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Recently, a general theoretical description of multi-hole collimators has been proposed by one of the authors. In this work, the theory was used to study the artifacts, which are caused by the hole-array pattern of high-energy collimators. The relevant outcomes of the study were that the response to a sheet source does not depend on its distance from the collimator, whereas the response depends on the gap between the collimator and the image plane that is located in the scintillation crystal; the dependence of hole-array artifacts on this gap was assessed and the existence of optimal values for which they are minimized was found. A large field of view camera, equipped with a high-energy general purpose collimator for [sup.131]I with hexagonal holes placed in an hexagonal lattice, was used. A series of images of a flood source placed on the external face of the collimator was acquired. Each image was acquired with a different value of the crystal-collimator distance. The analysis of the images clearly showed the existence of optimal crystal-collimator distances to reduce the hole-array artifacts. These distances have been found in agreement with the values predicted by the theory. This work may be useful for the design of new collimators, for the optimal arrangement of camera-collimator system and it may also foster the study of some artifacts compensation methods. Index Terms--Hole-array artifacts, multihole collimators.

Peer review can be used as a teaching methodology to improve students' learning and critical thinking. However, teachers have many concerns about the reliability and validity of students' grading. The paper describes the application of peer review as a teaching strategy to the large course of Biomedical Informatics in the School of Medicine at the University of Florence. The aim of the study was twofold: (I) assessing the validity of students' reviews, calculating the correlation between students' assigned score and instructor's assigned score; (II) assessing the validity of student's self-evaluation, calculating the correlation between student's assigned score and teacher's assigned score. To this aim a statistical analysis was performed. The results showed a moderate concordance between the marks assigned by peers and those assigned by the instructor. Nevertheless, the comparison between the teacher median and the peer-review median shows a minimal difference that has almost no effect on changing the final grade. Instead, there was poor concordance between the marks attributed by the instructor and those relating to the student's self-evaluation. Even if further studies are needed, the promising results can begin to dispel teachers' concerns about students' grading skills that prevent the application of peer review. On such basis, the use of peer review systems can streamline the application of peer review in classes with a high number of students reducing the workload on the teacher. [ABSTRACT FROM AUTHOR]

Focuses on the study which developed a method to calculate weighting factors from the theoretical knowledge of the geometric response of a multihole collimator. Discretization of the blurred Radon transform; Derivation of the collimator geometric transfer function; Calculation of the weighting factors; Theory; Conclusions.

In this article the present knowledge of the collimators used for single photon imaging in Nuclear Medicine is recapped. In the introduction, the classic formulas to describe the performances of multihole collimator are presented with an overview of the basic concepts. In the subsequent sections, the tools available for the characterization of multihole collimators are described. The characterization is divided into aspects related to the geometrical response and aspects related to effects due to interaction of photons with the collimator. Nowadays a rather complete theoretical description of the collimator geometrical response is available. In section Collimator characterization: theory a general formulation that is valid for a wide range of collimators is outlined. In section Collimator characterization: numerical methods the kind of knowledge that can be derived today from numerical computing methods about the penetration and scatter effects is presented. The problem of performances optimization in collimator design is described in section Collimator design. Difficulties and some new perspectives on avoiding both penetration and hole pattern artifacts while designing high-energy collimators are mentioned.

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.

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 (64x64) slices could be reconstructed from a set of 90 (64x64) 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.

An accurate model of the nonstationary geometrical response of a camera-collimator system is discussed. The algorithm is compared to three other algorithms that are specialized for region-of-interest evaluation, as well as to the conventional method for summing the reconstructed quantity over the regions of interest. For noise-free data and for regions of accurate shape, least-squares estimates were unbiased within roundoff errors. For noisy data, estimates were still unbiased but precision worsened for regions smaller than resolution: simulating typical statistics of brain perfusion studies performed with a collimated camera, the estimated standard deviation for a 1-cm-square region was 10% with an ultra-high-resolution collimator and 7% with a low-energy all-purpose collimator. Conventional region-of-interest estimates show comparable precision but are heavily biased if filtered backprojection is used for image reconstruction. Using the conjugate-gradient iterative algorithm and the model of nonstationary geometrical response, bias of estimates decreased on increasing the number of iterations, but precision worsened, thus achieving an estimated standard deviation of more than 25% for the same 1-cm region.

Regional cerebral blood flow was studied using 99mTc-HM-PAO and single photon emission computed tomography (SPECT) in five healthy subjects and in eight patients with clinical and computed tomography (CT) features of subcortical arteriosclerotic encephalopathy. All the patients and controls underwent magnetic resonance imaging (MRI). Circumscribed cortical hypoperfusion was observed in three patients, and in one case there was no evidence of cortical or sulcal alterations on MRI. In all the patients, a reduction of the white matter perfusion was found in periventricular and/or supraventricular regions. This pattern was substantially correlated with the typical white matter hyperintensities shown by MRI scans. These findings support the hypothesis that white matter hypoperfusion is the most prominent functional abnormality in subcortical arteriosclerotic encephalopathy.

A method of estimating the 131I-hippuran clearance of each kidney during renal sequential scintigraphy is presented. The clearance rates were obtained from the elaboration of the renal histograms and of a blood curve recorded on the precordium, directly compared with the injected dose. A function simulating the progressive deformation of the radioactive bolus as kidney perfusion is impaired was introduced into the computation. The method was tested on 23 patients who underwent simultaneous determination of the clearance rate with the Sapirstein method, the results being corrected to 1.73 m2 of body surface area. The correlation coefficient r between the obtained data (the sum of the clearance rates of the two sides) and the results of the Sapirstein method was 0.89.