Gallstone lithotripsy: relevant physical principles and technical issues. A basic understanding of shock wave generation is essential for the radiologist who performs gallstone lithotripsy. Shock waves differ from ordinary acoustic waves in that they have a rapid rise time, a positive pressure component that gives rise to compressive forces approaching 1,000 atm, and a low-amplitude sustained negative pressure (rarefactive) component. Shock waves are created by means of three different types of shock wave generators: spark-gap, electromagnetic, and piezoelectric. The authors describe and compare these three types of shock wave generators with regard to equipment selection. Regardless of how shock waves are generated, they share common interactions with tissue. These interactions are reviewed along with the proposed mechanisms of stone fragmentation. 