Figure 1
source compared to the surrounding biological tissue. (ii,iii) However, due to the large difference in acoustic impedance between the gold and the tissue, most of the pressure waves will reflect back at the boundary and decay quickly as they superpose within the marker. Only waves with a frequency equal to the resonant frequency of the marker survives in the marker and spill outward into the surrounding tissue. This resonant frequency is 1.62 MHz for a marker with a diameter of 2 mm. (c) Schematic drawing of the SPIRE detection and range estimation. The SPIREs are observed by a hydrophone less than 100 μs after the beam delivery. Since the SPIRE amplitude is correlated with the residual beam range, Rres, at the marker and the marker position is constantly monitored by the fluoroscopy system in clinical cases, the beam range can be estimated if the relation between the SPIRE amplitude and Rres is known beforehand.
Proton beam range detection using SPIRE measurement. (a) Schematic representation of patient positioning during proton beam delivery using a gold fiducial marker. Spherical gold markers of 2 mm diameter are inserted in or near to the tumour and constantly monitored by X-ray fluoroscopy during beam delivery. If two sets of fluoroscopies are used, the marker’s three-dimensional coordinates can be obtained. (b) Schematic drawing of the SPIRE emission. (i) Upon the injection of the proton beam, the material properties of the gold act as a strong pressure
