A plunger head for a medication injection device includes a first ultrasonic transducer coupled to emit ultrasonic signals, where the first ultrasonic transducer is oriented within the plunger head to send the ultrasonic signals along a length of a barrel of the medication injection device when the plunger head is disposed in the barrel. A second ultrasonic transducer is disposed within the plunger head and oriented to receive reflections of the ultrasonic signals. A controller is disposed in the plunger head and coupled to the first ultrasonic transducer and the second ultrasonic transducer. The controller includes logic that when executed by the controller causes the controller to perform operations including instructing the first ultrasonic transducer to emit the ultrasonic signals.