Ultrasound

ULTRASOUND

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Originally an outgrowth of defense efforts and research during WWII, Ultrasound (or Sonography as it is also known) is one of the up and coming disciplines in the modern Radiology department. Since it uses sound waves to image the human body, it is the only modality other than MRI which does not depend on some form of ionizing radiation; yet at the same time the equipment is small enough to be made portable unlike the pieces of equipment that make up the standard MRI scanner. Indeed, Ultrasound may even move into the special procedure, or interventional radiology suite soon thanks to efforts to develop transducers small enough to be mounted on the end of an Angiography catheter.

Still, the biggest use of ultrasound is also the one most people think of when they hear the word, namely in pregnancy. However before I talk about that, it would be best if we first looked at how ultrasound works and a little bit of history in connection with it as well.

As I mentioned in the first paragraph, ultrasound is an outgrowth of Naval experiments that also gave us Sonar, hence it's alternative name of Diagnostic Medical Sonagraphy. The thing that makes ultrasound work is the fact that when sound waves are passed through the body, some of the sound waves are reflected back when they hit the surface between two layers of tissue while the rest passes on through until they hit the next surface where the same thing happens again. At the same time as this is going on, the different tissues will change the sound slightly so that the sound that is bounced back to the transducer (the piece of equipment that emits the sound waves and then picks it up again when it returns) is not exactly the same as the sound that was originally sent out. By feeding this information back into a relatively simple computer, the time elapse and changes in the sound or frequency can be used to give a "picture" or the body part through which the sound has traveled. This picture can take one of two forms.

The first form, or Display Mode is called an A-mode display, and looks kind of like the waves shown on an oscilloscope or heart monitor. The larger a reflection of sound waves at the surface between two layers of tissue is, the larger the spike shown on the monitor will be. I'm told that this can often be extremely useful when monitoring a patient's heart or the heart of a fetus since each time the heart beats, the blood that is pumped during that beat will cause just such spike to appear which can then be used to estimate the rate the heart is beating at.

The second form, and the most commonly used today is the B-mode display. In this method, the display from an A-mode tracing is basically turned 90 degrees towards the viewer resulting in a two dimensional, gray scale image that is much easier to read and understand than the more singular dimension display of an A-mode only devise. Also, so called Doppler studies, which are basically collorized ultrasound studies, normally of blood vessels, use this display mode as does the classic abdominal and pelvic ultrasounds used to check on unborn babies or Fetuses.

The final thing I think I should mention on this page is that, like it's naval "sister" technology of sonar, ultrasound requires fluids (water) in order to work. For this reason many of the earliest versions used a large tank of water in which the patient was seated or submerged as the examining chamber. Fortunately the human body is made up of somewhere between 60% and 80% water, depending upon who you listen to, so the use of a water tank was readily done away with. Still, for ultrasound exams of the abdomen and/or pelvis, it may be necessary for you, the patient, to have a full bladder in order to get good results (especially if a transabdominal study is the method to be used as opposed to a transvaginal or a transrectal). If the exam is a routine study that has been scheduled several days in advance, this problem is easy to take care of. You simply drink a large amount of water (1 to 2 quarts) the day of the exam and avoid going to the bathroom until after the test is over. However if you are an emergency patient in need of an ultrasound exam right now and a transabdominal study is the only method practical for one reason or another, then the doctor will have two choices. Either send you off with an empty bladder and except the loss of quality an empty bladder will cause, or find some way of filling up your bladder such as inserting a Foley catheter and using it to fill your bladder with sterile water. Personally, I've seen both methods used at various hospitals I've worked at, but I am not qualified to speculate on the various advantages and disadvantages of each method.

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