الفهرس 
Physics of UltrasoundOnly 14 pages are availabe for public view
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Abstract
Ultrasound is an acoustic wave with a frequency above the audible range of 20 KHz. The principle of ultrasonic investigation in ophthal¬mology is to deliver a series of ultra¬sonic pulses to the eye by a `transducer probe’ which are reflected like an echo by any solid or semisolid surface within the eye and are received by the same probe, rectified, amplified and indicat¬ed on screen.
One of the most significant advances in ophthalmic ultrasound is the development of ultrasound biomicroscopy (UBM). In 1990 Pavlin and colleagues described the first high frequency ultrasound (50-100 MHz) in ophthalmology. At this frequency, microscopic images of tissues are obtained at a magnification approaching that of light microscopy. However, high resolution is won against the loss of penetration. It has many applications in diagnosis of various types of glaucoma, intraocular tumors, inflammations, trauma and corneal diseases.
Recently, the VHF ultrasound (Artemis) was developed to provide a very high resolution ultrasound B-scan imaging of the anterior segment including high-precision 3-D mapping of individual corneal layers, and axial length .The Artemis is designed to scan in an arc of adjustable radius, thus following the curved surfaces. Also, it uses a digital processing technology and reverse immersion technique. It is widely used in preoperative and postoperative preparation of LASIK patients and many other applications.
Development of duplex scanners and color Doppler instruments in the 1980s has facilitated their use in ophthalmology. Power Doppler (PD) has three times the sensitivity of conventional Color Doppler in detecting blood flow and it is very useful in imaging of vascular lesions of the globe and orbit.
Three-dimensional (3D) ultrasound is a promising technology. The 3-D image is constructed from a series of ordered B-scan planes that are processed by special software. It has many advantages and clinical applications such as outlining the intraocular masses and calculating their volumes.
Other promising new developments in the field of diagnostic ultrasound include Tissue characterization and Echo contrast agents. Tissue characterization technology analyzes the echo (reflected amplitude in relation to used frequency) and calculates the scatter size and concentration of a given tissue, presenting them by false colors (each color assigned to special scatter size ad concentration) in the original b-scan, just like histological sections. Echo contrast agents (such as gas microbubbles) act by increasing the backscatter effect of ultrasound in vascular regions. They can be used with ”harmonic imaging” to image the contrast agent maximally while inhibiting surrounding artifacts. Therefore, it is very useful in imaging small vessels and vascular lesions.
Other developments in ultrasound in general include refinement in transducer’s material and technology, leading to improvements in sensitivity and bandwidth. 20-MHz transducer and annular array are examples of this development.