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Recent advances in positron emission tomography (PET) have allowed to perform brain scans of freely moving animals by using rigid motion correction. One of the current challenges in these scans is that, due to the PET scanner spatially variant point spread function (SVPSF), motion corrected images have a motion dependent blurring since animals can move throughout the entire field of view (FOV). We developed a method to calculate the image-based resolution kernels of the motion dependent and spatially variant PSF (MD-SVPSF) to correct the loss of spatial resolution in motion corrected reconstructions. The resolution kernels are calculated for each voxel by sampling and averaging the SVPSF at all positions in the scanner FOV where the moving object was measured. In resolution phantom scans, the use of the MD-SVPSF resolution model improved the spatial resolution in motion corrected reconstructions and corrected the image deformation caused by the parallax effect consistently for all motion patterns, outperforming the use of a motion independent SVPSF or Gaussian kernels. Compared to motion correction in which the SVPSF is applied independently for every pose, our method performed similarly, but with more than two orders of magnitude faster computation time. Importantly, in scans of freely moving mice, brain regional quantification in motion-free and motion corrected images was better correlated when using the MD-SVPSF in comparison with motion independent SVPSF and a Gaussian kernel. The method developed here allows to obtain consistent spatial resolution and quantification in motion corrected images, independently of the motion pattern of the subject. [ABSTRACT FROM AUTHOR]

To avoid the confounding effects of anesthesia and immobilization stress in rat brain positron emission tomography (PET), motion tracking-based unrestrained awake rat brain imaging is being developed. In this paper, we propose a fast and accurate rat headmotion tracking method based on small PET point sources. PET point sources (3-4) attached to the rat’s head are tracked in image space using 15–32-ms time frames. Our point source tracking (PST) method was validated using a manually moved microDerenzo phantom that was simultaneously tracked with an optical tracker (OT) for comparison. The PST method was further validated in three awake [18F]FDG rat brain scans. Compared with the OT, the PST-based correction at the same frame rate (31.2 Hz) reduced the reconstructed FWHM by 0.39–0.66 mm for the different tested rod sizes of the microDerenzo phantom. The FWHM could be further reduced by another 0.07–0.13 mm when increasing the PST frame rate (66.7 Hz). Regional brain [18F]FDG uptake in the motion corrected scan was strongly correlated ( $p<0.0001$ ) with that of the anesthetized reference scan for all three cases ( $0.94 < r < 0.97$ ). The proposed PST method allowed excellent and reproducible motion correction in awake in vivo experiments. In addition, there is no need of specialized tracking equipment or additional calibrations to be performed, the point sources are practically imperceptible to the rat, and PST is ideally suitable for small bore scanners, where optical tracking might be challenging. [ABSTRACT FROM PUBLISHER]

Total variation (TV) methods have been proposed to improve the image quality in count-reduced images, by reducing the variation between neighboring pixels. Although very easy to implement and fast to compute, TV-based methods may lead to a loss of texture information when applied to images with complex textures, such as high-resolution abdominal CT images. Here, we investigate the use of another regularization approach in the context of medical images based on multiresolution transformations. One such transformation is the shearlet transform, which is optimally sparse for images that are C^2 except for discontinuities along C^2 curves, and has better directional sensitivity than most other, related, wavelet transform approaches. We propose to solve the convex problem using the split-Bregman (augmented Lagrangian) approach. One of the primary advantages of the split-Bregman approach, is that the shearlet transform can easily be incorporated into the sparse-view CT reconstruction. The required sparsity prior is the \ell 1 norm of the shearlet coefficients. Results are shown for this method in comparison to the same framework with TV as the regularization term on simulated data. The noise-resolution performance is investigated at different contrast levels. At equal image noise, TV-based regularization outperforms shearlet-based regularization. However, when image texture is analyzed on measured mouse data, shearlets outperform TV, which suffers from staircasing effects. Our results show that there are benefits in using shearlets in CT imaging: texture is reconstructed more accurately compared to when TV is used, without biasing the image towards a piecewise constant image model. However, due to the larger support of the basis functions, our results suggest that uncareful usage of shearlets may lead to wavy artifacts, which can be equally unwanted as staircasing effects. [ABSTRACT FROM AUTHOR]

A maximum likelihood reconstruction algorithm for gated dynamic cardiac PET studies was developed and evaluated. A two dimensional tensor product spline basis spanning the time and gate domain is proposed. The activity variations introduced by the biochemical kinetics and cardiac motion are modeled as conic combinations of the B-spline basis functions. The basis explicitly takes the cyclic nature of the heart motion into account. We make use of the expectation-maximization (EM) algorithm to derive a closed form iteration scheme for the reconstruction. The proposed algorithm is validated through computer simulations of the dynamic NCAT beating heart phantom and the kinetics of 13N-ammonia uptake. We used the Monte Carlo simulator GATE to simulate a typical PET scanner. We qualitatively found that a reconstruction using cubic splines resulted in smoother images while better delineating the myocardial wall. Fourth order spline modeling reduced the mean squared error (MSE) of binned reconstruction by 56% whereas conventional Gaussian filtering reduced the MSE by 38%. Spline modeling in the gate domain reduced the MSE by 72% compared to a reduction of 47% obtained with filtering. Quantitative evaluation of the reconstructed motion information suggested that the number of basis functions in the gate domain could be reduced from 16 in a framed approach to 5 for reconstructions using a higher order spline interpolation. [ABSTRACT FROM AUTHOR]

A theoretical formulation of the effective point spread function of a slat collimator on a strip detector has been derived. The used technique to obtain the geometric transfer function, which is the Fourier transform of the effective point source image, is based on the geometric characteristics of a single gap and it accurately describes the performance of the collimation system for system analysis. Valuable conclusions resulting from this geometric transfer function could be made on the sensitivity, on the spatial resolution, and on the line spread function of the imaging system. We found that the sensitivity was dependent on the angle of incidence and on the distance to the detector. The spatial resolution was constant in a plane at a fixed distance to the detector and closed analytical expressions were derived for transaxial line spread profiles. These results were confirmed by the appropriate Monte Carlo simulations. The presented formulation of the geometric transfer function will be useful with usage in iterative reconstruction algorithms to incorporate distance dependent effects. [ABSTRACT FROM AUTHOR]

Nonuniform patient attenuation maps can be acquired using an axially moving point source of a high energy isotope that emits a fanbeam of photons. We simulated the Beacon attenuation correction tool attached to multiheaded Single Photon Emission Computed Tomography (SPECT) cameras which uses this approach. We investigated the scatter order of the photons reaching the detector, and the scatter contributions from the different detector components were evaluated for different energy windows. In case of simultaneous emission and transmission scanning the spatial signals are electronically windowed to allow differentiation between photons from the attenuation and from the emission source. However, an additional correction needs to be applied for cross-contamination. When applying the Beacon device in hybrid mode [gammacamera-Positron Emission Tomography (PET)] there are no degrees of freedom for crystal and collimator. The inter-window contamination was thus examined in detail to derive possible protocol optimizations for that dedicated setup. For the case of applying Beacon-SPECT, we performed multiple types of simulations including different crystal thicknesses and different collimators to evaluate the inter-window contamination. The main conclusion of this work is that a thick crystal detector coupled to a Low Energy High Resolution (LEHR) collimator is the best solution for acquiring attenuation maps in low energy applications. For medium energy studies attenuation maps have to be rescaled to account for the low sensitivity near the center of the patient. Fully Monte Carlo simulating the system matrix for medium energy studies on low energy collimators in order to replace the Medium Energy General Purpose (MEGP) collimators by the LEHR variants appeared to be a more valuable approach. This last method is penalized by a computational burden but results in an improved image quality after reconstruction. [ABSTRACT FROM AUTHOR]