Doppler ultrasound is a type of ultrasound that depends on the Doppler Effect, a change in the wave's frequency, which can occur from the motion of a reflector, such a red blood cell. We experience the Doppler Effect when, for example, an ambulance approaches us, passes and then drives away.
Definition of PLACODERM. : any of a class (Placodermi) of extinct chiefly Devonian fishes with an armor of bony plates and primitive jaw structures.ADVERTISEMENT. New Latin Placodermi, ultimately from GK plak-, plax + derma skin â more at derm-. any of a class (Placodermi) of extinct chiefly Devonian fishes with an armor of bony plates and primitive jaw structures. ADVERTISEMENT.
By Jennifer Kennedy. Definition: Placoid scales (dermal denticles) are tough scales that cover the skin of elasmobranchs (sharks and rays). Even though placoid scales are similar to the scales of bony fish, they are really modified teeth and are covered with a hard enamel.Placoid scales are packed tightly together and grow with their tips facing backwards.his dentine is covered by enamel-like vitrodentine. Scales in bony fish grow as the fish gets larger, but placoid scales stop growing after they reach a certain size, and then more scales are added as the fish grows.