Ultrasound Transducer Frequency: Adjusting the machine for finer detail
©Walter Rasmussen, R.D.C.S.
Ultrasound transducer frequency is perhaps the most important adjustment available on an ultrasound machine because it determines the over-all resolution power of the two-dimensional image. The higher the frequency, the more detail obtained from the image because the wavelength is shorter and it therefore produces reflections from smaller objects.
Because ultrasound interacts with more tissue at higher frequencies it is thus absorbed or reflected at a higher rate than at lower frequencies and the resulting image may look fainter, requiring an increase in overall gain. If the image is too faint and changes to dynamic range are not effective to compensate for its grayness, the sonographer should switch to the next lower frequency. As frequency is lowered, the ultrasound beam is able to penetrate deeper and the image usually looks brighter however, the resolution is noticeably decreased due to less interaction with the tissue.
Despite the impressive power of a lower frequency, it is important to begin every ultrasound exam at the highest available frequency in order to attempt to obtain the maximum amount of diagnostic information. Simply setting the machine at a low frequency in order to get strong ultrasound images is poor technique and while it saves time and effort, it significantly reduces the quality of the diagnostic information.
Some ultrasound machine manufacturers do not refer to frequency units on their control panels. Instead, they reference a change in the degree of penetration or degree of resolution. While operating these machines, it becomes apparent which controls are increasing or decreasing frequencies. The high penetration setting causes a loss in resolution, making the larger structures more visible but with less detail, while smaller structures loose detail and appear thicker. A sonographer should be constantly re-adjusting the frequency throughout the exam in order to get the best performance under the conditions present.
Here is a strategy for determining when to change frequencies:
- Start at the highest possible frequency.
- Focus the beam at the primary depth of interest (e.g. mitral valve or left ventricle).
- Adjust the overall gain (auto-gain if available, first, then manually adjust).
- Adjust the TGC slider pots, to make the near to far field appear evenly-lit.
- Adjust the Dynamic Range or Compression to control weak echoes.
- Use Higher frequencies for heart valves, suspected vegetations, thrombi or tumors.
If the image is still faint, with poor detail or excessive background noise after the above adjustments, the sonographer should move to a lower frequency and repeat the process.
Current ultrasound machines utilize, “tissue harmonics” imaging for increasing both penetration and resolution at various frequency settings. Harmonic imaging enables transducers to emit a spectrum of lower frequencies, thus achieving better penetration, while processing the higher frequency harmonics of the returning signal, resulting in an image that is of higher resolution than it would have been at the fundamental frequency.