HPP Activity 55v1
Give me an A(-scan). Give a B(-scan). What does that spell?
Exploration
We are now in a position to understand how an ultrasound image is created. There are several
kinds of ultrasound imaging systems. The earliest device, which also forms the basis for later
systems, is called the A-scan. The "A" stands for amplitude.
Let's get an idea of how the A-scan works by considering an echo measuring experiment. You
may have done this experiment in a previous unit. The set-up is shown below.
Figure 1.
This shows an air-filled tube, closed at one end, a microphone, and an oscilloscope. We can
snap our fingers and record the snap and the echo, after the sound pulse travels the length of the
tube and back. Try the simulation out.
GE 1.
The figure below shows an example of the simulated oscilloscope display
from an experiment done at room temperature. The vertical axis shows the
amplitude of the signal and the horizontal axis shows the time (each square on
the display is 2 milliseconds wide).
Activity Guide
 2010 The Humanized Physics Project
Supported in part by NSF-CCLI Program under grants DUE #00-88712 and DUE #00-88780
HPP Activity 55v1
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1. What change could you make to this picture so that it can show the object
position encountered by the sound pulse?
2. How would the display change if a tube 2.4 m long (50% longer) were used
instead.
3. Suppose the tube had been filled with an additional barrier, at 0.80 [m]
from the end. If the barrier allowed transmission of sound, as well as
reflection, how might the oscilloscope output look when the echo experiment
is done?
Invention
The basic idea behind the A-scan is to use timing data on sound echoes to fix the distance of
objects from the ultrasound transducer. By assuming a constant speed of sound, we can calculate
the distance to an object by using the time between the initial pulse (the snap in this case) and the
echo. If t is the time between sound emission and detection of the echo, and v is the speed of
sound, then the distance, D, to the object is
D
v t
2
(1)
Equation (1) can be used to convert the amplitude versus time picture given on the oscilloscope
to an amplitude versus position picture. An A-scan instrument would do this calculation
automatically. The A-scan gives a one-dimensional picture of what is in front of the transducer.
It tells the observer where there are interfaces along a line drawn out from the transducer.
Activity Guide
 2010 The Humanized Physics Project
HPP Activity 55v1
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Application
GE 2.
Suppose you must design instrumentation for making an A-scan image of the
eye.
1.What would you assume the speed of sound is in the eye? Explain.
2.Will your assumption produce any errors in the A-scan image? Hint: Will
the speed of sound be the same in all tissues in the eye? If so, make an order
of magnitude estimate of the (systematic) error introduced. Show your work/
3. Obtain some anatomical information on the eye and estimate how much
time the A-scan instrument must accommodate (wait for an echo) to obtain an
image of the various interfaces inside the eye? Show your work.
Invention
Usually a physician would prefer at least a two-dimensional picture of a body's internal structure,
rather than the simple one-dimensional picture of interface locations along a line that the A-scan
gives. If the transducer that produces the A-scan can be moved, then two-dimensional
information can be constructed. A B-scan instrument can do this. The "B" stands for brightness.
The amplitude of an echo from the A-scan is converted into the brightness of a point on a CRT
display or possibly a computer terminal. The larger the amplitude of the echo, the brighter is the
spot displayed.
View the Flash simulation UltraSoundImagingwithoutObject2.swf. This simulation allows you
to compare the A-scan and B-scan screens. Note that the transducer and the data it generates is
exactly the same for both scans. The data is just presented in a more visual format by the B-scan.
By moving the transducer, a two-dimensional image of the interfaces can be constructed.
GE 3.
1. Why might it be easier to produce a 2D image more easily from a series of
B-scans than from a series of A-scans?
View the flash animation UltraSoundImagingwithObject2.html. This simulation shows how the
B-scan information can be built up into a 2D image by moving the transducer.
Activity Guide
 2010 The Humanized Physics Project
HPP Activity 55v1
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Instead of moving the transducer by hand, as you do in the simulation, a B-scan instrument
would either move (or simply rotate) the transducer using an automated motor, or would use an
array of transducers that are spatially separated.
Application
The B-scan image shows a two-dimensional picture of interfaces within the body. We would
expect that interfaces between the same two tissue types should lead to the same brightness on
the image display. Unfortunately, this is not the case and a correction must be applied to the
ultrasound instrument.
GE 4.
1. What happens to the intensity of sound waves as they travel through a
medium? Think about how well you can hear a normal conversation when you
are standing next to the people talking and when you are 10, 20, 30, or more
feet away. What will happen to the brightness of the echoes for interfaces that
are farther away from the transducer?
2. Suggest a way to correct the ultrasound image for this problem.
Ultrasound imaging devices, including the ultrasonic motion detector you may
have used in the past, correct for the attenuation of the echo amplitude. The
ultrasound transducer uses the piezoelectric effect to both generate the sound
pulse and to detect the echo. When an oscillating voltage is applied to a
piezoelectric crystal, the crystal expands and contracts at the same frequency
as the applied voltage producing a sound wave. Conversely, when a sound
wave strikes a piezoelectric crystal, the crystal expands and contracts because
of the pressure differences. The crystal generates a voltage with an amplitude
that is related to the amount of deformation caused by the sound wave and a
frequency that is the same as the sound wave. It is that voltage that is
amplified to produce the ultrasound image.
Activity Guide
 2010 The Humanized Physics Project