Performance evaluation of MAP algorithms with different penalties, object geometries and noise levels

A new algorithm (LBFGS-B-PC) which combines ideas of two existing convergent reconstruction algorithms, relaxed separable paraboloidal surrogate (SPS) and limited-memory Broyden-Fletcher-Goldfarb-Shanno with boundary constraints (LBFGS-B), is proposed. Its performance is evaluated in terms of log-posterior value and regional recovery ratio. The results demonstrate the superior convergence speed of the proposed algorithm to relaxed SPS and LBFGS-B, regardless of the noise level, activity distribution, object geometry, and penalties

Performance evaluation of MAP algorithms with different penalties, object geometries and noise levels

A new algorithm (LBFGS-B-PC) which combines ideas of two existing convergent reconstruction algorithms, relaxed separable paraboloidal surrogate (SPS) and limited-memory Broyden-Fletcher-Goldfarb-Shanno with boundary constraints (LBFGS-B), is proposed. Its performance is evaluated in terms of log-posterior value and regional recovery ratio. The results demonstrate the superior convergence speed of the proposed algorithm to relaxed SPS and LBFGS-B, regardless of the noise level, activity distribution, object geometry, and penalties

Evaluation of a partial ring design for the INSERT SPECT/MRI system

The aim of the INSERT project is to develop a SPECT insert for a commercial MRI system, for performing simultaneous SPECT/MRI brain studies in humans. We have previously investigated various design options for the detector system, based on a complete ring of detectors. We are now considering a partial ring, due to space limitations. We have investigated the degradation in image quality with a partial ring as compared to a full ring, and the possibility of addressing the limitations by utilising MRI data during reconstruction. Noise-free data were generated by forward-projecting a cylindrical phantom with spherical inserts for a full-ring and a partial ring system, equipped with multislit- slat (MSS) and multi-pinhole (MPH) collimators. Poisson noise was added and images were reconstructed using ML-EM and MAP-EM with a smoothing prior and an anatomical prior. Contrast-recovery (CR) was calculated for the spheres in the lower part of the phantom compared to the top ones. Background CoV was also calculated. With noise-free data, CR was 77-84% for the MSS and 82-88% for the MPH partial-ring system with 400-1600 iterations. For noisy data and MAP-EM with a smoothing prior, CR was 78-80% and 81-82%, and CoV 22-28% and 26-31%, for the MSS and MPH systems, respectively. With the anatomical prior, CR was 85-89% and 87-91%, respectively. With the partial ring-systems, the transaxial resolution in the lower part of the image is reduced. The degradation is slightly larger with MSS than MPH collimators, but the MSS collimator results in a lower noise-level. Some resolution can be recovered with more iterations, but the improvement is limited when regularisation is included. The anatomical prior offers both qualitative and quantitative improvement in image quality

Design optimization and evaluation of a human brain SPECT-MRI insert based on high-resolution detectors and slit-slat collimators

Multi-modality scanners have become increasingly popular in recent years. While PET/CT, SPECT/CT and PET/MRI systems are now commercially available, SPECT/MRI systems are still in a development phase. The aim of this work was to design a SPECT insert for a conventional MRI scanner in order to perform simultaneous SPECT/MRI studies. The SPECT system will consist of a stationary ring of detectors equipped with multi-pinhole or slit-slat collimators. The detector ring will contain multiple small scintillation detectors with Si-based optical readout units. We have investigated a series of design options using analytical calculations of resolution and sensitivity as well as simulations based on various digital phantoms. Our results show that the highest sensitivity is achieved with a multi-pinhole collimator with 2x2 pinholes per detector. However, due to artifacts appearing as a result of cross-talk related to the parallax effect, a better solution could be the 2-slit slit-slat system, with only 5% lower sensitivity. With a target resolution of 10 mm FWHM, the mean sensitivity over a central ROI for this system was 6.30·10-4

Variance prediction in SPECT reconstruction based on the Fisher information using a novel angular blurring algorithm for computation of the system matrix

In this paper we propose a Gaussian decomposition approach to compute the SPECT imaging system matrix. The flexibility of our method allows the implementation of a wide range of imaging systems. With the system matrix, we predict the variance in reconstructed images using the Fisher information matrix and local block circulant approximation. We present results of noise prediction for 3 multi-pinhole systems and 3 multi-slit slit-slat systems, each of them being designed to be inserted in an MRI system. Results show that for a particular phantom 2x2 multi-pinhole and 2 slits slit-slat systems achieve lowest variance. 2013 IEEE

Evaluation of a direct motion estimation/correction method in respiratory-gated PET/MRI with motion-adjusted attenuation

Purpose Respiratory motion compensation in PET/CT and PET/MRI is essential as motion is a source of image degradation (motion blur, attenuation artifacts). In previous work, we developed a direct method for joint image reconstruction/motion estimation (JRM) for attenuation-corrected (AC) respiratory-gated PET, which uses a single attenuation-map (μ-map). This approach was successfully implemented for respiratory-gated PET/CT, but since it relied on an accurate μ-map for motion estimation, the question of its applicability in PET/MRI is open. The purpose of this work is to investigate the feasibility of JRM in PET/MRI and to assess the robustness of the motion estimation when a degraded μ-map is used. Methods We performed a series of JRM reconstructions from simulated PET data using a range of simulated Dixon MRI sequence derived μ-maps with wrong attenuation values in the lungs, from −100% (no attenuation) to +100% (double attenuation), as well as truncated arms. We compared the estimated motions with the one obtained from JRM in ideal conditions (no noise, true μ-map as an input). We also applied JRM on 4 patient datasets of the chest, 3 of them containing hot lesions. Patient list-mode data were gated using a principal component analysis method. We compared SUVmax values of the JRM reconstructed activity images and non motion-corrected images. We also assessed the estimated motion fields by comparing the deformed JRM-reconstructed activity with individually non-AC reconstructed gates. Results Experiments on simulated data showed that JRM-motion estimation is robust to μ-map degradation in the sense that it produces motion fields similar to the ones obtained when using the true μ-map, regardless of the attenuation errors in the lungs (\u3c 0.5% mean absolute difference with the reference motion field). When using a μ-map with truncated arms, JRM estimates a motion field that stretches the μ-map in order to match the projection data. Results on patient datasets showed that using JRM improves the SUVmax values of hot lesions significantly and suppresses motion blur. When the estimated motion fields are applied to the reconstructed activity, the deformed images are geometrically similar to the non-AC individually reconstructed gates. Conclusion Motion estimation by JRM is robust to variation of the attenuation values in the lungs. JRM successfully compensates for motion when applied to PET/MRI clinical datasets. It provides a potential alternative to existing methods where the motion fields are pre-estimated from separate MRI measurements

Fast Quasi-Newton Algorithms for Penalized Reconstruction in Emission Tomography and Further Improvements via Preconditioning

This paper reports on the feasibility of using a quasi-Newton optimization algorithm, limited-memory Broyden- Fletcher-Goldfarb-Shanno with boundary constraints (L-BFGSB), for penalized image reconstruction problems in emission tomography (ET). For further acceleration, an additional preconditioning technique based on a diagonal approximation of the Hessian was introduced. The convergence rate of L-BFGSB and the proposed preconditioned algorithm (L-BFGS-B-PC) was evaluated with simulated data with various factors, such as the noise level, penalty type, penalty strength and background level. Data of three 18F-FDG patient acquisitions were also reconstructed. Results showed that the proposed L-BFGS-B-PC outperforms L-BFGS-B in convergence rate for all simulated conditions and the patient data. Based on these results, L-BFGSB- PC shows promise for clinical application

Novel collimation for simultaneous SPECT/MRI

The aim of this work is to develop an optimal collimator design for a stationary, compact SPECT insert for an MRI system, intended to perform simultaneous brain SPECT/MRI in humans. The SPECT insert will consist of a single ring of 25 SiPM-based detectors, insensitive to magnetic fields (5 by 10cm, 0.8mm intrinsic resolution). Compactness is needed due to limited space inside the MRI bore. Here we introduce the concept of an interior slit in the slat component of a slit-slat collimator, providing the possibility of having longer slats extending beyond the slit collimator. We also explore the use of mini-slit arrays to obtain improved angular sampling of the object - we call this an MSS collimator.We compared various multi-pinhole and multi-slit slit-slat configurations, using analytical calculations of sensitivity for a target resolution of 10mm FWHM and simulations with digital phantoms. All slit-slat configurations provided higher sensitivity when compared to the corresponding pinhole designs. The highest sensitivity was obtained for the 2-slit configuration (3.8x10-4). Simulations with a uniform phantom showed reduced sampling artefacts for both the MSS and the corresponding multi-pinhole configuration, in comparison to the other geometries. With a Derenzo phantom, better reconstructed uniformity was observed for the same configurations, with slightly better resolution for the pinhole configuration. With a Defrise phantom, better axial resolution was observed for the slit-slat as compared to the pinhole collimators, and also a more uniform axial coverage. The proposed MSS design demonstrates good reconstructed uniformity and sensitivity, and less sampling artefacts when compared to other collimator configurations, and is therefore the design of choice for the SPECT/MRI insert

Maximum-likelihood joint image reconstruction and motion estimation with misaligned attenuation in TOF-PET/CT

This work is an extension of our recent work on joint activity reconstruction/motion estimation (JRM) from positron emission tomography (PET) data. We performed JRM by maximization of the penalized log-likelihood in which the probabilistic model assumes that the same motion field affects both the activity distribution and the attenuation map. Our previous results showed that JRM can successfully reconstruct the activity distribution when the attenuation map is misaligned with the PET data, but converges slowly due to the significant cross-talk in the likelihood. In this paper, we utilize time-of-flight PET for JRM and demonstrate that the convergence speed is significantly improved compared to JRM with conventional PET data

PETPVC: a toolbox for performing partial volume correction techniques in positron emission tomography

Positron emission tomography (PET) images are degraded by a phenomenon known as the partial volume effect (PVE). Approaches have been developed to reduce PVEs, typically through the utilisation of structural information provided by other imaging modalities such as MRI or CT. These methods, known as partial volume correction (PVC) techniques, reduce PVEs by compensating for the effects of the scanner resolution, thereby improving the quantitative accuracy. The PETPVC toolbox described in this paper comprises a suite of methods, both classic and more recent approaches, for the purposes of applying PVC to PET data. Eight core PVC techniques are available. These core methods can be combined to create a total of 22 different PVC techniques. Simulated brain PET data are used to demonstrate the utility of toolbox in idealised conditions, the effects of applying PVC with mismatched point-spread function (PSF) estimates and the potential of novel hybrid PVC methods to improve the quantification of lesion