In the Beginning...
I wish to begin this article by wish all our readers out there in digital and print and wonderful and prosperous New Year and hopes that your holidays were joyous and peaceful. Now on to ultrasound, I want to go back to the basics to get new readers up to speed and as a refresher for our more seasoned visitors.
Ultrasound physics may not be the most glamorous subjects but without a clear understanding, you do not have the foundation all other ultrasound service knowledge is built upon.
So how do these miraculous devices work? Essentially, a system generates a high voltage pulse with a transmit board which then travels to the transducer where it vibrates (a) crystal(s). This transforms the electrical energy to mechanical (sound) which enters the body. These waves are reflected off various parts of the body and return to the transducer where the crystals are excited by the wave and the energy is transformed back into electrical. This signal is amplified by the receive section of the system and shades of gray are assigned to various echo intensities and displayed as images. There are many factors which contribute to image formation and quality but we will begin with the basics.
Let’s examine frequency and its relation to ultrasound image formation. When you increase the transmit frequency of a transducer, two things happen; image quality or resolution increases and penetration decreases. The opposite occurs when reducing the frequency of a probe. When I began ultrasound service in 1986, all transducers had a single frequency and to change, you had to enable a different probe. Today, probes have many frequencies to suit most body parts and types. The general rule is to use the highest frequency possible for the body part/type you are interrogating.
Other adjustments related to penetration are gain and transmit power. The FDA regulates how much mechanical energy or transmitted power a device can couple into the body and, like most radiological devices, you want to follow ALARA (As Low As Reasonably Achievable) when adjusting transmit power. Gain adjustments are made on the other side of the equation by raising or lowering the amplification of the received signal from the body. The cool thing is most systems automatically increase gain as transmit power is decreased. In fact, I have performed experiments on my abdomen by seeing how low I can go on the transmit side and still get a diagnostic image and for me the result was only 25% of maximum output (Your Mileage May Vary).
We have just touched on the basics here but we are happy to present a more comprehensive overview to you or your team on basics, ultrasound image formation / applications or anything ultrasound. For more information, please contact me at firstname.lastname@example.org. Happy New Year!