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It might be that this is more a physical question but as the application is medical I firstly want to adress it here:

What are the limits of ultra-sonics in regard to diagnosis respectively why are x-rays still being used?

I guess this is a mixture of physical and medical reasons. The reason why I wonder is that x-rays are potentially damaging tissues while ultra-sonics do not (at all?). I'm only aware of the functional principles so I can differ them but I can't see in which way x-rays are more promising. Finally, it is probably about signal-to-noise - but why, when, how?

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    Quick comment because I don’t have the time to write an answer: X-rays are quick, easy and cheap and great for detecting skeletal anomalies, lung infections and with an adequate contrast agent anomalies in the digestive system. Ultra-sound is quick and easy and able to detect visceral anomalies, but is relatively useless when air is involved (lungs, digestive system) or when there are bones (sceleton). MRI is the most superior imaging technique (you can basically spot anything), it is free of radiation but takes 30mins (unsuitable for emergencies) and is very expensive. CT Scans are quicker – Narusan Sep 4 at 13:30
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    [cont‘d] (5 mins), have a similar resolution as MRI but come with a dose of radiation. // Basically, you can not substitute X-ray with ultrasound or the other way round because they provide you with entirely different information (look up pictures and compare them), but MRI and CTs are the more superior imaging technique for either. However, they are time-consuming and costly, so sometimes there‘s just no need for it. – Narusan Sep 4 at 13:32
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    Thanks a lot! I assume the ultrasound technology is developing, so I know at least that 3d ultrasound is also possible. Doesn't solve physical limits but some years ago we also thought that the optical resolution is at its maximum. – Ben Sep 4 at 13:41
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    @Pj30 What makes ultrasound more subjective than xrays? – D.Tan Sep 4 at 19:05
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    I think what pj30 is trying to say is that there are high chances of false positive results and hence the interpretations might vary with the evaluator. You may read this-cancerquest.org/patients/detection-and-diagnosis/ultrasound – Ojasvi Sep 5 at 7:56
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X-rays are ionizing radiation, so they introduce a small risk of harm such as cancer. The risk is negligible at the individual level, especially compared with the benefit of imaging when you're sick. Ultrasound is a form of sound, so it's not ionizing radiation. High intensities of ultrasound can be used to intentionally break up tissue (therapeutic ultrasound), but this doesn't occur with the intensities used in imaging.

X-rays give contrast based on the density and atomic number of the material, so they show bones clearly. They don't work well at showing the structure of soft tissues. To an x-ray, all soft tissue tends to look like an equivalent mass of water, which is just a weak absorber.

Ultrasound gives contrast based on changes in acoustic impedance, which is a different property of matter. Different types of soft tissue that would have almost the same x-ray absorption can have different enough impedances that they can be distinguished. Ultrasound is easily reflected and refracted by body structures, not just absorbed, so the images can look more distorted and messy.

Ultrasound has a resolution that's limited by its wavelength. It will diffract around structures comparable in size to the wavelength. X-rays are limited in resolution by other factors such as the finite width of the beam emitted by the x-ray tube. X-rays either get absorbed or travel straight.

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