In longitudinal scans, the longest diameter of the oval probe face is perpendicular to the limbus and the marking always faces towards the limbus. The orientation of B scan probe may be longitudinal, transverse or axial. Conventionally the marking denotes the upper part of B scan. The brightness scan gives a 2 dimensional display showing the size and echotexture of a lesion. The horizontal distance between 2 echoes can be used to measure the distance between 2 structures as in measurement of ocular axial length using A-scan probe during ocular biometry before cataract surgery. The 35MHz probe has a higher penetration and can evaluate the ciliary body and ciliary processes in detail.Īmplitude scan shows the amplitude of the echoes as vertical height from a baseline as the strength of the echoes. Ultrasound biomicroscopy (UBM) uses 35MHz and 50MHz probes. Probe using 20MHz has been used for high resolution imaging of the retina including detection of cystoid macular edema, central serous chorioretinopathy and evaluation of choroidal echoarchitecture. When the frequency of probe increases - the resolution of the image increases but the depth of penetration of the ultrasound decreases. Ophthalmic ultrasound probe usually uses 7.5, 8, 10 (MHz) megahertz or 12 MHz probes contrary to the 2.5MHz or 3.5MHz probes used for abdominal ultrasound. Intraocular and orbital soft tissue, blood When the speed is slowed (eg silicone oil), the machine erroneously shows a long axial length. The distance or axial length is calculated from the speed and time (required by the sound to reach the lesion and come back from the lesion to be detected by the piezoelectric material). The speed of sound varies according to the media through which it passes. The echodensity of the object determines the amount of reflected sound wave received by the probe and hence the intensity of whiteness in B scan and the amplitude in the A scan. Thus if the velocity of sound in the media is known, the distance of the object from the probe can be measured. The piezoelectric crystal picks up the reflected sound and converts it to electric signals which give the image of the echogram. The ultrasonic wave goes and gets reflected from any echodense object. The ultrasound uses piezoelectric lead-zirconate-titanate crystals which generate ultrasonic (inaudible, more than 20kilohertz) sound waves from electricity. 9.12.1 Retained lens material (posterior dislocation of lens fragments).
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