Chapter three : CARDIAC ULTRASOUND TUTORIAL SERIES

Terminology of the Physics of Ultrasound

by

Dr. Z. Gooding

One way of dealing with this chapter is to present it in the form of Q & A as follows:

1.What is ULTRASOUND exactly ?

To answer this question, one must realize that the term "ultrasound" contains in fact 2 words: "ultra" and "sound" . Physics teaches us that in order for a sound to be detected by the human ear, it has to occur with a certain "pitch" or "frequency" (proper terminology). In other words any sound occurring with a lower (infra) or higher (ultra) frequency is not going to be detected by the human ear. The frequency range for sound is known to be between 20 Hz and 20.000 Hz ( Hz being the abbreviation of Hertz, Physicist who gave his name as a the unit of frequency). Consequently, the frequency of ultrasound has got to be higher than 20.000 Hz and that of infrasound below 20 Hz as illustrated in the diagram below:



Infrasound---------SOUND--------ULTRASOUND

2. What's "frequency" ?

In the broad sense of the term, it is the "number-of-times", "how often" an event takes place. Therefore, it is relatively safe to assume that the "event" in question is potentially "repetitive".
In "Physics of Ultrasound" "frequency" is the number of "cycles" per "unit of time", which brings us to the very definition of "hertz" (e.g. the number of cycles per second). As we will see, in our particular context of "ultrasound", the "frequency" pertains to the number of cycles per second an ultrasound "pulse" is generated (transmitted).

Pearls : now putting together the above information, it is fairly safe to state that ultrasound consists of sound waves that occur (cyclically) with a frequency above 20.000 Hz (upper limit of human hearing range).

3. Using the above terminology (however "small" it might seem), how do we go to the next step and tie "things" up in such a way that makes these two terms more meaningful to our review of the "Physics of Ultrasound" ?

As a general "rule" (mine), in "Physics" all terms are somehow "tied" together, that is "interrelated" in essence through "mathematical" equations, that allow for even more meaningful "expression" and explanation of natural phenomena, such as "ultrasound". In other words, if "ultrasound" is not a "tangible" entity per se, it nevertheless lends itself to one that can be "characterized" by "variables" that can explain it in a more "concrete" , realistic way, which brings us to the following statement that should "sum" things up for us at this point:

"Ultrasound waves" are defined by their "physical" "characteristics", having to do with "how-often" ("frequency") they occur, and "how-they-can-be" represented "graphically" in a way that sonographers can "visually" understand, and also apply to develop and improve their diagnostic skills. This statement leads us to a new and meaningful definition of "ultrasound" that hopefully should "tie" things up for sonographers (see pearl below).

Pearls:

As ultrasound waves " deploy" in a "medium"(body organs), they are characterized by their :

1. Frequency (expressed in hertz or number of cycles per second);

2. Wavelength (distance, represented by the greek letter "lambda" between 2 consecutive peaks on the sound wave "profile") (see graph below);

3. Intensity (strength of the ultrasound);

4. Velocity (speed of propagation or distance covered by ultrasound per second);

The above characteristics are all represented by symbols (more of that later) as well as "mathematically" interrelated. To this end, velocity for instance is equal to the product of "frequency" and "wavelenth" (v=frequency x wavelength).

At this point, the next best "move" would be to place the above characteristics within the context of "General Physics" itself , and see how they can apply to "ultrasound" (chapter four).