Ultrasound, well it's a large part of this web-site so this page should deal mostly with general topics related to it. I received an e-mail through this site from someone asking why ultrasound shows in blacks and whites and shades of gray and whether there was a "contrast" equivalent usable with ultrasound like the injectable types of contrast used in x-ray. I was forced to organize my thoughts for the reply (which I'm not sure got through to the guy who wrote me the e-mail).
Anyway, I thought I'd post it here in order to; let newcomers have a simplified view of the principles of ultrasound, and to let those more knowledgeable in the ways of ultrasound see the extent of my ignorance.
Firstly, I'm not an expert on ultrasound principle or theory, there are plenty of good explanations of ultrasound to be found on the internet or in the library.
Having said this, I will try to tell you what I know. Ultrasound is high frequency sound generated in specific frequency ranges and sent through tissues. Penetration into tissue is based in large part on the range of the frequency produced. Lower frequencies (e.g- 2Mhz) penetrate deeper than high freq. (e.g.- 10mHz). As the sound passes through tissues it is either absorbed, reflected or allowed to pass through, depending on the density ("echo"-density) of the tissue. All ultrasound dissipates in tissue producing heat. The "listening" part of the probe (a piezo-electric crystal just like the generating part of the probe) "listens" for reflections (echos) of the sound waves sent out and passes the information to the processing unit. Time between sending and receiving equals distance.
The amount of energy reflected (not absorbed or propagated) equals density. Substances containing a lot of water (cerebro-spinal fluid, blood) are very good conductors of sound and reflect very little, they are called echo-lucent. Since they reflect very little of the sound they appear as dark areas. Substances which contain little water (or made up of material that otherwise is a poor sound conductor) such as bone or a worse conductor, (e.g.- air), reflect almost all the energy and appear very bright. Substances which conduct sound to a degree in between these 2 extremes, appear darker to lighter depending on the amount of energy they reflect. Most useful reflections (echos) occur at boundaries between tissues of differing densities. Mostly we are seeing outlines of things which have different reflecting properties in ultrasound.
Remember also that just like regular sound and light can reflect off of objects in their path, they don't necessarily reflect at an angle which returns to their source. (this is the principle behind much of armed forces "stealth" designs which reflect very little of the incident radar (light) energy back of the source). So tissue surfaces which may be echo-reflective but lie at in a non-perpendicular plane to the "listening" probe will have little if any energy returning from them and therefore won't show up on ultrasound.
Obviously, there are many other issues involved in producing an image but I can sum it up in this way, the implementation of the theory and physics of ultrasound to produce a meaningful, near-real-time, image of the inside of a living object is my idea of a miracle of human engineering and the neatest thing since sliced bread. This is mostly due to a part I glossed neatly over with the words "processing unit" in the second paragraph. The information contained in the "return signal", the echo, is heavily (and cleverly) processed to reduce the amount of artifact and produce a picture which corresponds to what we expect to see.
Contrast in Ultrasound
Contrast in ultrasound terms can be kind of a problem in some applications. The greatest "contrast" will be introducing air into the scene. However, while injecting air into tissue (muscle, connective tissue, subcutaneous tissue, etc.) will definitely show you where you are, air will continue to reflect energy until it is absorbed which can take some time. During that time you won't be able to see past the "contrast".
If you are doing vascular ultrasound, specifically cardiac preferably on the right side of the heart, cold IV solution is briskly agitated with a small amount of air and injected in the venous side of the heart. The small air bubbles come out solution in the warmer blood and appear as bright specks in the blood on ultrasound. These bubbles are carried to the vascular system of the lungs and are harmlessly diffused in to the air passages and blown off through respiration. Aerated solution could possibly be used in tissues I suppose, might have to try that. Mostly for the effect of contrast, to find your needle position, you are left mostly with moving the needle slightly and watching on ultrasound for the tissue to move, or to inject a small amount of saline solution to see the change in tissue density resulting from the introduction of an echo-lucent substance to the field of vision (so you would see tissue deformation from the solution bubble moving things and the bubble of solution itself should appear a bit darker).