Tiantian Dai

High accuracy geometrical calibration for half-mm animal SPECT imaging

In this study we investigate a specific animal SPECT imaging system with a clinical SPECT detector and a multipinhole collimator insert. High accuracy geometrical calibration is a crucial technique in terms of achieving the high SPECT imaging resolution. The calibration steps include: 1) The SPECT system is modeled with a geometrical projection equation; 2) The centroids of the point source projection are calculated from the acquired experimental data; and 3)The geometrical parameters is determined by minimizing a nonlinear least square cost function. With this calibration technique, Experimental studies shows the best achievable reconstruction resolution is 0.5 mm. Furthermore, the uniqueness of the calibration solution is investigated with a SVD based approach. By experimentally calculating centriod variance of point source projections, quantitative calibration accuracy of the proposed calibration method is studied and suggested minimum projection number of the point source experiments to achieve desired calibration accuracy is given.

Design and feasibility studies of a high-resolution and low-cost small animal SPECT system

In this study we investigate the feasibility of performing small animal SPECT imaging on a clinical SPECT scanner with a dedicate pinhole collimator. Analytical formulae are used to calculate spatial resolution and absolute sensitivity given a pinhole collimator design scheme. Two optimal pinhole designs are proposed in terms of achieving the best trade-off between spatial resolution and absolute sensitivity. One consists of one single pinhole aperture and the other one has 7 pinholes. In the 7-pinhole design scheme, the pinholes are arranged in the way that each pinhole only covers part of the region of the object to maximize the sensitivity in the central slice of field of view (FOV) and to make use of the entire detection area without much overlapping. Monte Carlo simulation studies confirm the predicted spatial resolution values from analytical formulae. Imaging reconstruction results of a simulated ultra-micro hotrod phantom demonstrate that the 0.5 mm hot rods are clearly identified with the 7-pinhole collimator. Despite that the intrinsic resolution of the SPECT detector is only 3.55 mm, its large detection area allows large magnification and good spatial resolution. We conclude that high-resolution animal SPECT imaging can be performed through the proposed approach with relatively low cost for clinical SPECT users comparing to purchasing a dedicated small animal SPECT.

Design and sampling completeness evaluation of scanning orbits in multi-pinhole small animal SPECT imaging

For a multi-pinhole SPECT system, it is important to choose an optimal scanning orbit to fulfill the sampling completeness requirement. In this study, we propose a novel four-degree-of-freedom (4-DOF) orbit which combines a conventional helical scanning path and a superimposed periodic reciprocating 3×3-pioint shift movement in transaxial plane. An Orlov-sphere-based numerical evaluation method was used and a sampling completeness percentage (SCP) index was introduced to access the sampling completeness for the merits of scanning orbits over entire field of view (FOV) in terms of sampling completeness. By comparing distribution of sampling completeness over the FOV of conventional circular orbit, helical orbits, and 4-DOF orbit applied to an experimental small animal SPECT system, it is shown that the novel 4-DOF orbit has better sampling performance than conventional orbits. The evaluation results of different orbits and the mouse whole-body imaging experiment demonstrate that this method is feasible and applicable for guiding the design of scanning orbits and determining the merits of the orbits.

Crystal Identification in Dual-Layer-Offset DOI-PET Detectors Using Stratified Peak Tracking Based on SVD and Mean-Shift Algorithm

An Anger-logic based pixelated PET detector block requires a crystal position map (CPM) to assign the position of each detected event to a most probable crystal index. Accurate assignments are crucial to PET imaging performance. In this paper, we present a novel automatic approach to generate the CPMs for dual-layer offset (DLO) PET detectors using a stratified peak tracking method. In which, the top and bottom layers are distinguished by their intensity difference and the peaks of the top and bottom layers are tracked based on a singular value decomposition (SVD) and mean-shift algorithm in succession. The CPM is created by classifying each pixel to its nearest peak and assigning the pixel with the crystal index of that peak. A Matlab-based graphical user interface program was developed including the automatic algorithm and a manual interaction procedure. The algorithm was tested for three DLO PET detector blocks. Results show that the proposed method exhibits good performance as well as robustness for all the three blocks. Compared to the existing methods, our approach can directly distinguish the layer and crystal indices using the information of intensity and offset grid pattern.

Development of a small animal SPECT and CT dual function imager with a microcolumnar CsI(Tl) and CCD based detector

We report on the development of a small animal SPECT/CT dual function imager with a single detector that consists of a 300 μm thick micro-columnar structured CsI(TI) phosphor, a light image intensifier (LII), and a CCD image sensor. The detector was originally designed for x-ray micro-angiographic fluoroscopy (MAF) imaging but used here for both x-ray and gamma-ray detection in the dual function imager. The large variable gain of the LII enables the detector to meet the very different SPECT and x-ray CT sensitivity requirements. The prototype imager allowed SPECT and CT imaging on a common image-subject movement platform. A detachable 5-pinhole tungsten collimator enabled switching between the two modes. Several technology modules including projection preprocessing, imager geometrical calibration, and iterative reconstruction, were developed to generate SPECT and CT images. A 10-mm diameter rod phantom and a 25-gram mouse were scanned with the imager in both SPECT and CT modes. The images showed the expected system resolution and sensitivity performance.

Development and assessment of statistical iterative image reconstruction for CT on a small animal SPECT/CT dual-modality system

We developed a statistical iterative CT image reconstruction software for a newly constructed high-resolution small animal SPECT/CT dual-modality system, and assessed its performance at different radiation exposure levels. The objective of this work was to preserve or improve reconstructed image quality at either the same or reduced animal x-ray radiation exposure. The SPECT/CT system used a single detector for both the CT and SPECT modalities that consists of a micro-columnar CsI(TI) phosphor, a light image intensifier (LII) and a CCD sensor. The CT reconstruction software was based on the ordered-subset-convex (OSC) algorithm, and the system matrix was calculated through a ray-driven approach. A self-calibration method was implemented to calculate the offset of the axis of rotation (AOR), an important geometry parameter of the system. An endovascular stent was imaged to evaluate the high resolution performance of the statistical reconstructed image. A sacrificed mouse was scanned at different exposure levels to assess the effect of statistical noise on the image. The mouse studies were reconstructed with both the statistical reconstruction software and a filtered back-projection (FBP) program. The images were assessed and compared by contrast to noise ratio (CNR) in the region of interest. The images yielded by the statistical reconstruction software were artifact free and show superior noise performance to those from FBP reconstruction at different radiation exposure levels. The statistical reconstructed images with reduced exposure showed obviously higher image quality than those from FBP reconstruction at full exposure.

Feasibility studies of animal SPECT imaging with a stationary multi-pinhole collimator inserted to animal PET detector ring

In this study, we design a stationary multi-pinhole SPECT collimator inserted to a small animal PET scanner and investigate the feasibility of SPECT imaging in this system with Monte Carlo simulation studies. Compared to preceding approaches, the unique aspect of the proposed design is that it enables SPECT imaging only by axial translation of the imaging object to obtain sufficient angular sampling, rather than rotation of either the object or the collimator. This reduces the difficulty of geometrical calibration and facilitates accurate system matrix derivation. In order to minimize the axial overlap, the transverse section of the pinhole is designed to be elliptical. The sampling completeness of different scanning orbits is studied by a numerical approach based on Tuy sampling criteria. Results show that in the reconstruction images of a simulated Derenzo phantom, 1.35 mm hot rods are distinguishable. Image quality with only object translation is comparable to those with both object translation and rotation. A practical calibration method of the system geometrical parameters is developed and tested with simulation data. Finally, we incorporate the TV prior in OS-EM algorithm and the reconstruction results show the noise is efficiently suppressed. We conclude that we present a practical way to implement SPECT imaging on existing PET scanner without object or collimator rotation and desired spatial resolution is achievable with this design.

Half-millimeter animal SPECT imaging on a clinical SPECT scanner with highly flexible collimator design

In order to develop animal SPECT imaging technology on clinical SPECT scanners, we propose a highly flexible multi-pinhole collimator design scheme. The proposed design allows individual adjustment of important system parameters, so that the collimator device is adaptable to various SPECT systems, and one can also optimize different specific aspects of system performance (e.g. improving spatial resolution, maximizing sensitivity or increasing FOV) for different application purposes. This work is focused on achieving half-mm SPECT imaging resolution on a single-head clinical SPECT scanner. A seven-pinhole collimator and a four-degree-offreedom motion control stage are assembled. High accuracy geometrical calibration is performed by scanning a point source in a hybrid orbit, which consists of three circular orbits in different axial positions and a helical orbit. The system matrix is derived from Monte-Carlo simulation and de-noised by fitting each point spread function (PSF) to a 2D Gaussian function. Dual-point-source studies show that 0.5 mm reconstruction resolution is achievable in the studied system. Preliminary ultra micro hot rod phantom experiments demonstrate 0.6 mm hot rods are identifiable clearly. The proposed technology is attractable for its low cost, high flexibility, high feasibility and reproducibility.

Quantitative accuracy assessment and optimization of SPECT geometrical calibration

Abstract Accurate geometrical calibration is critical to obtaining high resolution and artifact free reconstructed images for modern animal single photon emission computed tomography (SPECT) systems. Although there have been many published works on the calibration of various SPECT systems, few studies have been done to evaluate the efficacy of the proposed calibration methods in a quantitative manner. This paper presents a numerical method to assess both the uniqueness and the quantitative accuracy of SPECT calibration, which is based on analyzing the singular value decomposition (SVD) components of the Jacobian matrix from a least-square cost function of the calibration. The proposed method is firstly validated by applying it to the calibration of a single pinhole SPECT system and comparing the results with those derived using a published method, and is then used to optimize the calibration setup for a slit-slat SPECT system. With the proposed method, a minimum required number of point source projections to achieve the desired calibration accuracy can be estimated and used as figure-of-merit to evaluate the goodness of a calibration setup. An inverse-square relationship between the calibration accuracy and the number of sampled projections is revealed. Optimal calibration setup is determined through an exhaustive search among all the possibilities of point source arrangements under certain conditions. We demonstrate that for the studied system, the best calibration accuracy is achieved by arranging the point source over the edge of FOV with evenly-spaced angular positions. Point source experiments were conducted to validate the proposed method.

Crystal identification for a dual-layer-offset LYSO based PET system via Lu-176 background radiation and mean shift algorithm

Crystal identification is a critical procedure for PET systems. In this paper, we proposed a crystal identification method for a dual-layer-offset LYSO based animal PET system via Lu-176 background radiation. Single event data of Lutetium background radiation were acquired in list-mode for one hour to generate a single photon flood histogram (SPH). Coincidence events were retrieved from the same data using time information to generate a coincidence flood histogram (CFH). The coincidence flood map was employed to identify the peaks of the inner layer responses using average peaks calculation and mean-shift algorithm. The response of the inner layer was removed from the SPH using the CFH and then the peaks of the outer layer were also identified using mean-shift algorithm. At last, the crystal position map was generated using these peaks with the distance criteria. Results show that the proposed method can be employed for the dual-layer offset PET system to implement crystal identification instead of using external radiation sources.