Question

3. Describe how an ultrasound transducer works? (2 page limit)

Include a discussion of the energy form used to create images and how that interacts with tissues in the body

Answer 1

An ultrasound transducer is the handheld device that the technician or doctor moves on or over the body of the patient. A cord connects it to a computer. The device sends sound waves and receives the echoes as they bounce off the body tissue and organs of the patient. These echoes are used by the computer to create an image.

1. 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.

The basic principle of ultrasound

Ultrasound or sonography is based on the same basic principle used by bats. An ultrasound machine measures the echoes bouncing back to the transducer from the body of the patient to form an image. Bats ‘hear’ echoes and measure them to determine how far away the object is that caused the echo. They use what’s called ‘echolocation’ to fly around at night without bumping into anything.

In the transducer probe are piezoelectric crystals that change shape when an electrical current is applied to them. The vibrations or shape changes create sound waves that move outward. When they are directed at the human body, they pass right through the skin and into the internal anatomy.

As the waves encounter tissues with different characteristics and densities, they produce echoes that reflect back to the crystals. This happens more than a thousand times a second. The returning echoes are converted to electrical signals, and the computer uses them as points of brightness on the image, corresponding to the anatomic position and strength of the reflecting echoes.

A transducer contains a large array of crystals which allow it to make a series of image lines that together form a complete image frame called a sonogram. All the crystals are repeatedly activated many times in such a way that a complete image frame is formed around 20 times per second. This means that ‘real-time- motion is displayed in the ultrasound image.

As images are captured in real-time, they can show how the blood is moving through the vessels and how an internal organ is moving. This is why they are useful during pregnancy as they can be used to observe the structure and movement of the fetus. They are especially useful when it comes to seeing the interface between spaces that are solid and those that are filled with fluid. The field of view depends on the shape of the probe, and the frequency of the emitted sound waves determines the depth to which they penetrate.