“Seeing” Inside the Body

The Use of Ultrasound for Medical Diagnosis

1. How to “See” Inside the Body…

1. A common clinical situation concerns a woman in early pregnancy. She needs to have an accurate dating of the pregnancy so as to estimate reliably the birth date and to help assess proper growth during the pregnancy. Several length measurements can be made on the fetus to help determine its gestational age, including the crown-rump length, the bi-parietal diameter (ear to ear distance), and the femur length. What are some ways you can imagine making these length measurements in a minimally invasive way? Brainstorm to see what you come up with.

2. You have probably had a physical at the doctor's office. One examination method used in a routine physical is "percussing". The doctor listens to the echoes to identify different density regions in the body. Give it a try if you like: Stand up. Place two fingers of one hand on the side of your abdomen, preferably where there is a cavity. Tap these fingers with two fingers from your other hand. (Physicians use one finger from each hand, but they are better at this procedure than most of us.) Move your fingers to a higher position that is more filled. Is there a difference in what you hear?

3. Would percussing give us some or all of the information we want to have about the fetus? What might it give us and what might it not? What are its limitations?

2. Ultrasound as a Diagnostic Tool…

1. Ultrasounds (high frequency sound waves) have been an important diagnostic tool in obstetrics and other areas of medicine since the 1940's. Why might waves be useful for getting information about what is inside the human body, in a way that, say, matter is not?

2. Ultrasound waves are not “matter.” They are not “beads on a string.” Nevertheless, they carry information. What sorts of information can such waves carry? How do we know that they carry this information?

3. Ultrasound devices have two settings, a ‘signal-sending’ setting and a ‘signal receiving’ setting. Only one setting is on at a time. Why do you think that might be? Why not just send and receive signals continuously?

4. Ultrasound is useful because sound reflects upon encountering an interface, which is an abrupt change in the medium carrying the sound wave. This encounter produces an echo, just as a person standing in front of a brick wall would hear an echo when he or she claps. The echo is the result of sound reflecting from the wall. Suppose you had a sensitive enough clock that you could measure the time between the sending and receiving of a sound wave. How then might you use ultrasound waves to determine the distance of a fetus from the surface of the body?

3. All Body Parts Do Not “Sound” the Same…

1. Ultrasound is a little trickier than clapping in front of a brick wall, because the body is made of bone and tissue and fat, etc. Sound travels at different speeds in each of these materials. What physical properties of a medium do you think might be important in determining the speed at which sound travels through it? In other words, for the brick wall to form an interface, sound must travel at different speeds in air and brick… what is it about brick and air that are different and that therefore causes the sound to travel at different speeds in each?

2. As we have seen with waves on a string, the “springiness” of a material (its “tension”) is one property that goes toward determining the speed at which waves travel through it. We also saw last week that the mass density of the material is important. In fluids or soft tissue the "springiness" of the material is described by the bulk modulus B, defined as B = - ΔP / (ΔV/V),  where ΔP is a change in pressure and ΔV/V is the corresponding proportional change in volume. (Does a large bulk modulus correspond to more or less “springiness”?) The mass density in fluids or soft tissue is just the density ρ = m/V. Working by analogy from what we know about the speed of a wave on a string, what do you think the expression might be for the speed of sound in a fluid or soft medium, expressed in terms of B and ρ?

The following table shows some densities and sound speeds through different parts of the human body:

Table 1. from Diagnostic Ultrasound, Matthew Hussey, Blackie & Son Limited (London, 1975).

3. When a pregnant woman undergoes an ultrasound procedure, a gel is often applied to the woman’s abdomen prior to applying the ultrasound paddle device. Looking at the last row in the table above, why do you think that such a gel might be useful?

4. Putting It All Together…

1. Now that we have a sense of the relevant factors involved in performing an ultrasound, let’s put the pieces together. Describe how one might combine the information in the table above with the time elapsed between the sending and receiving of ultrasound signals to form a 2-dimensional image of a fetus. What other information, if any, might one want to have?

2. One could imagine creating a 3-dimensional image of a fetus using ultrasound technology as well. Rather than using a single paddle to apply the ultrasound waves, one could imagine forming a ring of paddles around the woman’s abdomen and sending signals from all directions at once, to construct a 3-dimensional image. Why do you think such a procedure is not normally done?

Ben Geller and Ben Dreyfus 8/12/16