The basic principles and terminology employed in emergency ultrasonography are straightforward but differ in important respects from that of other imaging techniques. The ultrasound image is created electronically from high-frequency sound waves generated by the transducer (or probe), which also receives the reflected waves. The time required for the reflection of each structure determines its depth on the image, and the intensity of the reflection determines its shade on a black-to-white scale. A perfect reflector of ultrasound waves appears white and is referred to as hyperechoic. A perfect transmitter of ultrasound waves has no reflection and appears black, or anechoic. A great advantage of ultrasound is that most structures, particularly those not well visualized by standard x-ray, have intermediate echogenicity that is quite characteristic and allows for identification of normal and abnormal organs and tissues.
An important factor in determining the quality of the image is the frequency of the transducer employed: the lower the frequency, the greater the depth of penetration but the lower the resolution. For all the primary ED indications (except endovaginal ultrasound), a general-purpose probe of approximately 3.5 MHz is appropriate and will allow visualization even in obese patients. A 5 MHz probe can be used in thin adults or pediatric patients, providing better resolution. Some of the specialized applications, including many vascular and procedural uses (Xablein295:2.), require higher-frequency probes for adequate sonographic examination.
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