A tri-frequency probe emits three different frequencies. Probes with tri-frequency capabilities allow a wide range of scanning applications from a single probe.

See also Multi-Frequency Probe and Dual Frequency Phased Array Transducer.

Usually, multiple probes are used because most transducers are only able to emit one frequency because the piezoelectric ceramic or crystals within it have a certain inherent frequency.
Multi-frequency probes have multiple crystals with different frequencies and the desired specific frequency can be selected. Advanced probes can emit sound waves at different frequencies for the near and far fields. The disadvantage is that multi-frequency (multifrequency) probes have slower frame rates and therefore they are only useful for imaging of static structures.

See also Dual Frequency Phased Array Transducer and Tri-Frequency Probe.

A rectal probe is a small ultrasound transducer with high frequency that is inserted directly into the rectum. Transrectal ultrasound is a well established examination for rectal or prostate carcinoma assessment. Rectal probes (e.g., a microconvex probe) generate a much sharper image, caused by the close proximity to the prostate or rectum, and the higher sensitivity.

A vaginal probe is a small ultrasound transducer (e.g. a biplane probe) that is inserted directly into the vagina. A sterile cover is slipped over the handheld transducer (also handle or probe), which is then covered with lubricating gel and placed in the vagina.
This technique produces a sharp image, not only because of the close proximity to the uterus, but also because this transducer is more sensitive.