There are many situations in which ultrasound is performed.
Perhaps you are pregnant, and your obstetrician wants you to
have an ultrasound to check on the developing baby or
determine the due date. Maybe you are having problems with blood
circulation in a limb or your heart, and
your doctor has requested a Doppler ultrasound to look at the
blood flow. Ultrasound has been a popular medical imaging
technique for many years.
Photo courtesy Philips Research
Ultrasound examination during
In this edition of How Stuff
Works, we will look at how ultrasound works, what type
of ultrasound techniques are available and what each technique
can be used for.
What is Ultrasound?
ultrasonography is a medical imaging technique that
uses high frequency sound waves and their echoes. The
technique is similar to the echolocation used by bats, whales
and dolphins, as well as SONAR used by submarines.
In ultrasound, the following events happen:
- The ultrasound machine transmits high-frequency (1 to 5
megahertz) sound pulses into your body using a probe.
- The sound waves travel into your body and hit a boundary
between tissues (e.g. between fluid and soft tissue, soft
tissue and bone).
- Some of the sound waves get reflected
back to the probe, while some travel on further until they
reach another boundary and get reflected.
- The reflected waves are picked up by the probe and
relayed to the machine.
- The machine calculates the distance from the probe to
the tissue or organ (boundaries) using the speed of sound in
tissue (5,005 ft/s or1,540 m/s) and the time of the each
echo's return (usually on the order of millionths of a
- The machine displays the distances and intensities of
the echoes on the screen, forming a two dimensional image
like the one shown below.
Photo courtesy Karim and Nancy
Ultrasound image of a
growing fetus (approximately 12 weeks old) inside a
mother's uterus. This is a side view of the baby,
showing (right to left) the head, neck, torso and
In a typical ultrasound, millions of pulses and echoes are
sent and received each second. The probe can be moved along
the surface of the body and angled to obtain various views.
The Ultrasound Machine
basic ultrasound machine has the following parts:
Photo courtesy Dynamic Imaging
with various transducer
- transducer probe - probe that sends and receives
the sound waves
- central processing unit (CPU) - computer that
does all of the calculations and contains the electrical
power supplies for itself and the transducer probe
- transducer pulse controls - changes the
amplitude, frequency and duration of the pulses emitted from
the transducer probe
- display - displays the image from the ultrasound
data processed by the CPU
- keyboard/cursor - inputs data and takes
measurements from the display
- disk storage device (hard, floppy, CD) - stores
the acquired images
- printer - prints the image from the displayed
The transducer probe is the main part of the
ultrasound machine. The transducer probe makes the sound waves
and receives the echoes. It is, so to speak, the mouth and
ears of the ultrasound machine. The transducer probe generates
and receives sound waves using a principle called the
piezoelectric (pressure electricity) effect, which was
discovered by Pierre and Jacques Curie in 1880. In the probe,
there are one or more quartz
crystals called piezoelectric crystals. When an
electric current is applied to these crystals, they change
shape rapidly. The rapid shape changes, or vibrations, of the
crystals produce sound waves that travel outward. Conversely,
when sound or pressure waves hit the crystals, they emit
electrical currents. Therefore, the same crystals can be used
to send and receive sound waves. The probe also has a sound
absorbing substance to eliminate back reflections from the
probe itself, and an acoustic lens to help focus the emitted
Transducer probes come in many shapes and sizes, as shown
in the photo above. The shape of the probe determines its
field of view, and the frequency of emitted sound waves
determines how deep the sound waves penetrate and the
resolution of the image. Transducer probes may contain one or
more crystal elements; in multiple-element probes, each
crystal has its own circuit. Multiple-element probes have the
advantage that the ultrasounc beam can be "steered" by
changing the timing in which each element gets pulsed;
steering the beam is especially important for cardiac
ultrasound (see Basic
Principles of Ultrasound for details on transducers). In
addition to probes that can be moved across the surface of the
body, some probes are designed to be inserted through various
openings of the body (vagina, rectum, esophagus) so that they
can get closer to the organ being examined (uterus, prostate
gland, stomach); getting closer to the organ can allow for
more detailed views.
The parts of an ultrasound machine
Central Processing Unit
The CPU is the brain of the ultrasound
machine. The CPU is basically a computer that contains the microprocessor,
amplifiers and power supplies for the microprocessor and
transducer probe. The CPU sends electrical currents to the
transducer probe to emit sound waves, and also receives the
electrical pulses from the probes that were created from the
returning echoes. The CPU does all of the calculations
involved in processing the data. Once the raw data are
processed, the CPU forms the image on the monitor. The CPU can
also store the processed data and/or image on disk.
The transducer pulse controls allow the
operator, called the ultrasonographer, to set and
change the frequency and duration of the ultrasound pulses, as
well as the scan mode of the machine. The commands from the
operator are translated into changing electric currents that
are applied to the piezoelectric crystals in the transducer
The display is a
monitor that shows the processed data from the CPU.
Displays can be black-and-white or color, depending upon the
model of the ultrasound machine.
Ultrasound machines have a
and a cursor, such as a trackball, built in. These devices
allow the operator to add notes to and take measurements from
processed data and/ or images can be stored on disk. The disks
can be hard
disks, compact discs
(CDs) or digital video
discs (DVDs). Typically, a patient's ultrasound scans are
stored on a floppy disk and archived with the patient's
machines have thermal printers that can be used to capture a
hard copy of the image from the display.
Different Types of Ultrasound
that we have described so far presents a two dimensional
image, or "slice," of a three dimensional object (fetus,
organ). Two other types of ultrasound are currently in use,
3D ultrasound imaging and Doppler ultrasound.
3D Ultrasound Imaging
the past two years, ultrasound machines capable of
three-dimensional imaging have been developed. In these
machines, several two-dimensional images are acquired by
moving the probes across the body surface or rotating inserted
probes. The two-dimensional scans are then combined by
specialized computer software to form 3D images.
Photo courtesy Philips Research
3D ultrasound images
3D imaging allows you to get a better look at the organ
being examined and is best used for:
- Early detection of cancerous and benign tumors
- examining the prostate gland for early detection of
- looking for masses in the colon and rectum
- detecting breast lesions for possible biopsies
- Visualizing a fetus to assess its development,
especially for observing abnormal development of the face
- Visualizing blood flow in various organs or a fetus
Doppler ultrasound is based upon the Doppler
Effect. When the object reflecting the ultrasound
waves is moving, it changes the frequency of the echoes,
creating a higher frequency if it is moving toward the probe
and a lower frequency if it is moving away from the probe. How
much the frequency is changed depends upon how fast the object
is moving. Doppler ultrasound measures the change in frequency
of the echoes to calculate how fast an object is moving.
Doppler ultrasound has been used mostly to measure the rate of
flow through the heart and
Photo courtesy Philips Research
Doppler ultrasound used to measure blood flow
through the heart. The direction of blood flow is shown
in different colors on the screen.
Major Uses of Ultrasound
Ultrasound has been
used in a variety of clinical settings, including obstetrics
and gynecology, cardiology and cancer detection. The main
advantage of ultrasound is that certain structures can be
observed without using radiation.
Ultrasound can also be done much faster than X-rays or other
radiographic techniques. Here is a short list of some uses for
addition to these areas, there is a growing use for ultrasound
as a rapid imaging tool for diagnosis in emergency rooms.
- Obstetrics and Gynecology
- measuring the size of the fetus to determine the due
- determining the position of the fetus to see if it is
in the normal head down position or breech
- checking the position of the placenta to see if it is
improperly developing over the opening to the uterus
- seeing the number of fetuses in the uterus
- checking the sex of the baby (if the genital area can
be clearly seen)
- checking the fetus's growth rate by making many
measurements over time
- detecting ectopic pregnancy, the life-threatening
situation in which the baby is implanted in the mother's
Fallopian tubes instead of in the uterus
- determining whether there is an appropriate amount of
amniotic fluid cushioning the baby
- monitoring the baby during specialized procedures -
ultrasound has been helpful in seeing and avoiding the
baby during amniocentesis (sampling of the amniotic fluid
with a needle for genetic testing). Years ago, doctors use
to perform this procedure blindly; however, with
accompanying use of ultrasound, the risks of this
procedure have dropped dramatically.
- seeing tumors of the ovary and breast
- seeing the inside of the heart to identify abnormal
structures or functions
- measuring blood flow through the heart and major blood
- measuring blood flow through the kidney
- seeing kidney stones
- detecting prostate cancer early
Dangers of Ultrasound
There have been many concerns about the safety of
ultrasound. Because ultrasound is energy, the question becomes
"What is this energy doing to my tissues or my baby?" There
have been some reports of low birthweight babies being born to
mothers who had frequent ultrasound examinations during
pregnancy. The two major possibilities with ultrasound are as
However, there have been no
substantiated ill-effects of ultrasound documented in studies
in either humans or animals. This being said, ultrasound
should still be used only when necessary (i.e. better to be
- development of heat - tissues or water absorb the
ultrasound energy which increases their temperature locally
- formation of bubbles (cavitation) - when
dissolved gases come out of solution due to local heat
caused by ultrasound
An Ultrasound Examination
For an ultrasound
exam, you go into a room with a technician and the ultrasound
machine. The following happens:
- You remove your clothes (all of your clothes or only
those over the area of interest).
- The ultrasonographer drapes a cloth over any exposed
areas that are not needed for the exam.
- The ultrasonographer applies a mineral oil-based jelly
to your skin -- this jelly eliminates air between the probe
and your skin to help pass the sound waves into your body.
- The ultrasonographer covers the probe with a plastic
- He/she passes the probe over your skin to obtain the
required images. Depending upon the type of exam, the probe
may be inserted into you.
- You may be asked to change positions to get better looks
at the area of interest.
- After the images have been acquired and measurements
taken, the data is stored on disk. You may get a hard copy
of the images.
- You are given a towelette to clean up.
- You get dressed.
The Future of Ultrasound
As with other
computer technology, ultrasound machines will most likely get
faster and have more memory for storing data. Transducer
probes may get smaller, and more insertable probes will be
developed to get better images of internal organs. Most
likely, 3D ultrasound will be more highly developed and become
more popular. The entire ultrasound machine will probably get
smaller, perhaps even hand-held for use in the field (e.g.
paramedics, battlefield triage). One exciting new area of
research is the development of ultrasound
imaging combined with heads-up/virtual reality-type
displays that will allow a doctor to "see" inside you as
he/she is performing a minimally invasive or non-invasive
procedure such as amniocentesis or biopsy.
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