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Biomedical subjects

R S Baker

Publications and source records attributed to R S Baker.

154 records · Page 9Linked to original sources
PubMedDate not supplied

A convolution algorithm for brachytherapy dose computations in heterogeneous geometries.

Currently-available brachytherapy dose computation algorithms ignore heterogeneities such as tissue-air interfaces, shielded gynecological colpostats, and tissue-composition variations in 125I implants despite dose computation errors as large as 40%. To calculate dose in the presence of tissue and applicator heterogeneities, a computer code has been developed that describes scatter dose as a 3-D spatial integral which convolves primary photon fluence with a dose-spread array. The dose-spread array describes the distribution of dose due to multiple scattering about a single primary interaction site and is precomputed by the Monte Carlo method. To correct for heterogeneities traversed by the primary photons, the dose-spread array is renormalized to reflect the density and composition of the element, and the distance to the point of interest is scaled by the path-length of the intervening medium. Convolution calculations for 125I and 137Cs point sources in the presence of finite phantoms, air voids and high-density shields have been compared to the corresponding Monte Carlo calculations. The convolution code absolute and relative dose rate predictions are shown to agree with Monte Carlo calculations within 3%. Direct evaluation of the 3-D spatial convolution integral using 1-D adaptive integration reveals efficiency gains of 20-50 relative to Monte Carlo photon-transport calculations.

Algorithms↗
PubMedDate not supplied

Middle ear implantable hearing device: ongoing animal and human evaluation.

The first five patients have been permanently implanted with an electromagnetic middle ear implantable hearing device. Hearing tests were performed at the time of operation and at 8 weeks postoperatively with a coil held at the isthmus of the ear canal. All patients reported clear, high fidelity sound, as proven by speech discrimination scores. Improvements were seen in all frequencies, including 4,000 Hz. Improvement in pure tones as tested with an audiometer monitoring sounds amplified by a 3-V sound processor was as high as 50 dB sound pressure level. That which remains to be done is the final design of a compact, wearable sound processor with filtering and signal-processing capabilities to meet the needs of the sensorineural hearing-impaired population.

Animals↗
PubMedDate not supplied

Acquired Brown's syndrome from blunt orbital trauma.

Blunt trauma to the supero-medial orbit resulted in an isolated Brown's syndrome in two patients. One recovered spontaneously in three weeks; the other had a good outcome after surgery two years post trauma. Recognition of the characteristic motility abnormality and forced duction testing will differentiate this condition from other, more common, motility disorders caused by orbital trauma. Spontaneous resolution may occur but in persistent deviations a superior oblique muscle weakening procedure should be considered.

Adult↗
PubMedDate not supplied

Histologic consequences of inferior oblique anastomosis to denervated lateral rectus muscle.

Secondary muscular neurotization has been proposed as a means of restoring contractility to paretic extraocular muscle. We studied this technique by anastomosing healthy inferior oblique muscle to lateral rectus muscle that had been denervated either orbitally or intracranially in 20 dogs. Nerve and muscle fiber growth from the inferior oblique to the lateral rectus was demonstrated but no new neuromuscular junctions were formed. Regeneration of the lesioned sixth nerve occurred frequently and may explain the restoration of function claimed after this procedure.

Animals↗
PubMedDate not supplied

Myoblast transfer therapy in the treatment of ptosis: a preliminary study.

Congenital ptosis with poor levator function is now managed by frontalis suspension techniques. While this procedure is better than those used in the past, serious shortcomings exist. A technique producing more normal lid function would be a beneficial addition to surgical management. Since congenital ptosis is thought to be a focal myopathy, we investigated the potential of myoblast transfer therapy in myopathic levator palpebrae superioris. Satellite cells harvested from temporalis muscle were grown as clones, labeled with Dil, and transplanted into experimentally myopathic levator muscle of the same animal. Within 2 weeks, the injected cells were found to be incorporated into muscle fibers within the levator basal lamina. The control side appeared myopathic with very little muscle regeneration. The presence of Dil labeled muscle fibers in the experimental muscles strongly suggests their origin from the injected cells. Electron microscopy of nearby sections showed these fibers to be maturing striated muscle. We feel that the development of this technique may make autogenous myoblast transfer therapy a useful treatment for congenital ptosis and other focal myopathies.

Animals↗
PubMedDate not supplied

Dichoptic luminance beat visual evoked potentials in the assessment of binocularity in children.

Direct evidence of a distinct cortical binocular pathway has been provided by the production of nonlinear (difference) beats from dichoptic luminance stimulation in stereonormal adults and the absence or diminution of these beats in stereoblind subjects. We have investigated a clinically useful application of this technique in a pediatric population with potentially abnormal binocular vision. We recorded dichoptic luminance beat visual evoked potentials (VEPs) from 20 children (ages 7 months to 8 years) with abnormal binocular ability secondary to strabismus and/or amblyopia and compared this to a control group of 20 children with normal binocularity. Stereoblind children generated significantly lower dichoptic signal-to-noise ratios than stereonormal children (P < .001). Responses to monoptic multifrequency flicker were not significantly different between the two groups (P = .936). This dichoptic VEP can be performed quickly and easily on young children and gives a quantitative assessment of cortical binocularity that may not be determinable by standard clinical methods. This technique may also prove useful for the preoperative gradation of binocular potential and prediction of postoperative binocular fusion.

Amblyopia↗
PubMedDate not supplied

Large recession nystagmus surgery in albinos: effect on acuity.

PURPOSE: To evaluate the effect of large muscle recessions on visual acuity in albinos with nystagmus. METHODS: This is a retrospective, unmasked, chart review of 12 patients with ocular or oculocutaneous albinism demonstrating nystagmus who underwent four horizontal rectus muscle retroequatorial recessions. Pre- and postoperative visual acuity were measured. RESULTS: Best corrected visual acuity improved postoperatively by two or more Snellen lines in 7 of 12 patients (58%). Subjective improvement of vision and nystagmus was noted by patients and examiners. Follow up was performed between 3 and 41 months; 25% of patients required reoperation for ocular alignment. CONCLUSION: Large four-muscle horizontal rectus recessions may improve uniocular and binocular visual acuities in albinos with nystagmus as measured by Snellen acuity. Patients may note a qualitative decrease in the severity of their nystagmus.

Albinism, Ocular↗