Occupational and patient exposure as well as image quality for full spine examinations with the EOS imaging system.

PURPOSE: EOS (EOS imaging S.A, Paris, France) is an x-ray imaging system that uses slot-scanning technology in order to optimize the trade-off between image quality and dose. The goal of this study was to characterize the EOS system in terms of occupational exposure, organ doses to patients as well as image quality for full spine examinations. METHODS: Occupational exposure was determined by measuring the ambient dose equivalents in the radiological room during a standard full spine examination. The patient dosimetry was performed using anthropomorphic phantoms representing an adolescent and a five-year-old child. The organ doses were measured with thermoluminescent detectors and then used to calculate effective doses. Patient exposure with EOS was then compared to dose levels reported for conventional radiological systems. Image quality was assessed in terms of spatial resolution and different noise contributions to evaluate the detector's performances of the system. The spatial-frequency signal transfer efficiency of the imaging system was quantified by the detective quantum efficiency (DQE). RESULTS: The use of a protective apron when the medical staff or parents have to stand near to the cubicle in the radiological room is recommended. The estimated effective dose to patients undergoing a full spine examination with the EOS system was 290 µSv for an adult and 200 µSv for a child. MTF and NPS are nonisotropic, with higher values in the scanning direction; they are in addition energy-dependent, but scanning speed independent. The system was shown to be quantum-limited, with a maximum DQE of 13%. The relevance of the DQE for slot-scanning system has been addressed. CONCLUSIONS: As a summary, the estimated effective dose was 290 µSv for an adult; the image quality remains comparable to conventional systems.

Lipid deposition in rat aortas with intraluminal hemispherical plug stenosis. A morphological and biophysical study.

A new method was devised to create a stenosis in the rat abdominal aorta. To restrict blood flow, a hemispherical plug was inserted into the aorta through a renal artery. This type of intrinsic (intraluminal) stenosis minimizes possible intramural effects associated with external compression or ligation which severely deform the arterial wall. In the aorta of hypercholesterolemic rats, lipid deposits were distributed in crescent-shaped patches proximal and distal to the plug, whereas lipid deposition in the opposite aortic wall was inhibited. Based on enlarged physical scale models used to study the flow field, the regions of lipid deposition were found to coincide with regions of low shear stress, stagnation, and recirculation. Shear stress was elevated at the wall opposite the plug. These results show that when confounding mural effects are minimized, lipid deposition is promoted in regions of low shear stress with recirculation and inhibited in regions of elevated shear stress.

Lipid deposition and intimal stress and strain. A study in rats with aortic stenosis.

These experiments were designed to study the topography of lipid deposition in the stenotic aorta of hypercholesterolemic rats, and to correlate it with flow conditions and intimal stresses and strains studied in a scale biophysical model and in a computer model. A 69% +/- 5% stenosis was produced with a U-shaped metal clip. One month to 8 months later, the aorta was studied en face by light microscopy after fixation and lipid staining. The intima in the throat of the stenosis was almost completely free of lipid, whereas symmetric lipid deposits occurred as bands just above and especially just below the stenosis; elsewhere lipid deposits appeared to be random. The flow data obtained from the scale model showed that the intima in the throat of the stenosis was subjected to an increase of as much as 20 times in shear stress, whereas the lipid deposits just above and just below the stenosis were associated with asymmetric flow conditions: the proximal area corresponded to a region of rapidly increasing shear stress, the distal area to a region of low to normal shear stress and separated flow. A finite element computer model based on the aortic deformations indicated that the endothelium at the inlet and outlet of the stenosis is subjected to a symmetric pattern of elevated stresses and strains. These results indicate that 1) the pattern of lipid deposition can not be adequately explained by a hypothesis based solely on flow conditions, and 2) lipid deposits can develop in areas of increased fluid shear stress, decreased fluid shear stress, and increased intimal strains.

Quantitation of oil red O staining of the aorta in hypercholesterolemic rats.

A technique is described which provides morphologic and quantitative data on the amount of oil red O (ORO) staining in thoracic aortas of rats fed a high cholesterol diet. Samples are stained with ORO, the dye is extracted, and the concentration of ORO in the extract is measured colorimetrically. Wistar rats fed ad libitum either standard chow (control group: n = 15) or chow supplemented with 4% cholesterol, 1% cholic acid, and 0.5% thiouracil (CCT group: n = 23) were maintained on these diets for 1, 3, 6, 9, or 12 months. Plasma cholesterol levels averaged overall 87 and 737 mg/dl for the control and CCT groups, respectively. Animals were killed under anesthesia by perfusion fixation with formalin or glutaraldehyde, and samples of thoracic aorta were stained with ORO. After microscopic study en face and measurement of surface area, the ORO was extracted in chloroform-methanol (2:1). Concentrations of ORO (microM) were determined from a standard curve and expressed as microM/mm2 of aorta. Aortas of CCT animals showed progressive diet- and time-dependent increases in the amount of ORO staining compared to controls. We conclude that this method yields reliable quantitative data applicable to studying atherosclerosis in small animals.

Endothelial adaptations in aortic stenosis. Correlation with flow parameters.

A 69 +/- 5% stenosis was produced in the rat aorta, with the purpose of correlating endothelial changes with local flow patterns and with levels of shear stress; the hydrodynamic data were obtained from a scaled-up model of the stenosed aorta. In the throat of the stenosis, where shear stress values were 15-25 times normal, the endothelium was stripped off within 1 hour. It regenerated at half the rate of controls but modulated into a cell type that could withstand the increased shear stress. Adaptations included changes in cell orientation, number, length, width, thickness, stress fibers, and anchoring structures, as well as changes in the length, argyrophilia, and permeability of the junctions. Areas of either elongated or "polygonal" cells consistently developed at the same sites in relation to the stenosis, but the hydrodynamic data showed that they did not always correspond (as had been anticipated) to high and low shear, respectively. It is concluded that endothelial cell shape in the living artery must be determined by some other factor(s) in addition to shear stress.

The significance of endothelial stomata and stigmata in the rat aorta. An electron microscopic study.

Perfusion of arteries with dilute silver nitrate produces in the endothelium (a) a pattern of pericellular black lines, which we earlier interpreted as a marker of the physiological electrolyte pathway (Zand et al. 1982), and (b) focal black deposits on or between the cells, either ring-shaped (stomata) or solid (stigmata). The purpose of this study was to clarify the nature and significance of these controversial structures. A glutaraldehyde-fixed normal rat aorta was perfused with silver nitrate; 17 typical stomata and stigmata were photographed en face, then studied on ultrathin serial sections. When seen en face, they fell into three groups: (I) 4 stomata in endothelial cells; (II) 6 stigmata in endothelial cells; (III) 7 stigmata on intercellular junctions. By electron microscopy, (I) all the stomata in endothelial cells corresponded to myoendothelial herniae. (II) Of the 6 stigmata in endothelial cells, 4 corresponded again to myoendothelial herniae, 2 corresponded to blebs (it seemed likely that these blebs had existed in vivo, but the possibility of a fixation artefact could not be excluded). (III) Of the 7 stigmata on intercellular junctions, one corresponded to the diapedesis of a mononuclear cell; the other 6 did not correspond to visible endothelial changes and are best interpreted as points of normally higher permeability. We conclude that stomata and stigmata (under the conditions of our experiments) can be explained in at least 4 different ways, depending in part on their location (in cells, on junctions). These ancient terms therefore remain useful for descriptive purposes, as long as it is realized that their significance in any given case must be determined by electron microscopic study.

Studies on the pathogenesis of atherosclerosis. I. Adhesion and emigration of mononuclear cells in the aorta of hypercholesterolemic rats.

In rats with diet-induced hypercholesterolemia, two concomitant changes began to occur within 1 week and persisted for 1 year: an increase in total plasma cholesterol and an increase in the number of mononuclear cells adhering to the aortic intima (up to values 50 times normal). Adherent cells were approximately 90% monocytes and approximately 10% lymphocytes. Adhesion was focal, with some preference for ostia of aortic branches; it was followed by migration into the subendothelial space. The subendothelial monocytes/macrophages progressively became foam cells, thus giving rise to microscopic "fatty streaks." Ultimately, typical atherosclerotic plaques were formed. Four possible mechanisms of increased cell adhesion are suggested. Endothelial changes were mild; myelin figures arising from the endothelial surface were seen by electron microscopy. Endothelial denudation was never observed, neither in light-microscopic preparations stained with AgNO3 nor by ultrastructure. Platelet participation was minimal. It is concluded that in this model atherosclerotic plaques are initiated by mononuclear cell adhesion and emigration; endothelial denudation is not a necessary step in their pathogenesis.

Hydrodynamic injury of the endothelium in acute aortic stenosis.

The acute effects of increased shear stress on the endothelium were studied by reducing the lumen of the rat aorta to 20-25% of normal by means of metal clips. Intimal damage in the stenotic area was assessed by light microscopy after perfusion with AgNo3 and study of the endothelium en face. Most of the endothelium was lost within 3 minutes; the extent of the damage was not increased after 1 hour. Electron-microscopic examination showed that some endothelial cells became permeable to tracers (thorium dioxide and horseradish peroxidase); platelets adhered to the exposed internal elastic membrane. Focal endothelial changes were represented by myelin figures of various kinds arising from the luminal surface and by "cellular ulcers," superficial erosions of the endothelial cells accompanied by localized cytoplasmic changes. These "ulcers" occurred more frequently over the nucleus and near junctions; they have not been described in other forms of arterial injury.

Endothelium and "silver lines". An electron microscopic study.

The significance of endothelial "silver lines" was studied by TEM in rat aortas after perfusion with glutaraldehyde followed by silver nitrate. Standard TEM technique proved unsatisfactory (coarse silver granules, imprecise localization, artefacts). Exposure of the silver-treated aortas to photographic fixer markedly improved the image of the deposits leaving fine, stable, uniform "residual granules" about 100 A in diameter. Most of these granules were localized along the intercellular junctions; they also tended to pool in the basement membrane beneath each junction. This image suggests that the Ag+ ions pass through the junction, and react with its contents as well as with the basement membrane beyond it. A scheme is proposed to explain the reaction of Ag+ ions with anions and negatively charged radicals within the junction. It is concluded that the "silver lines" represent not only a histochemical effect, but also the visualization of a transendothelial electrolyte pathway.