Answers to some basic questions about ultrasound transducers.
Ultrasound “Transducer”? Or Ultrasound “Probe”?
Which term is correct: ultrasound transducer or ultrasound probe?
The truth is that both terms are correct, although the term “probe” has become less used in recent years, for reasons unknown (perhaps because it sounds more invasive?).
You may hear the terms “transducer” and “probe” used interchangeably to this day. At Sonosite, we tend to use the term “transducer”, although some of our customers prefer to say “probe.”
How Do Ultrasound Transducers Work?
Ultrasound transducers send ultrasonic sound waves into the body. The waves bounce off tissues in the body and travel back to the transducer.
A ceramic piezoelectric element inside the transducer receives the echoing sound waves and turns them into an electric current. This current is then read by the ultrasound machine’s computer, and translated into an onscreen image.
What is Piezoelectricity?
Ultrasound transducers work much in the same way that echolocation does in bats. The transducer emits ultrasonic waves; for medical ultrasound, the sound waves range from 1-20 MHz, which is much higher than humans can hear. The sound waves encounter varying tissues in the body, and the way that the waves bounce back, or don’t bounce back, depends on the echogenicity (see below) of the tissue that the waves hit.
A ceramic piezoelectric sensor (also called a piezoelectric element) inside the ultrasound transducer receives the sound waves that bounce back from the body, and turns them into an electric current. This current is then read by the ultrasound machine’s computer, and translated into an onscreen image.
What is Echogenicity?
Echogenicity refers to the tendency of a material to reflect ultrasound waves.
Echogenicity varies greatly by tissue characteristic (hard or soft, solid or hollow, etc.), which is great, because it’s the reason that ultrasound systems can differentiate the various types of internal structures (and anatomical abnormalities/pathologis) using sound.
A structure can be as hyperechoic (appears white on the screen), hypoechoic (appears gray on the screen), or anechoic (appears black on the screen).
- Bone absorbs almost no sound waves at all, so sound waves rebound very quickly—bone is consider hyperechoic, meaning, essentially “really echo-y"
- In contrast, fat absorbs sound waves and reflects back very few, so it is considered hypoechoic
- A urine-filled bladder qualifies as anechoic
What does the MHz refer to in an ultrasound transducer?
Megahertz (abbreviated at MHz) is the frequency of the ultrasound wave (basically, how many times the wave completes a cycle during a given amount of time).
The higher the number of the frequency, the shorter the wavelength (15 MHz is a higher frequency, and a shorter wavelength, than 5MHz). Shorter wavelenghts are absorbed faster, and can't penetrate as deeply.
The lower the frequency, the longer the wavelength; longer ultrasound wavelengths offer deeper penetration of tissue.
Medical grade ultrasound transducers usually have more than one operating frequency, so you will often see a range of MHz listed with a transducer.
If you are looking for a detailed scientific explanation of how ultrasound transducers work, Wiley Online Library features a highly information publication, Ultrasound Transducer Selection in Clinical Imaging Practice in full.