Tomographic phase and attenuation extraction for a sample composed of unknown materials using x-ray propagation-based phase-contrast imaging.

Propagation-based phase-contrast x-ray imaging (PB-PCXI) generates image contrast by utilizing sample-imposed phase-shifts. This has proven useful when imaging weakly attenuating samples, as conventional attenuation-based imaging does not always provide adequate contrast. We present a PB-PCXI algorithm capable of extracting the x-ray attenuation  ß and refraction  δ, components of the complex refractive index of distinct materials within an unknown sample. The method involves curve fitting an error-function-based model to a phase-retrieved interface in a PB-PCXI tomographic reconstruction, which is obtained when Paganin-type phase retrieval is applied with incorrect values of δ and ß. The fit parameters can then be used to calculate true δ and ß values for composite materials. This approach requires no a priori sample information, making it broadly applicable. Our PB-PCXI reconstruction is single-distance, requiring only one exposure per tomographic angle, which is important for radiosensitive samples. We apply this approach to a breast-tissue sample, recovering the refraction component  Î´, with 0.6-2.4% accuracy compared with theoretical values.

Accurate measures of changes in regional lung air volumes from chest x-rays of small animals.

Objective. To develop a robust technique for calculating regional volume changes within the lung from x-ray radiograph sequences captured during ventilation, without the use of computed tomography (CT).Approach. This technique is based on the change in transmitted x-ray intensity that occurs for each lung region as air displaces the attenuating lung tissue.Main results. Lung air volumes calculated from x-ray intensity changes showed a strong correlation (R2= 0.98) against the true volumes, measured from high-resolution CT. This correlation enables us to accurately convert projected intensity data into relative changes in lung air volume. We have applied this technique to measure changes in regional lung volumes from x-ray image sequences of mechanically ventilated, recently-deceased newborn rabbits, without the use of CT.Significance. This method is suitable for biomedical research studies,enabling quantitative regional measurement of relative lung air volumes at high temporal resolution, and shows great potential for future clinical application.

The projection approximation and edge contrast for x-ray propagation-based phase contrast imaging of a cylindrical edge.

We examine the projection approximation in the context of propagation-based phase contrast imaging using hard x-rays. Specifically, we consider the case of a cylinder or a rounded edge, as a simple model for the edges of many biological samples. The Argand-plane signature of a propagation-based phase contrast fringe from the edge of a cylinder is studied, and the evolution of this signature with propagation. This, along with experimental images obtained using a synchrotron source, reveals how propagation within the scattering volume is not fully described in the projection approximation's ray-based approach. This means that phase contrast fringes are underestimated by the projection approximation at a short object-to-detector propagation distance, namely a distance comparable to the free-space propagation within the volume. This failure of the projection approximation may become non-negligible in the detailed study of small anatomical features deep within a large body. Nevertheless, the projection approximation matches the exact solution for a larger propagation distance typical of those used in biomedical phase contrast imaging.

Assessment of the use of a diffuser in propagation-based x-ray phase contrast imaging.

A rotating random-phase-screen diffuser is sometimes employed on synchrotron x-ray imaging beamlines to ameliorate field-of-view inhomogeneities due to electron-beam instabilities and beamline optics phase artifacts. The ideal result is a broader, more uniformly illuminated beam intensity for cleaner coherent x-ray images. The spinning diffuser may be modeled as an ensemble of transversely random thin phase screens, with the resulting set of intensity maps over the detector plane being incoherently averaged over the ensemble. Whilst the coherence width associated with the source is unaffected by the diffuser, the magnitude of the complex degree of second-order coherence may be significantly reduced [K. S. Morgan, S. C. Irvine, Y. Suzuki, K. Uesugi, A. Takeuchi, D. M. Paganin, and K. K. W. Siu, Opt. Commun. 283, 216 (2010)]. Through use of a computational model and experimental data obtained on x-ray beamline BL20XU at SPring-8, Japan, we investigate the effects of such a diffuser on the quality of Fresnel diffraction fringes in propagation-based x-ray phase contrast imaging. We show that careful choice of diffuser characteristics such as thickness and fiber size, together with appropriate placement of the diffuser, can result in the ideal scenario of negligible reduction in fringe contrast whilst the desired diffusing properties are retained.

Dynamic In Vivo Chest X-ray Dark-Field Imaging in Mice.

X-ray grating interferometry is a powerful emerging tool in biomedical imaging, providing access to three complementary image modalities. In addition to the conventional attenuation modality, interferometry provides a phase modality, which visualizes soft tissue structures, and a dark-field modality, which relates to the number and size of sub-resolution scattering objects. A particularly strong dark-field signal originates from the alveoli or air sacs in the lung. Dark-field lung radiographs in animal models have already shown increased sensitivity in diagnosing lung diseases, such as lung cancer or emphysema, compared to conventional X-ray chest radiography. However, to date, X-ray dark-field lung imaging has either averaged information over several breaths or has been captured during a breath hold. In this paper, we demonstrate the first time-resolved dark-field imaging of a breath cycle in a mechanically ventilated mouse, in vivo, which was obtained using a grating interferometer. We achieved a time resolution of 0.1 s, visualizing the changes in the dark-field, phase, and attenuation images during inhalation and exhalation. These measurements show that the dark-field signal depends on the air volume and, hence, the alveolar dimensions of the lung. Conducting this type of scan with animal disease models would help to locate the optimum breath point for single-image diagnostic dark-field imaging and could indicate if the changes in the dark-field signal during breath provide a diagnostically useful complementary measure.

Phase contrast x-ray velocimetry of small animal lungs: optimising imaging rates.

Chronic lung diseases affect a vast portion of the world's population. One of the key difficulties in accurately diagnosing and treating chronic lung disease is our inability to measure dynamic motion of the lungs in vivo. Phase contrast x-ray imaging (PCXI) allows us to image the lungs in high resolution by exploiting the difference in refractive indices between tissue and air. Combining PCXI with x-ray velocimetry (XV) allows us to track the local motion of the lungs, improving our ability to locate small regions of disease under natural ventilation conditions. Via simulation, we investigate the optimal imaging speed and sequence to capture lung motion in vivo in small animals using XV on both synchrotron and laboratory x-ray sources, balancing the noise inherent in a short exposure with motion blur that results from a long exposure.

Cataract induction in rabbits with the Nd-YAG laser.

The study of occlusion amblyopia and its therapy has involved animal models of stimulus deprivation achieved by various means, none of which closely simulates human congenital cataract. The authors used the Nd-YAG laser as a means of inducing cataracts in rabbit eyes. Twenty rabbit eyes were treated at various frequency and power settings. High energy YAG laser pulses of 10 mJ produced cataracts that at first resembled large bubbles. Over several days, these bubbles coalesced into smaller opacities. With multiple treatments (usually 150-200 pulses), the authors could create a cataract of a specific size and position. Six months later, 100% of these laser-treated rabbit eyes showed persistent cataracts that resembled human congenital cataracts. It appears that the YAG laser can provide a reproducible and reliable method of inducing specific types of cataracts in animal eyes without damaging other ocular structures.

Clinical evaluation of a commercial photorefractor.

The Visiscreen 100 is a system designed to detect ocular anomalies photographically. Photorefractions obtained in 63 consecutive 3-month-old to 8-year-old patients were compared in a masked fashion with results obtained by conventional techniques, including cycloplegic refraction. In 87% of the 57 patients with analyzable photographs, photorefractive and examination results correlated for the detection of constant tropias. The photorefractor detected all ocular media opacities. In 77% of the cases, its refraction was within 2.5 diopters or 33% of the cycloplegic spherical equivalent. In detecting abnormal ocular status, the Visiscreen had a 91% sensitivity and a 74% specificity. The overall agreement rate in distinguishing abnormal from normal status was 84%. Most errors were related to inaccuracies in fixation and positioning, rather than the optical principles utilized. This device offers a rapid technique for the detection of factors predisposing preverbal children to amblyopia.

Interface opacities in epikeratophakia.

Although small, peripheral, cystic, or putty-gray infiltrates were observed along the suture tracks in 20 of 100 pediatric patients after epikeratophakia at Louisiana State University Medical Center, New Orleans, they progressed to clinically significant opacities in only three. In one, the interface material was curetted and the lenticule was reattached. The removed material was identified histologically as epithelial cells in varying states of degeneration. In a second case, the graft became hazy and the opacity involved most of the interface. The lenticule was removed, and epikeratophakia was again performed. On the posterior surface of the removed lenticule was a multilayered epithelium that had infiltrated into adjacent stromal lamellae. In the third case, a clear cyst resolved without intervention over a five-year period. These cases illustrate the importance of meticulous removal of all surface epithelium and the necessity for copious irrigation of epithelial debris intraoperatively. Epithelial ingrowth should not be confused with bacterial infection. Such opacities can be treated by curettage or aspiration of the invading material or removal of the epikeratophakia lenticule if spontaneous resolution does not occur.

Keratometric and refractive results of pediatric epikeratophakia.

Keratometric and refractive results of pediatric epikeratophakia showed that patients under 1 year of age had steeper corneas preoperatively and required more correction, as estimated by the Sanders-Retzlaff-Kraff regression formula. The average spherical equivalent of refractive error six months postoperatively was +6.92 +/- 4.67 diopters in patients under 1 year, and -0.72 +/- 4.22 D in patients over 1 year. Three of 14 younger patients and 35 of 54 older patients were within 3 D of emmetropia. Since March 1982, significant undercorrection has occurred only in patients 6 months old or younger. Younger children achieved an average of 46% of the predicted change in corneal curvature, while older children achieved 85%. Also, the Sanders-Retzlaff-Kraff formula may be inaccurate in estimating powers for younger children. Therefore, we recommend at this time that epikeratophakia be used as a secondary procedure in neonates with congenital cataracts.

Long-term follow-up of refractive and keratometric results of pediatric epikeratophakia.

To examine the long-term stability of epikeratophakia and its effect on the development of young eyes, the change in corneal curvature and refractive error over time were reviewed in pediatric patients who underwent epikeratophakia for optical correction of aphakia. Over an average follow-up time of 28.0 months, corneal curvature flattened by 1.15 diopters, or 0.53 D/yr. The average change in refractive error was -2.81 D over 23.6 months, or 1.49 D/yr. Patients younger than 1 year of age at surgery became significantly more myopic over the period of follow-up (4.75 D) than patients older than 1 year of age at surgery (2.24 D). These changes are consistent with the corneal flattening and increase in axial length expected from normal growth. Aphakic infants corrected to emmetropia with epikeratophakia will likely require correction of myopia in later childhood.

Polyvinyl alcohol as a protective coating on intraocular lenses.

Sixty-eight intraocular lenses were implanted in cats to compare the efficacy of a new polyvinyl alcohol coating in protecting the corneal endothelium from lens-cell contact damage. The mean endothelial density after implantation of the coated lens was significantly higher than the cell density in those eyes receiving the uncoated lens. We saw no clinical signs of unusual inflammation; whole-eye histopathologic study disclosed no signs of inflammation or toxicity related to the coating. Ten coated lenses were placed in balanced salt solution or perfused with balanced salt solution at 0.5 mL/min. The coating remained intact for 60 minutes, affording corneal endothelial protection for this period. In a rabbit anterior chamber toxicity study, using balanced salt solution as a control, no differencs in reaction in the anterior chamber or intraocular pressure were noted.

Preliminary visual results of pediatric epikeratophakia.

Although cataract surgery is feasible in children with unilateral cataracts, visual rehabilitation depends on optical correction and conscientiously maintained amblyopia therapy. Epikeratophakia for the correction of aphakia was performed in 47 children (50 grafts). Postoperative keratometry readings showed an average increase of 12.68 diopters in early patients. With new tissue-handling techniques, the last eight patients showed an average increase of +16.80 D, with -0.64-D overrefraction for emmetropia. Visual results indicate that epikeratophakia is an effective primary procedure for patients with unilateral traumatic cataracts and that it is superior to leaving vision uncorrected in children who have had unilateral congenital cataracts removed and are contact-lens intolerant. Further studies with younger patients will be necessary to define its role in neonates; however, individual results demonstrate that good vision is obtainable with epikeratophakia.

Safety and efficacy of timolol in pediatric glaucoma.

We reviewed the effect of timolol in 89 eyes (50 patients) with various types of pediatric glaucoma. Systemic side effects were observed in two patients (4%). Intraocular pressure (IOP) effect of timolol could only be observed in 18 eyes of 11 patients. The average decrease of IOP was 30.7% at the last follow-up.

Treatment of congenital glaucoma.

We reviewed 37 consecutive patients who had undergone at least one goniotomy, filtering surgery, and patching for amblyopia because of congenital glaucoma between 1969 and 1979. Of 32 eyes treated for uncomplicated congenital glaucoma, 25 (78%) achieved satisfactory control of intraocular pressure. Seven of the 12 eyes (58%) for which Snellen visual acuities could be obtained had visual acuities of 6/15 (20/50) or better. Of eight eyes treated for complicated congenital glaucoma, four (50%) achieved satisfactory control of intraocular pressure, but the visual acuities of these patients could not be tested.

The nationwide study of epikeratophakia for aphakia in adults.

In the nationwide study of epikeratophakia, 154 ophthalmic surgeons who had attended a training course performed 519 procedures for the correction of aphakia in adults: 310 of the eyes had 30 or more days of follow-up after suture removal. Of 229 eyes, 172 (75%) were within 3 diopters of emmetropia after surgery. Of 259 eyes, 245 (95%) demonstrated improved uncorrected visual acuity; 138 (53%) improved by four or more Snellen lines. Of 265 eyes, 209 (78%) achieved within two lines or improved their best corrected visual acuity. Of the 119 patients who achieved or improved their preoperative best corrected visual acuity, 110 (92%) were within two Snellen lines or better by 30 to 60 days after suture removal. Of the 127 patients with more than three months of follow-up after suture removal, 124 (98%) of those between 18 and 70 years of age but only 13 of 23 (54%) of those between 81 and 87 years of age achieved within two lines or better of their best corrected visual acuity. Corneal astigmatism measured by keratometry changed from a preoperative mean (+/- S.D.) of 2.1 +/- 1.8 diopters to a postoperative mean of 2.7 +/- 2.6 diopters. Of the 519 tissue lenses, 22 (4%) were removed, and one third of these patients underwent a second, successful epikeratophakia procedure.

The nationwide study of epikeratophakia for aphakia in children.

In the nationwide study of epikeratophakia, 97 surgeons performed a total of 335 procedures in 314 eyes for the correction of aphakia in children under the age of 8 years 1 month. Fifteen children underwent bilateral surgery. Thirty-six tissue lenses were removed and 21 of these eyes underwent a second epikeratophakia procedure. Overall, the success rate for procedures was 89%, and with repeated surgery it was 95% for eyes. Seventy-three percent of the patients were within 3 diopters of emmetropia after surgery. Visual acuity results in patients able to provide verbal responses to the illiterate E, Allen card, or Snellen line chart testing showed improvement in most cases. The safety of epikeratophakia makes it a desirable option for the correction of aphakia in children who are spectacle or contact-lens intolerant, and the permanence of the correction eliminates the problem of optical noncompliance.

The nationwide study of epikeratophakia for myopia.

In the nationwide study of epikeratophakia, 116 ophthalmic surgeons performed 352 procedures for the correction of myopia; 256 of the eyes had 30 or more days of follow-up after suture removal. Of 204 eyes, 153 (75%) were within 30% of emmetropia after surgery. Of 208 eyes, 202 (97%) had postoperative best corrected visual acuity within two Snellen lines or better of their preoperative visual acuity. All but one patient improved uncorrected visual acuity. Of the 120 patients who equaled or improved their preoperative best corrected visual acuity, 116 (97%) were within two Snellen lines of their preoperative measurement between 30 and 60 days after suture removal. Corneal astigmatism changed from a preoperative mean (+/- S.D.) of 1.4 +/- 0.8 diopters to a postoperative mean of 2.6 +/- 2.1 diopters. Of 352 tissue lenses, 36 (10%) were removed, largely as a result of inaccurate power, decay, or failure to reepithelialize, and 17 of these eyes underwent a second successful epikeratophakia procedure.

Off-axis photorefractive eye screening in children.

PURPOSE: To determine the utility of mass photoscreening for detecting visual anomalies in young, school-age children. SETTING: Seventy elementary schools in Alabama, Florida, Georgia, North Carolina, and Virginia, USA. METHODS: An off-axis photorefractive technique was used to screen 14,591 children in kindergarten, first, and fourth grades in 70 schools in 5 southeastern states in the United States. RESULTS: A total of 14,075 (96.5%) of the screenings yielded analyzable photographs. Overall, 647 (4.6%) children had significant ocular findings; significant was defined as hyperopia 2.50 diopters (D) or more, myopia 1.00 D or more, anisometropia 1.00 D or more, astigmatism 2.00 D or more, media opacity 1.0 mm or more in diameter, or 5 degrees or more or 10.00 prism diopters or more of ocular misalignment. Another 950 (6.7%) had possibly significant findings, defined as the sign being present but in an amount less than that considered significant. The most common refractive error was myopia (4.5%), followed by anisometropia (3.1%), hyperopia (2.9%), and astigmatism (1.0%). Alignment problems were found in 1.7% and media opacities in 0.1%. Among the 1.0% with other ocular anomalies were cases of anisocoria greater than 2.0 mm or irregular pupils, ptosis greater than 2.0 mm, and subconjunctival hemorrhages. In all, 202 children had significant anomalies thought to predispose to amblyopia, including 196 with significant anisometropia and 6 with significant lens opacities. At a screening cost of $5 per child for the whole study, the cost of identifying these children with potentially blinding eye disease was $361 each. CONCLUSION: Mass photoscreening may be used to detect amblyogenic conditions in children early enough to prevent blindness with timely and appropriate treatment.

Update on epikeratophakia in children.

Epikeratophakia continues to be an extremely attractive option for younger children with unilateral aphakia who are noncompliant users of contact lenses but who are young enough to benefit from amblyopia therapy. The epikeratophakia procedure is much safer than IOL implantation. The epikeratophakia tissue lens is especially useful for children with traumatic aphakia and corneal lacerations because the lens can strengthen and smooth the cornea as well as correct the aphakia. This allows much quicker rehabilitation than could be accomplished with contact lenses. The epikeratophakia procedure may be combined with a cataract extraction and should be in those children with acquired cataracts who demonstrate contact lens noncompliance in an office trial of contact lens insertion before operation. Epikeratophakia should be used with caution in neonates and young infants because of the rapid growth of the eye. Extended-wear contact lenses are a safer option for these children, and epikeratophakia can be performed as a secondary procedure if and when problems with contact lens compliance arise. Surface ocular problems such as uncontrolled dry eyes or severe blepharitis will continue to be incompatible with the survival of epikeratophakia tissue lenses. Children who are treated with high doses of radiation for orbital tumors such as rhabdomyosarcomas invariably develop radiation cataracts, which can occur before the onset of radiation keratitis. These children do not do well with epikeratophakia tissue lenses. Likewise, children with severe metabolic disturbances who are not healthy or gaining weight have a diminished chance of graft healing, as do children with poor vision in whom oculodigital autostimulation produces persistent epithelial defects, which prevent survival of the tissue lens.(ABSTRACT TRUNCATED AT 250 WORDS)