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PubMed1993

Prospects for using an IVEM with a FEG for imaging macromolecules towards atomic resolution.

Specimen preparation and imaging techniques for biological macromolecules have been improved to the point where attention to the electron-optical imaging conditions becomes a significant factor for achieving high resolution. A field emission gun (FEG) can provide an illumination source with a better spatial and temporal coherence suitable for imaging near atomic resolution. Our computational analysis of carbon film images taken between Scherzer focus and 1.1 microns underfocus (20x Scherzer focus) with the Hitachi 200 kV microscope with a cold field emission gun shows detectable contrast beyond 3.5 A resolution. In biological imaging, a large defocus is often used to optimize the low-resolution contrast in order to facilitate the subsequent steps in computer reconstruction. An intermediate-voltage electron microscope (IVEM) would optimize the contrast at high resolution by reducing the temporal coherent effects. In theory, the IVEM would give a greater depth of field so that large macromolecular assemblies such as viruses and cellular structures can be interpreted and reconstructed reliably using the projection approximation. These experimental and theoretical considerations provide a rationale for designing a future IVEM with a FEG suitable for biological macromolecule imaging close to atomic resolution.

Fourier Analysis↗

PubMed1994

Capillary zone electrophoresis of peptides: prediction of the electrophoretic mobility and resolution.

The determination of the pKa values of some selected peptides of similar size was performed by microtitration, which makes possible an accurate determination of the peptide charge as a function of the solution pH. Capillary zone electrophoresis separation of these peptides on modified capillaries at acidic pH showed that the electrophoretic mobility correlates with the peptide charge. This observation suggests that when an appropriate charge value is used, the basic electrophoretic equation is respected and, at least at a peptide charge value less than 1, the utilization of alternative semi-empirical predictions is not necessary. As a general rule, a peptide separation at acidic pH values is to be preferred to that at basic pH values. In fact, at basic pH a separation in the absence of both electroosmotic flow and of spurious interactions between the peptides and the inner wall of the capillary is difficult, owing to the instability of capillary modification. Further, from the differences in the peptide charge, a prediction of the best resolution as a function of the pH could be obtained; in fact, the resolution, for peptides of similar size and in the absence of electroosmotic flow, is connected to a simple equation, where the principal term depends on the effective charge of the peptides, which is a function of the pH of the solution and the pKa values of the peptides. The predictions of resolution at acidic pH agreed well with the experimental results; the spatial resolution measured in the separation of met- and leu-enkephalin was virtually coincident with the predicted resolution; in the case of a mixture of four model tetrapeptides of sequence GGNA, GGQA, GGDA and GGEA some anomalous results with respect to the predicted resolutions were observed. Nevertheless, an acceptable prediction can also be made in this case.

Amino Acid Sequence↗

PubMed1995

Poly(ethyleneoxide) for high-resolution and high-speed separation of DNA by capillary electrophoresis.

Capillary electrophoresis (CE) with polyacrylamide gels has already been demonstrated to allow single-base resolution of single-stranded DNA. However, linear polyacrylamide is not an ideal matrix because of a high viscosity and difficulties in preparing the polymer with well defined pore sizes. Alternatively, poly(ethyleneoxide) (PEO) with a large range of molecular masses from 300,000 to 8,000,000 is available commercially. In addition, it is easy to prepare homogeneous solutions to provide highly reproducible separation performance with sufficient resolution. Single-base resolution of double-stranded DNA between 123 and 124 base pairs can be achieved by the use of homogeneous matrices prepared from PEO (2.5% M(r) 8,000,000), and even better resolution is achieved by using mixed polymer matrices. With further work, it should be possible to change the fractions and the total amounts of polymers to achieve even higher resolution for different samples with different size ranges of fragments. Another advantage of mixed polymer matrices is that relatively high resolution can be obtained while maintaining a relatively low viscosity compared to linear polyacrylamide with identical contents of formamide and urea, which makes it easier to fill these matrices into small capillaries.

DNA↗

PubMed1993

Low resolution spin echo: a simple timesaving technique for MRI liver exams.

MR evaluation of the liver at mid-field strength requires relatively lengthy T2-weighted sequences (T2W) for differentiation of benign cavernous hemangiomas from malignant lesions. A short duration T2W, which maintains standard signal-to-noise ratio and also contrast relationships, can be easily implemented by increasing the pixel size in the phase-encoding direction (thus reducing spatial resolution) and proportionally decreasing the number of phase-encoding steps in the matrix (thus reducing acquisition time). Blinded interpretations of a quick (4 min), low resolution (3.4 mm x 1.7 mm pixel) T2W sequence (matrix 64 x 256, FOV 21.7 cm phase x 43.5 cm frequency) were compared to the 17 min standard resolution (1.7 mm x 1.7 mm) T2W sequence (256 x 256 matrix, FOV 43 x 43 cm) in 25 patients suspected of having liver metastasis. Lesions felt to be cavernous hemangiomas showed a 100% (24/24) agreement for interpreter "A" and 96% (22/23) agreement for interpreter "B" when 4 min low resolution T2W was compared to the standard 17 min sequence. Sensitivity (for all types of lesions) of the low resolution T2W sequence ranged from 100% (31/31) for interpreter "A" to 80% (28/35) for interpreter "B." Missed lesions (interpreter "B") were either partially obscured by excessive fat (wrap around) (N = 4), less than 1 cm in size (N = 2), or degraded by motion artifact (N = 1). Thus in many situations low resolution T2 may provide a substantial timesaving alternative to standard T2W particularly where T2W is used primarily for lesion classification in normal sized patients.

Adult↗

PubMed1994

Improved endocardial border resolution during dobutamine stress echocardiography with intravenous sonicated dextrose albumin.

OBJECTIVES: The purpose of this study was to determine whether intravenous sonicated dextrose albumin could improve endocardial border resolution during dobutamine stress echocardiography. BACKGROUND: Sonicated albumin improves endocardial border resolution in patients undergoing exercise stress echocardiography. Because a sonicated mixture of albumin with dextrose results in better transpulmonary passage than sonicated albumin alone, this agent could be utilized to further improve endocardial border resolution during dobutamine stress echocardiography. METHODS: We gave intravenous injections of sonicated dextrose and albumin to 50 patients undergoing dobutamine stress echocardiography. Left ventricular video intensity, contrast agent persistence (in seconds), number of contrast agent-enhanced cardiac cycles and improvement in endocardial border resolution were assessed from the apical four-chamber view at baseline, low dose (10 micrograms/kg body weight per min) and peak dobutamine infusion. RESULTS: There was significantly better left ventricular peak video intensity at peak dobutamine infusion than after the same injection at baseline (p < 0.005, analysis of variance). The number of contrast agent-enhanced cardiac cycles in the left ventricular cavity was also significantly longer at peak infusion. Endocardial border resolution was improved (mainly in lateral segments) in 93% of patients at low dose and 95% of patients at peak infusion. CONCLUSIONS: Intravenous sonicated dextrose albumin improves endocardial border resolution during dobutamine stress echocardiography.

Aged↗

PubMed1995

Time course of resolution of persistent air leak in spontaneous pneumothorax.

Persistent air leak (PAL) following tube drainage for spontaneous pneumothorax (SP) is a potentially troublesome complication. The optimum time to intervene surgically is not known. We therefore investigated the rate of resolution of PAL (defined as continued air leak at 2 days after tube insertion) in patients treated medically. We retrospectively reviewed the records of 214 patients presenting with SP over 5 yr. One hundred and forty-two (67%) had tube drainage and in 73 (51%) of these a PAL developed. Overall median time to resolution in the 43 patients with PAL treated medically was 8 days from tube insertion, but PAL resolution was longer in those with underlying lung disease (n = 19) than those without such disease (11 days vs. 7 days, P = 0.05). In patients with normal lungs 20/24 had resolved by 9 days but subsequently the rate of resolution was much slower. In those with abnormal lungs only 8/19 had resolved by 9 days (P < 0.05 compared with those with normal lungs) but the rate of resolution in such patients appeared to change little subsequently. The size of the initial SP did not influence the rate of resolution of PAL. For patients with PAL and normal lungs, surgery at 7-9 days from tube insertion would provide a reasonable opportunity for the PAL to resolve with medical treatment. For those with underlying lung disease there is no clear optimal time and decisions need to be taken on an individual basis.

Adolescent↗

PubMed1993

Two-dimensional crystallization of Escherichia coli-expressed bacteriorhodopsin and its D96N variant: high resolution structural studies in projection.

Highly ordered two-dimensional (2-D) crystals of Escherichia coli-expressed bacteriorhodopsin analog (e-bR) and its D96N variant (e-D96N) reconstituted in Halobacterium halobium lipids have been obtained by starting with the opsin protein purified in the denaturing detergent sodium dodecyl sulfate. These crystals embedded in glucose show electron diffraction in projection to better than 3.0 A at room temperature. This is the first instance that expressed bR or a variant has been crystallized in 2-D arrays showing such high order. The crystal lattice is homologous to that in wild-type bR (w-bR) in purple membranes (PM) and permit high resolution analyses of the structure of the functionally impaired D96N variant. The e-bR crystal is isomorphous to that in PM with an overall averaged fractional change of 12.7% (26-3.6-A resolution) in the projection structure factors. The projection difference Fourier map e-bR-PM at 3.6-A resolution indicates small conformational changes equivalent to movement of approximately < 7 C-atoms distributed within and in the neighborhood of the protein envelope. This result shows that relative to w-bR there are no global structural rearrangements in e-bR at this 3.6 A resolution level. The e-D96N crystal is isomorphous to the e-bR crystal with a smaller (9.2%) overall averaged fractional change in the structure factors. The significant structural differences between e-D96N and e-bR are concentrated at high resolution (5-3.6 A); however, these changes are small as quantified from the 3.6 A resolution e-D96N-e-bR Fourier difference map. The difference map showed no statistically significant peaks or valleys within 5 A in projection from the site of D96 substitution on helix C. Elsewhere within the protein envelope the integrated measure of peaks or valleys was < approximately 3 C-atom equivalents. Thus, our results show that for the isosteric substitution of Asp96 by Asn, the molecular conformation of bR in its ground state is essentially unaltered. Therefore, the known effect of D96N on the slowed M412 decay is not due to ground-state structural perturbations.

Bacteriorhodopsins↗

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↗

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↗

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↗