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PubMed2000

Resolution of inflammation.

Acute inflammatory reactions, in contrast to chronic inflammatory reactions, are usually self-limiting and resolve. We have investigated the resolving phase of a number of immune and non-immune inflammatory reactions induced in the pleural cavity of rats. COX-2 is expressed during resolution of these models. Using carrageenan pleurisy, we showed that this enzyme has a proinflammatory role as the reaction develops but an antiinflammatory role as the lesion resolves. This antiinflammatory role is associated with production of cyclopentenone prostaglandins and the absence of PGE2. Dual COX-1/COX-2 inhibitors or COX-2 inhibitors when given at the peak of the inflammatory response delay resolution, an effect reversed by replacing CyPGs into the pleural space. PGF2alpha like the CyPGs appears to have a role in resolving this reaction. Stress proteins are also induced in a variety of acute inflammatory models during resolution. Heme oxygenase-1 (HO-1) is one such protein so too are members of the hsp70 family. An inducer of HO-1 promotes resolution whereas an inhibitor is proinflammatory. In most cases it appears to be the macrophage that is the source of proteins necessary for resolution to occur. Understanding how proinflammatory pathways switch to the antiinflammatory pathways necessary for resolution to take place may eventually allow the exploitation of endogenous antiinflammatory pathways in the treatment of chronic inflammation.

Animals↗

PubMed1996

Accuracy of high-resolution computed tomography in cochlear implantation.

Multichannel cochlear implants are a proven method for the auditory rehabilitation of individuals who have severe-to-profound sensorineural hearing loss. These devices typically require insertion into the scala tympani of the cochlea to provide auditory stimulations. A patent scala provides the best chance for an adequate insertion of the electrode array. Preoperative high-resolution computed tomography imaging has traditionally been used to determine the patency of the scala tympani. Its ability to accurately predict the patency of the cochlea has been questioned in several retrospective studies. A prospective study was undertaken in 28 consecutive individuals undergoing cochlear implant surgery to compare the findings on high-resolution computed tomography with the surgical findings in an attempt to determine high-resolution computed tomography's accuracy. Cochlear obstruction caused by ossification was accurately predicted in six of six individuals but overestimated in the round window region in three individuals. High-resolution computed tomography accurately predicted patent cochleas in 19 individuals. No false-negative results were encountered. In this study sensitivity of high-resolution computed tomography was 100%, and specificity was 86%. High-resolution computed tomography appears to be more helpful than previously reported for determining cochlear patency.

Acoustic Stimulation↗

PubMed2000

Dietary essential fatty acids, long-chain polyunsaturated fatty acids, and visual resolution acuity in healthy fullterm infants: a systematic review.

BACKGROUND: Biologically active neural tissue is rich in docosahexaenoic acid (DHA), an omega-3 long-chain polyunsaturated fatty acid (LCPUFA). We conducted a systematic review to examine the nature of discordant results from studies designed to test the hypothesis that dietary DHA leads to better performance on visually-based tasks in healthy, fullterm infants. We also conducted a meta-analysis to derive combined estimates of behavioral- and electrophysiologic-based visual resolution acuity differences and sample sizes that would be useful in planning future research. STUDY DESIGN AND METHODS: Twelve empirical studies on LCPUFA intake during infancy and visual resolution acuity were identified through bibliographic searches, examination of monograph and review article reference lists, and written requests to researchers in the field. Works were reviewed for quality and completeness of information. Study design and conduct information was extracted with a standardized protocol. Acuity differences between groups consuming a source of DHA and groups consuming DHA-free diets were calculated as a common outcome from individual studies; this difference score was evaluated against a null value of zero and then used, with the method of DerSimonian and Laird (Meta-analysis in clinical trials. Control Clin Trials 1986;7:177-188), to derive combined estimates of visual resolution acuity differences within seven age categories. RESULTS OF RANDOMIZED COMPARISONS: The combined visual resolution acuity difference measured with behaviorally based methods between DHA-supplemented formula fed groups and DHA-free formula fed groups is 0.32+/-0.09 octaves (combined difference+/-S.E.M., P=0.0003) at 2 months of age. The direction of this value indicates higher acuity in DHA-fed groups. RESULTS OF NON-RANDOMIZED STUDY DESIGNS: The combined visual resolution acuity difference measured with behaviorally based methods between human milk fed groups and DHA-free formula fed groups is 0.49+/-0.09 octaves (P< or =0.000001) at 2 months of age and 0.18+/-0.08 octaves (P=0.04) at 4 months of age. Acuity differences for electrophysiologic-based measures are also greater than zero at 4 months (0.37+/-0.16 octaves, P=0.02). CONCLUSION: Some aspect of dietary n-3 intake is associated with performance on visual resolution acuity tasks at 2, and possibly, 4 months of age in healthy fullterm infants. Whether n-3 intake confers lasting advantage in the development of visually based processes is still in question.

Bottle Feeding↗

PubMed2000

The influence of the resolution and contrast on measuring the articular cartilage volume in magnetic resonance images.

The progression of OA in patients may be followed by measuring the volume of articular cartilage from MR images. We attempted to determine the reproducibility of volume measurements of articular cartilage made from magnetic resonance images of the knees and the dependence of the reproducibility on image resolution and contrast-to-noise. A fat-suppressed 3D technique was used to generate four image sets with different image resolution. Each patient was imaged twice to obtain image pairs at each resolution. To assess the dependence of reproducibility on noise we generated six image sets for each patient by adding noise to the original images and repeating the comparison. On each image set, the femoral, tibial, and patellar cartilage were outlined by a combination of computer and manual methods, and the images were used to calculate the volume of each cartilage plate. Comparing the coefficient of variance between the volume measurements made from the two visits, the volume measurements made from images with the highest resolution (0.275 x 0.275 x 1.0 mm) had the highest reproducibility. The high resolution images of the tibia and femur had the least partial-volume averaging and, as a result, better defined the boundaries between cartilage and adjacent tissues. A different trend was evident for the patella. For studies of osteoarthritis therapies, we recommend using MR images with the highest possible in-plane spatial resolution to provide the most reproducible volume measurements of knee cartilage.

Body Mass Index↗

PubMedDate not supplied

Examples of marginal resolution of NMR relaxation peaks using UPEN and diagnostics.

The multiexponential inversion program UPEN by the authors [J. Magn. Reson. 1998; 132: 65-77; Ibid. 2000;147:273-85] employs negative feedback to a regularization penalty to implement variable smoothing when both sharp and broad features appear on a single distribution of relaxation times. This allows a good fit to relaxation data that correspond to a sum of decaying exponentials plus random noise, but it usually does not give a good fit to data that are distorted by systematic errors from instrument problems, which can cause erroneous "resolution" or erroneous non-resolution of peaks. UPEN provides a series of diagnostic parameters to help identify such data problems that can lead to interpretation errors, and, in particular, to warn when a close call on the resolution or non-resolution of nearby peaks might be questionable. Examples are given from a series of T(2) data sets from desiccated bone samples, with examples where the presence of two peaks is required by good data, examples where the presence of two peaks is negated by good data, and examples where the resolution or non-resolution of peaks cannot be trusted because of instrumental distortions revealed by UPEN diagnostic parameters. It is suggested that processing relaxation data with UPEN in nearly real time could permit retaking data while a sample is still available if the diagnostic parameters show instrumental problems.

Bone and Bones↗

PubMed1997

Comparative prognostic significance of simultaneous versus independent resolution of ST segment depression relative to ST segment elevation during acute myocardial infarction.

OBJECTIVES: We sought to determine the prognostic significance of simultaneous versus independent resolution of ST segment depression that occurs concomitant with ST segment elevation during acute myocardial infarction (AMI). BACKGROUND: ST segment depression in leads other than those showing ST segment elevation during AMI is a common phenomenon. Whether this indicates adverse outcomes remains controversial. We hypothesized that the timing of ST segment depression resolution relative to ST segment elevation resolution might differentiate between a high risk group and a low risk group of patients. METHODS: Continuous 12-lead ST segment monitoring was performed after thrombolytic therapy for AMI in 413 patients, 261 of whom met technical criteria for analysis. Blinded analysis of ST segment depression resolution patterns was used to group patients as follows: 1) no ST segment depression at any time (control group); 2) ST segment depression resolving simultaneously with ST segment elevation (simultaneous group); and 3) ST segment depression persisting after ST segment elevation resolution (independent group). These patterns were correlated with the outcomes-recurrent angina, reinfarction, heart failure and death-using chi-square analysis and the Fisher exact test for categoric variables and the Wilcoxon rank-sum test for continuous variables. RESULTS: The incidence of recurrent angina, reinfarction and heart failure was similar among the three groups. In-hospital mortality, however, was significantly higher in the independent group (13%) than either the simultaneous group (1%, p < 0.001) or the control group (0%, p = 0.002). CONCLUSIONS: Continuous analysis of ST segment resolution identifies, among patients with AMI with concomitantly occurring ST segment elevation and depression, a subgroup with increased in-hospital mortality. The pathogenic mechanism of increased mortality is not currently known.

Aged↗

PubMed1988

Silicone intraocular lens resolution in air and in water.

The resolution efficiencies of 31 biconvex silicone intraocular lenses, ranging in power from 16.0 to 23.5 diopters, were tested in air and in water to see if a predictable relationship existed as previously reported with polymethylmethacrylate lenses. Resolution efficiency is defined as the percentage ratio of the actual resolving power of a lens to that of a perfect lens of the same focal length which is only limited in resolution by diffraction. The lenses ranged from 29% to 58% resolution efficiency in air. No lenses exhibiting multiple images were included. All 31 lenses achieved at least 73% resolution efficiency in water, and one lens achieved 82%. Based on these findings, a biconvex silicone lens that exceeds 30% resolution efficiency in air and does not produce multiple images can perform near its diffraction limit when implanted in the eye.

Air↗

PubMed1995

The 70S Escherichia coli ribosome at 23 A resolution: fitting the ribosomal RNA.

BACKGROUND: The ribosome--essential for protein synthesis in all organisms--has been an evasive target for structural studies. The best available structures for the 70S Escherichia coli ribosome or its 30S and 50S subunits are based on electron microscopical tilt experiments and are limited in resolution to 28-55 A. The angular reconstitution approach, which exploits the random orientations of particles within a vitreous ice matrix, can be used in conjunction with cryo-electron microscopy to yield a higher-resolution structure. RESULTS: Our 23 A resolution map of the 70S ribosome elucidates many structural details, such as an extensive system of channels within the 50S subunit and an intersubunit gap ideally shaped to accommodate two transfer RNA molecules. The resolution achieved is sufficient to allow the preliminary fitting of double-helical regions of an earlier three-dimensional ribosomal RNA model. CONCLUSIONS: Although we are still a long way from attaining an atomic-resolution structure of the ribosome, cryo-electron microscopy, in combination with angular reconstitution, is likely to yield three-dimensional maps with gradually increasing resolution. As exemplified by our current 23 A reconstruction, these maps will lead to progressive refinement of models of the ribosomal RNA.

Escherichia coli↗

PubMed1995

Fibrinogen and markers of fibrinolysis and endothelial damage following resolution of critical limb ischaemia.

OBJECTIVES: To assess the effects of resolution of critical limb ischaemia on the elevated plasma fibrinogen, cross-linked fibrin degradation products (FDP), and von Willebrand factor antigen (vWF) levels, reported in peripheral arterial occlusive disease. DESIGN: A prospective study of patients undergoing surgery for chronic critical limb ischaemia. SETTING: Two vascular surgery units providing tertiary referral services for the West of Scotland. MATERIALS: Venous blood samples were assayed for plasma fibrinogen, FDP D-dimer, and vWF levels, prior to surgery, together with fibrinolytic and rheological parameters, in 82 patients. Sampling was repeated 4 months after resolution of critical limb ischaemia. OUTCOME MEASURES: Levels of these parameters following successful resolution of critical limb ischaemia were compared with pre-operative levels, and with an age-matched random population sample. MAIN RESULTS: Plasma fibrinogen and vWF levels were significantly lower (both p < 0.005, Wilcoxon matched pairs) following successful resolution of critical limb ischaemia in the 56 patients available for review, although levels remained higher than in population controls (p < 0.01, Mann-Whitney U-test). FDP levels were unchanged following surgery, remaining higher than in age-matched population controls (p < 0.01). CONCLUSIONS: Resolution of critical limb ischaemia fails to reduce plasma fibrinogen, fibrin turnover, and vWF levels to those seen in population controls. This implies that increased fibrinogen and fibrin turnover in peripheral arterial disease is not solely a consequence of tissue ischaemia, while the persisting prothrombotic state following resolution of critical limb ischaemia has potentially important implications for graft and patient survival.

Aged↗

PubMed1998

High-resolution magnetic resonance imaging: three-dimensional trabecular bone architecture and biomechanical properties.

The purpose of this study was to use high-resolution magnetic resonance (MR) imaging combined with image analysis to investigate the three-dimensional (3D) trabecular structure, anisotropy, and connectivity of human vertebral, femoral, and calcaneal specimens. The goal was to determine whether: (a) MR-derived measures depict known skeletal-site-specific differences in architecture and orientation of trabeculae; (b) 3D architectural parameters combined with bone mineral density (BMD) improve the prediction of the elastic modulus using a fabric tensor formulation; (c) MR-derived 3D architectural parameters combined with BMD improve the prediction of strength using a multiple regression model, and whether these results corresponded to the results obtained using higher resolution depictions of trabecular architecture. A total of 94 specimens (12 x 12 x 12 mm cubes) consisting of trabecular bone only were obtained, of which there were 7 from the calcaneus, 15 from distal femur, 47 from the proximal femur, and 25 from the vertebral bodies. MR images were obtained using a 1.5 Tesla MR scanner at a spatial resolution of 117 x 117 x 300 microm. Additionally, BMD was determined using quantitative computed tomography (QCT), and the specimens were nondestructively tested and the elastic modulus (YM) was measured along three orthogonal axes corresponding to the anatomic superior-inferior (axial), medial-lateral (sagittal), and anterior-posterior (coronal) directions. A subset of the specimens (n=67) was then destructively tested in the superior-inferior (axial) direction to measure the ultimate compressive strength. The MR images were segmented into bone and marrow phases and then analyzed in 3D. Ellipsoids were fitted to the mean intercept lengths, using single value decomposition and the primary orientation of the trabeculae and used to calculate the anisotropy of trabecular architecture. Stereological measures were derived using a previously developed model and measures such as mean trabecular width, spacing, and number were derived. Because the spatial resolution of MR images is comparable to trabecular bone dimensions, these measures may be subject to partial volume effects and were thus treated as apparent measures, such as BV/TV, Tb.Sp, Tb.N, and Tb.Th rather than absolute measures, as would be derived from histomorphometry. In addition, in a subset of specimens, the Euler number per unit volume was determined to characterize the connectivity of the trabecular network. There were significant differences in the BMD, trabecular architectural measures, elastic modulus, and strength at the different skeletal sites. The primary orientation axes for most of the specimens was the anatomic superior-inferior (axial) direction. Using the fabric tensor formulation, in addition to BMD, improved the prediction of YM (SI), while including some of the architectural parameters significantly improved the prediction of strength. In comparing MR-derived 3D measures with those obtained from 20 microm optical images (n=18; 9 vertebrae, 9 femur specimens), good correlations were found for the apparent Tb.Sp and Tb.N, moderate correlation was seen for the apparent BV/TV, and poor correlation was found for the apparent Tb.Th. Using these higher resolution images, the fabric tensor formulation for predicting the elastic modulus also showed improved correlation between the measured and calculated modulus in the axial (SI) direction. In summary, high-resolution MR images may be used to assess 3D architecture of trabecular bone, and the inclusion of some of the 3D architectural measures provides an improved assessment of biomechanical properties. Further studies are clearly warranted to establish the role of architecture in predicting overall bone quality, and the role of trabecular architecture measures in clinical practice. (ABSTRACT TRUNCATED)

Anatomy, Cross-Sectional↗

PubMed1993

Ultrahigh resolution matrix-assisted laser desorption/ionization of small proteins by Fourier transform mass spectrometry.

Recent research has demonstrated that matrix-assisted laser desorption/ionization (MALDI) is feasible for Fourier transform mass spectrometric analysis of biomolecules with masses in excess of 50,000 Da. Here, the effects of electrostatic deceleration times and laser energy upon mass resolution are reported. It is demonstrated that optimum deceleration times for singly-charged MALDI-generated protein ions ranging in mass from 2627 to 29,000 Da are a linear function of m1/2 when a 9.5-V decelerating potential is used. Furthermore, higher resolution is obtained with laser fluences close to the threshold for MALDI. Slow metastable decay of molecular ions in the absence of co-matrix is demonstrated for melittin and bovine insulin. It appears that the resolution enhancing effect of co-matrix may result from slowing molecular ion unimolecular decomposition rates sufficiently to allow infrared emission to compete with metastable decay, thus providing the requisite population of long-lived ions for high mass resolution. A spectrum of bovine insulin molecular ion with mass resolution of 30,000 is presented, together with several spectra of lower mass proteins with mass resolution in excess of 100,000. Detection of a doubly-charged carbonic anhydrase trimer ion with a mass of 87,000 Da is reported.

Fourier Analysis↗

PubMed1999

Limiting spectral resolution in speech for listeners with sensorineural hearing loss.

Consonant recognition was measured as a function of the degree of spectral resolution of the speech stimulus in normally hearing listeners and listeners with moderate sensorineural hearing loss. Previous work (Turner, Souza, and Forget, 1995) has shown that listeners with sensorineural hearing loss could recognize consonants as well as listeners with normal hearing when speech was processed to have only one channel of spectral resolution. The hypothesis tested in the present experiment was that when speech was limited to a small number of spectral channels, both normally hearing and hearing-impaired listeners would continue to perform similarly. As the stimuli were presented with finer degrees of spectral resolution, and the poorer-than-normal spectral resolving abilities of the hearing-impaired listeners became a limiting factor, one would predict that the performance of the hearing-impaired listeners would then become poorer than the normally hearing listeners. Previous research on the frequency-resolution abilities of listeners with mild-to-moderate hearing loss suggests that these listeners have critical bandwidths three to four times larger than do listeners with normal hearing. In the present experiment, speech stimuli were processed to have 1, 2, 4, or 8 channels of spectral information. Results for the 1-channel speech condition were consistent with the previous study in that both groups of listeners performed similarly. However, the hearing-impaired listeners performed more poorly than the normally hearing listeners for all other conditions, including the 2-channel speech condition. These results would appear to contradict the original hypothesis, in that listeners with moderate sensorineural hearing loss would be expected to have at least 2 channels of frequency resolution. One possibility is that the frequency resolution of hearing-impaired listeners may be much poorer than previously estimated; however, a subsequent filtered speech experiment did not support this explanation. The present results do indicate that although listeners with hearing loss are able to use the temporal-envelope information of a single channel in a normal fashion, when given the opportunity to combine information across more than one channel, they show deficient performance.

Adult↗

PubMed2000

Supplemental oxygen improves resolution of injury-induced pneumothorax.

BACKGROUND/PURPOSE: Traditionally, supplemental oxygen is administered to patients with asymptomatic pneumothorax to accelerate spontaneous resolution. However, this practice is based on models that did not include injury to the visceral pleura and ongoing pleural air leak. This study evaluated the effects of increased inspired oxygen concentration on pneumothorax resolution in a visceral pleural injury model. METHODS: A total of 27 New Zealand white rabbits were divided randomly into 3 groups: room air (RA), 40% FIO2 (40%), and 60% FIO2 (60%). A complete unilateral pneumothorax was created in each animal by a thoracoscopically guided visceral pleural puncture. The animals were then housed in designated oxygen chambers, and observers were blinded to the inspired oxygen concentration. Cross table anteroposterior chest x-rays were obtained preoperatively, postoperatively, and twice a day until the pneumothorax resolved. Time to resolution between the 3 groups was analyzed with 1-way analysis of variance (ANOVA). RESULTS: Twenty of 27 (74%) of the animals completed the study successfully. Mean time to resolution in the RA group (n = 7; 111.2 +/- 30.8 hours) was longer than in the 40% group (n = 6; 71.8 +/- 22.3 hours) and the 60% group (n = 7; 39.4 +/- 14.2 hours). The time to resolution also was longer in the 40% group than in the 60% group. Seven rabbits died before completion of the study of tension pneumothorax (3 of 7) or anesthetic complications (4 of 7). CONCLUSIONS: Supplemental oxygen improves resolution of injury-induced pneumothorax. The tradition of administering supplemental oxygen to patients with asymptomatic pneumothorax should be continued even if there is ongoing pleural air leak.

Animals↗

PubMed1987

Video-endoscopes in comparison with fiberscopes: quantitative measurement of optical resolution.

The subject of this study was the optical performance of video-endoscopic systems (VE) in terms of maximal resolving power and resolvable picture elements. Olympus, Toshiba/Machida, Fujinon, and Welch Allyn video gastroscopes were tested. A GIF Q 10 fiberscope from Olympus was also included for comparison. The resolution measurements were made at various distances using two independent methods--electronic analysis of the TV signal, and visual evaluation of the resolution, of a standardized test target. The results obtained with the two methods were in perfect agreement. The resolution of fine details clearly depends on the distance between the distal end and the target because of decreasing image scale. Depending on the individual optical design, the various VE's show maxima at different distances. At shorter distances, the image is degraded by defocusing. An optimal distance which is as small as possible is desired for clinical routine. Apart from the fiberscope this requirement is best met by the Fuji system. The greatest resolution is obtained with the Toshiba system but at the cost of the viewing angle which is the smallest of all the systems. Fuji combines relatively high resolution with a large viewing angle. Because of the widely varying viewing angle a comparison based solely on resolution cannot represent the true imaging capability of the system. We therefore eliminated purely optical parameters and calculated the number of resolvable picture elements per line. We regard this number to be a fair characterization of both TV and fiber systems.

Fiber Optic Technology↗

PubMed1998

beta Recombinase catalyzes inversion and resolution between two inversely oriented six sites on a supercoiled DNA substrate and only inversion on relaxed or linear substrates.

The beta recombinase, in the presence of a chromatin-associated protein such as Hbsu, catalyzes DNA resolution or DNA inversion on supercoiled substrates containing two directly or inversely oriented six sites. Hbsu stabilizes the formation of the recombination complex (Alonso, J. C., Weise, F., and Rojo, F. (1995) J. Biol. Chem. 270, 2938-2945). In this study we show that resolution by beta recombinase strictly requires supercoiled DNA, but inversion does not. On a substrate with two inversely oriented six sites, beta recombinase catalyzed both resolution and inversion if the DNA was supercoiled but only inversion if the substrate was relaxed or linear. Hbsu was critical for the formation of synaptic complexes; its concentration relative to that of the supercoiled DNA substrate determined whether resolution or inversion products were preferentially formed. The results suggest that the beta recombinase forms unproductive short-lived synaptic complexes between two juxtaposed inversely oriented six sites; the presence of 3 to 13 Hbsu dimers per supercoiled DNA molecule would stabilize a synaptic complex with a relative geometry of the six sites allowing beta recombinase preferentially to achieve resolution. Supercoiling probably helps to overcome an energetic barrier, since resolution does not occur in relaxed DNA. The presence of >30 Hbsu dimers per DNA molecule probably favors the formation of a recombination complex with a different geometry since the reaction is directed preferentially toward DNA inversion.

Catalysis↗

PubMed1987

Influence of zoom factor on centre-of-rotation of the SPECT system and on the resolution of tomographic images.

The influence of zooming on centre-of-rotation (COR) of the rotating camera-based SPECT system and on the resolution of tomographic images was investigated. COR values were measured as a function of zoom factor varying from x1.0 to x3.0. Measurements were performed using two SPECT systems collimated with three collimators. In this study, the COR was observed to shift linearly with zoom factor. The influence of zooming on COR depends on the location and the deviation of the COR from the axis-of-rotation (AOR). We recommend that the deviation of the COR from the AOR be kept to within 0.4 pixels corresponding to 2.4 mm. Unlike zooming, the magnitude of COR shift is uncorrelated to the type of collimator used. Contrast and spatial resolution variations of the tomographic images were quantitated as a function of COR shift for clinical data acquired using no zoom factor and a zoom factor of 2.50. The resolution indices were observed to be more sensitive to COR shift for clinical data acquired using no zoom factor. The sensitivity appeared to be inversely dependent on the zoom factor utilized. However, zooming was found not to preserve the same resolution degradation. Optimum resolution images can be reconstructed by requiring the COR be determined and clinical study be acquired using the same zoom factor and collimator. For qualitative evaluation of tomographic images, the zoom factor requirement can be removed provided that the deviation of the COR from the AOR is within the recommended limit. This limit allows for a maximum spatial resolution degradation of 5%.(ABSTRACT TRUNCATED AT 250 WORDS)

Image Enhancement↗

PubMed1998

Modeling human eye aberrations and their compensation for high-resolution retinal imaging.

We introduced a mathematical eye model using Gullstrand's six-surface eye model modified by clinically measured aspherical data to study human eye aberrations and their compensation for high-resolution retinal imaging. Ray tracing was used to characterize aberrations and point spread functions (PSFs) of the eye model. By using the Zernike polynomial decomposition of the calculated pupil function, we quantified the wavefront aberrations. Based on calculated PSFs, we designed optical inverse filters to reduce the aberrations for a large pupil size and improve the resolution. Spherical aberration and oblique astigmatism were found to be in good agreement with published experimental measurements. Spherical aberration and defocus were the most significant aberrations for on-axis imaging, whereas oblique astigmatism and coma combined with spherical aberration and defocus were most significant for off-axis imaging. The best retinal image resolution occurred at 2- to 3-mm pupil diameter. After aberration correction for an 8-mm diameter pupil, the resolutions for on-axis or 9 degrees off-axis imaging points were very close to diffraction-limited resolutions. Over a limited field of view (FOV), retinal image resolution of the eye model can be greatly improved by aberration correction using aspheric and astigmatic lenses. For imaging large FOVs, space-variant compensation techniques will be required for aberration correction.

Astigmatism↗

PubMed1992

High-resolution scanning electron microscopy of frozen-hydrated cells.

Cryo-fixed yeast Paramecia and sea urchin embryos were investigated with an in-lens type field-emission SEM using a cold stage. The goal was to further develop and investigate the processing of frozen samples for the low-temperature scanning electron microscope (LTSEM). Uncoated frozen-hydrated samples were imaged with the low-voltage backscattered electron signal (BSE). Resolution and contrast were sufficient to visualize cross-fractured membranes, nuclear pores and small vesicles in the cytoplasm. It is assumed that the resolution of this approach is limited by the extraction depth of the BSE which depends upon the accelerating voltage of the primary beam (V0). In this study, the lowest possible V0 was 2.6 kV because below this value the sensitivity of the BSE detector is insufficient. It is concluded that the resolution of the uncoated specimen could be improved if equipment were available for high-resolution BSE imaging at 0.5-2 kV. Higher resolution was obtained with platinum cryo-coated samples, on which intramembranous particles were easily imaged. These images even show the ring-like appearance of the hexagonally arranged intramembranous particles known from high-resolution replica studies. On fully hydrated samples at high magnification, the observation time for a particular area is limited by mass loss caused by electron irradiation. Other potential sources of artefacts are the deposition of water vapour contamination and shrinkage caused by the sublimation of ice. Imaging of partially dehydrated (partially freeze-dried) samples, e.g. high-pressure frozen Paramecium and sea urchin embryos, will probably become the main application in cell biology. In spite of possible shrinkage problems, this approach has a number of advantages compared with any other electron microscopy preparation method: no chemical fixation is necessary, eliminating this source of artefacts; due to partial removal of the water additional structures in the cytoplasm can be investigated; and finally, the mass loss due to electron beam irradiation is greatly reduced compared to fully frozen-hydrated specimens.

Animals↗