Search

Age-related maculopathy is the leading cause of vision loss among adults in developed countries, and its prevalence is expected to double over the next decade. The two forms of age-related maculopathy--exudative and nonexudative--are capable of affecting the macula, with potential symptoms including decreased contrast sensitivity, decreased visual acuity, abnormal adaptation to the dark, and central or pericentral scotomas. When a central scotoma impairs the ability of the fovea to distinguish detail, one or more preferred retinal loci (PRL), also known as pseudo-foveas, naturally develop at eccentric retinal locations and become the new points of fixation. The location of the PRL varies from person to person, as does the ability to maintain steady fixation with the PRL and to use it for reading and activities of daily living. Fletcher and Schuchard (1997) reported the results of a study examining patterns of PRL ability and location relative to macular scotomas. They mapped dense scotomas and determined the location of the PRL in 1,339 eyes in 825 persons with a scanning laser ophthalmoscope during a standard low vision rehabilitation evaluation. Fletcher and Schuchard created a scoring system to measure the pursuit ability, fixation stability, and saccadic ability of the PRL. They determined the characteristics of dense scotomas within 2.5 degrees of the PRL or fovea. Of the 1,339 eyes, 84% (1,130) demonstrated an established PRL, ranging in size from 1.0 degrees to 9.0 degrees in diameter, and 82.5% had a central dense scotoma. The PRL had no dense scotomas on any border in 14.8% of the eyes. It had one, two, or three borders with a dense scotoma in 39.7%, 19.0%, and 9.0% of the eyes, respectively. Ring scotomas, bordering the PRL on four borders, were present in 17.4% of the eyes. Of the eyes with one scotomatous border, most had a field defect located either superior (39.0%) or to the right (33.7%). It was less common for the eyes to have field defects to the left or inferior, with 19.9% and 7.5% of the eyes, respectively. This report presents a 10-year comparison of these previous findings. It examines whether the PRL characteristics of persons with low vision remained steady over the ensuing decade. Changes in PRL and visual field characteristics among persons with low vision over time could have implications for the approaches of low vision rehabilitation professionals to training in eccentric viewing and other rehabilitation techniques. (Contains 4 figures and 1 table.)

Central scotomas--areas of the nonseeing retina within the central 20 degrees of the visual field--are present in approximately 90% of vision rehabilitation patients. They vary in size and shape and can be small or large, symmetrical or asymmetrical, round or irregularly shaped. Most central scotomas border fixation on one side and can be overcome with magnification and sometimes eccentric viewing. However, these strategies seldom suffice in the case of ring scotomas, which border three or four sides of fixation and have been present in 20%-47.5% of patients with eye conditions that affect central vision. Patients with ring scotomas generally have near-normal acuity, which does not reflect their functional impairment in reading and activities of daily living. They display a characteristic slow-fast-slow reading pattern when performing tests such as the MNREAD acuity chart. They read the largest print slowly, speed up with the middle print sizes as more letters fit within the central island of vision, and then slow again when the print drops below the size they can discern. Most patients with ring scotomas fixate centrally even for targets that are larger than the central island, possibly because reorganization of cortical visual processing only occurs in the absence of foveal vision. This article discusses the three principles that characterize the approach to intervention for patients with foveal-sparing scotomas: (1) education about the scotoma; (2) supporting the use of the limited central vision; and (3) maximizing the potential to use the peripheral retina. (Contains 1 figure.)

Older adults from underrepresented racial and ethnic backgrounds and with low vision have independently been found to be at an increased risk of depression. This retrospective cross-sectional pilot study found that non-White older adults with low vision reported significantly more depressive symptoms than White older adults with low vision (P =.005). When controlling for diagnosis and functional impairment, the relationship was no longer significant. Older adults with low vision from underrepresented racial and ethnic groups may be more at risk for depression than White older adults with low vision, with comorbid visual conditions potentially contributing to this relationship. [ABSTRACT FROM AUTHOR]

A study was conducted to investigate the interaction of a high current electron beam supporting dielectric loaded waveguides as a source of electromagnetic radiation. An integrating current transformer was utilized to monitor drive beam intensity while the duration of the rf pulse generated by a single drive bunch was limited by the length of the dielectric structure. Results showed that high rf power can be produced by dielectric wavefield devices.

Now more than ever, the lack of racial and ethnic diversity must be addressed within the health care system, specifically in occupational therapy. This change starts with the successful completion of educational programs by underrepresented minority (URM) occupational therapy students. To increase diversity in the profession, accrediting bodies should mandate support for students of all backgrounds to be successful in higher education. As addressed in the American Occupational Therapy Association 2020 Code of Ethics, the Vision 2025 statement and its pillars, current knowledge on health disparities and occupational therapy demographic data, and other health professional programs' accreditation standards, there is a need for an addition to, or revision of, the Accreditation Council for Occupational Therapy Education (ACOTE®) standards to support the recruitment and retention of URM occupational therapy students. What This Article Adds: This column provides an evidence-based rationale to address the need for an ACOTE standard requiring documented efforts to support racial and ethnic diversity within occupational therapy education. [ABSTRACT FROM AUTHOR]

The transformer ratio, characterizing the ratio of maximum accelerating field behind the drive bunch to the maximum decelerating field inside the drive bunch, is one of the key parameters for characterizing the performance of collinear wakefield accelerators. In this paper the use of electron drive bunches possessing a particular temporal profile (the double-triangular bunch) is shown to significantly increase the transformer ratio beyond 2, the limit for a symmetric bunch. The double-triangular bunch is generated using the emittance exchange technique [P. Emma et al., Phys. Rev. ST Accel. Beams 9, 100702 (2006)]PRABFM1098-440210.1103/PhysRevSTAB.9.100702. Complete beam simulations have been performed for a collinear wakefield acceleration experiment planned at the Argonne Wakefield Accelerator facility including the effects of the physical emittance increase of the bunch in the wakefield device. A transformer ratio of 6.5 and gradient of 20 MV/m is expected in this proof-of-principle experiment for a 3 nC double-triangular bunch traversing a 10 cm long, 200 GHz quartz based dielectric wakefield accelerator structure.

The wakefields of a relativistic electron beam passing through a waveguide loaded with an active medium with weak resonant dispersion are considered. For the calculations in this paper, the parameters of the medium are those of a solution of fullerene (C_{60}) in a nematic liquid crystal that exhibits activity in the X-band. It is shown that several of the TM accelerating modes can be amplified for the geometries under consideration; structures in which higher order modes are amplified exhibit essential advantages as PASERs. In particular, the amplification of the highest mode occurs in a structure loaded with a rather thick active medium layer that maximizes the energy stored by the active medium.

We report on the experimental demonstration of a novel wakefield acceleration technique where a short electron bunch excites a broadband harmonic frequency spectrum, in a cylindrical dielectric structure, to synthesize an accelerating waveform. The structure is designed to have its TM_{0n} modes nearly equally spaced so that the modes generated by a single short electron bunch constructively interfere in the neighborhood of integral multiples of the fundamental wavelength producing large acceleration gradients. Realization of a harmonic multimode structure requires more stringent design considerations than a single-mode structure, since the permittivity and loss tangent of the material should not change substantially over the bandwidth of the structure. In this experiment, a bunch train of four 5 nC electron bunches, separated by 760 ps (one net wavelength), were passed through a 60 cm long dielectric-lined cylindrical harmonic structure. Use of a train of drive bunches spaced by one wavelength reinforced the accelerating wakefield; observation of the energy loss of each bunch via a magnetic spectrometer served as a diagnostic of the wakefield. The measured energy spectrum of the four beams after passing through the waveguide was found to be in excellent agreement with the predictions of the analytic model. This result demonstrates that a dielectric can be fabricated which can synthesize the required wakefield. We also discuss potential advantages of this harmonic approach over conventional single-mode wakefield accelerators.

We report experimental results on the high peak current electron beam generated at the Argonne Wakefield Accelerator facility for wakefield applications. The facility produces bunch charges in the range 15–100 nC by using a photocathode based RF electron gun. The energy of the beam is 14 MeV. Our measurements show that for bunches of 100 nC, the pulse length can be as short as 30 ps. A detailed systematic study of pulse length versus charge is reported, as well as comparisons with numerical simulations. Dark current beam loading in the high field RF gun is also discussed.

Provides information on a study that focused on directly measuring the damping of wake-field deflection modes in a slow-wave accelerating structure consisting of a dielectric-lined waveguide. Reason for the desire in damping deflection modes; Capability of deflection modes; Configuration that is of specific interest to the Cerenkov wake-field accelerator program; Difficulty in obtaining severe enough damping in either configuration to eliminate the head-tail instability.