Figure 4
Variation in SPIRE amplitude with the residual range at the marker position. (a) The SPIRE amplitude, defined as the amplitude at the peak frequency (1.53 MHz), as measured for different values of the residual range at the marker by stacking the acrylic plates in front of the water phantom. (b) For each residual range (Rres of 0.4, 2.4, 4.4, and 8.4 mm), the calculated energy distributions in and nearby the gold marker are shown in (b-1)–(b-4), respectively. As shown in (a), the measurement results agree well with the results of the simulation after the application of a systematic correction of 0.8 mm to the measured residual range. The peak frequency for the simulation was 1.62 MHz. A sharp increase in the SPIRE amplitude (11.7%/mm) was observed for 0 mm < Rres < 9 mm (the region shaded in blue), which reflects the strong variation in the size of the energy deposition in the marker around the Bragg peak (shown in (b)). Given the relatively small SD of the signal, the beam range can be estimated with submillimetre accuracy as long as this experimental setup is used.
