Andreas Robert Formiconi

Compensation of spatial system response in SPECT with conjugate gradient reconstruction technique

A procedure for the determination of the system matrix in single photon emission tomography (SPECT) is described which uses the conjugate gradient reconstruction technique in order to take into account the variable system resolution of a camera equipped with parallel-hole collimators. The procedure involves the acquisition of the system line spread functions (LSF) in the region occupied by the object to be studied. Those data are used to generate a set of weighting factors based on the assumption that the LSFs of the collimated camera are of Gaussian shape with the full width at half maximum (FWHM) linearly dependent on the source depth in the span of image space. The factors are stored on a disc file for subsequent use in the reconstruction process. Afterwards the reconstruction is performed using the conjugate gradient method with the system matrix modified by the incorporation of these precalculated factors in order to take into account the variable geometrical system response. The set of weighting factors is regenerated whenever the acquisition conditions are changed (collimator, radius of rotation). In the case of an ultra high resolution (UHR) collimator 2000 weighting factors need to be calculated. The modification of the system matrix for the geometrical response allows the number of iterations to increase, considerably improving image definition without the appearance of noise artifacts. Moreover, phantom studies show that the number of iterations is less critical because of improved stability in the convergence to the solution. For brain studies of patients 10-15 iterations are usually performed. Studies with a single line source give a value between 7 and 8 mm for the FWHM of the point spread function (PSF) when the conjugate gradient method with modified system matrix is used on data acquired with a UHR collimator, whereas without the modification of the system matrix the result is 9 mm FWHM, if filtered backprojection (FBP) is used with the same filter as in the clinical studies the result is 15 mm FWHM. The results of this work show that proper definition of the system matrix using conjugate gradients influences the quality of the reconstruction remarkably. Nevertheless, further work has to be done in order to assess to what extent the system matrix is ill-conditioned and, eventually, to define a suitable regularization technique.

Interaction of caudate dopamine depletion and brain metabolic changes with cognitive dysfunction in early Parkinson's disease

Damage to nonmotor dopamine (DA)-mediated frontostriatal circuits has been proposed as the main pathophysiological basis of cognitive dysfunction in Parkinsons disease (PD). In the present study, 18 early nondemented drug naive PD patients were investigated, by dual-tracer N-ω-fluoropropyl-2β-carbomethoxy-3β-4-[123I]iodophenyl-nortropane ([123I]FP-CIT) single-photon emission computed tomography (SPECT)/[18F] fluoro-deoxyglucose (FDG) positron emission tomography (PET) imaging, to test whether an early and not yet treatment-modulated relation exists between cognitive functions, caudate nucleus (CN) DA impairment and brain metabolism (CMRglc) in associative frontostriatal circuits. Verbal fluency performance correlated with DA impairment in CN, and with CMRglc in dorsolateral prefrontal cortex (DLPFC) and anterior cingulate cortex (ACC). Further, CMRglc in orbitofrontal cortex, DLPFC, and ACC was shown to be early modulated by the level of DA impairment in CN. The present study demonstrates in vivo the early functional disruption of nonmotor frontostriatal circuits in PD. The effect of CN DA impairment on DLPFC and ACC metabolism is proposed as a possible early pathophysiological and functional substrate for executive dysfunction in PD.

Geometrical response of multihole collimators

A complete theory of camera multihole collimators is presented. The geometrical system response is determined in closed form in frequency space. This closed form accounts for the known efficiency and resolution formulae for parallel beam, fan beam, cone beam and astigmatic collimators as well as for the most frequent hole array patterns and hole shapes. The point spread function in the space domain for a certain collimator and source position can be calculated via a discrete fast Fourier transform. Beside the complete theoretical definition of the response of multihole collimators, this theory allows the definition of accurate models of the geometrical response for SPECT reconstruction and it is suitable for designing new collimators.

Brain metabolic correlates of dopaminergic degeneration in de novo idiopathic Parkinson’s disease

PurposeThe aim of the present study was to evaluate the reciprocal relationships between motor impairment, dopaminergic dysfunction, and cerebral metabolism (rCMRglc) in de novo Parkinson’s disease (PD) patients.MethodsTwenty-six de novo untreated PD patients were scanned with 123I-FP-CIT SPECT and 18F-FDG PET. The dopaminergic impairment was measured with putaminal 123I-FP-CIT binding potential (BP), estimated with two different techniques: an iterative reconstruction algorithm (BPOSEM) and the least-squares (LS) method (BPLS). Statistical parametric mapping (SPM) multiple regression analyses were performed to determine the specific brain regions in which UPDRS III scores and putaminal BP values correlated with rCMRglc.ResultsThe SPM results showed a negative correlation between UPDRS III and rCMRglc in premotor cortex, and a positive correlation between BPOSEM and rCMRglc in premotor and dorsolateral prefrontal cortex, not surviving at multiple comparison correction. Instead, there was a positive significant correlation between putaminal BPLS and rCMRglc in premotor, dorsolateral prefrontal, anterior prefrontal, and orbitofrontal cortex (p < 0.05, corrected for multiple comparison).ConclusionsPutaminal BPLS is an efficient parameter for exploring the correlations between PD severity and rCMRglc cortical changes. The correlation between dopaminergic degeneration and rCMRglc in several prefrontal regions likely represents the cortical functional correlate of the dysfunction in the motor basal ganglia-cortical circuit in PD. This finding suggests focusing on the metabolic course of these areas to follow PD progression and to analyze treatment effects.

Kinetic parameter estimation from renal measurements with a three-headed SPECT system: a Simulation study

We present here a direct least-squares estimation (DLSE) method for the determination of renal kinetic parameters from sequences of very fast acquisitions performed with a three-headed single photon emission computed tomography (SPECT) system. A simple linear model for the behavior of the radiopharmaceutical, as well as a spatial model for its spatial distribution are defined. The model enables one to estimate the kinetic parameters directly from the projections, once the plasma concentration function is known. A new technique for the accurate reconstruction of time-radioactivity curves based on the direct reconstruction of the region-of-interest contents from a series of data from three-projections is presented. The technique is used to determine the plasma concentration function with a sub-second time resolution. The spatially-variant geometrical response is also included in the model to compensate for the spatial resolution of the SPECT system. Results obtained from simulations are presented. Basic spatial and time features of the simulations are derived from a patient study. Noise and segmentation errors are also simulated. The DLSE method is compared with the conventional one of deriving kinetic parameters from the time series of reconstructed images. The standard deviation of results given by DLSE is less than 2%, whereas with the conventional method it is between 5% and 6%. Within the limit of statistical fluctuations, DLSE results are unbiased whereas those of the conventional method are overestimated by 24%.

Physical modelling (geometrical system response, Compton scattering and attenuation) in brain SPECT using the conjugate gradients reconstruction method

A technique for the simultaneous compensation of spatial system response, Compton scattering and attenuation in brain SPECT using 99Tcm is described here. The procedure involves the acquisition of experimental information relative to spatial response, attenuation and scatter from capillary sources as well as by means of a special experimental set-up. The latter has been accurately designed to isolate the Compton scattering part of the point spread function, by blocking the acquisition of primary gamma rays. The formulation of a physical model allowed a set of weighting factors, which were stored in a disk file, to be calculated. Data were then reconstructed by means of the conjugate gradients iterative least-squares technique, with the system matrix modified by the incorporation of the precalculated weighting factors. Experimental projection data relative to phantoms with constant activity along the rotation axis as well as experimental projection data relative to one phantom with variable axial activity, were acquired. Reconstruction of these data shows that a high degree of compensation for attenuation and scatter in brain SPECT is achieved. With respect to the reconstruction algorithms based upon the backprojection of analytically filtered experimental projections, higher spatial resolution together with very good quantitative activity ratios are the main accomplishments of this reconstruction technique. The method has also been applied to in vivo study reconstructions.

A direct ROI quantification method for inherent PVE correction: accuracy assessment in striatal SPECT measurements

PurposeThe clinical potential of striatal imaging with dopamine transporter (DAT) SPECT tracers is hampered by the limited capability to recover activity concentration ratios due to partial volume effects (PVE). We evaluated the accuracy of a least squares method that allows retrieval of activity in regions of interest directly from projections (LS-ROI).MethodsAn Alderson striatal phantom was filled with striatal to background ratios of 6:1, 9:1 and 28:1; the striatal and background ROIs were drawn on a coregistered X-ray CT of the phantom. The activity ratios of these ROIs were derived both with the LS-ROI method and with conventional SPECT EM reconstruction (EM-SPECT). Moreover, the two methods were compared in seven patients with motor symptoms who were examined with N-3-fluoropropyl-2-β-carboxymethoxy-3-β-(4-iodophenyl) (FP-CIT) SPECT, calculating the binding potential (BP).ResultsIn the phantom study, the activity ratios obtained with EM-SPECT were 3.5, 5.3 and 17.0, respectively, whereas the LS-ROI method resulted in ratios of 6.2, 9.0 and 27.3, respectively. With the LS-ROI method, the BP in the seven patients was approximately 60% higher than with EM-SPECT; a linear correlation between the LS-ROI and the EM estimates was found (r = 0.98, p = 0.03). ConclusionThe LS-ROI PVE correction capability is mainly due to the fact that the ill-conditioning of the LS-ROI approach is lower than that of the EM-SPECT one. The LS-ROI seems to be feasible and accurate in the examination of the dopaminergic system. This approach can be fruitful in monitoring of disease progression and in clinical trials of dopaminergic drugs.

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.

IntroOpenEd 2007: an experience on Open Education by a virtual community of teachers

In Fall 2007 David Wiley, professor at Utah State University held a course about Open Education. That time, however, Dr Wiley’s course was followed by a rather unusual group of students. The Fall 2007 edition, in fact, was available to anybody, free of charge, all over the world. The only requisite required was the possession of a blog for the completion of the weekly assignments. The present paper, whose authors attended the course completing it successfully, is an account of the experience they had. It can be considered an innovating experience from many different viewpoints and can be regarded as an example of how the world of the formal education can meet the demands of the informal one, in the broader landscape of professional training and lifelong learning.

High-performance computing and networking as tools for accurate emission computed tomography reconstruction.

It is well known that the quantitative potential of emission computed tomography (ECT) relies on the ability to compensate for resolution, attenuation and scatter effects. Reconstruction algorithms which are able to take these effects into account are highly demanding in terms of computing resources. The reported work aimed to investigate the use of a parallel high-performance computing platform for ECT reconstruction taking into account an accurate model of the acquisition of single-photon emission tomographic (SPET) data. An iterative algorithm with an accurate model of the variable system response was ported on the MIMD (Multiple Instruction Multiple Data) parallel architecture of a 64-node Cray T3D massively parallel computer. The system was organized to make it easily accessible even from low-cost PC-based workstations through standard TCP/IP networking. A complete brain study of 30 (64×64) slices could be reconstructed from a set of 90 (64×64) projections with ten iterations of the conjugate gradients algorithm in 9 s, corresponding to an actual speed-up factor of 135. This work demonstrated the possibility of exploiting remote high-performance computing and networking resources from hospital sites by means of low-cost workstations using standard communication protocols without particular problems for routine use. The achievable speed-up factors allow the assessment of the clinical benefit of advanced reconstruction techniques which require a heavy computational burden for the compensation effects such as variable spatial resolution, scatter and attenuation. The possibility of using the same software on the same hardware platform with data acquired in different laboratories with various kinds of SPET instrumentation is appealing for software quality control and for the evaluation of the clinical impact of the reconstruction methods.