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What are the differences between using x-rays and ultrasounds for medical diagnosis?

From a physical point of view I am aware that x-rays are able to reach deeper into the material - but I think this is just a matter of power in case of ultrasounds, or? As it is for x-rays as well. The physics behind both principles is quite different as ultrasounds are mechanical waves propagating through matter while x-rays are electromagnetic waves. In case of the latter, the absorption through the body is technically measured while in case of ultrasounds, the reflection is measured. So what are the benefits and differences compared to each other?

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  • $\begingroup$ Re, "just a matter of power in case of ultrasounds." Ultrasound is not benign. This company makes an ultrasound device that is used to destroy cancerous tissue. You can't just crank up the power as high as you like and expect no side effects. $\endgroup$
    – Solomon Slow
    Commented Sep 4, 2020 at 13:10

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Ultrasound furnishes useful contrast in soft tissue, whereas X-rays generally do not. Ultrasound hence lets you accurately image things like heart valves, and can also be used to measure fluid flow in real time within a beating heart by using doppler detection.

X-rays furnish contrast in bone for finding cracks and breaks which ultrasound generally does not, and support the use of contrast agents like barium salts for real-time imaging of occlusion of small arteries in a beating heart.

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  • $\begingroup$ Thanks, but I can't see a physical explanation, e.g., why ultrasounds are not able to diagnose bone cracks? I don't see why they shouldn't be able to do so. $\endgroup$
    – Ben
    Commented Sep 4, 2020 at 6:07
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    $\begingroup$ because the density difference between soft tissue and bone is so great that the ultrasound bounces off without penetrating the bone. $\endgroup$
    – niels nielsen
    Commented Sep 4, 2020 at 6:17
  • $\begingroup$ Well, that makes sense. Does that finally imply that ultrasounds are only useful in regions without bones like neck and stomach? $\endgroup$
    – Ben
    Commented Sep 4, 2020 at 6:28
  • $\begingroup$ @Ben, Conventional X-ray images have vastly higher resolution than any hand-held ultrasound probe can produce. That enables the radiologist to diagnose even very fine fractures. $\endgroup$
    – Solomon Slow
    Commented Sep 4, 2020 at 13:12

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