Ultrasound is diagnostically useful in medicine two modalities, continuous energy and pulsed energy:
Remember that sonic energy is not identical to electromagnetic radiation and that while they share some of the same properties, sound can behave differently, particularly at extreme ends of the spectrum, when passing through complex media, or when interfered with by conflicting sounds.
Consider an object that generates a sound. At rest, the sound frequency is constant. If the object moves towards you, the sound that you hear will seem a little higher in frequency. If the object moves away from you, the sound will have a lower frequency.
Fetal heart beat detectors generate a constant sound. Some of the sound is reflected back toward the transducer. The frequency of the outgoing sound and incoming sound is the same, UNLESS the object is moving (either toward the transducer or away from it. Blood passing through the heart or major placental vessels will reflect back sound that is a slightly different frequency (higher or lower) than the frequency generated by the machine. Because they are of a slightly different frequency, they never line up evenly, except every now and then when the both incoming and outgoing sound energies line up perfectly. The "beat frequency" happens to be in the audible range (less than 10,000 Hz), and can be detected and amplified.
So when you are listening to a fetal heart beat, you are not actually hearing the sound of the heart. You are hearing the beat frequency generated by the interference between the outgoing ultrasound frequency and the incoming ultrasound frequency, that are slightly different because of the movement of the heart wall and blood flowing through the heart and large vessels.
If you could accurately measure the time it took from your handclap to the time you heard the returning echo, you could calculate how far the sound has traveled, and by inference, how far away the wall is from you.
distance = (time) x (speed of sound in air)
Of course, you have to remember that the distance traveled by the sound is twice the distance to the wall...the sound had to travel out to the wall and then back to you (a round trip).
The initial spike from the clap would be followed some time later by the echo. The earliest "A-Mode" ultrasound machines worked in this way. You could know how far the echo has traveled, and how loud the echo was when it got back to you.
This image becomes recognizable, particularly with practice. The recognizable image can then be evaluated for abnormalities, and measured.
B-mode imaging was the first practical application of ultrasound for diagnostic purposed.
Real Time Imaging
A real-time image is still a 2-dimensional view, but one that is constantly updated. This then becomes 3-dimensional imaging (height, width, and time).
Real-time sonography is most useful when the visualized object is moving (like a fetal heart), but is also valuable when the transducer beam is swept through the object, enhancing the operator's appreciation for details, texture, and shape.
OB-GYN 101: Introductory
Obstetrics & Gynecology
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