ABOUT REAL TIME ULTRASOUND

• Overview
• Background information
• What is ultrasound imaging?
• Scope of practice issues

WHAT IS ULTRASOUND IMAGING?

Ultrasound refers to sound waves with a frequency greater than 20,000 Hz. Ultrasound imaging involves the propagation of sound waves into the body. These waves reflect off the tissues that they encounter and the reflections are the collected and then displayed as a dynamic or static image. Ultrasound imaging allows medical practitioners to observe the movement of a tissue (for instance a muscle or muscle group) as it occurs in real time.

Ultrasound waves are generated by short electrical impulses being passed across crystalline structures referred to as transducers. The electrical impulses excite and vibrate the crystalline structures and are correspondingly converted into very high frequency sound waves. This effect of conversion of electrical signals into high ultrasonic sound wave is termed a reverse piezoelectric effect. The distinction between therapeutic and imaging units is that the transducer on the former is capable of collecting the reflection of the sound waves from the various tissue boundaries that it has encountered and converting it back into an electrical signal (piezoelectric effect) which can be used to create an image.

Once generated ultrasound waves behave similar to any sound wave. If you were to bang on a drum you would produce a sound wave, how far the sound could be heard and the properties of its echo would depend on how the drum was hit, the properties of the medium that it had to travel through and the number, shape and properties of the objects that the waves encounter as well as how far they where located from the source of the sound. Just as the sound waves from a drum, ultrasound waves will travel through some mediums better than others and can virtually bounce off of and be partially absorbed by anything.

Ultimately the sound waves are absorbed into (as heat), or bounce off the tissues encountered along their path. The sound waves that bounce off and return back to the ultrasound probe are referred to as an echo. When the echo or reflection is received by the transducer it is converted back into an electrical signal and then through a process of calculating the time taken for the echo to return, the distance of the echo generating structure from the transducer is determined, the signal amplified, and an image is displayed.

The depth that an ultrasound wave penetrates depends on its frequency, wavelength, as well as the properties of the medium or tissue it is traveling through. The higher the frequency of the source of the sound the less the emerging sound wave diverges. Consequently ultrasound waves, due to their high frequency (1-10 MHz), are very cohesive (similar to a light beam leaving a laser pointer) and can be used to selectively expose a limited target area. 3.5 – 10 MHz are the most common frequencies utilized with ultrasound imaging and the frequency is determined by the probe used.

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