT imaging. The intrinsic radioactivity of (176)Lu in the LSO crystals, however, contaminates the SPECT data, and can generate image artifacts and introduce quantification error. The objectives of this study were to evaluate the effectiveness of a LSO background subtraction method, and to estimate the minimal detectable target activity (MDTA) of image object for SPECT imaging. For LSO background correction, the LSO contribution in an image study was estimated based on a pre-measured long LSO background scan and subtracted prior to the image reconstruction. The MDTA was estimated in two ways. The empirical MDTA (eMDTA) was estimated from screening the tomographic images at different activity levels. The calculated MDTA (cMDTA) was estimated from using a formula based on applying a modified Currie equation on an average projection dataset. Two simulated and two experimental phantoms with different object activity distributions and levels were used in this study. The results showed that LSO background adds concentric ring artifacts to the reconstructed image, and the simple subtraction method can effectively remove these artifacts-the effect of the correction was more visible when the object activity level was near or above the eMDTA. For the four phantoms studied, the cMDTA was consistently about five times of the corresponding eMDTA. In summary, we implemented a simple LSO background subtraction method and demonstrated its effectiveness. The projection-based calculation formula yielded MDTA results that closely correlate with that obtained empirically and may have predicative value for imaging applications.

A SVD-Based Method to Assess the Uniqueness and Accuracy of SPECT Geometrical Calibration

Geometrical calibration is critical to obtaining high resolution and artifact-free reconstructed image for SPECT and CT systems. Most published calibration methods use analytical approach to determine the uniqueness condition for a specific calibration problem, and the calibration accuracy is often evaluated through empirical studies. In this work, we present a general method to assess the characteristics of both the uniqueness and the quantitative accuracy of the calibration. The method uses a singular value decomposition (SVD) based approach to analyze the Jacobian matrix from a least-square cost function for the calibration. With this method, the uniqueness of the calibration can be identified by assessing the nonsingularity of the Jacobian matrix, and the estimation accuracy of the calibration parameters can be quantified by analyzing the SVD components. A direct application of this method is that the efficacy of a calibration configuration can be quantitatively evaluated by choosing a figure-of-merit, e.g., the minimum required number of projection samplings to achieve desired calibration accuracy. The proposed method was validated with a slit-slat SPECT system through numerical simulation studies and experimental measurements with point sources and an ultra-micro hot-rod phantom. The predicted calibration accuracy from the numerical studies was confirmed by the experimental point source calibrations at ~ 0.1 mm for both the center of rotation (COR) estimation of a rotation stage and the slit aperture position (SAP) estimation of a slit-slat collimator by an optimized system calibration protocol. The reconstructed images of a hot rod phantom showed satisfactory spatial resolution with a proper calibration and showed visible resolution degradation with artificially introduced 0.3 mm COR estimation error. The proposed method can be applied to other SPECT and CT imaging systems to analyze calibration method assessment and calibration protocol optimization.

A novel method to calibrate DOI function of a PET detector with a dual‐ended‐scintillator readout

The detection of depth-of-interaction (DOI) is a critical detector capability to improve the PET spatial resolution uniformity across the field-of-view and will significantly enhance, in particular, small bore system performance for brain, breast, and small animal imaging. One promising technique of DOI detection is to use dual-ended-scintillator readout that uses two photon sensors to detect scintillation light from both ends of a scintillator array and estimate DOI based on the ratio of signals (similar to Anger logic). This approach needs a careful DOI function calibration to establish accurate relationship between DOI and signal ratios, and to recalibrate if the detection condition is shifted due to the drift of sensor gain, bias variations, or degraded optical coupling, etc. However, the current calibration method that uses coincident events to locate interaction positions inside a single scintillator crystal has severe drawbacks, such as complicated setup, long and repetitive measurements, and being prone to errors from various possible misalignments among the source and detector components. This method is also not practically suitable to calibrate multiple DOI functions of a crystal array. To solve these problems, a new method has been developed that requires only a uniform flood source to irradiate a crystal array without the need to locate the interaction positions, and calculates DOI functions based solely on the uniform probability distribution of interactions over DOI positions without knowledge or assumption of detector responses. Simulation and experiment have been studied to validate the new method, and the results show that the new method, with a simple setup and one single measurement, can provide consistent and accurate DOI functions for the entire array of multiple scintillator crystals. This will enable an accurate, simple, and practical DOI function calibration for the PET detectors based on the design of dual-ended-scintillator readout. In addition, the new method can be generally applied to calibrating other types of detectors that use the similar dual-ended readout to acquire the radiation interaction position.

Derivation of System Matrix From Simulation Data for an Animal SPECT With Slit-Slat Collimator

We developed SPECT imaging capability on an animal PET system. Our goal was to provide animal PET users the SPECT capability at a low cost and facilitate potential PET/SPECT dual modality imaging applications. The SPECT function was enabled with a slit-slat collimator insert and by acquiring data in singles mode. The focus of this paper is to establish a method for deriving the system matrix for the SPECT system from Monte Carlo simulation. With the Monte Carlo package GATE, we simulated a uniform cylinder source which filled the SPECT field of view (FOV). To reduce the size of the original large and sparse system matrix, the detectors that were exposed to individual emission elements were selectively included for system matrix derivation and storage. The axial symmetry of the system was exploited so that only the base-axial volume was used for deriving system response. The system matrix derived was validated with point source measurements at known positions and implemented in an iterative reconstruction algorithm. The imaging performance of the system matrix was evaluated with experimental phantom studies. Reconstructed phantom images were artifact free and demonstrated expected spatial resolution. The method presented in this work is generally applicable to other SPECT imaging systems.

Initial studies of PET-SPECT dual-tracer imaging

Simultaneous PET and SPECT dual-tracer imaging with common detector and bed will potentially open the door for many new clinical and preclinical applications, such as two receptor binding studies or perfusion-metabolism differential imaging of neurodegenerative diseases, and can be technically advantageous in certain aspects compared to conventional SPECT dual-tracer or PET dual-tracer imaging. In this feasibility study, PET and SPECT data from concurrently existing Tc-99m and F-18 radiotracers were acquired with the same acquisition hardware setup: a microPET and a collimator inside the gantry. The collimator is a slit-slat type, consists of knife-edge slits formed by Lead plates and septa formed by annular Tungsten sheets to form 2D fan beam geometry. SPECT data were acquired in list-mode with 135-145 keV energy window. With the collimator stayed inside the gantry, PET data were acquired and processed to correct collimator effects. A hot-rod phantom was filled with Tc-99m and F-18 tracers in separated rods so that possible contamination between the PET and SPECT acquisitions can be evaluated. The rods are 0.9-2.3 mm in diameters and separated by roughly twice of the rod diameter. The ratio of activity concentration between the two tracers was close to 1:1. Since there is no corresponding simultaneous data acquisition mode available in the current system, PET and SPECT data were acquired successively with the same hardware setup and phantom position to mimic the simultaneous imaging. The rod images corresponding to PET and SPECT were well separated without severe artifacts. The data corrections, including LSO background and down-scatter from 511 keV photons to the SPECT acquisition were applied to the corresponding images. In addition, F-18 PET images of a phantom acquired with and without collimator reveal slight degradation in image quality due to the attenuation and extra data correction. Initial PET-SPECT dual-tracer mouse images were acquired and fused with mixed Tc-99m MDP and F-18 FDG injection. These studies have demonstrated the feasibility to acquire simultaneous PET-SPECT dual-tracer images with a microPET and a collimator insert.

Initial Evaluation of LabPET/SPECT Dual Modality Animal Imaging System

We developed SPECT imaging capability on a LabPET animal scanner to provide an integrated PET/SPECT dual-modality imaging system. The add-on SPECT sub-system was enabled by 1) mechanically integrating a multiple-pinhole collimator in the PET detector ring, and 2) configuring the detectors to acquire singles events in the 120-160 keV range. We report on the design parameters, data acquisition protocols and initial performance assessment of this cost-effective SPECT imaging solution. Phantom and animal images demonstrating the relevance of the system for various imaging tasks in preclinical research are presented.

Assessment of a three-dimensional line-of-response probability density function system matrix for PET

To achieve optimal PET image reconstruction through better system modeling, we developed a system matrix that is based on the probability density function for each line of response (LOR-PDF). The LOR-PDFs are grouped by LOR-to-detector incident angles to form a highly compact system matrix. The system matrix was implemented in the MOLAR list mode reconstruction algorithm for a small animal PET scanner. The impact of LOR-PDF on reconstructed image quality was assessed qualitatively as well as quantitatively in terms of contrast recovery coefficient (CRC) and coefficient of variance (COV), and its performance was compared with a fixed Gaussian (iso-Gaussian) line spread function. The LOR-PDFs of three coincidence signal emitting sources, (1) ideal positron emitter that emits perfect back-to-back γ rays (γγ) in air; (2) fluorine-18 (¹⁸F) nuclide in water; and (3) oxygen-15 (¹⁵O) nuclide in water, were derived, and assessed with simulated and experimental phantom data. The derived LOR-PDFs showed anisotropic and asymmetric characteristics dependent on LOR-detector angle, coincidence emitting source, and the medium, consistent with common PET physical principles. The comparison of the iso-Gaussian function and LOR-PDF showed that: (1) without positron range and acollinearity effects, the LOR-PDF achieved better or similar trade-offs of contrast recovery and noise for objects of 4 mm radius or larger, and this advantage extended to smaller objects (e.g. 2 mm radius sphere, 0.6 mm radius hot-rods) at higher iteration numbers; and (2) with positron range and acollinearity effects, the iso-Gaussian achieved similar or better resolution recovery depending on the significance of positron range effect. We conclude that the 3D LOR-PDF approach is an effective method to generate an accurate and compact system matrix. However, when used directly in expectation-maximization based list-mode iterative reconstruction algorithms such as MOLAR, its superiority is not clear. For this application, using an iso-Gaussian function in MOLAR is a simple but effective technique for PET reconstruction.