What is 3D/4D ultrasound?

When diagnostic ultrasound was in its infancy, it was actually only a single, one dimensional line (A-mode). Later, it became possible to show changes across this line over time (M-mode), a modality which is still used in some applications (e.g. some cardiac applications) today.

The ability to move the ultrasound beam itself, either mechanically (as with the MSU) or later with the introduction of arrays of crystals (as with the KX5600v, Scan Pad or SonoScape A5v), allowed 2D images to be generated. The sweeping of this beam across the transducer face is what leads to that familiar ‘pie-shaped’ sector image we’re all so familiar with today.

With the advent of linear arrays, manufacturers quickly began to introduce another dimension. Imagine a row of 60 people, all standing in a line. These people represent the elements in a transducer. But you can also then line up three or four more people behind each person in the row. Now, you have a set of elements which is 60 elements across and 4 elements deep. The advantage of this is that it allows a degree of focusing to occur in the ‘elevation’ plane, helping to keep the slices we are taking in our images as thin as possible. This helps to reduce imaging artefacts (particularly slice thickness artefacts).

Most linear and convex probes are indeed multi-row element arrays. However, if the number of elements deep matches the number of elements across (i.e. you have 59 other people standing behind that first row of 60 people), it is possible to obtain a 3D volume.

 

Why is this technology still so expensive?

The most valuable part of any scanner is the ultrasound transducer. The average price of an ultrasound probe is £1000 for an entry-level machine, up to around £3500 for machines like the SonoScape S9v. However, this would typically be for a transducer which is, say, 60 elements across, and 4 deep (240 elements in total).

Now go back to our 3D probes, which might be 60 elements across by 60 elements deep. That’s a total of 3600 elements in a single transducer – 15x that of the 2D example above. It is not surprising, therefore, that 3D technology is exponentially more expensive than 2D.

 

Is there anyone performing 3D/4D veterinary ultrasound?

3D ultrasound refers to the acquisition of a full 3D volume; 4D refers to the added dimension of time. The implementation of this technology in the veterinary world is currently only in its academic and experimental stages. A paper by Hildebrandt et al. (2009) looked at 3D pregnancy monitoring in dogs (five Labradors, two Chihuahuas, one Collie) and cats (Bengals). They concluded that this technology could provide valuable information in the future, as improvements in technology and pricing makes it practical to implement.

The biggest limitation (beside price) in 3D/4D imaging is that it requires relatively long acquisition times (remember ultrasound is generated by the sending and receiving of ultrasound pulses. The more pulses you send, the longer you need to wait for one full acquisition to be performed). This is why 3D veterinary echocardiography is practically unheard of at the present time (even human 3D echo requires exceptionally good imaging windows and breath-holding from the patient).

In 3D pregnancy scanning, good images depend upon the mother keeping still for 3-4 seconds. Whilst this is unproblematic in human pregnancy scanning, it can be a challenge in the veterinary world.

Potentials of 3D scanning in animal pregnancy scanning include:

- Foetal sexing

- Easier checking for foetal viability

- Easier number counting in smaller litters

- Learning more about the behaviour of the unborn animal

 

In summary, it may be possible to obtain high-quality 3D and even 4D datasets in pregnant domestic animals at the present time, but current limitations of the technology (i.e. the fact that it would not be practical to use on the majority of patients/clients) and its high price tag make it unfeasible in the veterinary ultrasound market at the present time.

Of note, the canine and feline reabsorption rate in the Hildebrandt et al. study were 10%, all occurring before 30 days (noted by 26 days). This is why it is so important not to offer a pregnancy confirmation scanning service for bitches or queens earlier than the 30 day mark.