Ultrasound Principles, Ultrasound Biometry Instrumentation, Accuracy and Standard Dimensions. A problem should be suspected if a difference between the 2 eyes of more than 0. In these instances, consult the patient's history to see if a medical reason exists for a difference or macular pathology that could explain the variation on the same eye (eg, posterior staphyloma). The most common error in the contact technique is corneal compression. This inevitably occurs because the eye is pliable and the cornea is indented even with minimal pressure from the probe tip. The lower the intraocular pressure the softer the eye and the more significant the corneal compression. Therefore, the amount of compression can vary even with the same technician. If the contact technique must be used, the anterior chamber depth must be monitored and the shallower anterior chamber depths deleted even if the spikes appear to be of high quality. Of course, the immersion method completely avoids corneal compression, which is why the contact method is becoming obsolete. See the image below. The second most common error is misalignment, either by not obtaining perpendicularity to the macular surface or by not directing the sound beam through the visual axis. Ultrasound is defined by the American National Standards Institute as 'sound at frequencies greater than 20 kHz.' Perception Humans. The upper frequency limit in humans (approximately 20 kHz) is due to limitations of the. Perpendicularity to the macular surface is achieved when the retinal spike and scleral spike are of high amplitude, and the retinal spike arises steeply from baseline. No sloping of the retinal spike should be present and no jags, humps, or steps should be present on the ascending edge of that spike. See the image below. If either the posterior or anterior lens spike are not of high amplitude, the sound beam could be misaligned at an angle through the lens and, therefore, not through the visual axis. The posterior lens spike may be slightly shorter than the anterior lens spike because the convex curvature of the posterior lens is steeper than the convex curvature of the anterior lens surface, allowing for reflection of the echoes away from the probe tip. Also, if a dense nuclear sclerotic cataract is present, more sound absorption could occur within the lens, causing the posterior lens spike to be shorter. In these instances, the gain can be increased to obtain better posterior lens and retinal/scleral spikes. See the image below. BASIC PHYSICAL PRINCIPLES OF ULTRASOUND Basic physics of ultrasound imaging John E. Aldrich, PhD, FCCPM T o accurately interpret ultra-. A comparison with those pertaining to radiographic imaging illus-trates. BME 1/20 DTU Elektro Medical diagnostic ultrasound - physical principles and imaging. Wilhjelm, Andreas Illum, Martin Kristensson and Ole Trier Andersen Biomedical Engineering, DTU Elektro Technical University of. Manual of diagnostic ultrasound . 100% 48dB ZD4 4.0cm 11B/s Z THI CF5.1MHz PRF1102Hz F-Mittel 70dB ZD6 DF5.5MHz PRF5208Hz. Obstetric Ultrasound Principles And Techniques PDF Obstetric Ultrasound Principles And Techniques.pdf DOWNLOAD HERE 39th AnnuAl Advanced ultrasound Seminar - AIUM Course Description An intense 3-day seminar will be presented. Misalignment along the optic nerve is an error that is easily recognized, since the scleral spike will be absent. The retinal spike will be present and of high amplitude and can even appear steeply rising, but, if the scleral spike is not as high in amplitude as the retina, the sound beam is misaligned along the nerve. No sclera is present at the optic nerve; the sound beam is passing through the nerve cord with only short amplitude echoes present, because the sound beam is striking blood vessels within the nerve cord. In the normal eye, there will generally not be a great difference in axial length when aligned along the optic nerve, but, in cases of a full optic disc, papilledema, or optic disc drusen, this will result in an erroneously short axial length measurement. In cases of optic nerve cupping, as seen in glaucomatous eyes, this error results in an erroneously long axial length measurement. See the image below. Another possible error in the contact method is a fluid meniscus between the probe tip and the cornea caused from ointment use, methylcellulose in the eye from previous examinations, or abnormally thick tear film. If either of these is suspected, rinse the eye with sterile saline prior to biometry. Extremely dense cataracts can be a challenge because of absorption of the sound beam as it passes through the lens. A higher gain setting may be necessary to achieve high- amplitude spikes from the retina and sclera. Improper gate placement also can occur easily, because a dense cataract produces multiple spikes within the lens. The posterior lens gate may erroneously align along one of the echoes within the lens nucleus, resulting in an erroneously thin lens thickness and erroneously long vitreous length, which results in an error in the total length of the eye. In this case, manually realign the gate to the correct posterior lens spike, and if the equipment does not allow for manual gate placement, repeat scans until the gates automatically align properly. Posterior staphylomas are among the greatest biometry challenges. These occur primarily in high myopes, where the globes are so elongated and thin that the posterior uvea bulges outward into the sclera, most commonly in the posterior pole. This causes the macula to be sloped in configuration, which in turn causes reflection of the sound beam away from the probe tip and a poor retinal spike. It is impossible to obtain perpendicularity to a macular surface that is sloped; thus, it is impossible to obtain a proper retinal spike. Also because of the sloped surface, the measurements will be not only long but extremely variable. Patients must be alerted that because their eye is misshapen, they have a higher risk of the postoperative result not being as accurate as a patient with a normally shaped, round globe. In these cases, a B- scan examination is necessary, with a horizontal macular scan performed and the axial length measured from the B- scan. The proper B- scan probe position for this measurement is to have the patient in primary gaze with the B- scan probe face (using a generous amount of gel- type tear solution) centered on the corneal vertex and the probe marker aimed nasally. The macula will lie centered on the right, about 4. Simply place calipers on the vertex of the epithelial corneal echo and on the macula to measure the axial length at average sound velocity of 1. Compare this axial length measurement to the various biometry measurements, and use the measurement that has the most comparable vitreous length in the IOL calculation, preferably within 0. See the image below. Optical coherence biometry has been shown to be beneficial in the case of the highly myopic globe because it measures to the fixation point of the patient and the lack of perpendicularity is not prohibitive. But again, optical methods are not always usable if the patient has a dense cataract or other media opacity or the inability to fixate, so ultrasound techniques still need to be used in these situations. Known macular retinal detachments are yet another challenging situation. In retina practices, the physician may decide to remove the cataract while the patient is undergoing retinal detachment repair and will need accurate IOL calculations. In these cases, the retinal spike will appear further to the left in the vitreous cavity, depending on the elevation of the macula. In these instances, the retinal gate should be moved from the detached retina to the next more posterior spike, since the retina should lie back into this position once repaired. The examiner should inquire if the surgeon plans to place a scleral buckle around the globe to repair the detachment, and if so manually add another 0.
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