Technical Terminology Practice Test

In the context of the passage, coincidence detection means...

A technical description of PET scanners explains how they reject random background events. The passage states that when annihilation occurs, two gamma photons are produced nearly 180° apart. PET scanners use coincidence detection, meaning the system records an event only when two detectors on opposite sides register photons within a very short time window. This timing requirement suggests the photons came from the same annihilation, not from unrelated emissions or electronic noise. By accepting only these paired detections, the scanner improves spatial localization and reduces false counts. The passage contrasts coincidence detection with single-photon counting, which would be more vulnerable to scatter and random events.

In the context of the passage, free radicals means...

A biology lecturer connects radiation physics to chemistry. When ionizing radiation passes through cells, it often interacts with water, the most abundant cellular molecule. The passage states that this interaction can split water into highly reactive fragments, including free radicals, which are species that possess an unpaired electron. Because electrons prefer to exist in pairs, an unpaired electron makes the molecule eager to steal an electron from nearby molecules, initiating chain reactions. The lecturer notes that free radicals can attack lipids in membranes and bases in DNA, creating lesions that cells must repair. Antioxidants are mentioned as molecules that can donate electrons to radicals without becoming dangerously reactive themselves, thereby “quenching” radical activity. The passage uses this context to explain why radiation damage is not only direct hits to DNA but also indirect chemical attack mediated by radicals.