Neutron imaging of laser fusion targets.

Along-term goal of inertial-confinement fusion research is the generation of energy by imploding capsules containing deuterium-tritium fuel. Progress in designing the capsules is aided by accurate imaging of the fusion burn. Penumbral coded-aperture techniques have been used to obtain neutron images that are a direct measurement of the fusion burn region in the capsules.

Aperture tolerances for neutron-imaging systems in inertial confinement fusion.

Neutron-imaging systems are being considered as an ignition diagnostic for the National Ignition Facility (NIF) [Hogan et al., Nucl. Fusion 41, 567 (2001)]. Given the importance of these systems, a neutron-imaging design tool is being used to quantify the effects of aperture fabrication and alignment tolerances on reconstructed neutron images for inertial confinement fusion. The simulations indicate that alignment tolerances of more than 1 mrad would introduce measurable features in a reconstructed image for both pinholes and penumbral aperture systems. These simulations further show that penumbral apertures are several times less sensitive to fabrication errors than pinhole apertures.

Mechanical stability of the cornea after radial keratotomy and photorefractive keratectomy.

BACKGROUND: Corneal refractive surgical procedures alter the shape and structure of the tissue, possibly compromising its mechanical stability. METHODS: One or both eyes of 87 subjects were studied: 26 of these 87 had been treated for myopia by excimer laser ablation and 36 by radial keratotomy; 25 without previous corneal surgery functioned as controls. Corneal topography was evaluated by TMS-1 videophotokeratography before and after 180 degrees tilting of the patient, and the pressure-induced ring-wise changes in spherical equivalent power (axial power) were calculated. Changes in corneal shape also were evaluated by computing the instantaneous radius of curvature. The intraocular pressure was measured before and after tilting by a hand-held applanation tonometer. RESULTS: The mean intraocular pressure increased from 13.9 +/- 2.3 mm Hg before tilting to 30.0 +/- 3.8 mm Hg during tilting. In the control eyes, the mean power of the central cornea during tilting decreased 0.187 +/- 0.045 diopters (D) (p < .05); in the excimer-laser-ablated eyes, 0.038 +/- 0.056 D (not statistically significant); and in those treated with radial keratotomy, 0.523 +/- 0.054 D (p < .01). After radial keratotomy, the cornea steepened outside the clear zone in response to pressure loading, whereas it did not change significantly in the laser-ablated or control eyes. CONCLUSIONS: Pressure-induced deformation of normal and excimer laser-ablated human cornea is small, whereas radially incised corneas have significantly decreased mechanical stability.

Streak camera measurement of subnanosecond plastic scintillator properties.

A streak camera technique with temporal resolution of 20 ps has been used to measure the fluorescence properties of several subnanosecond plastic scintillators. The method employs a vacuum light pipe coupled to an optical streak camera. The scintillators are excited by a 200-ps x-ray pulse generated by a 1.06-microm Nd:YAG laser focused onto an iron target. The time history of the low-energy x-ray pulse is measured with an x-ray streak camera. Results are given for NElll plastic scintillators doped with benzophenone or acetophenone, for PVT doped with butyl-PBD, and for a ZnO phosphor doped with Ga.

[Measuring intraocular pressure in keratoconus. Effect of the changed biomechanics]. / Messung des intraokularen Druckes bei Keratokonus. Einfluss der veränderten Biomechanik.

BACKGROUND: Keratoconus is associated with changes in the corneal structure, such as defects of Bowman's layer, a decrease of corneal thickness etc. They result in alterations of some of the biomechanical parameters of the cornea, namely, rigidity and elasticity. The present study was performed to examine how impression tonometry and applanation tonometry for determination of intraocular pressure (IOP) are affected by the changed biomechanical parameters associated with keratoconus. PATIENTS AND METHODS: We examined 20 normal subjects (40 eyes) and 17 keratoconus patients (25 eyes). The corneal thickness was measured by ultrasound pachymetry in the corneal center. In the keratoconus patients, an additional measurement was made at the conus peak. The corneal curvature was determined using the TMS keratoscope. The IOP measurements were made with the Schiötz tonometer (10 g). For comparison, additional IOP measurements in the corneal center and, in the keratoconus group, on the conus peak were made with the applanation tonometer. RESULTS: The normal subjects had a central corneal thickness of 548 +/- 30 microns, compared to 505 +/- 42 microns in the corneal center and 425 +/- 41 microns on the conus peak in keratoconus patients. The average corneal curvature was 43.3 +/- 1.8 D in the normal subjects and 47.8 +/- 4.1 D in keratoconus patients. Applanation tonometry produced results on 11.33 +/- 1.43 mm Hg in the normal group (corneal center) compared to values of 12.00 +/- 2.55 mm Hg (corneal center) and 7.30 +/- 1.95 mm Hg (conus peak) in the keratoconus cohort. The coefficient of rigidity was 0.0236 +/- 0.0026 microliter-1 in the normal subjects, compared to 0.0173 +/- 0.0050 microliter-1 in the keratoconus patients. CONCLUSION: The morphological changes associated with keratoconus may cause tonometry errors.

[Optical coherence tomography for macular edema. Classification, quantitative assessment, and rational usage in the clinical practice]. / OCT bei Makulaödem. Klassifizierung, quantitative Beurteilung und rationaler Einsatz in der Praxis.

Optical coherence tomography (OCT) allows for morphological assessment of macular edema of various origins by producing two-dimensional images of the retina. In addition, retinal thickness can be measured quantitatively. In diabetic maculopathy, damage of the blood-retinal barrier can be indirectly assessed by locating the intraretinal fluid accumulation and the progress of maculopathy from diffuse to cystoid edema and even rupture of the retinal structure can be described as well as the formation of neurosensory retinal detachment with subretinal fluid. Increase of retinal thickness correlates with decrease in visual acuity. Detailed interpretation of OCT images can replace fluorescein angiography in certain cases, as has been shown for uveitis. In order to obtain correct classification of macular disease in routine clinical use, all images have to be completely assessed and compared to previous investigations and to the measurements of the fellow eye. More accurate normal values of retinal thickness and volume can be expected in the future. The importance of OCT in routine clinical assessment of macular edema will most probably continue to grow.

[Keratomileusis with a lamellar microkeratome and the excimer laser]. / Keratomileusis mit einem lamellären Mikrokeratom und einem Excimer-Laser.

In a combined prospective study with E. Arnott's department, intrastromal excimer keratomileusis on the lenticle stroma was performed in 15 eyes of 15 patients (follow-up 6 months). The primary keratectomy was performed using the lamellar automatic rotating microkeratome, followed by refractive modification by the excimer laser on the excised corneal discs. For the laser ablation, a repetition rate of 10 Hz and a fluence on the corneal plane of 180 mJ/cm2 was used. Following surgery, all treated eyes were subjected to guttae of Maxitrol q.i.d. for 12 weeks. Excimer laser intrastromal keratomileusis effectively combines corneal microsurgery and photoablation. The predictability of the refractive correction is better than with the other surgical keratomileusis techniques.

[Thrombophilia factors as inducers of retinal vascular occlusion]. / Thrombophiliefaktoren als Auslöser retinaler Gefässverschlüsse.

PURPOSE: Thrombophilic abnormalities and defects in the anticoagulant system, such as protein C or protein S deficiency, activated protein C resistance and factor V Leiden mutation, may produce retinal vascular occlusions. PATIENTS: Blood samples from 66 consecutive retinal vascular occlusion patients were obtained and analysed for protein C resistance and fibrinolysis activity. Other thrombophilic and standard laboratory coagulant tests were also carried out. RESULTS: An increased activity of factor VIII was found in 72% of the collective. Furthermore, 24% of all patients and 32% of patients younger than 45 years old were resistant to activated protein C. Nevertheless only one patient showed a homozygous factor V Leiden mutation. CONCLUSION: Thrombophilic activity in cases of protein C resistance or factor V Leiden mutation may result in severe thrombotic manifestations in ocular vessels. Compared to the elderly, younger people showed a higher activated protein C resistance which seems be one of the most common causes for retinal vascular occlusion in this age group.

[Biomechanical study of corneal stability after photorefractive keratectomy]. / Biomechanische Untersuchung der Hornhautstabilität nach photorefraktiver Keratektomie.

BACKGROUND: Photorefraktive keratectomy (PRK) is the most frequent refractive surgical procedure worldwide. The central corneal thickness is reduced due to removal of the anterior stroma, including Bowman's layer, with a laser beam. This procedure results in considerable alterations of the corneal structure. What does this mean for the mechanical properties of the cornea? METHODS: Intraocular pressure was increased via a 180 degrees tilt. Before and during this procedure, corneal topography was measured by photokeratoscopy. We examined 26 patients after PRK and 25 controls who had not undergone any surgical procedure. RESULTS: The corneal center flattened by 0.038 +/- 0.05 dpt (P > 0.05) in the PRK patients and by 0.187 +/- 0.045 dpt (P < 0.05) in the control group. PRK patients within 1 year after operation showed a minimal central corneal steepening, whereas PRK patients after more than 1 year showed a reaction similar to that in normal corneas (P < 0.05). CONCLUSION: Corneal stability is altered after PRK. After 1 year corneal stability seems to normalize due to stromal remodelling.

[Corneal reinnervation after lamellar refractive corneal surgery]. / Korneale Reinnervation nach lamellärer refraktiver Hornhautchirurgie.

In a retrospective study corneal sensitivity after refractive autokeratoplasty for correction of myopia was measured. Thirty-eight patients operated on for keratomileusis in situ and 18 patients for keratomileusis myopica were examined with the Draeger esthesiometer between 1 and 23 months after surgery. Three measurements points were on the refractive disc, two others on the untouched cornea. After keratomileusis myopica, the corneal reinnervation is significantly delayed compared to keratomileusis in situ. In the first group sensitivity is normal after 2 years and in the second group after 1 year. Even refraction and visual acuity depend on metabolism and reorganization of the tissue after surgery. Besides retarded cell repopulation and disturbed stromal metabolism--caused by toxic and cryopreservation effects--the dissection depth of the refractive ablation is responsible for these phenomena.

Structural changes of the retina in retinal vein occlusion--imaging and quantification with optical coherence tomography.

BACKGROUND AND OBJECTIVE: To evaluate the potential of optical coherence tomography (OCT) for imaging and quantifying structural changes in the retinal architecture following venous occlusive disease. PATIENTS AND METHODS: We studied 20 patients with retinal venous occlusive disease: 5 patients suffered from branch retinal vein occlusion (BRVO), and 15 from central retinal vein occlusion (CRVO). Patients with CRVO were studied within 72 hours after the first onset of symptoms. Patients were examined by OCT after a complete ophthalmologic examination including fluorescein angiography. A standard set of linear scans through the center of fixation and individual scans over areas of special interest were performed. Scans were qualitatively evaluated and quantitative measurements were performed on single A-scans at 5 locations of linear scans. Measurements were taken in the center of fixation, at the foveal rim, and at the edge of the scan. We measured total retinal thickness, thickness of cystoid spaces, and thickness of the highly reflective outer band. RESULTS: OCT produced detailed images of retinal thickening, intra- and subretinal hemorrhage, intra- and subretinal fluid accumulation, and formation of intraretinal cystoid spaces. Quantitative evaluation revealed a mean central retinal thickness of 274 +/- 181 microm (72-760 microm) in patients with central retinal vein occlusion. Visual acuity did not correlate with central or extrafoveal retinal thickness. CONCLUSION: Optical coherence tomography is a useful imaging technique for studying changes of retinal architecture following venous occlusive disease. Even minor changes like localized subretinal fluid accumulation or beginning epiretinal membrane formation can be visualized in detail. In contrast to other diseases like diabetic maculopathy, quantitative assessment of macular thickness does not correlate to visual acuity. Nevertheless, because of detailed imaging, OCT might become a useful tool in the follow-up and treatment of individual patients.

Neutron spectrometry--an essential tool for diagnosing implosions at the National Ignition Facility (invited).

DT neutron yield (Y(n)), ion temperature (T(i)), and down-scatter ratio (dsr) determined from measured neutron spectra are essential metrics for diagnosing the performance of inertial confinement fusion (ICF) implosions at the National Ignition Facility (NIF). A suite of neutron-time-of-flight (nTOF) spectrometers and a magnetic recoil spectrometer (MRS) have been implemented in different locations around the NIF target chamber, providing good implosion coverage and the complementarity required for reliable measurements of Y(n), T(i), and dsr. From the measured dsr value, an areal density (ρR) is determined through the relationship ρR(tot) (g∕cm(2)) = (20.4 ± 0.6) × dsr(10-12 MeV). The proportionality constant is determined considering implosion geometry, neutron attenuation, and energy range used for the dsr measurement. To ensure high accuracy in the measurements, a series of commissioning experiments using exploding pushers have been used for in situ calibration of the as-built spectrometers, which are now performing to the required accuracy. Recent data obtained with the MRS and nTOFs indicate that the implosion performance of cryogenically layered DT implosions, characterized by the experimental ignition threshold factor (ITFx), which is a function of dsr (or fuel ρR) and Y(n), has improved almost two orders of magnitude since the first shot in September, 2010.

Algorithm for precision subsample timing between Gaussian-like pulses.

Moderately priced oscilloscopes available for the NIF power sensors and target diagnostics have 6 GHz bandwidths at 20-25 Gsamples/s (40 ps sample spacing). Some NIF experiments require cross timing between instruments be determined with accuracy better than 30 ps. A simple analysis algorithm for Gaussian-like pulses such as the 100-ps-wide NIF timing fiducial can achieve single-event cross-timing precision of 1 ps (1/50 of the sample spacing). The midpoint-timing algorithm is presented along with simulations that show why the technique produces good timing results. Optimum pulse width is found to be ∼2.5 times the sample spacing. Experimental measurements demonstrate use of the technique and highlight the conditions needed to obtain optimum timing performance.

National Ignition Facility neutron time-of-flight measurements (invited).

The first 3 of 18 neutron time-of-flight (nTOF) channels have been installed at the National Ignition Facility (NIF). The role of these detectors includes yield, temperature, and bang time measurements. This article focuses on nTOF data analysis and quality of results obtained for the first set of experiments to use all 192 NIF beams. Targets produced up to 2×10(10) 2.45 MeV neutrons for initial testing of the nTOF detectors. Differences in neutron scattering at the OMEGA laser facility where the detectors were calibrated and at NIF result in different response functions at the two facilities. Monte Carlo modeling shows this difference. The nTOF performance on these early experiments indicates that the nTOF system with its full complement of detectors should perform well in future measurements of yield, temperature, and bang time.

The National Ignition Facility neutron time-of-flight system and its initial performance (invited).

The National Ignition Facility (NIF) successfully completed its first inertial confinement fusion (ICF) campaign in 2009. A neutron time-of-flight (nTOF) system was part of the nuclear diagnostics used in this campaign. The nTOF technique has been used for decades on ICF facilities to infer the ion temperature of hot deuterium (D(2)) and deuterium-tritium (DT) plasmas based on the temporal Doppler broadening of the primary neutron peak. Once calibrated for absolute neutron sensitivity, the nTOF detectors can be used to measure the yield with high accuracy. The NIF nTOF system is designed to measure neutron yield and ion temperature over 11 orders of magnitude (from 10(8) to 10(19)), neutron bang time in DT implosions between 10(12) and 10(16), and to infer areal density for DT yields above 10(12). During the 2009 campaign, the three most sensitive neutron time-of-flight detectors were installed and used to measure the primary neutron yield and ion temperature from 25 high-convergence implosions using D(2) fuel. The OMEGA yield calibration of these detectors was successfully transferred to the NIF.

[Radial optic neurotomy in ischemic central retinal vein occlusion]. / Radiäre Optikusneurotomie bei ischämischer Zentralvenenthrombose.

BACKGROUND: Loss of visual acuity due to ischemic retinal vein occlusion (RVO) is still a major problem in ophthalmology. Prognosis is poor and loss of vision is a severe risk. New approaches for treatment like systemic fibrinolysis and surgical procedures have been suggested. PATIENTS AND METHODS: In a clinical trial 8 patients with ischemic CRVO underwent surgical decompression. Strict criteria of inclusion were maintained. Radial optic neurotomy (RON) was performed 0.25-5 months after retinal vein occlusion. Follow up-time was 3 months. Visual acuity and incidence of typical complications after RVO were the main points of interest in our scientific evaluation. RESULTS: Visual acuity improved significantly after the surgical procedure. For ischemic CRVO EDTRS charts increased from logMAR 1.0 (decimal 0.17) to 0.68 (0.30) at 3 months after surgery. Surgical or early complications did not occur within 3 months. The recovery of retinal blood flow during fluorescein angiography was investigated in 75 % of the patients. A resolution of non perfusion-areas could be detected in 50 % of the eyes. CONCLUSIONS: For patients with retinal vein occlusion RON seems to be a safe and feasible procedure. The results indicate the potential to improve visual acuity while typical complications due to surgery or vein occlusion did not occur during the first three months.

[Arteriovenous sheathotomy in venous thrombosis]. / Adventitiaspaltung bei Venenastthrombose.

BACKGROUND: The pathogenesis of branch retinal vein occlusion (BRVO) seems to differ from that of central retinal vein occlusion (CRVO). Arteriosclerotic and anatomic aspects suggest that arteriovenous crossing of vessels may play a significant role. New procedures like arteriovenous decompression have been proposed to be a suitable form of treatment. PATIENTS AND METHODS: In a clinical trial, 12 patients with ischemic BRVO underwent surgical decompression. Strict criteria of inclusion were maintained. Arteriovenous sheathotomy (AVS) was performed 0.5 - 6 months after retinal vein occlusion. Follow up-time was 3 months. Visual acuity and incidence of typical complications after RVO were the main aspects of interest in the scientific evaluation. RESULTS: After surgical AVS, visual acuity increased significantly from logMAR 0.74 (decimal 0.18) to 0.56 (0.32) in EDTRS charts. Surgical or early complications did not occur during the 3-month control period. Improvement of retinal blood flow during angiography was demonstrated in 75 % of the patients. In 50 % of the patients all non-perfusion areas had disappeared. CONCLUSIONS: For patients with retinal vein occlusion, AVS seems to be a safe and feasible procedure according to the pathogenesis of branch occlusion. Our results suggest that AVS has the potential to improve visual acuity while typical complications due to surgery or vein occlusion do not occur during the first three months.

Treatment of retinal arterial occlusion with local fibrinolysis using recombinant tissue plasminogen activator.

OBJECTIVE: Retinal arterial occlusion is one of the most dramatic problems faced by ophthalmologists because of its sudden onset and the severe consequences it may have on the visual system. In this study, local intra-arterial fibrinolysis (LIF) using recombinant tissue plasminogen activator (rTPA) as a new technique for the treatment of retinal arterial occlusion was investigated. DESIGN: Retrospective, noncomparative case series. PARTICIPANTS: Strict inclusion and exclusion criteria were used to select patients for treatment. Fifty-three patients with central retinal artery occlusion (n = 46) or branch retinal arterial occlusion (n = 7) were enrolled. INTERVENTION: For a maximum of 3 hours, 10- to 20-mg rTPA per hour in 50-ml sodium chloride was infused transfemorally by catheterization of the ophthalmic artery with a variable stiffness microcatheter. MAIN OUTCOME MEASURES: The best-corrected visual acuity for distance by an 18-line logarithmic table was measured on admission, at 24 hours, and at 3 months after intervention. RESULTS: At 3 months, visual acuity had improved in 35 (66%) of 53 patients. Twenty-five (47.2%) patients showed an improvement of more than 2 lines, and in 10 (18.8%) patients, improvements of 1 to 2 lines were observed. No change in visual acuity occurred in 12 (22.6%) patients, and in 6 (11.3%) patients, the visual acuity deteriorated. The mean occlusion time was 14 hours (range, 3-50 hours). No statistically significant correlation was found between occlusion time and visual outcome (P > 0.22). In two patients, a temporary slight hemiplegia was observed during catheterization, and in one patient, a hypertensive crisis after LIF treatment was observed. CONCLUSIONS: The high success rate of LIF using rTPA in patients suffering from retinal arterial occlusion is supposedly due to a causal effect of rTPA on primary platelet-fibrin emboli and secondary thrombi. The local fibrinolytic therapy with rTPA involves little risk for patients selected by strict inclusion and exclusion criteria. It may be used for the treatment of retinal arterial occlusion even later than 8 hours after the acute visual loss. However, a successful outcome of the therapy depends on the prompt referral by well-informed ophthalmologists; a speedy execution of all internal, neurologic, and ophthalmologic diagnostic measures; and a prompt therapy.

[Biomechanical study of corneal stability after radial keratotomy]. / Biomechanische Untersuchung der Hornhautstabilität nach radiärer Keratotomie.

BACKGROUND: Radial Keratotomy is one of the most frequent refractive surgical procedures performed worldwide. It results in considerable alterations of the corneal structure. What does this mean for the mechanical properties of the cornea? METHODS: Intraocular pressure was increased via a 180 degrees tilt. Before and during this procedure, corneal topography was measured by photokeratoscopy. We examined 36 patients after RK and 25 controls who had not undergone any surgical procedure. RESULTS: The corneal center flattened by 0.523 +/- 0.054 dpt (p < 0.01) in the RK patients and by 0.187 +/- 0.045 dpt (p < 0.05) in the control group. CONCLUSION: Even several years after radial keratotomy, corneal stability is still decreased.

[Corneal reinnervation after keratomileusis in situ and keratomileusis myopica--a comparison]. / Hornhautreinnervation nach Keratomileusis in situ und Keratomileusis myopica--Ein Vergleich.

BACKGROUND: In a retrospective study corneal sensitivity after keratomileusis in situ and keratomileusis myopica was measured. METHOD: 5 points on the cornea were measured with the Draeger aesthesiometer at 18 eyes after keratomileusis myopica and at 66 eyes after keratomileusis in situ. RESULTS: The ingrowth of corneal nerve fibres is delayed after keratomileusis myopica compared to keratomileusis in situ because of delayed stromal repopulation of keratocytes and disturbances of stromal metabolism due to toxic effects of the cryopreservation solution.