This brief reviews high-voltage pulsers for ultrasound imaging systems with a focus on energy efficiency. Most ultrasound imaging systems have been developed with the aim of achieving a high signal-to-noise ratio for good image quality. As miniaturization and portability of the systems are pursued recently, high energy efficiency is of great need. In the ultrasonic imaging systems, the most power-hungry block is typically the pulser that drives the ultrasound transducer at a high voltage. To better understand the pulser's operation and power consumption, an equivalent circuit model of the transducer is introduced. Based on the model, we investigate the fundamentals of pulser driving methods and review the conventional class-D pulser as well as state-of-the-art pulsers with dynamic power loss reduction techniques. Their operation principles, strengths, and limitations are analyzed and discussed in depth.