Exploitation of realistic computational anthropomorphic phantoms for the optimization of nuclear imaging acquisition and processing protocols.

Monte Carlo (MC) simulations play a crucial role in nuclear medical imaging since they can provide the ground truth for clinical acquisitions, by integrating and quantifing all physical parameters that affect image quality. The last decade a number of realistic computational anthropomorphic models have been developed to serve imaging, as well as other biomedical engineering applications. The combination of MC techniques with realistic computational phantoms can provide a powerful tool for pre and post processing in imaging, data analysis and dosimetry. This work aims to create a global database for simulated Single Photon Emission Computed Tomography (SPECT) and Positron Emission Tomography (PET) exams and the methodology, as well as the first elements are presented. Simulations are performed using the well validated GATE opensource toolkit, standard anthropomorphic phantoms and activity distribution of various radiopharmaceuticals, derived from literature. The resulting images, projections and sinograms of each study are provided in the database and can be further exploited to evaluate processing and reconstruction algorithms. Patient studies using different characteristics are included in the database and different computational phantoms were tested for the same acquisitions. These include the XCAT, Zubal and the Virtual Family, which some of which are used for the first time in nuclear imaging. The created database will be freely available and our current work is towards its extension by simulating additional clinical pathologies.

Development and evaluation of QSPECT open-source software for the iterative reconstruction of SPECT images.

OBJECTIVE: In this study open-source software (QSPECT) suitable for the iterative reconstruction of single-photon emission computed tomography (SPECT) data is presented. QSPECT implements maximum likelihood expectation maximization and ordered subsets expectation maximization algorithms in a user-friendly graphical interface. The software functionality is described and validation results are presented. METHODS: Maximum likelihood expectation maximization and ordered subsets expectation maximization algorithms are implemented in C++. The Qt toolkit, a standard C++ framework for developing high-performance cross-platform applications, has been used for the graphical user interface development. QSPECT is tested using original projection data from two clinical SPECT systems: (i) APEX SPX-6/6HR and (ii) Millennium MG. Phantom experiments were carried out to evaluate the quality of reconstructed images in terms of (i) spatial resolution, (ii) sensitivity to activity variations, and (iii) the presence of scatter media. A cardiac phantom was used to simulate a normal and abnormal scenario. Finally, clinical cardiac SPECT images were reconstructed. In all cases, QSPECT results were compared with the clinical systems reconstruction software that uses the standard filtered backprojection algorithm. RESULTS: The reconstructed images show that QSPECT, when compared with standard clinical reconstruction, provides images with higher contrast, reduced background, and better separation of small sources located in small distances. In addition, reconstruction with QSPECT provides more quantitative images, and reduces the background created by scatter media. Finally, the phantom and clinical cardiac images are reconstructed with similar quality. CONCLUSION: QSPECT is a freely distributed, open-source standalone application that provides real-time, high-quality SPECT images. The software can be further modified to improve reconstruction algorithms, and include more correction techniques, such as, scatter and attenuation correction.

An efficient analytical calculation of probability matrix in 2D SPECT.

Expectation Maximization iterative reconstruction algorithms are being widely used in PET and SPECT imaging. The system probability matrix is usually calculated by using Monte Carlo simulations, since the analytical calculation is a rather complicated problem, especially in 3D reconstruction; however, realistic Monte Carlo simulations in 3D are time consuming and simplifications are necessary. In this paper, the probability matrix in the case of 2D SPECT is analytically calculated, using only two basic parameters: (i) the total number of image pixels and (ii) the number of projection angles. In the more general phase three more parameters will be taken into account: (i) the distance between the detectors and the object to be imaged; (ii) the collimator parameters; (iii) scintillator cells parameters (in the case of pixilated scintillators) and relative position between scintillator cell and collimator hole. It is shown that the accuracy of the probability matrix affects the quality of reconstructed images, especially in the case of pixilated scintillators, which are used in many dedicated SPECT systems. The methods presented here can be extended to 3D SPECT and also 2D and 3D PET. In addition, this analytically calculated matrix can be a reference matrix in order to be compared with Monte Carlo generated matrices.

Comparative evaluation of linear and cyclic 99mTc-RGD peptides for targeting of integrins in tumor angiogenesis.

Cell adhesion molecules, such as integrins, play a vital role in angiogenesis, a key pathway for tumor growth, invasion and metastasis. The integrin alpha(v)beta(3), which recognizes the RGD sequence (Arg-Gly-Asp), may provide a target for in vivo tumor imaging. A linear and a cyclic RGD peptide derivative (RGDfK-His and cRGDfK-His, respectively), labelled via the precursor [99mTc(H2O)3(CO)3]+, were comparatively evaluated and their radiobiological properties were assessed in normal and tumor-bearing mice. Biodistribution studies showed non-specific uptake in all organs, rapid blood clearance and elimination via the hepatobiliary and urinary systems. Tumor uptake was higher for the cyclic radiolabelled derivative, as the both biodistribution and imaging studies suggested. The cRGDfK-His, labelled via the fac-[99mTc(CO)3]-core, may prove to be a useful tool for early tumor detection.