Today our topic of discussion is Ultrasound in Obstetrics.
Ultrasound in Obstetrics
ULTRASOUND IN OBSTETRICS
The ultrasound is a sound wave beyond the audible range of frequency greater than 2 MHz cycles per second). The commonly used frequency range in obstetrics is 3.5-5 MHz. SONAR stands for “Sound, Navigation and Ranging” In clinical practice, two main varieties of ultrasound (sound that is produced at a very high pitch) are used depending upon whether the reflected waves give audible or visual signals.
- The apparatus, which interprets the audible signals- doptone and sonicaid are easy to carry and simple to use even with batteries. It can detect fetal heartbeats as early as 10thweek of gestation.
- The apparatus for interpretation of visible signals-the sonar system, which is a much more sophisticated and bulky apparatus, issued in three forms:
- The A-scan, that gives a one-dimensionalpicture
- The B-scan, that gives a composite two-dimensional picture
- The real-time scanner that depicts movements-to display cardiac and breathing activity.
How Does Ultrasound Work?
Scanners are used to produce static pictures. The picture is built up as a single crystal transducer (a thin disk to which a wire is attached) is moved backward and forward across the area scanned. When the transducer is placed on the body and as it encounters a structure, a fraction of that sound is reflected back. The echo is detected electronically and transmitted on to the screen as dots. The amount of sound from each organ varies according to the type of tissue encountered:
- Strong echoes give bright dots, e.g. bone
- Weaker echoes give various shades of gray according to their strength
- Fluid-filled areas cause no reflexion and give rise to a black image.
The real-time scanners are so called because it produces a moving picture on the screen as opposed to scanner that give static picture. The real-time scanner can have several types of transducers attached to it, which are interchangeable and are used according to the type of image needed and the part of the anatomy to be examined.
Types of transducers in common use include the linear array, the curved linear array, the sector and the vaginal probe. Instead of a single crystal, all these types of transducers have many crystals that fire off electrical energy and collect the echoes very rapidly, thus producing the moving picture.
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