  With their wide fields of view and often relatively long coverage of any
position in the sky in imaging survey mode, modern radio telescopes provide a
data stream that is naturally suited to searching for rare transients. However,
Radio Frequency Interference (RFI) can show up in the data stream in similar
ways to such transients, and thus the normal pre-treatment of filtering RFI
(flagging) may also remove astrophysical transients from the data stream before
imaging. In this paper we investigate how standard flagging affects the
detectability of such transients by examining the case of transient detection
in an observing mode used for Low Frequency Array (LOFAR; \citep{LOFAR})
surveys. We quantify the fluence range of transients that would be detected,
and the reduction of their SNR due to partial flagging. We find that transients
with a duration close to the integration sampling time, as well as bright
transients with durations on the order of tens of seconds, are completely
flagged. For longer transients on the order of several tens of seconds to
minutes, the flagging effects are not as severe, although part of the signal is
lost. For these transients, we present a modified flagging strategy which
mitigates the effect of flagging on transient signals. We also present a script
which uses the differences between the two strategies, and known differences
between transient RFI and astrophysical transients, to notify the observer when
a potential transient is in the data stream.
