Functionality of the Acousto-Optic Delay Line Beyond the Cutoff Frequency

For processing signals in the time area an efficient tool is the acousto-optic delay line (AODL). The smoothly controlled delay of signals in a broad time interval permits to build high-performance radiolocation simulators. In the work, the design of the AODL has been considered, and the parameters, determining the limit of using its potential have been noted. The features of photoelastic interaction in AODL have been considered for the case when the duration of the input pulse is shorter than the time of crossing the optical beam by an elastic wave packet. It has been found that under these conditions the duration of the output response is determined by the time of crossing the optical beam by an elastic wave packet and does not depend on the duration of the input action. It has been shown that the AODL response to the input action in the form of a rectangular pulse is determined as the sum of three terms. In this case, the process of the entry of the elastic wave packet into the optical beam determines the first term, the second one – by the process of propagation of the elastic wave packet in the optical beam aperture, and the third – by the process of the exit of the elastic wave packet from the optical beam aperture. The corresponding equations have been obtained for calculating the pulse parameters at the AODL output. It has been shown that for a sufficiently short input pulse duration, the output signal parameters contain the information on the energy-geometric characteristics of the laser radiation. The results of numerical simulation have been tested experimentally on AODL layout with the direct detection. A comparative analysis of the results of theoretical and experimental studies have unambiguously has confirmed that AODL can also be used at frequencies above the cutoff frequency, both in terms of its main functional purpose and for solving a number of other engineering problems.

The Dosimetric Impact of Shifts in Patient Positioning during Boron Neutron Capture Therapy for Brain Tumors

Unlike conventional photon radiotherapy, sophisticated patient positioning tools are not available for boron neutron capture therapy (BNCT). Thus, BNCT remains vulnerable to setup errors and intra-fractional patient motion. The aim of this study was to estimate the impact of deviations in positioning on the dose administered by BNCT for brain tumors at the Tsing Hua open-pool reactor (THOR). For these studies, a simulated head model was generated based on computed tomography (CT) images of a patient with a brain tumor. A cylindrical brain tumor 3 cm in diameter and 5 cm in length was modeled at distances of 6.5 cm and 2.5 cm from the posterior scalp of this head model (T6.5 cm and T2.5 cm, respectively). Radiation doses associated with positioning errors were evaluated for each distance, including left and right shifts, superior and inferior shifts, shifts from the central axis of the beam aperture, and outward shifts from the surface of the beam aperture. Rotational and tilting effects were also evaluated. The dose prescription was 20 Gray-equivalent (Gy-Eq) to 80 % of the tumor. The treatment planning system, NCTPlan, was used to perform dose calculations. The average decreases in mean tumor dose for T6.5 cm for the 1 cm, 2 cm, and 3 cm lateral shifts composed by left, right, superior, and inferior sides, were approximately 1 %, 6 %, and 11 %, respectively, compared to the dose administered to the initial tumor position. The decreases in mean tumor dose for T6.5 cm were approximately 5 %, 11 %, and 15 % for the 1 cm, 2 cm, and 3 cm outward shifts, respectively. For a superficial tumor at T2.5cm, no significant decrease in average mean tumor dose was observed following lateral shifts of 1 cm. Rotational and tilting up to 15° did not result in significant difference to the tumor dose. Dose differences to the normal tissues as a result of the shifts in positioning were also minimal. Taken together, these data demonstrate that the mean dose administered to tumors at greater depths is potentially more vulnerable to deviations in positioning, and greater shift distances resulted in reduced mean tumor doses at the THOR. Moreover, these data provide an estimation of dose differences that are caused by setup error or intra-fractional motion during BNCT, and these may facilitate more accurate predictions of actual patient dose in future treatments.