The following procedure was performed for sonicating each of the phantoms. First, the rod holding the transducer, the tissue phantom axis of symmetry and the z-axis of the positioning system were placed parallel to each other (Fig. 3(a)). The HIFU beam axis was thus parallel to the axis of symmetry of tissue phantom, while the TC wires were perpendicular to the beam axis. T1 and T2 TC wires were parallel to the y-axis while T3 and T4 wires were parallel to the x-axis of the positioning system (Fig. 3(b)). In order to find the T1 junction, the beam was moved inside the tissue phantom, using the 3D positioning system, until the maximum temperature rise for T1 was observed during a brief sonication period (10 s). After positioning the beam on the T1 junction (Fig. 3(b)), the transducer was activated in a continuous-wave mode at a time of 5 s for a period of 30 s. The temperature on the full array, i.e., T1, T2, T3, and T4, was recorded using an OMB-DAQ-56 (Omega Engg. Inc., Stamford, CT) data acquisition system over the 5 s presonication period, 30 s heating period, and 40 s cooling period (total time = 75 s). The temporal resolution of the temperature measurements was 0.5 s. A heating period of 30 s was chosen because it yielded a temperature just short of boiling when the highest power and highest mNPs concentration were used. Three transducer acoustic powers of 5.15 W, 9.17 W, and 14.26 W were used. These powers correspond to values at which the transducer was previously calibrated using a radiation-force balance. Three trials (n = 3) were performed for each power level. After each trial, the tissue phantoms were allowed to cool down to the ambient water temperature.