Spectral Reconstruction Algorithm Research Articles

Fast Monte Carlo-simulator with full collimator and detector response modelling for SPECT

Monte Carlo (MC)-simulations have proved to be a valuable tool in studying SPECT-reconstruction algorithms. Despite their popularity, the use of Monte Carlo-simulations is still often limited by their large computation demand. This is especially true in situations where full collimator and detector modelling with septal penetration, scatter and X-ray fluorescence needs to be included. This paper presents a rapid and simple MC-simulator, which can effectively reduce the computation times. The simulator was built on the convolution-based forced detection principle, which can markedly lower the number of simulated photons. Full collimator and detector response look-up tables are pre-simulated and then later used in the actual MC-simulations to model the system response. The developed simulator was validated by comparing it against (123)I point source measurements made with a clinical gamma camera system and against (99m)Tc software phantom simulations made with the SIMIND MC-package. The results showed good agreement between the new simulator, measurements and the SIMIND-package. The new simulator provided near noise-free projection data in approximately 1.5min per projection with (99m)Tc, which was less than one-tenth of SIMIND's time. The developed MC-simulator can markedly decrease the simulation time without sacrificing image quality.

Slow‐rotation dynamic SPECT with a temporal second derivative constraint

Dynamic tracer behavior in the human body arises as a result of continuous physiological processes. Hence, the change in tracer concentration within a region of interest (ROI) should follow a smooth curve. The authors propose a modification to an existing slow-rotation dynamic SPECT reconstruction algorithm (dSPECT) with the goal of improving the smoothness of time activity curves (TACs) and other properties of the reconstructed image. The new method, denoted d2EM, imposes a constraint on the second derivative (concavity) of the TAC in every voxel of the reconstructed image, allowing it to change sign at most once. Further constraints are enforced to prevent other nonphysical behaviors from arising. The new method is compared with dSPECT using digital phantom simulations and experimental dynamic 99mTc -DTPA renal SPECT data, to assess any improvement in image quality. In both phantom simulations and healthy volunteer experiments, the d2EM method provides smoother TACs than dSPECT, with more consistent shapes in regions with dynamic behavior. Magnitudes of TACs within an ROI still vary noticeably in both dSPECT and d2EM images, but also in images produced using an OSEM approach that reconstructs each time frame individually, based on much more complete projection data. TACs produced by averaging over a region are similar using either method, even for small ROIs. Results for experimental renal data show expected behavior in images produced by both methods, with d2EM providing somewhat smoother mean TACs and more consistent TAC shapes. The d2EM method is successful in improving the smoothness of time activity curves obtained from the reconstruction, as well as improving consistency of TAC shapes within ROIs.

Drag coefficient accuracy improvement by means of particle image velocimetry for a transonic NACA0012 airfoil

A method to improve the reliability of the drag coefficient computation by means of particle image velocimetry measurements is made using experimental data acquired on a NACA0012 airfoil tested in the transonic regime, using the combination of a variable pulse separation with a new high-order Poisson spectral pressure reconstruction algorithm.

Enhancing the resolution of spectral images from the Advanced Electro-Optical System Spectral Imaging Sensor

This research develops a model-based spectral image reconstruction (MBSIR) algorithm to reconstruct images collected from the Advanced Electro-Optical System (AEOS) Spectral Imaging Sensor (ASIS). The development of the algorithm requires two key elements: 1. the statistics of the photon arrival and 2. estimates of the spatial and spectral transfer functions. With these two elements, the MBSIR algorithm can, through image postprocessing, dramatically increase the resolution of the images as well as give insight into the performance of the imaging sensor itself. The MBSIR algorithm is designed to simultaneously improve both the spatial and spectral resolution, and is derived for the general case of a spectrally variant imaging system. While MBSIR algorithms can be developed for any spectral imaging system, this research focuses on ASIS, a new spectral imaging sensor installed with the 3.6-m AEOS telescope at the Maui Space Surveillance Complex (MSSC). The primary purpose of ASIS is to take spatially resolved spectral images of space objects. The low-light levels and object motion inherent in imaging some objects in space, such as satellites, lead to a sensor design with less spectral resolution than required for image analysis. However, by applying MBSIR to the collected data, the sensor will be capable of achieving a higher resolution, allowing for better spectral analysis. The algorithm is shown to work with simulated ASIS data and measured data from an ASIS-like sensor.

Implementation of an iterative scatter correction, the influence of attenuation map quality and their effect on absolute quantitation in SPECT

We investigated the accuracy of qSPECT, a quantitative SPECT reconstruction algorithm we have developed which employs corrections for collimator blurring, photon attenuation and scatter, and provides images in units of absolute radiotracer concentrations (kBq cm−3). Using simulated and experimental phantom data with characteristics similar to clinical cardiac perfusion data, we studied the implementation of a scatter correction (SC) as part of an iterative reconstruction protocol. Additionally, with experimental phantom studies we examined the influence of CT-based attenuation maps, relative to those obtained from conventional SPECT transmission scans, on SCs and quantitation. Our results indicate that the qSPECT estimated scatter corrections did not change appreciably after the third iteration of the reconstruction. For the simulated data, qSPECT concentrations agreed with images reconstructed using ideal, scatter-free, simulated data to within 6%. For the experimental data, we observed small systematic differences in the scatter fractions for data using different combinations of SCs and attenuation maps. The SCs were found to be significantly influenced by errors in image coregistration. The reconstructed concentrations using CT-based corrections were more quantitatively accurate than those using attenuation maps from conventional SPECT transmission scans. However, segmenting the attenuation maps from SPECT transmission scans could provide sufficient accuracy for most applications.

Progress in SPECT/CT Imaging of Prostate Cancer

Prostate cancer is the most common type of cancer (other than skin cancer) among men in the United States. Although prostate cancer is one of the few cancers that grow so slowly that it may never threaten the lives of some patients, it can be lethal once metastasized. Indium-111 capromab pendetide (ProstaScint, Cytogen Corporation, Princeton, NJ) imaging is indicated for staging and recurrence detection of the disease, and is particularly useful to determine whether or not the disease has spread to distant metastatic sites. However, the interpretation of 111In-capromab pendetide is challenging without correlated structural information mostly because the radiopharmaceutical demonstrates nonspecific uptake in the normal vasculature, bowel, bone marrow, and the prostate gland. We developed an improved method of imaging and localizing 111In-Capromab pendetide using a SPECT/CT imaging system. The specific goals included: i) development and application of a novel iterative SPECT reconstruction algorithm that utilizes a priori information from coregistered CT; and ii) assessment of clinical impact of adding SPECT/CT for prostate cancer imaging with capromab pendetide utilizing the standard and novel reconstruction techniques. Patient imaging studies with capromab pendetide were performed from 1999 to 2004 using two different SPECT/CT scanners, a prototype SPECT/CT system and a commercial SPECT/CT system (Discovery VH, GE Healthcare, Waukesha, WI). SPECT projection data from both systems were reconstructed using an experimental iterative algorithm that compensates for both photon attenuation and collimator blurring. In addition, the data obtained from the commercial system were reconstructed with attenuation correction using an OSEM reconstruction supplied by the camera manufacturer for routine clinical interpretation. For 12 sets of patient data, SPECT images reconstructed using the experimental algorithm were interpreted separately and compared with interpretation of images obtained using the standard reconstruction technique. The experimental reconstruction algorithm improved spatial resolution, reduced streak artifacts, and yielded a better correlation with anatomic details of CT in comparison to conventional reconstruction methods (e.g., filtered back-projection or OSEM with attenuation correction only). Images produced with the experimental algorithm produced a subjective improvement in the confidence of interpretation for 11 of 12 studies. There were also changes in interpretations for 4 of 12 studies although the changes were not sufficient to alter prognosis or the patient treatment plan.

An FDK-like cone-beam SPECT reconstruction algorithm for non-uniform attenuated projections acquired using a circular trajectory

In this paper, Novikov's inversion formula of the attenuated two-dimensional (2D) Radon transform is applied to the reconstruction of attenuated fan-beam projections acquired with equal detector spacing and of attenuated cone-beam projections acquired with a flat planar detector and circular trajectory. The derivation of the fan-beam algorithm is obtained by transformation from parallel-beam coordinates to fan-beam coordinates. The cone-beam reconstruction algorithm is an extension of the fan-beam reconstruction algorithm using Feldkamp–Davis–Kress's (FDK) method. Computer simulations indicate that the algorithm is efficient and is accurate in reconstructing slices close to the central slice of the cone-beam orbit plane. When the attenuation map is set to zero the implementation is equivalent to the FDK method. Reconstructed images are also shown for noise corrupted projections.

Application of FT‐Raman and FTIR measurements using a novel spectral reconstruction algorithm

AbstractRecently, an advanced spectral reconstruction algorithm based on information entropy was developed to identify individual compounds contained in mixture spectra without recourse to any library or any a priori knowledge. In this study, standard mixtures containing various polycyclic aromatic hydrocarbons (PAHs) and α,ω‐dicarboxylic acids were measured by solid‐state FT‐Raman and FTIR spectroscopy, and this was followed by the application of the aforementioned algorithm to recover all pure component spectra contained therein. The results demonstrate that the developed algorithm successfully recovered the spectra of individual species even though only a very limited number of mixture spectral measurements were made. FT‐Raman measurements coupled with chemometric analysis provides satisfactory pure spectral results for both PAHs and α,ω‐dicarboxylic acids. Additionally, FT‐Raman measurements together with FTIR measurements provided even better spectral recovery for some of the α,ω‐dicarboxylic acids when using band‐target entropy minimization (BTEM). This study demonstrates that chemometric analysis without a priori information provides a promising solution and simplified approach to identify individual components from samples containing a complex composition. In particular, this is desired for a better understanding of environmental samples. Copyright © 2003 John Wiley & Sons, Ltd.

Influence of initial electron beam characteristics on Monte Carlo calculated absorbed dose distributions for linear accelerator electron beams

The least known parameters in a Monte Carlo simulation of a linear accelerator treatment head are often the properties of the initial electron beam directed onto the exit vacuum window. Several initial beams with different spatial fluence distributions, angular divergences and energy spectra have been transported through the geometry of a scattering foil accelerator. The electron beam characteristics (energy spectrum and angular distribution) at the phantom surface and the subsequent relative absorbed dose distribution in a water phantom were calculated. The dose distribution was found to be insensitive to the geometrical properties of the initial beam. Furthermore, the lateral dose profiles are unaffected by the energy spectrum of the initial beam. The effect on the depth–dose curve is negligible if the initial energy spectrum is symmetric (e.g., Gaussian shaped) and its full width at half maximum (FWHM) is less than approximately 10% of the most probable energy. A larger FWHM will decrease the normalized dose gradient, but will not affect the dose in the build-up region. An asymmetric wedge shaped spectrum with a low-energy extension simultaneously increases the dose in the build-up region and decreases the dose gradient. The relationship between the energy spectral width and the normalized dose gradient is, however, smaller than published analytical expressions indicate. Some well-established energy-range relationships were shown to be accurate for most of the initial beams studied. The energy spectrum at the phantom surface was also derived from a measured depth–dose curve through different methods. The extracted spectrum depends on the beam model and the spectral reconstruction algorithm. Even though the depth–dose curve is fairly independent of initial beam characteristics, a correct description of the low-energy tail of the energy spectrum is important to obtain good agreement between measured and Monte Carlo calculated doses in the build-up region.

A new SPECT reconstruction algorithm based on theNovikov explicit inversion formula

We present a new reconstruction algorithm for single-photonemission computed tomography. The algorithm is based on theNovikov explicit inversion formula for the attenuated Radontransform with non-uniform attenuation. Our reconstructiontechnique can be viewed as a generalization of both the filteredbackprojection algorithm and the Tretiak-Metz algorithm. Wetest the performance of the present algorithm in a variety ofnumerical experiments. Our numerical examples show that thealgorithm is capable of accurate image reconstruction even inthe case of strongly non-uniform attenuation coefficient,similar to that occurring in a human thorax.

Absolute quantitation of myocardial activity in phantoms

We have developed a new technique for compensating myocardial SPECT images for partial volume errors using co-registered X-ray CT images. The CT-derived myocardial mass defines a template that can be assigned a unit activity and mathematically projected with a realistic physical model of the radionuclide imaging process, including non-ideal collimation and incorporating an object-specific attenuation map from CT. The template projections then are reconstructed using a SPECT reconstruction algorithm to obtain a pixel-by-pixel partial-volume correction for the myocardial SPECT image. Experiments in phantoms demonstrate that this technique substantially improves the absolute quantitation of myocardial radionuclide concentration, reducing the accuracy error from approximately 50% to less than 8%. This method also can by used for correcting background effects such as "spill-in" of background counts from uptake in the liver.

Theoretical determination of the collimator geometrical transfer function for the reconstruction of SPECT data

Iterative SPECT reconstruction algorithms allow for the inclusion of information related to the geometric system response with accuracy and flexibility. Since the experimental setup of a camera-collimator system may vary considerably, it is necessary to fit the restoration potential of the algorithm to the actual acquisition conditions. In order to avoid any experimental procedure, the authors developed a method to calculate weighting factors from the theoretical knowledge of the geometric system response of a multihole collimator. The calculation of the weighting factors was based on an appropriate discretization of the Radon transform whose kernel was modified to describe the variable system response of the collimator.

Pulse shape analysis of signals from a CsI(T1)/photodiode detector

Use of 1 cm 2 CsI(T1) scintillators with photodiode readout in the hard X-ray band is currently limited to above ∼ 40 keV by the lack of intrinsic gain in the photodiode and the electronic noise in the readout system. However, below this energy the photodiode itself becomes a practical X-ray detector. We report on the use of a pulse-shape analysis technique in an attempt to separate the signals from direct X-ray interactions in the photodiode and from scintillation events in the CsI(T1). We have demonstrated that separation of the two types of event is certainly possible, and the use of an appropriate spectral reconstruction algorithm enables us to normalise the signals from the two detectors. In this way we have constructed a hybrid detector capable of operating in the energy range 10 keV to 1 MeV. We discuss, using both simulations and laboratory measurements, the optimisation which can be carried out by changing the thickness of the silicon photodiode. We present details of the pulse-shape analysis system, demonstrate the identification and separation of the two types of signal, and present energy spectra for a range of energies.

A cone beam SPECT reconstruction algorithm with a displaced center of rotation

A filtered backprojection (FBP) algorithm is derived based on Feldkamp's FBP algorithm for a cone beam geometry that has a displaced center of rotation. In cone beam single photon emission computed tomography (CB-SPECT) the center of rotation displacement can degrade the reconstructed images. The center of rotation displacement of interest is mechanical shift, which is the displacement of the midplane of the cone beam collimator off the rotation center. Mechanical shift is characterized by two orthogonal components: the shift of the midplane of the cone beam collimator along the direction of the axis of rotation, and the distance between the midline of the cone beam collimator and the axis of rotation. This new algorithm corrects mechanical shift directly by incorporating mechanical shift into the algorithm. This new algorithm is evaluated using both Monte Carlo simulated data and experimentally acquired data. The results demonstrate that this algorithm is able to correct for blurring and the "doughnut" type artifacts caused by system mechanical shift and improve the image resolution.

Graphics applied to medical image registration

Software that has been developed to automate and standardize comparison of 3D images is discussed. The major approaches to image registration are examined. The authors' registration and reconstruction software is described. Studies carried out to determine the relative accuracy of the present registration methods are reported. The main goal of the authors is to improve on commercial SPECT reconstruction algorithms and provide quantification accuracy sufficient for numerical assessment of tracer distribution. >