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I have a basic question concerning ultrasound A-scans. Such an A-scan might typically look as follows (source: https://img.medscapestatic.com/pi/meds/ckb/18/7418tn.jpg):

enter image description here

I understand that the x-axis is the time axis but I'm unsure about the y-axis. In the literature, I can find that the y-axis and the amplitudes of the signal represent the "intensity of reflected sound" but what's the unit of this intensity? Or to put it in other words: What would an amplitude value of, say, 0.2 actually mean?

Thank you very much.

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  • \$\begingroup\$ Knowing nothing about that type of equipment I would have guessed that the vertical axis is the reflection time delay and, therefore, a measure of distance to target. However, I can't think how multiple layers would be shown. \$\endgroup\$ – Transistor Jul 30 '19 at 14:16
  • \$\begingroup\$ It is dependent on the system. Whatever units your signal is in, and sometimes that is not an easy answer since you have signal processing and amplification. I would just call them units. \$\endgroup\$ – MadHatter Jul 30 '19 at 14:17
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The amplitude is just arbitrary units for relative measurements only. The absolute values don't really matter all that much.

The peaks in the graph represent changes in tissue properties — specifically, changes in acoustical impedance, which cause energy to be reflected back toward the source. It is the horizontal position (representing distance) and width of these peaks that is more relevant than their amplitude. Indeed, you can see that some of them "saturate" the receiver — they have flat tops.

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  • \$\begingroup\$ Thank you very much for this explanation. This helped a lot. \$\endgroup\$ – Hagbard Aug 2 '19 at 10:07
  • \$\begingroup\$ At some level, it has to be related to force or pressure, probably AC. \$\endgroup\$ – Scott Seidman Aug 21 '19 at 14:52
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On top of the previous question, I would also add that the scale depends on the digitizer on the receiver side, and that the envelope signal that you see is being shaped by non only the signal from the sensor, but also from a variable gain amplifier, different filters, and possibly the envelope detection circuit per se.

If you want to have a look at a signal in practice, you may want to explore this experiment - the scale in itself is at first a voltage from the ADC, but becomes an absolute number when you do your envelope detection magic.

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