If you’ve attended one of our ultrasound courses, you’ll know how important it is to set a focal point – or multiple focal points – at the depth you are looking at. Wave fronts propagate out from your probe and act like the beam of a torch (that’s a ‘flashlight’ to our increasing number of American readers!), spreading out with increasing depth. This means that the further the ultrasound beam travels through the tissue, the poorer your image resolution will be, because instead of that nice thin beam you started with, your beam has spread out too widely to be able to detect objects which fall within the beam as separate. How often have you scanned a bitch and noticed that the puppies at the bottom of your screen are fuzzy and difficult to make out, but the ones at the top are sharp and crystal clear?

What setting a focal point is actually doing is telling your probe to send out ultrasound waves earlier from the areas of your probe furthest from your target (usually the sides), and delay sending out the ultrasound waves from the points closest to your target (Thrush, 2014).

If you’re finding this difficult to visualise, imagine a line of archers in a field, with a target in front of them at the other end of the field. If you drew a line from the target to each archer, you’d form a triangle shape. The shortest line would be the one joining the target with the archer in the middle, directly opposite the target. The longest lines would be the two long sides of the triangle.

So, if all the archers were to all fire their arrows simultaneously, the archers straight in front of the target would hit the target first. The arrows fired by archers on the far edges would have further to travel, so would hit later (albeit only a second or a fraction of a second).

What you would hear would be barrage of thumps as each arrow hit at a slightly different time.

However, if it were possible to work out the added time the arrows of those on the edges would take to reach the target, and to tell the archers on the edges to fire their arrows this fraction of a second before the ones standing closer to the target, you could ensure that all of the arrows hit the target at precisely the same time. What you would hear would be one very loud thump, as twenty arrows simultaneously hit into the target.

This is precisely what your ultrasound probe does when you set a focal point: it times all of the pulses to reach the target together, amplifying your signal in this area.

When you set multiple focal points, however, the scanner has to send out more than one set of pulses (or barrage of arrows). It has to send out one set for the furthest away focal point (target), and another for the closer one(s). Clearly, the machine can’t send both barrages at the same time, or when the returning echoes come back, it won’t know from which target they’ve returned. This is why increasing the number of focal points slows down your frame rate; your scanner now has to send out multiple distinct signals, and wait for all of these signals to return back, before it can send out its next set of signals.

Incidentally, this same principle applies to depth settings: the greater the go and return time for each pulse (i.e. the more you increase your depth range), the slower your frame rate will be (Hoskins et al, 2010). For this reason, you should always keep your imaging depth as shallow as is necessary if you’re after higher frame rates – that means, if the gestation sacs are all at the top of your screen, don’t have a big long sector stretching out below them. Bring your depth up so that your puppies are pretty much filling your viewing area.

Of course, a fast frame rate is not always necessary in abdominal work, but it can be important if you feel your scanner is ‘lagging’ or you want to image a fast-moving structure like the foetal heart.

 

References

Hoskins, P., Martin, K., Thrush, A. (2010). Diagnostic Ultrasound: Physics and Equipment.

Thrush, Abigail (2014). MSc Medical Ultrasound (Echocardiography). Accessed 23/10/14.