Ultrasound waves have different abilities to travel through different mediums.
This quality of mediums or tissues is called resistance or impedance.
Travel well though liquid: blood, urine.
Travel less well through solid organs: liver, spleen.
Not at all though bone (high resistance).
The more a substance reflect energy back towards the probe the brighter the image appears on screen. On the other hand the less it reflects, the darker the image.
If the probe is not at right angle to surface of interest scatter (deflection) of ultrasound waves occurs, especially when scanning aorta.
Appears black. No resistance therefore no echo and no energy.
Blood filled structures: heart or blood vessels.
Urine filled bladder.
Cystic structures: gallbladder.
In normal location fluid is contained and surrounded by echogenic membrane.
Free fluid  (internal haemorrhage or ascites) has no membrane and has irregular shapes.
Can function as "acoustic window", which allow deeper penetration into the body. Fluid filled bladder has the same function as well.
Bone reflects back almost 100% of the ultrasound waves. It appears as bright white.
No waves traverse beyond bone therefore no energy after that. Areas behind the bone appears black.
Bone can impede the view. Also it can be useful serving as landmark, i.e.spine.
Almost always bowel gas.
Subcutaneous emphysema and pathologic gas-forming process in structure can also be the case.
Appear as grey, "snow storm" appearance, which provide no information.
Gas or air between the probe and the body can cause problem. Apply generous amount of gel to create better acoustic contact.
Ultrasound: physics and knobology
Ultrasound: Signs
Ultrasound: Probe selection
Nerve Block: Median
Ultrasound: Probe orientation
Ultrasound: Tendons
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