There are many advantages to imaging the body with ultrasound. Most importantly, there is no ionizing radiation as with X-rays, so that ultrasound is used extensively during pregnancy. ( More on the safety of ultrasound later in the unit.) Furthermore, soft tissues, such as the liver, spleen, kidneys and pancreas can be imaged directly without the injection of any sort of radio-opaque substances or isotopes to make them visible. In addition, the entire abdomen and pelvis can be rapidly scanned while the patient is lying on the table and photographs can be made of the area in question.
What are the drawbacks to ultrasound? Probably the most serious is the fact that sound is not able to travel through certain organs; their surfaces reflect almost 100% of soundwaves, so that the interiors of these organs and those lying directly beneath them cannot be imaged. Organs filled with air such as the lungs, stomach and intestines are opaque to sound as are hard tissues such as bone. (Acoustic impedance is explained in Beverly Stern’s unit.)
However, an organ like the liver is ideal for the sonographer to work with. It is a very large homogeneous soft organ that permits sound pulses to move through with only small amounts of reflection so that the spleen, pancreas or kidney which lie beneath the liver can be imaged as well. For this reason, the liver is called an “acoustic window”.
Formerly it was almost impossible to view the cervix and lower uterus because they lay under the air-filled intestines which reflected most of the sound. Then doctors discovered that they could create an ideal acoustic window if they had the patient drink 32 ounces of fluid one hour before an ultrasound exam. The distended bladder pushed the air-filled intestines out of the way and permitted sound to reach the reproductive organs in the lower pelvis.
Another barrier to sound is the air layer between the patient’s skin and the transducer. In order to overcome the reflections at this level, the sonographer liberally smears mineral oil (or another lubricant) on the patient’s skin before she begins the scanning. If this sounds like a messy exam consider the set-up of ultrasound machines in the 1950’s where the patient had to be immersed in a water bath:.
To understand how scanning is done and sonograms produced, we will follow the examination of a patient who is to have a liver scan. He lies on a table with his abdomen liberally covered with mineral oil. Next to him is a large console with a television screen at the top, and connected to this is a hinged arm with a transducer at the end. This transducer contains the crystal which both emits sound waves and then listens for the returning echoes. These echoes are converted into electric signals which are sent to the console and form a picture on the screen. See diagram below.
Ultrasound Equipment with Patient
(figure available in print form)
The sonographer asks the patient to take a deep breath and hold it. This brings the top of the liver out from under the ribs. Then the transducer is either moved in a straight line in a longitudinal or transverse direction, or else rocked back and forth in one place. Either procedure will produce a two-dimensional scan on the screen. The sonographer scans the rest of the liver and takes pictures of areas of interest.
According to Dr. Kenneth Taylor, at Yale-New Haven Hospital ultrasound technicians are trained for 12 months on the job. They must be able to read the scan immediately to decide whether whether the patient needs to have more pictures taken.
It may be difficult for high school students to understand the orientation of the sonograms that they are shown. First they have to realize that sonograms are two-dimensional sections cut through three-dimensional organs. The relationship of the organs in space will probably not make much sense to students unless they have either dissected frogs or studied three-dimensional models of the human body. In Lessons I and II (at the back of the unit) students are given exercises in visualizing how the internal organs would look if the human body were cut at different levels, both in longitudinal and transverse sections. Once they understand this they can be told that, conventionally, longitudinal sonograms are oriented with the patient’s head at the left and feet at the right, whereas transverse sonograms are viewed from the patient’s feet so that left and right sides are reversed. (See diagram above.)