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There’s a lot of confusion about the correct imaging frequency to use for a particular scan and unfortunately there is no 100% fool proof answer. But there are general rules of thumb that you can follow. It’s important to understand that you will at some point during a scan have to make some compromises with regards to the image quality to enable you to have a better depth of penetration.
OK, here’s the basics:
Higher imaging frequency = better image quality at the expense of depth of penetration.
Lower imaging frequency = better depth of penetration at the expense of image quality.
I’m often asked why don’t I just use the highest frequency that my probe will allow me to use, as this will allow the best image quality? If you keep the probe at its highest frequency, you will get a very good superficial image and this will be fine if your study is superficial, but as soon as you start looking deeper into the body the image will drop off quickly when using a higher frequency. When you see the image fade the deeper it goes (and you know it’s not a ‘gain’ issue) you will need to drop or lower the frequency of the probe to allow the extra depth of penetration. And this is where the compromise is; you will also lower the image quality. But at least you will be able to image the deeper structures.
Fluids within the body can often allow you to use a higher imaging frequency as it helps transmit the soundwaves through the body. Conversely, tissues such as fat can attenuate and absorb the beam meaning you will need to use a lower frequency. It’s important to realise when to change the frequency to optimise your image.
Typical probe frequency ranges in veterinary scanning applications:
Linear Probe
Normally higher frequency probes with ranges of 7.0-14.0 MHz, although some will have slightly different ranges, great for superficial studies and give the best resolution.
Microconvex Probe
These ‘jack of all trades’ probes tend to have frequencies in the range of 4.0-9.0 MHz, great for general abdominal scans as well as some detailed superficial work. You will find that you will struggle with depth of penetration when scanning deeper abdomens, say over 12cm.
Convex Probe
larger footprint probe, normally lower frequency, 1.0-5.0 MHz and great for scanning when you require more depth of penetration. Don’t be put off by the large footprint of these probes, they are optimised for larger cases which means the probe size is less of an issue than you would expect.
Phased Array Probes
These are cardiac probes which are available in different frequency ranges to suit the scan being carried out. Typically, the probes available would have ranges 1.0-5.0MHz for very large breeds and some subcostal views, 3.0-8.0 MHz for mid sizes breeds and 8.0-12.0 MHz for smaller breeds, puppies and cats. Normally the higher frequency probes will be smaller footprint too.