How Ultrasound Works.

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How Ultrasound Works

by Craig C. Freudenrich, Ph.D.

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 pregnancy

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?

Ultrasound or 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.

2. The sound waves travel into your body and hit a boundary between tissues (e.g.

between fluid and soft tissue, soft tissue and bone).

3. Some of the sound waves get reflected back to the probe, while some travel on

further until they reach another boundary and get reflected.

4. The reflected waves are picked up by the probe and relayed to the machine.

5. 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

second).

6. 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 Nice

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 legs.

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

Photo courtesy Dynamic

Imaging Limited

Ultrasound machine with

various transducer

probes

A basic ultrasound machine has the following parts:

* 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
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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 data

Transducer Probe

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 ...

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