Bladder ultrasound - 76775 vs 51798

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Our nurse will typically perform this before a patient's office visit, and this would be the most likely dictation (in addition to remainder of OV):

BLADDER ULTRASOUND: Using a Toshiba Nemio XG unit and a 3.75 MHz probe, the bladder was scanned in transverse and longitudinal planes. Postvoid residual was 300 ml. There was no further bladder pathology identified.

Which is the appropriate code? I would assume the 51798, but what would make this a 76775?

Thank you!