Research On Ocular Biometry In Ultrasound Method

The Ocular Biometry is the application of relevant inspection methods for measuring structural parameters of the eyeball, such as the measurement of the axial length, corneal curvature, corneal thickness, anterior chamber depth, thickness, glass lens body cavity length, the eye muscle thickness, optic nerve diameter and the thickness of the orbit periosteum. These measurements provide the basis for diagnosis and treatment of the eye diseases. Many eye diseases are relevant to the measurement of structural parameters of the eyeball, such as congenital glaucoma, angle-closure glaucoma, nearsightedness, far-sighted, etc, and these clinical manifestations are changes of the axial length or other biological measuring parameters. So the sooner discovery of these changes, the better for early diagnosis of these diseases.Along with the development of the new treatment technology in the ophthalmology, such as cataract extraction with implantation of the intraocular artificial lens and corneal refraction surgery, the measurements technology of the eyeball get the attention from the clinical doctors more and more. How to get the accurate parameters of each component in the eyeball has been a matter of concern, because any tiny error can make unsatisfactory effect in the perfect operation. In the surgery of cataract extraction with implantation of the intraocular artificial lens preoperative, it can get more ideal refractive status through the biological measurements about axial length, anterior chamber depth and related parameters, and the accurate calculation about the lens power which is implanted inside the eye.This topic studies the biological measurement method in the eyeball. At present the main biological measurement method has two kinds:the optical method and the ultrasonic method. Optical coherence biological measurements (IOL-Master) technology is emerging in recent years. It uses the principle of laser interference and provides a new means of biological measurement for clinical. The optical method is based on intra-ocular light propagation. Light rays impact surfaces with different refractive indices. Reflected rays interfere with each other. As in the first case, the analysis of the interference ray provides knowledge of the axial position. IOL-Master uses optical principle for the measurement of the axial length; it can still simultaneously measure corneal curvature, anterior chamber depth, level corneal diameter, which is the necessary data for intraocular lens power calculation. IOL-Master is a kind of new non-contact measurement method for eyeball; it expands the application fields of the optical coherence imaging technology. Because of a variety of the measurement parameters and the characteristics of contact, no damage, convenient operation, it is more and more widely used in the clinical application. The ultrasonic method for measuring the length of the axial is mainly based on the principle of the reflection of the ultrasonic echo wave. The wave impacts a surface which separates two media with different velocity characteristics. A part of the wave is refracted through the surface and the second part is reflected back. The analysis of the reflected wave enables us to describe the axial position. As a result of different density of each, the speeds of ultrasonic in different organizations are also different. According to the different speeds of the ultrasonic within the different organizations, we can calculate the lengths of different organizations. The instrument used for biological measurement with the ultrasonic method is type A ultrasonic, there are two different approaches for setting the measuring probe. The probe can be placed on the cornea (the contact method) or we can make artificial surroundings where we immerse the probe (the immersion method). The immersion method is more accurate than the contact method. The contact method causes applanation of the cornea. The applanation reduces the anterior chamber depth and also influences a proportion of the axial length. Type A ultrasound for eye biological measurements has a number of factors which can make errors, such as the different ways of examination, the sound waves direction and the view axis direction is same or not, the choice of the speed in different medium and so on. This paper makes some theory analysis about several of those factors which can make errors, and draw the following several conclusions. The immersion method is considered to be more accurate. The immersion method tended to register the error caused by the probe movement. It was important to consider that the impact point further from the central position by more than2mm does not allow us to catch the echo. The method which reaches this limit sooner is more precise owing to the invalidity of the distorted information from the periphery. The greater sensitivity of the immersion method was more visible in the case of the probe tilt:if we tilted the probe using the immersion method, we got a larger deviation of echo than when using the contact method. The transducer catches only echoes from the central area (closer to the optical axis). Therefore these echoes contain precise information. The contact method allows processing from the peripheral area with its associated information distortion. This situation can be compared to the keyhole effect. The further from the cornea the probe is (immersion method), the more strictly the condition of the central and coincidence position of the probe has to be kept. Approaching the probe to the cornea allows larger misalignment from the central position and more distorted information. This allows us to proceed under the assumption of the greater accuracy of the immersion method. In the actual measurement, according to the different conditions, we should select section measurement method or average velocity measurement method, making the velocity of the ultrasonic corresponding to the medium. The errors in the biological measurements are unavoidable. In order to minimize the errors and make the measurement result more accurate, we should do something following in the inspection:the complete measurement process should be operated by professionally trained doctors; it is best to use the immersion method in order to avoid oppression to the cornea; each eye should be measured three times above, the differences of the three times’measurements results should be within0.1mm, the differences in the two eyes of the axial length measurements should be within0.3mm, if not, it should be measured again. Any changes about probe selection, sound choices and measuring technology can control the accuracy of the measurement results. Optical biological measurements——IOL-Master is a new developed tool for biological measurement, it has not yet widely used in clinical in our country. The traditional measuring method for biological measurements is still ultrasound biological measurements, which is also known as the gold standard for the biological measurements. Comparing with these two methods, the advantages of IOL-Master are that it is a non-contact check, which can avoid the damage to the cornea, and it shortens the time of preparation and check, improving the efficiency of the check. It also shows its advantage in some special eyes because lights travel faster than sounds, and are not easily affected. Its accuracy of the biological measurements in the eyeball is higher than traditional type A ultrasound. Of course, there are also insufficient in the IOL-Master:in the IOL-Master measurement, it requires that patients could be able to keep watching a bit in the vision and keep a few seconds. Because of this, those who have poor vision or turbid refraction medium can’t be measured. For these patients, we need type A ultrasound to perform complete measurement. Therefore, IOL-Master also cannot completely replace type A ultrasound, it is better to serve for patients only when they help each other.This paper makes some researches about the application of one parameter in the biological measurement. A method of predicting the postoperative anterior chamber depth value (ACD) with the evaluation standard of contrast sensitivity curve (CSF) is put forward. We build the personalized eye models, replace the natural lens with the artificial lens, realize the tracing method with ZEMAX software, simulate pseudophakic optical system, through these steps, we can obtain the best MTF curve through the optical optimization, and then calculate CSF to conclude the best contrast sensitivity (CS) curve. The best ACD value is acquired according to the CSF curve. We use10cases with no retinal diseases cataract patients’eye data as objects of study. We predict the postoperative anterior chamber depth value separately with Holladay, Hoffer Q, SRK/T and the method above, and carry on the analysis comparison. To analysis the results, we make the consistency analysis. The consistency analysis of the results are made by Bland-Altman figure, the graph shows that all points are fall in95%consistency within the boundaries, the differences of the largest absolute value of three formula and the method above are0.73mm,0.65mm,0.68mm and the calculation results of the mean value are5.846mm,5.804mm,5.825mm, the two methods of calculating results has good consistency in clinical range, so the method above can be used to forecast the postoperative ACD. The application of the method above establish a connection with contrast sensitivity curve and the anterior chamber depth, make the doctor better satisfy the postoperative refraction requirements of patient. So the quality of life of the patients can be improved.