IMPROVING LATERAL RESOLUTION

The VIS Facebook group is a wonderful resource for budding ultrasound technicians to share scans, information, and get help from others.

A common topic discussed there is image optimisation, particularly in tricky situations. With some pregnancy scans, it seems you can literally touch the skin with the probe and you have crystal clear pregnancy sacs jumping out at you. Other times – and often without any apparent reason or warning – scans can be extremely challenging, and no amount of angle changing or knob twiddling seems to make a difference.

It is in cases like these that understanding some basic ultrasound physics in order to optimise your image is so important.

Here’s an example of a difficult scan, taken by Jennie Lill on the TH80v (reproduced with permission):

Jennie has correctly located the bladder, and it looks as though she may have located a gestation sac beyond it, but we’re unable to see the area in sufficiently high resolution to be able to say for sure. This is because we lose lateral resolution with depth.

The reason for this is that lateral resolution is dependent upon beam width (as well as scan line density – discussed another time), and beam width diverges with depth. Imagine shining a red laser pointer onto a piece of paper a few centimetres in front of you. What will you see? You will see a small, sharp red dot. If you now drop the paper and let the beam hit the wall several metres away, how will your dot appear now? It will be larger and fuzzier.

Beam width behaves in a similar way, widening out the further it travels. Crucially, ultrasound machines are unable to resolve multiple points within the same beam as separate. If you're short-sighted, you may find this easier to understand: just think about two dots drawn very very closely together, held several metres away from you. If your shortsightedness is severe enough, most likely your eyes just see one big blob - you can't distinguish them as being two separate entities. So it is with a large beam width; the scanner simply knows it's getting a signal back, but it cannot distinguish individual objects falling within the same beam width as separate.

Your ultrasound machine also assumes that any point falling within a beam has originated from the centre of the beam only, and displays it as such on your monitor.

Hopefully you can now appreciate that closest to your transducer, where your beam width is narrow, three points side by side are likely to all fall within separate beams and be correctly displayed on your monitor as three distinct points. At depth, however, your beams may have spread out so much that these same three points, were they now 10cm deeper, might now all fall within one scan line! Your ultrasound scanner doesn’t realise that there are three points; it assumes that the signal it has received originated from one single point, located at the centre of the scan line, so what should be shown as three points is actually only shown to you as one.

This is one of the main reasons why lateral resolution deteriorates at depth.

So, what can we do about it?

The first important point to note is that higher frequencies result in narrower beam widths. This is one of the reasons why higher frequencies provide better image quality. It is therefore very important to use the highest frequency possible each time you scan – start high, and you can always adjust lower if you’re losing penetration.

Another very important factor is setting your focal point correctly. Some scanners (such as the VIS Scan Pad) automatically adjust the focus as the operator changes the depth (this can be overridden, however), but on the vast majority of machines, this is fully manual and you should always pay attention to where your focal point is set. By setting your focal point, you are greatly increasing lateral resolution at that depth. To learn more about how focal points work, click here.

If we go back to Jennie's image above, we can see that two focal points have been set. However, these are not at the level of the object we are wanting to see most clearly, and for this reason may actually be more of a hindrance than a help. This is due to a little-known fact that whilst setting a focal point improves lateral resolution at the depth you are focusing at, it actually decreases resolution past this point even more than if you had no focal point at all.

Finally, it is worth noting that resolution is highest at the centre of your imaging area. If you’re struggling to make something out, tilt or move your probe so that the object is in the very centre of the screen, rather than off to the side. In Jennie's example, it would have been particularly advantagous to adjust the positioning so that the area of interest was in the centre and below the bladder on the screen, as the bladder does not attenuate ultrasound waves as much as tissue and for this reason causes a phenomenon known as acoustic enhancement (notice how bright the signal is below the bladder on Jennie's image).

Scanning tips:

Jennie’s image has been used to demonstrate the above points, because she was faced with a very challenging scan. However, it’s important to practice optimising your image every single time. If you pop your probe on a bitch’s tummy and immediately see all of those beautiful sacs at the top of the screen, wonderful – by all means save some images. After this, however, instead of thinking “well, that was easy… what am I going to do for the next 15 minutes to give my customer value for money?”, pick an area of interest – probably one individual puppy - and try to optimise your image for that one area. Ask yourself:

• * How can I increase lateral resolution?
• – Can I increase my frequency without losing penetration?
• – Is this puppy in the centre of my imaging area?
• – Have I set my focal point correctly?

Once you've mastered these, also consider:

• * How can I increase temporal resolution (frame rate – important when looking at heartbeats)?
• – Is this puppy filling my imaging area? Click here to learn more about depth, sector width and temporal resolution.
• – Do I need multiple focal points, or can I achieve my goal with just one? Read more about focal points here.

By asking yourself these questions every time, and pushing yourself to optimise your image each and every time you scan (not just when it's difficult!), these things will become second nature, saving you time and stress the next time you are presented with a difficult case.