As with many things, Risk-based PM is "the answer".
In my experience, PM on low priority (low risk) equipment rarely (if ever) gets completed* according to schedule (due to lack of time, man-power or other resources).
The trouble here though is that the probe obviously has "patient contact", so you might have to look again (and probably juggle) your risk criteria in order for it to be awarded a Low priority status.
What matters most with ultrasound scanning is "image quality". But "quality" of the image is subjective, and so (in my opinion) if the user is satisfied with the results being obtained, that should suffice in this case. To be honest, I can't see an absolute need for testing this device on a phantom.
If the probe is in doubt, then testing it with a tester of the FirstCall type will probably prove or disprove the probe (if you are lucky enough to have test kit like that).
*
If PM on Low priority items gets missed (slips) time after time, then a decent database system will (should) upgrade the priority (increase the "risk") to ensure it gets looked at during the next schedule cycle. Once PM'd, it can revert to Low priority.
