Snell’s Law

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    Snell’s Law

    For some reason, most clinicians seem to grasp x-ray and CT scan imaging reasonably well. Denser structures are white, less dense are black, water dense structures are grey.

    Thus, when novice ultrasound users attempt to discern images created with sound, it can be confusing that bone and air both create bright white signal as well as shadow. The purpose of this brief post is to describe very subjectively how sound behaves as it crosses media of different densities. In the real world of physics this would be referred to as Snell’s Law (unless you want to give more credit to Ibn Sahl or Descartes).

    For a very concise and well-animated description of Snell’s Law please see Dr. Dan Russells’ excellent website. The basic premise is that  sound (like light) will bend depending on the density of the medium it is traveling in. The greater the change in density from one medium to another, the greater the bend. For our purposes, that also means the more scattering of ultrasound waves back towards the transducer and less acoustic energy propagating forwards.

    For practical purposes, we always start with liquid density in clinical sonography. That is because the transducer and acoustic gel are roughly water-dense, and so is the skin (bear with this oversimplification a moment).  Thus, we really only have three scenarios to think about. Going from liquid to air, liquid to liquid, and liquid to bone.

    As illustrated above, the great density differences from liquid to air or bone create lots of scatter (and therefore bright white signal on the screen), and leave little or no acoustic energy to travel deeper into the tissue (thus the distal shadowing). When liquid-dense structures are encountered, relatively little energy is lost (attenuated), and the beam continues to send signal deeper into the body. Thus, liquid structures such as liver, spleen, kidney, bladder make good acoustic windows. They allow lots of ultrasound energy to propagate into the body. Bone and air make poor windows, as it is difficult to see past them.

     

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