Eye Phantom Research Articles

Dynamic eye phantom for retinal oximetry measurements.

Measurements of oxygen saturation and flow in the retina can yield information about eye health and the onset of eye pathologies such as diabetic retinopathy. Recently, we developed a multiaperture camera that uses the division of the retinal image into several wavelength-sensitive subimages to compute retinal oxygen saturation. The calibration of such instruments is particularly difficult due to the layered structure of the eye and the lack of alternative measurement techniques. For this purpose, we realize an in vitro model of the human eye composed of a lens, the retina vessel, and three layers: the choroid, the retinal pigmented epithelium, and the sclera. The retinal vessel is modeled with a microtube connected to a micropump and a hemoglobin reservoir in a closed circulatory system. Hemoglobin oxygenation in the vessel could be altered using a reversible fuel cell. The sclera is represented by a Spectralon slab. The optical properties of the other layers are mimicked using titanium dioxide as a scatterer, ink as an absorber, and epoxy as a supporting structure. The optical thickness of each layer of the eye phantom is matched to each respective eye layer.

A Comparison of Two and Three Dimensional Dipole Antennas for an Implantable Retinal Prosthesis

The feasibility is investigated using three dimensional folded dipole antennas as a data-telemetry implantable receiving antenna in a dual-unit retinal prosthesis to restore partial vision to the blind. Three dimensional designs are explored in an effort to enhance certain antenna characteristics such as bandwidth and maximum gain while reducing the planar footprint size in comparison to its two dimensional equivalent. The current vector alignment between the three dimensional layers are examined through folding and rotating the dipole arms with respect to each other to fully optimize the antenna's characteristics. The performance of the 2D and 3D antennas were compared in simulations and further examined by fabricating and characterizing the performance in a transmit/ receive system in air and inside eye phantoms. Results show that three-dimensional antennas can provide larger bandwidth while being physically smaller than the correspondent two-dimensional ones, thus providing larger channel capacity that could lead to a system with an increased number of stimulating electrodes.

Thermoluminescence dosimetry using TL-readers equipped with CCD cameras

A prototype laboratory-type 2-D TL reader equipped with a sensitive 640 × 480 pixel charge coupled device (CCD) camera and a heater planchet of 72 mm diameter was developed at the Institute of Nuclear Physics (IFJ PAN) in Krakow, Poland. ETFE (tetrafluoroethylene and ethylene copolymer) TL foils of sizes up to 200 × 200 mm 2 containing LiF:Mg,Cu,P (MCP-N) powder were manufactured. A clinical version of the 2-D TL reader, incorporating a 1024 × 1024 CCD camera and 200 × 200 mm 2 heater, is also being developed. The laboratory 2-D TL reader and 40 × 40 mm 2 ETFE foils were used to measure 2-D dose distributions from a well-collimated 6 MV radiotherapy beam, around small brachytherapy sources ( 106Ru, 226Ra, X-ray needle), for 62 MeV proton beam dosimetry in an eye phantom, to obtain TL images of a raster-scanned 12C radiotherapy beam and in X-ray microbeam dosimetry. This TL reader was also used to distinguish between static and dynamic exposures of personal dosemeters exposed to X-rays and for Quality Control of TLD detectors and of chemical vapor deposition (CVD) diamond wafers. While the uniformity of our presently manufactured TL foils remains to be further improved, the novel CCD-reader and ETFE foil, the developed 2-D TL system is a promising tool for clinical and personal dosimetry.

2-D dosimetry of a proton radiotherapy beam using large-area LiF:Mg,Cu,P TL detectors

A two-dimensional (2-D) thermoluminescence (TL) dosimetry system, consisting of LiF:Mg,Cu,P (MCP-N)-based TL foils and a TLD reader equipped with a CCD camera, was developed at the Institute of Nuclear Physics (IFJ PAN) in Krakow, Poland. We applied this system to verify 2-D dose distributions in the 62 MeV proton ocular radiotherapy beam at INFN, Catania. TL foils placed inside a specially designed PMMA eye phantom were irradiated and read out to determine 40 × 40 mm 2 planar ( X – Y and X – Z ) dose distributions for doses up to 16 Gy. Dose and energy response for LiF:Mg,Cu,P were calculated using the one-hit detector microdosimetric model and applied to correct the detector response. TLD-measured and MCNPX-calculated distal ranges agreed to within 0.3 mm, while TLD-measured transverse beam cross-sections were about 0.8 mm wider than those calculated using the MCNPX Monte Carlo code.

Two-dimensional dosimetry of radiotherapeutical proton beams using thermoluminescence foils

In modern radiation therapy such as intensity modulated radiation therapy or proton therapy, one is able to cover the target volume with improved dose conformation and to spare surrounding tissue with help of modern measurement techniques. Novel thermoluminescence dosimetry (TLD) foils, developed from the hot-pressed mixture of LiF:Mg,Cu,P (MCP TL) powder and ethylene-tetrafluoroethylene (ETFE) copolymer, have been applied for 2-D dosimetry of radiotherapeutical proton beams at INFN Catania and IFJ Krakow. A TLD reader with 70 mm heating plate and CCD camera was used to read the 2-D emission pattern of irradiated foils. The absorbed dose profiles were evaluated, taking into account correction factors specific for TLD such as dose and energy response. TLD foils were applied for measuring of dose distributions within an eye phantom and compared with predictions obtained from the MCNPX code and Eclipse Ocular Proton Planning (Varian Medical Systems) clinical radiotherapy planning system. We demonstrate the possibility of measuring 2-D dose distributions with point resolution of about 0.5 x 0.5 mm(2).

Ultrasound Quantitative Evaluation of Human Eye Cataract

A technique to improve an eye cataract early detection and quantitative evaluation of maturity using ultrasound was investigated. A broadband coherent signal, backscattered from an eye lens tissue, was digitized, recorded and processed. A new parameter - lens quality was proposed for the human eye cataract quantitative evaluation. Lens quality reflects two phenomena of ultrasound interaction with lens tissue - attenuation and scattering. Digital technique for echo-signal energy and time frequency analysis was applied, ultrasound waves scattering strength and spectral slope was calculated. Experimental statistical investigations performed with signals divided into five groups - mature cataract, immature form of cataract, incipience cataract phase, healthy lenses and human eye phantom. Investigations have showed that value of specific quality in the test groups vary in the wide range from 1 to 60. This feature allows theoretically differentiate eye lenses cataract in different classes with defined boundaries. Presented results show that we with high reliability can differentiate lenses into three groups: healthy lenses (Q L>50), lenses with incipient or immature cataract (Q L=2-20) and lenses with mature cataract (Q L<1). The investigated method can be used for an eye lens classification and for early cataract detection This technique was used at the Department of Ophthalmology, Institute for Biomedical Research, Kaunas University of Medicine.

Novel low-kVp beamlet system for choroidal melanoma

BackgroundTreatment of choroidal melanoma with radiation often involves placement of customized brachytherapy eye-plaques. However, the dosimetric properties inherent in source-based radiotherapy preclude facile dose optimization to critical ocular structures. Consequently, we have constructed a novel system for utilizing small beam low-energy radiation delivery, the Beamlet Low-kVp X-ray, or "BLOKX" system. This technique relies on an isocentric rotational approach to deliver dose to target volumes within the eye, while potentially sparing normal structures.MethodsMonte Carlo N-Particle (MCNP) transport code version 5.0(14) was used to simulate photon interaction with normal and tumor tissues within modeled right eye phantoms. Five modeled dome-shaped tumors with a diameter and apical height of 8 mm and 6 mm, respectively, were simulated distinct positions with respect to the macula iteratively. A single fixed 9 × 9 mm2 beamlet, and a comparison COMS protocol plaque containing eight I-125 seeds (apparent activity of 8 mCi) placed on the scleral surface of the eye adjacent to the tumor, were utilized to determine dosimetric parameters at tumor and adjacent tissues. After MCNP simulation, comparison of dose distribution at each of the 5 tumor positions for each modality (BLOKX vs. eye-plaque) was performed.ResultsTumor-base doses ranged from 87.1–102.8 Gy for the BLOKX procedure, and from 335.3–338.6 Gy for the eye-plaque procedure. A reduction of dose of at least 69% to tumor base was noted when using the BLOKX. The BLOKX technique showed a significant reduction of dose, 89.8%, to the macula compared to the episcleral plaque. A minimum 71.0 % decrease in dose to the optic nerve occurred when the BLOKX was used.ConclusionThe BLOKX technique allows more favorable dose distribution in comparison to standard COMS brachytherapy, as simulated using a Monte Carlo iterative mathematical modeling. Future series to determine clinical utility of such an approach are warranted.

MR anatomy and small lesions of the eye: improved delineation with a special surface coil.

To develop an improved investigation protocol for MRI studies of intraocular lesions, imaging with a small surface coil (diameter 6 cm) was compared with a standard surface coil (diameter 11 cm). Both coils were assessed initially on an eye phantom and then by studying 22 patients with uveal melanoma and similar lesions of the eye. The influence of bandwidth and field or view (FOV) were systematically studied and evaluated quantitatively. A smaller bandwidth improved image quality independent of surface coil size. The subsequent secondary increase in chemical shift artefact was acceptable. Smaller FOVs (60-80 mm) necessitated the use of a smaller surface coil. A smaller bandwidth also proved to be advantageous with the use of the smaller surface coil. In conclusion, a smaller-diameter surface coil improves MR imaging of ocular lesions. Pulse sequences with a small bandwidth maintain an acceptable signal-to-noise ratio when the FOV is reduced.

Dosimetric evaluation of lead and tungsten eye shields in electron beam treatment

Purpose : The purpose of this study is to report that commercially available eye shields (designed for orthovoltage x-rays) are inadequate to protect the ocular structures from penetrating electrons for electron beam energies equal to or greater than 6 MeV. Therefore, a prototype medium size tungsten eye shield was designed and fabricated. The advantages of the tungsten eye shield over lead are discussed. Methods and Materials : Electron beams (6–9 MeV) are often used to irradiate eyelid tumors to curative doses. Eye shields can be placed under the eyelids to protect the globe. Film and thermoluminescent dosimeters (TLDs) were used within a specially constructed polystyrene eye phantom to determine the effectiveness of various commercially available internal eye shields (designed for orthovoltage x-rays). The same procedures were used to evaluate a prototype medium size tungsten eye shield (2.8 mm thick), which was designed and fabricated for protection of the globe from penetrating electrons for electron beam energy equal to 9 MeV. A mini-TLD was used to measure the dose enhancement due to electrons backscattered off the tungsten eye shield, both with or without a dental acrylic coating that is required to reduce discomfort, permit sterilization of the shield, and reduce the dose contribution from backscattered electrons. Results : Transmission of a 6 MeV electron beam through a 1.7 mm thick lead eye shield was found to be 50% on the surface (cornea) of the phantom and 27% at a depth of 6 mm (lens). The thickness of lead required to stop 6–9 MeV electron beams is impractical. In place of lead, a prototype medium size tungsten eye shield was made. For 6 to 9 MeV electrons, the doses measured on the surface (cornea) and at 6 mm (lens) and 21 mm (retina) depths were all less than 5 % of the maximum dose of the open field (4 × 4 cm). Electrons backscattered off a tungsten eye shield without acrylic coating increased the lid dose from 85 to 123 % at 6 MeV and 87 to 119 % at 9 MeV. For the tungsten eye shield coated with 2∼3 m of dental acrylic, the lid dose was increased from 85 to 98.5% at 6 MeV and 86 to 106% at 9 MeV. Conclusion : Commercially available eye shields were evaluated and found to be clearly inadequate to protect the ocular structures for electron beam energies equal to or greater than 6 MeV. A tungsten eye shield has been found to provide adequate protection for electrons up to 9 MeV. The increase in lid dose due to electrons backscattered off the tungsten eye shield should be considered in the dose prescription. A minimum thickness of 2 mm dental acrylic on the beam entrance surface of the tungsten eye shield was found to reduce the backscattered electron effect to acceptable levels.

125I eye plaque dose distribution including penumbra characteristics.

The two main purposes of this work are (1) to determine the penumbra characteristics for 125I eye plaque and the relative influence of the plaque and eye-air interface on the dose distribution, and (2) to initiate development of a treatment planning algorithm for clinical dose calculations. Dose was measured in a newly designed solid water eye phantom for an 125I (6711) seed at the center of a 20 mm COMS eye plaque using thermoluminescent dosimeter (TLD) "cubes" and "minichips" inside and outside the eye, in the longitudinal and transverse central planes. TLD cubes were used in most locations, except for short distances from the seed and in the penumbra region. In the presence of both the plaque and the eye-air interface, the dose along the central axis was found to be reduced by 10% at 1 cm and up to 20% at 2.5 cm, relative to the bulk homogeneous phantom case. In addition, the overall dose reduction was greater for larger off-axis coordinates at a given depth. The penumbra characteristics due to the lip collimation were quantified, particularly the dependence of penumbra center and width on depth. Only small differences were observed between the profiles in the transverse and longitudinal planes. In the bulk geometry (without the eye-air interface), the dose reduction due to the presence of the plaque alone was found to be 7% at a depth of 2.5 cm. The additional reduction of 13% observed, with the presence of eye-air interface (20% combined), can be attributed to the lack of backscattering from the air in front of the eye. The dose-reduction effect due to the anterior air interface alone became unnoticeable at a depth of 1.1 cm (1.5 cm from the eye-air interface). An analytic fit to measured data was developed for clinical dose calculations for a centrally loaded seed. The central axis values of the dose rates multiplied by distance squared, Dr2, were fitted with a double exponential function of depth. The off-axis profile of Dr2, at a given depth, was parametrized by a modified Fermi-Dirac function to model both the penumbra characteristics due the plaque lip collimation and the effect of oblique filtration by silastic.

Anomalous reflectance of laser-induced retinal lesions

We investigated anomalies in the reflectance change of retinal lesions resulting from laser photocoagulation. These anomalies deviate from the predicted pattern and could cause misleading postoperative assessment of lesion depth and problems with proposed automated lesion control algorithms. We found that for certain irradiation parameters which produce super-threshold lesions, lesions exhibiting annular reflectance patterns are induced in an albumen eye phantom. The correlation between central reflectance and actual lesion depth thus becomes invalid for these anomalous lesions. Evidence of anomalous lesions in vivo was also found. Assumptions regarding retinal lesion formation must consider annular reflectance patterns. These patterns are produced by vaporization in the albumen eye phantom; vaporization may also play a role in anomalous retinal lesions along with the well known effect of minor retinal hemorrhage.

Radiation effects on the optic nerve observed after brachytherapy of choroidal melanomas with 106Ru/106Rh plaques

From 1966 to 1990 a total of 93 juxtapapillary choroidal melanomas were treated using 106Ru/106Rh plaques with a notch for the optic nerve. The choroidal melanoma was controlled after brachytherapy in 79 cases (85%). Fourteen eyes (15%) had to be enucleated because of tumor regrowth. Eye and optic nerve phantoms were fabricated, loaded with small-volume thermoluminescent dosimeters, treated with active plaques, and the radiation dose determined at the optic disc and along the optic nerve. The median dose within the anterior optic nerve was 51.2 Gy (range 10.3-60.5 Gy). The probability of developing complete radiation optic neuropathy (RON) was 23% and 53% at 5 and 10 years, respectively. The probability of developing partial RON was 66% at 5 years and 82% at 10 years. The probability of retaining visual acuity better than 0.5 was 38% at 5 years and 26% at 10 years. No dose-response relationship could be established from the ophthalmological, morphological and functional findings. Eyes following plaque irradiation with 50 Gy or more in the center of the optic nerve experienced significant radiation optic neuropathy, other eyes did not.

Dosimetry for 125I seed (model 6711) in eye plaques.

The effect of eye plaque materials (gold backing and silastic seed-carrier insert) on the dose distribution around a single 125I seed has been measured, using cubic lithium fluoride thermoluminescent dosimeters (TLDs) 1 mm on an edge, in a solid water eye phantom embedded in a solid water head phantom. With an 125I seed (model 6711) positioned in the center slot of the silastic insert for a 20-mm plaque of the design used in the collaborative ocular melanoma study (COMS), dose was measured at 2-mm intervals along the plaque central axis (the seed's transverse axis) and at various off-axis points, both with and without the COMS gold backing placed over the insert. Monte Carlo calculations (MORSE code) were performed, as well, for these configurations and closely the same geometry but assuming a large natural water phantom. Additional Monte Carlo calculations treated the case, both for 20- and 12-mm gold plaques, where the silastic insert is replaced by natural water. Relative to previous measurements taken in homogeneous medium of the same material (without the eye plaque), the dose reduction found by both Monte Carlo and TLD methods was greater at points farther from the seed along the central axis and, for a given central-axis depth, at larger off-axis distances. Removal of the gold backing from the plaque did not make measurable difference in the dose reduction results (10% at 1 cm).

Optimal electron-beam treatment planning for retinoblastoma using a new three-dimensional Monte Carlo-based treatment planning system.

Electron-beam treatment planning for retinoblastoma was investigated and an optimal treatment plan was devised for a particular case using a new three-dimensional Monte Carlo-based treatment planning system known to be capable of correctly predicting dose perturbations caused by body surface obliquities and tissue heterogeneities. Computed tomography (CT) data files were used to construct a three-dimensional eye phantom representing the anatomy of a child's orbit. Dose distributions in sagittal, transverse, and coronal planes were predicted with 1-mm resolution. Study of these distributions led to an optimal treatment plan consisting of an anterior-lateral pair, with the anterior field being a 10-MeV, 30-mm-diam circular field, centrally blocked by a 10-mm-diam lucite lens shield and the lateral field being a 16-MeV, 30 x 25-mm D-shaped field. The anterior field delivers a therapeutic dose to the ora serrata, but it underdoses the posterior retinal surface behind the lens shield; the lateral field provides the necessary boost dose to the posterior retinal surface. An equally weighted combination of the two fields produces a dose distribution in which the entire retinal surface receives a therapeutic dose, with less than 10% of that dose being delivered to the lens, brain, and the contralateral orbit.

Dosimetry and physical treatment planning for iodine eye plaque therapy

The dosimetry of eye plaques loaded with iodine-125 seeds (type 6702) was performed by means of computer calculations and measurements with thermoluminescent dosimeters (TLD). Measurements of the depth dose distribution (2–25.5 mm) along the transverse axis of a single seed were performed in water equivalent phantom material. The transverse axis attenuation and geometry factor F(r) was obtained by applying a least squares fit to the measured data. Based on the resulting radial dose function, a computer program was developed which calculates dose distributions within the eye for arbitrary loading and placement of the eye plaque. The computational results were verified by TLD measurements in an eye phantom.

Determination of the Dose Distribution of an Ophthalmic 106Ru Irradiator with TLDs and an Eye Phantom

Abstract Two 106Ru applicators of the type CCB as manufactured by the Zentralinstitut für Isotopen- und Strahlenforschung (Berlin, GDR) were investigated. The homogeneity of the relative dose distribution at the surface of the applicators was determined. For the evaluation of the data extremely small TLD crystals (2x1x1 mm3) had to be manufactured as well as an almost tissue-equivalent acrylic eye phantom. In order to ensure reproducibility of the applicator positioning, the plaque suture eyelets had to be placed on locating pins. 120 measurement points located within the active plaque area of 20 mm diameter were evaluated. The results show that the area to be used in tumour treatment is about 20 to 25% less than expected. One plaque shows a considerable inhomogeneity with respect to dose distribution (up to 40%). A modified eye phantom was used for the determination of the central axis depth dose distribution.

Determination of the Dose Distribution of an Ophthalmic 106Ru Irradiator with TLDs and an Eye Phantom

Determination of the Dose Distribution of an Ophthalmic 106Ru Irradiator with TLDs and an Eye Phantom

Dosimetric Evaluation in Heterogeneous Tissue of Anterior Electron Beam Irradiation for Treatment of Retinoblastoma: Medical Physics, Volume 14, No. 5, September/October 1987, 772–779

A dosimetric study of anterior electron beam irradiation for treatment of retinoblastoma was performed to evaluate the influence of tissue heterogeneities on the dose distribution within the eye and the accuracy of the dose calculated by a pencil beam algorithm. Film measurements were made in a variety of polystyrene phantoms and in a removable polystyrene eye incorporated into a tissue substitute phantom constructed from a human skull. Measurements in polystyrene phantoms were used to demonstrate the algorithm's ability to predict the effect of a lens block placed in the beam, as well as the eye's irregular surface shape. The eye phantom was used to measure dose distributions within the eye in both the sagittal and transverse planes in order to test the algorithm's ability to predict the dose distribution when bony heterogeneities are present. Results show (1) that previous treatment planning conclusions based on flat, uniform phantoms for central-axis depth dose are adequate; (2) that a three-dimensional heterogeneity correction is required for accurate dose calculations; and (3) that if only a two-dimensional heterogeneity correction is used in calculating the dose, it is more accurate for the sagittal than the transverse plane.

Dosimetric evaluation in heterogeneous tissue of anterior electron beam irradiation for treatment of retinoblastoma.

A dosimetric study of anterior electron beam irradiation for treatment of retinoblastoma was performed to evaluate the influence of tissue heterogeneities on the dose distribution within the eye and the accuracy of the dose calculated by a pencil beam algorithm. Film measurements were made in a variety of polystyrene phantoms and in a removable polystyrene eye incorporated into a tissue substitute phantom constructed from a human skull. Measurements in polystyrene phantoms were used to demonstrate the algorithm's ability to predict the effect of a lens block placed in the beam, as well as the eye's irregular surface shape. The eye phantom was used to measure dose distributions within the eye in both the sagittal and transverse planes in order to test the algorithm's ability to predict the dose distribution when bony heterogeneities are present. Results show (1) that previous treatment planning conclusions based on flat, uniform phantoms for central-axis depth dose are adequate; (2) that a three-dimensional heterogeneity correction is required for accurate dose calculations; and (3) that if only a two-dimensional heterogeneity correction is used in calculating the dose, it is more accurate for the sagittal than the transverse plane.

Spatial localization of tissue metabolites by phosphorus-31 NMR rotating-frame zeugmatography

Surface coils are employed for rotating-frame zeugmatography experiments to accomplish one-dimensional 31P metabolite mapping in phantoms and bovine eyes. The spatially dependent β 1 field of the surface coil provides the necessary magnetic field gradient, and the experiments are designed to acquire 15–32 spectra obtained with rf pulses of equally incremented durations. Regions of the sample are distinguished on the basis of the differing magnetization precessional rates about the spatially dependent β 1 field. Thus the mapping dimension corresponds to changes in the transverse ( x′y′) magnetization components of the β 1 field, which correspond to successive curved surfaces of constant β 1 field strength. Two-dimensional Fourier transform processing of the data results in a metabolite map composed of a suite of spectra displayed as a function of the mapping dimension. Experiments with bovine eye phantoms reveal that T 1 discrimination can produce a loss of resolution due to smearing in the mapping dimension. Resonance offset effects are also apparent at distances removed from the coil. Elimination of the T 1 discrimination-induced smearing is demonstrated through the incorporation of a saturation pulse into the experiment. Application of the method to the intact bovine eye is shown to result in metabolite maps delineating contributions from constituent ocular tissues of the bovine eye.