We generally think of mechanical sector scanners as outdated technology. Yes; they’re still on the market and popular with people needing an ultrasound machine for pregnancy scanning on a limited budget, people who are new to scanning and unaware of the limitations of this technology, and a handful of loyal affectionados who simply want to stick with what they know. Compared to when I first began working with ultrasound ten years ago, though, they’ve very much fallen out of fashion.
Even their last remaining stronghold – farm use – is in sharp decline. Old school sheep scanning professionals with their trusty twenty year old machines are beginning to retire, and veterinarians who have scanned for decades have almost all updated their ultrasound machines at this point.
It came as a surprise to me, therefore, to read in the journal ‘Theriogenology’ that a mechanical sector rectal probe outperformed a linear array for ovum pickup in cows. Granted, the paper was published in 2004 and the resolution of electronic transducers is now far better, but even so, the fact that a mechanical probe allowed visualisation of a greater number of follicles under 5mm in size was surprising. How is this possible, when the spatial resolution of probes with electronic beamforming is invariably superior to fixed-focus mechanical probes?
Rather than having better resolution, it is likely that the mechanical sector probe is simply more practical for this application due to its wider field of view, allowing for better positioning and therefore visualisation of the ovary. Indeed, the authors report that previous studies found “a restricted view of some portions of the ovary using a linear array transducer.” Thus, the lower follicle count is likely not due to resolution, but to the ease of interrogation of the structure.
It is also worth noting that the researchers controlled for frequency, i.e. they set both transducers at 5MHz. However, one of the main strengths of electronic transducers is that they have a range of frequencies, and increasing the frequency on the linear array transducer may have improved performance.
What this study does highlight, however, is the fact that there is still a place for mechanical sector technology for niche applications, even if they’ve lost their place for mainstream uses like canine pregnancy scanning. Mechanical sector scanners like the MSU3 offer a tough, robust option for pig pregnancy scanning, for example – and also operate at just the right frequencies for this job.
Bols et al., 2004. A comparison of a mechanical sector and a linear array transducer for ultrasound-guided transvaginal oocyte retrieval (OPU) in the cow. Theriogenology 62(5):906-14.