Search PubMed⌕ Search

PubMed · 12072841

Collimators.

Abstract

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.

Explore related subjects

Keep this discovery

Explore connections, maps & timelines

BibTeXRIS

A R Formiconi. 2002. Collimators.. https://pubmed.ncbi.nlm.nih.gov/12072841/

Cite the original work for its findings. Save a collection to share your selection of sources.

KEEP EXPLORING

Related citations

Optimization stacking by transient pseudo-isotachophoresis for capillary electrophoresis: example analysis of plasma glutathione.

Low concentrations of both reduced form glutathione (GSH) and oxidized form glutathione (GSSG) in diabetic nephropathy (DN) patient's plasma were measured with transient pseudo-isotachophoresis. The plasma samples were deproteined with acetonitrile and centrifuged. The method was performed at constant voltage of 5 kV using a 300 mM borate buffer (pH 8.0), with a fused-silica capillary of 21 cm x 75 microm. The sample length can reach 25% of the efficient length of the capillary, and the sensitivities of GSH and GSSG increased 15-20-fold. The method was also systematically optimized, and the results show that this type of stacking offers good repeatability for routine clinical assay of glutathione in DN plasmas.

Calibration↗

Density functional study of the electric hyperfine interactions and the redox-structural correlations in the cofactor of nitrogenase. Analysis of general trends in (57)Fe isomer shifts.

The influence of the interstitial atom, X, discovered in a recent crystallographic study of the MoFe protein of nitrogenase, on the electric hyperfine interactions of (57)Fe has been investigated with density functional theory. A semiempirical theory for the isomer shift, delta, is formulated and applied to the cofactor. The values of delta for the relevant redox states of the cofactor are predicted to be higher in the presence of X than in its absence. The analysis strongly suggests a [Mo(4+)4Fe(2+)3Fe(3+)] oxidation state for the S = 3/2 state M(N). Among C(4-), N(3-), and O(2-), oxide is found to be the least likely candidate for X. The analysis suggests that X should be present in the cofactor states M(OX) and M(R) as well as in the alternative nitrogenases. The calculations of the electric field gradients (EFGs) indicate that the small values for DeltaE(Q) in M(N) result from an extensive cancellation between valence and ligand contributions. X emerges from the analysis of the hyperfine interactions as an ionically bonded species. Its major effect is on the asymmetry parameters for the EFGs at the six equatorial sites, Fe(Eq). A spin-coupling scheme is proposed for the state [Mo(4+)4Fe(2+)3Fe(3+)] that is consistent with the measured (57)Fe A-tensors and DeltaE(Q) values for M(N) and identifies the unique site exhibiting the small A value with the terminal Fe site, Fe(T). The optimized structure of a cofactor model has been calculated for several oxidation states. The study reveals a contraction in the average Fe-Fe distance upon increasing the number of electrons stored in the cluster, in accord with extended X-ray absorption fine structure studies. The reliability of the adopted methodology for predicting redox-structural correlations is tested for cuboidal [4Fe-4S] clusters. The calculations reveal a systematic increase in the S...S sulfide distances, in quantitative agreement with the available data. These trends are rationalized by a simple electrostatic model.

Calibration↗

Determination of quercetin and kaempferol in human urine after orally administrated tablet of ginkgo biloba extract by HPLC.

A sensitive, simple and accurate method was developed for determination of quercetin and kaempferol in human urine by reversed phase high performance liquid chromatography. The urine samples were analyzed on C18 column. Quercetin and kaempferol were analyzed simultaneously with good separation. UV detector was set at 380 nm. There was a linear relationship between chromatographic area of analytes and concentration of analytes over the concentration range 1.638-81.90 and 1.872-93.60 ng/ml for quercetin and kaempferol, respectively. The recovery of the assay was 99.7+/-6.2 and 97.4+/-7.2% for quercetin and kaempferol, respectively. The within-day and between-day coefficients of variation were less than 9.7 and 16.5% (RSD), respectively. The limit of detection was 1.0 ng/ml for quercetin and 1.1 ng/ml for kaempferol. The limit of quantitation was 1.61+/-0.11 ng/ml (n=5) for quercetin and 1.85+/-0.11 ng/ml (n=5) for kaempferol. The method developed has been applied to determine quercetin and kaempferol after orally administrated tablet of Ginkgo biloba extract in human urine.

Calibration↗