PubMed Health⌕ Search

Biomedical subjects

A Seret

Publications and source records attributed to A Seret.

11 recordsLinked to original sources
PubMed1999

Maximum-likelihood reconstruction with ordered subsets in bone SPECT.

UNLABELLED: This study was aimed at determining whether the ordered-subset expectation maximum (OSEM) is more effective than filtered backprojection (FBP) for bone SPECT in the routine clinical context. METHODS: Fifty-seven consecutive bone SPECT studies were analyzed. They included pelvic and lumbar spine, thoracolumbar spine, head and neck, feet and shoulders. A 64-projection SPECT study was acquired over 360 degrees by single-head cameras 2-3 h after the injection of 750 MBq 99mTc-methylene diphosphonate. Three observers compared the OSEM and FBP reconstructed images. RESULTS: Streak artifacts, always present with FBP, were rarely generated with the OSEM. When present (n = 24), artifacts associated with negative values near hyperactivities in FBP were not generated with the OSEM in 67% of the cases (n = 16), permitting a satisfactory interpretation of these regions. In half of the other cases (17%, n = 4/24), interpretation was precluded. In only one case did the three observers agree that more hyperactivities were seen with the OSEM. Ninety-six percent of the OSEM pictures were superior or equal to FBP for anatomic resolution and were clearly better in 12% of the cases. The extent of the lesion with the OSEM seemed better or equally defined in 96% and clearly better in 14% of the cases. The low-activity regions were better or equally visualized in all cases and were clearly better seen in 23% of the cases. The quality of the pictures was found to be better or superior with the OSEM in 98% of the cases and definitely better in 65% of the cases. CONCLUSION: Replacement of FBP by the OSEM in bone SPECT would be beneficial to clinical practice.

Artifacts↗
PubMed1998

Median root prior and ordered subsets in Bayesian image reconstruction of single-photon emission tomography.

Median root prior allows Bayesian image reconstruction without any a priori knowledge of the final solution. It limits the noise generated by maximum likelihood-expectation maximization, including when the ordered subsets accelerating procedure is used. Therefore the number of iterations can be optimized to obtain the best resolution for cold lesions. Moreover, the higher the number of subsets, the better the contrast, with optimal results for subsets containing between four and eight projections.

Bayes Theorem↗
PubMed1993

Destruction of stearic acid nitroxyl radicals mediated by photoexcited merocyanine 540 in liposomal and micellar systems.

Fatty acid spin labels have been included into liposomes and micelles, in order to study the photochemical behavior of merocyanine 540 toward nitroxyl radicals situated at various depths in the bilayer or the surfactant layer. Visible illumination of the dye, either free in ethanol or bound to liposomes or micelles, leads to the reduction of the electron spin resonance signal of the label. The efficiency of the interaction between merocyanine 540 and spin labels depends on the depth at which the nitroxyl moiety is localized in the micelle or vesicle. Fluorescence measurements indicate that the first excited singlet state of merocyanine 540 is not directly implicated in the reaction mechanism. Flash photolysis experiments conducted in aqueous solutions of hexadecyltrimethylammonium bromide micelles show that the presence of nitroxyl radical decreases the rate constant of triplet decay in a concentration-dependent fashion. The corresponding quenching rate constant (kq) is determined for the different spin labels. The kq values and the reduction rates of ESR signal show the same dependence on the localization of the nitroxyl moiety in the micelles.

Cetrimonium↗
PubMed1992

Singlet oxygen quantum yield of sulfur and selenium analogs of psoralen.

The quantum yield of singlet oxygen production by eight newly synthesized sulfur and selenium analogs of psoralen irradiated with UV-A (366 nm) has been determined in CCl4 with the help of the steady state luminescence technique. The new psoralen derivatives are generally better singlet oxygen producers than psoralen itself. In particular, the replacement of selenophene for furan and/or of thiopyrone for pyrone induces an important enhancement of the singlet oxygen quantum yield.

Furocoumarins↗
PubMed1990

Triplet yield of merocyanine 540 in water is wavelength dependent.

Monomers and aggregates of Merocyanine 540 (MC540) in water are able to photoisomerize. The shape of the photoisomer absorption spectrum is very similar to that of the ground state. Triplet state of MC540 in water has been produced by energy transfer from triplet anthracene and displays a broad absorption spectrum between 600 and 700 nm. The triplet state may also be produced by direct excitation of MC540 with UV light. However, when the dye is excited by visible light, no triplet state absorbance in the red could be detected so that the triplet yield of MC540 in water seems to be excitation wavelength dependent.

Kinetics↗
PubMed1988

Singlet oxygen production and photoisomerization: two competitive processes for merocyanine 540 irradiated with visible light.

The quantum yields of singlet oxygen production by merocyanine 540 have been measured during visible light irradiation performed in methanol and ethanol. These appear to be one hundred times smaller than the quantum yield for rose bengal measured under the same conditions. Flash photolysis experiments demonstrate the ability of merocyanine 540 molecules to isomerize under visible light irradiation: the isomerization quantum yields are about 0.65 in both ethanol and methanol. This information combined with the fluorescence quantum yield data account for the low values for singlet oxygen production.

Isomerism↗