Phase shift droplets that can be vaporized in situ by acoustic stimulation offer a number of advantages over microbubbles as contrast agents due to their higher stability and smaller size distribution. The acoustic vaporization threshold (ADV) of droplets with perfluoropentane (PFP) core has been investigated extensively via optical and acoustical means. However, there are noticeable discrepancies among reported ADV thresholds between the two methods. In this study, we thoroughly discuss the criteria and the experimental methodology of determining the ADV threshold. In addition, we explain the possible reasons for the discrepancies between the optical and acoustical studies of the droplet vaporization. The ADV threshold was measured as a function of the excitation frequency by examining the scattered signal from PFP droplets (400-3000 nm). The threshold increases with frequency¾2 MPa at 2.25 MHz, 2.5 MPa at 5 MHz, and 3 MPa at 10 MHz. The scattered response from droplets was also compared with the scattered response from a microbubble at the corresponding excitation pressure and frequency. We found the ADV threshold to increase with frequency. The ADV threshold values determined here are in agreement with past values obtained using an optical technique.