Measurement of the temporal latency of a respiratory gating system using two distinct approaches.

PURPOSE: To develop a methodology that can be used to measure the temporal latency of a respiratory gating system. METHODS: The gating system was composed of an automatic gating interface (Response) and an in-house respiratory motion monitoring system featuring an optically tracked surface marker. Two approaches were used to measure gating latencies. A modular approach involved measuring separately the latency of the gating system's complementary metal-oxide-semiconductor tracking camera, tracking software, and a gating latency of the LINAC. Additionally, an end-to-end approach was used to measure the total latency of the gating system. End-to-end latencies were measured using the displacement of a radiographic target moving at known velocities during the gating process. RESULTS: Summing together the latencies of each of the modular components investigated yielded a total beam-on latency of 1.55 s and a total beam-off latency of 0.49 s. End-to-end beam-on and beam-off latency was found to be 1.49 and 0.34 s, respectively. In each case, no statistically significant differences were found between the end-to-end latency of the gating system and the summation of the individually measured components. CONCLUSION: Two distinct approaches to quantify gating latencies were presented. Measuring the end-to-end latency of the gating system provided an independent means of validating the modular approach. It is expected that the beam-on latencies reported in this work could be reduced by altering the control system configuration of the LINAC. The modular approach can be used to decouple the individual latencies of the gating system, but future improvements in the temporal resolution of the service graphing feature are needed to reduce the uncertainty of LINAC-related gating latencies measured using this approach. Both approaches are generalizable and can be used together when designing a quality assurance program for a respiratory gating system.

Fast negative breakdown in thunderstorms.

Thunderstorms are natural laboratories for studying electrical discharges in air, where the vast temporal, spatial, and energy scales available can spawn surprising phenomena that reveal deficiencies in our understanding of dielectric breakdown. Recent discoveries, such as sprites, jets, terrestrial gamma ray flashes, and fast positive breakdown, highlight the diversity of complex phenomena that thunderstorms can produce, and point to the possibility for electrical breakdown/discharge mechanisms beyond dielectric breakdown theory based mainly on laboratory experiments. Here we present one such confounding discovery, termed fast negative breakdown, that does not fit with our current understanding of dielectric breakdown. Our adaptation of radio astronomy imaging techniques to study extremely transient lightning-associated events confirms that electrical breakdown in thunderstorms can begin with oppositely-directed fast breakdown of negative polarity, similar and in addition to fast positive breakdown expected from conventional dielectric theory and recent observations. The discovery of fast negative breakdown calls for an addendum to the physical description of electrical discharges in air.

Observations of narrow bipolar events reveal how lightning is initiated in thunderstorms.

A long-standing but fundamental question in lightning studies concerns how lightning is initiated inside storms, given the absence of physical conductors. The issue has revolved around the question of whether the discharges are initiated solely by conventional dielectric breakdown or involve relativistic runaway electron processes. Here we report observations of a relatively unknown type of discharge, called fast positive breakdown, that is the cause of high-power discharges known as narrow bipolar events. The breakdown is found to have a wide range of strengths and is the initiating event of numerous lightning discharges. It appears to be purely dielectric in nature and to consist of a system of positive streamers in a locally intense electric field region. It initiates negative breakdown at the starting location of the streamers, which leads to the ensuing flash. The observations show that many or possibly all lightning flashes are initiated by fast positive breakdown.