Материал: part17

•​RGB LUT Transfer Function = TABLE​ •​Alpha LUT Transfer Function = TABLE​

•​Red, Green, Blue, and Alpha Palette Color Lookup Table Descriptors and Data included​

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RROphthalmicRefractiveReportsUseCases​ (Informative)​

RR.1 Introduction​

Refractive instruments are the most commonly used instruments in eye care. At present many of them have the capability for digital​ output, but their data is most often addressed by manual input into a paper or electronic record.​

Refractive instruments address the power of a lens or of a patient's eye to bend light. In order for a patient to see well light must be​ focused on the retina in the back of the eye. If the natural optics of a patient's eye do not accomplish this, corrective lenses can bend​ incident light so that it will be focused on the retina after passing through the optics of the eye. The power of an optical system such​ as a spectacle lens or the eye is measured by its ability to bend light, and is measured in diopters (D). In practical clinical applications,​ this is measured to 3 decimal points, in increments of 0.125 D. The power of a lens is measured in at least two major meridians. A​ spherical lens power occurs when the power is the same in all meridians (0-180 degrees). A cylindrical lens power occurs when there​ is a difference in lens power across the various meridians. The shape of the anterior surface of the eye largely determines what type​ of correcting lens is needed. An eye that requires only spherical lens power is usually shaped spherically, more like a ball, while an​ eye that requires cylindrical lens power is ellipsoid and shaped more like a football.​

Lensescanalsobendlightwithoutchangingitsfocaldistance.Thistypeofrefractionsimplydisplacesthepositionoftheimagelaterally.​ The power of a prism to bend light is measured in prism diopters. In practical clinical applications this is measured to 1 decimal point,​ in increments of 0.5 prism diopters. Prism power is required in a pair of spectacles most commonly when both eyes are not properly​ aligned with the object of regard. Clinical prisms are considered to bend all light coming in from the lens either up, down, in toward​ the nose, or out away from the nose, in order to compensate for ocular misalignment.​

Visual acuity is measured in various scales, all of which indicate a patient's vision as a fraction of what a reference standard patient​ would see at any given distance. For example, if a patient has 20/30 vision it means that he sees from a distance of 20 feet what a​ referencestandardpatientwouldseefromadistanceof30feet.Thesemeasurementsaredeterminedbypresentationofstandardized​ objects or symbols (optotypes) of varying sizes calibrated to reference standard vision (20/20). The smallest discernible optotype​ defines the patient's visual acuity expressed in a variety of formats (letters, numbers, pictures, tumbling E, Landolt C, etc).​

Visual acuity is measured in two categories of viewing distances: distance, and near. Distance visual acuity is measured at 20' or six​ meters. This distance is roughly equivalent to optical infinity for clinical purposes. The near viewing distance can vary from 30cm to​ 75 cm depending on a variety of other conditions, but most commonly is measured at 40 cm.​

Visual acuity is measured under several common viewing conditions: 1) Uncorrected vision is measured using the autoprojector to​ project the above mentioned optotypes for viewing, with no lenses in front of the patient's eyes. The line of smallest optotypes of​ which the patient can see more than half is determined, and that information is uploaded to a computer system. 2) The patient's vision​ using habitual correction is measured in a similar fashion using whichever vision correction the patient customarily wears. 3) Pinhole​ vision is measured in a similar fashion, with the patient viewing the optotypes through a pinhole occluder held in front of the eye.​ Pinholevisualacuitytestingreducesretinalblur,providinganapproximationofwhatthepatient'svisionshouldbewiththebestpossible​ refractive correction (spectacles) in place. 4) Best corrected visual acuity is the visual acuity with the best refractive correction in​ place.5)Crowdingvisualacuitymeasuresthepresenceandamountofdisparityinacuitybetweensingleoptotypeandmultipleoptotype​ presentations.​

Apatient'sspectacleprescriptionmayormaynotrepresentthesamelensesthatprovidedbestcorrectedvisualacuityinhisrefraction.​ Subjective comfort plays a role in determining the final spectacle prescription.​

1.​Autolensometer: an autolensometer is used to measure the refractive power of a patient's spectacles. This is done by the auto-​ matic analysis of the effect of the measured lens upon a beam of light passing through it. Output from an autolensometer can be​ uploadedtoaphoroptertoprovideabaselineforsubjectiverefraction(discussedbelow),anditcanbeuploadedtoacomputerized​ medical record. Lenses may also be measured to confirm manufacturing accuracy.​

2.​Autorefractor: an autorefractor is used to automatically determine, without patient input, what refractive correction should provide​ best corrected visual acuity. Output from an autorefractor can be uploaded to a phoropter to provide a baseline for subjective​ refraction (discussed below), and it can be uploaded to a computerized medical record.​

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3.​Phoropter (or phoroptor):an instrument containing multiple lenses, that is used in the course of an eye exam to determine the​ individual's subjective response to various lenses (subjective refraction) and the need for glasses or contact lenses.. The patient​ looks through the phoropter lenses at an eye chart that may be at 20 ft or 6m or at a reading chart that may be at 40 cm. Inform-​ ation from the subjective refraction can be uploaded from an autophoropter to a computer. The best corrected vision that was​ obtained is displayed in an autoprojector, and that information can also be uploaded to a computer.​

4.​Autokeratometer: an autokeratometer is used to measure the curvature, and thus the refractive power, of a patient's cornea. Two​ measurements are generally taken, one at the steepest and one at the flattest meridian of the cornea. The meridian measured​ is expressed in degrees, whole integers, in increments of 1 degree. If the measurement is expressed as power, the unit of​ measurement is diopters, to 3 decimal points, in increments of 0.125D. If the measurement is expressed as radius of curvature,​ the unit of measurement is millimeters, to 2 decimal points, in increments of 0.01 mm.​

RR.2 Reference Tables For Equivalent Visual Acuity Notations​

RR.2.1 Background​

Visual acuity is defined as the reciprocal of the ratio between the letter size that can just be recognized by a patient, relative to the​ size just recognized by a standard eye. If the patient requires letters that are twice as large (or twice as close), the visual acuity is​ said to be 1/2; if the letters need to be 5x larger, visual acuity is 1/5, and so on.​

Note that the scales in the tables extend well above the reference standard (1.0, 20/20, the ability to recognize a letter subtending a​ visual angle 5 min. of arc), since normal acuity is often 1.25 (20/16), 1.6 (20/12.5) or even 2.0 (20/10).​

Today, the ETDRS chart and ETDRS protocol, established by the National Eye Institute in the US, are considered to represent the​ de facto gold standard for visual acuity measurements The International Council Of Ophthalmology, Visual Standard, Aspects and​ Ranges of Vision Loss (April, 2002) is a good reference document.​

The full ETDRS protocol requires a wide chart, in the shape of an inverted triangle, on a light box, and cannot be implemented on the​ limited screen of a projector (or similar) chart.​

Formostroutineclinicalmeasurementsprojectorchartsortraditionalchartswitharectangularshapeareused;thesenon-standardized​ tools are less accurate than ETDRS measurements.​

This appendix contains two lookup tables, one for traditional charts and one for ETDRS measurements.​

RR.2.2 Notations​

Various notations may be used to express visual acuity. Snellen (in 1862) used a fractional notation in which the numerator indicated​ the actual viewing distance; this notation has long been abandoned for the use of equivalent notations, where the numerator is​ standardized to a fixed value, regardless of the true viewing distance. In Europe the use of decimal fractions is common (1/2 = 0.5,​ 1/5 = 0.2); in the US the numerator is standardized at 20 (1/2 = 20/40, 1/5 = 20/100), while in Britain the numerator 6 is common (1/2​ = 6/12, 1/5 = 6/30).​

The linear scales on the right side of the tables are not meant for clinical records. They are required for statistical manipulations, such​ ascalculationofdifferences,trendsandaveragesandpreferredforgraphicalpresentations.Theyconvertthelogarithmicprogression​ of visual acuity values to a linear one, based on Weber-Fechner's law, which states that proportional stimulus increases lead to linear​ increases in perception.​

The logMAR scale is calculated as log (MAR) = log (1/V) = - log (V). LogMAR notation is widely used in scientific publications. Note​ that it is a scale of vision loss, since higher values indicate poorer vision. The value "0" indicates "no loss", that is visual acuity equal​ to the reference standard (1.0, 20/20). Normal visual acuity (which is better than 1.0 (20/20) ) is represented by negative logMAR​ values.​

The VAS scale (VAS = Visual Acuity Score) serves the same purpose. Its formula is: 100 - 50 x logMAR or 100 + 50 x log (V). It is​ more user friendly, since it avoids decimal values and is more intuitive, since higher values indicate better vision. The score is easily​ calculated on ETDRS charts, where 1 point is credited for each letter read correctly. The VAS scale also forms the basis for the cal-​ culation of visual impairment ratings in the AMA Guides to the Evaluation of Permanent Impairment.​

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RR.2.3 Use of The Lookup Table​

Data input: Determine the notation used in the device and the values of the lines presented. No device will display all the values listed​ in each of the traditional columns. Convert these values to the decimal DICOM storage values shown on the left of the same row.​ DICOMvaluesarenotmeantfordatadisplay.Inthetable,theyarelistedinscientificnotationtoavoidconfusionwithdisplaynotations.​

In the unlikely event that a value must be stored that does not appear in the lookup table, calculate the decimal equivalent and round​ to the nearest listed storage value.​

Data display: If the display notation is the same as the input notation, convert the DICOM storage values back to the original values.​ If the notation chosen for the display is different from the input notation, choose the value on the same row from a different column.​ In certain cases this may result in an unfamiliar notation; unfortunately, this is unavoidable, given the differences in size progressions​ between different charts. If a suffix (see Attribute "Visual Acuity Modifiers" (0046,0135) ) is present, that suffix will be displayed as it​ was recorded.​

Suffixes: Suffixes may be used to indicate steps that are smaller than a 1 line difference. On traditional charts, such suffixes have no​ definednumericalvalue.Suffixes+1,+2,+3and-1,-2,-3maybeencountered.Thesesuffixesdonotcorrespondtoadefinednumber​ of rows in the table.​

RR.2.4 Traditional Charts​

The Traditional charts used in clinical practice are not standardized; they have an irregular progression of letter sizes and a variable​ number of characters per line. Measurement accuracy may further suffer from hidden errors that cannot be captured by any recording​ device,suchasaninconsistent,non-standardizedprotocol,inaccurateviewingdistance,inaccurateprojectoradjustmentandcontrast​ lossfromroomillumination.Therefore,thedifferencebetweentworoutineclinicalmeasurementsshouldnotbeconsideredsignificant,​ unless it exceeds 5 rows in the table (1 line on an ETDRS chart).​

Table RR-1 contains many blank lines to make the vertical scale consistent with that used in Table RR-2. Notations within the same​ gray band are interchangeable for routine clinical use, since their differences are small compared to the clinical variability, which is​ typically in the order of 5 rows (1 ETDRS line).​

Table RR-1. Reference Table for Use with Traditional Charts​

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