Ultrasound tranducers, often called probes, come in different shapes and sizes for use in different scanning situations. For example, in an obstetric scan the probe used is usually one that looks like a curved soap bar ( more accurately known as a convex-array transducer, and the one with a flat surface a linear-array transducer ) which can be slided over the maternal abdomen while maintaining good surface contact to the abdominal surface for the whole width of the probe. In a vaginal scan, the probe has to be a long and slender piece to fit into the vagina.
Pulsed ultrasound, because of it's high frequency can be aimed in a specific direction and obeys the laws of geometric optics with regard to reflection, transmission and refraction. When an ultrasound wave meets an interface of differing echogenicity, the wave is reflected, refracted or absorbed. Reflected sound waves are processed. The transducer, though emitting ultrasound in rapid pulses, acts as a receiver most of the time. The ultrasound images can be displayed on an oscilloscope screen or a video monitor (via what is known as a scan converter) and can be recorded on videotape, thermal paper or radiographic film.
sound waves are propagated at a speed of approximately 1540 m/sec in soft tissues. The thickness, size and location of various soft tissue structures in relation to the origin of the ultrasound beam are calculated at any point in time. The strength of the reflected sound wave depends on the difference in "acoustic impedance" between adjacent structures. The acoustic impedance of a tissue is related to its density; the greater the difference in acoustic impedance between two adjacent tissues the more reflective will be their boundary. Higher frequency ultrasound waves have a longer near field and less divergence in the far field; they permit better resolution of small structures. More energy however is absorbed and scattered by the soft tissues so that higher frequencies have less penetrating ability. Conversely, a transducer producing lower frequencies will provide greater depth of penetration but less well defined images. Focusing and aperture control technology are often emplyed to narrow the beam along it's entire path to acheive maximum right-and-left (lateral) resolution.
The transducer of a realtime scanner typically contains over 300 crystals( piezo electric devices) arranged in a row where each emits and receives an ultrasound beam in rapid succession to form a sweep. The part of the abdomen under the probe is "swept" about 30 times (frames) a second and a moving picture (a realtime picture) will be formed (not unlike the principle in a movie projector). Beam density and dynamic range control technologies are futher being incoporated into each scanner's design to optimize the resultant image.
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