CHAPTER 1: Ultrasound

DEFINITION OF ULTRASOUND

• Sound frequencies above the audible range.

• Familiar example: A dog whistle.

TRANSDUCER FREQUENCY (PITCH)

• Units: MHz—million (mega) cycles (Hz) per second.

• Examples of typical clinical transducers:

  • - Low-frequency transducers are physically bigger (“woofers”) and emit around 2 MHz.

  • - High-frequency transducers are physically smaller (“tweeters”) and emit around 5 or 7.5 MHz.

• Frequency is picked by manually choosing the transducer and/or by setting the frequency electronically in the chosen transducer.

• The frequency choice affects image quality:

  • - Lower frequency penetrates deeper, but the images are more “coarse.”

  • - Higher frequency provides better resolution but less penetration.

  • - The settings of modern multifrequency transducers are electronically altered by the user for desired results: deeper penetration versus better resolution.

ULTRASOUND TRANSDUCERS

• Ultrasound transducers are used for echocardiographic imaging. Electricity is converted to ultrasound by a transducer and transmitted to the body.

• Reflected sound from cardiac tissue is converted back to electrical signals by the same transducer. The reflected signals are used to create a visual display of cardiac anatomy, and to perform Doppler calculations of cardiac physiology.

• Familiar transducer back-and-forth example using audible sound:

  • - Sound is converted to electricity by a microphone.

  • - Electricity is converted back to sound by sending it to a speaker.

• Ultrasound transducers use the piezoelectric effect: A ceramic material vibrates and produces ultrasound waves when an electrical current is passed through it.

TRANSDUCER MANIPULATION

• There are five basic transducer movements:

  • - Sliding, rocking, tilting, rotation, and compression.

  • - Transducer motion that is perpendicular to the visualized plane is called cross-plane motion.

  • - Rotating the transducer from 11 o’clock to 2 o’clock switches from long to short axis.

Source

• American Institute of Ultrasound in Medicine. Transducer manipulation for echocardiography. J Ultrasound Med. 2005; 24:733–736.

PHYSICAL PROPERTIES OF ULTRASOUND

• Sound is a mechanical wave.

• In order for sound to propagate, it must travel through a medium.

• Sound starts with a vibrating object that pushes and pulls adjacent molecules.

• Sound spreads by propagating these vibrations to still farther molecules.

• The movie Alien was promoted with the slogan “In space no one can hear you scream.” As opposed to light, sound cannot travel through the vacuum of outer space.

• The direction of sound waves is longitudinal. In other words, sound makes the medium vibrate back and forth in the direction that it travels. Think of the visible to-and-fro movements made by the membrane of a large sound speaker, and how it compresses and decompresses the air in front of it.

• Sound propagation results in regions of compression and regions of rarefaction (decompression) within the medium through which the sound is traveling.

• When sound travels through tissues, there can be mechanical and thermal effects on the tissue.